Inhibition of TGF-β Signaling for the Treatment of Tumor Metastasis and Fibrotic Diseases

Transforming growth factor-beta (TGF-beta) is a cytokine involved in cell proliferation, apoptosis, differentiation, angiogenesis, cell adhesion, migration, extracellular matrix deposition, wound healing and immune regulation. The cellular response to TGF-beta depends on the cell type and cell microenvironment. TGF-beta supresses tumor growth in the early phase of neoplasia, while promotes tumor progression and metastasis in later phases. Thus, many malignant cells produce large amounts of TGF-beta, but are resistant to its growth inhibitory effects. TGF-beta produced by tumors depresses antitumor immune responses and diminishes cancer immunotherapy. TGF-beta initiates the epithelial-to-mesenchymal transition (EMT) associated specifically with tumor invasiveness and metastasis and also with the generation of fibroblasts associated with accumulation of extracellular matrix in chronic fibrotic disorders. There are several possibilities for the disruption of TGF-beta signaling: 1) targeting the expression and function of TGF-beta by a small interfering RNA strategy, by a neutralizing TGF-beta monoclonal antibody or by decreasing the enzymatic activity of furin, a TGF-beta activating protease, 2) inhibiting TGF-beta receptor kinase 1, named also activin receptor-like kinase (ALK5), activity by pyridinylimidazoles (SB-431542, SB-505124, SB-525334, A-83-01), by 2,4-disubstituted pteridine (SD-208) or by a quinazoline-derived inhibitor (SD-093) which all interact with the ATP-binding site of ALK5; 3) inhibiting of Smad 2 and Smad 3 signaling by overexpression of their physiological inhibitor (Smad 7) or by using thioredoxin as an Smad anchor disambling Smad from activation; 4) inducing an immune response by administration of TGF-beta-resistant cytotoxic T-lymphocytes or by the treatment with a small-molecule ALK5 inhibitor.