Immune regulation of corpus luteum function

Possible role of interferon tau on the bovine corpus luteum and neutrophils during the early pregnancy

When pregnancy is established, interferon tau (IFNT), a well-known pregnancy recognition signal in ruminants, is secreted by embryonic trophoblast cells and acts within the uterus to prepare for pregnancy. IFNT acts as an endocrine factor on the corpus luteum (CL) to induce refractory ability against the luteolytic action of PGF2α. Hypothesising that IFNT may influence not only the uterine environment but also the CL in cows via local or peripheral circulation, we investigated qualitative changes in the CL of pregnant cows during the maternal recognition period (day 16) and the CL of non-pregnant cows. The CL of pregnant animals had a higher number of neutrophils, and the expression of interleukin 8 (IL8) mRNA and its protein was higher as well as compared with the CL of non-pregnant animals. Although IFNT did not affect progesterone (P4) secretion and neutrophil migration directly, it stimulated IL8 mRNA expression on luteal cells (LCs), influencing the neutrophils, resulting in the increased migration of IFNT-activated neutrophils. Moreover, both IFNT-activated neutrophils and IL8 increased P4 secretion from LCs in vitro. Our novel finding was the increase in neutrophils and IL8 within the CL of pregnant cows, suggesting the involvement of IFNT function within the CL toward establishment of pregnancy in cows. The present results suggest that IFNT upregulates neutrophil numbers and function via IL8 on LCs in the CL of early pregnant cows and that both neutrophils and IL8, stimulated by IFNT, are associated with an increase in P4 concentrations during the maternal recognition period in cows.

Endocrine and local control of the primate corpus luteum

The primate corpus luteum is a transient endocrine gland that differentiates from the ovulatory follicle midway through the ovarian (menstrual) cycle. Its formation and limited lifespan is critical for fertility, as luteal-derived progesterone is the essential steroid hormone required for embryo implantation and maintenance of intra-uterine pregnancy until the placenta develops. It is well-established that LH and the LH-like hormone, CG, are the vital luteotropic hormones during the menstrual cycle and early pregnancy, respectively. Recent advances, particularly through genome analyses and cellular studies, increased our understanding of various local factors and cellular processes associated with the development, maintenance and repression of the corpus luteum. These include paracrine or autocrine factors associated with angiogenesis (e.g., VEGF), and that mediate LH/CG actions (e.g., progesterone), or counteract luteotropic effects (i.e., local luteolysis; e.g., PGF2α). However, areas of mystery and controversy remain, particularly regarding the signals and events that initiate luteal regression in the non-fecund cycle. Novel approaches capable of gene "knockdown" or amplification", in vivo as well as in vitro, should identify novel or underappreciated gene products that are regulated by or modulate LH/CG actions to control the functional lifespan of the primate corpus luteum. Further advances in our understanding of luteal physiology will help to improve or control fertility for purposes ranging from preservation of endangered primate species to designing novel ovary-based contraceptives and treating ovarian disorders in women.

Emerging roles of immune cells in luteal angiogenesis

In the mammalian ovary, the corpus luteum (CL) is a unique transient endocrine organ displaying rapid angiogenesis and time-dependent accumulation of immune cells. The CL closely resembles 'transitory tumours', and the rate of luteal growth equals that of the fastest growing tumours. Recently, attention has focused on multiple roles of immune cells in luteal function, not only in luteolysis (CL disruption by immune responses involving T lymphocytes and macrophages), but also in CL development (CL remodelling by different immune responses involving neutrophils and macrophages). Neutrophils and macrophages regulate angiogenesis, lymphangiogenesis, and steroidogenesis by releasing cytokines in the CL. In addition, functional polarisation of neutrophils (proinflammatory N1 vs anti-inflammatory N2) and macrophages (proinflammatory M1 vs anti-inflammatory M2) has been demonstrated. This new concept concurs with the phenomenon of immune function within the luteal microenvironment: active development of the CL infiltrating anti-inflammatory N2 and M2 versus luteal regression together with proinflammatory N1 and M1. Conversely, excessive angiogenic factors and leucocyte infiltration result in indefinite disordered tumour development. However, the negative feedback regulator vasohibin-1 in the CL prevents excessive tumour-like vasculogenesis, suggesting that CL development has well coordinated time-dependent mechanisms. In this review, we discuss the physiological roles of immune cells involved in innate immunity (e.g. neutrophils and macrophages) in the local regulation of CL development with a primary focus on the cow.

Downregulation of lymphatic vessel formation factors in PGF2α-induced luteolysis in the cow

Prostaglandin F2α (PGF2α) induces luteolysis in cows and causes infiltration of immune cells, which resembles inflammatory immune response. Since the general immune response is mediated by the lymphatic system, we hypothesized that luteolysis is associated with generation of an immune response that involves lymphatic vessels in the bovine corpus luteum (CL). The CL was obtained from Holstein cows at the mid-luteal phase (days 10-12, ovulation = day 0) by ovariectomy at various time points after PGF2α injection. Lymphatic endothelial cell (LyEC) marker, endothelial hyaluronan receptor 1 (LYVE1), levels decreased significantly 12 h after PGF2α injection. Podoplanin, another LyEC marker, decreased from 15 min after PGF2α injection. PGF2α also diminished mRNA expression of lymphangiogenic factors, such as vascular endothelial growth factor (VEGF) C, VEGFD and VEGF receptor 3 (VEGFR3). During PGF2α-induced luteolysis, the levels of mRNA expression of tumor necrosis factor α (TNFα; the major pro-inflammatory cytokine) and chemokine (C-X-C motif) ligand 1 (neutrophil chemokine) were increased. On the other hand, chemokine (C-C motif) ligand 21, which regulates outflow of immune cells from tissues via the lymphatic vessels during an immune response, was decreased. This study demonstrated that the lymphatic network in the CL is disrupted during luteolysis and suggests that during luteolysis, immune cells can induce a local immune response in the CL without using the lymphatic vessels.

Vascular and immune regulation of corpus luteum development, maintenance, and regression in the cow

The bovine corpus luteum (CL) is a unique, transient organ with well-coordinated mechanisms by which its development, maintenance, and regression are effectively controlled. Angiogenic factors, such as vascular endothelial growth factor A and basic fibroblast growth factor, play an essential role in promoting progesterone secretion, cell proliferation, and angiogenesis. These processes are critically regulated, through both angiogenic and immune systems, by the specific immune cells, including macrophages, eosinophils, and neutrophils, that are recruited into the developing CL. The bovine luteolytic cascade appears to be similar to that of general acute inflammation in terms of time-dependent infiltration by immune cells (neutrophils, macrophages, and T lymphocytes) and drastic changes in vascular tonus and blood flow, which are regulated by luteal nitric oxide and the vasoconstrictive factors endothelin-1 and angiotensin II. Over the period of maternal recognition of pregnancy, the maternal immune system should be well controlled to accept the semiallograft fetus. The information on the presence of the developing embryo in the genital tract is suggested to be transmitted to the ovary by both the endocrine system and the circulating immune cells. In the bovine CL, the lymphatic system, but not the blood vascular system, is reconstituted during early pregnancy, and interferon tau from the embryo could trigger this novel phenomenon. Collectively, the angiogenic and vasoactive factors produced by luteal cells and the time-dependently recruited immune cells within the CL and their interactions appear to play critical roles in regulating luteal functions throughout the life span of the CL.

Rapid accumulation of polymorphonuclear neutrophils in the Corpus luteum during prostaglandin F(2α)-induced luteolysis in the cow

Prostaglandin F_2α (PGF_2α) induces luteolysis within a few days in cows, and immune cells increase in number in the regressing corpus luteum (CL), implying that luteolysis is an inflammatory-like immune response. We investigated the rapid change in polymorphonuclear neutrophil (PMN) numbers in response to PGF_2α administration as the first cells recruited to inflammatory sites, together with mRNA of interleukin-8 (IL-8: neutrophil chemoattractant) and P-selectin (leukocyte adhesion molecule) in the bovine CL. CLs were collected by ovariectomy at various times after PGF_2α injection. The number of PMNs was increased at 5 min after PGF_2α administration, whereas IL-8 and P-selectin mRNA increased at 30 min and 2 h, respectively. PGF_2α directly stimulated P-selectin protein expression at 5–30 min in luteal endothelial cells (LECs). Moreover, PGF_2α enhanced PMN adhesion to LECs, and this enhancement by PGF_2α was inhibited by anti-P-selectin antibody, suggesting that P-selectin expression by PGF_2α is crucial in PMN migration. In conclusion, PGF_2α rapidly induces the accumulation of PMNs into the bovine CL at 5 min and enhances PMN adhesion via P-selectin expression in LECs. It is suggested that luteolytic cascade by PGF_2α may involve an acute inflammatory-like response due to rapidly infiltrated PMNs.

Five different phenotypes of endothelial cell cultures from the bovine corpus luteum: present outcome and role of potential dendritic cells in luteolysis

Progress in understanding the background of structural luteolysis depends on insights into the physiological function of innate immunity (INIM), in particular the presence of dendritic cells (DCs) in the corpus luteum (CL). For this reason, the cultures of five endothelial cell-like phenotypes derived from the bovine CL and their long-lasting analysis (morphology, function, and origin) become important. Types 1 and 2 represent microvascular endothelial cells with cytokeratin (CK) expression, assumed to be danger-sensing cells. Types 3 and 4 express features of common endothelial cells. Type 5 indicates a steroidogenic cell type, which could be derived from steroidogenic CK(+) cells in the CL of development after loss of CK expression. Type 5 is a promising candidate to become a mature DC. It might act with the microvascular CK(+) cell/type 1 like a luteovascular unit, which connects INIM with adaptive/cell-mediated immunity (ADIM) in structural luteolysis.

Local effects of the sphingosine 1-phosphate on prostaglandin F2alpha-induced luteolysis in the pregnant rat

Since the regression of the corpus luteum (CL) occurs via a tightly controlled apoptotic process, studies were designed to determine if local administration of the antiapoptotic agent sphingosine 1-phosphate (S1P) effectively blocks the luteolytic action of prostaglandin F-2alpha (PGF-2alpha). On day 19 of pregnancy, 2 hr before systemic PGF-2alpha administration, rats were injected intrabursa with either S1P or vehicle (control). The activity of four caspases, which contribute to the initial (caspase-2, -8, and -9) and final (caspase-3) events in apoptosis was measured in pooled CL from four individual ovaries at 0 and 4 hr after PGF-2alpha injection. The expression of the phosphorylated form of AKT (pAKT) and tumor necrosis factor-alpha (TNF-alpha) was analyzed by ELISA. In addition, cell death was evaluated by electronic microscopy (EM) in CL 4 and 36 hr after PGF-2alpha injection. The activity of caspase-2, -3, and -8 was significantly greater by 4 hr after PGF-2alpha, but not caspase-9 activity. In contrast, expression of pAKT and TNF-alpha decreased significantly. Administration of S1P suppressed (P < 0.05) these effects, decreasing caspase activities and increasing pAKT and TNF-alpha expression. The administration of S1P also significantly decreased the percentage of luteal apoptotic cells induced by PGF-2alpha. PGF-2alpha treatment increased the prevalence of luteal cells with advanced signs of apoptosis (i.e., multiple nuclear fragments, chromatin condensation, or apoptotic bodies). S1P treatment suppressed these changes and increased the blood vessel density. These results suggest that S1P blocks the luteolytic effect of the PGF-2alpha by decreasing caspase-2, -3, and -8 activities and increasing AKT phosphorylation and TNF-alpha expression.

Expression and regulation of tumor necrosis factor (TNF) and TNF-receptor family members in the macaque corpus luteum during the menstrual cycle

Members of the tumor necrosis factor (TNF)-receptor (R) family may be involved in the tissue remodeling that occurs in the primate corpus luteum (CL) during development and regression. As a first step towards addressing this issue, studies assessed TNF ligand-R expression and regulation in CL collected from monkeys during the early (ECL, Days 3-5), mid (MCL, Days 7-8), mid-late (MLCL, Days 10-11), late (LCL, Days 14-16), and very late (VLCL, menses) luteal phase of the menstrual cycle. CL were also collected after gonadotropin and/or steroid ablation and replacement (with hLH and the progestin R5020) for 3 days at mid-late luteal phase. TNF-alpha, -beta, FAS ligand (FASL), and TNF-R1 mRNA levels were two- to sixfold greater (P < 0.05) at the MLCL or LCL phase as compared to earlier (ECL, MCL). In contrast, TNF-R2 and FAS mRNA levels did not change during the luteal phase. Immunohistochemical staining for TNF-beta, TNF-R1, TNF-R2, FAS, and FASL was observed in luteal cells, whereas only TNF-beta staining was observed in endothelial cells. Several TNF-R components were influenced by LH and/or steroid ablation; notably, steroid ablation reduced (P < 0.05) luteal TNF-alpha, but not TNF-beta, mRNA levels, which was prevented by progestin treatment. In contrast, steroid ablation increased (P < 0.05) luteal cell immunostaining for FAS and FASL, which was reduced by progestin treatment. Thus, several members of the TNF R-ligand family are expressed in the primate CL in an LH- and/or progestin-dependent manner. Peak expression in the late luteal phase may signify a role for the TNF-R system in death receptor-mediated apoptosis during luteolysis.

Regulation of functional and regressing stages of corpus luteum development in mice. Role of reactive oxygen species

The endocrine and immune systems modulate ovarian function. The aim of the present work was to compare the status of various modulating factors in two well-defined stages of corpus luteum (CL) development (the functional stage and the regressing stage) by means of a gonadotropin-synchronised mouse model. At the regressing stage of CL development, we found that ovarian tissue showed increased prostaglandin (PG) F(2alpha) and diminished PGE levels concomitantly with enhanced protein abundance of ovarian cyclooxygenase 2, the inducible isoform of the limiting enzyme of PG synthesis. We also found both enhanced lipid peroxidation and enhanced total superoxide dismutase activity, as well as inhibited catalase activity and inhibited total hydroxyl radical scavenger capacity, when compared with ovaries at the functional stage. In addition, at the regressing stage we observed an increased percentage of CD8+ (cytotoxic/suppressor) T-cells and a decreased percentage of CD4+ (helper) T-cells from ovarian-draining lymph nodes. Also, the serum interleukin (IL)-2, IL-4 and IL-10 were diminished as compared with the functional stage. We conclude that a pro-oxidant status together with a pro-inflammatory response is responsible for the loss of luteal function.

Expression of caspase-2, -3, -8 and -9 proteins and enzyme activity in the corpus luteum of the rat at different stages during the natural estrous cycle

Apoptosis is associated with the regression of the corpus luteum (CL) in many species. Since caspases play a central role in apoptosis, we studied several initiators (-2, -8, and -9) and the main effector (-3) caspase in the CL during the estrous cycle of the rat. Two different populations of CL (old and new) were identified on ovaries at estrus and diestrus II (DII). Diminished (P< 0.05) luteal progesterone content and P450scc levels suggested that functional luteolysis occurred between the new CL at DII and old CL at estrus, whereas the decline (P< 0.05) in luteal weight indicated that structural regression was occurring between old CL at estrus to DII. Immunostaining for caspase-2 in luteal and endothelial cells appeared to increase as the luteal phase progressed, peaking at DII in the old CL. However, caspase-8 and -9 immunostaining showed little change with a slight increase at estrus in the old population. Notably, caspase-3 staining appeared to peak at DII in the new CL. Enzyme activity of caspase-9 increased (P< 0.05) in the new CL at DII, followed by that of caspase-2 and -3 in old CL at estrus. Caspase-8 activity did not change at any stage. The number of apoptotic cells increased at DII in the old CL. These results suggest an important role for this protease family during early events of luteolysis in the rat estrous cycle.

Corpus Luteum Development: Lessons from Genetic Models in Mice

The corpus luteum is a transient endocrine gland that produces essentially progesterone, a required product for the establishment and maintenance of early pregnancy. In the absence of pregnancy, the corpus luteum will cease to produce progesterone, and the structure itself will regress in size over time. The life span and function of the corpus luteum is regulated by complex interactions between stimulatory (luteotrophic) and inhibitory (luteolytic) mediators. Although the process of luteal formation and regression has been studied for several decades, many of the regulatory mechanisms involved in loss of function and involution of the structure are incompletely understood. In rodents, prolactin is the major luteotrophic hormone by maintaining the structural and functional integrity of the corpus luteum for several days after mating. Other factors involved in steroidogenesis, control of cell cycle, apoptosis, and tissue remodeling have been shown to play a role in corpus luteum development and maintenance. Especially, PGF2alpha seems to be the most potent luteolytic hormone. One of the most important advances in the study of mammalian genes has been the development of techniques to obtain defined mutations in mice. These tools enable us to target specific genes and to analyze the impact of their loss on cell fate and function. With these approaches, several receptors, transcription factors, enzymes, and other factors have been linked to corpus luteum development and maintenance. These models are helping to define mechanisms of reproductive function and to identify potential new contraceptive targets and genes involved in the pathophysiology of reproductive disorders.

The matrix metalloproteinase system: changes, regulation, and impact throughout the ovarian and uterine reproductive cycle

The ovary and uterus undergo extensive tissue remodeling throughout each reproductive cycle. This remodeling of the extracellular environment is dependent upon the cyclic hormonal changes associated with each estrous or menstrual cycle. In the ovary, tissue remodeling is requisite for growth and expansion of the follicle, breakdown of the follicular wall during the ovulatory process, transformation of the postovulatory follicle into the corpus luteum, as well as the structural dissolution of the corpus luteum during luteal regression. In the uterus, there is extraordinary turnover of the endometrial connective tissue matrix during each menstrual cycle. This turnover encompasses the complete breakdown and loss of this layer, followed by its subsequent regrowth. With implantation, extensive remodeling of the uterus occurs to support placentation. These dynamic changes in the ovarian and uterine extracellular architecture are regulated, in part, by the matrix metalloproteinase (MMP) system. The MMP system acts to control connective tissue remodeling processes throughout the body and is comprised of both a proteolytic component, the MMPs, and a regulatory component, the associated tissue inhibitors of metalloproteinases. The current review will highlight the key features of the MMPs and tissue inhibitors of metalloproteinases, focus on the changes and regulation of the MMP system that take place throughout the estrous and menstrual cycles, and address the impact of the dynamic tissue remodeling processes on ovarian and uterine physiology.

Reactive oxygen species mediate leukocyte-endothelium interactions in prostaglandin F2alpha -induced luteolysis in rats

We investigated the contribution of neutrophils to prostaglandin (PG)F(2 alpha)-induced luteolysis and the role of reactive oxygen species (ROS) as potential mediators of neutrophil accumulation in regressing corpora lutea of superovulated rats. On day 8 of pseudopregnancy, subcutaneous injection of PGF(2 alpha) (500 microg) significantly increased rhodamine 6G-labeled leukocyte adhesion in luteal venules, as observed by intravital microscopy. Neutrophil accumulation was confirmed by significantly increased myeloperoxidase (MPO) activity. Pretreatment with anti-leukocyte antibody (CD18-directed monoclonal antibody, WT-3) significantly inhibited the PGF(2 alpha)-induced increases in adherent leukocytes and MPO activity. Anti-leukocyte antibody also maintained serum progesterone concentrations. Pretreatment with oxygen free radical scavengers, superoxide dismutase (50,000 U/kg) and catalase (90,000 U/kg), also attenuated these PGF(2 alpha)-induced alterations. Corpora lutea preloaded with dichlorodihydrofluorescein diacetate succinimidyl ester, a fluorescent indicator for determining intracellular ROS generation, exhibited an increase in fluorescence after PGF(2 alpha) treatment. These findings suggest that leukocyte-endothelium interactions mediated by ROS generation are important in PGF(2 alpha)-induced luteolysis in rats.

Cyclic changes in the matrix metalloproteinase system in the ovary and uterus

With each estrous or menstrual cycle, extensive alterations occur in the extracellular matrix and connective tissue of the ovary and uterus. In the ovary, these changes occur during follicular development, breakdown of the follicular wall and extrusion of the oocyte, as well as during the formation and regression of the corpus luteum. In the uterus, the endometrium undergoes dramatic connective tissue turnover associated with tissue breakdown and subsequent regrowth during each menstrual cycle. These changes in the ovarian and uterine extracellular architecture are regulated, in part, by the matrix metalloproteinase (MMP) system. This system is comprised of both a proteolytic component, the MMPs, and associated inhibitors, and it is involved in connective tissue remodeling processes throughout the body. The current review highlights the key features of the MMP system and focuses on the changes in the MMPs and the tissue inhibitors of metalloproteinases during the dynamic remodeling that takes place in the ovary and uterus during the estrous and menstrual cycles.

Role of tumor necrosis factor alpha and its type I receptor in luteal regression: induction of programmed cell death in bovine corpus luteum-derived endothelial cells

The role of tumor necrosis factor alpha (TNF alpha) and its type I receptor (TNFRI) in structural luteolysis was investigated. A semiquatitative reverse-transcription polymerase chain reaction (RT-PCR) was used to characterize the pattern of TNFRI mRNA expression within the corpus luteum (CL) throughout the estrous cycle and its cellular distribution. Increase in TNFRI mRNA levels was recorded both in regressed luteal tissue and in CL of cows injected with prostaglandin F(2 alpha). All three major cell types composing the CL, steroidogenic (large and small) and endothelial cells expressed the TNFRI gene. A densitometric analysis of TNFRI mRNA expression revealed that resident endothelial cells had significantly higher levels of TNFRI mRNA than steroidogenic luteal cells. The physiological effects associated with TNFRI expression were investigated in the various luteal cell types. TNF alpha-induced programmed cell death (PCD) in dose- and time-dependent manners of cultured luteal endothelial cells (LECs) but not of in vitro luteinized steroidogenic cells. Several lines of evidence are provided to show that progesterone regulates luteal cell survival: 1) CL and LECs express progesterone receptor mRNA, 2) physiological levels of the steroid abolished TNF alpha-induced PCD of LECs, and 3) progesterone-producing cells are protected from PCD. In conclusion, this study suggests that TNF alpha-induced PCD during structural luteolysis is mediated by TNFRI, primarily affects endothelial cells, and that the decline in progesterone, preceding structural luteolysis, is a prerequisite for the initiation of apoptosis in endothelial cells.

Evidence for nitric oxide acting as a luteolytic factor in the human corpus luteum

The aims of the present study were to characterize the expression and cellular localization of isoforms of nitric oxide synthase (NOS) in the human corpus luteum (CL) and to determine the effects of nitric oxide (NO) on CL steroidogenesis. Immunoblotting analyses revealed that endothelial NOS (eNOS) is the most abundant isoform in human CL with highest values during the late luteal phase. Immunoreactive eNOS was localized predominantely in the theca lutein layer, being particularly abundant in endothelial cells, but with positive staining also in some steroidogenic cells. Immunoreactive inducible NOS (iNOS) was also detected, but to lesser degree, and did not display apparent phase-specific changes. The effect of NO on CL steroid synthesis was examined using human chorionic gonadotrophin (HCG)-stimulated dispersed CL cells cultured in vitro. Progesterone production was significantly decreased (P < 0.05) by the NO donor spermine NONOate (10(-5) mol/l) in cells of the late, but not mid-, luteal phase. To investigate a potential link between NO and the local prostaglandins (PG), concentrations of PGF(2alpha) and PGE(2) were measured in culture medium. NO significantly increased (P < 0.05) concentrations of both PGF(2alpha) and PGE(2) during the late luteal phase. It is concluded that NO may be luteolytic in the human CL of menstruation.

Phase-dependent influence of nonsteroidogenic cells on steroidogenesis and prostaglandin production by the human corpus luteum

OBJECTIVE

To test the hypothesis that paraluteal cells in the human corpus luteum (CL) modulate steroidogenesis and prostaglandin production by the CL.

DESIGN

In vitro cell culture study using human luteal cells.

SETTING AND PATIENT(S)

Women (n = 7) with normal menstrual cycles who were undergoing operations for benign, nonovarian conditions during the midluteal phase (5-9 days after ovulation) or the late luteal phase (10-14 days after ovulation) at a university hospital.

INTERVENTION(S)

Steroidogenic and nonsteroidogenic human CL cells were isolated by mechanical and enzymatic digestion and density sedimentation. The cells were cultured (75,000 cells per well) for 24 hours either as a crude sample of all CL cells or as an enriched fraction of steroidogenic CL cells.

MAIN OUTCOME MEASURE(S)

Levels of progesterone, E2, prostaglandins F2alpha, E2, and I2 in conditioned medium.

RESULT(S)

Higher concentrations of progesterone, E2, and prostaglandins F2alpha, E2, and I2 were released into the media of the crude sample of all CL cells than into the enriched fraction of steroidogenic CL cells from the midluteal phase. No such difference was noted in CL cells from the late luteal phase.

CONCLUSION(S)

The paraluteal cells in the human CL stimulated progesterone and E2 synthesis. This may be mediated by an increase in prostaglandin production in the midluteal phase.

Differentiation-dependent prolactin responsiveness and stat (signal transducers and activators of transcription) signaling in rat ovarian cells

PRL activates an important cytokine signaling cascade that is obligatory for maintaining luteal cell function in the rat ovary. To determine when specific components of this cascade are expressed and can be activated by PRL, we analyzed the expression of receptor subtypes (short, PRL-R(s), and long, PRL-R(L)), the presence and kinetics of Stat (signal transducer and activator of transcription) activation using the PRL-response element (PRL-RE) of the alpha2M (alpha2-macroglobulin) gene, and the content and hormonal regulation of three specific modulators of cytokine signaling; the tyrosine phosphatases (SHP-1 and SHP-2), and the protein inhibitor of activated Stat3 (PIAS-3). These components were analyzed in differentiating granulosa/ luteal cells of hypophysectomized (H) rats and in corpora lutea of pregnant rats. Levels of PRL-R mRNAs increased as granulosa cells differentiated and reached maximal levels in luteal cells of pregnant rats where levels of PRL-R(s) approached those of PRL-R(L). The relative concentrations shifted from a 27-fold excess of PRL-R(L) in preovulatory granulosa cells to a 3.7-fold difference in luteal cells during midgestation. Despite the increased PRL-R(L) expression in differentiated granulosa cells, PRL did not stimulate detectable activation of Stats. Rather PRL activation of Stat5, principally Stat5b, occurred in association with luteinization. In contrast, granulosa cells of untreated immature and H rats contained a high level of DNA binding activity, which was shown to be comprised entirely of activated, phosphorylated Stat3. Treatment with estrogen and FSH reduced the amount of phosphorylated Stat3 and abolished its ability to bind DNA, an effect temporally related to increased PIAS-3. Expression of SHP-1 (but not SHP-2) was also hormonally regulated; SHP-1 mRNA and protein were high in granulosa cells of H rats, decreased by estrogen and FSH, and subsequently increased dramatically with luteinization. Of particular note, SHP-1 was localized in cytoplasm of granulosa cells in atretic follicles but was distinctly nuclear in luteal cells, indicative of different functional roles. Collectively, these results indicate that Stat3 and Stat5 are activated by distinct cytokine-signaling pathways modulated through differentiation-dependent transcriptional regulation of signaling pathway components and mediate distinct functional processes in the rat ovary: early follicle growth and atresia vs. luteinization.