Insulin promotes the biosynthesis and secretion of apolipoprotein B-48 by altering apolipoprotein B mRNA editing.

Abstract

Long-term insulin treatment selectively stimulates secretion of the truncated form of apolipoprotein B (apoB), apoB-48, from primary rat hepatocytes in culture. Chronic treatment with insulin at 400 ng/ml causes a 3-fold increase in total apoB secretion, with apoB-48 making up about 75% of that increase. apoB-48 is the protein product generated by translation of full-length apoB mRNA which has been modified by a posttranscriptional editing mechanism. Editing changes codon 2153 in the middle ofthe apoB-100 coding region from CAA, coding for glutamine, to UAA, a translation stop signal. We therefore examined the effect ofinsulin treatment on the ratio of edited to nonedited apoB mRNA in RNA isolated from primary rat hepatocyte cultures. There was a dramatic shift in the ratio of edited versus nonedited forms of apoB mRNA, from about 1:1 in untreated cells to 7:1 in insulintreated cells. Insulin exerted a dose-dependent effect on apoB secretion and apoB mRNA editing over the range of insulin concentrations studied (0.4-400 ng/ml). In contrast, oleic acid, which also increased apoB (B-48 and B-100) secretion, had no significant effect on the ratio of apoB-48 to apoB-100 particles secreted and no effect on the proportion of edited apoB mRNA. Neither insulin nor oleic acid affects total apoB mRNA levels as assayed by Northern blot analysis. These data strongly suggest that insulin stimulates biosynthesis and secretion of apoB-48 in rat hepatocytes by regulating the proportion of edited apoB mRNA.Apolipoprotein B (apoB) is an essential structural component of triglyceride-rich very low density lipoproteins (VLDLs) and chylomicrons. The VLDLs are the metabolic precursors of the low density lipoprotein (LDL), the main carrier of cholesterol in humans. apoB is one of the ligands involved in clearance of LDL through binding to the LDL receptor; thus apoB is central to cholesterol homeostasis in the circulation. apoB is also essential for triglyceride (TG) transport to peripheral tissues (1).There are two forms of apoB, which are coded for by a single gene. The full-length translation product is apoB-100; it is a 512,000-Da polypeptide synthesized in the liver and secreted as an obligatory component of VLDL. The shorter form, apoB-48, is a 250,000-Da polypeptide identical to the amino-terminal 48% of apoB-100 and synthesized in the intestine as an integral part of the chylomicrons. In the rat, apoB-48 is also synthesized in the liver and secreted in VLDL. The mechanism for generating these two forms of apoB is a post-transcriptional RNA modification in which the CAA codon for glutamine-2153 in apoB-100 is changed to UAA, resulting in an in-frame translation stop signal (2). The polypeptide translation product of this edited RNA is apoB-48.As part of studies to define the role of insulin in the pathogenesis of dyslipidemia in the hyperinsulinemic state, we examined the effects oflong-term (5-day) insulin exposure on apoB secretion from primary cultures of rat hepatocytes. Chronic exposure to insulin selectively stimu...

Dexamethasone enhanced by insulin, but not by thyroid hormones stimulates apolipoprotein B mRNA editing in cultured rat hepatocytes depending on the developmental stage

“…Our findings, that insulin alone had no significant effect on apoB mRNA editing but acted in a synergistic manner together with dexamethasone, are noteworthy with regard to the results of others [8,28]. Pan and Koontz [28] analysed the effect of insulin on glucocorticoid receptor-mediated transcription using minimal artificial promoters.…”

“…No significant differences were observed between the corresponding ratio of edited apoB mRNA to edited plus unedited apoB mRNA and the ratio of secreted apoB-48 radioactivity to secreted apoB-48 plus apoB-100 radioactivity. Therefore, the editing is the major determinant of the form of apoB secreted not only in adult rats [7,8,27], but also during the developmental period.…”

“…Addition of insulin for a longer period increased the apoB mRNA editing in primary rat hepatocytes in culture [8], just like supplementation of 3,3',5-triiodothyronine (T3) in hypothyroid rats [9]. However, the factors governing the increase in hepatocellular apoB production and apoB mRNA editing up to the weaning period are still unknown.…”

Role of insulin in hepatic fatty acid partitioning: emerging concepts

“…There also seems to be disagreement as to whether insulin only affects apoB degradation [139], or whether it also affects apoB synthesis [140]. Moreover, prolonged exposure (5 days) of cultured rat hepatocytes to insulin enhanced apoB %) secretion by promoting apoB mRNA editing [141].…”

Interaction between growth hormone and insulin in the regulation of lipoprotein metabolism in the rat

“…The interaction of insulin and GH in the regulation of lipid and lipoprotein metabolism is of special interest, because similar effects of GH and chronic hyperinsulinemia have been observed (3,4,25,42,46,47). GH treatment in vivo increases editing of apoB mRNA, production of apolipoprotein (apo)B-48, and very lowdensity lipoprotein (VLDL) secretion from isolated hepatocytes (42, 43) and perfused liver (10).…”