Apobec-1 Complementation Factor Modulates Liver Regeneration by Post-transcriptional Regulation of Interleukin-6 mRNA Stability

Valerie Blanc,Kimberly J Sessa,Susan Kennedy,Jianyang Luo,Nicholas O Davidson

doi:10.1074/jbc.m110.115147

Abstract

Apobec-1 complementation factor (ACF) is the RNA binding subunit of a core complex that mediates C to U RNA editing of apolipoprotein B (apoB) mRNA. Targeted deletion of the murine Acf gene is early embryonic lethal and Acf(-/-) blastocysts fail to implant and proliferate, suggesting that ACF plays a key role in cell growth and differentiation. Here we demonstrate that heterozygous Acf(+/-) mice exhibit decreased proliferation and impaired liver mass restitution following partial hepatectomy (PH). To pursue the mechanism of impaired liver regeneration we examined activation of interleukin-6 (IL-6) a key cytokine required for induction of hepatocyte proliferation following PH. Peak induction of hepatic IL-6 mRNA abundance post PH was attenuated >80% in heterozygous Acf(+/-) mice, along with decreased serum IL-6 levels. IL-6 secretion from isolated Kupffer cells (KC) was 2-fold greater in wild-type compared with heterozygous Acf(+/-) mice. Recombinant ACF bound an AU-rich region in the IL-6 3'-untranslated region with high affinity and IL-6 mRNA half-life was significantly shorter in KC isolated from Acf(+/-) mice compared with wild-type controls. These findings suggest that ACF regulates liver regeneration following PH at least in part by controlling the stability of IL-6 mRNA. The results further suggest a new RNA target and an unanticipated physiological function for ACF beyond apoB RNA editing.

Highlights

ApoB RNA editing is mediated by a multicomponent protein complex with a minimal core containing Apobec-1, the catalytic cytidine deaminase [3] and Apobec-1 complementation factor (ACF),2 the RNA binding subunit [4, 5]
These findings demonstrate that ACF binds to the AU-rich region of IL-6 3Ј-untranslated region (UTR) in vitro and confers stability to IL-6 mRNA in primary Kupffer cells (KC) cells, providing a mechanism to account for the loss-of-function phenotype observed in Acfϩ/Ϫ mice
The central findings of this report are that ACF, the RNA binding subunit of the mammalian C to U RNA editing holoenzyme, demonstrates high affinity binding to an AU-rich region in IL-6 3Ј-UTR with functional consequences for post-transcriptional regulation of IL-6 gene expression and the response of Acfϩ/Ϫ mice to partial hepatectomy (PH)

Summary

Introduction

ApoB RNA editing is mediated by a multicomponent protein complex with a minimal core containing Apobec-1, the catalytic cytidine deaminase [3] and Apobec-1 complementation factor (ACF), the RNA binding subunit [4, 5]. Questions concerning potential RNA targets were first raised with the demonstration that ACF is expressed in tissues that do not express apoB RNA [4, 5, 7] and assumed greater significance with the surprising finding that germline deletion of murine Acf resulted in early embryonic lethality [8]. These observations, coupled with the finding that ACF is expressed at high levels in human and mouse liver [8], suggested the possibility that a loss-of-function phenotype might be elicited in the liver of adult Acfϩ/Ϫ (heterozygous knock-out) mice. The findings collectively suggest a new RNA target and expanded the functional role for ACF

Methods

Findings

Conclusion