Event: 289

Key Event Title


Decrease, Translocator protein (TSPO)

Short name


Decrease, Translocator protein (TSPO)

Key Event Component


Process Object Action
gene expression translocator protein decreased

Key Event Overview

AOPs Including This Key Event




Level of Biological Organization


Biological Organization

Cell term


Cell term
steroid hormone secreting cell

Organ term


Taxonomic Applicability


Term Scientific Term Evidence Link
Homo sapiens Homo sapiens Strong NCBI
rat Rattus norvegicus Strong NCBI
Mus musculus Mus musculus Moderate NCBI

Life Stages


Sex Applicability


How This Key Event Works


Biological state

Translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), is a mitochondrial outer membrane protein implicated in cholesterol import to the inner mitochondrial membrane (Besman et al. 1989).

Biological compartments

The TSPO is present in virtually all mammalian peripheral tissues (Zisterer and Williams 1997), however highly prominent TSPO protein expression has been identified in steroidogenic tissues (R. R. Anholt et al. 1985), (Wang, Fan, and Papadopoulos 2012). The presence of TSOP has been confirmed in Leydig and Sertoli cells (Morohaku, Phuong, and Selvaraj 2013), granulosa cells (Amsterdam and Suh 1991) and to a lesser extent in thecal cells (Morohaku, Phuong, and Selvaraj 2013). In subcellular fractions, binding sites for the TSOP have been identified to be present in the outer mitochondrial membrane (OMM) (R. R. Anholt et al. 1985), (R. Anholt et al. 1986). Transcriptional regulation of TSPO genes has been examined and recently reviewed (Morohaku, Phuong, and Selvaraj 2013).

General role in biology: regulation of lipid transport

TSPO mediates the delivery of the substrate cholesterol to the inner mitochondrial side chain cleavage enzyme P450scc (Besman et al. 1989). TSPO ligands stimulate steroidogenesis and induce cholesterol movement from the outer mitochondrial membrane (OMM) to the inner mitochondrial membrane (IMM) (Besman et al. 1989).

How It Is Measured or Detected


TSPO levels can be assayed by standard methods for assessment of gene expression levels like qPCR or direct protein levels by Western blot.

The level of TSPO as well as other steroidogenic protein can be measured in vitro cultured Leydig cells. The methods for culturing Leydig cells can be found in the Database Service on Alternative Methods to animal experimentation (DB-ALM): Leydig Cell-enriched Cultures [1], Testicular Organ and Tissue Culture Systems [2].

Uncertainties and Inconsistencies

This information needs to be moved to a key event relationship page.

TSPO -knockout mice have shown embryonic lethality (Lacapère and Papadopoulos 2003); in contrast recent findings have shown no effect on viability of foetuses (Tu et al. 2014). Aberrant TSPO levels have been linked to multiple diseases, including cancer, endocrine disorders, brain injury, neurodegeneration, ischemia-reperfusion injury and inflammatory diseases (Wang, Fan, and Papadopoulos 2012). However, recent studies have shown opposite results. Peripheral benzodiazepine receptor/translocator protein global knock-out mice are viable and show no effects on steroid hormone biosynthesis (Tu et al. 2014), (Morohaku et al. 2014). As stated in a recent review "At this point in time, a functional designation for TSPO is still actively being sought" (Selvaraj, Stocco, and Tu 2015).

Evidence Supporting Taxonomic Applicability


TSPO is a protein that shows high DNA sequence conservation from bacteria to mammals. It is expressed ubiquitously, but most abundant in steroidogenic cells (Yeliseev, Krueger, and Kaplan 1997).



Amsterdam, A. & Suh, B.S., 1991. An inducible functional peripheral benzodiazepine receptor in mitochondria of steroidogenic granulosa cells. Endocrinology, 129(1), pp.503–10.

Anholt, R. et al., 1986. The peripheral-type benzodiazepine receptor. Localization to the mitochondrial outer membrane. J. Biol. Chem., 261(2), pp.576–583.

Anholt, R.R. et al., 1985. Peripheral-type benzodiazepine receptors: autoradiographic localization in whole-body sections of neonatal rats. The Journal of pharmacology and experimental therapeutics, 233(2), pp.517–26.

Besman, M.J. et al., 1989. Identification of des-(Gly-Ile)-endozepine as an effector of corticotropin-dependent adrenal steroidogenesis: stimulation of cholesterol delivery is mediated by the peripheral benzodiazepine receptor. Proceedings of the National Academy of Sciences of the United States of America, 86(13), pp.4897–901.

Lacapère, J.J. & Papadopoulos, V., 2003. Peripheral-type benzodiazepine receptor: structure and function of a cholesterol-binding protein in steroid and bile acid biosynthesis. Steroids, 68(7-8), pp.569–85.

Morohaku, K. et al., 2014. Translocator protein/peripheral benzodiazepine receptor is not required for steroid hormone biosynthesis. Endocrinology, 155(1), pp.89–97. Morohaku, K., Phuong, N.S. & Selvaraj, V., 2013. Developmental expression of translocator protein/peripheral benzodiazepine receptor in reproductive tissues. W. Yan, ed. PloS one, 8(9), p.e74509.

Papadopoulos, V. et al., 1997. Targeted disruption of the peripheral-type benzodiazepine receptor gene inhibits steroidogenesis in the R2C Leydig tumor cell line. The Journal of biological chemistry, 272(51), pp.32129–35.

Tu, L.N. et al., 2014. Peripheral benzodiazepine receptor/translocator protein global knock-out mice are viable with no effects on steroid hormone biosynthesis. The Journal of biological chemistry, 289(40), pp.27444–54.

Wang, H.-J., Fan, J. & Papadopoulos, V., 2012. Translocator protein (Tspo) gene promoter-driven green fluorescent protein synthesis in transgenic mice: an in vivo model to study Tspo transcription. Cell and tissue research, 350(2), pp.261–75.

Yeliseev, A.A., Krueger, K.E. & Kaplan, S., 1997. A mammalian mitochondrial drug receptor functions as a bacterial “oxygen” sensor. Proceedings of the National Academy of Sciences of the United States of America, 94(10), pp.5101–6. Zisterer, D.M. & Williams, D.C., 1997. Peripheral-type benzodiazepine receptors. General pharmacology, 29(3), pp.305–14.