Leydig cell aging: Steroidogenic acute regulatory protein (StAR) and cholesterol side-chain cleavage enzyme

Primary points of control in steroidogenesis are the transport of cholester ol from intracellular stores to the inner mitochondrial membrane, and the s ubsequent conversion of cholesterol to pregnenolone by the cholesterol side -chain cleavage enzyme (P450(scc)). Testosterone production has been shown to decline in Brown Norway rat Leydig cells as the rats age. To better unde rstand the mechanism by which aging Leydig cells lose steroidogenic functio n, we examined the effect of aging on steroidogenic acute regulatory protei n (StAR), an important Leydig cell cholesterol transfer protein. and on P45 0(scc). Leydig cells isolated from middle-aged (14 months) and old (24 mont hs) rats produced significantly less testosterone than cells from young (4 months) rats. StAR mRNA (1.7 kilobase [kb]) was significantly reduced in Le ydig cells from middle-aged and old rats, by 26% and 52%, respectively. Sig nificant reductions also were seen in the steady-state levels of mRNA for P 450(scc), of 29% and 50%, respectively. Western blots revealed significant reductions in StAR protein, by 47% and 74%, respectively, and in P450(scc), protein, by 38% and 54%, respectively. In response to LH stimulation in vi tro, testosterone production by Leydig cells in young, middle-aged, and old rats increased by 30-, 40-, and 33-fold, respectively, although the amount s of testosterone produced by the young cells significantly exceeded that p roduced by the middle-aged and old cells. StAR protein also increased in re sponse to LH by 1.4-3-, and Ii-fold, respectively, whereas P450(scc) protei n remained unchanged. These results are consistent with the conclusion that compromise of StAR-mediated cholesterol transport may play a key role in a ge-related reductions in Leydig cell steroidogenesis. However, because P450 (scc) is reduced in old Leydig cells, the reaction catalyzed by this enzyme would be rate-limiting under circumstances in which saturating amounts of cholesterol entered the mitochondria.