The cellular response to O2 (oxygen) is a central process in animal cells and figures prominently in the pathophysiology of several diseases, including cancer, cardiovascular disease, and stroke. This process is coordinated by the HIF (Hypoxia-Inducible Factor) and its regulator, the pVHL (Von Hippel-Lindau tumor suppressor protein). HIF1 is a basic helix-loop-helix transcription factor that transactivates genes encoding proteins that participate in homeostatic responses to hypoxia. It induces expression of proteins controlling glucose metabolism, cell proliferation, and vascularization. Several genes involved in cellular differentiation are directly or indirectly regulated by hypoxia. These include Epo (Erythropoietin), LDHA (Lactate Dehydrogenase-A), ET1 (Endothelin-1), transferrin, transferrin receptor, VEGF (Vascular Endothelial Growth Factor), Flk1, FLT1 (Fms-Related Tyrosine Kinase-1), PDGF-β (Platelet-Derived Growth Factor-β), bFGF (basic Fibroblast Growth Factor), and others genes affecting glycolysis (Ref.1).
HIF1 consists of a heterodimer of two basic helix-loop-helix PAS (Per-ARNT-Sim) proteins, HIF1-α, and HIF1-β. HIF1-α accumulates under hypoxic conditions whereas HIF1-β is constitutively expressed. HIF1-α is an important mediator of the hypoxic response of tumor cells and controls the up-regulation of a number of factors important for solid tumor expansion including the angiogenic factor VEGF. HIF1-β is the ARNT (Aryl hydrocarbon Receptor Nuclear Translocator), an essential component of the xenobiotic response (Ref.2).
In the presence of O2, HIF is targeted for destruction by an E3 ubiquitin ligase containing the pVHL. Human pVHL binds to a short HIF-derived peptide when a conserved proline residue at the core of this peptide is hydroxylated. The human genome contains EGL9 (Egg Laying Nine-9) homologues that are named EGLN1, EGLN2, and EGLN3 (also called PHD2, PHD1, and PHD3 (Prolyl Hydroxylase Domain-Containing Proteins) respectively). Prolyl hydroxylase post-translationally modifies HIF1-α, allowing it to interact with the VHL complex. Prolyl hydroxylase contains an iron moiety, so iron chelation inhibits this activity. All three proteins of Prolyl hydroxylase can hydroxylate HIF1-α at one of two proline sites within the ODD (Pro-402 and Pro-564). Analogous prolyl residues are present in HIF2-α and HIF3-α. In the presence of oxygen, the EGLN proteins are active and hydroxylate the ODD domain of HIF1-α, which allows pVHL to bind and polyubiquitinate HIF (Ref.3). VHL is part of a larger complex that includes Elongin-B, Elongin-C, Cul2, RBX1 (Ring-Box 1) and a ubiquitin-conjugating enzyme (E2). This complex, together with a ubiquitin-activating enzyme (E1), mediates the Ub (Ubiquitylation) of HIF1-α. The Ub modification targets HIF1-α for degradation, which can be blocked by proteasome inhibitors. Under hypoxic conditions the HIF1-α subunits are not recognized by pVHL, and they consequently accumulate and dimerize with HIF1-β and translocates to the nucleus, where they interacts with cofactors such as CBP (CREB Binding Protein)/p300 and the Pol II (DNA polymerase II) complex to bind to HREs (Hypoxia-Responsive Element) and activate transcription of target genes. HIF1-α-activated genes include VEGF, which promotes angiogenesis; GLUT1 (Glucose Transporter-1), which activates glucose transport; LDHA (Lactate Dehydrogenase), which is involved in the glycolytic pathway; and Epo, which induces erythropoiesis. HIF1-α also activates transcription of NOS (Nitric Oxide Synthase), which promotes angiogenesis and vasodilation. ARNT2 and MOP3 (Member of Pas superfamily-3) are other proteins that have been shown to heterodimerize with HIF1-α (Ref.4). HIF1-α can also be regulated by ERK2, which phosphorylate HIF1-α. HIF1-α also associates with the molecular chaperone HSP90 (Heat Shock Protein-90). HSP90 antagonists also inhibited HIF1-α transcriptional activity and dramatically reduced both hypoxia-induced accumulation of VEGF mRNA and hypoxia-dependent angiogenic activity. Recently, a factor inhibiting HIF1-α activation, FIH (Factor Inhibiting HIF1-α), has been described, representing a further level of HIF regulation.
Hypoxia also induces p53 protein accumulation. p53 directly interacts with HIF1-α and limits hypoxia-induced expression of HIF1-α by promoting MDM2-mediated ubiquitination and proteasomal degradation under hypoxic conditions. Furthermore, the degradation of HIF1-α by p53 in a hypoxic condition is inhibited by direct interaction with the JAB1 (Jun Activation domain Binding protein-1) and the ODD domain by blocking the interaction with p53. HIF1-α also associates with HNF4α2 (Hepatocyte Nuclear Factor-4-α 2), which activates the Epo gene in concert with HIF1-α in response to hypoxic conditions. Hypoxia contributes significantly to the pathophysiology of major categories of human disease, including myocardial and cerebral ischemia, cancer, pulmonary hypertension, congenital heart disease and chronic obstructive pulmonary diseases (Ref.5).
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