RESUMO
Transforming growth factor-beta (TGF-ß) has a dual role in epithelial malignancies, including head and neck squamous cell carcinoma (HNSCC). Attenuation of canonical TGF-ß signaling enhances de novo tumor development, whereas TGF-ß overexpression and signaling paradoxically promotes malignant progression. We recently observed that TGF-ß-induced growth arrest response is attenuated, in association with aberrant activation of nuclear factor-κB (NF-κB), a transcription factor, which promotes malignant progression in HNSCC. However, what role cross-talk between components of the TGF-ß and NF-κB pathways plays in altered activation of these pathways has not been established. Here, we show TGF-ß receptor II and TGF-ß-activated kinase 1 (TAK1) are predominantly expressed in a subset of HNSCC tumors with nuclear activation of NF-κB family member RELA (p65). Further, TGF-ß1 treatment induced sequential phosphorylation of TAK1, IKK, IκBα and RELA in human HNSCC lines. TAK1 enhances TGF-ß-induced NF-κB activation, as TAK1 siRNA knockdown decreased TGF-ß1-induced phosphorylation of IKK, IκB and RELA, degradation of IκBα, RELA nuclear translocation and DNA binding, and NF-κB-induced reporter and target gene transcription. Functionally, TAK1 siRNA inhibited cell proliferation, migration and invasion. Celastrol, a TAK1 inhibitor and anti-inflammatory compound used in traditional Chinese medicine, also decreased TGF-ß1-induced phosphorylation of TAK1 and RELA, and suppressed basal, TGF-ß1- and tumor necrosis factor-alpha (TNF-α)-induced NF-κB reporter gene activity. Celastrol also inhibited cell proliferation, while increasing sub-G0 DNA fragmentation and Annexin V markers of apoptosis. Furthermore, TGF-ß and RELA activation promoted SMAD7 expression. In turn, SMAD7 preferentially suppressed TGF-ß-induced SMAD and NF-κB reporters when compared with constitutive or TNF-α-induced NF-κB reporter gene activation. Thus, cross-talk by TGF-ß via TAK1 and NF-κB promotes the malignant phenotype of HNSCC. Moreover, NF-κB may contribute to the downstream attenuation of canonical TGF-ß signaling through increased SMAD7 expression. Celastrol highlights the therapeutic potential of agents targeting TAK1 as a key node in this pro-oncogenic TGF-ß-NF-κB signal pathway.
