Haem synthesis from exogenous 5-aminolaevulinate in cultured chick-embryo hepatocytes. Effects of inducers of cytochromes P-450

Dynamic regulation of NADPH oxidase 5 by intracellular heme levels and cellular chaperones

NADPH oxidase 5 (NOX5) is a transmembrane signaling enzyme that produces superoxide in response to elevated cytosolic calcium. In addition to its association with numerous human diseases, NOX5 has recently been discovered to play crucial roles in the immune response and cardiovascular system. Details of NOX5 maturation, and specifically its response to changes in intracellular heme levels have remained unclear. Here we establish an experimental system in mammalian cells that allows us to probe the influence of heme availability on ROS production by NOX5. We identified a mode of dynamic regulatory control over NOX5 activity through modulation of its heme saturation and oligomeric state by intracellular heme levels and Hsp90 binding. This regulatory mechanism allows for fine-tuning and reversible modulation of NOX5 activity in response to stimuli.

Glyceraldehyde-3-phosphate dehydrogenase is a chaperone that allocates labile heme in cells

Cellular heme is thought to be distributed between a pool of sequestered heme that is tightly bound within hemeproteins and a labile heme pool required for signaling and transfer into proteins. A heme chaperone that can hold and allocate labile heme within cells has long been proposed but never been identified. Here, we show that the glycolytic protein glyceraldehyde-3-phosphate dehydrogenase (GAPDH) fulfills this role by acting as an essential repository and allocator of bioavailable heme to downstream protein targets. We identified a conserved histidine in GAPDH that is needed for its robust heme binding both in vitro and in mammalian cells. Substitution of this histidine, and the consequent decreases in GAPDH heme binding, antagonized heme delivery to both cytosolic and nuclear hemeprotein targets, including inducible nitric-oxide synthase (iNOS) in murine macrophages and the nuclear transcription factor Hap1 in yeast, even though this GAPDH variant caused cellular levels of labile heme to rise dramatically. We conclude that by virtue of its heme-binding property, GAPDH binds and chaperones labile heme to create a heme pool that is bioavailable to downstream proteins. Our finding solves a fundamental question in cell biology and provides a new foundation for exploring heme homeostasis in health and disease.

Enhanced heme function and mitochondrial respiration promote the progression of lung cancer cells

Lung cancer is the leading cause of cancer-related mortality, and about 85% of the cases are non-small-cell lung cancer (NSCLC). Importantly, recent advance in cancer research suggests that altering cancer cell bioenergetics can provide an effective way to target such advanced cancer cells that have acquired mutations in multiple cellular regulators. This study aims to identify bioenergetic alterations in lung cancer cells by directly measuring and comparing key metabolic activities in a pair of cell lines representing normal and NSCLC cells developed from the same patient. We found that the rates of oxygen consumption and heme biosynthesis were intensified in NSCLC cells. Additionally, the NSCLC cells exhibited substantially increased levels in an array of proteins promoting heme synthesis, uptake and function. These proteins include the rate-limiting heme biosynthetic enzyme ALAS, transporter proteins HRG1 and HCP1 that are involved in heme uptake, and various types of oxygen-utilizing hemoproteins such as cytoglobin and cytochromes. Several types of human tumor xenografts also displayed increased levels of such proteins. Furthermore, we found that lowering heme biosynthesis and uptake, like lowering mitochondrial respiration, effectively reduced oxygen consumption, cancer cell proliferation, migration and colony formation. In contrast, lowering heme degradation does not have an effect on lung cancer cells. These results show that increased heme flux and function are a key feature of NSCLC cells. Further, increased generation and supply of heme and oxygen-utilizing hemoproteins in cancer cells will lead to intensified oxygen consumption and cellular energy production by mitochondrial respiration, which would fuel cancer cell proliferation and progression. The results show that inhibiting heme and respiratory function can effectively arrest the progression of lung cancer cells. Hence, understanding heme function can positively impact on research in lung cancer biology and therapeutics.

Neurite degeneration induced by heme deficiency mediated via inhibition of NMDA receptor-dependent extracellular signal-regulated kinase 1/2 activation

The early stages of many neurodegenerative diseases and age-related degeneration are characterized by neurite damage and compromised synaptic function that precede neuronal cell death. We investigated the signaling mechanisms underlying neurite degeneration using cortical neuron cultures. Inhibition of heme synthesis caused neurite damage, without neuronal death, and was mediated by reduced NMDA receptor (NMDAR) expression and phosphorylation. The signaling toward the degenerative phenotype involved suppression of the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway, and electrophysiological recording showed that the neurodegeneration is accompanied by reduced NMDAR current and Ca2+ influx, as well as reduced voltage-gated sodium currents, consistent with compromised neurite integrity. Rescue from the degenerative phenotype by heme replacement was dependent on restoration of NR2B subunit phosphorylation and expression of NMDAR currents with higher Ca2+ permeability, consistent with triggering prosurvival ERK1/2 signaling to maintain and extend neurites. This study demonstrated a new mechanism of neurodegeneration in which impaired heme synthesis led to NMDAR signaling dysfunction, suppression of the prosurvival ERK1/2 pathway, and progressive fragmentation of neuronal projections.

Levetiracetam in Idiopathic Generalised Epilepsy and Porphyria??Cutanea Tarda

We report the case of a 50-year-old male patient with idiopathic generalised epilepsy and porphyria cutanea tarda. Valproic acid and phenobarbital monotherapy controlled seizures but exacerbated porphyric symptomatology, while clobazam, clonazepam and lamotrigine monotherapy were well tolerated as regards porphyric disturbances but did not completely control seizures. Tonic- clonic seizures were eventually controlled by a combination of clonazepam (9 mg/day) and lamotrigine (150 mg/day), but absences persisted and this treatment caused significant adverse effects consisting of sedation and memory disturbances. Levetiracetam monotherapy (3 g/day) was accompanied by complete control of seizures; memory disturbances and sedation also resolved, and no porphyrinogenetic activity of levetiracetam was observed. This is the first report of the safe use of levetiracetam in porphyria cutanea tarda.

Heme Deficiency Is Associated with Senescence and Causes Suppression ofN-Methyl-d-aspartate Receptor Subunits Expression in Primary Cortical Neurons

Heme is a crucial component of many pharmacological and toxicological processes, and studies have suggested that heme deficiency may play a role in cellular ageing. A model of ageing neurons was established using prolonged cultures of BALB/c mouse primary cortical neurons. Aged neurons displayed a senescent phenotype and a marked up-regulation of cathepsin-L expression. Down-regulation of the candidate neuron-specific genes for N-methyl-D-aspartate (NMDA) receptor subunits (NMDAzeta1 and -epsilon2) and neurofilament light peptide (NF-L) were found to be characteristic of the aging process as reported in vivo (Brain Res 907:71-83, 2001; Brain Res Mol Brain Res 99:40-45, 2002). In contrast, the genes for the controlling enzymes of heme synthesis and degradation (5-aminolevulinate synthase 1 and heme oxygenase 1, respectively) were up-regulated, implying depletion of a regulatory heme pool. Inhibition of heme synthesis (by 70-80%) at different enzymic steps by succinyl acetone and N-methylprotoporphyrin IX resulted in the earlier lowered expression of NMDAzeta1 and -epsilon2 and NF-L. Exogenous hemin added to heme-depleted cells rescued the expression of these neuron-specific genes. Culture of cortical neurons from BALB/c Fech(m1Pas) mutant mice demonstrating depressed heme synthesis showed premature senescence and reduced expression of NMDAzeta1 and -epsilon2 receptor subunits and NF-L compared with wild-type cells. Our findings suggest that reduced availability of heme in neurons associated with senescence may have significant effects on synaptic function.

Heme deficiency suppresses the expression of key neuronal genes and causes neuronal cell death

Defective heme synthesis may cause acute porphyrias, which are associated with a wide array of neurological disturbances involving both the central and peripheral nervous systems. Thus, the understanding of the roles of heme in neuronal cell function may provide insights into the molecular events underlying the pathogenesis of neuropathies associated with defective heme synthesis. In this report, we use rat pheochromocytoma (PC12) clonal cells as a model system for studying the role of heme in neuronal cell survival. We examined the effects of inhibition of heme synthesis on signaling pathways and gene expression in nerve growth factor (NGF)-induced PC12 cells. We found that succinyl acetone-induced heme deficiency selectively caused apoptosis in NGF-induced PC12 cells. Further, we found that in succinyl acetone-treated, NGF-induced cells, the pro-survival Ras-ERK1/2 signaling pathway was inactivated and the pro-apoptotic JNK signaling pathway was activated. In these cells, the activation of caspase and the cleavage of nuclear poly (ADP-ribose) polymerase (PARP) were also evident. Importantly, microarray gene expression analysis showed that more than 20 key neuronal genes that were induced by NGF were suppressed by succinyl acetone. These genes include those encoding survival motor neuron protein, synaptic vesicle protein SVOP, and neural cell adhesion molecule NCAM. These results indicate that heme is important for neuronal cell signaling and the proper functioning of neuronal cells.

Heme deficiency causes apoptosis but does not increase ROS generation in HeLa cells

Mitochondria provide cellular energy supply via respiration and are the major sites for the generation of reactive oxygen species (ROS). Mitochondria also play a fundamental role in apoptosis. Heme is a key factor in mitochondrial function. Defective heme synthesis or altered heme metabolism is associated with numerous diseases. Here we investigated the molecular mechanism by which heme promotes HeLa cell growth and survival. We found that heme deficiency-induced apoptosis involves the release of cytochrome c and the activation of caspase 3. However, heme deficiency-induced apoptosis appears to occur by a unique mechanism distinct from those known to mediate mitochondrial-dependent apoptosis. Specifically, our data show that heme deficiency causes apoptosis in a pathway that is independent of ROS generation and the collapse of mitochondrial membrane potential. These results provide insights into how defective heme synthesis or altered heme metabolism causes diseases and how heme may control cell growth and cell death.

Heme controls the expression of cell cycle regulators and cell growth in HeLa cells

Heme plays a central role in oxygen utilization and in the generation of cellular energy. Here we examined the effect of heme and heme deficiency on cell cycle progression and the expression of key regulators in HeLa cells. We found that inhibition of heme synthesis causes cell cycle arrest and induces the expression of molecular markers associated with senescence and apoptosis, such as increased formation of PML nuclear bodies. Our data show that succinyl acetone-induced heme deficiency increases the protein levels of the tumor suppressor gene product p53 and CDK inhibitor p21, and decreases the protein levels of Cdk4, Cdc2, and cyclin D2. Further, we found that heme deficiency diminishes the activation/phosphorylation of Raf, MEK1/2, and ERK1/2-components of the MAP kinase signaling pathway. Our results show that heme is a versatile molecule that can effectively control cell growth and survival by acting on multiple regulators.

An examination of heme action in gene expression: heme and heme deficiency affect the expression of diverse genes in erythroid k562 and neuronal PC12 cells

The aim of this study was to identify key genes whose expression is altered by heme and heme deficiency in the human erythroleukemia K562 cells and in the NGF-induced rat pheochromocytoma neuronal PC12 cells, respectively. By quantitative RT-PCR, Northern blotting, and Western blotting analyses, we found that the expression of the CDK inhibitors p18 and p21 was upregulated at the early and late stages of heme-induced erythroid differentiation of K562 cells, respectively, while the expression of cyclin D1 was downregulated. Data from succinyl acetone and desferrioxamine treatments suggest that these effects of heme in K562 cells were specific. Further, by microarray expression analysis, we found that inhibition of heme synthesis by succinyl acetone in NGF-induced PC12 cells drastically altered the expression of several groups of important neuronal genes, including the structural genes encoding neurofilament proteins and synaptic vesicle proteins, regulatory genes encoding signaling components beta-arrestin and p38 MAPK, and stress-response genes encoding hsp70. These results show that heme and heme deficiency affect the expression of diverse genes in a cell-type specific manner in mammalian cells, and that heme, although needed at different levels, is critical for both erythropoiesis and neurogenesis. These studies provide insights into how heme may act to control diverse regulatory processes in mammals.

Association between CYP1A2 polymorphism and susceptibility to porphyria cutanea tarda

Individuals with the most common form of the porphyrias, porphyria cutanea tarda (PCT), are believed to be genetically predisposed to development of clinically overt disease through mutations and polymorphisms in genes associated with known precipitating factors. In this study, we have examined a group of Danish patients with PCT for the presence of the C/A polymorphism in intron 1 of CYP1A2. The results demonstrate that the frequency of the highly inducible A/A genotype is increased in both familial and sporadic PCT. This suggests that inheritance of this genotype is a susceptibility factor in development of PCT.

Physiological oxygen tensions modulate expression of the mdr1b multidrug-resistance gene in primary rat hepatocyte cultures

P-Glycoprotein transporters encoded by mdr1 (multidrug resistance) genes mediate extrusion of an array of lipophilic xenobiotics from the cell. In rat liver, mdr transcripts have been shown to be expressed mainly in hepatocytes of the periportal region. Since gradients in oxygen tension (pO(2)) may contribute towards zonated gene expression, the influence of arterial and venous pO(2) on mRNA expression of the mdr1b isoform was examined in primary rat hepatocytes cultured for up to 3 days. Maximal mdr1b mRNA levels (100%) were observed under arterial pO(2) after 72 h, whereas less than half-maximal mRNA levels (40%) were attained under venous pO(2). Accordingly, expression of mdr protein and extrusion of the mdr1 substrate rhodamine 123 were maximal under arterial pO(2) and reduced under venous pO(2). Oxygen-dependent modulation of mdr1b mRNA expression was prevented by actinomycin D, indicating transcriptional regulation. Inhibition of haem synthesis by 25 microM CoCl(2) blocked mdr1b mRNA expression under both oxygen tensions, whereas 80 microM desferrioxamine abolished modulation by O(2). Haem (10 microM) increased mdr1b mRNA levels under arterial and venous pO(2). In hepatocytes treated with 50 microM H(2)O(2), mdr1b mRNA expression was elevated by about 1.6-fold at venous pO(2) and 1.5-fold at arterial pO(2). These results support the conclusion that haem proteins are crucial for modulation of mdr1b mRNA expression by O(2) in hepatocyte cultures and that reactive oxygen species may participate in O(2)-dependent signal transduction. Furthermore, the present study suggests that oxygen might be a critical modulator for zonated secretion of mdr1 substrates into the bile.

Erythropoietin induction in Hep3B cells is not affected by inhibition of heme biosynthesis

Erythropoietin (Epo) is one of the physiologically important genes whose transcription is up-regulated by hypoxia. Our laboratory previously proposed that the sensor of this event is a heme protein which turns over rapidly. We have investigated the effects of four inhibitors of heme synthesis (4,6-dioxoheptanoic acid (DHA), isoniazid (INH), N-methyl protoporphyrin IX (MPP), and deferoxamine mesylate (DSF)) on hypoxia-, cobalt-, and DSF-induced erythropoietin (Epo) mRNA expression, heme biosynthesis, and cell viability in Hep3B cells. DHA (0.1-1.0 mM) inhibited heme biosynthesis more than 85%, but did not suppress Epo mRNA expression. Epo mRNA expression was inhibited only at higher concentrations of DHA (2, 4 mM) which also inhibited cell viability. No suppression of Epo mRNA expression by INH was observed at doses known to inhibit heme biosynthesis. MPP did not suppress Epo mRNA expression although it showed an inhibitory effect on heme biosynthesis without any decreased cell viability. 130 microM DSF, a dose which inhibited heme biosynthesis without cell toxicity, suppressed hypoxia-induced Epo mRNA expression, but enhanced cobalt-induced Epo mRNA expression. These results show that although the oxygen sensor is probably a heme protein it does not turn over rapidly. Therefore, cobalt is unlikely to act by substituting for heme iron.

Mimicry in primary rat hepatocyte cultures of the in vivo perivenous induction by phenobarbital of cytochrome P-450 2B1 mRNA: role of epidermal growth factor and perivenous oxygen tension

Treatment of male rats with phenobarbital (PB) results in a perivenous and mid-zonal pattern of cytochrome P-450 (CYP)2B1 mRNA expression within the liver acinus. The mechanism of this zonated induction is still poorly understood. In this study sinusoidal gradients of oxygen and epidermal growth factor (EGF) besides those of the pituitary-dependent hormones growth hormone (GH), thyroxine (T4), and triiodothyronine (T3) were considered to be possible determinants for the zonated induction of the CYP2B1 gene in liver. Moreover, heme proteins seem to play a key role in oxygen sensing. Therefore, the influence of arterial (16% O2) and venous (8% O2) oxygen tension (pO2), and of the heme synthesis inhibitors CoCl2 and desferrioxamine (DSF) on PB-dependent CYP2B1 mRNA induction as well as the repression by EGF and, for comparison, by GH, T4, and T3, of the induction under arterial and venous pO2 were investigated in primary rat hepatocytes. Within 3 days, phenobarbital induced CYP2B1 mRNA to maximal levels under arterial pO2 and to about 40% of maximal levels under venous pO2. CoCl2 annihilated induction by PB under both oxygen tensions, whereas desferrioxamine and heme abolished the positive modulation by O2, suggesting that heme is a necessary component for O2 sensing. EGF suppressed CYP2B1 mRNA induction by PB only under arterial but not under venous pO2, whereas GH, T4, and T3 inhibited induction under both arterial and venous pO2. Thus, in hepatocyte cultures, an O2 gradient in conjunction with EGF mimicked the perivenous induction by PB of the CYP2B1 gene observed in the liver in vivo.

Involvement of a local fenton reaction in the reciprocal modulation by O2 of the glucagon-dependent activation of the phosphoenolpyruvate carboxykinase gene and the insulin-dependent activation of the glucokinase gene in rat hepatocytes

H2O2 mimicked the action of periportal pO2 in the modulation by O2 of the glucagon-dependent activation of the phosphoenolpyruvate carboxykinase (PCK) gene and the insulin-dependent activation of the glucokinase (GK) gene. H2O2 can be converted in the presence of Fe2+ in a Fenton reaction into hydroxyl anions and hydroxyl radicals (.OH). The hydroxyl radicals are highly reactive and might interfere locally with transcription factors. It was the aim of the present study to investigate the role of and to localize such a Fenton reaction. Hepatocytes cultured for 24 h were treated under conditions mimicking periportal or perivenous pO2 with glucagon or insulin plus the iron chelator desferrioxamine (DSF) or the hydroxyl radical scavenger dimethylthiourea (DMTU) to inhibit the Fenton reaction. PCK mRNA was induced by glucagon maximally under conditions of periportal pO2 and half-maximally under venous pO2. GK mRNA was induced by insulin with reciprocal modulation by O2. DSF and DMTU reduced the induction of PCK mRNA to about half-maximal and increased the induction of GK mRNA to maximal under both O2 tensions. Hydroxyl radical formation was maximal under arterial pO2. Perivenous pO2, DSF and DMTU each decreased the formation of .OH to about 70% of control. The Fenton reaction could be localized in a perinuclear space by confocal laser microscopy and three-dimensional reconstruction techniques. In the same compartment, iron could be detected by electron-probe X-ray microanalysis. Thus a local Fenton reaction is involved in the O2 signalling, which modulated the glucagon- and insulin-dependent PCK gene and GK gene activation.

Photosensitization of experimental hepatocellular carcinoma with protoporphyrin synthesized from administered delta-aminolevulinic acid: studies with cultured cells and implanted tumors

BACKGROUND/AIMS

Photodynamic therapy using porphyrins or related compounds and laser light is an investigational treatment for neoplasms. The aim of this study was to establish whether this might be applicable for hepatocellular carcinoma using protoporphyrin synthesized in the tissue from administered delta-aminolevulinic acid.

METHODS

We measured porphyrin accumulation in normal rat hepatocytes and Morris hepatoma cells in culture, and in subcutaneously implanted hepatomas and other tissues of the rat after administration of delta-aminolevulinic acid, and assessed cell and tissue damage after application of laser light.

RESULTS

Porphyrin accumulation after delta-aminolevulinic acid was added to the medium was greater and continued to increase for a longer period of time in hepatoma cells than in hepatocytes (1337+/-42 vs 513+/-31 fluorescence units/cell at 8 h, means+/-SE, p<0.001). After intraperitoneal injection of delta-aminolevulinic acid to rats with subcutaneously growing hepatomas, porphyrin content in tumor and liver was similar at 4 h but was higher in tumor at 6 h. Laser light caused necrosis of normal and malignant liver cells in culture and subcutaneous hepatomas in vivo.

CONCLUSIONS

We conclude from these in vitro and in vivo studies that porphyrin accumulation after administration of delta-aminolevulinic acid in this hepatoma is substantial and time dependent, and delivery of laser light locally can cause tumor photosensitization and necrosis.

Arterial oxygen partial pressures reduce the insulin-dependent induction of the perivenously located glucokinase in rat hepatocyte cultures: mimicry of arterial oxygen pressures by H2O2

Liver glucokinase (GK) is localized predominantly in the perivenous zone. GK mRNA was induced by insulin maximally under venous O2 partial pressure (pO2) and only half-maximally under arterial pO2. CoCl2 and desferrioxamine mimicked venous pO2 and enhanced the insulin-dependent induction of GK mRNA under arterial pO2. H2O2 mimicked arterial pO2 and reduced insulin-induced GK mRNA under venous pO2 to the lower arterial levels. Thus the zonal O2 gradient in liver seems to have a key role in the heterogenous expression of the GK gene.

Modification of the photodynamic action of delta-aminolaevulinic acid (ALA) on rat pancreatoma cells by mitochondrial benzodiazepine receptor ligands

We have shown that addition of exogenous delta-aminolaevulinic acid (ALA) to rat pancreatoma AR4-2J cells in culture leads to the increased production of porphobilinogen (PBG) and the accumulation of photoactive protoporphyrin IX (PPix) in these cells. Exposure to light (lambda > 400 nm) at an intensity of 0.2 mW cm-2 for 8 min resulted in an ALA dose-dependent cytolysis of the cells, with an EC50 of 6.6 +/- 0.7 microM. This cytolytic effect was light intensity dependent, with greater cell destruction after exposure to light at an intensity of 0.47 mW cm-2 than at 0.2 mW cm-2; it was also dependent on the duration of illumination, cell survival decreasing with increasing illumination times. The photodestruction of the AR4-2J cells following exposure to ALA can be attributed to the production of endogenous PPix, a photoactive porphyrin that we have shown to generate singlet oxygen upon illumination, whereas ALA itself does not. Further investigation of the molecular mechanisms underlying the photodynamic action of ALA demonstrated the involvement of the mitochondrial (peripheral) benzodiazepine receptor (MBR), a high-affinity recognition site for dicarboxylic porphyrins, and especially PPix. The centrally acting benzodiazepine compounds clonazepam and flumazenil, which have negligible affinities for the MBR, had no effect on ALA-mediated phototoxicity. In contrast, both the isoquinoline carboxamide PK11195 and the benzodiazepine Ro 5-4864 ligands, displaying a high affinity for the MBR, did affect ALA-mediated phototoxicity, each markedly increasing the EC50 for cell photodestruction and thus exerting a photoprotective effect. It is concluded that the MBR may play an important role in the expression of ALA-mediated PPix phototoxicity and that MBR ligands, by diminishing the actions of endogenous PPix, have the potential to rescue cells from porphyrin-induced photolysis.

Oxygen-dependent expression of the erythropoietin gene in rat hepatocytes in vitro

Since in juvenile rats the liver is the predominant site of erythropoietin (EPO) gene expression, we have used primary cultures of juvenile rat hepatocytes to establish and in vitro system for investigation of oxygen-dependent EPO formation. When isolated hepatocytes were incubated at reduced oxygen tensions for 18-48 h, we found increased secretion of EPO protein and elevated levels of EPO mRNA, as determined by RNas protection. This increase was maximal at 3% O2, where EPO mRNA levels after 18 h were approximately 15-fold higher than at 20% O2. The increase in EPO mRNA at low oxygen tensions was specific insofar as [3H]uridine incorporation, as a measure of total RNA synthesis, was reduced by approximately 50% at 3% O2, and it appeared to involve gene transcription since it was abolished in the presence of actinomycin D (35 microM). Significant increases in EPO mRNA were also observed in cells kept at 20% oxygen in the presence of cobalt chloride (50 microM) and nickel chloride (400 microM), but EPO mRNA levels achieved under these conditions were less than 7% of those in cells incubated at 3% oxygen. No increase in EPO mRNA levels was observed in cultures incubated at 20% O2 in the presence of cyclic dibutyryl-AMP (10 microM-3 mM), cyclic 8-bromoGMP (10 microM-1 mM), cyclohexyladenosine (1 microM), 5'-N-ethylcarboxamidoadenosine (1 microM) and phorbol 12-myristate 13-acetate (3 nM). In the presence of 10% carbon monoxide, used to block haem proteins in their oxy conformation, EPO mRNA levels in hepatocytes incubated at low oxygen tensions were reduced to 63%.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of the erythropoietin gene

Erythropoietin (Epo), the hormone that stimulates red blood cell production, is induced by hypoxia. We have utilized the human hepatoma cell line, Hep3B, to investigate the regulation of the Epo gene. We present evidence that the oxygen sensor in Hep3B cells is a heme protein. Hypoxic and cobalt induction of Epo protein is paralleled by a 50- to 100-fold increase in Epo mRNA which we have accurately quantified by means of an assay based on competitive polymerase chain reaction. This increase in Epo mRNA is due primarily to increased transcription. Transfection experiments utilizing the sensitive luciferase reporter gene show that the minimal portions of the Epo gene required for hypoxic induction include a 53 bp promoter element and a 43 bp enhancer located downstream from the polyadenylation site. Gel shift experiments show that these two regions cross-compete for specific DNA binding proteins. The enhancer contains a hexanucleotide direct repeat with a two bp insert which footprints with nuclear extracts from Hep3B cells and, when mutated, results in loss of hypoxic induction. This sequence is likely to bind to a member of the steroid/thyroid hormone receptor family of DNA binding proteins. These enhancer and promoter elements appear to cooperate in enabling the Epo gene to respond to hypoxia in a physiologically appropriate manner.

Selective accumulation of endogenously produced porphyrins in a liver metastasis model in rats

The possibility of using the porphyrin precursor 5-aminolevulinic acid to cause selective porphyrin accumulation in tumors was examined. Syngeneic colon carcinomas CC531 were implanted in the livers of Wag/Rij rats. Groups of three to six animals each were given 2 mg/mL of 5-aminolevulinic acid in drinking water from the 8th, 14th, or 17th day after tumor implantation. Two other groups received either 2.5 or 5 mg/kg of Photofrin II (Photomedica Inc., Raritan, NJ) intravenously on day 17. On day 19 the livers were removed and porphyrin concentrations were measured in normal livers and tumors by solvent extraction and high-performance liquid chromatography. Protoporphyrin accumulated progressively in tumors with increasing duration of 5-aminolevulinic acid administration (P = 0.0001), whereas no increase was found in normal livers. After 11 days of 5-aminolevulinic acid administration the porphyrin concentration ratio between tumors and livers was 4:1. In contrast, after Photofrin II administration the concentration was higher in normal livers than in tumors (1:3 ratio, tumor to liver). Enzyme measurements showed a threefold decrease in ferrochelatase activity in tumors compared with livers (P less than 0.001). In conclusion, oral administration of 5-aminolevulinic acid results in progressive accumulation of protoporphyrin in a transplantable colon carcinoma without accumulation in the surrounding liver tissue. This selective accumulation of porphyrins appears to be caused by a relative ferrochelatase deficiency in malignant tissue. 5-Aminolevulinic acid administration may be a suitable approach to photosensitizing liver tumors for photodynamic therapy or to early detection of tumors by fluorescence in ultraviolet light.

Differential potency of atropisomers of polychlorinated biphenyls on cytochrome P450 induction and uroporphyrin accumulation in the chick embryo hepatocyte culture

The atropisomers of 2,2',3,4,6-pentachlorobiphenyl (PeCB), 2,2',3,4,4',6-hexachlorobiphenyl (HeCB), and 2,2',3,3',4,4',6,6'-octachlorobiphenyl (OCB) were studied in the chick embryo hepatocyte culture to determine if chirality plays a role in the recognition events associated with the induction of cytochromes P450 and the accumulation of uroporphyrin (URO). Concentration-related induction of cytochrome P450 content, ethoxyresorufin-O-deethylase (EROD) and benzphetamine N-demethylase (BPDM) activities were measured. The rank order of potency for total cytochrome P450 induction was HeCB greater than OCB greater than or equal to PeCB. The (+)- and (-)-enantiomers of PeCB and OCB were of equal potencies as inducers of cytochromes P450, whereas the (+)-HeCB was greater than the (-)-HeCB. HeCB was a much more potent inducer of EROD activity than was either PeCB or OCB. EROD activity was induced to a much greater extent by the (+)-enantiomers of all compounds, with the (-)-enantiomers of PeCB and OCB being inactive. BPDM activity was induced by all three compounds in the order of OCB greater than or equal to HeCB greater than PeCB. The (-)-enantiomers were more potent inducers of BPDM activities than were the (+)-enantiomers, except for HeCB, in which the (+)- was more potent than the (-)-enantiomer. Analysis of porphyrin accumulation in cultures treated with delta-aminolevulinic acid revealed that (+)-HeCB caused the greatest percent URO accumulation, which also correlated with the greatest increase in EROD activity. All other enantiomers caused up to 47% URO accumulation, which did not correlate with an increase in EROD activity.

Human colon tumor cell line LS174T drug metabolizing system

The effects of culture variables on the specific content and activity of various enzymes of the drug metabolizing system were assessed in colon tumor cell line LS174T. The NADH reduced cytochrome b5 (cyt b5)4 spectrum of these cells was similar to rat liver cyt b5. When released from the membrane by trypsin and concentrated, the cyt b5 was found to cross react with rabbit antibody to rat liver cyt b5 and human liver cyt b5. The enzyme activities were found stable over limited cell passages with control values of 0.03 and 0.13 mumol/min/mg protein for NADPH and NADH cytochrome c (cyt c) reducing activity, 0.05 nmol cyt b5 and 0.013 nmol cytochrome P450 per milligram of microsomal protein. Phenobarbital/hydrocortisone showed a consistent, but not always significant increase in the NADPH and NADH cyt c reduction and benzanthracene an increase in the NADH cyt c reducing activity and cyt b5 content. Griseofulvin lowered the NADH cyt c reducing activity. Delta-aminolevulinic acid (0.5 mM) caused a significant decrease in the specific activity of all enzymes, as judged by a student's t test, with a p less than 0.001.

Effects of polychlorinated biphenyls on cytochrome P450 induction in the chick embryo hepatocyte culture

The effects of pure synthetic polychlorinated biphenyl (PCB) congeners on the induction of cytochrome P450 and associated activities were examined in cultured chick embryo hepatocytes. Dose-response effects for the induction of total cytochrome P450 ethoxyresorufin-O-deethylase (EROD) activity, and benzphetamine demethylase (BPDM) activity were studied using 10 selected tetra- to hexachlorinated PCB congeners. These studies revealed that PCBs caused effects in the chick hepatocyte culture different from previously observed effects in rat liver. Based on their effects in chick hepatocytes, the PCBs could be categorized into two groups. The first group (consisting of 3,3',4,4'-PCB, 3,3',4,4',5-PCB, 3,3',4,4',5,5'-PCB, 2',3,3',4,5-PCB, 2,3,3',4,4',5'-PCB, and 2,3,4,4',5-PCB) induced total cytochrome P450 2.4- to 2.9-fold and EROD activity from 1-2 pmol/min/mg protein to 162-247. There was marked variation in potency, but all these congeners had a maximal inducing dose above which cytochrome P450 concentrations and EROD activities declined. BPDM activities were increased only slightly (1.2- to 1.6-fold) at the maximal cytochrome P450 inducing dose. The second group of congeners (consisting of 2,2',4,5,5'-PCB. 2,2',4,4',5,5'-PCB, and 2,2',3,4,4',6-PCB) induced total cytochrome P450 concentrations 4.0-fold and BPDM activities 2.2- to 2.6-fold with greatest activity occurring at the highest doses which could be added (10-50 microM). However, EROD activities were also increased by these congeners to 60-112 pmol/min/mg protein with declining activities seen at the highest PCB doses (i.e., resembling EROD induction patterns of the first group). The EROD induction patterns with these latter PCB congeners are noteworthy since these PCBs do not induce EROD activity in the rat. For both groups of PCB congeners, EROD induction was associated with increased accumulation of uroporphyrin in cultures exposed to exogenous 5-aminolevulinate. Studies investigating the reason for the depression of cytochrome P450 concentrations and/or EROD activities by high doses of the PCBs revealed that with the first group there was slightly decreased total protein synthesis, decreased total cell heme concentrations, and decreased accumulation of radiolabeled heme synthesized from 5-[14C]aminolevulinate. These changes might represent nonspecific toxic effects of the first group of PCBs. However, since these changes were not seen with the second group of PCBs, it is unlikely that either inhibition of heme synthesis or toxicity cause the depression of EROD activity with high PCB doses.

Regulation of the erythropoietin gene: evidence that the oxygen sensor is a heme protein

Erythropoietin (Epo), the hormone that stimulates red blood cell production, is synthesized in the kidney and liver in response to hypoxia. The human hepatoma cell line Hep3B regulates its production of Epo in a physiologic manner. Either hypoxia or cobalt chloride markedly increases expression of Epo mRNA as well as production of biologically active and immunologically distinct Epo protein. New protein synthesis is required before the induction of increased levels of hypoxia- or cobalt-induced Epo mRNA. Hypoxia, cobalt chloride, and nickel chloride appear to stimulate Epo production through a common pathway. The inhibition of Epo production at low partial pressures of oxygen by carbon monoxide provides evidence that a heme protein is integrally involved in the oxygen-sensing mechanism. This hypothesis is further supported by the finding that when heme synthesis is blocked, hypoxia-, cobalt-, and nickel-induced Epo production are all markedly inhibited. A model is proposed in which a ligand-dependent conformational change in a heme protein accounts for the mechanism by which hypoxia as well as cobalt and nickel stimulate the production of Epo.

Effect of serum proteins on haem uptake and metabolism in primary cultures of liver cells

A role of haemopexin in transporting haem to hepatocytes for degradation has been inferred from the high affinity of haemopexin for haem. We have examined this question in primary cultures of chick-embryo and adult rat liver cells. We present here the results of four sets of experiments which indicate that haemopexin retarded haem uptake by hepatocytes in culture. (1) Haem bound to bovine serum albumin is known to repress the activity of delta-aminolaevulinate synthase in chick cultures as indicated by decreased porphyrin accumulation. When haem-albumin was added in the presence of excess purified or freshly secreted chicken haemopexin, no haem-mediated repression of porphyrin production was observed. The haem-mediated repression of porphyrin accumulation was partially prevented when human, but not chicken, albumin was added to cultures. This finding reflected the higher affinity of human albumin for haem compared with that of chicken albumin. (2) Haemopexin inhibited the ability of haem to be incorporated into cytochrome P-450 induced in the chick cultures in the presence of the iron chelator desferrioxamine. (3) The rate of association of [55Fe]haem with cultured rat hepatocytes when [55Fe]haem-haemopexin was added was one-eighth of the rate observed when [55Fe]haem-bovine serum albumin was used as the haem donor. (4) The presence of haemopexin also diminished the catabolism of haem by both rat and chick-embryo liver cell cultures. It is concluded that the uptake and subsequent metabolic effects of haem are inhibited in cultured hepatocytes by proteins such as haemopexin which have a high affinity for haem.

Regulation of hepatic haem metabolism. Disparate mechanisms of induction of haem oxygenase by drugs and metals

We studied drug- and metal-mediated increases in activity of haem oxygenase, the rate-controlling enzyme for haem breakdown, in chick-embryo hepatocytes in ovo and in primary culture. Phenobarbitone and phenobarbitone-like drugs (glutethimide, mephenytoin), which are known to increase concentrations of an isoform of cytochrome P-450 in chick-embryo hepatocytes, were found to increase activities of haem oxygenase as well. In contrast, 20-methylcholanthrene, which increases the concentration of a different isoform of cytochrome P-450, had no effect on activity of haem oxygenase. Inhibitors of haem synthesis, 4,6-dioxoheptanoic acid or desferrioxamine, prevented drug-mediated induction of both cytochrome P-450 and haem oxygenase in embryo hepatocytes in ovo or in culture. Addition of haem restored induction of both enzymes. These results are interpreted to indicate that phenobarbitone and its congeners induce haem oxygenase by increasing hepatic haem formation. In contrast, increases in haem oxygenase activity by metals such as cobalt, cadmium and iron were not dependent on increased haem synthesis and were not inhibited by 4,6-dioxoheptanoic acid. We conclude that (1) induction of hepatic haem oxygenase activity by phenobarbitone-type drugs is due to increased haem formation, and (2) induction of haem oxygenase by drugs and metals occurs by different mechanisms.