- Open Access
Oxygen-regulated transcription factors and their role in pulmonary disease
© Current Science Ltd 2000
- Received: 27 September 2000
- Accepted: 19 October 2000
- Published: 9 November 2000
The transcription factors nuclear factor interleukin-6 (NF-IL6), early growth response-1 (EGR-1) and hypoxia-inducible factor-1 (HIF-1) have important roles in the molecular pathophysiology of hypoxia-associated pulmonary disease. NF-IL6 controls the production of interleukin (IL)-6 in vascular endothelial cells, which may have anti-inflammatory activity by counteracting effects of IL-1 and IL-8. EGR-1 controls the production of tissue factor by macrophages, which triggers fibrin deposition in the pulmonary vasculature. HIF-1 activates the expression of the vasoconstrictor endothelin-1 in vascular endothelial cells. Angiotensin II induces HIF-1 expression and hypertrophy of pulmonary arterial smooth muscle cells. HIF-1 might therefore have multiple roles in the pathogenesis of pulmonary vascular remodeling.
- vascular remodeling
Hypoxia-inducible gene expression
Adenylate kinase 3
HMG I(Y), NF-κB
Insulin-like growth factor-2 (IGF-2)
Lactate dehydrogenase A
Nitric oxide synthase-2
Plasminogen activator inhibitor-1
Prolyl-4-hydroxylase α (I)
Pyruvate kinase M
Transforming growth factor β3
Vascular endothelial growth factor (VEGF)
VEGF receptor FLT-1
NF-IL6 (also known as C/EBPβ, for CCAAT-enhancer-binding protein-β) is a basic-leucine-zipper transcription factor that activates IL-6 gene transcription in hypoxic pulmonary vascular endothelial cells (ECs) . The NF-IL6-binding site from the IL-6 promoter was sufficient to direct the expression of a lacZ gene in the heart, kidney, and lung, but not the liver, of transgenic mice subjected to hypoxia or ischemia . The basis for the tissue-specific induction of NF-IL6 expression in vascular ECs is not known. Because IL-6 is a cytokine with anti-inflammatory properties, it might counteract the effects of proinflammatory cytokines. For example, the expression of IL-1 and IL-8 are also induced in hypoxic ECs [4,5] and promote the expression of adherence molecules that recruit activated leukocytes to the vessel wall, leading to vascular leakage and/or thrombus formation . Thus, NF-IL6 might protect the integrity of the pulmonary vasculature under conditions of hypoxia or ischemia.
EGR-1 (also known as ZIF268) is a zinc-finger transcription factor that is expressed in hypoxic mononuclear phagocytes . EGR-1-mediated production of tissue factor by monocytes leads to hypoxia-induced fibrin deposition in the pulmonary vasculature . In the lungs of mice subjected to hypoxia, EGR-1 and tissue factor are co-induced in bronchial and vascular smooth muscle and alveolar macrophages . When wild-type mice are exposed an ambient O2 to concentration of 6% (reduced from the 21% O2 in room air), EGR-1 and tissue factor expression are induced in the lung and marked vascular deposition of fibrin is observed, whereas none of these responses occur in knockout mice lacking expression of the gene encoding protein kinase C-β . The mechanism by which hypoxia activates protein kinase C-β activity in mononuclear phagocytes is unknown but this activity seems to be crucial for the induction of EGR-1 activity and tissue factor production. EGR-1 also mediates tissue factor production by endothelial cells in response to stimulation by vascular endothelial growth factor (VEGF) , the hypoxia-induced expression of which is mediated by HIF-1 (see below), indicating that hypoxia induces tissue factor expression by two different mechanisms that both involve EGR-1. Thus, in contrast to NF-IL6, hypoxia-induced EGR-1 activity promotes pulmonary vascular thrombosis.
Although the pathophysiology of pulmonary hypertension is complex , a major component of this process is the elaboration of peptides, such as endothelin-1 (ET-1) and angiotensin II, that induce smooth muscle cell (SMC) contraction and hypertrophy. ET-1 expression is induced within the pulmonary vasculature of hypoxic rats , and ETA-receptor antagonists prevent/reverse chronic hypoxia-induced pulmonary hypertension . A HIF-1-binding site in the ET-1 gene promoter is required for hypoxia-induced transcription , suggesting that ET-1 mRNA expression by hypoxic pulmonary vascular ECs is mediated by HIF-1.
Expression of angiotensin-converting enzyme (ACE), which converts angiotensin I to angiotensin II, is also induced within the pulmonary vasculature of hypoxic rats . The administration of captopril, an ACE inhibitor, or losartan, a type 1 angiotensin receptor antagonist, also attenuates the development of hypoxic pulmonary hypertension . Angiotensin II, which induces vascular SMC hypertrophy, has recently been shown to induce HIF-1α expression . These results suggest that HIF-1 might be required for the angiotensin-induced hypertrophy of vascular SMCs in the hypoxic lung.
In addition to SMC hypertrophy, hypoxia is associated with increased pulmonary vascular tone, which also reduces luminal diameter. One determinant of vascular tone is the production by ECs of ET-1, a potent SMC vasoconstrictor. The activity of voltage-gated potassium (KV) channels is another important determinant of SMC membrane potential and pulmonary vasomotor tone. Acute hypoxia inhibits KV channel activity, resulting in the depolarization and vasoconstriction of pulmonary artery SMCs. Chronic hypoxia is associated with downregulation of KV1.2 and KV1.5 mRNA expression , but the involvement of HIF-1 or another hypoxia-induced transcription factor in this process has not yet been demonstrated.
NF-IL6, EGR-1, and HIF-1 mediate important physiological responses to chronic hypoxia in macrophages and pulmonary vascular ECs and SMCs. NF-IL6 might mediate adaptive anti-inflammatory responses, but definitive studies with knockout mice have not been performed. In contrast, analysis of Egr1 -/- and Hif1a +/- mice has provided definitive evidence that EGR-1 and HIF-1 mediate pathophysiologic responses to chronic hypoxia, suggesting that pharmacologic inhibition of these factors might be useful in the prevention or treatment of hypoxia-induced pulmonary vascular pathology.
Work in the author's laboratory is supported by grants from the National Institutes of Health (R01-DK39869 and R01-HL55338).
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