Functional Genomic Analysis in Arthritis-Affected Cartilage: Yin-Yang Regulation of Inflammatory Mediators by α5β1 and αVβ3 Integrins

Osteoarthritis-affected cartilage exhibits enhanced expression of fibronectin (FN) and osteopontin (OPN) mRNA in differential display and bioinformatics screen. Functional genomic analysis shows that the engagement of the integrin receptors α5β1 and αvβ3 of FN and OPN, respectively, have profound effects on chondrocyte functions. Ligation of α5β1 using activating mAb JBS5 (which acts as agonist similar to FN N-terminal fragment) up-regulates the inflammatory mediators such as NO and PGE2 as well as the cytokines, IL-6 and IL-8. Furthermore, up-regulation of these proinflammatory mediators by α5β1 integrin ligation is mediated via induction and autocrine production of IL-1β, because type II soluble IL-1 decoy receptor inhibits their production. In contrast, αvβ3 complex-specific function-blocking mAb (LM609), which acts as an agonist similar to OPN, attenuates the production of IL-1β, NO, and PGE2 (triggered by α5β1, IL-1β, IL-18, or IL-1β, TNF-α, plus LPS) in a dominant negative fashion by osteoarthritis-affected cartilage and activated bovine chondrocytes. These data demonstrate a cross-talk in signaling mechanisms among integrins and show that integrin-mediated “outside in” and “inside out” signaling very likely influences cartilage homeostasis, and its deregulation may play a role in the pathogenesis of osteoarthritis.