Rates of low density lipoprotein uptake and cholesterol synthesis are regulated independently in the liver

The relationship between rates of hepatic sterol synthesis and rates of hepatic low density lipoprotein (LDL) uptake (clearance) was studied in animals with high (rats), low (female hamsters), and very low (male hamsters) basal rates of hepatic sterol synthesis. In rats and female hamsters, rates of hepatic sterol synthesis were varied over a 110-fold range by feeding cholesterol or cholestyramine; nevertheless, rates of hepatic LDL clearance remained essentially unchanged as did plasma LDL-cholesterol concentrations. In contrast, in male hamsters, which have a very limited capacity to synthesize cholesterol in the liver, cholestyramine feeding increased rates of hepatic LDL uptake by 2.5-fold and this was associated with a 50% reduction in plasma LDL-cholesterol concentrations. The observed increase in LDL uptake was due to an increase in receptor-dependent LDL transport while receptor-independent lipoprotein uptake remained constant. These studies suggest that rates of hepatic cholesterol synthesis and receptor-dependent LDL uptake are regulated independently. Furthermore, the primary response of the liver to changes in cholesterol availability is regulation of sterol synthesis and only when the capacity of this compensatory mechanism is exceeded is the rate of LDL transport altered.