Induction of heme oxygenase-1 by phenylarsine oxide. Studies in cultured primary liver cells

Reactive oxygen species generated by thiol-modifying phenylarsine oxide stimulate the expression of protein L-isoaspartyl methyltransferase

Expression of the repair enzyme protein L-isoaspartyl methyltransferase (PIMT) has been reported to play important roles in brain. However, little is known about the regulation of PIMT expression following protein damage by oxidation in brain. Phenylarsine oxide (PAO) is an arsenical compound that alters proteins by forming disulfide bond with vicinal cysteinyl residues. Here we report that PIMT was rapidly up-regulated by PAO in U-87 human astroglioma cells. We also confirmed that PIMT up-regulation by PAO was mediated by the reaction with vicinal cysteines. Furthermore, we showed that PIMT induction by PAO was dependent on formation of reactive oxygen species (ROS). Crucially, both ROS formation and PIMT induction by PAO were inhibited by antioxidant N-acetyl-L-cysteine and NADPH oxidase inhibitor diphenyleneiodonium chloride. Importantly, down-regulation of PIMT by siRNA strikingly enhanced PAO-induced ROS. Together, these results highlight that PIMT expression is regulated by ROS and could primarily act as an antioxidant enzyme.

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.

Identification of key elements that are responsible for heme-mediated induction of the avian heme oxygenase-1 gene

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. In recent work, we had identified a metalloporphyrin-responsive element (MPRE) that localized at -3.7 kb upstream of the transcription start site of the chick HO-1 gene. Here, we identify four additional heme-responsive elements (HeREs), which are "expanded" AP-1 sites, in the 5'-flanking region of the chick HO-1 gene. These sites, located at -4675, -4599, -3660, and -3625 bp from the transcription start site of the gene, were necessary and sufficient for up-regulation of luciferase reporter gene expression in the presence of heme and cobalt protoporphyrin (CoPP), but not several other metalloporphyrins. Site-directed mutagenesis was carried out using pcHO7.1Luc or pcHO7.1-4.6Luc as templates. Single and multiple mutations of HeREs and MPRE significantly abrogated the heme- and CoPP-dependent up-regulation of reporter gene expression in transient or stable transfection experiments.

CONCLUSIONS

The chick HO-1 promoter region contains "expanded" AP-1 sites that are important for up-regulation of the gene by heme and CoPP, but not other metalloporphyrins. These key regulatory elements consist of consensus AP-1 binding sites that have been extended by three base pairs.

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.

Analysis of Heme oxygenase isomers in rat

AIM: To purify and identify heme oxygenase (HO) isomers which exist in rat liver, spleen and brain treated with hematin and phenylhydrazine and in untreated rat liver and to investigate the characteristics of HO isomers, to isolate and confirm the rat HO-1 cDNA that actually encodes HO-1 by expressing cDNA in monkey kidney cells (COS-1 cells), to prepare the rat heme oxygenase-1 (HO-1) mutant and to detect inhibition of HO-1 mutated enzyme.

METHODS: First, rat liver, spleen and brain microsomal fractions were purified by DEAE-Sephacel and hydroxylapatite. The characteristics including activity, immunity and inducibility of two isomers (HO-1 and HO-2), and their apparent molecular weight were measured by detecting enzymatic activities, SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting analysis, respectively. Second, plasmid pcDNA3HO1 containing native rat HO-1 cDNA and pcDNA3HO1D25 carrying mutated rat HO-1 cDNA (His25Ala) were constructed by site-directed mutagenesis. COS-1 cells transfected with pcDNA3HO1 and pcDNA3HO1D25 were collected and disrupted by sonication, the microsomes were prepared by ultracentrifugation. Third, the inhibition of rat HO-1 mutant was analyzed.

RESULTS: Two isomers were purified and identified in treated rat liver, spleen, brain and untreated rat liver. HO-1 was the predominant form with a ratio of 2.0:1 and 3.2:1 of HO-1 and HO-2 in liver and spleen, respectively, but only the activity of HO-2 in the brain and untreated liver could be detected. The apparent molecular weights of HO-1 and HO-2 were about M_r 30000 and M_r 36000 under reducing conditions, respectively. The antiserum against liver HO-2 was employed in Western blotting analysis, the reactivity of HO-1 in the liver was not observed. The plasmid pcDNA3HO1 was highly expressed in endoplasmic reticulum of transfected COS-1 cells. The specific activity was ≈5-fold higher than that of the control. However, the enzyme activity of mutated HO-1 declined. While an equal amount of mutant was added to the enzyme reaction system, the levels of bilirubin decreased 42%.

CONCLUSION: The studies suggest that HO-1 and HO-2 exist in the hematin and phenylhydrazine treated rat liver and spleen, but only HO-2 in the brain and untreated liver. Two constitutive forms are different in molecular weight, inducibility and immunochemical properties. The activity of expressed HO-1 in COS-1 cells is higher than that of purified enzyme from rat spleen tissue. It suggests that this clone has an insert of 1030 base-pairs encodes HO-1. His25Ala mutant reduced the formation of bilirubin and it suggests that the mutant could competely bind the heme with native enzyme.

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.