Hypoxia-inducible Factor-1 Mediates Transcriptional Activation of the Heme Oxygenase-1 Gene in Response to Hypoxia

Abstract

Exposure of rats to hypoxia (7% O2Low cellular oxygen tension is a feature of both physiological conditions, such as adaptations to high altitude and physical endurance exercise (1), and pathophysiological conditions including ischemia, fibrosis (2), and neoplasia (3). Mammalian cells respond to hypoxia in part by increased expression of several genes that encode both tissue-specific and ubiquitous proteins (4). These proteins participate in diverse biological processes including erythropoiesis, which enhances the oxygen carrying capacity of the blood; angiogenesis, which permits delivery of oxygen carrying blood to hypoxic sites; glycolysis, as a means of energy production; xenobiotic detoxification; and cellular adaptation to stress. Hypoxia-inducible proteins within these respective categories include erythropoietin (EPO) 1 (5), vascular endothelial growth factor (6), glycolytic enzymes (7-9), NAD(P)H:quinone oxidoreductase (10), and heat shock proteins (11,12). Where examined, increased expression of specific proteins in response to hypoxia is regulated primarily at the level of gene transcription (although posttranscriptional mechanisms have also been characterized).Another stress-associated protein whose expression is stimulated by hypoxia is heme oxygenase-1 (HO-1) (13, 14). HO-1, a microsomal membrane enzyme, catalyzes the first and ratelimiting reaction in heme catabolism, the oxidative cleavage of b-type heme molecules to yield equimolar quantities of biliverdin, carbon monoxide (CO), and iron. Biliverdin is subsequently converted to bilirubin by the action of biliverdin reductase. The expression of HO-1 is dramatically induced not only by the substrate, heme, but a variety of stress-associated agents, including heavy metals, hyperthermia, and UV irradiation (reviewed in Maines (15)). A common feature among these inducers, including heme, is that they generate reactive oxygen species and/or diminish glutathione levels. This correlation and the observation that bilirubin functions as an antioxidant (16) has led to the hypothesis that induction of HO-1 is part of a general response to oxidant stress and that this enzyme plays a protective role during such conditions (17)(18)(19).Stimulation of HO-1 expression by most if not all inducers is controlled primarily at the level of gene transcription and in our studies on the regulation of the mouse HO-1 gene, we have identified two 5Ј distal enhancer regions, SX2 and AB1, that mediate gene activation by a variety of pro-oxidants including heme, heavy metals, TPA, hydrogen peroxide, and LPS (20 -23). The mechanism of HO-1 induction by hypoxia has not been investigated and because this induction has been proposed to occur as a consequence of oxidative stress (13), we examined the role of the SX2 and AB1 enhancers in hypoxia-dependent gene activation. In this report we show that these enhancers do not mediate transcriptional activation of the HO-1 gene in response to hypoxia. Rather, this induction is mediated by a 163-bp fragment located directly downstream of ...

Reciprocal Effects of Micro-RNA-122 on Expression of Heme Oxygenase-1 and Hepatitis C Virus Genes in Human Hepatocytes

“…Multiple lines of evidence support the idea that biliverdin, bilirubin, and CO contribute to the physiological functions of HO-1, including anti-oxidative, anti-inflammatory, anti-proliferative, and anti-apoptotic effects (21)(22)(23)(24)(25)(26)(27). HO-1 is widely distributed in tissues and robustly induced in many cells by its substrate heme (28)(29)(30) and by numerous stressful stimuli such as ultraviolet radiation, heat shock, inflammation, hypoxia, and various oxidative agents (31)(32)(33)(34)(35)(36)(37). The physiological function of HO-1 has been confirmed by observations in HO-1 knockout mice (38;39) and in one child, the product of a consanguineous mating, with severe genetic deficiency of HO-1 activity (40;41).…”

Heme: a versatile signaling molecule controlling the activities of diverse regulators ranging from transcription factors to MAP kinases

“…The regulation of HO-1 expression by these factors is not only cell type-dependent but also species-dependent [162,164]. For example, hypoxia induces HO-1 expression in rodent, bovine and monkey cells, but represses HO-1 expression in several human cell lines, including lung cancer A549 cells, umbilical vein endothelial cells and glioblastoma cells [72,[165][166][167][168][169][170]. A detailed discussion of HO-1 regulation can be found in an excellent review written by Sikorki et al [162].…”

Hypoxia upregulates hypoxia inducible factor (HIF)-3α expression in lung epithelial cells: characterization and comparison with HIF-1α

“…By contrast, under hypoxia or in the presence of iron chelators, the degradation of HIF-1α is prevented [7,11,12]. As a result, this stabilization initiates a multi-step pathway of activation of HIF-1α that includes hypoxia-dependent nuclear translocation and dimerization with ARNT to interact with hypoxia responwww.cell-research.com | Cell Research Qi Fang Li et al 549 npg sive element (HRE) of target genes such as erythropoietin (EPO) [13], vascular endothelial growth factor (VEGF) [14], inducible nitric-oxide synthase [15], heme oxygenase 1 [16], and so on. Taken together, it comes as no surprise that the HIF-dependent hypoxic response pathway plays a prominent role in mediating the consequences of many disease states, including cerebral and myocardial ischemia, pulmonary hypertension, and tumorigenesis [17].…”