Expression of sterol carrier protein 2 (SCP2) in human adrenocortical tissue

Sterol carrier protein 2 (SCP2) has been implicated in adrenal steroidogenesis by in vitro studies. In order to clarify the clinical significance of SCP2 in human steroidogenesis, we investigated the expression of SCP2 mRNA in various types of adrenocortical tissue and one testis and examined the correlation between the amounts of SCP2 and other values such as the free cholesterol content and the cholesterol side-chain cleavage (SCC) activity in the tissue mitochondria. The types of adrenocortical tissue examined included adrenocortical carcinomas (N = 3), adrenocortical adenomas from patients with Conn's syndrome (N = 3) and from patients with Cushing's syndrome (N = 3), non-functioning adrenocortical adenomas (N = 2) and normal adrenal glands (N = 2). Northern blot hybridization predominantly revealed a 1.8-kb SCP2 mRNA in all tissue specimens examined. The mRNA concentrations of SCP2 in two out of three adrenocortical carcinomas were relatively lower than those in other types of tissue. No other special tendency was observed regarding the mRNA expression levels in various tissue specimens. The mRNA concentrations of SCP2 correlated significantly with mitochondrial contents of free cholesterol (r = 0.67, p < 0.01), but was not correlated with the SCC activities in mitochondria measured by an in vitro enzyme assay. The mitochondrial SCC activities, however, were correlated significantly with the protein levels of mitochondrial P-450 scc determined by a Western blot analysis (r = 0.79, p < 0.01). The significant positive correlation between mRNA concentrations of SCP2 and the mitochondrial content of free cholesterol suggests that the central role of SCP2 in human steroidogenic tissues may be in part a translocation of cytoplasmic free cholesterol to the mitochondria, as demonstrated previously by in vitro studies.