Expression of steroidogenic acute regulatory protein (StAR) and LH receptor in MA-10 cells.

LH stimulation is mediated by its own receptor at the first step of the cascade, after which intracellular cAMP increases to stimulate the transcription of steroidogenic acute regulatory protein (StAR) in mouse MA-10 Leydig tumor cells. StAR mediates the rate-limiting step of steroidogenesis, which is the transfer of cholesterol to the inner mitochondrial membrane. Northern blot analysis consistently revealed two major transcripts, of about 3.6 kb and 1.6 kb, that hybridized with rat StAR mRNA. In this culture, treatment with hCG led StAR mRNA levels to rapidly and strongly increase by 3 h. Parallel increases were observed in transcripts of both sizes. Compared to StAR mRNA expression, LH receptor mRNA levels gradually decreased and declined to 50% of control values between 6 and 12 h incubation. Compared to the control, StAR mRNA levels increased and LH receptor mRNA levels decreased in a dose-dependent manner in the presence of increasing concentrations of hCG (3-100 ng/ml) and of increasing concentrations of 8-Br-cAMP (0.2-2 mM) after 4 h incubation. Since the over production of steroid hormones might be toxic to the own cells, the LH signal transduction that stimulates steroidogenesis might concomitantly decrease the responsiveness of steroidogenesis to LH stimulation at the receptor level. This result should be further investigated to clarify the mechanism of LH receptor regulation and steroidogenesis.