The better adaptation of native Tibetans to hypoxia is thought to be partly due to improved umbilical circulation, which results in reduced pre- and postnatal fatalities. We hypothesized that the difference in umbilical circulation between native Tibetans and other high-altitude inhabitants was due to differences in the expression of hypoxia-induced factor (HIF-1) and its target genes vascular endothelial growth factor (VEGF) and inducible nitric oxide synthase (iNOS). We tested this hypothesis by examining the effect of hypoxia on the expression of HIF- 1a, VEGF, and iNOS in cultured umbilical venous endothelial cells (UVECs) from native Tibetans and immigrant Hans. UVECs were collected and cultured under hypoxic (0.5% oxygen) or normoxic conditions for 2, 4, 12 and 24 h. The mRNA levels of HIF-1a, VEGF, endothelialnitricoxidesynthase(eNOS)andiNOSandtheproteinlevelofHIF-1aweredeterminedwithRT-PCRandWesternblotanalyses, respectively. In both immigrant Han and Tibetans, HIF-1a mRNA was constitutively expressed under normoxic condition, and remained constant after hypoxic exposure. In contrast, HIF-1a protein was undetectable under normoxic condition, but underwent dynamic changes in response to hypoxia. It was induced at 4 h, peaked at 12 h, and remained elevated at 24 h. Concurrent with the induction of HIF-1a protein, the mRNA levels of VEGF and iNOS were also up-regulated whereas that of eNOS was down-regulated. The lack of a hypoxia-related difference in the expression of HIF-1a and its target genes suggests that HIF-1a does not play a critical role in high altitude adaptation. Alternative mechanisms may be responsible for the better adaptation of native Tibetans. D 2005 Elsevier Inc. All rights reserved.