Regulation of steroidogenic acute regulatory protein and luteinizing hormone receptor messenger ribonucleic acid in hen granulosa cells

The regulation of steroidogenic acute regulatory protein (StAR) in vitro by gonadotropins was investigated in granulosa cells from prehierarchal and p reovulatory hen follicles. Basal levels of StAR messenger RNA (mRNA) in und ifferentiated granulosa cells from prehierarchal (6- to 8-mm) follicles wer e consistently low, but detectable, and were significantly increased by tre atment with 8-bromo-cAMP and FSH (but not LH) within 3-6 h of culture. Afte r 20 h of culture, 8-bromo-cAMP, FSH, and LH each increased StAR mRNA level s above those in control cultured cells, and the delayed response to LH tre atment was associated with increased levels of LH receptor (LH-R) mRNA. On the other hand, inhibition of mitogen-activated protein (MAP) kinase signal ing, using the MAP kinase kinase inhibitors U0126 and PD98059, in the prese nce of FSH further increased StAR mRNA and protein levels, LH-R mRNA levels , and progesterone synthesis compared with those in cells cultured with FSH alone. The highest basal expression of StAR mRNA during follicle developme nt was found in granulosa from the largest (F1) preovulatory follicle, with comparatively lower levels in granulosa from less mature (F2 plus F3) preo vulatory follicles, Treatment with LH rapidly increased StAR mRNA and prote in (but not LH R mRNA) expression in cultures of F1 granulosa and in combin ed F2 plus F3 granulosa within 3 h, although the magnitude of stimulation w as greater in F2 plus F3 granulosa. Compared with results from granulosa ce lls from prehierarchal follicles cultured for 20 h, inhibition of MAP kinas e signaling in the presence of LH for 1 h failed to further enhance levels of StAR or LH-R expression or progesterone production in F2 plus F3 follicl e granulosa compared with the effect of LH treatment alone. These results d emonstrate that StAR expression in the hen ovary is upregulated by gonadotr opins at least in part via cAMP signaling. The ability of MAP kinase kinase inhibitors to potentiate gonadotropin-induced StAR and LH-R expression plu s progesterone synthesis in prehierarchal follicle granulosa cells in vitro suggests that inhibition of paracrine or autocrine factor-mediated MAP kin ase signaling in vivo may be a prerequisite for the full potentiation of gr anulosa cell steroidogenesis that occurs after recruitment into the preovul atory hierarchy. Finally, these results fail to support a role for MAP kina se signaling in acutely modulating LH-mediated StAR expression or progester one production in hierarchal follicles, such as occurs during the preovulat ory surge of progesterone.