Hypoxia-inducible factor-1 mediates adaptive developmental plasticity of hypoxia tolerance in zebrafish,Danio rerio

In recent years, natural and anthropogenic factors have increased aquatic hypoxia the world over. In most organisms, the cellular response to hypoxia is mediated by the master regulator hypoxia-inducible factor-1 (HIF-1). HIF-1 also plays a critical role in the normal development of the cardiovascular system of vertebrates. We tested the hypothesis that hypoxia exposures which resulted in HIF-1 induction during embryogenesis would be associated with enhanced hypoxia tolerance in subsequent developmental stages. We exposed zebrafish (Danio rerio) embryos to just 4 h of severe hypoxia or total anoxia at 18, 24 and 36 h post-fertilization (hpf). Of these, exposure to hypoxia at 24 and 36 hpf as well as anoxia at 36 hpf activated the HIF-1 cellular pathway. Zebrafish embryos that acutely upregulated the HIF-1 pathway had an increased hypoxia tolerance as larvae. The critical window for hypoxia sensitivity and HIF-1 signalling was 24 hpf. Adult male fish had a lower critical oxygen tension (P_crit) compared with females. Early induction of HIF-1 correlated directly with an increased proportion of males in the population. We conclude that mounting a HIF-1 response during embryogenesis is associated with long-term impacts on the phenotype of later stages which could influence both individual hypoxia tolerance and population dynamics.     

Hypoxic but not anoxic stabilization of HIF-1alpha requires mitochondrial reactive oxygen species

The molecular mechanisms by which cells detect hypoxia (1.5% O2), resulting in the stabilization of hypoxia-inducible factor 1alpha (HIF-1alpha) protein remain unclear. One model proposes that mitochondrial generation of reactive oxygen species is required to stabilize HIF-1alpha protein. Primary evidence for this model comes from the observation that cells treated with complex I inhibitors, such as rotenone, or cells that lack mitochondrial DNA (rho(0)-cells) fail to generate reactive oxygen species or stabilize HIF-1alpha protein in response to hypoxia. In the present study, we investigated the role of mitochondria in regulating HIF-1alpha protein stabilization under anoxia (0% O2). Wild-type A549 and HT1080 cells stabilized HIF-1alpha protein in response to hypoxia and anoxia. The rho(0)-A549 cells and rho(0)-HT1080 cells failed to accumulate HIF-1alpha protein in response to hypoxia. However, both rho(0)-A549 and rho(0)-HT1080 were able to stabilize HIF-1alpha protein levels in response to anoxia. Rotenone inhibited hypoxic, but not anoxic, stabilization of HIF-1alpha protein. These results indicate that a functional electron transport chain is required for hypoxic but not anoxic stabilization of HIF-1alpha protein.     

Anoxia-Reoxygenation Regulates Mitochondrial Dynamics through the Hypoxia Response Pathway,SKN-1/Nrf,and Stomatin-Like Protein STL-1/SLP-2

Many aerobic organisms encounter oxygen-deprived environments and thus must have adaptive mechanisms to survive such stress. It is important to understand how mitochondria respond to oxygen deprivation given the critical role they play in using oxygen to generate cellular energy. Here we examine mitochondrial stress response in C. elegans, which adapt to extreme oxygen deprivation (anoxia, less than 0.1% oxygen) by entering into a reversible suspended animation state of locomotory arrest. We show that neuronal mitochondria undergo DRP-1-dependent fission in response to anoxia and undergo refusion upon reoxygenation. The hypoxia response pathway, including EGL-9 and HIF-1, is not required for anoxia-induced fission, but does regulate mitochondrial reconstitution during reoxygenation. Mutants for egl-9 exhibit a rapid refusion of mitochondria and a rapid behavioral recovery from suspended animation during reoxygenation; both phenotypes require HIF-1. Mitochondria are significantly larger in egl-9 mutants after reoxygenation, a phenotype similar to stress-induced mitochondria hyperfusion (SIMH). Anoxia results in mitochondrial oxidative stress, and the oxidative response factor SKN-1/Nrf is required for both rapid mitochondrial refusion and rapid behavioral recovery during reoxygenation. In response to anoxia, SKN-1 promotes the expression of the mitochondrial resident protein Stomatin-like 1 (STL-1), which helps facilitate mitochondrial dynamics following anoxia. Our results suggest the existence of a conserved anoxic stress response involving changes in mitochondrial fission and fusion.     

Specific inhibition of hypoxia inducible factor 1 exaggerates cell injury induced by in vitro ischemia through deteriorating cellular redox environment

Hypoxia inducible factor 1 (HIF-1) has been suggested to play a critical role in the fate of cells exposed to hypoxic stress. However, the mechanism of HIF-1-regulated cell survival is still not fully understood in ischemic conditions. Redox status is critical for decisions of cell survival, death and differentiation. We investigated the effects of inhibiting HIF-1 on cellular redox status in SH-SY5Y cells exposed to hypoxia or oxygen and glucose deprivation (OGD), coupled with cell death analyses. Our results demonstrated that inhibiting HIF-1α expression by HIF-1α specific small interfering RNA (siRNA) transfection increased reactive oxygen species generation, and transformed the cells to more oxidizing environments (low GSH/GSSG ratio, low NADPH level) under either hypoxic or OGD exposure. Cell death increased dramatically in the siRNA transfected cells, compared to non-transfected cells after hypoxic/OGD exposures. In contrast, increasing HIF-1α expression by desferrioxamine, a metal chelator and hydroxylase inhibitor, induced a more reducing environment (high GSH/GSSG ratio, high NADPH level) and reduced cell death. Further studies showed that HIF-1 regulated not only glucose transporter-1 expression, but also the key enzymes of the pentose phosphate pathway such as glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. These enzymes are important in maintaining cellular redox homeostasis by generating NADPH, the primary reducing agent in cells. Moreover, catalase significantly decreased cell death in the siRNA-transfected cells induced by hypoxia and OGD. These results suggest that maintenance of cellular redox status by HIF-1 protects cells from hypoxia and ischemia mediated injuries.     

多种翻译后修饰对低氧诱导因子-1α稳定性调控的机制

低氧诱导因子-1（hypoxia-inducible factor-1, HIF-1）是组织细胞对缺氧感应和调控的一类关键转录因子,在机体中广泛表达.作为细胞低氧应答反应中的重要调节因子,HIF-1能够调节100多种涉及低氧应激下细胞适应和存活的靶基因. HIF-1是由氧依赖的α亚基和细胞内稳定表达的β亚基构成的异源二聚体.其中α亚基对氧浓度变化敏感,是HIF-1的功能性亚基,它的表达活性决定了HIF-1的生物学活性.近期研究发现,HIF-1α的一系列翻译后修饰可改变其稳定性,进而调控其转录激活活性,从而参与肿瘤、低氧性肺动脉高压以及心血管疾病等的发生与发展.本文主要就HIF-1α的一列系翻译后修饰,如羟基化、泛素化、磷酸化、乙酰化、SUMO化修饰作一综述.     

HIF-1α的可逆性SUMO化修饰

低氧诱导因子1(hypoxia inducible factor-1, HIF-1)是参与调节机体氧平衡的重要转录因子,在细胞低氧应答反应中起核心作用,能调节100多种涉及低氧应激下细胞适应和存活的靶基因．HIF-1由氧敏感的α亚基和在细胞内稳定表达的β亚基组成．其中α亚基可受到多种翻译后化学修饰作用,如在常氧下,HIF-1α通过泛素化蛋白酶修饰并导致其快速降解．最近几年发现的泛素样蛋白家族成员小泛素蛋白样修饰蛋白（SUMO）也能与HIF-1α共价结合．SUMO是一种分子量约为12 kD的小蛋白,从拟南芥到人类普遍存在．SUMO可共价结合许多靶底物蛋白,并对其进行翻译后修饰,该过程称为SUMO化．与泛素化蛋白酶体途径不同的是,SUMO化修饰能在常氧和相对低氧的条件下调节HIF-1α蛋白的稳定性,从而改变其转录活性．SUMO化是一个可逆的动态过程,可被特异性蛋白酶ULP/SENP将其从底物上去除．本文主要就HIF-1α的可逆性SUMO化修饰作一综述．