Dinitrosyl Iron Complexes and their Role in Patho-physiological Conditions

Nitric oxide (•NO) is a small free radical known to play an important role in cancer. Inside the cell, one of its primary interactions occurs with iron and thiols leading to the formation of dinitrosyl iron complexes (DNIC). Formation of these complexes renders iron inactive towards the various physiological functions it performs. Recently, N-myc downstream regulated gene 1 (NDRG1), a well known metastasis suppressor gene, was shown to be up-regulated by iron chelation and hypoxia inducible factor 1 alpha (HIF-1α) stabilization. We also observed HIF-1α stabilization in cancer cells on exposure to •NO. Given the similarities between NDRG1 regulation and the actions of •NO in cancer cells, we hypothesized that exposure of nitric oxide to cells results in NDRG1 upregulation, in an iron dependent manner. We observed time (0-24 hours) and dose (0-1mM) dependent increase in NDRG1 expression at both mRNA and protein level in cancer cells treated with •NO. NDRG1 protein, once expressed, was stable for a prolonged period of time (~48hours). As protein is a functional product of the gene, these results show that NDRG1 could be a clinically significant target for nitric oxide. We observed that, •NO-mediated NDRG1 up-regulation was an iron dependent process, possibly via formation of DNIC. In addition, we observed ~50% decrease in migration/invasion potential without affecting the viability of cancer cells on exposure to •NO. In another set of studies, we examined the role of DNIC in iron mediated oxidative stress. We observed that cells treated with •NO were partially protected against ROS mediated cytotoxicity (H2O2 exposure). Furthermore, pretreatment with •NO markedly decreases the oxidative stress (proportional to •OH generation) induced by peroxide treatment. Nitric oxide concentrations at which we observe cytoprotective effects correlated with the concentration-dependent increase in DNIC formation. We observed that iron supplemented cells showed increased oxidative stress and cytotoxicity on peroxide treatment, and higher doses of •NO-donors were required to circumvent this toxicity. These results support the hypothesis that •NO can act as an antioxidant by its ability to sequester cellular iron. In conclusion, these results show a novel mechanism for nitric oxide mediated effects in a biological system. It also signifies the importance of DNIC in the intracellular environment.