Brief Review |
From the Departments of Physiology and Cellular Biophysics, Surgery and Medicine, College of Physicians and Surgeons of Columbia University, New York, NY (S.-F.Y., D.M.S., D.J.P.), the Departments of Immunology and Vascular Biology, The Scripps Research Institute and La Jolla Cancer Research Foundation, La Jolla, CA (N.M.), and the Departments of Pathology and Biochemistry, University of New Mexico School of Medicine, Albuquerque, NM (W.K.).
Correspondence to Dr Shi-Fang Yan/Dr David J. Pinsky, Departments of Physiology and Cellular Biophysics, P&S 17-401, College of Physicians and Surgeons of Columbia University, 630 West 168th Street, New York, New York. E-mail djp5{at}columbia.edu
| Abstract |
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Key Words: tissue factor hypoxia ischemia Egr-1 PAI-1
| Introduction |
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Nonetheless, links between thrombosis and hypoxemia/stasis have remained strong even in the absence of mechanistic explanations. Studies in a canine model of limb immobilization showed stasis to be associated with a rapid fall in venous oxygen tension to virtually undetectable levels in the affected extremity.2 Furthermore, hypoxemia was most severe in proximity to venous valve cusps, and nascent thrombi appeared to form on the apparently intact vessel wall at the parietal aspect of the valve cusp (this occurred during the period of hypoxemia). These accumulations of fibrin contained peripheral blood cells and erythrocytes and were speculated to result from hypoxemia-induced perturbation of the vessel wall. In fact, when untraumatized canine leg veins were exposed in vivo to intermittent periods of hypoxemia and reoxygenation, venous thrombi were shown to occur.9 Therefore, although in selected circumstances hypoxemia alone is sufficient to cause venous thrombosis, blood stasis results in ischemia that is associated with a myriad of changes in the vascular microenvironment, including diminished aerobic metabolism and accumulation of waste products. Thus the ischemic environment induces cell stress on many levels in addition to a contribution resulting from diminished blood oxygen. Another setting in which hypoxia has been considered to have a central role concerns vascular perturbation in response to high altitude, termed hypobaric hypoxia. Mountain climbing and other activities at high altitude are associated with vascular dysfunction, both prothrombotic events and increased vascular permeability (high altitude pulmonary and cerebral edema),10 11 12 13 and these have been recently documented in graphic detail in the lay press.14
In the setting of ischemia, the most striking tissue injury occurs during reperfusion when leukocytes pour into the previously unperfused zone. We propose that mechanisms underlying pathologic events whose consequences are exaggerated during reperfusion are set in motion during the previous period of hypoxemia. An example of this principle is the critical period during organ preservation, when hypoxemia and stasis prime the graft vasculature for damage during reperfusion.15 Such tissue injury occurring as soon as blood flow to the graft is reestablished is limited by strategies to diminish preservation-induced vascular dysfunction with more optimal preservation solutions during the period of organ storage. Our brief review will focus on mechanisms through which hypoxemia promotes fibrin formation in vivo and the results of recent studies to elucidate the bases for such activation of the procoagulant pathway.
| A Model of Hypoxia-Induced Fibrin Deposition |
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| Hypoxia-Associated Expression of Tissue Factor |
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To test this hypothesis, tissue factor expression in hypoxic
murine lung was analyzed.20 Northern
analysis showed
20-fold increased tissue factor transcripts
in hypoxic lung from wild-type animals compared with normoxic controls
(Figure 2A
). Immunohistochemical studies
demonstrated increased tissue factor in hypoxic lung, colocalized with
mononuclear phagocytes20 from wild-type mice, compared
with normoxic controls. In vitro studies confirmed that mononuclear
phagocytes placed in an hypoxic environment demonstrate transcriptional
upregulation of tissue factor mRNA.20 These data
implicating tissue factor expression in hypoxia-driven
thrombosis are especially provocative, given recent data
showing the important role tissue factor plays in maintaining normal
hemostasis. When tissue factor is expressed at very low levels in
genetically altered mice, they exhibit a hemorrhagic
tendency.21
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In depth exploration of the mechanism by which hypoxia
increases tissue factor transcription led to the identification of
Egr-1 as the primary driving motif underlying hypoxia-induced
tissue factor transcription. These data are in concordance with recent
studies identifying an association between Egr-1 and vascular
injury.22 23 Using double-stranded DNA probes from the
tissue factor promoter, electrophoretic mobility shift assays with
extracts from mononuclear phagocytes showed an apparent increase in Egr
DNA binding activity with no change in the DNA binding activity for Sp1
sites. The most striking data were obtained using consensus probes for
Sp1 or Egr-120 ; nuclear extracts showed no increase in the
gel shift band with Sp1 in response to hypoxia. In contrast,
similar experiments with the radiolabeled consensus Egr probe showed a
gel shift band (supershifted with antiEgr-1 antibody) in nuclear
extracts from hypoxic cultures, although not in normoxic counterparts.
Finally, transient transfection experiments were performed with
constructs promoter-reporter gene constructs derived from the serum
response region of the tissue factor promoter (Figure 2B
). Increased expression of the luciferase reporter was seen
with the wild-type construct, in which Sp1 and Egr-1 sites were intact,
whereas mutational inactivation of Egr-1 sites blocked
hypoxia-enhanced gene expression. Inactivation of Sp1 sites
prevented both basal and hypoxia-induced expression of
luciferase. Transfection studies with various promoter-luciferase
reporter constructs showed that only constructs with an intact Egr site
displayed increased expression in hypoxia.20 These
data emphasized the role of Egr-1 in hypoxia-modulated
expression of tissue factor.
In vivo studies confirmed the biological importance of
Egr-1driven tissue factor transcription by hypoxia;
homozygous Egr-1 null animals placed in a hypoxic environment showed a
much smaller increment in tissue factor mRNA, with virtually no change
in tissue factor antigen (Figure 2A
) and virtually no fibrin
deposition compared with their counterparts left in a normoxic
environment.20 Studies are in progress to trace the
sequence of events increasing Egr-1 activation in hypoxia, and
preliminary results indicate that oxygen deprivation
enhances de novo Egr-1 synthesis and that this is driven by binding of
ternary complex factor to ets/SRE sites in the Egr-1
promoter.24
| Hypoxia-Associated Suppression of Fibrinolysis |
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| Conclusion/Hypothesis |
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Previous in vitro studies suggested a central role for Egr-1 in multiple cellular homeostatic events. Initially, Egr-1 was believed to play a role in macrophage differentiation,36 although this has been called into question by a more recent report.37 These possibilities were markedly drawn into focus after production of Egr-1 null mice, which appear to develop normally, and have only shown a phenotype after environmental stress. For example, female Egr-1 null mice are sterile due to luteinizing hormone-ß deficiency, an effect due to decreased transcription of the gene. Our studies have shown a central role for Egr-1 in monocyte expression of tissue factor after induction of hypoxia. In view of the multiple stimuli that have been shown to activate Egr-1, an immediate-early gene, it is unlikely that oxygen deprivation directly induces activation of Egr-1. Rather, hypoxic stress is likely to generate conditions leading to Egr-1 activation and triggering of downstream mechanisms, including tissue factor transcription-translation. In contrast to the apparently positive impact of HIF-1mediated events on adaptation to hypoxia, the beneficial effects of tissue factor expression in monocytes and smooth muscle cells are less clear. Although local promotion of clotting would serve to isolate an ischemic area, the negative impact of vascular fibrin deposition eventuating in occlusive thrombosis can have obvious deleterious consequences. Egr-1 expression in hypoxia might be predicted to impact on cell-cell interactions through changes in the expression of cell adherence molecules and inflammatory mechanisms through effects on proinflammatory cytokines,38 although definite proof of Egr-1 involvement in these events remains to be tested in Egr-1 null mice. Thus Egr-1-initiated mechanisms may subserve a quite different facet of the adaptive response to oxygen deprivation compared with that associated with HIF-1. However, recognition of a role for Egr-1 in potentially pathologic changes associated with hypoxia could point to novel approaches to limiting these events, ie, preventing activation of coagulation in hypoxia by suppressing Egr-1 activation rather than by interfering directly with the procoagulant mechanism.
The data presented in our brief review provide the
outline of a pathway through which hypoxia triggers de novo
expression of tissue factor in mononuclear phagocytes and smooth muscle
cells (Figure 4
). Lungs, which possess
tissue-based (alveolar) macrophages in great abundance, are
likely to recruit even more macrophages to intravascular
locations through expression of the monocyte chemoattractant
JE/monocyte chemoattractant protein-1 (MCP-1).
Hypoxia-associated activation of Egr-1 causes transcription and
subsequent cell surface expression of tissue factor in both vascular
smooth muscle cells and macrophages, which can initiate the
local procoagulant response. Further amplification of the amount of
fibrin deposited could be due to alterations in the balance of
fibrinolytic mediators, such as may occur by increased expression of
plasminogen activator inhibitor-1,
thereby attenuating removal of vascular fibrin. It might be expected
that any prothrombotic tendency, such as that in mice that have a
targeted point mutation in the thrombomodulin gene,16
would display an enhanced procoagulant response in this model. Thus
patients with the Factor V Leiden mutation, those deficient in protein
S, or those with other defects manifest as diminished
endogenous antithrombotic activity would be vulnerable to
enhanced fibrin deposition at hypoxemic sites in the vasculature.
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This initial picture of hypoxia-associated triggering of the procoagulant pathway raises many questions. For example, by what mechanisms are monocytes attracted and retained in hypoxemic vasculature? In a previous study, we observed that hypoxia increased transcripts for the macrophage chemoattractant JE/MCP-1 in cultured endothelial cells35 and that JE/MCP-1 antigen was increased in hypoxic lung (Lawson et al, unpublished observation, 1995). Thus it is possible that endothelial production of JE/MCP-1 draws in macrophages and serves as a cofactor in their activation and retention. JE/MCP-1 produced by hypoxic endothelium could also have an integral role in increased expression of tissue factor levels observed in smooth muscle cells39 40 in hypoxemic vasculature. Aside from these questions of the basic biology of the procoagulant response, recognition of the relative vulnerability of the lung to hypoxemia-associated fibrin deposition in the mouse may also provide important insights. Although reasons for this are unclear, the combination of the lung's rich vasculature and the intensity of the vasoconstrictor response to hypoxia, in contrast to vasodilation in the systemic circulation, may serve to magnify activation of coagulation. Taken together, the series of events causing expression of tissue factor in the hypoxemic vasculature, especially in mononuclear phagocytes and smooth muscle cells, provides a new biologic context to consider mechanisms underlying and possible interventions to prevent pathologic thrombosis in settings of oxygen scarcity.
| Acknowledgments |
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Received September 16, 1998; accepted February 8, 1999.
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