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 typ es. Although corticosterone is secreted by the adrenal cortex, its dir ect effect on the growth of different zones is poorly determined. We s tudied the effects of corticosterone on cell proliferation and cytochr ome P450 cholesterol side-chain cleavage enzyme (P450(scc); the rate-l imiting step in adrenal steroidogenesis) messenger RNA (mRNA) accumula tion in primary cultures of fetal rat adrenals. Adrenocortical cells, grown in the absence of ACTH for 3 weeks, possess typical features of tons glomerulosa cells. These cells differentiate into fasciculata-typ e cells and undergo biphasic proliferation when stimulated with ACTH. The primary antimitogenic phase of 24 h is followed by rapidly increas ed bromodeoxyuridine incorporation after 72 h of ACTH treatment. If th e treatment is continued, the proliferation decreases again, but remai ns higher than the proliferation of the untreated cells. Undifferentia ted zona glomerulosa-type cells secrete very low amounts of corticoste rone. The 10% basal proliferation was not affected if exogenous cortic osterone was added. However, if corticosterone was combined with ACTH for 3 days, it blocked the stimulatory growth effect of ACTH dose depe ndently. Etomidate, an inhibitor of steroidogenic enzymes, completely blocked corticosterone secretion. In our cultures it inhibited 50% of the proliferation of the tons glomerulosa-type cells. However, its eff ect was totally opposite in long term ACTH-treated cultures; in these fasciculata-type cells, etomidate stimulated the proliferation rate 3- fold. P450(scc) gene expression was low in undifferentiated zona glome rulose-like cells. ACTH stimulation increased P450(scc) mRNA expressio n 10-fold. Exogenous corticosterone inhibited ACTH-induced P450(scc) m RNA accumulation by 50%, whereas etomidate doubled it. Our data sugges t that a low corticosterone concentration supports the proliferation o f undifferentiated zone glomerulosa-type cells, whereas a high cortico sterone concentration inhibits the proliferation of differentiated zon a fasciculata-type cells. In addition, a high corticosterone concentra tion may inhibit steroidogenesis by reducing P450(scc) expression. Thu s, corticosterone may be an important modulator of adrenocortical cell proliferation and steroidogenesis in different zones of the adrenal c ortex.