Renal tubular cell or hepatocyte hyperplasia is not associated with tumor promotion by di(2-ethylhexyl)phthalate in B6C3F1 mice after transplacental initiation with N-nitrosoethylurea

Abstract

B6C3F1 mice of both sexes that had been exposed transplacentally on day 18 of gestation to 0.5 mmole N-nitrosoethylurea (NEU) were fed either normal diets or diets containing di(2-ethylhexyl)phthalate (DEHP) at 6,000 ppm beginning at 6 wk of age and continuing to 78 wk of age. At 52 and 78 wk of age, 6-26 mice from each group received a single injection of 5-bromo-2'-deoxyuridine (Brdu) at 200 mg/kg i.p. and were sacrificed 1 h later for determination of the levels of renal and hepatic DNA synthesis by the Brdu immunohistochemical technique. No differences occurred in incidences of gross or microscopic renal tubular cell tumors between the NEU (males 15%, females 21%) and NEU-DEHP groups (males 10%, females 15%) at 78 wk. The labelling index (LI) of renal cortical tubular cells was significantly increased at 78 wk (22.3 +/- 3.7/mm2 for males, 21.8 +/- 1.2 for females) in mice given NEU and DEHP as compared with NEU alone (9.7 +/- 1.0 for males, 6.9 +/- 0.7 for females). The number and sizes of focal hepatocellular proliferative lesions (FHPL), including hyperplastic foci, hepatocellular adenomas and carcinomas, were quantified by image analysis and stereology. DEHP significantly enhanced the mean volume and volume % of FHPL, including liver tumors, but not numbers of FHPL/liver. Hepatocyte LI was also not affected, at least as detected by the technique used, while FHPL had significantly increased LI (14.5-48.3) as compared with normal hepatocytes (0.5-2.4). This study provides some evidence that enhanced chronic cell replication in the kidney may not always be associated with renal carcinogenesis of tumor promotion, while tumor promotion in liver may be a consequence of increased DNA synthesis in initiated or focus cells rather than in nonproliferative parenchymal hepatocytes, which may not be target cells of some tumor promoters.