Abstract 1200: Destabilization of the ABCG5 ABCG8 Sterol Transporter in db/db Mice
ATP binding cassette transporters ABCG5 (G5) and ABCG8 (G8) form a functional complex (G5G8) that limits intestinal absorption and promotes biliary excretion of neutral sterols. Formation of the G5G8 complex occurs in the endoplasmic reticulum (ER) and is dependent on N-linked glycans present on G8 and the ER chaperone, calnexin. ER stress is associated with diminished levels of calnexin, suggesting that under conditions of ER stress G5G8 dimer formation may be compromised. Recent reports indicate that obesity is associated with both hepatic ER and oxidative stress. We examined G5 and G8 protein levels in a rodent model of obesity and type 2 diabetes (db/db). In db/db mice, immunoreactive G5 and G8 were reduced by ~50% compared to heterozygous controls. Calnexin levels were reduced by ~40% while levels of GRP78 (Bip) were unchanged. This reduction correlated with a decrease in biliary cholesterol concentrations (7.9±5.6 vs. 2.7±1.2). However, the reduction in G5 and G8 protein occurred in the absence of changes in mRNA for each half-transporter, indicating that changes in G5 and G8 protein levels occurred post-transcriptionally. The presence of ER and oxidative stress in the livers of these mice would be expected to reduce efficiency of G5G8 dimer formation in the ER and reduce stability of the G5G8 heterodimer at the cell surface, respectively. In vitro, we examined the effects of ER and oxidative stress on G5 and G8 protein levels in hepatocytes that stably express each half transporter. Treatment with hydrogen peroxide resulted in a dose-dependent decrease in G5, but not G8, within 4 hours, indicating that the G5G8 complex was destabilized by oxidative stress. In conclusion, G5 and G8 protein levels and biliary cholesterol concentrations are reduced in db/db mice. The mechanism by which G5 and G8 are reduced in db/db mice appears to involve both ER and oxidative stress.