Rheumatoid arthritis is a disease that, pathologically, is characterized by the progressive growth and invasion of the synovial pannus into the surrounding cartilage and bone. Many cytokines, including transforming growth factor beta1 (TGFbeta1), have been implicated in this process, but their mode of action is incompletely understood. The goal of the present study was to better understand the downstream signaling pathways of TGFbeta in fibroblasts.
The role of phosphatidylinositol 3-kinase (PI 3-kinase) was determined by chemical inhibition with LY294002 or wortmannin. Activation of protein kinase B (Akt), c-Jun N-terminal kinases (JNKs), and extracellular signal-regulated kinases (ERKs) was evaluated by Western blot analysis using phospho-specific antibodies.
Exposure of fibroblasts to TGFbeta rapidly induced activation of a kinase, Akt, that is known to inhibit apoptosis by a variety of pathways. Activation of Akt was blocked by the specific PI 3-kinase inhibitor, LY294002, indicating that TGFbeta-mediated phosphorylation of Akt was dependent on PI 3-kinase activation. This activation pathway was relatively selective for Akt, since inhibition of PI 3-kinase failed to substantially modify activation of ERKs or JNKs in synovial fibroblasts. Inhibition of the PI 3-kinase/Akt pathway resulted in impaired proliferation of synovial fibroblasts and partial attenuation of the protective effect of TGFbeta on Fas-mediated apoptosis.
TGFbeta exerts its growth and antiapoptotic effects on fibroblasts, at least in part, by activation of the PI 3-kinase/Akt pathway.