Expression of steroidogenic acute regulatory protein (StAR) in the human ovary

Expression of steroidogenic acute regulatory protein (StAR) messenger ribonucleic acid (mRNA) in human ovary and cultured proliferating granulosa-lutein cells, theca interna cells, and luteinized granulosa cells was examined. The StAR transcripts were restricted in situ to theca of preovulatory follicles and luteinized granulosa and thecal cells of the corpus luteum. The cyclic nucleotide analog, 8-bromo-cAMP (8-Br-cAMP), increased StAR mRNA in all cell types studied by a process requiring on-going RNA and protein synthesis. Phorbol myristate acetate prevented the stimulatory effects of 8-Br-cAMP. In proliferating granulosa-lutein cells, 8-Br-cAMP increased StAR gene transcription and did not significantly affect StAR mRNA stability. Forskolin treatment was also found to increase the expression of a human StAR proximal promoter-luciferase fusion gene transfected into the proliferating granulosa-lutein cells. We conclude that 1) the StAR gene is expressed in the most steroidogenic compartments of the human ovary; 2) induction of StAR gene transcription by cAMP, produced in response to the LH surge, accounts for the appearance of StAR transcripts in luteinized granulosa cells; and 3) the effects of cAMP are antagonized by activators of protein kinase C.