Protective effect of acetaminophen against colon cancer initiation effects of 3,2'-dimethyl-4-aminobiphenyl in rats.

A previous investigation demonstrated the anticarcinogenicity of acetaminophen (APAP) against colon carcinogenesis in rats induced by 3,2'-dimethyl-4-aminobiphenyl (DMAB). DMAB was selected as a structurally related surrogate for heterocyclic amines, formed during cooking of protein, which are believed to be involved in human colon cancer. The objective of the present study was to ascertain whether the early initiating effects of this colon carcinogen are inhibited by APAP. Six groups of male F344 rats were treated over a 6-week period as follows: (1) vehicle (corn oil) for 6 weeks; (2) APAP in the diet at 1000 ppm daily for 6 weeks; (3) 50 mg/kg DMAB by gavage once a week for the last 4 weeks; (4) 5 mg/kg DMAB as for (3); (5) 1000 ppm APAP for 6 weeks and 50 mg/kg DMAB for the last 4 weeks; and (6) 1000 ppm APAP and 5 mg/kg DMAB as for (5). Colonic tissue was within normal limits in the control and APAP groups. In the APAP only group, apical enterocytic hypertrophy and hyperaemia over the entire surface epithelium was present. In the high-dose DMAB group, in the lower third of the crypts, foci of enlarged glands with hypertrophic cells exhibiting karyomegaly and anisokaryosis (FHE) of 3+ degree of severity were evident in 100% of the animals. Also, there were increases in periglandular fibrocytes, matrix and mononuclear cells (PF). In the low-dose DMAB group both FHE and PF changes with the same degree of severity were reduced. In rats given the low dose of DMAB plus APAP, FHE and PF with the same degree of severity (3+) was absent. Both DMAB exposures increased significantly the replicating fraction of colonic enterocytes in an exposure-related fashion and the replicating fractions were significantly reduced by APAP. In 32P-postlabelling of colon, liver and urinary bladder DNA, high-dose DMAB produced 2-6 distinct dose-related spots reflecting DNA adducts. These spots were reduced or were no longer detectable in all three tissues when APAP was given 2 weeks before and during DMAB exposure. Using immunohistochemical detection of DMAB adducts in the colon, a dose-related colour intensity was present for both doses of DMAB. APAP reduced this by 94-fold. Thus, APAP produced a marked protective effect in colonic enterocytes against several parameters of neoplastic development by the carcinogen.