Effects of an ultraviolet B radiation absorber on photocarcinogenesis in hairless albino mice.

Uvinul T 150, a UVB absorber, was administered (concentration 5%) in a vehicle to the skin of hairless albino mice before ultraviolet radiation (UVR) exposure for 5 days per week in a photocarcinogenicity study. Uvinul T 150 prolonged the latency period to 50% skin tumor incidence (controls: 21-22 weeks; Uvinul T 150: 36 weeks in males and 31 weeks in females). When Uvinul T 150 was applied in an alternating-exposure procedure (3 days/week before and 2 days/week after UVR), the inhibition of photocarcinogenesis was less marked (latency period 28-30 weeks). The vehicle formulation had no effect (latency period 20-21 weeks). The sensitivity of the test system was demonstrated by a positive control (8-methoxy-psoralene). Although UVB absorption was shown to inhibit photocarcinogenesis, the results also suggest that UVA radiation makes a contribution to skin tumor formation.

The effects of styrene on lung cells in female mice and rats.

Styrene has been shown to cause an increase in the incidence of lung tumors in CD-1 mice following chronic exposure at 40 and 160 ppm, whereas no treatment-related increase in tumors in any organ was seen in rats chronically exposed to up to 1000 ppm styrene. So far most of the mechanistic studies have been performed with male animals. The aim of the present study was to further elucidate the target cell population in mouse lungs exposed to styrene, and to investigate possible differential in vivo effects (e.g., glutathione depletion, increased lipid peroxidation, and oxidative DNA damage). Groups of female CD-1 mice were exposed to styrene at concentrations of 0, 172 or 688 mg/m3 (0, 40 or 160 ppm) for 6 h per day on 1 day, 5 consecutive days or for 20 days during a 4 week period. Groups of female Crl:CD rats were exposed to styrene at concentrations of 0, 688 or 2150 mg/m3 (0, 160 or 500 ppm) for a single 6 h period or for 6 h per day on 5 consecutive days. No signs of lung toxicity were observed in rats. The cytology of cells in lung lavage fluid provided no signs of an inflammatory response in either rats or mice. In mice, both exposure levels caused decreased CC16 protein concentrations in lung lavage fluid after 1 and 5 exposures and in mouse blood serum throughout the study, suggesting that styrene may cause destruction of Clara cells in mice. Degenerative lesions in mouse Clara cells (vacuolar cell degeneration, cell necrosis) were revealed by electronmicroscopy. After 5 and 20 exposures of mice at 160 ppm, cellular crowding, expressed as an irregular epithelial lining and indicative of a very early hyperplasia was noted. Although a depletion of glutathione was noted in mouse lung homogenates after 20 exposures, there was no evidence of oxidative stress as indicated by unchanged concentrations of 8-OH-deoxyguanosine. Malondialdehyde, an indicator of lipid peroxidation, was slightly increased in mice after 1 exposure at 160 ppm only.

Quantitative aspects of chemical carcinogenesis and tumor promotion in liver.

Chronic exposure of rodents to high dose levels of drugs, food additives and environmental chemicals frequently results in liver enlargement. Several of these compounds have been found to enhance the incidence of liver tumors in animals briefly exposed previously to hepatocarcinogens. Accordingly, it has been advanced that these agents act as tumor promoters. This contention has remained subject of controversy following reports that these substances may also cause liver tumors in noncarcinogen-treated rodents, particularly in those characterized by a relatively high incidence of "spontaneous" liver tumors. Since many of these chemicals are in common use, a crucial question would seem to be whether such effects are due to facilitation of the expression of pre-existing oncogenic potential, i.e., to tumor promotion, or to the synergistic action of weakly carcinogenic agents. As a result of mechanistic differences tumor promotion and syn-carcinogenesis must exhibit different dose-time-response characteristics, and, accordingly, it should be possible, in principle, to discriminate between these phenomena. However, since tumor manifestation periods in low-dose groups frequently exceed the animals average lifespan, this approach may not always yield conclusive data, unless a sensitive early marker of carcinogenic activity can be employed. There is evidence that enzyme-deficient preneoplastic areas in liver can be used for this purpose. A strong quantitative correlation between carcinogen dose, the extent of ATPase deficient areas, and the subsequent appearance of tumors has now been established for a number of hepatocarcinogens. Experimental data are consistent with the concept that two critical events (hits) are required for induction of ATPase deficiency in hepatocytes. The first hit is carcinogen-dependent, whereas the second hit would seem to be due to time-dependent event(s). Tumor-promoters, such as phenobarbital, were found to accelerate and increase formation of preneoplastic islets. This evidence, together with data indicating that the compound is devoid of carcinogenic potential, suggests that phenobarbital may be operative at relatively early stages of hepatocarcinogenesis by increasing the probability of the occurrence of the time-dependent second hit. Such effects are dose-dependent and appear to be related to the induction of liver enlargement. The changes in hepatocellular ploidy status and atypical nuclear figures observed during phenobarbital treatment and cessation thereof, suggest that this compound might induce abnormal redistributions of genetic material. It is postulated that these cytological changes may result in phenotypical manifestation of recessive oncogenic information.

Dose-response analysis of the enhancement of liver tumour formation in CF-1 mice by dieldrin.

The current study was undertaken to investigate the dose-response characteristics of dieldrin-mediated enhancement of liver tumour formation in CF-1 mice. The median time to tumour development was established in controls, and in dieldrin-treated animals at six levels of continuous exposure (0.1, 1, 2.5, 5, 10 and 20 p.p.m.). The results of the analysis, which was based on liver tumour data from two parallel chronic feeding studies involving 1800 mice, are at variance with those reported by Druckrey for various established chemical carcinogens. In a double-logarithmic system of coordinates there was no linear relationship between the median total dose or the median time to tumour formation and the daily dieldrin exposure level. These results suggest that the tumourigenicity of this compound in CF-1 mouse liver is determined not by the sum of all consecutive doses, but rather by the level of daily exposure, and, presumably, the duration of treatment. This concept is consistent with the observed dose-dependency and reversible nature of dieldrin-induced subcellular changes in mouse liver. These considerations, together with evidence that dieldrin and its mammalian metabolites possess neither genotoxic activity nor potential, are not inconsistent with the concept that this compound is devoid of initiating potential, and operates by enhancing the effect of a genetically linked oncogenic factor in CF-1 mouse liver.

Effects of dieldrin, diet, and bedding on enzyme function and tumor incidence in livers of male CF-1 mice.

The effects of naturally occurring microsomal enzyme inducers on hepatocellular drug-metabolizing enzyme systems and also upon the incidence of "spontaneous" liver tumors in CF-1 mice were investigated, using animals maintained on semisynthetic diet and filter-paper bedding as controls. The administration of dieldrin, a potent microsomal enzyme inducer with tumorigenic properties in livers of CF-1 mice, to some of the experimental treatment groups served as a positive control. Conventional diet and sawdust bedding caused induction of the liver monooxygenase system, although this effect was far less pronounced than that produced by dieldrin. The incidence of liver tumors in mice exposed to conventional diet and sawdust bedding was similar to that seen in the control group. The incidence of liver tumors was significantly increased in dieldrin-treated mice, including those maintained on semisynthetic diet and filter-paper bedding. Both benign and malignant tumors were found in dieldrin-treated mice, the latter type of lesion showing evidence of lung metastasis. These results, together with evidence that dieldrin and its mammalian metabolites possess neither genotoxic activity nor potential, are consistent with the concept that dieldrin exacerbates or facilitates the expression of a preexisting oncogenic factor which is genetically linked and possibly viral in origin.