The Receptor Tyrosine Kinase FGFR4 Negatively Regulates NF-kappaB Signaling

Background: NF kappa B signaling is of paramount importance in the regulation of apoptosis, proliferation, and inflammatory responses during human development and homeostasis, as well as in many human cancers. Receptor Tyrosine Kinases (RTKs), including the Fibroblast Growth Factor Receptors (FGFRs) are also important in development and disease. However, a direct relationship between growth factor signaling pathways and NFkB activation has not been previously described, although FGFs have been known to antagonize TNF alpha-induced apoptosis. Methodology/Principal Findings: Here, we demonstrate an interaction between FGFR4 and IKKb (Inhibitor of NF kappa B Kinase beta subunit), an essential component in the NF kappa B pathway. This novel interaction was identified utilizing a yeast two-hybrid screen [1] and confirmed by coimmunoprecipitation and mass spectrometry analysis. We demonstrate tyrosine phosphorylation of IKKb in the presence of activated FGFR4, but not kinase-dead FGFR4. Following stimulation by TNF alpha (Tumor Necrosis Factor a) to activate NF kappa B pathways, FGFR4 activation results in significant inhibition of NF kappa B signaling as measured by decreased nuclear NF kappa B localization, by reduced NFkB transcriptional activation in electophoretic mobility shift assays, and by inhibition of IKK beta kinase activity towards the substrate GST-I kappa B alpha in in vitro assays. FGF19 stimulation of endogenous FGFR4 in TNF alpha-treated DU145 prostate cancer cells also leads to a decrease in IKKb activity, concomitant reduction in NF kappa B nuclear localization, and reduced apoptosis. Microarray analysis demonstrates that FGF19 + TNF alpha treatment of DU145 cells, in comparison with TNFa alone, favors proliferative genes while downregulating genes involved in apoptotic responses and NF kappa B signaling. Conclusions/Significance: These results identify a compelling link between FGFR4 signaling and the NF kappa B pathway, and reveal that FGFR4 activation leads to a negative effect on NF kappa B signaling including an inhibitory effect on proapoptotic signaling. We anticipate that this interaction between an RTK and a component of NF kappa B signaling will not be limited to FGFR4 alone.