Induction of terminal enzymes for heme biosynthesis during differentiation of mouse erythroleukemia cells

Different effects of an N-phenylimide herbicide on heme biosynthesis between human and rat erythroid cells

Rat developmental toxicity including embryolethality and teratogenicity (mainly ventricular septal defects and wavy ribs) were produced by S-53482, an N-phenylimide herbicide that inhibits protoporphyrinogen oxidase (PPO) common to chlorophyll and heme biosynthesis. The sequence of key biological events in the mode of action has been elucidated as follows: inhibition of PPO interferes with normal heme synthesis, which causes loss of blood cells leading to fetal anemia, embryolethality and the development of malformations. In this study we investigated whether the rat is a relevant model for the assessment of the human hazard of the herbicide. To study effects on heme biosynthesis, human erythroleukemia, human cord blood, and rat erythroleukemia cells were treated with the herbicide during red cell differentiation. Protoporphyrin IX, a marker of PPO inhibition, and heme were determined. We investigated whether synchronous maturation of primitive erythropoiesis, which can contribute to massive losses of embryonic blood, occurs in rats. The population of primitive erythroblasts was observed on gestational days 11 through 14. Heme production was suppressed in rat erythroid cells. In contrast, heme reduction was not seen in both human erythroid cells when PPO was inhibited. Rats underwent synchronous maturation in primitive erythropoiesis. Our results combined with epidemiological findings that patients with deficient PPO are not anemic led us to conclude that human erythroblasts are resistant to the herbicide. It is suggested that the rat would be an inappropriate model for assessing the developmental toxicity of S-53482 in humans as rats are specifically sensitive to PPO inhibition by the herbicide.

Heme biosynthesis and the porphyrias

Porphyrias, is a general term for a group of metabolic diseases that are genetic in nature. In each specific porphyria the activity of specific enzymes in the heme biosynthetic pathway is defective and leads to accumulation of pathway intermediates. Phenotypically, each disease leads to either neurologic and/or photocutaneous symptoms based on the metabolic intermediate that accumulates. In each porphyria the distinct patterns of these substances in plasma, erythrocytes, urine and feces are the basis for diagnostically defining the metabolic defect underlying the clinical observations. Porphyrias may also be classified as either erythropoietic or hepatic, depending on the principal site of accumulation of pathway intermediates. The erythropoietic porphyrias are congenital erythropoietic porphyria (CEP), and erythropoietic protoporphyria (EPP). The acute hepatic porphyrias include ALA dehydratase deficiency porphyria, acute intermittent porphyria (AIP), hereditary coproporphyria (HCP) and variegate porphyria (VP). Porphyria cutanea tarda (PCT) is the only porphyria that has both genetic and/or environmental factors that lead to reduced activity of uroporphyrinogen decarboxylase in the liver. Each of the 8 enzymes in the heme biosynthetic pathway have been associated with a specific porphyria (Table 1). Mutations affecting the erythroid form of ALA synthase (ALAS2) are most commonly associated with X-linked sideroblastic anemia, however, gain-of-function mutations of ALAS2 have also been associated with a variant form of EPP. This overview does not describe the full clinical spectrum of the porphyrias, but is meant to be an overview of the biochemical steps that are required to make heme in both erythroid and non-erythroid cells.

Synthesis, delivery and regulation of eukaryotic heme and Fe-S cluster cofactors

In humans, the bulk of iron in the body (over 75%) is directed towards heme- or Fe-S cluster cofactor synthesis, and the complex, highly regulated pathways in place to accomplish biosynthesis have evolved to safely assemble and load these cofactors into apoprotein partners. In eukaryotes, heme biosynthesis is both initiated and finalized within the mitochondria, while cellular Fe-S cluster assembly is controlled by correlated pathways both within the mitochondria and within the cytosol. Iron plays a vital role in a wide array of metabolic processes and defects in iron cofactor assembly leads to human diseases. This review describes progress towards our molecular-level understanding of cellular heme and Fe-S cluster biosynthesis, focusing on the regulation and mechanistic details that are essential for understanding human disorders related to the breakdown in these essential pathways.

Ferrochelatase catalyzes the formation of Zn-protoporphyrin of dry-cured ham via the conversion reaction from heme in meat

Ferrochelatase (FECH), the enzyme at the last step of the heme-biosynthetic pathway, is involved in the formation of Zn-protoporphyrin via an iron-removal reaction of heme. To improve the efficacy of the formation of Zn-protoporphyrin from heme, the use of recombinant FECHs from porcine, yeast, and bacteria was examined. Incubation of FECH with myoglobin in the presence of ascorbic acid and cysteine resulted in the efficient conversion of myoglobin-heme to Zn-protoporphyrin. Exogenously added recombinant yeast FECH facilitates the production of Zn-protoporphyrin from myoglobin-heme and heme in meat, via the replacement of iron in the protoporphyrin ring by zinc ions. A large amount of Zn-protoporphyrin was also generated by the catalysis of FECH using an intact piece of meat as a substrate. These findings can open up possible approaches for the generation of a nontoxic bright pigment, Zn-protoporphyrin, to shorten the incubation time required to produce dry-cured ham.

Genome-wide identification of TAL1's functional targets: insights into its mechanisms of action in primary erythroid cells

Coordination of cellular processes through the establishment of tissue-specific gene expression programs is essential for lineage maturation. The basic helix-loop-helix hemopoietic transcriptional regulator TAL1 (formerly SCL) is required for terminal differentiation of red blood cells. To gain insight into TAL1 function and mechanisms of action in erythropoiesis, we performed ChIP-sequencing and gene expression analyses from primary fetal liver erythroid cells. We show that TAL1 coordinates expression of genes in most known red cell-specific processes. The majority of TAL1's genomic targets require direct DNA-binding activity. However, one-fifth of TAL1's target sequences, mainly among those showing high affinity for TAL1, can recruit the factor independently of its DNA binding activity. An unbiased DNA motif search of sequences bound by TAL1 identified CAGNTG as TAL1-preferred E-box motif in erythroid cells. Novel motifs were also characterized that may help distinguish activated from repressed genes and suggest a new mechanism by which TAL1 may be recruited to DNA. Finally, analysis of recruitment of GATA1, a protein partner of TAL1, to sequences occupied by TAL1 suggests that TAL1's binding is necessary prior or simultaneous to that of GATA1. This work provides the framework to study regulatory networks leading to erythroid terminal maturation and to model mechanisms of action of tissue-specific transcription factors.

GATA-1 binding sites in exon 1 direct erythroid-specific transcription of PPOX

We investigated erythroid-specific expression of the human PPOX gene. This gene encodes protoporphyrinogen oxidase, which is involved in synthesizing heme for red blood cells and heme as a cofactor for the respiratory cytochromes. In vitro luciferase transfection assays in human uninduced and hemin induced erythroleukemic K562 cells showed that the presence of exon 1 increased promoter activity fourfold as compared to reporter constructs lacking this exon. This transcriptional regulation was mediated by two GATA-1 sites in exon 1. Electrophoretic mobility shift and chromatin immunoprecipitation assays demonstrated that both GATA sites were able to bind GATA-1 in vitro and in vivo. Exon 1 did not affect promoter activity in human hepatoma HepG2 cells and U937 monocytic cells but its presence decreased promoter activity in HeLa human cervical carcinoma cells. We conclude that the GATA-1 binding sites in exon 1 constitute key regulatory elements in differential expression of PPOX in erythroid and non-erythroid cells.

Methotrexate used in combination with aminolaevulinic acid for photodynamic killing of prostate cancer cells

Photodynamic therapy (PDT) using 5-aminolaevulinic acid (ALA) to drive production of an intracellular photosensitiser, protoporphyrin IX (PpIX), is a promising cancer treatment. However, ALA-PDT is still suboptimal for thick or refractory tumours. Searching for new approaches, we tested a known inducer of cellular differentiation, methotrexate (MTX), in combination with ALA-PDT in LNCaP cells. Methotrexate alone promoted growth arrest, differentiation, and apoptosis. Methotrexate pretreatment (1 mg l⁻¹, 72 h) followed by ALA (0.3 mM, 4 h) resulted in a three-fold increase in intracellular PpIX, by biochemical and confocal analyses. After exposure to 512 nm light, killing was significantly enhanced in MTX-preconditioned cells. The reverse order of treatments, ALA-PDT followed by MTX, yielded no enhancement. Methotrexate caused a similar relative increase in PpIX, whether cells were incubated with ALA, methyl-ALA, or hexyl-ALA, arguing against a major effect upon ALA transport. Searching for an effect among porphyrin synthetic enzymes, we found that coproporphyrinogen oxidase (CPO) was increased three-fold by MTX at the mRNA and protein levels. Transfection of LNCaP cells with a CPO-expressing vector stimulated the accumulation of PpIX. Our data suggest that MTX, when used to modulate intracellular production of endogenous PpIX, may provide a new combination PDT approach for certain cancers.

Biosynthesis of heme in mammals

Most iron in mammalian systems is routed to mitochondria to serve as a substrate for ferrochelatase. Ferrochelatase inserts iron into protoporphyrin IX to form heme which is incorporated into hemoglobin and cytochromes, the dominant hemoproteins in mammals. Tissue-specific regulatory features characterize the heme biosynthetic pathway. In erythroid cells, regulation is mediated by erythroid-specific transcription factors and the availability of iron as Fe/S clusters. In non-erythroid cells the pathway is regulated by heme-mediated feedback inhibition. All of the enzymes in the heme biosynthetic pathway have been crystallized and the crystal structures have permitted detailed analyses of enzyme mechanisms. All of the genes encoding the heme biosynthetic enzymes have been cloned and mutations of these genes are responsible for a group of human disorders designated the porphyrias and for X-linked sideroblastic anemia. The biochemistry, structural biology and the mechanisms of tissue-specific regulation are presented in this review along with the key features of the porphyric disorders.

Dual targeting of spinach protoporphyrinogen oxidase II to mitochondria and chloroplasts by alternative use of two in-frame initiation codons

Protoporphyrinogen oxidase (Protox) is the final enzyme in the common pathway of chlorophyll and heme biosynthesis. Two Protox isoenzymes have been described in tobacco, a plastidic and a mitochondrial form. We isolated and sequenced spinach Protox cDNA, which encodes a homolog of tobacco mitochondrial Protox (Protox II). Alignment of the deduced amino acid sequence between Protox II and other tobacco mitochondrial Protox homologs revealed a 26-amino acid N-terminal extension unique to the spinach enzyme. Immunoblot analysis of spinach leaf extract detected two proteins with apparent molecular masses of 57 and 55 kDa in chloroplasts and mitochondria, respectively. In vitro translation experiments indicated that two translation products (59 and 55 kDa) are produced from Protox II mRNA, using two in-frame initiation codons. Transport experiments using green fluorescent protein-fused Protox II suggested that the larger and smaller translation products (Protox IIL and IIS) target exclusively to chloroplasts and mitochondria, respectively.

Regulation of the expression of human ferrochelatase by intracellular iron levels

Mammalian ferrochelatase, the terminal enzyme of the heme biosynthetic pathway, catalyzes the insertion of a ferrous ion into protoporphyrin and contains a labile [2Fe-2S] cluster center at the C-terminus. To clarify the roles of the iron-sulfur cluster in the expression of mammalian ferrochelatase, enzyme activity in human erythroleukemia K562 cells under iron-depleted conditions was examined. Treatment of cells with an iron chelator, desferrioxamine, resulted in a decrease in enzyme activity, in a dose- and time-dependent manner. Heme content decreased during desferrioxamine treatment of the cells. Addition of ferric ion-nitrilotriacetate [Fe (III)NTA] to desferrioxamine-containing cultures led to restoration of the reduction in the enzyme activity. While RNA blots showed that the amount of ferrochelatase mRNA remained unchanged during these treatments, the amount of ferrochelatase decreased with a concomitant decrease in enzyme activity. When full-length human ferrochelatase was expressed in Cos7 cells, the activity was found mainly in the mitochondria and was decreased markedly by treatment with desferrioxamine. The activity in Cos7 cells expressing human ferrochelatase in cytoplasm decreased with desferrioxamine, but to a lesser extent. When Escherichia coli ferrochelatase, which lacks the iron-sulfur cluster, was expressed in Cos7 cells, the activity did not change following any treatment. Conversely, the addition of Fe (III)NTA to the culture of K562 and Cos7 cells led to an increase in ferrochelatase activity. These results indicate that the expression of mammalian ferrochelatase is regulated by intracellular iron levels, via the iron-sulfur cluster center at the C-terminus, and this contributes to the regulation of the biosynthesis of heme at the terminal step.

Analysis of ferrochelatase expression during hematopoietic development of embryonic stem cells

Ferrochelatase, the last enzyme in the heme pathway, chelates protoporphyrin IX and iron to form heme and is mutated in protoporphyria. The ferrochelatase gene is expressed in all tissues at low levels to provide heme for essential heme-containing proteins and is up-regulated during erythropoiesis for the synthesis of hemoglobin. The human ferrochelatase promoter contains 2 Sp1 cis-elements and GATA and NF–E2 sites, all of which bind their cognatetrans-acting factors in vitro. To investigate the role of these elements during erythropoiesis, we introduced expression of the green fluorescent protein (EGFP) transgenes driven by various ferrochelatase promoter fragments into a single locus in mouse embryonic stem cells. EGFP expression was monitored during hematopoietic differentiation in vitro using flow cytometry. We show that a promoter fragment containing the Sp1 sites, the NF–E2 and GATA elements, was sufficient to confer developmental-specific expression of the EGFP transgene, with an expression profile identical to that of the endogenous gene. In this system the −0.275 kb NF–E2 cis-element is required for erythroid-enhanced expression, the GATA cis-element functions as a stage-specific repressor and enhancer, and elements located between −0.375kb and −1.1kb are necessary for optimal levels of expression. Ferrochelatase mRNA increased before the primitive erythroid-cell stage without a concomitant increase in ferrochelatase protein, suggesting the presence of a translational control mechanism. Because of the sensitivity of this system, we were able to assess the effect of an A-to-G polymorphism identified in the promoters of patients with protoporphyria. There was no effect of the G haplotype on transcriptional activity of the −1.1 kb transgene.

Analysis of ferrochelatase expression during hematopoietic development of embryonic stem cells

Ferrochelatase, the last enzyme in the heme pathway, chelates protoporphyrin IX and iron to form heme and is mutated in protoporphyria. The ferrochelatase gene is expressed in all tissues at low levels to provide heme for essential heme-containing proteins and is up-regulated during erythropoiesis for the synthesis of hemoglobin. The human ferrochelatase promoter contains 2 Sp1 cis-elements and GATA and NF–E2 sites, all of which bind their cognatetrans-acting factors in vitro. To investigate the role of these elements during erythropoiesis, we introduced expression of the green fluorescent protein (EGFP) transgenes driven by various ferrochelatase promoter fragments into a single locus in mouse embryonic stem cells. EGFP expression was monitored during hematopoietic differentiation in vitro using flow cytometry. We show that a promoter fragment containing the Sp1 sites, the NF–E2 and GATA elements, was sufficient to confer developmental-specific expression of the EGFP transgene, with an expression profile identical to that of the endogenous gene. In this system the −0.275 kb NF–E2 cis-element is required for erythroid-enhanced expression, the GATA cis-element functions as a stage-specific repressor and enhancer, and elements located between −0.375kb and −1.1kb are necessary for optimal levels of expression. Ferrochelatase mRNA increased before the primitive erythroid-cell stage without a concomitant increase in ferrochelatase protein, suggesting the presence of a translational control mechanism. Because of the sensitivity of this system, we were able to assess the effect of an A-to-G polymorphism identified in the promoters of patients with protoporphyria. There was no effect of the G haplotype on transcriptional activity of the −1.1 kb transgene.

Homozygous variegate porphyria in South Africa: genotypic analysis in two cases

Variegate porphyria is an autosomal dominant disorder of heme metabolism which results from decreased activity of the enzyme protoporphyrinogen oxidase. Clinically, the disease manifests postpubertally and is characterized by photocutaneous sensitivity and/or acute neurovisceral crises. However, in homozygous variegate porphyria, onset of the disease usually occurs in infancy with severe skin manifestations. The molecular basis of variegate porphyria in two severely affected probands in two South African families is described. Mutation detection included combined SSCP-heteroduplex analysis followed by direct sequencing. The unrelated probands both had the common R59W mutation while the other lesion was Y348C or R138P (both novel mutations), causing homozygous variegate porphyria.

Acylation stabilizes a protease-resistant conformation of protoporphyrinogen oxidase, the molecular target of diphenyl ether-type herbicides

Protein acylation is an important way in which a number of proteins with a variety of functions are modified. The physiological role of the acylation of cellular proteins is still poorly understood. Covalent binding of fatty acids to nonintegral membrane proteins is thought to produce transient or permanent enhancement of the association of the polypeptide chains with biological membranes. In this paper, we investigate the functional role for the palmitoylation of an atypical membrane-bound protein, yeast protoporphyrinogen oxidase, which is the molecular target of diphenyl ether-type herbicides. Palmitoylation stabilizes an active heat- and protease-resistant conformation of the protein. Palmitoylation of protoporphyrinogen oxidase has been demonstrated to occur in vivo both in yeast cells and in a heterologous bacterial expression system, where it may be inhibited by cerulenin leading to the accumulation of degradation products of the protein. The thiol ester linking palmitoleic acid to the polypeptide chain was shown to be sensitive to hydrolysis by hydroxylamine and also by the widely used serine-protease inhibitor phenylmethylsulfonyl fluoride.

Structure and transcriptional regulation of the mouse ferrochelatase gene

Ferrochelatase (EC.4.99.1.1), the final step in the biosynthesis of heme, is widely expressed in various tissues and is induced in erythroid cells. We determined the structure of the mouse ferrochelatase gene after isolation and characterization of lambda phage clones mapping discrete regions of the cDNA. The gene spans about 25 kb and consists of 11 exons. The exon/intron boundary sequences conform to consensus acceptor (GTn)/donor (nAG) sequences, and exons in the gene encode functional protein domains. The promoter region contains multiple Sp1 sites, a CACCC box and GATA-1 binding sites. Function analysis of the promoter by transient transfection assay demonstrated that one Sp1 binding site located at -37/-32 is essential for basic expression of the ferrochelatase gene in both mouse erythroleukemia (MEL) and non-erythroid EL4 cells. In addition, the region (-66/-51) containing a CACCC box and the neighboring GC box partly contributes to the inducible activity of the reporter in MEL cells upon induction with dimethylsulfoxide. It appears that at least two promoter regions of the mouse ferrochelatase gene function in basic and inducible expression.

Molecular characterization of a newly identified heme-binding protein induced during differentiation of urine erythroleukemia cells

A heme-binding protein with a molecular mass of 22 kDa, termed p22 HBP, was purified from mouse liver cytosol, using blue Sepharose CL-6B. We identified a cDNA encoding p22 HBP, and sequence analysis revealed that p22 HBP comprises 190 amino acid residues (Mr 21,063) and has no homology to any other known heme-binding protein. The p22 HBP mRNA (approximately 1.0 kilobases) is ubiquitously expressed in various tissues and is extremely abundant in the liver. cDNA allows for expression of active p22 HBP, with a high affinity for 55Fe-hemin, with a Kd of 26 +/-1.8 nM. The Bmax of hemin binding to p22 HBP was 0.55 +/- 0.021 mol/mol of protein, a value consistent with one heme molecule binding per molecule of protein. The order of potency of different ligands to compete against 55Fe-hemin binding to p22 HBP was hemin = protoporphyrin IX > coproporphyrin III > bilirubin > palmitic acid > all-trans-retinoic acid. Treatment of mouse erythroleukemia (MEL) cells with dimethyl sulfoxide or hemin resulted in an increase in p22 HBP mRNA. The immunoblot analysis showed that p22 HBP increased with time in dimethyl sulfoxide- and hemin-induced MEL cells. Conversely, transfer of antisense oligonucleotides to p22 HBP cDNA resulted in a decrease of p22 HBP in dimethyl sulfoxide-treated MEL cells, and the heme content in these cells decreased to 66-71% of sense oligonucleotides-transferred cells. Thus, this newly identified heme-binding protein, p22 HBP, may be involved in heme utilization for hemoprotein synthesis and even be coupled to hemoglobin synthesis during erythroid differentiation.

The domain structure of protoporphyrinogen oxidase, the molecular target of diphenyl ether-type herbicides

Protoporphyrinogen oxidase (EC 1-3-3-4), the 60-kDa membrane-bound flavoenzyme that catalyzes the final reaction of the common branch of the heme and chlorophyll biosynthesis pathways in plants, is the molecular target of diphenyl ether-type herbicides. It is highly resistant to proteases (trypsin, endoproteinase Glu-C, or carboxypeptidases A, B, and Y), because the protein is folded into an extremely compact form. Trypsin maps of the native purified and membrane-bound yeast protoporphyrinogen oxidase show that this basic enzyme (pI > 8.5) was cleaved at a single site under nondenaturing conditions, generating two peptides with relative molecular masses of 30,000 and 35,000. The endoproteinase Glu-C also cleaved the protein into two peptides with similar masses, and there was no additional cleavage site under mild denaturing conditions. N-terminal peptide sequence analysis of the proteolytic (trypsin and endoproteinase Glu-C) peptides showed that both cleavage sites were located in putative connecting loop between the N-terminal domain (25 kDa) with the betaalphabeta ADP-binding fold and the C-terminal domain (35 kDa), which possibly is involved in the binding of the isoalloxazine moiety of the FAD cofactor. The peptides remained strongly associated and fully active with the Km for protoporphyrinogen and the Ki for various inhibitors, diphenyl-ethers, or diphenyleneiodonium derivatives, identical to those measured for the native enzyme. However, the enzyme activity of the peptides was much more susceptible to thermal denaturation than that of the native protein. Only the C-terminal domain of protoporphyrinogen oxidase was labeled specifically in active site-directed photoaffinity-labeling experiments. Trypsin may have caused intramolecular transfer of the labeled group to reactive components of the N-terminal domain, resulting in nonspecific labeling. We suggest that the active site of protoporphyrinogen oxidase is in the C-terminal domain of the protein, at the interface between the C- and N-terminal domains.

Differentiation-specific increase in ALA-induced protoporphyrin IX accumulation in primary mouse keratinocytes

A treatment regimen that takes advantage of the induction of intracellular porphyrins such as protoporphyrin IX (PPIX) by exposure to exogenous 5-amino-laevulinic acid (ALA) followed by localized exposure to visible light represents a promising new approach to photodynamic therapy (PDT). Acting upon the suggestion that the effectiveness of ALA-dependent PDT may depend upon the state of cellular differentiation, we investigated the effect of terminal differentiation upon ALA-induced synthesis of and the subsequent phototoxicity attributable to PPIX in primary mouse keratinocytes. Induction of keratinocyte differentiation augmented intracellular PPIX accumulation in cells treated with ALA. These elevated PPIX levels resulted in an enhanced lethal photodynamic sensitization of differentiated cells. The differentiation-dependent increase in cellular PPIX levels resulted from several factors including: (a) increased ALA uptake, (b) enhanced PPIX production and (c) decreased PPIX export into the culture media. Simultaneously, steady-state levels of coproporphyrinogen oxidase mRNA increased but aminolaevulinic acid dehydratase mRNA levels remained unchanged. From experiments using 12-o-tetradecanoylphorbol-13-acetate, transforming growth factor beta 1 and calcimycin we demonstrated that the increase in PPIX concentration in terminally differentiating keratinocytes is calcium- and differentiation specific. Stimulation of the haem synthetic capacity is seen in primary keratinocytes, but not in PAM 212 cells that fail to undergo differentiation. Interestingly, increased PPIX formation and elevated coproporphyrinogen oxidase mRNA levels are not limited to differentiating keratinocytes; these were also elevated in the C2C12 myoblast and the PC12 adrenal cell lines upon induction of differentiation. Overall, the therapeutic implications of these results are that the effectiveness of ALA-dependent PDT depends on the differentiation status of the cell and that this may enable selective targeting of several tissue types.

Homozygous variegate porphyria: identification of mutations on both alleles of the protoporphyrinogen oxidase gene in a severely affected proband

Homozygous variegate porphyria is a severe skin and neurologic disease manifesting in early infancy, and characterized by markedly reduced levels of the penultimate enzyme in the heme biosynthetic pathway, protoporphyrinogen oxidase. We investigated the molecular basis of variegate porphyria, usually an autosomal dominantly inherited trait, in a severely affected female proband and her parents. The mutation detection strategy included heteroduplex analysis, automated sequencing, and allele specific oligonucleotide hybridization. We identified two underlying missense mutations in the protoporphyrinogen oxidase gene, consisting of a G-to-A transition in exon 6 (G169E), and a G-to-A transition in exon 10 (G358R). Our study establishes the molecular basis of "homozygous" variegate porphyria for the first time, in demonstrating that this patient is a compound heterozygote for two different missense mutations in the protoporphyrinogen oxidase gene.

Variegate porphyria: identification of a nonsense mutation in the protoporphyrinogen oxidase gene

The porphyrias are disorders of porphyrin metabolism that result from inherited or acquired aberrations in the control of the heme biosynthetic pathway. Variegate porphyria is characterized by a partial reduction in the activity of protoporphyrinogen oxidase. In this study, we identified the first nonsense mutation in a family with variegate porphyria. The mutation consisted of a previously unreported G-to-T transversion in exon 5 of the protoporphyrinogen oxidase gene, resulting in the substitution of glutamic acid by a nonsense codon, designated E133X. Our investigation establishes that a nonsense mutation in the protoporphyrinogen oxidase gene is the underlying mutation in this family with variegate porphyria.

Molecular basis of variegate porphyria: a missense mutation in the protoporphyrinogen oxidase gene

Variegate porphyria (VP) is an autosomal dominant disorder characterised by a partial defect in the activity of protoporphyrinogen oxidase (PPO), and has recently been genetically linked to the PPO gene on chromosome 1q22-23 (Z=6.62). In this study, we identified a mutation in the PPO gene in a patient with VP and two unaffected family members. The mutation consisted of a previously unreported T to C transition in exon 13 of the PPO gene, resulting in the substitution of a polar serine by a non-polar proline (S450P). This serine residue is evolutionarily highly conserved in man, mouse, and Bacillus subtilis, attesting to the importance of this residue. Interestingly, the gene for Gardner's syndrome (FAP) also segregates in this family, independently of the VP mutation. Gardner's syndrome or familial adenomatous polyposis (FAP) is also an autosomal dominantly inherited genodermatosis, and typically presents with colorectal cancer in early adult life secondary to extensive adenomatous polyps of the colon. The specific gene on chromosome 5 that is the site of the mutation in this disorder is known as APC (adenomatous polyposis coli), and the gene has been genetically linked to the region of 5q22.

Molecular mechanism of heme biosynthesis

Two of the major organs producing heme are bone marrow and the liver. delta-Aminolevulinate synthase (ALAS) plays the key role to regulate heme biosynthesis in hepatocytes as well as in erythroid cells. In the liver, nonspecific (or housekeeping) isozyme of ALAS (ALAS-N) is expressed to be regulated by its end product, heme, in a negative feedback manner. The way to regulate ALAS-N in the liver is suitable to supply a constant level of heme for a family of drug metabolizing enzymes, cytochrome P-450 (CYP). In erythroid tissues, not only erythroid-specific isozyme of ALAS (ALAS-E) but also ALAS-N are expressed, and regulated by distinctive manners. Although heme regulates ALAS-N in a negative feedback manner even in erythroid cells, ALAS-E is upregulated by induced heme concentration. ALAS-N in undifferentiated erythroid cells, therefore, is suggested to produce heme for CYP, whereas heme for accumulating hemoglobin (Hb) in cells undergoing differentiation is synthesized via ALAS-E. In this article, we describe the molecular mechanisms to regulate heme biosynthesis in non-erythroid as well as in erythroid tissues, and discuss the pathological significance of the mechanisms in patients with inherited disorders, porphyrias.

Cloning and characterization of a plastidal and a mitochondrial isoform of tobacco protoporphyrinogen IX oxidase

Protoporphyrinogen IX oxidase is the last enzyme in the common pathway of heme and chlorophyll synthesis and provides precursor for the mitochondrial and plastidic heme synthesis and the predominant chlorophyll synthesis in plastids. We cloned two different, full-length tobacco cDNA sequences by complementation of the protoporphyrin-IX-accumulating Escherichia coli hemG mutant from heme auxotrophy. The two sequences show similarity to the recently published Arabidopsis PPOX, Bacillus subtilis hemY, and to mammalian sequences encoding protoporphyrinogen IX oxidase. One cDNA sequence encodes a 548-amino acid residues protein with a putative transit sequence of 50 amino acid residues, and the second cDNA encodes a protein of 504 amino acid residues. Both deduced protein sequences share 27.2% identical amino acid residues. The first in vitro translated protoporphyrinogen IX oxidase could be translocated to plastids, and the approximately 53-kDa mature protein was detected in stroma and membrane fraction. The second enzyme was targeted to mitochondria without any detectable reduction in size. Localization of both enzymes in subcellular fractions was immunologically confirmed. Steady-state RNA analysis indicates an almost synchronous expression of both genes during tobacco plant development, greening of young seedlings, and diurnal and circadian growth. The mature plastidal and the mitochondrial isoenzyme were overexpressed in E. coli. Bacterial extracts containing the recombinant mitochondrial enzyme exhibit high protoporphyrinogen IX oxidase activity relative to control strains, whereas the plastidal enzyme could only be expressed as an inactive peptide. The data presented confirm a compartmentalized pathway of tetrapyrrole synthesis with protoporphyrinogen IX oxidase in plastids and mitochondria.

Isolated Bacillus subtilis HemY has coproporphyrinogen III to coproporphyrin III oxidase activity

Oxidation of coproporphyrinogen III to coproporphyrin III is found in extracts of Escherichia coli cells containing the Bacillus subtilis HemY protein (M. Hansson and L. Hederstedt, J. Bacteriol. 176, 5962-5970). We have analysed whether this activity is due to the heterologous expression system, since it in vivo would lead to disruption of the heme biosynthetic pathway. B. subtilis hemY was fused in its 3'-end to a polynucleotide encoding six histidine residues and expressed from plasmids in both E. coli and B. subtilis. The His6-tagged HemY protein extracted from membranes using non-ionic detergent was purified by Ni2+ affinity chromatography. Isolated HemY fusion protein synthesised in E. coli and B. subtilis oxidised coproporphyrinogen III to coproporphyrin III. No direct formation of protoporphyrin IX from coproporphyrinogen III could be detected. Our results suggest that the coproporphyrinogen III to coproporphyrin III activity of HemY is either avoided in B. subtilis in vivo or that coproporphyrin III is a heme biosynthetic intermediate in this bacterium.

Involvement of the transcriptional factor GATA-1 in regulation of expression of coproporphyrinogen oxidase in mouse erythroleukemia cells

Coproporphyrinogen oxidase (CPO; EC 1.3.3.3), the sixth enzyme of heme biosynthesis, transcribed from a single promoter is markedly induced during erythroid differentiation. CPO is ubiquitously expressed in all cells. To determine cis-acting elements of the human CPO gene, the promoter region of the gene was isolated, and three potential GATA-1 motifs and four GC boxes were found within this fragment. In a functional analysis of various deletion mutants, we found that the GATA-1 binding site at -143 to -138 was essential for basic and inducible expressions of the CPO gene in mouse erythroleukemia (MEL) cells. Gel mobility shift assay revealed that GATA-1 bound to the region is required for the expression and this was confirmed by observations that the nuclear protein bound to the GATA-1 motif was supershifted with anti GATA-1 antibody, by gel mobility shift assay. Furthermore, co-expression of mouse GATA-1 in MEL cells led to an increase in the promoter activity, which was markedly increased by dimethyl sulfoxide-treatment. These results indicate that GATA-1 plays an important role in regulation of transcription of the CPO gene in erythroid cells.

Molecular cloning and characterization of a cDNA that encodes protoporphyrinogen oxidase of Arabidopsis thaliana

A cDNA encoding protoporphyrinogen oxidase (PPOX), the last enzyme common to the biosynthetic pathways for chlorophylls and hemes, was obtained from a library of Arabidopsis thaliana cDNA constructed in a lambda vector by screening for complementation of a hemG mutant of Escherichia coli. Extracts of E. coli cells transformed with the Arabidopsis PPOX cDNA had high PPOX activity, and this activity was markedly inhibited by acifluorfen, a specific inhibitor of PPOX. Sequence analysis revealed that the cDNA for Arabidopsis PPOX encodes a protein of 537 amino acids (aa) with a calculated molecular mass of 57.7 kDa. The deduced aa sequence exhibited similarity to sequences of PPOX from Bacillus subtilis, mouse, and human. However, the PPOX of Arabidopsis contained a putative leader peptide for import into mitochondria (mt). southern analysis indicated that the PPOX whose cDNA we cloned is encoded by a single gene in Arabidopsis. Northern blot analysis showed that the level of expression of the gene in Arabidopsis leaves was high. whereas it was low in roots and floral buds. To our knowledge, this is the first report for the cloning of a cDNA for a plant PPOX.

Cloning and characterization of the yeast HEM14 gene coding for protoporphyrinogen oxidase, the molecular target of diphenyl ether-type herbicides

Protoporphyrinogen oxidase, which catalyzes the oxygen-dependent aromatization of protoporphyrinogen IX to protoporphyrin IX, is the molecular target of diphenyl ether type herbicides. The structural gene for the yeast protoporphyrinogen oxidase, HEM14, was isolated by functional complementation of a hem14-1 protoporphyrinogen oxidase-deficient yeast mutant, using a novel one-step colored screening procedure to identify heme-synthesizing cells. The hem14-1 mutation was genetically linked to URA3, a marker on chromosome V, and HEM14 was physically mapped on the right arm of this chromosome, between PRP22 and FAA2. Disruption of the HEM14 gene leads to protoporphyrinogen oxidase deficiency in vivo (heme deficiency and accumulation of heme precursors), and in vitro (lack of immunodetectable protein or enzyme activity). The HEM14 gene encodes a 539-amino acid protein (59,665 Da; pI 9.3) containing an ADP- beta alpha beta-binding fold similar to those of several other flavoproteins. Yeast protoporphyrinogen oxidase was somewhat similar to the HemY gene product of Bacillus subtilis and to the human and mouse protoporphyrinogen oxidases. Studies on protoporphyrinogen oxidase overexpressed in yeast and purified as wild-type enzyme showed that (i) the NH2-terminal mitochondrial targeting sequence of protoporphyrinogen oxidase is not cleaved during importation; (ii) the enzyme, as purified, had a typical flavin semiquinone absorption spectrum; and (iii) the enzyme was strongly inhibited by diphenyl ether-type herbicides and readily photolabeled by a diazoketone derivative of tritiated acifluorfen. The mutant allele hem14-1 contains two mutations, L422P and K424E, responsible for the inactive enzyme. Both mutations introduced independently in the wild-type HEM14 gene completely inactivated the protein when analyzed in an Escherichia coli expression system.