Submitted to: Journal of Nutrition
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/27/2004
Publication Date: 6/1/2004
Citation: Johnson, W.T., Demars, L.C. 2004. Increased heme oxygenase-1 expression during copper deficiency in rats results from increased mitochondrial generation of hydrogen peroxide. Journal of Nutrition. 134:1328-1333. Interpretive Summary: Mitochondria are responsible for metabolizing most of the oxygen taken up by cells. In order to efficiently metabolize oxygen, mitochondria must have a fully operative set of enzymes called respiratory complexes. One of the enzymes that is part of the set of mitochondrial respiratory complexes is cytochrome c oxidase. Cytochrome c oxidase is a copper-containing enzyme whose activity in liver, heart and brain is reduced by dietary copper deficiency. Even though cytochrome c oxidase is the only respiratory complex that contains copper, it was found that copper deficiency also lowered the activities of the other respiratory complexes as well. When the activities of the respiratory complexes are impaired as they are during copper deficiency, mitochondria do not completely convert the oxygen they consume to water, but instead, convert more oxygen to potentially harmful oxidants. Copper deficiency increased the release of hydrogen peroxide from liver mitochondria. The stimulation of hydrogen peroxide release from mitochondria during copper deficiency was accompanied by an increase in the expression of heme oxygenase in the liver. Copper deficiency also increased the expression of heme oxygenase in the heart. Heme oxygenase is a member of a class of proteins that help protect cells against damage caused by stress, including the stress caused by harmful oxidants. This finding indicates that copper deficiency causes a general impairment in mitochondrial function that leads to the over production of oxidants like hydrogen peroxide. They also indicate that oxidants released from mitochondria serve as signals that stimulate genes in the cell nucleus responsible for regulating the expression of proteins that help moderate oxidative stress in the liver and heart during copper deficiency. Together, the results of this study indicate that the impairment of mitochondrial respiratory complex activities during copper deficiency causes over production of oxidants that serve as intermediates in signaling pathways between the mitochondria and nucleus. Although oxidants produced by the mitochondria can serve as signals to trigger the induction of proteins that protect the cell against oxidative damage, overproduction of oxidants can cause localized damage to mitochondrial proteins and DNA and impair mitochondrial function. Thus, the induction of heme oxygenase through overproduction of mitochondrial oxidants during copper deficiency suggests that the amounts of oxidants being generated may be sufficient to damage the mitochondria. Oxidative damage to mitochondria may contribute to the development of degenerative diseases of the heart, brain, and muscles, and this study suggests that copper is an important nutrient for lowering the severity of mitochondrial oxidative stress and possibly slowing the development of degenerative diseases.
Technical Abstract: The activity of hepatic heme oxygenase (HO) in rats is elevated in response to copper deficiency. However, the mechanism responsible for the increase in HO activity is poorly understood. Oxidative stress is a common denominator for many of the signals that induce HO-1, the inducible isoform of HO. The present study evaluated the role of H2O2 and the mitochondrial electron transport chain as a potential mechanism for the induction of HO-1 during copper deficiency. Mitochondria isolated from the livers of young, male rats fed a copper-deficient diet for 5 weeks exhibited significantly reduced levels of NADH:cytochrome c reductase (31% reduction), succinate:cytrochrome c reductase (42% reduction), and cytochrome c oxidase (70% reduction) activities and increased production of H2O2 (48% increase) when glutamate was used as a substrate. Hepatic levels of HO-1 protein and mRNA were also elevated (48% and 20%, respectively) in copper-deficient rats indicating that copper-deficiency stimulated the expression of the HO-1 gene. Furthermore, the variation in hepatic HO-1 protein content was significantly correlated to variations in mitochondrial NADH:cytochrome c reductase and succinate:cytochrome c reductase activites (R2=0.54, P<0.02). Hydrogen peroxide is a known inducer of HO-1 and our results suggest that increased mitochondrial H2O2 production resulting from inhibition of respiratory complex activities contributes to the induction of HO-1 during copper deficiency. The levels of HO-1 protein and mRNA were also elevated (85% and 95%, respectively) in hearts from copper-deficient rats indicating that the effects of copper deficiency on HO-1 gene expression are not limited to hepatic tissue.