Peripheral-type benzodiazepine receptor: structure and function of a cholesterol-binding protein in steroid and bile acid biosynthesis

Abstract

Cholesterol transport from the outer to the inner mitochondrial membrane is the rate-determining step in steroid and bile acid biosyntheses. Biochemical, pharmacological and molecular studies have demonstrated that the peripheral-type benzodiazepine receptor (PBR) is a five transmembrane domain mitochondrial protein involved in the regulation of cholesterol transport. PBR gene disruption in Leydig cells completely blocked cholesterol transport into mitochondria and steroid formation, while PBR expression in bacteria, devoid of endogenous PBR and cholesterol, induced cholesterol uptake and transport. Molecular modeling of PBR suggested that cholesterol might cross the membrane through the five helices of the receptor and that synthetic and endogenous ligands might bind to common sites in the cytoplasmic loops. A cholesterol recognition/interaction amino acid consensus (CRAC) sequence in the cytoplasmic carboxy-terminus of the PBR was identified by mutagenesis studies. In vitro reconstitution of PBR into proteoliposomes demonstrated that PBR binds both drug ligands and cholesterol with high affinity. In vivo polymeric forms of PBR were observed and polymer formation was reproduced in vitro, using recombinant PBR protein reconstituted into proteoliposomes, associated with an increase in drug ligand binding and reduction of cholesterol-binding capacity. This suggests that the various polymeric states of PBR might be part of a cycle mediating cholesterol uptake and release into the mitochondria, with PBR functioning as a cholesterol exchanger against steroid product(s) arising from cytochrome P450 action. Taking into account the widespread presence of PBR in many tissues, a more general role of PBR in intracellular cholesterol transport and compartmentalization might be considered.

Introduction

In 1977, Braestrup and Squires searching for specific benzodiazepine receptors observed the presence of high density radiolabeled diazepam-binding sites in the kidney [1]. Since then, numerous publications have described this type of benzodiazepine-binding site in many tissues, including the central nervous system (CNS). Due to their abundance in peripheral tissues and in order to distinguish them from the central benzodiazepine receptors (GABA_A), this type of benzodiazepine-binding receptor was named “peripheral-type benzodiazepine receptor” or PBR [2], [3], [4], [5]. PBR is found in most tissues in the rat body [6], although it is expressed in high levels in steroid forming tissues and to a lesser extent in liver [5], [6], [7]. Subcellular fractionation studies demonstrated that PBR is primarily localized on the outer mitochondrial membrane [6], [8] and concentrated at the outer/inner mitochondrial membrane contact sites [9]. PBR, however, is not restricted to this organelle and other locations for this receptor have been identified [8], [10], [11]. The pharmacological, biochemical and molecular characteristics of PBR were reviewed in detail in recent publications [12], [13], [14]. The present review will focus exclusively on the data accumulated on the function of PBR in cholesterol transport and associated functions in various tissues.