Cholesterol trafficking in steroidogenic cells. Reversible cycloheximide-dependent accumulation of cholesterol in a pre-steroidogenic pool.

Peptide hormones activate steroid hormone biosynthesis in responsive tissues by stimulating the delivery of cholesterol to a steroidogenic pool, thought to be located in the inner mitochondrial membrane. At this site, it is metabolized to pregnenolone, the precursor of the steroid hormones, by side-chain-cleaving cytochrome P-450 (cytochrome P-450scc). In the presence aminoglutethimide (an inhibitor of cytochrome P-450scc) and an activating stimulus, cholesterol accumulates in the steroidogenic pool, and increased pregnenolone generation is observed upon removal of the inhibitor. Using Y-1 adrenocortical cells and MA-10 Leydig tumor cells, we now provide evidence for a distinct, functionally relevant cholesterol pool which precedes the steroidogenic pool, which we designate the pre-steroidogenic pool. This pool was defined by activating the cells with 8-bromo-adenosine 3',5'-cyclic monophosphoric acid in the presence of cycloheximide, an inhibitor of steroidogenesis. Following a wash procedure, which removed 8-bromo-adenosine 3',5'-cyclic monophosphoric acid and cycloheximide, augmented pregnenolone synthesis was observed. Unlike synthesis from the steroidogenic pool, pregnenolone formation from pre-steroidogenic pool in Y-1 cells indicates that this pool is somewhat smaller than the steroidogenic pool. The results support a cholesterol-trafficking model in which cycloheximide-sensitive transport from the pre-steroidogenic pool to the steroidogenic pool precedes metabolism, and is regulated by cAMP.