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

Nitric oxide (NO) is believed to play an important, but as yet undefined, role in regulating hypoxia inducible gene expression. Recently, we have reported evidence suggesting that the human insulin-like growth factor-binding protein-1 (IGFBP-1) gene is directly regulated by hypoxia through the hypoxia-inducible factor-1 pathway. The goal of the current study was to investigate NO regulation of hypoxic induction of IGFBP-1 gene expression using HepG2 cells, a model system of hepatic gene expression. We report that a NO generator, 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 different inhibitors, LY 83583, a specific inhibitor of guanylate cyclase, and KT 5823, a protein kinase G inhibitor, had no effect on IGFBP-1 induction by hypoxia. Hypoxic induction of a reporter gene containing four tandemly ligated hypoxia response elements was completely blocked by sodium nitroprusside, but not by 8-bromo-cGMP, an analog of cGMP. These results suggest that NO blocks hypoxic induction of IGFBP-1 by a guanylate cyclase/cGMP-independent pathway, possibly at the level of oxygen sensing. The impaired hypoxia regulation of IGFBP-1 by nitric oxide may play a key role in the hyperinduction of IGFBP-1 observed in pathophysiological conditions such as fetal hypoxia and preeclampsia where dysregulation of NO has been observed.