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

Neurosteroids are important endogenous regulators of gamma-aminobutryic aci d (GABA(A)) and N-methyl-D-aspartate (NMDA) receptors and also influence ne uronal morphology and function. Neurosteroids are produced in the brain usi ng many of the same enzymes found in the adrenal and gonad. The crucial enz yme for the synthesis of DHEA (dehydroepiandrosterone) in the brain is cyto chrome P450c17, The transcriptional strategy for the expression of P450c17 is clearly different in the brain from that in the adrenal or gonad. We pre viously characterized a novel transcriptional regulator from Leydig MA-10 c ells, termed StF-IT-1, that binds at bases -447/-399 of the rat P450c17 pro moter, along with the known transcription factors COUP-TF (chicken ovalbumi n upstream promoter transcription factor), NGF-IB (nerve growth factor indu cible protein B), and SF-1 (steroidogenic factor-1). We have now purified a nd sequenced this protein from immature porcine testes, identifying it as t he nuclear phosphoprotein SET; a role for SET in transcription was not esta blished previously. Binding of bacterially expressed human and rat SET to t he 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 hum an NT2 neuronal precursor cells, implicating a role in neurosteroidogenesis . Immunocytochemistry and in situ hybridization in the mouse fetus show tha t the ontogeny and distribution of SET in the developing nervous system are consistent with SET being crucial for initiating P450c17 transcription. SE T'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 transcrip tional regulation of P450c17 and consequently, in the production of DHEA an d sex steroids.