Heme oxygenase-1 is an inducible cytoprotective gene, although its induction by environmental factors is not completely understood. This study aimed to ascertain if specific nutritive factors or related compounds influence heme oxygenase-1 expression. In HCT-116 cells, cadmium increased heme oxygenase-1 enzymatic activity. This effect of cadmium was weaker in cells made iron-deficient with the iron chelator, desferrioxamine, which was associated with repression of heme oxygenase-1 protein and mRNA expression. The repression by desferrioxamine of cadmium-induced heme oxygenase-1 upregulation was reversed upon iron replenishment of the cells. Additionally, it was found that thiol antioxidants inhibited the heme oxygenase-1 upregulation caused by cadmium and also by ethacrynic acid, which each decreased intracellular glutathione as did buthionine sulfoxamine. Interestingly, cadmium and ethacrynic acid increased nuclear translocation of Nrf2 and subsequent heme oxygenase-1 expression, but buthionine sulfoxamine did not. Furthermore, NADPH oxidase inhibitors (diphenyleneiodonium and apocynin, and a superoxide scavenger (Tiron) inhibited cadmium-induced upregulation of heme oxygenase-1. Diphenyleneiodonium was the most potent and inhibited NADPH-cytochrome P450 reductase as well, whereas apocynin and Tiron did not. It is concluded that adequate amounts of iron, which at the atomic level can serve as the pivotal element of heme in NADPH oxidase, must be present in cells to permit what appears to be thiol redox-sensitive, NADPH oxidase-dependent upregulation of heme oxygenase-1. Thus, these findings are significant because they suggest that cells without adequate iron would be unable to fully express the stress gene, heme oxygenase-1, when confronted with the toxic metal, cadmium.

Trace metals can have contrasting biological effects. Iron and cadmium are normally regarded as a crucial micronutrient and noxious environmental agent, respectively. However, it is unknown if iron is required to help defend cells against cadmium. Thus, the effect of cadmium on cytoprotective heme oxygenase‐1 (HO‐1) during cellular iron deficiency caused by iron chelators (deferoxamine and 2,2′‐dipyridyl) was investigated in HCT‐116 cells. Exposing cells to cadmium induced HO‐1 enzyme activity and increased HO‐1 protein expression, which were attenuated by thiol antioxidants and also by chemical inhibitors of the flavoheme protein, NADPH oxidase (NOX). In iron‐deficient cells exposed to cadmium, induction of HO‐1 mRNA, protein, and enzyme activity were all blunted. Also, nuclear translocation of the redox‐sensitive transcription factor, Nrf2, which is known to activate the HO‐1 gene, was diminished. But, after treating the iron‐deficient cells with ferrous sulfate to restore positive iron balance, all of the above molecular events were not impaired upon exposing the rescued cells to cadmium. Zinc or copper treatment was ineffective. These results suggest that iron, being a critical element of the NOX catalytic subunit, plays a pivotal indirect role in the capacity of cadmium and potentially other xenobiotics to optimally upregulate HO‐1 gene expression. Supported by NRI USDA CSREES (2006‐35200‐16578).