Abnormal angiogenesis and responses to glucose and oxygen deprivation in mice lacking the protein ARNT

Abstract

The arylhydrocarbon-receptor nuclear translocator (ARNT) is a member of the basic-helix-loop-helix-PAS family of heterodimeric transcription factors which includes the arylhydrocarbon receptor (AHR), hypoxia-inducible factor-1alpha (HIF-1alpha) and the Drosophila single-minded protein (Sim). ARNT forms heterodimeric complexes with the arylhydrocarbon receptor, HIF-1alpha, Sim and the PAS protein Per. In response to environmental pollutants, AHR-ARNT heterodimers regulate genes involved in the metabolism of xenobiotics, whereas ARNT-HIF-1alpha heterodimers probably regulate those involved in the response to oxygen deprivation. By generating a targeted disruption of the Arnt locus in the mouse, we show here that Arnt-/- embryonic stem cells fail to activate genes that normally respond to low oxygen tension. Arnt-/- ES cells also failed to respond to a decrease in glucose concentration, indicating that ARNT is crucial in the response to hypoxia and to hypoglycaemia. Arnt-/- embryos were not viable past embryonic day 10.5 and showed defective angiogenesis of the yolk sac and branchial arches, stunted development and embryo wasting. The defect in blood vessel formation in Arnt-/- yolk sacs is similar to the angiogenic abnormalities reported for mice deficient in vascular endothelial growth factor or tissue factor. On the basis of these findings, we propose a model in which increasing tissue mass during organogenesis leads to the formation of hypoxic/nutrient-deprived cells, the subsequent activation of ARNT, and a concomitant increase in the expression of genes (including that encoding vascular endothelial growth factor) that promote vascularization of the developing yolk sac and solid tissues.

The role of oxygen availability in embryonic development and stem cell function

“…While the role of mTOR and mTOR-associated proteins (raptor, rictor, and mLST8) are critical for embryonic development, their connection to changes in embryonic O 2 availability is not conclusive at this time. However, given the similarity of phenotypes between HIF-deficient and mTORC2-deficient embryos 22,26,101 , it seems plausible that mTOR is sensing O 2 in the early embryo to also regulate cardiovascular differentiation. By contrast, O 2 sensation by soluble guanylate cyclases appears to regulate neuronal function as opposed to neuronal development in nematodes.…”

“…This hypothesis was tested by generating HIF-1β (also known as aryl hydrocarbon receptor nuclear translocator [ARNT]) -deficient mice. These animals show lethality by embryonic day (E)10.5, due to vascular defects in the yolk sac, branchial arches, cranium, somites, and placenta 22,23 . Reminiscent of the scenario in tracheal morphogenesis, the initial development of vascular beds is intact in Arnt −/− embryos, but vessel remodelling is subsequently compromised.…”

“…Reminiscent of the scenario in tracheal morphogenesis, the initial development of vascular beds is intact in Arnt −/− embryos, but vessel remodelling is subsequently compromised. Furthermore, ARNT -deficient embryos have decreased VEGF mRNA and protein levels, which implies that VEGF secretion is modulated by naturally low O 2 microenvironments in the early conceptus [22][23][24] . Consistent with these findings, Iyer et al demonstrated that HIF-1α protein can be detected in E8-E18 mouse embryos 25 ; HIF-1α -deficient embryos show similar phenotypes to Arnt −/− mice, with defects in blood vessel formation and neural fold closure 25,26 .…”

Functional analyses of the TEL-ARNT fusion protein underscores a role for oxygen tension in hematopoietic cellular differentiation

“…The HIF1a/ARNT dimer activates transcription through the hypoxia response element (HRE: TACGTG) in the promoter or enhancer region of target genes that include vascular endothelial growth factor (VEGF), erythropoietin and phosphoglycerate kinase (PGK). In mammals, HIF1 is required for the establishment and utilization of the placenta, cartilage, erythroid, vascular, cardiac and respiratory systems (Maltepe et al, 1997;Iyer et al, 1998;Ryan et al, 1998;Kotch et al, 1999;Kline et al, 2002;Kojima et al, 2002;Ramirez-Bergeron et al, 2004). In keeping with this fundamental role in normal development, HIF1 activity has also been associated with human diseases.…”

Activation of the hypoxia‐inducible factor pathway induced by prolyl hydroxylase domain 2 deficiency enhances the effect of running training in mice

“…1997). These heterodimers activate the transcription of numerous hypoxia‐inducible genes (Wang & Semenza 1993, Semenza et al .…”