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Inhibitory effects of prostaglandin F2α on mammary carcinogenesis induced by ethyl methanesulphonate in rats
Cancer Letters, 56 (1991)
225-230
225
Elsevier Scientific Publishers Ireland Ltd.
Inhibitory effects of prostaglandin F2a on mammary carcinogenesis induced by ethyl methanesulphonate in rats H. Ueoa, H. Matsuokaa, “Department
T. Akiyoshia and R. Takakib
of Surgery, Medical Institute of Bioregulation,
First Department
of Internal
(Received 4 October 9 January
Medicine,
Oita Medical
Beppu 874,
to: H. Ueo, Department
Japan.
Tsurumihara.
Beppu 874 and h7%e
(Japan)
1991)
The effect of prostaglandin F,CY(PGF,(r) on mammary carcinogens is was examined for a new system in female rats, using ethyl methanesulphonate (EMS). The rats were given EMS orally for a period of 12 weeks starting at age 4 weeks. Mammary carcinomas were first detected at the 16th week and were found in all surviving rats at the 32nd week. The concomitant administration of PGF2~ for 8 weeks made the development of tumor retarded; i.e., the carcinomas were first detected at the 30th week and final tumor incidence at the 44th week was 61.1% The incidence of developing mammary carcinomas and multiplicity (number of mammary carcinomas per rat) were significantly lower in PGF,(r treated rats than in those given EMS alone. The inhibitory effect of PGF,cY on tumor development was apparent when PGF,a was concomitantly given to the rats with EMS at age 4 weeks, while PGF*cY injections after oral administration of EMS at age 16 weeks did not significantly retard tumor development. Histologically, no significant difference in morphology was observed between Correspondence
Oita 879
1991)
Summary
stitute of Bioregulation,
Kyushu University. 4546
College,
1990)
(Revision received 7 January (Accepted
H. Inouea,
of Surgery, Medical In-
Kyushu University,
4546
Tsurumihara.
PGF,cr-treated and PGF,cr-non-treated rats in either the cancerous or noncancerous mammary tissues. The finding demonstrates that PGF,(r inhibits the development of EMS-induced mammary carcinomas when given to younger rats, presumably by affecting the hormonal status rather than by direct action on the mammary glands. Keywords: prostaglandin F,cY; rat mammary carcinoma; ethyl methanesulphonate Introduction The relevance of prostaglandin F,cr (PGF,(r) to the biological characteristics of mammary carcinoma has been extensively investigated both in human and laboratory animals. Human breast cancer has been shown to produce PGF,cY in vitro [5] and the tissue content of PGF,a was higher in mammary carcinomas than that in noncancerous mammary tissues [3, lo]. It has also been shown that human breast tumors with elevated prostaglandins production might have a high metastatic potential [ 1,5]. In the animal experiments, Jubit et al. [Z] reported the inhibitory effects of PGF,a on the growth of a cell line of hormone-dependent mammary adenocarcinoma in rats. Thus, PGF,a, as well
03043835/91/$03.50 0 1991 El sevier Scientific Publishers Ireland Ltd Published and Printed in Ireland
226
as other prostaglandins, appears to contribute to the growth and metastatic potential of mammary carcinoma. The effects of PGF,cY on mammary carcinogenesis, however, have yet to be clearly elucidated even in animal experiments. We previously developed a new system for the induction of mammary carcinomas in female rats, using ethyl methanesulphonate (EMS), an alkylating agent and a chemical carcinogen [7,8]. Mammary carcinogenesis by EMS is dependent on ovarian and adrenal hormones, i.e., tumor induction was completely inhibited by an ovariectomy and partially by an adrenalectomy [91. Since PGF,(r exerts several effects on both the ovarian and gonadal hormones in rats [6], the present study was undertaken to examine the effects of PGF,ar on EMS-induced mamThe possible role of mary carcinogenesis. PGF,(r in inhibiting the development of mammary carcinomas was also discussed. Materials
and methods
Female Wistar King A rats were purchased from CLEA Japan Inc., Osaka. The rats were freely fed with commercial pellet food (Nippon CLES Co., Tokyo) and tap water during the experiments. Mammary carcinomas were induced by the oral administration of EMS (Eastman Kodak Co., Rochester, New York, U.S.A.) in the same manner as described in our previous studies [7-g]. In brief; 10-3 M of EMS solution in tap water was orally administered for 12 weeks with the renewal of the solution every 2 days. PGF,o (Cayman Chemical Co.) was diluted in sesame oil and the solution containing PGF,(r (1 mg/kg) was intramuscularly injected to the thigh muscle twice a week for 8 weeks. Equal volumes of sesame oil were administered to the control group at the same time. One hundred and twenty female rats, 4 weeks old, were divided into 6 groups according to the following treatments: Group A, EMS administration alone for 12 weeks; Group B, concomitant treatment of PGF,ar for 8 weeks with EMS
administration; Group C, PGF,cr treatment for 8 weeks subsequent to EMS administration for 12 weeks; Group D, PGF,(r treatment alone for 8 weeks; Group E, sesame oil treatment alone for 8 weeks as a control, and Group F, no treatment control. The rats were then examined weekly for palpable tumors. The occurrence of tumors was dated from the initiation of EMS administration and all surviving rats were autopsied at the 44th week. All tumors were fixed in a 10% formalin solution for histological examination. The statistical significance of the tumor incidence and mean numbers of tumors per rat were determined by the x2-test and Wilcoxon test, respectively. P-values less than 0.05 were considered to be significant. Results The incidence of mammary carcinomas in the 6 groups is summarized in Table I. In the Group A rats, given EMS alone, the earliest mammary carcinomas were detected at the 16th week after the initiation of EMS administration and the tumor incidence rapidly increased as the observation period proceeded, i.e., 11.1% at the 16th week, 66.7% at the 24th week and 100% at the 32nd week. In the Group B rats, concomitantly given EMS and PGF,a, the development of mammary carcinomas was retarded, i.e., no tumor was found up to the 30th week and the tumor incidences during the observation period were significantly lower than that in Group A. However, tumor incidence was not significantly affected by PGF,ar in the Group C rats given PGF,ar after EMS administration at age 16 weeks. Mammary carcinomas were not evoked in the control group rats, the Group D rats given PGF,(r alone, the Group E rats given sesame oil alone, or the Group F rats given tap water. The multiplicity of the mammary carcinomas, expressed as the mean number per rat (carcinomas/rat) , was significantly decreased by the concomitant administration of PGF,(r in Group B, as compared with Group A (Table II). In the rats in Group C, which were consecutively ad-
18 18 19 18 20 20
EMS alone EMS + PGF,c$ EMS - PGF,cr’ PGF,cr alone Sesame oil No treatment
A B C D E F
0 0 0 0 0 0
11.1% 0 0 0 0 0
EMS + PGF,cr
EMS -
B
C
l
aMeans f S.D. ‘P < 0.01, ‘P < 0.05;
PGF,(r
EMS alone
A
significantly
0
0
0
14
f
16
different
0
0
0.11 0.32
induced
by EMS.
0.26 0.81
0
0.33 0.49
f
0.53 0.58”
0
0.89 zt 0.83
24
from Group A.
f
f
20
at the following period
carcinoma
Carcinomas/rat
Multiplicity of mammary
Treatment
II.
Groups
Table
l
0
1.50 1.10
0.78 zt 0.88”
f
28
(weeks)”
66.7% 0 26.3’ 0 0 0
33.3% 0 10.5 0 0 0 l
1.50 1.10
83.3% 0 55.6 0 0 0
28
f
1.35 1.17
0.28 zt 0.46’
f
30
(weeks)”
24
16
14
period
20
at the following
by EMS.
Incidence
induced
aPercentage of rats with mammary carcinoma among survivors. bEMS and PGR,(r were concomitantly administered. ‘Arrow indicates ‘followed by’. lP c 0.05; significantly different from Group A. ‘P c 0.01,
No. of rats
carcinoma
Treatment”
of mammary
Group
Table 1. Incidence
0.39 0.70’ 1.71 ztz 1.36
f
2.00 zt 1.46
32
83.3% 27.8’ 76.5 0 0 0
30
f
f
l
36
100% 27.8’ 76.5 0 0 0
32
2.06 1.49
0.67 0.77’
2.44 1.20
2.89 1.28
61.1’ 87.5 0 0 0
40
l
2.25 1.44
0.94 zt 0.87’
f
40
50.0’ 87.5 0 0 0
36
2.89 1.28
f
2.38 1.45
1.22 zt 1.06’
f
44
61.1’ 87.5 0 0 0
44
228
Histological features of mammary carcinoma developed in Group B rats given EMS and PGF,a! concomitantFig. 1. ly. Predominant features were moderately differentiated medullary type adenocarcinoma, which was similar with those of Group A rats given EMS alone.
ministered EMS for 12 weeks and PGF,(r for 8 weeks, the multiplicity of mammary carcinomas was not consistently reduced by PGF,a treatment. Histologically, there was no significant difference in morphology in either the developed mammary carcinomas or the non-cancerous mammary tissues among Groups A, B and C. Fig. 1 shows the histology of the mammary carcinomas which developed in the Group B rats. The predominant features of the mammary tumors were moderately-differentiated adenocarcinomas of a medullary type containing glandular structure focal areas, which were similar to those in Group A. No evident morphological change in the mammary glands was
observed in the Group D rats given PGF,o alone in comparison with those in the control Groups E and F. Discussion An increasing amount of prostaglandins has been found in human breast cancer tissues [3, lo] and elevated prostaglandin synthesis appears to play a part in tumor spreading and bone metastasis [1,5]. However, there has been little information available concerning the role of PGF,(r on the carcinogenic process [4]. Lupulescu [4] reported that PGF,(r enhanced the induction of mouse skin tumors by 3-methylcholanthrene, in which concomitant in-
229
jections of PGF,(r with chemical carcinogen stimulated the DNA, RNA and protein synthesis in hyperplastic epidermal cells and resulted in a shortening of the latent period. These findings suggest that PGF,cr tends to enhance the development of primary tumors and the growth of mammary carcinomas. The present results, however, indicated that the concomitant administration of PGFp inhibited the induction of mammary carcinomas by EMS. In the Group B rats, given EMS plus PGF,(r at age 4 weeks, tumor appearance was markedly retarded by the 30th week and the final tumor incidence at the 44th week was 61.1%) whereas in the Group A rats, given EMS alone, the tumors were first detected at the 16th week and the tumor incidence reached 100% at the 32nd week. However, PGF,a injections after EMS administration in the Group C rats at age 16 weeks did not significantly retard the development of tumors. The putative mechanisms behind the inhibitory effects of PGF,cY on EMS-induced mammary carcinogenesis may involve the direct action of PGF,cY on the mammary glands and/or the influence on the environmental circumstances such as hormonal status. Histological examination of the mammary glands revealed that PGF,a did not cause any significant morphological difference either in cancerous or noncancerous mammary tissues. This finding did not support the direct effect of PGF,cY on the mammary glands as being responsible for the inhibition of the mammary carcinogenesis. The difference in inhibitory effect of PGF,(r on EMS-induced mammary carcinogenesis between younger and older female rats was compatible with that of ablative oophorectomy in the same experimental system [9]. That is, oophorectomy at age 4 weeks prior to EMS administration completely prevented tumor induction, while oophorectomy at age 16 weeks after EMS administration only delayed the tumor production. Since the presence of estrogen receptors was confirmed in EMS-induced mammary carcinomas [9], we previously proposed that the circulating levels of ovarian hormones and the
concentration of ER in mammary glands in rats at different ages might be responsible for the different susceptibility to cancer induction by EMS and to ablative oophorectomy. These findings raised the possibility that PGF,a affected the hormonal status of the rats and elicited the same effect as oophorectomy. Thus, the inhibitory effects of PGF,a on mammary carcinogenesis in this experiment seemed to involve the following mechanisms; (i) In Group B rats, PGF,a inhibited the ovarian and/or adrenal functions [6] and had prevented the development of mammary carcinomas during PGF,a injections. After terminating PGF,o administration, the EMS-sensitized mammary glands began to develop tumors in the presence of ovarian hormones, thus resulted in the prolongation of the latent period; (ii) In the Group C rats, given EMS at age 4 weeks and PGF,a at age 16 weeks, the mammary glands had already been sentitiied by EMS in the presence of ovarian hormones and the ablative effect of PGF,cY was diminished at this age. These speculations may be supported by the findings of Jubii et al. [Z] , in which PGF,(r inhibited the growth of hormonedependent rat mammary carcinoma, which was associated with a marked reduction in serum progesterone levels. The inhibitory effect of PGF,cY on tumor growth was abolished by a supplement of progesterone. They also speculated that the site of PGF,cY action might be the ovaries. Further investigation is needed for a better understanding of the biological mechanisms in which prostaglandins affect mammary carcinogenesis, such as defining the chronological change of their contents in developing tumors and their relation to the hormone receptor status. Therefore rat mammary carcinoma induced by EMS, possessing hormone receptors and sensitive to PGFP treatment, may serve as an useful model for the research on mammary carcinogenesis in relevance to the effect of prostaglandins. Acknowledgments
We thank Mr. Brian Quinn of Kyushu University for his comments on the manuscript.
230
References
(1978)
The sequence
of hormonal
changes
during pro-
staglandin induced luteolysis of the superluteintzed rat ovary. Bennett,
A. (1983)
hibitors in cancer. 237-247.
Prostaglandins In: Leukotrienes
and their synthesis inand Prostacyclin,
pp.
7
ASI Series, Plenum Press, New York.
998-
rat
tumor.
Cancer
Res.,
8
by
Ueo,
H.,
Takaki,
oral
H. and Lefevre,
parameters
S. (1983)
status. Br. J. Cancer,
A. (1978)
prostaglandins. Rolland, P.H.,
48,
Enhancement
Nature,
272,
by
Prostaglandin
R., Yamagami,
H. and Sugimachi,
Determination
affecting tumor induction.
Ueo, H., Matsuoka,
H., Akiyoshi,T.,
in human breast can-
rat mammary 10
Vergote,
carcinoma.
I., Laekeman,
Cancer G.,
151G.,
De Roy, G. and Herman,
cells. J. Natl. Cancer
benign and malignant breast tumors.
Inst., 64,
1061-1070
R., Haug,
Sugimachi,
Lett., 51,
broeck. F., Vanderheyden,
Dahlin,
of some of the
Carcinogenesis,
Keersmaekers,
duction is a marker of hiih metastatic portential for neoplastic P.A.,
K.
2,
K. and
docrine ablative surgery in ethyl methanesulphonate-induced J., Rolland, A.M.
cer: evidence suggesting that an elevated prostaglandin pro-
Torjesen,
ethyl
Takaki, R. (1990) Estrogen receptor status and effects of en-
634-636.
Martin, P.M., Jacquemur,
and Toga, M. (1980b)
9
689-696
of carcinogenesis
of
79, 79-84.
1223-1228.
Prostaglandins in breast cancer: relationship to disease state and hormone
S.. Okeda,
administration
Cancer Lett.,
of ethyl methanesulphonate.
R., Welt, S., Thaler,
Lupulescu,
carcinoma
H.. Nakano,
High incidence of rat mam-
(1981) Mammary carcinoma induced by oral administration
39,
1000.
Karmali,
K. (1979)
methanesulphonate.
Inhibitory ef-
F20 on the growth of a hormone-
mammary
R., Yamagami,
mary
Jubiz, W., Frailey, J. and Smith, J.B. (1979) fect of prostaglandin
87, 617-624.
Ueo, H., Takaki, T. and Sakakibara,
Editors: F. Berti, G. Folco. and G. Velo. NATO
dependent
Acta Endocrinol.,
E., and Aakvaag,
A.
827-836.
155. Uytten-
J., Albertyn, G., Haensch, A. (1985)
Prostaglandin
C.,
Fzo in
Br. J. Cancer,
51,
225-230
225
Elsevier Scientific Publishers Ireland Ltd.
Inhibitory effects of prostaglandin F2a on mammary carcinogenesis induced by ethyl methanesulphonate in rats H. Ueoa, H. Matsuokaa, “Department
T. Akiyoshia and R. Takakib
of Surgery, Medical Institute of Bioregulation,
First Department
of Internal
(Received 4 October 9 January
Medicine,
Oita Medical
Beppu 874,
to: H. Ueo, Department
Japan.
Tsurumihara.
Beppu 874 and h7%e
(Japan)
1991)
The effect of prostaglandin F,CY(PGF,(r) on mammary carcinogens is was examined for a new system in female rats, using ethyl methanesulphonate (EMS). The rats were given EMS orally for a period of 12 weeks starting at age 4 weeks. Mammary carcinomas were first detected at the 16th week and were found in all surviving rats at the 32nd week. The concomitant administration of PGF2~ for 8 weeks made the development of tumor retarded; i.e., the carcinomas were first detected at the 30th week and final tumor incidence at the 44th week was 61.1% The incidence of developing mammary carcinomas and multiplicity (number of mammary carcinomas per rat) were significantly lower in PGF,(r treated rats than in those given EMS alone. The inhibitory effect of PGF,cY on tumor development was apparent when PGF,a was concomitantly given to the rats with EMS at age 4 weeks, while PGF*cY injections after oral administration of EMS at age 16 weeks did not significantly retard tumor development. Histologically, no significant difference in morphology was observed between Correspondence
Oita 879
1991)
Summary
stitute of Bioregulation,
Kyushu University. 4546
College,
1990)
(Revision received 7 January (Accepted
H. Inouea,
of Surgery, Medical In-
Kyushu University,
4546
Tsurumihara.
PGF,cr-treated and PGF,cr-non-treated rats in either the cancerous or noncancerous mammary tissues. The finding demonstrates that PGF,(r inhibits the development of EMS-induced mammary carcinomas when given to younger rats, presumably by affecting the hormonal status rather than by direct action on the mammary glands. Keywords: prostaglandin F,cY; rat mammary carcinoma; ethyl methanesulphonate Introduction The relevance of prostaglandin F,cr (PGF,(r) to the biological characteristics of mammary carcinoma has been extensively investigated both in human and laboratory animals. Human breast cancer has been shown to produce PGF,cY in vitro [5] and the tissue content of PGF,a was higher in mammary carcinomas than that in noncancerous mammary tissues [3, lo]. It has also been shown that human breast tumors with elevated prostaglandins production might have a high metastatic potential [ 1,5]. In the animal experiments, Jubit et al. [Z] reported the inhibitory effects of PGF,a on the growth of a cell line of hormone-dependent mammary adenocarcinoma in rats. Thus, PGF,a, as well
03043835/91/$03.50 0 1991 El sevier Scientific Publishers Ireland Ltd Published and Printed in Ireland
226
as other prostaglandins, appears to contribute to the growth and metastatic potential of mammary carcinoma. The effects of PGF,cY on mammary carcinogenesis, however, have yet to be clearly elucidated even in animal experiments. We previously developed a new system for the induction of mammary carcinomas in female rats, using ethyl methanesulphonate (EMS), an alkylating agent and a chemical carcinogen [7,8]. Mammary carcinogenesis by EMS is dependent on ovarian and adrenal hormones, i.e., tumor induction was completely inhibited by an ovariectomy and partially by an adrenalectomy [91. Since PGF,(r exerts several effects on both the ovarian and gonadal hormones in rats [6], the present study was undertaken to examine the effects of PGF,ar on EMS-induced mamThe possible role of mary carcinogenesis. PGF,(r in inhibiting the development of mammary carcinomas was also discussed. Materials
and methods
Female Wistar King A rats were purchased from CLEA Japan Inc., Osaka. The rats were freely fed with commercial pellet food (Nippon CLES Co., Tokyo) and tap water during the experiments. Mammary carcinomas were induced by the oral administration of EMS (Eastman Kodak Co., Rochester, New York, U.S.A.) in the same manner as described in our previous studies [7-g]. In brief; 10-3 M of EMS solution in tap water was orally administered for 12 weeks with the renewal of the solution every 2 days. PGF,o (Cayman Chemical Co.) was diluted in sesame oil and the solution containing PGF,(r (1 mg/kg) was intramuscularly injected to the thigh muscle twice a week for 8 weeks. Equal volumes of sesame oil were administered to the control group at the same time. One hundred and twenty female rats, 4 weeks old, were divided into 6 groups according to the following treatments: Group A, EMS administration alone for 12 weeks; Group B, concomitant treatment of PGF,ar for 8 weeks with EMS
administration; Group C, PGF,cr treatment for 8 weeks subsequent to EMS administration for 12 weeks; Group D, PGF,(r treatment alone for 8 weeks; Group E, sesame oil treatment alone for 8 weeks as a control, and Group F, no treatment control. The rats were then examined weekly for palpable tumors. The occurrence of tumors was dated from the initiation of EMS administration and all surviving rats were autopsied at the 44th week. All tumors were fixed in a 10% formalin solution for histological examination. The statistical significance of the tumor incidence and mean numbers of tumors per rat were determined by the x2-test and Wilcoxon test, respectively. P-values less than 0.05 were considered to be significant. Results The incidence of mammary carcinomas in the 6 groups is summarized in Table I. In the Group A rats, given EMS alone, the earliest mammary carcinomas were detected at the 16th week after the initiation of EMS administration and the tumor incidence rapidly increased as the observation period proceeded, i.e., 11.1% at the 16th week, 66.7% at the 24th week and 100% at the 32nd week. In the Group B rats, concomitantly given EMS and PGF,a, the development of mammary carcinomas was retarded, i.e., no tumor was found up to the 30th week and the tumor incidences during the observation period were significantly lower than that in Group A. However, tumor incidence was not significantly affected by PGF,ar in the Group C rats given PGF,ar after EMS administration at age 16 weeks. Mammary carcinomas were not evoked in the control group rats, the Group D rats given PGF,(r alone, the Group E rats given sesame oil alone, or the Group F rats given tap water. The multiplicity of the mammary carcinomas, expressed as the mean number per rat (carcinomas/rat) , was significantly decreased by the concomitant administration of PGF,(r in Group B, as compared with Group A (Table II). In the rats in Group C, which were consecutively ad-
18 18 19 18 20 20
EMS alone EMS + PGF,c$ EMS - PGF,cr’ PGF,cr alone Sesame oil No treatment
A B C D E F
0 0 0 0 0 0
11.1% 0 0 0 0 0
EMS + PGF,cr
EMS -
B
C
l
aMeans f S.D. ‘P < 0.01, ‘P < 0.05;
PGF,(r
EMS alone
A
significantly
0
0
0
14
f
16
different
0
0
0.11 0.32
induced
by EMS.
0.26 0.81
0
0.33 0.49
f
0.53 0.58”
0
0.89 zt 0.83
24
from Group A.
f
f
20
at the following period
carcinoma
Carcinomas/rat
Multiplicity of mammary
Treatment
II.
Groups
Table
l
0
1.50 1.10
0.78 zt 0.88”
f
28
(weeks)”
66.7% 0 26.3’ 0 0 0
33.3% 0 10.5 0 0 0 l
1.50 1.10
83.3% 0 55.6 0 0 0
28
f
1.35 1.17
0.28 zt 0.46’
f
30
(weeks)”
24
16
14
period
20
at the following
by EMS.
Incidence
induced
aPercentage of rats with mammary carcinoma among survivors. bEMS and PGR,(r were concomitantly administered. ‘Arrow indicates ‘followed by’. lP c 0.05; significantly different from Group A. ‘P c 0.01,
No. of rats
carcinoma
Treatment”
of mammary
Group
Table 1. Incidence
0.39 0.70’ 1.71 ztz 1.36
f
2.00 zt 1.46
32
83.3% 27.8’ 76.5 0 0 0
30
f
f
l
36
100% 27.8’ 76.5 0 0 0
32
2.06 1.49
0.67 0.77’
2.44 1.20
2.89 1.28
61.1’ 87.5 0 0 0
40
l
2.25 1.44
0.94 zt 0.87’
f
40
50.0’ 87.5 0 0 0
36
2.89 1.28
f
2.38 1.45
1.22 zt 1.06’
f
44
61.1’ 87.5 0 0 0
44
228
Histological features of mammary carcinoma developed in Group B rats given EMS and PGF,a! concomitantFig. 1. ly. Predominant features were moderately differentiated medullary type adenocarcinoma, which was similar with those of Group A rats given EMS alone.
ministered EMS for 12 weeks and PGF,(r for 8 weeks, the multiplicity of mammary carcinomas was not consistently reduced by PGF,a treatment. Histologically, there was no significant difference in morphology in either the developed mammary carcinomas or the non-cancerous mammary tissues among Groups A, B and C. Fig. 1 shows the histology of the mammary carcinomas which developed in the Group B rats. The predominant features of the mammary tumors were moderately-differentiated adenocarcinomas of a medullary type containing glandular structure focal areas, which were similar to those in Group A. No evident morphological change in the mammary glands was
observed in the Group D rats given PGF,o alone in comparison with those in the control Groups E and F. Discussion An increasing amount of prostaglandins has been found in human breast cancer tissues [3, lo] and elevated prostaglandin synthesis appears to play a part in tumor spreading and bone metastasis [1,5]. However, there has been little information available concerning the role of PGF,(r on the carcinogenic process [4]. Lupulescu [4] reported that PGF,(r enhanced the induction of mouse skin tumors by 3-methylcholanthrene, in which concomitant in-
229
jections of PGF,(r with chemical carcinogen stimulated the DNA, RNA and protein synthesis in hyperplastic epidermal cells and resulted in a shortening of the latent period. These findings suggest that PGF,cr tends to enhance the development of primary tumors and the growth of mammary carcinomas. The present results, however, indicated that the concomitant administration of PGFp inhibited the induction of mammary carcinomas by EMS. In the Group B rats, given EMS plus PGF,(r at age 4 weeks, tumor appearance was markedly retarded by the 30th week and the final tumor incidence at the 44th week was 61.1%) whereas in the Group A rats, given EMS alone, the tumors were first detected at the 16th week and the tumor incidence reached 100% at the 32nd week. However, PGF,a injections after EMS administration in the Group C rats at age 16 weeks did not significantly retard the development of tumors. The putative mechanisms behind the inhibitory effects of PGF,cY on EMS-induced mammary carcinogenesis may involve the direct action of PGF,cY on the mammary glands and/or the influence on the environmental circumstances such as hormonal status. Histological examination of the mammary glands revealed that PGF,a did not cause any significant morphological difference either in cancerous or noncancerous mammary tissues. This finding did not support the direct effect of PGF,cY on the mammary glands as being responsible for the inhibition of the mammary carcinogenesis. The difference in inhibitory effect of PGF,(r on EMS-induced mammary carcinogenesis between younger and older female rats was compatible with that of ablative oophorectomy in the same experimental system [9]. That is, oophorectomy at age 4 weeks prior to EMS administration completely prevented tumor induction, while oophorectomy at age 16 weeks after EMS administration only delayed the tumor production. Since the presence of estrogen receptors was confirmed in EMS-induced mammary carcinomas [9], we previously proposed that the circulating levels of ovarian hormones and the
concentration of ER in mammary glands in rats at different ages might be responsible for the different susceptibility to cancer induction by EMS and to ablative oophorectomy. These findings raised the possibility that PGF,a affected the hormonal status of the rats and elicited the same effect as oophorectomy. Thus, the inhibitory effects of PGF,a on mammary carcinogenesis in this experiment seemed to involve the following mechanisms; (i) In Group B rats, PGF,a inhibited the ovarian and/or adrenal functions [6] and had prevented the development of mammary carcinomas during PGF,a injections. After terminating PGF,o administration, the EMS-sensitized mammary glands began to develop tumors in the presence of ovarian hormones, thus resulted in the prolongation of the latent period; (ii) In the Group C rats, given EMS at age 4 weeks and PGF,a at age 16 weeks, the mammary glands had already been sentitiied by EMS in the presence of ovarian hormones and the ablative effect of PGF,cY was diminished at this age. These speculations may be supported by the findings of Jubii et al. [Z] , in which PGF,(r inhibited the growth of hormonedependent rat mammary carcinoma, which was associated with a marked reduction in serum progesterone levels. The inhibitory effect of PGF,cY on tumor growth was abolished by a supplement of progesterone. They also speculated that the site of PGF,cY action might be the ovaries. Further investigation is needed for a better understanding of the biological mechanisms in which prostaglandins affect mammary carcinogenesis, such as defining the chronological change of their contents in developing tumors and their relation to the hormone receptor status. Therefore rat mammary carcinoma induced by EMS, possessing hormone receptors and sensitive to PGFP treatment, may serve as an useful model for the research on mammary carcinogenesis in relevance to the effect of prostaglandins. Acknowledgments
We thank Mr. Brian Quinn of Kyushu University for his comments on the manuscript.
230
References
(1978)
The sequence
of hormonal
changes
during pro-
staglandin induced luteolysis of the superluteintzed rat ovary. Bennett,
A. (1983)
hibitors in cancer. 237-247.
Prostaglandins In: Leukotrienes
and their synthesis inand Prostacyclin,
pp.
7
ASI Series, Plenum Press, New York.
998-
rat
tumor.
Cancer
Res.,
8
by
Ueo,
H.,
Takaki,
oral
H. and Lefevre,
parameters
S. (1983)
status. Br. J. Cancer,
A. (1978)
prostaglandins. Rolland, P.H.,
48,
Enhancement
Nature,
272,
by
Prostaglandin
R., Yamagami,
H. and Sugimachi,
Determination
affecting tumor induction.
Ueo, H., Matsuoka,
H., Akiyoshi,T.,
in human breast can-
rat mammary 10
Vergote,
carcinoma.
I., Laekeman,
Cancer G.,
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