Adenovirus-mediated overexpression of microsomal triglyceride transfer protein (MTP): Mechanistic studies on the role of MTP in apolipoprotein B-100 biogenesis

The intracellular concentration of the microsomal triglyceride transfer pro tein large subunit; (lMTP), the abetalipoproteinemia gene product, is tight ly controlled. To date, attempts at overexpressing lMTP in vivo or in vitro have been unsuccessful. We successfully overexpressed lMTP in HepG2 cells using an: adenoviral: vector containing an lMTP cDNA, AdMTP; AdMTP-transduc ed HepG2 cells overexpressed MTP activity. They secreted increased amounts. of apoB-100 lipoproteins with LDL and HDL density into the medium. lMTP ov erexpression alone minimally changed the density profile of apoB-containing lipoproteins, but addition of,oleic acid shifted the profile toward lower densities. Oleic acid had a greater stimulatory effect on apoB-100 secretio n-in control HepG2 cells than in AdMTP-transduced cells, because (i) adenov iral transduction per se suppressed protein: synthesis, affecting apoB-100 and albumin equally, and (ii) adenoviral transduction partially attenuated the increase in triglyceride synthesis in response to oleic acid supplement ation. AdMTP treatment greatly diminished the intracellular degradation of apoB-100, but in comparison with recombinant virus containing luciferase CD NA (AdLuc), it caused no change in its; biosynthetic rate. It greatly reduc ed, but did not eliminate, its proteasomal degradation. Our study constitut es the initial demonstration that adenovirus-mediated transfer of lMTP mark edly stimulates MTP expression which in turn stimulates apoB-100 production . The mechanism involves a downregulation of ubiquitin-proteasome-mediated degradation without any change in synthetic rate.