Analysis of the innate immune system through the ASK1-p38 MAP kinase pathway in mucosal immunity

在这项研究中，我们首次发现，果蝇ASK 1（DASK 1）的组成型活性表达上调黑色素的生物合成，通过诱导酪氨酸羟化酶（TH）在p38通路依赖的方式。我们还发现，p38诱导的NR 4A核因子的磷酸化在DASK 1对TH表达的调节中起着关键作用。此外，p38诱导的NR 4A核因子磷酸化也在哺乳动物细胞中起作用。结合黑色素生物合成在果蝇先天免疫中的重要性，通过磷酸化调节NR 4A因子似乎通过ASK 1-p38途径在先天免疫应答中发挥重要作用。另一方面，我们鉴定了与ASK 1高度同源的ASK 2作为ASK 1的结合蛋白。通过与ASK 1形成异聚体复合物，发现ASK 2通过蛋白酶体逃避蛋白水解，因此稳定。ASK 2作为MAP 3 K发挥作用，仅在与ASK 1的复合物中可能被氧化应激激活。还发现ASK 2通过直接磷酸化诱导ASK 1活化，这表明存在一种机制，ASK 1和ASK 2通过不同的机制相互促进其活性。由于ASK 2的表达优先在组织中检测到，包括具有相对高周转的上皮，如皮肤和肠道，ASK 2可能作为ASK 1-p38通路的调节剂在粘膜免疫中发挥关键作用。

In this research, we first found that expression of constitutively active Drosophila ASK1 (DASK1) in flies upregulated melanin biosynthesis through the induction of tyrosine hydroxylase (TH) in a p38 pathway-dependent manner. We also found that p38-induced phosphorylation of NR4A nuclear factors was critically involved in the regulation of TH expression by DASK1. Moreover, p38-induced phosphorylation of NR4A nuclear factors operated also in mammalian cells. Taken together with the evidence showing the importance of melanin biosynthesis in Drosophila innate immunity, the regulation of NR4A factors by phosphorylation appears to play important roles in innate immune response through the ASK1-p38 pathway. On the other hand, we identified ASK2, which was highly homologous to ASK1, as a binding protein of ASK1. By forming a heteromeric complex with ASK1, ASK2 was found to escape from the proteolysis through proteasome and therefore stabilize. ASK2 functioned as a MAP3K that was potentially activated by oxidative stress only in the complex with ASK1. ASK2 was also found to induce ASK1 activation by direct phosphorylation, suggesting the existence of a mechanism by which ASK1 and ASK2 facilitate their activities to each other by distinct mechanisms. Since ASK2 expression was detected preferentially in the tissues including epithelium with relatively high turnover such as skin and gut, ASK2 may play pivotal roles in mucosal immunity as a regulator of the ASK1-p38 pathway.