Phytohemagglutinin inhibits phorbol diester promotion of UV-irradiation initiated transformation in Syrian hamster embryo cells.

Phytohemagglutinin (PHA) or either of its isolectins, erythroagglutinin or leukoagglutinin, causes a dose-dependent decrease in 12-0-tetradecanoylphorbol 13-acetate (TPA)-promoted transformation of Syrian hamster embryo cells, but has no effect on transformation induced by ultraviolet irradiation. The ineffectiveness of concanavalin A indicates that not every lectin inhibits TPA. Galactose, a dominant sugar in receptors for PHA binding, reverses the inhibition of TPA promotion caused by PHA but galactose does not inhibit TPA promotion itself. Therefore, the TPA and PHA binding sites are functionally discrete. The PHA inhibition of TPA-promoted transformation is reversible because PHA is only effective if present with TPA, whereas lymphotoxin, an immunologic hormone, has a persistent anti-carcinogenic effect, regardless of whether it is added before or after TPA. PHA in conjunction with lymphotoxin causes additional inhibition of TPA-promoted transformation. PHA and lymphotoxin affect the biological activity of TPA by diverse mechanisms. Lymphotoxin alters the physiological state of the cell, causing a change in the cellular response to TPA. PHA may affect either the binding of TPA to a critical cellular receptor for promotion or a later step in promotion.