REPR and complementation factor(s) interact to modulate rat apolipoprotein B mRNA editing in response to alterations in cellular cholesterol flux.

Y Inui,F Giannoni,T Funahashi,N O Davidson

doi:10.1016/s0022-2275(20)40089-6

Abstract

Apolipoprotein B (apoB) mRNA editing is a post-transcriptional cytidine deamination involving several protein factor(s), one of which has recently been cloned. We have examined the effects of alterations in cellular cholesterol flux in the rat liver and small intestine as a means of dissecting the physiologic mechanisms regulating apoB mRNA editing, both in vivo and in isolated S-100 extracts. Hepatic cholesteryl ester accumulation was produced by feeding rats a high cholesterol diet, alone, or in combination with either ethinyl estradiol treatment, or after induction of hypothyroidism. Endogenous hepatic apoB mRNA editing decreased in parallel with the increase in cellular cholesteryl ester content (r = -0.948, P < 0.001). None of these conditions altered endogenous intestinal apoB mRNA editing. Hepatic S-100 extracts demonstrated decreased in vitro apoB RNA editing activity, in parallel with the changes observed in vivo. By contrast, the activity of intestinal S-100 extracts demonstrated a paradoxical increase in hypothyroid rats and a similar, paradoxical decrease in hyperthyroid rats, when compared to controls. Hepatic REPR mRNA, quantitated by RNase protection assay, showed a 25-50% decrease in cholesterol-fed rats. The editing activity of hepatic S-100 extracts prepared from cholesterol-fed, hypothyroid rats was restored to control levels with REPR supplementation but not with chicken intestinal S-100 extracts, suggesting that changes in REPR, but not complementation activity, may play a critical role in the regulation of apoB mRNA editing in rat liver. By contrast, the editing activity of intestinal S-100 extracts prepared from hyperthyroid animals was unaltered by supplementation with REPR, but was restored to control levels after the addition of chicken intestinal S-100 extracts. Taken together, the data suggest that tissue-specific factors regulate apoB mRNA editing in the rat and that the complex interplay of REPR and complementation factor(s) may be modulated in response to alterations in cholesterol flux, in vivo.

Highlights

Apolipoprotein B mRNA editing is a posttranscriptional cytidine deamination involving several protein factor(s), one of which has recently been cloned
ApoB mRNA editing demonstrated an inverse correlation with hepatic free cholesterol content (Fig. 2B, P < O.Ol), and to a lesser extent with triglyceride content (Fig. 2A, P < 0.05). These results demonstrate a reduction in endogenous hepatic Apolipoprotein B (apoB) mRNA editing in the rat liver following modulations that increase hepatic cholesterol ester accumulation
Several important observations emerged from these studies, among them, that endogenous hepatic apoB mRNA editing is decreased in response to cholesterol accumulation and in a manner that was recapitulated using isolated S-100 extracts and a synthetic apoB RNA template

Summary

Introduction

Apolipoprotein B (apoB) mRNA editing is a posttranscriptional cytidine deamination involving several protein factor(s), one of which has recently been cloned. The editing activity of hepatic S-100 extracts prepared from cholesterol-fed, hypothyroid rats was restored to control levels with REPR supplementation but not with chicken intestinal S-100 extracts, suggesting that changes in REPR, but not complementation activity, may play a critical role in the regulation of apoB mRNA editing in rat liver. Animals treated with pharmacologic doses of estrogen were found to have a decrease in hepatic apoB mRNA editing [10] These conditions are recognized to have numerous effects on hepatic lipid metabolism, the common denominator responsible for the modulation of hepatic apoB mRNA editing is unknown. Recent work has demonstrated that modulation of hepatic lipid metabolism in the rat is not necessarily accompanied by alterations in apoB mRNA editing, as animals treated with increasing doses of dexamethasone were found to have progressive, intrahepatic steatosis with no effects on apoB mRNA abundance or editing [11]

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