Vascular smooth muscle cells (SMCs) are major precursors contributing to osteochondrogenesis and calcification in atherosclerosis. Runt-related transcription factor-2 (Runx2) has been found essential for SMC differentiation to an osteochondrogenic phenotype and subsequent calcification in vitro. A recent study using a conditional targeting allele that produced a truncated Runx2 protein in SMCs of ApoE^−/− mice showed reduced vascular calcification, likely occurring via reduction of receptor activator of nuclear factor-κB ligand (RANKL), macrophage infiltration, and atherosclerotic lesion formation. Using an improved conditional Runx2 knockout mouse model, we have elucidated new roles for SMC-specific Runx2 in arterial intimal calcification (AIC) without effects on atherosclerotic lesion size.

We used an improved targeting construct to generate LDLr^−/− mice with floxed-Runx2 alleles ( LDLr^−/− :Runx2 ^f/f ) such that Cre-mediated recombination ( LDLr^−/− :Runx2 ^ΔSM ) does not produce functional truncated Runx2 protein, thereby avoiding off-target effects. We found that both LDLr^−/− :Runx2 ^f/f and LDLr^−/− :Runx2 ^ΔSM mice fed with a high fat diet developed atherosclerosis. SMC-specific Runx2 deletion did not significantly reduce atherosclerotic lesion size, macrophage number, or expression of RANKL, MCP-1, and CCR2. However, it significantly reduced AIC by 50%. Mechanistically, Sox9 and type II collagen were unaltered in vessels of LDLr^−/− :Runx2 ^ΔSM mice compared to LDLr^−/− :Runx2 ^f/f counterparts, while type X collagen, MMP13 and the osteoblastic marker osteocalcin were significantly reduced.

SMC autonomous Runx2 is required for SMC differentiation towards osteoblast-like cells, SMC-derived chondrocyte maturation and AIC in atherosclerotic mice. These effects were independent of systemic lipid metabolism, RANKL expression, macrophage infiltration, and atheromatous lesion progression.