TRPV4 Channel-Regulated ATP Release Contributes to γ-Irradiation-Induced Production of IL-6 and IL-8 in Epidermal Keratinocytes

External stimuli, such as radiation, induce inflammatory cytokine and chemokine production in skin, but the mechanisms involved are not completely understood. We previously showed that the P2Y11 nucleotide receptor, p38 mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-kappa B) all participate in interleukin (IL)-6 production induced by gamma-irradiation. Here, we focused on the transient receptor potential vanilloid 4 (TRPV4) channel, which is expressed in skin keratinocytes and has been reported to play a role in inflammation. We found that irradiation of human epidermal keratinocytes HaCaT cells with 5 Gy of gamma-rays (Cs-137: 0.75Gy/min) induced IL-6 and IL-8 production. HaCaT cells treated with TRPV4 channel agonist GSK1016790A also showed increased IL-6 and IL-8 production. In both cases, IL-6/IL-8 production was not increased at 24 h after stimulation, but was increased at 48h. ATP was released from cells exposed to gamma-irradiation or TRPV4 channel agonist, and the release was suppressed by TRPV4 channel inhibitors. The gamma-irradiation-induced increase in IL-6 and IL-8 production was suppressed by apyrase (ecto-nucleotidase), NF157 (selective P2Y11 receptor antagonist) and SB203580 (p38 MAPK inhibitor). GSK1016790A-induced inhibitor of kappa B-alpha (I kappa B alpha) decomposition, which causes NF-kappa B activation was suppressed by NF157 and SB203580, and gamma-irradiation-induced I kappa B alpha decomposition was suppressed by TRPV4 channel inhibitors. Our results suggest that gamma-irradiation of keratinocytes induces ATP release via activation of the TRPV4 channel, and then ATP activates P2Y11 receptor and p38 MAPK-NF-kappa B signaling, resulting in IL-6/IL-8 production.