Variation in inducibility of cytochrome P-450c and aryl hydrocarbon hydroxylase in rat liver, lung, kidney, pancreas and nasopharynx

Mechanisms of exocrine pancreatic toxicity induced by oral treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin in female Harlan Sprague-Dawley Rats

In previous 2-year studies of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) conducted by the National Toxicology Program on female Harlan Sprague-Dawley rats, acinar-cell vacuolation, atrophy, inflammation, and arteritis developed at high incidence, and a rare occurrence of pancreatic acinar-cell adenomas and carcinomas was noted. In this investigation, we sought to identify the mechanism involved in the early formative stages of acinar-cell lesions. Pancreas from animals treated for 14 and 31 weeks with 100 ng TCDD/kg body weight or corn oil vehicle was examined immunohistochemically and/or morphometrically. Acinar-cell kinetics were analyzed using staining with hematoxylin and eosin and proliferating cell nuclear antigen. Expressions of cytochrome P450 (CYP) 1A1 and aryl hydrocarbon receptor (AhR) were evaluated to assess direct effects of TCDD exposure. The cholecystokinin-A receptor (CCK-A receptor; CCKAR) and duodenal cholecystokinin 8 (CCK) revealed the associations of dioxin treatment with hormonal changes. Amylase localization showed acinar structural changes that could affect enzymatic production. Increased apoptotic activity in acinar cells occurred in 14- and 31-week-treated animals, with an increase in proliferative activity in the latter. Also in the latter, in the vacuolated acinar cells, CYP1A1 was overexpressed, and statistically significant decreases in expressions of AhR, CCKAR, and amylase occurred. The intensity of CCKAR expression increased in nonvacuolated acinar cells; a decrease in the size of CCK-positive epithelial cells occurred in duodenum. Our findings indicate that dioxin-induced acinar-cell lesions might be related to a direct effect of TCDD on the pancreas. Increase in CYP1A1 and decrease in CCKAR expressions in vacuolated acinar cells may be involved in the pathogenesis of pancreatic lesions. Changes in the expression of CYP or CCKAR may have induced the acinar-cell tumors by initiating proliferation.

A transgenic mouse expressing human CYP1A2 in the pancreas

A transgenic mouse line expressing the human cytochrome P450 CYP1A2 in the pancreas under the control of the mouse elastase promoter was established. The expression of CYP1A2 was specific to the transgenic pancreas and was not found in the control wild-type mouse pancreas. The level of CYP1A2 expressed in pancreatic microsomes from transgenic mice was comparable to that of the endogenously expressed CYP1A2 protein in the liver, as judged by western blotting analyses. Estrone metabolism was used to determine the activity of CYP1A2 expressed in the pancreas of the transgenic mouse. The transgenic pancreas exhibited almost one-third to one-half of the activity of wild-type or CYP1A2 transgenic mouse liver, whereas the wild-type pancreas demonstrated no activity. The addition of NADPH-cytochrome P450 oxidoreductase to the reaction mixture containing pancreatic microsomes from the transgenic mice did not increase the estrone metabolism activity significantly. This transgenic mouse line provides another useful tool to study human CYP1A2 and its relation to chemical toxicity and carcinogenesis.

Inducibility of cytochromes P-4502E1 and P-4501A1 in the rat pancreas

Cytochrome P-450 (CYP) isoenzymes have been incriminated in the toxicity and carcinogenicity of various xenobiotics in different tissues, but prior measurements of their activity in pancreatic microsomes have been disappointing. We now applied new isolation methods and a highly sensitive procedure to assay for the metabolism of p-nitrophenol and 7-ethoxyresorufin, specific substrates for CYP2E1 (2E1) and CYP1A1 (1A1), respectively. 2E1 and 1A1 content was estimated with high-resolution chemiluminescent Western blots using recombinant 2E1 and 1A1 as standards. We found that p-nitrophenol hydroxylase activity was 5.07 +/- 0.66 and 1.50 +/- 0.26 pmol/ min/mg of protein in pancreatic microsomes of ethanol-fed and control rats, respectively. Chronic ethanol treatment increased 2E1 content in pancreatic microsomes 3.6-fold. Activity and content of 2E1 were also assessed in hepatic microsomes: specific activity (expressed per 2E1 content) was similar in pancreatic and hepatic microsomes. There was also an inductive effect of 3-methylcholanthrene (MC) on 1A1 in pancreatic microsomes. Pancreatic microsomal 7-ethoxyresorufin-O-dealkylation activity in MC-treated rats was 19.6 +/- 1.7 pmol/min/mg of protein, 61-fold higher than in controls. MC treatment increased the 1A1 content in pancreatic microsomes 42-fold. These results demonstrate that, in pancreatic microsomes, ethanol and MC exert striking inductive effects on 2E1 and 1A1 activities, which could play a role in the pathogenesis of pancreatitis and/or pancreatic cancer.

Induction of cytochrome CYPIA1 and formation of toxic metabolites of benzo[a]pyrene by rat aorta: a possible role in atherogenesis

Cigarette smoking is a leading risk factor for atherosclerosis. Endothelial injury may be the initial event in this process. The carcinogenic metabolites of the polycyclic aromatic hydrocarbons found in cigarette smoke tars could cause this injury. We tested this model by examining the effect of 3-methylcholanthrene administration on aortic polycyclic aromatic hydrocarbon metabolism. Immunoblotting with a monoclonal antibody (mAb 1-7-1) specific for cytochromes CYPIA1 and CYPIA2 showed that aortic microsomes from treated, but not from control, animals contained CYPIA1; the CYPIA1 was primarily in the endothelium. Aortic microsomes from induced animals metabolized benzo[a]pyrene (BaP) to the 7R,8S,9,10-tetrahydrotetrol-, 7,8-dihydrodiol-, 1,6 quinone-, 3,6 quinone-, 6,12 quinone-, 3-hydroxy-, and 9-hydroxy-BaP. mAb 1-7-1 inhibited the formation of the tetrahydrotetrol, the dihydrodiol-BaP, and the 3-hydroxy-BaP but did not inhibit the quinones or the 9-hydroxy-BaP. Arachidonic acid did not affect metabolism. These data suggest that the aortas of induced animals metabolize the BaP in cigarette smoke to carcinogenic and toxic products and that this metabolism may initiate vessel injury and lead to the accelerated atherosclerosis seen in cigarette smokers.