Oxidized low-density lipoprotein promotes osteoblast differentiation in primary cultures of vascular smooth muscle cells by up-regulating Osterix expression in an Msx2-dependent manner.

We have previously shown that oxidized low-density lipoproteins (oxLDLs) act synergistically with ß-glycerophosphate to induce the osteogenic differentiation of primary bovine aortic smooth muscle cells (BASMCs). In the present study, we attempt to resolve the mechanism responsible for this effect by examining the expression of several osteoblast-specific transcription factors. Thus, by culturing BASMCs in the absence or presence of ß-glycerophosphate and/or oxLDL, we demonstrate that ß-glycerophosphate induces both Runx2 and Osterix (Osx) expression. In contrast, oxLDL has no effect on Runx2 expression but rather it enhances ß-glycerophosphate-induced osteoblast differentiation by further up-regulating Osx expression. In an attempt to elucidate the mechanism responsible for this latter effect, we examined the ability of oxLDL to affect Msh homeobox 2 (Msx2) expression. Similar to its effect on Osx expression, oxLDL was found to synergistically enhance ß-glycerophosphate-induced Msx2 expression in an extracellular signal-regulated kinase 1 and 2 (Erk 1 and 2)-dependent manner. Furthermore, oxLDL's ability to enhance both ß-glycerophosphate-induced Osx expression and alkaline phosphatase activity was prevented when the BASMCs were first transfected with Msx2-specific siRNA. Taken together, these findings suggest a plausible mechanism by which oxLDL may promote osteoblast differentiation and vascular calcification in vivo.