Sustained expression of heme oxygenase-1 alters iron homeostasis in nonerythroid cells
ABSTRACT Heme oxygenases initiate the catabolism of heme, releasing carbon monoxide, iron, and biliverdin. Sustained induction of heme oxygenase-1 (HO-1) in nonerythroid cells plays a key role in many pathological processes, yet the effect of long-term HO-1 expression on cellular iron metabolism in the absence of exogenous heme is poorly understood. Here we report that in a model nonerythroid cell, both transient and stable HO-1 expression increased heme oxygenase activity, but total cellular heme content was decreased only with transient enzyme expression. Sustained HO-1 activity increased the expression of both the mitochondrial iron importer mitoferrin-2 and the rate-limiting enzyme in heme synthesis, aminolevulinate synthase-1, and it augmented the mitochondrial content of heme. Also, the expression of transferrin receptor-1 and the activities of iron-regulatory proteins 1 and 2 decreased, whereas total labile iron and the regulatory activity of the heme-binding transcription factor Bach1 were unaltered. In addition, stable, but not transient, HO-1 expression decreased the activities of aconitase, as well as increasing proteasomal degradation of ferritin. Together, our results reveal a novel and coordinated adaptive response of nonerythroid cells to sustained HO-1 induction that has an impact on cellular iron homeostasis.
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ABSTRACT: Induction or ectopic overexpression of heme oxygenase 1 (HO-1) protects against a wide variety of disorders. These protective effects have been variably ascribed to generation of carbon monoxide (released during cleavage of the alpha methene bridge of heme) and/or to production of the antioxidant, bilirubin. We have investigated HO-1 overexpressing A549 cells and find that, as expected, HO-1 overexpressing cells are resistant to killing by hydrogen peroxide. Surprisingly, these cells have ~2x the normal amount of intracellular iron which usually tends to amplify oxidant killing. However, HO-1 overexpressing cells contain only ~25% as much 'loose' (probably redox active) iron. Indeed, inhibition of ferritin synthesis (via siRNA directed at the ferritin heavy chain) sensitizes the HO-1 overexpressing cells to peroxide killing. It appears that HO-1 overexpression leads to enhanced destruction of heme, consequent 2-3 fold induction of ferritin, and compensatory increases in transferrin receptor expression and heme synthesis. However, there is no functional heme deficiency because cellular oxygen consumption and catalase activity are similar in both cell types. We conclude that, at least in many cases, the cytoprotective effects of HO-1 induction or forced overexpression may derive from elevated expression of ferritin and consequent reduction of redox active 'loose' iron.Biochemical Journal 09/2012; 449. DOI:10.1042/BJ20120936 · 4.78 Impact Factor
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ABSTRACT: BTB and CNC homology 1 (Bach1) is a transcriptional repressor of anti-oxidative enzymes, such as heme oxygenase-1 (HO-1). Oxidative stress is reportedly involved in insulin secretion impairment and obesity-associated insulin resistance. However, the role of Bach1 in the development of diabetes is unclear. HO-1 expression in the liver, white adipose tissue and pancreatic islets was markedly up-regulated in Bach1-deficient mice. Unexpectedly, glucose and insulin tolerance tests showed no differences in obese wild-type (WT) and obese Bach1-deficient mice, after high-fat diet loading for 6 weeks, suggesting minimal roles of Bach1 in the development of insulin resistance. In contrast, Bach1 deficiency significantly suppressed alloxan-induced pancreatic insulin content reduction and the resultant glucose elevation. Furthermore, TUNEL-positive cells in pancreatic islets of Bach1-deficient mice were markedly decreased, by 60%, compared with those in WT mice. HO-1 expression in islets was significantly up-regulated in alloxan-injected Bach1-deficient mice, while expression of other anti-oxidative enzymes, catalase, superoxide dismutase and glutathione peroxidase, was not changed by either alloxan administration or Bach1 deficiency. Our results suggest that Bach1 deficiency protects pancreatic β-cells from oxidative stress-induced apoptosis and that the enhancement of HO-1 expression plays an important role in this protection.AJP Endocrinology and Metabolism 07/2013; 305(5). DOI:10.1152/ajpendo.00120.2013 · 4.09 Impact Factor