Oxygen-regulated transferrin expression is mediated by hypoxia-inducible factor-1

Identification of a hypoxia response element in the transferrin receptor gene

Expression of the transferrin receptor, which mediates iron uptake from transferrin, is negatively regulated post-transcriptionally by intracellular iron through iron-responsive elements in the 3'-untranslated region of the transferrin receptor mRNA. Transcriptional mechanisms are also involved in receptor expression, but these are poorly understood. In this study we have characterized the transferrin receptor promoter region and identified a functional hypoxia response element that contains a binding site for hypoxia-inducible factor-1 (HIF-1). Exposure of K562 and HeLa cells to hypoxia for 16 h resulted in a 2- to 3-fold increase in transferrin receptor mRNA expression. A motif with multipartite organization similar to the hypoxia response element of a number of hypoxia-inducible genes such as erythropoietin was identified within a 100-base pair sequence upstream of the transcriptional start site. Mutation of a site similar to the consensus HIF-binding site (HBS) in this motif attenuated the hypoxic response by 80%. Transient co-expression of the two HIF-1 subunits (HIF-1alpha and HIF-1beta) enhanced the wild type transferrin receptor promoter activity, but that which contained a mutated HBS yielded no such response. Electrophoretic mobility shift assays revealed that HIF-1 was stimulated and bound to the transferrin receptor HBS upon hypoxic challenge. Our results indicate that the transferrin receptor is a target gene for HIF-1.

Regulation of cardiovascular development and physiology by hypoxia-inducible factor 1

Hypoxia is an essential pathophysiologic component of ischemic cardiovascular disease. A better understanding of the molecular mechanisms underlying adaptive responses to hypoxia may lead to novel therapeutic strategies. Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic-helix-loop-helix-PAS domain transcription factor that mediates changes in gene expression in response to changes in O2 concentration. Genes that are transcriptionally activated by HIF-1 in hypoxic cells encode proteins that increase O2 delivery or allow metabolic adaptation to limited O2 availability. HIF-1 target genes include those encoding vascular endothelial growth factor (VEGF), erythropoietin, glucose transporters, and glycolytic enzymes. In anemic fetal sheep, increased myocardial vascularization was associated with concomitant increases in the expression of HIF-1 and VEGF. Expression of HIF-1 target genes was not induced by hypoxia in embryonic stem cells lacking expression of the O2-regulated HIF-1 alpha subunit. Mouse embryos lacking HIF-1 alpha expression arrested in their development by E9.0 and died by E10.5 with cardiovascular malformations and massive cell death throughout the embryo. These studies indicate that HIF-1 functions as a master regulator of O2 homeostasis that controls the establishment of essential physiologic systems during embryogenesis as well as their subsequent utilization during fetal and postnatal life.

Impaired physiological responses to chronic hypoxia in mice partially deficient for hypoxia-inducible factor 1alpha

Chronic hypoxia induces polycythemia, pulmonary hypertension, right ventricular hypertrophy, and weight loss. Hypoxia-inducible factor 1 (HIF-1) activates transcription of genes encoding proteins that mediate adaptive responses to hypoxia, including erythropoietin, vascular endothelial growth factor, and glycolytic enzymes. Expression of the HIF-1alpha subunit increases exponentially as O2 concentration is decreased. Hif1a-/- mouse embryos with complete deficiency of HIF-1alpha due to homozygosity for a null allele at the Hif1a locus die at midgestation, with multiple cardiovascular malformations and mesenchymal cell death. Hif1a+/- heterozygotes develop normally and are indistinguishable from Hif1a+/+ wild-type littermates when maintained under normoxic conditions. In this study, the physiological responses of Hif1a+/- and Hif1a+/+ mice exposed to 10% O2 for one to six weeks were analyzed. Hif1a+/- mice demonstrated significantly delayed development of polycythemia, right ventricular hypertrophy, pulmonary hypertension, and pulmonary vascular remodeling and significantly greater weight loss compared with wild-type littermates. These results indicate that partial HIF-1alpha deficiency has significant effects on multiple systemic responses to chronic hypoxia.

Oxygen tension modulates beta-globin switching in embryoid bodies

Little is known about the factors influencing the hemoglobin switch in vertebrates during development. Inasmuch as the mammalian conceptus is exposed to changing oxygen tensions in utero, we examined the effect of different oxygen concentrations on beta-globin switching. We used an in vitro model of mouse embryogenesis based on the differentiation of blastocyst-derived embryonic stem cells to embryoid bodies (EBs). Cultivation of EBs at increasing oxygen concentrations (starting at 1% O2) did not influence the temporal expression pattern of embryonic (betaH1) globin compared to the normoxic controls (20% O2). In contrast, when compared to normoxically grown EBs, expression of fetal/adult (betamaj) globin in EBs cultured at varying oxygen concentrations was delayed by about 2 days and persisted throughout differentiation. Quantitation of hemoglobin in EBs using a 2,7-diaminofluorene-based colorimetric assay revealed the appearence of hemoglobin in two waves, an early and a late one. This observation was verified by spectrophotometric analysis of hemoglobin within single EBs. These two waves might reflect the switch of erythropoiesis from yolk sac to fetal liver. Reduced oxygenation is known to activate the hypoxia-inducible factor-1 (HIF-1), which in turn specifically induces expression of a variety of genes among them erythropoietin (EPO). Although EBs increased EPO expression upon hypoxic exposure, the altered beta-globin appearance was not related to EPO levels as determined in EBs overexpressing EPO. Since mRNA from both mouse HIF-1alpha isoforms was detected in all EBs tested at different differentiation stages, we propose that HIF-1 modulates beta-globin expression during development.

Temporal, spatial, and oxygen-regulated expression of hypoxia-inducible factor-1 in the lung

Hypoxia-inducible factor (HIF)-1 is a basic helix-loop-helix transcription factor that transactivates genes encoding proteins that participate in homeostatic responses to hypoxia. Several of these downstream gene products, such as erythropoietin, vascular endothelial growth factor, heme oxygenase-1, and inducible nitric oxide synthase, may contribute to the pathogenesis of pulmonary hypertension. Previous studies demonstrated increased HIF-1 mRNA levels in rats and mice subjected to hypoxia. In this study, we have demonstrated spatial, temporal, and O2-dependent expression of HIF-1 protein. Immunoblot analysis revealed hypoxic induction of HIF-1 in all cultured pulmonary cell types assayed, including those derived from pulmonary arterial endothelium and smooth muscle, bronchial epithelium, alveolar macrophages, alveolar epithelium, and microvascular endothelium. In contrast to all other cell types, pulmonary arterial smooth muscle cells expressed HIF-1 under nonhypoxic conditions. Immunohistochemistry and immunoblot analysis of ferret lungs demonstrated pulmonary expression of HIF-1 in vivo. HIF-1 protein expression was induced maximally when lungs were ventilated with 0 or 1% O2 for 4 h. On reoxygenation, HIF-1 was rapidly degraded, with a half-life of <1 min. These findings demonstrate that HIF-1 expression is tightly coupled to O2 concentration in vivo and are consistent with the involvement of HIF-1 in the physiological and pathophysiological responses to hypoxia in the lung.

Hypoxia-inducible factor 1: master regulator of O2 homeostasis

Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that mediates essential homeostatic responses to reduced O2 availability in mammals. Recent studies have provided insights into the O2-dependent regulation of HIF-1 expression, target genes regulated by HIF-1, and the effects of HIF-1 deficiency on cellular physiology and embryonic development.

Mouse Hypoxia-Inducible Factor-1α Is Encoded by Two Different mRNA Isoforms: Expression From a Tissue-Specific and a Housekeeping-Type Promoter

Hypoxic induction of erythropoietin (Epo) and other oxygen-dependent genes is mediated by the hypoxia-inducible factor-1 (HIF-1), a heterodimeric transactivator consisting of an α and a β subunit. We previously found that the mouse gene encoding HIF-1α harbors two alternative first exons (I.1 and I.2), giving rise to two different HIF-1α mRNA isoforms. Here, we show by RNase protection analysis that the exon I.1-derived mRNA isoform is differentially expressed in mouse tissues, being highest in kidney, tongue, stomach, and testis, but undetectable in liver, whereas the exon I.2 mRNA isoform is ubiquitously expressed. Sequence and methylation analysis showed that, in contrast to exon I.1, exon I.2 resides within a region showing typical features of a CpG island, known to be associated with the 5′ end of housekeeping genes. We identified a 232-bp minimal exon I.2 promoter that strongly induced reporter gene expression in mouse L929 fibroblasts and Hepa1 hepatoma cells. In contrast to L929 cells, the exon I.1 promoter was inactive in Hepa1 cells and hypoxic exposure (1% O2) markedly reduced exon I.2 promoter activity in Hepa1 cells. Prolonged exposure of mice to hypoxia (7.5% O2 for up to 72 hours) also caused a decrease in liver HIF-1α mRNA, whereas aldolase mRNA levels increased. These findings might be related to the relatively low Epo levels in the adult liver.

Mouse Hypoxia-Inducible Factor-1α Is Encoded by Two Different mRNA Isoforms: Expression From a Tissue-Specific and a Housekeeping-Type Promoter

Hypoxic induction of erythropoietin (Epo) and other oxygen-dependent genes is mediated by the hypoxia-inducible factor-1 (HIF-1), a heterodimeric transactivator consisting of an α and a β subunit. We previously found that the mouse gene encoding HIF-1α harbors two alternative first exons (I.1 and I.2), giving rise to two different HIF-1α mRNA isoforms. Here, we show by RNase protection analysis that the exon I.1-derived mRNA isoform is differentially expressed in mouse tissues, being highest in kidney, tongue, stomach, and testis, but undetectable in liver, whereas the exon I.2 mRNA isoform is ubiquitously expressed. Sequence and methylation analysis showed that, in contrast to exon I.1, exon I.2 resides within a region showing typical features of a CpG island, known to be associated with the 5′ end of housekeeping genes. We identified a 232-bp minimal exon I.2 promoter that strongly induced reporter gene expression in mouse L929 fibroblasts and Hepa1 hepatoma cells. In contrast to L929 cells, the exon I.1 promoter was inactive in Hepa1 cells and hypoxic exposure (1% O2) markedly reduced exon I.2 promoter activity in Hepa1 cells. Prolonged exposure of mice to hypoxia (7.5% O2 for up to 72 hours) also caused a decrease in liver HIF-1α mRNA, whereas aldolase mRNA levels increased. These findings might be related to the relatively low Epo levels in the adult liver.