TGF-beta promotes cancer progression through the xIAP:TAB1:TAK1:IKK axis in mammary epithelial cells

The research presented here focuses on molecular mechanisms coupling TGF-β signaling to the NF-κB pathway and the consequences of pro-inflammatory gene expression during the conversion of TGF-β from tumor suppressor to tumor promoter. TGF-β activated the NF-κB pathway in metastatic murine MECs while it repressed activity in normal MECs. Inhibition of TAK1 and IKK was capable of inhibiting TGF-β mediated NF-κB activation in metastatic MECs. Comparison of normal and metastatic MECs revealed that the TAB1 interacted with IKKβ in metastatic MECs but not within normal MECs. TAB1:IKKβ complexes could only be detected in normal MECs following their induction of EMT, after which TGF-β was also able to induce NF-κB activity. Targeted inhibition of TAK1 and TAB1 inhibited TGF-β-stimulated NF-κB activation, invasion, and growth of metastatic cells both in vitro and in vivo. Taken together, our findings define a novel TAB1:TAK1:IKKβ:NF-κB signaling axis that forms aberrantly in breast cancer cells, and consequently, enables oncogenic signaling by TGF-β.;xIAP was also determined to participate in this signaling axis. Over-expression of xIAP, induced TAB1:IKKβ complex formation, increased Smad signaling and NF-κB activity while inhibiting TGF-β-mediated repression of NF-κB activity. Ubiquitination of TAK1 and induction of TAK1:IKKγ/NEMO interactions within this axis were facilitated by the ubiquitin ligase activity of xIAP. xIAP-mediated NF-κB activation was blocked by the expression of K63R ubiquitin. Loss or shRNA-mediated reduction of xIAP confirmed the involvement of xIAP in TGF-β signaling and regulation of pro-inflammatory and mesenchymal genes. Finally, the ubiquitin ligase function and expression of xIAP in metastatic MECs was required for TGF-β-mediated NF-κB activation in addition to its contribution to anchorage-independent survival and invasiveness.;Pro-inflammatory COX-2 expression was greater in metastatic MECs and could also be induced through TGF-β-mediated EMT, a process coupled with reduction of the Smad3 expression. Increased COX-2 resulted in loss of Smad3 activity while potentiating other signaling pathways. The repression of Smad3 activity occurred through a PGE2:EP receptor signaling axis. Lastly, shRNA-mediated reduction of COX-2 inhibited PGE-2 production, TGF-β-mediated EMT, and potentiated TGF-β-mediated Smad2/3 nuclear translocation and transcriptional activity.;These alterations represent key changes in oncogenic TGF-β signaling.