Andrianifahanana, Mahefatiana

Transforming growth factor β (TGFβ) has significant profibrotic activity both in vitro and in vivo . This reflects its capacity to stimulate fibrogenic mediators and induce the expression of other profibrotic cytokines such as platelet‐derived growth factor (PDGF) and epidermal growth factor (EGF/ErbB) ligands. Here we address both the mechanisms by which TGFβ induced ErbB ligands and the physiological significance of inhibiting multiple TGFβ‐regulated processes. The data document that ErbB ligand induction requires PDGF receptor (PDGFR) mediation and engages a positive autocrine/paracrine feedback loop via ErbB receptors. Whereas PDGFRs are essential for TGFβ‐stimulated ErbB ligand up‐regulation, TGFβ‐specific signals are also required for ErbB receptor activation. Subsequent profibrotic responses are shown to involve the cooperative action of PDGF and ErbB signaling. Moreover, using a murine treatment model of bleomycin‐induced pulmonary fibrosis we found that inhibition of TGFβ/PDGF and ErbB pathways with imatinib plus lapatinib, respectively, not only prevented myofibroblast gene expression to a greater extent than either drug alone, but also essentially stabilized gas exchange (oxygen saturation) as an overall measure of lung function. These observations provide important mechanistic insights into profibrotic TGFβ signaling and indicate that targeting multiple cytokines represents a possible strategy to ameliorate organ fibrosis dependent on TGFβ.—Andrianifahanana, M., Wilkes, M. C., Gupta, S. K., Rahimi, R. R., Repellin, C. E., Edens, M., Wittenberger, J., Yin, X., Maidl, E., Becker, J., Leof, E. B. Profibrotic TGFβ responses require the cooperative action of PDGF and ErbB receptor tyrosine kinases. FASEB J . 27, 4444‐4454 (2013). www.fasebj.org

Abstract Engagement of the transforming growth factor-β (TGF-β) receptor complex activates multiple signaling pathways that play crucial roles in both health and disease. TGF-β is a key regulator of fibrogenesis and cancer-associated desmoplasia; however, its exact mode of action in these pathologic processes has remained poorly defined. Here, we report a novel mechanism whereby signaling via members of the ERBB or epidermal growth factor family of receptors serves as a central requirement for the biological responses of fibroblasts to TGF-β. We show that TGF-β triggers upregulation of ERBB ligands and activation of cognate receptors via the canonical SMAD pathway in fibroblasts. Interestingly, activation of ERBB is commonly observed in a subset of fibroblast but not epithelial cells from different species, indicating cell type specificity. Moreover, using genetic and pharmacologic approaches, we show that ERBB activation by TGF-β is essential for the induction of fibroblast cell morphologic transformation and anchorage-independent growth. Together, these results uncover important aspects of TGF-β signaling that highlight the role of ERBB ligands/receptors as critical mediators in fibroblast responses to this pleiotropic cytokine. Cancer Res; 70(19); 7421–30. ©2010 AACR.

Abstract Transforming growth factor-β (TGF-β) promotes a multitude of diverse biological processes, including growth arrest of epithelial cells and proliferation of fibroblasts. Although the TGF-β signaling pathways that promote inhibition of epithelial cell growth are well characterized, less is known about the mechanisms mediating the positive response to this growth factor. Given that TGF-β has been shown to promote fibrotic diseases and desmoplasia, identifying the fibroblast-specific TGF-β signaling pathways is critical. Here, we investigate the role of mammalian target of rapamycin (mTOR), a known effector of phosphatidylinositol 3-kinase (PI3K) and promoter of cell growth, in the fibroblast response to TGF-β. We show that TGF-β activates mTOR complex 1 (mTORC1) in fibroblasts but not epithelial cells via a PI3K-Akt-TSC2–dependent pathway. Rapamycin, the pharmacologic inhibitor of mTOR, prevents TGF-β–mediated anchorage-independent growth without affecting TGF-β transcriptional responses or extracellular matrix protein induction. In addition to mTORC1, we also examined the role of mTORC2 in TGF-β action. mTORC2 promotes TGF-β–induced morphologic transformation and is required for TGF-β–induced Akt S473 phosphorylation but not mTORC1 activation. Interestingly, both mTOR complexes are necessary for TGF-β–mediated growth in soft agar. These results define distinct and overlapping roles for mTORC1 and mTORC2 in the fibroblast response to TGF-β and suggest that inhibitors of mTOR signaling may be useful in treating fibrotic processes, such as desmoplasia. [Cancer Res 2009;69(1):84–93]