Enhancement of epidermal growth factor signaling and activation of SRC kinase by gangliosides.

In a recent study, inhibition of cellular ganglioside synthesis blocked growth factor-induced fibroblast proliferation. Conversely, enrichment of cell membrane gangliosides by ganglioside preincubation enhanced growth factor-elicited cell proliferation. In the absence of serum and growth factors, NeuNAcalpha2-3Galbeta1-3GalNAcbeta1-4(NeuNAcalpha2-3)Galbeta1-4Glcbeta1-1Cer (G(D1a)) acted like a growth factor when cells were pretreated with the ganglioside, stimulating proliferation of normal human dermal fibroblasts and Swiss 3T3 fibroblasts. In contrast, growth inhibition was observed when high concentrations of gangliosides were continuously present in the culture medium during incubation of fibroblasts with growth factors (Li, R., Manela, J., Kong, Y., and Ladisch, S. (2000) J. Biol. Chem. 275, 34213-34223). Here, we investigated the mechanisms whereby gangliosides elicit proliferation-coupled signaling in normal human dermal fibroblasts. Incubation of the fibroblasts with G(D1a) enhanced epidermal growth factor (EGF) receptor autophosphorylation and Ras and MAPK activation in a dose-dependent manner. Exposure of the cells to G(D1a) also enhanced the phosphorylation of Elk-1 by the activated MAPK. Brief pretreatment of the cells with PD98059 blocked the enhancing effect of gangliosides on EGF-induced MAPK activation. In the absence of serum and growth factors, G(D1a) incubation induced phosphorylation of Src kinase, Ras activation, and phosphorylation of MAPK and Elk-1 in a dose-dependent manner. The activation of Src kinase was confirmed by enhanced Src kinase activity. Brief treatment of the cells with PP1 blocked the activation of Src kinase and MAPK. Again, PD98059 treatment inhibited ganglioside-elicited MAPK phosphorylation. Among the gangliosides tested, G(D1a), was the most active molecule, whereas lactosylceramide was the least active one, indicating relative structural specificity of the ganglioside action. In conclusion, gangliosides promote fibroblast proliferation through enhancement of growth factor signaling and activation of Src kinase.