Inhibition of DNA adduct formation of PhIP in female F344 rats by dietary conjugated linoleic acid

Abstract

The dietary mutagen 2‐amino‐1‐methyl‐6‐phenylimidazo[4,5‐b]pyridine (PhIP) is a mammary carcinogen in the female Fischer (F344) rat and a colon carcinogen in the male F344 rat. To exert its carcinogenicity, it is believed that PhIP needs to form adducts with DNA, a process requiring N‐hydroxylation of PhIP by cytochromes P‐450 1A1 and/or 1A2 (CYP 1A1 and/or 1A2), as well as further esterification of the hydroxylamine thus formed. Dietary conjugated linoleic acid (CIA) inhibits chemical carcinogenesis in various experimental models. We have examined the effect of dietary CLA on PhIP‐DNA adduct formation in female F344 rats. Four‐week‐old animals were maintained on AIN‐76A diet without or with CLA (1%, 0.5%, and 0.1% wt/wt) for 57 days. PhIP was added to the diets (0.04% wt/wt) from Days 14–42. Animals were killed (4/group) on Days 43, 50, and 57. DNA isolated from liver, mammary epithelial cells (MEC), colon, and white blood cells (WBC) was analyzed for PhIP‐DNA adducts by ³²P‐postlabeling assays. On Day 43, CIA inhibited adduct formation in the liver (up to 58%) in a dose‐dependent manner. CIA also inhibited hepatic adduct levels (29–39%) on Day 50 (at 1.0% and 0.5% CIA) and on Day 57 (53% at 0.5% CLA). CLA significantly reduced adduct levels in the WBC on Day 50 (63–70%). Adducts in MEC and the colon were not affected by dietary CIA. On Day 57, adduct levels in MEC, liver, colon, and WBC were 0–30.3%, 8.6–41.7%, 21.5–50.7%, and 7.5–11.8%, respectively, of those on Day 43. Northern blot analysis of liver RNA showed that dietary CIA did not affect steady‐state levels of CYP 1A1 or 1A2 mRNA. It is concluded that dietary CLA inhibits PhIP‐DNA adduct formation in liver and WBC but that those in MEC and the colon are unaffected when a low‐level dietary regimen of carcinogen and inhibitor was used. In inhibiting PhIP‐DNA adduct formation, CIA does not appear to act by inhibiting CYP 1A1 or 1A2 expression.