Abstract 14857: Hepatic Insulin Signaling Posttranscriptionally Regulates Ldl Receptor via Pcsk9
The levels of hepatic LDLRs play a key role in the regulation of LDL levels and atherogenesis. Recently, we proposed that the increased VLDL, normal LDL levels typically associated with Type 2 diabetes can be explained by the dual effect of increased hepatic insulin signaling to increase VLDL secretion, while increasing hepatic LDLRs. This study was undertaken to elucidate potential mechanisms linking hepatic insulin signaling to regulation of LDLR levels and the possible involvement of PCSK9, a key factor involved in the post-transcriptional down-regulation of hepatic LDLR protein levels that has emerged as therapeutic target. Insulin receptor (IR) knockdown by shRNA resulted in increased PCSK9 mRNA and protein (91% and 52%, respectively, p<0.05) , no change in LDLR mRNA and reduced hepatic LDLR protein (57%, p<0.05) in mice. Interestingly, IR shRNA failed to affect LDLR levels in Pcsk9-/- mice. Conversely, in ob/ob mice with hyperinsulinemia and increased hepatic AKT activity, Pcsk9 mRNA was reduced by 40%, and LDLR protein was increased by 30%. Overexpression of constitutively active AKT increased LDLR expression in WT mice. Next we investigated different signaling pathways downstream of AKT, and ruled out a role of Foxo1 and GSK pathways. However, the LDLR levels were reduced by 21% (p<0.05) and plasma cholesterol levels increased by 1.5 fold_in mice treated with Rapamycin (2mg/kg), an inhibitor of the mTORC1 kinase, for 7 days. Rapamycin did not change Ldlr mRNA level, but increased HNF1α levels and activity on the Pcsk9 promoter (shown by ChiP assay) and increased plasma cholesterol level in a SREBP2-independent manner. This suggests a crucial role of insulin signaling via PI3K/AKT/mTOR in the post-transcriptional up-regulation of hepatic LDLR levels via suppression of HNF-1 and PCSK9 and suggests that hepatic insulin signaling should be considered when the disruption of PCSK9 is used as therapeutic application.
- © 2010 by American Heart Association, Inc.