Hypoxia-inducible factor-1 (HIF-1) alpha is the major transcription factor involved in the adaptive response to hypoxia. The purpose of this study was to investigate whether HIF 1-alpha protects HP75 cells, pituitary adenoma cell line from hypoxia induced apoptosis. HP75 was transfected with siRNA targeting HIF 1-alpha mRNA sequences or scrambled RNA duplexes, followed by subjected to hypoxia (1% oxygen) for 24 h, compared with normoxia (21%). The efficacy of RNAi was assessed via real-time RT-PCR and immunohistochemistry. Apoptosis was determined by Tdt-mediated dUTP nick end-labeling (TUNEL) assay and agarose gel electrophoresis. Membrane cDNA microarray was examined to detect gene profiling among the cell in normoxia, hypoxia, or hypoxia following the RNAi. A significantly greater proportion of HP75 cells transfected with specific siRNA duplexes and subsequently exposed to hypoxia demonstrated apoptosis to a large extent when compared with non-transfected cells. Transfection with specific siRNA duplexes knocked down HIF 1-alpha mRNA and protein expression in hypoxia-exposed cells by approximately 80%, whereas transfection with scrambled siRNA duplexes had no noticeable effect on HIF 1-alpha expression. Microarray analysis indicated that HIF1-alpha down-regulated caspase-10. These findings strongly suggest that HIF 1-alpha exerts an antiapoptotic role in HP75 in hypoxia.