Differential effects of metalloporphyrins on messenger RNA levels of delta-aminolevulinate synthase and heme oxygenase. Studies in cultured chick embryo liver cells

Update on heme biosynthesis, tissue-specific regulation, heme transport, relation to iron metabolism and cellular energy

Heme is a primordial macrocycle upon which most aerobic life on Earth depends. It is essential to the survival and health of nearly all cells, functioning as a prosthetic group for oxygen-carrying proteins and enzymes involved in oxidation/reduction and electron transport reactions. Heme is essential for the function of numerous hemoproteins and has numerous other roles in the biochemistry of life. In mammals, heme is synthesised from glycine, succinyl-CoA, and ferrous iron in a series of eight steps. The first and normally rate-controlling step is catalysed by 5-aminolevulinate synthase (ALAS), which has two forms: ALAS1 is the housekeeping form with highly variable expression, depending upon the supply of the end-product heme, which acts to repress its activity; ALAS2 is the erythroid form, which is regulated chiefly by the adequacy of iron for erythroid haemoglobin synthesis. Abnormalities in the several enzymes of the heme synthetic pathway, most of which are inherited partial enzyme deficiencies, give rise to rare diseases called porphyrias. The existence and role of heme importers and exporters in mammals have been debated. Recent evidence established the presence of heme transporters. Such transporters are important for the transfer of heme from mitochondria, where the penultimate and ultimate steps of heme synthesis occur, and for the transfer of heme from cytoplasm to other cellular organelles. Several chaperones of heme and iron are known and important for cell health. Heme and iron, although promoters of oxidative stress and potentially toxic, are essential cofactors for cellular energy production and oxygenation.

Heme status affects human hepatic messenger RNA and microRNA expression

AIM: To assess effects of heme on messenger RNA (mRNA) and microRNA (miRNA) profiles of liver cells derived from humans.

METHODS: We exposed human hepatoma cell line Huh-7 cells to excess iron protoporphyrin (heme) (10 μmol/L) or induced heme deficiency by addition of 4, 6-dioxoheptanoic acid (500 μmol/L), a potent inhibitor of aminolevulinic acid dehydratase, for 6 h or 24 h. We harvested total RNA from the cells and performed both mRNA and miRNA array analyses, with use of Affymetrix chips, reagents, and instruments (human genome U133 plus 2.0 and miRNA 2.0 arrays). We assessed changes and their significance and interrelationships with Target Scan, Pathway Studios, and Ingenuity software.

RESULTS: Changes in mRNA levels were most numerous and striking at 6 h after heme treatment but were similar and still numerous at 24 h. After 6 h of heme exposure, the increase in heme oxygenase 1 gene expression was 60-fold by mRNA and 88-fold by quantitative reverse transcription-polymerase chain reaction. We found striking changes, especially up-regulation by heme of nuclear erythroid-2 related factor-mediated oxidative stress responses, protein ubiquitination, glucocorticoid signaling, P53 signaling, and changes in RNAs that regulate intermediary metabolism. Fewer mRNAs were down-regulated by heme, and the fold decreases were less exuberant than were the increases. Notable decreases after 24 h of heme exposure were patatin-like phospholipase domain-containing protein 3 (-6.5-fold), neuronal PAS domain protein 2 (-1.93-fold), and protoporphyrinogen oxidase (-1.7-fold).

CONCLUSION: Heme excess exhibits several toxic effects on liver and kidney, which deserve study in humans and in animal models of the human porphyrias or other disorders.

Reduction in porphyrin excretion as a sensitive indicator of lead toxicity in primary cultures of adult rat hepatocytes

Alterations of specific metabolic pathways can be used as sensitive indicators of toxicity by chemicals and can give valuable information on the mechanism(s) involved. Short-term effects of lead on hepatic haem biosynthesis were studied in an in vitro system. Primary cultures of adult rat hepatocytes were exposed for 24-48 hr to lead (0.024-3.6 mm), and excreted and intracellular porphyrins were measured in untreated and lead-treated cultures. Cytotoxicity, as estimated by enzyme leakage, and morphological alterations were also evaluated. Control hepatocytes produced porphyrins at a rate of 387 pmol/mg cellular protein/day. Most of the released and intracellular porphyrins were protoporphyrins, although uro- and coproporphyrins were also detected in lower amounts. After 24 hr of exposure to 0.1-3.6 mm Pb(2+) , excreted porphyrins decreased by 24-92% and intracellular porphyrins by 36-60%, while 48 hr of exposure to 0.024-3.6 mm Pb(2+) caused a progressive reduction of 77-97% in porphyrin excretion and of 49-67% in intracellular porphyrins. Lead exposure also produced a differential decrease of proto-, copro- and uro-porphyrin excretion. These lead effects can be explained mainly by inhibition of the enzyme 5-aminolaevulinate dehydratase, resulting in a decreased monopyrrole supply for porphyrin biosynthesis, and probably by inhibition of the enzyme uroporphyrinogen decarboxylase. Morphological alterations and enzyme leakage were detected only after 24 hr of exposure to 2.4 mm and 48 hr of exposure to 3.6 mm Pb(2+), respectively. The results show that changes in porphyrin production, and particularly in their excretion, in cultured rat hepatocytes are useful indicators of lead toxicity, since they are more sensitive than enzyme leakage and can give preliminary information on the enzyme(s) that could be affected. They also suggest the potential benefits of the use of this method for the evaluation of compounds that alter haem biosynthesis.

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.

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.

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

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

CONCLUSIONS

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

Hepatic porphyrias: diagnosis and management

Porphyrias are a group of metabolic disorders in which there are defects in the normal pathway for the biosynthesis of heme, the critical prosthetic group for numerous hemoproteins. The clinical manifestations of the porphyrias can be highly varied, and patients may present to general physicians and be referred to a wide variety of subspecialists because of these manifestations. However, two major clinical forms are represented by the so-called "acute" porphyrias, in which patients suffer recurrent bouts of pain, especially pain in the abdomen, and the "cutaneous" porphyrias, in which patients have painful skin lesions. Knowledge of the factors chiefly responsible for regulating the rate of synthesis of heme has helped to explain how drugs and other factors may cause porphyria. Knowledge of the physical and chemical properties of porphyrins also forms an important part of the foundation for understanding the clinical manifestations of these diseases. Thus, the porphyrias can best be understood after reviewing the chemical properties of porphyrins and heme and the control of their biosynthesis.

Effects of modulators of protein phosphorylation on heme metabolism in human hepatic cells: induction of delta-aminolevulinic synthase mRNA and protein by okadaic acid

Effects of modulators of protein phosphorylation on delta-aminolevulinic acid (ALA) synthase and heme oxygenase-1 mRNA were analyzed in the human hepatic cell lines Huh-7 and HepG2 using a quantitative RNase protection assay. Okadaic acid was found to induce ALA synthase mRNA in a concentration-dependent fashion in both Huh-7 and HepG2 cells. The EC(50) for induction of ALA synthase mRNA in Huh-7 cells was 13.5 nM, with maximum increases occurring at okadaic acid concentrations of 25-50 nM. The EC(50) for induction of ALA synthase mRNA in HepG2 cells was 35.5 nM, with maximum increases occurring at okadaic acid concentrations of 50 nM. Concentration-dependent induction of ALA synthase mRNA paralleled the increase in ALA synthase protein. Maximum induction of ALA synthase was observed between 5 and 10 h post-treatment in both cell lines. Induction of ALA synthase mRNA in Huh-7 cells, but not HepG2 cells, was associated with an increase in ALA synthase mRNA stability. Okadaic acid also induced heme oxygenase-1 mRNA in both cell lines, but the magnitude of induction was only twofold, and was rapid and transient. Okadaic acid and phorbol 12-myristate 13-acetate significantly decreased heme-mediated induction of heme oxygenase-1 mRNA in both Huh-7 and HepG2 cells. Wortmannin diminished the heme-mediated induction of heme oxygenase-1 mRNA in HepG2 cells, but not Huh-7 cells. These results report a novel property of okadaic acid to affect heme metabolism in human cell lines.

Benefits of chronic plasmapheresis and intravenous heme-albumin in erythropoietic protoporphyria after orthotopic liver transplantation

Erythropoietic protoporphyria (EPP) is characterized by a deficiency of ferrochelatase the final enzyme of the heme biosynthetic pathway. Patients with EPP may overproduce protoporphyrin IX, chiefly in developing erythrocytes. In some, protoporphyrin accumulates and causes toxicity, particularly to the skin and liver. Orthotopic liver transplantation (OLT) treats the severe liver disease that sometimes occurs in EPP; however, it does not correct the underlying metabolic disorder. We recently reported a patient with EPP who was improved with plasmapheresis and i.v. heme-albumin before OLT. Subsequently he developed histological and biochemical evidence of recurrent hepatotoxicity from protoporphyrin in the graft liver. We now report successful treatment of the patient with additional plasmapheresis and heme-albumin with improvement of hepatic histological and biochemical abnormalities. We conclude that plasmapheresis and heme-albumin are of benefit in EPP complicated by hepatotoxicity before and after liver transplantation.

Regulation of heme metabolism in rat hepatocytes and hepatocyte cell lines: delta-aminolevulinic acid synthase and heme oxygenase are regulated by different heme-dependent mechanisms

Regulation of delta-aminolevulinic acid (ALA) synthase and heme oxygenase was analyzed in primary rat hepatocytes and in two immortalized cell lines, CWSV16 and CWSV17 cells. ALA synthase was induced by 4,6-dioxohepatnoic acid (4,6-DHA), a specific inhibitor of ALA dehydratase, in all three systems; however, the induction in CWSV17 cells was greater than in either of the other two systems. Therefore, CWSV17 cells were used to explore the regulation of both enzymes by heme and 4,6-DHA. Data obtained from detailed concentration curves demonstrated that 4,6-DHA induced the activity of ALA synthase once ALA dehydratase activity became rate-limiting for heme biosynthesis. Heme induced heme oxygenase activity with increases occurring at concentrations of 10 microM or greater. Heme blocked the 4,6-DHA-dependent induction of ALA synthase with an EC50 of 1.25 microM. Heme-dependent decreases of ALA synthase mRNA levels occurred more quickly and at lower concentrations than heme-dependent increases of heme oxygenase mRNA levels. ALA synthase mRNA remained at reduced levels for extended periods of time, while the increases in heme oxygenase mRNA were much more transient. The drastic differences in concentrations and times at which heme-dependent effects were observed strongly suggest that two-different heme-dependent mechanisms control the ALA synthase and heme oxygenase mRNAs. In CWSV17 cells, heme decreased the stability of ALA synthase mRNA from 2.5 to 1.3 h, while 4,6-DHA increased the stability of the mRNA to 5.2 h. These studies demonstrate that regulation of ALA synthase mRNA levels by heme in a mammalian system is mediated by a change in ALA synthase mRNA stability. The results reported here demonstrate the function of the regulatory heme pool on both ALA synthase and heme oxygenase in a mammalian hepatocyte system.

The Porphyrias

The porphyrias are a diverse group of metabolic diseases. Major manifestations are episodic neurovisceral attacks of pain or other neurologic features, and/or dermatologic abnormalities. It is essential that a clear diagnosis be established prior to planning management. In our experience, most patients referred with a presumptive diagnosis of "porphyria" do not have true porphyria at all, but rather have syndromes of other etiologies associated with mild, nonspecific increases in urinary porphyrin excretion (secondary porphyrinurias). The management of the acute or inducible porphyrias depends upon prevention and prompt, aggressive management of acute attacks. The latter includes nutrition (at least 300 g/d carbohydrate plus adequate protein), analgesia, and intravenous heme. The management of active porphyria cutanea tarda involves iron depletion by therapeutic phlebotomy and cessation of precipitating or exacerbating factors, especially alcohol and estrogens. When chronic hepatitis C and/or HIV infection are present, they should also be treated. The management of protoporphyria involves ensuring adequate iron stores, and avoidance of hepatotoxic or cholestatic factors. Liver transplantation may be life-saving in the small minority of patients who develop progressive protoporphyric liver disease. A few patients with congential erythropoietic porphyria (Günther's disease) have been treated successfully by transplantation of bone marrow from a normal donor. In the future, this and other forms of porphyria may be treated by specific gene therapy. Such efforts are now under development, but they are not yet ready for human trials in the US.

Induction of the heme oxygenase-1 gene by metalloporphyrins

Induction of expression of heme oxygenase-1 (HO-1) has been studied in primary cultures of chick embryo liver cells and in the LMH line of avian hepatoma cells. Cells were transiently transfected with selected constructs containing portions of the 5'-untranslated (promoter) region of the HO-1 gene linked to luciferase as reporter gene. LMH cells that had been stably transfected with selected wild type or mutant constructs were also studied. Metalloporphyrins, especially Fe protoporphyrin (heme) and Co protoporphyrin strongly induced luciferase expression in both types of transfected cells. Low concentrations of Zn mesoporphyrin, an inhibitor of HO activity, exerted a synergistic effect on heme-, but not Co protoporphyrin-dependent induction. The antioxidant and &bond;SH donor N-acetyl cysteine had little effect on the metalloporphyrin-dependent inductions of HO-1, in contrast to its marked inhibitory effect on the sodium arsenite-dependent induction of the HO-1 gene. Deletional analysis showed that the key element(s) required for the metalloporphyrin-dependent induction of HO-1 is located between -3.6 and -5.6 kb upstream of the transcription starting point. Data from electrophoretic mobility shift and site-directed mutagenesis experiments excluded a role for consensus AP-1 binding elements at -1576, -3647, or -4578 in the inductions produced by heme or Co protoporphyrin.

Upstream regulatory elements in chick heme oxygenase-1 promoter: a study in primary cultures of chick embryo liver cells

Previously, chick heme oxygenase-1 (cHO-1) gene was cloned by us and two regions important for induction by sodium arsenite were identified. These two regions were found to contain consensus sequences of an AP-1 (-1580 to -1573) and a MRE/cMyc complex (-52 to -41). In the current study, the roles of these two elements in mediating the sodium arsenite or cobalt chloride dependent induction of cHO-1 were investigated further. DNA binding studies and site-directed mutagenesis studies indicated that both the AP-1 and MRE/cMyc elements are important for the sodium arsenite induction, while cobalt chloride induction involves only the AP-1 element. Electrophoretic mobility shift assays showed that nuclear protein binding to the AP-1 element was increased by both sodium arsenite or cobalt chloride treatment, whereas the binding of proteins to the MRE/cMyc element showed a high basal expression in untreated cells and the binding activity was only slightly increased by sodium arsenite treatment. Site-directed mutagenesis studies showed that, to completely abolish sodium arsenite induction, both the AP-1 and MRE/cMyc elements must be mutated; mutation of either element alone resulted in only a partial effect. In contrast, a single mutation at AP-1 element was sufficient to reduce the cobalt chloride induction almost completely. The MRE/cMyc complex plays a major role in the basal level expression, and shares some similarities to the upstream stimulatory factor element (USF) identified in the promoter regions of mammalian HO-1 genes and other stress regulated genes. Because sodium arsenite is known to cause oxidative stress and because activation of AP-1 proteins has been shown to be a key step in the oxidative stress response pathway, we also explored the possibility that the induction of the cHO-1 gene by sodium arsenite is mediated through oxidative stress pathway(s) by activation of AP-1 proteins. We found that pretreatment with antioxidants (N-acetyl cysteine or quercetin) reduced the induction of the endogenous cHO-1 message or cHO-1 reporter construct activities induced by sodium arsenite or cobalt chloride. These antioxidants also reduced the protein binding activities to the AP-1 element in the electrophoretic mobility shift assays. In summary, induction of the cHO-1 gene by sodium arsenite or cobalt chloride is mediated by activation of the AP-1 element located at -1,573 to -1,580 of the 5'UTR.

Heme biosynthesis in a chicken hepatoma cell line (LMH): comparison with primary chick embryo liver cells (CELC)

5-Aminolevulinic acid synthase (ALA synthase), the rate-controlling enzyme of hepatic heme biosynthesis, is feed-back repressed by heme. In the liver, chemicals such as barbiturates markedly induce ALA synthase, especially in the presence of partial defects of heme biosynthesis. The inducibility and regulation of ALA synthase have been investigated using a variety of models, including intact animals and liver cell culture systems. A widely used model that closely approximates what occurs in vivo and in humans is that of primary cultures of chick embryo liver cells (CELCs). However, CELCs have some limitations: the cells obtained are somewhat heterogeneous; isolation and culture must be repeated every week resulting in weekly variations; and cells are short-lived limiting the feasibility of time-course and transfection studies. The aim of this study was to determine if LMH cells, a chick hepatoma cell line, are a good model comparable to that of CELCs. In both cells similar patterns of response of, ALA synthase activities and mRNA levels, and of porphyrin accumulation were obtained following treatments known to affect heme biosynthesis. Similarly, heme repressed ALA synthase mRNA levels in both cell types and ALA synthase activities in LMH cells. We conclude that LMH cells are a useful model for the study of hepatic heme biosynthesis and regulation of ALA synthase.

Effects of selected antihypertensives and analgesics on hepatic porphyrin accumulation: implications for clinical porphyria

When patients with acute porphyrias are treated with antihypertensives and analgesics, they could be placed at increased risk of developing porphyric attacks, since little is known about the potential for many of these drugs to induce these attacks. We used primary chick embryo liver cells, which maintain intact heme synthesis and regulation, to study the effects of antihypertensives and analgesics on porphyrin accumulation. Cells were treated with desferrioxamine to block heme synthesis partially, simulating conditions encountered in porphyric patients. Typically, cells were treated for 20 hr with the test drugs (3.16 to 1000 microM), along with desferrioxamine. Porphyrins were measured spectrofluorometrically, as uro-, copro,- and protoporphyrin. The evaluated drugs included six antihypertensives (two calcium channel blockers, an angiotensin receptor antagonist, and three inhibitors of angiotensin converting enzyme) and eight analgesics. Of the calcium channel blockers tested, nifedipine greatly increased porphyrin accumulation, whereas diltiazem caused only a slight increase. Losartan (an angiotensin receptor antagonist), captopril, or lisinopril (two angiotensin converting enzyme inhibitors) produced only small increases in porphyrin accumulation. In contrast, enalapril (another angiotensin converting enzyme inhibitor) substantially increased porphyrin accumulation when given in high concentrations. Among the analgesics tested, fentanyl and tramadol produced the highest porphyrin accumulations. Nalbuphine, hydrocodone, oxycodone, and dezocine were moderately or weakly porphyrogenic, whereas buprenorphine and morphine did not increase porphyrin accumulation. These studies suggest that patients with acute porphyrias may be at greater risk for developing porphyric attacks when treated with nifedipine (compared with diltiazem), enalapril (compared with captopril or lisinopril), and tramadol (compared with the other analgesics).

Nitric oxide mediates the lipopolysaccharide dependent upregulation of the heme oxygenase-1 gene expression in cultured rat Kupffer cells

BACKGROUND/AIMS

Heme oxygenase catalyzes the rate-limiting enzymatic step of heme degradation. The inducible isoform of heme oxygenase, heme oxygenase-1, is expressed at a low level in most tissues and is upregulated by its substrate heme and various stress stimuli. Kupffer cells which represent the largest population of the body's tissue macrophages serve physiological functions in the defense against various pathogens such as lipopolysaccharide. The goal of the present study was to investigate the heme oxygenase-1 gene expression in Kupffer cells of rat liver and in isolated Kupffer cell cultures during treatment with lipopolysaccharide.

METHODS

Cryostat sections of normal rat liver were investigated by immunofluorescence double-staining using specific antibodies for rat heme oxygenase-1 and ED2. Isolation and cell culture of Kupffer cells and primary hepatocytes from rat liver, as well as Northern and Western blot analysis, were performed with standard protocols.

RESULTS

Heme oxygenase-1 protein was highly expressed in large sinusoidal cells of normal rat liver, which were identified as Kupffer cells by staining with the macrophage surface marker ED2. By contrast, no expression of heme oxygenase-1 was detected in liver parenchymal cells. High expression of heme oxygenase-1 was also found in isolated Kupffer cells in culture by immunocytochemical staining as well as by Western and Northern blot analysis. After treatment of Kupffer cells cultures with lipopolysaccharide, heme oxygenase-1 was upregulated on the protein and mRNA level in a time- and dose-dependent manner. This increase in heme oxygenase-1 expression by lipopolysaccharide was prevented by the nitric oxide inhibitor N(G)-monomethyl-L-arginine which was reversed by an excess of L-arginine. Various nitric oxide donors up-regulated heme oxygenase-1 mRNA expression in Kupffer cells.

CONCLUSIONS

The lipopolysaccharide-dependent upregulation of the heme oxygenase-1 gene which is highly expressed in Kupffer cells is mediated by a nitric oxide-dependent mechanism.

Mechanism of sodium arsenite-mediated induction of heme oxygenase-1 in hepatoma cells. Role of mitogen-activated protein kinases

Heme oxygenase-1 is an inducible enzyme that catalyzes heme degradation and has been proposed to play a role in protecting cells against oxidative stress-related injury. We investigated the induction of heme oxygenase-1 by the tumor promoter arsenite in a chicken hepatoma cell line, LMH. We identified a heme oxygenase-1 promoter-driven luciferase reporter construct that was highly and reproducibly expressed in response to sodium arsenite treatment. This construct was used to investigate the role of mitogen-activated protein (MAP) kinases in arsenite-mediated heme oxygenase-1 gene expression. In LMH cells, sodium arsenite, cadmium, and heat shock, but not heme, induced activity of the MAP kinases extracellular-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38. To examine whether these MAP kinases were involved in mediating heme oxygenase-1 gene expression, we utilized constitutively activated and dominant negative components of the ERK, JNK, and p38 MAP kinase signaling pathways. Involvement of an AP-1 site in arsenite induction of heme oxygenase-1 gene expression was studied. We conclude that the MAP kinases ERK and p38 are involved in the induction of heme oxygenase-1, and that at least one AP-1 element (located -1576 base pairs upstream of the transcription start site) is involved in this response.

Molecular cloning, characterization, and expression of the chicken heme oxygenase-1 gene in transfected primary cultures of chick embryo liver cells

Using chick heme oxygenase-1 (cHO-1) cDNA as a probe, three independent clones were identified from screening a lambda FixII chick genomic library. Genomic Southern blots using this cDNA probe or a cHO-1 5' specific probe showed that cHO-1 is a single-copy gene. Based on restriction enzyme analysis, Southern blots, polymerase chain reaction analysis and DNA sequencing, it was confirmed that the three overlapping clones isolated cover the entire cHO-1 gene, as well as approximately 10 kb of the flanking regions on both ends. As with mammalian HO-1x, cHO-1 has five exons and four introns. Computer analysis of the DNA sequence obtained identified consensus sequences corresponding to numerous transcription factor recognition elements. These include AP-1, AP-2, NF-kB, C/EBP, c-Myc and a metal-responding element identified in the promoter region, and two Sp-1 elements in intron 1. Transient expression studies in transfected primary cultures of chick embryo liver cells showed that a CAT reporter gene construct containing 2.8 kb of the cHO-1 promoter region responded to sodium arsenite, H2O2, transition metals and 12-0-tetradecanoylphorbol 13-acetate, but not to heme. Studies with deletion mutants, consisting of various lengths of the cHO-1 promoter region, indicated that there are two regions important for sodium arsenite induction, one located between residues -1642 and -1293, and the second located in the first 263 base pairs of the cHO-1 promoter. DNA binding studies by electrophoretic mobility shift assay showed that nuclear protein isolated from primary cultures of chick embryo liver cells bound to the oligonucleotide probe containing an AP-1 element identified at -1573 to -1580. In addition, such binding was increased by cobalt or sodium arsenite treatment.

Regulation of expression of the human heme oxygenase-1 gene in transfected chick embryo liver cell cultures

Induction of heme oxygenase (HO) has been proposed as a protective cellular mechanism against oxidative damage. In previous work (Tyrrell et al., Carcinogenesis [1993] 14, 761-765), portions of the 5' promoter region of the human HO-1 gene linked to the reporter gene chloramphenicol acetyl transferase (CAT), had been transiently expressed in HeLa cells. To extend the study of human HO gene expression into primary liver cells, these reporter gene fusion constructs, containing 121 or 1416 base pairs of the untranscribed 5'-upstream sequences of the human HO-1 gene, were used along with pSV beta-Gal plasmid to dually transfect primary cultures of chick embryo liver cells (CELC). The transfected cells were treated with selected metals, heme, phorbol ester, and chemical agents that produce oxidative stress (H2O2 or sodium arsenite). Reporter gene activities were measured 18-20 h later. Our major findings are: (1) these HO-CAT constructs were expressed in CELC; (2) unlike HeLa cells, the expression of CAT was detected in CELC without the need for the SV40 enhancer; (3) sodium arsenite and cobalt chloride induced the expression of the HO-CAT constructs whereas heme had no effect on or decreased CAT expression for all of the transfected constructs; (4) study of endogenous chick HO-1 gene expression in CELC showed that HO-1 responded to sodium arsenite treatment in a dose-dependent fashion, and the response was rapid and transient. We conclude that, in chick liver cell cultures, induction of the HO-1 gene by heme is fundamentally different from that produced by transition metals or sodium arsenite. Furthermore, the results suggest that expression of the HO-1 gene is highly conserved across species.

5-Aminolevulinic acid-based photodynamic therapy. Clinical research and future challenges

BACKGROUND

Photodynamic therapy (PDT) for cancer patients has developed into an important new clinical treatment modality in the past 25-years. PDT involves administration of a tumor-localizing photosensitizer or photosensitizer prodrug (5-aminolevulinic acid [ALA], a precursor in the heme biosynthetic pathway) and the subsequent activation of the photosensitizer by light. Although several photosensitizers other than ALA-derived protoprophyrin IX (PpIX) have been used in clinical PDT, ALA-based PDT has been the most active area of clinical PDT research during the past 5 years. Studies have shown that a higher accumulation of ALA-derived PpIX in rapidly proliferating cells may provide a biologic rationale for clinical use of ALA-based PDT and diagnosis. However, no review updating the clinical data has appeared so far.

METHODS

A review of recently published data on clinical ALA-based PDT and diagnosis was conducted.

RESULTS

Several individual studies in which patients with primary nonmelanoma cutaneous tumors received topical ALA-based PDT have reported promising results, including outstanding cosmetic results. However, the modality with present protocols does not in general, appear to be superior to conventional therapies with respect to initial complete response rates and long term recurrence rates, particularly in the treatment of nodular skin tumors. Topical ALA-PDT does have the following advantages over conventional treatments: it is noninvasive; it produces excellent cosmetic results; it is well tolerated by patients; it can be used to treat multiple superficial lesions in short treatment sessions; it can be applied to patients who refuse surgery or have pacemakers and bleeding tendency; it can be used to treat lesions in specific locations, such as the oral mucosa or the genital area; it can be used as a palliative treatment; and it can be applied repeatedly without cumulative toxicity. Topical ALA-PDT also has potential as a treatment for nonneoplastic skin diseases. Systemic administration of ALA does not seem to be severely toxic, but the advantage of using this approach for PDT of superficial lesions of internal hollow organs is still uncertain. The ALA-derived porphyrin fluorescence technique would be useful in the diagnosis of superficial lesions of internal hollow organs.

CONCLUSIONS

Promising results of ALA-based clinical PDT and diagnosis have been obtained. The modality has advantages over conventional treatments. However, some improvements need to be made, such as optimization of parameters of ALA-based PDT and diagnosis; increased tumor selectivity of ALA-derived PpIX; better understanding of light distribution in tissue: improvement of light dosimetry procedure; and development of simpler, cheaper, and more efficient light delivery systems.

Mechanism of induction of heme oxygenase by metalloporphyrins in primary chick embryo liver cells: evidence against a stress-mediated response

Heme oxygenase catalyzes the first and rate-controlling step in heme catabolism. One of the two forms of heme oxygenase (heme oxygenase-1) has been shown to be increased by heme, metals, and in some systems, by certain environmental stresses. However, it remains uncertain whether heme induces hepatic heme oxygenase-1 by a general stress response, or a specific heme-dependent cellular response. The work communicated here explores this issue by examining possible mechanisms whereby heme and other metalloporphyrins induce heme oxygenase-1 in normal liver cells. Primary cultures of chick embryo liver cells were tested for their ability to increase heme oxygenase mRNA after exposure to selected metalloporphyrins (heme, chromium mesoporphyrin, cobalt protoporphyrin and manganese protoporphyrin). The ability of antioxidants to decrease metalloporphyrin-mediated induction of heme oxygenase-1 mRNA was also tested. Our results indicate that: 1) the increase in heme oxygenase-1 mRNA mediated by heme or other metalloporphyrins may involve a short-lived protein(s) since the increase was prevented by several inhibitors of protein synthesis; and 2) in normal liver cells, heme-dependent oxidative stress does not play a key role in the heme-mediated induction of heme oxygenase-1. We conclude that heme and other non-heme metalloporphyrins induce heme oxygenase-1 through a mechanism requiring protein synthesis, not because metalloporphyrins increase cellular oxidative or other stress.

Gene regulation of HBP 23 by metalloporphyrins and protoporphyrin IX in liver and hepatocyte cultures

Heme-binding protein 23 kDa (HBP23) belongs to the antioxidant family of peroxiredoxins and binds heme with high affinity. In vivo treatment of rats with heme induced expression of HBP23 mRNA levels in liver coordinately with that of the heme degrading enzyme heme oxygenase-1 (HO-1). In primary rat hepatocyte cultures Sn-, Co-, and Zn-metalloprotoporphyrin as well as the heme precursor protoporphyrin IX increased the HBP23 mRNA expression to a level similar to that elicited by heme. Heme-dependent induction of HBP23 mRNA was prevented by pretreatment with actinomycin D, indicating a transcriptional mechanism of gene induction. The results suggest that the coordinate gene regulation pattern of HBP23 and HO-1 plays a physiological role against oxidative stress.

Hepatic 5-aminolevulinic acid synthase mRNA stability is modulated by inhibitors of heme biosynthesis and by metalloporphyrins

Hepatic 5-aminolevulinic acid synthase, the first and normally rate-controlling enzyme of heme biosynthesis, is regulated by heme. One of the known mechanisms whereby increased cellular heme regulates 5-aminolevulinic acid synthase is by decreasing the stability of its mRNA. In primary cultures of chick embryo liver cells, we tested whether a decrease in cellular heme might increase 5-aminolevulinic acid synthase mRNA stability and whether heme or other metalloporphyrins could reverse this stabilization. We found that: (a) The stability of 5-aminolevulinic acid synthase mRNA was markedly increased by inhibitors of heme biosynthesis, namely, 4,6-dioxoheptanoic acid or deferoxamine; (b) This increased stability of 5-aminolevulinic acid synthase mRNA was reversed by the addition of heme (10 microM) or by the combination of zinc mesoporphyrin (50 nM), an inhibitor of heme oxygenase, and heme (200 nM); (c) Repression of 5-aminolevulinic acid synthase mRNA levels by zinc mesoporphyrin (10 microM) was due to inhibition of heme oxygenase, rather than a direct, heme-like, effect of zinc mesoporphyrin on 5-aminolevulinic acid synthase mRNA; (d) Among the several non-heme metalloporphyrins tested, only zinc mesoporphyrin and chromium mesoporphyrin significantly decreased 5-aminolevulinic acid synthase mRNA without increasing heme oxygenase mRNA.

Regulation of metallothionein gene expression. Studies in transfected primary cultures of chick embryo liver cells

To study the regulation of expression of the metallothionein gene in normal liver cells, we transfected chick embryo liver cells in primary cultures with constructs containing luciferase or chloramphenicol acetyl transferase (as reporter genes) under the control of differing lengths of the 5'-promoter region of the chick metallothionein gene (containing 30, 122, 190, or 623 base pairs upstream of the transcriptional start site). We controlled for efficiency of transfection by co-transfections with a plasmid containing a bacterial beta-galactosidase gene under the control of the SV 40 promoter and enhancer. Treatment of the transfected cells with transition metallic ions (cadmium, cobalt, and zinc) or sodium arsenite produced increases in activities of luciferase or chloramphenicol acetyl transferase, relative to beta-galactosidase, and this activity mapped to the first 122 base pairs of the promoter. Although heme has recently been reported to induce the endogenous metallothionein gene in chick embryo liver cells, 10-50 microM heme did not increase reporter gene activities in transfected cells. Nevertheless, the heme-dependent induction of endogenous heme oxygenase-1 in these cells was normal. We conclude that the heme-dependent induction of the liver metallothionein gene depends upon DNA region(s) outside the regulatory region of the chick metallothionein gene studied here and that elements within the first 122 base pairs of the metallothionein promoter are sufficient to confer responsiveness to transition metals or sodium arsenite.

Modulation of corneal heme oxygenase expression by oxidative stress agents

Heme oxygenase, the rate-limiting enzyme in the degradation of heme to bile-pigments and carbon monoxide, is induced in response to increased oxidative stress and is believed to provide a cytoprotective effect. We investigated the role of heme oxygenase in cultured rabbit corneal epithelial cells (RCE), and its potential to alleviate oxidative stress-induced cell damage. Heme oxygenase in RCE was effectively and potently induced by most metals tested, including tin, silver, and gold, and cytokines such as IL-6, and TGF beta. Stannous chloride and heme-induced heme oxygenase mRNA by 40 and 100 fold within 1-3 hours and increased enzyme activity by 9.2- and 10-fold, respectively, over a 24 hour period. IL-6, TGF beta and H2O2 induced heme oxygenase by 2-3 fold. Zinc protoporphyrins were effective inhibitors of heme oxygenase activity in vitro. However, when incubated with cells for 24 h they induced heme oxygenase mRNA but decreased or had no effect on its activity. Administration of heme, SnCl2, and H2O2 resulted in some degree of glutathione perturbation (GSH/GSSG). However, in all cases, depletion of glutathione was exacerbated if heme oxygenase was simultaneously inhibited. Conversely, perturbation of glutathione levels was minimized if heme oxygenase was induced by heme or stannous chloride. These results demonstrate that RCE cells exhibit functional heme oxygenase activity which is inducible in response to inflammatory cytokines and oxidative stress agents and suggest a cytoprotective role for heme oxygenase against cell injury.