Hypoxia-inducible factor-1 (HIF-1) is a master regulator of cellular adaptive responses to hypoxia. Levels of the HIF-1alpha subunit increase under hypoxic conditions. Exposure of cells to certain nitric oxide (NO) donors also induces HIF-1alpha expression under nonhypoxic conditions. We demonstrate that exposure of cells to the NO donor NOC18 or S-nitrosoglutathione induces HIF-1alpha expression and transcriptional activity. In contrast to hypoxia, NOC18 did not inhibit HIF-1alpha hydroxylation, ubiquitination, and degradation, indicating an effect on HIF-1alpha protein synthesis that was confirmed by pulse labeling studies. NOC18 stimulation of HIF-1alpha protein and HIF-1-dependent gene expression was blocked by treating cells with an inhibitor of the phosphatidylinositol 3-kinase or MAPK-signaling pathway. These inhibitors also blocked NOC18-induced phosphorylation of the translational regulatory proteins 4E-BP1, p70 S6 kinase, and eIF-4E, thus providing a mechanism for the modulation of HIF-1alpha protein synthesis. In addition, expression of a dominant-negative form of Ras significantly suppressed HIF-1 activation by NOC18. We conclude that the NO donor NOC18 induces HIF-1alpha synthesis under conditions of NO formation during normoxia and that hydroxylation of HIF-1alpha is not regulated by NOC18.
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Abstract Hypoxia‐inducible factor 1 (HIF‐1) is a transcriptional activator that mediates adaptive responses to hypoxia. HIF‐1 activity is increased in the majority of human cancers as a result of genetic alterations and intratumoral hypoxia. HIF‐1 activates the transcription of genes that increase O 2 availability by stimulating angiogenesis or that reprogram cellular metabolism to adapt to reduced O 2 availability. Proof of principle studies in mouse models suggests that inhibition of HIF‐1 activity may have therapeutic effects, especially in combination with other anticancer drugs. © 2008 IUBMB IUBMB Life, 60(9): 591–597, 2008
Hypoxia-inducible factor 1 (HIF-1) is a basic-helix-loop-helix transcription factor that plays essential roles in mammalian development and physiology. HIF-1 is a heterodimer composed of HIF-1alpha and HIF-1beta subunits. The expression and activity of the HIF-1alpha subunit are tightly regulated by cellular O2 concentration. Under hypoxic conditions, HIF-1 activates the transcription of genes encoding erythropoietin, glucose transporters, glycolytic enzymes, vascular endothelial growth factor, and other genes whose protein products increase O2 delivery or facilitate metabolic adaptation to hypoxia. HIF-1 is essential for embryonic vascularization and survival, neovascularization in ischemic myocardium, hypoxia-induced pulmonary vascular remodeling, and tumor vascularization. HIF-1alpha is overexpressed in the majority of common human cancers and their metastases, due to the presence of intratumoral hypoxia and as a result of mutations in genes encoding oncoproteins and tumor suppressors. Pharmacologic manipulation of HIF-1 levels may provide a novel therapeutic approach to diseases that represent the most common causes of mortality in Western society, including cancer, chronic lung disease, and myocardial ischemia.
Homeostatic responses to reduced O(2) availability are regulated by the transcriptional activator hypoxia-inducible factor 1 (HIF-1) in all metazoan species. An essential adaptation to sustained hypoxia is an active repression of mitochondrial respiration. In mouse embryo fibroblasts, HIF-1 induces expression of BNIP3, which triggers selective mitochondrial autophagy. When exposed to hypoxia, HIF-1-deficient cells do not induce BNIP3 or autophagy, do not decrease mitochondrial mass or downregulate respiration, and die within 72 hours due to toxic levels of reactive oxygen species. These studies indicate that mitochondrial autophagy represents an adaptive metabolic response to hypoxia that is necessary to maintain redox homeostasis and cell survival.
Hypoxia-inducible factor 1 (HIF-1) is a critical mediator of physiological responses to acute and chronic hypoxia. First, HIF-1 is required for the development of the systems that mediate these responses, including the heart, blood and blood vessels. Mice with complete HIF-1alpha deficiency manifest developmental defects that involve all three components of the circulatory system. Second, HIF-1 mediates changes in gene expression that underlie physiological responses to chronic hypoxia, such as increased erythropoiesis and angiogenesis. Hif1a(+/-) mice, which are partially HIF-1alpha deficient, manifest impaired hypoxia-induced pulmonary vascular remodelling. Smooth muscle cells from pulmonary arteries (PASMCs) of wild-type mice subjected to chronic hypoxia manifest hypertrophy, depolarization, increased [Ca2+]i, and decreased voltage-gated K+ currents. These responses are impaired in PASMCs from Hif1a(+/-) mice. Carotid bodies isolated from Hif1a(+/-) mice are unresponsive to hypoxia despite normal histology and normal responses to cyanide stimulation. Rat PC12 cells share properties with O2-sensing glomus cells of the carotid body, including hypoxia-inducible expression of tyrosine hydroxylase, the rate limiting enzyme for catecholamine biosynthesis. In PC12 cells subjected to intermittent hypoxia, Ca2+/calmodulin-dependent kinase activity leads to HIF-1 transcriptional activity and tyrosine hydroxylase mRNA expression. Thus, HIF-1 regulates both acute and chronic responses to continuous and intermittent hypoxia.
Analysis of DNA from the beta-globin gene cluster in an Albanian family identified a novel RsaI site approximately 550 base pairs 5' to the beta-globin gene. In this family, two chromosomes carrying otherwise identical beta-globin haplotypes were found to differ at the RsaI site. Population screening demonstrated the presence and absence of the site in DNA from individuals of northern European, Mediterranean, Middle Eastern, Southeast Asian, African, and Asian Indian descent, indicating that this site is a DNA polymorphism common in many ethnic groups. The polymorphism is also present in DNA from individuals carrying different beta-globin alleles. Additional nucleotide sequence changes identified in an RsaI (+) genomic clone in the region immediately 3' to the RsaI site suggest a mechanism for the randomization of the site with respect to haplotype.
Tumor progression occurs as a result of the clonal selection of cells in which somatic mutations have activated oncogenes or inactivated tumor suppressor genes leading to increased proliferation and/or survival within the hypoxic tumor microenvironment. Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that mediates adaptive responses to reduced O2 availability, including angiogenesis and glycolysis. Expression of the O2-regulated HIF-1α subunit and HIF-1 transcriptional activity are increased dramatically in hypoxic cells. Recent studies indicate that many common tumor-specific genetic alterations also lead to increased HIF-1α expression and/or activity. Thus, genetic and physiologic alterations within tumors act synergistically to increase HIF-1 transcriptional activity, which appears to play a critical role in the development of invasive and metastatic properties that define the lethal cancer phenotype.
A bstract : Hypoxia is an essential pathophysiologic component of ischemic cardiovascular disease. A better understanding of the molecular mechanisms underlying adaptive responses to hypoxia may lead to novel therapeutic strategies. Hypoxia‐inducible factor 1 (HIF‐1) is a heterodimeric basic‐helix‐loop‐helix‐PAS domain transcription factor that mediates changes in gene expression in response to changes in O 2 concentration. Genes that are transcriptionally activated by HIF‐1 in hypoxic cells encode proteins that increase O 2 delivery or allow metabolic adaptation to limited O 2 availability. HIF‐1 target genes include those encoding vascular endothelial growth factor (VEGF), erythropoietin, glucose transporters, and glycolytic enzymes. In anemic fetal sheep, increased myocardial vascularization was associated with concomitant increases in the expression of HIF‐1 and VEGF. Expression of HIF‐1 target genes was not induced by hypoxia in embryonic stem cells lacking expression of the O 2 ‐regulated HIF‐1αa subunit. Mouse embryos lacking HIF‐1α expression arrested in their development by E9.0 and died by E10.5 with cardiovascular malformations and massive cell death throughout the embryo. These studies indicate that HIF‐1 functions as a master regulator of O 2 homeostasis that controls the establishment of essential physiologic systems during embryogenesis as well as their subsequent utilization during fetal and postnatal life.
Human erythropoietin gene expression in liver and kidney is inducible by anemia or hypoxia. DNase I-hypersensitive sites were identified 3' to the human erythropoietin gene in liver nuclei. A 256-base-pair region of 3' flanking sequence was shown by DNase I protection and electrophoretic mobility-shift assays to bind four or more different nuclear factors, at least two of which are induced by anemia in both liver and kidney, and the region functioned as a hypoxia-inducible enhancer in transient expression assays. These results provide insight into the molecular basis for the regulation of gene expression by a fundamental physiologic stimulus, hypoxia.
The existence of multicellular organisms is based on the efficient capture of solar energy by plants through photosynthesis, a process by which carbon dioxide (CO2) and water are converted into glucose and oxygen (O2). These are subsequently utilized by all eukaryotic organisms to generate ATP and, as by-products, CO2 and water, thus completing the circle of life on our planet (Figure 29.1). The highly efficient recovery of the energy contained within the chemical bonds of glucose through the process of oxidative phosphorylation provides the power necessary to assemble and maintain complex multicellular machines such as Homo sapiens, in which more than 100,000,000,000,000 parts (i.e., cells) are assembled and organized into a functional unit (i.e., organism). A requirement for the efficient generation of ATP is the continuous delivery of O2 to every cell in the body. In postnatal life, this requirement is met through the concerted action of the lungs, blood, heart, and vessels. The lungs of an adult human take in 5 to 6 liters of air per minute or approximately 8,000 liters per day. Within the lungs, O2 is bound by hemoglobin present within erythrocytes, which are pumped by the heart through blood vessels that represent the transportation infrastructure for O2 and glucose delivery to the tissues and for the removal of CO2, hydrogen ions (H+), potassium ions (K+), and other toxic metabolites.
