The proteasome inhibitor MG132 promotes accumulation of the steroidogenic acute regulatory protein (StAR) and steroidogenesis

StAR, a protein synthesized in the cytoplasm and subsequently imported into mitochondria, regulates the rate-determining step in steroidogenesis, the transport of cholesterol from the outer to the inner mitochondrial membrane . The active form of StAR is the 37 kDa pre-protein, which has a short half -life. To determine whether proteasomes participate in the turnover of StAR , we incubated primary cultures of preovulatory rat granulosa cells and imm ortalized human granulosa cells in the presence of MG132, a specific inhibi tor to proteasome catalysis, This treatment caused accumulation of StAR in unstimulated cells. Moreover, incubation of the cells with MG132 in the pre sence of forskolin (FK), luteinizing hormone/chorionic gonadotropin or foll icular stimulating hormone augmented the accumulation of both the 37 kDa cy toplasmic protein and the 30 kDa mature mitochondrial protein, compared to cells incubated with FK or the gonadotropic hormones alone. Concomitantly, progesterone production was enhanced, In contrast no elevation in the 37 kD a StAR intracellular levels or progesterone production was observed followi ng incubation of the cells with the cysteine protease inhibitor E-64. The i ncrease of the 37 kDa StAR protein was evident after 15 min and 30 min of i ncubation with MG132 (143% and 187% of control values, respectively) with n o significant elevation of the 30 kDa protein. Accumulation of the intermed iate mitochondrial 32 kDa protein was evident after 1-2 h and the accumulat ion of the 30 kDa protein was evident only after 4 h of incubation with MG1 32, In contrast, no elevation in adrenodoxin, a component of the cytochrome P450scc enzyme system, was found, These data suggest that StAR protein is either directly or indirectly degraded by the proteasome which may explain, in part, its short half-life. Moreover, it seems that the cytosolic 37 kDa protein, which is responsible for the steroidogenic activity of StAR, is t he primary proteasomal substrate and that the inhibition of its degradation by MG132 causes the up-regulation of progesterone production, (C) 2001 Fed eration of European Biochemical Societies. Published by Elsevier Science B. V. All rights reserved.