Attenuation of signal flow from P2Y6 receptor by protein kinase C‐α in SK‐N‐BE(2)C human neuroblastoma cells

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Abstract
Extracellular nucleotides exert a variety of biological actions through several kinds of P2 receptors in many tissues and cell types. We found that treatment with nucleotides increases intracellular Ca2+ concentration ([Ca2+]i) in SK‐N‐BE(2)C human neuroblastoma cells with a following order of potency: UDP > UTP > ADP >> ATP. Reverse transcription‐polymerase chain reaction (RT‐PCR) analysis showed that specific mRNAs coding for human P2Y1, P2Y4, and P2Y6 receptors were expressed in the cells, but Northern blot analysis revealed that P2Y6 receptors were the predominant type. Activation of protein kinase C‐α by treatment with 1 µm phorbol 12‐myristate 13‐acetate dramatically inhibited both the UDP‐induced [Ca2+]i rise and inositol 1,4,5‐trisphosphate (IP3) generation, whereas incubation with pertussis toxin had little effect on the responses. The UDP‐induced [Ca2+]i rise and IP3 production were maintained up to 30 min after stimulation, while bradykinin‐induced responses rapidly decreased to the basal level within 5 min of stimulation. Pretreatment of cells with the maximal effective concentration of UDP reduced the subsequent carbachol‐ or bradykinin‐induced [Ca2+]i rise without inhibition of IP3 generation. Neuronal differentiation of the cells by treatment with retinoic acid for 7 days did not change the expression level of P2Y6 receptors. Taken together, the data indicate that P2Y6 receptors highly responsive to diphosphonucleotide UDP are endogenously expressed in the human neuroblastoma SK‐N‐BE(2)C cells and that they are involved in the modulation of other phospholipase C‐coupled receptor‐mediated Ca2+ mobilization by depleting the IP3‐sensitive Ca2+ stores.