Cell Reports (Nov 2024)

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

  • Mei-Yan Ma,
  • Gang Deng,
  • Wen-Zhuo Zhu,
  • Ming Sun,
  • Lu-Yi Jiang,
  • Wei-Hui Li,
  • Yuan-Bin Liu,
  • Lin Guo,
  • Bao-Liang Song,
  • Xiaolu Zhao

Journal volume & issue
Vol. 43, no. 11
p. 114912

Abstract

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Summary: 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.

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