Growth hormone and prolactin receptors in adipogenesis: STAT-5 activation, suppressors of cytokine signaling, and regulation of insulin-like growth factor I

Growth hormone GH stimulates lipolysis in mature adipocytes and primary preadipocytes but promotes adipogenesis in preadipocyte cell lines. The lactogenic hormones (prolactin PRL and placental lactogen) also stimulate adipogenesis in preadipocyte cell lines but have variable lipolytic and lipogenic effects in mature adipose tissue. We hypothesized that differences in expression of GH receptors GHR and PRL receptors PRLR during adipocyte development might explain some of the differential effects of the somatogens and lactogens on fat metabolism. To that end, we compared: (a) the expression of GHR and PRLR mRNAs in 3T3-L1 preadipocytes during the course of adipocyte differentiation; (b) the induction of STAT-5 activity by GH and PRL during adipogenesis; and (c) the acute effects of GH and PRL on the suppressors of cytokine signaling (SOCS-1-3 and cytokine-inducible SH2-domain-containing protein CIS) and IGF-I. In confluent, undifferentiated 3T3-L1 cells, the levels of GHR mRNA were approximately 250-fold higher than the levels of PRLR mRNA. Following induction of adipocyte differentiation the levels of PRLR mRNA rose 90-fold but GHR mRNA increased only 0.8-fold. Expression of both full-length (long) and truncated (short) isoforms of the PRLR increased during differentiation but the long isoform predominated at all time points. Mouse GH mGH stimulated increases in STAT-5a and 5b activity in undifferentiated as well as differentiating 3T3-L1 cells; mouse PRL mPRL had little or no effect on STAT-5 activity in undifferentiated cells but stimulated increases in STAT-5a and 5b activity in differentiating cells. mGH stimulated increases in SOCS-2 and SOCS-3 mRNAs in undifferentiated cells and SOCS-1-3 and CIS mRNAs in differentiating cells; mPRL induced CIS in differentiating cells but had no effect on SOCS-1-3. mPRL and mGH stimulated increases in IGF-I mRNA in differentiating cells but not in undifferentiated cells; the potency of mGH (3-6-fold increase, p < 0.01) exceeded that of mPRL (40-90% increase, p < 0.05). Our findings reveal disparities in the expression of PRLR and GHR during adipocyte development and differential effects of the hormones on STAT-5, the SOCS proteins, CIS, and IGF-I. These observations suggest that somatogens and lactogens regulate adipocyte development and fat metabolism through distinct but overlapping cellular mechanisms.     

NGF gene expression and secretion in white adipose tissue: regulation in 3T3-L1 adipocytes by hormones and inflammatory cytokines

The sympathetic nervous system plays a central role in lipolysis and the production of leptin in white adipose tissue (WAT). In this study, we have examined whether nerve growth factor (NGF), a target-derived neurotropin that is a key signal in the development and survival of sympathetic neurons, is expressed and secreted by white adipocytes. NGF mRNA was detected by RT-PCR in the major WAT depots of mice (epididymal, perirenal, omental, mesenteric, subcutaneous) and in human fat (subcutaneous, omental). In mouse WAT, NGF expression was observed in mature adipocytes and in stromal vascular cells. NGF expression was also evident in 3T3-L1 cells before and after differentiation into adipocytes. NGF protein, measured by ELISA, was secreted from 3T3-L1 cells, release being higher before differentiation. Addition of the sympathetic agonists norepinephrine, isoprenaline, or BRL-37344 (beta(3)-agonist) led to falls in NGF gene expression and secretion by 3T3-L1 adipocytes, as did IL-6 and the PPARgamma agonist rosiglitazone. A substantial decrease in NGF expression and secretion occurred with dexamethasone. In contrast, LPS increased NGF mRNA levels and NGF secretion. A major increase in NGF mRNA level (9-fold) and NGF secretion (相似文献   

Dopamine receptors in human adipocytes: expression and functions

IntroductionDopamine (DA) binds to five receptors (DAR), classified by their ability to increase (D1R-like) or decrease (D2R-like) cAMP. In humans, most DA circulates as dopamine sulfate (DA-S), which can be de-conjugated to bioactive DA by arylsulfatase A (ARSA). The objective was to examine expression of DAR and ARSA in human adipose tissue and determine whether DA regulates prolactin (PRL) and adipokine expression and release.MethodsDAR were analyzed by RT-PCR and Western blotting in explants, primary adipocytes and two human adipocyte cell lines, LS14 and SW872. ARSA expression and activity were determined by qPCR and enzymatic assay. PRL expression and release were determined by luciferase reporter and Nb2 bioassay. Analysis of cAMP, cGMP, leptin, adiponectin and interleukin 6 (IL-6) was done by ELISA. Activation of MAPK and PI3 kinase/Akt was determined by Western blotting.ResultsDAR are variably expressed at the mRNA and protein levels in adipose tissue and adipocytes during adipogenesis. ARSA activity in adipocyte increases after differentiation. DA at nM concentrations suppresses cAMP, stimulates cGMP, and activates MAPK in adipocytes. Acting via D2R-like receptors, DA and DA-S inhibit PRL gene expression and release. Acting via D1R/D5R receptors, DA suppresses leptin and stimulates adiponectin and IL-6 release.ConclusionsThis is the first report that human adipocytes express functional DAR and ARSA, suggesting a regulatory role for peripheral DA in adipose functions. We speculate that the propensity of some DAR-activating antipsychotics to increase weight and alter metabolic homeostasis is due, in part, to their direct action on adipose tissue.     

Genistein restricts leptin secretion from rat adipocytes

The isoflavones--genistein and daidzein -- compounds found in high concentrations in soy play an important role in prevention of many diseases and affect some metabolic pathways. In the performed experiment it was demonstrated that genistein (5mg/kg b.w.) administered intragastrically for three days to male Wistar rats substantially diminished blood leptin level. Studies with isolated rat adipocytes revealed that this phytoestrogen strongly restricted leptin secretion from these cells. These effects were not accompanied by any changes in leptin gene expression in adipocytes. Daidzein-- an analogue of genistein -- used at similar concentrations did not affect blood leptin concentration, leptin secretion and expression of its gene. To determine the influence of genistein and daidzein on leptin release, adipocytes isolated from the epididymal fat tissue were incubated for 2h in Krebs--Ringer buffer. Leptin secretion stimulated by glucose with insulin was significantly diminished by genistein (0.25--1mM). This effect of genistein may arise from several aspects of its action in adipocytes documented in the literature such as the inhibition of glucose transport and metabolism, the attenuation of insulin signalling, the inhibition of cAMP phosphodiesterase and the stimulation of lipolysis. However, the bypassing of the restrictive action of genistein on glucose transport and glycolysis (by the use of alanine instead of glucose) and on insulin action (by the use of nicotinic acid) was not sufficient to restore leptin secretion from isolated adipocytes. It was also demonstrated that the restriction of the stimulatory influence of genistein on cAMP/protein kinase A (PKA) pathway (by the inhibition of PKA activity) did not improve leptin release. Results obtained in our experiments point at the restriction of glucose metabolism following formation of pyruvate as the pivotal reason of the inhibitory action of genistein on leptin release.     

Differences in prolactin receptor (PRLR) in mouse and human fallopian tubes: evidence for multiple regulatory mechanisms controlling PRLR isoform expression in mice

The anterior pituitary-derived hormone prolactin (PRL) signals through the PRL receptor (PRLR) and is important for female reproductive function in mammals. In contrast to the extensive studies of PRLR expression and regulation in human and mouse ovary and uterus, the mechanisms controlling the regulation of PRLR isoform expression in the fallopian tube are poorly understood. Because dynamic interaction of hormonal signaling in gonadal tissue and the pituitary or in gonadal tissues themselves in mammals suggests endocrine or paracrine regulation of PRLR expression, we questioned whether differential regulation of PRLR isoforms by PRL ovarian-derived estrogen (E(2)) and progesterone (P(4)) exists in the fallopian tube and pituitary of prepubertal female mice. Western blot analysis showed distinct molecular separation of PRLR isoforms in mouse and human fallopian tubes, and cellular localization was found in mouse and human tubal epithelia but not in mouse tubal smooth muscle cells. These data support the concept of an isoform- and cell type-specific expression of PRLR in human and mouse fallopian tubes. Moreover, expression of the long form of PRLR decreased after PRL treatment and increased after blockage of endogenous PRL secretion by bromocriptine (an inhibitor of PRL secretion) in a time-dependent manner in mouse fallopian tube. The opposite regulation was observed in the pituitary. Treatment with exogenous E(2) or P(4) led to changes in PRLR expression in the fallopian tube similar to those of PRL treatment. However, E(2) and P(4) did not affect PRLR expression in the pituitary. Estrogen had no effect on the long form of PRLR expression, whereas P(4) regulated the long form of PRLR in the fallopian tube, as did PRL. Taken together, the data from our comparative study provide evidence that PRLR can be regulated by an interplay of two different mechanisms, PRL or ovarian steroid hormones independently or in combination in a tissue-specific manner. Furthermore, we found that ovarian steroid hormones selectively suppress the expression of PRLR isoforms in mouse fallopian tubes. These findings may contribute to our understanding of the mechanisms controlling PRLR isoform expression in the fallopian tube (in addition to ovary and uterus), with implications for female reproduction.     

Growth Hormone Treatment of Hypophysectomized Rats Increases Catecholamine-induced Lipolysis and the Number of β-Adrenergic Receptors in Adipocytes: No Differences in the Effects of Growth Hormone on Different Fat Depots

Growth hormone (GH) has a lipolytic effect in adipose tissue but this effect may differ in adipose tissue from various fat depots. This latter possibility was investigated in the present study, in which the effects of GH in vivo on catecholamine-induced lipolysis and the number of β-adrenergic receptors in isolated adipocytes from different fat depots of hypophysectomized rats were investigated. Female and male Sprague-Dawley rats were hypophysectomized or sham-operated at 45 days of age. One week after the operation, hormonal replacement therapy with L-thyroxine and hydrocortisone acetate was given. In addition, groups of rats were treated with GH (1.33 mg/kg per day, given as two daily subcutaneous injections). After 1 week of hormonal treatment, adipocytes were isolated from the parametrial, epididymal and inguinal fat pads, and glycerol release after catecholamine-stimulation and ¹²⁵I-cyanopindolol binding were measured. Hypophysectomy resulted in a marked decrease in the lipolytic response to catecholamines. GH treatment significantly increased catecholamine-induced lipolysis with similar effects in adipocytes from parametrial or epididymal and inguinal fat depots in both female and male rats. There were no differences between norepinephrine compared with isoproterenol-induced responses. ¹²⁵I-cyanopindolol binding was reduced after hypophysectomy and normalized by GH treatment, without differences between parametrial and inguinal adipose tissue regions. We conclude that the lipolytic effects of GH in the rat may partly be mediated by a stimulatory effect on β-adrenergic receptors in adipocytes. In addition, GH exerted similar effect on catecholamine-induced lipolysis and β-adrenergic receptors in adipocytes from parametrial, epididymal and inguinal fat depots.     

泌乳素及其受体的研究进展

泌乳素(PRL)又名催乳素,是由腺垂体及一些垂体外器官如乳腺、胸腺、脾脏等合成的一种多肽类激素,以内分泌、自分泌、旁分泌的形式发挥作用,其广泛参与机体生长发育、物质代谢、性腺功能调节、应激反应、免疫调节等。泌乳素通过与其受体(PRL receptor,PRLR)在靶细胞的细胞膜表面结合,激活下游的信号转导通路,发挥其生物学作用。由于泌乳素在人体内复杂的生物学效应,泌乳素及其受体又与乳腺癌、泌乳素瘤等多种疾病的发生发展及其预后密切相关。本文就泌乳素发挥效应时对其受体的激活、受体激活后的信号转导机制以及泌乳素同相关疾病的联系进行了综述,相信泌乳素及其受体的研究将为这些疾病的治疗提供重要的方向。     

Prolactin receptor signaling is essential for perinatal brown adipocyte function: a role for insulin-like growth factor-2

Background

The lactogenic hormones prolactin (PRL) and placental lactogens (PL) play central roles in reproduction and mammary development. Their actions are mediated via binding to PRL receptor (PRLR), highly expressed in brown adipose tissue (BAT), yet their impact on adipocyte function and metabolism remains unclear.

Methodology/Principal Findings

PRLR knockout (KO) newborn mice were phenotypically characterized in terms of thermoregulation and their BAT differentiation assayed for gene expression studies. Derived brown preadipocyte cell lines were established to evaluate the molecular mechanisms involved in PRL signaling on BAT function. Here, we report that newborn mice lacking PRLR have hypotrophic BAT depots that express low levels of adipocyte nuclear receptor PPARγ2, its coactivator PGC-1α, uncoupling protein 1 (UCP1) and the β3 adrenoceptor, reducing mouse viability during cold challenge. Immortalized PRLR KO preadipocytes fail to undergo differentiation into mature adipocytes, a defect reversed by reintroduction of PRLR. That the effects of the lactogens in BAT are at least partly mediated by Insulin-like Growth Factor-2 (IGF-2) is supported by: i) a striking reduction in BAT IGF-2 expression in PRLR KO mice and in PRLR-deficient preadipocytes; ii) induction of cellular IGF-2 expression by PRL through JAK2/STAT5 pathway activation; and iii) reversal of defective differentiation in PRLR KO cells by exogenous IGF-2.

Conclusions

Our findings demonstrate that the lactogens act in concert with IGF-2 to control brown adipocyte differentiation and growth. Given the prominent role of brown adipose tissue during the perinatal period, our results identified prolactin receptor signaling as a major player and a potential therapeutic target in protecting newborn mammals against hypothermia.     

Inhibition of MAPK by prolactin signaling through the short form of its receptor in the ovary and decidua: involvement of a novel phosphatase

Prolactin (PRL) is essential for normal reproduction and signals through two types of receptors, the short (PRL-RS) and long (PRL-RL) form. We have previously shown that transgenic mice expressing only PRL-RS (PRLR(-/-)RS) display abnormal follicular development and premature ovarian failure. Here, we report that MAPK, essential for normal follicular development, is critically inhibited by PRL in reproductive tissues of PRLR(-/-)RS mice. Consequently, the phosphorylation of MAPK downstream targets are also markedly inhibited by PRL without affecting immediate upstream kinases, suggesting involvement of MAPK specific phosphatase(s) in this inhibition. Similar results are obtained in a PRL-responsive ovary-derived cell line (GG-CL) that expresses only PRL-RS. However, we found the expression/activation of several known MAPK phosphatases not to be affected by PRL, suggesting a role of unidentified phosphatase(s). We detected a 27-kDa protein that binds to the intracellular domain of PRL-RS and identified it as dual specific phosphatase DUPD1. PRL does not induce expression of DUDP1 but represses its phosphorylation on Thr-155. We also show a physical association of this phosphatase with ERK1/2 and p38 MAPK. Using an in vitro phosphatase assay and overexpression studies, we established that DUPD1 is a MAPK phosphatase. Dual specific phosphatase inhibitors as well as siRNA to DUPD1, completely prevent PRL-mediated MAPK inhibition in ovarian cells. Our results strongly suggest that deactivation of MAPK by PRL/PRL-RS contributes to the severe ovarian defect in PRLR(-/-)RS mice and demonstrate the novel association of PRL-RS with DUPD1 and a role for this phosphatase in MAPK deactivation.     

Rat heart is a site of leptin production and action

Leptin, the 16-kDa peptide hormone product of the ob gene, is produced primarily by adipocytes and was initially thought to exert its effects exclusively through actions on the hypothalamus via distinct leptin receptors termed OB-R. However, recent data show that leptin is produced elsewhere and that receptors are present in many other tissues. Using real-time PCR, we determined whether leptin and its receptors are present in the rat heart and demonstrated regional distribution patterns and gender differences as well as the effect of ischemia and reperfusion. Gene expression of leptin and its receptors (OB-Ra, OB-Rb, and OB-Re) was identified in myocytes and whole heart homogenates from all regions of the heart of male and female rats, with the highest abundance in left and right atria of male and female rats, respectively. No differences in regional distribution of OB-R were evident in male rat hearts. In female rats, expression was highest in right atria for all three isoforms and was significantly greater than in male rats. Ischemia and reperfusion significantly downregulated leptin and OB-R expression, although this was more pronounced in male rat hearts. Leptin release in the coronary effluent was also detected using ELISA, although this was generally unaffected by global ischemia and reperfusion. Our results demonstrate for the first time the presence of the leptin system, including the peptide and its receptors, in all regions of the rat heart. In view of emerging evidence for cardiac effects of leptin, it is proposed that the heart is a target for leptin action and that the peptide modulates function through a paracrine- or autocrine-dependent manner.     

Chronic ethanol and triglyceride turnover in white adipose tissue in rats: inhibition of the anti-lipolytic action of insulin after chronic ethanol contributes to increased triglyceride degradation

Chronic ethanol consumption disrupts whole-body lipid metabolism. Here we tested the hypothesis that regulation of triglyceride homeostasis in adipose tissue is vulnerable to long-term ethanol exposure. After chronic ethanol feeding, total body fat content as well as the quantity of epididymal adipose tissue of male Wistar rats was decreased compared with pair-fed controls. Integrated rates of in vivo triglyceride turnover in epididymal adipose tissue were measured using (2)H(2)O as a tracer. Triglyceride turnover in adipose tissue was increased due to a 2.3-fold increase in triglyceride degradation in ethanol-fed rats compared with pair-fed controls with no effect of ethanol on triglyceride synthesis. Because increased lipolysis accompanied by the release of free fatty acids into the circulation is associated with insulin resistance and liver injury, we focused on determining the mechanisms for increased lipolysis in adipose tissue after chronic ethanol feeding. Chronic ethanol feeding suppressed beta-adrenergic receptor-stimulated lipolysis in both in vivo and ex vivo assays; thus, enhanced triglyceride degradation during ethanol feeding was not due to increased beta-adrenergic-mediated lipolysis. Instead, chronic ethanol feeding markedly impaired insulin-mediated suppression of lipolysis in conscious rats during a hyperinsulinemic-euglycemic clamp as well as in adipocytes isolated from epididymal and subcutaneous adipose tissue. These data demonstrate for the first time that chronic ethanol feeding increased the rate of triglyceride degradation in adipose tissue. Furthermore, this enhanced rate of lipolysis was due to a suppression of the anti-lipolytic effects of insulin in adipocytes after chronic ethanol feeding.     

Distribution of prolactin receptors suggests an intraductal role for prolactin in the mouse and human mammary gland,a finding supported by analysis of signaling in polarized monolayer cultures

Despite the important role of prolactin (PRL) in mammary gland development and function, little is known about the distribution of the different forms of the prolactin receptor (PRLR) under various physiological circumstances. Here, the distribution of the long (LF) and the short (S3 in mouse) receptor common to both mice and rats was determined by immunofluorescence on frozen sections of virgin, pregnant and lactating mouse mammary gland. Myoepithelial cells were consistently and intensely stained for both receptors. For luminal cells at all stages (ducts and alveoli), a large proportion of PRLR staining was unexpectedly present on the apical face. In the non-lactating state, no basal staining of luminal cells was detectable. During lactation, a proportion of both receptors moved to the basolateral surface. In vitro, HC11 cells showed constitutive expression of LF but expression of S3 only upon the formation of adherent junctions. Tight junction formation was accelerated by incubation in pseudo-phosphorylated PRL, as measured by transepithelial resistance and the expression and placement of the tight junction protein, zonula occludens-1. Once an intact monolayer had formed, all LF and S3 receptors were apical (akin to the non-lactating state) and only apical application of PRL activated the Jak2-STAT5 and ERK pathways. By contrast, basolateral application of PRL resulted in a reduction in basal ERK phosphorylation, suggesting an involvement of a dual specificity protein phosphatase. Normal human breast samples also showed apical PRLRs. These results demonstrate important contextual aspects of PRL-PRLR interactions with implications for the analysis of the role of PRL in breast cancer.     

Prolactin increases tumor necrosis factor alpha expression in peripheral CD14 monocytes of patients with rheumatoid arthritis

Tumor necrosis factor (TNF)-α is one of the major proinflammatory mediators of rheumatic arthritis (RA); the regulatory factors for TNF-α release is not fully understood. This study aims to investigate the role of prolactin receptor (PRLR) activation in regulating the expression and release of TNF-α from CD14⁺ monocytes. The results showed that the expression of PRLR was detectable in CD14⁺ monocytes of healthy subjects, which was markedly increased in RA patients. Exposure to PRL in the culture increased the expression and release of TNF-α from CD14⁺ monocytes, which was abolished by the PRLR gene silencing or blocking the mitogen activated protein (MAPK) pathway. We conclude that exposure to PRL increases TNF-α release from CD14⁺ monocytes of RA patients, which can be abolished by PRLR gene silencing or treating with MAPK inhibitor.     

Novel form of lipolysis induced by leptin.

Hyperleptinemia causes disappearance of body fat without a rise in free fatty acids (FFA) or ketones, suggesting that leptin can deplete adipocytes of fat without releasing FFA. To test this, we measured FFA and glycerol released from adipocytes obtained from normal lean Zucker diabetic fatty rats (+/+) and incubated for 0, 3, 6, or 24 h in either 20 ng/ml recombinant leptin or 100 nM norepinephrine (NE). Whereas NE increased both FFA and glycerol release from adipocytes of +/+ rats, leptin increased glycerol release in +/+ adipocytes without a parallel increase in FFA release. In adipocytes of obese Zucker diabetic fatty rats (fa/fa) with defective leptin receptors, NE increased both FFA and glycerol release, but leptin had no effect on either. Leptin significantly lowered the mRNA of leptin and fatty acid synthase of adipocytes (FAS) (p < 0.05), and up-regulated the mRNA of peroxisome proliferator-activated receptor (PPAR)-alpha, carnitine palmitoyl transferase-1, (CPT-1), and acyl CoA oxidase (ACO) (p < 0.05). NE (100 nM) also lowered leptin mRNA (p < 0.05) but did not affect FAS, PPARalpha, ACO, or CPT-1 expression. We conclude that in normal adipocytes leptin directly decreases FAS expression, increases PPARalpha and the enzymes of FFA oxidation, and stimulates a novel form of lipolysis in which glycerol is released without a proportional release of FFA.     

A non-catalytic function of the Src family tyrosine kinases controls prolactin-induced Jak2 signaling

The cytokine prolactin (PRL) plays important roles in the proliferation and differentiation of the mammary gland and it has been implicated in tumorigenesis. The prolactin receptor (PRLR) is devoid of catalytic activity and its mitogenic response is controlled by cytoplasmic tyrosine kinases of the Src (SFK) and Jak families. How PRLR uses these kinases for signaling is not well understood. Previous studies indicated that PRLR-induced Jak2 activation does not require SFK catalytic activity in favor of separate signaling operating on this cellular response. Here we show that, nevertheless, PRLR requires Src-SH2 and -SH3 domains for Jak2 signaling. In W53 lymphoid cells, conditional expression of two c-Src non-catalytic mutants, either SrcK295M/Y527F or Src?K, whose SH3 and SH2 domains are exposed, controls Jak2/Stat5 activation by recruiting Jak2, avoiding its activation by endogenous active SFK. In contrast, the kinase inactive SrcK295M mutant, with inaccessible SH3 and SH2 domains, does not. Furthermore, all three mutants attenuate PRLR-induced Akt and p70S6K activation. Accordingly, PRLR-induced Jak2/Stat5 signaling is inhibited in MCF7 breast cancer cells by Src depletion, expression of SrcK295M/Y527F or active Src harboring an inactive SH2 (SrcR175L) or SH3 domain (SrcW118A). Finally, Jak2/Stat5 pathway is also reduced in Src^?/? mice mammary glands. We thus conclude that, in addition to Akt and p70S6K, SFK regulate PRLR-induced Jak2 signaling through a kinase-independent mechanism.     

Prolactin Induces Apoptosis of Lactotropes in Female Rodents

Anterior pituitary cell turnover occurring during female sexual cycle is a poorly understood process that involves complex regulation of cell proliferation and apoptosis by multiple hormones. In rats, the prolactin (PRL) surge that occurs at proestrus coincides with the highest apoptotic rate. Since anterior pituitary cells express the prolactin receptor (PRLR), we aimed to address the actual role of PRL in the regulation of pituitary cell turnover in cycling females. We showed that acute hyperprolactinemia induced in ovariectomized rats using PRL injection or dopamine antagonist treatment rapidly increased apoptosis and decreased proliferation specifically of PRL producing cells (lactotropes), suggesting a direct regulation of these cell responses by PRL. To demonstrate that apoptosis naturally occurring at proestrus was regulated by transient elevation of endogenous PRL levels, we used PRLR-deficient female mice (PRLRKO) in which PRL signaling is totally abolished. According to our hypothesis, no increase in lactotrope apoptotic rate was observed at proestrus, which likely contributes to pituitary tumorigenesis observed in these animals. To decipher the molecular mechanisms underlying PRL effects, we explored the isoform-specific pattern of PRLR expression in cycling wild type females. This analysis revealed dramatic changes of long versus short PRLR ratio during the estrous cycle, which is particularly relevant since these isoforms exhibit distinct signaling properties. This pattern was markedly altered in a model of chronic PRLR signaling blockade involving transgenic mice expressing a pure PRLR antagonist (TG^{Δ1–9-G129R-hPRL}), providing evidence that PRL regulates the expression of its own receptor in an isoform-specific manner. Taken together, these results demonstrate that i) the PRL surge occurring during proestrus is a major proapoptotic signal for lactotropes, and ii) partial or total deficiencies in PRLR signaling in the anterior pituitary may result in pituitary hyperplasia and eventual prolactinoma development, as observed in TG^{Δ1–9-G129R-hPRL} and PRLRKO mice, respectively.