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

在这项研究中，我们首次发现在果蝇中表达具有结构性活性的果蝇ASK1(DASK1)通过诱导酪氨酸羟化酶(TH)以p38途径依赖的方式上调黑色素的生物合成。我们还发现，p38诱导的NR4A核因子的磷酸化在DASK1对TH表达的调控中起着关键作用。此外，p38诱导的NR4A核因子的磷酸化也在哺乳动物细胞中发挥作用。综上所述，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NR4A因子似乎通过ASK1-p38途径在先天免疫反应中发挥重要作用。另一方面，我们发现与ASK1高度同源的ASK2是ASK1的结合蛋白。通过与ASK1形成异构体复合体，发现ASK2通过蛋白酶体逃脱蛋白水解酶，从而稳定。ASK2作为MAP3K发挥作用，只有在与ASK1的复合体中才有可能被氧化应激激活。ASK2也被发现通过直接磷酸化诱导ASK1激活，这表明ASK1和ASK2通过不同的机制促进各自的活性。由于ASK2优先在皮肤和肠道等更替较高的组织中表达，因此ASK2可能作为ASK1-p38途径的调节因子在黏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.