Corticosterone regulates cell proliferation and cytochrome P450 cholesterol side-chain cleavage enzyme messenger ribonucleic acid expression in primary cultures of fetal rat adrenals.

Glucocorticoids are known to inhibit growth in many different cell types. Although corticosterone is secreted by the adrenal cortex, its direct effect on the growth of different zones is poorly determined. We studied the effects of corticosterone on cell proliferation and cytochrome P450 cholesterol side-chain cleavage enzyme (P450scc; the rate-limiting step in adrenal steroidogenesis) messenger RNA (mRNA) accumulation in primary cultures of fetal rat adrenals. Adrenocortical cells, grown in the absence of ACTH for 3 weeks, possess typical features of zona glomerulosa cells. These cells differentiate into fasciculata-type cells and undergo biphasic proliferation when stimulated with ACTH. The primary antimitogenic phase of 24 h is followed by rapidly increased bromodeoxyuridine incorporation after 72 h of ACTH treatment. If the treatment is continued, the proliferation decreases again, but remains higher than the proliferation of the untreated cells. Undifferentiated zona glomerulosa-type cells secrete very low amounts of corticosterone. The 10% basal proliferation was not affected if exogenous corticosterone was added. However, if corticosterone was combined with ACTH for 3 days, it blocked the stimulatory growth effect of ACTH dose dependently. Etomidate, an inhibitor of steroidogenic enzymes, completely blocked corticosterone secretion. In our cultures it inhibited 50% of the proliferation of the zona glomerulosa-type cells. However, its effect was totally opposite in long term ACTH-treated cultures; in these fasciculata-type cells, etomidate stimulated the proliferation rate 3-fold. P450scc gene expression was low in undifferentiated zona glomerulosa-like cells. ACTH stimulation increased P450scc mRNA expression 10-fold. Exogenous corticosterone inhibited ACTH-induced P450scc mRNA accumulation by 50%, whereas etomidate doubled it. Our data suggest that a low corticosterone concentration supports the proliferation of undifferentiated zona glomerulosa-type cells, whereas a high corticosterone concentration inhibits the proliferation of differentiated zona fasciculata-type cells. In addition, a high corticosterone concentration may inhibit steroidogenesis by reducing P450scc expression. Thus, corticosterone may be an important modulator of adrenocortical cell proliferation and steroidogenesis in different zones of the adrenal cortex.