Structure of a mouse erythroid 5-aminolevulinate synthase gene and mapping of erythroid-specific DNAse I hypersensitive sites

Molecular evolution of multiple-level control of heme biosynthesis pathway in animal kingdom

Adaptation of enzymes in a metabolic pathway can occur not only through changes in amino acid sequences but also through variations in transcriptional activation, mRNA splicing and mRNA translation. The heme biosynthesis pathway, a linear pathway comprised of eight consecutive enzymes in animals, provides researchers with ample information for multiple types of evolutionary analyses performed with respect to the position of each enzyme in the pathway. Through bioinformatics analysis, we found that the protein-coding sequences of all enzymes in this pathway are under strong purifying selection, from cnidarians to mammals. However, loose evolutionary constraints are observed for enzymes in which self-catalysis occurs. Through comparative genomics, we found that in animals, the first intron of the enzyme-encoding genes has been co-opted for transcriptional activation of the genes in this pathway. Organisms sense the cellular content of iron, and through iron-responsive elements in the 5′ untranslated regions of mRNAs and the intron-exon boundary regions of pathway genes, translational inhibition and exon choice in enzymes may be enabled, respectively. Pathway product (heme)-mediated negative feedback control can affect the transport of pathway enzymes into the mitochondria as well as the ubiquitin-mediated stability of enzymes. Remarkably, the positions of these controls on pathway activity are not ubiquitous but are biased towards the enzymes in the upstream portion of the pathway. We revealed that multiple-level controls on the activity of the heme biosynthesis pathway depend on the linear depth of the enzymes in the pathway, indicating a new strategy for discovering the molecular constraints that shape the evolution of a metabolic pathway.

The major splice variant of human 5-aminolevulinate synthase-2 contributes significantly to erythroid heme biosynthesis

The initial step of the heme biosynthetic pathway in erythroid cells is catalyzed by an erythroid-specific isoform of 5-aminolevulinate synthase-2 (ALAS2). Previously, an alternatively spliced mRNA isoform of ALAS2 was identified although the functional significance of the encoded protein was unknown. We sought to characterize the contribution of this ALAS2 isoform to overall erythroid heme biosynthesis. Here, we report the identification of three novel ALAS2 mRNA splice isoforms in addition to the previously described isoform lacking exon 4-derived sequence. Quantitation of these mRNAs using ribonuclease protection experiments revealed that the isoform without exon 4-derived sequence represents approximately 35-45% of total ALAS2 mRNA while the newly identified transcripts together represent approximately 15%. Despite the significant amounts of these three new transcripts, their features indicate that they are unlikely to substantially contribute to overall mitochondrial ALAS2 activity. In contrast, in vitro studies show that the major splice variant (lacking exon 4-encoded sequence) produces a functional enzyme, albeit with slightly reduced activity and with affinity for the ATP-specific, beta subunit of succinyl CoA synthase, comparable to that of mature ALAS2. It was also established that the first 49 amino acids of the ALAS2 pre-protein are necessary and sufficient for translocation across the mitochondrial inner membrane and that this process is not affected by the absence of exon 4-encoded sequence. We conclude that the major splice isoform of ALAS2 is functional in vivo and could significantly contribute to erythroid heme biosynthesis and hemoglobin formation.

Identification of the xenosensors regulating human 5-aminolevulinate synthase

Heme is an essential component of numerous hemoproteins with functions including oxygen transport, energy metabolism, and drug biotransformation. In nonerythropoietic cells, 5-aminolevulinate synthase (ALAS1) is the rate-limiting enzyme in heme biosynthesis. Upon exposure to drugs that induce cytochromes P450 and other drug-metabolizing enzymes, ALAS1 is transcriptionally up-regulated, increasing the rate of heme biosynthesis to provide heme for cytochrome P450 hemoproteins. We used a combined in silico-in vitro approach to identify sequences in the ALAS1 gene that mediate direct transcriptional response to xenobiotic challenge. We have characterized two enhancer elements, located 20 and 16 kb upstream of the transcriptional start site. Both elements respond to prototypic inducer drugs and interact with the human pregnane X receptor NR1I2 and the human constitutive androstane receptor NR1I3. Our results suggest that the fundamental mechanism of drug induction is the same for cytochromes P450 and ALAS1. Transcriptional activation of the ALAS1 gene is the first step in the coordinated up-regulation of apoprotein and heme synthesis in response to exogenous and endogenous signals controlling heme levels. Understanding the direct effects of drugs on heme synthesis is of clinical interest, particularly in patients with hepatic porphyrias.

Distinct domains of erythroid Krüppel-like factor modulate chromatin remodeling and transactivation at the endogenous beta-globin gene promoter

Characterization of the mechanism(s) of action of trans-acting factors in higher eukaryotes requires the establishment of cellular models that test their function at endogenous target gene regulatory elements. Erythroid Krüppel-like factor (EKLF) is essential for beta-globin gene transcription. To elucidate the in vivo determinants leading to transcription of the adult beta-globin gene, functional domains of EKLF were examined in the context of chromatin remodeling and transcriptional activation at the endogenous locus. Human EKLF (hEKLF) sequences, linked to an estrogen-responsive domain, were studied with an erythroblast cell line lacking endogenous EKLF expression (J2eDeltaeklf). J2eDeltaeklf cells transduced with hEKLF demonstrated a dose-dependent rescue of beta-globin transcription in the presence of inducing ligand. Further analysis using a series of amino-terminal truncation mutants of hEKLF identified a distinct internal domain, which is sufficient for transactivation. Interestingly, studies of the chromatin structure of the beta-promoter revealed that a smaller carboxy-terminal domain generated an open promoter configuration. In vitro and in vivo binding studies demonstrated that this region interacted with BRG1, a component of the SWI/SNF chromatin remodeling complex. However, further study revealed that BRG1 interacted with an even smaller domain of EKLF, suggesting that additional protein interactions are required for chromatin remodeling at the endogenous beta-promoter. Taken together, our findings support a stepwise process of chromatin remodeling and coactivator recruitment to the beta-globin promoter in vivo. The J2eDeltaeklf inducible hEKLF system will be a valuable tool for further characterizing the temporal series of events required for endogenous beta-globin gene transcription.

Alternative splicing and tissue-specific transcription of human and rodent ubiquitous 5-aminolevulinate synthase (ALAS1) genes

The rate of haem synthesis in non-erythroid mammalian tissues is controlled by the ubiquitous isoform of 5-aminolevulinate synthase (ALAS1). In order to explore the regulation of mammalian ALAS1 genes, we have investigated the transcription of the human and rat genes. The 17 kb human gene differs from the rat gene in containing an additional untranslated exon that is alternatively spliced to produce a longer, minor mRNA transcript. Relative amounts of the two transcripts were similar in all tissues examined. Analysis of mRNA transcripts in human and rat tissues revealed tissue-specific differences in the use of transcription start sites by closely similar core promoters. In brain, initiation was from sites within and upstream from the TATA box, including an initiator-like element. In liver, initiation was TATA-driven from a single downstream site that appeared to be used exclusively for induction by drugs. Intermediate patterns were observed in other tissues and cell lines. Mutation of the TATA box did not impair transcription in transfected HeLa cells but activated upstream start sites, recapitulating the brain pattern. Our findings indicate that the conformation of the core ALAS1 promoter that directs assembly of the transcription pre-initiation complex may vary between tissues and have implications for understanding the tissue-specific regulated expression of this gene.

Human uroporphyrinogen-III synthase: genomic organization, alternative promoters, and erythroid-specific expression

Uroporphyrinogen-III (URO) synthase is the heme biosynthetic enzyme defective in congenital erythropoietic porphyria. The approximately 34-kb human URO-synthase gene (UROS) was isolated, and its organization and tissue-specific expression were determined. The gene had two promoters that generated housekeeping and erythroid-specific transcripts with unique 5'-untranslated sequences (exons 1 and 2A) followed by nine common coding exons (2B to 10). Expression arrays revealed that the housekeeping transcript was present in all tissues, while the erythroid transcript was only in erythropoietic tissues. The housekeeping promoter lacked TATA and SP1 sites, consistent with the observed low level expression in most cells, whereas the erythroid promoter contained GATA1 and NF-E2 sites for erythroid specificity. Luciferase reporter assays demonstrated that the housekeeping promoter was active in both erythroid K562 and HeLa cells, while the erythroid promoter was active only in erythroid cells and its activity was increased during hemin-induced erythroid differentiation. Thus, human URO-synthase expression is regulated during erythropoiesis by an erythroid-specific alternative promoter.

A novel endoproteolytic processing activity in mitochondria of erythroid cells and the role in heme synthesis

The erythroid isoform of aminolevulinate synthase (eALAS) protein is a major control point in erythroid heme synthesis and hemoglobin formation. Erythroid cells were extracted from mouse blood and bone marrow and metabolically labeled with 35S-methionine. This was followed by immunoprecipitation of eALAS protein products. The results show that the N-terminus of the expected full-length 59-kd form of the eALAS protein is truncated in bone marrow erythroid cells by approximately 7 kd. More differentiated erythroid cells in the peripheral blood exhibit very little of this protein truncation. Erythroid cells from the bone marrow were isolated using monoclonal antibody TER-119 and were shown to contain a unique endoprotease activity that could cleave the eALAS protein to the shorter form in vitro. With or without the mitochondrial signal sequence, the eALAS protein could serve as a substrate for the cleavage. This cleavage renders a functional eALAS protein and only removes a domain of unclear function, which has previously been reported to vary in size as a result of alternative RNA splicing. The protease activity was enriched from the membranes of mitochondria from bone marrow cells and was shown to be different from mitochondrial processing peptidase, medullasin, and other known proteases. Apart from the mitochondrial processing peptidase that cleaves the import signal sequence, this is the first description of a mitochondrially located site-specific processing protease activity.

A novel endoproteolytic processing activity in mitochondria of erythroid cells and the role in heme synthesis

The erythroid isoform of aminolevulinate synthase (eALAS) protein is a major control point in erythroid heme synthesis and hemoglobin formation. Erythroid cells were extracted from mouse blood and bone marrow and metabolically labeled with 35S-methionine. This was followed by immunoprecipitation of eALAS protein products. The results show that the N-terminus of the expected full-length 59-kd form of the eALAS protein is truncated in bone marrow erythroid cells by approximately 7 kd. More differentiated erythroid cells in the peripheral blood exhibit very little of this protein truncation. Erythroid cells from the bone marrow were isolated using monoclonal antibody TER-119 and were shown to contain a unique endoprotease activity that could cleave the eALAS protein to the shorter form in vitro. With or without the mitochondrial signal sequence, the eALAS protein could serve as a substrate for the cleavage. This cleavage renders a functional eALAS protein and only removes a domain of unclear function, which has previously been reported to vary in size as a result of alternative RNA splicing. The protease activity was enriched from the membranes of mitochondria from bone marrow cells and was shown to be different from mitochondrial processing peptidase, medullasin, and other known proteases. Apart from the mitochondrial processing peptidase that cleaves the import signal sequence, this is the first description of a mitochondrially located site-specific processing protease activity.

Transcriptional regulation of the murine erythroid-specific 5-aminolevulinate synthase gene

5-Aminolevulinate synthase (ALAS) catalyzes the first step of the heme biosynthetic pathway in mammalian cells. Separate genes encode the two isoforms: ubiquitously expressed ALAS (ALAS1) and erythroid-specific ALAS (ALAS2). Transcription of the ALAS2 gene is only activated during erythroid cell differentiation. This stimulation allows for the formation of hemoglobin-specific heme. The 5'-flanking region of the mouse ALAS2 gene was studied in order to define its erythroid-specific function in transcriptional activation. Putative binding sites for the erythroid-specific nuclear factors GATA-1, NF-E2, and EKLF were identified within the first 300bp region of the mouse ALAS2 5'-flanking region. However, this 300bp region alone did not efficiently activate transient expression in erythroid MEL and K562 cell lines. Additional DNA regulatory sequences found within 300-718bp upstream of the transcription start site were required for maximal transcriptional activation, even though these regions stimulated similar expression in the non-erythroid HeLa and NIH/3T3 cells. This suggests that cis-acting elements present in the 5'-flanking region are not responsible for maintenance of transcriptional silencing in non-erythroid cell lines and that tissue-specific regulation of ALAS2 depends on other regions of the gene or on chromatin remodeling. A putative hypoxia inducible factor 1 (HIF-1) response element was identified within the 300-718bp upstream region. Significantly, two proximal GATA-1-binding sites (-118/-113 and -98/-93) and a region located within -518 to -315bp of the mouse ALAS2 promoter were essential for transcriptional activation during chemically induced differentiation of MEL cells, implying their importance in conferring erythroid specificity to the ALAS2 transcriptional activation. This is the first study to delimit the cis-acting region responsible for activation of the ALAS2 promoter upon dimethyl-sulfoxide induction in MEL cells.

Structure and regulated expression of the delta-aminolevulinate synthase gene from Drosophila melanogaster

The structure of the single copy gene encoding the putative housekeeping isoform of Drosophila melanogaster delta-aminolevulinate synthase (ALAS) has been determined. Southern and immunoblot analyses suggest that only the housekeeping isoform of the enzyme exists in Drosophila. We have localized a critical region for promoter activity to a sequence of 121 base pairs that contains a motif that is potentially recognized by factors of the nuclear respiratory factor-1 (NRF-1)/P3A2 family, flanked by two AP4 sites. Heme inhibits the expression of the gene by blocking the interaction of putative regulatory proteins to its 5' proximal region, a mechanism different from those proposed for other hemin-regulated promoters. Northern and in situ RNA hybridization experiments show that maternal alas mRNA is stored in the egg; its steady-state level decreases rapidly during the first hours of development and increases again after gastrulation in a period where the synthesis of several mRNAs encoding metabolic enzymes is activated. In the syncytial blastoderm, the alas mRNA is ubiquitously distributed and decreases in abundance substantially through cellular blastoderm. Late in embryonic development alas shows a specific pattern of expression, with an elevated mRNA level in oenocytes, suggesting an important role of these cells in the biosynthesis of hemoproteins in Drosophila.

Regulation of erythroid 5-aminolevulinate synthase expression during erythropoiesis

Erythroid tissue is the major site of heme production in the body. The synthesis of heme and globin chains is coordinated at both the transcriptional and post-transcriptional levels to ensure that virtually no free heme or globin protein accumulates. The key rate-controlling enzyme of the heme biosynthetic pathway is 5-aminolevulinate synthase (ALAS) and an erythroid-specific isoform (ALAS2) is up-regulated during erythropoiesis. Differentiation of embryonic stem cells with a disrupted ALAS2 gene has established that expression of this gene is critical for erythropoiesis and cannot be compensated by expression of the ubiquitous isoform of the enzyme (ALAS1). Interestingly, heme appears to be important for expression of globin and other late erythroid genes and for erythroid cell differentiation although the mechanism of this effect is not clear. Transcriptional control elements that regulate the human gene for ALAS2 have been identified both in the promoter and in intronic enhancer regions. Subsequent translation of the ALAS2 mRNA is dependent on an adequate iron supply. The mechanism by which transcription of the gene for ALAS2 is increased by erythropoietin late in erythropoiesis remains an interesting issue. Erythropoietin action may result in altered levels of critical erythroid transcription factors or modulate the phosphorylation/acetylation status of these factors. Defects in the coding region of the gene for ALAS2 underlie the disease state X-linked sideroblastic anemia. In this review, we focus on the regulation and function of erythroid-specific 5-aminolevulinate synthase during erythropoiesis and its role in the X-linked sideroblastic anemia.

Comparison of the beluga whale (Delphinapterus leucas) expressed genes for 5-aminolevulinate synthase with those in other vertebrates

The cDNA and inferred amino acid sequences were determined for beluga whale (Delphinapterus leucas) erythroid (E) and housekeeping (H) forms of 5-aminolevulinate synthase (ALS), and they were compared with known sequences for five other vertebrates with particular attention to regulatory features. The cDNAs for whale ALS-E and -H encode, respectively, proteins of 582 and 640 amino acids. Sequence alignments suggest that the whale ALS-H, like those for rat and chicken, has an N-terminal mitochondrial targeting sequence of 56 amino acids. There is a high degree of amino acid conservation between the beluga whale proteins and those of other vertebrates, including regulatory elements and functional residues that have been defined in other ALSs. Both whale proteins contain three heme regulatory motifs suggesting that mitochondrial uptake may be regulated by heme. The ALS-E mRNA contains an iron responsive element in its 5'-untranslated region indicating that its expression may be post-transcriptionally regulated by cellular iron. This extensive structural similarity and the presence of the same regulatory elements found in other ALSs indicate that regulation of ALS in beluga whale is similar to that in other vertebrates.

Identification and characterization of a conserved erythroid-specific enhancer located in intron 8 of the human 5-aminolevulinate synthase 2 gene

Thirty five kilobases of sequence encompassing the human erythroid 5-aminolevulinate synthase (ALAS2) gene have been determined. Analysis revealed a very low GC content, few repetitive elements, and evidence for the insertion of a reverse-transcribed mRNA sequence and a neighboring gene. We have investigated whether introns 1, 3, and 8, which correspond to DNase I-hypersensitivity sites in the structurally related mouse ALAS2 gene, affect expression of the human ALAS2 promoter in transient expression assays. Whereas intron 3 was marginally inhibitory, introns 1 and 8 of the human gene stimulated promoter activity. Intron 8 harbored a strong erythroid-specific enhancer activity which was orientation-dependent. Deletion analysis of this region localized enhancer activity to a fragment of 239 base pairs. Transcription factor binding sites clustered within this region include GATA motifs and CACCC boxes, critical regulatory sequences of many erythroid cell-expressed genes. These sites were also identified in the corresponding intron of both the murine and canine ALAS2 genes. Mutagenesis of these conserved sites in the human intron 8 sequence and transient expression analysis in erythroid cells established the functional importance of one GATA motif and two CACCC boxes. The GATA motif bound GATA-1 in vitro. The two functional CACCC boxes each bound Sp1 or a related protein in vitro, but binding of the erythroid Krüppel-like factor and the basic Krüppel-like factor could not be detected. The intron 8 enhancer region was not activated by GATA-1 together with Sp1 in transactivation experiments in COS-1 cells indicating the involvement of a related Sp1 protein or of another unidentified erythroid factor. Overall, these results demonstrate that a GATA-1-binding site and CACCC boxes located within the human ALAS2 intron 8 are critical for the erythroid-specific enhancer activity in transfected erythroid cells, and due to the conserved nature of these binding sites across species, it seems likely that these sites play a functional role in the tissue-restricted expression of the gene in vivo.

Transcriptional regulation of the human erythroid 5-aminolevulinate synthase gene. Identification of promoter elements and role of regulatory proteins

We have characterized the 5'-flanking region of the human erythroid-specific 5-amino levulinate synthase (ALAS) gene (the ALAS2 gene) and shown that the first 300 base pairs of promoter sequence gives maximal expression in erythroid cells. Transcription factor binding sites clustered within this promoter sequence include GATA motifs and CACCC boxes, critical regulatory sequences of many erythroid cell-expressed genes. GATA sites at -126/-121 (on the noncoding strand) and -102/-97 were each recognized by GATA-1 protein in vitro using erythroid cell nuclear extracts. Promoter mutagenesis and transient expression assays in erythroid cells established that both GATA-1 binding sites were functional and exogenously expressed GATA-1 increased promoter activity through these sites in transactivation experiments. A noncanonical TATA sequence at the expected TATA box location (-30/-23) bound GATA-1- or TATA-binding protein (TBP) in vitro. Conversion of this sequence to a canonical TATA box reduced expression in erythroid cells, suggesting a specific role for GATA-1 at this site. However, expression was also markedly reduced when the -30/-23 sequence was converted to a consensus GATA-1 sequence (that did not bind TBP in vitro), suggesting that a functional interaction of both factors with this sequence is important. A sequence comprising two overlapping CACCC boxes at -59/-48 (on the noncoding strand) was demonstrated by mutagenesis to be functionally important. This CACCC sequence bound Sp1, erythroid Krüppel-like factor, and basic Krüppel-like factor in vitro, while in transactivation experiments erythroid Krüppel-like factor activated ALAS2 promoter expression through this sequence. A sequence at -49/-39 with a 9/11 match to the consensus for the erythroid specific factor NF-E2 was not functional. Promoter constructs with 5'-flanking sequence from 293 base pairs to 10.3 kilobase pairs expressed efficiently in COS-1 cells as well as in erythroid cells, indicating that an enhancer sequence located elsewhere or native chromatin structure may be required for the tissue-restricted expression of the gene in vivo.

Genetic regulation of delta-aminolevulinate dehydratase during erythropoiesis

In an effort to understand how the heme biosynthetic pathway is uniquely regulated in erythroid cells, we examined the structure of the gene encoding murine delta-aminolevulinate dehydratase (ALAD; EC4.2.1.24), which is the second enzyme of the pathway. The gene contains two first exons, named 1A and 1B, which are alternatively spliced to exon 2, where the coding region begins. Each first exon has its own promoter. The promoter driving exon 1A expression is TATA-less and contains many GC boxes. In contrast, the exon 1B promoter bears regulatory sequences similar to those found for beta-globin and other erythroid-specific genes. Tissue distribution studies reveal that ALAD mRNA containing axon 1A is ubiquitous, whereas mRNA containing axon 1B is found only in erythroid tissues. This finding, together with our further observation that GATA-1 mRNA levels increase 3-fold during maturation of murine erythroid progenitor cells, may help explain simultaneous 3-fold increases in exon 1B expression. The unexpected result that axon 1A expression also increases 3-fold during CFU-E maturation may be attributable to the action of NF-E2, since there is a potential binding site in a position analogous to the NF-E2 site in the locus control region of the beta-globin gene cluster.

Regulation of the aminolevulinate synthase gene in the Syrian hamster Harderian gland: changes during development and circadian rhythm and role of some hormones

The Syrian hamster Harderian gland has been advocated as a model to study the porphyrin biosynthetic pathway, since it shows by far the highest porphyrin concentration known to date. Another particular characteristic is the sexual dimorphism at both the morphological and the biochemical levels. We found a variation in the ALV-S (aminolevulinate synthase) gene expression according to sex, with females exhibiting much higher mRNA levels than do males. After castration, ALV-S mRNA rose considerably in males, this increase being inhibited by darkness or treatment with melatonin. Treatment with hCG or progesterone did not vary the ALV-S mRNA levels in females. Castrated males, however, showed a much larger increase when they were treated with hCG. No variations have been found in the expression of the ALV-S gene in female HG throughout the estrous cycle. During development, males and females showed similar ALV-S mRNA levels until they were 20 days old. Afterwards, they started showing gender-associated differences. In females, ALV-S mRNA levels rose during the first 3 months of life, and thereafter they decreased progressively with aging. A circadian rhythm has been found in the gene expression of ALV-S mRNA in females, showing very low levels in the morning and reaching a peak during the first hours of darkness. It was an endogenous rhythm, probably regulated at the transcriptional level. It is proposed that the light-dark period duration modulates this rhythm through the suprachiasmatic nucleus which in turn acts on the pineal secretion of melatonin that regulates ALV-S gene expression.

Molecular defects of erythroid 5-aminolevulinate synthase in X-linked sideroblastic anemia

The erythroid-specific isozyme of 5-aminolevulinate synthase (ALAS2), the first and rate-limiting enzyme of heme biosynthesis, is expressed concomitantly with the differentiation and maturation of the erythroid cell in order to accommodate generation of the large amounts of heme required for hemoglobin production. During the past few years the ALAS2 gene and its transcript have been characterized and the amino acid sequence of the enzyme deduced. The human genetic disorder X-linked sideroblastic anemia, previously postulated to be caused by defects of ALAS, has now been analyzed at the molecular and tissue-specific level. A heterogeneous group of point mutations in the catalytic domain of the ALAS2 enzyme has been found to cause the disorder. Impaired activity of recombinant mutant ALAS2 enzymes has also been demonstrated. Characterization of molecular defects in individuals with X-linked sideroblastic anemia has provided improved diagnosis for at-risk family members.

5-Aminolevulinate synthase and the first step of heme biosynthesis

5-Aminolevulinate synthase catalyzes the condensation of glycine and succinyl-CoA to yield 5-aminolevulinate. In animals, fungi, and some bacteria, 5-aminolevulinate synthase is the first enzyme of the heme biosynthetic pathway. Mutations on the human erythroid 5-aminolevulinate synthase, which is localized on the X-chromosome, have been associated with X-linked sideroblastic anemia. Recent biochemical and molecular biological developments provide important insights into the structure and function of this enzyme. In animals, two aminolevulinate synthase genes, one housekeeping and one erythroid-specific, have been identified. In addition, the isolation of 5-aminolevulinate synthase genomic and cDNA clones have permitted the development of expression systems, which have tremendously increased the yields of purified enzyme, facilitating structural and functional studies. A lysine residue has been identified as the residue involved in the Schiff base linkage of the pyridoxal 5'-phosphate cofactor, and the catalytic domain has been assigned to the C-terminus of the enzyme. A conserved glycine-rich motif, common to all aminolevulinate synthases, has been proposed to be at the pyridoxal 5'-phosphate-binding site. A heme-regulatory motif, present in the presequences of 5-aminolevulinate synthase precursors, has been shown to mediate the inhibition of the mitochondrial import of the precursor proteins in the presence of heme. Finally, the regulatory mechanisms, exerted by an iron-responsive element binding protein, during the translation of erythroid 5-aminolevulinate synthase mRNA, are discussed in relation to heme biosynthesis.

Structure and regulation of the chicken erythroid delta-aminolevulinate synthase gene

Erythroid cells regulate heme biosynthesis in a manner that is distinct from all other cell types. While heme negatively regulates the synthesis of the housekeeping delta-aminolevulinate synthase (ALAS-N) in all non-erythroid cells, the expression of an erythroid-specific isozyme (ALAS-E) is developmentally regulated in red blood cells. As a first step towards understanding the molecular basis for the transcriptional regulation of ALAS-E during erythropoiesis, we cloned and characterized the chicken ALAS-E locus. This gene spans 18 kbp and is composed of eleven exons. The intron/exon structure of erythroid ALAS was found to be conserved among several vertebrate species. Direct RNA sequencing identified a 5' untranslated region that is derived from two continuous exons and is predicted to form a very stable stem-loop structure that bears resemblance to the ferritin iron-responsive element. Tissue-specific expression of the ALAS-E gene was analyzed by transient transfection assays in hematopoietic cells of both erythroid and non-erythroid origins. These experiments identified distal (-784 to -505 bp) and proximal (-155 to +21 bp) promoter elements which are required for high level, erythroid-specific transcription.

Regulation of transferrin receptor mRNA expression. Distinct regulatory features in erythroid cells

In proliferating non-erythroid cells, the expression of transferrin receptors (TfR) is negatively regulated by the amount of intracellular iron. Fe-dependent regulation of TfR occurs post-transcriptionally and is mediated by iron-responsive elements (IRE) located in the 3' untranslated region of the TfR mRNA. IREs are recognized by a specific cytoplasmic binding protein (IRE-BP) that, in the absence of Fe, binds with high affinity to TfR mRNA, preventing its degradation. While TfR numbers are positively correlated with proliferation in non-erythroid cells, in hemoglobin-synthesizing cells, their numbers increase during differentiation and are, therefore, negatively correlated with proliferation. This suggests a distinct regulation of erythroid TfR expression and evidence, as follows, for this was found in the present study. (a) With nuclear run-on assays, our experiments show increased TfR mRNA transcription following induction of erythroid differentiation of murine erythroleukemia (MEL) with Me2SO. (b) Me2SO treatment of MEL cells does not increase IRE-BP activity which is, however, increased in uninduced MEL cells by Fe chelators. (c) Following induction of MEL cells, there is an increase in the stability of TfR mRNA, whose level is only slightly affected by iron excess. (d) Heme-synthesis inhibitors, such as succinylacetone and isonicotinic acid hydrazide, which inhibit numerous aspects of erythroid differentiation, also inhibit TfR mRNA expression in induced MEL cells. However, heme-synthesis inhibition does not lead to a decrease in TfR mRNA levels in uninduced MEL cells. Thus, these studies indicate that TfR gene expression is regulated differently in hemoglobin synthesizing as compared to uninduced MEL cells.

Biphasic ordered induction of heme synthesis in differentiating murine erythroleukemia cells: role of erythroid 5-aminolevulinate synthase

During dimethyl sulfoxide (DMSO)-stimulated differentiation of murine erythroleukemia (MEL) cells, one of the early events is the induction of the heme biosynthetic pathway. While recent reports have clearly demonstrated that GATA-1 is involved in the induction of erythroid cell-specific forms of 5-aminolevulinate synthase (ALAS-2) and porphobilinogen (PBG) deaminase and that cellular iron status plays a regulatory role for ALAS-2, little is known about regulation of the remainder of the pathway. In the current study, we have made use of a stable MEL cell mutant (MEAN-1) in which ALAS-2 enzyme activity is not induced by DMSO, hexamethylene bisacetamide (HMBA), or butyric acid. In this cell line, addition of 2% DMSO to growing cultures results in the normal induction of PBG deaminase and coproporphyrinogen oxidase but not in the induction of the terminal two enzymes, protoporphyrinogen oxidase and ferrochelatase. These DMSO-treated cells did not produce mRNA for beta-globin and do not terminally differentiate. In addition, the cellular level of ALAS activity declines rapidly after addition of DMSO, indicating that ALAS-1 must turn over rapidly at this time. Addition of 75 microM hemin alone to the cultures did not induce cells to terminally differentiate or induce any of the pathway enzymes. However, the simultaneous addition of 2% DMSO and 75 microM hemin caused the cells to carry out a normal program of terminal erythroid differentiation, including the induction of ferrochelatase and beta-globin. These data suggest that induction of the entire heme biosynthetic pathway is biphasic in nature and that induction of the terminal enzymes may be mediated by the end product of the pathway, heme. We have introduced mouse ALAS-2 cDNA into the ALAS-2 mutant cell line (MEAN-1) under the control of the mouse metallothionein promoter (MEAN-RA). When Cd and Zn are added to cultures of MEAN-RA in the absence of DMSO, ALAS-2 is induced but erythroid differentiation does not occur and cells continue to grow normally. In the presence of metallothionein inducers and DMSO, the MEAN-RA cells induce in a fashion similar to that found with the wild-type 270 MEL cells. Induction of the activities of ALAS, PBG deaminase, coproporphyrinogen oxidase, and ferrochelatase occurs. In cultures of MEAN-RA where ALAS-2 had been induced with Cd plus Zn 24 h prior to DMSO addition, onset of heme synthesis occurs more rapidly than when DMSO and Cd plus Zn are added simultaneously. This study reveals that induction of ALAS-2 alone is not sufficient to induce terminal differentiation of the MEAN-RA cells, and it does not appear that ALAS-2 alone is the rate-limiting enzyme of the heme biosynthetic pathway during MEL cell differentiation.

Biphasic ordered induction of heme synthesis in differentiating murine erythroleukemia cells: role of erythroid 5-aminolevulinate synthase

During dimethyl sulfoxide (DMSO)-stimulated differentiation of murine erythroleukemia (MEL) cells, one of the early events is the induction of the heme biosynthetic pathway. While recent reports have clearly demonstrated that GATA-1 is involved in the induction of erythroid cell-specific forms of 5-aminolevulinate synthase (ALAS-2) and porphobilinogen (PBG) deaminase and that cellular iron status plays a regulatory role for ALAS-2, little is known about regulation of the remainder of the pathway. In the current study, we have made use of a stable MEL cell mutant (MEAN-1) in which ALAS-2 enzyme activity is not induced by DMSO, hexamethylene bisacetamide (HMBA), or butyric acid. In this cell line, addition of 2% DMSO to growing cultures results in the normal induction of PBG deaminase and coproporphyrinogen oxidase but not in the induction of the terminal two enzymes, protoporphyrinogen oxidase and ferrochelatase. These DMSO-treated cells did not produce mRNA for beta-globin and do not terminally differentiate. In addition, the cellular level of ALAS activity declines rapidly after addition of DMSO, indicating that ALAS-1 must turn over rapidly at this time. Addition of 75 microM hemin alone to the cultures did not induce cells to terminally differentiate or induce any of the pathway enzymes. However, the simultaneous addition of 2% DMSO and 75 microM hemin caused the cells to carry out a normal program of terminal erythroid differentiation, including the induction of ferrochelatase and beta-globin. These data suggest that induction of the entire heme biosynthetic pathway is biphasic in nature and that induction of the terminal enzymes may be mediated by the end product of the pathway, heme. We have introduced mouse ALAS-2 cDNA into the ALAS-2 mutant cell line (MEAN-1) under the control of the mouse metallothionein promoter (MEAN-RA). When Cd and Zn are added to cultures of MEAN-RA in the absence of DMSO, ALAS-2 is induced but erythroid differentiation does not occur and cells continue to grow normally. In the presence of metallothionein inducers and DMSO, the MEAN-RA cells induce in a fashion similar to that found with the wild-type 270 MEL cells. Induction of the activities of ALAS, PBG deaminase, coproporphyrinogen oxidase, and ferrochelatase occurs. In cultures of MEAN-RA where ALAS-2 had been induced with Cd plus Zn 24 h prior to DMSO addition, onset of heme synthesis occurs more rapidly than when DMSO and Cd plus Zn are added simultaneously. This study reveals that induction of ALAS-2 alone is not sufficient to induce terminal differentiation of the MEAN-RA cells, and it does not appear that ALAS-2 alone is the rate-limiting enzyme of the heme biosynthetic pathway during MEL cell differentiation.

Expression of ferrochelatase mRNA in erythroid and non-erythroid cells

Ferrochelatase, which catalyses the last step in haem biosynthesis, i.e. the insertion of Fe(II) into protophorphyrin IX, is present in all cells, but is particularly abundant in erythroid cells during haemoglobinization. Using mouse ferrochelatase cDNA as a probe two ferrochelatase transcripts, having lengths of 2.9 kb and 2.2 kb, were found in extracts of mouse liver, kidney, brain, muscle and spleen, the 2.9 kb transcript being more abundant in the non-erythroid tissues and the 2.2 kb transcript more predominant in spleen. In mouse erythroleukemia cells the 2.9 kb ferrochelatase transcript is also more abundant; however, following induction of erythroid differentiation by dimethyl sulphoxide there is a preferential increase in the 2.2 kb transcript, which eventually predominates. With mouse reticulocytes, the purest immature erythroid cell population available, over 90% of the total ferrochelatase mRNA is present as the 2.2 kb transcript. Since there is probably only one mouse ferrochelatase gene, the occurrence of two ferrochelatase transcripts could arise from the use of two putative polyadenylation signals in the 3' region of ferrochelatase DNA. This possibility was explored by using a 389 bp DNA fragment produced by PCR with synthetic oligoprimers having sequence similarity with a region between the polyadenylation sites. This fragment hybridized only to the 2.9 kb ferrochelatase transcript, indicating that the two transcripts differ at their 3' ends and suggesting that the 2.2 kb transcript results from the utilization of the upstream polyadenylation signal. The preferential utilization of the upstream polyadenylation signal may be an erythroid-specific characteristic of ferrochelatase gene expression.

Regulation of heme biosynthesis: distinct regulatory features in erythroid cells

Our previous research has demonstrated that in hemoglobin-synthesizing cells, as compared with nonerythroid cells, a step in iron transport from transferrin localized between the transferrin receptor and ferrochelatase is rate-limiting for the synthesis of heme. In this communication we report our more recent studies on the mechanisms involved in the regulation of the transferrin receptors and ferrochelatase in differentiating erythroid cells. Our studies indicate that transferrin receptor gene expression is regulated differently in hemoglobin synthesizing as compared with uninduced murine erythroleukemia (MEL) cells: 1) With nuclear run-on assays our experiments showed increased transferrin receptor mRNA transcription cells of MEL following induction of erythroid differentiation with dimethylsulfoxide (DMSO). 2) DMSO treatment of MEL cells does not increase iron-responsive element binding protein (IRE-BP) activity which is, however, increased in uninduced MEL cells by Fe chelators. 3) Following induction of MEL cells there is an increase in the stability of transferrin receptor mRNA whose level is only slightly affected by iron excess. Using murine ferrochelatase cDNA as a probe, two ferrochelatase transcripts having lengths of 2.9 kb and 2.2 kb were found in extracts of mouse liver, kidney, brain, muscle and spleen, the 2.9 kb transcript being more abundant in nonerythroid tissues and the 2.2 more predominant in spleen. In MEL cells, the 2.9 ferrochelatase transcript is also more abundant; however, following induction of erythroid differentiation by DMSO there is a preferential increase in the 2.2 kb transcript which eventually predominates. With mouse reticulocytes, the purest immature erythroid cell population available, over 90% of the total ferrochelatase mRNA is present as the 2.2 kb transcript. Our further experiments indicate that the 2.2 kb transcript results from the utilization of the upstream polyadenylation signal and suggest that the preferential utilization of the upstream polyadenylation signal may be an erythroid-specific characteristic of ferrochelatase gene expression. These results provide further evidence for the idea that iron metabolism and heme synthesis are controlled by distinct mechanisms in erythroid versus nonerythroid cells.

Regulation by heme of mitochondrial protein transport through a conserved amino acid motif

A conserved motif, termed the heme regulatory motif (HRM), was identified in the presequences of the erythroid delta-aminolevulinate synthase precursors and was shown to be involved in hemin inhibition of transport of these proteins into mouse mitochondria in vitro. When the HRM was inserted into the presequence of the ornithine transcarbamoylase precursor, a normally unregulated mitochondrial protein, it conferred hemin inhibition on the transport of the chimeric protein. The conserved cysteine within the HRM was shown by site-directed mutagenesis to be required for hemin inhibition.

Position is the critical determinant for function of iron-responsive elements as translational regulators

At least two groups of eukaryotic mRNAs (ferritin and erythroid 5-aminolevulinate synthase) are translationally regulated via iron-responsive elements (IREs) located in a conserved position within the 5' untranslated regions of their mRNAs. We establish that the spacing between the 5' terminus of an mRNA and the IRE determines the potential of the IRE to mediate iron-dependent translational repression. The length of the RNA spacer rather than its nucleotide sequence or predicted secondary structure is shown to be the primary determinant of IRE function. When the position of the IRE is preserved, sequences flanking the IRE in natural ferritin mRNA can be replaced by altered flanking sequences without affecting the regulatory function of the IRE in vivo. These results define position as a critical cis requirement for IRE function in vivo and imply the potential to utilize transcription start site selection to modulate the function of this translational regulator.

Position is the critical determinant for function of iron-responsive elements as translational regulators

At least two groups of eukaryotic mRNAs (ferritin and erythroid 5-aminolevulinate synthase) are translationally regulated via iron-responsive elements (IREs) located in a conserved position within the 5' untranslated regions of their mRNAs. We establish that the spacing between the 5' terminus of an mRNA and the IRE determines the potential of the IRE to mediate iron-dependent translational repression. The length of the RNA spacer rather than its nucleotide sequence or predicted secondary structure is shown to be the primary determinant of IRE function. When the position of the IRE is preserved, sequences flanking the IRE in natural ferritin mRNA can be replaced by altered flanking sequences without affecting the regulatory function of the IRE in vivo. These results define position as a critical cis requirement for IRE function in vivo and imply the potential to utilize transcription start site selection to modulate the function of this translational regulator.

Enzymatic defect in "X-linked" sideroblastic anemia: molecular evidence for erythroid delta-aminolevulinate synthase deficiency

Recently, the human gene encoding erythroid-specific delta-aminolevulinate synthase was localized to the chromosomal region Xp21-Xq21, identifying this gene as the logical candidate for the enzymatic defect causing "X-linked" sideroblastic anemia. To investigate this hypothesis, the 11 exonic coding regions of the delta-aminolevulinate synthase gene were amplified and sequenced from a 30-year-old Chinese male with a pyridoxine-responsive form of X-linked sideroblastic anemia. A single T----A transition was found in codon 471 in a highly conserved region of exon 9, resulting in an Ile----Asn substitution. This mutation interrupted contiguous hydrophobic residues and was predicted to transform a region of beta-sheet structure to a random-coil structure. Prokaryotic expression of the normal and mutant cDNAs revealed that the mutant construct expressed low levels of enzymatic activity that required higher concentrations of pyridoxal 5'-phosphate to achieve maximal activation than did the normal enzyme. The amino acid substitution occurred in the exon containing the putative pyridoxal 5'-phosphate binding site and may account for the reduced ability of the cofactor to catalyze the formation of delta-aminolevulinic acid.

Structure of the human ferrochelatase gene. Exon/intron gene organization and location of the gene to chromosome 18

We have determined the structure of the human ferrochelatase gene after isolation and characterization of lambda phage clones mapping discrete regions of the cDNA. This gene was assigned to human chromosome 18 at region q21.3, by fluorescent in situ hybridization. The gene contains a total of 11 exons and has a minimum size of about 45 kb. The exon/intron boundary sequences conform to consensus acceptor (GTn) and donor (nAG) sequences, and the exons in the gene appear to encode functional protein domains. A major site of the transcription initiation, determined by S1 nuclease mapping, was assigned to an adenine base 89 bases upstream from the adenine base of the translation initiation ATG. The promoter region contains a potential binding site for Sp1, NF-E2 and erythroid-specific transcriptional factor GATA-1, but not a typical TATAA or CCAAT sequence. Analysis of primer extension showed that the transcription starts at the same position between hepatoma HepG2 and erythroleukemia K562 cell mRNA, thereby suggesting that there can be a single transcript in erythroid and non-erythroid cells.

Eclectic mechanisms of heme regulation of hematopoiesis

Regulatory features of heme (ferroprotoporphyrin IX) on hematopoietic growth/differentiation and related processes are reviewed. It is emphasized that expressions of specific erythroid and nonerythroid heme biosynthetic and degradatory enzymes are required, and the regulatory processes whereby this occurs is considered. The specificity of heme, relationship to cellular events such as differentiation, response to growth factors, oncogene and receptor expression, and how heme counteracts toxic effects such as viral growth are all discussed. The significance of heme in the hemopoietic bone marrow microenvironment and growth factor network are considered. Finally, the third pathway for arachidonic acid metabolism via the heme-cytochrome P450 monooxygenase system, in addition to cyclooxygenase and lipoxygenase, by bone marrow adherent cells and its role in cellular differentiation is briefly reviewed.

Molecular regulation--biological role of heme in hematopoiesis

Heme synthesis and degradation play pivotal roles in the regulation of growth and differentiation of erythroid and non-erythroid cells. Heme synthesis in mammalian cells involves eight enzymes which are localized in mitochondrial and cytoplasmic compartments. These enzymes have been well-characterized and cDNAs for six of the enzymes has been cloned. Two enzymes in the enzymes of the heme biosynthetic pathway, delta-aminolevulinic acid synthase (ALAS) and porphobilinogen deaminase (PBG-D) have special features and may have regulatory functions in heme synthesis by hematopoietic cells. ALAS exists as two isozymes which are encoded by non-erythroid and erythroid-specific genes, respectively. By contrast, PBG-D, which also exists as two isozymes, arises from a single gene comprised of two overlapping transcriptional units, each with its own promoter. Transcription from one or the other of these promoters gives rise through differential splicing to two distinct mRNA species which encode the distinct nonerythroid and erythroid isoforms. On the other hand, heme catabolism is determined by the levels of the heme oxygenase system. The enzyme has been purified and the cDNA for heme oxygenase has been cloned. Repression of heme oxygenase in erythroid progenitor cells may initiate differentiation. In addition, recent evidence has suggested that heme may have a broader role in hematopoiesis and in the network of cytokine production by adherent stromal cells.

Initiation of transcription of the erythroid promoter of the porphobilinogen deaminase gene is regulated by a cis-acting sequence around the cap site

Although the erythroid-specific promoter of human porphobilinogen deaminase [PBGD] gene has no TATA box, transcription is initiated at a single nucleotide. Using 5' and 3' deletions and point mutations, we have identified an element, located around the initiation site, which is necessary and sufficient for 'in vitro' accurate initiation of transcription. This 15 bp element extends 1 bp 5' and 14 bp 3' from the initiation site. It is composed of two regions, a proximal region centred on the cap site and a distal region which bears homology with the TdT initiator element. We show that a nuclear factor, present both in erythroid and non erythroid cells, binds the distal PBGD initiator element. Lack of heat inactivation suggests that initiation of transcription mediated by this element is not TFIID dependent. By transfection into erythroid cells, we also show that the proximal PBGD initiator element is essential for the selection of the initiation site but not for the regulation of transcription of the PBGD erythroid promoter during erythroid differentiation.