Analysis of the inhibitory effects of VP-16-213 (etoposide) and podophyllotoxin on thymidine transport and metabolism in Ehrlich ascites tumor cells in vitro.
Analysis of the inhibitory effects of VP-16-213 (etoposide) and podophyllotoxin on thymidine transport and metabolism in Ehrlich ascites tumor cells in vitro.
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发表时间：
1984-03
期刊：
影响因子：
11.2
通讯作者：
Jack C. Yalowich;I. Goldman
中科院分区：
文献类型：
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作者：
Jack C. Yalowich;I. Goldman
这些研究探讨了表鬼臼毒素对埃利希腹水瘤细胞中胸苷的膜转运和代谢的影响。细胞暴露于[3 H]-胸苷后3 H摄取的特征为初始速度快，接近膜转运，随后摄取速率较慢，与细胞内胸苷磷酸化衍生物（主要是三磷酸胸苷）的蓄积平行。细胞内胸苷转运相对于胸苷代谢为三磷酸的高速率随着细胞外核苷浓度的降低而降低，这是由于膜转运的降低比随后的代谢步骤大得多。因此，随着细胞外胸苷减少，转运变得越来越速率限制细胞内的代谢。VP-16-213（依托泊苷）或鬼臼毒素抑制胸苷的初始摄取速率，因此抑制细胞内三磷酸胸苷的形成。当细胞外胸苷高时，对转运的抑制作用是短暂的，药物处理细胞内胸苷三磷酸积累的净速率迅速接近与对照细胞相当的速度，表明VP-16-213或鬼臼毒素对核苷和核苷酸磷酸化没有直接影响。当细胞外胸苷减少，使得转运对代谢具有速率限制时，VP-16-213对三磷酸胸苷形成的抑制作用持续时间延长。VP-16-213的继发性效应在与[3 H]胸苷孵育10 min后变得明显，胸苷掺入酸沉淀物中几乎完全停止，三磷酸胸苷水平无任何变化。在鬼臼毒素中未观察到这种晚期效应，表明VP-16-213对DNA合成的直接效应不同于对胸苷磷酸化的早期抑制效应，后者继发于膜转运。
These studies explore the effects of an epipodophyllotoxin on the membrane transport and metabolism of thymidine in Ehrlich ascites tumor cells. Uptake of 3H after exposure of cells to [3H]-thymidine is characterized by a rapid initial velocity that approximates membrane transport followed by a slower rate of uptake that parallels the accumulation of phosphorylated derivatives of thymidine, primarily thymidine triphosphate, within the cell. The high rate of thymidine transport relative to thymidine metabolism to the triphosphate within the cell decreases as the extracellular nucleoside concentration is reduced due to a much greater decrease in membrane transport than the subsequent metabolic step. Hence, as extracellular thymidine is decreased, transport becomes increasingly rate limiting to metabolism within the cell. VP-16-213 (etoposide) or podophyllotoxin inhibits the initial uptake rate for thymidine and, as a consequence, inhibits the intracellular formation of thymidine triphosphate. When extracellular thymidine is high, inhibitory effects on transport are transient, and the net rate of thymidine triphosphate accumulation within drug-treated cells rapidly approaches a velocity comparable to that of control cells, indicating no direct VP-16-213 or podophyllotoxin effect on nucleoside and nucleotide phosphorylation. When extracellular thymidine is reduced so that transport is rate limiting to metabolism, the duration of the inhibitory effects of VP-16-213 on thymidine triphosphate formation is prolonged. A secondary effect of VP-16-213 becomes manifest beyond 10 min of incubation with [3H]thymidine with the virtual complete cessation of thymidine incorporation into the acid precipitate without any change in the thymidine triphosphate level. This late effect is not observed with podophyllotoxin and indicates a direct effect of VP-16-213 on DNA synthesis that is distinct from the earlier inhibitory effect on thymidine phosphorylation, which is secondary to membrane transport.