Upstream regulatory elements in chick heme oxygenase-1 promoter: a study in primary cultures of chick embryo liver cells

BACH1, the master regulator gene: A novel candidate target for cancer therapy

BACH1 (BTB and CNC homology 1, basic leucine zipper transcription factor 1) is a transcriptional factor and a member of cap 'n' collar (CNC) and basic region leucine zipper factor family. In contrast to other bZIP family members, BACH1 appeared as a comparatively specific transcription factor. It acts as transcription regulator and is recognized as a recently hypoxia regulator and functions as an inducible repressor for the HO-1 gene in many human cell types in response to stress oxidative. In regard to studies lately, although, BACH1 has been related to the regulation of oxidative stress and heme oxidation, it has never been linked to invasion and metastasis. Recent studies have showed that BACH1 is involved in bone metastasis of breast cancer by up-regulating vital metastatic genes like CXCR4 and MMP1. This newly discovered aspect of BACH1 gene provides new insight into cancer progression study and stands on its master regulator role in metastasis process, raising the possibility of considering it as a potential target for cancer therapy.

MicroRNA-196 represses Bach1 protein and hepatitis C virus gene expression in human hepatoma cells expressing hepatitis C viral proteins

Hepatitis C virus (HCV) directly induces oxidative stress and liver injury. Bach1, a basic leucine zipper mammalian transcriptional repressor, negatively regulates heme oxygenase 1 (HMOX1), a key cytoprotective enzyme that has antioxidant and anti-inflammatory activities. microRNAs (miRNAs) are small noncoding RNAs ( approximately 22 nt) that are important regulators of gene expression. Whether and how miRNAs regulate Bach1 or HCV are largely unknown. The aims of this study were to determine whether miR-196 regulates Bach1, HMOX1, and/or HCV gene expression. HCV replicon cell lines (Con1 and 9-13) of the Con1 isolate and J6/JFH1-based HCV cell culture system were used in this study. The effects of miR-196 mimic on Bach1, HMOX1, and HCV RNA, and protein levels were measured by way of quantitative real-time polymerase chain reaction (qRT-PCR) and Western blotting, respectively. The Dual Glo Luciferase Assay System was used to determine reporter activities. miR-196 mimic significantly down-regulated Bach1 and up-regulated HMOX1 gene expression and inhibited HCV expression. Dual luciferase reporter assays demonstrated that transfection of miR-196 mimic resulted in a significant decrease in Bach1 3'-untranslated region (UTR)-dependent luciferase activity but not in mutant Bach1 3'-UTR-dependent luciferase activity. Moreover, there was no detectable effect of mutant miR-196 on Bach1 3'-UTR-dependent luciferase activity.

CONCLUSION

miR-196 directly acts on the 3'-UTR of Bach1 messenger RNA and translationally represses the expression of this protein, and up-regulates HMOX1. miR-196 also inhibits HCV expression in HCV replicon cell lines (genotype 1b) and in J6/JFH1 (genotype 2a) HCV cell culture system. Thus, miR-196 plays a role in both HMOX1/Bach1 expression and the regulation of HCV expression in human hepatocytes. Overexpression of miR-196 holds promise as a potential novel strategy to prevent or ameliorate hepatitis C infection, and to protect against liver injury in chronic HCV infection.

Heme degradation and vascular injury

Heme is an essential molecule in aerobic organisms. Heme consists of protoporphyrin IX and a ferrous (Fe(2+)) iron atom, which has high affinity for oxygen (O(2)). Hemoglobin, the major oxygen-carrying protein in blood, is the most abundant heme-protein in animals and humans. Hemoglobin consists of four globin subunits (alpha(2)beta(2)), with each subunit carrying a heme group. Ferrous (Fe(2+)) hemoglobin is easily oxidized in circulation to ferric (Fe(3+)) hemoglobin, which readily releases free hemin. Hemin is hydrophobic and intercalates into cell membranes. Hydrogen peroxide can split the heme ring and release "free" redox-active iron, which catalytically amplifies the production of reactive oxygen species. These oxidants can oxidize lipids, proteins, and DNA; activate cell-signaling pathways and oxidant-sensitive, proinflammatory transcription factors; alter protein expression; perturb membrane channels; and induce apoptosis and cell death. Heme-derived oxidants induce recruitment of leukocytes, platelets, and red blood cells to the vessel wall; oxidize low-density lipoproteins; and consume nitric oxide. Heme metabolism, extracellular and intracellular defenses against heme, and cellular cytoprotective adaptations are emphasized. Sickle cell disease, an archetypal example of hemolysis, heme-induced oxidative stress, and cytoprotective adaptation, is reviewed.

Simvastatin-induced heme oxygenase-1 increases apoptosis of Neuro 2A cells in response to glucose deprivation

Heme oxygenase-1 (HO-1) has been suggested as an important mediator of the cholesterol-independent cytoprotection actions of statins, which may be of benefit for the treatment of degenerative neurological diseases and for reduction of infarct volume after cerebral ischemia. Overexpression of HO-1, however, has dual effects under oxidative stress, and the release of ferric iron from heme under these conditions may result in detrimental rather than cytoprotective effects. This study was designed to investigate the effect of simvastatin-induced HO-1 on Neuro 2A cells in response to glucose deprivation. We demonstrated that simvastatin induced a dose- and time-dependent upregulation of HO-1 protein expression in Neuro 2A cells. The induction of HO-1 after simvastatin treatment was mediated by nuclear factor erythroid 2-related factor 2 (Nrf2), which was expressed by Western blots of nuclear fractions and retarded complex formation in the electrophoretic mobility shift assay reaction. In addition, simvastatin activated the extracellular signal-regulated kinase and p38, but not the phosphorylation of c-Jun N-terminal kinase and Akt. Glucose deprivation in the cells pretreated with simvastatin induced more HO-1 expression, and the transcript could be decreased by small interfering RNA for Nrf2. This upregulation of HO-1 was significantly associated with increased apoptosis, manifested as expression at the protein level of 17-kDa cleaved caspase-3 and increased percentage of apoptotic cells shown by flow cytometry. The increased cleaved caspase-3 expression and percentage of apoptotic cells was significantly reduced by the HO inhibitor zinc protoporphyrin. Addition of the iron chelator desferrioxamine also resulted in blockade of the aggravated apoptosis, which implies that iron production from HO-1 activity may play an important role in the increased apoptosis in response to glucose deprivation in neuronal cells pretreated with simvastatin.

Heme oxygenase-1 as a potential therapeutic target for hepatoprotection

Heme oxygenase (HO), the rate limiting enzyme in the breakdown of heme into carbon monoxide (CO), iron and bilirubin, has recently received overwhelming research attention. To date three mammalian HO isozymes have been identified, and the only inducible form is HO-1 while HO-2 and HO-3 are constitutively expressed. Advances in unveiling signal transduction network indicate that a battery of redox-sensitive transcription factors, such as activator protein-1 (AP-1), nuclear factor-kappa B (NF-kappaB) and nuclear factor E2-related factor-2 (Nrf2), and their upstream kinases including mitogen-activated protein kinases play an important regulatory role in HO-1 gene induction. The products of the HO-catalyzed reaction, particularly CO and biliverdin/bilirubin have been shown to exert protective effects in several organs against oxidative and other noxious stimuli. In this context, it is interesting to note that induction of HO-1 expression contributes to protection against liver damage induced by several chemical compounds such as acetaminophen, carbon tetrachloride and heavy metals, suggesting HO-1 induction as an important cellular endeavor for hepatoprotection. The focus of this review is on the significance of targeted induction of HO-1 as a potential therapeutic strategy to protect against chemically-induced liver injury as well as hepatocarcinogenesis.

Role of Bach1 and Nrf2 in up-regulation of the heme oxygenase-1 gene by cobalt protoporphyrin

Heme oxygenase (HO) catalyzes the conversion of heme to biliverdin with the release of iron and carbon monoxide. HO-1 is highly inducible by a large number of physical and chemical factors. CoPP is known to be a potent and effective inducer of HO-1 activity in many tissues. Here we report that CoPP up-regulates HO-1 via Bach1 and Nrf2 in human liver cells. CoPP did not influence hepatic Bach1 or Nrf2 mRNA levels, but markedly reduced Bach1 protein levels by increasing degradation of Bach1 protein (t(1/2) from 19 h to 2.8 h), and increased Nrf2 by decreasing degradation of Nrf2 protein (t(1/2) from 2.5 h to 9 h). Silencing Bach1 by Bach1-siRNA significantly increased levels of HO-1 mRNA and protein, and addition of CoPP up-regulated HO-1 mRNA and protein further. However, silencing Nrf2 mRNA by Nrf2-siRNA did not significantly change baseline HO-1 mRNA or protein levels, but significantly decreased 5-10 microM CoPP-mediated up-regulation of HO-1 mRNA levels compared with CoPP alone. Transfection with equal amounts of non-Bach1 or non-Nrf2 related control siRNA did not reduce Bach1 or Nrf2 mRNA or protein, confirming the specificity of Bach1- and Nrf2-siRNA in Huh-7 cells. We conclude that the pathway of CoPP-mediated induction of HO-1 involves the repression of Bach1 and up-regulation of the Nrf2 protein by post-transcriptional site(s) of action. Because CoPP, unlike heme, is neither a prooxidant nor a substrate for HO-1, it might be considered as a potential therapeutic agent in situations where up-regulation of HO-1 is desired.

How many transcription factors does it take to turn on the heme oxygenase-1 gene?

The ability to communicate with the environment and respond to changes--particularly those of an adverse nature--within that environment is critical for cell function and survival. A key component of the overall cellular stress response includes adjustments in the gene expression program in favor of proteins that manifest activities capable of frustrating and eventually eliminating the molecular constituents of the stress condition. One protein providing such cytoprotective activity is heme oxygenase-1 (HO-1), an enzyme that catalyzes the rate-limiting reaction in heme catabolism (i.e., the oxidative cleavage of b-type heme molecules to yield equimolar quantities of biliverdin IXalpha, carbon monoxide, and iron). Because of the potent antioxidant, anti-inflammatory, and signaling properties of the reaction products, the HO-1 gene (hmox1) is frequently activated under a variety of cellular stress conditions. Cells use multiple signaling pathways and transcription factors to fine-tune their response to a specific circumstance. Among these factors, members of the heat-shock factor, nuclear factor-kappaB, nuclear factor-erythroid 2, and activator protein-1 families are arguably the most important regulators of the cellular stress response in vertebrates. Although there is functional overlap between individual families, each broadly regulates different aspects of the cellular stress response and thus, with some exceptions, modulates the expression of different sets of targets genes. To the best of our knowledge, hmox1 is unique in that it is proposed to be directly regulated by all four of these stress-responsive transcription factors. In this article we provide a review and analysis of the data supporting this proposition.

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

Molecular basis of heme oxygenase-1 induction: implications for chemoprevention and chemoprotection

Heme oxygenase (HO)-1, involved in the heme degradation process, is an important antioxidant enzyme. The induction of HO-1 gene expression, in response to diverse oxidative stimuli, represents a critical event in adaptive cellular response. Experimental models of various diseases, including acute inflammation, atherosclerosis, degenerative diseases, and carcinogenesis, have demonstrated that the induction of HO-1 can prevent or mitigate the symptoms associated with these ailments. Recent progress in our understanding of cellular signaling networks as critical modulators of gene transcription sheds light on the molecular basis of HO-1 gene expression. A panel of redox-sensitive transcription factors such as activator protein-1, nuclear factor- kappaB, and nuclear factor E2-related factor-2, and some of the upstream kinases have been identified as regulators of HO-1 gene induction. The scope of this review is limited to focus on molecular mechanisms underlying HO-1 expression and the significance of targeted induction of HO-1 as a strategy to achieve chemoprevention and chemoprotection.

Role of Bach-1 in regulation of heme oxygenase-1 in human liver cells: insights from studies with small interfering RNAS

Heme oxygenase-1 is an antioxidant defense enzyme that converts heme to biliverdin, iron, and carbon monoxide. Bach-1 is a bZip protein that forms heterodimers with small Maf proteins and was reported recently to down-regulate the HO-1 gene in mice. Using small interfering RNAs targeted to human Bach-1 mRNA, we investigated whether modulation of human hepatic Bach-1 expression by small interfering (si)RNA technology influences heme oxygenase-1 gene expression. We found that Bach-1 siRNAs transfected into Huh-7 cells significantly reduced Bach-1 mRNA and protein levels approximately 80%, compared with non siRNA-treated cells. In contrast, transfection with the same amounts of nonspecific control duplexes or LaminB2-duplex did not reduce Bach-1 mRNA or protein levels, confirming the specificity of Bach-1 siRNA. Expression of the heme oxygenase-1 gene in Bach-1 siRNA-transfected cells was up-regulated 7-fold, compared with cells without Bach-1 siRNA. The effect of increasing concentrations of heme to up-regulate levels of heme oxygenase-1 was more pronounced when Bach-1 siRNA was present. Taken together, these results indicated that Bach-1 has a specific and selective ability to repress expression of human hepatic heme oxygenase-1. Silencing of Bach-1 by siRNAs is a useful method for up-regulating HO-1 gene expression. Exogenous heme produces additional up-regulation, beyond that produced by Bach-1 siRNAs, suggesting that heme does not act solely through its effects on Bach-1.

Blockage of multidrug resistance-associated proteins potentiates the inhibitory effects of arsenic trioxide on CYP1A1 induction by polycyclic aromatic hydrocarbons

Arsenic is a toxic metalloid known to interact with drug-metabolizing enzymes. In the present study, we investigated the effects of arsenic trioxide (As2O3), recently used as an anticancer drug, on the expression of human cytochrome P450 (P450) 1A1, which bioactivates polycyclic aromatic hydrocarbons into mutagenic metabolites. Clinically relevant concentrations (0.25-5 microM) of As2O3 were demonstrated to inhibit CYP1A activity in primary human hepatocytes and hepatoma Hep3B and HepG2 cells coexposed to 3-methylcholanthrene (3MC), benzo(a)pyrene, or dioxin and the metalloid for 24 h. Inhibition reached 50 and 90% in Hep3B cells treated with 1 and 5 microM As2O3, respectively, and was not due to direct interaction of the metalloid with CYP1A1. As2O3 (2.5-5 microM) was demonstrated to markedly reduce induction of CYP1A1 mRNA and apoprotein levels and gene promotor activity in 3MC-treated Hep3B cells, whereas lower concentrations (0.25-1 microM) were ineffective. These effects of As2O3 were abrogated by N-acetylcysteine. Surprisingly, this agent was found 1) to block cellular arsenic uptake when coincubated with the metalloid and 2) to increase arsenic efflux through multidrug resistance-associated proteins. In addition, blockade of these transporters was shown to enhance intracellular amounts of metalloid and to potentiate its effects on CYP1A1 gene. Finally, our results have demonstrated that As2O3, at low concentrations routinely reached in As2O3-treated patients, prevents induction of human CYP1A1 gene expression and that such an effect is increased by blocking multidrug resistance-associated proteins.

Heme oxygenase-1: redox regulation and role in the hepatic response to oxidative stress

Heme oxygenase (HO) catalyzes the oxidative cleavage of the alpha-mesocarbon of Fe-protoporphyrin-IX yielding equimolar amounts of biliverdin-IXalpha, free divalent iron, and carbon monoxide (CO). Among the three isoenzymes cloned to date, only HO-1 can be induced by a variety of seemingly disparate stimuli, most of which are linked by their ability to provoke oxidative stress. Although constitutive expression of HO-1 in the liver is restricted to Kupffer cells, the gene is inducible in nonparenchymal as well as in parenchymal liver cells. HO-1 induction potentially confers protection against oxidative stress in a variety of experimental models, such as liver ischemia/reperfusion secondary to transplantation or hemorrhage/resuscitation. Induction of HO-1 may protect the cell against oxidative injury by (a) controlling intracellular levels of "free" heme (a prooxidant), (b) producing biliverdin (an antioxidant), (c) improving nutritive perfusion via CO release, and (d) fostering the synthesis of the Fe-binding protein ferritin. Although protective effects of up-regulation of the HO pathway--presumably through production of bile pigments and CO--have been reported for a variety of cells and tissues, including the liver, evidence suggests that the protective action might be restricted to a rather narrow threshold of overexpression. High levels of HO-1 may even sensitize the cell to oxidative stress, e.g., through release of reactive iron. Transcriptional activation of the HO-1 gene is an integral part of the cellular response to oxidative stress, but its induction seems to be neither exclusively cytoprotective nor exclusively cytotoxic.

Multiple basic-leucine zipper proteins regulate induction of the mouse heme oxygenase-1 gene by arsenite

The mechanism of heme oxygenase-1 (ho-1) gene activation by arsenite was examined. Arsenite-stimulated expression of a ho-1 promoter/luciferase chimera in a dose-dependent manner in mouse hepatoma (Hepa) cells. Mutation analyses identified the arsenite-responsive sequence as the stress-response element (StRE), which resembles the binding sites for the AP-1 superfamily of basic-leucine zipper factors. In electrophoretic mobility shift assays, up to seven specific StRE-protein complexes were routinely detected using extracts from untreated Hepa cells whereas a single complex was typically observed after treatment with arsenite. Antibody "supershift" experiments identified Nrf2, JunD, and ATF3 in control complexes and the amount of these factors increased significantly in the arsenite-induced complex. MafG, ATF2, FosB, and JunB were also detected in the arsenite complex. Activation of a StRE-dependent luciferase gene by arsenite was inhibited to varying degrees by dominant-negative mutants of Nrf2, MafK, c-Fos, and CREB but most strongly with the latter. Together, these results implicate multiple basic-leucine zipper transcription factors in ho-1 gene activation by arsenite.

Mapping of the chick heme oxygenase-1 proximal promoter for responsiveness to metalloporphyrins

Heme oxygenase (HO) catalyzes the rate-controlling step of physiologic heme catabolism, namely, the oxidation of the alpha-methene bridge of the macrocycle with formation of CO, Fe, and biliverdin. HO-1, the first isoform of HO to be identified, is highly inducible by a large number of physical and chemical factors. Many of these factors cause oxidative or other stresses to cells. In this work, we have studied the regulation of the chick HO-1 gene, using selected promoter--reporter constructs of the gene transiently or stably transfected into primary cultures of chick embryo liver cells or into the LMH line of chicken hepatoma cells. By use of deletional and mutational analyses, DNase protection, and electromobility shift DNA-binding assays, we identified a heretofore undefined regulatory region in the 5'-UTR of the chick HO-1 gene which confers up-regulation of reporter gene (luciferase) expression in the presence of heme and other selected metalloporphyrins. This new metalloporphyrin-responsive element (MPRE) was localized to a 200-bp region 3.8 to 3.6 kb upstream of the transcription starting point of the chick HO-1 gene. It responded particularly to heme and cobalt protoporphyrin with maximal inductions at 10-15 microM concentrations and 15-18 h of exposure. In contrast, sodium arsenite, a prototypical stress-type inducer of HO-1, led to down-regulation of the reporter gene down stream of MPRE. DNase analysis identified an 18-mer oligonucleotide that was required for the metalloporphyrin response (5'-(-3711)TATTGCAGCTGTGTGGGG-3'). Mutations at any of four sites within this oligonucleotide abrogated the metalloporphyrin-dependent up-regulation of reporter gene expression. Nuclear protein extracts of cells treated with heme or cobalt protoporphyrin showed specific enhanced binding to this 18-mer. We conclude that the chick HO-1 promoter region contains a unique sequence that subserves up-regulation of the gene by metalloporphyrins and propose the name "metalloporphyrin-responsive element" for this sequence.