Differential modulation of basic fibroblast and epidermal growth factor receptor activation by ganglioside GM3 in cultured retinal M ller glia

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Abstract
Polypeptide growth factors and membrane‐bound gangliosides are involved in cell signaling, including that observed in cells of neural origin. To analyze possible interactions between these two systems, we investigated the modulation of short‐ and long‐term responses to basic fibroblast and epidermal growth factor (bFGF and EGF, respectively) in cultured retinal Müller glial cells following experimental modification of their ganglioside composition. These glial cells readily incorporated exogenously administered GM3 ganglioside, which was not substantially metabolized within 24 h. Such treatments significantly inhibited bFGF‐induced DNA replication and cell migration, while having much less effect on analogous EGF‐mediated behaviors. To explore GM3/growth factor interactions further, different aspects of glial metabolism in response to bFGF or EGF stimulation were examined: membrane fluidity, growth factor binding, global and individual changes in growth factor‐induced phosphotyrosine levels, and growth factor‐induced activation of mitogen‐activated protein kinase. GM3 reduced the intensity of immunocytochemical labeling of phosphotyrosine‐containing proteins within bFGF‐stimulated cells and down‐regulated FGF receptor activation and tyrosine phosphorylation of its cellular substrates, whereas similar parameters in EGF‐stimulated cells were much less affected. Hence the data reveal a complex relationship in normal neural cells between polypeptide growth factors and membrane‐bound gangliosides, which may participate in retinal cellular physiology in vivo.