ARE LEYDIG-CELL STEROIDOGENIC ENZYMES DIFFERENTIALLY REGULATED WITH AGING

Previous studies have shown that the ability of Brown Norway rat Leydi g cells to produce testosterone declines significantly with age. To ad dress the possible mechanism(s) by which aging Leydig cells lose stero idogenic function, we determined the effect of age on the steady-state levels of the mRNAs for the steroidogenic enzymes P450 cholesterol si de-chain cleavage (P450(scc)), Delta(5)-3 beta-hydroxysteroid dehydrog enase/Delta(5)-Delta 4-isomerase (3 beta-HSD), and 17 alpha-hydroxylas e/C-17-20 lyase (P450(17 alpha)), and on the levels of immunoreactive steroidogenic enzyme proteins and enzyme activities. Northern blot ana lysis revealed that the levels of P450(scc) and P450(17 alpha) mRNAs i n Leydig cells isolated from the testes of aged (22-month-old) Brown N orway rats were reduced from their levels in young (4-month-old) rats, but that 3 beta-HSD mRNA was not reduced. Western blot analysis, howe ver, revealed that cellular levels of each of the P450(scc), P450(17 a lpha), and 3 beta-HSD proteins were reduced with aging. The activities of the steroidogenic enzymes, assessed by incubating Leydig cells in culture with substrate and then summing all steroidogenic reaction pro ducts through testosterone, similarly revealed that P450(scc), 3 beta- HSD, P450(17 alpha), and additionally 17 beta-hydroxysteroid dehydroge nase (17 beta-HSD), were all reduced with aging. We conclude that age- related loss of steroidogenic function results at least in part from r eductions in the levels and activities of each of the steroidogenic en zymes responsible for converting cholesterol to testosterone, and not by differential regulation of these enzymes.