We have previously demonstrated that tumor necrosis factor alpha (TNF-alpha), a multifunctional cytokine mainly produced by activated monocytes, inhibits the ACTH-induced production of cortisol in cultures of human fetal adrenals. To clarify the molecular basis of this suppression, we investigated the effect of recombinant TNF-alpha (rTNF-alpha) on the messenger RNAs (mRNAs) for adrenal cytochrome P450 oxidases, P450scc (cholesterol side-chain cleavage enzyme/20.22-desmolase), P450c11 (11 beta-hydroxylase/18-hydroxylase/18-methyl oxidase), P450c17 (17 alpha-hydroxylase/17,20-lyase), and P450c21 (21-hydroxylase). Northern and dot blot experiments showed that 36 h incubation of primary cultures of human fetal adrenals with ACTH (200 ng/ml) increased the levels of all P450 enzymes severalfold. Preincubation of the cultures with rTNF-alpha at concentrations ranging from 0.1-100 ng/ml produced a dose-dependent inhibition of the ACTH-induced accumulation of all P450 mRNAs. The decrease in the expression of genes for steroidogenic enzymes was accompanied by a similar decrease in the production of cortisol but not in that of dehydroepiandrosterone sulphate nor androstenedione. Neither the basal expression of P450 enzymes nor the basal secretion of the steroids was significantly altered by 10 ng/ml of rTNF-alpha. rTNF-alpha did not affect the level of actin mRNA, the cell viability, nor the cell number. All the effects brought about by rTNF-alpha could be neutralized by addition of monoclonal anti-TNF-alpha antibody. These results show that TNF-alpha suppresses the synthesis of cortisol and shifts the steroid secretory pattern towards androgen production at least partly by suppressing the accumulation of mRNAs for adrenal cytochrome P450 oxidases.