Epithelial-mesenchymal transition (EMT) plays a critical role in airway injury, repair, and structural remodeling. IκB kinase (IKK)-NFκB signaling regulates late EMT-associated gene expression. However, IKK-mediated mesenchymal transition occurs earlier than NFκB/RelA subunit-dependent EMT gene expression, leading us to investigate the hypothesis that IKK plays an independent mechanism in transforming growth factor-β (TGFβ)-induced EMT. Time-resolved dissection of early proteome and phosphoproteome changes in response to TGFβ and a specific IKK inhibitor, BMS-345541, revealed that IKK regulates cascades of 23 signaling pathways essential in EMT, including TGFβ signaling, p38 mitogen associate protein kinase (MAPK), Toll receptor signaling, and integrin pathways. We identified early IKK-dependent phosphorylation of core regulatory proteins in essential EMT signaling cassettes, including ATF2, JUN, NFKB1/p105, and others. Interestingly, we found that IKKβ directly complexes with and phosphorylates the spliced X-box-binding protein 1 (XBP1s). XBP1s is an arm of the unfolded protein response (UPR) that activates the hexosamine biosynthetic pathway (HBP), a pathway that mediates protein N-glycosylation and survival from ER stress-induced apoptosis in EMT. We found that inhibition of IKK activity abolishes the phosphorylation of XBP1-T48, blocks XBP1s nuclear translocation, and inhibits the activation of HBP. Our study elucidates a previously unrecognized IKKβ-XBP1s-HBP crosstalk pathway that couples inflammation and glucose metabolic reprogramming in ETM. Because XBP1-HBP controls N-glycosylation of the extracellular matrix (ECM) in EMT, this novel IKKβ-XBP1-HBP pathway may contain therapeutic targets whose inhibition could prevent ECM remodeling in lung fibrosis or other airway remodeling diseases.
Keywords: IκB kinase-NFκB; N-glycosylation; epithelial−mesenchymal transition; extracellular matrix; hexosamine biosynthetic pathway; proteomics; unfolded protein response.