The expression of steroidogenic acute regulatory protein (StAR) in bovine adrenocortical cells

StAR protein may facilitate rapid transfer of cholesterol from the outer to the inner mitochondrial membrane, the site at which cholesterol is convert ed to pregnenolone by the cholesterol side chain cleavage complex. We have studied the effect of ACTH treatment on StAR mRNA and protein levels in bov ine adrenocortical cells in primary culture. Cells were initially cultured for 3 days after isolation, and then treated with ACTH (10(-8) M) for vario us times up to 24 hours. Northern analysis of total BAC mRNA, using a [alph a(32)P]-labelled cDNA probe encoding a 5' region of bovine StAR mRNA, revea led two principal hybridising species of 1.6 and 3.0 kb. Western immunoblot analysis revealed a principal band at 30 kDa. Levels of both StAR mRNA and protein showed an increase at 1 hour, reached a maximum at around 6 hours and declined to basal levels at 24 hours. Cortisol secretion (measured by R IA) showed a similar change over the same period. From these results it app ears that StAR mRNA and protein levels in BAC are acutely regulated in conc ert with ACTH-stimulated cortisol secretion.