Inhibition of Hypoxia-inducible Factor 1 Activation by Carbon Monoxide and Nitric Oxide

Abstract

It has been proposed that cells sense hypoxia by a heme protein, which transmits a signal that activates the heterodimeric transcription factor hypoxia-inducible factor 1 (HIF-1), thereby inducing a number of physiologically relevant genes such as erythropoietin (Epo). We have investigated the mechanism by which two heme-binding ligands, carbon monoxide and nitric oxide, affect oxygen sensing and signaling. Two concentrations of CO (10 and 80%) suppressed the activation of HIF-1 and induction of Epo mRNA by hypoxia in a dose-dependent manner. In contrast, CO had no effect on the induction of HIF-1 activity and Epo expression by either cobalt chloride or the iron chelator desferrioxamine. The affinity of CO for the putative sensor was much lower than that of oxygen (Haldane coefficient, ~0.5). Parallel experiments were done with 100 μM sodium nitroprusside, a nitric oxide donor. Both NO and CO inhibited HIF-1 DNA binding by abrogating hypoxia- induced accumulation of HIF-1α protein. Moreover, both NO and CO specifically targeted the internal oxygen-dependent degradation domain of HIF-1α, and also repressed the C-terminal transactivation domain of HIF- 1α. Thus, NO and CO act proximally, presumably as heme ligands binding to the oxygen sensor, whereas desferrioxamine and perhaps cobalt appear to act at a site downstream.