Transcriptional control of the rat heme oxygenase gene by a nuclear protein that interacts with adenovirus 2 major late promoter

Heme oxygenase-1 induction by heat shock in rat hepatoma cell line is regulated by the coordinated function of HSF1, NRF2, AND BACH1

The mechanism of heme oxygenase-1 (HO-1) induction by heat shock (HS) loading remains unclear. Here, we investigated the contribution of transcription factors to HS-induced HO-1 expression, using a rat hepatoma cell line (H-4-II-E). Our results demonstrated that HS treatment resulted in a marked induction of HO-1. Immunohistochemical analysis showed a slight mismatch in the expression levels of HO-1 and HSP70 by HS among cells, suggesting a conflict between multiple induction mechanisms. We observed HS-induced nuclear localization of, not only phosphorylated HSF1, but also NRF2, which is a typical transcription factor activated by oxidative stress. HSF1 knockdown in H-4-II-E markedly reduced HO-1 induction by HS, while NRF2 knockdown resulted in a partial effect. The chromatin immunoprecipitation assay demonstrated that HS loading resulted in significant binding of HSF1 to the HSE in the promoter proximal region of HO-1 gene and another HSE located close to the MARE in the -4 kb upstream enhancer region 1, where NRF2 also bound, together with BACH1, a negative transcription factor of HO-1. These observations indicate that HO-1 induction by HS is mainly mediated by HSF1 binding to the proximal HSE. NRF2 binding to MARE by HS is predominantly suppressed by an increased binding of BACH1.

Upstream stimulatory factor 2 and hypoxia-inducible factor 2α (HIF2α) cooperatively activate HIF2 target genes during hypoxia

While the functions of hypoxia-inducible factor 1α (HIF1α)/aryl hydrocarbon receptor nuclear translocator (ARNT) and HIF2α/ARNT (HIF2) proteins in activating hypoxia-inducible genes are well established, the role of other transcription factors in the hypoxic transcriptional response is less clear. We report here for the first time that the basic helix-loop-helix-leucine-zip transcription factor upstream stimulatory factor 2 (USF2) is required for the hypoxic transcriptional response, specifically, for hypoxic activation of HIF2 target genes. We show that inhibiting USF2 activity greatly reduces hypoxic induction of HIF2 target genes in cell lines that have USF2 activity, while inducing USF2 activity in cells lacking USF2 activity restores hypoxic induction of HIF2 target genes. Mechanistically, USF2 activates HIF2 target genes by binding to HIF2 target gene promoters, interacting with HIF2α protein, and recruiting coactivators CBP and p300 to form enhanceosome complexes that contain HIF2α, USF2, CBP, p300, and RNA polymerase II on HIF2 target gene promoters. Functionally, the effect of USF2 knockdown on proliferation, motility, and clonogenic survival of HIF2-dependent tumor cells in vitro is phenocopied by HIF2α knockdown, indicating that USF2 works with HIF2 to activate HIF2 target genes and to drive HIF2-depedent tumorigenesis.

Human haem oxygenase-1 induction by nitro-linoleic acid is mediated by cAMP, AP-1 and E-box response element interactions

Nitro-fatty acid products of oxidative inflammatory reactions mediate anti-inflammatory cell signalling responses. LNO2 (nitrolinoleic acid) induces expression of HO-1 (haem oxygenase-1), an enzyme that catabolizes haem into products exhibiting potent anti-inflammatory properties. In the present manuscript, the molecular mechanisms underlying HO-1 induction by LNO2 were examined in HAEC (human aortic endothelial cells), HEK-293 (human embryonic kidney 293) cells, and in transcription factor-deficient MEF (mouse embryonic fibroblasts). LNO2 induced HO-1 expression in Nrf2 [NF-E2 (nuclear factor-erythroid 2)-related factor 2]-deficient MEF and in HEK-293 cells transfected with Nrf2-specific shRNA (small-hairpin RNA), supporting the fact that LNO2-mediated HO-1 induction can be regulated by Nrf2-independent mechanisms. LNO2 activated expression of a -4.5 kb human HO-1 promoter construct, whereas a -4.0 kb construct with deletion of 500 bp from the 5' region was unresponsive. Site-directed mutagenesis of a CRE (cAMP-response element) or of a downstream NF-E2/AP-1 (activating protein-1) element, individually, within this 500 bp region modestly reduced activation of the HO-1 promoter by LNO2. Mutations of both the CRE and the NF-E2/AP-1 site also attenuated LNO2-mediated HO-1 promoter expression, whereas the addition of a third mutation in the proximal E-box sequence completely abolished LNO2-induced HO-1 expression. Chromatin immunoprecipitation assays confirmed CREB (CRE-binding protein)-1 binding to the CRE (located at -4.0 kb) and E-box regions (located at -44 bp) of the human HO-1 promoter. A 3C (Chromosome Conformation Capture) assay of intact cells showed LNO2-induced interactions between the CRE- and E-box- containing regions. These observations indicate that regulation of human HO-1 expression by LNO2 requires synergy between CRE, AP-1 and E-box sequences and involves the participation of CREB-1.

Opposite expression of the antioxidant heme oxygenase-1 in primary cells and tumor cells: regulation by interaction of USF-2 and Fra-1

Heme oxygenase-1 is the rate-limiting enzyme for the degradation of the prooxidant heme. Previously, we showed that an E-box within the HO-1 promoter is crucial for the regulation of HO-1 expression in primary hepatocytes. Further to investigate the importance of this E-box, we determined the regulatory capacity of the E-box-binding factor USF-2 in primary cells in comparison with transformed cell lines. We found that HO-1 expression was inhibited by USF-2 in primary cells, whereas it was induced in tumor cell lines. Mutation of either the E-box or the AP-1 site within the HO-1 promoter only partially affected the USF-dependent regulation. However, this regulation was dramatically reduced in tumor cells and completely abolished in primary cells transfected with an HO-1 promoter construct containing mutations in both the E-box and the AP-1 site, suggesting that AP-1 factors and USF-2 may act in a cooperative manner. Indeed, protein-protein interaction studies revealed that USF proteins interacted with Fra-1. Further, the USF-dependent HO-1 promoter activity was not detectable with an USF-2 mutant lacking residues of the USF-specific region (USR) or the transactivation domain encoded by exon 4. Together, these data suggest that USF-2 has opposite regulatory roles for HO-1 gene expression in primary cells and tumor cell lines.

Hypoxia and heme oxygenases: oxygen sensing and regulation of expression

Heme is an essential molecule for life, as it is involved in sensing and using oxygen. Heme must be synthesized and degraded within an individual nucleated cell. Physiologic heme degradation is catalyzed by two functional isozymes of heme oxygenase, heme oxygenase-1 (HO-1) and HO-2, yielding carbon monoxide, iron, and biliverdin, an immediate precursor to bilirubin. HO-1 is an inducible enzyme, but the expression level of HO-2 is maintained in a narrow range. Characteristically, human HO-1 contains no Cys residue, whereas human HO-2 contains three Cys residues, each of which might be involved in heme binding. These features suggest separate physiologic roles of HO-1 and HO-2. Recent studies have shown that the expression levels of HO-1 and HO-2 are reduced under hypoxia, depending on the cell types. Moreover, we have proposed HO-2 as a potential O(2) sensor, because HO-2-deficient mice show hypoxemia and a blunted hypoxic ventilatory response with normal hypercapnic ventilatory response. HO-2-deficient mice also show hypertrophy of the pulmonary venous myocardium and enlargement of the carotid body. These morphometric changes are attributable to chronic hypoxemia. Here, we update the understanding of the regulation of HO-1 and HO-2 expression and summarize the regulatory role of HO-2 in the intercellular communication.

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.

Upstream stimulating factors: highly versatile stress-responsive transcription factors

Upstream stimulating factors (USF), USF-1 and USF-2, are members of the eucaryotic evolutionary conserved basic-Helix-Loop-Helix-Leucine Zipper transcription factor family. They interact with high affinity to cognate E-box regulatory elements (CANNTG), which are largely represented across the whole genome in eucaryotes. The ubiquitously expressed USF-transcription factors participate in distinct transcriptional processes, mediating recruitment of chromatin remodelling enzymes and interacting with co-activators and members of the transcription pre-initiation complex. Results obtained from both cell lines and knock-out mice indicates that USF factors are key regulators of a wide number of gene regulation networks, including the stress and immune responses, cell cycle and proliferation, lipid and glucid metabolism, and in melanocytes USF-1 has been implicated as a key UV-activated regulator of genes associated with pigmentation. This review will focus on general characteristics of the USF-transcription factors and their place in some regulatory networks.

Upstream stimulatory factors, USF1 and USF2, bind to the human haem oxygenase-1 proximal promoter in vivo and regulate its transcription

The human HO-1 (haem oxygenase-1) gene encodes a microsomal enzyme responsible for the breakdown of haem, and is also cytoprotective in response to various cellular insults. HO-1 transcription is induced by a vast array of compounds including, but certainly not limited to, haem and heavy metals such as cadmium. In the present study, we show that upstream stimulatory factors, USF1 and USF2, ubiquitous proteins belonging to the basic helix-loop-helix-leucine zipper family of transcription factors, constitutively bind to the class B E-box located in the proximal promoter of the human HO-1 gene and are responsible for the enhancement of HO-1 gene transcription in human renal proximal tubular epithelial cells. Dimethylsulphate in vivo footprinting studies have identified three protected guanine residues in the E-box of the HO-1 proximal promoter. One of these guanine contact points is essential for USF binding, and when mutated mimics a deletion mutation of the entire E-box palindrome sequence encompassing all three guanine contact points. Binding of USF1 and USF2 to the HO-1 E-box was confirmed by chromatin immunoprecipitation and gel-shift assays. Furthermore, we show that overexpression of USF1 or USF2 enhances the basal expression of HO-1 and that expression of a USF dominant negative form reduces its expression. These results demonstrate for the first time that USF proteins bind to the human HO-1 promoter in vivo and are required for high-level expression of HO-1 by haem and cadmium in human renal epithelial cells.

The heme oxygenase dilemma in cellular homeostasis: new insights for the feedback regulation of heme catabolism

Heme must be synthesized and degraded within an individual nucleated cell. Heme degradation is catalyzed by the two isozymes of heme oxygenase, heme oxygenase-1 (HO-1) and HO-2, eventually yielding biliverdin/bilirubin, CO, and iron. These products possess important physiological roles but are potentially toxic to cells. Characteristically, human HO-1 contains no Cys residues, whereas HO-2 contains the potential heme-binding motifs of the Cys-Pro dipeptide. Expression of HO-1 is inducible or repressible, depending on cell types or cellular microenvironments, but expression levels of HO-2 are fairly constant. Thus, the main regulation of heme catabolism is a problem of the balance between induction and repression of HO-1. Notably, HO-1 expression is induced by heme in all mammalian cells examined, but is repressed by hypoxia in certain types of cultured human cells. The recent discovery of Bach1 as a heme-regulated and hypoxia-inducible repressor for transcription of the HO-1 gene has provided a missing link in the feedback control of heme catabolism. On the other hand, the human HO-1 gene promoter contains the (GT)n repeat polymorphism and a single nucleotide polymorphism (-427A --> T), both of which may contribute to fine-tuning of the transcription. Importantly, long (GT)n alleles are associated with susceptibility to smoking-induced emphysema or coronary artery disease, but may provide with resistance to cerebral malaria. The latter finding suggests a novel therapeutic strategy with inhibitors of HO-1 for the treatment of cerebral malaria. We discuss the potential regulatory role of Bach1 and HO-2 in heme catabolism and update the understanding of the regulation of HO-1 expression.

Bach1 functions as a hypoxia-inducible repressor for the heme oxygenase-1 gene in human cells

Heme oxygenase 1 (HO-1) catalyzes heme breakdown, eventually releasing iron, carbon monoxide, and bilirubin IXalpha. HO-1 is induced by its substrate heme and various environmental factors, which represents a protective response against oxidative stresses. Here we show that hypoxia represses HO-1 expression in three human cell types but induces it in rat, bovine, and monkey cells, indicating the inter-species difference in the hypoxic regulation of HO-1 expression. The hypoxia-mediated repression of HO-1 expression is consistently associated with the induction of Bach1, a heme-regulated transcriptional repressor, in human cells. Bach1 is a basic leucine zipper protein, forming a heterodimer with a small Maf protein. Expression of HO-1 was also reduced in human cells when exposed to interferon-gamma or an iron chelator desferrioxamine, each of which induced Bach1 expression. In contrast, induction of HO-1 expression by CoCl(2) is associated with reduced expression of Bach1 mRNA. Thus, expression of HO-1 and Bach1 is inversely regulated. We have identified a Maf recognition element in the human HO-1 gene that is required for repression of a reporter gene by hypoxia and targeted by Bach1. Therefore, Bach1 functions as a hypoxia-inducible repressor for the HO-1 gene, thereby contributing to fine-tuning of oxygen homeostasis in human cells.

Heme degradation and human disease: diversity is the soul of life

We all depend on molecular oxygen and heme for our life, as evident from the pigments in blood and daily wastes. About 80% of serum bilirubin is derived from hemoglobin of senescent erythrocytes, which have finished their mission of 120 days and have been phagocytized by macrophages in the reticuloendothelial system. Here we present an overview of the heme degradation processes and relevant disorders by focusing on heme oxygenase-1 (HO-1), a key enzyme in heme catabolism. HO-1 cleaves the porphyrin macrocycle of heme at the expense of molecular oxygen to release a linear tetrapyrrole biliverdin, carbon monoxide, and ferrous iron; biliverdin is rapidly reduced to bilirubin. Bilirubin is transported to the liver (hepatocytes), conjugated with glucuronic acid by bilirubin UDP-glucuronosyltransferase, and excreted into bile. Genetic diversity, a strategy in the host defense, is seen in the human ho-1 and UDP-glucuronosyltransferase genes. Moreover, striking interspecies variations are noted in the regulation of HO-1 expression by hypoxia, heat shock, or interferon-gamma, each of which mainly represses HO-1 expression in human cells. Implications of such a variety are discussed in relevance to the pathogenesis of severe malaria caused by Plasmodium falciparum, the most ancient foe of humans.

Urea and hypertonicity increase expression of heme oxygenase-1 in murine renal medullary cells

Epithelial cells derived from the mammalian kidney medulla are responsive to urea at the levels of signal transduction and gene regulation. Hybridization of RNA harvested from control- and urea-treated murine inner medullary collecting duct (mIMCD3) cells with a cDNA expression array encoding stress-responsive genes suggested that heme oxygenase (HO)-1 mRNA was upregulated by urea. RNase protection assay confirmed this upregulation; hypertonicity also increased HO-1 mRNA expression but neither hypertonic NaCl nor urea were effective in the nonrenal 3T3 cell line. The effect on HO-1 expression appeared to be transcriptionally mediated on the basis of mRNA half-life studies and reporter gene analyses using the promoters of both human and chicken HO-1. Although urea signaling resembles that of heavy metal signaling in other contexts, the effect of urea on HO-1 transcription was independent of the cadmium response element in this promoter. Urea-inducible HO-1 expression was sensitive to antioxidants but not to scavengers of nitric oxide. Urea also upregulated HO-1 protein expression and pharmacological inhibition of HO-1 action with zinc protoporphyrin-sensitized mIMCD3 cells to the adverse effects of hypertonicity but not to urea. Coupled with the prior observation of others that HO-1 expression increases along the renal corticomedullary gradient, these data suggest that HO-1 expression may comprise an element of the adaptive response to hypertonicity and/or urea in renal epithelial cells.

Identification of a cis-regulatory element for delta 12-prostaglandin J2-induced expression of the rat heme oxygenase gene

We recently reported that delta 12-prostaglandin (PG) J2 caused various cells to synthesize heme oxygenase, HO-1 (Koizumi, T., Negishi, M., and Ichikawa, A. (1992) Prostaglandins 43, 121-131). Here we examined the molecular mechanism underlying the delta 12-PGJ2-induced HO-1 synthesis. delta 12-PGJ2 markedly stimulated the promoter activity of the 5'-flanking region of the rat HO-1 gene from -810 to +101 in rat basophilic leukemia cells. From functional analysis of various deletion mutant genes we found that the delta 12-PGJ2-responsive element was localized in a region from -690 to -660, containing an E-box motif, which was essential for the delta 12-PGJ2-stimulated promoter activity. When the region containing the delta 12-PGJ2-responsive element was combined with a heterologous promoter, SV40 promoter, in the sense and antisense direction, the element showed an enhancer activity in response to delta 12-PGJ2. Gel mobility shift assays demonstrated that delta 12-PGJ2 specifically stimulated the binding of two nuclear proteins to the E-box motif of this region. These results indicate that delta 12-PGJ2 induces the expression of the rat HO-1 gene through nuclear protein binding to a specific element having an E-box motif.

Identification of a second region upstream of the mouse heme oxygenase-1 gene that functions as a basal level and inducer-dependent transcription enhancer

A 161-base pair fragment (AB1) approximately 10 kilobase pairs upstream of the transcription start site of the mouse heme oxygenase-1 gene functions as a basal level and inducer-dependent enhancer. AB1/chloramphenicol acetyltransferase fusion genes stably transfected into mouse hepatoma (Hepa) cells or L929 fibroblasts were activated 7-8- or 17-22-fold, respectively, after treatment of the cells with either CdCl2 or heme. The AB1 fragment is composed largely of three tandem repeats containing two conserved core elements, A and B. Part of core element A (TCCGGAGCTGTG) resembles the consensus-binding site for transcription factor AP-4, whereas core element B (GCTGAGTCANGG) includes the consensus-binding site (TGAGTCA) for the AP-1 family of transcription factors. Nuclear proteins from Hepa cells did not bind to any of the core A elements, but bound to all three copies of the core B element. AB1 derivatives with one or two mutant AP-1-binding elements exhibited reduced but measurable inducer-dependent enhancer activity, but mutation of all three AP-1-binding sites abolished activation by CdCl2 and heme and also by mercury chloride, zinc chloride, H2O2, sodium arsenate, and 12-O-tetradecanoylphorbol-13-acetate. Pretreatment of stably transfected L929 cells with protein kinase C inhibitors, but not with tyrosine kinase inhibitors or N-acetylcysteine, abrogated 12-O-tetradecanoylphorbol-13-acetate-dependent activation of the AB1/chloramphenicol acetyltransferase fusion gene. Induction by H2O2 was unaffected by the kinase inhibitors, but completely abolished by N-acetylcysteine. Heme-dependent induction was not significantly affected by any of these chemicals.

Nuclear factor kappa B binding activity in mouse L1210 cells following photofrin II-mediated photosensitization

Clinical photodynamic therapy (PDT) uses the photosensitizer photofrin II to produce singlet molecular oxygen and other reactive oxygen intermediates for localized tumor tissue cytotoxicity. In this report, we show that PDT enhances the DNA binding activity of nuclear factor kappa B (NF kappa B), a transactivator of cytokine gene expression. Photosensitization following a 16 h incubation of photofrin II induced NF kappa B binding activity in mouse leukemia L1210 cells 10-fold above that observed in exponentially growing cultures. Serum starvation, as well as drug-alone and light-alone controls, elevated basal NF kappa B binding activity two- to three-fold. Upstream stimulatory factor binding activity was not modulated by any of the cell treatments and was used to standardize gel mobility shift data. This study identifies porphyrin-mediated PDT as an inducer of NF kappa B binding activity, extending recent findings that NF kappa B activation is a general response to oxidative stress.

Identification of a nuclear protein that constitutively recognizes the sequence containing a heat-shock element. Its binding properties and possible function modulating heat-shock induction of the rat heme oxygenase gene

Heme oxygenase is an essential enzyme in heme catabolism, and also known as a 32-kDa heat-shock protein in rat. The rat heme-oxygenase gene promoter contains a functional heat-shock element (HSE) designated as HSE1 (-290 to -276 from the transcriptional initiation site), which consists of three copies of a 5-bp unit (5'-NGAAN-3';-->) in alternating orientation. Here we identified a putative HSE (-221 to -212), designated as HSE2, consisting of an inverted repeat of this 5-bp unit (<==>). Using transient expression assays, we show that HSE1 is sufficient to confer the heat-inducibility (a three fold to fourfold increase) on the reporter gene located downstream from the rat heme-oxygenase gene promoter, but HSE2 alone is not, suggesting that HSE2, a HSE of a tail-to-tail configuration, is not functional in vivo. However, the presence of both HSE1 and HSE2 in the promoter region increased the heat-mediated induction of the reporter-gene expression by more than 15-fold. Gel mobility-shift assays indicate that both HSE1 and HSE2 are recognized by activated heat-shock factor present only in heat-shocked rat glioma cells. Interestingly, the sequence containing HSE2 is also bound by a protein that is present in nuclear extracts prepared from either heat-shocked or non-shocked glioma cells, but this nuclear protein is unable to bind to HSE1. We suggest that a protein binding to the sequence containing HSE2 may be involved in transcriptional regulation of the rat heme oxygenase gene under thermal stress.

Tributyltin is a potent inducer of the heat shock response in human diploid fibroblasts

Submicromolar concentrations of tributyltin (TBT), a commercially used organotin compound, were found to induce the expression of several stress proteins, most notably HSP89 and HSP70, in IMR-90 human diploid fibroblasts in a time- and dose-dependent manner. This induction can be demonstrated by quantitation of 1) synthesis of the heat shock proteins (HSPs), 2) relative abundance of mRNA of hsp70, and 3) transient expression of a human hsp70 promoter driven reporter gene. TBT also increased the abundance of mRNA of heme oxygenase, whereas heat shock was without effect. Analysis of protein binding to a consensus heat shock element (HSE) by electrophoretic mobility shift assay suggests that the induction of the heat shock response by TBT was attributable to activation of the heat shock transcription factor (HSTF).

Expression of heme oxygenase and its RNA in mouse liver after injection of heme and splenectomy

Heme is known to activate the HO (heme oxygenase) gene in cultured cells, but little is known about the effect of heme on the HO gene in intact organisms. The expressions of HO and its RNA in mouse liver were measured using mouse HO cDNA and HO antibody after injection of heme or splenectomy. The antibody was prepared against a beta-galactosidase-HO hybrid protein made in Escherichia coli. The HO mRNA level increased to a maximum 15 h after heme injection. In contrast, expression of HO was maximal about 45 h after heme injection. Essentially the same results were obtained in mice after splenectomy. These results suggest that the HO gene in mouse liver was activated by the injection of heme and splenectomy.

A human binding site for transcription factor USF/MLTF mimics the negative regulatory element of human immunodeficiency virus type 1

Transcriptional regulation of the proviral form of the human immunodeficiency virus type 1 (HIV-1) is exerted by its 5' long terminal repeat (LTR), which contains recognition sites for several cell factors. By gel retardation and DNase I footprinting experiments we have identified a binding site for a human nuclear protein between nucleotides -152 to -174 upstream of transcription start site, in a region previously recognized as a negative regulator of transcription (negative regulatory element, NRE). The recognized sequence contains the dyad symmetry element CACGTG, which represents a binding motif, very conserved through evolution, present in a putative human DNA replication origin (pB48), in the upstream element of the major late promoter (MLP-UE) of adenovirus, and, as transcriptional element, upstream of many eukaryotic genes. Common binding activities exist in human nuclear extracts for pB48, MLP-UE and the HIV-1 LTR; at least three protein species recognize the LTR sequence, of 44 (corresponding to transcription factor USF/MLTF), 70, and 110 kDa, respectively. Chloramphenicol acetyltransferase assays suggest that the USF/MLTF binding site located in the HIV-1 LTR acts as a negative regulator of transcription, and that it contributes to the overall negative function exerted by the NRE. An oligonucleotide corresponding to another characterized human USF/MLTF binding site can functionally replace part of the activity of the NRE. This negative function is exerted both in presence or absence of tat transactivation, in different cell lines, and after PMA stimulation.

Co-regulation of heme oxygenase and erythropoietin genes

The mechanism responsible for the accumulation of heme oxygenase and erythropoietin (epo) transcripts due to cobalt chloride (CoCl2) administration was investigated in rat kidney using a rat heme oxygenase and mouse epo probes. We found an increase of heme oxygenase transcripts in kidney in response to CoCl2. Quantitative evaluation of the heme oxygenase mRNA changes, by scanning densitometry, indicated that the levels of mRNA encoding heme oxygenase were increased by about fiftyfold in rat kidney after administration of CoCl2. That the increase in heme oxygenase mRNA levels resulted from enhanced transcription of the heme oxygenase gene was confirmed by nuclear runoff using isolated rat kidney nuclei after CoCl2 administration. Transcription of the heme oxygenase gene is greatly increased in rat kidney within 1 hr of administration of CoCl2 as evidenced from the levels of 32P-UTP incorporation into the specific transcript. Time course studies showed that stimulation of transcription was increased about fortyfold 3 hr after CoCl2 administration. This stimulation is the most rapid transcriptional response to heavy metals yet described. In addition, Northern blot analysis demonstrated that epo mRNA was first detected 4 hr following CoCl2 administration and reached a maximum at 5 hr. On the other hand, PCR analysis indicated that epo mRNA was increased as early as 1 hr following CoCl2 administration. The fact that CoCl2 caused increased transcription of both the epo and heme oxygenase genes suggests that a common mechanism may be involved in the regulation of these two genes by the heavy metal ion.

Characterization of the murine Hox-2.3 promoter: involvement of the transcription factor USF (MLTF)

The murine homeobox-containing gene Hox-2.3 contains a basal promoter in a 210-bp region upstream of the transcription start site. In vitro studies of DNA-protein interactions in this region, and in a 1.3-kb upstream region which is known to play a role in tissue specific expression in vivo, led to the identification of DNA elements interacting with nuclear proteins from embryocarcinoma cells. Among the factors binding to the basal promoter is the upstream stimulating factor (USF), also known as major late transcription factor (MLTF). A single point mutation in its binding site abolishes binding in vitro and leads to 50% reduction of the transcriptional activity as measured in receptor gene experiments, showing that it is an activator of Hox-2.3 expression.

Structural studies on bovine spleen heme oxygenase. Immunological and structural diversity among mammalian heme oxygenase enzymes

Heme oxygenase is an Mr 32,000 microsomal enzyme which catalyzes the rate-limiting step in the oxidative catabolism of heme to yield equimolar quantities of biliverdin IX alpha, carbon monoxide, and iron. In the present investigation, evidence is presented suggesting that immunochemical and structural differences exist between bovine spleen heme oxygenase and heme oxygenase enzymes from other mammalian species. Using an antibody directed against bovine spleen heme oxygenase, enzyme-linked immunosorbent assays, Western blotting experiments, and cell-free translation immunoprecipitation studies showed that bovine spleen heme oxygenase is only weakly immunochemically related to heme oxygenase from rat spleen. This observation was supported by the fact that a rat spleen heme oxygenase cDNA probe did not hybridize significantly to bovine spleen heme oxygenase mRNA in Northern analyses nor to restriction fragments containing the bovine heme oxygenase gene in Southern analyses. Tryptic peptides were prepared from bovine spleen heme oxygenase and the amino acid sequences of nine peptides comprising 94 amino acid residues were determined, providing the first information on the primary structure of bovine spleen heme oxygenase. Comparison of the sequences of these tryptic peptides with regions of the deduced amino acid sequences of rat spleen and human macrophage heme oxygenase revealed sequence similarities ranging from 55 to 100%. Several peptides displaying the highest degree of sequence similarity were found to occur in regions of the heme oxygenase molecule postulated to contain the heme binding site, indicating that despite the immunochemical and apparent structural differences between bovine spleen heme oxygenase and the rat and human enzymes, functionally important amino acid residues have been conserved in the evolution of mammalian heme oxygenase genes.

Interaction of upstream stimulatory factor with the human heme oxygenase gene promoter

Upstream stimulatory factor (USF), originally identified in HeLa cells, interacts with the upstream promoter sequence of adenovirus 2 major late promoter (Ad2MLP) and activates its transcription. USF is present in uninfected HeLa cells and appears to be involved in the transcription of cellular genes related to stress. Recently, we have proposed that the rat heme oxygenase gene, newly identified heat-shock protein gene, is regulated at least in partly by a rat homolog of USF [Sato, M., Fukushi, Y., Ishizawa, S., Okinaga, S., Muller, R.M. & Shibahara, S. (1989) J. Biol. Chem. 264, 10251-10260]. We therefore confirm that the heme oxygenase gene is expressed in HeLa cells and its expression is increased by cadmium, suggesting that human heme oxygenase is a stress protein similar to the metallothioneins. Using partially purified USF from HeLa cells, we show that USF binds to the human heme oxygenase gene promoter and stimulates its cell-free transcription. The cis-acting element, identified as CACGTGACCCG, is located 34 bp upstream from the transcription initiation site, and contains the core sequence of the upstream promoter sequence of Ad2MLP. We propose that USF contributes to the transcription of the human heme oxygenase gene.

Regulation of the genes for heme pathway enzymes in erythroid and in non-erythroid cells

There are eight enzymes in the heme biosynthetic pathway and three enzymes in the heme catabolic pathway. Enzymatic defects in heme biosynthesis lead to clinical conditions termed porphyrias. cDNAs for five of the eight enzymes in the heme biosynthetic pathway and two of the three enzymes in the heme catabolic pathway have been cloned and characterized in mammalian cells. At least two enzymes exist as isozymes between erythroid and non-erythroid tissues. One is delta-aminolevulinic acid synthase (ALAS), and the erythroid and hepatic isozymes are coded by two separate genes. The other is porphobilinogen deaminase (PBGD), and both the erythroid and the non-erythroid PBGD mRNA are transcribed from a single PBGD gene by alternate transcription and splicing. There is also a significant tissue-specific control of expression of the uroporphyrinogen decarboxylase gene which is expressed as a unique mRNA in all tissues.

Haem as a multifunctional regulator

Haem has long been known as the prosthetic group of haemoproteins such as haemoglobin, catalase and the cytochromes. Its biosynthesis is regulated by feedback mechanisms that ensure its adequate production but prevent its overaccumulation, which is highly deleterious as diseases such as porphyrias attest. However, recent years have seen rapid strides in our understanding of how haem (or more accurately haemin, its oxidized form) itself acts as an intracellular regulator of a variety of other metabolic pathways for systems that utilize oxygen.