The absence of γ‐glutamyltransferase activity in transport‐dependent methotrexate‐resistant hepatoma cells

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Abstract
A cell line derived from H35 hepatoma cells resistant to methotrexate (MTX) as a result of a defective transport system for MTX has been examined to determine how closely the variant resembles the parent cells with regard to other biochemical properties. The capacity of extracts of resistant cells to catalyze the poly‐γ‐glutamylation of MTX was approximately twice as great as that of wild‐type cell extracts. Evidence of similarity between wild‐type and H35 R0.3 cells was derived from the equitoxic activity to both cell lines of non‐classical antifolates and other miscellaneous antineoplastics which act by a variety of mechanisms. Two phenotypic markers of hepatic cell function, α‐aminoisobutyric acid (AIB) transport and tyrosine aminotransferase (TAT) activity inducibility, were present in both cell types, demonstrating the maintenance of these phenotypic properties in the H35 R0.3 cells. γ‐Glutamyltransferase (GGT, EC 2.3.2.2) activity differed in that it was present in wild‐type cells and barely detectable in H35 R0.3 cells. The GGT activity reappeared in the H35 cells when they regained MTX sensitivity after incubation for 14–20 weeks in MTX‐free media. Although defective MTX transport appeared to be correlated with the disappearance of GGT activity in an H35 variant cell line, no functional relationship between them is apparent at this time. It is possible that a lack of GGT activity may be evidence of a more differentiated phenotype in the transport‐resistant cell line.

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