Novel role for the nuclear phosphoprotein SET in transcriptional activation of P450c17 and initiation of neurosteroidogenesis

Neurosteroids are important endogenous regulators of gamma-aminobutryic acid (GABA(A)) and N-methyl-D-aspartate (NMDA) receptors and also influence neuronal morphology and function. Neurosteroids are produced in the brain using many of the same enzymes found in the adrenal and gonad. The crucial enzyme for the synthesis of DHEA (dehydroepiandrosterone) in the brain is cytochrome P450c17, The transcriptional strategy for the expression of P450c17 is clearly different in the brain from that in the adrenal or gonad. We previously characterized a novel transcriptional regulator from Leydig MA-10 cells, termed StF-IT-1, that binds at bases -447/-399 of the rat P450c17 promoter, along with the known transcription factors COUP-TF (chicken ovalbumin upstream promoter transcription factor), NGF-IB (nerve growth factor inducible protein B), and SF-1 (steroidogenic factor-1). We have now purified and sequenced this protein from immature porcine testes, identifying it as the nuclear phosphoprotein SET; a role for SET in transcription was not established previously. Binding of bacterially expressed human and rat SET to the DNA site at -418/-399 of the rat P450c17 gene transactivates P450c17 in neuronal and in testicular Leydig cells. We also found SET expressed in human NT2 neuronal precursor cells, implicating a role in neurosteroidogenesis. Immunocytochemistry and in situ hybridization in the mouse fetus show that the ontogeny and distribution of SET in the developing nervous system are consistent with SET being crucial for initiating P450c17 transcription. SET's developmental pattern of expression suggests it may participate in the early ontogenesis of the nervous, as well as the skeletal and hematopoietic, systems. These studies delineate an important new factor in the transcriptional regulation of P450c17 and consequently, in the production of DHEA and sex steroids.