The role of non-esterified cholesterol concentration in endoplasmic-reticular membranes in the regulation of hydroxymethylglutaryl-CoA reductase

Probucol enhances cholesterol transport in cultured rat hepatocytes

Probucol has been investigated extensively in vivo and ex vivo. However, little information is available on direct treatment of cultured cells. Treatment of cultured hepatocytes by 1 to 10 microM probucol for 16 to 20 hours had no apparent deleterious effect on the cell, and in fact decreased lactic dehydrogenase leakage. Also, total cellular cholesterol was increased, cholesterol synthesis was decreased, and cholesterol esterification was increased. The increase in cellular cholesterol was not the result of increased lipoprotein uptake but appeared to be the result of an interaction between cell and lipoprotein in which the cell became enriched and the lipoprotein depleted in cholesterol. This type of change in lipoprotein composition after treatment is also observed clinically.

The activity of 3-hydroxy-3-methylglutaryl-CoA reductase and acyl-CoA: cholesterol acyltransferase in hepatic microsomes from male, female and pregnant rats. The effect of cholestyramine treatment and the relationship of enzyme activity to microsomal lipid composition

The relationship of microsomal cholesterol and phospholipid fatty acid composition to the activities of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and acyl-CoA: cholesterol acyltransferase was investigated in male, female virgin and pregnant rats when hepatic cholesterogenesis was stimulated by cholestyramine. Cholestyramine increased HMG-CoA reductase activity in both sexes but had no effect on microsomal free cholesterol level or acyl-CoA: cholesterol acyltransferase activity. The data suggest that during cholestyramine treatment high rates of bile acid synthesis are supported by preferential channelling of cholesterol into this pathway, whilst the substrate pool and activity of acyl-CoA:cholesterol acyltransferase are maintained unaltered. The lack of a consistent relationship among enzyme activities and microsomal lipid composition infers that HMG-CoA reductase and acyl-CoA:cholesterol acyltransferase are regulated in vivo by independent mechanisms which are unlikely to involve modulation by the physical properties of the microsomal lipid.

Polychlorinated biphenyls interfere with the regulation of hydroxymethylglutaryl-coenzyme A reductase activity in rat liver via enzyme-lipid interaction and at the transcriptional level

Feeding a 0.05% polychlorinated biphenyl-supplemented diet to rats resulted in an increase of liver 3-hydroxy-3-methylglutaryl coenzyme A reductase activity within 9 days, followed by a decrease towards normal levels. Polychlorinated biphenyls were incorporated into the microsomal membrane. There was a concomitant decrease in the cholesterol/phospholipid ratio in the microsomal fraction. Immunotitration studies strongly suggested that polychlorinated biphenyls modulate preexisting 3-hydroxy-3-methylglutaryl coenzyme A reductase activity by changing the lipid environment of the enzyme. 32P-labeled cDNA probes were used to study the levels of mRNA coding for 3-hydroxy-3-methylglutaryl coenzyme A reductase during polychlorinated biphenyl feeding. Northern dot hybridization experiments showed an increase in the amount of this mRNA. This stimulation correlated with the increase in the activity of the enzyme but was more pronounced. The data suggest that polychlorinated biphenyls act on the regulation of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase activity by enzyme-lipid interaction and at the transcriptional level.