27-Hydroxylated low density lipoprotein (LDL) cholesterol can be converted to 7 alpha,27-dihydroxy-4-cholesten-3-one (cytosterone) before suppressing cholesterol production in normal human fibroblasts - Evidence that an altered metabolism of LDL cholesterol can underlie a defective feedback control in malignant cells

The formation of oxysterols in cultured human fibroblasts and their physiological roles as intracellular regulators of cholesterol production have been investigated. In the presence of low density lipoproteins (LDL), normal fibroblasts converted LDL cholesterol to 27-hydroxycholesterol, which was further metabolized to 7 alpha,27-dihydroxycholesterol, 7 alpha,27-dihydroxy-4-cholesten-3-one, and 7 alpha-hydroxy-3-oxo-4-cholestenoic acid. Autooxidation products of cholesterol contaminating the lipoproteins were also metabolized in the cells. 7 alpha-Hydroxycholesterol was converted to 7 alpha-hydroxy-3-cholesten-3-one prior to 27-hydroxylation and further oxidation to 7 alpha-hydroxy-3-oxo-4-cholestenoic acid. 7 beta-Hydroxycholesterol and 7-oxocholesterol were 27-hydroxylated and then oxidized to C-27-acids. Oxidation of the 7 beta-hydroxy group also occurred. 25-Hydroxycholesterol was 7 alpha-hydroxylated and further oxidized to 7 alpha,25-dihydroxy-4-cholesten-3-one. 25-Hydroxylation of sterols was observed only under specific conditions. In contrast, only small amounts of oxysterols were formed in virus-transformed human fibroblasts when incubated with lipoproteins. This was due to very low activities of the 27- and 7 alpha-hydroxylating enzymes. The rate of oxidation at C-3 was also decreased moderately. A defective suppression of 3-hydroxy-3-methylglutaryl coenzyme A reductase by LDL and autooxidation products of cholesterol observed in the transformed fibroblasts could be caused by the deficiencies of the sterol-metabolizing enzymes, since these cells responded normally to the sterol metabolites 7 alpha,27-dihydroxy-4-cholesten-3-one, 7 alpha,25-dihydroxy-4-cholesten-3-one, and 27-hydroxy-7-oxo-cholesterol. These metabolites, which all possessed an oxo group with a conjugated double bond in the steroid nucleus and a hydroxyl group in the side chain, did not seem to require further metabolism in order to be active. An impaired response to LDL was also seen in other human tumor cells, including breast carcinoma, colonic carcinoma, and malignant melanoma cells. Common to all the malignant cells was an intracellular shortage of 7 alpha,27-dihydroxy-4-cholesten-3-one caused by a decreased formation or an increased metabolism.