J. Biol. Chem., Vol. 261, Issue 14, 6285-6292, May, 1986
Fluorescent methotrexate labeling and flow cytometric analysis of cells containing low levels of dihydrofolate reductase
P Gaudray, J Trotter and GM Wahl
Previous studies have demonstrated the usefulness of flow cytometry in the
analysis of dihydrofolate reductase (EC 1.5.1.3) gene amplification.
However, this powerful and potentially sensitive method for analyzing gene
expression in individual cells has not seen widespread use. This is due in
part to the difficulty of producing fluorescent methotrexate (Fluo-MTX),
which is needed to label dihydrofolate reductase in vivo, in yields higher
than 1% and of sufficient purity to give low nonspecific backgrounds by the
published procedures. We have significantly improved the synthesis of
Fluo-MTX to obtain rapidly a chromatographically pure product in 20%
yields. In addition, we have found that cell volume is a variable which
makes direct comparisons of fluorescence intensity between cell lines
difficult. In order to circumvent this problem, we have improved flow
cytometric analysis to measure the fluorescence specific intensity of
individual cells. A survey of various cells commonly used for gene transfer
shows a significant variability in the efficiency with which they are
labeled with Fluo-MTX, which appears to be due to variations in their
ability to transport this reagent.