Steroidogenic acute regulatory protein mRNA expression in adrenal tumours

The rate limiting step in steroidogenesis is cholesterol transport through the outer to the inner mitochondrial membrane and the cytochrome P450 side chain cleavage (P450scc) complex. The protein factor responsible for this t ransport, and as such necessary for regulating the acute production of ster oids, has been identified and named the steroidogenic acute regulatory prot ein (StAR). We investigated the expression of StAR in functional and non-fu nctional adrenal neoplasms and compared the expression with that of P450scc . Poly A RNA was extracted from normal adrenal glands (NAG, n = 5), aldoste rone producing adenomas (APA, n = 4), cortisol producing adenomas (CPA, n = 5), adrenocortical carcinomas (ACC, n = 6) and non-functional adenomas (NF A, n = 3), electrophoresed through a 1% agarose gel, blotted and hybridised with a PCR-generated cDNA labelled with [P-32]CTP. The blots were stripped and re-hybridised with a P450scc cDNA and a mouse beta-actin probe. Compar ed with P450scc, StAR mRNA expression showed little variability in the magn itude of expression and did not correlate with the endocrine profiles (NAG: StAR 100 +/- 16%, P450scc 100 +/- 14%; APA: StAR 80 +/- 3%, P450scc 94 +/- 13%: CPA: StAR 71 +/- 10%, P450scc 109 +/- 15%: NFA: StAR 64 +/- 9.5%, P45 0scc 18 +/- 5%; means +/- S.E.M.). ACC expressed low levels of both genes p robably as a result of dedifferentiation (StAR 29 +/- 9%, P450scc 46 +/- 18 %). Incubation of the NCI-h295 tumour cell line with 10 nmol ACTH and 10 mu mol forskolin induced an increase in the abundance of StAR and P450scc mRN A, demonstrating gene regulation by the cAMP protein kinase A pathway. Furt hermore, we incubated the NCI-h295 tumour cell line with the adrenostatic c ompounds, aminoglutethimide and metyrapone. We could not detect an effect o n the expression of StAR mRNA, whereas the expression of P450scc mRNA was s ignificantly reduced. We conclude that, in contrast to P450scc, StAR seems to be evenly expressed in adrenocortical adenomas, Therefore, the endocrine activity of a given tumour cannot be explained by the abundance of StAR ex pression. In ACC, both StAR and P450scc expression is low explaining the re latively inefficient steroid production of these tumours.