Regulation of steroid hormone biosynthesis in R2C and MA-10 Leydig tumor cells: Role of the cholesterol transfer proteins StAR and PBR

The MA-10 mouse Leydig tumor cell line produces large amounts of steroids only in response to hormonal stimulation while the R2C rat Leydig tumor cell line is constitutively steroidogenic in nature. In an effort to uncover the potential reasons for constitutive steroidogenesis in R2C cells, we have recently shown that compared to MA-10 cells, R2C cells express much higher levels of the Scavenger Receptor Class B type 1 which results in a higher capacity for cholesteryl ester uptake through the selective uptake pathway. We also found an enhanced expression of Hormone Sensitive Lipase and the Steroidogenic Acute Regulatory protein in these cells and reasoned that they may further facilitate the conversion of cholesteryl esters to free cholesterol and its mobilization to the inner mitochondrial membrane, thus rendering them constitutively steroidogenic. Given the proposed role of the peripheral-type benzodiazepine receptor (PBR) in conferring a constitutively steroidogenic phenotype to the R2C cells, the current study was conducted to investigate the relationship between its expression in MA-10 and R2C cells and correlate it with the constitutive nature of R2C cell steroidogenesis. Our studies show that PBR expression as measured by PK 11195 ligand binding and Western analysis is much higher in MA-10 cells than R2C cells. We also determined that the affinity of ligand binding to the PBR is comparable in the two cell lines, suggesting that PBR is unlikely to be solely responsible for the constitutive nature of R2C cell steroidogenesis.