Progenitor Cells Derived From Injured Cartilage Surface Respond to Damage Associated Molecular Patterns

Abstract

Objective: To elucidate how chondrogenic progenitor cells (CPCs) originated from mechanically injured cartilage surface respond to proinflammatory endogenous damage-associated-molecular-patterns (DAMPs). Design: Passage 1 bovine CPCs and non-CPCs isolated from injured articular cartilage either by blunt impaction or by scratches were treated with mitochondrial DAMPs (MTDs) composed of fMLF and CpG DNA, or HMGB1 (a nuclear DAMP), or IL-1b for 24 hrs. At the end of the experiments, the expression levels of matrix metalloproteinases (MMPs), chemokines, and cytokines that are associated with cartilage degeneration was examined with Western blotting and quantitative PCR. Results: Both HMGB1 and MTDs remarkably up-regulated expression level of pro-MMP-13 protein in CPCs while showed weak effect on non-CPCs. Compared to non-CPCs, CPCs expressed significantly higher baseline mRNA levels of MMP-13, CXCL12, and IL-6. MTDs further increased the expression levels of MMP-13 and IL-6 in CPCs while HMGB1 did not show such effect. When treated with MTDs, CPCs also expressed significantly higher levels of IL-8 mRNA than did non-CPCs. However, compared to non-CPCs, CPCs expressed much lower levels of MMP-3 and active MMP-13 proteins as well as lower mRNA levels of CCL2 and IL-6 in response to IL-1b. Conclusions: CPCs were more sensitive than non-CPCs in response to DAMPs, especially MTDs, to up-regulate expression of MMP-13, IL-6 and -8. When IL-1b was present, CPCs were less responsive than non-CPCs in terms of up-regulating MMP-3, CCL2, and IL-6 expression. The proinflammatory nature of CPCs implied their critical role in the early phase of PTOA development.