Effects of low dose mixtures of four N-nitroso compounds on hepatic foci development in the rat

Modifying effects of liver tumor promotion in rats subjected to co-administration of indole-3-carbinol and phenobarbital

Indole-3-carbinol (I3C) and phenobarbital (PB) are cytochrome P450 (CYP) 1A and CYP2B inducers, respectively, and have liver tumor-promoting effects in rats. In this study, we investigated the modifying effects on tumor promotion by I3C and PB co-administration. Six-week-old male F344 rats received a single intraperitoneal injection of N-diethylnitrosamine for initiation treatment. Two weeks after the initiation, rats were given no tumor-promoting agents (DEN alone), I3C (2,500 or 5,000 ppm in diet), PB (60 or 120 ppm in drinking water), or 2,500 ppm I3C + 60 ppm PB for 6 weeks. One week after the I3C/PB treatments, all animals underwent a two-thirds partial hepatectomy. The number and area of liver cell foci positive for glutathione S-transferase placental form (GST-P(+) foci) were not significantly fluctuated in the PB+I3C group in the isoadditive statistical model. On the contrary, the mRNA levels of Cyp2b1/2 and Nqo1 were suppressed and enhanced, respectively, in the PB+I3C group in the isoadditive model, but there was no enhancement in the microsomal reactive oxygen species (ROS) production, thiobarbituric acid-reactive substance levels, and Ki-67(+) cell ratio in this group. The results suggest that the co-administration of I3C and PB causes no modifying effects in liver tumor promotion in rats.

Suppressive effect of liver tumor-promoting activities in rats subjected to combined administration of phenobarbital and piperonyl butoxide

Phenobarbital (PB) is a cytochrome P450 (CYP) 2B inducer, and piperonyl butoxide (PBO) is a CYP1A/2B inducer. These inducers have liver tumor-promoting effects in rats. In this study, we performed a rat two-stage liver carcinogenesis bioassay to examine the tumor-promoting effect of PB and PBO co-administration. Male rats received an intraperitoneal injection of N-diethylnitrosamine (DEN) for initiation. Two weeks after DEN administration, rats were given PB (60 or 120 ppm in drinking water), PBO (1,250 or 2,500 ppm in diet) or 60 ppm PB+1,250 ppm PBO for 6 weeks. One week after the PB/PBO treatment, all rats were subjected to a two-thirds partial hepatectomy. To evaluate the effect of the combined administration, we used two statistical additive models. In the isoadditive model, the average values of the area of GST-P positive foci in the PB+PBO group were significantly lower than those in the High PB or High PBO groups. In the heteroadditive model, the net values of Cyp1a1 mRNA level and microsomal reactive oxygen species (ROS) production in the PB+PBO group were significantly lower than the sum of those in the Low PB or Low PBO groups. On the contrary, there was no interactive effect in the PCNA-positive hepatocyte ratio, mRNA levels of Cyp2b1/2, Gstm3, Gpx2 and Nqo1, and the level of thiobarbituric acid-reactive substances in the PB+PBO group. These results suggest that PB and PBO co-administration causes suppressive effects in liver tumor-promoting activity in rats resulting from inhibited microsomal ROS production because of suppression of CYP1A induction.

Differential microbial transformation of nitrosamines by an inducible propane monooxygenase

The toxicity of N-nitrosamines, their presence in drinking and environmental water supplies, and poorly understood recalcitrance collectively necessitate a better understanding of their potential for bioattenuation. Here, we show that the bacterial strain Rhodococcus jostii RHA1 can biotransform N-nitrosodiethylamine (NDEA), N-nitrosodi-n-propylamine (NDPA), N-nitrosopyrrolidine (NPYR), and possibly N-nitrosomorpholine (NMOR) in addition to N-nitrosodimethylamine (NDMA). Growth of cells on propane as the sole carbon source greatly enhanced degradation rates when contrasted with cells grown on complex organics. Propane-induced rates in order of fastest to slowest were NDMA > NDEA > NDPA > NPYR > NMOR at concentrations <2000 μg/L. Removal rates for linear functional groups scaled inversely with mass and cyclic nitrosamines were more recalcitrant than linear nitrosamines. Controls demonstrated significant NDEA and NDPA losses independent of biomass, suggesting abiotic processes may play a role in attenuation of these two compounds under experimental conditions tested here. In contrast to NDMA, a transition from first to zero order kinetics was not observed for the other nitrosamines included in this study over a concentration range of 20-2000 μg/L. A genetic knockout for the propane monooxygenase enzyme (PrMO) confirmed the role of this enzyme in the biotransformation of NDEA and NPYR. This study furthers our understanding of environmental nitrosamine attenuation by revealing an enzymatic mechanism for the biotransformation of multiple nitrosamines, their relative recalcitrance to transformation, and potential for abiotic loss.

Enhanced liver tumor promotion activity in rats subjected to combined administration of phenobarbital and orphenadrine

Phenobarbital (PB) and orphenadrine (ORPH) are cytochrome P450 (CYP) 2B inducers and have liver tumor-promoting effects in rats. In this study, we performed a rat two-stage liver carcinogenesis bioassay to examine the tumor-promoting effect of PB and ORPH co-administration. Twelve male rats per group were given an intraperitoneal injection of N-diethylnitrosamine (DEN) for initiation. Two-week after DEN administration, rats were given PB (60 or 120 ppm in drinking water), ORPH (750 or 1,500 ppm in diet) or 60 ppm PB+750 ppm ORPH for 6-week. One-week after the PB/ORPH treatment, all rats were subjected to two-thirds partial hepatectomy. To evaluate the effect of the combined administration, we used two statistical models: a heteroadditive model and an isoadditive model. In the heteroadditive model, the net values of the number and area of glutathione S-transferase placental form (GST-P) positive foci, Cyp2b1/2, Gstm3 and Gpx2 mRNA levels, microsomal reactive oxygen species (ROS) production and thiobarbituric acid-reactive substances level in the PB+ORPH group were significantly higher than the sum of the net values of those in the Low PB and Low ORPH groups. In the isoadditive model, the average values of the area of GST-P positive foci and PCNA positive hepatocyte ratio and Gstm3 mRNA level in the PB+ORPH group were significantly higher than the average values of those in the High PB and High ORPH groups. These results suggest that PB and ORPH co-administration causes synergistic effects in liver tumor-promoting activity in rats resulting from oxidative stress due to enhanced microsomal ROS production.

Combined effects of organochlorine pesticides heptachlor and hexachlorobenzene on the promotion stage of hepatocarcinogenesis in rats

We aimed to investigate the combined effect of organochlorine pesticides heptachlor (HEP) and hexachlorobenzene (HCB) by using a medium-term rat liver bioassay. Male F344 rats were initially administered diethylnitrosamine (DEN, 200mg/kgi.p.); after a 2-week non-dosing period, they were given diets containing HEP (5 or 25ppm), HCB (70 or 350ppm), or their mixtures (5 and 70ppm or 25 and 350ppm) for 6weeks. All rats were subjected to partial hepatectomy at week 3 and killed at week 8. We observed additive or synergistic effects of HEP and HCB in groups treated with mixtures of these pesticides. Number and area of preneoplastic foci positive for glutathione S-transferase placental form (GST-P) were consistently higher in these groups than the sum of individual values in the groups treated with HEP or HCB alone. Consistent with these findings, HEP and HCB had additive or synergistic effects on cell proliferation induction within the preneoplastic foci and cytochrome P450 (CYP) 2B1 and 3A1 induction, which may lead to more efficient metabolic activation of HEP and HCB. On the basis of these findings, we conclude that HEP and HCB have additive and synergistic effects on the development of GST-P-positive foci and that higher risks are associated with a combination of residual organochlorine pesticides in foods than with individual residual organochlorine pesticides.

Enhanced liver tumor promotion but not liver initiation activity in rats subjected to combined administration of omeprazole and β-naphthoflavone

Omeprazole (OPZ) and β-naphthoflavone (BNF) are cytochrome P450 (CYP)1A inducers and have liver tumor promoting effects. In this study, we investigated the co-promoting and co-initiating effects of OPZ and BNF in rats. In Experiment 1, male rats were subjected to partial hepatectomy (PH), and given oral doses of 138 or 276 mg/kg OPZ, 0.125% or 0.25% BNF or 138 mg/kg OPZ+0.125% BNF (n = 9~12) for 6 weeks after N-diethylnitrosamine (DEN) initiation. In Experiment 2, male rats were treated with oral doses of 138 or 276 mg/kg OPZ, 0.03% or 0.06% BNF or 138 mg/kg OPZ+0.03% BNF (n = 11~12) for 9 days starting 1 week before initiating treatment. As an initiating treatment, 2-Amino-3,4-dimethylimidazo[4,5-f]quinolone (MeIQx) was orally administered 12 hr after PH. The rats were fed a basal diet for 15 days, followed by a diet containing 0.015% 2-acetylaminofluorene for the next 10 days with a single oral dose of carbon tetrachloride. In Experiment 1, the number and area of glutathione S-transferase placental form-positive foci in the OPZ+BNF group were significantly higher than the average values of the High OPZ or the High BNF group. The expression of cyclooxygenase-2 (Cox-2) and COX-2 protein in the liver significantly increased in the OPZ+BNF group. In Experiment 2, liver initiation activity was not enhanced by the co-administration of OPZ+BNF. The results of our studies suggest that the co-administration of OPZ and BNF results in synergistic effects in the liver tumor promotion probably owing to increased COX-2 expression, but no modifying effect in the liver initiation activity of MeIQx in rats.

Fate of N-nitrosodimethylamine in recycled water after recharge into anaerobic aquifer

Laboratory and field experiments were undertaken to assess the fate of N-nitrosodimethylamine (NDMA) in aerobic recycled water that was recharged into a deep anaerobic pyritic aquifer, as part of a managed aquifer recharge (MAR) strategy. Laboratory studies demonstrated a high mobility of NDMA in the Leederville aquifer system with a retardation coefficient of 1.1. Anaerobic degradation column and (14)C-NDMA microcosm studies showed that anaerobic conditions of the aquifer provided a suitable environment for the biodegradation of NDMA with first-order kinetics. At microgram per litre concentrations, inhibition of biodegradation was observed with degradation half-lives (260±20 days) up to an order of magnitude greater than at nanogram per litre concentrations (25-150 days), which are more typical of environmental concentrations. No threshold effects were observed at the lower ng L(-1) concentrations with NDMA concentrations reduced from 560 ng L(-1) to <6 ng L(-1) over a 42 day 14C-NDMA aerobic microcosm experiment. Aerobic (14)C-NDMA microcosm studies were also undertaken to assess potential aerobic degradation, likely to occur close to the recharge bore. These microcosm experiments showed a faster degradation rate than anaerobic microcosms, with a degradation half-life of 8±2 days, after a lag period of approximately 10 days. Results from a MAR field trial recharging the Leederville aquifer with aerobic recycled water showed that NDMA concentrations reduced from 2.5±1.0 ng L(-1) to 1.3±0.4 ng L(-1) between the recharge bore and a monitoring location 20 m down gradient (an estimated aquifer residence time of 10 days), consistent with data from the aerobic microcosm experiment. Further down gradient, in the anaerobic zone of the aquifer, NDMA degradation could not be assessed, as NDMA concentrations were too close to their analytical detection limit (<1 ng L(-1)).