Molecular mechanisms regulating the assembly and secretion of apolipoprotein B in hamster liver and its overproduction in insulin resistant states

grantor: University of TorontoInsulin-resistant states are commonly associated with lipoprotein abnormalities that are risk factors for coronary heart disease. Hepatic overproduction of apolipoprotein B (apoB)-containing lipoproteins is the hallmark of metabolic dyslipidemia in insulin resistance. We used Syrian golden hamsters to study the mechanisms of hepatic overproduction of apoB-containing lipoproteins, as their lipoprotein metabolism closely resembles that of humans. Our studies revealed that hamster hepatocytes efficiently assemble and secrete apoB-containing very low density lipoprotein (VLDL) particles. We induced an insulin resistant state in the hamster by fructose feeding and studied the effects of insulin resistance on apoB biogenesis. Cellular stability of apoB was significantly increased in insulin resistant hepatocytes, possibly linked to marked suppression of ER-60, an apoB-associated ER cysteine protease. Furthermore, hepatic mRNA levels, protein mass, and activity of microsomal triglyceride transfer protein (MTP), a key enzyme in assembly of apoB-containing lipoproteins were dramatically increased. Overall, VLDL-apoB overproduction in fructose-fed hamsters appeared to result from enhanced post-translational stability of nascent apoB and an enhanced expression of MTP, leading to facilitated assembly and secretion of VLDL. We also investigated hepatic insulin signaling in the fructose-fed hamster model to examine the potential contribution of attenuated insulin signaling to the observed VLDL overproduction. Reduced tyrosine phosphorylation of insulin receptor (IR), and its substrates, reduced PI 3-kinase activity, and attenuated serine/threonine phosphorylation of Akt/PKB confirmed induction of hepatic insulin resistance. Elevation of protein mass and activity of protein tyrosine phosphatase (PTP)-1B appeared to be partly responsible for induction of hepatic insulin resistance. Chronic exposure of control hepatocytes to high insulin attenuated IR phosphorylation and increased PTP-1B levels, coinciding with marked suppression of ER-60 and apoB oversecretion. Improvements in phosphorylation status of the IR, in vitro by treatment with sodium orthovanadate, or in vivo by rosiglitazone treatment, attenuated secretion of apoB from fructose-fed hamster hepatocytes. In summary, induction of whole body and hepatic insulin resistance in the hamster by fructose-feeding is accompanied by a marked hepatic overproduction of VLDL-apoB. The VLDL-apoB overproduction may arise from interplay between hepatic insulin resistance and changes in several key components of the VLDL assembly and secretion process including MTP and ER-60.Ph.D