Submitted to: Journal of Nutritional Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/1996
Publication Date: N/A
Citation: Interpretive Summary: Folate, a water soluble B vitamin, is essential for the metabolism and maintenance of the genetic building block, DNA. Methotrexate, a potent folate inhibitor, has an established role in the treatment of several types of cancer but has little effectiveness in the treatment of colon cancer. On the other hand, 5-FU, another folate inhibitor, is the most effective agent in the treatment of colon cancer. To date, the reason for the poor response to methotrexate is unclear. To investigate this, we examined the effect of these agents on colon cancer cells by measuring the DNA synthesis, which needs folate. We also compared the effect of methotrexate and 5-FU in various delivery systems. Our studies convincingly demonstrate that methotrexate is an effective inhibitor of colon cancer cell. These observations provide the ground work for the investigation of these agents effect on biopsies or surgical specimens from colon cancer patients and the poor response of methotrexate in colon cancer.
Technical Abstract: Although colon cancers respond poorly to chemotherapeutic agents, 5- fluorouracil (5-FU) is considered the most effective single agent in the treatment of advanced colon cancer, whereas methotrexate has been reported as an ineffective agent. But both 5-FU and methotrexate inhibit de novo thymidylate synthesis. In the present study we assessed the sensitivity of folate-dependent thymidylate synthesis to methotrexate and 5-FU in Caco-1, a human colon carcinoma cell line. Sensitivity was assessed indirectly, by the deoxyuridine suppression test and directly, by the degree of inhibition of [3H] incorporation into deoxyribonucleic acid (DNA). Methotrexate or 5-fluorodexyuridine resulted in a significant decrease in suppression of [3H]thymidine incorporation by exogenous deoxyuridine. [3H]deoxyuridine incorporation was also inhibited by the two agents. Inhibition was dose dependent and 50% inhibition occurred at about 2.5 micromol/L 5-FU. In a second study, the effect of methotrexate and 5-FU on [3H]deoxyuridine incorporation into DNA was assessed under conditions in which a Caco-2 cell monolayer was exposed to the agents either at the apical or at the basolateral membrane side. Under these conditions, inhibition was also dose dependent and cells were more sensitive to basolateral exposure to both methotrexate and 5-FU (P<0.05). The data suggest that both methotrexate and 5-FU are effective inhibitors of thymidylate synthesis in Caco-2 cells. Determining the degree of inhibition of deoxyuridine incorporation into DNA is an effective method for evaluating these agents effect on de novo thymidylate synthesis.