Inhibition of methionine transport by methotrexate in mitogen-stimulated human lymphocytes

Mitogen stimulation of human lymphocytes was used to study the expression of several amino acid transport systems in response to cell growth. Resting lymphocytes possess detectable neutral amino acid transport systems which can be activated by the mitogen, phytohemagglutinin (PHA). Sodium-independent transport of 2-aminobicyclo (2,2,1) heptane 2-carboxylic acid was similar in resting and stimulated lymphocytes. However, sodium-dependent uptake of methyl aminoisobutyric acid and alpha-aminoisobutyric acid increased threefold to eightfold with stimulation by PHA. Methionine uptake was primarily sodium-independent in resting lymphocytes, whereas methionine uptake increased fourfold to eightfold in stimulated cells; most of this increase was attributable to sodium-dependent transport. Uptake of methotrexate (MTX) was minimal in resting lymphocytes but was maximal 72 hrs after stimulation with PHA; this increase in MTX uptake was accompanied by inhibition of sodium-dependent methionine transport. These results suggest that PHA activates sodium-dependent transport of methionine and that this activation is blocked by the folate antagonist, MTX.