116 Chapter 5 immunodetection studies revealed that the distribution pattern of TSPO between rodents and humans is similar; secretory and glandular tissues were particularly rich in TSPO. These studies also indicated that although TSPO is present in most tissues in most species at various levels of expression, it is most abundant in steroid-synthesizing adrenal and gonadal tissues. The heart and kidney express intermediate levels of TSPO, while lower levels are found in the liver and brain. Interestingly, when considering the mitochondrial content of each tissue, there is not always a clear correlation between TSPO levels and mitochondrial content. This finding suggests that tissue- and cell-specific factors regulating TSPO gene expression are driving TSPO content rather than factors driving mitochondria formation. Steady-state mRNA profiling shows that TSPO mRNA is present in all tissues, and correlates well with reported protein expression levels24. Moreover, the expression patterns of mouse TSPO, were found to be well-correlated and similar to that reported for human TSPO33,34. TSPO levels were found to be elevated in cancer cell lines and numerous cancers suggesting a role for TSPO in cell proliferation and carcinogenesis23,24,28,35-37. Increased TSPO levels in cancer are due to gene amplification; Sp1, Sp3, and Sp4 transcription factor regulation of constitutive TSPO expression; and epigenetic modifications of the proximal promoter and first intron24,38,39. TSPO is an integral outer mitochondrial membrane protein spanning the membrane through its 5 α-helical domains40-53. While TSPO is a nuclear encoded protein, unlike most mitochondrial proteins, TSPO does not possess a mitochondrial targeting sequence, although it contains information on the C-terminus that is essential for its mitochondrial import54. After integration into the OMM, TSPO forms dimers and sometimes polymers19,55,56 at the outer and inner mitochondrial membrane contact sites where it becomes part of a larger protein complex57. This complex includes the OMM voltage-dependent anion channel 1 (VDAC1); ATPase family AAA domain-containing protein 3 (ATAD3), a protein that spans across the mitochondrial membranes, and in steroidogenic cells; and the inner mitochondrial membrane cytochrome P450 side-chain cleavage enzyme (CYP11A1), among others57,58. In addition, cytosolic, endoplasmic reticulum and Golgi proteins have been shown to associate with TSPO to form functional complexes58-60. When assembled together, these proteins function as a signal transduction complex, or “transduceosome” mediating the transmission of information to mitochondrial TSPO. Although 95 % of TSPO is found in the mitochondria, the protein can be found in intracellular locations other than mitochondria, such as the (peri)nuclear region and plasma membrane, likely playing different functions. Nevertheless, non-mitochondrial TSPO42,61 has received little attention so far. Characteristics and effects of endogenous and exogenous ligands TSPO is involved primarily in the mitochondria of steroid synthesizing cells. Steroidogenesis in the mitochondria begins with the transport of substrate cholesterol from intracellular stores into the mitochondria. Therefore, the role or roles of TSPO in mitochondrial steroidogenesis and cholesterol transport, in particular, were investigated. With the availability of high-affinity TSPO ligands, the function of TSPO in various tissues was explored, aiming to assess whether these ligands could affect mitochondrial function, including steroid production. Several TSPO ligands were found to affect mitochondrial
RkJQdWJsaXNoZXIy MTk4NDMw