Suppression of Idol expression is an additional mechanism underlying statin-induced up-regulation of hepatic LDL receptor expression

Recent studies have identified proprotein convertase subtilisin/kexin type 9 (PCSK9) and Idol as negative regulators of low density lipoprotein receptor (LDLR) protein stability. While the induction of PCSK9 transcription has been recognized as a limitation to the statin cholesterol-lowering efficacy at higher doses, it is unknown whether Idol is involved in the statin-mediated up-regulation of the hepatic LDLR. Here we report that statins exert opposite effects on PCSK9 and Idol gene expression in human hepatoma-derived cell lines and primary hepatocytes isolated from hamsters and rats. While PCSK9 expression was induced, the level of Idol mRNA rapidly declined in statin-treated cells in a dose-dependent manner. This differs from the effect of the liver X receptor ligand, GW3965, which increased the expression of both PCSK9 and Idol. We further show that cellular depletion of Idol by siRNA transfection did not change PCSK9 expression levels in control and statin-treated cells; however, the basal level of LDLR protein increased by 60% in Idol siRNA transfected HepG2 cells. More importantly, the increase in LDLR protein abundance by rosuvastatin and atorvastatin treatment was compromised by Idol siRNA transfection. Collectively, our present findings suggest that the suppression of Idol gene expression in liver cells is an additional mechanism underlying the statin-induced up-regulation of hepatic LDLR expression. This may contribute to the hypocholesterolemic effects of statins observed in clinical settings.