INHIBITION OF TRANSCRIPTION AFFECTS SYNTHESIS OF STEROIDOGENIC ACUTE REGULATORY PROTEIN AND STEROIDOGENESIS IN MA-10 MOUSE LEYDIG TUMOR-CELLS

Hormonal induction of steroidogenesis in the adrenal and gonads is dep endent on the synthesis and function of the steroidogenic acute regula tory protein (StAR). As a first approach to investigate the role of tr anslation in the control of StAR expression, are examined StAR protein synthesis and steroid production in MA-10 mouse Leydig tumor cells in the presence of the transcriptional inhibitor, actino-mycin D. We sho w that human CG (hCG)-induced StAR synthesis, as determined by radiola beling MA-10 cells with [S-35]methionine and immunoprecipitation of St AR, is blocked by actinomycin D. The rate of hCG-stimulated progestero ne production is also decreased, but not completely blocked, suggestin g a possible StAR-independent mechanism that may contribute approximat ely 10-20% of the acute steroidogenic potential of the cells. When MA- 10 cells were pretreated with hCG to increase StAR messenger RNA level s and then the proteins radiolabeled in the presence of hCG or hCG plu s actinomycin D, no difference was observed in the amount of the 30-kD a StAR protein synthesized. However, a 50% increase in the precursor f orm of StAR protein was detected with hCG treatment alone. These data suggest that ongoing StAR protein synthesis is not inhibited by actino mycin D, but that continued synthesis requires transcriptional activit y. Progesterone production was inhibited by actinomycin D in the hCG-p retreated cells, supporting the proposal that maintaining StAR protein synthesis is required for optimal steroid production in MA-10 mouse L eydig tumor cells.