Defects in CYB5A and CYB5B impact sterol-C4 oxidation in cholesterol biosynthesis and demonstrate regulatory roles of dimethyl sterols.

Cytochrome b5 (CYB5) is a hemoprotein crucial for electron transfer to oxygenases. Although microsomal CYB5A is required for sterol C4-demethylation in vitro, cholesterol biosynthesis remains intact in Cyb5a knockout mice. Here, we show that knockout of mitochondrial CYB5B, rather than CYB5A, blocks cholesterol biosynthesis at the sterol-C4 oxidation step in HeLa cells, causing an accumulation of testis meiosis-activating sterol (T-MAS) and dihydro-T-MAS. Surprisingly, liver-specific Cyb5b knockout (L-Cyb5b^-/-) mice exhibit normal cholesterol metabolism. Further knockdown of Cyb5a in L-Cyb5b^-/- (L-Cyb5b^-/-/short hairpin [sh]Cyb5a) mice leads to a marked accumulation of T-MAS and dihydro-T-MAS, indicating that either CYB5A or CYB5B is required for sterol C4-demethylation. The L-Cyb5b^-/-/shCyb5a mice are largely normal, with lower sterol regulatory element-binding protein (SREBP)-target gene expression during refeeding and higher liver triglyceride levels while fasting, as T-MAS and dihydro-T-MAS inhibit the SREBP pathway and activate the PPARγ pathway. In summary, CYB5A and CYB5B compensate for sterol C4-demethylation, and T-MAS and dihydro-T-MAS can modulate the SREBP and PPARγ pathways.