Syntaxin 4 Mediates NF-kB Signaling and Chemokine Ligand Expression via Specific Interaction With IkBb

Abstract

Enrichment of human islets with syntaxin 4 (STX4) improves functional b-cell mass through a nuclear factorkB (NF-kB)–dependent mechanism. However, the detailed mechanisms underlying the protective effect of STX4 are unknown. For determination of the signaling events linking STX4 enrichment and downregulation of NF-kB activity, STX4 was overexpressed in human islets, EndoC-bH1 and INS-1 832/13 cells in culture, and the cells were challenged with the proinflammatory cytokines interleukin-1b, tumor necrosis factor-a, and interferon-g individually and in combination. STX4 expression suppressed cytokineinduced proteasomal degradation of IkBb but not IkBa. Inhibition of IKKb prevented IkBb degradation, suggesting that IKKb phosphorylates IkBb. Moreover, the IKKb inhibitor, as well as a proteosomal degradation inhibitor, prevented the loss of STX4 caused by cytokines. This suggests that STX4 may be phosphorylated by IKKb in response to cytokines, targeting STX4 for proteosomal degradation. Expression of a stabilized form of STX4 further protected IkBb from proteasomal degradation, and like wild-type STX4, stabilized STX4 coimmunoprecipitated with IkBb and the p50-NF-kB. This work proposes a novel pathway wherein STX4 regulates cytokine-induced NF-kB signaling in b-cells via associating with and preventing IkBb degradation, suppressing chemokine expression, and protecting islet b-cells from cytokine-mediated dysfunction and demise.