Identical changes in Bax expression, but not Fas ligand expression, occur in structural luteolysis in gonadotropin releasing hormone agonist- and prolactin-treated superovulated rats

Interplay between nitric oxide and gonadotrophin-releasing hormone in the neuromodulation of the corpus luteum during late pregnancy in the rat

BACKGROUND

Nitric oxide and GnRH are biological factors that participate in the regulation of reproductive functions. To our knowledge, there are no studies that link NO and GnRH in the sympathetic ganglia. Thus, the aim of the present work was to investigate the influence of NO on GnRH release from the coeliac ganglion and its effect on luteal regression at the end of pregnancy in the rat.

METHODS

The ex vivo system composed by the coeliac ganglion, the superior ovarian nerve, and the ovary of rats on day 21 of pregnancy was incubated for 180 min with the addition, into the ganglionic compartment, of L-NG-nitro arginine methyl ester (L-NAME), a non-selective NO synthase inhibitor. The control group consisted in untreated organ systems.

RESULTS

The addition of L-NAME in the coeliac ganglion compartment decreased NO as well as GnRH release from the coeliac ganglion. In the ovarian compartment, and with respect to the control group, we observed a reduced release of GnRH, NO, and noradrenaline, but an increased production of progesterone, estradiol, and expression of their limiting biosynthetic enzymes, 3β-HSD and P450 aromatase, respectively. The inhibition of NO production by L-NAME in the coeliac ganglion compartment also reduced luteal apoptosis, lipid peroxidation, and nitrotyrosine, whereas it increased the total antioxidant capacity within the corpora lutea.

CONCLUSION

Collectively, the results indicate that NO production by the coeliac ganglion modulates the physiology of the ovary and luteal regression during late pregnancy in rats.

Type-dependent differences in Fas expression and phagocytes distribution in rat corpora lutea during natural regression: an immunohistochemical evidence

Though Fas/Fas ligand (FasL) system-dependent apoptosis is considered to be the primary form of cell death in regressing corpus luteum (CL), the cellular identity and regulation of expression of the ligand and receptor molecules are not fully understood. Here, we focused on immunohistochemical determination of Fas expression during natural regression with comparison of three different types of rat CLs. Detected Fas was in good spatial association with cleaved caspase-3 and FasL proteins and with macrophages and neutrophils. In CLs of the cycle and pseudopregnancy, Fas-positive cell types included large and small luteal (steroidogenic) cells and capillary endothelial cells mainly, and blood-derived immune cells occasionally. Fas signals were abundant at multiple focal inflammatory-like sites. In contrast, Fas signals in CL of pregnancy did not localize in steroidogenic cells, but almost exclusively in endothelial cells and granulocytes. The signals scattered evenly throughout the CL tissue as phagocytes also did. In all CLs types, the numbers of Fas-expressing cells increased transiently after functional inactivation and at the early phase of structural regression. This observation revealed spatio-temporally regulated expression of Fas that was highly associated with apoptotic and phagocytotic systems and type-dependent differences in Fas expression and phagocytes dynamics in naturally regressing CL of rats.

Gonadotropin-releasing hormone-regulated prohibitin mediates apoptosis of the gonadotrope cells

GnRH regulates circulating levels of the gonadotropins but has also been implicated in establishing the gonadotrope cell population. Consistent with this, GnRH induces proliferation of partially differentiated gonadotropes, while reducing the numbers of fully differentiated cells. We have previously reported that the proapoptotic protein, prohibitin (PHB) is expressed more abundantly in gonadotrope-derived LβT2 cells than in partially differentiated αT3-1 gonadotrope precursor cells, suggesting a possible role for PHB in GnRH-induced apoptosis. We show here that PHB is required for GnRH-induced apoptosis in mature gonadotropes. PHB expression and activity are regulated by GnRH: its transcription is via c-Jun NH2-terminal kinase, whereas its nuclear export follows activation of ERK. Moreover, PHB levels are down-regulated by microRNA27, which is expressed at lower levels in mature gonadotropes, possibly explaining the switch to an apoptotic response with development. PHB is required for mitochondrial import of the proapoptotic BAX, whose expression is also induced by GnRH-activated c-Jun NH2-terminal kinase, as is expression of the BH3-only protein, HRK, and this too plays a role in GnRH-induced apoptosis. Finally, we show that gonadotrope-specific PHB-knockout mice display reproductive abnormalities, including a larger gonadotrope population, increased LH levels, reduced fertility, and altered gonad development. We thus demonstrate a role for PHB in GnRH-induced cell death in mature gonadotropes, which is crucial for the normal development and function of the reproductive axis.

The molecular control of corpus luteum formation, function, and regression

The corpus luteum (CL) is one of the few endocrine glands that forms from the remains of another organ and whose function and survival are limited in scope and time. The CL is the site of rapid remodeling, growth, differentiation, and death of cells originating from granulosa, theca, capillaries, and fibroblasts. The apparent raison d'etre of the CL is the production of progesterone, and all the structural and functional features of this gland are geared toward this end. Because of its unique importance for successful pregnancies, the mammals have evolved a complex series of checks and balances that maintains progesterone at appropriate levels throughout gestation. The formation, maintenance, regression, and steroidogenesis of the CL are among the most significant and closely regulated events in mammalian reproduction. During pregnancy, the fate of the CL depends on the interplay of ovarian, pituitary, and placental regulators. At the end of its life span, the CL undergoes a process of regression leading to its disappearance from the ovary and allowing the initiation of a new cycle. The generation of transgenic, knockout and knockin mice and the development of innovative technologies have revealed a novel role of several molecules in the reprogramming of granulosa cells into luteal cells and in the hormonal and molecular control of the function and demise of the CL. The current review highlights our knowledge on these key molecular events in rodents.

Significance of matrix metalloproteinases in the pathophysiology of the ovary and uterus

Matrix metalloproteinases (MMP) are capable of degrading a variety of extracellular matrix (ECM) proteins and are also involved in the processing of a number of bioactive molecules. Our findings indicate that the functions of MMP in the ovary and uterus are organ-specific and time-dependently vary during the reproductive cycle. Prolactin induces structural luteolysis indicated by loss of luteal weight, protein and DNA within 36 h after pretreatment with ergot alkaloid. MMP activation appears crucial for the selective depletion of protein during luteal involution, which entails loss of ECM accompanied by apoptosis. During GnRHagonist-induced luteolysis, this response was also associated with marked increases in MMP-2, which degraded collagen type IV, and MT1-MMP, which in addition to activating MMP-2 also degrades collagen type I, III and V. We also found that the level of MT1-MMP and MMP-2 expression in the human CL is greater during the late luteal phase than during either the early mid luteal phases or during gestation, respectively. That dehydroepiandrosterone (DHEA) treatment caused the formation of cysts from antral follicles in the ovaries of immature rats while depressing MMP-2 collagenolytic activity and enhancing lysyl oxidase expression highlights the importance of collagen degradation in the process of ovulation and suggests that changes in the activities of these enzymes play a key role in ovarian cystogenesis in polycystic ovary syndrome patients. Furthermore, immunohistochemical analyses showed that MT1-MMP and FasL co-localize with TdT-mediated dUTP-biotin nick end-labeling (TUNEL)-positive apoptotic granulosa cells in rats treated with DHEA, that the Fas/FasL/Caspase-8 (death receptor-dependent) pathway is pivotal for follicular atresia and that increased levels of MT1-MMP likely play an important role in tissue remodeling during follicular atresia. After parturition, the uterus undergoes involution, a conspicuous feature characterized by a rapid reduction in the collagen content mediated by degradation of extracellular collagen bundles. Our findings strongly suggest that MT1-MMP, MMP-2 and MMP-9 are each time-dependently regulated and play important roles in tissue remodeling during postpartum uterine involution. (Reprod Med Biol 2006; 5: 235-243).