In vitro studies on the role of the peripheral-type benzodiazepine receptor in steroidogenesis

In vitro studies using isolated cells, mitochondria and submitochondrial fr actions demonstrated that in steroid synthesizing cells, the peripheral-typ e benzodiazepine receptor (PBR) is an outer mitochondrial membrane protein, preferentially located in the outer/inner membrane contact sites, involved in the regulation of cholesterol transport from the outer to the inner mit ochondrial membrane, the rate-determining step in steroid biosynthesis. Mit ochondrial PER ligand binding characteristics and topography are sensitive to hormone treatment suggesting a role of PER in the regulation of hormone- mediated steroidogenesis. Targeted disruption of the PER gene in Leydig cel ls in vitro resulted in the arrest of cholesterol transport into mitochondr ia and steroid formation; transfection of the mutant cells with a PER cDNA rescued steroidogenesis demonstrating an obligatory role for PER in cholest erol transport. Molecular modeling of PER suggested that it might function as a channel for cholesterol. This hypothesis was tested in a bacterial sys tem devoid of PER and cholesterol. Cholesterol uptake and transport by thes e cells was induced upon PER expression. Amino acid deletion followed by si te-directed mutagenesis studies and expression of mutant PBRs demonstrated the presence in the cytoplasmic carboxy-terminus of the receptor of a chole sterol recognition/interaction amino acid consensus sequence. This amino ac id sequence may help for recruiting the cholesterol coming from intracellul ar sites to the mitochondria. (C) 1999 Elsevier Science Ltd. All rights res erved.