Activation of nuclear factor-κB by depolarization and Ca²⁺ influx in MIN6 insulinoma cells

The purpose of the current study was to determine whether nuclear factor-κB (NF-κB) activation is a component of the depolarization/Ca²⁺-dependent signaling in β-cells. MIN6 cells were transfected with a plasmid containing five tandem repeats of NF-κB binding sites linked to a luciferase reporter. The results of these experiments showed that KCl induced depolarization-activated NF-κB-dependent transcription (3.8-fold at 45 mmol/l, P < 0.01) in a concentration-dependent manner. Tumor necrosis factor-α (TNF-α), a known inducer of NF-κB signaling, activated this construct by 3.4-fold (P < 0.01). The response of NF-κB to depolarization was inhibited by the Ca²⁺-channel blocker verapamil and by the mitogen-activated protein kinase kinase (MEK) inhibitor PD98059 (70 and 62%, respectively). TNF-α, glucose, and KCl treatment resulted in inhibitory κBα degradation by Western blot analysis. TNF-α treatment and depolarization activation of NF-κB differed significantly in that TNF-α activation was not blocked by PD98059. Transfection with PKA, MEK, and MEK kinase induced NF-κB-dependent transcription by 20-, 90-, and 300-fold, respectively, suggesting that these pathways contribute to the activation in the depolarization response. These findings demonstrate that depolarization/Ca²⁺ influx, as well as TNF-α treatment, can activate NF-κB-dependent transcription in pancreatic β-cells, but by different signaling pathways. The current studies show that Ca²⁺ signals in pancreatic β-cells can activate transcription factors involved in the regulation of cell cycle and apoptosis. These findings now add NF-κB to the list of depolarization-induced transcription factors in pancreatic β-cells.