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