Estrogen withdrawal induces macrophage invasion in the rabbit corpus luteum

The interface of the immune and reproductive systems in the ovary: lessons learned from the corpus luteum of domestic animal models

The dynamic changes that characterize the female reproductive system are regulated by hormones. However, local cell-to-cell interactions may mediate responsiveness of tissues to hormonal signals. The corpus luteum (CL) is an excellent model for understanding how immune cells are recruited into tissues and the role played by those cells in regulating tissue homeostasis or demise. Leukocytes are recruited into the CL throughout its lifespan, and leukocyte-derived cytokines have been found in corpora lutea of all species examined. The proinflammatory cytokines inhibit gonadotropin-stimulated steroidogenesis, profoundly stimulate prostaglandin synthesis by luteal cells, and promote apoptosis. However, there is mounting evidence that leukocytes and luteal cells communicate in different ways to maintain homeostasis within the functional CL. Domestic animals have provided important information regarding the presence and role of immune cells in the CL.

Activity and expression of different members of the caspase family in the rat corpus luteum during pregnancy and postpartum

Studies were designed to examine the expression and activity of four caspases that contribute to the initial (caspases-2, -8, and -9) and final (caspase-3) events in apoptosis in the rat corpus luteum (CL) during pregnancy (days 7, 17, 19, and 21 of gestation), postpartum (days 1 and 4), and after injection (0, 8, 16, 24, and 36 h) of the physiological luteolysin PGF2alpha. In addition, the temporal relationship of caspase expression/activity relative to steroid production and luteal regression was evaluated. During pregnancy, the activity of all four caspases was significantly greater on day 19, before a decline in CL progesterone (P) and CYP11A1 levels at day 21 of gestation. The levels of the caspase-3 active fragment (p17, measured by Western blot) also increased at days 19 and 21 of pregnancy. Immunohistochemical analyses detected specific staining for the caspases in luteal cells (large and small) as well as in endothelial cells. However, the percentage of apoptotic cells did not increase in the CL until postpartum. Following PGF2alpha injection, there was a significant decrease in CL P by 24 h, although the activity of all four caspases did not increase until 36 h posttreatment. The active p17 fragment of caspase-3 also significantly increased at 36 h post-PGF2alpha. These results suggest that an increase in the activity of caspases-2, -8, -9, and -3 is associated with the early events of natural luteolysis at the end of pregnancy. Also, the exogenous administration of the luteolysin PGF2alpha may regulate members of the caspase family.

Cell death induced by serum deprivation in luteal cells involves the intrinsic pathway of apoptosis

The corpus luteum is a transient endocrine gland specializing in the production of progesterone. The regression of the corpus luteum involves an abrupt decline in its capacity for producing progesterone followed by its structural involution, which is associated with apoptosis of the luteal cells. An in vitro experimental approach is needed to study the molecular mechanisms underlying hormonal regulation of luteal cell death under defined experimental conditions. In this study, we investigated simian virus-40-transformed luteal cells to determine whether they can be driven to apoptosis and, if so, to define the intracellular pathway involved. Luteal cells were cultured in the presence or absence of fetal bovine serum for 24 or 48 h. Under serum starvation conditions, the luteal cells underwent growth arrest accompanied by cell death as evaluated by dye exclusion, and confirmed by two-color fluorescence cell viability/cytotoxicity assay. We next studied whether serum starvation-induced death of luteal cells occurred by apoptosis. Morphologic features of apoptosis were observed in cells stained with hematoxylin after being subjected to serum starvation for 48 h. The apoptotic nature was further confirmed by in situ 3'-end labeling and fragmentation of genomic DNA. Apoptosis of serum-deprived luteal cells was dependent upon caspase activation. Serum starvation induced cleavage of poly (ADP-ribose) polymerase (PARP), suggesting that caspase-3 had been activated under the stress of withdrawal of growth factors. This was confirmed by cleavage of full-length procaspase-3. Finally, the fact that serum starvation promoted the cleavage of full-length procaspase-9 and the decrease in the expression of endogenous Bid, a BH-3-only proapoptotic protein of the Bcl-2 family, indicates that the intrinsic (i.e., mitochondrial) pathway of apoptosis was activated. In summary, we have characterized an in vitro experimental model of luteal cell death that can be utilized to evaluate the role of hormones in apoptosis of luteal cells under defined culture conditions, and to study the mechanism of luteal regression.

The human corpus luteum: which cells have progesterone receptors?

Studies comparing the regressing corpus luteum with the rescued corpus luteum have demonstrated that human chorionic gonadotrophin (hCG) has effects on cell types that do not express hCG receptors. As progesterone synthesis is hCG dependent and the corpus luteum has been shown to express genomic progesterone receptors, progesterone is a candidate molecule for these paracrine effects. This study aimed to define the cellular localisation of progesterone receptors in the human corpus luteum using dual-staining immunohistochemistry for genomic progesterone receptors and specific cellular markers. Well-characterised corpora lutea (n = 12) from different stages of the luteal phase were studied. The same distribution was observed in all corpora lutea examined. The steroidogenic cells (3β-hydroxysteroid dehydrogenase positive) and both thecalutein (17α-hydroxylase positive) and granulosalutein (aromatase positive) express progesterone receptors, as do stromal fibroblasts (vimentin positive, fibroblast antigen positive). Vascular endothelial cells (CD31 positive), pericytes (α-smooth muscle actin positive), macrophages (CD68 positive) and fibroblasts within the central clot do not express nuclear progesterone receptors. Progesterone is a candidate messenger molecule for the effects of hCG on the matrix metalloproteinase-producing stromal fibroblasts. Some of the effects of hCG on steroidogenic cells may be mediated by progesterone, but its effects on blood vessels and macrophages require alternate paracrine signalling mechanisms. In addition, there appears to be at least two fibroblast populations in the corpus luteum.

Possible actions of tumor necrosis factor-alpha in ovarian function

Tumor necrosis factor-alpha (TNFalpha) is a multifunctional cytokine that was first described as a tumoricidal factor produced by activated macrophages. Extensive research over the last two decades has suggested that TNFalpha has physiologically diverse actions in ovarian function in a variety of species. TNFalpha and its specific receptors are present in the ovaries of many species. Furthermore, TNFalpha plays multiple and probably important roles in corpus luteum (CL) function as well as ovarian cell function throughout the estrous cycle. This review focuses on recent studies documenting TNFalpha in ovarian follicles and CL in several mammals. In addition, possible roles of TNFalpha in ovarian function throughout the estrous cycle and in the gestation period are discussed.

Expression patterns of cytokines, p53 and nitric oxide synthase isoenzymes in corpora lutea of pseudopregnant rabbits during spontaneous luteolysis

The gene expressions for macrophage chemoattractant protein-1 (MCP-1), interleukin (IL)-1β, IL-2 and p53 were examined by semi-quantitative RT-PCR in corpora lutea (CL) of rabbits during spontaneous luteolysis at days 13, 15, 18 and 22 of pseudopregnancy. In the same luteal tissue, total activity of nitric oxide (NO) synthase (NOS) and genes for both endothelial (eNOS) and inducible (iNOS) isoforms were also analysed. From day 13 to 15, MCP-1 and IL-1βmRNA levels rose (P≤ 0.01) almost 2-fold, and the transcript for p53 almost 8-fold, but then all dropped (P≤ 0.05) from day 18 onward. IL-2 mRNA abundance was higher (P≤ 0.01) on day 13 and then gradually declined. During luteolysis, eNOS mRNA decreased 40% (P≤ 0.05) by day 15, but thereafter remained unchanged, while iNOS mRNA was barely detectable and did not show any clear age-related pattern throughout the late luteal stages. Total NOS activity progressively increased (P≤ 0.01) from day 13 to 18 of pseudopregnancy and then dropped to the lowest (P≤ 0.01) levels on day 22. Luteal progesterone content also declined during CL regression from 411 to 17 pg/mg found on days 13 and 22 respectively, in parallel with the decrease in blood progesterone concentrations. These data further support a physiological role of NO as modulator of luteal demise in rabbits. Locally, luteal cytokines may be involved in the up-regulation of NOS activity, while downstream NO may inhibit steroroidogenesis and induce expression of p53 gene after removal of the protective action of progesterone.

Multiple roles of TNF super family members in corpus luteum function

The main function of the corpus luteum (CL) is the production of progesterone. Adequate luteal progesterone is crucial for determining the physiological duration of the estrous cycle and for achieving a successful pregnancy. The CL is regulated not only by hypophyseal gonadotropin, but also by a number of cytokines that are locally produced. Tumor necrosis factor-alpha (TNF) and its specific receptors (TNFR) are present in the CL of many species. TNF plays multiple and likely important roles in CL function throughout the estrous cycle. TNF appears to have luteotropic and luteolytic roles in the CLs. In contrast, Fas ligand (Fas L), another member of TNF super family (TNF-SF), is primarily recognized for its apoptotic actions. Presumably, Fas L binds its cognate receptor (Fas) to induce structural luteolysis. This review is designed to focus on recent studies documenting the expression of TNF and Fas L, their receptors, and intracellular signaling mechanisms in the CL.

Secretion of monocyte chemoattractant protein-1 by endothelial cells of the bovine corpus luteum: regulation by cytokines but not prostaglandin F2alpha

Information regarding the regulation of monocyte chemoattractant protein-1 (MCP-1) in regression of the corpus luteum (CL) is limited. This study tested the hypothesis that endothelial cells derived from bovine CL are a source of MCP-1, and that proinflammatory cytokines, prostaglandin F2alpha (PGF2alpha), and progesterone regulate MCP-1 expression. Endothelial cells were treated without (Control) or with PGF2alpha (1 micro M), TNFalpha (100 ng/ml), interferon-gamma (IFNgamma, 200 IU/ml), and TNFalpha + IFNgamma for 24 and 48 h in the absence or presence of progesterone (P4, 250 ng/ml). Increases in MCP-1 mRNA and protein were observed in response to TNFalpha within 24 and 48 h of culture, respectively (P < 0.05). Interferon-gamma stimulated (P < 0.05) both MCP-1 mRNA and protein after 24 h of culture, and this effect was also sustained through 48 h of culture (P < 0.05). Cotreatment of cultures with TNFalpha + IFNgamma lead to further increases (P < 0.05) in MCP-1 in both 24- and 48-h cultures. Surprisingly, neither PGF2alpha nor P4 affected MCP-1 production. Subsequent experiments revealed that the endothelial cells lacked prostaglandin F2alpha receptor mRNA, and the MAPK pathway, although present and responsive to growth factor stimulation, was unresponsive to PGF2alpha stimulation. In summary, endothelial cells derived from bovine CL respond to TNFalpha and IFNgamma stimulation with an increase in MCP-1 secretion. In contrast, neither PGF2alpha nor P4 directly influenced endothelial expression of MCP-1. These results suggest that cytokines stimulate the synthesis of MCP-1 observed during PGF2alpha-induced luteal regression.

The fate of corpora lutea in the cyclic golden hamster

Structural luteolysis shows striking interspecies differences. Morphological changes in the corpus luteum (CL) of the cyclic hamster have been studied alongside the potential involvement of known luteolytic hormones. Ovaries from intact Syrian golden hamsters killed at 1100 h on days 1 and 2 and at 1100 and 1700 h on days 3 and 4 of the estrous cycle were dissected for histological study. The day of ovulation, the day of estrus, was arbitrarily designated day 1 of the estrous cycle. Steroidogenic cells in the CL were scarcely luteinized on day 1 and reached full luteinization on day 2. On the morning of day 3, initial regressive changes (accumulation of lipid droplets, invasion by neutrophils, and accumulation of phagocytic cells) were observed. These regressive changes increased progressively and apoptotic cells as well as phagocytic cells containing phagocytized apoptotic cells were abundant on the evening of day 3. On the morning of day 4, apoptotic cells/bodies and phagocytic cells containing phagocytized material were extremely abundant throughout the CL. However, steroidogenic cells with intact nuclei and well-preserved blood vessels were also found. Surviving cells in the CL showed progressive morphological changes. These cells showed morphological features intermediate between luteal and interstitial cells in the evening of day 4 and were virtually indistinguishable from interstitial cells on day 1 of the following cycle. Additional animals were injected at 1100 h on day 2 with: (a) the dopaminergic agonist CB154 (0.4 mg) to block prolactin secretion, (b) the anti-estrogen LY117018 (1.6 mg) or the anti-androgen Flutamide (3 mg) to block estrogen or androgen receptors, respectively, and (c) progesterone (2 mg) to prevent the fall in serum progesterone concentrations. Ovaries from these animals were collected at 1700 h on day 3 and at 1000 h on day 4. The luteolytic process was not affected by any treatment. These data indicate that, in contrast to its close relatives (e.g., the rat), structural luteolysis in the hamster is independent of the apoptotic inducing luteolytic hormones. In addition, differences in the cellular mechanisms responsible for CL elimination were also present. In the hamster, part of the luteal cells do not undergo apoptosis and seemed to progress through another developmental path giving rise to interstitial-like cells.

The proestrous prolactin surge is not the sole initiator of regressive changes in corpora lutea of normally cycling rats

During the estrous cycle, secretion of prolactin is largely restricted to a surge on proestrus. We investigated whether this proestrous prolactin surge initiates regression of the corpora lutea of the preceding cycle. Adult rats were killed prior to the prolactin surge (Proestrus group), following the prolactin surge (Estrus group), after chemical blockade of the prolactin surge with bromocryptine (Estrus+BRC group), and after blockade of the prolactin surge and administration of prolactin (Estrus+BRC+PRL group). Corpora lutea of the current (proestrus) or preceding (estrus) cycle were dissected out, weighed, and sectioned for immunohistochemistry or cultured for examination of in vitro progestin production. Numbers of luteal monocytes/macrophages, differentiated macrophages, and apoptotic nuclei per high-power field were greater for Estrus and Estrus+BRC+PRL than for Estrus+BRC, which in turn had greater numbers than Proestrus (P< 0.05). In contrast, BRC completely reversed the decline in luteal weight observed between Proestrus and Estrus (P<0.05). Number of major histocompatibility complex II-positive cells was not different between groups (P>0.05). Finally, progestin production by corpora lutea in vitro was lower for Proestrus than for the other groups (P<0.05). The results indicate that the prolactin surge alone is not responsible for initiation of apoptosis or immune cell infiltration in regressing corpora lutea of the estrous cycle, although prolactin increases these markers of regression. Prolactin does cause a decline in luteal weight; however, the corpora lutea retain the capacity for steroidogenesis. We conclude that although prolactin has a role in luteal regression, it is not solely responsible for the initiation of this process.