Are Leydig cell steroidogenic enzymes differentially regulated with aging?

Previous studies have shown that the ability of Brown Norway rat Leydig cells to produce testosterone declines significantly with age. To address the possible mechanism(s) by which aging Leydig cells lose steroidogenic function, we determined the effect of age on the steady-state levels of the mRNAs for the steroidogenic enzymes P450 cholesterol side-chain cleavage (P450(scc)), Δ⁵-3β-hydroxysteroid dehydrogenase/Δ⁵-Δ⁴-isomerase (3β- HSD), and 17α-hydroxylase/C_17-20 lyase (P450(17α)), and on the levels of immunoreactive steroidogenic enzyme proteins and enzyme activities. Northern blot analysis revealed that the levels of P450(scc) and P450(17α) mRNAs in Leydig cells isolated from the testes of aged (22-month-old) Brown Norway rats were reduced from their levels in young (4-month-old) rats, but that 3β-HSD mRNA was not reduced. Western blot analysis, however, revealed that cellular levels of each of the P450(scc) P450(17α), and 3β-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 products through testosterone, similarly revealed that P450(scc), 3β-HSD, P450(17α), and additionally 17β-hydroxysteroid dehydrogenase (17β-HSD), were all reduced with aging. We conclude that age-related loss of steroidogenic function results at least in part from reductions in the levels and activities of each of the steroidogenic enzymes responsible for converting cholesterol to testosterone, and not by differential regulation of these enzymes.