The transforming growth factor type beta-1 (TGF-beta) signaling pathway is a major tumor suppressor during early carcinogenesis, and its growth-suppressive activity is commonly lost during early tumor progression. IkappaB kinase alpha (IKKalpha) also acts as a tumor suppressor in stratified epithelia, and its expression and nuclear localization are progressively down-regulated during malignant progression of squamous cell carcinoma (SCC) and acquisition of an invasive phenotype. A critical role for IKKalpha in TGF-beta signaling in stratified epithelia was identified recently during normal keratinocyte differentiation, and both IKKalpha and components of the TGF-beta signaling pathway are required for induction of antiproliferative Myc antagonists in such cells. We now describe that the interaction between IKKalpha and the TGF-beta signaling pathway is also important in a subset of SCCs. In SCCs that are unable to shuttle IKKalpha to the nucleus, defective TGF-beta-induced growth arrest was rescued by introduction of a constitutively nuclear IKKalpha variant. These results suggest that the tumor-suppressive activity of IKKalpha in stratified epithelia may be exerted in part via the TGF-beta signaling pathway.