Smad7 is overexpressed in 50% of human pancreatic cancers. COLO-357 pancreatic cancer cells engineered to overexpress Smad7 are resistant to the actions of transforming growth factor-β1 (TGF-β1) with respect to growth inhibition and cisplatin-induced apoptosis but not with respect to modulation of gene expression. To delineate the mechanisms underlying these divergent consequences of Smad7 overexpression, we studied the effects of Smad7 on TGF-β1-dependent signaling pathways and cell cycle regulating proteins. TGF-β1 induced the phosphorylation of MAPK, p38 MAPK, and AKT2 irrespective of the levels of Smad7, and inhibitors of these pathways did not alter TGF-β1 actions on cell growth. By contrast, Smad7 overexpression interfered with TGF-β1-mediated attenuation of cyclin A and B levels, inhibition of cdc2 dephosphorylation and CDK2 inactivation, up-regulation of p27, and the maintenance of the retinoblastoma protein (RB) in a hypophosphorylated state. Smad7 also suppressed TGF-β1-mediated inhibition of E2F activity but did not alter TGF-β1-mediated phosphorylation of Smad2, the nuclear translocation of Smad2/3/4, or DNA binding of the Smad2/3/4 complex. Although Smad7 did not associate with the type I TGF-β receptor (TβRI), SB-431542, an inhibitor of the kinase activity of this receptor, blocked TGF-β1-mediated effects on Smad-2 phosphorylation. These findings point toward a novel paradigm whereby Smad7 acts to functionally inactivate RB and de-repress E2F without blocking the activation of TβRI and the nuclear translocation of Smad2/3, thereby allowing for TGF-β1 to exert effects in a cancer cell that is resistant to TGF-β1-mediated growth inhibition.
Bcl-xL Blocks Transforming Growth Factor-β1-induced Apoptosis by Inhibiting CytochromecRelease and Not by Directly Antagonizing Apaf-1-dependent Caspase Activation in Prostate Epithelial Cells
