Inhibition of the activity of a neuronal κB-binding factor by glutamate

Activation of transcription factors with affinity for κB enhancers is generally correlated with enhanced survival of neurons. In an apparent exception, excitotoxic concentrations of glutamate have been reported to elevate the activity of one such factor, nuclear factor-κB (NF-κB). Our data indicate that the constitutive neuronal κB-binding factor (NKBF) is distinct from bona fide NF-κB (RelA/p50 heterodimer). Therefore, we analyzed glutamate's effects on κB-binding activity in highly enriched primary neuronal cultures and in mixed neuron/glia cocultures. Electrophoretic mobility shift assays indicated that a 30-60-min exposure to 50-500 μM glutamate reduced NKBF activity by as much as 70%. Subtoxic doses of glutamate had little or no effect on this DNA-binding activity. Selective antagonists of either NMDA or AMPA [(RS)-α-amino-3-hydroxy-5-methyl-4- isoxazolepropionate]/kainate receptors inhibited the influence of glutamate on NKBF activity. The effect of glutamate was mimicked by calcium ionophore, and it was blocked by lowering extracellular calcium concentrations or by cyclosporin A. Bona fide NF-κB was found only in cocultures containing significant numbers of glia, where it could be activated by glutamate. These data suggest that the primary influence of excitatory amino acids on neuronal κB-binding activity is an inhibitory one, strengthening the correlation between this transcriptional parameter and neuronal survival.