The hydrogen peroxide (H₂O₂)-producing NADPH oxidase Nox4, forming a heterodimer with p22^phox, is expressed in a variety of cells including those in the heart to mediate adaptive responses to cellular stresses such as hypoxia. Since Nox4 is constitutively active, H₂O₂ production is controlled by its protein abundance. Hypoxia-induced Nox4 expression is observed in various types of cells and generally thought to be regulated at the transcriptional level. Here we show that hypoxia upregulates the Nox4 protein level and Nox4-catalyzed H₂O₂ production without increasing the Nox4 mRNA in rat H9c2 cardiomyocytes. In these cells, the Nox4 protein is stabilized under hypoxic conditions in a manner dependent on the presence of p22^phox. Cell treatment with the proteasome inhibitor MG132 results in a marked decrease of the Nox4 protein under both normoxic and hypoxic conditions, indicating that the proteasome pathway does not play a major role in Nox4 degradation. The decrease is partially restored by the autophagy inhibitor 3-methyladenine. Furthermore, the Nox4 protein level is upregulated by the lysosome inhibitors bafilomycin A1 and chloroquine. Thus, in cardiomyocytes, Nox4 appears to be degraded via an autophagy-related pathway, and its suppression by hypoxia likely stabilizes Nox4, leading to upregulation of Nox4-catalyzed H₂O₂ production.

中文翻译：

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缺氧通过抑制自噬相关的溶酶体降解来稳定心肌细胞中产生 H2O2 的氧化酶 Nox4

产生过氧化氢 (H ₂ O _{2 ) 的 NADPH 氧化酶 Nox4 与 p22} ^phox形成异二聚体，在包括心脏在内的多种细胞中表达，介导对缺氧等细胞应激的适应性反应。由于Nox4 具有组成型活性，因此H ₂ O ₂的产生受其蛋白质丰度控制。在各种类型的细胞中都观察到缺氧诱导的 Nox4 表达，并且通常认为其在转录水平上受到调节。在这里，我们发现缺氧会上调Nox4蛋白水平和Nox4催化的H ₂ O ₂产生，而不增加大鼠H9c2心肌细胞中的Nox4 mRNA。^{在这些细胞中，Nox4 蛋白在缺氧条件下以依赖于 p22 phox}存在的方式稳定。在常氧和低氧条件下，用蛋白酶体抑制剂 MG132 处理细胞会导致 Nox4 蛋白显着减少，表明蛋白酶体途径在 Nox4 降解中并不起主要作用。自噬抑制剂 3-甲基腺嘌呤可以部分恢复这种减少。此外，溶酶体抑制剂巴弗洛霉素 A1 和氯喹会上调 Nox4 蛋白水平。因此，在心肌细胞中，Nox4 似乎通过自噬相关途径降解，缺氧对其的抑制可能稳定 Nox4，导致 Nox4 催化的 H ₂ O ₂产生上调。