26-Hydroxycholesterol: synthesis, metabolism, and biologic activities

Abstract

Cholest-5-ene-3 beta,26-diol (26-hydroxycholesterol) is synthesized by a mitochondrial P-450 enzyme that appears to be widely distributed in tissues. Together with other C-27 steroid intermediates it is transported to the liver and metabolized to bile acids. Although 26-hydroxycholesterol is transported in plasma lipoproteins mostly as the fatty acid ester, neither its assembly and orientation within lipoproteins nor its mechanism of transport across the sinusoidal liver membrane is known. Cell culture studies indicate that 26-hydroxycholesterol can inhibit both cholesterol synthesis and low density lipoprotein (LDL) receptor activity. Inhibition of DNA synthesis also occurs and may not be related to the reduction in HMG-CoA reductase activity. The relationship of these in vitro activities to the physiologic role(s) of 26-hydroxycholesterol remains to be clarified. A clue to its biologic role is the knowledge that markedly decreased 26-hydroxylase activity appears to be the molecular basis of cerebrotendinous xanthomatosis, an inborn error of metabolism characterized by a significant decrease in 26-hydroxycholesterol and bile acid synthesis and an increase in cholesterol synthesis.