Attenuation of the TGF-β-Smad signaling pathway in pancreatic tumor cells confers resistance to TGF-β-induced growth arrest

We have investigated the mechanism whereby tumor cells become resistant to the antiproliferative effects of transforming growth factor (TGF)-β, while maintaining other responses that can lead to increased malignancy and invasiveness. TGF-β signaling results in nuclear accumulation of active Smad complexes which regulate transcription of target genes. Here we show that in two pancreatic carcinoma cell lines, PT45 and Panc-1, that are resistant to TGF-β-induced growth arrest, the TGF-β-Smad signaling pathway is attenuated compared with epithelial cells that are sensitive to the antiproliferative effects of TGF-β (HaCaT and Colo-357). In PT45 and Panc-1 cells, active Smad complexes remain nuclear for only 1–2 h compared with more than 6 h in HaCaT and Colo-357 cells. The attenuated pathway in PT45 and Panc-1 cells correlates with low levels of TGF-β type I receptor and results in an altered expression profile of TGF-β-inducible genes required for cell cycle arrest. Most significantly, expression of the CDK inhibitor, p21Cip1/WAF1, which is required for TGF-β-induced growth arrest in these cells, is not maintained. Moreover, we show that artificially attenuating the TGF-β-Smad signaling pathway in HaCaT cells is sufficient to prevent TGF-β-induced growth arrest. Our results demonstrate that the duration of TGF-β-Smad signaling is a critical determinant of the specificity of the TGF-β response.