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% (
<0.01). 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 (
<0.01). 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.
These findings suggest that binding and retention by Grp78 may play a critical role in proteasomal targeting and the ER quality-control of misfolded apoB. Interaction with core lipoprotein lipids may facilitate apoB transport out of the ER by reducing Grp78-mediated ER retention.