Calcification of the pericellular matrix in articular cartilage is highly prevalent in osteoarthritis (OA) and aging. The deposited crystals of calcium pyrophosphate dihydrate (CPPD) and/or hydroxyapatite (HA) can be released from the cartilage matrix and can trigger inflammation and promote the expression of connective tissue degrading enzymes, thereby contributing to further cartilage degradation. The expression of C-X-C chemokines including IL-8 and its receptors CXCR-1 and CXCR-2 has recently been demonstrated in chondrocytes, and these mediators all are up regulated in OA cartilage in vivo. Our preliminary studies demonstrated that IL-8 induces MMP-13, and promotes features of hypertrophic differentiation (type X collagen expression and transglutaminase activation) associated with heightened matrix calcification in vitro in articular chondrocytes. Base on our new data on IL-8 signaling and function in leukocytes and articular chondrocytes, we propose to advance understanding of how IL-8 signaling via CXCR1 and/or CXCR2 transduces hypertrophy and matrix calcification in articular chondrocytes. We will test the following hypothetical model: 1) Activation of p38 MAPK is essential for induction of the features of hypertrophic differentiation, matrix calcification and MMP-13 by IL-8 in articualr chondrocytes. 2) Pyk2 tyrosine kinase, associated with a Src family tyrosine kinase, plays a central role in activation of p38 MAPK in mediating such effects of IL-8 in articular chondrocytes. 3) CXCR1 and CXCR2 signal differentially to transduce the stimulatory effect of IL-8. 4) Phospholipase C (PLC) transduces CXCR1 and CXCR2 signaling to induce intracellular Ca2+ increase and protein kinase C (PKC) activation, which in turn, activate Pyk2 and alpha5beta1 integrin, respectively. 5) FAK, activated through alpha5-beta1 integrin, and Pyk2 differentially mediate CXCR1 and CXCR2 signaling to induce hypertrophic differentiation, matrix calcification and MMP-13 in articular chondrocytes. The completion of this study has the potential to provide a foundation for novel chemokine and signal transduction-based therapeutic strategies for treatment of chondrocalcinosis in OA and aging.
