Tissue-specific induction of SOCS gene expression by PRL

SOCS1 and SOCS3 as key checkpoint molecules in the immune responses associated to skin inflammation and malignant transformation

SOCS are a family of negative inhibitors of the molecular cascades induced by cytokines, growth factors and hormones. At molecular level, SOCS proteins inhibit the kinase activity of specific sets of receptor-associated Janus Activated Kinases (JAKs), thereby suppressing the propagation of intracellular signals. Of the eight known members, SOCS1 and SOCS3 inhibit activity of JAKs mainly induced by cytokines and can play key roles in regulation of inflammatory and immune responses. SOCS1 and SOCS3 are the most well-characterized SOCS members in skin inflammatory diseases, where their inhibitory activity on cytokine activated JAKs and consequent anti-inflammatory action has been widely investigated in epidermal keratinocytes. Structurally, SOCS1 and SOCS3 share the presence of a N-terminal domain containing a kinase inhibitory region (KIR) motif able to act as a pseudo-substrate for JAK and to inhibit its activity. During the last decades, the design and employment of SOCS1 and SOCS3-derived peptides mimicking KIR domains in experimental models of dermatoses definitively established a strong anti-inflammatory and ameliorative impact of JAK inhibition on skin inflammatory responses. Herein, we discuss the importance of the findings collected in the past on SOCS1 and SOCS3 function in the inflammatory responses associated to skin immune-mediated diseases and malignancies, for the development of the JAK inhibitor drugs. Among them, different JAK inhibitors have been introduced in the clinical practice for treatment of atopic dermatitis and psoriasis, and others are being investigated for skin diseases like alopecia areata and vitiligo.

Transcript Abundances of the Prolactin Receptor, the Leptin Receptor and their Major Suppressor in the Sheep Mammary Gland During Pregnancy and Lactation

This study aimed to expand the knowledge of the interactions between prolactin (PRL) and leptin in the ovine mammary gland during pregnancy and lactation; we examined the mRNA expression of prolactin receptor (PRLR), the long form of the leptin receptor (LRb) and suppressor of cytokine signaling (SOCS)-3 in mammary gland biopsies collected on days 60, 90 and 120 of pregnancy and on days 30, 60 and 90 of lactation (n = 6 for each time point), along with the plasma PRL and leptin concentrations. The PRL concentrations were stable throughout pregnancy and increased during lactation. The plasma leptin concentrations were comparable among nonpregnant, early-pregnant, late-pregnant and lactating ewes, but this metric peaked during mid-pregnancy. Expression of PRLR and SOCS-3 in the mammary gland fluctuated during the transition from pregnancy to lactation, and differences in LRb expression occurred during the late stages of lactation. The LRb transcript abundance was approximately 31 times higher in ewes on day 60 of lactation than in early-lactating ewes. Expression of SOCS-3 mRNA in biopsies gradually decreased over the course of pregnancy and reached a minimum value during late pregnancy. After lambing, the transcript level of SOCS-3 increased and peaked on day 60 of lactation. During pregnancy, the plasma PRL concentration positively correlated with the abundances of PRLR (r = 0.971, P<0.01) and SOCS-3 (r = 0.818, P<0.05). Positive correlations were also observed between the transcript abundances of SOCS-3 and LRb (r = 0.854, P<0.05). The variations observed in the plasma PRL and leptin concentrations and the changes in expression of key leptin and PRL signal transduction pathway components, such as PRLR, LRb and SOCS-3, indicate that the efficacies of both hormone actions are modulated in a multilevel manner throughout pregnancy and lactation. These interactions may regulate the ability of the mammary gland to respond to current energy requirements and challenges, thus affecting milk yield and lactation duration.

SOCS3 expression in SF1 cells regulates adrenal differentiation and exercise performance

Many hormones/cytokines are secreted in response to exercise and cytokine signaling may play a pivotal role in the training adaptations. To investigate the importance of cytokine signaling during vertical ladder climbing, a resistance exercise model, we produced mice lacking SOCS3 protein exclusively in steroidogenic factor-1 (SF1) cells (SF1 Socs3 KO mice). SF1 expression is found in steroidogenic cells of the adrenal cortex and gonads, as well as in neurons of the ventromedial nucleus of the hypothalamus. Histological markers of the fetal adrenal zone (or X-zone in rodents) were still present in adult males and postpartum SF1 Socs3 KO females, suggesting a previously unrecognized effect of SOCS3 on the terminal differentiation of the adrenal gland. This change led to a distinct distribution of lipid droplets along the adrenal cortex. Under basal conditions, adult SF1 Socs3 KO mice exhibited similar adrenal weight, and plasma ACTH and corticosterone concentrations. Nonetheless, SF1 Socs3 KO mice exhibited a blunted ACTH-induced corticosterone secretion. The overall metabolic responses induced by resistance training remained unaffected in SF1 Socs3 KO mice, including changes in body adiposity, glucose tolerance and energy expenditure. However, training performance and glucose control during intense resistance exercise were impaired in SF1 Socs3 KO mice. Furthermore, a reduced counter-regulatory response to 2-deoxy-d-glucose was observed in mutant mice. These findings revealed a novel participation of SOCS3 regulating several endocrine and metabolic aspects. Therefore, cytokine signaling in SF1 cells exerts an important role to sustain training performance possibly by promoting the necessary metabolic adjustments during exercise.

Dysregulation of SOCS-Mediated Negative Feedback of Cytokine Signaling in Carcinogenesis and Its Significance in Cancer Treatment

Suppressor of cytokine signaling (SOCS) proteins are major negative feedback regulators of cytokine signaling mediated by the Janus kinase (JAK)-signal transducer and activator of transcription signaling pathway. In particular, SOCS1 and SOCS3 are strong inhibitors of JAKs and can play pivotal roles in the development and progression of cancers. The abnormal expression of SOCS1 and SOCS3 in cancer cells is associated with the dysregulation of cell growth, migration, and death induced by multiple cytokines and hormones in human carcinomas. In addition, the mechanisms involved in SOCS1- and SOCS3-regulated abnormal development and activation of immune cells in carcinogenesis, including T cells, macrophages, dendritic cells, and myeloid-derived suppressor cells, are still unclear. Therefore, this study aims to further discuss the molecules and signal pathways regulating the expression and function of SOCS1 and SOCS3 in various types of cancers and elucidate the feasibility and efficiency of SOCS-based target therapeutic strategy in anticancer treatment.

Maternal dietary restriction during the periconceptional period in normal-weight or obese ewes results in adrenocortical hypertrophy, an up-regulation of the JAK/STAT and down-regulation of the IGF1R signaling pathways in the adrenal of the postnatal lamb

Maternal dietary restriction during the periconceptional period results in an increase in adrenal growth and in the cortisol stress response in the offspring. The intraadrenal mechanisms that result in the programming of these changes are not clear. Activation of the IGF and the signal transducer and activator of transcription (STAT)/suppressors of cytokine signaling (SOCS) pathways regulate adrenal growth. We have used an embryo transfer model in sheep to investigate the impact of exposure to either dietary restriction in normal or obese mothers or to maternal obesity during the periconceptional period on adrenal growth and function in the offspring. We assessed the adrenal abundance of key signaling molecules in the IGF-I and Janus kinase/STAT/SOCS pathways including IGF-I receptor, IGF-II receptor, Akt, mammalian target of rapamycin, ribosomal protein S6, eukaryotic translation initiation factor 4E-binding protein 1, eukaryotic translation initiation factor 4E, STAT1, STAT3, STAT5, SOCS1, and SOCS3 in female and male postnatal lambs. Maternal dietary restriction in the periconceptional period resulted in the hypertrophy of the adrenocortical cells in the zona fasciculata-reticularis and an up-regulation in STAT1, phospho-STAT1, and phospho-STAT3 (Ser727) abundance and a down-regulation in IGF-I receptor, Akt, and phospho-Akt abundance in the adrenal cortex of the postnatal lamb. These studies highlight that weight loss around the time of conception, independent of the starting maternal body weight, results in the activation of the adrenal Janus kinase/STAT pathway and adrenocortical hypertrophy. Thus, signals of adversity around the time of conception have a long-term impact on the mechanisms that regulate adrenocortical growth.

Transcriptional down-regulation of suppressor of cytokine signaling (SOCS)-3 in chronic obstructive pulmonary disease

BACKGROUND

Tobacco is a leading environmental factor in the initiation of respiratory diseases and causes chronic obstructive pulmonary disease (COPD). Suppressor of cytokine signaling (SOCS) family members are involved in the pathogenesis of many inflammatory diseases and SOCS-3 has been shown to play an important role in the regulation, onset and maintenance of airway allergic inflammation indicating that SOCS-3 displays a potential therapeutic target for anti-inflammatory respiratory drugs development. Since chronic obstructive pulmonary disease (COPD) is also characterized by inflammatory changes and airflow limitation, the present study assessed the transcriptional expression of SOCS-3 in COPD.

METHODS

Real-time PCR was performed to assess quantitative changes in bronchial biopsies of COPD patients in comparison to unaffected controls.

RESULTS

SOCS-3 was significantly down-regulated in COPD at the transcriptional level while SOCS-4 and SOCS-5 displayed no change.

CONCLUSIONS

It can be concluded that the presently observed inhibition of SOCS-3 mRNA expression may be related to the dysbalance of cytokine signaling observed in COPD.

SOCS3-mediated blockade reveals major contribution of JAK2/STAT5 signaling pathway to lactation and proliferation of dairy cow mammary epithelial cells in vitro

Suppressor of cytokine signaling 3 (SOCS3) is a cytokine-induced negative feedback-loop regulator of cytokine signaling. More and more evidence has proved it to be an inhibitor of signal transducers and activators of transcription 5 (STAT5). Here, we used dairy cow mammary epithelial cells (DCMECs) to analyze the function of SOCS3 and the interaction between SOCS3 and STAT5a. The expression of SOCS3 was found in cytoplasm and nucleus of DCMECs by fluorescent immunostaining. Overexpression and inhibition of SOCS3 brought a remarkable milk protein synthesis change through the regulation of JAK2/STAT5a pathway activity, and SOCS3 expression also decreased SREBP-1c expression and fatty acid synthesis. Inhibited STAT5a activation correlated with reduced SOCS3 expression, which indicated that SOCS3 gene might be one of the targets of STAT5a activation, DCMECs treated with L-methionine (Met) resulted in a decrease of SOCS3 expression. SOCS3 could also decrease cell proliferation and viability by CASY-TT detection. Together, our findings indicate that SOCS3 acts as an inhibitor of JAK2/STAT5a pathway and disturbs fatty acid synthesis by decreasing SREBP-1c expression, which validates its involvement in both milk protein synthesis and fat synthesis. In aggregate, these results reveal that low SOCS3 expression is required for milk synthesis and proliferation of DCMECs in vitro.

Region-, neuron-, and signaling pathway-specific increases in prolactin responsiveness in reproductively experienced female rats

Pregnancy and lactation cause long-lasting enhancements in maternal behavior and other physiological functions, along with increased hypothalamic prolactin receptor expression. To directly test whether reproductive experience increases prolactin responsiveness in the arcuate, paraventricular, and supraoptic nuclei and the medial preoptic area, female rats experienced a full pregnancy and lactation or remained as age-matched virgin controls. At 5 wk after weaning, rats received 2.5, 100, or 4000 ng ovine prolactin or vehicle intracerebroventricularly. The brains underwent immunohistochemistry for the phosphorylated forms of signal transducer and activator of transcription 5 (pSTAT5) or ERK1/2 (pERK1/2). There was a marked increase in pSTAT5 and pERK1/2 in response to prolactin in the regions examined in both virgin and primiparous rats. Primiparous rats exhibited approximately double the number of prolactin-induced pSTAT5-immunoreactive cells as virgins, this effect being most apparent at the higher prolactin doses in the medial preoptic area and paraventricular and supraoptic nuclei and at the lowest prolactin dose in the arcuate nucleus. Dual-label immunohistochemistry showed that arcuate kisspeptin (but not oxytocin or dopamine) neurons displayed increased sensitivity to prolactin in reproductively experienced animals; these neurons may contribute to the reduction in prolactin concentration observed after reproductive experience. There was no effect of reproductive experience on prolactin-induced pERK1/2, indicating a selective effect on the STAT5 pathway. These data show that STAT5 responsiveness to prolactin is enhanced by reproductive experience in multiple hypothalamic regions. The findings may have significant implications for understanding postpartum disorders affecting maternal care and other prolactin-associated pathologies.

Higher expression levels of SOCS 1,3,4,7 are associated with earlier tumour stage and better clinical outcome in human breast cancer

BACKGROUND

Suppressors of cytokine signaling (SOCS) are important negative feedback regulators of the JAK/STAT signaling pathway, and have been recently investigated for their role in the development of different cancers. In this study, we examined the expression of SOCS1-7 genes in normal and breast cancer tissue and correlated this with several clinico-pathological and prognostic factors.

METHODS

SOCS1-7 mRNA extraction and reverse transcription were performed on fresh frozen breast cancer tissue samples (n = 127) and normal background breast tissue (n = 31). Transcript levels of expression were determined using real-time PCR and analyzed against TNM stage, tumour grade and clinical outcome over a 10 year follow-up period.

RESULTS

SOCS1,4,5,6 and 7 expression decreased with increased TNM stage (TNM1 vs. TNM3 p = 0.039, TNM1 vs. TNM4 p = 0.016, TNM2 vs. TNM4 p = 0.025, TNM1 vs. TNM3 p = 0.012, and TNM1 vs. TNM3 p = 0.044 respectively). SOCS2 and 3 expression decreased with increased Nottingham Prognostic Index (NPI) (NPI1 vs. NPI3 p = 0.033, and NPI2 vs. NPI3 p = 0.041 respectively). SOCS7 expression decreased with higher tumour grade (Grade 3 vs. Grade 2 p = 0.037). After a median follow up period of 10 years, we found higher levels of SOCS1,2 and 7 expression among those patients who remained disease-free compared to those who developed local recurrence (p = 0.0073, p = 0.021, and p = 0.039 respectively). Similarly, we found higher levels of SOCS 2,4, and 7 expression in those who remained disease-free compared to those who developed distant recurrence (p = 0.022, p = 0.024, and p = 0.033 respectively). Patients who remained disease-free had higher levels of SOCS1 and 2 expression compared to those who died from breast cancer (p = 0.02 and p = 0.033 respectively). The disease free survival (DFS) and overall survival (OS) curves showed that higher levels of SOCS1, 3 and 7 were significant predictors of higher DFS (p = 0.015, p = 0.024 and 0.03 respectively) and OS (p = 0.005, p = 0.013 and p = 0.035 respectively). Higher levels of SOCS 4 were significant in predicting better OS (p = 0.007) but not DFS. Immunohistochemical staining of representative samples showed a correlation between SOCS1, 3, 7 protein staining and the SOCS1, 3, 7 mRNA expression.

CONCLUSION

Higher mRNA expression levels of SOCS1, 3, 4 and 7 are significantly associated with earlier tumour stage and better clinical outcome in human breast cancer.

Ovarian hormones are not required for PRL-induced mammary tumorigenesis, but estrogen enhances neoplastic processes

Epidemiologic studies have demonstrated that increased prolactin (PRL) exposure raises the risk of invasive estrogen receptor alpha (ERalpha)-positive breast cancer in women. However, the mechanism(s) whereby this occurs and the interactions with estrogen itself in this disease remain poorly understood. In order to investigate the role of ovarian hormones in the disease process, we employed a transgenic model neu-related lipocalin (NRL)-PRL in which transgenic PRL is directed to mammary epithelial cells by the PRL- and estrogen-insensitive NRL promoter, mimicking the endogenous PRL expression within the breast observed in women. This high local exposure leads to mammary lesion development and eventually carcinomas. Ovariectomy (ovx), shortly after puberty, did not alter the incidence or latency of PRL-induced mammary carcinomas, consistent with the independence of PRL from circulating estrogens as a risk factor for invasive breast cancer in women. However, chronic estrogen administration to ovx NRL-PRL females decreased the latency of both ERalpha-positive and -negative tumors. We identified multiple mechanisms that may underlie this observation. Elevated estrogen exposure cooperated with PRL to increase epithelial proliferation and myoepithelial abnormalities, increasing the incidence of preneoplastic lesions. Critical components of the extracellular matrix secreted by the myoepithelium were reduced with age, and transgenic PRL raised transcripts for tenascin-C and maspin, both associated with tumor progression and poor prognosis in subclasses of clinical breast tumors. Mammary pERK1/2 and pAkt, but not phosphorylated Stat5, were markedly elevated by local PRL. Together, these findings indicate that PRL employs multiple mechanisms to promote mammary tumorigenesis.

Hypothalamic prolactin receptor messenger ribonucleic acid levels, prolactin signaling, and hyperprolactinemic inhibition of pulsatile luteinizing hormone secretion are dependent on estradiol

Hyperprolactinemia can reduce fertility and libido. Although central prolactin actions are thought to contribute to this, the mechanisms are poorly understood. We first tested whether chronic hyperprolactinemia inhibited two neuroendocrine parameters necessary for female fertility: pulsatile LH secretion and the estrogen-induced LH surge. Chronic hyperprolactinemia induced by the dopamine antagonist sulpiride caused a 40% reduction LH pulse frequency in ovariectomized rats, but only in the presence of chronic low levels of estradiol. Sulpiride did not affect the magnitude of a steroid-induced LH surge or the percentage of GnRH neurons activated during the surge. Estradiol is known to influence expression of the long form of prolactin receptors (PRL-R) and components of prolactin's signaling pathway. To test the hypothesis that estrogen increases PRL-R expression and sensitivity to prolactin, we next demonstrated that estradiol greatly augments prolactin-induced STAT5 activation. Lastly, we measured PRL-R and suppressor of cytokine signaling (SOCS-1 and -3 and CIS, which reflect the level of prolactin signaling) mRNAs in response to sulpiride and estradiol. Sulpiride induced only SOCS-1 in the medial preoptic area, where GnRH neurons are regulated, but in the arcuate nucleus and choroid plexus, PRL-R, SOCS-3, and CIS mRNA levels were also induced. Estradiol enhanced these effects on SOCS-3 and CIS. Interestingly, estradiol also induced PRL-R, SOCS-3, and CIS mRNA levels independently. These data show that GnRH pulse frequency is inhibited by chronic hyperprolactinemia in a steroid-dependent manner. They also provide evidence for estradiol-dependent and brain region-specific regulation of PRL-R expression and signaling responses by prolactin.

Use it or lose it: Enhancing milk production efficiency by frequent milking of dairy cows1

In the past century, great strides have been made toward optimizing milk production efficiency of dairy cows. One of the key findings that has emerged is that the milk yield of dairy cows is responsive to demands of offspring or milk removal; hence, milk production can be increased by frequent milking. Early studies illustrated the galactopoietic effect of frequent milking during the entire lactation, with 3 times daily milking increasing milk yield by up to 20% relative to twice daily milking. Later studies reported that cows produced more milk during the entire lactation if they were allowed to suckle a calf for the first 3 to 4 mo of lactation. The results of these experiments laid the groundwork for current research, which has identified a time during early lactation wherein the mammary gland of dairy cows is especially receptive to the stimulus of frequent milking. This window of time has been slowly whittled down from the first 10 wk of lactation to the first 6 wk, and it was subsequently established that frequent milking for a short duration within the first 3 wk of lactation can increase milk production through the remainder of lactation [corrected] In addition, there is strong evidence that this milk yield response is locally regulated. Consequently, the concept of "use it or lose it" is becoming more clearly established; that is, the stimulus of frequent milking during early lactation permanently increases the milk production capacity of the mammary gland. Exciting research opportunities now present themselves, and ongoing experiments seek to identify the local factors that are involved in the regulation of milk production efficiency of dairy cows.

Effects of short day photoperiod on prolactin signaling in dry cows: a common mechanism among tissues and environments?

Photoperiodic manipulation has dramatic physiological and production effects in dairy cows. During lactation, exposure to long day photoperiod (LDPP) increases milk yield and circulating IGF-I and prolactin (PRL) concentrations. Conversely, cows housed under a short day photoperiod (SDPP) during the dry period produce more milk in the subsequent lactation than cows exposed to LDPP or natural photoperiod. Exposure to SDPP depresses PRL secretion but increases PRL receptor mRNA levels in mammary, immune, and hepatic tissues. In dry cows under SDPP, PRL signaling is a potential mechanism to drive more extensive mammary cell differentiation and growth relative to LDPP. In mammary biopsies taken during the dry period and into lactation, the amount of IGF-II mRNA was greater in SDPP vs. LDPP cows during the dry period, whereas IGFBP-5 mRNA increased in both groups during lactation even though photoperiodic treatments ended at parturition and all cows were on an ambient lighting schedule when lactating. Levels of IGF-I mRNA did not differ over time or between treatments; however, during the dry period, lower IGFBP-5 and increased IGF-II expression in SDPP cows may enhance mammary cell growth and survival. Key among the potential modulators of PRL signaling is the suppressors of cytokine signaling (SOCS) family. Mammary transcription of mRNA for SOCS proteins was low during the dry period but increased in lactation. During the dry period, SOCS mRNA level in the mammary gland of cows on SDPP was reduced compared with cows on LDPP, which may enhance PRL-induced proliferation and subsequent milk production. However, improved mammary capacity and immune function alone are likely insufficient to support increased milk yield. Using improved milk yield as a functional indicator of greater animal well-being during the transition, it is clear that some metabolic accommodation is necessary for expression of that capacity. Emerging evidence supports a link between PRL signaling and hepatic lipid metabolism, with decreases in PRL being beneficial to lipid metabolism. Extending that concept to broad environmental responses, it can be speculated that altered PRL signaling impairs lipid metabolism, mammary growth, and immune function under conditions of stress (e.g., heat stress) also. Thus, shifts in gene expression related to PRL signaling may provide an environmentally mediated mechanism to alter production and health in cows as they transition into lactation.

Characterization of the SOCS3 promoter response to prostaglandin E2 in T47D cells

Suppressor of cytokine signaling 3 (SOCS3), a negative regulator of cytokine signaling, is expressed in breast cancer cells where it can modify sensitivity and responsiveness to cytokine signaling through the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathways. Although it is widely accepted that SOCS3 expression is in itself regulated by STATs, we and others have shown that prostaglandins can also up-regulate SOCS3 expression. Here we used T47D breast cancer cells treated with prostaglandin E2 (PGE2) to examine this pathway. T47D cells responded to PGE2 stimulation with a significant increase in SOCS3 mRNA that was independent of de novo protein synthesis. PGE2 stimulation resulted in STAT3 serine and tyrosine phosphorylation, although mutation of either of the two previously characterized STAT response elements on the SOCS3 promoter did not affect SOCS3 promoter activation by PGE2. In addition, overexpression of STAT3 wild-type, constitutively active or dominant-negative constructs did not affect PGE2-induced SOCS3 promoter activation, indicating that STATs are unlikely mediators of this pathway in these cells. PGE2 is a known activator of the cAMP/protein kinase A (PKA) pathway, and in T47D cells, up-regulation of SOCS3 mRNA by PGE2 was abolished by pretreatment with H89, a PKA inhibitor and increased by cAMP and forskolin treatment. Consistent with this, PGE2 treatment increased cAMP response element (CRE)-binding protein serine phosphorylation. However, mutation of the activator protein 1/CRE on the promoter did not affect basal or PGE2-stimulated activation, suggesting a role for cAMP/PKA that is independent of CRE-binding protein binding. Mutation of the GC-rich region of the SOCS3 promoter, a putative Sp1/Sp3 binding site, abolished both basal and PGE2-stimulated activation. Gel-shift assays showed increased complex formation after treatment, and this was inhibited by the addition of an Sp1 antibody or pretreatment with PKA inhibitor. Chromatin immunoprecipitation assay verified Sp1 binding to the promoter in response to PGE2. Sp1 overexpression increased SOCS3 promoter activation, and both basal and PGE2-induced SOCS3 mRNA expression was prevented by mithramycin, an inhibitor of Sp1 DNA binding. Finally, a physiological role for PGE2 was demonstrated with PGE2 pretreatment reducing lipopolysaccharide-induced STAT3 activation. Collectively, this study details a novel mechanism of SOCS3 up-regulation by PGE2 in breast cancer cells that appears to be STAT independent and involve Sp1 binding to the promoter. This process has possible implications for cytokine responsiveness and tumor progression.

The declining phase of lactation: peripheral or central, programmed or pathological?

In most species the functional activity of the mammary gland during lactation follows a biphasic developmental pattern. This pattern starts with a rapid increase in milk output that occurs with secretory activation and continues with a more gradual increase until the point of peak lactation is reached. Following this gain-of-function phase, the ability of the gland to produce milk decreases. This decrease occurs even if the lactation is prolonged by the presence of continued suckling stimulus and complete milk removal. This review describes the current state of our knowledge concerning the factors that regulate milk synthesis capacity by the mammary gland during the lactation cycle. The review describes four potential alternatives as mechanisms governing the process, which we refer to as secretory diminution. These alternatives are not presented as mutually exclusive of each other or other possible mechanisms, but are proposed as potential contributing mechanisms.

Opposing roles of gp130-mediated STAT-3 and ERK-1/ 2 signaling in liver progenitor cell migration and proliferation

Gp130-mediated IL-6 signaling may play a role in oval cell proliferation in vivo. Levels of IL-6 are elevated in livers of mice treated with a choline-deficient ethionine-supplemented (CDE) diet that induces oval cells, and there is a reduction of oval cells in IL-6 knockout mice. The CDE diet recapitulates characteristics of chronic liver injury in humans. In this study, we determined the impact of IL-6 signaling on oval cell-mediated liver regeneration in vivo. Signaling pathways downstream of gp130 activation were also dissected. Numbers of A6(+ve) liver progenitor oval cells (LPCs) in CDE-treated murine liver were detected by immunohistochemistry and quantified. Levels of oval cell migration and proliferation were compared in CDE-treated mouse strains that depict models of gp130-mediated hyperactive ERK-1/2 signaling (gp130(deltaSTAT)), hyperactive STAT-3 signaling (gp130(Y757F) and Socs-3(-/deltaAlb)) or active ERK-1/2 as well as active STAT-3 signaling (wild-type). The A6(+ve) LPC numbers were increased with IL-6 treatment in vivo. The gp130(Y757F) mice displayed increased A6(+ve) LPCs numbers compared with wild-type and gp130(deltaSTAT) mice. Numbers of A6(+ve) LPCs were also increased in the livers of CDE treated Socs-3(-/deltaAlb) mice compared with their control counterparts. Lastly, inhibition of ERK-1/2 activation in cultured oval cells increased hyper IL-6-induced cell growth. For the first time, we have dissected the gp130-mediated signaling pathways, which influence liver progenitor oval cell proliferation.

CONCLUSION

Hyperactive STAT-3 signaling results in enhanced oval cell numbers, whereas ERK-1/2 activation suppresses oval cell proliferation.

Mammary Response to Exogenous Prolactin or Frequent Milking During Early Lactation in Dairy Cows

Frequent milking of dairy cows during early lactation results in a persistent increase in milk yield; however, the mechanism underlying this effect is unknown. We hypothesized that increased exposure of the mammary gland to prolactin (PRL) mediates the milk yield response. Fifteen multiparous Holstein cows were assigned to 3 treatments for the first 3 wk of lactation: twice daily milking with (2x + PRL) or without (2x) supplemental exogenous PRL, or 4 times daily milking (4x). Mammary biopsies were obtained at 7 DIM, and rates of [(3)H]thymidine incorporation into DNA in vitro were determined. Mammary expression of suppressors of cytokine signaling (SOCS)-1, -2, and -3; the long form of PRL-receptor; and alpha-lactalbumin mRNA was measured by real-time reverse-transcription PCR. Incorporation of [(3)H]thymidine into DNA was not affected by frequent milking or PRL treatment; however, analysis of autoradiograms revealed that stromal cell proliferation was greater in 4x cows. Mammary expression of SOCS-1 was not affected by milking frequency or PRL treatment. Expression of SOCS-2 mRNA was increased with frequent milking or PRL treatment, whereas expression of SOCS-3 mRNA was reduced by frequent milking or exogenous PRL. Abundance of PRL-receptor mRNA was reduced, whereas alpha-lactalbumin mRNA was increased with PRL treatment. These results demonstrate that the bovine mammary gland is responsive to exogenous PRL during early lactation. In addition, differences in the response to frequent milking or exogenous PRL during early lactation indicate distinct effects of PRL and milk removal on the mammary function of dairy cows.

c-myc as a mediator of accelerated apoptosis and involution in mammary glands lacking Socs3

Suppressor of cytokine signalling (SOCS) proteins are critical attenuators of cytokine-mediated signalling in diverse tissues. To determine the importance of Socs3 in mammary development, we generated mice in which Socs3 was deleted in mammary epithelial cells. No overt phenotype was evident during pregnancy and lactation, indicating that Socs3 is not a key physiological regulator of prolactin signalling. However, Socs3-deficient mammary glands exhibited a profound increase in epithelial apoptosis and tissue remodelling, resulting in precocious involution. This phenotype was accompanied by augmented Stat3 activation and a marked increase in the level of c-myc. Moreover, induction of c-myc before weaning using an inducible transgenic model recapitulated the Socs3 phenotype, and elevated expression of likely c-myc target genes, E2F-1, Bax and p53, was observed. Our data establish Socs3 as a critical attenuator of pro-apoptotic pathways that act in the developing mammary gland and provide evidence that c-myc regulates apoptosis during involution.

Prolactin and the expression of suppressor of cytokine signaling-3 in the sheep adrenal gland before birth

The fetal pituitary-adrenal axis plays a key role in the fetal response to intrauterine stress and in the timing of parturition. The fetal sheep adrenal gland is relatively refractory to stimulation in midgestation (90–120 days) before the prepartum activation, which occurs around 135 days gestation (term = 147 ± 3 days). The mechanisms underlying the switch from adrenal quiescence to activation are unclear. Therefore, we have investigated the expression of suppressor of cytokine signaling-3 (SOCS-3), a putative inhibitor of tissue growth in the fetal sheep adrenal between 50 and 145 days gestation and in the adrenal of the growth-restricted fetal sheep in late gestation. SOCS-3 is activated by a range of cytokines, including prolactin (PRL), and we have, therefore, determined whether PRL administered in vivo or in vitro stimulates SOCS-3 mRNA expression in the fetal adrenal in late gestation. There was a decrease ( P < 0.005) in SOCS-3 expression in the fetal adrenal between 54 and 133 days and between 141 and 144 days gestation. Infusion of the dopaminergic agonist, bromocriptine, which suppressed fetal PRL concentrations but did not decrease adrenal SOCS-3 mRNA expression. PRL administration, however, significantly increased adrenal SOCS-3 mRNA expression ( P < 0.05). Similarly, there was an increase ( P < 0.05) in SOCS-3 mRNA expression in adrenocortical cells in vitro after exposure to PRL (50 ng/ml). Placental and fetal growth restriction had no effect on SOCS-3 expression in the adrenal during late gestation. In summary, the decrease in the expression of the inhibitor SOCS-3 after 133 days gestation may be permissive for a subsequent increase in fetal adrenal growth before birth. We conclude that factors other than PRL act to maintain adrenal SOCS-3 mRNA expression before 133 days gestation but that acute elevations of PRL can act to upregulate adrenal SOCS-3 expression in the sheep fetus during late gestation.

Suppression of prolactin-induced signal transducer and activator of transcription 5b signaling and induction of suppressors of cytokine signaling messenger ribonucleic acid in the hypothalamic arcuate nucleus of the rat during late pregnancy and lactation

During late pregnancy and lactation, the tuberoinfundibular dopamine (TIDA) neurons that regulate prolactin secretion by negative feedback become less able to produce dopamine in response to prolactin, leading to hyperprolactinemia. Because prolactin-induced activation of dopamine synthesis in these neurons requires the Janus kinase/signal transducer and activator of transcription 5b (STAT5b) signaling pathway, we investigated whether prolactin-induced STAT5b signaling is reduced during lactation and whether induction of suppressors of cytokine signaling (SOCS) mRNAs occur at this time and in late pregnancy. During lactation, the ability of exogenous prolactin to induce STAT5 phosphorylation and STAT5b nuclear translocation was markedly reduced when compared with diestrous rats. In nonpregnant female rats, acute treatment with ovine prolactin markedly increased levels of SOCS-1 and -3 and cytokine-inducible SH2-containing protein mRNA in arcuate nucleus micropunches. On gestation d 22, SOCS-1 and SOCS-3 mRNA levels were 10-fold that on G20. SOCS-1 and -3 and cytokine-inducible SH2-containing protein mRNA levels were also elevated on lactation d 7. At these times, dopaminergic activity was decreased and the rats were hyperprolactinemic. The high levels of SOCS mRNA were prevented by bromocriptine pretreatment (gestation d 22) or pup removal (lactation d 7), which suppressed circulating prolactin to basal levels. These results demonstrate that around the end of pregnancy, prolactin loses the ability to activate STAT5b, associated with an increase in SOCS mRNAs. The loss of this stimulating pathway may underlie the reduced tuberoinfundibular dopamine neuron dopamine output and hyperprolactinemia that characterizes late pregnancy and lactation. The high maternal levels of SOCS mRNAs appear to be dependent on prolactin, presumably acting through an alternative signaling pathway to STAT5b.

Reproductive experience increases prolactin responsiveness in the medial preoptic area and arcuate nucleus of female rats

The experience of pregnancy plus lactation produces long-term enhancements in maternal behavior as well as reduced secretion of prolactin, a key hormone for the initial establishment of maternal care. Given that prolactin acts centrally to induce maternal care as well as regulate its own secretion, we tested whether prolactin receptors in brain regions known to regulate behavioral and neuroendocrine processes were up-regulated and more responsive to prolactin in reproductively experienced females. Diestrous primiparous (8 wk after weaning) and age-matched virgin rats were treated with 250 microg ovine prolactin sc or vehicle and the brains collected 2 h later for measurement of mRNA for genes involved in prolactin signaling. Reproductively experienced rats had lower serum prolactin concentrations, compared with virgin rats, suggesting enhanced prolactin feedback on the arcuate neurons regulating prolactin secretion. In the medial preoptic area and arcuate nucleus (regions involved in regulating maternal behavior and prolactin secretion, respectively), the level of long-form prolactin receptor mRNA was higher in primiparous rats, and prolactin treatment induced a further increase in receptor expression in these animals. In the same regions, suppressors of cytokine signaling-1 and -3 mRNA levels were also markedly increased after prolactin treatment in reproductively experienced but not virgin rats. These results support the idea that reproductive experience increases central prolactin responsiveness. The induction of prolactin receptors and enhanced prolactin responsiveness as a result of pregnancy and lactation may help account for the retention of maternal behavior and shifts in prolactin secretion in reproductively experienced females.

The Genes Induced by Signal Transducer and Activators of Transcription (STAT)3 and STAT5 in Mammary Epithelial Cells Define the Roles of these STATs in Mammary Development

Prolactin and leukemia inhibitory factor (LIF) have different roles in the adult mammary gland, which are mediated in part by the signal transducers and activators of transcription (STAT)5 and STAT3. In vivo studies have shown that STAT5 contributes to prolactin-dependent lobuloalveolar development and lactation whereas STAT3 mediates LIF-dependent epithelial apoptosis during postlactational involution. To understand the molecular basis of these STAT-dependent pathways, we demonstrate the ligand-independent effects of STAT5 and STAT3 in mammary epithelial cells in vitro and also identify the genes regulated by these related transcription factors. Thus, using conditionally active STAT3- or STAT5a-GyraseB fusion proteins, we observed that enforced and specific dimerization of STAT3 induced apoptosis whereas STAT5 induced differentiation of mammary epithelial cells. Furthermore, STAT5 attenuated apoptosis mediated by LIF, the physiological inducer of STAT3. Microarray analysis of STAT3- and STAT5-induced genes using this system demonstrated a marked specificity, which reflected their different physiological effects in vitro and in vivo. STAT5-specific gene targets included the milk protein genes alpha-casein and kallikrein-8 and the survival factors prosaposin and Grb10. STAT3-specific genes included the apoptosis regulators CCAAT enhancer binding protein-delta, phosphatidylinositol 3-kinase-regulatory subunits, purine nucleoside phosphorylase, and c-fos. These data illustrate that specific activation of STAT3 and STAT5 alone is sufficient to induce and suppress apoptosis, respectively, and that these transcription factors elicit their actions by inducing distinct subsets of target genes in mammary epithelial cells.

Mechanisms underlying the diminished sensitivity to prolactin negative feedback during lactation: reduced STAT5 signaling and up-regulation of cytokine-inducible SH2 domain-containing protein (CIS) expression in tuberoinfundibular dopaminergic neurons

Hyperprolactinaemia during lactation is a consequence of the sucking stimulus and in part due to reduced prolactin (PRL) negative feedback. To date, the mechanisms involved in this diminished sensitivity to PRL feedback are unknown but may involve changes in PRL signal transduction within tuberoinfundibular dopaminergic (TIDA) neurons. Therefore, we investigated signal transducers and activators of transcription (STAT) 5 signaling in the TIDA neurons of lactating rats. Dual-label confocal immunofluorescence studies were used to determine the intracellular distribution of STAT5 within TIDA neurons in the dorsomedial arcuate nucleus. In lactating rats with pups removed for 16 h, injection of ovine PRL significantly (P < 0.05) increased the STAT5 nuclear/cytoplasmic ratio compared with vehicle-treated mothers. In contrast, ovine PRL injection did not increase the STAT5 nuclear/cytoplasmic ratio in lactating mothers with pups, demonstrating that PRL signal transduction through STAT5 is reduced in TIDA neurons in the presence of pups. To investigate possible mechanisms involved in reduced PRL signaling, we examined the expression of suppressors of cytokine signaling (SOCS) proteins. Northern analysis on whole hypothalamus showed that CIS (cytokine-inducible SH2 domain-containing protein), but not SOCS1 or SOCS3, mRNA expression was significantly (P < 0.01) up-regulated in suckled lactating rats. Semiquantitative RT-PCR on arcuate nucleus micropunches also showed up-regulation of CIS transcripts. Immunofluorescence studies demonstrated that CIS is expressed in all TIDA neurons in the dorsomedial arcuate nucleus, and the intensity of CIS staining in these neurons is significantly (P < 0.05) increased in lactating rats with sucking pups. Together, these results support the hypothesis that loss of sensitivity to PRL-negative feedback during lactation is a result of increased CIS expression in TIDA neurons.

Differential regulation of suppressor of cytokine signaling-3 in the liver and adipose tissue of the sheep fetus in late gestation

It is unknown whether the JAK/STAT/suppressor of cytokine signaling-3 (SOCS-3) intracellular signaling pathway plays a role in tissue growth and metabolism during fetal life. We investigated whether there is a differential profile of SOCS-3 expression in the liver and perirenal adipose tissue during the period of increased fetal growth in late gestation and the impact of fetal growth restriction on SOCS-3 expression in the fetal liver. We also determined whether basal SOCS-3 expression in the fetal liver and perirenal adipose tissue is regulated by endogenous fetal prolactin (PRL). SOCS-3 mRNA abundance was higher in the liver than in the pancreas, spleen, and kidney of the sheep fetus during late gestation. In the liver, SOCS-3 mRNA expression was increased (P < 0.05) between 125 (n = 4) and 145 days (n = 7) gestation and lower (P < 0.05) in growth-restricted compared with normally grown fetal sheep in late gestation. The relative expression of SOCS-3 mRNA in the fetal liver was directly related to the mean plasma PRL concentrations during a 48-h infusion of either a dopaminergic agonist, bromocriptine (n = 7), or saline (n = 5), such that SOCS-3 mRNA expression was lower when plasma PRL concentrations decreased below approximately 20 ng/ml [y = 0.99 - (2.47/x) + (4.96/x(2)); r(2) = 0.91, P < 0.0001, n = 12]. No relationship was shown between the abundance of phospho-STAT5 in the fetal liver and circulating PRL. SOCS-3 expression in perirenal adipose tissue decreased (P < 0001) between 90-91 (n = 6) and 140-145 days (n = 9) gestation and was not related to endogenous PRL concentrations. Thus SOCS-3 is differentially expressed and regulated in key fetal tissues and may play an important and tissue-specific role in the regulation of cellular proliferation and differentiation before birth.

SOCS3 promotes apoptosis of mammary differentiated cells

Growth and function of the mammary gland is regulated by cytokines and modulated by suppressor of cytokine signalling (SOCS) proteins. In vitro experiments demonstrated that SOCS3 can inhibit PRL induction of milk protein gene expression and STAT5 activation. We explored the SOCS3 expression pattern during mouse mammary development and its regulation by PRL and GH in wild-type and STAT5a-null mammary tissue. Our results suggest that, in vivo, PRL stimulates SOCS3 expression in stromal adipocytes, independently of STAT5a stimulation. In mammary epithelial cells, SOCS3 expression appears to be related to STAT3 activation. Together, our results are consistent with a role of SOCS3 in the mammary gland by promoting apoptosis of differentiated cells (adipocytes during gestation and epithelial cells during involution).

Short Communication: Short-Day Photoperiod During the Dry Period Decreases Expression of Suppressors of Cytokine Signaling in Mammary Gland of Dairy Cows

Suppressors of cytokine signaling (SOCS) are induced by prolactin and act through negative feedback to inhibit cytokine signaling. We hypothesized that lower prolactin concentrations in cows exposed to short-day photoperiod (SD; 8 h light:16 h dark) lead to decreased expression of SOCS, which mediate the effects of SD on mammary proliferation during the dry period. Multiparous Holstein cows were dried off 60 d before expected calving and were assigned to long-day photoperiod (LD; 16 h light:8 h dark) or SD during the dry period. Mammary biopsies were obtained at -40, -20, -10, and +10 d relative to expected calving, and expression of SOCS-1, SOCS-2, SOCS-3, and cytokine-inducible SH2-containing protein (CIS) mRNA was assessed by real-time, quantitative, reverse transcription-PCR. Expression of all SOCS increased over time and expression of SOCS-3, SOCS-2, and CIS mRNA was lower in mammary gland of SD cows. These data suggest that lower SOCS expression in cows exposed to SD during the dry period may enhance prolactin signaling to the mammary gland, thereby augmenting mammary development during pregnancy and milk production in the subsequent lactation. Changes in SOCS expression during pregnancy and lactation imply that SOCS may regulate mammary gland development and function in dairy cows.

STAT 5a expression in various lesions of the breast

The seven signal transducer and activator of transcription (STAT) molecules are effectors of hormonal or cytokine stimulation through receptors. STAT 5a, isolated from prolactin-stimulated mammary cells, contributes to normal proliferation and is essential for mammary gland differentiation. Using a monoclonal antibody, we tested 100 formalin-fixed, paraffin-embedded breast tissues representing everything from simple hyperplasia to invasive carcinoma for the expression of STAT 5a in comparison to normal breast epithelial cells. Immunohistochemical analysis was performed following heat treatment in a pressure cooker. STAT 5a was found in endothelial cells, adipocytes, and leukocytes as well as in the cytoplasm and nucleus of normal epithelial cells, usual ductal hyperplasia, and benign lesions such as fibroadenoma. Myoepithelial cells and stromal fibroblasts failed to demonstrate any STAT 5a in addition to most atypical proliferations including in situ and invasive carcinomas. A few examples of lobular intraepithelial neoplasia and invasive carcinoma demonstrated some reactivity, albeit comparatively reduced. The absence of STAT 5a in the abnormal breast epithelial cells may indicate a defect contributory to the abnormal state.

Differential hypermethylation of SOCS genes in ovarian and breast carcinomas

Suppressor of cytokine signaling (SOCS) proteins have emerged as critical attenuators of cytokine-mediated processes, suggesting a role in the suppression of tumorigenesis. In the ovary and mammary gland, cytokines such as prolactin and IL-6 are important regulators of growth and differentiation. We have investigated whether silencing or inactivation of SOCS genes occurs in ovarian and breast carcinomas. The SOCS1 and SOCS2 CpG islands were found to be hypermethylated in 23 and 14% of primary ovarian cancers, respectively, whereas only SOCS1 was methylated in breast cancers (9%). Methylation of these genes did not occur in normal tissues. No correlation was apparent between methylation and loss of heterozygosity, and no somatic mutations were found in a large panel of carcinomas. Aberrant methylation of these SOCS genes correlated with transcriptional silencing in ovarian and breast cancer cell lines, since expression was induced by the demethylating agent 5-azadeoxycytidine. SOCS3 was not hypermethylated in either cancer type. Consistent with this data, SOCS1 and SOCS2 but not SOCS3 suppressed the growth of ovarian and breast cancer cells. Hypermethylation and silencing of specific SOCS genes in the ovary, and to a lesser extent in breast, may augment cytokine responsiveness in these tissues, thereby contributing to oncogenesis.

Ovarian insufficiency and early pregnancy loss induced by activation of the innate immune system

We describe a murine model of early pregnancy failure induced by systemic activation of the CD40 immune costimulatory pathway. Although fetal loss involved an NK cell intermediate, it was not due to lymphocyte-mediated destruction of the fetus and placenta. Rather, pregnancy failure resulted from impaired progesterone synthesis by the corpus luteum of the ovary, an endocrine defect in turn associated with ovarian resistance to the gonadotropic effects of prolactin. Pregnancy failure also required the proinflammatory cytokine TNF-alpha and correlated with the luteal induction of the prolactin receptor signaling inhibitors suppressor of cytokine signaling 1 (Socs1) and Socs3. Such links between immune activation and reproductive endocrine dysfunction may be relevant to pregnancy loss and other clinical disorders of reproduction.

Identification of Taxreb107 as a lactogenic hormone responsive gene in mammary epithelial cells

Mammary gland development and differentiation is regulated by a number of growth factors and hormones. Milk protein gene expression represents a hallmark of functional mammary epithelial differentiation and is coordinated by the lactogenic hormone prolactin and glucocorticoids. To date, few 'early-response' genes transcriptionally activated by lactogenic hormones have been described. We have used representational difference analysis (RDA) to search for lactogenic-responsive genes in SCp2 mouse mammary epithelial cells. One of the cDNAs identified encoded the DNA-binding protein Taxreb107, originally identified as a HTLV-I Tax responsive element binding protein. Increased Taxreb107 expression was confirmed following prolactin and dexamethasone-induced differentiation of SCp2 and HC11 mammary epithelial cells. Taxreb107 RNA levels were developmentally regulated in the mouse mammary gland, where levels increased substantially during mid- and late pregnancy and persisted during lactation. Overexpression of an antisense Taxreb107 cDNA construct or antisense oligonucleotide in HC11 mammary epithelial cells attenuated milk protein gene expression following prolactin and dexamethasone treatment. These findings indicate a role for Taxreb107 as a lactogenic hormone-responsive gene during differentiation of the mammary gland.

Suppressor of cytokine signalling gene expression is elevated in breast carcinoma

Cytokines are important for breast cell function, both as trophic hormones and as mediators of host defense mechanisms against breast cancer. Recently, inducible feedback suppressors of cytokine signalling (SOCS/JAB/SSI) have been identified, which decrease cell sensitivity to cytokines. We examined the expression of SOCS genes in 17 breast carcinomas and 10 breast cancer lines, in comparison with normal tissue and breast lines. We report elevated expression of SOCS-1-3 and CIS immunoreactive proteins within in situ ductal carcinomas and infiltrating ductal carcinomas relative to normal breast tissue. Significantly increased expression of SOCS-1-3 and CIS transcripts was also shown by quantitative in situ hybridisation within both tumour tissue and reactive stroma. CIS transcript expression was elevated in all 10 cancer lines, but not in control lines. However, there was no consistent elevation of other SOCS transcripts. CIS protein was shown by immunoblot to be present in all cancer lines at increased levels, mainly as the 47 kDa ubiquitinylated form. A potential proliferative role for CIS overexpression is supported by reports that CIS activates ERK kinases, and by strong induction in transient reporter assays with an ERK-responsive promoter. The in vivo elevation of SOCS gene expression may be part of the host/tumour response or a response to autocrine/paracrine GH and prolactin. However, increased CIS expression in breast cancer lines appears to be a specific lesion, and could simultaneously shut down STAT 5 signalling by trophic hormones, confer resistance to host cytokines and increase proliferation through ERK kinases.

A prostaglandin f(2alpha) analog induces suppressors of cytokine signaling-3 expression in the corpus luteum of the pregnant rat: a potential new mechanism in luteolysis

PRL and placental lactogen (PL) play key roles in maintaining the rodent corpus luteum through pregnancy. Suppressors of cytokine signaling (SOCS) have been shown to decrease cell sensitivity to cytokines, including PRL, and so here we have addressed the issue of whether luteolysis induced by prostaglandin F(2alpha) (PGF(2alpha)) might up-regulate SOCS proteins to inhibit PRL signaling. In d 19 pregnant rats, cloprostenol, a PGF(2alpha) analog, rapidly induced transcripts for SOCS-3 and, to a lesser extent, SOCS-1. We also found increased SOCS-3 protein in the ovary by immunoblot and in the corpus luteum by immunohistochemistry. Increased SOCS-3 expression was preceded by an increase in STAT3 tyrosine phosphorylation 10 min after cloprostenol injection and was maintained for 4 h, as determined by gel shift and immunohistochemistry. Induction of SOCS-3 was accompanied by a sharp decrease in active STAT5, as determined by gel-shift assay and by loss of nuclear localized STAT5. Four hours after cloprostenol administration, the corpus luteum was refractory to stimulation of STAT5 by PRL administration, and this was not due to down-regulation of PRL receptor. Therefore, induction of SOCS-3 by PGF(2alpha) may be an important element in the initiation of luteolysis via rapid suppression of luteotropic support from PL.