Hypoxia-lnducible Factor 1: Systemic and Cellular Physiologic Responses to Hypoxia
Hypoxia can also occur as a local disorder in the context of ischemia due to hypoperfusion, most notably of the cerebral and coronary circulations. When hypoxia is local and vascular in nature, a variety of vasoactive molecules are produced that function either to increase flow through existing vessels (vasodilation) or to promote the growth of new blood vessels (angiogenesis). Examples include the production of the vasodilators nitric oxide and carbon monoxide by the enzymes inducible nitric oxide synthase (iNOS) and heme oxygenase-1 (HO-1), respectively, and the production of vascular endothelial growth factor (VEGF), the primary regulator of angiogenesis. Increased expression of the iNOS, HO-1,and VEGF genes in response to hypoxia and/or ischemia has been demonstrated both in cultured myocardial and vascular cells and in the heart and lungs in vivo.
Regardless of whether hypoxia is systemic or local in nature, individual cells must undergo metabolic adaptation in order to maintain homeostasis under conditions of limited oxygen availability. The classic intracellular response to hypoxia is the transition from oxidative phosphorylation to glycolysis as the principal pathway for adenosine triphosphate generation. Under hypoxic conditions, increased expression of messenger RNA (mRNA) and protein for both glucose transporters and glycolytic enzymes has been documented in a number of different tissue culture cell types.
Molecular Responses to Hypoxia
Underlying the systemic and cellular physiologic adaptations to hypoxia such as erythropoiesis, angiogenesis, and glycolysis described above are changes in gene expression. Changes in the steady-state levels of the mRNA product of a given gene may reflect changes in the rate of either RNA transcription or RNA degradation. Analysis of the transcriptional regulation of hypoxia-inducible genes revealed the presence of short (<100 bp) DNA sequences within promoter or enhancer sequences that have been designated hypoxia response elements. Representative exam-pies of hypoxia response elements from the genes encoding EPO, VEGF, HO-1, and the glycolytic enzymes enolase-1 (ENO-1), lactate dehydrogenase A (LDHA), and phosphoglycerate kinase-1 (PGK-1) are shown in Figure 1. These elements all have in common the presence of one or more binding sites for hypoxia-inducible factor-1 (HIF-1).
Figure 1. Hypoxia response elements. DNA sequences from the 3′-flanking region of the human EPO gene and from the 5′-flanking region of the human VEGFJ mouse lactate dehydrogenase A (LDHA),’15 mouse phosphoglycerate kinase 1 (PGK1)’16 mouse HO-1, and human enolase 1 (ENOl ) genes are shown. Functionally essential HIF-1 binding sites on the sense or antisense strand are indicated by the arrows either above or below the sequence, respectively. All HIF-1 sites shown contain the core sequence 5′-CGTG-3\ Other sequences (5′-[A/ C] AC AG-3′) in the EPO and VEGF hypoxia-response elements that are also functionally essential are overscored. Whereas complete EPO and VEGF hypoxia-response elements are shown, only the LDHA, PGK1, HOI, and ENOl sequences encompassing functional HIF-1 bindings sites are shown.