The TGF-β signaling pathway plays a dual role in epithelial malignancies. While attenuated function of TGF-β signaling enables the development of epithelial cancers, activation of alternative TGF-β signaling in established cancer promotes a more invasive and metastatic phenotype. While the loss of its tumor suppressor function may be due to a variety of defects in the TGF-β Smad-dependent signal transduction pathway, the mechanisms responsible for the pro-oncogenic effects are less well characterized. Since aberrant activation of transcription factor Nuclear Factor-kappaB (NF-κB) promotes the malignant phenotype in head and neck squamous cell carcinomas (HNSCC), we examined whether TGF-β cross-activates NF-κB. Here, we show that TGFβ-1 treatment induced NF-κB activation and gene expression in HNSCC lines. TGFβ-1 and TNFα-induced NF-κB activation was mediated through TGF-β-activated kinase 1 (TAK1), since knocking down of TAK1 using siRNA decreased both TGFβ-1 and TNFα induced NF-κB-dependent reporter gene activity. Furthermore, TAK1 knockdown decreased degradation of IκBα, the inhibitor of NF-κB, and promoted nuclear translocation of the transactivating NF-κB subunit p65 (RELA). Consequently, p65 DNA binding activity and transcription of NF-κB downstream genes was attenuated. As TGFβ-1 treatment of HNSCC cells with mutated TGF-β Receptor II (TβRII) did not affect NF-κB-dependent reporter gene expression, functional TβRII signaling was required for activation of TAK1. Transient transfection and expression of the inhibitor SMAD7 was able to attenuate NF-κB activation as shown by NF-κB-dependent reporter gene activity, however SMAD7 preferentially decreased TGFβ-1 over TNFα induced NF-κB activation. Conversely, knocking down of SMAD7 using siRNA increased TGFβ-1 induced NF-κB activation to a greater extent than that induced by TNFα. Furthermore, TGFβ-1 treatment increased SMAD7 mRNA and overexpression of SMAD7 nearly abrogated TGF-β-induced reporter gene activity, indicating SMAD7 provides a negative feedback loop preferentially suppressing canonical TGF-β signaling. Knockdown of the NF-κB subunit p65 using siRNA decreased SMAD7 mRNA, indicating that NF-κB activation further contributes to the expression and greater inhibitory effect of SMAD7 upon TGF-β signaling. In conclusion, we show that both TGFβ-1 and TNFα can induce NF-κB activation through TAK1. Although SMAD7 was able to attenuate this TAK1/NF-κB activation, the inhibitory effect on canonical TGF-β signaling was more potent, favoring increased NF-κB-mediated cell growth and survival signaling in HNSCC. This study helps explain the dual role of TGF-β signaling, which involves activation of NF-κB and suppression of canonical TGF-β signaling in HNSCC.(Supported by NIDCD intramural project ZIADC-000016)

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5052.