Molecular cloning of partial cDNAs for rabbit liver apolipoprotein B and the regulation of its mRNA levels by dietary cholesterol.

Abstract

Apolipoprotein B (apoB) is the major protein of plasma very low density lipoprotein (VLDL) and low density lipoprotein (LDL). Here we report the molecular cloning of cDNAs for rabbit liver apoB, by use of the expression vector .lambda.gt11, and the use of these cDNAs to study the regulation of apoB mRNA levels by dietary cholesterol. The .beta.-galactosidase-apoB fusion proteins expressed by recombinant clones were identified with guinea pig ant-rabbit LDL antibodies. The cloned cDNAs hybridized to an 18-kilobase mRNA that was present in liver and intestine. Slot blot analysis showed that this mRNA was not present in other tissues studied, with the possible exception of kidney. When rabbits are fed a high-cholesterol diet, they develop severe hypercholesterolemia. Most of the excess cholesterol is contained in .beta.-VLDL, a cholesteryl ester-rich lipoprotein that contains apoB and apoE. We addressed the question of whether increased apoB mRNA levels, and by inference increased apoB synthetic rates, are responsible for the accumulation of .beta.-VLDL. A comparison of apoB mRNA levels showed that cholesterol-fed rabbits had lower liver apoB mRNA levels than control rabbits. We suggest that the accumulation of plasma .beta.-VLDL in cholesterol-fed rabbits is not due to an increased production of .beta.-VLDL but solely due to a suppression of hepatic LDL receptors.