Modulation of chemical carcinogenesis by xenobiotics

FUNDAMENTAL

AND

APPLIED

Modulation

TOXICOLOGY

4,

325-344 (1984)

of Chemical Carcinogenesis GARY

by Xenobiotics’

M. WILLIAMS

American Health Foundation, Naylor Dana Institute for Disease Prevention, Dana Road, Valhalla, New York IO595

Modulation of Chemical Carcinogenesis by Xenobiotics. WILLIAMS, G. M. (1984). Fundam. Appl. Toxicol. 4, 325-344. Xenobiotics enhance and inhibit chemical carcinogenesis by a variety of mechanisms through effects at different steps in the overall process, including modification of carcinogen availability, biotransformation, reactive interactions, expression of celhtlar alteration, and neoplastic development. Importantly, the same agent can be both an enhancer or an inhibitor depending upon the circumstance of its interaction with the carcinogen.

Induction of cancer by chemicals in experimental animals is a complex process comprised of several distinct steps which may be divided into two mechanistically separate sequences-neoplastic conversion of the cell and neoplastic development (Fig. 1). Neoplastic conversion is the process in which a normal cell is transformed into a neoplastic cell and thus corresponds to initiation as defined by Berenblum (1974). This process, however, is not identical to initiation as used by other investigators; for example, Foulds (1969) preferred the term to designate “incipient neoplasia” in which the tissue acquired a new capacity for neoplastic development, which might or might not entail the generation of neoplastic cells. Others have specifically restricted initiation to events, which are short of the emergence of neoplastic cells, such as the development of hyperplasia (Farber, 1979). Therefore, for precision in the present discussion, the process of the alteration of a normal cell into a fully neoplastic cell will be designated as neoplastic conversion. This sequence may proceed through stages, of which initiation as used by some would be one. The second sequence in carcinogenesis,

neoplastic development, is the process of growth of the neoplastic cell into a tumor and the further acquisition of abnormal properties by that tumor. Promotion was defined by Berenblum ( 1974) as the facilitation of growth of dormant neoplastic cells into tumors and, thus, neoplastic development includes promotion but is more comprehensive than that process. Chemicals operate in a variety of ways in both of these sequences and facilitation of either sequence by a chemical can increase the occurrence of neoplasms. However, in the case of an agent that acts exclusively in facilitating neoplastic development, in order for it to increase cancer, preexisting neoplastic cells would have to be present. This situation apparently obtains in strains of rodents that develop spontaneous neoplasms (Williams, 1980a). Consequently, chemicals that are carcinogenic in animals by the broad definition of increasing the occurrence of neoplasms are NEOPLASTIC CONVERSION CHEMICAL CARCINOGEN YETABOLIC ICTIVATION 1 ULTIMATE CARCINOGEN + DNA 1 ALTERED RECEPTOR EXPRESSION LJ

’ Presented at the 22nd Annual Meeting of the Society of Toxicology, March 10, 1983, Las Vegas, Nev. as part of a symposium entitled “Biological Effects of Chemical Interactions.”

IIEOPLASTIC DEVELOPMENT NEOPLASTIC CELL GROWTH PROMOTION I OIFFERENTIATED NEOPLASM PROGRESSION 1 UNDIFFERENTIATED CANCER

FIG. 1. Events in chemical carcinogenesis. 325

0272-0590/84 $3.00 Copyright Q 1984 by the Society of Toxicology. All rights of reproduction in any form reserved.

326

GARY M. WILLIAMS

an extremely diverse group of agents. In an effort at mechanistic distinction, they have been categorized into two basic types, genotoxic carcinogens that damage DNA through direct chemical interaction and epigenetic carcinogenic agents that do not interact with DNA but produce some other biological effect that underlies their carcinogenicity (Williams, 1980b; Weisburger and Williams, 1980; Stott and Watanabe, 1982). This distinction is fundamental to the mechanistic understanding of modifying effects which will be discussed. Genotoxic carcinogens by definition react with DNA and presumably thereby induce neoplastic conversion of the cell. For some genotoxic carcinogens a single exposure is sufficient to ultimately result in cancer, indicating that neoplastic conversion was readily produced. Others require prolonged administration which may be necessary for the production of neoplastic conversion or may be required to facilitate neoplastic development in some way, perhaps through an additional promoting action. Nongenotoxic agents produce neoplasms through a variety of mechanisms; they may enhance the process of neoplastic conversion by indirectly producing genetic alterations (Williams, 1983); or they may op erate through epigenetic effects in the second sequence of events to increase neoplastic development of preexisting neoplastic cells. As a consequence of enhancement of neoplastic development, certain epigenetic carcinogenic agents can augment the effects of genotoxic carcinogens and, hence, they have also been described as carcinogenesis enhancers (ClayTABLE I MODIFYING

FACTORS

IN CHEMICAL

CARCIN~GENESIS

Intrinsic Genetics Sex and endocrine status Age Extrinsic Mode and regimen of exposure Housing Nutrition Other chemicals

TABLE 2 MECHANISMS

OF MODIFICATION CHEMICAL

OF EFFECTS

OF

CARCINOGENS

(A) Modification of effects of carcinogens I. Alteration of bioavailability 2. Alteration of biotransformation 3. Depletion of active molecular species 4. Alteration of excretion (B) Modification of response of test subject 1. Alteration of susceptibility to biological effect(s) of test agent 2. Imposition of biological effects that ameliorate or exacerbate those of the test agent

son, 198 1). Thus, an important group of agents that modify the effects of genotoxic carcinogens is comprised of chemicals that themselves are capable of increasing the occurrence of cancer under specific conditions in the absence of any other intentional carcinogen administration. The carcinogenic process can be modified by various intrinsic and extrinsic factors (Table 1) impinging at different steps in the two sequences (Weisburger and Williams, 1982a). Among these factors, xenobiotics, both naturally occurring and synthetic, modify the carcinogenic effects of genotoxic agents through a variety of mechanisms (Table 2), which will be discussed in detail. Importantly, the same agent can exert several effects and consequently a given chemical may through one action be an inhibitor at one step and through another action be an enhancer at a different step. Moreover, the concentrations or amounts available in specific tissues can likewise determine what kind of effects occur. Additionally, a chemical can be an inhibitor of the carcinogenicity of one agent, but an enhancer of another. Thus, the influences of a xenobiotic on the genesis of cancer are complex and vary with the specific circumstances of its interaction with a carcinogen. ALTERATION OF CARCINOGEN AVAILABILITY , An agent that modifies the absorption from the point of entry in animals or uptake into

CHEMICAL

CARCINOGENESIS

cells of a chemical carcinogen can thereby modulate carcinogenicity. A possible example of an effect on uptake is the study by Hornburger and Tregier (1969) of subcutaneous carcinogenesis in mice by benzo[rstlpentaphene in different vehicles. The latent period for neoplasm development by the same dose was 16 weeks in peanut oil, 37 weeks in lipoprotein, and 62 weeks in Ringer’s solution. Similarly, in a’study of gastric carcinogenesis by iV-methyl-N’nitro-N-nitrosoguanidine, Hirono and Shibuya (1972) used either saline or olive oil as the vehicle for intragastric instillation of the carcinogen. The incidence of gastric tumors with olive oil was 70% and with saline was 36%. The mechanism(s) by which these vehicle effects are produced has not been determined, but modification of carcinogen availability is a likely possibility. Cocarcinogenesis Originally, cocarcinogenesis was broadly defined as the augmentation of neoplasm induction brought about by noncarcinogenic factors which operate in conjunction with a carcinogen, designated as the initiating agent (Berenblum, 1974; Sivak, 1979). Cocarcinogenesis thus comprised several kinds of enhancement including promotion in which the enhancing agent facilitates tumor development after completion of initiation (see below). Commonly, however, cocarcinogenesis has been distinguished operationally from promotion as the enhancement of carcinogenicity resulting from application of a modifier either just before or together with a carcinogen, while promotion has referred to enhancement produced by an agent given after a carcinogen. This distinction is useful to allow a conceptual differentiation between the enhancement of the sequence of neoplastic conversion by cocarcinogenesis and the enhancement of neoplastic development by promotion. From a mechanistic perspective, enhancement of carcinogenesis by an agent given after a genotoxic carcinogen, but while the DNA damage pro-

AND XENOBIOTICS

327

duced by the carcinogen is still persistent, most likely results from increased neoplastic conversion and should be considered a cocarcinogenic action unless proven otherwise. Therefore, in the present discussion, cocarcinogenesis will be defined as the enhancement of carcinogenesis resulting from effects produced either immediately before or during carcinogen exposure or at a time after carcinogen exposure when chemical damage is still persistent. Promoters can also augment cancer development when given together with a carcinogen. Under these circumstances, it is difficult to determine whether the enhancement of carcinogenesis is due to a cocarcinogenic or promoting action. Consequently, such agents are probably best classified as promoters unless it can be demonstrated that the step of neoplastic conversion is facilitated. Chemical cocarcinogens could operate through a variety of nongenotoxic mechanisms (Table 3). In studies on the induction of lung neoplasms in hamsters, Safhotti et al. ( 1968) demonstrated that benzo[a]pyrene combined with ferric oxide was more effective than benzo[a]pyrene alone, thereby establishing a cocarcinogenic effect of ferric oxide. Subsequent studies provided evidence that bronchial epithelial cells (Kennedy and Little, 1974) and pulmonary alveolar macrophages (Autrup et al., 1979) eluted benzo[a]pyrene from the carrier, indicating that the cocarcinogenicity of the latter probably resulted from facilitated cellular uptake of the carcinogen. However, ferric oxide also enhanced the carcinogenesis of diethylnitrosamine (Montesano

TABLE 3 POSSIBLEMECHANISMS OF COCARCINOGENEW Increase uptake of carcinogen Increase proportion of carcinogen activated Deplete competing nucleophiles Inhibit rate or fidelity of DNA repair Enhance conversion of DNA lesions to permanent alterations

G.RY M. I.... . 1.. _” W ILLlAM>

328

et al., 1970; Feron et al., 1972), a water-soluble carcinogen, and therefore other actions such as enhancement of the effects of the interaction of the carcinogen with cellular constituents may be involved. There are several important examples of probable cocarcinogenic effects in humans: enhancement of lung cancer in cigarette smokers by asbestos (Saracci, 1977; Selikoff 1977), enhancement of cancer of the upper alimentary tract in cigarette smokers by alcohol abuse (see Groupe and Salmoiraghi, 1979), and the high incidence of liver cancer seen in populations with endemic hepatitis and exposure to mycotoxins (see Davis and Rosenfeld, 1979). The augmentation of lung cancer by asbestos in cigarette smokers may have a basis similar to the effect of ferric oxide. Recent studies have shown that in cell culture, asbestos enhances cell transformation (DiPaolo et al., 1982) and mutagenicity (Reiss et al., 1983) induced by benzo[a]pyrene. Since asbestos is known to adsorb polycyclics (Lakowicz and Hylden, 1978), it is probable that asbestos, like ferric oxide, acts, at least in part, as a vehicle to carry the carcinogen into cells. In addition, asbestos exerts several other effects (see Wagner, 1980), such as stimulation of cell proliferation, which are undoubtedly relevant to both its cocarcinogenic and suggested promoting effects (Topping and Nettesheim, 1980). MODIFICATION OF BIOTRANSFORMATION Many genotoxic chemical carcinogens are procarcinogens that require activation by enzyme systems to reactive species to produce their carcinogenic effects (Miller and Miller, 198 1). The enzyme systems involved are numerous (Weisburger and Williams, 1982b), but the most important is the cytochrome P-450 monooxygenase system localized in the endoplasmic reticulum. The induction of enzyme activities associated with this system was elucidated by Conney and the Millers (Brown et al., 1954; Conney et al., 1956). Subsequently, a wide variety of stimulators and in-

hibitors of this system have been described (Snyder and Remmer, 1979), including exogenous hormones and substances that modify the endocrine system. Modification of the enzyme systems involved in biotransformation can either decrease or increase the carcinogenicity of genotoxic procarcinogens. One of the earlier examples of inhibition of carcinogenesis was the demonstration by Kensler et al. (1941) that riboflavin protected against the liver carcinogenicity of aminoazo dyes. Subsequently, it was shown that a hepatic azo reductase that cleaved azo dyes required riboflavin as a cofactor. The preponderance of metabolism of most activation-dependent carcinogens is toward detoxified metabolites (Weisburger and Williams, 1982b) and for such carcinogens enzyme induction usually, but not always, reduces carcinogenicity (Table 4). The enhancement of activities of type II conjugating systems such as glutathione S-transferase and uridinediphosphate-glucuronosyl transferase that serve to detoxify carcinogens seems in particular to be involved in this inhibition (Hesse et al., 1982; Spamins et al., 1982). However, enhancement of detoxification in one organ can result in increased carcinogenicity in another as seen with butylated hydroxytoluene inhibition of liver carcinogenicity and enhancement of bladder carcinogenicity (Williams et al., 1983). It is noteworthy that certain genotoxic carcinogens such as the polycyclic aromatic hydrocarbons when administered together with other activation-dependent genotoxins can actually reduce their carcinogen&y as a result of effects on biotransformation. Reduction of the effects of activation-dependent carcinogens can also be produced by inhibitors of enzymes involved in carcinogen metabolism (Table 5). However, in some situations inhibition of metabolism increases carcinogenic effects (Kotin et al., 1962; Wheatley, 1968; Kinoshita and Gelboin, 1972; Argus et al., 1978). Moreover, enzyme inhibitors, even when reducing overall carcinogenicity, can produce an increase of neoplasms

CHEMICAL

CARCINOGENESIS

329

AND XENOBIOTICS

TABLE 4 EXAMPLES

OF INHIBITION

OF CARCIN~GENESIS

BY INDUCERS

Inducer

Carcinogen inhibited

Polycyclic hydrocarbons

3’-Methyl-4-dimethylaminoazobenzene 2-Acetylaminofluorene 7,12-Dimethylbenz[a]anthracene

Polychlorinated biphenyls

Benzoflavones DDT Butylated hydroxyanisole Butylated hydroxytoluene

in secondary sites for the carcinogen (Fiala et al., 198 1; Wong et al., 1982), probably as a consequence of increased availability of the carcinogen in other organs resulting from inhibition of biotransformation in the major organ for metabolism. The mechanisms of enzyme inhibition leading to reductions of car-

BIOTRANSFORMATION

Authors

Organ

4-Dimethylaminostilbene 4-Dimethylaminoazobenzene Urethan 2-Acetylaminofluorene Aflatoxin 3’-Methyl-4.dimethylaminoazobenzene 2-Acetylaminofluorene Diethylnitrosamine 7,12-Dimethylbenz[a]anthracene benzo[a]pyrene 7,12-Dimethylbenz[n]anthracene Benzo[a]pyrene 7,12-Dimethylbenz[a]anthracene Benzo[a]pyrene 2-Acetylaminofluorene

Barbiturates

OF XENOBIOTIC

Liver

Richardson et al. (1952)

Liver Skin Breast Earduct Liver Lung Liver Breast Liver

Miller et al. (1958) Hill et ul. (1951) Huggins et al. (1964) Tawhc (1965) Ishidate et al. (1967) Yamamoto et al. (1971) Peraino et al. ( I97 1) McLean and Marshall ( 197 1) Makiura et al. (1974)

Liver Liver Skin Lung Breast Forestomach Lung Forestomach Liver

Makiura et al. (1974) Makiura et al. (1974) Wattenberg and Leong (1968) Wattenberg and Leong ( 1970) Silinskas and Okey (1975) Wattenberg ( 1972) Wattenberg ( 1973) Wattenberg ( 1972) Ulland et al. (1973)

cinogenicity are discussed by Fiala (1981). However, the exact basis for the effects of certain inhibitors is not always clear because some are also enzyme inducers (Wiebel, 1980). Depletion of cellular substrates involved in activation reactions can also result in inhibition of carcinogenesis. A classic example was

TABLE 5 EXAMPLES

OF INHIBITION

Inhibitor 7,8Benzoflavone Disulfiram Aminoacetonitrile Pyrazole

OF CARCIN~GENESIS

Carcinogen inhibited 7,12-Dimethylbenz[a]anthracene Benzo[u]pyrene 7,12Dimethylbenz[a]anthracene 1,2-Dimethylhydrazine Dimethylnitrosamine 1,2-Dimethylhydrazine Azoxymethane

Pregnenolone- 16-w carbonitrile

BY INHIBITORS

Dimethylnitrosamine

OF XENOBIOTIC

BIOTRANSFORMATION

Organ

Authors

Skin Stomach Breast Large intestine Liver Large intestine, blood vessels Large intestine, blood vessels Liver

Gelboin et al. (1970) Wattenberg ( 1974) Marquardt et al. (1974) Wattenberg (1975) Hadjiolov ( 197 1) Moriya et al. (1982) Moriya et al. (1982) Argus et al. (1978)

330

GARY M. WILLIAMS

delineated in the series of studies by the Weisburgers based on the observation that chloramphenicol inhibited liver carcinogenesis by iV-2-fluorenyldiacetamide (Puron and Firminger, 1965). In these studies, it was shown that acetanilide, which bears some structural similarity to chloramphenicol, decreased the liver carcinogenicity of N-2-fluorenylacetamide by reducing formation of the ultimate carcinogenic metabolite, the sulfate ester, largely through reduction of available sulfate by binding to phydroxyacetanilide (Yamamoto et al., 1968; Weisburger et al., 1972). A variety of other inhibitors has been described (Wattenberg, 1978), but their modes of action are not yet clear, albeit in most instances altered levels of enzymes controlling the ratio of activation to detoxification reactions may be involved. Chemical cocarcinogens can inhibit detoxification or enhance activation of genotoxic carcinogens and, thereby enhance neoplastic conversion and carcinogenicity (Table 3). An example of the first mechanism is the enhancement of carcinogenicity of polycyclic aromatic hydrocarbons by trichloropropene oxide (Berry et al., 1977) which inhibits the epoxide hydrolase that inactivates the reactive epoxides of the carcinogens. Only a few agents known to be enzyme inducers have increased the carcinogenicity of activation-dependent carcinogens (Table 6). Of these, ethanol, interestingly, increased the hepatocarcinogenicity of vinyl chloride and N-nitrosopyrrolidine, but had no effect on N’-nitrosonomicotine (McCoy et al., 1981) and decreased the carcinogenicity of dimethylnitrosamine (Gel-

lert et al., 1980) and diethylnitrosamine (Habs and Schmlhl, 198 1). Studies of subcellular preparations from livers of ethanol treated rats have revealed increased activation of dimethylnitrosamine to mutagenic metabolites (Garro et al., 198 I), but in vivo studies have not shown an increase in DNA alkylation by dimethylnitrosamine following chronic ethanol consumption by rats (Belinsky et al., 1982). Thus, the mode of action of ethanol as a cocarcinogen in human cancer is not yet clear, particularly since the major effect of alcohol in humans is enhancement of head and neck cancer in smokers, not of liver cancer. Another means by which increased activation of a carcinogen could occur is through competition by the agent for detoxification systems. This has been documented in in vitro systems (Ashby and Styles, 1980) and it is possible that enhancement of carcinogenicity by ethanol may result from such competition. An important effect of chemicals on carcinogen metabolism is the permanent alteration of enzyme systems, known as imprinting, which is produced by chemical exposures during the developmental period (Einarson et al., 1973; Lucier, 1976). Rats exposed to synthetic hormones in the neonatal period have displayed an altered response to N-hydroxyN-2-fluorenylacetamide (Weisburger et al., 1966, 1968) and 7,12-dimethylbenz[a]anthracene (Rustia and Shubik, 1979) later in life. This is a potentially important type of modulation that has received relatively little attention as regards carcinogenesis. It may be involved in part in the transplacental carcinogenicity of diethylstilbestrol (For&erg, 1974).

TABLE 6 EXAMPLES

Inducer Ethanol

Phenobarbital

OF ENHANCEMENT

OF CARCIN~CENESIS

Carcinogen enhanced Benzo[a]pyrene 7,12-Dimethylbenz[a]anthracene Vinyl chloride N-Nitrosopyrrolidine Safrole

BY INDUCERS

OF XENOBIOTIC

OCWn Oral cavity Oral cavity Liver Liver Liver

BICYRANSFORMATION

Authors Protzel et al. ( 1964) Elzay ( 1966) Radiki et al. (1977) McCoy ez al. (1981) Wislocki et al. (1977)

CHEMICAL

CARCINOGENESIS

MODIFICATION OF REACTIVE INTERACTIONS The reactive type of carcinogen is capable of forming an electrophile or radical cation (Miller and Miller, 198 1). These molecular species bind to various cellular nucleophiles, ultimately resulting in neoplastic conversion of the cell (Fig. 1). A critical target for such reactions appears to be DNA, hence the designation of carcinogens with this property as genotoxic. In addition to producing neoplastic conversion, genotoxic carcinogens may be selfpromoting through genotoxic or cytotoxic effects resulting in selective proliferation of altered cells (Farber, 1979). Also, nongenotoxic agents may indirectly generate reactive species such as activated oxygen (Mason and Chingnell, 1982; Moody and Hassan, 1982) and 9 adenosylmethionine (Barrows and Shank, 198 1; Barrows and Magee, 1982), leading to indirect genotoxicity (Williams, 1983a). An agent that competes with relevant macromolecules for the binding of a reactive carcinogen or indirectly generated reactive species could decrease genotoxicity and carcinogenicity. Certain nucleophiles, particularly those containing sullhydryl groups, have been suggested to be capable of inhibiting carcinogenesis on this basis (Miller and Miller, 1972). Reduction of mutagenesis in vitro by means of electrophile trapping has been demonstrated with cysteine (Rosin and Stich, 1978) and a variety of thiols (Friedman et al., 1982), but modification of carcinogenesis in viva through this effect of an exogenous substance has not yet been clearly documented, although it is possible that antioxidants could act as inhibitors of carcinogenesis in this manner (Wattenberg, 1978). Glutathione, a nucleophile known to bind reactive metabolites of carcinogens, has been reported to inhibit the liver carcinogenicity of aflatoxin B, (Novi, 1981), but the basis for this effect has not been established. Conversely, agents that reduce levels of cellular nucleophiles could act as enhancers of carcinogenesis. For example, diethyl maleate

AND XENOBIOTICS

331

which depletes liver glutathione (Boyland and Chasseud, 1969) increases the hepatotoxicity of aflatoxin B1 (MgBodile et al., 1975). MODIFICATION CELLULAR

OF EXPRESSION ALTERATION

OF

The adducts formed in DNA by genotoxic carcinogens can be removed by enzymatic DNA repair systems and this restoration of DNA reduces mutagenic and carcinogenic effects. If repair is not complete before the cell replicates its damaged genome, the persisting adducts can give rise to mispairing of bases and probably other genetic effects such as rearrangements and translocations of segments of DNA, including oncogenes. Thus, an agent that alters repair processes or the rate of cell proliferation can affect the frequency of neoplastic conversion of carcinogen-damaged cells. The repair of certain kinds of alkylation damage in DNA by carcinogens appears to be inducible (Montesano et al., 1979; Swenberg et al., 1982), but reduction of the effects of a genotoxic carcinogen by this action of another xenobiotic has not been demonstrated. &carcinogens could produce their effects as a result of inhibition of DNA repair (Table 3). However, most agents that retard repair produce a nonspecific effect on DNA synthesis which is also accompanied by a decrease in replicative synthesis (Cleaver and Painter, 1975). As a result, the overall effect is that a longer interval is available for repair prior to replication and, consequently, the inhibition of repair under these conditions has not been shown to result in a greater yield of permanent alterations in DNA. Recently, agents that inhibit processes specific for DNA repair, such as the poly(ADP-ribose) polymerase, have been identified (Purnell and Whish, 1980). One of these 3-aminobenzamide has enhanced the effect of a liver carcinogen (Takahashi et al., 1982). Agents that retard cell proliferation in a tissue can under appropriate conditions reduce

332

GARY

M.

the carcinogenicity of chemicals as a consequence of providing a greater interval for the repair of DNA damage. Conversely, agents that enhance proliferation, such as occurs following the production of cell necrosis, can enhance carcinogenesis (Pound et al., 1973; Mori et al., 1977), presumably because the time available for DNA repair is reduced relative to the utilization of the damaged template during replicative DNA synthesis. Increased cell proliferation may account in part for the cocarcinogenicity of ferric oxide (Saffiotti et al., 1968) and asbestos (Saracci, 1977) in lung carcinogenesis. Likewise the production of sustained liver cell proliferation resulting from cell death in chronic hepatitis may contribute to an augmentation of the human liver carcinogenicity of mycotoxins. Syncarcinogenic Efects The additive or synergistic effect of two or more carcinogens in neoplasm production is defined as syncarcinogenesis (Nakahara, 1970; Schmahl, 1970). Syncarcinogenesis can occur either when two carcinogens are administered concurrently (MacDonald et al., 1952) or when they are administered one after the other (Odashima, 1959). Since these two types may have different mechanisms and since the latter type must be clearly identified in order to be distinguished from initiation-promotion, they will be designated as combination syncarcinogenesis and sequential syncarcinogenesis, respectively. Syncarcinogenesis has been demonstrated in various organs including liver (MacDonald et al., 1952), skin (Steiner, 1955), and bladder (Deichmann et al., 1965). Generally, the effect occurs only when the two carcinogens have the same target organ (Schmahl, 1980). In such situations, syncarcinogenesis can be most striking when weakly carcinogenic dosages are used. Nevertheless, as noted above, concomitant or sequential administration of two carcinogens can conversely result in reduction of carcinogenicity (Richardson et al., 1952; Hoch-Ligeti et al., 1968) when one carcinogen or the first carcinogen in the case of sequential exposures is also a potent enzyme inducer.

WILLIAMS

Several pairs of genotoxic carcinogens have produced syncarcinogenic effects of the combination (MacDonald et al., 1952; Steiner, 1955; Deichmann et al., 1965; Schmahl, 1970; Davis et al., 198 1; Angsubhakorn et al., 198 1) or sequential (Odashima, 1959; Nakahara and Fukuoka, 1960; Takayama and Imaizumi, 1969; Tatematsu et al., 1977; Williams et al., 198 1) types. Enhancement of carcinogenesis by multiple exposures also characterizes cocarcinogenesis and neoplasm promotion. These have been distinguished from syncarcinogenesis on the basis that the cocarcinogen or promoter is ostensibly noncarcinogenic, although “weak” carcinogenic effects are often overlooked in that assumption. Mechanistically, the critical difference appears to be that cocarcinogens and promoters are not genotoxic. Moreover, in sequential syncarcinogenesis the order of administration can be reversed and the effect still occurs (Williams and Furuya, 1984), whereas with cocarcinogenesis and promotion this is not the case unless the agent has both cocarcinogenesis and promoting properties and the exposure to the chemical is proximate to that of the carcinogen so that cocarcinogenesis can be produced. It is essential that these phenomena be carefully distinguished in protocols of sequential administration to properly ascertain the mechanism of an enhancing effect (see below). Syncarcinogenesis is believed to result from a summation of the irreversible effects of the two carcinogens (Nakahara, 1970). These irreversible effects now appear to be the DNA alterations produced by the agents. In addition, several studies have shown that administration of one carcinogen can inhibit repair of the DNA damage produced by a second one (Kleihues and Margison, 1976; Pegg, 1978). Thus, syncarcinogenesis could result both from summation of the DNA damage produced by both agents as well as from the action of one carcinogen in inhibiting the repair of the DNA damage produced by another. Also, in the case of sequential syncarcinogenesis, the enhancement could result from a promoting action of the second carcinogen. In fact, it might be possible to assessthe pro-

CHEMICAL

CARCINOGENESIS

moting activity of a chemical, as distinct from its production of neoplastic conversion by determining its relative enhancing effect in two sequences of administration. An interesting syncarcinogenic effect is that produced by a genotoxic and a nongenotoxic carcinogen. For example, the nongenotoxic liver carcinogen clofibrate, a hypolipidemic drug, which is a perioxisome proliferator, enhances the hepatocarcinogenicity of previously administered diethylnitrosamine (Reddy and Rao, 1978; Mochizuki et al., 1982). Likewise, the nongenotoxic liver carcinogen methapyrilene, an antihistamine, which induces mitochondrial proliferation, enhances the liver carcinogenicity of N-2-fluorenylacetamide when given either after or before the genotoxin (Furuya and Williams, 1984; Furuya et al., 1983). These studies suggest that the indirect gentoxic effects postulated to be produced by these types of agents (Reddy et al., 1980; Reznik-Schtiller and Lijinsky, 198 1) can summate with the DNA damage produced by genotoxic carcinogens.

AND

XENOBIOTICS

333

inhibitors is the nonsteroidal antiinflammatory drugs, such as indomethacin (Kudo et al., 1980; Pollard and Luckert, 1980), which stimulate prostaglandin synthesis and may thereby affect cell proliferation. The neoplastic process is influenced by the immune system in a variety of ways (Melief and Schwartz, 1982). Certain immunosup pressants such as azathioprine have caused the development of lymphomas and leukemias in experimental animals and humans and increased the frequency of cancers at a few other sites (Casey, 1960; Weisburger, 1977; Mitrou et al., 1979). In addition, azathioprine has acted synergistically with N-nitrosobutylurea in the induction of leukemias (Imamura et al., 1973). However, effects of chemical immunomodulators on the carcinogenicity of other chemicals in nonhematopoietic tissues has not been established (Fraenkel et al., 1970; Dargent et al., 1972). Promotion

of Neoplasia

Promotion was originally defined conceptually as the encouragement of dormant neoplastic cells to develop into growing tumors MODIFICATION OF NEOPLASTIC (Berenblum, 1974). Operationally, the pheDEVELOPMENT nomenon is usually demonstrated by the enFollowing neoplastic conversion of a cell, hancing effect on carcinogenesis by a modifier further development and proliferation is re- administered after a carcinogen (Boutwell, quired for the formation of a tumor (Fig. 1). 1974). An important concept that requires The progeny of initiated or neoplastic cells greater attention is whether any enhancement can remain latent for very long intervals in of carcinogenesis by an agent administered the skin (Van Duuren et al., 1975; Stenback, subsequent to a carcinogen can be considered 1978) and other tissues (Wheelock et al., promotion or whether the phenomenon 198 1), indicating that host homeostatic factors should be restricted to the facilitation of are able to control such abnormal cells. Thus, growth and development of dormant neoagents that enhance the differentiation of neo- plastic cells as proposed by Berenblum. In most “promotion” studies, the second plastic cells or facilitate their control by tissue factors could inhibit neoplastic development. agent is given shortly after a carcinogen. Under Vitamin A and related retinoids appear to in- these conditions unrepaired DNA adducts are hibit carcinogenesis through effects on cell dif- likely to be present such that stimulation of ferentiation (Sporn, 1980). A variety of pro- cell proliferation could enhance neoplastic tease inhibitors have also inhibited carcinoconversion of cells (Fig. 1) ultimately resulting genesis, particularly in the mouse skin two in increased neoplasm formation. This repstage system (Rossman and Troll, 1980). For resents, in fact, a kind of cocarcinogenesis these agents, the potential modes of action are which is very different in nature from enmultiple, including effects on cellular prolifhanced neoplasm development produced by eration and differentiation. Another class of agents acting on neoplastic cells generated by

334

GARY M. WILLIAMS

the carcinogen. Moreover, as described above, two carcinogens given in sequence can produce a syncarcinogenic effect. Therefore, when the mechanism of enhancement is uncertain, it would be preferable to use a general term such as carcinogenesis enhancement. In the present discussion promotion will be restricted to the facilitation of development of neoplastic cells into a growing tumor (Fig. 1). Neoplasm promotion was first established for skin carcinogenesis (Twort and Twort, 1939; Rous and Kidd, 194 1; Berenblum, 194 1) and the most detailed studies of the phenomenon have been conducted with the phorbol ester series of compounds (Boutwell, 1974; Hecker, 1978; Diamond et al., 1980), which are promoters in mouse skin carcinogenesis. A variety of chemical promoters for other organs such as breast, colon, liver, and bladder have now been described (see monographs edited by Slaga et al., 1978, Hecker et al., 1982, and the International Symposium on Tumor Promotion, 1983), although in most cases the demonstration has not been rigorous. In particular, the reverse sequence administrations required to distinguish promotion from syncarcinogenesis have rarely been done (Williams and Furuya, 1984), apart from studies in mouse skin carcinogenesis (Boutwell, 1974; Iversen and Iversen, 1982). In skin carcinogenesis studies, it has been shown that treatment with croton oil or phorbol ester promoters before or together with carcinogen application enhances tumor formation (Iversen and Iverse.n, 1982). In these studies, the biological effects of the agents have been persistent at the time of carcinogen exposure and, thus, it is possible that enhanced susceptibility of the tissue to neoplastic conversion was prevailing; that is, a kind of cocarcinogenic effect may have occurred. Indeed, most skin tumor promoters are also cocarcinogens. The situation can be quite different in other tissues. For example, in a study of liver carcinogenesis in which a 4-week interval was allowed between treatments, phenobarbital enhanced the effects of N-Zfluorenylacetamide when given after, but not before, the

carcinogen (Williams and Furuya, 1984). Moreover, when phenobarbital is given together with such activation-dependent carcinogens, it inhibits carcinogenicity (Table 4). The distinction between cocarcinogenicity and promotion has application in the design of protocols for the study of enhancing effects. For example, in many studies of enhancement of liver carcinogenesis (Leonard et al., 1982; Pereira, 1982; Tatematsu et al., 1983) the enhancing agents are administered shortly after a carcinogen, but before abnormal populations have evolved. Thus, both cocarcinogenic and promoting effects may contribute to enhancement. In a different type of protocol (Watanabe and Williams, 1978; Kitagawa and Sugano, 1978; Williams et al., 198 1; Mori et at., 198 1; Mazue et al., 1982) liver altered foci are induced first and then after an interval, the effect of the enhancing agent is delineated on these abnormal cells. Providing that carcinogen induced DNA damage has been repaired, such protocols should measure promoting effects. A number of cellular effects have been demonstrated for promoting agents (Weinstein et al., 1979; Colburn, 1980). Those that could facilitate the growth of neoplastic cells into tumors are listed in Table 7. Promotion could be a consequence of any of these actions alone or in combination. Although these effects are nongenetic, it is also possible that genotoxic carcinogens may exert a promoting action through nongenotoxic effects. A current hypothesis of promotion based mainly on studies with phorbol esters is that TABLE 7 POSSIBLEMECHANISMSOF TUMOR PROMOTION

Enhancementof expressionof neoplasticphenotype Inhibition of differentiation Stimulation of cell proliferation Cytotoxicity Hormone effects Cell membrane effects Induction of proteases Inhibition of intercellular communication Immunosuppression

CHEMICAL

CARCINOGENESIS

promoters in some way, perhaps through generation of active oxygen species produce genetic damage that completes the neoplastic conversion of the cell (Kinsella and Radman, 1978; Emerit and Cerutti, 1982). Such an effect would be more properly designated as cocarcinogenesis, or even syncarcinogenesis, as discussed above. Nevertheless, phorbol esters do have promoting activity in the sense used here on neoplastic development. Whether this activity can be attributed to effects of oxygen radicals needs to be reconciled with the fact that potent antioxidants, such as butylated hydroxytoluene have promoting activity (Peraino et al., 1978; Wits&i and Lock, 1978; Maeura and Williams, 1984). As a basis for the action of promoters in enhancing neoplastic development (Fig. l), an attractive hypothesis is that some promoters may operate by inhibiting intercellular communication. According to this concept, neoplastic cells may be restrained by interactions with normal cells and disruption of these exchanges would thereby release dormant neoplastic cells from tissue constraints, allowing them to proliferate according to their altered genome (Trosko et al., 198 1; Williams, 198 1). One form of cell-to-cell communication involves intercellular transfer of molecules which could be the means by which growth regulating signals are transmitted to neoplastic cells. A variety of promoters have now been shown to produce inhibition of molecular transfer in culture (Murray and Fitzgerald, 1979; Yotti et al., 1979; Umeda et al., 1980; Williams, 1980b; Trosko et al., 1981; Williams et al., 198 lb; Telang et al., 1982) supporting this hypothesis. Promotion was identified at a time when promoting agents were not rigorously tested for carcinogenicity and thus promoters were considered to be noncarcinogenic, although even the earliest studies on skin neoplasm promotion with croton oil and phorbol esters revealed a weak carcinogenic effect of these agents (Van Duuren, 1969). It is now established that many agents with promoting activity will increase neoplasia in the absence of

AND XENOBIOTICS

335

induced initiation when tested for long duration under appropriate conditions (Foulds, 1969; Williams et al., 198 la). Nevertheless, they do not seem to be able to produce DNA damage directly, and therefore have been categorized as a type of epigenetic carcinogen (Williams, 198Ob; Weisburger and Williams, 1980). It seems likely that their “carcinogenic” effects are due to a promoting action on cryptogenically arising neoplastic cells, whose occurrence appears to be the basis for development of spontaneous neoplasms. Nevertheless, indirect genotoxicity leading to neoplastic conversion could be involved for the phorbol esters (Kinsella and Radman, 1978; Emerit and Cerutti, 1982), in which case they might better be described as indirect genotoxic carcinogens. Among promoters, those that enhance liver carcinogenesis are receiving increased attention because they include a number of pharmaceuticals, food additives, and pesticides (Peraino et al., 1978; Pitot et al., 1982; Williams, 1983b). Exogenous hormones and hormonally active substances are also important (Williams, 1982a, Yager, 1983). Many of these substances are also carcinogenic upon long-term administration (Williams, 1980a; 198 1). In the case of phenobarbital, the absence of an enhancing effect when it precedes a genotoxin (Williams and Furuya, 1984) argues against indirect genotoxicity and in favor of a promoting action as the basis for its carcinogenicity. It is noteworthy that some effects of promoters, such as the enzyme induction produced by certain liver promoters including phenobarbital and butylated hydroxytoluene, make them inhibitors of the effects of procarcinogens when the two are administered simultaneously. Inhibition of the promoting effect of phorbol esters on mouse skin was described in several early studies (Van Duuren and Melchionne, 1969; Falk, 197 1). Presently, a wide variety of agents are known to be inhibitory to skin promotion, including protease inhib itors (Rossman and Troll, 1980) and anti-inflammatory steroids (Slaga, 1980).

336

GARY M. WILLIAMS

Neoplastic Progression Neoplastic progression was defined by Foulds ( 1969) as the stepwise development of a neoplasm through qualitatively different stages. So defined, progression includes both neoplastic conversion and neoplastic development as discussed here. However, progression is often used in a more restricted sense to denote the change of a neoplasm from benign to malignant or from low grade to high grade malignancy (Pitot, 1978). These latter processes would be part of the sequence of neoplastic development (Fig. 1). Little is known about the evolution of qualitative changes in a neoplasm, although emergence of new subpopulations seems likely. Proposed mechanisms for the evolution of new cell types within a neoplasm include infidelity of DNA polymerases (Springgate and Loeb, 1973) and hybridization of normal and neoplastic cells (Goldenberg et al., 1974). The observation that metals interfere with the fidelity of DNA polymerases (Sirover and Loeb, 1976) suggests that exogenous elements could effect progession. CONCLUSIONS The carcinogenic process is modulated in many ways, and factors that modify the process play an important and even decisive role in determining the eventual outcome of exposures to carcinogens. A variety of enhancing

effects have been described in chemical carcinogenesis; these include cocarcinogenesis, promotion, and syncarcinogenesis. To appreciate the mechanisms by which enhancement is produced as a result of combined exposures, the different types of enhancement must be clearly distinguished (Table 8). The cancer-enhancing effects of xenobiotics may account for their carcinogenicity in experimental animals under specific conditions in the absence of any other deliberate exposure. For example, the ability of liver neoplasm promoters to produce neoplasms in this organ in strains of rodents that develop spontaneous liver tumors has been attributed to their promoting effect on neoplastic cells arising from inherited genetic defects or from unknown environmental exposures (Williams, 1980a, 198 1). A key property of such carcinogens is that they do not directly damage DNA through chemical interactions. This distinguishes them from genotoxic carcinogens which appear to exert their carcinogenic effects through alterations in DNA. The nature of the hazard to humans from these two types of carcinogens appears in many cases to be quite different (Weisburger and Williams, 198 1). Specifically for promoters, many require high levels of exposure or prolonged intervals of exposure to produce their effects. Such conditions are apparently achieved by the promoting factors

TABLE 8 DISTINCTION

BETWEEN TYPES OF MULTIPLE

EXPOSURE

Emcrs

IN CHEMICAL

CARCIN~GENESIS

Process

Operational characteristics

Mechanistic differences

Combination syncarcinogenesis

Two carcinogens acting together

Cocarcinogenesis

Enhancer acting before or together with carcinogen; or when carcinogen effects still persistent Two carcinogens acting in sequence; sequence reversible Enhancer acting after effects of carcinogen have been completed

Both carcinogens genotoxic Enhancer facilitates neoplastic conversion; enhancer nongenotoxic and noncarcinogenic Both carcinogens genotoxic Enhancer facilitates neoplastic development; enhancer may also be a carcinogen, but is nongenotoxic

Sequential syncarcinogenesis Promotion

CHEMICAL

CARCINOGENESIS

resulting from high consumption of dietary fat or cigarette smoking (Weisburger and Williams, 1982a). However, synthetic promoters are usually not present in the environment at the high levels typically used in animal studies. Furthermore, the action of promoters is reversible when their concentration is lowered, or when they are omitted. This fact underlies the successin lowering the risk of cancer when cigarette smokers give up the habit. An increase of human cancer has been produced by several types of naturally occurring substances with carcinogenesis-enhancing properties. These include the enhancing factors in cigarette smoke and those arising from high level consumption of food components such as fat (Newbeme, 1976; Reddy et al., 1980). These factors appear to be decisive in the production of the majority of cancers in the United States (Weisburger and Williams, 1982a) and, therefore, require intensive attention in efforts at cancer control. The concept of anticarcinogenesis stemmed from experiments reported more than 75 years ago by Lathrup and Loeb (19 16). Much of the considerable early literature has been reviewed by Van Duuren and Melchionne ( 1969) and Falk (197 1). Presently, a spectrum of xenobiotics has been proposed as potentially useful for chemoprevention of human cancer (see Zedeck and Lipkin, 198 1). While this ap preach certainly deserves evaluation, the situation of adding an agent to the human environment as opposed to eliminating one, entails major compexities. Specifically, as described, the fact that inhibitors of carcinogenesis in one organ can enhance the disease in another and that an inhibitor at one stage of carcinogenesis can be an enhancer at another indicate the need for detailed research prior to human application. At present, the reduction of enhancing factors stemming from lifestyle exposures is more practical, REFERENCES ANGSUBHAKORN, S., BHAMARAPRAVATI, N., ROMRUEN, K., AND SAHAPHONG, S. (198 1). Enhancing effects of

AND XENOBIOTICS

337

dimethylnitrosamine on aflatoxin Bi hepatocarcincgenesis in rats. ht. J. Cancer 28, 621-626. ARGUS, M. F., HOCH-LIGETI, C., ARcos, J. C., AND CONNEY, A. H. (1978). Differential effects of &naphthotIavone and pregnenolone- 16y-carbonitrile oo dimethylnitrosamine-induced hepatocarcinogenesis. J. Nat. Cancer Inst. 61,441-449. ASHBY, J., AND Sn~es, J. A. (1980). Carcinogenic synergism and its reflection in vitro. Brit. Med. Bull. 36, 63-70. AUTRUP, H., HARRIS, C. C., SCHAFER,P. W., TRUMP, B. F., STONER, G. D., AND Hsu, I. (1979). Uptake of benzo(a)pyrene-fenic oxide particulates by human pulmonary macropha8es and release of benzo(a)pyrene and its metabolites (40536). Proc. Sot. Exp. Biol. Med. 161, 280-284. BARROWS, L. R., AND SHANK, R. C. (1981). Aberrant methylation of liver DNA in rats during hepatotoxicity. Toxicol.

Appl. Pharmacol.

60, 334-345.

BARROWS,L. R., AND MAGEE, P. N. (1982). Nonenxymatic methylation of DNA by S-adenosylmethionine in vitro. Carcinogenesis 3, 349-35 1. BELINSKY, S. A., BEDELL, M. A., AND SWENBERG,J. A. (1982). Effect of chronic ethanol diet on the replication, alkylation, and repair of DNA from hepatocytes and nonparenchymal cells following dimethylnitrosamine administration. Curcinogenesis 3, 1293-1297. BERENBLUM, I. (1941). The mechanism of carcinogenesis. A study of the significance of cocarcinogenic action and related phenomena. Cancer Res. 1, 807-8 14. BERENBLUM, I. (1974). Carcinogenesis as a Biological Problem (A. Neuberger and E. L. Tatum, eds.). Amer. Elsevier, New York. BERRY, D. L., SLAGA, T. J., VIAJE, A., WILSON, N. M., DIGIOVANNI, J., JUCHAU, M. R., AND SELKIRK, J. K. (1977). Effect of trichloropropene oxide on the ability of polyaromatic hydrocarbons and their “K-region” oxides to initiate skin tumors in mice and to bind to DNA in vitro. J. Nat. Cancer Inst. 58, 1051-1055. BOUTWELL, R. K. (1974). The functions of mechanism of promoters of carcinogenesis. CRC Crit. Rev. Toxicol. 2,419-433. BOYLAND, E., AND CHASSEAUD, L. F. (1969). The role of glutathione and glutathione-S-transferases in mercapturic acid biosynthesis. Adv. Enzymol. 32,173-2 19. BROWN, R. R., MILLER, J. A., AND MILLER, E. C. (1954). The metabolism of methylated animo azo dyes. IV. Dietary factors enhancing demethylation in vivo. J. Biol. Chem.

209,2

11-222.

CASEY, T. P. (1968). The development of lymphomas in mice with autoimmune disorders treated with azathioprine. Blood 31, 396-39. COLBURN, N. H. ( 1980). Tumor promotion and preneoplastic progression. In Carcinogenesis, Vol. 5, Modl$ers of Chemical Carcinogenesis, (T. J. Slaga, ed.), pp. 3356. Raven Press, New York.

GARY M. WILLIAMS

338

CLAYSON, D. B. (198 1). Carcinogens and carcinogenesis enhancers. Mutation Res. 86, 2 1l-229. CLEAVER, J. E., AND PAINTER, R. B. (1975). Absence of specificity in inhibition of DNA repair replication by DNA-binding agents, cocarcinogens, and steroids in human cells. Cancer Res. 35, 1773-1778. CONNEY,A. H., MILLER, E. C., AND MILLER, J. A. (1956). The metabolism of methylated animoazo dyes. V. Evidence for induction of enzyme synthesis in the rat by 3-methylcholanthrene. Cancer Res. 16, 450-459. DARGENT, M., BOURGOIN, J.-J, NOEL, P., AND WEISSBROD, R. (1972). The influence of azathioprine on tumorigenesis by 7,12dimethylbenz(a)anthracene in rats. Eur. J. Cancer

8, 605-609.

DAVIS, W., AND ROSENFELD,C. (eds.) (1979). Carcinogenic Risks/Strategies for Intervention. IARC Scientific Publications, No. 25, International Agency for Research on Cancer, Lyon. DAVIS, W. E., JONES,D. C. L., ROSEN, V. J., AND SASMORE, D. P. ( 198 1). Rapid hepatoma induction in rats by dipentylnitrosamine and its modification by other carcinogens. ht. J. Cancer 21, 249-253. DEICHMANN, W. B., SCOTTI, T., RADOMSKI, J., BERNAL, E., COPLAN, M., AND WOODS, F. (1965). Synergism among oral carcinogens. II. Results of the simultaneous feeding of bladder carcinogens to dogs. Toxicol. Appl. Pharrnacol.

I, 657-659.

DIAMOND, L., O’BRIEN, T. G., AND BAIRD, W. M. ( 1980). Tumor promoters and the mechanism of tumor promotion. In Advances in Cancer Research (G. Klein and S. Weinhouse, eds.), Vol. 32, pp. l-74, Academic Press, New York. DIPAOLO, J. A., DEMARINIS, A. J., AND DONIGER, J. (1982). Asbestos and benzo(a)pyrene synergism in the transformation of Syrian hamster embryo cells. J. Environ.

Pathol.

Toxicol.

5, 535-543.

EINARSON, K., GUSTAFSSON, J., AND STENBERG, A. (1973). Neonatal imprinting of liver microsomal hydroxylation and reduction of steroids. J. Biol. Chem. 248,4987-4997.

ELZ,AY,R. P. ( 1966). Local effectof alcohol in combination DMBA and hamster cheek pouch. J. Dent. Res. 45, 1788-1795. EMERIT, I., AND CERUTTI, P. A. (1982). Tumor promoter phorbol 12-my&ate 13-acetate induces a clastogenic factor in human lymphocytes. Proc. Nat. Acad. Sci. 79, 1509-1513.

FALK, H. L. (1971). Anticarcinogenesis-an alternative. Prog. Exp. Tumor Res. 14, 105-137. FARBER, E. (1979). Reversible and irreversible lesions in processes of cancer development. In Molecular and Cellular Aspects of Carcinogen Screening Tests (R. Montesano, H. Bartsch, L. Tomatis, and W. Davis, eds.), pp. 143-151. FERON, V. J., EMMELOT, P., AND VOSSENAAR,T. ( 1972). Lower respiratory tract turnouts in syrian golden hamsters after intratracheal instillations of diethylnitrosa-

mine alone and with ferric oxide. Eur. J. Cancer 8, 445-449.

FIALA, E. S. (1981). Inhibition of carcinogen metabolism and action by disulfiram, pyrazole, and related compounds. In Inhibition of Tumor Induction and Development (M. S. Zedeck and M. Lipkin, eds.), pp. 2369. Plenum, New York. FIALA, E. S., WEISBURGER,J. H.. KATAYAMA, S., CHANDRASEKARAN,V., AND WILLIAMS, G. M. (198 1). The effect of disulfiram on the carcinogenicity of 3,2’dimethyl&aminobiphenyl in Syrian golden hamsters and rats. Carcinogenesis 2, 965-969. FORSBERG,J. G. (1974). Induction of condition leading to cancer in the genital tract by estrogen during the differentiation phase of the genital epithelium. Adv. Biosci. 13, 139-151. FOULDS,L., ed. ( 1969). Neoplastic Development. Academic Press, New York. FRANKEL, H. H., YAMAMOTO, R. S., WEISBURGER, E. K., AND WEISBURGER,J. K. (1970). Chronic toxicity of azatbioprine and the effectof this immunosuppmasant on liver tumor induction by the carcinogen N-hydroxyN-2-fluorenylacetamide. Toxicol. Appl. Pharmacol. 17, 462-480.

FRIEDMAN, M., WEHR, C. M., SCHADE,J. E., AND MACGREGOR, J. T. (1982). Inactivation of aflatoxin Bi mutagenicity by thiols. Food Chem. Toxicol. 20,887-892. FURUYA, K., AND WILLIAMS, G. M. (1984). Neoplastic conversion in rat liver by the antihistamine methapyrilene demonstrated by a sequential syncarcinogenic effect with N-2-fluroenylacetamide. Toxicol. Appl. Pharmacol. (in press). FURUYA, K., MORI, H., AND WILLIAMS, G. M. (1983). An enhancing effect of the antihistamine drug methapyrilene on rat liver carcinogenesis by previously administered N-2-fluorenylacetamide. Toxicol. Appl. Pharmacol.

70,49-56.

GARRO, A. J., SEITZ, H. K., AND LIEBER, C. S. (1981). Enhancement of dimethylnitrosamine metabolism and activation to a mutagen following chronic ethanol consumption. Cancer Res. 41, 120-l 24. GELBOIN, H. V., WIEBEL, F., AND DIAMOND, L. (1970). Dimethylbenzanthracene tumorigenesis and aryl hydrocarbon hydroxylase in mouse skin: Inhibition by 7,8benzoflavone. Science 170, 169- I7 1. GELLERT, J., MORENO, F., HAYDIN, M., OLDIGES, H., FRENZEL, H., TESCHKE, R., AND STROHMEYER, F. ( 1980). Decreased hepatotoxicity of dimethylnitrosamine folIowing chronic alcohol consumption. Adv. Exp. Med.

Biol.

132, 231-243.

GOLDENBERG, D. M., PAVIA, R. A., AND TSAO, M. C. ( 1974). In vivo hydridization of human tumor and normal hamster cells. Nature 250, 649-65 1. GROUPE, V., AND SALMOIRAGHI, G. C. (eds.) (1979). Alcohol and cancer workshop. Cancer Res. 39, 28162908.

HABS, M., AND SCH~~HL, D. (1981). Inhibition of the

CHEMICAL

CARCINOCENESIS

hepatocarcinogenic activity of diethylnitrosamine by ethanol in rats. Acta Hepatogastroenterol. 28,242-244. HADJIOLOV, D. (197 1). The inhibition of dimethylnitrosamine carcinogenesis in rat liver by aminoacetonitrile. Z. Krebsforsch. 76, 9 l-92. HECKER, E. (1978). Structure-activity relationships in diterpene esters irritant and cocarcinogenic to mouse skin. In Carcinogenesis A Comprehensive Survey (T. J. Slaga, A. Sivak, and R. K. Boutwell, eds.), Vol. 2, pp. 1l-48, Raven Press, New York. HECKER, E., FUSENIG,N. E., KUNZ, W., MARKS, F., AND THIELMANN, H. W., eds. (1982). Cocarcinogenesis and Biological Efects of Tumor Promoters. Raven Press, New York. HESSE, S., JERNSTROM, G., MARTINEZ, M., MOLDELJS, P., CHRISTODOULIDES,L., AND KETTERER, B. (1982). Inactivation of DNA-binding metabolites of benzo(a)pyrene and benzo(a)-pyrene-7,8dihydrodiol by glutathione and glutathione S-transferase. Carcinogenesis 3, 757-760.

HILL, W. T., STANGER, D. W., PIZZO, A., RIEGEL, B., SHUBIK, P., AND WARTMAN, W. B. (195 1). Inhibition of 9, IO-dimethyl- 1,2benzanthracene skin carcinogenesis in mice by polycyclic hydrocarbons. Cancer Res. 11, 892-897.

HIRONO, I., AND SHIBUYA, C. (1972). Induction of stomach cancer by a single dose of N-methyl-N-nitro-Nnitrosoguanidine through a stomach tube. In Topics of Chemical Carcinogenesis, (W. Nakahara, S. Takayama, T. Sugimura, and S. Odashima, eds.), pp. 121-131. Univ. of Tokyo Press, Japan. HOCH-LIGETI, C., ARGUS, M. F., AND ARCOS,J. C. (1968). Combined carcinogenic effectsof dimethylnitrosamine and 3-methylcholanthrene in the rat. J. Nat. Cancer Inst. 40, 535-549.

HOMBURGER, F., AND TREGIER, A. ( 1969). Modifiers of carcinogenesis. In Progress in Experimental Tumor Research (F. Homburger, ed.), Vol. 11, pp. 86-99. Karger, New York. HUGGINS,C., L~RRA~, G., AND FUKUNISHI,R. ( 1964). Aromatic influences on the yields of mammary cancers following administration of 7,12-dimethylbenz(a)anthracene. Proc. Nat. Acad. Sci. USA 51,737742. IMAMURA, N., NAKANO, M., KAWASE, A., KAWAMURA, Y., AND YOKORO, K. (1973). Synergistic action of Nnitrosobutylurea and azathioprine in induction of leukemia in C57BL mice. Gann 64,493-498. International Symposium on Tumor Promotion. (1983). Environ.

Health

Perspect.

50, 3-368.

ISHIDATE,M., WATANABE, M., AND ODASHIMA, S. ( 1967). Effect of barbital on carcinogenic action and metabolism of 4diethylaminoazobenzene. Gann 58, 267-28 1. IVERSEN, 0. H., AND IVERSEN, U. M. (1982). Must initiators come tlrst? Tumorigenic and carcinogenic effects on skin of 3-methylcholanthrene and TPA in various sequences. Brit. J. Cancer 45, 912-920.

AND XENOBIOTICS

339

KENNEDY, A. R., AND LITTLE, J. B. (1974). The transport and localization of benzo(a)pyrene-hematite and hematite- po in the hamster lung following intracheal instillation. Cancer Res. 34, 1344- 1352. KENSLER, C. J., SUGIURA, K., YOUNG, N. F., HALTER, C. R., AND RHOADS, C. P. (1941). Parial protection of rats by ribotlavin with casein against liver cancer caused by dimethylaminoazobenzene. Science 93, 308-3 10. K~NOSHITA, N., AND GELBOIN, G. V. (1972). Aryl hydrocarbon hydroxylase and polycyclic hydrocarbon tumorigenesis: Effect of the enzyme inhibitor 7,8&nzoflavone on tumorigenesis and macromolecule binding. Proc. Nat. Acad.

Sci. 69, 824-828.

K~NSELLA, A. R., AND RADMAN, M. (1978). Tumor promoter induces sister chromatid exchanges: Relevance to mechanisms of carcinogenesis. Proc. Nat. Acad. Sci. 75,6149-6153. KITAGAWA, T., AND SUGANO,H. (1978). Enhancing effect of phenobarbital on the development of enzymealtered islands and hepatocellular carcinomas initiated by 3’methyl-4-(dimethylamino)azobenzene or diethylnitrosoamine. Gann 69, 679-687. KLEIHUES, P., AND MARGISON, G. (1976). Exhaustion and recovery of repair excision of 06-methylguanine from rat liver DNA. Nature (London) 259, 153-l 55. KOTIN, P., FALK, H. L., AND MILLER, A. (1962). Effect of carbon tetrachloride intoxication on metabolism of benzo(a)pyrene in rats and mice. .I. Nat. Cancer Inst. 28, 725-745. KUDO, T., NARISAWA, T., AND ABO, S. (1980). Antitumor activity of indomethacin on methylazoxymethanol-induced large bowel tumors in rats. Gann 71, 260-264. LAKOWICZ, J. R., AND HYLDEN, J. L. (1978). Asbestosmediated membrane uptake of benzo(a)pymne observed by fluorescence spectroscopy. Nature (London) 275, 446-448. LATHROP, A. E. C., AND LQEB, L. (1916). Further investigation on the origin of tumors in micee III on the part played by internal secretion on the spontaneous development of tumors. J. Cancer Res. 1, I-20. LEONARD, T. B., DENT, J. G., GRAICHEN, E., LYGHT, O., AND POPP, J. A. (1982). Comparison of hepatic carcinogen initiation-promotion systems. Carcinogenesis 3, 851-856. LUCIER, G. W. (1976). Perinatal development of conjugative enzyme systems. Environ. Health Perspect. 29, 7-16. MACDONALD, J. C., MILLER, E. C., AND MILLER, J. A. (1952). The synergistic action of mixtures of certain hepatic carcinogens. Cancer Res. 12, 50-54. MAEURA, Y., AND WILLIAMS, G. M. (1984). Enhancing effect of butylated hydroxytoluene on the development of liver altered foci and neoplasms induced by N-2Quorenylacetamide in rats. Food Chem. Toxicol., in Press. MAKIURA, S., AOE, H., SUGIHARA, S., HIRAO, K., MASAYUKI, A., AND 1~0, N. (1974). Inhibitory effect of

340

GARY M. WILLIAMS

polychlorinated biphenyls on liver tumorigenesis in rats treated with 3’-methyl-4dimethylaminoazobenzene, N2-fluorenylacetamide and diethylnitrosamine. J. Nut. Cancer. Inst. 53, 1253-1257. MARQUARDT, H., SAFQZINK, M., AND ZEDECK, M. (1974). Inhibition by cysteamine-HCl of oncogenesis induced by 7,12-dimethylbenz(a)anthracene without affecting toxicity. Cancer Res. 34, 3387-3390. MASON, R. P., AND CHINGNELL, C. P. ( 1982). Free radicals in pharmacology and toxicology. Pharmacol. Rev. 33, 189-212. MAZUE, G., REMANDET, B., GOUY, D., BERTHE, J., RONCUCCI,R., AND WILLIAMS, G. M. (1982). Limited in vivo bioassays on some benzodiazepines: Lack of experimental initiating or promoting effect of the benzodiazepine tranquilizers diazepam, clorazepate, oxazepam and clorazepam. Arch. Znt. Pharmacodyn. Ther. 257, 59-65.

MCCOY, G. D., HECHT, S. S., KATAYAMA, S., AND WYNDER, E. L. (1981). Differential effect of chronic ethanol consumption on the carcinogenicity of N-nitrosopyrrolidine and N’-nitrosonomicotine in male Syrian golden hamsters. Cancer Res. 41, 2849-2854. MCLEAN, A. E., AND MARSHALL, A. (1971). Reduced carcinogenic effects of aflatoxin in rats given phenobarbitone. Brit. J. Exp. Pathol. 52, 322-329. MELIEF, C. J. M., AND SCHWARTZ, R. S. (1982). Immunocompetence and malignancy. In Cancer A Comprehensive Treatise (F. F. Becker, ed.), Vol. 1, pp. 16 l199. Plenum, New York. MGBODILE, M. U. K., HOLSCHER, M., AND NEAL, R. A. (1975). A possible protective role for reduced glutathione in aflatoxin B, toxicity: Effect of pretreatment of rats with phenobarbital and 3-methylcholanthrene on aflatoxin toxicity. Toxicol. Appl. Pharmacol. 34, 128142.

MILLER, E. C., MILLER, J. A., BROWN, R. R., AND MACDONALD, J. C. (1958). On the protective action ofcertain polycyclic aromatic hydrocarbons against carcinogenesis by aminoazodyes and 2-acetylaminofluorene. Cancer Res. 18, 464-477.

MILLER, E. C., AND MILLER, J. A. (1972). Approaches to the mechanism and control of chemical carcinogenesis.In Environment and Cancer Twenty-fourth Annual Symposium on Fundamental Cancer. pp. 5-39. The University of Texas, M. D. Anderson Hospital and Tumor Institute, Williams & Wilkins, Maryland. MILLER, E. C., AND MILLER, J. A. (1981). Mechanisms of chemical carcinogenesis. Cancer 47, 1055-1064. MITROU, P. S., FISCHER,M., MITROU, G., R~TTGER, P., AND HOLTZ, G. (1979). The oncogenic effect of immunosuppressive (cytotoxic) agents in (NZB X NZW) mice. I. Long-term treatment with azathioprine and ifosfamide. Arzneim.-Forsch. Drug Res. 29, 483-488. MOCHIZUKI, Y., FURUKAWA, K., AND SAWADA, N. (1982). Effects of various concentrations of ethyl-a-p chlorophenoxyisobutyrate (clofibrate) on diethylnitro-

samine-induced hepatic tumorigenesis in the rat. Car3, 1027-1029. MONTESANO, R., BRESIL, H., AND MARGISON, G. P. (1979). Increased excision of 06-methylguanine from rat liver DNA after chronic administration of dimethylnitrosamine. Cancer Res. 39, 1798- 1802. MONTESANO, R., SAFFIOTTI, U., AND SHUBIK, P. (1970). The role of topical and systemic factors in experimental respiratory carcinogenesis. In Inhalation Carcinogenesis (M. G. Hanna, P. Nettesheim, and J. R. Gilbert, eds.), p. 353. Proc. Biol. Div., Oakridge Nat. Laboratory Oakridge, Tenn. MWDY, C. S., AND HASSAN, H. M. (1982). Mutagenicity of oxygen free radicals. Proc. Nat. Acad. Sci. 79,2855cinogenesis

2859.

MORI, H., USHIMARU, Y., TANAKA, T., AND HIRONO, I. (1977). Effect of carbon tetrachloride on carcinogenicity of p&sites japonicus and transplantability of induced tumors. Gann 68, 841-845. MORI, H., TANAKA, T., NISHIKAWA, A., TAKAHASHI, M., AND WILLIAMS, G. M. (1981). Enhancing effect of barbital on N-2-fluorenylacetamide induced rat liver lesions. Gann 72,798-80 1. MORIYA, M., HARADA, T., AND SHIRASU, Y. (1982). Inhibition of carcinogenicities of 1,Zdimethylhydrazine and azoxymethane by pyrazole. Cancer Lett. 17, 147152. MURRAY, A. W., AND FITZGERALD, D. J. (1979). Tumor promoters inhibit metabolic cooperation in co-cultures of epidermal and 3T3 cells. Biochem. Biophys. Res. Common. 91, 385-401. NAKAHARA, W. (1970). Mode of origin and characterization of cancer. In Chemical Tumor Problems (W. Nakaham, ed.), pp. 287-330. Japanese Society for the Promotion of Science, Tokyo. NAKAHARA, W., AND FUKUOKA, F. (1960). Summation of carcinogenic effects of chemically unrelated carcinogens, 4-nitroquinoline N-oxide and 20-methylcholanthrene. Gann 51, 125-137. NEWBERNE, P. M. (1976). Environmental modifiers of susceptibility to carcinogenesis. Cancer Detec. Prev. 1, 129-173. NOVI, A. M. (1981). Regression of aflatoxin B1-induced hepatocellular carcinomas by reduced glutathione. Science 212, 541-542.

ODASHIMA, S. (1959). Development of liver cancers in the rat by 20-methylcholanthrene painting following initial 4-dimethylaminoazobenzene feeding. Gann SO, 321-345. PEGG, A. E. (1978). Dimethylnitrosamine inhibits enzymatic removal of 06-methylguanine from DNA. Nature (London) 274, 182- 184. PERAINO, C., FRY, R. J. M., AND STAFFELM, E. (197 1). Reduction and enhancement of phenobarbital of hepatocarcinogenesis induced in the rat by 2-actylaminofluorene. Cancer Res. 31, 1506-l 5 15. PERAINO, C.. FRY, R. J. M., AND GRUBE, D. D. (1978).

CHEMICAL

CARCINOGENESIS

Drug-induced enhancement of hepatic tumorigenesis. In Carcinogenesis, A Comprehensive Survey (T. J. Slaga, A. Sivak, and R. K. Boutwell, eds.), Vol. 2, pp. 421432, Raven Press, New York. PEREIRA, M. A. (1982). Rat liver foci bioassay. J. Amer. Coil. Toxicol. 1, 101-117. PITOT, H. C., ed. (1978). Fundamentals of Oncology. Dekker, New York. PITOT, H. C., GOLDSWORTHY, T., MORAN, S., SIRICA, A. E., AND WEEKS, J. (1982). Properties of incomplete carcinogens and promoters in hepatocarcinogenesis. In Carcinogenesis, A Comprehensive Survey, (T. J. Slaga, A. Sivak, and R. K. Boutwell, eds.), Vol. 7, pp. 85-98. Raven Press, New York. POLLARD, M., AND LUCKERT, P. H. (1980). Indomethacin treatment of rats with dimethylhydrazine-induced intestinal tumors. Cancer Treat. Rep. 64, 1323-1327. POUND, A. W., LAWSON, T. A., AND HORN, L. (1973). Increased carcinogenic action of dimethylnitrosamine after prior administration of carbon tetrachloride. Brit. J. Cancer 27,45 l-459. PROTZEL, M., GLARDINA, A. C., AND ALBANO, U. H. ( 1964). The effectof liver imbalance on the development of oral tumors in mice following the applications of benzpyrene or tobacco tar. Oral. Surg Oral Med. Oral Pathol. 18, 622-634. PURNELL, M. R., AND WHISH, W. J. D. (1980). Novel inhibitors of poly(ADP-ribose)synthetase. B&hem. J. 185, 775-777. PLJRON, R., AND FIRMINGER, H. I. (1965). Protection against induced cirrhosis and hepatocellular carcinoma in rats by chloramphenicol. J. Nat. Cancer Inst. 35, 29-37. RADIKE, M. S., STEMMER, K. L., BROWN, P. G., LARSON, E., AND BINGHAM, E. (1977). Effect of ethanol and vinyl chloride on the induction of liver tumours: Preliminary report. Environ. Health Perspect. 21, 153- 155. REDDY, J. K., AND RAO, M. S. (1978). Enhancement by WY-14,643, a hepatic perioxisome proliferator, of diethylnitrosamineinitiated hepatic tumorigenesis in the rat. Brit. J. Cancer 38, 537-543. REDDY, J. K., AZARNOFF, D. L., AND HIGNITE, C. E. ( 1980). Hypolipidaemic hepatic peroxisome proliferators form a novel class of chemical carcinogens. Nature (London)

283, 397-398.

REDDY, B. S., COHEN, L. A., MCCOY, G. D., HILL, P., WEISBURGER,J. H., AND WYNDER, E. L. (1980). Nutrition and its relationship to cancer. Adv. Cancer Res. 32, 237-345. REISS,B., TONG, C., AND WILLIAMS, G. M. (1983). Enhancement of benzo(a)pyrene mutagenicity by chrysotile asbestos in rat liver epithelial cells. Environ. Res. 31, 100-104. REZNIK-SCH~LLER, H. M., AND LLIINSKY, W. (1981). Morphology of early changes in liver carcinogenesis induced by methapyrilene. Arch. Toxicol. 49, 79-83. RICHARDSON,H. L., STEIN, A. R., AND BORSOS-NACHT-

AND XENOBIOTICS

341

NEBEL, E. (1952). Tumor inhibition and adrenal histologic responses in rats in which 3’-methyl-4dimethylaminoazobenzene and 20-methylcholanthrene were simultaneously administered. Cancer Res. 12,356-37 1. ROSIN, M. P., AND STICH, H. F. (1978). The inhibitory effect of cysteine on the mutagenic activities of several carcinogens. Mutation Res. 54, 73-8 1. ROSSMAN, T. G., AND TROLL, W. (1980). Protease inhibitors in carcinogenesis: Possible sites of action. In Carcinogenesis, Vol. 5, ModiJiers of Chemical Carcinogenesis (T. J. Slaga, ed.), pp. 127- 143. Raven Press, New York. Rous, P., AND KIDD, J. G. (194 1). Conditional neoplasms and subthreshold neoplastic states. A study of the tar tumors of rabbits. J. Exp. Med. 73, 365-390. RUSTIA, M., AND SHUBIK, P. (1979). Effects of transplacental exposure to diethylstilbestrol on carcinogenic susceptibility during postnatal life in hamster. Cancer Res. 39,4636-4644.

SAFF~OTTI, U., CEFIS, F., AND KOLB, L. H. (1968). A method for the experimental induction of bronchogenic carcinoma. Cancer Res. 28, 104-124, 1968. SARACCI,R. (1977). Asbestos and lung cancer: An analysis of epidemiological evidence on the asbestos-smoking interaction. Int. J. Cancer 20, 323-331. SCHMAHL, D. ( 1970). Syncarcinogenesis: Experimental investigations. In Chemical Tumor Problems (W. Nakahara, ed.), pp. I-18. Japanese Society for the Promotion of Science, Japan. SCHMAHL, D. (1980). Combination effects in chemical carcinogenesis. Arch. Toxicol. Suppl. 4, 29-40. SELIKOFT, I. J. (1977). Cancer Risk of Asbestos Exposure. In Origins of Human Cancer, Vol. 4, Human Risk Assessment (H. H. Hiatt, J. D. Watson, and J. A. Winsten, eds.), pp. 1765-1784, Cold Spring Harbor Laboratory, Cold Spring Harbor, New York. SILINSKAS, K. C., AND OKEY, A. B. (1975). Protection by 1,l ,-trichloro-2,2-bis(pchlorophenyl)ethane (DDT) against mammary tumors and leukemia during prolonged feeding of 7,12-dimethylbenz(a)anthracene to female rats. J. Natl. Cancer Inst. 55, 653-657. SIROVER,M. A., AND LOEB, L. A. (1976). Metal-induced infidelity during DNA synthesis. Proc. Nat. Acad. Sci. 73,2331-2335. SIVAK, A. (1979). Cocarcinogenesis. Biochim. Biophys. Acta 560, 67-89.

SLAGA, T. J. (1980). Antiintlammatory steriods: Potent inhibitors oftumor promotion. In Carcinogenesis, Vol. 5, Modijiers of Chemical carcinogenesis (T. J. Slaga, ed.), pp. 11 I-126. Raven Press, New York. SLAGA, T. J., SIVAK, A., AND BOUTWELL, R. K. @Is.) (1978). In Carcinogenesis, A Comprehensive Survey, Vol. 2, Mechanisms of Tumor Promotion and Cocarcinogenesis, pp. 588. Raven Press, New York. SNYDER,R., AND REMMER, H. (1979). Classesof hepatic microsomal mixed function oxidase inducers. Pharmacol.

Ther. 7, 203-244.

GARY M. WILLIAMS

342

SPARNINS,V. L., VENEGAS, P. L., AND WATTENBERG, L. W. (1982). Glutathione S-transferase activity by compounds inhibiting chemical carcinogenesis and by dietary constituents. J. Natl. Cancer Inst. 68,493-496. SPORN,M. B. (1980). Retinoids and cancer prevention. In Carcinogenesis, Vol. 5, Modifiers of Chemical Carcinogenesis (T. J. Slaga, ed.), pp. 99-109. Raven Press, New York. SPRINGGATE, C. F., AND LQEB, S. A. (1973). Mutagenic DNA polymerase in human leukemic cells. Proc. Natl. Acad. Sci. 10, 245-249.

STEINER,P. E. (1955). Carcinogenicity of multiple chemicals simultaneously administered. Cancer Res. 15,632635.

STENBKCK, F. (1978). Tumor persistence and regression in skin carcinogenesis. Z. Krebsforsch. 91, 249-259. STOTT, W. T., AND WATANABE, P. G. (1982). Differentiation of genetic versus epigenetic mechanisms of toxicity and its application to risk assessment.Drug. Metab. Rev. 13, 853-873.

SWENBERG,J. A., BEDELL, M. A., BILLINGS, K. C., UMBENHAUER,D. R., AND PEGG, A. E. (1982). Cell-specific differences in 06-alkylguanine DNA repair activity during continuous exposure to carcinogen. Proc. Nat. Acad. Sci. 79, 5499-5502.

TAKAHASHI, S., OHNISHI, T., DENDA, A., AND KONISHI, Y. (1982). Enhancing effect of 3-aminobenzamide on induction of y-glutamyl transpeptidase positive foci in rat liver. Chem-Biol Interact. 39, 363-368. TAKAYAMA, S., AND IMAIZUMI, T. (1969). Sequential effects of chemically different carcinogens, dimethylnitrosamine and 4dimethylaminoazobenzene, on hepatocarcinogenesis in rats. Int. J. Cancer 4, 373-383. TATEMATSU, M., HASEGAWA, R., IMAIDA, K., TSUDA, H., AND ITO, N. (1983). Survey of various chemicals for initiating and promoting activities in a short-term in vivo systembased on generation of hyperplastic liver nodules in rats. Carcinogenesis 4, 381-386. TATEMATSU, M., MIYATA, Y., MIZUTANI, M., HANANOUCHI, M., MIROSE, M., AND ITO, N. (1977). Summation effect of N-butyl-N-(4-hydroxybutyl) nitrosamine, N[-4-(5-nitro-2-furyl)-2-thiazolyl]-formamide, N2-fluorenylacetamide, 33’dichlorobenzidine on urinary bladder carcinogenesis in rats. Gann 68, 183-202. TAWF~C,H. N. (1965). Studies on ear duct tumors in rats. Part I: Inhibitory effects of methylcholanthrene and 1,2-benzanthracene on tumor formation by 4-dimethylaminostilbene. Acta Pathol. Japon. 15, 255-260. TELANG, S., TONG, C., AND WILLIAMS, G. M. (1982). Epigenetic membrane effects of a possible tumor promoting type on cultured liver cells by the nongenotoxic organochlorine pesticides chlordane and heptachlor. Carcinogenesis 3, 1175-l 178. TOPPING, D. C., AND NETTEZSHEIM,P. (1980). Two-stage carcinogenesis studies with asbestos in Fischer 344 rats. J. Nat. Cancer

Inst. 65, 627-630.

TROSKO, J. E., YO-ITI, L. P., DAWSON, B., AND CHANG, C. C. (1981). In vitro assay for tumor promoters. In Short-Term Tests for Chemical Carcinogens (H. F. Stich and R. H. C. San, eds.), pp. 420-427. Springer-Verlag, New York. TWORT, S. M., AND TWORT, C. C. (1939). Comparative activity of some carcinogenic hydrocarbons. Amer. J. Cancer

35, 80-85.

ULLAND, B., WEISBURGER, J., YAMMAMOTO, R., AND WEISBURGER,E. (1973). Antioxidants and carcinogenesis: Butylated hydroxytoluene but not diphenyl-pphenylenediamine, inhibits cancer induction by N-2fluorenylacetamide and by N-hydroxy-N-2-fluorenylacetamide in rats. Food Cosmet. Toxicol. 11, 199-207. UMEDA, M., NODA, K., AND ONO, T. (1980). Inhibition of metabolic cooperation in Chinese hamster cells by various chemicals including tumor promoters. Gann 71, 614-620.

VAN DUUREN, B. L. (1969). Tumor-promoting agents in two-stage carcinogenesis. Prog. Exp. Tumor Res. 11, 3 l-68.

VAN DUUREN, B. L., AND MELCHIONNE, S. (1969). Inhibition of tumorigenesis. Prog. Exp. Tumor Res. 12, 5.5-94.

VAN DUUREN, B. L., SIVAK, A., KATZ, C., SEIDMAN, I., AND MELCHIONNE, S. (1975). The effect of aging and interval between primary and secondary treatment in two stage carcinogenesis on mouse skin. Cancer Res. 35, 502-505.

WAGNER, J. C. (Ed.) ( 1980). Biological Effects of Mineral Fibres, Vols. 1 and 2. International Agency for Research on Cancer, Lyon. WATANABE, K., AND WILLIAMS, G. M. (1978). The enhancement of rat hepatocellular altered foci by the liver tumor promoter phenobarbital: Evidence that foci are precursors of neoplasm and that the promoter acts on carcinogen-induced lesions. J. Nat. Cancer. Inst. 61, 131 I-1314. WATTENBERG, L. W. (1972). Inhibition of carcinogenic and toxic effectsof polycyclic hydrocarbons by phenohc antioxidants and ethoxyquin. J. Nat. Cancer Inst. 48, 1425-1430. WATTENBERG, L. W. (1973). Inhibition of chemical carcinogen-induced pulmonary neoplasia by butylated hydroxyanisole. J. Nat. Cancer Inst. SO, 1541-1544. WATTENBERG, L. W. (1974). Inhibition of carcinogenic and toxic effects of polycyclic hydrocarbons by several sulfur-containing compounds. J. Nat. Cancer Inst. 52, 1583-1587. WATTENBERG, L. W. (1975). Brief communication: Inhibition of dimethylhydrazine-induced neoplasia of the large intestine by disulfiram. J. Nat. Cancer Inst. 54, 1005-1006. WATTENBERG, L. W. (1978). Inhibitors of chemical carcinogenesis. In Advances in Cancer Research (G. Klein

CHEMICAL

CARCINOGENESIS

and S. Weinhouse, eds.), Vol. 26, pp. 197-226. Academic Press, New York. WATTENBERG, L. W., AND LEONG, J. L. (1968). Inhibition of the carcinogenic action of 7,1Zdimethylbenz(a)anthracene by beta-naphthoflavone. Proc. Sot. Exp. Biol. Med. 128, 940-943. WATTENBERG, L. W., AND LEONG, J. L. (1970). Inhibition of the carcinogenic action of benzo(a)pyrene by flavones. Cancer Res. 30, 1922-1925. WEINSTEIN, I. B., YAMASAKI, H., WIGLER, M., LEE, L. S., FISHER, P. B., JEFFREY,A. M., AND GRUNBERGER, D. ( 1979). Molecular and cellular events associated with the action of initiating carcinogens and tumor promoters. In Carcinogens, Identification and Mechanisms ofAction (A. C. Griffin and C. R. Shaw, eds.), pp. 399418. Raven Press, New York. WEISBURGER, E. K. (1977). Bioassay program for carcinogenic hazards of cancer chemotherapeutic agents. Cancer 40, 1935-1951. WEISBURGER, J. H., YAMAMOTO, R. S., WILLIAMS, G. M., GRANTHAM, P. H., MATSUSHIMA, T., AND WEISBURGER,E. K. ( 1972). On the sulfate eater of N-hydroxyN-2-lluorenylacetamide as a key ultimate hepatocarcinogen in the rat. Cancer Res. 32,491-500. WEISBURGER, J. H., AND WILLIAMS, G. M. (1980). Chemical carcinogens. In Toxicology The Basic Science of Poisons (J. Doull, C. D. Klaassen, and M. 0. Amdur, eds.), 2nd ed., pp. 84-138. Macmillan Co., New York. WEISBURGER,J. H., AND WILLIAMS, G. M. (198 I). Basic requirements of health risk analysis: the decision point approach for systematic carcinogen testing. In Proceedings of the Third L$e Sciences Symposium on Health Risk Analysis, pp. 249-247, Franklin Press, Philadelphia. WEISBURGER,J. H., AND WILLIAMS, G. M. (1982a). Metabolism of chemical carcinogens. In Cancer: A Comprehensive Treatise (F. F. Becker, ed.), 2nd ed., pp. 241-333. Plenum, New York. WEISBURGER, J. H., AND WILLIAMS, F. M. (1982b). Chemical carcinogenesis. In Cancer Medicine (J. F. Holland and F. Frei, eds.), 2nd ed., pp. 42-95. Lea & Febiger, Philadelphia. WEISBURGER,J. H., YAMAMOTO, R. S., KORZIS, J., AND WEISBURGER,E. K. (1966). Liver cancer: Neonatal estrogen enhances induction by a carcinogen. Science 154, 673-674.

WEISBURGER,E. K., YAMAMOTO, R. S., GLASS, R. M., GRANTHAM, P. H., AND WEISBURGER, J. H. (1968). Effect of neonatal androgen and estrogen injection on liver tumor induction by N-hydroxy-N-2-thtorenylacetamide and on the metabolism of the carcinogen in rats. Endocrinology 82, 685-692. WHEATLEY, D. N. (1968). Enhancement and inhibition of the induction by 7,12dimethylbenz(a)anthracene of mammary tumors in female spraguedawley rats. Brit. J. Cancer 22, 787-792.

AND XENOBIOTICS

343

WHEELOCK, E. F., WEINHOLD, K. J., AND LEVICH, J. (1981). The tumor dormant state. Adv. Cancer Res. 34, 107-140.

WIEBEL, F. J. ( 1980). Activation and inactivation of carcinogens by microsomal monooxygenases: Modification of benzoflavones and polycychc aromatic hydrocarbons. In Carcinogenesis, Vol. 5, Modifiers of Chemical Carcinogenesis (T. J. Slaga, ed.), pp. 57-84, Raven Press, New York. WILLIAMS, G. M. (1980a). The pathogenesis of rat liver cancer caused by chemical carcinogens. B&hem. Biophys. Acta Rev. Cancer 605, 167-189. WILLIAMS, G. M. (198Ob). Classification of genotoxic and epigenetic hepatocarcinogens using liver culture assays. Ann. N.Y. Acad. Sci. 349, 273-282. WILLIAMS, Cl. M. (198 1). Liver carcinogenesis: The role for some chemicals of an epigenetic mechanism of liver tumor promotion involving modification of the cell membrane. Food Cosmet. Toxicol. 19, 577-583. WILLIAMS, G. M., KATAYAMA, S., AND OHMORI, T. (198 la). Enhancement of hepatocarcinogenesis by sequential administration of chemicals: Summation versus promotion effects. Carcinogenesis 2, 1111-l 117. WILLIAMS, G. M., TELANG, S., AND TONG, C. (1981b). Inhibition of intercellular communication between liver cells by the liver tumor promoter 1,1, I-trichloro-2,2bis (P-chlorophenyl)ethane (DDT). Cancer Lett. 11, 339-344.

WILLIAMS, G. M. (1982a). Sex hormones and liver cancer. Lab. Invest. 46, 352-354. WILLIAMS, G. M. (1982b). Phenotypic properties of preneoplastic rat liver lesions and applications to detection of carcinogens and tumor promoters. Toxicologic Pathol. 10, 3-10. WILLIAMS, G. M. (1983a). Genotoxic and epigenetic carcinogens: Their identification and signiticance. Ann. N. Y. Acad. Sci. 407, 328-333. WILLIAMS, G. M. (1983b). Epigenetic effectsof liver tumor promoters and implications for health effects.Environ. Health Perspect. SO, 177-183. WILLIAMS, G. M., AND FURUYA, K. (1984). Distinction between liver neoplasm promoting and syncaminogenic effects demonstrated by reversing the order of administering phenobarbital and diethylnitrosamine either before or after N-2-fluorenylacetamide. Carcinogenesis 5, 171-174. WILLIAMS, G. M., MAEURA, Y., AND WEISBURGER, J. H. (1983). Simultaneous inhibition of liver carcinogenicity and enhancement of bladder carcinogenicity of N-2-fluoreny1acetamide by butylated hydroxytoluene. Cancer Lett. 19, 55-60. WISLOCKI, P. G., MILLER, E. C., MILLER, J. A., McCoy, E. C., AND ROSENKRANZ, H. S. (1977). Carcinogenic and mutagenic activities of safrole, I’-hydroxysafrole, and some known or possible metabolites. Cancer Res. 37, 1883-1891.

344

GARY M. WILLIAMS

WITSCHI, H., AND LOCK, S. (1978). Butylated hydroxytoluene: A possible promoter of adenoma formation in mouse lung. In Carcinogenesis, Mechanisms of Tumor Promotion and Cocarcinogenesis (T. J. Slaga, A. Sivak, and R. K. Boutwell, eds.), Vol. 2, pp. 465-474, Raven Press, New York. WONG, L. C. K., WINSTON, J. M., HONG, C. B., AND PLOTNICK, H. (I 982). Carcinogenicity and toxicity of I ,2dibromoethane in the rat. Toxicol. Appl. Pharmacol. 63,155-165.

YAGER, J. D., JR. (1983). Oral contraceptive steroids as promoters or complete carcinogens for liver in female Sprague-Dawley rats. Environ. Health Perspect. 50, 109-112.

YAMAMOTO, R. S., GLASS, R. M., FRANKEL, H. H.,

WEISBURGER,E. K., AND WEISBURGER,J. H. (1968). Inhibition of the toxicity and carcinogenicity of N-2fluorenylacetamide by acetanilide. Toxicol. Appl. Pharmacol.

13,108-117.

YAMAMOTO, R. S., WEISBURGER,J. H., AND WEISBURGER, E. K. (197 1). Controlling factors in urethane carcinogenesis in mice: Effects of enzyme inducers and metabolic inhibitors. Cancer Res. 31, 483-486. YOTTI, L. P., CHANG, C. C., AND TROSKO, J. E. (1979). Elimination of metabolic cooperation in Chinese hamster cells by a tumor promoter. Science 206, 10891091. ZEDECK, M. S., AND LIPKIN, M., (Eds.) (198 1). Inhibition of Tumor Induction and Development. Plenum Press, New York.