Resistance to methotrexate is associated with selective changes of alpha 2,6- and alpha 2,3-sialyltransferase activities toward N-acetyllactosaminic sequences in human colon cancer cell line HT-29.

In previous works we established that the alpha 2,6-sialyltransferase acting on N-acetyllactosaminic sequences [alpha 2,6(N)ST, E.C. 2.4.99.1] behaves, in colonic cells, as an oncodevelopmentally regulated enzyme. Subpopulations of the human colon cancer cell line HT-29 adapted to grow in 10(-5) M methotrexate (MTX), permanently retain the ability to differentiate as mucus-secreting cells when kept confluent for extended periods of time [Lesuffleur et al. (1991) J. Cell Biol. 115, 1409-1418]. In this study we have compared the activities of five sialyltransferases acting on N- or O-linked chains of glycoproteins in parental HT-29 and in the 10(-5) M MTX-resistant variant. Both cell lines were studied during the exponential phase of growth as well as after a long period of postconfluent culture (28-30 days). Regardless the culture conditions, resistance to 10(-5) M MTX is associated with a virtual disappearance of alpha 2,6(N)ST activity. This change results in a dramatic reduction of the reactivity of cell membranes with the fluorescent lectin from Sambucus nigra, specific for alpha 2,6-sialylated structures. The activity of the alpha 2,3-sialyltransferase which acts on N-acetyllactosaminic sequences increases about two times in postconfluent cultures of 10(-5) M MTX-resistant cells, suggesting a close relationship with the differentiation degree. No significative changes were observed in the activity of other sialyltransferases.