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The effects of cadmium on heme oxygenase-1 in HCT116 human colon epithelial cells with or without iron deficiency

UNCG Author/Contributor (non-UNCG co-authors, if there are any, appear on document)
Chengzhi Lai (Creator)
Institution
The University of North Carolina at Greensboro (UNCG )
Web Site: http://library.uncg.edu/
Advisor
George Loo

Abstract: Metals may have contrasting biological effects. Iron, a component of a variety of iron-containing proteins, plays an important role in maintaining a healthy human body. In contrast, cadmium, a contaminant and carcinogen, has been considered one of the most toxic elements in the environment. As a defending mechanism against the exposure to cadmium, cells increase expression of cytoprotective genes such as heme oxygenase-1 (HO-1). Although both the importance of iron and the toxicity of cadmium are well known, it is not clear whether iron is required for the defending mechanism (that is, the upregulation of HO-1) against the toxicity of cadmium. In my thesis project, the effects of cadmium on HO-1 in HCT116 human colon epithelial cells with or without iron deficiency were investigated. It was found that cadmium upregulated HO-1 mRNA and protein expression and enzyme activity, but these effects were decreased by desferoxamine (DFO), an iron chelator, suggesting iron plays a critical role in cadmium-induced upregulation of HO-1. This conclusion was supported by two other observations: 1) another iron chelator, 2',2'-dipyridyl (DPD), also decreased the upregulating effects of cadmium on HO-1 mRNA and protein expression; 2) iron sulfate, but not zinc sulfate and copper sulfate, restored the upregulating effects of cadmium on HO-1 mRNA and protein expression and enzyme activity in iron-deficient cells caused by the pretreatments with iron chelators, DFO or DPD. Further experiments were conducted to help explain the observations. There were two primary findings: 1) cadmium decreased intracellular glutathione levels, similar in the effects of glutathione inhibitors, ethacrynic acid (EA) and buthionine sulfoximine (BSO); however, only cadmium and EA, but not BSO, increased the expression of HO-1 mRNA and the nuclear expression of nuclear factor-E2-related factor-2 (Nrf-2), suggesting that cadmium may have the same effect as EA to directly react with intracellular glutathione; 2) being similar in effect to iron chelators, NADPH oxidase (NOX) inhibitors such as apocynin and diphenyleneiodonium (DPI), and superoxide scavenger, tiron, decreased the upregulating effects of cadmium on HO-1 mRNA and protein expression and enzyme activity. Compiled together, the results suggest that NOX-produced ROS play an important role in cadmium-induced HO-1 upregulation; cadmium induces intracellular accumulation of ROS by depleting intracellular glutathione, and increases nuclear Nrf-2 expression, which all lead to the upregulation of HO-1 expression. Moreover, it is possible that iron chelators, DFO and DPD, deplete iron contained in heme, a component of NOX, decreasing NOX-produced ROS, therefore decreasing the upregulating effect of cadmium on HO-1. In conclusion, the results imply that cadmium could be more toxic to iron-deficient cells than to iron-sufficient cells, suggesting that cadmium exposure could result in more severe consequences in an iron-deficient population than in a healthy population.

Additional Information

Publication
Thesis
Language: English
Date: 2009
Keywords
Cadmium, Heme oxygenase-1, Toxicity, Iron
Subjects
Cadmium $x Physiological effect.
Iron in the body $x Physiological effect.
Cadmium $x Toxicology.
Oxygenases.