Folate-binding protein mRNA is decreased in methotrexate-resistant KB cells

Folate-binding protein (FBP), a high-affinity folate receptor expressed in certain malignant and normal cell lines and tissues, is responsible for cellular transport of folate and structurally related antifolates such as methotrexate in human KB (nasopharyngeal carcinoma) cells. We have developed a sensitive polymerase chain reaction (PCR) assay to quantitate folate-binding protein mRNA levels. This assay was tested in KB and methotrexate (MTX)-resistant KB (KB1BT) cell lines. Based on assay results, we conclude there are 245 molecules of folate-binding protein (FBP) mRNA per KB cell, and there is an approximately 3-fold decrease in FBP mRNA levels per KB1BT cell. The data obtained by quantitative PCR correlate well with northern blot analysis of mRNA levels and membrane-associated folate-binding protein (M-FBP) levels. Moreover, when KB1BT cells were grown in the absence of MTX for 8 months, the levels of FBP and M-FBP mRNA did not change, and there was a substantial decrease in MTX uptake, compared with KB cells.