Insulin-resistant states are characterized by hypertriglyceridemia, predominantly because of overproduction of hepatic very low density lipoprotein particles. The additional contribution of intestinal lipoprotein overproduction to the dyslipidemia of insulin-resistant states has not been previously appreciated. Here, we have investigated intestinal lipoprotein production in a fructose-fed hamster model of insulin resistance previously documented to have whole body and hepatic insulin resistance, and hepatic very low density lipoprotein overproduction. Chronic fructose feeding for 3 weeks induced significant oversecretion of apolipoprotein B48 (apoB48)-containing lipoproteins in the fasting state and during steady state fat feeding, based on (a) in vivo Triton WR1339 studies of apoB48 production as well as (b) ex vivo pulse-chase labeling of intestinal enterocytes from fasted and fed hamsters. ApoB48 particle overproduction was accompanied by increased intracellular apoB48 stability, enhanced lipid synthesis, higher abundance of microsomal triglyceride transfer protein mass, and a significant shift toward the secretion of larger chylomicron-like particles. ApoB48 particle overproduction was not observed with short-term fructose feeding or in vitro incubation of enterocytes with fructose. Secretion of intestinal apoB48 and triglyceride was closely linked to intestinal enterocyte de novo lipogenesis, which was up-regulated in fructose-fed hamsters. Inhibition of fatty acid synthesis by cerulenin, a fatty acid synthase inhibitor, resulted in a dose-dependent decrease in intestinal apoB48 secretion. Overall, these findings further suggest that intestinal overproduction of apoB48 lipoproteins should also be considered as a major contributor to the fasting and postprandial dyslipidemia observed in response to chronic fructose feeding and development of an insulin-resistant state.The metabolic syndrome, which is characterized by fasting hypertriglyceridemia, insulin resistance, glucose intolerance, hypertension, and obesity (1), appears also to include impaired postprandial lipoprotein metabolism (2, 3). Postprandial triglyceride-rich lipoproteins and especially chylomicron remnants have been implicated as risk factors for atherosclerosis and progression of coronary artery disease, based on both experimental work (4, 5) and cross-sectional studies (6 -8). Emerging evidence suggest that intestinal lipoproteins may be particularly atherogenic in diabetes (9). We (10, 11) and others (12-15) have shown previously that there is an elevation of postprandial triglyceride (TG) 1 -rich lipoproteins in subjects with insulin resistance and type 2 diabetes and that fasting hypertriglyceridemia predicts this abnormal postprandial response to a fat load. A strong correlation also exists between plasma insulin and the postprandial TG response to a fat meal, and the postprandial levels of large VLDLs and chylomicron remnants (3, 16). In the fasting state, plasma insulin, a marker of insulin resistance, is also related to fasting plas...

Objective-To investigate the role of glucosamine-mediated endoplasmic reticulum (ER) stress and Grp78 (BiP) in the intracellular degradation of apolipoprotein B100 (apoB100) in cultured hepatocytes. Methods and Results-Glucosamine treatment (2.5 to 10 mmol/L) of HepG2 cells increased levels of the ER chaperones, 78-kDa glucose-regulated protein (Grp78) and Grp94, in a dose-dependent manner and led to significant decreases in both cellular and secreted apoB100 by up to 97% (PϽ0.01). In contrast, no changes were observed in ER resident (ER60, PTP-1B) or secretory (albumin, apoE) control proteins. Glucosamine-induced apoB degradation was similarly observed in primary hamster hepatocytes and McA-RH7777 cells. Glucosamine treatment led to reduced tranlocational efficiency of apoB100 in the ER and enhanced its ubiquitination and proteasomal degradation. Adenoviral overexpression of Grp78 also led to significantly decreased levels of newly synthesized apoB100 in a dose-dependent manner (PϽ0.01). Grp78-induced downregulation of apoB100 was sensitive to inhibition by the proteasome inhibitor, lactacystin, but not lysosomal protease inhibitors, E64 and leupeptin, suggesting that overexpression of Grp78 selectively induced proteasomal degradation of apoB100. Conclusion-These findings suggest that binding and retention by Grp78 may play a critical role in proteasomal targetingand the ER quality-control of misfolded apoB. Key Words: apolipoprotein B Ⅲ degradation Ⅲ glucosamine Ⅲ Grp78 Ⅲ proteasome H epatic biogenesis of apoB is a complex process involving regulation by multiple post-transcriptional control mechanisms. 1-3 Intracellular availability of core lipoprotein lipid substrates, particularly triglyceride, appears to dictate the intracellular fate of newly synthesized apoB protein. 4 In the absence of lipid, a significant proportion of newly synthesized apoB100 is degraded in cultured hepatoma cells, 5 as well as in primary hepatocytes from hamsters, 6 rats, 7 and rabbits. 8 The bulk of apoB degradation appears to be mediated by the ubiquitin-proteasome degradative system. 9 High exogenous free fatty acid flux (particularly in cultured hepatocytes) or SREBP1-mediated de novo lipogenesis appear to protect apoB from proteasomal degradation. 10 Ubiquitination and proteasomal degradation of apoB begins cotranslationally and involves the interaction of misfolded apoB with cytosolic chaperones, Hsp70 and Hsp90. 11 Association with these cytosolic chaperones may be important in unfolding and subsequent targeting of the apoB polypeptide to the ubiquitin-proteasome pathway. 11 Mechanisms that target misfolded apoB or luminal lipoprotein-associated apoB to either proteasomal or nonproteasomal degradative pathways are currently unknown. Ample evidence is available showing the association of newly synthesized apoB polypeptide with endoplasmic reticulum (ER) chaperones. Several laboratories have observed that apoB100 is tightly associated with the 78-kDa glucoseregulated protein/immunoglobulin heavy chain-binding protein (Grp78/BiP...