THE IMPACT OF MICROSOMAL TRIGLYCERIDE TRANSFER PROTEIN ON INTRACELLULAR HEPATIC NEUTRAL LIPID DISTRIBUTION AND TRAFFICKING

Abstract

Microsomal triglyceride transfer protein (MTP) is a multifunctional protein necessary for conversion of apolipoprotein B into precursor lipoproteins and bulk triglyceride (TG) movement into the endoplasmic reticulum (ER) for precursor lipoprotein expansion. Invertebrate forms of MTP (e.g. Drosophila; dMTP) are capable of phospholipid (PL) transfer, whereas vertebrate forms (e.g. human; hMTP) engage in PL and TG transfer. We hypothesized that PL transfer is the primordial activity of MTP necessary for precursor lipoprotein particle formation, whereas TG transfer is a vertebrate adaptation necessary for trafficking of TG into the ER for 2nd-step assembly. Hence, we assessed hMTP's ability to promote TG trafficking into the ER of transiently transfected or stable, Dox-inducible, non-hepatic cell lines. In cell lines that expressed hMTP, a 2.5 - 5.0-fold increase in microsomal TG content was observed. The lipid-transfer activity of MTP was necessary for this function as in the presence of a potent MTP inhibitor, BMS-212122, microsomal TG content was similar to mock-transfected cells. To determine if the TG-transfer activity of MTP facilitated TG translocation we compared the TG content of microsomes from hMTP and dMTP expressing cells. Compared to mock-transfected cells, hMTP increased microsomal TG content by ~2.5-fold while cells transfected with dMTP demonstrated no increase in microsomal TG content. These data indicate that MTP promotes TG trafficking into the ER and that the TG-transfer activity of MTP is essential for this function.