Uroporphyria in mice: Thresholds for hepatic CYP1A2 and iron

Abstract

In mice treated with 5-aminolevulinic acid (ALA) and polyhalogenated aromatic compounds, the levels of both hepatic cytochrome P450 (CYP)1A2 and iron-which can be quite different among inbred strains-are critical in causing experimental uroporphyria. Here we investigate the development of uroporphyria as a function of CYP1A2 and iron levels in the liver of mice having a common C57BL/6 genetic background. We compared Cyp1a2(؊/؊) knockout mice, Cyp1a2(؉/؊) heterozygotes, Cyp1a2(؉/؉) wild type, and Cyp1a2(؉/؉) mice pretreated with a low dose of 3,3 ,4,4 ,5-pentachlorobiphenyl (PCB126) (4 g/kg). Cyp1a2(؉/؊) mice contain about 60% of the hepatic CYP1A2 content of Cyp1a2(؉/؉) mice, and the PCB126-pretreated Cyp1a2(؉/؉) mice have about twice the wild-type levels of CYP1A2. ALA-and iron-treated Cyp1a2(؉/؉) mice are known to accumulate hepatic uroporphyrin; this accumulation was increased 7-fold by pretreatment with the low dose of PCB126. ALA-and iron-treated Cyp1a2(؉/؊) heterozygote mice accumulated no uroporphyrin in 4 weeks, but by 8 weeks accumulated significant amounts of uroporphyrin. As previously reported, the ALA-and iron-treated Cyp1a2(؊/؊) knockout mouse has no CYP1A2 and exhibits no detectable uroporphyrin accumulation. Iron dose-response curves in ALA-and PCB126-treated Cyp1a2(؉/؉) mice showed that hepatic iron levels greater than 850 g/g liver were required to produce significant uroporphyrin accumulation in the liver. Other measures of hepatic effects of iron (iron-response element-binding protein [IRP]-iron response element [IRE]binding activity and accumulation of protoporphyrin from ALA) decreased when the level of iron was considerably lower than 850 g/g liver. At low iron doses, accumulation of iron was principally in Kupffer cells, whereas at the higher doses (required to stimulate uroporphyrin accumulation), more iron was found in parenchymal cells. We conclude that small changes in hepatic CYP1A2 levels can dramatically affect uroporphyria in C57BL/6 mice, providing the animals have been sufficiently loaded with iron; these data might be clinically relevant to acquired (sporadic) porphyria cutanea tarda, because humans show greater than 60-fold genetic differences in hepatic basal CYP1A2. (HEPATOLOGY 2002;35:912-921.)

Effect of an oral iron chelator or iron-deficient diets on uroporphyria in a murine model of porphyria cutanea tarda

“…Hepatic URO accumulation is highly dependent on the level of CYP1A2, especially in the range of its constitutive expression. 7,26 Therefore, hepatic CYP1A2 was assessed in L1-treated mice to ensure that the chelation of iron did not decrease CYP1A2. Two specific microsomal enzyme activities, MROD and UROX, 26 were used to assess CYP1A2.…”

“…7,26 Therefore, hepatic CYP1A2 was assessed in L1-treated mice to ensure that the chelation of iron did not decrease CYP1A2. Two specific microsomal enzyme activities, MROD and UROX, 26 were used to assess CYP1A2. Even at the highest L1 concentration used in this study (3 mg/mL), there was no effect of the L1 treatment on CYP1A2-catalyzed enzyme activities (Fig.…”

“…This conclusion is supported by previous studies with wild-type B6 mice treated with increasing doses of iron dextran where considerable hepatic iron loading was necessary before URO accumulation occurred. 10,26 Hfe(Ϫ/Ϫ) 129 mice were raised on a chow diet for 4-5 weeks after weaning. At this time, some mice were maintained on chow, and others were maintained on a defined Harlan Teklad diet without (#80396) or with (#01583) ferrous sulfate (200 mg Fe/kg diet), as indicated, for 5 weeks.…”

“…26 In iron dextran-treated mice maintained on the defined diets, there was no URO accumulation, but in iron dextran-treated mice maintained on the chow diet, there was massive URO accumulation (Table 2). Even though no hepatic URO accumulated in mice on the defined diet administered iron dextran, there was a large accumulation of iron in the livers of these animals.…”

“…UROX is an activity shown previously to be primarily catalyzed by CYP1A2. 26 MROD activity can be catalyzed by CYP1A1, in addition to CYP1A2, but there is no CYP1A1 in uninduced mice. The values are means and SDs for 4 pools of 2 mice each.…”