Regulation of Angiogenesis by Hypoxia and Hypoxia‐Inducible Factors
Introduction
In the absence of a functional blood supply, the growth of both multicellular organisms and tumors is limited by the ability of oxygen to diffuse to cells from blood vessels. Decreased oxygen levels, or hypoxia, can develop within rapidly proliferating tissues or as the result of occlusion of blood vessels. Hypoxia leads to insufficient cellular energy production as oxygen is essential for oxidative phosphorylation. At the same time, however, excessively high levels of oxygen are detrimental and can result in the production of reactive oxygen species (ROS) that damage cellular organelles and DNA. Therefore, it is imperative that oxygen concentrations be tightly regulated. At the systemic level, oxygen tension is detected by highly sensitive tissues, such as the carotid body, to effect a rapid physiological response to acute hypoxia that includes increased ventilation and cardiac output. More prolonged hypoxia is also sensed at the cellular level, leading to the activation of molecular pathways to cope with this stress. The key mediators of this response are members of the hypoxia‐inducible factor (HIF) family of proteins. The hypoxic response and the HIF pathway are conserved from Caenorhabitis elegans and Drosophila melanogaster to mice and humans, emphasizing its importance in the maintenance of oxygen homeostasis. These proteins function as transcriptional regulators that stimulate the expression of a multitude of genes important for adaptation to hypoxia, including those encoding glucose transporter‐1 (Glut‐1), which increases cellular glucose uptake, and glycolytic enzymes, which mediate enhanced glycolysis to maintain ATP production in the face of lower oxygen levels. In addition, HIF‐stimulated erythropoietin (Epo) expression improves the oxygen carrying capacity of the blood by enhancing the production of erythrocytes.
Another mechanism by which cells can alleviate the increasing metabolic demands presented by hypoxia is via new blood vessel formation through vasculogenesis and angiogenesis. These processes are complex and occur in a stepwise fashion. Angiogenesis is regulated by a balance of positive‐ and negative‐acting growth factors and by physiological stresses such as alterations in oxygen levels. Hypoxia stimulates the expansion and remodeling of the existing vasculature to enhance blood flow to oxygen‐deprived tissues. This is accomplished primarily through the activation of HIF target genes involved in various steps of angiogenesis such as vascular endothelial growth factor (VEGF). This regulation of angiogenesis by hypoxia and HIFs is essential for proper embryonic development and recovery after ischemic injury. In addition, numerous lines of evidence suggest that tumors develop regions of hypoxia and that the HIF pathway is an important component of tumor growth and angiogenesis.
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Mechanisms of Angiogenesis
The vascular network mediates the delivery of oxygen and nutrients to all cells of an organism and is essential for normal development and survival. Figure 1 outlines the mechanisms by which the vasculature is formed and highlights several of the key molecules involved in regulating these processes. Mesodermal progenitor cells known as hemangioblasts represent bipotential precursor cells that give rise to both endothelial and hematopoietic cells (Sabin, 1920). A putative hemangioblast, the
Structure of HIF Proteins
HIF proteins are members of a larger, evolutionarily conserved group of proteins known as bHLH‐PAS (basic helix loop helix‐Per ARNT Sim) proteins (Crews, 1998). These proteins function as sensors of environmental stimuli and activate the expression of genes important for angiogenesis, as well as circadian rhythms, xenobiotic detoxification, and adaptation to hypoxia (Crews 1999, Gu 2000, Kewley 2004). Each member of this family contains an N‐terminal bHLH domain that mediates binding to
The Role of HIF in Developmental Angiogenesis
Hypoxia is an important feature of embryonic development: as the embryo grows and develops, it quickly outstrips the oxygen and nutrients provided by diffusion alone (Maltepe and Simon, 1998). These naturally occurring hypoxic gradients trigger the expression of genes critical for the formation of a complex network of blood vessels to provide adequate oxygenation of the dividing tissues. A number of HIF target genes has been shown to be essential for proper vascular development, including Vegf
The Role of HIF in Adult Tissues
In addition to its role in development, HIF is important for the physiological hypoxic response in the adult. Mice heterozygous for Hif1a have an impaired response to chronic hypoxic exposure, including delayed erythropoiesis and decreased pulmonary vascular remodeling (Yu et al., 1999). HIF1α is induced in keratinocytes during wound healing of the skin, overlapping to some extent with VEGF expression (Elson et al., 2000). Several groups have used conditional gene targeting of a floxed Hif1a
HIF and Ischemic Injury
Levels of HIF1α mRNA and protein increase following ischemic insults in a number of tissues, including the retina, heart, brain, and lung, in both mice and humans (Bergeron 1999, Lee 2000, Ozaki 1999, Yu 1998). This finding supports a role for the HIF pathway in pathophysiological angiogenesis. For example, HIF activity has been associated with ischemia in the retina that is associated with diabetic retinopathy and retinopathy of prematurity (ROP), in which retinal vessels become ischemic due
Tumor Angiogenesis and Hypoxia
The induction of angiogenesis is essential for tumor growth, survival, and progression to an invasive phenotype (Hanahan and Folkman, 1996). One of the critical factors responsible for mediating the activation of the so‐called angiogenic switch is the presence of hypoxia within solid tumors. As tumors proliferate beyond 1–2 mm3, oxygen and nutrients become limiting, resulting in the development of hypoxia. Tumor hypoxia correlates with aggressive disease and a poor prognosis in patients, and
Conclusions
The HIF pathway is an essential effector of the cellular response to changes in oxygen concentration. Activation of HIF under hypoxia results in the induction of a broad program of gene expression that is necessary for adaptation to low oxygen. HIF signaling mediates the hypoxic response through cell autonomous mechanisms, for example, by regulating proliferation, apoptosis, and metabolism, as well as through cell nonautonomous effects on angiogenesis. HIF‐stimulated expression of proangiogenic
Acknowledgments
We thank members of the lab, especially Dr. Brian Keith, for helpful discussions and proofreading assistance. We apologize to those colleagues whose work could not be cited directly.
References (266)
- et al.
Genetic evidence for a tumor suppressor role of HIF‐2alpha
Cancer Cell
(2005) - et al.
Multilineage embryonic hematopoiesis requires hypoxic ARNT activity
Genes Dev.
(1999) - et al.
Placental cell fates are regulated in vivo by HIF‐mediated hypoxia responses
Genes Dev.
(2000) - et al.
Expression of hypoxia‐inducible factor‐1alpha: A novel predictive and prognostic parameter in the radiotherapy of oropharyngeal cancer
Cancer Res.
(2001) - et al.
Bacteriolytic therapy can generate a potent immune response against experimental tumors
Proc. Natl. Acad. Sci. USA
(2004) - et al.
c‐Jun and hypoxia‐inducible factor 1 functionally cooperate in hypoxia‐induced gene transcription
Mol. Cell. Biol.
(2002) - et al.
Stabilization of wild‐type p53 by hypoxia‐inducible factor 1alpha
Nature
(1998) - et al.
An essential role for p300/CBP in the cellular response to hypoxia
Proc. Natl. Acad. Sci. USA
(1996) - et al.
Phosphatidylinositol 3‐kinase/Akt signaling is neither required for hypoxic stabilization of HIF‐1 alpha nor sufficient for HIF‐1‐dependent target gene transcription
J. Biol. Chem.
(2002) - et al.
Isolation of putative progenitor endothelial cells for angiogenesis
Science
(1997)