Regulation of translocator protein 18-kDa (Tspo) gene expression through a protein kinase C (PKC) epsilon signal transduction pathway

Translocator Protein (18-kDa) TSPO is a high-affinity drug- and cholesterol-binding protein. TSPO regulates the transport of the steroid hormone precursor, cholesterol, to the inner mitochondrial membrane, the rate-determining step in steroidogenesis. The ubiquitous expression of TSPO, taken with its ability to bind cholesterol with high affinity suggests that, in non-steroidogenic cells, TSPO may regulate mitochondrial cholesterol compartmentalization and membrane biogenesis, events critical for mitochondrial function and multiple cellular processes. In search of the factors regulating Tspo expression and to understand the differential expression of TSPO in steroidogenic and non-steroidogenic cells, I analyzed Tspo transcriptional responses to the protein kinase C (PKC) activator and tumor promoter, phorbol-12-myristate 13-acetate (PMA), in cells with varying TSPO levels. PMA induced Tspo promoter activity and Tspo mRNA levels in poor-in-TSPO non-steroidogenic cells (NIH-3T3 fibroblasts and COS-7 kidney), but not in rich-in-TSPO steroidogenic cells (MA-10 Leydig) with high basal Tspo transcriptional activity. We localized the stimulatory effect of PMA to the 805-515-bp region upstream of the transcription start site. PMA exerted its effect on Tspo gene promoter through the PKC epsilon isoform. PKC epsilon activation by PMA drives inducible TSPO expression in non-steroidogenic cells, likely through activator protein 1 (AP1) and v-ets erythroblastosis virus E26 oncogene homolog (Ets) sites in this promoter region, while high levels of TSPO in steroidogenic cells are likely due to high constitutive expression of PKC epsilon. In addition to AP1 and Ets binding sites in the Tspo promoter, there are binding sites for signal transducer and activation of transcription 3 (STAT3) transcription factor, reported downstream targets of PKC epsilon. PKC epsilon overexpression induced STAT3 phosphorylation and c-Jun synthesis in NIH-3T3 cells while PKC epsilon knockdown reduced c-Jun new synthesis in NIH-3T3 and STAT3 phosphorylation in Leydig cells. Furthermore, we identified a MAPK (Raf-1-MEK1/2-ERK1/2) to be the signaling transduction pathway through which PKC epsilon mediates its effect to regulate Tspo gene expression in steroidogenic and nonsteroidogenic cells acting at least in part through c-Jun and STAT3 transcription factors.