Objective: Although biglycan (BGN) and oxidized low-density lipoprotein (oxLDL) accumulation has been observed in calcific, stenotic aortic valves, their role in the pathogenesis of calcific aortic valve disease is poorly understood. We hypothesized that soluble BGN induces the osteogenic response in human aortic valve interstitial cells via Toll-like receptor (TLR) 2 and TLR4 and mediates the proosteogenic effect of oxLDL.
Methods and results: Aortic valve interstitial cells of stenotic valves express higher levels of BGN. Stimulation of cells from normal valves with BGN increased the expression of bone morphogenetic protein-2 (BMP-2) and alkaline phosphatase (ALP) among the chondrogenic/osteogenic markers examined and caused accumulation of calcium deposits. TLR2 silencing, but not TLR4 silencing, reduced BMP-2 and ALP levels after BGN stimulation although coimmunoprecipitation revealed that BGN interacts with both TLR2 and TLR4. BGN induced the phosphorylation of extracellular signal-regulated protein kinase-1/2, p38 mitogen-activated protein kinase and nuclear factor-κB. Inhibition of extracellular-regulated kinase-1/2 markedly reduced the upregulation of BMP-2 and ALP expression by BGN whereas inhibition of p38 mitogen-activated protein kinase or nuclear factor-κB had a moderate effect. Stimulation of aortic valve interstitial cells with oxLDL upregulated BGN expression and release. Knockdown and neutralization of BGN reduced the effect of oxLDL on BMP-2 and ALP expression.
Conclusions: Extracellular soluble BGN induces the expression of BMP-2 and ALP in human aortic valve interstitial cells primarily via TLR2 and contributes to the proosteogenic effect of oxLDL. These findings highlight the potential role of soluble BGN and oxLDL in the development of calcific aortic valve disease.