Transcription factor HIF-1 is a key determinant of oxygen-dependent gene regulation. Suppression of HIF-1α is important for exploring HIF-1-dependent processes and for interfering with hypoxia-induced pathophysiological events. This study applied RNA-interference targeting HIF-1α to the human lung A549 cell line. Transfection of HIF-1α-siRNA reduced HIF-1α synthesis as measured on mRNA and protein level by realtime RT-PCR, Western blot, and immuncytochemistry. A time kinetic for hypoxic stabilization of HIF-1α protein and its inhibition by HIF-1α-siRNA is included. Hypoxic induction of HIF-1-controlled target genes as heme oxygenase I (HO-1), phosphoglycerate kinase (PGK), and vascular endothelial growth factor (VEGF) was markedly attenuated by HIF-1α-siRNA treatment. Correspondingly, gene activation via hypoxia-responsive-element, as shown by reporter gene assay, was inhibited by HIF-1α-siRNA. Moreover, this approach was found to suppress the shift from from S-phase to G 1-phase observed in A549 cells in response to hypoxia, supporting a role of HIF-1α in oxygen-dependent cell cycle regulation. © 2003 Elsevier Inc. All rights reserved.