ABNORMAL ANGIOGENESIS AND RESPONSES TO GLUCOSE AND OXYGEN DEPRIVATIONIN MICE LACKING THE PROTEIN ARNT

The arylhydrocarbon-receptor nuclear translocator (ARNT) is a member o f the basic-helix-loop-helix-PAS family of heterodimeric transcription factors which includes the arylhydrocarbon receptor (AHR), hypoxia-in ducible factor-1 alpha (HIF-1 alpha) and the Drosophila single-minded protein (Sim)(1-4), ARNT forms heterodimeric complexes with the arylhy drocarbon receptor, HIF-1 alpha, Sim and the PAS protein Per(2,4-6). I n response to environmental pollutants, AHR-ARNT heterodimers regulate genes involved in the metabolism of xenobiotics(7-9), whereas ARNT-HI F-1 alpha heterodimers probably regulate those involved in the respons e to oxygen deprivation(10-13). By generating a targeted disruption of the Arnt locus in the mouse, we show here that Arnt(-/-) embryonic st em cells fail to activate genes that normally respond to low oxygen te nsion, Arnt(-/-) ES cells also failed to respond to a decrease in gluc ose concentration, indicating that ARNT is crucial in the response to hypoxia and to hypoglycaemia. Arnt(-/-) embryos were not viable past e mbryonic day 10.5 and showed defective angiogenesis of the yolk sac an d branchial arches, stunted development and embryo wasting, The defect in blood vessel formation in Arnt(-/-) yolk sacs is similar to the an giogenic abnormalities reported for mice deficient in vascular endothe lial growth factor(14,15) or tissue factor(16). On the basis of these findings, we propose a model in which increasing tissue mass during or ganogenesis leads to the formation of hyoxic/nutrient-deprived cells, the subsequent activation of ARNT, and a concomitant increase in the e xpression of genes (including that encoding vascular endothelial growt h factor) that promote vascularization of the developing yolk sac and solid tissues.