Dose-response relationships in hexachlorobenzene-induced porphyria

Toxicogenomics of subchronic hexachlorobenzene exposure in Brown Norway rats

Hexachlorobenzene (HCB) is a persistent environmental pollutant with toxic effects in man and rat. Reported adverse effects are hepatic porphyria, neurotoxicity, and adverse effects on the reproductive and immune system. To obtain more insight into HCB-induced mechanisms of toxicity, we studied gene expression levels using DNA microarrays. For 4 weeks, Brown Norway rats were fed a diet supplemented with 0, 150, or 450 mg HCB/kg. Spleen, mesenteric lymph nodes (MLN), thymus, blood, liver, and kidney were collected and analyzed using the Affymetrix rat RGU-34A GeneChip microarray. Most significant (p < 0.001) changes, compared to the control group, occurred in spleen, followed by liver, kidney, blood, and MLN, but only a few genes were affected in thymus. This was to be expected, as the thymus is not a target organ of HCB. Transcriptome profiles confirmed known effects of HCB such as stimulatory effects on the immune system and induction of enzymes involved in drug metabolism, porphyria, and the reproductive system. In line with previous histopathological findings were increased transcript levels of markers for granulocytes and macrophages. New findings include the upregulation of genes encoding proinflammatory cytokines, antioxidants, acute phase proteins, mast cell markers, complements, chemokines, and cell adhesion molecules. Generally, gene expression data provide evidence that HCB induces a systemic inflammatory response, accompanied by oxidative stress and an acute phase response. In conclusion, this study confirms previously observed (immuno)toxicological effects of HCB but also reveals several new and mechanistically relevant gene products. Thus, transcriptome profiles can be used as markers for several of the processes that occur after HCB exposure.

Heme metabolism after discontinued hexachlorobenzene administration in rats: possible irreversible changes and biomarker for hexachlorobenzene persistence

The aim of the present study was to determine whether short-term administration of hexachlorobenzene (HCB) (1 g/kg body wt., suspended in water, 5 days/week), could cause and maintain marked porphyria in the absence of the exogenous drug, and whether porphyria parameters can be useful as biomarkers of HCB persistence in rats. Hepatic uroporphyrinogen decarboxylase activity, its inhibitor formation, porphyrin content and composition were studied in Wistar rats treated with the fungicide for 1, 2, 3, or 4 weeks and then withdrawn for a 20-week period. The time course of urinary porphyrin excretion was studied for 7 weeks either by continuous treatment for the entire period, or a 1-week HCB administration. The degree of porphyria achieved by rats after 20 weeks of suspended HCB administration was severe, independent of the length of the treatment, and even higher than that observed in animals analysed immediately at the end of each treatment. Rats treated with HCB for 1 week showed a modest decrease in uroporphyrinogen decarboxylase and low inhibitor formation, and exhibited a greater enzyme inhibition, inhibitor formation, hepatic porphyrin accumulation, and an altered pattern of porphyrin composition in the absence of the exogenous drug. Independent of the treatment, urinary porphyrins rose after a delay of 5 weeks. Substantial amounts of HCB were still found in fat of rats treated with HCB for 1 week, after a withdrawal period of 20 weeks. These results suggest that the high persistence of HCB in tissues acts as a continuous source of the xenobiotic, and stimulus for heme biosynthesis derangement. The alterations induced by HCB within 1 week of treatment could be regarded as an initial trigger for irreversible damage on heme metabolism. Thus, abnormalities in heme biosynthesis can be considered effective markers of HCB persistence in rats or of irreversible HCB-induced damage. Taking into account the delayed and enhanced metabolic effects of HCB, it is advisable that porphyria parameters should be evaluated not only immediately after exposure, but also some time afterwards, especially in susceptible and occupationally-exposed populations.

Studies on the mechanism of uroporphyrinogen decarboxylase inhibition in hexachlorobenzene-induced porphyria in the female rat

Hexachlorobenzene (HCB)-induced porphyria occurs in female, but not male, rats after a delay of 35 days following HCB treatment. Uroporphyrinogen decarboxylase (UROD) inhibition has been proposed as a primary causative event. To determine whether there also exists a delay phase and a sexual dimorphism for UROD inhibition, groups of male and female rats were given HCB (100 mg/kg/day) from Days 1 to 5. Hepatic uroporphyrin III was markedly increased only after Day 33. Liver cytosol UROD activity in HCB-treated female rats with porphyria at Days 33, 40, 47, 54, and 100 was decreased by over 70% compared to concurrent control, whereas treated male rats as well as nonporphyric female rats had UROD activity comparable to control levels at Days 6, 12, 19, 26, 33, 40, 47, and 54. Level of immunoreactive UROD in cytosol of porphyric rats was not modified by HCB. No gender-related differences in liver cytosol radiolabel level ([14C]HCB given as the fifth dose) were found at Days 6 and 30. Chromatography of liver cytosol showed nonspecific binding of radiolabel to proteins for males, porphyric and nonporphyric females, and loss of UROD activity did not correlate with the amount of radiolabel in the UROD-containing fractions. Thus, the gender-specific decrease in UROD activity observed when porphyria develops in female rats (delay of about 4 weeks), as well as the persistence of low activity and porphyria for months, suggests that UROD inhibition was causally related to porphyria.

Comparative toxicity of polychlorinated biphenyls to Japanese quail (Coturnix c. japonica) and American kestrels (Falco sparverius)

Polychlorinated biphenyls (PCBs) and related halogenated hydrocarbons bioaccumulate to high concentrations in top predators, such as raptorial birds, yet little is known of PCB toxicity to such species. This study explored several aspects of both the acute and chronic response of American kestrels (Falco sparverius) to three purified PCB congeners and a commercial mixture, Aroclor 1254, and compared the response to that of the Japanese quail (Coturnix c. japonica), a more studied species known to be PCB sensitive. In one experiment, adult female birds were given single oral doses of either Aroclor 1254, 3,3',4,4'-TCB (PCB 77, IUPAC nomenclature), 3,3',4,4',5-PCB (PCB 126) or 2,2',4,4',5,5'-HCB (PCB 153) and sacrificed after 5 d. In kestrels, neither the pure compounds nor the mixture affected hepatic or renal porphyrin levels. There was slight but significant hepatic and renal ethoxyresorufin O-deethylase (EROD) induction in birds dosed with PCBs 77 and 126. A cytochrome P-4501A (CYP1A) cross-reactive protein was detected in liver and kidney of kestrels given PCBs 77 and 126, but not in Aroclor 1254-dosed birds. In quail, an acute dose of Aroclor 1254 caused significant liver weight increases, hepatic and renal EROD and aminopyrine n-demethylase (APND) induction, and dose-related hepatic and renal porphyria. Quail treated with PCB 126 developed hepatic and renal porphyria; EROD and APND were also induced. Administration of PCB 77 caused only slight induction of hepatic EROD activity. PCB 153 caused some hepatic and renal porphyria and induced EROD to the same degree as PCB 126. A hepatic CYP1A cross-reactive protein was induced about 200-fold in all individual quail that exhibited significant EROD induction and was also induced in kidney of 1 quail given Aroclor 1254. A second experiment examined chronic exposure to Aroclor 1254 by feeding adult females of both species a daily dose of 7 mg/kg/d for 4-, 8-, and 12-wk periods. There were no effects on hepatic porphyrins in kestrels. APND and aldrin epoxidase (AE) were induced; EROD was not induced, although a hepatic CYP1A-like protein was detected in 1 kestrel dosed for 12 wk. Chronic exposure of quail to Aroclor 1254 caused highly significant increases in mean hepatic porphyrin levels and in activity of EROD, APND, and 4-chlorobiphenyl hydroxylase; a CYP1A-like protein was also induced about 200-fold. In both studies, Aroclor 1254 residues accumulated in tissues of both species, but there was no significant relationship between residue levels and effects. In conclusion, adult American kestrels were relatively insensitive to the effects of PCBs, from both acute and chronic exposure, on hepatic and renal porphyrin levels. Although concentrations of a CYP1A-like protein were increased in some kestrels given PCBs, EROD activity was only marginally increased, suggesting that catalytic activity of this protein differed among the two species.

Effect of diphenyl ether herbicides and oxadiazon on porphyrin biosynthesis in mouse liver, rat primary hepatocyte culture and HepG2 cells

The effects of the herbicides fomesafen, oxyfluorfen, oxadiazon and fluazifop-butyl on porphyrin accumulation in mouse liver, rat primary hepatocyte culture and HepG2 cells were investigated. Ten days of herbicide feeding (0.25% in the diet) increased the liver porphyrins in male C57B1/6J mice from 1.4 +/- 0.6 to 4.8 +/- 2.1 (fomesafen) 16.9 +2- 2.9 (oxyfluorfen) and 25.9 +/- 3.1 (oxadiazon) nmol/g wet weight, respectively. Fluazifop-butyl had no effect on liver porphyrin metabolism. Fomesafen, oxyfluorfen and oxadiazon increased the cellular porphyrin content of rat hepatocytes after 24 h of incubation (control, 3.2 pmol/mg protein, fomesafen, oxyfluorfen and oxadiazon at 0.125 mM concentration 51.5, 54.3 and 44.0 pmol/mg protein, respectively). The porphyrin content of HepG2 cells increased from 1.6 to 18.2, 10.6 and 9.2 pmol/mg protein after 24 h incubation with the three herbicides. Fluazifop-butyl increased hepatic cytochrome P450 levels and ethoxy- and pentoxyresorufin O-dealkylase (EROD and PROD) activity, oxyfluorfen increased PROD activity. Peroxisomal palmitoyl CoA oxidation increased after fomesafen and fluazifop treatment to about 500% of control values both in mouse liver and rat hepatocytes. Both rat hepatocytes and HepG2 cells can be used as a test system for the porphyrogenic potential of photobleaching herbicides.

Porphyrin synthesis by human hepatocytes and HepG2 cells--effects of enzyme inducers and delta-aminolevulinic acid

Experiments were carried out to investigate the possibility of inducing porphyria in human hepatocytes and HepG2 cells in culture. After treatment with hexachlorobenzene, 3-methylcholanthrene, phenobarbital or dimethyl sulfoxide, protoporphyrin was the predominating porphyrin accumulating in presence of delta-aminolevulinic acid. The typical uroporphyrin accumulation, as is seen in hexachlorobenzene-induced porphyria in vivo, was absent. In HepG2 cells, the activities of uroporphyrinogen decarboxylase and porphobilinogen deaminase were not influenced by cytochrome P-450 inducers, hexachlorobenzene or dimethyl sulfoxide during 48 h of culture. Therefore, the use of these cells in the study of porphyria cutanea tarda does not seem promising.