CELL-PROLIFERATION AND GLOBAL METHYLATION STATUS CHANGES IN MOUSE-LIVER AFTER PHENOBARBITAL AND OR CHOLINE-DEVOID, METHIONINE-DEFICIENT DIET ADMINISTRATION/

Our laboratory is testing the hypothesis that hypomethylation of DNA [ a decreased content of 5-methylcytosine (5MeC) compared with cytosine] facilitates aberrant oncogene expression involved in tumorigenesis, u sing a model system of mouse strains with differing susceptibilities t o liver tumorigenesis. The B6C3F1 (C57BL/6 x C3H/He) mouse serves as t he relatively susceptible strain and C57BL/6 serves as the relatively resistant strain, Phenobarbital (PB) and/or administration of a cholin e-devoid, methionine-deficient diet (CMD) were employed as non-genotox ic hepatocarcinogens. We have examined hepatocyte and nonhepatocyte pr oliferation in conjunction with an assessment of global methylation ch anges in liver DNA of B6C3F1 and C57BL/6 mice following these promoter treatments, Bromodeoxyuridine incorporation into DNA, used to measure cell proliferation indirectly, was visualized by immunohistochemistry and quantified by a Macintosh-based image analysis system, Increased hepatocyte proliferation was demonstrated following all three treatmen ts, This increase was larger in C57BL/6 (the relatively resistant stra in) as compared with B6C3F1. In contrast, global hypomethylation was e vident to a larger extent in the B6C3F1 mouse, as compared with C57BL/ 6, PB led to hypomethylation (>20% decrease as compared with controls) at weeks 1, 2 and 4 in B6C3F1, but not in C57BL/6 at the same time po ints, CMD diet administration led to hypomethylation in both strains, At week 1, 21 and 9% decreases in global methylation status were obser ved in B6C3F1 and C57BL/6 respectively, Evaluation of these data sugge sts that the heightened sensitivity of the B6C3F1 mouse compared with the C57BL/6 is due, in part, to a decreased capacity for, or fidelity of, maintaining normal methylation status, The relatively resistant st rain is better able to maintain the normal methylation status of DNA i n the face of a higher level of cell proliferation.