Activation of the prolactin receptor but not the growth hormone receptor is important for induction of mammary tumors in transgenic mice

Inhibition of prolactin (PRL)-induced proliferative signals in breast cancer cells by a molecular mimic of phosphorylated PRL, S179D-PRL

Posttranslational modifications of prolactin (PRL), including phosphorylation, vary with physiologic state and alter biologic activity. In light of the growing evidence for a role for PRL in proliferation in mammary cancer, we examined the ability of a mimic of phosphorylated human PRL, S179D-PRL, to initiate signals to several pathways in mammary tumor cells alone and in combination with unmodified PRL. Unmodified PRL employed multiple pathways to increase cellular proliferation and cyclin D1 levels in PRL-deficient MCF-7 cells. S179D-PRL was a weak agonist compared with unmodified PRL with regard to cellular proliferation, cyclin D1 levels, and phosphorylation of signal transducer and activator of transcription 5 and ERKs. However, S179D-PRL was a potent antagonist of unmodified PRL to these endpoints. In contrast to the reduced levels of the long isoform of the PRL receptor observed in response to a 3-d incubation with unmodified PRL, S179D-PRL up-regulated expression of this isoform, 4-fold. These studies support the utility of this mutant as a PRL antagonist to proliferative signals in mammary epithelial cells, including a potential role in breast cancer therapeutics.

Development of pure prolactin receptor antagonists

Prolactin (PRL) promotes tumor growth in various experimental models and leads to prostate hyperplasia and mammary neoplasia in PRL transgenic mice. Increasing experimental evidence argues for the involvement of autocrine PRL in this process. PRL receptor antagonists have been developed to counteract these undesired proliferative actions of PRL. However, all forms of PRL receptor antagonists obtained to date exhibit partial agonism, preventing their therapeutic use as full antagonists. In the present study, we describe the development of new human PRL antagonists devoid of agonistic properties and therefore able to act as pure antagonists. This was demonstrated using several in vitro bioassays, including highly sensitive assays able to detect extremely low levels of receptor activation. These new compounds also act as pure antagonists in vivo, as assessed by analyzing their ability to competitively inhibit PRL-triggered signaling cascades in various target tissues (liver, mammary gland, and prostate). Finally, by using transgenic mice expressing PRL specifically in the prostate, which exhibit constitutively activated signaling cascades paralleling hyperplasia, we show that these new PRL analogs are able to completely revert PRL-activated events. These second generation human PRL antagonists are good candidates to be used as inhibitors of growth-promoting actions of PRL.

Development of new prolactin analogs acting as pure prolactin receptor antagonists

Prolactin (PRL) promotes tumor growth, as recently highlighted by the spontaneous appearance of prostate hyperplasia and mammary neoplasia in PRL transgenic mice. Increasing experimental evidence argues for the involvement of autocrine PRL in this process. Human (h)PRL receptor antagonists have been developed to counteract these undesired proliferative actions of PRL. However, all PRL receptor antagonists obtained to date exhibit partial agonism, limiting their therapeutic use as full antagonists. This is the case for the first generation antagonists (the prototype of which is G129R-hPRL) that we developed ten years ago, which display antagonistic activity in some, but not all in vitro bioassays, and fail to inhibit PRL activity in transgenic mice expressing this analog. We recently developed new human PRL antagonists devoid of agonistic properties, and therefore able to act as pure antagonists. This was demonstrated using several in vitro bioassays, including assays able to detect extremely low levels of receptor activation. These new compounds also act as pure antagonists in vivo, as demonstrated by their ability to competitively inhibit PRL-triggered signaling cascades in various target tissues (liver, mammary gland and prostate). Finally, using transgenic mice specifically expressing PRL in the prostate, which have constitutively activated signaling cascades and prostate hyperplasia, these new PRL analogs are able to completely revert PRL-activated events to basal levels. These second generation antagonists are good candidates to be used as inhibitors of the growth-promoting actions of hPRL.

Prolactin induces ERalpha-positive and ERalpha-negative mammary cancer in transgenic mice

The role of prolactin in human breast cancer has been controversial. However, it is now apparent that human mammary epithelial cells can synthesize prolactin endogenously, permitting autocrine/paracrine actions within the mammary gland that are independent of pituitary prolactin. To model this local mammary production of prolactin (PRL), we have generated mice that overexpress prolactin within mammary epithelial cells under the control of a hormonally nonresponsive promoter, neu-related lipocalin (NRL). In each of the two examined NRL-PRL transgenic mouse lineages, female virgin mice display mammary developmental abnormalities, mammary intraepithelial neoplasias, and invasive neoplasms. Prolactin increases proliferation in morphologically normal alveoli and ducts, as well as in lesions. The tumors are of varied histotype, but papillary adenocarcinomas and adenosquamous neoplasms predominate. Neoplasms can be separated into two populations: one is estrogen receptor alpha (ERalpha) positive (greater than 15% of the cells stain for ERalpha), and the other is ERalpha- (<3%). ERalpha expression does not correlate with tumor histotype, or proliferative or apoptotic indices. These studies provide a mouse model of hormonally dependent breast cancer, and, perhaps most strikingly, a model in which some neoplasms retain ERalpha, as occurs in the human disease.

Prolactin expression and secretion by human breast glandular and adipose tissue explants

Prolactin (PRL) is a 23-kDa hormone produced by the pituitary and extrapituitary sites. The main target of PRL is the breast, where it affects cellular growth, differentiation, and milk production. Recent evidence suggests that locally produced PRL plays a role in breast tumorigenesis. Our objective was to examine PRL synthesis/release in different tissues of the human breast and determine the effect of ovarian steroids. Breast tissue, obtained from women undergoing mastectomy or breast reduction, was separated into glandular (nonmalignant) and adipose explants and incubated for 10 d. Conditioned media were analyzed for PRL by a bioassay. PRL release from glandular explants decreased by 60% from d 1-3, followed by a 4-fold increase on d 10. PRL release from adipose explants was unchanged from d 1-3 and increased more than 10-fold by d 10. PRL gene expression, determined by RT-PCR, was low on d 0 and markedly increased on d 10 in both types of explants. De novo synthesis of PRL was confirmed by metabolic labeling. Progesterone suppressed PRL release from glandular explants without affecting adipose explants. Estradiol did not alter PRL release from either tissue. In conclusion, the human breast produces and releases bioactive PRL, with a higher release rate by adipose than glandular tissue. The time-dependent rise in PRL release suggests removal from inhibitory control. Progesterone may be one of the factors that suppresses PRL production in the glandular compartment, whereas the factor(s) that regulate adipose PRL are unknown. These data suggest an autocrine/paracrine role for PRL in human glandular and adipose breast tissue.

Prolactin regulates mammary epithelial cell proliferation via autocrine/paracrine mechanism

Prolactin (PRL) is essential for a number of developmental events in the mammary gland. Work with PRL and PRL receptor knockout mice has shown that PRL indirectly regulates ductal side branching during puberty and directly controls lobuloalveolar development and lactogenesis during pregnancy. Anterior pituitary or placental PRL is thought to be responsible for these functions via an endocrine mechanism; however, PRL is also produced in a number of extrapituitary sites including the mammary gland. The physiologic relevance of mammary PRL remains unknown. In this study we utilized mammary recombination in Rag1(-/-) hosts, to determine whether mammary PRL plays a role in the regulation of mammary gland development. Mammary glands formed with the PRL gene deleted from either the epithelium, stroma, or both displayed normal development, on the basis of whole mount and hematoxylin and eosin histology, during puberty and lactation. At the end of pregnancy, a 2.8-fold decrease in bromodeoxyuridine incorporation was observed in the epithelial cells of mammary glands formed using PRL knockout epithelium compared with those formed using wildtype epithelium. No balancing alteration in the rates of apoptosis was detected. Thus, mammary-derived PRL influences mammary epithelial cell proliferation via an autocrine/paracrine mechanism, establishing a physiologic function for mammary PRL during mammopoiesis.

The role of prolactin in mammary carcinoma

The contribution of prolactin (PRL) to the pathogenesis and progression of human breast cancer at the cellular, transgenic, and epidemiological levels is increasingly appreciated. Acting at the endocrine and autocrine/paracrine levels, PRL functions to stimulate the growth and motility of human breast cancer cells. The actions of this ligand are mediated by at least six recognized PRL receptor isoforms found on, or secreted by, human breast epithelium. The PRL/PRL receptor complex associates with and activates several signaling networks that are shared with other members of the cytokine receptor superfamily. Coupled with the recently identified intranuclear function of PRL, these networks are integrated into the in vitro and in vivo actions induced by ligand. These findings indicate that antagonists of PRL/PRL receptor interaction or PRL receptor-associated signal transduction may be of considerable utility in the treatment of human breast cancer.

Caveolin-1 mutations (P132L and null) and the pathogenesis of breast cancer: caveolin-1 (P132L) behaves in a dominant-negative manner and caveolin-1 (-/-) null mice show mammary epithelial cell hyperplasia

Caveolin-1 (Cav-1) is the principal structural protein of caveolae membranes that are found in most cells types, including mammary epithelial cells. Recently, we mapped the human CAV1 gene to a suspected tumor suppressor locus (7q31.1/D7S522) that is deleted in a variety of human cancers, as well as mammary tumors. In addition, the CAV1 gene is mutated (P132L) in up to approximately 16% of human breast cancers. The mechanism by which deletion or mutation of the Cav-1 gene contributes to mammary tumorigenesis remains unknown. To understand the role of the Cav-1 (P132L) mutation in the pathogenesis of human breast cancers, we generated the same mutation in wild-type (WT) Cav-1 and studied its behavior in cultured cells. Interestingly, the P132L mutation leads to formation of misfolded Cav-1 oligomers that are retained within the Golgi complex and are not targeted to caveolae or the plasma membrane. To examine whether the Cav-1 (P132L) mutant behaves in a dominant-negative manner, we next co-transfected cells with Cav-1 (P132L) and WT Cav-1, and evaluated their caveolar targeting. Our results indicate that Cav-1 (P132L) behaves in a dominant-negative manner, causing the mislocalization and intracellular retention of WT Cav-1. Virtually identical results were obtained when Cav-1 (P132L) was stably expressed at physiological levels in a nontransformed human mammary epithelial cell line (hTERT-HME1). These data provide a molecular explanation for why only a single mutated CAV1 allele is found in patients with breast cancer. Thus, we next investigated if functional inactivation of Cav-1 gene expression leads to mammary tumorigenesis in vivo. For this purpose, we performed mammary gland analysis on Cav-1-deficient mice (-/-) that harbor a targeted disruption of the Cav-1 gene (a null mutation). Interestingly, we show that inactivation of Cav-1 gene expression leads to mammary epithelial cell hyperplasia, even in 6-week-old virgin female mice. These data clearly implicate loss of functional Cav-1 in the pathogenesis of mammary epithelial cell hyperplasia, and suggest that Cav-1-null mice represent a novel animal model to study premalignant mammary disease.

Estradiol stimulates expression of two human prolactin receptor isoforms with alternative exons-1 in T47D breast cancer cells

Human prolactin receptor (hPRLR) expression is regulated by estradiol-17beta (E(2)) in vivo in animal tissues, and in vitro in normal human endometrial cells and in MCF7 human breast cancer cells. The objective of this study was to determine the effect of E(2) on the expression of two recently described hPRLR isoforms with distinct exons-1, hE1(3) and hE1(N1) that are transcribed from the generic hPIII promoter, also present in the rat and mouse, and the human-specific promoter hP(N1), respectively. Also, to determine the effect of estradiol on the hPIII promoter activity in cancer cells. T47D breast cancer cells were examined using quantitative competitive RT-PCR for the level of expression of two alternative non-coding exon-1 transcripts, hE1(3) and hE1(N1) following incubation with E(2) in presence or absence of the E(2) receptor antagonist ICI 182,780. The effects of estradiol were also evaluated in cells transiently transfected with constructs of hPIII promoter luciferase reporter gene. E(2) significantly increased the expression of both hPRLR mRNA transcripts, hE1(3) and hE1(N1). In transfection studies E(2) activated the hPIII promoter. This effect of estradiol was markedly inhibited by coincubation with the E(2) receptor antagonist. Our results demonstrate a stimulatory effect of estradiol on the expression of hPRLR mRNA species with alternative exons-1, hE1(3) and hE1(N1) possibly through activation of their corresponding promoters. The lack of a formal ERE in these promoters suggested that the effect of estradiol is mediated through association of the activated ER with relevant DNA binding transfactor(s). These findings support the role of E(2) in the regulation of hPRLR expression in human breast cancer cell lines.

Reproductive disturbances, pituitary lactotrope adenomas, and mammary gland tumors in transgenic female mice producing high levels of human chorionic gonadotropin

To assess the consequences of prolonged exposure to elevated levels of LH/human chorionic gonadotropin (hCG) in the female, we developed a transgenic (TG) mouse model (hCGbeta+) that overexpresses the hCGbeta-subunit cDNA. Because of the promoter used, ubiquitin C, the transgene is expressed in multiple tissues, including the pituitary gland, in which coupling with the endogenous common alpha-subunit results in synthesis of high levels of bioactive hCG. The TG females presented with precocious puberty, infertility, enhanced ovarian steroidogenesis, and abnormal uterine structure. Pituitary enlargement was evident from the age of 2 months, which progressed to adenomas by the age of 10-12 months. Immunohistochemical studies and electron microscopy demonstrated lactotrope origin for the adenomas, associated with severe hyperprolactinemia. The mammary glands of TG females showed marked lobuloalveolar development followed by mammary tumors with characteristics of adenocarcinoma at the age of 9-12 months. More than 90% of penetrance and high frequency of metastasis (47%) was observed. Formation of the pituitary and mammary gland tumors was totally abolished by ovariectomy despite persistently elevated hCG levels. Taken together, these findings suggest that the hCG-induced aberrations of ovarian function are clearly responsible for the extragonadal tumors observed in these TG mice.

Mammary gland development in transgenic male mice expressing human P450 aromatase

We recently generated a transgenic mouse strain that expresses the human aromatase gene under the ubiquitin C promoter (AROM+). We have previously shown that in these mice the serum estradiol concentration is highly elevated, whereas the testosterone concentration is decreased. In the present study we examined mammary gland development in AROM+ male mice at different ages and found that the mammary glands of AROM+ males undergo ductal and alveolar development morphologically resembling that of terminally differentiated female mammary glands, expressing mRNA for a milk protein gene (beta-casein). The male mammary glands also express multiple hormone receptors typical for female mammary gland: estrogen receptor alpha and beta, progesterone receptor, and PRL receptor. Furthermore, data showed activation of the Stat5 (signal transducer and activator of transcription 5) signaling pathway in the AROM+ male mammary gland. Interestingly, the phenotype observed is in part reversible. Treatment with finrozole, a specific aromatase inhibitor, caused an involution of the differentiated phenotype of the mammary gland, marked by the disappearance of alveolar structures and the majority of the tertiary side branches of the ducts. The present animal model is a valuable tool for better understanding the cellular and molecular mechanisms involved in the development of gynecomastia.

Ovarian hyperstimulation by LH leads to mammary gland hyperplasia and cancer predisposition in transgenic mice

Many risk factors for breast cancer are associated with hormonally regulated events. Although numerous mouse models of mammary cancer exist, few address the roles of hormones in spontaneous tumor formation. Here we report that transgenic mice that overexpress LH, resulting in ovarian hyperstimulation, undergo precocious mammary gland development. A significant increase in proliferation leads to ovary-dependent mammary gland hyperplasia. Transgenic glands morphologically mimic those of wild-type pregnant mice and expression levels of multiple milk protein genes are comparable with what is observed at d 14 of pregnancy. In addition to sustained hyperplasia, spontaneous mammary tumors were observed with a mean latency of 41 wk, indicating that chronic hormonal stimulation causes mammary cancer. Although hormonally induced, these tumors lack expression of progesterone receptor, suggesting that following initiating events, the tumors may become hormone independent. This mouse model likely holds great potential as a tool for discovery of hormone-mediated mechanisms of breast cancer and identification of future targets for breast cancer prevention and treatment.

Effects of hyperactive Janus kinase 2 signaling in mammary epithelial cells

Prolactin, the Janus kinase 2 (JAK2) and the signal transducer and activator of transcription 5 (STAT5) are important for mammary gland development and have also been implicated in development and growth of breast tumors. In the present study we have investigated the role for JAK2 in proliferation, differentiation, and apoptosis of the mammary epithelial cell line HC11 by stably overexpressing two hyperactive JAK2 mutants. Cells expressing a JAK2 mutant consisting of only the kinase domain had high amount of nuclear STAT5 protein with low DNA-binding activity, which was rapidly induced to a DNA-binding state by prolactin treatment. Cells expressing JAK2 deleted of the kinase-like domain showed increased sensitivity to prolactin treatment compared to wild type cells. Proliferation was not affected by any of the mutants whereas the ability to undergo apoptosis was decreased implicating a transforming potential of the JAK2 mutants.

Prolactin as an autocrine/paracrine growth factor in human cancer

Prolactin (PRL) has a dual function -- as a circulating hormone and as a cytokine. This understanding is based on PRL production and distinct regulation in extrapituitary sites, its binding to membrane receptors of the cytokine receptor superfamily, and activation of signaling pathways that promote cell growth and survival. There is increasing evidence that PRL plays a role in several types of cancer in reproductive and non-reproductive tissues via local production or accumulation. The expression of both PRL and its receptor in human cancer cell lines of diverse origin lends further support to its action as an autocrine/paracrine growth factor. Establishment of PRL as an active participant in tumorigenesis should inspire the development of novel therapies aimed at reducing tumor growth by suppressing PRL production or by blocking its receptors.

Prolactin: the new biology of an old hormone

Prolactin (PRL) is a paradoxical hormone. Historically known as the pituitary hormone of lactation, it has had attributed to it more than 300 separate actions, which can be correlated to the quasi-ubiquitous distribution of its receptor. Meanwhile, PRL-related knockout models have mainly highlighted its irreplaceable role in functions of lactation and reproduction, which suggests that most of its other reported target tissues are presumably modulated by, rather than strictly dependent on, PRL. The multiplicity of PRL actions in animals is in direct opposition to the paucity of arguments that suggest its involvement in human pathophysiology other than effects on reproduction. Although many experimental data argue for a role of PRL in the progression of some tumors, such as breast and prostate cancers, drugs lowering circulating PRL levels are ineffective. This observation opens new avenues for research into the understanding of whether local production of PRL is involved in tumor growth and, if so, how extrapituitary PRL synthesis is regulated. Finally, the physiological relevance of PRL variants, such as the antiangiogenic 16K-like PRL fragments, needs to be elucidated. This review is aimed at critically discussing how these recent findings have renewed the manner in which PRL should be considered as a multifunctional hormone.

Basal activation of transcription factor signal transducer and activator of transcription (Stat5) in nonpregnant mouse and human breast epithelium

Transcription factor Stat5 (signal transducer and activator of transcription) is essential for PRL-induced terminal differentiation of mouse mammary epithelial cells during pregnancy and lactation and has been implicated in mammary tumorigenesis. A new and sensitive immunological method to detect active, tyrosine phosphorylated Stat5 in situ revealed that Stat5 is continuously activated in luminal epithelial cells of mouse and human breast, not only during pregnancy and lactation, but also outside of pregnancy. Examination of virgin Stat5a or Stat5b null mice suggested that Stat5a was the primary isoform activated in mammary epithelial cells. Basal activation of Stat5 in mammary epithelium of virgin wild-type mice was continuous throughout estrous cycle and was also detected in 17 of 17 normal human breast tissue specimens analyzed. PRL was identified as the principal factor maintaining basal activation of Stat5 in mammary epithelium of nonpregnant mice based on several lines of evidence. First, administration of PRL, but not GH or epidermal growth factor, uniformly enhanced basal activation of Stat5 in luminal mammary epithelial cells. Second, hypophysectomy disrupted basal activation of Stat5, an effect that was completely reversed by administration of PRL, but only partially by GH. Third, knock-out of the PRL receptor gene markedly reduced basal activation of Stat5, an effect that was maintained in a normalized endocrine environment after transplanting PRL receptor null mammary epithelium into wild-type mice. Continuous activation of Stat5 indicates a role of this transcription factor in normal, nonpregnant breast epithelial cells, and may shed new light on Stat5 involvement in breast tumor promotion.

Effects of prolactin on hematopoiesis

The presence of extra-pituitary prolactin and its cognitive receptors in the hematopoietic micro-environment raises the question of whether prolactin plays a role in lympho-hematopoiesis and under what conditions. Current studies suggest that endogenous prolactin does not play a significant role under normal steady-state conditions. Rather, prolactin has been implicated as a 'stress hormone', functioning to restore hematopoietic homeostasis under conditions of dysregulation. The stress response of prolactin as well as its complex relationship with other hormones and factors has resulted in conflicting reports in the literature regarding prolactin's role in lympho-hematopoiesis. A review of this literature is provided as well as discussion of conditions under which lymphohematopoietic activity of prolactin may be evident.

Role of tyrosine kinase Jak2 in prolactin-induced differentiation and growth of mammary epithelial cells

Genetic studies in mice have established a critical role for prolactin receptors and transcription factor Stat5 in mammary gland differentiation. However, the enzymatic coupling between prolactin receptors and Stat5 in this process has not been established. In addition to Jak2, several other tyrosine kinases reportedly also are associated with prolactin receptors and may phosphorylate Stat5. Because Jak2 null mice die in utero, we targeted Jak2 in an ex vivo model of prolactin-induced mammary epithelial cell differentiation to determine the role of Jak2 in regulation of cell differentiation and growth. Two independent targeting strategies were used to suppress Jak2 in immortalized HC11 mouse mammary epithelial cells: 1) stable expression of a specific Jak2 antisense construct and 2) adenoviral delivery of a dominant-negative Jak2 gene. We now demonstrate that Jak2 is essential for prolactin-induced differentiation and activation of Stat5 in normal mouse mammary epithelial cells. Furthermore, suppression of Jak2 in HC11 cells was associated with constitutive activation of oncoprotein Stat3 and a hyperproliferative phenotype characterized by increased mitotic rate, reduced apoptosis, and reduced contact inhibition. Collectively, our data suggest that Jak2 is differentiation-inducing and growth-inhibitory in normal mammary epithelial cells, observations that may shed new light on the role of the Jak2-Stat5 pathway in breast cancer.

Prolactin negatively regulates caveolin-1 gene expression in the mammary gland during lactation, via a Ras-dependent mechanism

Caveolin-1 is a 22-kDa integral membrane protein that has been suggested to function as a negative regulator of mitogen-stimulated proliferation in a variety of cell types, including mammary epithelial cells. Because much of our insight into caveolin-1 function has come from the study of human breast tumor-derived cell lines in culture, the normal physiological regulators of caveolin-1 expression in the mammary gland remain unknown. Here, we examine caveolin-1 expression in mice at different stages of mammary gland development. We show that caveolin-1 expression is significantly down-regulated during late pregnancy and lactation. Upon weaning, mammary gland expression of caveolin-1 rapidly returns to non-pregnant "steady-state" levels. Injection of virgin mice with a battery of hormones normally up-regulated during lactation demonstrates that prolactin is the main mediator of caveolin-1 down-regulation. Virtually identical results were obtained with human mammary epithelial cells (hTERT-HME1) in culture. In addition, we demonstrate that prolactin-mediated down-regulation of caveolin-1 expression occurs at the level of transcriptional control and via a Ras-dependent mechanism. Interestingly, in the mammary gland, both mammary epithelial cells and the surrounding mammary adipocytes show prolactin-mediated down-regulation of caveolin-1. This hormone-dependent regulation of caveolin-1 expression is specific to the mammary fat pad. Finally, we employed HC11 cells, a well-established model of mammary epithelial cell differentiation, to study the possible functional effects of caveolin-1 expression. In the presence of lactogenic hormones, recombinant expression of caveolin-1 in HC11 cells dramatically suppresses the induction of the promoter activity and the synthesis of beta-casein, an established reporter of lactogenic differentiation and milk production. These findings may explain why caveolin-1 levels are normally down-regulated during lactation. This report is the first demonstration that caveolin-1 levels are down-regulated during a normal physiological event in vivo, i.e. lactation, because previous reports have only documented that down-regulation of caveolin-1 occurs during cell transformation and tumorigenesis.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

PRL receptor-mediated effects in female mouse adipocytes: PRL induces suppressors of cytokine signaling expression and suppresses insulin-induced leptin production in adipocytes in vitro

PRL has been reported to regulate fat metabolism in several species. We recently reported PRL receptor (PRLR) expression in mouse adipocytes and increased levels of PRLR expression in the adipose tissue of lactating and PRL-transgenic mice compared with controls. These results suggest PRLR-mediated effects in adipose tissue. However, to date most studies have been performed in vivo, and it is unclear whether PRL has direct effects on adipocytes. The PRLR belongs to the cytokine receptor family, and a family of suppressors of cytokine signaling (SOCS) was recently identified. The present study was performed to investigate whether PRL has direct effects on adipocytes. The expression of cytokine-inducible SH2-domain-containing protein (CIS), SOCS-3, and SOCS-2 mRNA and protein was analyzed using ribonuclease protection assay and immunoblotting, respectively. Ovine PRL induced CIS mRNA expression and a combination of oPRL and insulin induced SOCS-3 mRNA expression in adipocytes cultured in vitro for 0-240 min, demonstrating PRLR-mediated direct effects in these cells. Furthermore, CIS, SOCS-3, and SOCS-2 mRNA and protein were all transiently expressed in adipose tissue obtained from female mice stimulated with oPRL (1 microg/g BW) for 0-24 h. In adipose tissue of female mice with endogenously high PRL levels, PRL-transgenic mice, only SOCS-2 expression was increased. The level of SOCS-2 mRNA was also increased in adipose tissue during pregnancy and lactation compared with that in wild-type virgin female mice. A possible reason for increased SOCS-2 expression after prolonged PRL exposure during lactation and in the PRL transgenes could be to restore the sensitivity of adipose tissue to PRL. In addition, the direct effect of PRL on leptin production was investigated in adipocytes cultured in vitro for 6 h. PRL inhibited insulin-induced leptin production in vitro. However, PRL had no effect on leptin production in the absence of insulin. In contrast, serum leptin concentrations were increased in PRL-transgenic females compared with control mice. In conclusion, our results demonstrate functional PRLRs in mouse adipocytes and suggest a role for CIS, SOCS-3, and SOCS-2 in regulating PRL signal transduction in adipose tissue.

Increased resistin expression in the adipose tissue of male prolactin transgenic mice and in male mice with elevated androgen levels

The aim of this study was to investigate the regulation of resistin, a recently identified adipocyte-secreted peptide, in the adipose tissue of prolactin (PRL)-transgenic (tg) mice using ribonuclease protection assay. The level of resistin mRNA increased 3.5-fold in the adipose tissue of untreated male PRL-tg mice compared to controls. However, there was no difference in resistin expression in the adipose tissue of female PRL-tg mice compared to control mice. PRL-tg male mice have elevated serum testosterone levels and we therefore analyzed the effects of testosterone alone on resistin mRNA expression. Furthermore, the effects of elevated androgen levels on PRL receptor (PRLR) mRNA expression in the adipose tissue were investigated. Resistin mRNA increased 2.6-fold in the adipose tissue of control male mice with elevated serum androgen levels. In addition, PRLR mRNA expression was increased in the adipose tissue of male mice with elevated testosterone. These results suggest testosterone to be a regulator of resistin and PRLR mRNA expression in the adipose tissue of male mice.

Mutational analysis of the PRL receptor gene in human breast tumors with differential PRL receptor protein expression

PRL is a major growth and differentiating hormone in the human breast, with activation of the PRL-PRL receptor complex increasingly recognized as an important mechanism in the induction and progression of mammary tumors. Although constitutive activation of various hormone and growth factor receptors is newly recognized as a common cause of tumor development, the PRL receptor gene has not been analyzed for similar aberrations in breast and other tumors. Therefore, using bacterial artificial chromosomes containing the PRL receptor gene and intron-spanning PCR, we determined the exon-surrounding intron sequences providing primers for the first analysis of the entire coding region of the human PRL receptor gene. We examined the presence of PRL receptor in 41 breast tumors by immunohistochemistry and attempted a correlation of its expression to pathological grading of the disease. Then tumor cells were isolated by laser capture microdissection to examine DNA from 30 patients for PRL receptor mutations. The PRL receptor immunoreactive score did not correlate to the tumor size, histopathological grading, age, or family history of patients. PRL receptor immunoreactivity was predominantly found in steroid hormone receptor-positive tumors, but without overall correlation of immunoreactive score. In both PRL receptor-positive and PRL receptor- negative breast cancer cells, direct sequencing of the coding sequence of the PRL receptor gene did not detect any somatic or hereditary gene aberrations. In conclusion, PRL receptor mutations do not appear to be common in human breast cancer, suggesting that constitutive activation of the PRL receptor can be excluded as a major cause of mammary tumor genesis. The molecular structure of the PRL receptor seems to remain intact in tumor tissue, and systemic and local production of PRL may participate in tumor cell growth and proliferation through functional receptors.

Pituitary prolactin-secreting macroadenoma combined with bilateral breast cancer in a 45-year-old male

We describe an unusual case of bilateral breast cancer synchronous with pituitary macroprolactinoma in a young male. Up to date, only very few of such cases have been described worldwide and to our knowledge this is the first one in which both breast cancer and pituitary macroadenoma have been found together at the time of presentation. A 45-year-old male was diagnosed as having a pituitary macroprolactinoma and bilateral breast cancer on the basis of hypogonadism (testosterone 2.9 pmol/l) with very high levels of prolactin (33,100 U/l), typical neuroradiologic finding of a pituitary macroadenoma, marked bilateral gynecomastia with mammographic pattern highly suspected for cancer and subsequent hystological confirmation. Bilateral mastectomy was performed and medical therapy with bromocriptine 10 mg/day was started. After 2-year follow-up the patient is disease-free. Hormonal, neuroradiological and oncological patterns are all negative or markedly improved. We stress the importance of prolactin for its possible biological effects on breast cancer induction or growth. Moreover in any case of hyperprolactinemia we suggest a mammographic examination and, in the case of breast cancer, at least a baseline hormonal profile.

Growth hormone receptor is expressed in human breast cancer

Several clinical observations and experimental studies indicate that pituitary hormones, including growth hormone, play a role in the development of human breast cancer. We analyzed 48 human breast carcinomas using reverse transcription polymerase chain reaction, immunohistochemistry, and Western blotting techniques to assess growth hormone receptor expression. In 17 of these cases, adjacent normal breast tissue was similarly analyzed. These analyses revealed that growth hormone receptor (GHR) is expressed in human breast cancer and appears to be up-regulated compared to adjacent normal breast tissue. GHR expression correlated inversely with tumor grade and MIB-1 index. Progesterone receptor expression correlated positively with GHR expression. These findings, along with our observation of GHR expression in breast cancer stromal cells and previous reports of local production of growth hormone in breast carcinoma, suggest that GHR-mediated signaling pathways are involved in the development of human breast cancer, possibly via autocrine or paracrine mechanisms.

A reduction in blood insulin levels as a host endocrine response during tumor development

It has been previously reported that endogenous insulin levels decrease during tumor growth. We have now studied whether this host endocrine response is independent of the way in which the tumor is induced. For this purpose, animals transplanted with tumor cells induced by 3-methylcholanthrene (MCA) or 7,12-dimethylbenz(a) anthracene (DMBA), or with EL-4 lymphoma cells, and animals that develop autochthonous tumors induced by MCA or the murine mammary tumor virus (MMTV) were used. These procedures result in the induction of tumors of different histologic types: fibrosarcoma, mammary adenocarcinoma and lymphoma. The results obtained showed that a reduction in insulin levels preceded the overt appearance of tumors in all models of syngeneic or autochthonous tumors studied but not when DMBA-induced tumor cells were administered into allogeneic recipients. Reduced levels of insulin before tumor detection appeared to affect the onset of MCA-induced tumors. Indeed, those mice with a late tumor onset were those that had a more pronounced decrease in insulin blood levels during the induction phase of autochthonous MCA-induced tumors. Soluble factors associated with tumor growth seem to mediate the reduction in insulin blood levels in mice transplanted with EL-4 tumor cells. The results obtained indicate that the reduction in insulin levels detected is a consequence of the recognition of tumor cells by the host, and seems to be independent of the histologic type of the neoplastic cells that develop. Pharmacological interventions at the levels of mechanisms that control insulin output should clarify the relevance of decreased levels of this hormone for tumor development.

Does the GH-IGF axis play a role in cancer pathogenesis?

Recent case-controlled studies have found increases in the serum levels of insulin-like growth factor-I (IGF-I) in subjects who had, or who eventually developed, prostate or premenopausal breast cancers. Since growth hormone (GH) increases IGF-I levels, concern has been raised regarding its potential role as a cancer initiation factor. The epidemiological studies, which indicate an association between serum IGF-I levels and cancer risk, have not established causality. In fact, several alternative explanations for the elevated serum IGF-I levels in cancer patients may be proposed based on human and animal models. First, an effect of IGF-I causing symptomatic benign tissue hyperplasia may result in an ascertainment bias leading to an initiation of procedures resulting in the diagnosis of asymptomatic cancers. Second, elevated serum IGF-I in cancer patients may originate within the tumor (as suggested by some animal studies). Thirdly, serum IGF-I may actually be a surrogate marker of tissue IGF-I levels or of nutritional factors, which are not under GH control and may be involved in cancer initiation. The role of GH in cancer initiation is further negated by the fact that in acromegaly, the incidence of cancer, other than possibly colonic neoplasia does not appear to be significantly increased. Furthermore, GH transgenic mice, with high IGF-I levels, do not develop breast, prostate, or colonic malignancies. It is known that IGFBP-3 can inhibit IGF action on cancer cells in vitro and also can induce apoptosis via an IGF-independent mechanism. Importantly, in addition to increasing IGF-I levels, GH also increases the serum levels of IGFBP-3 and serum IGFBP-3 levels have been shown to be negatively correlated with the risk of cancer in the above mentioned epidemiological studies and in a similar study on colon cancer. These studies suggest that cancer risk is increased in individuals in whom both high IGF-I levels and low IGFBP-3 levels are present. In subjects treated with GH, IGF-I and IGFBP-3 levels both rise together and are not within the elevated cancer-risk range, based on published studies. Long-term studies are needed to assess the potential risks, including the long-term cancer risk associated with GH therapy. These should take into account several factors, including the duration of exposure, the risk magnitude associated with the degree of serum IGF-I elevation, and the adjusted risk based on a concomitant increase in IGFBP-3 levels. Since GH treated patients often have sub-normal IGF-I serum levels, which normalize on therapy, one might predict that their cancer risk on GH therapy should not increase above the normal population. Until further research in the area dictates otherwise, on-going cancer surveillance and routine monitoring of serum IGF-I and IGFBP-3 levels in GH-recipients should be the standard of care. At present, the data that are available do not warrant a change in our current management of approved indications for GH therapy.

Constitutive tyrosine phosphorylation of ErbB-2 via Jak2 by autocrine secretion of prolactin in human breast cancer

Overexpression of the oncogene for ErbB-2 is an unfavorable prognostic marker in human breast cancer. Its oncogenic potential appears to depend on the state of tyrosine phosphorylation. However, the mechanisms by which ErbB-2 is constitutively tyrosine-phosphorylated in human breast cancer are poorly understood. We now show that human breast carcinoma samples with ErbB-2 overexpression have higher proliferative and metastatic activity in the presence of autocrine secretion of prolactin (PRL). By using a neutralizing antibody or dominant negative (DN) strategies or specific inhibitors, we also show that activation of Janus kinase Jak2 by autocrine secretion of PRL is one of the significant components of constitutive tyrosine phosphorylation of ErbB-2, its association with Grb2 and activation of mitogen-activated protein (MAP) kinase in human breast cancer cell lines that overexpress ErbB-2. Furthermore, the neutralizing anti-PRL antibody or erbB-2 antisense oligonucleotide or DN Jak2 or Jak2 inhibitor or DNRas or MAP kinase kinase inhibitor inhibits the proliferation of both untreated and PRL-treated cells. Our results indicate that autocrine secretion of PRL stimulates tyrosine phosphorylation of ErbB-2 by Jak2, provides docking sites for Grb2 and stimulates Ras-MAP kinase cascade, thereby causing unrestricted cellular proliferation. The identification of this novel cross-talk between ErbB-2 and the autocrine growth stimulatory loop for PRL may provide new targets for therapeutic and preventive intervention of human breast cancer.

Human prolactin (hPRL) antagonists inhibit hPRL-activated signaling pathways involved in breast cancer cell proliferation

The involvement of human prolactin (hPRL) in breast cancer has been recently reconsidered based on its autocrine/paracrine proliferative effect described in human mammary tumor epithelial cells. Therefore, there is growing interest in the development of potent hPRL antagonists that may inhibit this effect. We previously designed hPRL analogs displaying antagonistic properties in a human transcriptional bioassay. We now report that the most potent of those analogs, G129R-hPRL, antagonizes all hPRL-induced effects analysed in various breast cancer cell lines, including cell proliferation. The analog per se lacks intrinsic agonistic activity on PRL receptor-activated signaling cascades, cell proliferation and apoptosis, indicating that its mode of action only occurs through competitive inhibition of hPRL. We provide some molecular basis of this antagonistic effect by demonstrating that G129R-hPRL competitively inhibits hPRL-activation of the JAK-STAT and MAPK pathways, two signaling cascades involved in the mitogenic effect of hPRL in mammary epithelial cells. This competitive inhibition persists for at least 48 h, as evidenced by long term analysis of STAT5b activation or of progression through cell cycle. These results are the first demonstration at the molecular level that hPRL antagonists interfering with receptor dimerization disrupt signaling events in breast cancer cells, which prevents hPRL-induced cell proliferation.

Effects of growth hormone and prolactin on hematopoiesis

The use of the neuroendocrine hormones growth hormone (GH) and prolactin (PRL) in preclinical models, demonstrating promotion of hematopoietic recovery and immune function, offers promise for several clinical situations. These hormones do not appear to produce the same extent of immune/hematopoietic effects when compared to conventional hematopoietic and immune stimulating cytokines (i.e. G-CSF or interleukin-2). However, their pleiotropic effects and limited toxicity after systemic administration makes them attractive to test in myeloablative situations. More work needs to be performed to understand the mechanism(s) of GH and PRL action, particularly with regard to hematopoietic progenitor cell expansion and differentiation both in normal and pathologic situations.

Endocrine host responses during early and late phases of tumor development

It is well established that hormones affect tumor growth. Conversely, inoculation of cells obtained from tumors that had been transplanted for many generations causes changes in the concentration of different hormones before and after tumor detection. We aimed at answering the question of whether hormonal alterations also occur during the development of primary tumors and following transplantation of tumors from early generations. Primary tumors were induced in mice by either the carcinogenic agent 3-methylcholanthrene, which produces fibrosarcomas, or the milk-transmitted mammary tumor virus, which induces adenocarcinomas. The results showed that (i) in both models, an early reduction in plasma insulin and prolactin levels occurred, and in the case of insulin, this reduction was sustained for a prolong period prior to tumor detection, indicating that recognition by the host of emergent tumor cells triggers an endocrine response; (ii) in contrast with multiply transplanted tumors, cells from early transplant generations produced no significant endocrine changes during latency; (iii) irrespective of whether they were primary or transplanted, large tumor burdens caused similar hormonal alterations, consisting of increased corticosterone and growth hormone and decreased insulin, thyroxin, prolactin and sex steroid levels in blood. Our comprehensive longitudinal study demonstrates host endocrine responses during different stages of neoplastic development.

Characterization and modulation of a prolactin receptor mRNA isoform in normal and tumoral human breast tissues

The role of prolactin (PRL) and its specific receptor (R-PRL) in human breast tumorigenesis remains unclear. We have investigated here the presence of extracellular-deleted hPRL-R isoforms in normal human breast, fibrocystic disease, primary breast carcinoma (ductal carcinoma, ductulo-lobular and lobular) and breast cancer cell lines (T47-D and MCF-7). RT-PCR and Southern blot analysis demonstrated the expression of full-length hPRL-R transcript in all samples tested. We also detected a hPRL-R transcript generated by alternative exon 6 splicing. This isoform has a 170 bp deletion in its extracellular sub-domain that induces a frameshift. Thus, the predicted amino-acid sequence should encode a putative soluble protein with the N-terminal sub-domain of the hPRL-R and 10 additional carboxy-terminal residues. This isoform should not bind PRL as previously demonstrated by other experiments. Moreover, the ratio of full-length to deleted form of hPRL-R transcripts differs from normal to tumoral breast tissue. This ratio is higher in tumoral mammary gland than in normal tissue. Our data suggest that the alternative splicing of the hPRL-R gene towards the deleted transcript may be a mechanism to down- or up-regulate the expression of the native transcript of hPRL-R in accordance to the physiological or pathological state of the mammary gland.

The role of prolactin and growth hormone in breast cancer

This review will focus on the role for prolactin (PRL) and growth hormone (GH) in mammary tumor formation. Much attention has previously been focused on circulating levels of GH/PRL in relation to mammary tumor formation. We will review data demonstrating that these ligands also could be produced locally in different organs, including the mammary gland and mammary tumors, and suggest that this local production may be of importance for pathological conditions. We will also discuss mechanisms for crosstalk between steroids and GH/PRL. A crosstalk between GH- and PRL response is possible at multiple levels. In the human, GH can activate both the prolactin receptor (PRLR) and the growth hormone receptor (GHR). We have demonstrated that activation of the PRLR, but not the GHR, is inducing mammary tumors in transgenic mice. Furthermore, the elevated levels of insulin-like growth factor 1 (IGF-I) seen in the GHR activating transgenic mice is not sufficient for tumor induction. The induced tumors express functionally active prolactin that could be of importance for the tumor formation. Paracrine/aurocrine stimulation by PRL may be more important than PRL transported via the circulation. In women, the role for stimulation of the PRLR and/or the GHR in mammary tumor formation has not been proven, although experiments from primates suggest that the PRLR could be of importance.

Involvement of prolactin in breast cancer: redefining the molecular targets

The mammary gland is the major target tissue of prolactin (PRL) in mammals. Although this pituitary hormone has been long suspected to be involved in the progression of human breast cancer, the failure of clinical improvement by treatment with dopamine agonists (which lower circulating levels of PRL) rapidly reduced the interest of oncologists concerning a potential role of PRL in the development of breast cancer. Within the last few years, however, several studies reported first, that PRL is also synthesized by mammary epithelial cells, and second that it may exert a proliferative action in an autocrine/paracrine manner. In agreement with a recent epidemiological study, these observations have led to a reconsideration of the role of PRL as an active participant in breast cancer, and are an impetus to redefine the molecular targets of anti-prolactin strategies since dopamine analogs are assumed to be inefficient on extrapituitary PRL synthesis. In this review, we briefly summarize the current knowledge of PRL effects on both normal and tumor mammary cells, and we discuss the most relevant articles supporting the autocrine-paracrine action of PRL in the breast. With the aim of defining putative new molecular targets, we propose an overview of the main PRL receptor signaling cascades known to be triggered by PRL in mammary epithelial cells or, when not available, in other cell types. Finally, because proteolytic fragments of rat PRL have been shown to inhibit the angiogenic process, which may be relevant for preventing the progression of solid tumors such as breast tumors, we discuss the hypothesis that the enzymatic cleavage of human PRL could also represent a new molecular target in the search for alternative strategies in the treatment of breast cancer.

The 20-kilodalton (kDa) human growth hormone (hGH) differs from the 22-kDa hGH in the effect on the human prolactin receptor

Previously we have demonstrated that 20-kDa human GH (20K-hGH) is a full agonist for hGH receptor (hGHR) even though its complex formation with hGHR and hGH-binding protein differs from that of 22-kDa human GH (22K-hGH). In this study, we focused on the effect of 20K-hGH on human PRL receptor (hPRLR). To elucidate the effects of 20K-hGH on hPRLR and compare them with those of 22K-hGH, we prepared two cells stably expressing full-length hPRLR, Ba/F3-hPRLR and CHO-hPRLR. In the proliferation of Ba/F3-hPRLR cells, which can grow in a dose-response to lactogenic hormones, both 20K- and 22K-hGH exhibited bell-shaped curves in the absence of exogenous zinc ion (Zn2+); however, the curve of 20K-hGH was shifted to a 10-fold higher concentration than that of 22K-hGH in view of EC50 value (the EC50 of 20K- and 22K-hGH were 15 nM and 1.5 nM, respectively). Addition of Zn2+ up to 25 microM increased the activities of both 20K- and 22K-hGH; however, the enhancement by Zn2+ was greater in 20K-hGH than in 22K-hGH, thereby the activities of both hGH isoforms reached the same level at 25 microM Zn2+. Nevertheless, in the presence of 0.25-1 microM free Zn2+, which is equal in human serum, the activity of 20K-hGH was still lower than that of 22K-hGH. The modest effect of 20K-hGH on activating hPRLR in the absence of Zn2+ was confirmed in the rat serine protease inhibitor 2.1 (Spi2.1) gene promoter activation and JAK2/Stat5 tyrosine phosphorylation in CHO-hPRLR. In addition, in human breast cancer cell T-47D, 20K-hGH was proved to stimulate Stat5 tyrosine phosphorylation to much lower degree than 22K-hGH via not hGHR but hPRLR. Taken together, our data suggest that 20K-hGH may be a weaker agonist for hPRLR than 22K-hGH in the human body; therefore 20K-hGH may alleviate the hPRLR-mediated side-effects such as breast cancer when administered to human body.

Should prolactin be reconsidered as a therapeutic target in human breast cancer?

Although prolactin (PRL) has been long suspected to be involved in the progression of human breast cancer, the failure of clinical improvement by treatment with dopamine agonists, which lower circulating levels of PRL, rapidly reduced the interest of oncologists concerning a potential role of this pituitary hormone in the development of breast cancer. Within the last few years, however, several studies reported first, that PRL is also synthesized in the mammary gland, and second that it exerts its proliferative action in an autocrine/paracrine manner. These observations have led to a reconsideration of the role of PRL as an active participant in breast cancer and are an impetus to search for alternative strategies aimed at inhibiting the proliferative effects of PRL on tumor mammary cells. In this report, we discuss the three possible levels that can be targeted for this purpose: the mammary synthesis of PRL, the interaction of the hormone with its receptor at the surface of mammary cells, and the intracellular signaling cascades triggered by the activated receptor. For each of these steps, we discuss the molecular event(s) that can be targeted, our understanding of the mechanisms involving these putative targets as well as the tools currently available for their inhibition. Besides its proliferative effect, PRL is also involved in the control of angiogenesis through one of its cleaved fragments, named PRL 16K, which has been shown to inhibit the angiogenic process. In view of this biological activity, we discuss first the cleavage of PRL with respect to the human mammary gland and, second, the hypothesis speculating that a balance between the proliferative effect of intact PRL and the anti-angiogenic activity of its 16K-like fragments might be physiologically relevant in the evolution of mammary tumors. If true, our hypothesis would suggest that the enzymatic cleavage of PRL could represent a new molecular target in the search for alternative strategies in the treatment of breast cancer.

Retinoic acid modulates prolactin receptor expression and prolactin-induced STAT-5 activation in breast cancer cells in vitro

Two recent papers demonstrate that prolactin plays an important role in the induction and progression of mammary tumours. Retinoids have been shown to be potent inhibitors of breast carcinogenesis. We studied expression of prolactin receptor mRNA in human breast cancer cell lines MCF-7, SKBR-3, T47D and BT-20 treated with and without retinoids using Northern blot and a quantitative polymerase chain reaction (PCR) method. In all cell lines, all-trans- and 9-cis-retinoic acid, as well as the retinoic acid receptor gamma (RAR-gamma) selective agonists CD2325 and CD437 (1 microM), were able to down-regulate prolactin receptor. After 1 h, a significant reduction was detectable and maximal effect was achieved after 24 h of treatment. Pretreatment with retinoic acid also reduced the prolactin-/prolactin receptor-dependent signal transduction and activation of transcription 5 (STAT-5) activation in T47D cells. Cycloheximide failed to abrogate the retinoic acid-induced decline in prolactin receptor mRNA levels, indicating that this effect was not dependent upon continuing protein synthesis. Similarly, no change in the stability of prolactin receptor mRNA was observed during 12 h of retinoic acid treatment. In conclusion, our results demonstrate that retinoids are able to inhibit the expression of prolactin receptor message, which encodes an important growth factor receptor in breast cancer cells. This action could be responsible for the anti-tumour effects of retinoids.