GM3 directly inhibits tyrosine phosphorylation and de-N-acetyl-GM3 directly enhances serine phosphorylation of epidermal growth factor receptor, independently of receptor-receptor interaction.

GM3 ganglioside (II3NeuAcLacCer) inhibits epidermal growth factor (EGF)-dependent receptor autophosphorylation and cell growth (Bremer, E.G., Schlessinger, J., and Hakomori, S. (1986) J. Biol. Chem. 261, 2434-2440), whereas de-N-acetyl-GM3 (deNAcGM3; II3NeuNH2Lac-Cer) promotes these processes (Hanai, N., Dohi, T., Nores, G. A., and Hakomori, S. (1988) J. Biol. Chem. 263, 6296-6301). Receptor-receptor interaction has been proposed as an essential initial mechanism for EGF-dependent activation of EGF receptor kinase (EGF-RK) (Schlessinger, J. (1988) Trends Biochem. Sci. 13, 443-447). We studied the effects of GM3 and deNAcGM3 on EGF-RK function and EGF-R dimerization, and observed that (i) EGF-dependent in vitro and in vivo (in situ) phosphorylation of A431 cells at both monomeric and dimeric forms of EGF-R was inhibited in a dose-dependent manner by GM3, but unaffected by GM1. (ii) Quantities of both forms of EGF-R remained constant regardless of addition of various quantities of GM3 or GM1, as revealed by blotting with antibodies directed to the C-terminal region of EGF-R, or by cell surface 125I-labeling followed by immunoprecipitation. (iii) DeNacGM3 in the absence as well as in the presence of a minimal quantity of detergent significantly enhanced EGF-R phosphorylation, particularly Ser phosphorylation. (iv) DeNAcGM3 was detected in a large variety of actively growing tumor cells. Findings i and ii above indicate that GM3 directly inhibits EGF-dependent Tyr phosphorylation but does not affect receptor-receptor interaction. Findings iii and iv suggest that deNAcGM3 strongly promotes serine phosphorylation (in addition to Tyr phosphorylation) of EGF-R and may function as a second messenger in the process of cell growth stimulation.