Conditional Intestinal Lipotoxicity in Apobec-1-/- Mttp-IKO Mice

Abstract

Mammalian small intestinal lipid absorption requires the coordinated interactions of apolipoprotein B (apoB) and the microsomal triglyceride transfer protein (Mttp). The observation that apoB100 displays greater dependence on Mttp availability than does apoB48 prompted us to examine the phenotype of Mttp deletion in an Apobec-1 ؊/؊ background (i.e. apoB100 Mttp-IKO). 20% apoB100 Mttp-IKO mice died on a chow diet, and >90% died following high fat feeding (versus 0 and 11% apoB48 Mttp-IKO mice, respectively). Intestinal adaptation occurred in apoB48 Mttp-IKO mice in response to high fat feeding, evidenced by increased bromodeoxyuridine incorporation and villus lengthening, changes that did not occur in apoB100 Mttp-IKO mice. There was an exaggerated unfolded protein response (UPR), which became more pronounced in apoB100 Mttp-IKO mice. To examine the role of endoplasmic reticulum stress and the UPR in the lipotoxic effects of Mttp deletion, we administered tauroursodeoxycholate to apoB100 Mttp-IKO mice upon initiation of high fat feeding. Tauroursodeoxycholate administration abrogated the UPR but produced an unexpected acceleration in the onset of lethality in apoB100 Mttp-IKO mice. The findings demonstrate that there is activation of the UPR with lethal lipotoxicity in conditional intestinal apoB100 Mttp-IKO mice. Together the data provide the first plausible biological evidence for a survival advantage for mammalian intestinal apoB mRNA editing. Lipid absorption and the secretion of triglyceride (TG)2 -rich lipoproteins (chylomicrons) from enterocytes of the mammalian small intestine are critically dependent on the coordinated interactions of two dominant genes. These include the structural protein apolipoprotein B (apoB) and the resident endoplasmic reticulum (ER) protein microsomal triglyceride transfer protein (Mttp) (1). Proper lipidation of apoB is achieved through the lipid transfer functions of Mttp, and this step is essential to maintaining the conformational integrity of the nascent apoB protein as it enters the luminal ER and initiates nascent lipoprotein biogenesis (1). Limiting lipid availability or defective expression and/or function of Mttp leads predictably to a failure of lipidation of the nascent apoB polypeptide, which in turn causes its misfolding and presecretory degradation (reviewed in Ref.2). The physiological relevance of this pathway is illustrated by the phenotype associated with abetalipoproteinemia where mutations in the MTTP gene are associated with failure of lipid export from both enterocytes and hepatocytes and serum levels of apoB are extremely low or undetectable (3).The requirements for Mttp-mediated lipidation of the apoB protein exhibit an important pattern of isoform dependence. Studies in hepatoma cells and in primary murine hepatocytes using pharmacological inhibitors of Mttp have demonstrated that apoB100 is dramatically more susceptible to presecretory degradation with limiting Mttp than is the shorter isoform, apoB48 (4, 5). These findings were reproduced in mice with ...

ApoB100 is required for increased VLDL-triglyceride secretion by microsomal triglyceride transfer protein in ob/ob mice

“…There is a substantial body of evidence indicating that limiting expression of Mttp impairs the assembly and secretion of TG-rich lipoproteins, both from the liver (12,13) and the small intestine (28,29). In addition, numerous observations point to a disproportionate effect of limiting Mttp expression or pharmacologic Mttp inhibition on the secretion of apoB100-containing lipoproteins, in comparison to particles containing apoB48 (8)(9)(10)(11).…”

Multiple functions of microsomal triglyceride transfer protein

“…Davidson and associates created intestine specific KO mice (I-MTP -/- ) after crossing MTTP fl(exon1)/fl(exon1) mice with mice that express Cre-recombinase under an inducible Villin promoter activated by tamoxifen [48,49]. Intestinal MTP gene deletion was associated with no weight gain and steatorrhea, gross lipid accumulation, presence of large lipid droplets in the apical portions of the enterocytes and absence of lipoproteins in the Golgi and ER [48].…”

“…Xie et al [49] further examined the role of intestinal MTP in the assembly and secretion of apoB48 and apoB100 containing lipoproteins. Enterocytes only synthesized apoB48 due to efficient and complete posttranscriptional editing of the apoB mRNA by Apobec-1 enzyme [50,51].…”

Gut triglyceride production

“…However, the role of MTP in intestinal lipid absorption has been studied using intestine-specific Mttp KO ( I-Mttp −/− ) mice. Using I-Mttp −/− mice, Davidson's lab showed that CM secretion was reduced dramatically in vivo as well as there was 80% decrease in apoB48 secretion from primary enterocytes [107,108]. Additionally, using I-Mttp −/fl partially deficient mice, Iqbal et al showed that intestinal absorption of radiolabeled triolein was decreased by 63% compared with Mttp fl/fl mice [50].…”