Effect of bile acids on hepatic protoporphyrin metabolism in perfused rat liver

Hepatic farnesoid X receptor is necessary to facilitate ductular reaction and expression of heme biosynthetic genes

BACKGROUND

Bile, which contains bile acids, the natural ligands for farnesoid x receptor (FXR), moves from the liver to the intestine through bile ducts. Ductular reaction often occurs during biliary obstruction. A subset of patients with erythropoietic protoporphyria, an inherited genetic mutation in heme biosynthetic enzyme ferrochelatase, accumulate porphyrin-containing bile plugs, leading to cholestasis. Here, we examined the link between FXR, bile plug formation, and how heme biosynthesis relates to this connection.

METHODS

We treated female and male wild-type and global and tissue-specific Fxr knockout mice with a diet containing 3,5-diethoxycarbonyl-1,4-dihydrocollidine, an inhibitor of ferrochelatase, and examined the expression of heme biosynthetic genes. We mined FXR mouse ChIP-Seq data, performed biochemical and histological analysis, and tested HepG2 and primary human hepatocytes after treatment with obeticholic acid, an FXR agonist.

RESULTS

We observed that hepatic but not intestinal Fxr loss resulted in reduced bile plugs and ductular reaction in the liver. Then, we examined if FXR plays a regulatory role in heme biosynthesis and found significantly lower porphyrin accumulation in 3,5-diethoxycarbonyl-1, 4-dihydrocollidine-fed Fxr knockout mice. Gene expression and FXR mouse ChIP-Seq atlas analysis revealed that FXR orchestrates the expression of multiple heme biosynthetic enzymes. Finally, human HepG2 cells and primary human hepatocytes treated with obeticholic acid, showed increased expression of several heme biosynthetic genes.

CONCLUSIONS

Overall, our data show that hepatic Fxr is necessary to maintain ductular reaction and accumulation of bile plugs. FXR can direct the expression of multiple heme biosynthetic genes. Thus, modulating FXR activity in EPP patients may help alleviate its associated liver disease.

How we have learned about the complexity of physiology, pathobiology and pharmacology of bile acids and biliary secretion

During the last decades the concept of bile secretion as merely a way to add detergent components to the intestinal mixture to facilitate fat digestion/absorption and to eliminate side products of heme metabolism has evolved considerably. In the series of mini-reviews that the World Journal of Gastroenterology is to publish in its section of “Highlight Topics”, we will intend to give a brief but updated overview of our knowledge in this field. This introductory letter is intended to thank all scientists who have contributed to the development of this area of knowledge in gastroenterology.

Increased mitochondrial respiratory chain enzyme activities correlate with minor extent of liver damage in mice suffering from erythropoietic protoporphyria

Mitochondrial dysfunction might play a role in the pathogenesis of liver damage in erythropoietic protoporphyria (EPP). Changes in mitochondrial respiratory chain activities were evaluated in the Fech(m1pas)/Fech(m1pas) mouse model for EPP. Mice from different strains congenic for the same ferrochelatase germline mutation manifest variable degrees of hepatobiliary injury. Protoporphyric animals bred into the C57BL/6J background showed a higher degree of hepatomegaly and liver damage as well as higher protoporphyrin (PP) accumulation than those bred into the SJL/J and BALB/cJ backgrounds. Whereas mitochondrial respiratory chain activities remained unchanged in the liver of protoporphyric mice C57BL/6J, they were increased in protoporphyric mice from both SJL/J and BALB/cJ backgrounds, when compared to wild-type animals. Mitochondrial respiratory chain activities were increased in Hep G2 cell line after accumulation of PP following addition of aminolevulinic acid. As a direct effect of these elevated mitochondrial activities, in both hepatic cells from mutant mouse strains and Hep G2 cells, adenosine 5'-triphosphate (ATP) levels significantly increased as the intracellular PP concentration was reduced. These results indicate that PP modifies intracellular ATP requirements as well as hepatic mitochondrial respiratory chain enzymatic activities and further suggest that an increase of these activities may provide a certain degree of protection against liver damage in protoporphyric mice.

Liver transplantation for erythropoietic protoporphyria: report of a case with medium-term follow-up

A case of liver transplantation is described in a 35-year-old male with hepatic failure due to erythropoietic protoporphyria. Data regarding protoporphyrin levels in erythrocytes and faeces, before and after transplantation, seem to indicate that, in this case, protoporphyrin overproduction was, in part, due to liver synthesis. Four years after surgery, the patient is completely free of skin photosensitivity. Liver function tests are normal and there are no significant protoporphyrin deposits in the new liver. However, recurrence of the disease in the long-term cannot be excluded, since erythrocyte protoporphyrin levels have remained elevated after liver transplantation.

Relationship between biliary lipid and protoporphyrin secretion; potential role of mdr2 P-glycoprotein in hepatobiliary organic anion transport

BACKGROUND/AIMS

Erythropoietic protoporphyria, caused by ferrochelatase deficiency, leads to protoporphyrin accumulation in the liver. Therapeutic attempts to increase the secretion of this hydrophobic organic anion into bile are hampered by a lack of understanding of the secretory mechanism(s) involved. We have investigated biliary secretion of protoporphyrin in rats and mice, primarily targeted on the role of biliary lipids in this process.

METHODS

Gel permeation chromatography was applied to investigate the association of porphyrins with lipid fractions in bile. Secretion of endogenous porphyrins was studied in (GY mutant) rats and mdr2 P-glycoprotein deficient mice, under conditions of widely varying biliary lipid secretion rates.

RESULTS

Gel permeation chromatography revealed that, in native human and rat bile, protoporphyrin associated with cholesterol/phospholipid vesicles upon elution with bile salt-free buffer. In contrast, the more hydrophilic coproporphyrin isomers I and III were found only in bile salt/organic anion hybrid particles and smaller aggregates. Interruption of the enterohepatic circulation in normal Wistar rat resulted in parallel decrease of endogenous protoporphyrin-, lipid-, and bile salt secretion, but did not alter the secretion of coproporphyrin I and III. Uncoupling of lipid- from bile salt secretion by sulfated taurolithocholate resulted in impaired secretion into bile of protoporphyrin only. Conversely, secretion of coproporphyrin I and III, but not that of protoporphyrin, was impaired in mutant Groningen Yellow rats with defective ATP-dependent hepatobiliary organic anion transport. In mice homozygous for a disruption of the mdr2 P-glycoprotein gene, resulting in complete absence of phospholipids in bile and strongly reduced cholesterol output, secretion of protoporphyrin was reduced by 90%, whereas that of coproporphyrin I and III was affected to a much lesser extent.

CONCLUSIONS

Our data demonstrate a close association between protoporphyrin and lipid secretion into bile, indicating that these processes are, at least functioning coupled. This finding implicates a role of mdr2 P-glycoprotein activity in hepatobiliary removal of the hydrophobic organic anion protoporphyrin. Hence, it may be speculated that protoporphyrin secretion can be influenced by drugs, diet or other means that affect biliary lipid secretion.

Exogenous protoporphyrin inhibits Hep G2 cell proliferation, increases the intracellular hydrogen peroxide concentration and causes ultrastructural alterations

Ultrastructural hepatocellular abnormalities in early stages of erythropoietic protoporphyria lead to hepatobiliary changes that cause overt liver disease in 5-10% of patients, not infrequently progressing to fulminant hepatic failure. This cannot be attributed solely to protoporphyrin crystal deposition in the liver. Hepatic redox systems have therefore been postulated as an equivalent for the photoreaction of protoporphyrin. We studied the dark effects of protoporphyrin and hematoporphyrin on HL60 and Hep G2 cells. Cell proliferation and intracellular H2O2 concentrations were assessed and related to the ultrastructural morphology. The incubation with protoporphyrin and hematoporphyrin resulted in a dose- and time-dependent inhibition of proliferation indices of Hep G2 cells. Flow cytometric analyses of intracellular H2O2 concentrations demonstrated a dose-dependent increase in both cell lines upon incubation with protoporphyrin. Hep G2 cells displayed ultrastructural alteration of the endoplasmatic reticulum and plasma membranes. Also 'cell blebbing' occurred. We conclude that exogenous protoporphyrin increases the intracellular H2O2 concentration and exerts a cytotoxic dark effect. This may contribute to the liver injury observed in erythropoietic protoporphyria.

The effect of porphyrins on cellular redox systems: a study on the dark effect of porphyrins on phagocytes

Erythropoietic protoporphyria (EPP) and porphyria cutanea tarda (PCT) are characterized by skin morbidity, induced by pro-inflammatory reactive oxygen species generated by the photosensitizing properties of protoporphyrin IX and uroporphyrin I. How these porphyrins exert a toxic effect on the liver in the absence of light is poorly understood. We tested the hypothesis that porphyrins can interference with cellular redox systems, by studying the dark effects of protoporphyrin (PP), haematoporphyrin (HP), deuteroporphyrin (DP) and uroporphyrin (UP) on the cellular redox system of phagocytes, and on enzymatic oxyradical generating systems. Both in phagocytic cells and enzymatic systems, a dose-dependent inhibition of chemiluminescence was observed by all porphyrins added. Catalase and SOD-like activity of porphyrins was excluded by oxygraph and ferricytochrome c reduction. However, ferrocytochrome c oxidation was inhibited by porphyrins indicating ferrireductase-like activity. In a Fenton type reaction between H2O2 and PP, we could demonstrate the generation of .OH, or an electronically excited porphyrin species. No influence on phagocyte chemotaxis, phagocytosis and killing-capacity was observed. We conclude that porphyrins do interfere with (cellular) redox systems and can both inhibit and enhance oxygen free radical generation, dependent on the type of redox reaction. Porphyrins can thus affect cellular metabolism. Since H2O2 and PP both readily dissolve in biological membranes, their interaction in the presence of transition metals may contribute to the toxic dark effects of porphyrins as observed in patients with EPP and PCT.

Formation of biliary thrombi in protoporphyrin-induced cholestasis in perfused rat liver

The effect of bile acids on the formation of biliary thrombi in protoporphyrin-induced cholestasis was determined by perfusing isolated rat livers with taurocholate, chenodeoxycholate and ursodeoxycholate with and without protoporphyrin. Protoporphyrin-induced reduction of bile flow was similar in the presence of each bile acid. The cholestasis was greater at high doses (2,000 to 10,885 nmol) than at low doses (1,500 nmol) of protoporphyrin, unrelated to the amount of lactate dehydrogenase released into the perfusate, and it was not altered by increasing bile acid infusions. Bile acid excretion was inhibited by high protoporphyrin doses. Periportal birefringent pigment deposits were seen in canaliculi and ductules when the biliary protoporphyrin concentration exceeded 161 nmol/ml, 345 nmol/ml and 1,036 nmol/ml for ursodeoxycholate, chenodeoxycholate and taurocholate, respectively; or, when the protoporphyrin (nanomole) to bile acid (micromole) ratio exceeded 3.23, 7.03 and 23.43, respectively. The maximal ratio of ductular deposits to portal tract deposits examined was 0.9. Electron microscopy showed these deposits were associated with canalicular thrombi. Thus, biliary thrombi were produced by infusion of bile acids and protoporphyrin. The occurrence of thrombi varied with bile acid structure. Explanations for this finding are speculative. The presence of periportal thrombi, however, did not influence the degree of functional cholestasis.

Liver transplantation in a patient with protoporphyria

A 38-yr-old woman with liver disease due to protoporphyria underwent orthotopic liver transplantation. The resected liver was cirrhotic and contained a massive amount of protoporphyrin, with numerous birefringent pigment deposits. Transplantation was accomplished without difficulty following blood volume exchange to reduce the blood protoporphyrin level. Sequential biopsy specimens obtained through the 13th month after transplantation showed no accumulation of protoporphyrin pigment deposits in the new liver. Portal inflammation observed in the liver biopsy specimen at 6 mo after transplantation resolved spontaneously. Erythrocyte and serum protoporphyrin levels returned to values similar to those in the pretransplantation period when the patient had normal hepatic function; the fecal level was lower. Thus orthotopic liver transplantation can be successfully done in patients with protoporphyria who have severe liver disease. Prolonged follow-up is needed to determine the ultimate outcome, however, as the new liver remains susceptible to protoporphyrin-induced damage.