Regulation of insulin-like growth factor-binding protein-1 by nitric oxideunder hypoxic conditions

Nitric oxide (NO) is believed to play an important, but as yet undefined, r ole in regulating hypoxia inducible gene expression. Recently, we have repo rted evidence suggesting that the human insulinlike growth factor-binding p rotein-1 (IGFBP-1) gene is directly regulated by hypoxia through the hypoxi a-inducible factor-1 pathway. The goal of the current study was to investig ate NO regulation of hypoxic induction of IGFBP-1 gene expression using Rep G2 cells, a model system of hepatic gene expression. We report that a NO ge nerator, sodium nitroprusside, significantly diminishes hypoxic activation of IGFBP-1 protein and messenger ribonucleic acid expression. Furthermore, these effects are independent of guanylate cyclase/cGMP signaling, as two d ifferent inhibitors, LY 83583, a specific inhibitor of guanylate cyclase, a nd KT 5823, a protein kinase G inhibitor, had no effect on IGFBP-1 inductio n by hypoxia. Hypoxic induction of a reporter gene containing four tandemly Ligated hypoxia response elements was completely blocked by sodium nitropr usside, but not by 8-bromo-cGMP, an analog of cGMP. These results suggest t hat NO blocks hypoxic induction of IGFBP-1 by a guanylate cyclase/cGMP-inde pendent pathway, possibly at the level of oxygen sensing. The impaired hypo xia regulation of IGFBP-1 by nitric oxide may play a key role in the hyperi nduction of IGFBP-1 observed in pathophysiological conditions such as fetal hypoxia and preeclampsia where dysregulation of NO has been observed.