Developmental Regulation of Apolipoprotein B mRNA Editing Is an Autonomous Function of Small Intestine Involving Homeobox Gene Cdx1

Amy P Patterson,Zhigang Chen,Deborah C Rubin,Virginie Moucadel,Juan Lucio Iovanna,H Bryan Brewer,Thomas L Eggerman

doi:10.1074/jbc.m201601200

Abstract

Apolipoprotein B mRNA editing is developmentally regulated in the human and rodent small intestine, changing from <1% at day 14 to approximately 90% by day 20 in the rat fetus. This regulation is coincident with the developmental formation of the crypt-to-villus axis functional unit, a continuous and rapidly renewing system involving cell generation, migration, and differentiation. Utilizing small intestine isografts implanted into the subcutaneous tissue of adult recipients, apolipoprotein B mRNA editing was developmentally up-regulated, parallel to that seen with an intact control. In contrast, apoB mRNA expression remains nearly constant in the isograft, unlike the normal intact small intestine. Immunohistochemical analyses demonstrated that apoB-48 protein existed predominantly in well differentiated enterocytes along the villus surface whereas apoB-100 was in the lamina propria and crypts. ApoB mRNA editing levels were very low in the crypt-like rat intestinal cell line, IEC-6 ( approximately 0.3%), but very high in well differentiated enterocytes ( approximately 91.5%). The expression of homeobox gene Cdx1 increased 18-fold in small intestine in vivo during the same time course when apoB mRNA editing increased from approximately 2 to approximately 90%. The overexpression of Cdx1 in IEC-6 cells increased apoB mRNA editing over 10-fold compared with the vector control. This increase was associated with a significant increase of activating factor ACF, a component of the apoB mRNA editing complex. Taken together, these data suggest that the developmental regulation of apoB mRNA editing is an autonomous cytodifferentiation function of small intestine for which homeobox gene Cdx1 may play an important role.

Highlights

Dietary lipids are sources of energy and precursors of cell components
To assess whether the increased apolipoprotein B (apoB) mRNA editing observed during small intestinal development is because of endoderm cytodifferentiation or regulation from intraluminal contents including bile, local hormones, and neural intervention, segments of small intestine were taken from fetal Balb/c mice at age 15–16 days, prior to epithelial cytodifferentiation, and transplanted to the subcutaneous tissue of adult Balb/c recipients
The editing analyses showed that apoB mRNA editing in isografts increased sharply from ϳ22% at 16 days, the initial time point for transplantation, to ϳ93% at 6 days following transplantation, and the editing level remained thereafter (Fig. 1B)

Summary

Introduction

Dietary lipids are sources of energy and precursors of cell components. The small intestine plays a primary role in the absorption and transport of dietary lipids through the synthesis and secretion of lipoprotein particles, especially chylomi-. The human liver produces exclusively apoB-100, which is the major lipoprotein in very low density lipoproteins (VLDL) and low density lipoproteins (LDL) and functions as a ligand for the low density lipoprotein receptor. As our laboratory and others have shown [7, 8], during rat fetal development, apoB mRNA editing level in the rat small intestine increases from Ͻ1% at 14 days post-conception to 90% by day 20. Biliary lipid secretion occurs as early as 22 weeks [17] During this time, the human fetal intestine increasingly edits apoB transcripts from Ͻ10% at 11 weeks to an adult-like 85% level by 20 weeks of gestation [7, 9]. The axis of differentiation from the crypt to villus becomes first evident between 17 and 18 days of gestation when apoB mRNA editing increases rapidly to adult levels [7, 9, 18]

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Conclusion