In nonhepatic cells, cholesterol 7 alpha-hydroxylase induces the expression of genes regulating cholesterol biosynthesis, efflux, and homeostasis

CHO cells expressing the liver-specific gene product cholesterol-7 alpha-hy droxylase showed a 6-fold increase in the biosynthesis of [C-14]cholesterol from [C-14]acetate, as well as increased enzymatic activities of 3-hydroxy -3-methylglutarylcoenzyme A (HMG-CoA) reductase and squalene synthase, Cell s expressing cholesterol-7 alpha-hydroxylase contained less sterol response element-binding protein 1 (SREBP1) precursor, whereas the cellular content of mature SREBP1, as well as the mRNAs of cholesterol biosynthetic genes ( HMG-CoA reductase and squalene synthase), were all increased similar to 3-f old. Cells expressing cholesterol-7 alpha-hydroxylase displayed greater act ivities of luciferase reporters containing the SREBP-dependent promoter ele ments derived from HMG-CoA reductase and farnesyl diphosphate synthase, in spite of accumulating significantly more free and esterified cholesterol an d 7 alpha-hydroxycholesterol. While cells expressing cholesterol-7 alpha-hy droxylase displayed increased SREBP-dependent transcription, sterol-mediate d repression of SREBP-dependent transcription by LDL-cholesterol and exogen ous oxysterols was similar in both cell types. Cells expressing cholesterol -7 alpha-hydroxylase displayed greater rates of secretion of cholesterol as well as increased expression of the ABC1 cassette protein mRNA, Adding 25- hydroxycholesterol to the culture medium of both cell types increased the e xpression of ABC1 cassette protein mRNA.jlr The combined data suggest that in nonhepatic CHO cells multiple regulatory processes sensitive to cellular sterols act independently to coordinately maintain cellular cholesterol ho meostasis.