Involvement of the PI3K/Akt pathway in estrogen-mediated regulation of human CYP7B1: Identification of CYP7B1 as a novel target for PI3K/Akt and MAPK signalling

The steroid hydroxylase CYP7B1 metabolizes neurosteroids, cholesterol derivatives, and estrogen receptor (ER) ligands. Previous studies identified CYP7B1 as a target for regulation by estrogen. The present study examines the mechanism for estrogen-mediated regulation of the human CYP7B1 gene promoter. Treatment with LY294002, a specific inhibitor of phosphatidylinositol 3-kinase (PI3K), abolished ER-mediated up-regulation of a CYP7B1 promoter-luciferase reporter in HepG2 cells, whereas overexpression of PI3K or Akt significantly increased estrogenic up-regulation of CYP7B1. Overexpression of dominant-negative mutant Akt abolished ER-mediated stimulation of CYP7B1 in HepG2 cells. Data indicated no binding of ER to CYP7B1 promoter sequences, suggesting that ER interacts with the PI3K/Akt pathway without binding to the gene. At low ER levels, overexpression of Akt suppressed CYP7B1 promoter activity, suggesting that its effect on CYP7B1 is different when estrogens are absent. In HEK293 cells, CYP7B1 transcription was much less affected by Akt, indicating that the mechanism for up-regulation of CYP7B1 is different in different cell types. Other experiments indicated that MAPK signalling may affect basal CYP7B1 levels. The current results, indicating that regulation of CYP7B1 by ER can be mediated via the PI3K/Akt signal pathway, a regulatory pathway important for cellular survival and growth, suggest an important role for CYP7B1 in cellular growth, particularly in connection with estrogenic signalling.