Progesterone inhibits the estrogen-induced prolactin gene expression in the rat pituitary

mPRs represent a novel target for PRL inhibition in experimental prolactinomas

Membrane progesterone receptors are known to mediate rapid nongenomic progesterone effects in different cell types. Recent evidence revealed that mPRα is highly expressed in the rat pituitary, being primarily localized in lactotrophs, acting as an intermediary of P4-inhibitory actions on prolactin secretion. The role of mPRs in prolactinoma development remains unclear. We hypothesize that mPR agonists represent a novel tool for hyperprolactinemia treatment. To this end, pituitary expression of mPRs was studied in three animal models of prolactinoma. Expression of mPRs and nuclear receptor was significantly decreased in tumoral pituitaries compared to normal ones. However, the relative proportion of mPRα and mPRβ was highly increased in prolactinomas. Interestingly, the selective mPR agonist (Org OD 02-0) significantly inhibited PRL release in both normal and tumoral pituitary explants, displaying a more pronounced effect in tumoral tissues. As P4 also regulates PRL secretion indirectly, by acting on dopaminergic neurons, we studied mPR involvement in this effect. We found that the hypothalamus has a high expression of mPRs. Interestingly, both P4 and OrgOD 02-0 increased dopamine release in hypothalamus explants. Moreover, in an in vivo treatment, that allows both, pituitary and hypothalamus actions, the mPR agonist strongly reduced the hyperprolactinemia in transgenic females carrying prolactinoma. Finally, we also found and interesting gender difference: males express higher levels of pituitary mPRα/β, a sex that does not develop prolactinoma in these mice models. Taken together, these findings suggest mPRs activation could represent a novel tool for hyperprolactinemic patients, especially those that present resistance to dopaminergic drugs.

Participation of membrane progesterone receptor α in the inhibitory effect of progesterone on prolactin secretion

The membrane progesterone receptors (mPRα, mPRβ, mPRγ, mPRδ and mPRε) are known to mediate rapid nongenomic progesterone functions in different cell types. However, the functions of these receptors in the pituitary have not been reported to date. In the present study, we show that the expression of mPRα was the highest among the mPRs in the rat anterior pituitary gland. Immunostaining of mPRα was detected in somatotrophs, gonadotrophs and lactotrophs. Interestingly, 63% of mPRα-positive cells within the pituitary were lactotrophs, suggesting that mPRα is involved in controlling prolactin (PRL) secretion in the pituitary. To test this hypothesis, rat pituitaries were incubated (1 hour) with either progesterone (P4) or the mPRα-specific agonist Org OD 02-0. PRL secretion was then measured by radioimmunoassay. The results of this experiment revealed that both P4 and Org OD 02-0 decreased PRL secretion. Moreover, the results from the GH3 cell line (CCL-82.1) showed that P4 and Org OD 02-0 inhibited PRL release, although the nuclear PR agonist R5020 was ineffective. Our investigation of the cellular mechanisms behind mPRα activity indicated that both P4 and Org OD 02-0 decreased cAMP accumulation, whereas R5020 was ineffective. In addition, the Org OD 02-0-effect on PRL release was blocked by pretreatment with pertussis toxin, an inhibitor of Go/Gi proteins. Because transforming growth factor (TGF)β1 is a potent inhibitor of PRL secretion in lactotrophs, we lastly evaluated whether TGFβ1 was activated by progesterone and whether this effect was mediated by mPRα. Our results showed that P4 and Org OD 02-0, but not R5020, increased active TGFβ1 levels. This effect was not observed when cells were transfected with mPRα-small interfering RNA. Taken together, these data provide new evidence suggesting that mPRα mediates the progesterone inhibitory effect on PRL secretion through both decreases in cAMP levels and activation of TGFβ1 in the lactotroph population.

Developmental and Functional Effects of Steroid Hormones on the Neuroendocrine Axis and Spinal Cord

This review highlights the principal effects of steroid hormones at central and peripheral levels in the neuroendocrine axis. The data discussed highlight the principal role of oestrogens and testosterone in hormonal programming in relation to sexual orientation, reproductive and metabolic programming, and the neuroendocrine mechanism involved in the development of polycystic ovary syndrome phenotype. Moreover, consistent with the wide range of processes in which steroid hormones take part, we discuss the protective effects of progesterone on neurodegenerative disease and the signalling mechanism involved in the genesis of oestrogen-induced pituitary prolactinomas.

Opioid modulation of prolactin secretion induced by stress during late pregnancy. Role of ovarian steroids

BACKGROUND

The opioid system modulates prolactin release during late pregnancy. Its role and the participation of ovarian hormones in this modulation are explored in ether stress-induced prolactin release.

METHODS/RESULTS

Estrous, 3-day and 19-day pregnant rats were used. We administered the antagonist mifepristone (Mp) and tamoxifen to evaluate progesterone and estradiol action in naloxone (NAL, opioid antagonist) or saline treated rats. Ether stress had no effect on serum prolactin levels in controls but increased prolactin release in NAL-treated rats. Prolactin response to stress in NAL-treated rats was blocked by l-DOPA administration. Mp treatment on day 18 of pregnancy increased prolactin levels after stress without alterations by NAL. Tamoxifen on days 14 and 15 of pregnancy completely blocked Mp and NAL effects on prolactin release at late pregnancy. In contrast, stress significantly increased prolactin levels in estrous rats and pretreatment with NAL prevented this. On day 3 of pregnancy, at 6.00 p.m., stress and NAL treatment inhibited prolactin levels in saline-treated rat. No effect of stress or NAL administration was detected on day 3 of pregnancy at 9.00 a.m. icv administration of specific opioids antagonist, B-Funaltrexamine but not Nor-Binaltorphimine or Naltrindole, caused a significant increase in stress-induced prolactin release.

CONCLUSIONS

Opioid system suppression of prolactin stress response during late pregnancy was observed only after progesterone withdrawal, involving a different opioid mechanism from its well-established stimulatory role. This mechanism acts through a mu opioid receptor and requires estrogen participation. The opioid system and progesterone may modulate stress-induced prolactin release, probably involving a putative prolactin-releasing factor.

Estrogens induce expression of membrane-associated estrogen receptor α isoforms in lactotropes

Estrogens are key to anterior pituitary function, stimulating hormone release and controlling cell fate to achieve pituitary dynamic adaptation to changing physiological conditions. In addition to their classical mechanism of action through intracellular estrogen receptors (ERs), estrogens exert rapid actions via cell membrane-localized ERs (mERs). We previously showed that E2 exerts a rapid pro-apoptotic action in anterior pituitary cells, especially in lactotropes and somatotropes, through activation of mERs. In the present study, we examined the involvement of mERα in the rapid pro-apoptotic action of estradiol by TUNEL in primary cultures of anterior pituitary cells from ovariectomized rats using a cell-impermeable E2 conjugate (E2-BSA) and an ERα selective antagonist (MPP dihydrochloride). We studied mERα expression during the estrous cycle and its regulation by gonadal steroids in vivo by flow cytometry. We identified ERα variants in the plasma membrane of anterior pituitary cells during the estrous cycle and studied E2 regulation of these mERα variants in vitro by surface biotinylation and Western Blot. E2-BSA-induced apoptosis was abrogated by MPP in total anterior pituitary cells and lactotropes. In cycling rats, we detected a higher number of lactotropes and a lower number of somatotropes expressing mERα at proestrus than at diestrus. Acute E2 treatment increased the percentage of mERα-expressing lactotropes whereas it decreased the percentage of mERα-expressing somatotropes. We detected three mERα isoforms of 66, 39 and 22 kDa. Expression of mERα66 and mERα39 was higher at proestrus than at diestrus, and short-term E2 incubation increased expression of these two mERα variants. Our results indicate that the rapid apoptotic action exerted by E2 in lactotropes depends on mERα, probably full-length ERα and/or a 39 kDa ERα variant. Expression and activation of mERα variants in lactotropes could be one of the mechanisms through which E2 participates in anterior pituitary cell renewal during the estrous cycle.

Sexual dimorphism in the effect of concomitant progesterone administration on changes caused by long-term estrogen treatment in pituitary hormone immunoreactivities of rats

BACKGROUND

Since in clinical practice long-term estrogen (E) treatment is frequently applied, our aim was to study the effect of concomitant progesterone (P) administration on changes caused by long-term estrogen treatment in the secretion of LH, FSH, PRL and GH.

MATERIAL/METHODS

Diethylstilbestrol (DES), P or both in silastic capsules were implanted under the skin of prepubertal Sprague-Dawley male and female rats. Animals survived for two or five months. We have also studied whether the changed hormone secretion caused by DES can return to normal level 1 or 2 months after removing DES capsule.

RESULTS

1.) The males more rapidly responded than females with decreasing basal LH release upon treatments. The basal FSH release was decreased only in males. The effect of DES persisted in males; however, in females basal LH and FSH levels were upregulated after removal of DES capsule. 2.) The basal GH levels were low in each group. The body weight and length were depressed by DES in both genders and P little blunted this effect. The body weight and length in males remained low after removal of DES capsule, in females it was nearly similar to intact rats. 3.) There was no sexual dimorphism in the effect of steroids on PRL secretion. In both genders DES extremely enhanced the PRL levels, P prevented the effect of DES. PRL levels returned to intact value after removal of DES influence. 4.) Removal of DES capsule reversed the changes in the immunohistochemical appearance of hormone immunoreactivities.

CONCLUSIONS

There was sexual dimorphism in the change of basal gonadotropic hormone and GH secretion but not of PRL upon DES and DES+P treatments. P was basically protective and this role may be mediated by P receptors locally in the pituitary gland.

CNS-specific ablation of steroidogenic factor 1 results in impaired female reproductive function

The ventromedial hypothalamic nucleus (VMH) regulates a variety of homeostatic processes including female sexual behavior and reproduction. In the current study, we assessed the roles of steroidogenic factor 1 (SF-1) on reproductive function in the VMH using central nervous system-specific SF-1 knockout (SF-1 KO(nCre;F/-)) mice. Here we show that SF-1 KO(nCre;F/-) females exhibited marked impairment in female reproduction. Although male mice appeared to be normal in all aspects studied, including sexual behavior, SF-1 KO(nCre;F/-) females showed infertility or subfertility. Although adult SF-1 KO(nCre;F/-) females showed decreased or lacked corpora lutea, exogenous administration of gonadotropins induced the formation of multiple corpora lutea and induced normal ovulation, demonstrating that the ovaries are functionally intact. In addition, SF-1 KO(nCre;F/-) females stimulated with a synthetic GnRH agonist after priming exhibited markedly reduced LH secretion compared with wild-type littermates, arguing that disorganization in and around the VMH caused by SF-1 ablation interferes with the GnRH priming process or gonadotrope LH capacity. Furthermore, the SF-1 KO(nCre;F/-) females primed with estrogen benzoate and progesterone failed to induce steroid receptors around the VMH, consistent with impaired lordosis behavior in the SF-1 KO(nCre;F/-) females. Collectively, our results highlight that SF-1 in the VMH plays crucial roles in regulation of female reproductive function, presumably by organizing a precise neuronal connection and communication in and around the VMH.

Hormonal regulation of suppressors of cytokine signaling (SOCS) messenger ribonucleic acid in the arcuate nucleus during late pregnancy

Prolactin stimulates tuberoinfundibular dopamine neurons in the arcuate nucleus of the hypothalamus, mediated by signal transducer and activator of transcription 5b (STAT5b). During late pregnancy, these neurons become unresponsive to prolactin, with a loss of prolactin-induced activation of STAT5b and decreased dopamine secretion. Suppressors of cytokine signaling (SOCS) proteins inhibit STAT-mediated signaling, and SOCS mRNAs are specifically elevated in the arcuate nucleus during late pregnancy. We hypothesized that changes in circulating ovarian steroids during late pregnancy might induce expression of SOCS mRNAs, thus disrupting STAT5b-mediated prolactin signaling. Rats were ovariectomized on d 18 of pregnancy and treated with ovarian steroids to simulate an advanced, normal, or delayed decline in progesterone. Early progesterone withdrawal caused an early increase in prolactin secretion, and increased SOCS-1 and -3 and cytokine-inducible SH2-containing protein (CIS) mRNA levels in the arcuate nucleus. Prolonged progesterone treatment prevented these changes. To determine whether ovarian steroids directly alter SOCS mRNA levels, estrogen- and/or progesterone-treated ovariectomized nonpregnant rats were acutely injected with prolactin (300 microg sc) or vehicle. SOCS-1 and -3 and CIS mRNA levels in the arcuate nucleus were significantly increased by estrogen or prolactin, whereas progesterone treatment reversed the effect of estrogen. Results demonstrate that estrogen and prolactin can independently induce SOCS mRNA in the arcuate nucleus and that this effect is negatively regulated by progesterone. This is consistent with the hypothesis that declining progesterone and high levels of estrogen during late pregnancy induce SOCS in the tuberoinfundibular dopamine neurons, thus contributing to their insensitivity to prolactin at this time.

Upregulation of angiotensin II type 2 receptor expression in estrogen-induced pituitary hyperplasia

Recent evidence shows that reexpression and upregulation of angiotensin II (ANG II) type 2 (AT2) receptor in adult tissues occur during pathological conditions such as tissue hyperplasia, inflammation, and remodeling. In particular, expression of functional AT2 receptors in the pituitary and their physiological significance and regulation have not been described. In this study, we demonstrate that chronic in vivo estrogen treatment, which induces pituitary hyperplasia, enhances local AT2 expression (measured by Western blot and RT-PCR) concomitantly with downregulation of ANG II type 1 (AT1) receptors. In vivo progesterone treatment of estrogen-induced pituitary hyperplasia did not modify either the ANG II receptor subtype expression pattern or octapeptide-induced and AT1-mediated calcium signaling. Nevertheless, an unexpected potentiation of the ANG II prolactin-releasing effect was observed in this group, and this response was sensitive to both AT1 and AT2 receptor antagonists. These data are the first to document that ANG II can act at the pituitary level through the AT2 receptor subtype and that estrogens display a differential regulation of AT1 and AT2 receptors at this level.

Progestin regulation of galanin and prolactin gene expression in oestrogen-induced pituitary tumours

Galanin is a peptide widely distributed in the hypothalamic-pituitary axis. In the female rat pituitary, galanin is mainly present in lactotrophs, where it regulates their secretion and proliferation. Galanin expression is increased in oestrogen-induced prolactinomas, and it has been proposed that oestrogen effects on lactotroph function and proliferation could be mediated by galanin. Previous studies from our laboratory demonstrated that the synthetic progestin levonorgestrel antagonizes pituitary tumorigenesis of rats given oestrogen, reducing the number of proliferating cells and increasing cell death by nonapoptotic mechanism(s). To elucidate the role of galanin in levonorgestrel effects on the tumours, we examined galanin and prolactin mRNA and peptide expression in prolactinomas of rats receiving the progestin. Levonorgestrel reduced the pituitary weight and serum prolactin concentrations in oestrogen-treated rats. Galanin mRNA expression (determined by in situ hybridization), and the number of galanin expressing cells (determined by immunocytochemistry) were also reduced by the progestin in tumour-bearing rats. However, neither prolactin mRNA content, nor the number of prolactin-expressing cells, were modified by levonorgestrel treatment of oestrogen-receiving rats. The present study suggests that levonorgestrel controls pituitary growth by diminishing galanin expression. In contrast, changes in serum prolactin concentration seem to be more related to the reduction in tumour size, since the reduction in galanin expression was not large enough to regulate prolactin mRNA expression or the percentage of lactotrophs.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Differential gene expression response to gonadal hormones by preoptic regulatory factors-1 and -2 in the female rat brain

Steroids and neuropeptides interact in the central nervous system (CNS) to regulate reproductive function and behavior. The preoptic regulatory factors, PORF-1 and PORF-2, are unique neuropeptides for which roles in gender-related brain development and function have been proposed. PORF-1 and PORF-2 expression in rat brain are age, region and gender dependent, and castration or hypophysectomy alter the metabolism of the PORF-1 and PORF-2 mRNAs in male rat brain and testes. If these two peptides have a role in gender-dependent brain function, then gonadal steroids might well affect their expression. The present study was designed to investigate the response of the PORF-1 and PORF-2 mRNAs to sex steroids in the female rat brain and to compare this response to that of two peptides whose roles in the neuroendocrinology of reproduction are well established, gonadotropin-releasing hormone (GnRH) and neuropeptide Y (NPY). Rats were ovariectomized and treated with placebo, estradiol (E2), progesterone (P4) or a combination of the two (E2/P4) and NPY, PORF-2, GnRH and PORF-1 mRNAs were quantified by nuclease protection assays. PORF-1, PORF-2 and GnRH mRNAs were also measured in intact rats during estrus and proestrus. Responses were compared in the preoptic anterior hypothalamus (POA), medial basal hypothalamus (MBH), cerebral cortex (CC) and hippocampus (HIPP). Expression of PORF-1 and PORF-2 was also confirmed in the female rat hypothalamus by in situ hybridization analysis. PORF-1 and PORF-2 mRNAs were detected in the adult female rat brain by both in situ hybridization and ribonuclease protection analyses. In situ hybridization analysis demonstrated that PORF-1 and PORF-2 mRNAs are expressed in hypothalamic neurons. RNase protection analysis showed that PORF-1, PORF-2 and NPY mRNAs were present in all four brain regions examined while GnRH expression was detected only in the MBH and POA. Estradiol alone upregulated expression of the PORF-1 and PORF-2 mRNAs in the ovariectomized rat in the POA and HIPP, and of NPY mRNA in the MBH and HIPP. Progesterone alone had a stimulatory effect on NPY mRNA in the MBH and HIPP. Treatment with a combination of E2/P4 downregulated PORF-2 mRNA in the POA as well as PORF-1, PORF-2 and NPY mRNAs in the CC. In contrast, E2/P4 upregulated the PORF-2 and NPY mRNAs in the HIPP and NPY mRNA in the MBH. In the cycling rat, PORF-1 mRNA levels were higher during proestrus than estrus in both the MBH and POA, while PORF-2 mRNA levels did not change. In contrast GnRH mRNA was lower in the POA and higher in the MBH during proestrus compared with estrus. Thus, intrinsic factors, most likely both ovarian and neuroendocrine, regulate PORF-1 and GnRH expression in the intact cycling rat CNS in a region-dependent manner. In the ovariectomized rat, PORF-1, PORF-2, NPY and GnRH mRNAs all respond in a region-specific manner to sex steroid treatment. These data support the role of PORF-1 and PORF-2 in gender-dependent brain function in the adult female rat.

Mechanisms in progestin antagonism of pituitary tumorigenesis

Chronic exposure of F344 rats to diethylstilbestrol (DES) induces pituitary tumors (DES-T) composed of proliferating lactotrophs. Presently, we studied the effect of progestins on parameters related to tumor growth and function, due to previous evidences of progesterone antagonism of pituitary tumorigenesis acting at pituitary and hypothalamic levels [Piroli, G., Grillo, C., Ferrini, M., Lux-Lantos, V. and De Nicola, A. F., Antagonism by progesterone of diethylstilbestrol-induced pituitary tumorigenesis in Fischer 344 rats: Effects on sex steroid receptors and tyrosine hydroxylase mRNA, Neuroendocrinology, 1996, 63, 530-539]. In search of a quantitatively more important effect, animals bearing DES-T were treated with synthetic progestins. Competition assays using DES-T as source of progestin receptors indicated that levonorgestrel (LNG), gestodene and R5020 showed higher affinities (IC50 1-2 nM) than progesterone, norethisterone and medroxyprogesterone (IC50 10-25 nM). Treatment with LNG reduced DES-T weight by 45%, and serum PRL by one half. Small (monomeric) and big (polymeric) PRL increased 5- and 2.5-fold, respectively, in DES-T in comparison with pituitaries of ovariectomized (OVX) rats. However, LNG produced no changes indicating that synthesis and storage of PRL was conserved in rats receiving both hormonal treatments. DES induced a 15-fold increase in cell proliferation, measured as bromodeoxyuridine incorporation into cell nuclei, in comparison to OVX rats, while LNG treatment of DES-T bearing rats reduced this index by 72%. Electron microscopic images showed that LNG markedly reduced hypertrophy and hyperplasia of lactotropes, increasing the proportions of degenerating cells and cells of high electronic density with alterations of cytoplasmic organelles. However, histopathological signs of apoptosis were absent. Therefore, reduced cell proliferation and non-apoptotic cell death are part of the mechanisms employed by progestins to antagonize tumorigenesis at the pituitary level. The results may open a new therapeutic strategy for treatment of PRL secreting adenoma in humans.

Endocrine defects in mice carrying a null mutation for the progesterone receptor gene

Mice carrying a null mutation of the progesterone receptor gene exhibit several reproductive abnormalities, including anovulation, attenuated lordotic behavior, uterine hyperplasia, and lack of mammary gland development. The hormonal correlates of these abnormalities are unknown, however, and were the focus of these studies. Serum samples from female wild-type (WT) and progesterone receptor knockout (PRKO) mice were obtained and analyzed by RIA for LH, FSH, PRL, estrogen (E2), and progesterone. Hypothalamic tissues were also processed for measurement of LHRH by RIA. Serum LH levels in PRKO mice were found to be elevated by approximately 2-fold over basal (metestrus) values in WT mice. By contrast, basal FSH levels were not different in PRKO and WT mice. Basal levels of E2 and progesterone in serum were likewise similar in the two groups, as were hypothalamic LHRH concentrations. Basal PRL levels were slightly higher in PRKO vs. WT mice. Ovariectomy of both groups of mice was accompanied by significant increases in both LH and FSH. At 5 days following ovariectomy, LH levels were elevated in both groups by 2-fold over PRKO basal and 4-fold over WT basal levels; however, by 10 days postovariectomy LH levels had continued to rise to a greater extent in PRKO mice than in WT animals. The FSH response to ovariectomy was greater for the PRKO mice at 5 days, but was no different from WT at 10 days. Of seven PRKO mice that were exposed to male odor, none exhibited preovulatory surges 3 days later, on the day of presumptive proestrus; this was in marked contrast with WT females, in which 100% exhibited robust LH surges. These results confirm the essential role of progesterone receptors in the regulation of hypothalamic and/or pituitary processes that govern gonadotropin secretion. The finding that basal LH levels are elevated in PRKO mice confirms that circulating progesterone normally conveys a significant portion of the total ovarian negative feedback control of the gonadotropin. That gonadotropin responses to ovariectomy are slightly enhanced in PRKO mice suggests that adrenal progesterone may contribute to the imposition of negative feedback control. The apparent inability of PRKO mice to respond to male odor suggests that anovulation in these mice may not be solely due to reproductive abnormalities within the ovary itself; rather, PRKO mice additionally harbor neuroendocrine defects that render them incapable of mounting normal preovulatory gonadotropin surges. It remains to be determined how the absence of PR in brain and pituitary of PRKO mice may produce this hormonal acyclicity and, conversely, how the presence of PR in brain and pituitary of WT mice may be obligatory in the generation of gonadotropin surges.

Pituitary Sex Steroid Receptors: Localization and Function

The pituitary contains estrogen receptor (ER), progesterone receptor (PR), and androgen receptor (AR). In accordance with immunocytochemistry, it is agreed that sex hormone receptors reside into the nucleus. All three receptors are found predominantly in gonadotrophs and lactotrophs, and less frequently in other cell types. ER plays a major role in prolactin (PRL) production and lactotroph proliferation, and protracted estrogen administration induces lactotroph hyperplasia and adenoma in rodents. Most research on PR and AR is focused on their role in the fine-tuning of gonadotropin secretion during estrous cycle. Contrary to the effect in nontumorous pituitary, estrogens can inhibit the proliferation of transplantable rat pituitary tumors and of cell lines derived from them. In humans, despite the presence of ER in all types of adenohypophysial tumors, the role of estrogen in tumor cell proliferation is still unclear. Few results indicate that tumor growth is stimulated by estrogen, and inhibited by progesterone and androgen. Novel data reveal that steroid hormones can act directly on plasma membrane or via other receptors, and interact with growth factors, oncogenes, and other transcription factors. The mechanisms by which steroid hormones control cell proliferation remain a major challenge for future research.

Relationship between stages of mammary development and sensitivity to gamma-ray irradiation in mammary tumorigenesis in rats

Mature Wistar-MS rats were ovariectomized and treated with estradiol benzoate and/or progesterone. Control animals were treated with olive oil. The rats were then exposed to gamma-rays and implanted with a pellet of diethylstilbestrol. The incidence of mammary tumors in rats treated with estradiol benzoate or with progesterone was significantly higher than in rats in the non-treated control group, whereas, in rats treated with both estradiol benzoate and progesterone, the incidence was not significantly different from that in the controls. Histological examination of the mammary tumors showed 2 types of neoplasm: adenocarcinoma and fibroadenoma. Interestingly, over half of all the tumors in the rats treated with estradiol benzoate were adenocarcinomas, while fibroadenomas were mainly induced in the rats treated with progesterone or with both estradiol benzoate and progesterone. The expression of estrogen and progesterone receptors in the tumor tissues showed some differences according to whether the groups were treated with estradiol benzoate or with progesterone. Morphologically, mammary glands at irradiation showed well-developed lobuloalveoli in both the estradiol-benzoate-treated rats and in those rats treated with both estradiol benzoate and progesterone. This was consistent with the higher incorporation of [3H]thymidine into the DNA in the mammary glands of rats in both of these groups. Our findings suggest that a more advanced developmental stage of the mammary glands, dependent upon ovarian hormones, is related to a higher incidence of mammary tumors induced by irradiation.