Abstract 431: Impact of Liver-Specific Transgenic Overexpression of Drosophila and Human MTP on Diet-Induced Hepatic Steatosis and Lipoprotein Secretion

Abstract

Microsomal triglyceride transfer protein (MTP) is essential for the formation of apolipoprotein B (apoB)-containing lipoproteins. Previous results established that invertebrate forms of MTP (e.g., Drosophila ) are capable of phospholipid (PL) transfer, whereas vertebrate forms of MTP (e.g., human) engage in both PL and triglyceride (TG) transfer. To determine, in vivo, whether the PL and TG transfer activities of MTP play different roles in lipoprotein formation and protection from hepatic steatosis, we created B6D2F1 mice that express human MTP (hMTP) or Drosophila MTP (dMTP) under the control of the apoE 5’ proximal promoter and 3’ hepatic control region in the plasmid pLIV11. Immunoblot analysis confirmed that transgenic expression was limited to the liver and that expression of the transgenes did not impact the abundance of endogenous mouse MTP. To assess the ability of MTP to protect mice from hepatic steatosis, dMTP transgenic mice, hMTP transgenic, and wild type (WT) littermates were placed on a high fat (45% from lard) and cholesterol (0.2%) diet for eight weeks. Mice displayed no significant differences in plasma TG or total plasma cholesterol; however, hMTP mice displayed a trend toward reduced liver TG content, indicating that chronic MTP overexpression may protect the liver from hepatic lipid accumulation. Surprisingly, however, a 2-fold increase in liver TG was observed in two separate founder lines of dMTP transgenic mice. H&E staining confirmed an increase in neutral lipid accumulation in mice expressing the dMTP transgene, relative to both WT and hMTP mice. Quantitative PCR analysis did not reveal changes in expression of genes responsible for hepatic lipogenesis or lipid oxidation. These data complement earlier studies using adenovirus-mediated expression, indicating that acute hMTP and dMTP expression in liver-specific MTP knockout mice can reverse hepatic steatosis caused by MTP deficiency. In contrast, chronic hepatic overexpression of dMTP in a wild type background appears to interfere with endogenous lipoprotein formation and secretion. Understanding the basis for this phenotype may provide insights into how the specific lipid transfer and other activities of MTP contribute to apoB lipoprotein formation.