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Fat Metabolism and Cancer
Nutrition and Cancer I
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Fat Metabolism and Cancer Philomena F. McAndrew, M.D. *
The cachexia seen in cancer is not fully explained by inadequate energy intake and increased energy expenditure. Studies in humans and animals suggest that depletion of body fat is out of proportion to protein loss and may account for the majority of weight loss seen with cancer. 1. 5. 9, 29, 35, 55 Specific abnormalities in lipid metabolism have been noted,46, 51, 55 but their pathogenesis has been poorly understood. Delineation of such altered lipid metabolism may lead to improved management of malnutrition in cancer patients.
THE ROLE OF LIPIDS IN NORMAL METABOLISM The oxidation of fatty acids is the major source of fuel for daily energy needs in mammals. Beta-oxidation of fatty acids occurs in the mitochondria, during which progressive enzymatic shortening of the fatty acid carbon chain produces acetyl coenzyme A, a high-energy intermediate leading to the formation of ATP. Five moles of ATP are produced per mole of oxygen used to produce acetyl coenzyme A in the Krebs cycle. 28 Fatty acids may also escape this oxidative fate and instead work in the formation of new cell membranes. The major source of lipid used as fuel is dietary fat (usually in the form of triglycerides), although lipids may be mobilized from adipose tissue that acts as a fuel reservoir. An adult man is estimated to have a total fuel reserve of approximately 166,000 kcal, 141,000 kcal of which are in the form of lipid. 8 Before the fatty acids stored in adipose tissue can be used as an energy source, they must first be hydrolyzed from triglycerides in the fat tissue. The rate-limiting *Assistant Professor of Medicine, Department of Medicine, UCLA School of Medicine, and Division of Hematology/Oncology, Wadsworth VA Medical Center, Los Angeles, California
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step in this process is an enzyme called hormone-sensitive lipase. This enzyme is activated by a cyclic nucleotide-triggered activation of a protein kinase, which in turn converts the enzyme from an inactive to an active phosphorylated form. Catecholamines, glucagon, and several peptide hormones (including ACTH, growth hormone, and TSH) can bind to receptors on the adipocyte and trigger the activation of hormone-sensitive lipase. 56 Most of the dietary lipid present in the intestinal lumen is in the form of emulsion droplets. Triglycerides are absorbed after enzymatic conversion to more polar compounds-free fatty acids (FF As) and 2-monoglyceride. 12 Once absorbed into the mucosal cell, the FFA and monoglyceride may pass through the plasma membrane by difIusion44 and into the endoplasmic reticulum, where triglyceride is again generated and combined with apolipoprotein B in chylomicrons. 6 , 20, 58 The triglyceride component of the chylomicron is removed by adipose tissue and muscle. Lipoprotein lipase is involved in the hydrolysis of triglycerides to FFA and glycerol and their uptake into the adipocyte in the process of fat deposition. 34 In the adipocyte, FFA and alpha-glycerophosphate are converted once again into triglyceride.
STARVATION VERSUS CANCER CACHEXIA During early fasting, fat mobilization is promoted by falling levels of insulin and increased sympathetic nerve activity to the adipose tissue. Early rapid proteolysis occurs with amino acid mobilization from muscle, gluconeogenesis, and production of urinary urea nitrogen. This would result in the loss of 75 gm of muscle protein daily. As a part of the life-saving adaptation to total starvation, the body gradually converts from a glucose and amino acid economy to a fat-derived fuel (ketone bodies) economy. This complex adaptation results in a decrease in oxygen consumption and a sparing of protein breakdown. 57 This starvation adaptation is accompanied by the ability of FFA to become the prime fuel for most tissues and the brain to adapt to the use of ketone bodies. 42 In patients with cancer cachexia these adaptations do not occur. 19, 45, 51, 52 The decrease in oxygen consumption and carbon dioxide production normally seen during starvation does not occur in patients with cancer. 45, 53, 57 Recycling oflactate and pyruvate occurs together, with an increase in the use of acetoacetate by tumor tissue in the cancer-bearing host. 23, 51-53
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HOST METABOLIC ABNORMALITIES IN CANCER Studies in humans 21 , 45 and animals 29-32 have shown that total host lipids decrease as tumor growth proceeds. When tumor weight in the rat bearing a Walker 256 carcinoma exceeded 7 per cent of body weight, significant decreases in total body lipids were already evident. As tumor mass increased further, there was an inverse linear relation between tumor weight and total carcass lipid. 27 This loss cannot be caused by decreased caloric intake alone, since pairfed animals do not lose as much fat as tumor-bearing animals. Force feeding a high-fat diet to these animals reduces but does not prevent the loss of carcass fat. 31, 32 Transplantable tumors have provided a model to study lipid mobilization in animals. Liebelt29 noted severe fat depletion in experimental animals with noninvasive tumors, which could be reproduced by transplanting frozen and thawed tumor extracts in animals. Some tumors caused rapid depletion of total lipid stores, whereas others had no significant effects on body lipid content. The validity of these animal studies is questioned because human tumors rarely exceed 5 per cent of total body weight. Several clinical investigators have noted increased FFA mobilization before marked losses of body weight occurred. 27, 52, 53 Kralovic and colleagues,27 using transplantable Walker 256 carcinoma in rats, noted an early mobilization of fat even when the tumor burden was less than 4 per cent of total body weight. In these studies, a two- to threefold elevation in the basal rate of lipolysis was noted with these relatively small tumors in animals. When these animals developed ascites, the cell-free fluid from the ascites was able to increase lipolysis in tumor-free control animals. This suggested the elaboration of a lipid mobilizing factor by the transplanted tumor and not a catechol or hormone that stimulated lipolysis via adenyl cyclase, since lipolysis is not inhibited by propranolol and prostaglandin E (PGE). Further evidence of increased lipolysis in animals is an elevation of plasma levels of FFAs in several experimental systems. 14, 22, 38 Costa and co-workers10 studied the lipid composition of muscle biopsy samples obtained from patients with cancer and found that the fat content was half that present in control subjects. Studies in patients with cancer have failed to confirm plasma elevation of FFA consistently.22, 38, 45 Oxidation rates of fatty acid increase in patients with cancer given isotopically labeled fatty acid substrate. 29, 54 Moreover, when glucose, which normally suppresses fatty acid oxidation, is infused, inhibition of fatty acid oxidation decreases in patients with cancer compared with control subjects. In the fasting state,
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although FFA levels are similar, slightly more FF A is oxidized to CO 2 in cancer patients. There are clear abnormalities in glucose metabolism in patients with cancer, and the failure of glucose to suppress fatty acid oxidation may be secondary to abnormalities in glucose metabolism. No futile cycles of lipid metabolites have been found in patients with cancer similar to the known increases in recycling of glucose and lactate known as the Cori cycle.
PATHOGENESIS OF LIPID ABNORMALITIES: POTENTIAL MECHANISMS Remodeling Remodeling of metabolic substrates prior to oxidation might redirect substances from fuel-efficient .pathways into energy-requiring paths prior to final oxidation. An increased contribution of fatty acid to oxidation when glucose is available has been noted by Waterhouse and others. 54 This continued use of fatty acids when preferred substrates are present may be secondary to remodeling of glucose prior to oxidation. Glucose can be oxidized to two-carbon fragments and synthesized into fatty acids in an energy-requiring process rather than proceeding through the energy-generating Krebs cycle. Tumors also undergo increased anaerobic glycolysis, allowing the recycling of lactate and pyruvate produced and their conversion to glucose in an energy-requiring pathway (Cori cycle). 11. 15,57
Increased Energy Expenditure Although increased energy requirements do not explain specific metabolic changes seen in cancer patients, they may play an important role in the degree of weight loss and disordered metabolism seen with some cancers. It is known that tumors can obtain substrates and energy necessary for their synthesis from the host at a significant energy cost. It has been hypothesized that tumors may produce substances that could uncouple oxidative phosphorylation. Menon and colleagues 37 have shown a difference in the beta-oxidation of palmitate by normal and lymphoma-bearing AKR mice. This strain of mice has an almost 100 per cent mortality from thymic lymphoma between the ages of 6 and 14 months, with a peak incidence at 9 to 10 months. Within the control group of mice, two distinct populations can be separated, one Clow controls") showing an overall decrease in beta-oxidation of palmitate at a subcellular level similar to that in mice with advanced lymphoma. Elimination of exogenous energy (ATP) from the medium causes a more dramatic decrease in oxidation by this group as compared with normal controls. Fragile tumor mitochondria having uncoupled oxidative phosphorylation with impaired ATP synthesis could explain such findings in tumor-bearing
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animals, whereas the "low controls" might indicate an early or preneoplastic state in which metabolic abnormalities precede detectable malignant morphologic changes.
Decreased Lipogenesis In a process analogous to the nitrogen trap, tumors are probably using host fatty acids for tumor lipid synthesis. Mice bearing a nonmetastatic tumor (ESR 586) were found to have lipid depletion. 46 The levels of lipoprotein lipase, the enzyme necessary for the storage of fat as triglycerides from circulating FF As, were measured in the fat tissues of these animals and in the tumor mass. The level of lipoprotein lipase in host fat tissue was decreased, as is normally seen during starvation, but compared with fat tissue levels of the control tumor-free animals, the tumor lipoprotein lipase levels were increased. A substance produced by tumors may modulate lipoprotein lipase levels in the host. Recently, tumor necrosis factor (TNF)/cachectin has been isolated, sequenced, and synthesized, and its effects on lipogenesis have been studied. Some of this initial work evolved from the recognition of significant metabolic abnormalities, specifically mobilization of triglycerides from adipose tissue, associated with certain infections. These abnormalities were found not to be caused by endotoxin itself but by a protein secreted from endotoxin-stimulated macrophages. Cachectin was found to be a 17,000 molecular weight subunit that forms non covalent multimers and is active at 45,000 molecular weight. A high degree of homology was found between cachectin and TNF. Purified cachectin possessed potent TNF activity in vivo (cytolysis), suggesting a single protein. 2--4, 49 Cachectin was found to modulate the metabolic activity of normal and neoplastic cells through interactions with specific high-affinity receptors. CachectinlTNF specifically suppressed the activity of lipoprotein lipase in vivo and in vitro using cultured preadipocytes 3T3-Ll. Cachectin also inhibited the synthesis of fatty acid synthetase and acetyl coenzyme A carboxylase in adipocytes. Treatment with cachectin was found to prevent the accumulation of adipose-inducible mRNAs as well as lipid accumulation in these cells; this effect was reversed by the removal of cachectin.
Insulin Deficiency/Resistance Abnormalities in insulin secretion, or sensitivity, could affect the rates of lipolysis and lipogenesis in patients with cancer. Insulin stimulates the uptake and storage of glucose and the storage of triglycerides in adipose tissue. Insulin acts in part by stimulating synthesis of lipoprotein lipase and alpha-glycerophosphate and by
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inhibiting hormone-sensitive lipase that breaks down triglycerides. Insulin probably inhibits hormone-sensitive lipase by modulating intracellular levels of cyclic AMP, although this has not been shown definitely.7, 13, 17,43,45 Insulin administered to tumor-bearing animals ameliorates the loss of fat that occurs, 50 suggesting that insulin deficiency may play a role in the increased fat mobilization seen in patients with cancer. Other work has revealed a decreased clearance of glucose from the blood after exogenous insulin administration, supporting a relative insulin resistance. 33 , 45
Specific Mediators of Altered Lipid Metabolism Tumors produce a variety of hormones and proteins that exert metabolic effects and well-known paraneoplastic syndromes. Examples include the production of ectopic ACTH, parathyroid hormone, and antidiuretic hormone. 41 Substances may also be produced that cross-react immunologically with a variety of hormones. 41 Tumorderived growth factors that regulate cell proliferation are another class of hormones extracted from various malignant and nonmalignant cell lines. 16, 18, 47, 48 These factors compete with normal growth factors for specific binding to receptors and thus may affect the metabolic responses of target cells to circulating growth factors. Several other tumor-associated factors have been examined for potential involvement in cancer-related cachexia. Nakahara and Fukuoka isolated from tumor tissues a factor they coined toxohormone. 39, 40 This factor caused lipid depletion in the host as well as immunosuppression and thymic involution. Toxohormone is a 75,000 dalton protein found in several tumors. Toxohormone-L was isolated by Masuno and coworkers 35 , 36 from sarcoma-bearing mice and patients with hepatoma. When injected into animals, this factor also caused lipid mobilization. Mead, Kitada, and co-workers at UCLA23-25 have used the AKR lymphoma-bearing mouse as a model to study lipid mobilization. Adipose tissue prelabeled with radioactive fat in vitro was implanted in control and tumor-bearing animals. The rate of lipolysis was then judged by the amount of radioactive carbon dioxide derived from fat oxidation and expired by the animals. They found increased rates of lipolysis and fatty acid oxidation. Compared with normal tissues of the cancer-bearing host and control animals, the tumor tissues had incorporated increased amounts of the fatty acids released by the adipose tissue. A simplified in vivo bioassay was developed to test for lipolysis: radiolabeled linoleic acid was injected into fat pads of tumor-bearing and control animals while expired carbon dioxide was again measured to assess fatty acid oxidation and lipid mobilization in the fasted state compared with the fed state. Tumor-bearing animals showed increased carbon dioxide evolution in both the fed
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and fasted states. Control animals given injections of crude tumor homogenates, or serum from lymphoma-bearing animals, increased their mobilization of radioactive fat from the injected fat pads. The serum of patients with a variety of advanced malignancies similarly caused increased lipid mobilization. Lipid mobilizing factor has been partially purified from thymic tissue of AKR lymphoma-bearing mice and from the supernatant of lymphoma cell lines derived from the AKR lymphoma. This factor is a small peptide with an apparent molecular weight of about 5000 daltons; it occurs in cell-free and serum-free fractions of supernatants of lymphoma cell lines. 26
SUMMARY Progressive weight loss and anorexia are frequent phenomena in cancer patients. Although cachexia is an expected occurrence in the terminal stages of nearly all malignancies, it may be a presenting sign when the tumor burden is quite small. Lipid depletion occurs out of proportion to the protein loss and accounts for most of the weight loss in cancer. Lipids, more specifically fatty acids, are the major source of fuel in mammals and may also be used in the synthesis of new cell products. Lipolysis and lipogenesis are under the influence of several important enzymes and peptide hormones that may be modulated by a variety of exogenous factors. There is evidence that cancer patients have lost the normal homeostatic responses to decreased energy intake or starvation that allow a decrease in oxygen consumption and protein sparing. An increase in Cori cycle activity or futile recycling of metabolic products occurs with a net energy expenditure rather than energy production. Clinical studies have shown that the body lipid depletion accompanying tumor progression is not solely secondary to decreased food intake and may be reproduced by the transplantation of certain noninvasive tumors to normal hosts. Elevated basal lipolysis has occasionally been seen early in tumor growth. Such findings suggest the presence of a tumor-associated factor responsible for this increase in lipid mobilization. Some of the potential mechanisms for the altered lipid metabolism seen in cancer have been discussed. Metabolic substrates may be remodeled and directed away from fuel-efficient into energyrequiri.ng pathways. An increased energy expenditure may occur as a result of the energy costs of tumor synthesis, an uncoupling of oxidative phosphorylation, or energy-requiring futile cycling. An overall depletion of lipid may be the final outcome of the inhibition of lipid deposition. TNF/cachectin has recently been found to sup-
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press the activity and synthesis of several key lipogenic enzymes, including lipoprotein lipase. Abnormalities in insulin secretion or sensitivity may be involved in the decrease of fat storage in malignancy. Insulin also exerts a significant antilipolytic effect by its antagonism of hormone-sensitive lipase. Mediators of lipolysis and abnormal lipid metabolism may occur in a number of clinical conditions and include ectopic hormone production, growth factors, and tumor-associated lipolytic factors (lipid mobilizing factor, toxohormone).
REFERENCES 1. Axelrod, L., and Costa, G.: Contribution of fat loss to weight loss in cancer. Nutr. Cancer, 2:81-83, 1980. 2. Beutler, B., Greenwald, D., Hulmes, J. D., et al.: Identity of tumor necrosis factor and the macrophage-secreted factor cachectin. Nature, 316:552-554, 1985. 3. Beutler, B., Mahoney, J., LeTrang, N., et al.: Purification of cachectin, a lipoprotein lipase suppressing hormone secreted by endotoxin induced FAW 264.7 cells. J. Exp. Med., 161:984-995, 1985. 4. Beutler, B., Mahoney, J., Pekala, P., et al.: The major endotoxin-inducible secretory product of cultured macrophages is a novel lipolytic hormone. (Abstract.) Blood, 64(Suppl. 1):65a, 1984. 5. Brenneman, D. E.: Characterization of the hyperlipidemia in mice bearing Erlich ascites tumor. Eur. J. Cancer Clin. Oncol., 11:223-230, 1975. 6. Brindley, D. N.: The intracellular phase offat absorption. Biomembranes, 48:621, 1974. 7. Butcher, R. W.: Role of cyclic AMP in hormone action. N. Engl. J. Med., 278:1378-1384, 1968. 8. Cahill, G. F., Jr.: Starvation in man. N. Engl. J. Med., 282:668, 1970. 9. Costa, G.: Cachexia, the metabolic component of metastatic disease. Cancer Res., 37:2327-2335, 1977. 10. Costa, G., Bewley, P., Aragon, M., et al.: Anorexia and weight loss in cancer patients. Cancer Treat. Rep., 65(Suppl. 5):13-17, 1981. 11. Cori, C. F.: Mammalian carbohydrate metabolism. Physiol. Rev., 11:143-275, 1931. 12. Densnuelle, P.: Pancreatic lipase. Adv. Enzymol., 23:129, 1961. 13. Fassina, G.: Mechanisms of lipid mobilization. Adv. Exp. Med. BioI., 109:209-220, 1978. 14. Frederick, G., and Begg, R. W.: Development oflipedemia during tumor growth in the rat. Proc. Am. Assoc. Cancer Res., 1:14-18, 1954. 15. Gold, J.: Proposed treatment of cancer by inhibition of gluconeogenesis. Oncology, 22:185-207, 1968. 16. Golde, D. W., Herschman, H. R., Lusis, A. J., et al.: Growth factors. Ann. Intern. Med., 92:650-662, 1980. 17. Goodlad, G. A.: Protein metabolism in tumor growth. In Monroe, N. H. (ed.): Mammalian Protein Metabolism. Volume 2. New York, Academic Press, 1964, pp. 415-444. 18. Gospodarowicz, D., and Moran, J. S.: Growth factors in mammalian cell cultures. Ann. Biochem., 45:531-557, 1976. 19. Guillino, P., and Grantham, F.: Relationship between O2 and glucose consumption by transplanted tumors in vivo. Cancer Res., 27:1041-1052, 1967. 20. Hamilton, R. L.: SyntheSiS and secretion of plasma lipoproteins. Adv. Exp. Med. BioI., 26:7, 1972. 21. Holroyde, C. P.: Carbohydrate metabolism in cancer cachexia. Cancer Treat. Rep., 65(Suppl. 5):55-59, 1981. 22. Holroyde, C. P., Gabuzda, T., Putnam, R., et al.: Carbohydrate metabolism in cancer cachexia. Cancer Res., 35:3710-3714, 1975.
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23. Kitada, S., Hays, E. F., and Mead, J. F.: Characterization of lipid mobilizing factor from tumors. Prog. Lipid Res., 20:823-826, 1981. 24. Kitada, S., Hays, E. F., and Mead, J. F.: Lipid mobilizing factor in serum of tumor bearing mice. Lipids, 15:168-174, 1980. 25. Kitada, S., McAndrew, P. F., Hays, E. F., et al.: Lipid mobilizing factor in serum of cancer patients. (Abstract.) Proc. AACR, 25:155, 1984. 26. Kitada, S., Hays, E. F., Mead, J. F., et al.: Lipolysis induction in adipocytes by a protein from tumor cells. J. Cell Biochem., 20:409-416, 1982. 27. Kralovic, R. C., Zepp, E. A., and Cenedella, R. J.: Studies on the mechanism of carcass fat depletion in experimental cancer. Eur. J. Cancer, 13:1071-1079, 1977. 28. Krebs, H. A.: Some aspects of regulation of fuel supply in omnivorous animals. Adv. Enzyme Regul., 10:397-420, 1972. 29. Liebelt, R. A.: Lipid mobilization and food intake in experimentally obese mice bearing transplanted tumor. Proc. Soc. Exp. BioI. Med., 138:482-490, 1971. 30. Liebelt, R. A., and Gehring, L.: Paraneoplastic syndromes in experimental animal model systems. Ann. N.Y. Acad. Sci., 230:547-564,1974. 31. Lundholm, K., Edstrom, S., Eckman, L., et al.: Metabolism in peripheral tissues in cancer patients. Cancer Treat. Rep., 65(Suppl. 5):79-83, 1981. 32. Lundholm, K., Edstrom, S., and Eckman, L.: Comparative study of influence of tumor on host metabolism in mice and men. Cancer, 42:453-461, 1978. 33. Marks, P. A., and Bishop, J. S.: The glucose metabolism of patients with malignant disease and of normal subjects as studied by means of intravenous glucose tolerance test. J. Clin. Invest., 36:254-264, 1957. 34. Masoro, E. J.: Overview of the process and regulation of lipid transport. In Masoro, E. J. (ed.): International Encyclopedia of Pharmacology and Therapeutics. Section 24, Pharmacology of Lipid Transport and Atherosclerotic Processes. Oxford, Pergamon Press, 1975, p. 1. 35. Masuno, H., Yamasaki, N., and Okuda, H.: Isolation of a lipolytic factor (ToxohormoneL) from ascites of patients with hepatoma. Cancer Res., 41 :284-288, 1981. 36. Masuno, H., Yamasaki, N., and Okuda, H.: Purification and characterization of a lipolytic factor (toxohormone-L) from cell-free fluid of ascites sarcoma 180. Eur. J. Cancer Clin. Oncol., 20:1174-1185, 1984. 37. Menon, N., Baker, N., and Mead, J. F.: Differences in beta-oxidation of palmitic acid by normal and lymphoma-bearing AKR mice. J. Nutr. Growth Cancer, 2:137-144, 1985. 38. Mueller, P. S., and Watkin, D.: Plasma unesterified fatty acid concentrations in neoplastic disease. J. Lab. Clin. Med., 57:95-108, 1961. 39. Nakahara, W. A.: Chemical basis for tumor host relations. JNCI, 24:77-86, 1960. 40. Nakahara, W., and Fukuoka, F.: Toxohormone. Jpn. Med. J., 1948; 1:271-277. 41. Odell, W. D., and Wolfsen, A. R.: Hormones from tumors: Are they ubiquitous? Am. J. Med., 68:317-318, 1980. 42. Owen, O. E., Morgan, A. P., Kemp, H. G., et al.: Brain metabolism during fasting. J. Clin. Invest., 46:1589-1595, 1967. 43. Randle, P. J., Garland, P., Hales, C., et al.: Interaction of metabolism and the physiologic role of insulin. Recent Prog. Horm. Res., 22:1-48,1966. 44. Sallee, V. L., and Dietschy, J. M.: Determination of intestinal mucosal uptake of short and medium chain fatty acids and alcohols. J. Lipid Res., 14:475, 1973. 45. Schein, P., Kisner, D., Haller, D., et al.: Cachexia of malignancy: Potential role of insulin in nutritional management. Cancer, 43:2070-2076, 1979. 46. Thompson, M., and Koons, J.: Modified lipoprotein lipase activities, rates of lipogenesis and lipolysis as factors leading to lipid depletion in C57BL mice bearing preputial gland tumor ESR 586. Cancer Res., 41 :3228-3232, 1981. 47. Todaro, G. I., and DeLarco, J. E.: Growth factors produced by sarcoma virus transformed cells. Cancer Res., 38:4147-4154, 1978. 48. Todaro, G. I., DeLarco, J. E., Fryling, C., et al.: Transforming growth factors (TGF's): Properties and possible mechanisms of activity. J. Supramolec. Struct. Cell. Biochem. 15:287-301, 1981. 49. Torti, F., Dieckmann, B., Beutler, B., et al.: A macrophage factor inhibits adipocyte gene expression: An in vitro model of cachexia. Science, 229:867-869, 1985. 50. Trew, J. A., and Begg, R. W.: In vitro incorporation of acetate V'C into adipose tissue from normal and tumor bearing rats. Cancer Res., 19:1014-1019, 1959.
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51. Waterhouse, C.: How tumors affect host metabolism. Ann. N.Y. Acad. Sci., 230:86--93, 1974. 52. Waterhouse, C., and Fenninger, L. D.: Nitrogen exchange and caloric expenditure in patients with malignant neoplasms. Cancer, 4:500--514, 1951. 53. Waterhouse, C., and Kemperman, J.: Carbohydrate metabolism in subjects with cancer. Cancer Res., 31:1273-1278, 1971. 54. Waterhouse, C.: Oxidation and metabolic interconversion in malignant cachexia. Cancer Treat. Rep., 65(Suppl. 5):61-69, 1981. 55. Weinhouse, S.: Fatty acids as metabolic fuel of cancer cells. Adv. Cancer Res., 3:269-325, 1955. 56. White, A., Handler, P., and Smith, E.: Lipid metabolism. In Principles of Biochemistry. New York, McGraw-Hill, 1973, pp. 551-555. 57. Young, V. R.: Energy metabolism and requirements in the cancer patient. Cancer Res., 37:2336--2341, 1977. 58. Zilversmit, D. B.: Formation and transport of chylomicrons. Fed. Proc., 26:1599, 1967. Wadsworth VA Medical Center W-l11-H Wilshire and Sawtelle Boulevards Los Angeles, California 90073
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Fat Metabolism and Cancer Philomena F. McAndrew, M.D. *
The cachexia seen in cancer is not fully explained by inadequate energy intake and increased energy expenditure. Studies in humans and animals suggest that depletion of body fat is out of proportion to protein loss and may account for the majority of weight loss seen with cancer. 1. 5. 9, 29, 35, 55 Specific abnormalities in lipid metabolism have been noted,46, 51, 55 but their pathogenesis has been poorly understood. Delineation of such altered lipid metabolism may lead to improved management of malnutrition in cancer patients.
THE ROLE OF LIPIDS IN NORMAL METABOLISM The oxidation of fatty acids is the major source of fuel for daily energy needs in mammals. Beta-oxidation of fatty acids occurs in the mitochondria, during which progressive enzymatic shortening of the fatty acid carbon chain produces acetyl coenzyme A, a high-energy intermediate leading to the formation of ATP. Five moles of ATP are produced per mole of oxygen used to produce acetyl coenzyme A in the Krebs cycle. 28 Fatty acids may also escape this oxidative fate and instead work in the formation of new cell membranes. The major source of lipid used as fuel is dietary fat (usually in the form of triglycerides), although lipids may be mobilized from adipose tissue that acts as a fuel reservoir. An adult man is estimated to have a total fuel reserve of approximately 166,000 kcal, 141,000 kcal of which are in the form of lipid. 8 Before the fatty acids stored in adipose tissue can be used as an energy source, they must first be hydrolyzed from triglycerides in the fat tissue. The rate-limiting *Assistant Professor of Medicine, Department of Medicine, UCLA School of Medicine, and Division of Hematology/Oncology, Wadsworth VA Medical Center, Los Angeles, California
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step in this process is an enzyme called hormone-sensitive lipase. This enzyme is activated by a cyclic nucleotide-triggered activation of a protein kinase, which in turn converts the enzyme from an inactive to an active phosphorylated form. Catecholamines, glucagon, and several peptide hormones (including ACTH, growth hormone, and TSH) can bind to receptors on the adipocyte and trigger the activation of hormone-sensitive lipase. 56 Most of the dietary lipid present in the intestinal lumen is in the form of emulsion droplets. Triglycerides are absorbed after enzymatic conversion to more polar compounds-free fatty acids (FF As) and 2-monoglyceride. 12 Once absorbed into the mucosal cell, the FFA and monoglyceride may pass through the plasma membrane by difIusion44 and into the endoplasmic reticulum, where triglyceride is again generated and combined with apolipoprotein B in chylomicrons. 6 , 20, 58 The triglyceride component of the chylomicron is removed by adipose tissue and muscle. Lipoprotein lipase is involved in the hydrolysis of triglycerides to FFA and glycerol and their uptake into the adipocyte in the process of fat deposition. 34 In the adipocyte, FFA and alpha-glycerophosphate are converted once again into triglyceride.
STARVATION VERSUS CANCER CACHEXIA During early fasting, fat mobilization is promoted by falling levels of insulin and increased sympathetic nerve activity to the adipose tissue. Early rapid proteolysis occurs with amino acid mobilization from muscle, gluconeogenesis, and production of urinary urea nitrogen. This would result in the loss of 75 gm of muscle protein daily. As a part of the life-saving adaptation to total starvation, the body gradually converts from a glucose and amino acid economy to a fat-derived fuel (ketone bodies) economy. This complex adaptation results in a decrease in oxygen consumption and a sparing of protein breakdown. 57 This starvation adaptation is accompanied by the ability of FFA to become the prime fuel for most tissues and the brain to adapt to the use of ketone bodies. 42 In patients with cancer cachexia these adaptations do not occur. 19, 45, 51, 52 The decrease in oxygen consumption and carbon dioxide production normally seen during starvation does not occur in patients with cancer. 45, 53, 57 Recycling oflactate and pyruvate occurs together, with an increase in the use of acetoacetate by tumor tissue in the cancer-bearing host. 23, 51-53
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HOST METABOLIC ABNORMALITIES IN CANCER Studies in humans 21 , 45 and animals 29-32 have shown that total host lipids decrease as tumor growth proceeds. When tumor weight in the rat bearing a Walker 256 carcinoma exceeded 7 per cent of body weight, significant decreases in total body lipids were already evident. As tumor mass increased further, there was an inverse linear relation between tumor weight and total carcass lipid. 27 This loss cannot be caused by decreased caloric intake alone, since pairfed animals do not lose as much fat as tumor-bearing animals. Force feeding a high-fat diet to these animals reduces but does not prevent the loss of carcass fat. 31, 32 Transplantable tumors have provided a model to study lipid mobilization in animals. Liebelt29 noted severe fat depletion in experimental animals with noninvasive tumors, which could be reproduced by transplanting frozen and thawed tumor extracts in animals. Some tumors caused rapid depletion of total lipid stores, whereas others had no significant effects on body lipid content. The validity of these animal studies is questioned because human tumors rarely exceed 5 per cent of total body weight. Several clinical investigators have noted increased FFA mobilization before marked losses of body weight occurred. 27, 52, 53 Kralovic and colleagues,27 using transplantable Walker 256 carcinoma in rats, noted an early mobilization of fat even when the tumor burden was less than 4 per cent of total body weight. In these studies, a two- to threefold elevation in the basal rate of lipolysis was noted with these relatively small tumors in animals. When these animals developed ascites, the cell-free fluid from the ascites was able to increase lipolysis in tumor-free control animals. This suggested the elaboration of a lipid mobilizing factor by the transplanted tumor and not a catechol or hormone that stimulated lipolysis via adenyl cyclase, since lipolysis is not inhibited by propranolol and prostaglandin E (PGE). Further evidence of increased lipolysis in animals is an elevation of plasma levels of FFAs in several experimental systems. 14, 22, 38 Costa and co-workers10 studied the lipid composition of muscle biopsy samples obtained from patients with cancer and found that the fat content was half that present in control subjects. Studies in patients with cancer have failed to confirm plasma elevation of FFA consistently.22, 38, 45 Oxidation rates of fatty acid increase in patients with cancer given isotopically labeled fatty acid substrate. 29, 54 Moreover, when glucose, which normally suppresses fatty acid oxidation, is infused, inhibition of fatty acid oxidation decreases in patients with cancer compared with control subjects. In the fasting state,
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although FFA levels are similar, slightly more FF A is oxidized to CO 2 in cancer patients. There are clear abnormalities in glucose metabolism in patients with cancer, and the failure of glucose to suppress fatty acid oxidation may be secondary to abnormalities in glucose metabolism. No futile cycles of lipid metabolites have been found in patients with cancer similar to the known increases in recycling of glucose and lactate known as the Cori cycle.
PATHOGENESIS OF LIPID ABNORMALITIES: POTENTIAL MECHANISMS Remodeling Remodeling of metabolic substrates prior to oxidation might redirect substances from fuel-efficient .pathways into energy-requiring paths prior to final oxidation. An increased contribution of fatty acid to oxidation when glucose is available has been noted by Waterhouse and others. 54 This continued use of fatty acids when preferred substrates are present may be secondary to remodeling of glucose prior to oxidation. Glucose can be oxidized to two-carbon fragments and synthesized into fatty acids in an energy-requiring process rather than proceeding through the energy-generating Krebs cycle. Tumors also undergo increased anaerobic glycolysis, allowing the recycling of lactate and pyruvate produced and their conversion to glucose in an energy-requiring pathway (Cori cycle). 11. 15,57
Increased Energy Expenditure Although increased energy requirements do not explain specific metabolic changes seen in cancer patients, they may play an important role in the degree of weight loss and disordered metabolism seen with some cancers. It is known that tumors can obtain substrates and energy necessary for their synthesis from the host at a significant energy cost. It has been hypothesized that tumors may produce substances that could uncouple oxidative phosphorylation. Menon and colleagues 37 have shown a difference in the beta-oxidation of palmitate by normal and lymphoma-bearing AKR mice. This strain of mice has an almost 100 per cent mortality from thymic lymphoma between the ages of 6 and 14 months, with a peak incidence at 9 to 10 months. Within the control group of mice, two distinct populations can be separated, one Clow controls") showing an overall decrease in beta-oxidation of palmitate at a subcellular level similar to that in mice with advanced lymphoma. Elimination of exogenous energy (ATP) from the medium causes a more dramatic decrease in oxidation by this group as compared with normal controls. Fragile tumor mitochondria having uncoupled oxidative phosphorylation with impaired ATP synthesis could explain such findings in tumor-bearing
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animals, whereas the "low controls" might indicate an early or preneoplastic state in which metabolic abnormalities precede detectable malignant morphologic changes.
Decreased Lipogenesis In a process analogous to the nitrogen trap, tumors are probably using host fatty acids for tumor lipid synthesis. Mice bearing a nonmetastatic tumor (ESR 586) were found to have lipid depletion. 46 The levels of lipoprotein lipase, the enzyme necessary for the storage of fat as triglycerides from circulating FF As, were measured in the fat tissues of these animals and in the tumor mass. The level of lipoprotein lipase in host fat tissue was decreased, as is normally seen during starvation, but compared with fat tissue levels of the control tumor-free animals, the tumor lipoprotein lipase levels were increased. A substance produced by tumors may modulate lipoprotein lipase levels in the host. Recently, tumor necrosis factor (TNF)/cachectin has been isolated, sequenced, and synthesized, and its effects on lipogenesis have been studied. Some of this initial work evolved from the recognition of significant metabolic abnormalities, specifically mobilization of triglycerides from adipose tissue, associated with certain infections. These abnormalities were found not to be caused by endotoxin itself but by a protein secreted from endotoxin-stimulated macrophages. Cachectin was found to be a 17,000 molecular weight subunit that forms non covalent multimers and is active at 45,000 molecular weight. A high degree of homology was found between cachectin and TNF. Purified cachectin possessed potent TNF activity in vivo (cytolysis), suggesting a single protein. 2--4, 49 Cachectin was found to modulate the metabolic activity of normal and neoplastic cells through interactions with specific high-affinity receptors. CachectinlTNF specifically suppressed the activity of lipoprotein lipase in vivo and in vitro using cultured preadipocytes 3T3-Ll. Cachectin also inhibited the synthesis of fatty acid synthetase and acetyl coenzyme A carboxylase in adipocytes. Treatment with cachectin was found to prevent the accumulation of adipose-inducible mRNAs as well as lipid accumulation in these cells; this effect was reversed by the removal of cachectin.
Insulin Deficiency/Resistance Abnormalities in insulin secretion, or sensitivity, could affect the rates of lipolysis and lipogenesis in patients with cancer. Insulin stimulates the uptake and storage of glucose and the storage of triglycerides in adipose tissue. Insulin acts in part by stimulating synthesis of lipoprotein lipase and alpha-glycerophosphate and by
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inhibiting hormone-sensitive lipase that breaks down triglycerides. Insulin probably inhibits hormone-sensitive lipase by modulating intracellular levels of cyclic AMP, although this has not been shown definitely.7, 13, 17,43,45 Insulin administered to tumor-bearing animals ameliorates the loss of fat that occurs, 50 suggesting that insulin deficiency may play a role in the increased fat mobilization seen in patients with cancer. Other work has revealed a decreased clearance of glucose from the blood after exogenous insulin administration, supporting a relative insulin resistance. 33 , 45
Specific Mediators of Altered Lipid Metabolism Tumors produce a variety of hormones and proteins that exert metabolic effects and well-known paraneoplastic syndromes. Examples include the production of ectopic ACTH, parathyroid hormone, and antidiuretic hormone. 41 Substances may also be produced that cross-react immunologically with a variety of hormones. 41 Tumorderived growth factors that regulate cell proliferation are another class of hormones extracted from various malignant and nonmalignant cell lines. 16, 18, 47, 48 These factors compete with normal growth factors for specific binding to receptors and thus may affect the metabolic responses of target cells to circulating growth factors. Several other tumor-associated factors have been examined for potential involvement in cancer-related cachexia. Nakahara and Fukuoka isolated from tumor tissues a factor they coined toxohormone. 39, 40 This factor caused lipid depletion in the host as well as immunosuppression and thymic involution. Toxohormone is a 75,000 dalton protein found in several tumors. Toxohormone-L was isolated by Masuno and coworkers 35 , 36 from sarcoma-bearing mice and patients with hepatoma. When injected into animals, this factor also caused lipid mobilization. Mead, Kitada, and co-workers at UCLA23-25 have used the AKR lymphoma-bearing mouse as a model to study lipid mobilization. Adipose tissue prelabeled with radioactive fat in vitro was implanted in control and tumor-bearing animals. The rate of lipolysis was then judged by the amount of radioactive carbon dioxide derived from fat oxidation and expired by the animals. They found increased rates of lipolysis and fatty acid oxidation. Compared with normal tissues of the cancer-bearing host and control animals, the tumor tissues had incorporated increased amounts of the fatty acids released by the adipose tissue. A simplified in vivo bioassay was developed to test for lipolysis: radiolabeled linoleic acid was injected into fat pads of tumor-bearing and control animals while expired carbon dioxide was again measured to assess fatty acid oxidation and lipid mobilization in the fasted state compared with the fed state. Tumor-bearing animals showed increased carbon dioxide evolution in both the fed
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and fasted states. Control animals given injections of crude tumor homogenates, or serum from lymphoma-bearing animals, increased their mobilization of radioactive fat from the injected fat pads. The serum of patients with a variety of advanced malignancies similarly caused increased lipid mobilization. Lipid mobilizing factor has been partially purified from thymic tissue of AKR lymphoma-bearing mice and from the supernatant of lymphoma cell lines derived from the AKR lymphoma. This factor is a small peptide with an apparent molecular weight of about 5000 daltons; it occurs in cell-free and serum-free fractions of supernatants of lymphoma cell lines. 26
SUMMARY Progressive weight loss and anorexia are frequent phenomena in cancer patients. Although cachexia is an expected occurrence in the terminal stages of nearly all malignancies, it may be a presenting sign when the tumor burden is quite small. Lipid depletion occurs out of proportion to the protein loss and accounts for most of the weight loss in cancer. Lipids, more specifically fatty acids, are the major source of fuel in mammals and may also be used in the synthesis of new cell products. Lipolysis and lipogenesis are under the influence of several important enzymes and peptide hormones that may be modulated by a variety of exogenous factors. There is evidence that cancer patients have lost the normal homeostatic responses to decreased energy intake or starvation that allow a decrease in oxygen consumption and protein sparing. An increase in Cori cycle activity or futile recycling of metabolic products occurs with a net energy expenditure rather than energy production. Clinical studies have shown that the body lipid depletion accompanying tumor progression is not solely secondary to decreased food intake and may be reproduced by the transplantation of certain noninvasive tumors to normal hosts. Elevated basal lipolysis has occasionally been seen early in tumor growth. Such findings suggest the presence of a tumor-associated factor responsible for this increase in lipid mobilization. Some of the potential mechanisms for the altered lipid metabolism seen in cancer have been discussed. Metabolic substrates may be remodeled and directed away from fuel-efficient into energyrequiri.ng pathways. An increased energy expenditure may occur as a result of the energy costs of tumor synthesis, an uncoupling of oxidative phosphorylation, or energy-requiring futile cycling. An overall depletion of lipid may be the final outcome of the inhibition of lipid deposition. TNF/cachectin has recently been found to sup-
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press the activity and synthesis of several key lipogenic enzymes, including lipoprotein lipase. Abnormalities in insulin secretion or sensitivity may be involved in the decrease of fat storage in malignancy. Insulin also exerts a significant antilipolytic effect by its antagonism of hormone-sensitive lipase. Mediators of lipolysis and abnormal lipid metabolism may occur in a number of clinical conditions and include ectopic hormone production, growth factors, and tumor-associated lipolytic factors (lipid mobilizing factor, toxohormone).
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