Synthesis and secretion of B-100 and A-I apolipoproteins in response to the changes of intracellular cholesteryl ester content in chick liver

A study of fatty liver disease and plasma lipoproteins in a kindred with familial hypobetalipoproteinemia due to a novel truncated form of apolipoprotein B (APO B-54.5)

BACKGROUND/AIMS

Familial hypobetalipoproteinemia (FHBL) is a co-dominant disorder characterized by reduced plasma levels of low-density lipoproteins. It can be caused by mutations in the gene encoding apolipoprotein B-100 (apo B), leading to the formation of truncated apo Bs which have a reduced capacity to export lipids from the hepatocytes as lipoprotein constituents. Case reports suggest the occurrence of liver disease in FHBL, but there are no studies of liver involvement in FHBL with defined apo B gene mutations. The presence of fatty liver disease was investigated in a large FHBL kindred.

METHODS

Plasma lipoprotein and apolipoprotein analysis, liver function tests, and apo B gene sequence were performed in 16 members of a FHBL kindred. The presence of fatty liver was assessed by ultrasound and computed tomography scanning.

RESULTS

The proband, a non-obese heavy drinker male with hypobetalipoproteinemia, had steatohepatitis with fibrosis. He was heterozygous for a novel non-sense mutation of apo B gene producing a truncated apo B of 2745 amino acids (designated apo B-54.5, having half the size of normal apo B-100). Seven other members of his kindred carried apo B-54.5. Although all of them were hypolipidemic, their lipid levels showed a large inter-individual variability not accounted for by polymorphisms of genes involved in apo B metabolism. Four carriers (two heavy drinkers and two teetotallers), irrespective of their plasma lipid levels, had ultrasonographic evidence of fatty liver. In the other four carriers no evidence of fatty liver was found.

CONCLUSIONS

In this kindred apo B-54.5 predisposes to fatty liver, which however may require some additional factors to become clinically relevant.

The role of factors that regulate the synthesis and secretion of very-low-density lipoprotein by hepatocytes

Lipoproteins are particles that contribute to overall metabolic homeostasis by transporting hydrophobic lipids in the blood plasma to and from different tissues in the body. Very-low-density lipoprotein (VLDL) is the principal vehicle for the transport of endogenous triglyceride (TG), and, ultimately, through its metabolic product, low-density lipoprotein (LDL), of cholesterol as well. It is synthesized mainly in hepatocytes, with small amounts also being produced by enterocytes in the fasting state. The mechanism of VLDL assembly is complex and is regulated at different levels by a variety of factors. The main structural protein of VLDL is called apolipoprotein B-100 (Apo B). Apo B formation and degradation therefore represent two major points of regulation of VLDL secretion. Hepatic levels of lipids such as phosphatidylcholine (PC), cholesteryl ester (CE), fatty acids (FA), and TG also affect VLDL synthesis. There are different views as to the specific mechanism by which each lipid class affects VLDL particle formation. In general, PC appears to promote the translocation of apo B from the cytosol to the lumen of the endoplasmic reticulum, a step that is crucial in the early stages of VLDL assembly. Apo B degradation is suppressed, and therefore VLDL secretion is enhanced, in the presence of elevated CE levels. For TG to be incorporated into the lipoprotein, it requires the action of a protein called microsomal triglyceride transfer protein (MTP). MTP might have a preference for TG comprised of FA with a certain degree of saturation. It becomes apparent that changes in diet that are accompanied by variations in the type of fats that are ingested affect VLDL formation and secretion. Regulation also occurs post-prandially in response to elevations in plasma insulin levels. Acute elevations in insulin inhibit VLDL secretion by promoting the degradation of apo B. This action is consistent with insulin's anabolic properties as it allows for the hepatic storage of lipid rather than for its distribution in VLDL to other tissues for fuel. Many studies have attempted to unravel the mechanisms of VLDL formation and secretion. The fact that so many factors are involved complicates the issue. The purpose of this article is to describe the relationship between different factors involved in VLDL assembly and secretion so that a better understanding of its metabolic regulation may be achieved.