Peroxisome proliferators are a group of nongenotoxic carcinogens which include a number of hypolipidemic drugs, solvents, and industrial plasticizers. Although the mechanism by which they cause cancer remains unknown, one likely possibility is that they act as tumor promoters by increasing cell proliferation. Hepatic Kupffer cells represent a rich source of mitogenic cytokines (e.g., tumor necrosis factor alpha, TNF alpha) and are stimulated by peroxisome proliferators. Since glycine prevents activation of Kupffer cells, these experiments were designed to test the hypothesis that a diet containing glycine could block the mitogenic effect of the peroxisome proliferator [[4-chloro-6-(2,3-xylidino)pyrimidinyl]thio]acetic acid (WY-14,643). The effects of a glycine-enriched diet on WY-14,643-induced increases in cell proliferation after a single dose or after feeding WY-14,643 in the diet for 3 weeks were assessed. As expected, 24 h after a single dose of WY-14,643, rates of cell proliferation increased from basal values of 0.7 +/- 0.3% to 5.1 +/- 0.5%. Glycine largely prevented the increase caused by WY-14,643 with proliferation only reaching 1.9 +/- 0.4% (p < 0.05). Acyl CoA oxidase increased from 1.4 +/- 0.1 to 3.5 +/- 0.6 nmol of H2O2 min-1 (mg of protein)-1 (p < 0.05) indicating that peroxisome-specific enzyme activity was induced about 2-fold in livers of WY-14,643-treated rats after 24 h. Unlike cell proliferation, however, acyl CoA oxidase was not affected by dietary glycine, consistent with the hypothesis that cell proliferation and peroxisome proliferation occur via different mechanisms. After 3 weeks, dietary glycine reduced basal rates of cell proliferation by about 50% and completely prevented the sustained 5-fold increase in cell proliferation caused by feeding WY-14,643. Moreover, the 3-fold increase in TNF alpha mRNA caused by WY-14,643 was blocked completely by the glycine-enriched diet. Similarly, immunohistochemical staining for TNF alpha was increased 6-fold by WY-14,643, an increase which was prevented by dietary glycine. However, the 6-fold increase in acyl CoA oxidase activity was unaffected by glycine under similar conditions demonstrating that a diet containing 5% glycine prevents increased hepatocyte proliferation caused by a potent peroxisome proliferator without affecting induction of peroxisomes. These data demonstrate that a glycine-enriched diet prevents stimulated cell proliferation most likely by inhibiting TNF alpha production and raise the possibility that dietary glycine will be effective in preventing cancer caused by nongenotoxic carcinogens such as WY-14,643.