Effect of glucose on the induction of delta-aminolevulinic acid synthase and ferrochelatase in isolated rat hepatocytes by allylisopropylacetamide

Porphyric neuropathy

Acute hepatic porphyrias are inherited metabolic disorders that may present with polyneuropathy, which if not diagnosed early can lead to quadriparesis, respiratory weakness, and death. Porphyric neuropathy is an acute to subacute motor predominant axonal neuropathy with a predilection for the upper extremities and usually preceded by a predominantly parasympathetic autonomic neuropathy. The rapid progression and associated dysautonomia mimic Guillain-Barré syndrome but are distinguished by the absence of cerebrospinal fluid albuminocytologic dissociation, progression beyond 4 wk, and associated abdominal pain. Spot urine test to assess the porphyrin precursors delta-aminolevulinic acid and porphobilinogen can provide a timely diagnosis during an acute attack. Timely treatment with intravenous heme, carbohydrate loading, and avoidance of porphyrinogenic medications can prevent further neurological morbidity and mortality.

Complicity of haem in some adverse drug-reactions

Genetic variants in haem metabolism enzymes can be predisposition factors for adverse reactions in some individuals. New areas of haem biology may also be associated with idiosyncratic effects which are yet to be identified.

Nutritional regulation of hepatic heme biosynthesis and porphyria through PGC-1alpha

Inducible hepatic porphyrias are inherited genetic disorders of enzymes of heme biosynthesis. The main clinical manifestations are acute attacks of neuropsychiatric symptoms frequently precipitated by drugs, hormones, or fasting, associated with increased urinary excretion of delta-aminolevulinic acid (ALA). Acute attacks are treated by heme infusion and glucose administration, but the mechanisms underlying the precipitating effects of fasting and the beneficial effects of glucose are unknown. We show that the rate-limiting enzyme in hepatic heme biosynthesis, 5-aminolevulinate synthase (ALAS-1), is regulated by the peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha). Elevation of PGC-1alpha in mice via adenoviral vectors increases the levels of heme precursors in vivo as observed in acute attacks. The induction of ALAS-1 by fasting is lost in liver-specific PGC-1alpha knockout animals, as is the ability of porphyrogenic drugs to dysregulate heme biosynthesis. These data show that PGC-1alpha links nutritional status to heme biosynthesis and acute hepatic porphyria.

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.

Transcriptional regulation of 5-aminolevulinate synthase by phenobarbital and cAMP-dependent protein kinase

5-Aminolevulinate synthase (ALA-S) is a mitochondrial matrix enzyme that catalyzes the first and rate-limiting step of the heme biosynthesis. There are two ALA-S isozymes encoded by distinct genes. One gene encodes an isozyme that is expressed exclusively in erythroid cells, and the other gene encodes a housekeeping isozyme that is apparently expressed in all tissues. In this report we examine the mechanisms by which phenobarbital and cAMP regulate housekeeping ALA-S expression. We have determined that cAMP and phenobarbital effects are additive and the combined action is necessary to observe the cAMP effect on ALA-S mRNA in rat hepatocytes. The role of the cAMP-dependent protein kinase (PKA) has been examined. A synergism effect on ALA-S mRNA induction is observed in rat hepatocytes treated with pairs of selective analogs by each PKA cAMP binding sites. A 870-bp fragment of ALA-S 5'-flanking region is able to provide cAMP and phenobarbital stimulation to chloramphenicol O-acetyltranferase fusion vectors in transiently transfected HepG2 cells. ALA-S promoter activity is induced by cotransfection with an expression vector containing the catalytic subunit of PKA. Furthermore, cotransfection with a dominant negative mutant of the PKA regulatory subunit impairs the cAMP analog-mediated increase, but the phenobarbital-mediated induction is not modified. Our data suggest that the transcription factor cAMP-response element binding protein (CREB) is probably involved in PKA induction of ALA-S gene expression. Finally, heme addition greatly decreases the basal and phenobarbital or cAMP analog-mediated induction of ALA-S promoter activity. The present work provides evidence that cAMP, through PKA-mediated CREB phosphorylation, and phenobarbital induce ALA-S expression at the transcriptional level, while heme represses it.

Insulin inhibits delta-aminolevulinate synthase gene expression in rat hepatocytes and human hepatoma cells

Insulin has been known to regulate intracellular metabolism by modifying the activity or location of many enzymes but it is only in the past few years that the regulation of gene expression is recognized to be a major action of this hormone. The present work provides evidences that insulin inhibits delta-aminolevulinate synthase (ALA-S) gene expression, the enzyme which governs the rate-limiting step in heme biosynthesis. The addition of 5 nM insulin to hepatocytes culture led to a significant decrease of both basal and phenobarbital-induced ALA-S mRNA in a dose-dependent manner, as measured by Northern and slot-blot analysis. Several clues as to how insulin regulates ALA-S transcription were determined. The inhibitory effect is achieved at physiological concentrations but much higher proinsulin doses are needed. Insulin's effect is rapid, quite specific, and protein synthesis is not required. Moreover, ALA-S mRNA half-life is not modified by the presence of the peptidic hormone. Our results demonstrate that the insulin effect is dominant; it overrides 8-CPT-cAMP plus phenobarbital-mediated induction. Also, insulin requires the activation of protein kinase C to exert its full effect. On the other hand, a 870-bp fragment of the ALA-S promoter region is able to sustain the inhibition of CAT expression in plasmid-transfected HepG2 cells. Thus, these results indicate that insulin plays an important role in regulating ALA-S expression by inhibiting its transcription.

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

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

Effects of age, phenobarbital, β-naphthoflavone and dexamethasone on rat hepatic δ-aminolevulinic acid synthase

delta-Aminolevulinic acid synthase is a key enzyme leading to the synthesis of cytochrome P-450, a compound of the mixed function oxidase system. Because of this, we have studied the effects of both age and three typical mixed function oxidase system induces on this enzyme activity in male Wistar rats. Our results suggest that delta-aninolevulinic acid synthase activity in intact rats increases up to maturity and then decreases being, however, higher in the oldest animals than in the youngest. Phenobarbital affects delta-aminolevulinic acid synthase activity very characteristically. This activity is high in 4-week-old animals and then increases slightly until the twelfth month of life. Phenobarbital enzymic inducibility tends to decrease with age. In the beta-naphthoflavone groups, the profile of delta-aminolevulinic acid synthase activity looks like that in the control ones. Generally, beta-naphthoflavone is a weaker delta-aminolevulinic acid synthase inducer than phenobarbital, but it seems to induce delta-aminolevulinic acid synthase like dexamethasone, which, being a physiological inducer of the mixed function oxidase system, is also an inducer of delta-aminolevulinic acid synthase and this inducibility is closely connected with age.

Variable phenotypic expression of genotypic abnormalities in the porphyrias

The inherited porphyrias are the consequence of inherited deficiencies of enzymes in the heme synthesis pathway; they exhibit classical Mendelian inheritance patterns. The acute porphyrias (acute intermittent, porphyria variegata, hereditary coproporphyria) result from 50% (approx.) deficiencies of specific enzymes, which demonstrate autosomal dominant inheritance. However, only approx. 10% of subjects who inherit a porphyrin enzyme deficiency develop the corresponding acute porphyria and in most instances there is no obvious reason why one patient with an enzyme deficiency is symptomatic whereas another is not. Control of heme synthesis is achieved by the repressor effect of heme on the enzyme ALA synthase. Acute attacks of porphyria can be precipitated in susceptible persons by drugs, ethanol, starvation, hormones, stress and infection. The mechanism is usually by induction of ALA synthase activity. The molecular biology of porphyria variegata and hereditary coproporphyria is large unexplored. Acute intermittent porphyria is due to a partial deficiency of the enzyme porphobilinogen deaminase in the liver. The location of the gene for this enzyme has been identified on the long arm of chromosome 11. Acute intermittent porphyria is a genetically heterogenous disease with the abnormality frequently being a point mutation affecting synthesis of the enzyme.

Studies on induction of delta-aminolevulinic acid synthase, ferrochelatase, cytochrome P-450 and cyclic AMP by phenformin. Chlorpropamide, allylisopropylacetamide and lead in hepatocytes from normal and experimental diabetic rats

The present work demonstrates that phenformin exerted an inducing effect on delta-aminolevulinic acid synthase (ALA-S) and ferrochelatase activities and on cytochrome P-450 content in isolated hepatocytes from rats with experimental diabetes. Similar results were obtained with respect to ALA-S activity and cytochrome P-450 content when chlorpropamide was used. The inducing effect exerted by allylisopropylacetamide (AIA) on ALA-S and ferrochelatase activities in diabetic hepatic cells was markedly greater than that observed in normal hepatocytes. This stimulatory response was not enhanced by adding dibutyryl cyclic AMP (cAMP). When phenformin was added to isolated rat hepatocytes of normal rats, induction of ALA-S and ferrochelatase activities and cytochrome P-450 content was observed only in the presence of added dibutyryl cAMP. Addition of chlorpropamide to this in vitro system did not exert an inducing effect on the same enzymes even in the presence of dibutyryl cAMP. The present results add more experimental evidence about the lability of the heme pathway of diabetic hepatocytes.

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

The effects of inducers of cytochrome P-450 on haem biosynthesis from 5-aminolaevulinate were examined by using cultured chick-embryo hepatocytes. Cultures treated with either 2-propyl-2-isopropylacetamide or 3-methylcholanthrene contained increased amounts of cytochrome P-450 and haem. After treatment for 3 h with 5-amino[4-14C]laevulinate, the relative amounts of radioactivity accumulating as haem corresponded to the relative amounts of total cellular haem, but not to increases in the amounts of cytochrome P-450. Treatment with 5-aminolaevulinate did not alter cellular haem or cytochrome P-450 concentrations in either control or drug-treated cultures. The mechanism of the enhanced accumulation of radioactivity in haem was investigated. Although 2-propyl-2-isopropylacetamide enhanced the uptake of 5-aminolaevulinate and increased the cellular concentration of porphobilinogen 1.5-fold, these changes did not account for the increases in haem radioactivity. The inducing drugs had no effect on the rates of degradation of radioactive haem, but appeared to enhance conversion of protoporphyrin into haem. This latter effect was shown by: (1) a decreased accumulation of protoporphyrin from 5-aminolaevulinate in cells treated with inducers, and (2) complete prevention of this decrease if the iron chelator desferrioxamine was present. We conclude that inducers of cytochrome P-450 may increase haem synthesis not only by increasing activity of 5-aminolaevulinate synthase, but also by increasing conversion of protoporphyrin into haem.

Studies on regulatory mechanisms of heme biosynthesis in hepatocytes from experimental-diabetic rats

Isolated hepatocytes from rats with experimental diabetes exhibit increased content of cytochrome P-450 and cyclic AMP and normal activities of the regulatory enzymes delta-aminolevulinic acid synthase and ferrochelatase. The inducing effect exerted by phenobarbital on cytochrome P-450, delta-aminolevulinic acid synthase and ferrochelatase biosynthesis and cyclic AMP content in diabetic hepatic cells is markedly greater than that observed in normal hepatocytes. This stimulatory response is neither enhanced by added dibutyryl cyclic AMP nor repressed by glucose. The present results suggest that the heme pathway of diabetic hepatocytes is more susceptible to porphyrinogenic factors.