Regulation of 5-aminolevulinate synthase mRNA in different rat tissues

Heme metabolism in nonerythroid cells

Heme is an iron-containing prosthetic group necessary for the function of several proteins termed "hemoproteins." Erythrocytes contain most of the body's heme in the form of hemoglobin and contain high concentrations of free heme. In nonerythroid cells, where cytosolic heme concentrations are 2 to 3 orders of magnitude lower, heme plays an essential and often overlooked role in a variety of cellular processes. Indeed, hemoproteins are found in almost every subcellular compartment and are integral in cellular operations such as oxidative phosphorylation, amino acid metabolism, xenobiotic metabolism, and transcriptional regulation. Growing evidence reveals the participation of heme in dynamic processes such as circadian rhythms, NO signaling, and the modulation of enzyme activity. This dynamic view of heme biology uncovers exciting possibilities as to how hemoproteins may participate in a range of physiologic systems. Here, we discuss how heme is regulated at the level of its synthesis, availability, redox state, transport, and degradation and highlight the implications for cellular function and whole organism physiology.

Classification of likely functional class for ligand binding sites identified from fragment screening

Fragment screening is used to identify binding sites and leads in drug discovery, but it is often unclear which binding sites are functionally important. Here, data from 37 experiments, and 1309 protein structures binding to 1601 ligands were analysed. A method to group ligands by binding sites is introduced and sites clustered according to profiles of relative solvent accessibility. This identified 293 unique ligand binding sites, grouped into four clusters (C1-4). C1 includes larger, buried, conserved, and population missense-depleted sites, enriched in known functional sites. C4 comprises smaller, accessible, divergent, missense-enriched sites, depleted in functional sites. A site in C1 is 28 times more likely to be functional than one in C4. Seventeen sites, which to the best of our knowledge are novel, in 13 proteins are identified as likely to be functionally important with examples from human tenascin and 5-aminolevulinate synthase highlighted. A multi-layer perceptron, and K-nearest neighbours model are presented to predict cluster labels for ligand binding sites with an accuracy of 96% and 100%, respectively, so allowing functional classification of sites for proteins not in this set. Our findings will be of interest to those studying protein-ligand interactions and developing new drugs or function modulators.

N-Methyl Protoporphyrin IX: An Understudied Porphyrin

N-Methyl protoporphyrin IX (NmePPIX) is a derivative of protoporphyrin IX (PPIX) and the lattice of heme. Certain xenobiotics strongly induce NmePPIX production in the liver. The existence of endogenous NmePPIX in untreated animal liver has also been reported. The detailed mechanisms of NmePPIX biosynthesis remain unclear, but cytochrome P450 enzymes are thought to be critical in xenobiotic-induced NmePPIX production. High levels of NmePPIX cause PPIX accumulation because NmePPIX is a potent inhibitor (K_i = 7 nM) of ferrochelatase, the last enzyme in the heme biosynthesis pathway that converts PPIX to heme. NmePPIX is also involved in several other physiological processes, including inhibition of nitric oxide production and promotion of lamin aggregation. Compared to the two well-characterized porphyrins, PPIX and heme, NmePPIX is understudied regarding the mechanism of formation, fate, and physiological functions. This Review summarizes the current understanding of NmePPIX and provides perspectives on areas of future research on NmePPIX.

The heme synthesis-export system regulates the tricarboxylic acid cycle flux and oxidative phosphorylation

Heme is an iron-containing porphyrin of vital importance for cell energetic metabolism. High rates of heme synthesis are commonly observed in proliferating cells. Moreover, the cell-surface heme exporter feline leukemia virus subgroup C receptor 1a (FLVCR1a) is overexpressed in several tumor types. However, the reasons why heme synthesis and export are enhanced in highly proliferating cells remain unknown. Here, we illustrate a functional axis between heme synthesis and heme export: heme efflux through the plasma membrane sustains heme synthesis, and implementation of the two processes down-modulates the tricarboxylic acid (TCA) cycle flux and oxidative phosphorylation. Conversely, inhibition of heme export reduces heme synthesis and promotes the TCA cycle fueling and flux as well as oxidative phosphorylation. These data indicate that the heme synthesis-export system modulates the TCA cycle and oxidative metabolism and provide a mechanistic basis for the observation that both processes are enhanced in cells with high-energy demand.

Human aminolevulinate synthase structure reveals a eukaryotic-specific autoinhibitory loop regulating substrate binding and product release

5'-aminolevulinate synthase (ALAS) catalyzes the first step in heme biosynthesis, generating 5'-aminolevulinate from glycine and succinyl-CoA. Inherited frameshift indel mutations of human erythroid-specific isozyme ALAS2, within a C-terminal (Ct) extension of its catalytic core that is only present in higher eukaryotes, lead to gain-of-function X-linked protoporphyria (XLP). Here, we report the human ALAS2 crystal structure, revealing that its Ct-extension folds onto the catalytic core, sits atop the active site, and precludes binding of substrate succinyl-CoA. The Ct-extension is therefore an autoinhibitory element that must re-orient during catalysis, as supported by molecular dynamics simulations. Our data explain how Ct deletions in XLP alleviate autoinhibition and increase enzyme activity. Crystallography-based fragment screening reveals a binding hotspot around the Ct-extension, where fragments interfere with the Ct conformational dynamics and inhibit ALAS2 activity. These fragments represent a starting point to develop ALAS2 inhibitors as substrate reduction therapy for porphyria disorders that accumulate toxic heme intermediates.

High penetrance of acute intermittent porphyria in a Spanish founder mutation population and CYP2D6 genotype as a susceptibility factor

BACKGROUND

Acute intermittent porphyria (AIP) is a low-penetrant genetic metabolic disease caused by a deficiency of hydroxymethylbilane synthase (HMBS) in the haem biosynthesis. Manifest AIP (MAIP) is considered when carriers develop typical acute neurovisceral attacks with elevation of porphyrin precursors, while the absence of attacks is referred to as latent AIP (LAIP). Attacks are often triggered by drugs, endocrine factors, fasting or stress. Although AIP penetrance is traditionally considered to be around 10-20%, it has been estimated to be below 1% in general population studies and a higher figure has been found in specific AIP populations. Genetic susceptibility factors underlying penetrance are still unknown. Drug-metabolizing cytochrome P450 enzymes (CYP) are polymorphic haem-dependent proteins which play a role in haem demand, so they might modulate the occurrence of AIP attacks. Our aim was to determine the prevalence and penetrance of AIP in our population and analyse the main hepatic CYP genes to assess their association with acute attacks. For this, CYP2C9*2, *3; CYP2C19*2; CYP2D6*4, *5; CYP3A4*1B and CYP3A5*3 defective alleles were genotyped in fifty AIP carriers from the Region of Murcia, a Spanish population with a high frequency of the HMBS founder mutation c.669_698del30.

RESULTS

AIP penetrance was 52%, and prevalence was estimated as 17.7 cases/million inhabitants. The frequency of defective CYP2D6 alleles was 3.5 times higher in LAIP than in MAIP. MAIP was less frequent among CYP2D6*4 and *5 carriers (p < 0.05). The urine porphobilinogen (PBG)-to-creatinine ratio was lower in these individuals, although it was associated with a lower prevalence of attacks (p < 0.05) rather than with the CYP2D6 genotype.

CONCLUSIONS

AIP prevalence in our region is almost 3 times higher than that estimated for the rest of Spain. The penetrance was high, and similar to other founder mutation AIP populations. This is very relevant for genetic counselling and effective health care. CYP2D6*4 and *5 alleles may be protective factors for acute attacks, and CYP2D6 may constitute a penetrance-modifying gene. Further studies are needed to confirm these findings, which would allow a further progress in clinical risk profile assessment based on the CYP genotype, leading to predictive personalized medicine for each AIP carrier in the future.

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.

Nuclear receptor 5A (NR5A) family regulates 5-aminolevulinic acid synthase 1 (ALAS1) gene expression in steroidogenic cells

5-Aminolevulinic acid synthase 1 (ALAS1) is a rate-limiting enzyme for heme biosynthesis in mammals. Heme is essential for the catalytic activities of P450 enzymes including steroid metabolic enzymes. Nuclear receptor 5A (NR5A) family proteins, steroidogenic factor-1 (SF-1), and liver receptor homolog-1 (LRH-1) play pivotal roles in regulation of steroidogenic enzymes. Recently, we showed that expression of SF-1/LRH-1 induces differentiation of mesenchymal stem cells into steroidogenic cells. In this study, genome-wide analysis revealed that ALAS1 was a novel SF-1-target gene in differentiated mesenchymal stem cells. Chromatin immunoprecipitation and reporter assays revealed that SF-1/LRH-1 up-regulated ALAS1 gene transcription in steroidogenic cells via binding to a 3.5-kb upstream region of ALAS1. The ALAS1 gene was up-regulated by overexpression of SF-1/LRH-1 in steroidogenic cells and down-regulated by knockdown of SF-1 in these cells. Peroxisome proliferator-activated receptor-γ coactivator-1α, a coactivator of nuclear receptors, also strongly coactivated expression of NR5A-target genes. Reporter analysis revealed that peroxisome proliferator-activated receptor-γ coactivator-1α strongly augmented ALAS1 gene transcription caused by SF-1 binding to the 3.5-kb upstream region. Finally knockdown of ALAS1 resulted in reduced progesterone production by steroidogenic cells. These results indicate that ALAS1 is a novel NR5A-target gene and participates in steroid hormone production.

Lon peptidase 1 (LONP1)-dependent breakdown of mitochondrial 5-aminolevulinic acid synthase protein by heme in human liver cells

5-Aminolevulinic acid synthase (ALAS-1) is the first rate controlling enzyme that controls cellular heme biosynthesis. Negative feedback regulation of ALAS-1 by the end product heme is well documented and provides the foundation for heme treatment of acute porphyrias, a group of diseases caused by genetic defects in the heme biosynthesis pathway and exacerbated by controlled up-regulation of ALAS-1. Heme is known to affect ALAS-1 activity by repressing gene transcription, accelerating mRNA degradation, and impeding pre-ALAS-1 mitochondrial translocation. In the current study, we examined the effect of heme on the rate of mature ALAS-1 protein turnover in human cells and tissues and explored the mediator involved in this new regulatory mechanism. We found that heme and other metalloporphyrins such as CoPP and CrPP decreased mitochondrial ALAS-1 protein through proteolysis. This degradative effect cannot be emulated by iron or free protoporphyrin, two major chemical components of the heme ring, and is independent of oxidative stress. Down-regulating the activity of mitochondrial LONP1, an ATP-dependent protease that controls the selective turnover of mitochondrial matrix proteins, with potent inhibitors and specific siRNA diminished the negative effect of heme on mitochondrial ALAS-1. Therefore, our data support the existence of a conserved heme feedback regulatory mechanism that functions on the mature form of ALAS-1 protein through the activity of a mitochondrial proteolytic system.

Aminolevulinic Acid (ALA) as a Prodrug in Photodynamic Therapy of Cancer

Aminolevulinic acid (ALA) is an endogenous metabolite normally formed in the mitochondria from succinyl-CoA and glycine. Conjugation of eight ALA molecules yields protoporphyrin IX (PpIX) and finally leads to formation of heme. Conversion of PpIX to its downstream substrates requires the activity of a rate-limiting enzyme ferrochelatase. When ALA is administered externally the abundantly produced PpIX cannot be quickly converted to its final product - heme by ferrochelatase and therefore accumulates within cells. Since PpIX is a potent photosensitizer this metabolic pathway can be exploited in photodynamic therapy (PDT). This is an already approved therapeutic strategy making ALA one of the most successful prodrugs used in cancer treatment.

Hypoxic induction of human erythroid-specific δ-aminolevulinate synthase mediated by hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF1) is a heterodimeric basic helix-loop-helix transcription factor that regulates many key genes. δ-Aminolevulinate synthase (ALAS) catalyzes the first and rate-limiting reaction in the heme biosynthetic pathway. In this study, we show that hypoxia-induced expression of erythroid-specific ALAS2 is mediated by HIF1 in erythroid cells. Under hypoxic conditions, significantly increased ALAS2 mRNA and protein levels were detected in K562 cells and erythroid induction cultures of CD34+ hematopoietic stem/progenitor cells. Enforced HIF1α expression increased the level of ALAS2 expression, while HIF1α knockdown by RNA interference decreased the level of ALAS2 expression. In silico analysis revealed three potential hypoxia-response elements (HREs) that are located 611, 621, and 741 bp downstream of the ALAS2 gene. The results from reporter gene and mutation analysis suggested that these elements are necessary for a maximal hypoxic response. Chromatin immunoprecipitation and polymerase chain reaction showed that the HREs could be recognized and bound by HIF1α in vivo. These results demonstrate that the upregulation of ALAS2 during hypoxia is directly mediated by HIF1. We hypothesize that HIF1-mediated ALAS2 upregulation promotes erythropoiesis to satisfy the needs of an organism under hypoxic conditions. This may be accomplished via increased heme levels and an interaction between ALAS2 and erythropoietin.

Cytochrome P450 regulation: the interplay between its heme and apoprotein moieties in synthesis, assembly, repair, and disposal

Heme is vital to our aerobic universe. Heme cellular content is finely tuned through an exquisite control of synthesis and degradation. Heme deficiency is deleterious to cells, whereas excess heme is toxic. Most of the cellular heme serves as the prosthetic moiety of functionally diverse hemoproteins, including cytochromes P450 (P450s). In the liver, P450s are its major consumers, with >50% of hepatic heme committed to their synthesis. Prosthetic heme is the sine qua non of P450 catalytic biotransformation of both endo- and xenobiotics. This well-recognized functional role notwithstanding, heme also regulates P450 protein synthesis, assembly, repair, and disposal. These less well-appreciated aspects are reviewed herein.

Indispensable function for embryogenesis, expression and regulation of the nonspecific form of the 5-aminolevulinate synthase gene in mouse

The first step of heme biosynthesis in animals is catalyzed by 5-aminolevulinate synthase (ALAS), which controls heme supply in various tissues. To clarify the roles that the nonspecific isoform of ALAS (ALAS-N) plays in vivo, we prepared a green fluorescent protein (GFP) knock-in mouse line in which the Alas1 gene (encoding ALAS-N) is replaced with a gfp gene. We found that mice bearing a homozygous knock-in allele (Alas1(GFP/GFP)) were lethal by embryonic day 8.5, demonstrating that ALAS-N is essential for early embryogenesis. Fluorescence microscopic and flow cytometric analyses of heterozygous mouse (Alas1(+/GFP)) tissues showed that the Alas1 expression level differs substantially in tissues; Alas1 is highly expressed in testis Leydig cells, exocrine glands (including submandibular and parotid glands), endocrine glands (such as adrenal and thyroid glands) and hematopoietic lineage cells (including neutrophils and eosinophils). Quantitative analyses of GFP mRNA and ALAS-N mRNA in various tissues of Alas1(+/GFP) mice suggested that the destabilization of ALAS-N mRNA was not uniform in the various tissues. These results thus lay bare that elaborate control of the endogenous heme supply operates in various mouse tissues through regulation of the ALAS-N expression level and that this control is essential for heme homeostasis in animals.

Differential regulation of human ALAS1 mRNA and protein levels by heme and cobalt protoporphyrin

5-Aminolevulinic acid synthase 1 (ALAS1) is the first and rate-controlling enzyme of heme biosynthesis. This study was to determine the effects of heme and selected nonheme metalloporphyrins on human ALAS1 gene expression in hepatocytes. We found that, upon heme and cobalt protoporphyrin (CoPP) treatments, ALAS1 mRNA levels were down-regulated significantly by ca. 50% or more. Measurement of mRNA in the presence of actinomycin D showed that these down-regulations were due to the decreases in mRNA half-lives. Furthermore, the levels of mitochondrial mature ALAS1 protein were down-regulated by 60-70%, but those of the cytosolic precursor protein were up-regulated by 2-5-fold. Measurement of protein in the presence of cycloheximide (CHX) suggests that elevation of the precursor form is due to the increase in protein half-lives. These results provide novel insights into the mechanisms of heme repressional effects on ALAS1 and provide a rationale for further investigation of CoPP as a therapeutic agent for acute porphyric syndromes.

Tissue-specific expression of ALA synthase-1 and heme oxygenase-1 and their expression in livers of rats chronically exposed to ethanol

5-Aminolevulinic acid synthase-1 (ALAS1) and heme oxygenase-1 (HO-1) are the rate-controlling enzymes for heme biosynthesis and degradation, respectively. Expression of these two genes showed tissue-specific expression pattern at both mRNA and protein levels in selected non-treated rat tissues. In the livers of rats receiving oral ethanol for 10 weeks, ALAS1 mRNA levels were increased by 65%, and the precursor and mature ALAS1 protein levels were increased by 1.8- and 2.3-fold, respectively, while no changes were observed in HO-1 mRNA and protein levels, compared with pair-fed controls. These results provide novel insights into the effects of chronic ethanol consumption on hepatic heme biosynthesis and porphyrias.

Induction of 5-aminolevulinate synthase by activators of steroid biosynthesis

Different cytochromes P450 are involved in steroid biosynthesis. These cytochromes have heme as the prosthetic group. We previously reported that ACTH, an activator of glucocorticoid biosynthesis in adrenal, requires heme biosynthesis for a maximal response. In the present study, we investigated the effect of ACTH, and the effect of two activators of the adrenal mineralocorticoid synthesis, endothelin-1 and low sodium diet on 5-aminolevulinate-synthase (ALA-s) mRNA. ALA-s is the rate-limiting enzyme in heme biosynthesis. It was found that infusion of rats with ACTH for 1 h caused an increase of adrenal ALA-s mRNA and activity accompanied by an increase in plasma corticosterone. CYP21, a cytochrome involved in the synthesis of both corticosterone and aldosterone, was not modified at the RNA level in adrenal glands by 1 h of ACTH infusion. Consistently, infusion of endothelin-1 for 1 h increased ALA-s mRNA and aldosterone content in adrenal gland without modifying CYP21 mRNA levels. To study if ALA-s is also regulated by the main physiological stimuli that increase adrenal mineralocorticoid secretion, we fed rats with low salt diet for 2 or 15 days. Low salt diet treatment increased adrenal gland ALA-s mRNA levels. On the other hand, the rapid stimulation of ALA-s mRNA by ACTH which acts through cyclic AMP was confirmed in H295R human adrenocortical cells, the only human adrenal cell line that has a steroid secretion pattern and regulation similar to primary cultures of adrenal cells. Our findings suggest that the acute activation of adrenal steroidogenic cytochromes by trophic hormones involves an increase in heme biosynthesis which will favor the production of active cytochromes.

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.

Haem repression of the housekeeping 5-aminolaevulinic acid synthase gene in the hepatoma cell line LMH

Haem is essential for the health and function of nearly all cells. 5-Aminolaevulinic acid synthase-1 (ALAS-1) catalyses the first and rate-controlling step of haem biosynthesis. ALAS-1 is repressed by haem and is induced strongly by lipophilic drugs that also induce CYP (cytochrome P450) proteins. We investigated the effects on the avian ALAS-1 gene promoter of a phenobarbital-like chemical, Glut (glutethimide), and a haem synthesis inhibitor, DHA (4,6-dioxoheptanoic acid), using a reporter gene assay in transiently transfected LMH (Leghorn male hepatoma) hepatoma cells. A 9.1 kb cALAS-1 (chicken ALAS-1) promoter-luciferase-reporter construct, was poorly induced by Glut and not by DHA alone, but was synergistically induced by the combination. In contrast, a 3.5 kb promoter ALAS-1 construct was induced by Glut alone, without any further effect of DHA. In addition, exogenous haem (20 microM) repressed the basal and Glut- and DHA-induced activity of luciferase reporter constructs containing 9.1 and 6.3 kb of ALAS-1 5'-flanking region but not the construct containing the first 3.5 kb of promoter sequence. This effect of haem was subsequently shown to be dependent on the -6.3 to -3.5 kb region of the 5'-flanking region of cALAS-1 and requires the native orientation of the region. Two deletion constructs of this approx. 2.8 kb haem-repressive region (1.7 and 1.1 kb constructs) retained haem-dependent repression of basal and drug inductions, suggesting that more than one cis-acting elements are responsible for this haem-dependent repression of ALAS-1. These results demonstrate that there are regulatory regions in the 5'-flanking region of the cALAS-1 gene that respond to haem and provide a basis for further investigations of the molecular mechanisms by which haem down-regulates expression of the ALAS-1 gene.

An alternatively-spliced exon in the 5'-UTR of human ALAS1 mRNA inhibits translation and renders it resistant to haem-mediated decay

Haem controls its own synthesis in non-erythroid cells primarily by regulation of ALAS1 mRNA stability. Alternative splicing of human ALAS1 generates two mRNAs with different 5'-UTRs: a major one, where exon 1B is omitted, and a minor form containing exon 1B. We show that, unlike the major ALAS1 mRNA, the minor form was resistant to haem-mediated decay. Furthermore, we demonstrate that the ALAS1 5'-UTR alone did not confer haem-mediated decay upon a heterologous mRNA and the inclusion of exon 1B inhibited translation. These data suggest that translation of ALAS1 mRNA itself might be required for destabilisation in response to haem.

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.

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.

Limited heme synthesis in porphobilinogen deaminase-deficient mice impairs transcriptional activation of specific cytochrome P450 genes by phenobarbital

Heme is not only a very important prosthetic group that modulates the structure and activity of heme proteins but also a regulatory molecule that controls metabolic pathways and the biosynthesis of various proteins. However, investigation into heme regulatory effects in higher vertebrates has been hampered by the lack of a suitable animal model. A knockout mouse with targeted disruption of porphobilinogen deaminase, the third enzyme of the heme pathway, has been generated in our laboratory and used in the present study as an in vivo model of heme deficiency to explore diverse heme regulatory properties. In this model with a defined heme disturbance, we observed a superinductive response of delta-aminolevulinate synthase, the first enzyme in heme synthesis, after phenobarbital treatment. We also found that limited heme is associated with decreased induction of cytochrome P450 by phenobarbital as a consequence of impaired gene transcription. This inhibitory effect is isoenzyme-specific, being significant for cyp2a5. The activity and mRNA level of this particular cytochrome P450 are significantly lower in the phenobarbital-induced porphobilinogen deaminase-deficient mice (55% and 43%, respectively), but its expression can be restored to normal values when exogenous heme is administered. Other heme proteins, namely neuronal nitric oxide synthase and soluble guanylate cyclase, function normally in mice with limited heme. Our results demonstrate that the expression of various heme proteins is differentially regulated in conditions of reduced heme availability. Moreover, our findings emphasize the importance of heme protein function in the genesis of pathophysiological manifestations in acute intermittent porphyria.

MPTP selectively induces haem oxygenase-1 expression in striatal astrocytes

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta with accompanying evidence of increased oxidative damage, deficits in mitochondrial function and iron deposition. Recently, haem oxygenase-1 levels were reported to be elevated in PD brains. Because this enzyme is involved in the response to oxidative stress and is critical for cellular haem and iron homeostasis, it could play a role in the pathogenesis of PD. Therefore, we investigated the expression of haem oxygenase isoform 1 (HO-1) in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. MPTP triggered a relatively rapid and persistent increase in HO-1 mRNA exclusively in the mouse striatum. In situ hybridization and immunohistochemistry showed HO-1 to be localized to striatal astrocytes. The induction of HO-1 by MPTP was blocked by selegiline and GBR-12909, indicating the protoxin had to be metabolized by monoamine oxidase B and taken up by dopaminergic neurons to exert its action in astrocytes. MPTP did not alter the expression of other enzymes of haem synthesis or degradation nor were the levels of mRNA for haem or iron-binding proteins changed. Thus, expression of HO-1 was not part of a cellular program involving haem biosynthesis or homeostasis. In addition, heat shock proteins were not induced by MPTP. Thus, MPTP elicited a selective transcriptional response in striatal astrocytes. This response appears to be mediated by molecules released from affected dopaminergic nerve terminals in the striatum acting upon neighbouring astrocytes. This signalling pathway and its potential relevance to PD are discussed.

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.

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.

Evidence that protein kinase C is involved in delta-aminolevulinate synthase expression in rat hepatocytes

There are many factors that regulate the rate of synthesis of delta-aminolevulinate synthase (ALA-S), the enzyme which governs the rate-limiting step in heme biosynthesis. In rat hepatocytes, phenobarbital increases ALA-S gene transcription and dibutyryl cAMP potentiates this induction, whereas insulin and glucose have the opposite effect. The present report provides evidence that protein kinase C (PKC) activation negatively influences ALA-S mRNA levels, as measured by Northern and slot-blot analysis. The addition of 1,2-dioctanoyl-sn-glycerol (DOG) or 12-O-tetradecanoylphorbol 13-acetate (TPA), a PKC activator that mimics diacylglycerol function, to cultures led to a significant decrease of both basal and phenobarbital-induced ALA-S mRNA levels in a dose-dependent manner. This TPA effect depends on the specific activation of PKC because the analog 4 alpha-phorbol 12,13-diacetate, a nonstimulatory PKC phorbol ester, is unable to inhibit ALA-S mRNA. Furthermore, the effect of TPA is blocked by the PKC inhibitors staurosporine and calphostin C. Desensitization of the PKC pathway by prolonged exposure to TPA abolished the subsequent action of the phorbol ester. On the other hand, neither TPA nor DOG modified the half-life of ALA-S mRNA. The study of the combinatorial action of TPA and cAMP revealed that the inhibitory effect of TPA overcomes dibutyryl cAMP induction. Thus, these results indicate that PKC plays an essential role in regulating ALA-S expression, probably at a transcriptional level.

Identification of an arginine452 to histidine substitution in the erythroid 5-aminolaevulinate synthetase gene in a large pedigree with X-linked hereditary sideroblastic anaemia

The coding region of the erythroid 5-aminolaevulinate synthetase gene (ALAS2) from a large pedigree with pyridoxine-responsive X-linked hereditary sideroblastic anaemia was examined for mutations. In three affected males from this pedigree, single strand conformational polymorphism (SSCP) analysis showed anomalous migration of a PCR product spanning exon 9. Sequencing of amplified genomic DNA from one of these affected males revealed a guanine to adenine transition at nucleotide 1407 of the cDNA sequence in exon 9 of the gene. This mutation results in the loss of an HhaI restriction enzyme digest site. An HhaI digest assay demonstrated the presence of this mutation in other affected males but not in unaffected males and unrelated individuals. The point mutation results in an arginine to histidine substitution at amino acid residue 452. The arginine residue is conserved in both the erythroid and housekeeping ALAS genes in all known vertebrate sequences. This arginine is located in the middle of a predicted alpha-helix.

Heme binding by a bacterial repressor protein, the gene product of the ferric uptake regulation (fur) gene of Escherichia coli

The fur gene product, Fur, of Escherichia coli is a repressor when it binds Fe(II). Since heme and iron metabolism are closely linked and Fur is rich in histidine, a ligand for heme, the binding of heme to Fur was investigated. The oxidized Fur-heme complex is stable and low spin with a Soret maximum at 404 nm and no 620-nm band. CO coordinates with the reduced heme-Fur complex, causing a shift from 412 nm to 410 nm, and stabilizes it, increasing the half-life from 5 to 15 min. Circular dichroism (CD) spectra in the Soret region show heme bound in an asymmetric environment in Fur, both in the oxidized and reduced-CO forms. Quenching of tyrosine fluorescence by heme revealed rapid, tight binding (Kd < 1 microM) with an unusual stoichiometry of 1 heme:1 Fur dimer. Fur binds Mn(II), a model ligand for the endogenous Fe(II), much more weakly (Kd > 80 microM). Far-ultraviolet CD spectroscopy showed that the alpha-helix content of apo-Fur decreases slightly with heme binding, but increases with Mn(II) binding. Competition experiments indicated that heme interacts with Fur dimers at the same site as Mn(II) and can displace the metal. In contrast to Mn(II), Zn(II) did not quench the tyrosine fluoroescence of Fur, affected the CD spectrum less than Mn(II), but did bind in a manner which prevented heme from binding. In sum, Fur not only binds heme and Zn(II) with sufficient affinity to be biologically relevant, but the interactions that occur between these ligands and their effects on Mn(II) binding need to be taken into account when addressing the biological function of Fur.

In heme catabolism C2 and C4 vinyl groups reduction of cobalt protoporphyrin forms cobalt mesoporphyrin and alters the nature of action of the metalloporphyrin in vivo

Heme is a tetrapyrrolic ring with iron as the central metal atom and acts as a prosthetic group for a number of enzymes, e.g. cytochromes and globins. It also plays an important role in the regulation of transcription, translation, protein translocation and erythroid differentiation. Thus, heme regulation is under strict control in the body. Our studies on the regulatory enzymes of heme anabolism, aminolevulinic acid synthetase (ALA-S), and of catabolism, heme oxygenase (HMOX), in the spleen has revealed that cobalt protoporphyrin acts as an inducer of HMOX. It is revealed that by alteration of side groups at C2 and C4 changes the nature of action of Co-protoporphyrin from an inducer to a strong inhibitor of HMOX activity. All the three analogues Co-protoporphyrin, Co-mesoporphyrin and Co-hematoporphyrin have been shown to induce the ALA-S activity to the similar extent. NADPH-cytochrome c reductase, a microsomal membrane bound enzyme, is required by HMOX for the enzymatic conversion of heme into biliverdin IXc and is also required for NADPH-dependent lipid peroxidation in the microsomes. It has been observed that Co-mesoporphyrin causes an inhibition of HMOX activity and consequently leads to an induced level of microsomal NADPH-dependent lipid peroxidation.

Cloning and sequencing of some genes responsible for porphyrin biosynthesis from the anaerobic bacterium Clostridium josui

The 6.2-kbp DNA fragment encoding the enzymes in the porphyrin synthesis pathway of a cellulolytic anaerobe, Clostridium josui, was cloned into Escherichia coli and sequenced. This fragment contained four hem genes, hemA, hemC, hemD, and hemB, in order, which were homologous to the corresponding genes from E. coli and Bacillus subtilis. A typical promoter sequence was found only upstream of hemA, suggesting that these four genes were under the control of this promoter as an operon. The hemA and hemD genes cloned from C. josui were able to complement the hemA and hemD mutations, respectively, of E. coli. The COOH-terminal region of C. josui HemA and the NH2-terminal region of C. josui HemD were homologous to E. coli CysG (Met-1 to Leu-151) and to E. coli CysG (Asp-213 to Phe-454) and Pseudomonas denitrificans CobA, respectively. Furthermore, the cloned 6.2-kbp DNA fragment complemented E. coli cysG mutants. These results suggested that both C. josui hemA and hemD encode bifunctional enzymes.

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.

Androgen regulation of gene expression in the Syrian hamster Harderian gland

The androgenic control of sexual dimorphism has been studied in the Harderian gland from Syrian hamster and compared to rat Harderian gland, a system without dimorphism. Hybridization in situ with a rat cDNA clone has revealed the presence of androgen receptor mRNA in all secretory cells from male and female hamster glands. Testosterone or 5-alpha-dihydrotestosterone administration to females both caused a 60% decrease in the levels of 5-aminolevulinate synthase mRNA after 1 day of treatment, but the resulting patterns of in vitro translation using RNA from glands treated with the two androgens are different. Testosterone alters the mRNA levels for androgen receptor and 5-aminolevulinate synthase in the glands only 6 h after its implantation in females, and the action is maintained up to 10 days of treatment. Finally, androgen administration to females or deprivation in males alter androgen receptor but not 5-aminolevulinate synthase mRNA levels in rat Harderian glands. Our results suggest that the androgen receptor from Harderian glands is responsible for the sexual dimorphism found in Syrian hamsters, whereas the lack of sexual dimorphism in rat seems to be due to a restricted effect of androgens in the glands.

Porphyrin accumulation in the harderian glands of female Syrian hamster results in mitochondrial damage and cell death

BACKGROUND

The Harderian glands of female Syrian hamsters contain very high concentrations of protoporphyrin (in the range of micrograms per mg of tissue) which accumulate in the tubulo-alveoli of the gland. We have studied the process of synthesis, accumulation, and secretion of this cyclic compound by the secretory cells of the hamster Harderian glands.

METHODS

The animals used were female Syrian hamster of 15, 35, 75, 180, and 360 days of age. Items first examined were (1) percentage of the "clear cells," (2) area occupied by intraluminal porphyrins, and (3) histological characteristics of "clear cells" by light and transmission electron microscopy (TEM). In a second study the total content of porphyrins was determined. Finally, the levels of mRNA for the enzyme aminolevulinate synthase (ALV-S) were measured.

RESULTS

In the glands of female hamsters, both the tissue concentration and the intraluminal area occupied by protoporphyrin correlate with the appearance of a special type of cell (clear cells) which show signs of cell degeneration. In addition, the expression of the gene for ALV-S, which is the limiting enzyme in porphyrin production, also parallels the relative number of clear cells. Analyzed under TEM, these clear cells display dilated mitochondria and short and swollen endoplasmic reticulum cisternae. In a late phase of necrosis, the nuclear envelope appears disorganized with scarce chromatin. The mitochondria undergo complete destruction, resulting in electron-dense bacillary formations which progressively coalesce in large and dense areas of protoporphyrin. The cell dies after this accumulation, being secreted by a "cytogen" mechanism.

CONCLUSIONS

In view of our results, the Harderian gland of female Syrian hamster may provide a useful model for the study of the mechanism by which the anomalous accumulation of protoporphyrin induces cell damage in human protoporphyria.

Mitochondrial biogenesis in striated muscle

Mitochondrial biogenesis (synthesis) has been observed to occur in skeletal muscle in response to chronic use. It also occurs in cardiac muscle during growth and hypertrophy, and it may be impaired during the aging process. This review summarizes the literature on the processes of mitochondrial biogenesis at the biochemical and molecular levels, with particular reference to striated muscles. Mitochondrial biogenesis involves the expression of nuclear and mitochondrial genes and the coordination of these two genomes, the synthesis of proteins and phospholipids and their import into the organelle, and the incorporation of these lipids and proteins into their appropriate locations within the matrix, inner or outer membranes. The emphasis is on the regulation of these events, with information derived in part from other cellular systems. Although descriptions of mitochondrial content changes in heart and skeletal muscle during altered physiological states are plentiful, much work is needed at the molecular level to investigate the regulatory processes involved. A knowledge of biochemical and molecular biology techniques is essential for continued progress in the field. This is a promising area, and potential new avenues for future research are suggested.

Heme biosynthesis in mammalian systems: evidence of a Schiff base linkage between the pyridoxal 5'-phosphate cofactor and a lysine residue in 5-aminolevulinate synthase

5-Aminolevulinate synthase is the first enzyme of the heme biosynthetic pathway in nonplant higher eukaryotes. Murine erythroid 5-aminolevulinate synthase has been purified to homogeneity from an Escherichia coli overproducing strain, and the catalytic and spectroscopic properties of this recombinant enzyme were compared with those from nonrecombinant sources (Ferreira, G.C. & Dailey, H.A., 1993, J. Biol. Chem. 268, 584-590). 5-Aminolevulinate synthase is a pyridoxal 5'-phosphate-dependent enzyme and is functional as a homodimer. The recombinant 5-aminolevulinate synthase holoenzyme was reduced with tritiated sodium borohydride and digested with trypsin. A single peptide contained the majority of the label. The tritiated peptide was isolated, and its amino acid sequence was determined; it corresponded to 15 amino acids around lysine 313, to which pyridoxal 5'-phosphate is bound. Significantly, the pyridoxyllysine peptide is conserved in all known cDNA-derived 5-aminolevulinate synthase sequences and is present in the C-terminal (catalytic) domain. Mutagenesis of the 5-aminolevulinate synthase residue, which is involved in the Schiff base linkage with pyridoxal 5'-phosphate, from lysine to alanine or histidine abolished enzyme activity in the expressed protein.

An examination of the immune system of the duck (Anas platyrhynchos) for factors resembling some defined mammalian cytokines

Duck lymphoblasts generated by phytohaemagglutinin (PHA) did not respond to recombinant or Jurkat cell line human interleukin (IL)-2 or possess surface antigens resembling mammalian IL-2 receptors or IL-1 beta. Supernatant fluids from normal and PHA-stimulated duck lymphocyte cultures, and normal and lipopolysaccharide (LPS)-stimulated monocytes, gave negative results in a range of assays for biological activity and immunochemical presence of factors resembling mammalian IL-1 and IL-2. However, supernatant fluids from LPS-stimulated duck monocytes contained IL-6-like activity (up to 35 units/mL) assessed on the 7TD-1 murine cell line. We were unable to demonstrate mRNA that would hybridize to cDNA probes for human IL-1 beta, IL-6, and tumour necrosis factor (TNF) in extracts of blood and lymphoid organs from normal and antigen-stimulated ducks. Because homologous serum or plasma is essential for duck lymphocytes and macrophages to respond to mitogens in vitro, we asked whether this growth-factor-like activity might be caused by substances resembling mammalian cytokines. Serum and plasma were examined for activity consistent with IL-1 and IL-6 on mammalian target cells. None was detected. Instead, both serum and plasma contained inhibitors of human IL-1 beta and IL-6, detected at dilutions up to 1:100. Inhibition by serum was heat (56 degrees C, 30 min) labile but inhibition by plasma was heat stable. The identities and biological functions of these inhibitors remain to be defined.

Gender-associated differences in the development of 5-aminolevulinate synthase gene expression in the harderian gland of Syrian hamsters

The mRNA levels for aminolevulinate synthase (ALV-S), the rate-limiting enzyme in porphyrin synthesis, were studied in male and female Syrian hamsters during postnatal development. Sex-associated differences in the expression of ALV-S gene were evident at the end of the third week of postnatal development. Serum levels of luteinizing hormone (LH), testosterone, cortisol, thyroid hormones and insulin-like growth factor were also studied in order to correlate their concentrations with the mRNA levels for ALV-S. Among these hormones, serum LH levels showed a positive correlation with the ALV-S mRNA levels. However, the expected negative correlation with testosterone levels was not clearly observed. Thus, in order to test the effects of testosterone on ALV-S gene expression, 11-day-old male and female Syrian hamsters and adult female hamsters were injected with 50 micrograms of testosterone for 4 days. Testosterone administration decreased the levels of ALV-S mRNA in the adult females but did not influence those of young females. The possible explanation for the insensitivity to testosterone during these postnatal stages might involve the maturational state of androgen receptors in the Harderian glands.

5'-Aminolevulinate synthase activity is decreased in skeletal muscle of anemic rats

Expression of the rate-limiting heme biosynthetic enzyme 5'-aminolevulinate synthase (ALAS) was investigated in skeletal muscle of 3-wk-old rats fed an iron-deficient diet. After 14 days, ALAS activity had declined 70% relative to control (2.1 +/- 0.2 vs. 0.6 +/- 0.1 nmol.h-1.g-1; P less than 0.005). Similar decreases were observed for blood hemoglobin (11.4 +/- 0.2 vs. 3.9 +/- 0.3 g/dl; P less than 0.005) and muscle cytochrome c (14.5 +/- 1.3 vs. 7.1 +/- 0.6 nmol/g; P less than 0.005). An iron-deficient diet decreased body and skeletal muscle growth by 15 (P less than 0.005) and 10% (P less than 0.05), respectively, whereas concentrations of protein, RNA, ALAS mRNA, and citrate synthase activity in muscle were not different from control. One mechanism by which heme biosynthesis may be slowed in muscle of young anemic rats is a decrease in ALAS activity. At a time when enzyme activity was decreased, ALAS mRNA expression was not affected by an iron-deficient diet, suggesting that steps after transcription of the ALAS gene may regulate the decrease in activity.

Mitochondrial actaptations to chronic muscle use: Effect of iron deficiency

1. The effects of chronic muscle use on mitochondrial structure, enzymes and gene expression is reviewed. The role of iron deficiency in modulating this adaptation is discussed. 2. Chronic muscle use and disuse alter mitochondrial composition and affect mitochondrial subpopulations differentially. This has implications for an understanding of organelle assembly. 3. Iron deficiency decreases mitochondrial functional mass within muscle by reducing the level of heme and non-heme iron-containing components. This alters the metabolic response during exercise and results in a reduced endurance performance. 4. Both iron deficiency and chronic muscle use represent contrasting experimental models for the study of mitochondrial function and biogenesis.

Effect of dexamethasone on mRNA levels for 5-aminolevulinate synthase in different rat tissues

5-Aminolevulinate synthase mRNA levels from different tissues were quantitated by Northern blot hybridization analysis utilizing the rat liver 5-aminolevulinate synthase cDNA clone as probe. A 5-aminolevulinate synthase mRNA species of size 2.3 kb was seen in all the tissues examined. Densitometric scanning of the autoradiographs demonstrated that the adrenal gland contained the largest amount of 5-aminolevulinate synthase mRNA. Levels corresponding to approximately 50% of this amount were found in the small intestine, lung, heart, muscle and testes. In the liver and kidney the level was approximately 25% of that found in the adrenal gland. These results demonstrate the housekeeping role of this gene. Dexamethasone treatment for 1 day or 5 days dramatically induced 5-aminolevulinate synthase mRNA levels in the liver and small intestine, and to a lesser extent in lung, heart, kidney and muscle. Nuclear run-off experiments suggest that a post-transcriptional mechanism predominantly contributes to the dexamethasone-induced increase in 5-aminolevulinate synthase mRNA levels observed in the liver. Interestingly, in the steroidogenic tissues of the adrenal gland and testes, there was a substantial decrease in 5-aminolevulinate synthase mRNA levels after dexamethasone administration but the mechanism of this control remains to be investigated.

Age and food restriction alter the porphyrin concentration and mRNA levels for 5-aminolevulinate synthase in rat Harderian gland

The effects of age and food restriction on the porphyrin concentration in Harderian glands were studied in male Fisher 344 rats. Harderian gland porphyrin concentrations increased with age; this was statistically significant in 20 month old animals compared with 3 month old animals. Food restriction (by 40%) prevented the age-associated rise in porphyrins; thus, in 20 month old food restricted rats had porphyrin concentrations similar to those found in young animals. In a second experiment, we correlated the age-associated rise in Harderian gland porphyrin concentrations with an increase in mRNA levels for 5-aminolevulinate synthase (ALV-S). Both the porphyrin concentration and ALV-S mRNA rose at 12 and 18 months of age, but decreased by 24 months of age. It is concluded that, a) porphyrin biosynthesis in the Harderian glands increases up to 20 months of age but decreases in rats that are 24 months old, and b) food restriction prevents the porphyrin rise associated with age in the Harderian gland of male Fisher 344 rats.