Rose M L, Germolec D, Arteel G E, Schoonhoven R, Thurman R G
Department of Pharmacology, University of North Carolina, Chapel Hill 27599-7365, USA.
Chem Res Toxicol. 1997 Oct;10(10):1198-204. doi: 10.1021/tx970079u.
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.
过氧化物酶体增殖剂是一类非遗传毒性致癌物，包括多种降血脂药物、溶剂和工业增塑剂。尽管它们致癌的机制尚不清楚，但一种可能的情况是，它们通过增加细胞增殖而作为肿瘤促进剂起作用。肝库普弗细胞是促有丝分裂细胞因子（如肿瘤坏死因子α，TNFα）的丰富来源，并受到过氧化物酶体增殖剂的刺激。由于甘氨酸可阻止库普弗细胞的激活，因此设计了这些实验来检验以下假设：含甘氨酸的饮食可阻断过氧化物酶体增殖剂[[4-氯-6-(2,3-二甲基苯胺基)嘧啶基]硫代]乙酸（WY-14,643）的促有丝分裂作用。评估了富含甘氨酸的饮食对单次给药后或在饮食中投喂WY-14,643 3周后WY-14,643诱导的细胞增殖增加的影响。正如预期的那样，单次给药WY-14,643后24小时，细胞增殖率从基础值0.7±0.3%增加到5.1±0.5%。甘氨酸在很大程度上阻止了WY-14,643引起的增加，增殖仅达到1.9±0.4%（p<0.05）。酰基辅酶A氧化酶从1.4±0.1增加到3.5±0.6 nmol H2O2 min-1（mg蛋白质）-1（p<0.05），表明在WY-14,643处理的大鼠肝脏中，过氧化物酶体特异性酶活性在24小时后诱导增加了约2倍。然而，与细胞增殖不同，酰基辅酶A氧化酶不受饮食甘氨酸的影响，这与细胞增殖和过氧化物酶体增殖通过不同机制发生的假设一致。3周后，饮食中的甘氨酸使基础细胞增殖率降低了约50%，并完全阻止了投喂WY-14,643导致的细胞增殖持续5倍增加。此外，富含甘氨酸的饮食完全阻断了WY-14,643引起的TNFα mRNA 3倍增加。同样，WY-14,643使TNFα的免疫组织化学染色增加了6倍，而这种增加被饮食中的甘氨酸阻止。然而，在类似条件下，酰基辅酶A氧化酶活性增加6倍不受甘氨酸影响，这表明含5%甘氨酸的饮食可阻止强效过氧化物酶体增殖剂引起的肝细胞增殖增加，而不影响过氧化物酶体的诱导。这些数据表明，富含甘氨酸的饮食最有可能通过抑制TNFα的产生来阻止刺激的细胞增殖，并增加了饮食中的甘氨酸对预防由WY-14,643等非遗传毒性致癌物引起的癌症有效的可能性。
