Abstract 5464: The Cell Proliferative Effect of LRP6 Mutation is Mediated by Increased PDGF Dependent Cyclin D1 Expression
We recently identified a disease causing mutation (R611C) at a highly conserved residue of the LDL receptor like protein 6 (LRP6) in a family with early onset coronary artery disease (CAD), and metabolic syndrome. This finding signifies the effect of a single gene defect in LRP6 on development of atherosclerosis and underscores emerging evidence implicating effect of altered Wnt signaling on atherosclerosis. The exact mechanisms linking LRP6 mutation with atherosclerosis are not well understood. mRNA expression analysis in cells from LRP6 mutation carriers has demonstrated that LRP6 mutation has a heterogeneous effect on Wnt signaling. Strikingly, cyclin D1, a target of Wnt signaling and a cell cycle activator is expressed at significantly higher levels in the mutant compared to the wildtype cells. Smooth muscle cell proliferation is a major component in the process of atherosclerosis development. We therefore dissected Wnt signaling and examined cell proliferation in aortic smooth muscle cell (SMC) cultures isolated from LRP6R593C knock-in and LRP6 +/− knockout mice. The proliferation rate of LRP6R593C SMCs was highest and those of the wildtype littermates were the lowest among the three, indicating that both haploinsufficiency as well as impaired LRP6 function result in cell cycle activation. GSK3β is an intermediate peptide of the Wnt signaling pathway that inhibits cyclin D1expression. PDGF inactivates GSK3β and promotes cellular proliferation by increasing the expression levels of cyclin D1. Our experiments showed that GSK3β is at baseline more activated in cells expressing the mutant vector compared to the wildtype cells. However, after stimulation with PDGF, GSK3β activation in the mutant cells diminishes to levels significantly lower than wildtype cells. This effect was mediated by higher expression levels of PDGF receptor in the mutant compared to the wildtype cells. Our result signifies the important role of LRP6 and its interaction with PDGF in the process of smooth cell proliferation and identifies LRP6 and GSK3β as potential targets for both prevention and pharmacotherapy of atherosclerosis.