Hormonal induction of steroidogenesis in the adrenal and gonads is dependent on the synthesis and function of the
steroidogenic acute regulatory protein (StAR). As a first approach to investigate the role of translation in the control of StAR expression, we examined StAR
protein synthesis and
steroid production in MA-10 mouse Leydig
tumor cells in the presence of the transcriptional inhibitor,
actinomycin D. We show that human CG (hCG)-induced StAR synthesis, as determined by radiolabeling MA-10 cells with [35S]
methionine and immunoprecipitation of StAR, is blocked by
actinomycin D. The rate of hCG-stimulated
progesterone production is also decreased, but not completely blocked, suggesting a possible StAR-independent mechanism that may contribute approximately 10-20% of the acute steroidogenic potential of the cells. When MA-10 cells were pretreated with hCG to increase StAR
messenger RNA levels and then the
proteins radiolabeled in the presence of hCG or hCG plus
actinomycin D, no difference was observed in the amount of the 30-kDa StAR
protein synthesized. However, a 50% increase in the precursor form of StAR
protein was detected with hCG treatment alone. These data suggest that ongoing StAR
protein synthesis is not inhibited by
actinomycin D, but that continued synthesis requires transcriptional activity.
Progesterone production was inhibited by
actinomycin D in the hCG-pretreated cells, supporting the proposal that maintaining StAR
protein synthesis is required for optimal
steroid production in MA-10 mouse Leydig
tumor cells.