Inflammatory cytokine signaling in insulin producing β-cells enhances the colocalization correlation coefficient between L-type voltage-dependent calcium channel and calcium-sensing receptor

The immunological processes in type 1 diabetes and metabolic/inflammatory disorder in type 2 diabetes converge on common signaling pathway(s) leading to β-cell death in these two diseases. The cytokine-mediated β-cell death seems to be dependent on voltage-dependent calcium channel (VDCC)-mediated Ca2+ entry. The Ca2+ handling molecular networks control the homeostasis of [Ca2+]i in the β-cell. The activity and membrane density of VDCC are regulated by several mechanisms including G protein-coupled receptors (GPCRs). CaR is a 123-kDa seven transmembrane extracellular Ca2+ sensing protein that belongs to GPCR family C. Tumor necrosis factor-α (TNF-α), is a cytokine widely known to activate nuclear factor-κB (NF-κB) transcription in β-cells. To obtain a better understanding of TNF-α-induced molecular interactions between CaR and VDCC, confocal fluorescence measurements were performed on insulin-producing β-cells exposed to varying concentrations of TNF-α and the results are discussed in the light of increased colocalization correlation coefficient. The insulin producing β-cells were exposed to 5, 10, 20, 30, and 50 ng/ml TNF-α for 24 h at 37°. The cells were then immunolabelled with antibodies directed against CaR, VDCC, and NF-κB. The confocal fluorescence imaging data showed enhancement in the colocalization correlation coefficient between CaR and VDCC in β-cells exposed to TNF-α thereby indicating increased membrane delimited spatial interactions between these two membrane proteins. TNF-α-induced colocalization of VDCC with CaR was inhibited by nimodipine, an inhibitor of L-type VDCC thereby suggesting that VDCC activity is required for spatial interactions with CaR. The 3-D confocal fluorescence imaging data also demonstrated that addition of TNF-α to RIN cells led to the translocation of NF-κB from the cytoplasm to the nucleus. Such molecular interactions between CaR and VDCC in tissues possibly provide control over Ca2+ channel activity via direct protein-protein contact.