Overexpression of NeuAcalpha2-3Galbeta1-4Glcbeta1-Cer (GM3), a major
ganglioside of cutaneous
tumor cell membranes, inhibits
ligand-dependent and
ligand-independent activation of the
epidermal growth factor (
EGF) receptor in normal and neoplastic epithelial cells. This leads to the suppression of Ras/
extracellular signal-regulated kinase (ERK) activation and, in the presence of
EGF or
fibronectin, inhibits cell proliferation. However, some
tumor cells show increased levels of GM3, and
vaccines that target GM3 can inhibit the growth of neoplastic cells in vivo, especially
melanomas. We report that in the presence of
urokinase plasminogen activator (uPA), overexpression of GM3 paradoxically increases the proliferation of
carcinoma cells by augmenting ERK-independent p70S6
kinase activation. Functional blockade of uPA
receptor (uPAR) or inhibition of p70S6
kinase, but not inhibition of Ras/ERK signaling, suppresses this GM3-induced stimulation of cell proliferation. The ERK-independent activation of p70S6
kinase involves phosphorylation at threonine-389,
threonine-421/
serine-424, and serine-411 sites with intermediate
phosphatidylinositol 3 kinase and
protein kinase C-zeta activation. These studies implicate
gangliosides as enhancers of uPAR-related signaling and suggest that the response to GM3 depends on the local concentration of uPA. Therapeutic modalities that target or supplement
gangliosides may require concomitant treatment that suppresses EGFR or uPAR signaling, respectively, to control neoplastic cell proliferation.