Hypoxic Regulation of Inducible Nitric Oxide Synthase via Hypoxia Inducible Factor-1 in Cardiac Myocytes

The relationship between hypoxia and regulation of nitric oxide synthase (NOS) in myocardial tissue is not well understood. We investigated the role of hypoxia inducible factor-1 (HIF-1) on expression of the inducible NOS (iNOS) in myocardial cells in vivo and in vitro. In situ hybridization in myocardial tissue from rats exposed to hypoxia for 3 weeks demonstrated increased iNOS mRNA expression. Northern analysis of RNA from hearts of those animals and from cells exposed to hypoxia for 12 hours in vitro demonstrated an increase of HIF-1 RNA expression. Electrophoretic mobility shift assays using oligonucleotides containing the iNOS HIF-1 DNA binding site and nuclear extracts from cardiac myocytes showed induction of specific DNA binding in cells subjected to hypoxia. Transient transfection of cardiac myocytes using the murine iNOS promoter resulted in a 3.43-fold increase in promoter activity under hypoxia compared with normoxia. Mutation or deletion of the HIF-1 site eliminated the hypoxic response. As cytokines have been shown to regulate iNOS expression in myocardial cells, cultured neonatal cardiac myocytes were stimulated with interleukin-1beta causing a dramatic induction of iNOS protein expression under normoxia, with further augmentation under hypoxia. Transient transfection of cells stimulated with interleukin-1beta showed an increased iNOS promoter activity under normoxic conditions compared with unstimulated cells, with a further increase in response to hypoxia, which was dependent on HIF-1. These results demonstrate that hypoxia causes an increase in iNOS expression in cardiac myocytes and that HIF-1 is essential for the hypoxic regulation of iNOS gene expression.