Transcription of cytochrome P450 46A1 in NIH3T3 cells is negatively regulated by FBS.

Extracellular administration of side-chain oxysterols, such as 24S-hydroxycholesterol (24S-HC), 27-hydroxycholesterol (27-HC) and 25-hydroxycholesterol (25-HC) to cells suppresses HMG-CoA reductase (Hmgcr) and CTP:phosphoethanolamine cytidylyltransferase (Pcyt2) mRNA levels. Oxysterols are enzymatically produced in cells from cholesterol by cytochrome P450 46A1 (Cyp46A1), Cyp27A1, Cyp3A11 and cholesterol 25-hydroxylase (Ch25h). We analyzed which of these oxysterol-producing enzymes are expressed in NIH3T3 cells and found that only Cyp46A1 was expressed. When Cyp46A1 was overexpressed in NIH3T3 cells, intrinsic oxysterols increased in the order 24S-HC > 25-HC > 27-HC. We investigated the mechanism regulating the production of endogenous oxysterols in NIH3T3 cells by Cyp46A1 and found that the mRNA, relative protein levels and enzymatic activity of Cyp46A1, and the amounts of 24S-HC, 25-HC and 27-HC significantly increased under serum-starved conditions, and these increases were suppressed by FBS supplementation. The aqueous phase of FBS obtained by the Bligh & Dyer method significantly suppressed Cyp46A1 mRNA levels. Fractionation of the aqueous phase by HPLC and analysis of the inhibiting fractions by nanoLC and TripleTOF MS/MS identified insulin-like factor-II (IGF-II). Cyp46A1 mRNA levels in serum-starved NIH3T3 cells were significantly suppressed by the addition of IGFs and insulin and endogenous oxysterol levels were decreased. CYP46A1 mRNA levels in the T98G human glioblastoma cell line were also increased by serum starvation but not by FBS supplementation, and the aqueous phase did not inhibit the increase. These results suggest that mRNA levels of Cyp46A1 are regulated by factors in FBS.