Glutamate is an excitatory neurotransmitter that activates ionotropic and metabotropic glutamate receptors (mGluRs) on neurons and glia to mediate such functions as synaptic plasticity, proliferation, and survival. mGluR5 activates the transcription factor nuclear factor κB (NF-κB), but the mechanisms involved are unclear. Sitcheran et al. performed gel shift assays to demonstrate that glutamate induced the binding of the p65 and p50 NF-κB subunits to DNA in the human glioma cell line, H4. The authors also examined the signaling of the epidermal growth factor receptor (EGFR), which is found on astrocytes and is also known to activate NF-κB. Reporter assays showed that glutamate-induced activation of NF-κB was comparable to that induced by EGF. Western blotting analyses demonstrated that glutamate induced the phosphorylation and activation of inhibitor of κB kinase α and β (IKKα and IKKβ) and of p65. Knockdown of both IKKα and IKKβ in H4 cells by small inhibitory RNA (siRNA) suppressed glutamate-induced phosphorylation of p65. In canonical NF-κB signaling, IKKβ phosphorylates IκBα, which leads to its degradation and the release of active NF-κB subunits. In this study, however, the authors found no evidence of glutamate-induced (or EGF-induced) phosphorylation or degradation of IκBα, although coimmunoprecipitation assays showed that glutamate substantially reduced the interaction between IκBα and p65 compared with that in control cells. Knockdown of EGFR blocked mGluR5-stimulated phosphorylation of p65. The authors found that mGluR5 stimulation led to the phosphorylation of specific tyrosine residues in EGFR and to its physical association with mGluR5. A cell-permeable Ca2+ chelator blocked mGluR5-mediated NF-κB activation, and an inhibitor of EGFR activity reduced mGluR5-stimulated Ca2+ signaling. Together, these data suggest that EGFR signaling is critical for the noncanonical activation of NF-κB by glutamate and that this crosstalk may have implications for the treatment of glioblastoma, in which EGFR abundance is increased.
R. Sitcheran, W. C. Comb, P. C. Cogswell, A. S. Baldwin, Essential role for epidermal growth factor receptor in glutamate receptor signaling to NF-κB. Mol. Cell. Biol. 28, 5061-5070 (2008). [Abstract] [Full Text]