The role of oxidative metabolism in hexachlorobenzene-induced porphyria and thyroid hormone homeostasis: a comparison with pentachlorobenzene in a 13-week feeding study.

Hexachlorobenzene (HCB) induces a broad spectrum of effects including disturbances in the heme synthesis (porphyria) and in thyroid hormone homeostasis. For most of its effects, biotransformation of the parent compound seems to be a prerequisite. The present study was designed to assess the relevance of the oxidative metabolites in HCB-induced toxicity, with special attention to the role of the reactive tetrachlorobenzoquinone (TCBQ). To this end, toxicity and biotransformation of HCB were compared with those of pentachlorobenzene (PCB), since this chemical is oxidized to the same products as HCB, i.e., pentachlorophenol (PCP) and TCBQ. Female Wistar rats received diets containing different dose levels of HCB or PCB for 13 weeks, with or without cotreatment with triacetyloleandomycin (TAO), a selective inhibitor of cytochrome P450IIIA1/2. Rats treated with HCB (high dose) had significantly elevated levels of urinary porphyrins from the 4th week on and had a significant hepatic accumulation of porphyrins at the end of the study. Both urinary porphyrin excretion and hepatic porphyrin accumulation were greatly inhibited in rats receiving cotreatment with HCB and TAO. However, the inhibition of HCB-induced porphyria by TAO cannot be explained by a diminished formation of the highly reactive TCBQ, since rats treated with a high dose of PCB, which had a several fold higher urinary excretion of PCP and TCHQ compared to a high dose of HCB, did not develop porphyria. Instead, the present study points to the involvement of a putative reactive intermediate in the primary oxidative step in HCB-induced porphyria, since based on paired observations of individual rats, the degree of porphyria was correlated to a high degree with excretion of PCP, whereas correlation of porphyria with early excretion of TCHQ was much weaker. This finding fits well with the fact that the mechanisms of oxidation of HCB to PCP and PCB to PCP are different. Both HCB and PCB were oxidized to PCP and tetrachlorohydroquinone (TCHQ), the reduced analog of TCBQ. Cytochrome P450IIIA1/2 appears to be involved in the conversion of HCB and PCB, since cotreatment of TAO resulted in a strongly diminished urinary excretion of PCP and TCHQ. Treatment with HCB as well as PCB results in disturbances of retinoid and thyroid hormone homeostasis. These effects, which have also been reported after exposure to polychlorinated biphenyls, originate from interference of hydroxylated metabolites (notably PCP) with the plasma thyroxine transport protein, transthyretine, and since this metabolite is formed from both HCB and PCB, this results in the same toxicity for both compounds.

On the mechanism of dialkyltin-induced thymus involution.

In relation to the thymolytic activity of dialkyltin compounds, the effects of di-n-butyltin (DBTC) and di-n-octyltin dichloride (DOTC) on the ultrastructure of the rat thymus, the proliferation of bone marrow stem cells and their interference with growth hormone production are analyzed. Ultrastructurally, depletion of small lymphocytes in the thymic cortex, without signs of lymphocyte destruction or macrophage activation, was the most prominent feature of DOTC treatment. The thymus cortex was maximally depleted 96 h after a single i.p. injection of 1 mg DOTC/kg body weight. 120 h after DOTC treatment a repopulation of the cortex with small lymphocytes was noted together with the presence of many pale large nuclei of large lymphocytes, suggestive of active blast transformation. These thymus effects are probably not caused by a diminished input of bone marrow stem cells into the thymus, since neither the spontaneous blastogenesis nor the colony formation of bone marrow cells isolated from DBTC-treated animals were affected. An indirect mechanism of thymus involution induced by interference with the hormonal system is also very unlikely. A stress-related increase of glucocorticosteroids by dialkyltins has already been excluded (Seinen and Willems, 1976). An interference of dialkyltins with growth hormone production, as indicated in this study, did not occur. Application of growth hormone in amounts that reversed the hypophysectomy-induced thymus atrophy did not modify the thymus involution induced by DOTC treatment of rats. However, an interference with the production of thymic humoral factors cannot be excluded yet, although it is not supported by morphological changes in thymic reticular cells. Only an increased vacuolization of reticular epithelial cells is seen when the thymus is markedly involuted, but this is considered to be a consequence rather than a cause of thymus atrophy. Most probably the dialkyltin-induced thymus involution is caused by an antiproliferative activity, which is strongly supported by an inhibition of thymidine incorporation of thymocytes isolated from DBTC-treated rats.

A carcinogenicity study with mutagenic organic concentrates of drinking-water in the Netherlands.

The carcinogenicity in male and female Wistar SSP TOX rats of organic drinking-water concentrates that are positive in the Ames test was studied at three doses. The organic mutagenic concentrates were prepared weekly from drinking-water from one location in The Netherlands by adsorption onto XAD-4/8 resins and elution with dimethylsulphoxide. The organic concentrates in dimethylsulphoxide were mixed with non-mutagenic drinking-water before exposure of the rats. Dose levels were based on multiples of expected human exposure levels. For the calculation the average human daily intake of drinking-water was taken as 2 litres for a body weight of 70 kg. There was no significant increase in tumour induction when male Wistar SSP TOX rats were exposed for 106 wk to 4.5, 14 or 40 times the expected human exposure level and females to 7,22 or 68 times the human level. The development and types of tumours were similar in the treated and control groups. The numbers of animals with tumours and of animals that died as a result of tumours in the exposed groups did not differ significantly from those in the control groups. These results suggest that these organic mutagenic drinking-water concentrates did not contain very potent carcinogens in effective concentrations.

Role of histamine in the antitumour activity of endotoxin.

The role of histamine in the antitumour activity of endotoxin against solid syngeneic Meth-A sarcoma in BALB/c mice was studied. Endotoxin induces haemorrhagic necrosis and regression of this tumour. Histamine and the selective H1 receptor agonist 2-pyridylethylamine mimicked the induction of necrosis but did not cause regression. The selective H2 receptor agonist dimaprit did not cause any tumour damage. The effect of histamine could be inhibited by the H1 receptor antagonists diphenhydramine and promethazine but not by the H2 receptor antagonist cimetidine. Endotoxin-induced necrosis was slightly affected by diphenhydramine, and the incidence of regression was reduced by both H1 antagonists. Cimetidine potentiated endotoxin-induced regression. Similar effects were observed concerning the effects of H-receptor antagonists on necrosis and regression induced by tumour necrosis serum (TNS). Histological examination revealed no marked additional effects of diphenhydramine or cimetidine on endotoxin-induced hyperaemia, haemorrhagic necrosis, and mitotic arrest of the tumour cells. Only cimetidine increased the extent of nonhaemorrhagic necrosis. The endotoxin-induced release of tumour necrosis factor and cytostatic activity in TNS was clearly reduced by diphenhydramine, but hardly affected by cimetidine. Data indicate that intact H1 receptors are required for the induction of tumour regression and antitumour factors by endotoxin. Concomitant H2 blockade may facilitate this by stimulating H1 receptor-mediated processes upon endotoxin-induced histamine release, although a cimetidine-induced inhibition of T-suppressor cell activation might also be involved.