Current Pharmaceutical Design, 2007, 13, 2385-2405 2385 1381-6128/07 $50.00+.00 © 2007 Bentham Science Publishers Ltd. Channel-Like Functions of the 18-kDa Translocator Protein (TSPO): Regulation of Apoptosis and Steroidogenesis as Part of the Host-Defense Response Leo Veenman 1 , Vassilios Papadopoulos 2, * and Moshe Gavish 1, * 1 Department of Pharmacology, Rappaport Family Institute for Research in the Medical Sciences, Technion-Israel Institute of Tech- nology, Haifa, Israel and 2 Department of Biochemistry and Molecular & Cellular Biology, Georgetown University Medical Center, 3900 Reservoir Road, NW, Washington, DC 20057, USA Abstract: Due to its channel-like properties, the peripheral-type benzodiazepine receptor (PBR) has been renamed the translocator pro- tein (TSPO). In eukaryotes, the TSPO is primarily located in the outer mitochondrial membrane. In prokaryotes, it is found in the cell membrane. A broad spectrum of functions has been attributed to the TSPO, including various host defense responses, developmental processes, and mitochondrial functions. In the present review, we focus on the role of TSPO in immunological responses, apoptosis, and steroidogenesis, to determine whether these functions may be governed by a common denominator including TSPO. At physiological concentrations (nM range), the TSPO specific ligands, PK 11195 and Ro5-4864, appear to be anti-apoptotic. Knockdown of TSPO by genetic manipulation, resulting a reduction by more than 50% in [ 3 H]PK 11195 binding, was reported to show anti-apoptotic effects, sug- gesting a potential pro-apoptotic function of TSPO. However, a reduction of more than 70% of TSPO abundance was found to cause cell death, possibly due to impairment of other essential cell functions. The pro-apoptotic function of TSPO may involve the modulation of the channel formed by the mitochondrial voltage-dependent anion channel (VDAC) and the adenine nucleotide transporter (ANT) [i.e., the mitochondrial permeability transition pore (MPTP)]. The frequently reported pro-apoptotic effects of PK 11195 and Ro5-4864 may be due to sites with low-affinity binding for these specific TSPO ligands, and not directly related to VDAC and ANT. Also at concentra- tions in the nM range, PK 11195 and Ro5-4864 appear to stimulate steroidogenesis. For this function TSPO by itself appears to suffice i.e. no involvement of VDAC and ANT. TSPO appears to operate as a translocator/channel to transfer cholesterol into mitochondria where it is converted to pregnenolone, a precursor of further steroidogenesis. Apoptosis and steroids play important roles in various as- pects of the host defense response. Thus, our review suggests that the involvement of TSPO and its ligands in such seemingly disparate biological functions as immunological responses, apoptosis, and steroidogenesis may have a common denominator in the multi- dimensional role of TSPO in the host-defense response to disease and injury. Key Words: Peripheral-type benzodiazepine receptor, PK 11195, Ro5-4864, apoptosis, steroidogenesis. INTRODUCTION In the 1970s, Braestrup and Squires [1] reported the presence of benzodiazepine-binding sites outside the central nervous system (CNS). One of the frequently used names for this benzodiazepine- binding site is the peripheral-type benzodiazepine receptor (PBR) as opposed to the central-type benzodiazepine receptor (CBR) that is part of the GABA receptor in the CNS [2,3]. The PBR, on the other hand, is unrelated to the GABA receptor. The peripheral ben- zodiazepine-binding site is part of a complex that consists of at least three components: an isoquinoline-binding protein (IBP; 18 kDa), a voltage-dependent anion channel (VDAC; 32 kDa); and an adenine nucleotide transporter (ANT; 30 kDa) [4]. Subsequently, many studies only considered the IBP component of the complex as the PBR [3,5], while several other studies considered the entire com- plex that consisted of IBP, VDAC, and ANT to be the PBR [2,4,6,7]. In addition, various other names were given to the PBR. Recently, a large assembly of researchers from widespread geo- graphical locations who study diverse topics regarding the research field of the PBR, addressed this problem of diverging nomenclature [3,7]. Finally, a consensus was reached where it was decided that the new name would be “translocator protein” with the acronym “TSPO” [3]. TSPO refers to the 18-kDa protein, named IBP above. This new name, TSPO, will be used throughout the rest of the pre- sent review. *Address correspondence to these authors at the Department of Pharmacol- ogy, Rappaport Family Institute for Research in the Medical Sciences, Technion-Israel Institute of Technology, P.O.B. 9649, Bat-Galim, Haifa 31096, Israel; Tel: 972-4-8295275; Fax: 972-4-8295271; E-mail: mgavish@techunix.technion.ac.il; Department of Biochemistry and Molecular & Cellular Biology, George- town University Medical Center, 3900 Reservoir Road, NW, Washington, DC 20057, USA; Tel: 202-687-8991; Fax: 202-687-7855; E-mail: papadopv@georgetown.edu GENERAL BACKGROUND TSPOs in Prokaryotes and Eukaryotes TSPOs are found throughout the animal kingdom, including insects, mollusks, pisces, amphibians, aves, and mammals [7-12]. Surprisingly, TSPOs have not been detected in reptiles to date [13- 15]. TSPO, as well as VDAC and ANT, shows a high degree of homology between various animal species [2,7,16,17]. A TSPO knockout by gene targeting resulted in early embryonic lethality, indicating the importance of TSPO [18]. TSPOs have also been detected in various plant species, such as Arabidopsis thaliana and Solanum tuberosum, where it may play a role in both steroidogenesis and development [5,19,20]. In addition, plant extracts, such as EGb 761 (EGb) from Ginkgo biloba leaves, were found to affect TSPO expression in animal tissues and cells, including steroidogenesis and cancer cell proliferation [21-26]. Ginkgolide B appeared to be one component of EGb 761 able to modulate TSPO expression [22]. It would be interesting to study this and other active compounds of EGb 761 and the detailed mechanisms by which they modulate TSPO function. In 1995, a 17-kDa protein with an unusually high content of aromatic amino acids in general and of L-tryptophan in particular was reported to be localized to the outer membrane of aerobically grown Rhodobacter sphaeroides 2.4.1 [27]. This tryptophan-rich sensory protein gene, TspO (formerly crtK, ORF160) is present in several-fold higher levels in photosynthetic as opposed to aerobi- cally grown cells [27]. TspO in R. sphaeroides 2.4.1 was shown to function as an oxygen sensor [28-30]. TspO also appeared to be important for pigment production and shows significant sequence homology and similarity to the TSPO from both mammalian and plant sources [19,27-29,31]. A homologue of TspO was also found in Sinorhizobium meliloti strain 1021 and Fremyella diplosiphon [32,33]. It has been suggested that TspO is involved in the bacterial porphyrin biosynthesis pathway, a function it may share with mammalian TSPO [3,34,35]. Not For Distribution