The endocrine system and its numerous hormones function to integrate genetically encoded developmental programs and environmental signals and regulate the myriad biochemical processes, such as cell proliferation, differentiation, and death, required to maintain homeostasis within boundaries compatible with life. It is becoming increasingly clear that aberrations in the control of these hormone-regulated processes contribute to the genesis of many forms of cancer. In specific instances, hormones also appear to act through nonreceptor-mediated mechanisms to contribute to carcinogenesis. Steroid hormones, in particular, are implicated in the etiology of cancers of the breast, female reproductive tract, and male reproductive tract, as well as benign tumors of the anterior pituitary gland. Both endogenous and exogenous steroid hormones appear to contribute to carcinogenesis. In addition, much effort is being focused on defining the potential of environmental chemicals, both naturally occurring and man made, to enhance or inhibit carcinogenesis via hormonal mimicry. This article summarizes our current knowledge of hormonal carcinogenesis, focusing on cancers of the breast and other tissues where hormones play primary, if not causative, roles in cancer etiology.
I. BREAST CANCER
A. Endogenous Estrogens, Progestins, and Breast Cancer Etiology
It is well documented that estrogens and progestins act through direct and indirect pathways to stimulate development of the breast at puberty and during pregnancy. In addition, the breast epithelium exhibits a cyclical pattern of cell proliferation and death throughout each menstrual cycle; cell proliferation is highest during the luteal phase of the menstrual cycle, when circulating estradiol is submaximally elevated and circulating progesterone is at its peak. Data from the study of mouse models in which the genes encoding either estrogen receptor (ER) α or progesterone receptor (PR) are disrupted through insertional mutagenesis indicate that ERα and PR are required for the growth-promoting actions of estrogens and progestins within the mammary epithelium.
The report of Cooper in 1836 that breast cancers regress in size at the beginning of each menstrual cycle and at menopause and the report of Beatson in 1896 that oophorectomy leads to regression of breast cancers are now recognized as providing the first demonstrated links between ovarian hormones and breast cancer. Subsequent epidemiologic and laboratory studies provide overwhelming evidence inextricably linking ovarian steroids to the genesis of breast cancer. Both early menarche and late onset of menopause are associated with an increased risk of breast cancer. Moreover, oophorectomy prior to menopause significantly reduces breast cancer risk, with the greatest reduction in risk being observed in women 35 years of age or younger. Several, but not all, studies have demonstrated a positive correlation between the level of circulating estrogens in postmenopausal women and breast cancer risk. Clinical trials have demonstrated that tamoxifen, a selective estrogen receptor modulator (SERM), reduces the risk of breast cancer in asymptomatic women at high risk of developing the disease. In addition, inhibitors of aromatase, the enzyme that catalyzes the production of estrogens from androgen precursors, are gaining use in the treatment of breast cancer and may provide additional effective agents for the prevention of these cancers. Together, these data indicate that lifetime exposure to endogenous ovarian hormones is directly correlated with breast cancer risk and demonstrate that these hormones, particularly the estrogens, play an important, if not causative, role in breast cancer development.
Several epidemiologic studies have attempted to identify associations between specific allelic variants of genes that encode enzymes involved in estrogen metabolism and breast cancer risk. The hypothesis being tested in these studies is that genes with variant alleles that impact expression and/or function of the encoded protein may be low penetrance, but high frequency, modifiers of breast cancer risk. Cytochrome P450c17 (CYP17) catalyzes two steps in the pathway leading to sex steroid biosynthesis. A variant allele encoding CYP17 has been identified that is associated with increased levels of circulating estradiol and progesterone. This variant CYP17 allele has been associated with increased breast cancer risk in some, but not other, studies.
Cytochrome P4501A1 (CYP1A1) catalyzes the hydroxylation of a variety of substrates, including polycyclic aromatic hydrocarbons and the estrogens, 17β- estradiol (E2) and estrone (E1). Variant alleles of CYP1A1 have been identified that may differ in activity and/or inducibility. The m1 variant, which exhibits a single nucleotide polymorphism in the 3'-untranslated region, has been associated with an increased breast cancer risk in African-American and Chinese women. However, no risk modification was associated with the m1 allele in other studies. The m2 variant, a single nucleotide polymorphism that results in a single amino acid change, Ile462Val, in the heme-binding domain, did not significantly modify breast cancer risk in the general population, but was associated with an increased risk in specific subsets of smokers. Estrogens are also substrates for cytochrome P4501B1, which catalyzes the hydroxylation of E2 and E1 preferentially on C-4 of the aromatic A ring.
Two single nucleotide polymorphisms have been identified that impact amino acid sequence in the heme-binding domain of CYP1B1: m1, Val432Leu; and m2, Asn453Ser. Neither the m1 nor the m2 variant was observed to impact significantly breast cancer risk in Caucasian or African-American populations. However, homozygosity for the m1 allelic variant of CYP1B1 was associated with an increased breast cancer risk in a Chinese population. Catechol-Omethyl-transferase (COMT) catalyzes the methylation of various catechols, including the 2-hydroxylated and 4-hydroxylated metabolites of E2 and E1. A single nucleotide polymorphism in exon 4 of COMT results in an amino acid change, Val158Met, that reduces COMT activity. Two studies have associated homozygosity for the low-activity COMT allele with an increased breast cancer risk in postmenopausal, but not premenopausal, women; another study demonstrated an association between the low-activity COMT allele and increased breast cancer risk in premenopausal, but not postmenopausal, women; and yet another study did not associate the COMT genotype to breast cancer risk. The failure of these genetic epidemiology studies to produce consensus regarding the associations of variant alleles for genes encoding these different enzymes in the etiology of breast cancer most probably results from several factors, including (1) het-erogeneity within or between the populations studied; (2) gene-gene and gene-environment interactions that occlude modifier effects; and (3) modest population sizes in several of the studies.
Additional studies will be required to establish firmly whether these genes act as low penetrance modifiers of breast cancer risk. In addition, it must ultimately be demonstrated that any associations between genotype at a particular genetic locus and cancer risk result from loss or gain of function of the gene residing at that locus or the protein encoded by that gene.
B. Exogenous Estrogens, Progestins, and Breast Cancer Etiology
Exogenous hormones also modify breast cancer risk. Several studies indicate that current or recent use of estrogen replacement therapies (estrogen alone) or hormone replacement therapies (estrogen and progestin) by menopausal women is associated with an increased breast cancer risk. The increase in breast cancer is greatest in women using regimens containing both estrogen and progestin. Breast cancer risk also appears to be slightly increased by the current use of oral contraceptives. Finally, use of the synthetic estrogen, diethylstilbestrol (DES), during pregnancy is associated with an increased breast cancer risk.
C. Estrogen Receptor and Breast Cancer Etiology
The ER, in particular ERα, is central to estrogen action in the breast. Much effort has been invested toward the identification of mutations and polymorphisms in the gene encoding ERα and attempts to correlate these genetic differences with breast cancer risk, pathology, ER status, or responsiveness to hormonal therapy. Although mutant forms of the ER have been identified that differ from the wild-type receptor in ligand-binding and/or transcriptional activation activity, little evidence exists to suggest that somatic mutations in the ER play a major role in breast cancer etiology. However, loss of ER expression does often occur during progression from a hormonedependent to a hormone-independent stage. Specific ERα polymorphisms have been associated with an increased breast cancer risk in two of four published studies, suggesting that variant forms of ERα may act as low penetrance modifiers of breast cancer risk.
D. Potential Mechanisms of Hormonal Carcinogenesis in the Breast
It is well established that estrogens and progestins interact in the regulation of cell proliferation and cell survival in the breast. By stimulating cell proliferation and/or enhancing cell survival within the breast epithelium, ovarian steroids may increase the probability of somatic mutations, as well as enhance the growth and/or survival of transformed cells. These mutations could arise through the actions of environmental or endogenous mutagens, exposure to ionizing radiation, or replication errors during DNA synthesis. Mammographic density correlates with the proportion of the breast volume composed of the epithelial and stromal parenchyma as well as with breast cancer risk. Use of estrogens with progestins in menopausal hormone replacement regimens is associated with increased mammographic density, an increased rate of cell proliferation within the breast epithelium, and an increased risk of breast cancer. Moreover, the observation that the SERM tamoxifen significantly reduces breast cancer incidence in women at increased risk indicates that ER-mediated pathways play important roles in breast cancer etiology. Together, these studies strongly link the actions of ovarian steroids in the regulation of cell proliferation and/or survival in the breast epithelium to the genesis of breast cancer. At this time, the mechanisms through which ovarian hormones regulate proliferation and survival within the breast epithelium are not completely understood.
Both estrogens and progestins induce expression of cyclin D1 in breast epithelial cells and may thereby directly stimulate progression of those cells through the G1 phase of the cell cycle. In addition, estrogens regulate the production of several growth factors, which in turn act through autocrine and/or paracrine mechanisms to stimulate cell cycle progression and/or enhance cell survival within the breast epithelium. It is becoming increasing clear that interactions between stromal and epithelial cells are required for normal development and function of the breast and that the estrogens and progestins act through the stroma to exert some of their stimulatory effects on the breast epithelium.
It is hypothesized that specific estrogen metabolites may be genotoxic carcinogens that contribute to the development of breast cancers and other cancers by inducing mutations in oncogenes and/or tumor suppressor genes. An often proposed mechanism of estrogen carcinogenesis is that E2 and/or E1 is hydroxylated at either the C-2 or the C-4 position to the corresponding catechol estrogen, which is subsequently oxidized through a semiquinone intermediate to the estrogen quinone, which could bind covalently to DNA. Alternatively, the catechol estrogen and/or estrogen semiquinone may undergo redox cycling and generate superoxide radicals, which could damage DNA. An enzyme(s) capable of generating the catechol estrogens 4-hydroxyestradiol and 4-hydroxyestrone is expressed in human breast cancers, and CYP1B1 has been identified as an estrogen-4-hydroxylase expressed in the MCF-7 breast cancer cell line.
However, the carcinogenicity of catechol estrogens in the rodent mammary gland has yet to be established, and estrone-3,4-quinone did not induce mammary cancers when injected directly into the mammary gland of female rats. In addition, several studies have demonstrated that estrogens and catechol estrogens lack mutagenic activity in standardized bacterial and mammalian cell assays. Therefore, the contribution of potentially genotoxic estrogen metabolites to the etiology of breast cancer remains to be defined.
Treatment of a mouse mammary epithelial cell line with 16α-hydroxyestrone induces anchorage independent growth, suggesting that this estrogen metabolite has the potential to induce neoplastic transformation in this mammary cell model. The ability of estrogens and estrogen metabolites to induce neoplastic transformation in vitro has been extensively studied by T. Tsutsui and J.C. Barrett using Syrian hamster embryo cells. Transformation of Syrian hamster embryo cells is associated with the induction of aneuploidy, which may result from binding of estrogens or specific estrogen metabolites to β-tubulin and disruption of microtubule assembly during mitosis. More recently, the transforming potential of different metabolites of E2, including the 2-hydroxy and 4- hydroxy catechols of E2 and E1, as well as 16α- hydroxyestrone, in Syrian hamster embryo cells has been associated with the induction of mutations, chromosomal aberrations, and/or aneuploidy.
In summary, estrogens and progestins most probably act through multiple mechanisms to induce and/or promote the development of breast cancers (Fig. 1). Pathways dependent on the ER are clearly implicated by the ability of tamoxifen to reduce breast cancer incidence in women at increased risk. It is also clear that the actions of these hormones in regulating cell proliferation and survival within the breast epithelium contribute to the genesis of breast cancers. Data are emerging to suggest that genes encoding enzymes involved in steroid hormone metabolism may be low penetrance modifiers of breast cancer risk. In this regard, breast cancer risk could be impacted by genetic variation in (1) the amounts of active hormones produced; (2) the specific types of hormones or metabolites produced; (3) the timing of hormone production; and/or (4) the location of hormone production. Estrogens may be metabolized to genotoxic forms that can bind to DNA and induce mutations. Alternatively, estrogens may induce aneuploidy or other forms of genomic instability. Additional studies in validated animal models are required to define precisely the mechanisms through which hormones contribute to breast cancer etiology.
FIGURE 1 Possible pathways of hormonal carcinogenesis in the breast. Genetic variation in genes encoding enzymes involved in estrogen biosynthesis (1) or estrogen excretion (2) may impact the type or amount of steroidal estrogens acting on target cells. Endogenous and exogenous estrogens may be metabolized to reactive forms that can bind macromolecules, including DNA, and proteins such as tubulin (3). Genetic variation in genes encoding specific detoxifying enzymes may impact the clearance of potentially genotoxic estrogen metabolites (4). Genetic variation in the genes encoding ERα or ERβ may impact the binding of estrogens to its receptor, receptor levels, or receptor signaling. (5) Although not as intensely studied, genetic variability in progesterone metabolism or signaling may also contribute to breast cancer etiology.
E. Animal Models of Estrogen-Induced Mammary Cancer
The first demonstration that estrogens induce mammary cancers in rodents was by Lacassagne in the 1930s. However, rodent models of estrogen-induced mammary carcinogenesis did not achieve the same degree of prominence in the research community as animal models in which mammary cancers are induced by carcinogens, such as dimethylbenz[a]anthracene (DMBA) or N-methyl-N-nitrosourea (MNU). Studies suggest that rat models of estrogen-induced mammary carcinogenesis can provide novel and physiologically relevant insights into the role of estrogens and other hormones in the etiology of breast cancer.
1. ACI Rat as a Model of Estrogen-Induced Mammary Cancer
The ACI rat was developed by Dunning and colleagues in 1926 from a cross between the August and the Copenhagen (COP) rat strains. Although these investigators first demonstrated the ability of DES to induce mammary cancers in the ACI rat in 1947, this model was used only sporadically over the next 50 years.
Data from our laboratory indicate that the female ACI rat exhibits a unique propensity to develop mammary cancers when treated continuously with physiologic levels of E2, released from subcutaneous Silastic tubing implants. Palpable mammary cancers begin to appear as early as 70 days following the initiation of E2 treatment, median latency is approximately 140 days, and virtually 100% of the treated animals exhibit palpable cancers within 210 days of treatment. Ovariectomy significantly inhibits the development of E2-induced mammary cancers, suggesting a requirement for another ovarian factor(s), possibly progesterone, in E2-induced mammary carcinogenesis. Focal regions of atypical epithelial hyperplasia are common in the mammary glands of E2- treated ACI rats, and these are probable precursors to carcinoma (Fig. 2). Relative to the surrounding mammary epithelium, cells within the focal regions of atypical hyperplasia and the mammary cancers exhibit an increased expression of progesterone receptor (PR). Carcinomas induced in ACI rats by E2 are estrogen dependent and commonly exhibit aneuploidy, two features that are common to breast cancers in humans.
Continuous E2 Treatment from Subcutaneous Implant
FIGURE 2 Histologic progression of estrogen-induced mammary carcinogenesis in the ACI rat. (A) The mammary gland of the ovary-intact, virgin female ACI rat is composed of ductal structures extending throughout the mammary fat pad. (B) Continuous treatment with E2 rapidly induces marked lobular development and hyperplasia. (C) Focal regions of atypical hyperplasia begin to appear within the hyperplastic epithelium following approximately 8 weeks of E2 treatment. (D) Palpable mammary carcinomas appear as early as 10 weeks following the initiation of E2 treatment, but the median latency to the appearance of the first palpable mammary carcinoma is 20 weeks. (E) Invasive features are observed in a fraction of the E2-induced mammary carcinomas.
Relative to the ACI rat strain, females of the COP strain, which is closely related genetically to ACI, and the Brown Norway (BN) strain, which is unrelated, are much less susceptible to E2-induced mammary cancers (Table I). These strain differences provide an avenue toward elucidation of the mechanisms through which estrogens induce mammary cancer development. For example, we have demonstrated that the proliferative response of the ACI mammary epithelium to E2 significantly exceeds that of the COP mammary epithelium, suggesting one possible mechanism for the differing susceptibilities of these rat strains to E2-induced mammary cancers. Genetic studies from our laboratory indicate that the highly susceptible ACI phenotype behaves as an incompletely dominant trait in crosses to either of the resistant COP or BN strains. Although most of the E2-treated ACI/COP F1 and ACI/BN F1 progeny develop mammary cancers when treated with E2, latency is significantly delayed relative to that observed in the parental ACI strain. We have mapped genetic modifiers of susceptibility to E2-induced mammary cancers to rat chromosome 5 in reciprocal crosses between ACI and COP rat strains and to chromosomes 5, 18, and 2 in a cross between ACI and BN strains. The gene encoding Cdkn2a is a candidate for the modifier residing on chromosome 5. Cdkn2a is an inhibitor of the cyclin D-dependent kinase Cdk4 and Cdk6 and would therefore be expected to inhibit the mitogenic actions of estrogens exerted through the cyclin D/Cdk/retinoblastoma pathway. We have demonstrated that the expression of Cdkn2a is markedly downregulated in both the focal regions of atypical hyperplasia and the mammary cancers induced in ACI rats by E2, relative to the surrounding epithelium. We anticipate that the ultimate identification of the genetic modifiers of susceptibility to E2-induced mammary cancers in this rat model will reveal significant information regarding the mechanisms through which estrogens contribute to breast cancer in humans. These data strongly suggest that the ACI rat provides a unique and physiologically relevant animal model for investigating the mechanisms of estrogen-induced mammary carcinogenesis, as well as for studying interactions among hormonal, genetic, and environmental determinants of breast cancer susceptibility.
TABLE I Unique Susceptibility of the Female ACI Rat to Estrogen-Induced Mammary Carcinogenesisa
|Rat strain||Mean latency (days)||Incidence (%)||Palpable tumors/rat|
aSilastic tubing implants containing 17β-estradiol were inserted subcutaneously when ovary intact rats of the indicated inbred rat strains were 9 weeks of age.
2. Noble Rat as Model of Estrogen/Androgen-Induced Mammary Cancer
The Noble (NBL) rat is another unique model for hormone-induced mammary carcinogenesis. Noble first demonstrated the ability of E1 to induce mammary cancers in males and females of this rat strain in 1940. In a subsequent study, Noble demonstrated that the susceptibility of the NBL rat to E1-induced mammary cancers declined markedly as a function of the age at which treatment was initiated. Whereas 100% of the E1- treated NBL rats developed mammary carcinoma when treatment was initiated at 2 weeks of age, less than 10% of animals treated at 9 weeks of age were tumor positive following 13 months of E1 treatment. More recently, the combination of E2 and testosterone propionate has been demonstrated by S.A. Li and colleagues to induce a high incidence of mammary cancers in male NBL rats, whereas mammary cancers did not develop in the same time period in rats treated with E2 or testosterone propionate alone. Similarly, studies by Y. C. Wong and colleagues indicate that E2 benzoate plus testosterone propionate induces a high incidence of mammary cancers in female NBL rats. These studies, in conjunction with those on the ACI rat summarized earlier, suggest that the NBL is less sensitive to mammary cancer induction by E2 or E2 benzoate relative to ACI rats treated with E2. Moreover, the requisite for coadministration of androgen and estrogen for rapid development of mammary cancers in the NBL rat may illustrate a fundamental mechanistic difference between the NBL and ACI models of E2-induced mammary carcinogenesis. Therefore, each of these inbred rat strains may ultimately reveal different insights into the mechanisms of hormonal carcinogenesis in the mammary gland.
II. CANCERS OF THE FEMALE REPRODUCTIVE TRACT
A. Endometrial Cancer
Estrogens stimulate cell proliferation within the uterine endometrium, and most cell proliferation in this tissue occurs during the follicular phase of the menstrual cycle, when circulating estrogens are increasing. An increased risk of endometrial cancer is associated with late menopause and obesity, both of which are generally believed to result in prolonged exposure of the uterus to estrogens. Use of combination oral contraceptives decreases the risk of endometrial cancer, whereas use of sequential oral contraceptive formulations is associated with an increased risk. Estrogen replacement therapy to alleviate symptoms of menopause significantly increases risk. However, hormone replacement regimens containing both estrogens and a progestin do not increase the risk of endometrial cancer. Moreover, tamoxifen exhibits agonistic properties in the uterus, and use of tamoxifen has been strongly associated with an increased risk of endometrial cancers. A recent genetic epidemiology study provides suggestive evidence that allelic variants in the gene encoding ERα may modify the risk of endometrial cancer. Together, these data strongly implicate both endogenous and exogenous estrogenic hormones in the etiology of cancers of the uterine endometrium.
In the mouse, disruption of the gene encoding ERα, but not ERβ, drastically alters normal development of the uterus and eliminates responsiveness of the uterine tissues to estrogens. Treatment of newborn CD-1 mice with DES has been shown to induce a 90% incidence of uterine adenocarcinoma by 18 months of age. Development of uterine cancers in this model requires a functional ovary. More recently, the estrogens E2 and 17α-ethinylestradiol, as well as the catechol estrogens 2-hydroxyestradiol and 4-hydroxyestradiol, have been demonstrated to induce uterine adenocarcinoma in this CD-1 mouse model. Similarly, tamoxifen has been demonstrated to induce uterine adenocarcinoma when administered to neonatal Wistar rats.
Data from these rodent models strongly suggest that estrogens play a direct role in inducing endometrial cancers and are consistent with the hypothesis that estrogens act through a genotoxic mechanism in the neonatal uterus to initiate carcinogenesis. Alternatively, the administered estrogens may act through an ER-mediated mechanism to trigger developmental changes in the neonatal uterine epithelium and/or stroma that predispose to adenocarcinoma in the adult animals.
B. Cervical and Vaginal Cancers
The synthetic estrogen DES has been inextricably linked to clear cell carcinoma of the cervix/vagina of young women who were exposed in utero when their mothers were treated with DES during the first trimester of pregnancy. This carcinogenic action appears to be quite specific because cancers at other sites are not significantly increased in women exposed to DES in utero. An association between exogenous estrogens and cervical cancers in mice has been demonstrated. For example, DES, administered prenatally or neonatally, induces vaginal adenocarcinoma in CD-1 mice. Endogenous ovarian steroid hormones, oral contraceptives, and hormone replacement regimens are not generally believed to be strong determinants of cervical cancer risk.
C. Myometrial Tumors and Cancers
Uterine leiomyoma is the most commonly observed uterine tumor in premenopausal women. Ovarian steroids have been linked to the genesis of these benign uterine tumors as well as to the development of malignant uterine leiomyosarcomas. Leiomyoma and, less frequently, leiomyosarcoma develop spontaneously in the Eker rat. Development of these tumors is markedly inhibited by ovariectomy or treatment with SERMs. Continuous treatment with E2 induces leiomyoma in guinea pigs. Ovariectomy increases the incidence of leiomyoma in this animal model and decreases the latency to tumor appearance. Tumor regression occurs in response to cessation of E2 treatment or treatment with the SERM raloxifene. Finally, treatment of Syrian hamsters with the combination of DES and testosterone propionate has been reported to induce uterine leiomyosarcoma. Together, these epidemiologic and laboratory studies indicate a role of estrogens in the development of tumors of the uterine myometrium.
III. PROSTATE CANCER
Abundant data link androgenic hormones to the etiology of prostate cancer. Orchiectomy leads to involution of the prostate and is recognized as an effective therapeutic regimen for the treatment of prostate cancers. Prospective studies have not demonstrated strong correlations between prostate cancer risk and circulating levels of testosterone, dihydrotestosterone, androstenedione, and several other androgenic hormones. However, several studies have demonstrated that variant alleles of the gene encoding the androgen receptor (AR) may modify prostate cancer risk. The AR gene contains within exon 1 a polymorphic CAG repeat that encodes a variable length glutamine tract that appears to impact the ability of the AR to activate gene transcription. Prostate cancer risk appears to be inversely correlated with the number of CAG repeats in the AR. A polymorphic GGC repeat in exon 1 of AR, encoding a variable length glycine tract, may also modify prostate cancer risk.
Evidence is emerging to suggest that genes encoding enzymes involved in androgen biosynthesis or catabolism may also modify prostate cancer risk. The enzyme 5α-reductase type 2 (SRD5A2) catalyzes the conversion of testosterone to the more active dihydrotestosterone, and the gene encoding this enzyme, SRD5A2, is expressed exclusively in the prostate. Finasteride, an inhibitor of SRD5A2, is currently being evaluated in a large cooperative trial for its ability to prevent prostate cancer. A variant SRD5A2 allele, which encodes threonine in place of alanine at codon 49 (Ala49Thr), is associated with a significantly increased risk of prostate cancer in African-American and Hispanic men. In addition, the Ala49Thr variant is associated with a more aggressive disease phenotype and poorer prognosis relative to the wild-type allele. Interestingly, the Ala49Thr variant of SRD5A2 encodes an enzyme with an increased capacity to produce dihydrotestosterone relative to the wild-type allele.
Two other SRD5A2 allelic variants, Val89Leu and a polymorphic dinucleotide repeat in the 3'- untranslated region, do not appear to significantly impact androgen metabolism or prostate cancer risk. As discussed in Section IA, CYP17 catalyzes two reactions in the biosynthetic pathway leading to androgens and estrogens. Data on CYP17 as a modifier of prostate cancer risk are inconsistent, with two studies associating the A2 allelic variant of CYP17 with an increased risk of prostate cancer and two studies associating the A1 allele with increased risk.
Epidemiologic studies associate vitamin D deficiency with prostate cancer, and numerous laboratory studies indicate a role of the hormonally active form of vitamin D, 1,25-dihydroxyvitamin D3, in the regulation of proliferation and survival of prostate cancer cell lines. 1,25-Dihydroxyvitamin D3 functions through a receptor that is a member of the nuclear receptor superfamily. Several allelic variants of the vitamin D receptor have been identified, and studies suggest that these variants may be associated with an altered prostate cancer risk. Although not all of these studies concur with regard to the associations between specific polymorphisms and risk, together they suggest that the gene encoding the vitamin D receptor may act as a low penetrance modifier of prostate cancer development.
Noble was the first to report that continuous treatment with either testosterone or testosterone in combination with estrogen induces the development of prostate cancers in the NBL rat. A series of studies by S.-M. Ho and colleagues indicates that treatment of intact NBL rats with testosterone and E2 for 16 weeks induces a high incidence of epithelial dysplasia in the dorsolateral lobe of the prostate. Relative to the surrounding epithelium, dysplastic lesions exhibit several changes in gene expression that may contribute to or result from their development, including increased expression of Ha-ras and progesterone receptor mRNAs. Induction of the dysplastic lesions is inhibited by treatment with bromocryptine, suggesting that prolactin contributes to their development. Interestingly, the location of the hormone-induced lesions within the prostate appears to be dependent on the amount and/or type of estrogen administered. For example, testosterone in combination with increased E2 induces atypical hyperplasia in the ventral lobe of the prostate. Similarly, treatment of NBL rats with testosterone plus DES induces dysplasia in the ventral prostate.
Studies by Bosland and colleagues indicate that long-term treatment of NBL rats with testosterone and E2 induces a 100% incidence of adenocarcinoma in the dorsolateral, but not ventral, prostate of the NBL rat. A DNA adduct of unknown structure was detected in the dorsolateral, but not ventral, prostate of NBL rats treated with testosterone and E2 for 16 weeks or longer. More recently, Y. C. Wong and colleagues have published a series of studies on the induction of carcinoma in the ventral prostate of the NBL rat by continuous treatment with testosterone and high-dose E2.
The mechanisms of prostate cancer induction by testosterone and estrogens are not currently known. It is clear that androgens play an important role in the regulation of cell proliferation and survival in the prostate gland of the human as well as rodent species, and it is probable that these actions contribute to the etiology of prostate cancers. For example, it is hypothesized that by stimulating proliferation within the prostate epithelium, androgens promote the development of cancers that are induced by endogenous or environmental carcinogens, possibly including genotoxic estrogen metabolites. The identification of specific mutations induced in the rat prostate as a consequence of combined testosterone and E2 treatment would provide strong support for this hypothesis. It is also hypothesized that the hormonal environment during embryonic development may contribute to the genesis of prostate cancers later in life. This hypothesis is based on data from numerous studies that indicate that prenatal exposure to naturally occurring estrogens, synthetic estrogens, or xenoestrogens impacts development of the male reproductive tract, including the prostate, in rats and mice. Each of these actions of androgenic and estrogenic hormones could contribute to prostate cancer development.
IV. PITUITARY TUMORS
Benign tumors of the anterior pituitary gland are common in humans, occurring in approximately 20-25% of individuals randomly evaluated at autopsy. Prolactin (PRL)-producing tumors, prolactinomas, represent the most common type of pituitary tumor. Estrogens have been implicated in the etiology of prolactinoma. The incidence of these tumors in females is approximately twice that of males, and several anecdotal observations link the development of prolactinoma to clinical or environmental exposures to estrogens. Supporting a role of estrogens in the etiology of prolactinoma are numerous reports that administered estrogens induce development of PRLproducing pituitary tumors in mice and rats. The literature on the role of estrogens in pituitary tumorigenesis in the human and rat has been reviewed by Spady and co-workers.
Different inbred rat strains exhibit marked differences in sensitivity to the pituitary tumor-inducing actions of estrogens. The Fischer 344 (F344) strain is the most sensitive of the inbred rat strains. Continuous treatment of male or female F344 rats with DES or E2 for 6 weeks or longer results in 10- to 20-fold increases in pituitary mass. This increase in pituitary mass results from increased proliferation and enhanced survival within the PRL-producing lactotroph population and correlates with both absolute lactotroph number and the level of PRL in the circulation. Estrogeninduced pituitary tumors are usually defined histologically as diffuse lactotroph hyperplasias, although adenomatous foci have been described in some studies.
In our laboratory, we have only rarely observed a pituitary carcinoma in a rat treated continuously with estrogen for long periods of time. Although less sensitive than F344, the Wistar-Furth, ACI, NBL, and COP rat strains also develop PRL-producing pituitary tumors when treated with estrogens. In contrast, the BN rat strain is highly insensitive to the pituitary growth-promoting actions of estrogens.
The differing sensitivities of different inbred rat strains to the pituitary tumor-inducing actions of estrogens allow the underlying mechanisms to be studied using genetic techniques. The first studies of this type, by Wiklund and Gorski, indicated that the high sensitivity of the F344 strain to DES resulted from the actions of multiple, independently segregating genes. Wendell and Gorski have subsequently mapped within the rat genome the locations of several modifiers of DES-induced pituitary growth in F344 x BN intercrosses. Our laboratory has mapped additional modifiers of DES-induced pituitary growth in ACI x COP and ACI x BN intercrosses. Together, these genetic studies indicate that the actions of estrogens in the regulation of lactotroph proliferation and the survival and development of PRL-producing pituitary tumors are controlled by many genes. The ultimate goals of these genetic studies are to identify these genes and establish their functions. It is possible that at least some of these genes will impact carcinogenesis in other estrogen-responsive tissues.
V. OTHER HORMONEINDUCED CANCERS
Numerous reports illustrate the ability of administered estrogens to induce cancers at a wide variety of tissue sites in rats, mice, hamsters, and other animal species. The incidence and type of cancers vary markedly as a function of species, genetic strain, type of estrogen used, route of estrogen administration, and timing of estrogen administration. For example, Noble described a wide variety of cancers or tumors that develop in NBL rats in response to continuous treatment with E1. The affected tissues include the adrenal cortex, mammary gland, pituitary gland, ovary, uterus, cervix, vagina, testis, prostate, thymus, adipose, pancreas, and salivary gland, as well as lymphoid tissues. Noble recognized that dependence on estrogens for growth is a common feature of most estrogen-induced tumors.
Some studies suggest that exogenous estrogens may increase the risk of liver cancers in humans. Estrogens are potent tumor promoters in the rat liver, and the SERM tamoxifen acts as a complete carcinogen in the rat liver.
Several, but not all, estrogens induce renal cancers in orchiectomized Syrian hamsters. These cancers appear to be of epithelial origin, and their induction by estrogens is markedly suppressed by testosterone, progesterone, and SERMs. Continuous estrogen treatment is associated with histologically evident cytotoxicity and compensatory regeneration in the hamster kidney. It has been suggested that these actions contribute significantly to estrogen-induced renal carcinogenesis. A role of potentially genotoxic estrogen metabolites has also been suggested to initiate the development of kidney cancers in this animal model.
VI. CONCLUDING REMARKS
Based on a wealth of data from epidemiologic and laboratory studies, steroid hormones can be considered to be the cause of at least a subset of specific cancers in humans. The evidence that endogenous hormones cause cancer is most compelling for cancers of the breast and prostate. Consequently, strategies based on blocking the actions of estrogens and androgens are currently being used or are being evaluated for the prevention of cancers of the breast and prostate, respectively. Exogenous hormones contribute significantly to the etiology of cancers of the breast, uterine endometrium, and uterine cervix. Despite many years of intense study, the mechanisms through which these hormones exert their carcinogenic actions are not well defined. Possible contributory mechanisms include, but are not limited to the (1) stimulation of cell proliferation; (2) inhibition of apoptosis; (3) alteration of hormonally regulated developmental programs; (4) genotoxic actions of specific hormone metabolites; and (5) abrogation of genome stability. It is probable that multiple and/or different mechanisms contribute to the genesis of different cancer types and that genetic background impacts on these mechanisms.
Currently existing animal models of hormoneinduced cancers should continue to serve as valuable tools for elucidating the mechanisms of hormonal carcinogenesis, and additional, physiologically relevant, animal models need to be developed. Geneticsbased approaches can be utilized to compare different inbred strains of rats and mice that exhibit marked differences in susceptibility to specific hormoneinduced cancers. Novel transgenic and knockout mouse models can be developed to evaluate the roles of specific genes as modifiers of susceptibility to hormone-induced cancers. It is critical to identify causative mutations for the different hormoneinduced cancers to establish firmly whether hormones or their metabolites act through genotoxic mechanisms to initiate carcinogenesis. The ongoing human, mouse, and rat genome projects are providing vast information as well as new tools to support these efforts. It is hoped that significant improvements in the prevention and treatment of hormone-associated cancers are on the horizon.
James D. Shull
University of Nebraska Medical Center
BREAST CANCER ; ENDOCRINE TUMORS ; ENDOMETRIAL CANCER ; ESTROGENS AND ANTIESTROGENS ; CHEMICAL MUTAGENESIS AND CARCINOGENESIS ; HORMONE RESISTANCE IN PROSTATE CANCER ; PROSTATE CANCER ; STEROID HORMONES AND HORMONE RECEPTORS
allele One form of a gene for which different forms exist in a population.
allelic variant One of two or more forms of a specific gene existing within a population.
cytochrome P450 A large family of enzymes that catalyze oxidation/reduction reactions on a wide variety of endogenous and exogenous substrates.
genetic modifier A gene that through the actions of its protein product alters cancer risk in a quantifiable manner.
genotoxic DNA damage or mutagenesis.
inbred strain A mouse or rat strain in which genetic variation is eliminated through multiple generations of stringent inbreeding.
low penetrance A gene for which a specific allelic variant does not uniformly confer the specific phenotype.
polymorphism Genetic variability within a population.
xenoestrogens Exogenous chemicals that can bind to the estrogen receptor and mimic the actions of estrogens.
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