Pregnenolone biosynthesis in C6-2B glioma cell mitochondria: regulation by a mitochondrial diazepam binding inhibitor receptor.

The C6-2B glioma cell line, rich in mitochondrial receptors that bind with high affinity to benzodiazepines, imidazopyridines, and isoquinolinecarboxamides (previously called peripheral-type benzodiazepine receptors), was investigated as a model to study the significance of the polypeptide diazepam binding inhibitor (DBI) and the putative DBI processing products on mitochondrial receptor-regulated steroidogenesis. DBI and its naturally occurring fragments have been found to be present in high concentrations in C6-2B glioma cells, to compete against specific isoquinolinecarboxamide or 4'-chlorodiazepam binding to mitochondrial recognition sites with high affinity, and to stimulate mitochondrial pregnenolone formation. These data suggest that this cell type may express both the receptor and the putative agonist ligand to regulate steroidogenesis. Therefore, we propose to term this mitochondrial receptor MDR (mitochondrial DBI receptor) to indicate its responsiveness to DBI in steroid biosynthesis. In the present work, we show that mitochondria of C6-2B cells convert (22R)-22-hydroxycholesterol to pregnenolone by a mechanism blocked by aminoglutethimide. Immunoblotting confirmed the presence of relatively high levels of cytochrome P-450 cholesterol side-chain-cleavage enzyme in C6-2B cell mitochondria. Furthermore, isoquinolinecarboxamide binding sites associated with the 18-kDa mitochondrial polypeptide subunit of the MDR are abundant in C6-2B glioma cell mitochondria (Bmax approximately 30 pmol/mg protein) and are coupled to the regulation of steroid biosynthesis. Occupancy of MDRs with nanomolar concentrations of the naturally occurring polypeptide, DBI, as well as its naturally occurring processing product tetratriacontaneuropeptide [DBI-(17-50)] increases pregnenolone formation. Clonazepam and octadecaneuropeptide [DBI-(33-50)], which exhibit a higher affinity for gamma-aminobutyric acid type A receptors but a low affinity for MDR, were ineffective in stimulating pregnenolone synthesis. These findings provide evidence that C6-2B cells exhibit a significant steroidogenic activity which resembles that found in peripheral endocrine organs and they suggest that MDRs and DBI are involved in the regulation of glial cell steroidogenesis.