PP60(C-SRC) IS A POSITIVE REGULATOR OF GROWTH FACTOR-INDUCED CELL SCATTERING IN A RAT BLADDER-CARCINOMA CELL-LINE

The NBT-II rat carcinoma cell line exhibits two mutually exclusive res ponses to FGF-I and EGF, entering mitosis at cell confluency while und ergoing an epithelium-to-mesenchyme transition (EMT) when cultured at subconfluency. EMT is characterized by acquisition of cell motility, m odifications of cell morphology, and cell dissociation correlating wit h the loss of desmosomes from cellular cortex. The pleiotropic effects of EGF and FGF-1 on NBT-II cells suggest that multiple signaling path ways may be activated. We demonstrate here that growth factor activati on is linked to at least two intracellular signaling pathways. One pat hway leading to EMT involves an early and sustained stimulation of pp6 0(c-src) kinase activity, which is not observed during the growth fact or-induced entry into the cell cycle. Overexpression of normal c-src c auses a subpopulation of cells to undergo spontaneous EMT and sensitiz es the rest of the population to the scattering activity of EGF and FG F-1 without affecting their mitogenic responsiveness. Addition of chol era toxin, a cAMP-elevating agent, severely perturbs growth factor ind uction of EMT without altering pp60(c-src) activation, therefore demon strating that cAMP blockade takes place downstream or independently of pp60(c-src). On the other hand, overexpression of a mutated, constitu tively activated form of pp60(c-src) does not block cell dispersion wh ile strongly inhibiting growth factor-induced entry into cell division . Moreover, stable transfection of a dominant negative mutant of c-src inhibits the scattering response without affecting mitogenesis induce d by the growth factors. Altogether, these results suggest a role for pp60(c-src) in epithelial cell scattering and indicate that pp60(c-src ) might contribute unequally to the two separate biological activities engendered by a single signal.