Bile Acids Induce IQ Containing GTPase Activating Protein 1 to Alleviate Liver Injury

Abstract

Bile acids (BA) are cholesterol metabolites synthesized in the liver. In addition to their role as detergents, bile acids can function as signaling molecules via the activation of nuclear and G-protein coupled receptors, FXR and TGR5. BA signaling is associated with inflammation, cell proliferation, as well as apoptosis in the liver. Excessive accumulation of bile acids in the liver (cholestasis) is observed in many diseased states. We found that the scaffold protein IQGAP1 was induced in mouse models of cholestasis and in patients. In vitro BA treatment of liver cell-line, HepG2 also induced IQGAP1 in a dose-dependent manner. To investigate the role of this IQGAP1 induction, we fed wild-type male mice 0.1% 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) diet for 3days, 1week, and 2weeks. Serum bile acid and hepatic IQGAP1 induction increased concomitantly with time. We next treated Iqgap1-/- male mice similarly with DDC and found a 3-fold increase in serum bile acid levels compared to treated wild-type mice. Liver injury markers ALT, AST, and bilirubin were also increased. Although Iqgap1-/- mice had similar gene expression of key BA regulators, Fxr and Shp as wild-type mice, the alteration in expression of BA transporters and reduced cell-cell junction protein levels may contribute to the elevated BAs levels. We quantified gene expression of pro-inflammatory and proliferative mediators using qRT-PCR and examined the liver histology with immuno-staining. We found Iqgap1-/- livers expressed increased pro-inflammatory mediators and proliferative genes like Il-6 and cyclin D1 along with MAPK signaling targets, Fos and Egr-1. To further examine how IQGAP1 functions as a mediator of bile acid signaling, we have generated IQGAP1 domain deleted constructs and are examining their protein-protein interactions with Mass Spec. Based on our results, IQGAP1 induction subsequent to accumulation of BAs is protective against injury in the liver.