PI3Kγ regulates cartilage damage in chronic inflammatory arthritis

The gamma isoform of phosphoinositide 3-kinase (PI3Kgamma) has been viewed as restricted to leukocytes mediating the regulation of chemokine-induced migration and recruitment of neutrophils, monocytes, and macrophages. In line with the observation that PI3Kgamma-deficient mice display defects in adaptive immunity, inhibition of PI3Kgamma reduces synovial inflammation in the collagen-induced arthritis mouse model of inflammatory arthritis [rheumatoid arthritis (RA)], which has been attributed to reduced influx of inflammatory cells. Challenging the concept of leukocyte-restricted PI3Kgamma function, we report here a novel, nonredundant function of PI3Kgamma as an important regulator of fibroblast-induced cartilage destruction during chronic destructive arthritis. We show that in human tumor necrosis factor transgenic mice, the loss of PI3Kgamma leads to a milder inflammatory arthritis. Interestingly, PI3Kgamma deficiency does not alter the recruitment of inflammatory cells, but significantly reduces cartilage damage through reduced expression of matrix metalloproteinases in fibroblasts and chondrocytes. In vitro analyses demonstrate that the decreased invasiveness of fibroblasts is mediated by reduced phosphorylation of Akt and extracellular signal-regulated kinase. Using a PI3Kgamma specific inhibitor, these data are confirmed in human synovial fibroblasts from patients with RA who exhibit a disease-specific up-regulation of PI3Kgamma. Our data indicate that in addition to mediating the recruitment of inflammatory cells, PI3Kgamma is an important regulator of fibroblast-mediated joint destruction in RA and suggest that specific inhibitors of PI3Kgamma will interfere with the activation of RA synovial fibroblasts and reduce cartilage destruction in RA.