Necessary role of phosphatidylinositol 3‐kinase in transforming growth factor β–mediated activation of Akt in normal and rheumatoid arthritis synovial fibroblasts

Abstract Objective 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 β1 (TGFβ1), 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 TGFβ in fibroblasts. Methods 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. Results Exposure of fibroblasts to TGFβ 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 TGFβ‐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 TGFβ on Fas‐mediated apoptosis. Conclusion TGFβ exerts its growth and antiapoptotic effects on fibroblasts, at least in part, by activation of the PI 3‐kinase/Akt pathway.