Interaction between methotrexate, "rescue" agents and cell proliferation as modulators of homocysteine export from cells in culture.

Clinical studies on cancer and psoriasis patients have shown that plasma and urinary homocysteine (Hcy) responds to methotrexate (MTX) therapy, indicating that Hcy in extracellular fluids may be an indicator of the antifolate effect. However, the clinical data indicate that the burden of proliferating cells, cytotoxicity and the folate status are also determinants of extracellular Hcy. To evaluate this further, we investigated the modulation of cellular Hcy egress by MTX, rescue agents, cell proliferation and cytotoxicity. Nontransformed and chemically transformed fibroblasts and murine lymphoma cells, which are characterized by different growth behavior and MTX response, were used. The Hcy export rate was correlated positively with the proliferation rate in all cell types. 5-Formyltetrahydrofolate or 5-methyltetrahydrofolate added to fibroblasts not exposed to MTX reduced the Hcy export rate, whereas the export from the lymphoma cells was not affected. All cells types exposed to MTX were rescued by thymidine + hypoxanthine, and this allowed the assessment of Hcy export during MTX exposure without interference from cytotoxicity. In the fibroblasts, MTX with thymidine + hypoxanthine rescue induced a marked increase in Hcy export, and the dose-response paralleled the cytotoxicity curves obtained for MTX without rescue. Nontoxic concentrations of MTX without rescue enhanced the Hcy export. When MTX concentration was increased further, Hcy export was stimulated initially, and then declined rapidly as cell death ensued. MTX did not enhance the Hcy export from the lymphoma cells and, in the absence of rescue, the Hcy export from these cells declined in proportion to inhibition of cell growth.(ABSTRACT TRUNCATED AT 250 WORDS)