Abstract
Objective- Vascular calcification is a common and severe complication in patients with atherosclerosis which is exacerbated by type 2 diabetes mellitus. Our laboratory recently reported that the collagen receptor discoidin domain receptor 1 (DDR1) mediates vascular calcification in atherosclerosis; however, the underlying mechanisms are unknown. During calcification, vascular smooth muscle cells transdifferentiate into osteoblast-like cells, in a process driven by the transcription factor RUNX2 (runt-related transcription factor 2). DDR1 signals via the phosphoinositide 3-kinase/Akt pathway, which is also central to insulin signaling, and upstream of RUNX2, and this led us to investigate whether DDR1 promotes vascular calcification in diabetes mellitus via this pathway. Approach and Results- Ddr1+/+ ; Ldlr-/- (single knock-out) and Ddr1-/- ; Ldlr-/- (double knock-out) mice were placed on high-fat diet for 12 weeks to induce atherosclerosis and type 2 diabetes mellitus. Von Kossa staining revealed reduced vascular calcification in the aortic arch of double knock-out compared with single knock-out mice. Immunofluorescent staining for RUNX2 was present in calcified plaques of single knock-out but not double knock-out mice. Primary vascular smooth muscle cells obtained from Ddr1+/+ and Ddr1-/- mice were cultured in calcifying media. DDR1 deletion resulted in reduced calcification, a 74% reduction in p-Akt levels, and an 88% reduction in RUNX2 activity. Subcellular fractionation revealed a 77% reduction in nuclear RUNX2 levels in Ddr1-/- vascular smooth muscle cells. DDR1 associated with phosphoinositide 3-kinase, and treatment with the inhibitor wortmannin attenuated calcification. Finally, we show that DDR1 is important to maintain the microtubule cytoskeleton which is required for the nuclear localization of RUNX2. Conclusions- These novel findings demonstrate that DDR1 promotes RUNX2 activity and atherosclerotic vascular calcification in diabetes mellitus via phosphoinositide 3-kinase/Akt signaling.