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

In vitro studies using isolated cells, mitochondria and submitochondrial fractions demonstrated that in steroid synthesizing cells, the peripheral-type 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 mitochondrial membrane, the rate-determining step in steroid biosynthesis. Mitochondrial 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 cells in vitro resulted in the arrest of cholesterol transport into mitochondria and steroid formation; transfection of the mutant cells with a PER cDNA rescued steroidogenesis demonstrating an obligatory role for PER in cholesterol transport. Molecular modeling of PER suggested that it might function as a channel for cholesterol. This hypothesis was tested in a bacterial system devoid of PER and cholesterol. Cholesterol uptake and transport by these cells was induced upon PER expression. Amino acid deletion followed by site-directed mutagenesis studies and expression of mutant PBRs demonstrated the presence in the cytoplasmic carboxy-terminus of the receptor of a cholesterol recognition/interaction amino acid consensus sequence. This amino acid sequence may help for recruiting the cholesterol coming from intracellular sites to the mitochondria. (C) 1999 Elsevier Science Ltd. All rights reserved.