Human
steroid biosynthesis depends on a specifically regulated cascade of
enzymes including 3β-hydroxysteroid
dehydrogenases (HSD3Bs). Type 2 HSD3B catalyzes the conversion of
pregnenolone, 17α-hydroxypregnenolone and
dehydroepiandrosterone to
progesterone, 17α-hydroxyprogesterone and
androstenedione in the human adrenal cortex and the gonads but the exact regulation of this
enzyme is unknown. Therefore, specific downregulation of
HSD3B2 at adrenarche around age 6-8 years and characteristic upregulation of
HSD3B2 in the ovaries of women suffering from the
polycystic ovary syndrome remain unexplained prompting us to study the regulation of
HSD3B2 in adrenal NCI-H295R cells. Our studies confirm that the
HSD3B2 promoter is regulated by
transcription factors GATA, Nur77 and SF1/LRH1 in concert and that the NBRE/Nur77 site is crucial for hormonal stimulation with cAMP. In fact, these three
transcription factors together were able to transactivate the
HSD3B2 promoter in placental JEG3 cells which normally do not express
HSD3B2. By contrast, epigenetic mechanisms such as methylation and acetylation seem not involved in controlling
HSD3B2 expression.
Cyclic AMP was found to exert differential effects on
HSD3B2 when comparing short (acute) versus long-term (chronic) stimulation. Short cAMP stimulation inhibited
HSD3B2 activity directly possibly due to regulation at co-factor or substrate level or posttranslational modification of the
protein. Long cAMP stimulation attenuated
HSD3B2 inhibition and increased
HSD3B2 expression through transcriptional regulation. Although PKA and MAPK pathways are obvious candidates for possibly transmitting the cAMP signal to
HSD3B2, our studies using PKA and MEK1/2 inhibitors revealed no such downstream signaling of cAMP. However, both signaling pathways were clearly regulating
HSD3B2 expression.