A unique pattern of hepatocyte proliferation in F344 rats following long-term exposures to low levels of a chemical mixture of groundwater contaminants

Abstract

Most exposures of humans to environmental agents involve mixtures of chemicals, rather than individual chemicals. Some chemicals can cause hepatocellular proliferation and act as neoplastic promoters. Little is known concerning hepatocellular proliferation caused by chemical mixtures such as those found in groundwater at hazardous waste sites. Therefore, a 6 month study was performed to investigate hepatocellular proliferation and histopathological changes in F344 rats after long-term, low-level exposure to a mixture of groundwater contaminants. The seven chemicals used are among the most frequently detected contaminants associated with hazardous waste sites; arsenic, benzene, chloroform, chromium, lead, phenol and trichloroethylene. Male F344 rats were exposed to this mixture, or submixtures of the organic or inorganic chemicals, via drinking water for 6 months. The study design included a time-course experiment (i.e. 3 and 10 days and 1, 3 and 6 months) and a dose-response experiment. Hepatocellular proliferation studies were performed by subcutaneously implanting osmotic mini-pumps to continuously deliver 5-bromo-2'-deoxyuridine for 7 days, which labeled nuclei of proliferating cells. In all groups, there were no differences in weight gain, body weight, liver weight ratios or liver-associated plasma enzymes. Light microscopic evaluation revealed no lesions related to the treatments in any animals. However, significant increases in hepatocellular labeling were observed at the 3 and 10 day and 1 month exposure time points after treatment with the full mixture, as well as the organic or inorganic submixtures. Proliferating hepatocytes expressed a unique labeling pattern surrounding large hepatic veins (0.5-2.0 mm), but not central veins. This did not appear to be a regenerative response due to cytotoxic mechanisms, as assessed by the absence of increased plasma enzyme activity and the absence of hepatocellular lesions.

Detection of Initiation Activity of 1,2-Dimethylhydrazine in in vivo Medium-Term Liver Initiation Assay Ssytem using 4-Week-Old Rats without Hepatocellular Proliferative Stimuli during the Test Chemical Treatment Period

“…Multiple low-dose administrations proved superior to the single high-dose administration when the margin of test chemical between toxic/lethal dose level and genotoxic/car- cinogenic dose level was small. Human exposure to environmental carcinogens is usually limited to low concentrations and human are generally exposed to such carcinogens over a long-term period [1,7,49]; therefore, multiple low-dose administration would be expected to be superior to single high-dose administration for extrapolation to carcinogenesis in humans. Generally, previous assay models using a cell proliferation stimulus tested only single or multiple doses over 1-day administration of test chemicals because the induction of hepatocyte proliferation could be maintained only for approximately 1 day [2,39].…”

An In Vivo Method for Using 5-Bromo-2'-deoxyuridine (BrdU) as a Marker of Chemically-Induced Hepatocellular Proliferation in the Japanese Medaka (Oryzias latipes)

“…There was also a statistically signi cant increase in the number of hepatocellular adenomas (single and multiple) and carcinomas in the 100-mg/L DEN sh as compared to control and 10 mg/L sh ( p < 0.0001), as measured by chi-square analysis. of a chemical mixture of groundwater contaminants in the drinking water (6). These proliferation increases occurred at 3 days, 10 days, and 1 month after the start of the exposure.…”

“…Proliferative indices have historically been quanti ed in rodents (6,19) and sh (22) by counting the number of labeled nuclei in a eld of 1,000 cells. In this study, we developed a timesaving area labeling index (ALI) using a computer-assisted image analysis system to quantify BrdU-labeled medaka cells.…”

Increased Rate of Apoptosis Correlates with Hepatocellular Proliferation in Fischer-344 Rats Following Long-Term Exposure to a Mixture of Groundwater Contaminants

“…Recently, we reported observing a unique pattern of hepatocellular proliferation around large hepatic veins in Fischer-344 (F-344) rats following repeated exposure, at low levels, to a mixture of common groundwater contaminants (13). Chemical mixture components and their concentrations were selected based on frequency of detection in groundwater around hazardous waste sites (42,44 Fig.…”

“…4. Treated animals exposed to the drinking water contaminant mixture for 3 days, 10 (13), by permission of Oxford University Press. crease in cell proliferation was observed before returning to background levels. A similar response was also observed when evaluating the time-course index of apoptosis (Fig.…”