Inhibition of N-methyl-N-nitrosourea- and 7,12-dimethylbenz[alpha]anthracene-induced rat mammary tumorigenesis by dietary cholesterol is independent of Ha-ras mutations

Dietary cholesterol has previously been shown to inhibit rat mammary tumori genesis but the mechanisms remain unclear, Uptake of serum low density lipo protein cholesterol by tissues leads to down-regulation of 3-hydroxy-3-meth ylglutaryl-CoA (HMG-CoA) reductase, the rate limiting enzyme in cholesterol biosynthesis that catalyzes the formation of mevalonate, In addition to be ing a precursor of cholesterol, mevalonate is necessary for DNA synthesis a nd cell proliferation. Isoprenoids, also derived from mevalonate, are requi red for the post-translational modification of Ras proteins that are mutate d in a number of carcinogen-induced rat mammary tumors. The purpose of this study, therefore, was to determine whether inhibition of tumorigenesis by cholesterol is dependent on the frequency of mutations in the Ha-ras gene. Female Sprague-Dawley rats (30/group) were given a single dose of either N- methyl-N-nitrosourea (MNU, 50 mg/kg i.p.) or 7,12-dimethylbenz[a]anthracene (DMBA, 100 mg/kg intragastrally), carcinogens that produce tumors with eit her a high (MNU) or low (DMBA) frequency of Ha-ras mutations in codon 12 or 61, respectively. Rats were fed either a control AIN-93G diet or the contr ol diet supplemented with 0.3% cholesterol for 14 weeks. Dietary cholestero l significantly decreased the final tumor incidence in rats given DMBA (83 versus 100%, P < 0.05) or MNU (53 versus 77%, P < 0.05). HMG-CoA reductase activity was higher in mammary tumors than in normal mammary glands, but th e activity of this enzyme was reduced by cholesterol feeding only in mammar y glands and not in tumors, Tumors induced by MNU had a high frequency of H a-ras mutations in both the control (65 %) and cholesterol-fed (68%) groups . Tumors induced by DMBA had a low frequency of Ha-ras mutations that also did not differ between the control (21%) and cholesterol-fed (18%) groups, These findings show that dietary cholesterol inhibits mammary tumorigenesis induced by either MNU or DMBA and that the inhibition is independent of th e type or extent of mutations in the Ha-ras gene.