Porphyrogenic effects and induction of heme oxygenase in vivo by delta-aminolevulinic acid

Chimeric Mouse With Humanized Liver Is an Appropriate Animal Model to Investigate Mode of Action for Porphyria-Mediated Hepatocytotoxicity

Porphyrinogenic compounds are known to induce porphyria-mediated hepatocellular injury and subsequent regenerative proliferation in rodents, ultimately leading to hepatocellular tumor induction. However, an appropriate in vivo experimental model to evaluate an effect of porphyrinogenic compounds on human liver has not been fully established. Recently, the chimeric mouse with humanized liver (PXB mice) became widely used as a humanized model in which human hepatocytes are transplanted. In the present study, we examined the utility of PXB mice as an in vivo experimental model to evaluate the key events of the porphyria-mediated cytotoxicity mode of action (MOA) in humans. The treatment of PXB mice with 5-aminolevulinic acid, a representative porphyrinogenic compound, for 28 days caused protoporphyrin IX accumulation, followed by hepatocyte necrosis, increased mitosis, and an increase in replicative DNA synthesis in human hepatocytes, indicative of cellular injury and regenerative proliferation, similar to findings in patients with porphyria or experimental porphyria models and corresponding to the key events of the MOA for porphyria-mediated hepatocellular carcinogenesis. We conclude that the PXB mouse is a useful model to evaluate the key events of the porphyria-mediated cytotoxicity MOA in humans and suggest the utility of PXB mice for clarifying the human relevancy of findings in mice.

5-Aminolevulinate dehydratase porphyria: Update on hepatic 5-aminolevulinic acid synthase induction and long-term response to hemin

BACKGROUND

5-Aminolevulinic acid dehydratase (ALAD) porphyria (ADP) is an ultrarare autosomal recessive disease, with only eight documented cases, all of whom were males. Although classified as an acute hepatic porphyria (AHP), induction of the rate limiting hepatic enzyme 5-aminolevulinic acid synthase-1 (ALAS1) has not been demonstrated, and the marrow may also contribute excess 5-aminolevulinic acid (ALA). Two patients have died and reported follow up for the others is limited, so the natural history of this disease is poorly understood and treatment experience limited.

METHODS

We report new molecular findings and update the clinical course and treatment of the sixth reported ADP patient, now 31 years old and the only known case in the Americas, and review published data regarding genotype-phenotype correlation and treatment.

RESULTS

Circulating hepatic 5-aminolevulinic acid synthase-1 (ALAS1) mRNA was elevated in this case, as in other AHPs. Gain of function mutation of erythroid specific ALAS2 - an X-linked modifying gene in some other porphyrias - was not found. Seven reported ADP cases had compound heterozygous ALAD mutations resulting in very low residual ALAD activity and symptoms early in life or adolescence. One adult with a germline ALAD mutant allele developed ADP in association with a clonal myeloproliferative disorder, polycythemia vera.

CONCLUSIONS

Elevation in circulating hepatic ALAS1 and response to treatment with hemin indicate that the liver is an important source of excess ALA in ADP, although the marrow may also contribute. Intravenous hemin was effective in most reported cases for treatment and prevention of acute attacks of neurological symptoms.

Heme biosynthesis and the porphyrias

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

Acute hepatic porphyrias: Current diagnosis & management

Each of the four acute hepatic porphyrias is due to mutation of an enzyme in the heme biosynthetic pathway. The accumulation of pathway intermediates that occur most notably when these diseases are active is the basis for screening and establishing a biochemical diagnosis of these rare disorders. Measurement of enzyme activities and especially DNA testing also are important for diagnosis. Suspicion of the diagnosis and specific testing, particularly measurement of urinary porphobilinogen, are often delayed because the symptoms are nonspecific, even when severe. Urinary porphyrins are also measured, but their elevation is much less specific. If porphobilinogen is elevated, second line testing will establish the type of acute porphyria. DNA testing identifies the familial mutation and enables screening of family members. Management includes removal of triggering factors whenever possible. Intravenous hemin is the most effective treatment for acute attacks. Carbohydrate loading is sometimes used for mild attacks. Cyclic attacks, if frequent, can be prevented by a GnRH analogue. Frequent noncyclic attacks are sometime preventable by scheduled (e.g. weekly) hemin infusions. Long term complications may include chronic pain, renal impairment and liver cancer. Other treatments, including RNA interference, are under development.

Protoporphyrin IX: the Good, the Bad, and the Ugly

Protoporphyrin IX (PPIX) is ubiquitously present in all living cells in small amounts as a precursor of heme. PPIX has some biologic functions of its own, and PPIX-based strategies have been used for cancer diagnosis and treatment (the good). PPIX serves as the substrate for ferrochelatase, the final enzyme in heme biosynthesis, and its homeostasis is tightly regulated during heme synthesis. Accumulation of PPIX in human porphyrias can cause skin photosensitivity, biliary stones, hepatobiliary damage, and even liver failure (the bad and the ugly). In this work, we review the mechanisms that are associated with the broad aspects of PPIX. Because PPIX is a hydrophobic molecule, its disposition is by hepatic rather than renal excretion. Large amounts of PPIX are toxic to the liver and can cause cholestatic liver injury. Application of PPIX in cancer diagnosis and treatment is based on its photodynamic effects.

Porphyria Diagnostics-Part 1: A Brief Overview of the Porphyrias

Porphyria diseases are a group of metabolic disorders caused by abnormal functioning of heme biosynthesis enzymes and characterized by excessive accumulation and excretion of porphyrins and their precursors. Precisely which of these chemicals builds up depends on the type of porphyria. Porphyria is not a single disease but a group of nine disorders: acute intermittent porphyria (AIP), hereditary coproporphyria (HCP), variegate porphyria (VP), δ-aminolevulinic acid dehydratase deficiency porphyria (ADP), porphyria cutanea tarda (PCT), hepatoerythropoietic porphyria (HEP), congenital erythropoietic porphyria (CEP), erythropoietic protoporphyria (EPP), and X-linked protoporphyria (XLP). Each porphyria results from overproduction of heme precursors secondary to partial deficiency or, in XLP, increased activity of one of the enzymes of heme biosynthesis. Taken together, all forms of porphyria afflict fewer than 200,000 people in the United States. Based on European studies, the most common porphyria, PCT, has a prevalence of 1 in 10,000, the most common acute porphyria, AlP, has a prevalence of ∼1 in 20,000, and the most common erythropoietic porphyria, EPP, is estimated at 1 in 50,000 to 75,000. CEP is extremely rare, with prevalence estimates of 1 in 1,000,000 or less. Only six cases of ADP are documented. The current porphyria literature is very exhaustive and a brief overview of porphyria diseases is essential in order for the reader to better appreciate the relevance of this area of research prior to undertaking biochemical diagnostics procedures. This unit summarizes the current knowledge on the classification, clinical features, etiology, pathogenesis, and genetics of porphyria diseases.

In vitro and in vivo porphyrin accumulation by C6 glioma cells after exposure to 5-aminolevulinic acid

Several malignant tissues synthesize endogenous porphyrins after exposure to 5-aminolevulinic acid (5-ALA). The present experiments have been designed to elucidate whether the C6 glioma cell, a model cell for human malignant glioma, similarly synthesizes porphyrins when exposed to 5-ALA, and whether specific synthesis occurs when C6 cells are inoculated into rat brains to form a tumor. In this situation the blood-brain barrier may interfere with 5-ALA availability, and spreading of porphyrins with edema outside the tumor may occur. Flow cytometry is used to determine the course of cell volume and porphyrin fluorescence intensities in cultured C6 cells which are incubated in 1 mM 5-ALA. For the induction of experimental brain tumors, 10(4) untreated C6 cells are inoculated into the brains of rats. After 9 days animals receive 100 mg 5-ALA/kg body weight. Brains are removed after 3, 6, or 9 h and frozen coronal sections obtained for H/E staining or fluorescence spectography. Cultured C6 cells show a linear increase of protoporphyrin IX fluorescence after exposure to 5-ALA, which begins to plateau after 85 min. Marked fluorescence is also observed in solid and infiltrating experimental tumor. However, faint fluorescence also occurs in normal tissue, basal pia, choroid plexus, and, more obviously, in white-matter tracts bordering the tumor (maximal distance: 1.5 +/- 0.7 mm). The observations demonstrate that C6 cells synthesize protoporphyrin IX after exposure to 5-ALA in vitro and in vivo. However, when utilizing 5-ALA for fluorescence detection or photodynamic therapy of brain tumors, attention should be paid to the possibility of protoporphyrin IX occurring outside the tumor.

Biochemical basis of 5-aminolaevulinic acid-induced protoporphyrin IX accumulation: a study in patients with (pre)malignant lesions of the oesophagus

Administration of 5-aminolaevulinic acid (ALA) leads to porphyrin accumulation in malignant and premalignant tissues, and ALA is used as a prodrug in photodynamic therapy (PDT). To understand the mechanism of porphyrin accumulation after the administration of ALA and to investigate whether ALA-induced protoporphyrin IX might be a suitable photosensitizer in Barrett's oesophagus and adenocarcinoma, we determined the activities of porphobilinogen deaminase (PBG-D) and ferrochelatase (FC) in various malignant and premalignant as well as in normal tissues of the human oesophagus. A PDT power index for ALA-induced porphyrin accumulation, the ratio of PBG-D to FC normalized for the normal squamous epithelium of the oesophagus, was calculated to evaluate intertissue variation in the ability to accumulate porphyrins. In malignant and premalignant tissue a twofold increased PBG-D activity and a marginally increased FC activity was seen compared with normal squamous epithelium. A significantly increased PDT power index in Barrett's epithelium and adenocarcinoma was found. Our results suggest that, after the administration of ALA, porphyrins will accumulate in a greater amount in Barrett's epithelium and adenocarcinoma of the oesophagus because of an imbalance between PBG-D and FC activities. The PDT power index here defined might be a useful indicative parameter for predicting the susceptibility of these tissues to ALA-PDT.

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.

Bilirubin production and conjugation from newly formed heme in isolated rat hepatocytes

1. Heme synthesis from delta-aminolevulinic acid (delta-ALA) in freshly isolated rat hepatocytes was maximal at 100 microM with a rate of approx. 7 nmol being synthesized per g wet weight cells. 2. Approximately 8% of synthesized heme was converted to bilirubin and 50% of the newly synthesized bilirubin was conjugated. 3. The ratio of di to monoconjugate was approx. 2.5. Incorporation of delta-ALA into bilirubin was increased by additional delta-ALA, heme and was also doubled in cells isolated from animals treated with CoCl2. 4. Bilirubin formation was inhibited approx. 90% by in vitro treatment with heme oxygenase inhibitors zinc and tin protoporphyrin.

Regulation of production of delta-aminolaevulinate synthase in tissues of chick embryos. Effects of porphyrogenic agents and of haem precursors

Studies conducted by several groups have established that porphyrogenic agents which caused elevations in chick-embryo liver delta-aminolaevulinate (ALA) synthase activity also increased the concentrations of the enzyme's RNA, and that haemin inhibited these elevations. We have determined in this study, using immune-blot analyses, that administration in ovo of allylisopropylacetamide (AIA) in combination with diethyl 1,4-dihydro-2,4,6-trimethyl,3,5-pyridinedicarboxylate (DDC) increased the mass of ALA synthase in intestine and kidney of chick embryos. Furthermore, the molecular mass of the subunit of the enzyme in those tissues appeared identical with that of liver ALA synthase. Using a synthetic oligonucleotide complementary to ALA synthase mRNA, we determined by solution hybridization and Northern-blot analyses that AIA and DDC also increased the concentrations of ALA synthase mRNA in intestine and kidney and that testosterone elevated the concentration of the RNA in kidney. In analyses of RNA obtained from chick-embryo liver, intestine, kidney, heart, brain and lung, the probe bound primarily in each case to a single 2.3 kb RNA. Finally, the haem precursors ALA and FeCl3, when injected together into the fluid surrounding embryos, inhibited both the elevations in ALA synthase mass and RNA concentration brought about by porphyrogenic agents in liver, kidney and intestine. Thus the results indicated that: (1) certain porphyrogenic agents increased ALA synthase mass and RNA in chick-embryo intestine and kidney, in addition to liver; (2) ALA and FeCl3 inhibited the elevations; and (3) the sizes of ALA synthase's subunit as well as the enzyme's mRNA appeared identical, in each case, in all tissues examined.

Decreased estrogen hydroxylation in male rat liver following cimetidine treatment

Administration of cimetidine (600 mumol/kg x 5) to adult male rats resulted in 55 and 25% decreases, respectively, in estradiol 2- and 16 alpha-hydroxylation. The same treatment also decreased the activities of ethylmorphine demethylase, aryl hydrocarbon hydroxylase, aniline hydroxylase and heme oxygenase but did not inhibit the activities of 7-ethoxycoumarin de-ethylase and delta-aminolevulinic acid synthase or decrease cytochrome P-450 content. In vitro addition of cimetidine (10-300 microM) also inhibited estradiol hydroxylations, and the effect was additive in rats pretreated with cimetidine in vivo; the other enzymic activities studied were completely unaffected by in vitro addition of cimetidine. In contrast, there was no effect of cimetidine either in vivo or in vitro on any of these activities in female rats. The results point to a wide variation in the susceptibilities of different isozymes of cytochrome P-450 to inhibition by cimetidine and suggest that such differential susceptibilities are also highly dependent on the sex of the animal.

Control of 5-aminolevulinate synthase in animals

The proposed mechanism by which hepatic ALV-synthase mitochondrial levels are regulated is outlined in Fig. 2. ALV-synthase catalyzes the first and rate-limiting step in the heme pathway and is normally present in low amounts. A cytosolic, regulatory free heme pool tightly controls the amount of ALV-synthase in two ways. In the primary mechanism of regulation, heme is proposed to inhibit the synthesis of ALV-synthase mRNA. Most likely this would be mediated through the action of specific heme-binding protein(s) which recognize regulatory control regions of the ALV-synthase gene. Gene activity therefore is significantly repressed most of the time. When there is an increased demand for heme by newly synthesized cellular hemoproteins, the free heme pool is reduced, leading to a derepression of ALV-synthase mRNA synthesis. Once the need for increased heme synthesis is satisfied, inhibitory heme levels build up again. When drugs such as phenobarbital are administered to animals, there is a rapid induction in the liver of both cytochrome P-450 and ALV-synthase. It is proposed that the heme pool governing ALV-synthase levels is lowered by the increased heme demand due to cytochrome P-450 apoprotein formation. The primary event in the drug induction of ALV-synthase is therefore the increased synthesis of cytochrome P-450 apoprotein. However, the mechanism by which this occurs is unknown, although drugs do increase the synthesis of mRNA for cytochrome P-450 (Fig. 2). (There is evidence that for the aromatic hydrocarbons a specific cytosolic receptor exists.) In the acute hepatic porphyria diseases, uncontrolled synthesis of hepatic ALV-synthase occurs. The various forms are characterized by reduced levels of one of the heme pathway enzymes other than ALV-synthase. Attacks of the disease are commonly precipitated by drugs which induce cytochrome P-450, and the uncontrolled accumulation of ALV-synthase which accompanies these attacks results from the combined action of the block in the heme pathway and the increased cytochrome P-450 levels. A major challenge which now exists is to understand at the molecular level how the genes for ALV-synthase and cytochrome P-450 are regulated in the liver and other tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

An experimental model of postnatal jaundice in the suckling rat. Suppression of induced hyperbilirubinemia by Sn-protoporphyrin

A model of experimental postnatal hyperbilirubinemia in the rat has been developed utilizing the heme precursor delta-aminolevulinic acid (ALA) to produce jaundice during a selective time period after birth. This time period is defined as that between 7 d postnatally, when the initial postpartum alterations of serum bilirubin and heme metabolism in the neonate have subsided, and 21 d, when the hepatic conjugation mechanism for the bile pigment appears fully developed. Administration of ALA in this time period led to a rapid, consistent, and significant dose-dependent increase in serum bilirubin levels in the newborn animals. Heme administration produced a qualitatively similar but enhanced effect. Both compounds, in addition, induced a dose-dependent increase in hepatic heme oxygenase activity concomitant with the increase in serum bilirubin levels. Neither compound increased serum bilirubin levels significantly when administered at or after 21 d postnatally. Administration of the synthetic metalloporphyrin, Sn-protoporphyrin, to ALA-treated neonates resulted in a dose-dependent decrease in serum bilirubin levels and hepatic heme oxygenase activity. Mn- and Zn-protoporphyrin in comparable doses did not significantly inhibit ALA-induced hyperbilirubinemia. Sn-protoporphyrin also inhibited the hyperbilirubinemia produced by heme in the suckling animals. ALA administration to newborn rats during the specific postnatal period described provides a simple and convenient model of experimental jaundice in the developing neonate which permits an examination of the potential ability of synthetic metalloporphyrins or other compounds to suppress induced hyperbilirubinemia in the newborn animal. The ability to induce a consistent and significant degree of jaundice in the postnatal rat by the method described may also be useful for other types of studies concerned with the biological disposition and effects of endogenously formed bilirubin in the neonate. The results of this study confirm in another model system the potent ability of Sn-protoporphyrin to suppress jaundice in the neonate, and suggest that suppression of heme oxidation by synthetic heme analogues may represent a useful therapeutic approach to the problem of severe hyperbilirubinemia in human premature newborn.

Regulation of heme metabolism and cytochrome P-450 levels in primary culture of rat hepatocytes in a defined medium

Liver cells were prepared from adult Sprague-Dawley rats and used for the determination of delta-aminolevulinic acid synthetase (ALAS) activity and cytochrome P-450 concentrations at different time intervals in tissue culture in a serum-free synthetic medium. During the first 24 hr in culture, the level of cytochrome P-450 decreased to 30-40% of the level in isolated liver cells from untreated animals. The disappearance of cytochrome P-450 was especially fast in hepatocytes obtained from female phenobarbital-treated rats where only 40% of the original cytochrome P-450 was present after 2 hr in culture and 80% had disappeared in 2 days. The activity of ALAS increased 3- to 4-fold when measured 2 hr after plating, and it reached the maximum level in 19-24 hr when its activity was about eight times the original activity. In 2-4 days in culture, the activity of ALAS was four to five times above the original level. When the amount of delta-aminolevulinic acid (ALA) in the medium was increased from 1 to 100 microM, a decrease in ALAS was obtained, but no significant increase in cytochrome P-450 level was observed. Addition of heme to the medium gave a dose-dependent decrease in the activity of ALAS. Our data indicate that during the first 24 hr in culture the increase of ALAS activity was prevented by exogenous heme. This effect may be due to inhibition of the catalytic activity, suppression of the synthesis of the enzyme, or accelerated breakdown of the enzyme by heme.

The cytochrome P-450-depleted animal: an experimental model for in vivo studies in chemical biology

An experimental method is described to deplete markedly in vivo the cytochrome P-450 content of liver for prolonged periods of time. The method uses the synthetic metalloporphyrin cobalt-heme (cobalt protoporphyrin IX), which possesses the dual biological properties of repressing delta-aminolevulinate synthase, the rate-limiting enzyme of heme biosynthesis, and of potently inducing microsomal heme oxygenase, the rate-limiting enzyme of heme catabolism. A single dose of cobalt-heme (125 mumol/kg of body weight) decreased within 48 hr hepatic cytochrome P-450 to approximately 20% of normal, at which level it remained for 10 days; normal levels were not achieved by 36 days. Periodic administration (total, six injections) of a smaller dose of cobalt-heme (50 mumol/kg of body weight) maintained the cytochrome P-450 content at levels approximately 15% of normal for greater than 90 days with concurrent profound impairment of mono-oxygenase reactions catalyzed by this heme protein. The ability of cobalt-heme to produce profound and prolonged depletion of cytochrome P-450 in vivo provides a valuable model for examining the role of cytochrome P-450-dependent metabolism in the biology of endogenous and exogenous chemicals.