Biology of HIF-1α
HIF-1α生物学

Investigation of the molecular mechanisms of one of the most striking responses to hypoxia – the induction of the hematopoietic growth hormone erythropoietin (EPO) – paved the way for the first identification of a hypoxia-inducible transcription factor. When the blood oxygen content is reduced in anemia or at high altitude, the EPO production in renal interstitial fibroblasts is rapidly turned on. The up to several 100-fold induction of EPO mRNA and protein induces erythropoietic responses that directly increase blood oxygen transport. Studies of DNA–protein interactions at the 3′ enhancer of the EPO gene identified a protein complex that only bound in hypoxia. It was designated hypoxia-inducible factor 1 (HIF-1) by Semenza and Wang.¹
对缺氧最引人注目的反应之一的分子机制的研究 - 造血生长激素促红细胞生成素 （EPO） 的诱导 - 为首次鉴定缺氧诱导转录因子铺平了道路。当贫血或高海拔地区血氧含量降低时，肾间质成纤维细胞中的 EPO 产生迅速启动。EPO mRNA 和蛋白质的高达 100 倍的诱导诱导红细胞生成反应，直接增加血氧运输。对 EPO 基因 3' 增强子处的 DNA-蛋白质相互作用的研究确定了一种仅在缺氧中结合的蛋白质复合物。它被 Semenza 和 Wang 命名为缺氧诱导因子 1 （HIF-1）。¹

It soon became obvious that the HIF system is a key regulator of a broad range of cellular and systemic responses to hypoxia and acts in all mammalian cells. Changes in gene expression directly or indirectly regulated by HIF extend to well over a 100 genes. HIF-mediated pathways influence metabolic adaptation, erythropoiesis, angiogenesis and vascular tone, cell growth and differentiation, survival and apoptosis, and thus are critical factors in development, physiology and disease (for review, see Maxwell et al.²).
很快就发现，HIF 系统是对缺氧的广泛细胞和全身反应的关键调节因子，并作用于所有哺乳动物细胞。受 HIF 直接或间接调节的基因表达变化延伸到 100 多个基因。HIF 介导的通路影响代谢适应、红细胞生成、血管生成和血管张力、细胞生长和分化、存活和细胞凋亡，因此是发育、生理学和疾病的关键因素（综述见 Maxwell 等人。²）.

HIF is a heterodimeric DNA-binding complex composed of two basic helix-loop-helix proteins of the PAS family (PER, AHR, ARNT and SIM family):³ the constitutive HIF-1β and one of either hypoxia-inducible α-subunits, HIF-1α or HIF-2α. In hypoxia, the α/β heterodimer binds to a core pentanucleotide sequence (RCGTG) in the hypoxia response elements (HREs) of target genes. HIF-β subunits are non-oxygen-responsive nuclear proteins that also have other roles in transcription, for example, in the xenobiotic response (for review, see Gu et al.⁴). In contrast, the HIF-α subunits are highly inducible by hypoxia.
HIF 是一种异二聚体 DNA 结合复合物，由 PAS 家族（PER、AHR、ARNT 和 SIM 家族）的两个碱性螺旋-环-螺旋蛋白组成：³组成型 HIF-1β和缺氧诱导型 α 亚基之一，HIF-1α 或 HIF-2α。在缺氧时，α/β 异二聚体与靶基因缺氧反应元件 （HRE） 中的核心五核苷酸序列 （RCGTG） 结合。HIF-β 亚基是非氧反应性核蛋白，在转录中也具有其他作用，例如，在外源性反应中（综述，参见 Gu 等人。^相比之下，HIF-α 亚基高度易受缺氧诱导。

Under normoxic conditions, HIF-α subunits have a very short half-life.⁵ Cells continuously synthesize and degrade HIF-α protein. However, under decreasing concentrations of oxygen, the degradation of HIF-α is retarded.⁶ The interface between oxygen and the HIF-α subunit is provided by distinct enzymatic reactions: the hydroxylation of two prolyl residues (Pro402 and Pro564 in human HIF-1α) in the oxygen-dependent degradation domain (ODDD) of the α-subunits.^{7, 8} This oxygen-dependent hydroxylation regulates the interaction with the von Hippel–Lindau tumor suppressor protein (pVHL). pVHL is the recognition component of an E3 ubiquitin ligase complex that targets HIF-α for proteolysis by the ubiquitin–proteasome pathway.^{9, 10} Other components of the complex are elongin B, elongin C, Rbx1 and Cul2, which also participate in other E3 ubiquitin ligase complexes. Under hypoxic conditions, prolyl hydroxylation is suppressed, HIF-α protein escapes proteasomal destruction and can accumulate. It translocates to the nucleus and dimerizes with HIF-1β. The heterodimeric transactivating complex HIF then binds to the HRE in promoter or enhancer sequences of target genes (Figure 1).
在常氧条件下，HIF-α 亚基的半衰期非常短。⁵ 细胞持续合成和降解 HIF-α 蛋白。然而，在氧气浓度降低的情况下，HIF-α 的降解被延缓。⁶ 氧和 HIF-α 亚基之间的界面由不同的酶促反应提供：α 亚基的氧依赖性降解结构域 （ODDD） 中两个脯氨酰残基（人 HIF-1α中的 Pro402 和 Pro564）的羟基化。^7、8这种氧依赖性羟基化调节与 von Hippel-Lindau 抑癌基因蛋白 （pVHL） 的相互作用。pVHL 是 E3 泛素连接酶复合物的识别组分，该复合物靶向 HIF-α，通过泛素-蛋白酶体途径进行蛋白水解。^9、10该复合物的其他组分是 elongin B、elongin C、Rbx1 和 Cul2，它们也参与其他 E3 泛素连接酶复合物。在缺氧条件下，脯氨酰羟基化受到抑制，HIF-α 蛋白逃避蛋白酶体破坏并可以积累。它易位到细胞核并与 HIF-1β二聚化。然后，异二聚体反式激活复合物 HIF 与靶基因启动子或增强子序列中的 HRE 结合（图 1）。

Regulation of HIF-1α protein by prolyl hydroxylation and proteasomal degradation. There are three hydroxylation sites in the HIF-1α subunit: two prolyl residues in the oxygen-dependent degradation domain (ODDD) and one asparaginyl residue in the C-terminal transactivation domain (C-TAD). In the presence of oxygen, prolyl hydroxylation is catalyzed by the Fe(II)-, oxygen- and 2-oxoglutarate-dependent PHDs. The hydroxylated prolyl residues allow capture of HIF-1α by the von Hippel–Lindau protein (pVHL), leading to ubiquitination and subsequent proteasomal degradation. Asparaginyl hydroxylation is catalyzed by an enzyme termed as factor-inhibiting HIF (FIH) at a single site in the C-TAD. This hydroxylation prevents cofactor recruitment. In the absence of hydroxylation due to hypoxia or PHD inhibition, HIF-1α translocates to the nucleus, heterodimerizes with HIF-1β and binds to hypoxia-response elements (HREs) in the regulatory regions of target genes
通过脯氨酰羟基化和蛋白酶体降解对 HIF-1α蛋白的调节。HIF-1α亚基中有三个羟基化位点：氧依赖性降解结构域 （ODDD） 中的两个脯氨酰残基和稷基末端反式激活结构域 （C-TAD） 中的一个天冬酰胺残基。在氧气存在下，脯氨酰羟基化由 Fe（II）、氧和 2-氧代戊二酸依赖性 PHD 催化。羟基化的脯氨酰残基允许 von Hippel-Lindau 蛋白 （pVHL） 捕获 HIF-1α，导致泛素化和随后的蛋白酶体降解。天冬酰胺酰羟基化在 C-TAD 中的单个位点由一种称为因子抑制 HIF （FIH） 的酶催化。这种羟基化阻止了辅因子的募集。在缺氧或 PHD 抑制导致没有羟基化的情况下，HIF-1α易位到细胞核，与 HIF-1β异二聚化，并与靶基因调控区域中的缺氧反应元件 （HRE） 结合

Full size image

The HIF-modifying enzymes were identified as related to egl-9 in Caenorhabditis elegans and termed prolyl hydroxylase domain (PHD) enzymes (PHD1–PHD3).^{11, 12} The PHDs are non-heme Fe(II)- and 2-oxoglutarate-dependent dioxygenases that split molecular oxygen. One oxygen atom is inserted into the HIF-α peptide at the prolyl residue, the other reacts with 2-oxoglutarate yielding succinate and CO₂ as products. In hypoxia, the HIF prolines remain unmodified. The effects of hypoxia can be mimicked by iron chelation, use of 2-oxoglutarate analogs such as dimethyloxalylglycine or substitution of Fe(II) by metal ions such as cobalt.
HIF 修饰酶被鉴定为与秀丽隐杆线虫中的 egl-9 相关，称为 prolyl hydroxylase domain （PHD） 酶 （PHD1-PHD3）。^11、12PHD 是非血红素 Fe（II） 和 2-氧代戊二酸依赖性双加氧酶，可分裂分子氧。一个氧原子在脯氨酰残基处插入 HIF-α 肽中，另一个氧原子与 2-氧代戊二酸反应，产生琥珀酸盐和 CO₂ 作为产物。在缺氧时，HIF 脯氨酸保持不变。缺氧的影响可以通过铁螯合、使用 2-氧代戊二酸类似物（如二甲基草酰甘氨酸）或用金属离子（如钴）取代 Fe（II） 来模拟。

The ODDD of the HIF-α subunit contains an N-terminal (N-ODDD) and a C-terminal portion (C-ODDD),^{13, 14} both of which can interact with pVHL independently.¹⁵ Furthermore, two transactivation domains (TADs) have been defined in HIF-1α and HIF-2α: an N-terminal TAD (which overlaps with the C-ODDD) and a C-terminal TAD.¹⁶ In contrast to regulation of HIF-α stability by proline modification in the ODDD, transcriptional activity is regulated by the hydroxylation of a C-terminal asparagine residue (Asn 803 in human HIF-1α).¹⁷ The hydroxylation reaction is carried out by an asparaginyl hydroxylase termed factor-inhibiting HIF (FIH) and this modification prevents interaction of the HIF-α C-TAD with the CH-1 domain of the transcriptional coactivator p300.^{18, 19} Thus, in contrast to the prolyl hydroxylation that enables protein–protein interaction, the asparaginyl hydroxylation prevents protein recruitment. FIH can also interact with pVHL,²⁰ which fits well with the observation that an almost complete upregulation of HIF target genes can be found in cells devoid of functional pVHL.^{9, 21} This suggests that in the absence of pVHL both the oxygen-dependent controls of stability and activity of HIF-α are diminished and thus the functions of VHL might not be limited to those of an E3 ligase.
HIF-α 亚基的 ODDD 包含一个 N 端 （N-ODDD） 和一个 C 端部分 （C-ODDD），^{13， 14}，两者都可以独立地与 pVHL 相互作用。¹⁵ 此外，HIF-1α 和 HIF-2α中定义了两个反式激活结构域 （TAD）：N 端 TAD（与 C-ODDD 重叠）和 C 端 TAD。¹⁶ 与 ODDD 中脯氨酸修饰对 HIF-α 稳定性的调节相反，转录活性受 C 末端天冬酰胺残基（人 HIF-1α中的 Asn 803）的羟基化调节。¹⁷ 羟基化反应由称为 f actor-i的天冬酰胺羟化酶抑制 HIF （FIH） 进行，这种修饰阻止了 HIF-α C-TAD 与转录共激活因子 p300 的 CH-1 结构域的相互作用。^18、19因此，与实现蛋白质-蛋白质相互作用的脯氨酰羟基化相反，天冬酰胺羟基化阻止了蛋白质募集。FIH 还可以与 pVHL 相互作用，²⁰ 这与在缺乏功能性 pVHL 的细胞中发现 HIF 靶基因几乎完全上调的观察结果非常吻合。^9、21这表明在没有 pVHL 的情况下，HIF-α 的稳定性和活性的氧依赖性控制都会减弱，因此 VHL 的功能可能不仅限于 E3 连接酶的功能。

In keeping with the complexity of the hypoxic response, three principal isoforms of HIF-α exist (HIF-1α, HIF-2α and HIF-3α). All are encoded by distinct gene loci and further diversity is generated by alternative promoter usage and splicing patterns. HIF-1α and HIF-2α share a similar domain architecture and undergo similar proteolytic regulation; however, the tissue expression of HIF-2α seems to be more limited.²² There is mounting evidence, both in vivo from animals with targeted disruption of HIF2-α and also in vitro, that both isoforms have non-redundant functions in the regulation of gene expression. The complexity, however, is illustrated by the fact that a functional overlap between the two exists, which varies from one cell type to another. The molecular mechanisms for the target gene specificity are a matter of intense investigation and are now beginning to be understood in more detail.^{23, 24}
为了与缺氧反应的复杂性保持一致，存在 HIF-α 的三种主要亚型 （HIF-1α、HIF-2α 和 HIF-3α）。所有基因均由不同的基因位点编码，并且通过替代启动子使用和剪接模式产生进一步的多样性。HIF-1α 和 HIF-2α具有相似的结构域结构，并经历相似的蛋白水解调节;然而，HIF-2α的组织表达似乎更加有限。²² 越来越多的证据表明，无论是靶向破坏 HIF2-α 的动物体内还是体外，这两种亚型在基因表达的调节中都具有非冗余功能。然而，两者之间存在功能重叠的事实说明了复杂性，这种重叠因细胞类型而异。靶基因特异性的分子机制是一个深入研究的问题，现在开始更详细地了解。^23、24 元

HIF-3α is less closely related and its role is not yet fully understood. Interestingly, alternative splicing of HIF-3α generates an inhibitory PAS domain protein that is composed of the N-terminal basic helix-loop-helix and PAS domains but lacks TAD.²⁵ It inhibits HIF response by forming transcriptionally inactive heterodimers with HIF-1α.
HIF-3α 的相关性较低，其作用尚不完全清楚。有趣的是，HIF-3α的选择性剪接会产生一种抑制性 PAS 结构域蛋白，该蛋白由 N 末端碱性螺旋-环-螺旋和 PAS 结构域组成，但缺乏 TAD。²⁵ 它通过与 HIF-1α形成转录失活的异二聚体来抑制 HIF 反应。

After the finding that the HIF system was widely operating in most cells and was not restricted to EPO regulation, critical HIF-1α-binding sites were identified in other genes encoding the glycolytic enzymes phosphoglycerate kinase-1 and lactate dehydrogenase A.²⁶ Successive studies identified more enzymes involved in this metabolic pathway that are upregulated by hypoxia, as are glucose transporters and also enzymes of the gluconeogenesis. The metabolism of glucose to CO₂ and water is oxygen dependent and very energy efficient. Glucose is transformed into pyruvate in the cytoplasm and, secondarily, pyruvate is catabolized through the tricarboxylic acid (TCA) cycle and oxidative phosphorylation (OXPHOS) in the mitochondria. In contrast to OXPHOS, O₂-deprived cells utilize the less energy-efficient metabolism of pyruvate to lactic acid, an effect described by Louis Pasteur in 1861. Interestingly, most cancer cells rely on this source of energy also in normoxia, as described by Otto Warburg et al.²⁷ more than 80 years ago. Multiple enzymes responsible for shifting the metabolism toward anaerobic glycolysis are directly controlled by HIF-1α. Since HIF-1α overexpression is a frequent feature of many tumors, the contribution of HIF-1α to this characteristic metabolic phenotype of tumors seems very likely.
在发现 HIF 系统在大多数细胞中广泛运作并且不限于 EPO 调节后，在编码糖酵解酶磷酸甘油酸激酶-1 和乳酸脱氢酶 ^A.26 的其他基因中确定了关键的 HIF-1 α结合位点连续的研究确定了更多参与该代谢途径的酶，这些酶因缺氧而上调， 葡萄糖转运蛋白和糖异生酶也是如此。葡萄糖向 CO₂ 和水的代谢依赖于氧气，并且非常节能。葡萄糖在细胞质中转化为丙酮酸，其次，丙酮酸通过线粒体中的三羧酸 （TCA） 循环和氧化磷酸化 （OXPHOS） 分解代谢。与 OXPHOS 相比，O₂ 剥夺细胞利用丙酮酸向乳酸的能量效率较低的代谢，路易斯·巴斯德 （Louis Pasteur） 在 1861 年描述了这种效应。有趣的是，正如 Otto Warburg 等人所描述的那样，大多数癌细胞在常氧状态下也依赖于这种能量来源。²⁷ 多年前 80 个。负责将代谢转向厌氧糖酵解的多种酶受 HIF-1α直接控制。由于 HIF-1α过表达是许多肿瘤的常见特征，因此 HIF-1α对肿瘤的这种特征性代谢表型的贡献似乎非常有可能。

In addition, it was discovered recently that HIF-1α also has a significant influence on mitochondrial respiration. HIF-1α induces the expression of pyruvate dehydrogenase kinase (PDK), which inhibits the enzyme pyruvate dehydrogenase by phosphorylation. Thus, pyruvate is not converted into acetyl-CoA, preventing pyruvate entry into the TCA cycle.^{28, 29} As a consequence, the mitochondrial oxygen-consumption is downregulated and hypoxic ROS generation is attenuated. HIF-1 also fine-tunes mitochondrial respiration by changing the composition of the cytochrome oxidase complex in hypoxia: a more efficient isoform is upregulated by HIF-1, and the isoform predominant in normoxia is degraded by the HIF-1α-regulated LON protease.³⁰ Finally, in VHL-defective renal carcinoma cell lines, mitochondrial biogenesis and metabolism are actively repressed in a HIF-1-dependent manner.³⁰ Thus, HIF-1α modulates key metabolic pathways to optimize glucose and O₂ utilization in hypoxia to generate sufficient amounts of ATP without producing excessive amounts of ROS by inhibition of the TCA cycle and mitochondrial respiration (Figure 2).
此外，最近发现 HIF-1α对线粒体呼吸也有显着影响。HIF-1α诱导丙酮酸脱氢酶激酶 （PDK） 的表达，PDK 通过磷酸化抑制丙酮酸脱氢酶。因此，丙酮酸不会转化为乙酰辅酶 A，从而阻止丙酮酸进入 TCA 循环。^28、29 元结果，线粒体耗氧量下调，缺氧 ROS 产生减弱。HIF-1 还通过改变缺氧时细胞色素氧化酶复合物的组成来微调线粒体呼吸：更有效的亚型被 HIF-1 上调，而常氧中占主导地位的亚型被 HIF-1α调节的 LON 蛋白酶降解。³⁰ 最后，在 VHL 缺陷的肾癌细胞系中，线粒体生物发生和代谢以 HIF-1 依赖性方式受到积极抑制。³⁰ 因此，HIF-1α调节关键代谢途径，以优化缺氧时葡萄糖和 O₂ 的利用，从而产生足够量的 ATP，而不会通过抑制 TCA 循环和线粒体呼吸产生过量的 ROS（图 2）。

HIF-1α controls metabolic and pH-regulating pathways. Cells respond to hypoxia by HIF-1α-mediated upregulation of glucose transporters (Glut-1 and Glut-3) and enzymes of glycolysis. Conversion of pyruvate to lactic acid is facilitated by the induction of lactate dehydrogenase (LDH). HIF-1α also induces pyruvate dehydrogenase kinase-1 (PDK-1), which inhibits the conversion of pyruvate into acetyl-CoA by pyruvate dehydrogenase (PDH), thus preventing entry of pyruvate into the TCA cycle. Subunit composition of cytochrome c oxidase (COX4) is influenced by HIF-1α in hypoxia: COX4-2 is induced and COX4-1 is reciprocally reduced by induction of the protease LON that degrades COX4-1. Switching the COX subunits ensures optimal efficiency of mitochondrial respiration in hypoxia. Furthermore, pH homeostasis is maintained by induction of carbonic anhydrase IX (CAIX) and the monocarboxylate transporter MCT 4 and the Na⁺/H⁺ exchanger NHE1
HIF-1α控制代谢和 pH 调节途径。细胞通过 HIF-1α介导的葡萄糖转运蛋白（Glut-1 和 Glut-3）和糖酵解酶的上调来响应缺氧。丙酮酸转化为乳酸是通过诱导乳酸脱氢酶 （LDH） 而促进的。HIF-1α还诱导丙酮酸脱氢酶激酶-1 （PDK-1），该激酶抑制丙酮酸脱氢酶 （PDH） 将丙酮酸转化为乙酰辅酶 A，从而阻止丙酮酸进入 TCA 循环。细胞色素 c 氧化酶 （COX4） 的亚基组成受缺氧时 HIF-1α的影响：COX4-2 被诱导，COX4-1 通过诱导降解 COX4-1 的蛋白酶 LON 而被相互还原。切换 COX 亚基可确保缺氧时线粒体呼吸的最佳效率。此外，通过诱导碳酸酐酶 IX （CAIX） 和单羧酸转运蛋白 MCT 4 和 Na ⁺ / H ⁺ 交换蛋白 NHE1 来维持 pH 稳态

Full size image

Apart from controlling key glycolytic enzymes, HIF-1α is also implicated in the regulation of intracellular pH. One consequence of cytosolic glucose metabolism is an increase in intracellular lactic acid concentration. For tumor cells to survive and proliferate, it is important to extrude these acids. HIF-1α has been demonstrated to regulate at least one member of the H⁺/lactate co-transporter family that excretes lactic acid from the cytoplasm, the monocarboxylate transporter (MCT 4 ³¹). In addition, H⁺ ions are transported out through the HIF-1α-regulated Na⁺/H⁺ exchanger NHE1.³² Furthermore, the carbonic anhydrases IX and XII are among the most highly induced HIF-1α targets: bound to the cell membrane they convert the metabolically generated CO₂ into carbonic acid. The base HCO₃⁻ re-enters the cell and contributes further to intracellular alkalinization. As a consequence, the extracellular tumor microenvironment is acidotic, which has been described for many tumors and is correlated with poor prognosis in cancer patients, even though the intracellular pH in these cells is maintained at a level that allows survival and proliferation.
除了控制关键的糖酵解酶外，HIF-1α还与细胞内 pH 值的调节有关。胞质葡萄糖代谢的一个后果是细胞内乳酸浓度增加。为了使肿瘤细胞存活和增殖，挤出这些酸很重要。HIF-1α已被证明可调节从细胞质中排泄乳酸的 H⁺/乳酸协同转运蛋白家族的至少一个成员，即单羧酸转运蛋白 （MCT 4 ³¹）。此外，H⁺ 离子通过 HIF-1α调节的 Na⁺/H⁺ 交换剂 NHE1 转运出去。³² 此外，碳酸酐酶 IX 和 XII 是诱导性最强的 HIF-1α靶标之一：它们与细胞膜结合，将代谢产生的 CO₂ 转化为碳酸。碱基 HCO₃⁻ 重新进入细胞并进一步促进细胞内碱化。因此，细胞外肿瘤微环境是酸性的，这已被描述为许多肿瘤，并且与癌症患者的不良预后相关，即使这些细胞中的细胞内 pH 值维持在允许存活和增殖的水平。

Taken together, the molecular mechanisms underlying the adaptation of cellular metabolism to hypoxia impact the metabolic phenotype of cancer cells, acting on the triad of increased glucose uptake, increased lactate production and decreased mitochondrial respiration.
综上所述，细胞代谢适应缺氧的分子机制会影响癌细胞的代谢表型，作用于葡萄糖摄取增加、乳酸生成增加和线粒体呼吸减少的三联征。

Beyond a certain size, simple diffusion of oxygen becomes inadequate to meet metabolic demands, especially for rapidly proliferating cells in embryos and in growing tumors. The three major processes involved in the formation of new blood vessels are referred to as vasculogenesis, angiogenesis and arteriogenesis. Early studies postulated that as the cell mass expands, angiogenic factors would be released,³³ but the triggers for the so-called angiogenic switch, a phenomenon in which a tumor progresses from a non-angiogenic to an angiogenic phenotype, remained obscure. The hypoxic microenvironment caused by the increased oxygen consumption of hyperplasia and/or hypertrophy and the decreased oxygen delivery due to the increase in diffusion distance was assumed to contribute to the angiogenic switch. An important link between hypoxia and angiogenesis was the discovery that the expression of the potent vascular endothelial growth factor (VEGF) was induced by hypoxia.³⁴
超过一定大小，氧的简单扩散就不足以满足代谢需求，特别是对于胚胎和生长中的肿瘤中快速增殖的细胞。新血管形成所涉及的三个主要过程称为血管生成、血管生成和动脉生成。早期研究假设，随着细胞质量的扩大，血管生成因子将被释放，³³ 但所谓的血管生成转换（一种肿瘤从非血管生成表型发展为血管生成表型的现象）的触发因素仍然不清楚。由增生和/或肥大的耗氧量增加引起的缺氧微环境以及由于弥散距离增加而导致的氧气输送减少被认为有助于血管生成转换。缺氧和血管生成之间的一个重要联系是发现强效血管内皮生长因子 （VEGF） 的表达是由缺氧诱导的。³⁴

Angiogenesis is essential for development, wound healing, tissue or organ regeneration, but it is also part of pathological processes, such as cancer and certain retinopathies. It is an intricate multistep and temporally ordered process that involves a great number of genes, modifiers and pathways. Many of these genes are directly induced by HIF-1α, such as nitric oxide synthases, angiogenic and vascular growth factors (VEGF) and genes regulating matrix metabolism (urokinase-type plasminogen activator receptor; uPAR). Others are independently regulated by hypoxia and might be influenced by secondary mechanisms, but a central role of HIF-1α is well established: it is required for proper vascularization of the mouse embryo³⁵ and for coordinating the complex cooperation of angiogenic growth factors. This was demonstrated in transgenic animals where the overexpression of VEGF alone led to hypervascularity with hyperpermeability in the skin,³⁶ whereas in contrast the vessels induced by a stable HIF-1α transgene driven by the same skin-specific promoter were not leaky.³⁷
血管生成对于发育、伤口愈合、组织或器官再生至关重要，但它也是病理过程的一部分，例如癌症和某些视网膜病变。这是一个错综复杂的多步骤和时间有序的过程，涉及大量的基因、修饰和途径。其中许多基因是由 HIF-1α直接诱导的，例如一氧化氮合酶、血管生成和血管生长因子 （VEGF） 以及调节基质代谢的基因（尿激酶型纤溶酶原激活剂受体;uPAR）。其他受缺氧独立调节，可能受次级机制影响，但 HIF-1α的核心作用已得到充分证实：它是小鼠胚胎正常血管化所必需的³⁵ 和协调血管生成生长因子的复杂合作。这在转基因动物中得到了证明，其中单独过表达 VEGF 会导致皮肤中的高血管化和高通透性，³⁶ 而相比之下，由相同的皮肤特异性启动子驱动的稳定的 HIF-1α转基因诱导的血管没有渗漏。³⁷ 元

The individual steps of angiogenesis require distinct changes to a variety of cells (e.g. endothelial cells or pericytes). Endothelial cells have to be transformed from a stable growth-arrested state to a plastic proliferative phenotype. The basement membrane has to be digested and the extracellular matrix remodeled so that the endothelial cells are able to migrate. HIF-1α signaling pathways have been demonstrated to influence factors such as uPAR, collagen prolyl-4-hydroxylases, matrix metalloproteinases (e.g. MMP-2) and tissue inhibitors of matrix metalloproteinases (TIMP-1) (Figure 3; for review, see Fukuda et al.³⁰).
血管生成的各个步骤需要对各种细胞（例如内皮细胞或周细胞）进行不同的改变。内皮细胞必须从稳定的生长停滞状态转变为可塑性增殖表型。必须消化基底膜并重塑细胞外基质，以便内皮细胞能够迁移。HIF-1α信号通路已被证明会影响 uPAR、胶原脯氨酰-4-羟化酶、基质金属蛋白酶（例如 MMP-2）和基质金属蛋白酶的组织抑制剂 （TIMP-1） 等因子（图 3;综述见 Fukuda 等人。³⁰）.

HIF-1α regulates factors involved in developmental and pathological angiogenesis. The steps of angiogenesis involve multiple gene products expressed by different cell types (e.g. endothelial cells and pericytes). A coordinated sequence of events is necessary with a tight balance of activators and inhibitors of angiogenesis. HIF-1α directly regulates genes involved in steps such as vasodilation, increased vascular permeability, extracellular matrix remodeling and proliferation (bold). Other genes (underlined) are also hypoxia-responsive; however, direct binding of HIF to regulatory regions in those genes has still to be defined
HIF-1α调节参与发育和病理血管生成的因子。血管生成步骤涉及由不同细胞类型（例如内皮细胞和周细胞）表达的多个基因产物。协调的事件序列是必要的，并且血管生成的激活剂和抑制剂必须保持紧密平衡。HIF-1α直接调节参与血管舒张、血管通透性增加、细胞外基质重塑和增殖等步骤的基因 （粗体）。其他基因（下划线）也对缺氧有反应;然而，HIF 与这些基因中调节区域的直接结合仍有待确定

Full size image

The hypoxic signaling is not restricted to mere target gene upregulation. A well-studied example of how multifaceted the hypoxic response can be is VEGF. VEGF is directly induced by HIF-1α³⁸ and biological activity is further increased by the hypoxic upregulation of VEGF receptor-1 (VEGFR-1/Flt-1).³⁹ VEGF mRNA stability is also increased under hypoxic conditions.⁴⁰ Deletion of HIF-1α in endothelial cells disrupts an autocrine signaling loop for hypoxic induction of VEGFR-2 by VEGF signaling through VEGFR-1, which ultimately leads to impaired vascularization of xenografts.⁴¹
缺氧信号传导不仅限于靶基因上调。VEGF 是低氧反应的多方面性的一个经过充分研究的例子。VEGF 由 HIF-1α³⁸ 直接诱导，生物活性通过 VEGF 受体-1 （VEGFR-1/Flt-1） 的缺氧上调进一步增加。³⁹ VEGF mRNA 在低氧条件下的稳定性也增加。⁴⁰ 内皮细胞中 HIF-1α 的缺失破坏了 VEGF 信号传导通过 VEGFR-1 诱导 VEGFR-2 的自分泌信号回路，最终导致异种移植物的血管化受损。⁴¹

Despite the multitude of insights into individual molecular pathways involved in angiogenesis, such as increased migration and tube formation, which may be predicted to induce angiogenesis in vitro, these analyses in isolated systems clearly have their limitations, especially when the large scale of interconnections and complexity involved in the process of angiogenesis in vivo are considered. Genetic studies have so far not been successful to establish a simple and comprehensive model of how HIF-1α promotes tumor angiogenesis and ultimately tumor growth.^{42, 43} And in xenograft models, which rely more on angiogenesis than naturally occurring tumors, the extent of angiogenesis is dependent on the site of implantation of the xenografts.⁴⁴
尽管对参与血管生成的单个分子途径有很多见解，例如迁移和管形成的增加，可以预测这会在体外诱导血管生成，但在孤立系统中进行的这些分析显然有其局限性，尤其是当体内血管生成过程涉及大规模的互连和复杂性时被考虑。迄今为止，遗传学研究尚未成功建立 HIF-1如何α促进肿瘤血管生成并最终促进肿瘤生长的简单而全面的模型。^42、43 元在异种移植模型中，与自然发生的肿瘤相比，它更依赖于血管生成，血管生成的程度取决于异种移植物的植入部位。⁴⁴

Many genes that are induced by HIF-1α are expressed at higher levels in cancer than in normal tissues, particularly angiogenic growth factors (such as VEGF) and enzymes of the glucose metabolism. As mentioned above, the hallmark of cancer metabolism is significantly influenced by HIF-1α: increased glucose uptake, lactate production and decreased respiration. HIF activation is a common feature of tumors,^{45, 46} is generally more pronounced in aggressive tumors⁴⁷ and can be an independent predictor of poor prognosis in certain types of cancer.⁴⁸ It has to be taken into account, however, that knowledge from developmental studies indicates that proliferation, hypoxia and angiogenesis are linked by physiological pathways originally designed to maintain oxygen supply.
许多由 HIF-1α诱导的基因在癌症中的表达水平高于正常组织，特别是血管生成生长因子（如 VEGF）和葡萄糖代谢酶。如上所述，癌症代谢的标志受到 HIF-1α的显着影响：葡萄糖摄取增加、乳酸产生和呼吸减少。HIF 激活是肿瘤的常见特征，^{45， 46} 通常在侵袭性肿瘤中更为明显⁴⁷，并且可以是某些类型癌症预后不良的独立预测因子。⁴⁸ 然而，必须考虑到，发育研究的知识表明，增殖、缺氧和血管生成与最初旨在维持氧气供应的生理途径有关。

In genetic models, HIF-1α has been identified as a positive factor for tumor growth⁴³ and increased HIF-1α activation was also correlated with the development of a more aggressive phenotype in a model of epidermal carcinogenesis.⁴⁹ Nevertheless, the mechanisms contributing to HIF activation in tumors are complex and difficult to dissect: HIF-α activity can be influenced by the hypoxic tumor microenvironment and also inactivation of tumor suppressor genes. The clearest example of the latter is loss of pVHL function with a subsequent activation of HIF-α (for review, see Ivan and Kaelin⁵⁰). Recently, research on another hereditary renal cell carcinoma caused by inactivation of the TCA cycle enzyme fumarate hydratase (FH) opened an interesting link between hypoxic and metabolic signaling. The Krebs cycle intermediate fumarate inhibits 2-oxoglutarate-dependent dioxygenases such as the HIF hydroxylases. Using a genetic approach, it was demonstrated that FH deficiency can indeed upregulate HIF-α and that the animals develop renal cysts but not overt cancer.⁵¹ However, the role of HIF in causing this tumor predisposition is less clearly established. Other dioxygenases that are inhibited by rising fumarate concentrations also have long-established functions in matrix formations. Thus, precise understanding of the underlying mechanisms is far from being complete and the distinction of cause and effect remains problematic.
在遗传模型中，HIF-1α 已被确定为肿瘤生长的阳性因素^43，HIF-1 α激活的增加也与表皮致癌模型中更具侵袭性的表型的发展相关。⁴⁹ 然而，导致肿瘤中 HIF 激活的机制复杂且难以剖析：HIF-α 活性可受缺氧肿瘤微环境以及肿瘤抑制基因失活的影响。后者最明显的例子是 pVHL 功能丧失，随后 HIF-α 激活（如需回顾，参见 Ivan 和 Kaelin⁵⁰）。最近，对另一种由 TCA 循环酶富马酸水合酶 （FH） 失活引起的遗传性肾细胞癌的研究揭示了缺氧和代谢信号之间的有趣联系。Krebs 循环中间体富马酸抑制 2-氧代戊二酸依赖性双加氧酶，例如 HIF 羟化酶。使用遗传方法，证明 FH 缺陷确实可以上调 HIF-α并且动物会发展为肾囊肿，但不会发展为明显的癌症。⁵¹ 然而，HIF 在导致这种肿瘤易感性中的作用尚不清楚。其他被富马酸盐浓度升高抑制的双加氧酶在基质形成中也具有长期确立的功能。因此，对潜在机制的准确理解远非完整，因果关系的区分仍然存在问题。

Other tumor suppressor genes that influence HIF-1α include p53⁵² and PTEN,⁵³ which suppresses hypoxic HIF-α induction and target gene activation, possibly via modulation of AKT. In addition to the effects of tumor suppressor genes, growth factors have a positive influence on the HIF system. HIF-1α protein synthesis can be induced by many different growth factors and cytokines, such as insulin,⁵⁴ insulin-like growth factors⁵⁵ or PDGF.⁵⁶ P42/44 mitogen-activated kinase (MAPK) has been implicated in phosphorylation of HIF-1α and activation of MAPK amplifies transcriptional response of HIF.⁵⁷
其他影响 HIF-1α的抑癌基因包括 p53⁵² 和 ^PTEN，53 它们可能通过调节 AKT 抑制缺氧 HIF-α 诱导和靶基因激活。除了肿瘤抑制基因的作用外，生长因子还对 HIF 系统有积极影响。HIF-1α蛋白合成可由许多不同的生长因子和细胞因子诱导，例如胰岛素 ⁵⁴、胰岛素样生长因子⁵⁵ 或 PDGF。⁵⁶ P42/44 丝裂原活化激酶 （MAPK） 与 HIF-1α磷酸化有关，MAPK 激活会放大 HIF 的转录反应。⁵⁷ 元

The tumor microenvironment has been demonstrated to impact HIF-1α stability. Radiation can induce HIF-1α through enhanced translation of HIF-1α mRNA released from stress granules upon reoxygenation.⁵⁸ This effect is dependent on free radical generation in vivo. Recently, it was reported that ionizing radiation requires NO produced from macrophages in the tumor microenvironment to stabilize HIF-1α through S-nitrosylation.⁵⁹ Apart from the molecular insights about how HIF-α protein stability can be regulated, another important aspect of these studies is that certain tumors respond to therapy by activating HIF-1α.
肿瘤微环境已被证明会影响 HIF-1α稳定性。放疗可通过增强 HIF-1α再氧合时应激颗粒释放的 mRNA 的翻译来诱导 HIF-1α。⁵⁸ 这种效果取决于体内自由基的产生。最近，据报道，电离辐射需要肿瘤微环境中巨噬细胞产生的 NO 才能通过 S-亚硝基化稳定 HIF-1α。⁵⁹ 除了关于如何调节 HIF-α 蛋白稳定性的分子见解外，这些研究的另一个重要方面是某些肿瘤通过激活 HIF-1α对治疗做出反应。

HIF-1α activation does not only modulate significant metabolic or angiogenic pathways but has also been implicated in tumor invasiveness and metastasis. These are complex multistep processes where tumor cells have to break away from the tumor, cross the basement membrane, migrate through extracellular matrix, invade into the vessels, extravasate and proliferate at a suitable site. These steps require a coordinated expression and interaction of numerous genes. HIF-1α plays a central role, as was recently shown in a transgenic mouse model of tumor progression and metastasis: HIF-1α null tumors exhibit retarded growth and reduced pulmonary metastases.⁶⁰ Multiple downstream genes might be involved in the development of this phenotype, which are under current investigation. In renal cancers, loss of the cell–cell adhesion molecule E-cadherin has been found to increase aggressiveness and invasiveness. HIF-1 appears to play a main role in mediating the downregulation of E-cadherin in the VHL-deficient background,⁶¹ however, HIF-2α also seems to be involved.⁶² The metastatic process is affected by several downstream hypoxia-induced genes: the chemokine-receptor CXCR4 is a direct HIF target.⁶³ Its ligand SDF-1 is also induced by hypoxia and regulates adhesion, migration and homing of CXCR4-expressing cells; this indicates that both the receptor and its ligand play important roles in different steps of metastasis. More recently, the extracellular matrix protein lysyl oxidase was identified to be induced by hypoxia and associated with lower metastasis-free and overall survival rates.⁶⁴ It regulates key steps such as invasion, migration and metastatic growth in distant organs.
HIF-1α激活不仅调节重要的代谢或血管生成途径，而且还与肿瘤侵袭和转移有关。这些是复杂的多步骤过程，其中肿瘤细胞必须脱离肿瘤，穿过基底膜，通过细胞外基质迁移，侵入血管，在合适的部位外渗和增殖。这些步骤需要许多基因的协调表达和相互作用。HIF-1α起着核心作用，正如最近在肿瘤进展和转移的转基因小鼠模型中所显示的那样：HIF-1α null 肿瘤表现出生长迟缓和肺转移减少。⁶⁰ 多个下游基因可能参与这种表型的发育，目前正在研究中。在肾癌中，已发现细胞间粘附分子 E-钙粘蛋白的缺失会增加侵袭性和侵袭性。HIF-1 似乎在 VHL 缺陷背景中介导 E-钙粘蛋白的下调中起主要作用，⁶¹ 然而，HIF-2α似乎也参与其中。⁶² 转移过程受几个下游缺氧诱导基因的影响：趋化因子受体 CXCR4 是直接的 HIF 靶标。⁶³ 它的配体 SDF-1 也由缺氧诱导，并调节表达 CXCR4 的细胞的粘附、迁移和归巢;这表明受体及其配体在转移的不同步骤中都起着重要作用。最近，细胞外基质蛋白赖氨酰氧化酶被鉴定为由缺氧诱导，并与较低的无转移生存率和总生存率相关。⁶⁴ 它调节远处器官的侵袭、迁移和转移生长等关键步骤。

Taken together, HIF-1α appears to be a highly involved factor in the development of a characteristic tumor phenotype influencing growth rate, invasiveness and metastasis. Investigation of xenograft tumors or genetically modified animals is indispensable, especially the former is influenced by the origin of the cells (e.g. renal or other), the genetic background (e.g. VHL status) or the site of implantation⁴⁴ to name only a few. And it has to be taken into account that HIF-1α does not only regulate actively downstream processes but is itself influenced by the tumor microenvironment in many different ways (Figure 4).
综上所述，HIF-1α似乎是影响生长速率、侵袭性和转移的特征性肿瘤表型发展的高度参与因素。异种移植肿瘤或转基因动物的研究是必不可少的，尤其是前者受细胞来源（例如肾脏或其他）、遗传背景（例如 VHL 状态）或植入部位的影响⁴⁴ 仅举几例。必须考虑到 HIF-1α不仅积极调节下游过程，而且本身以多种不同方式受到肿瘤微环境的影响（图 4）。

HIF-1α in cancer. In malignant tissue, different stimuli activate HIF-1α: local hypoxia due to increased proliferation or insufficient oxygen supply, inactivation of tumor suppressors such as VHL, oncogenes, growth factors, accumulation of TCA intermediates such as fumarate or succinate and therapeutic irradiation. Together with other cell types such as macrophages these factors contribute to a tumor microenvironment that is capable of modulating the HIF response itself. These complex interactions together influence the phenotype and the behavior of the tumor in terms of progression, invasiveness or metastatic potential
HIF-1 在癌症中α。在恶性组织中，不同的刺激激活 HIF-1α：由于增殖增加或氧气供应不足而导致的局部缺氧，VHL 等肿瘤抑制因子的失活，癌基因，生长因子，富马酸盐或琥珀酸盐等 TCA 中间体的积累和治疗性照射。这些因子与其他细胞类型（如巨噬细胞）一起，有助于形成能够调节 HIF 反应本身的肿瘤微环境。这些复杂的相互作用共同影响肿瘤的表型和行为，包括进展、侵袭性或转移潜力

Full size image

To better understand the role of HIF-1α in vivo, knockout animals have been generated. Consistent with the central role of HIF in the hypoxic response, targeted inactivation of HIF-1α^{35, 65} or HIF-1β⁶⁶ in the mouse leads to embryonic lethality due to abnormal vascular development. The defects in vasculature have been observed in the yolk sac as well as in the developing embryonic tissue and are associated with severe hypoxia in the HIF-1α^−/− embryos. Embryos with inactivation of VHL also die in utero at about E12 due to defects in placental development.⁶⁷ In contrast to the drastic effects, more subtle approaches utilizing mice with heterozygous defects or tissue-specific knockouts of HIF-1α provided fascinating insights into the physiology of HIF-1. Mice with heterozygous defects of HIF-1α have a reduced protective effect of hypoxic preconditioning in a model of cardiac ischemia,⁶⁸ and a dramatic effect on carotid body neural activity and ventilatory adaptation to chronic hypoxia.^{69, 70} Another elegant approach is the investigation of tissue-specific loss of HIF-1α function by the use of the Cre recombinase-loxP technology. After the generation of mice carrying a loxP-flanked exon 2 of HIF-1α,⁴³ various studies were performed to examine the consequences of the deletion of HIF-1α in different tissues. Taken together, the data demonstrate that HIF-1 is critical not only for hypoxic adaptation but also for physiological function in many cell types. Of particular interest is that HIF-1 is a key factor for the functional integrity and antimicrobial defense capacity of the immune system.^{71, 72} Other significant roles of HIF-1 were described in the myocardium,⁷³ in the colonic epithelium,⁷⁴ for chondrogenesis⁷⁵ and for osteoblast development.⁷⁶ Thus, using this genetic approach the physiology of the hypoxic response can be characterized elegantly in vivo with important implications on a wide range of different diseases.
为了更好地了解 HIF-1α 在体内的作用，已经产生了敲除动物。与 HIF 在缺氧反应中的核心作用一致，小鼠中 HIF-1α^35、65 或 HIF-1β⁶⁶ 的靶向失活导致血管发育异常导致胚胎致死。在卵黄囊和发育中的胚胎组织中观察到脉管系统的缺陷，并且与 HIF-1α^-/− 胚胎的严重缺氧有关。由于胎盘发育缺陷，VHL 失活的胚胎也在大约 E12 时在子宫内死亡。⁶⁷ 与剧烈的影响相反，利用具有杂合缺陷或 HIF-1 组织特异性敲除的小鼠的更微妙的方法α为 HIF-1 的生理学提供了引人入胜的见解。在心肌缺血模型中，具有 HIF-1 杂合缺陷的小鼠α缺氧预处理的保护作用降低，⁶⁸ 对颈动脉体神经活动和对慢性缺氧的通气适应有显着影响。^69、70 元另一种优雅的方法是通过使用 Cre 重组酶-loxP 技术研究 HIF-1α 的组织特异性丧失。在产生携带 HIF-1α 的 loxP 侧翼外显子 2 的小鼠后，⁴³ 进行了各种研究以检查不同组织中 HIF-1α缺失的后果。综上所述，数据表明 HIF-1 不仅对低氧适应至关重要，而且对许多细胞类型的生理功能也至关重要。 特别有趣的是 HIF-1 是免疫系统功能完整性和抗菌防御能力的关键因素。^71、72 元HIF-1 的其他重要作用在心肌中被描述⁷³，在结肠上皮中被描述⁷⁴，用于软骨形成⁷⁵ 和成骨细胞发育。⁷⁶ 因此，使用这种遗传方法可以在体内优雅地表征低氧反应的生理学，对各种不同的疾病具有重要意义。

The central role of HIF-1α in physiology and pathology makes it an attractive yet intricate target for pharmacological manipulations. Inhibitors of HIF could have some potential as anticancer therapeutics, whereas activators of HIF might be useful for the treatment of ischemic disease. The potential caveats of such manipulations are obvious: the great level of complexity of the HIF system is further complicated by the multifaceted changes imposed by disease states. Thus, a highly specific targeting of the organ or tissue of interest is mandatory since a general inhibition or activation of HIF will almost certainly generate pronounced side effects. Many of the compounds inhibiting or activating HIF-α still lack isoform specificity; therefore, all manipulations will affect both isoforms depending on the targeted tissue.
HIF-1α 在生理学和病理学中的核心作用使其成为药理学操作的有吸引力但复杂的靶标。HIF 抑制剂可能具有作为抗癌疗法的潜力，而 HIF 的激活剂可能有助于治疗缺血性疾病。这种操作的潜在警告是显而易见的：HIF 系统的高度复杂性因疾病状态施加的多方面变化而进一步复杂化。因此，必须对感兴趣的器官或组织进行高度特异性的靶向，因为 HIF 的一般抑制或激活几乎肯定会产生明显的副作用。许多抑制或激活 HIF-α 的化合物仍然缺乏亚型特异性;因此，所有操作都会根据目标组织影响两种亚型。

HIF-1 is often detected in cancer and HIF-1-regulated genes have impact on the cancer phenotype. HIF inhibitors for cancer therapy target the HIF pathway on different levels: they decrease HIF mRNA or protein levels, inhibit DNA binding or decrease HIF-mediated transactivation (for review, see Fukuda et al.³⁰). It is critical, however, to determine that HIF-1 is the cause for malignant predisposition in the particular tumor intended to be treated and not a marker of a physiological activation.
HIF-1 经常在癌症中检测到，HIF-1 调节基因对癌症表型有影响。用于癌症治疗的 HIF 抑制剂在不同水平上靶向 HIF 通路：它们降低 HIF mRNA 或蛋白质水平，抑制 DNA 结合或减少 HIF 介导的反式激活（综述，参见 Fukuda 等人。^然而，确定 HIF-1 是要治疗的特定肿瘤中恶性易感性的原因，而不是生理激活的标志物，这一点至关重要。

On the other hand, the physiological (and protective) HIF response is well documented in ischemic, hypoxic and inflammatory conditions. Different pharmacological compounds have been utilized to activate HIF, and the best studied so far are inhibitors of the prolyl-hydroxylases.^{77, 78}
另一方面，生理（和保护性）HIF 反应在缺血、缺氧和炎症情况下有很好的记录。不同的药理化合物已被用于激活 HIF，迄今为止研究最好的是脯氨酰羟化酶的抑制剂。^77、78 元

Thus, pharmacological approaches to exploit both aspects of the HIF response therapeutically are emerging from basic science laboratories. Some of them are currently under investigation in early-phase clinical trials. It will be critically important to select the right cohort of patients and the right regimen of the available substances to utilize the full potential of these compounds with the lowest possible side effects.
因此，基础科学实验室正在出现从治疗上利用 HIF 反应的两个方面的药理学方法。其中一些目前正在进行早期临床试验。选择正确的患者队列和正确的可用物质方案以利用这些化合物的全部潜力和尽可能低的副作用至关重要。

Although the understanding of the biology of the HIF system, and more precisely the biology of HIF-1α, at a molecular level has increased remarkably over the past years, the understanding of the physiological implications of the various pathways involved especially in angiogenesis and cancer has lagged behind. In contrast, the complexity of the system is reflected in sometimes unexpected results, and in some instances the mechanisms and interactions are even more intricate than originally thought: regulation of metabolic processes in hypoxia by HIF-1 go vastly beyond optimizing glucose utilization alone but influence biogenesis and function of mitochondria as well. The physiological role of HIF-1α in normoxia is more distinct than anticipated. However, the overlap of the extensive physiological responses with tumor development and progression complicates the distinction of cause and effect: tumor cells might share the physiologically linked pathways of HIF-1α activation and clearly more research is needed to determine the individual role of HIF in certain types of cancer. This is of crucial importance when the pharmacological manipulation of the HIF-pathway is intended as a therapeutical intervention.
尽管在过去几年中，在分子水平上对 HIF 系统生物学的理解，更准确地说是对 HIF-1α生物学的理解显着增加，但对涉及的各种途径的生理影响的理解，特别是对血管生成和癌症所涉及的各种途径的生理影响的理解却落后了。相比之下，系统的复杂性有时会反映在意想不到的结果上，在某些情况下，机制和相互作用甚至比最初想象的还要复杂：HIF-1 对缺氧代谢过程的调节远远超出了单独优化葡萄糖利用的范围，但也影响线粒体的生物发生和功能。HIF-1α 在常氧中的生理作用比预期的更明显。然而，广泛的生理反应与肿瘤发展的重叠使因果关系的区分复杂化：肿瘤细胞可能共享 HIF-1α激活的生理连接途径，显然需要更多的研究来确定 HIF 在某些类型癌症中的个体作用。当 HIF 通路的药理学操作旨在作为治疗干预时，这一点至关重要。