Critical Role of mTORC2-Akt Signaling in TGF-β1-Induced Myofibroblast Differentiation of Human Pterygium Fibroblasts

PURPOSE. Profibrotic activation is essential for pterygium development. In this study, we investigated the role of the mechanistic target of rapamycin (mTOR) in regulating TGF-beta 1-induced myofibroblastic responses in human pterygium fibroblasts (HPFs) and elucidated the relative contributions of mTOR signaling components. METHODS. HPFs were pretreated with the mTOR inhibitors rapamycin and Torin2, and TGF-beta 1-induced expression of profibrotic markers, including alpha-smooth muscle actin (alpha-SMA) and fibronectin, was evaluated. RNA interference-based approaches targeting raptor and rictor, regulatory subunits of mTOR complex 1 (mTORC1) and 2 (mTORC2), respectively, were used to determine the impact of each mTOR complex on HPFs. The contractile phenotype of HPFs was assessed by a collagen gel contraction assay. RESULTS. The mTOR active-site inhibitor Torin2, which suppresses both mTORC1 and mTORC2 activity in HPFs, inhibited TGF-beta 1-induced expression of alpha-SMA and fibronectin. The allosteric inhibitor rapamycin only partially suppressed mTORC1 activity and exhibited a minimal effect on the induction of profibrotic markers. The induction of alpha-SMA and fibronectin in HPFs was abrogated by RNA interference-mediated knockdown of rictor but was only moderately affected by raptor knockdown. Akt inhibition mimicked the effect of Torin2 and rictor knockdown on myofibroblast differentiation of HPFs. mTOR inhibition potently reduced the contractile ability of HPFs in collagen gel contraction assays. CONCLUSIONS. This study found that mTOR signaling promoted profibrotic activation of HPFs and confirmed the importance of the mTORC2-Akt axis in TGF-beta 1-induced myofibroblast differentiation. Therefore, our study may open up new avenues for the development of novel therapeutic strategies involving targeting of mTOR signaling to treat pterygium.