Abstract 4: Apolipoprotein A-iv Enhances Triglyceride Secretion and Reduces Hepatic Lipid Content by Promoting Apob Containing-lipoprotein Particle Expansion in Livers of Steatotic Mice

Abstract

Apolipoprotein A-IV (apoA-IV) is a lipid binding protein expressed in mammalian intestine, as well as rodent liver. While previous in vitro studies demonstrated that apoA-IV promotes cellular triglyceride (TG) secretion by modulating apoB secretory trafficking and lipoprotein size, evidence that apoA-IV affects TG secretion in vivo is lacking. To explore the possible role of apoA-IV in controlling hepatic VLDL-mediated lipid efflux we assessed whether apoA-IV is regulated by liver lipid accumulation and if its deficiency or overexpression affects hepatic TG secretion. Under conditions of high fat diet-induced hepatic steatosis and steatosis caused by transgenic overexpression of SREBP-1a (SREBP-1aTg), a ~40-fold induction of hepatic apoA-IV mRNA and protein was observed. In both systems there was a linear correlation between hepatic TG content and apoA-IV mRNA abundance (r2 = 0.8965). To establish whether the induction of apoA-IV impacted TG secretion, SREBP-1aTg mice were crossed with apoA-IV knock out mice (A4-KO). Based on the Triton block method, SREBP1aTg/A4-KO mice demonstrated a 24% reduction in hepatic TG secretion rate, relative to SREBP1aTg controls. The basis for the decrease in TG secretion was not due to decreased particle number, as apoB production was unaffected; however, negative stain electron microscopy revealed a 33% decrease in the abundance of large diameter (>120 nm) VLDL particles. To explore the impact of human apoA-IV overexpression, a recombinant apoA-IV adenovirus (AdA4) was developed. Mice infected with AdA4 demonstrated abundant hepatic expression of human apoA-IV, and a 50% increase in hepatic TG secretion rate, relative to the LacZ control. In addition, after only three days of expression AdA4 reduced liver TG content by 40%. Although again, apoA-IV expression did not impact apoB secretion, negative stain electron microscopy revealed a 1.8-fold increase in the abundance of large diameter VLDL particles. In conclusion, we demonstrate that in steatotic mice, hepatic apoA-IV expression increases TG secretion by promoting lipoprotein particle expansion, thereby dramatically reducing hepatic lipid burden without increasing atherogenic lipoprotein particle number.