The Drosophila MAPK p38c Regulates Oxidative Stress and Lipid Homeostasis in the Intestine

The p38 mitogen-activated protein (MAP) kinase signaling cassette has been implicated in stress and immunity in evolutionarily diverse species. In response to a wide variety of physical, chemical and biological stresses p38 kinases phosphorylate various substrates, transcription factors of the ATF family and other protein kinases, regulating cellular adaptation to stress. The Drosophila genome encodes three p38 kinases named p38a, p38b and p38c. In this study, we have analyzed the role of p38c in the Drosophila intestine. The p38c gene is expressed in the midgut and upregulated upon intestinal infection. We showed that p38c mutant flies are more resistant to infection with the lethal pathogen Pseudomonas entomophila but are more susceptible to the non-pathogenic bacterium Erwinia carotovora 15. This phenotype was linked to a lower production of Reactive Oxygen Species (ROS) in the gut of p38c mutants, whereby the transcription of the ROS-producing enzyme Duox is reduced in p38c mutant flies. Our genetic analysis shows that p38c functions in a pathway with Mekk1 and Mkk3 to induce the phosphorylation of Atf-2, a transcription factor that controls Duox expression. Interestingly, p38c deficient flies accumulate lipids in the intestine while expressing higher levels of antimicrobial peptide and metabolic genes. The role of p38c in lipid metabolism is mediated by the Atf3 transcription factor. This observation suggests that p38c and Atf3 function in a common pathway in the intestine to regulate lipid metabolism and immune homeostasis. Collectively, our study demonstrates that p38c plays a central role in the intestine of Drosophila. It also reveals that many roles initially attributed to p38a are in fact mediated by p38c.

p38丝裂原活化蛋白（mitogen-activated protein, MAP）激酶信号传导级联组件在进化上多样的物种中均参与应激与免疫调控过程。当受到多种物理、化学及生物应激刺激时，p38激酶可磷酸化包括ATF家族转录因子与其他蛋白激酶在内的多种底物，从而调控细胞对应激的适应能力。果蝇基因组编码三种p38激酶，分别命名为p38a、p38b与p38c。本研究针对p38c在果蝇肠道中的功能展开系统分析。p38c基因在中肠中表达，且在肠道感染时表达会上调。研究结果显示，p38c突变果蝇对致死性病原体嗜虫假单胞菌（Pseudomonas entomophila）的感染具有更强抗性，但对非致病菌胡萝卜软腐欧文氏菌15（Erwinia carotovora 15）更为易感。该表型与p38c突变体肠道内活性氧（Reactive Oxygen Species, ROS）生成量降低密切相关，具体表现为p38c突变果蝇中产ROS酶Duox的转录水平显著下调。我们的遗传分析表明，p38c与Mekk1、Mkk3共同参与一条信号通路，可诱导调控Duox表达的转录因子Atf-2发生磷酸化。有趣的是，p38c缺陷型果蝇的肠道内会出现脂质蓄积现象，同时其抗菌肽与代谢相关基因的表达水平显著升高。p38c在脂质代谢中的功能由转录因子Atf3介导调控。该结果提示，p38c与Atf3在果蝇肠道内通过共同的信号通路协同调控脂质代谢与免疫稳态。综上，本研究证实p38c在果蝇肠道中发挥核心调控功能，同时揭示了最初被认为由p38a承担的诸多生物学功能，实际上均由p38c介导。