Effects of disruption of the mitochondrial electrochemical gradient on steroidogenesis and the Steroidogenic Acute Regulatory (StAR) protein

The steroidogenic acute regulatory (StAR) protein, which mediates cholester ol delivery to the inner mitochondrial membrane and the P450scc enzyme, has been shown to require a mitochondrial electrochemical gradient for its act ivity in vitro. To characterize the role of this gradient in cholesterol tr ansfer, investigations were conducted in whole cells, utilizing the protono phore carbonyl cyanide m-chlorophenylhydrazone (m-CCCP) and the potassium i onophore valinomycin. These reagents, respectively, dissipate the mitochond rial electrochemical gradient and inner mitochondrial membrane potential. B oth MA-10 Leydig tumor cell steroidogenesis and mitochondrial import of StA R were inhibited by m-CCCP or valinomycin at concentrations which had only minimal effects on P450scc activity, m-CCCP also inhibited import and proce ssing of both StAR and the truncated StAR mutants, N-19 and C-28, in transf ected COS-1 cells. Steroidogenesis induced by StAR and N-47, an active N-te rminally truncated StAR mutant, was reduced in transfected COS-1 cells when treated with m-CCCP. This study shows that StAR action requires a membrane potential, which may reflect a functional requirement for import of StAR i nto the mitochondria, or more likely, an unidentified factor which is sensi tive to ionophore treatment. Furthermore, the ability of N-47 to stimulate steroidogenesis in nonsteroidogenic HepG2 liver tumor cells, suggests that the mechanism by which StAR acts may be common to many cell types. (C) 1999 Elsevier Science Ltd. All rights reserved.