Hypoxia-inducible factor 1 (HIF-1) is a key regulator of O(2) homeostasis, which regulates the expression of several genes linked to angiogenesis and energy metabolism. Tumor hypoxia has been shown to be associated with poor prognosis in a variety of tumors, and HIF-1 induced by hypoxia plays pivotal roles in tumor progression. The presence of putative HIF-1-binding sites on the promoter of human telomerase reverse transcriptase gene (hTERT) prompted us to examine the involvement of HIF-1 in the regulation of hTERT and telomerase in tumor hypoxia. The telomeric repeat amplification protocol (TRAP) assay revealed that hypoxia activated telomerase in cervical cancer ME180 cells, with peak induction at 24-48 h of hypoxia. Notably, hTERT mRNA expression was upregulated at 6-12 h of hypoxia, concordant with the elevation of HIF-1 protein levels at 6 h. hTERT protein levels were subsequently upregulated at 24 h and later. Luciferase assays using reporter plasmids containing hTERT core promoter revealed that hTERT transcription was significantly activated in hypoxia and by HIF-1 overexpression, and that the two putative binding sites within the core promoter are responsible for this activation. Chromatin immunoprecipitation assay identified the specific binding of HIF-1 to these sites (competing with c-Myc), which was enhanced in hypoxia. The present findings suggest that hypoxia activates telomerase via transcriptional activation of hTERT, and that HIF-1 plays a critical role as a transcription factor. They also suggest the existence of novel mechanisms of telomerase activation in cancers, and have implications for the molecular basis of hypoxia-induced tumor progression and HIF-1-based cancer gene therapy.