APOPTOSIS AND PROLIFERATION UNDER CONDITIONS OF DEOXYNUCLEOTIDE POOL IMBALANCE IN LIVER OF FOLATE METHYL DEFICIENT RATS/

Weanling male F344 rats were fed either a semi-purified diet low in me thionine and lacking in choline and folic acid (folate/methyl deficien t) or a supplemented control diet for periods of 2, 5, 7 days, 3 weeks , and 9 weeks, Two days after initiating the folate/methyl deficient d iet in weanling F344 rats, the incidence of apoptotic bodies, identifi ed by in situ end-labeling of 3'-OH DNA strand breaks, was significant ly increased in liver sections from the deficient rats, Apoptotic cell death was confirmed biochemically by an increase in nuclear Ca2+/Mg2-dependent endonuclease activity that paralleled the increase in apopt otic bodies over the 9-week feeding period, There was no morphologic e vidence of necrotic foci or necrosis-associated inflammatory response over the 9-week period, Confirming that cell turnover is chronically e levated in this model, the increase in apoptotic rate was accompanied by a sustained increase in the mitotic index (MI). The DNA repair-asso ciated enzyme, poly(ADPribose) polymerase (PARP), was similarly elevat ed and was associated with significant decreases in the substrate for ADPribose polymer synthesis, nicotinamide adenine dinucleotide (NAB), Because folate metabolites are essential for de novo purine and thymid ine biosynthesis, prolonged deficiency in folic acid can induce an imb alance in the deoxynucleotide precursors for DNA replication/repair an d negatively affect the fidelity of DNA synthesis, Using an HPLC metho d, hepatic deoxyuridine triphosphate (dUTP) levels were increased at 3 and 9 weeks after initiation of the deficient diet and levels of thym idine triphosphate (dTTP) were reduced, An increase in dUTP/dTTP ratio is consistent with a block in folate-dependent de novo thymidylate bi osynthesis and may predispose to uracil misincorporation and DNA repai r-related DNA strand breaks.