Molecular regulation of progesterone secretion by the human corpus luteum throughout the menstrual cycle

The present study examines the expression of the steroidogenic acute regulatory protein (StAR) within the human corpus luteum (CL) in conjunction with other molecules that regulate the apoptotic process of the CL. Our results indicate that the primary 1.6 kb StAR transcript occurs in greater abundance in early and mid-luteal phase compared with late luteal phase CL. Mature StAR protein (30 kDa) was present in lower amounts within late CL compared with early and mid-luteal phase. The pre-protein (37 kDa), which has been considered the active isoform to favor cholesterol translocation and subsequently steroid hormone synthesis, was also detected in lower amount in late CL. Several molecules, including pro-inflammatory cytokines, reactive oxygen species, steroids and inducible nitric oxide synthase (iNOS), have been linked as pro-apoptotic regulatory agents. Moreover, many of these molecules diminish progesterone synthesis in human cultured luteal cells. Interestingly, these molecules preferentially decrease progesterone biosynthesis in mid and late luteal cells in culture. These data suggest that the inhibitory effect of these molecules, as well as the amount of apoptotic cells in the CL are age dependent. The number of luteal apoptotic cells, as well as luteal cells stained positive for iNOS, increased from early to late CL. To examine the effects of hCG on StAR expression and apoptosis, we used two models-(1) in vivo hCG administration during the late luteal phase; and (2) in vitro incubation of explants of late CL with hCG. hCG increased both the level of StAR expression and the level of anti-apoptotic protein Bcl-2 within the late CL. We conclude that mRNA and protein expression of StAR and bcl-2 are important target elements for hCG during the CL rescue.