CONSTITUTIVE ACTIVATION OF C-MET IN LIVER METASTATIC B16 MELANOMA-CELLS DEPENDS ON BOTH SUBSTRATE ADHESION AND CELL-DENSITY AND IS REGULATED BY A CYTOSOLIC TYROSINE PHOSPHATASE-ACTIVITY

Serial selection in vivo for liver colonization of B16 murine melanoma cells consistently resulted in cell lines expressing elevated amounts of the hepatocyte growth factor/scatter factor receptor (c-Met), whic h is constitutively activated in the absence of its cognate ligand, In this paper we present evidence suggesting that c-Met constitutive act ivation in liver specific B16 melanoma cells depends on both receptor concentration on the cell surface and a cytosolic tyrosine phosphatase activity. In fact, c-Met constitutive activation is suddenly lost upo n detachment of the cells from the substrate and is dramatically decre ased in adherent cells plated at low density, The loss of tyrosine pho sphorylation of c-Met in suspension appears to depend, at least partly , on an increased cytosolic tyrosine phosphatase activity, Instead, lo wer activation of c-Met at low density mostly results from a decrease in receptor concentration on the membrane, Moreover, we show that c-Me t activation does not occur homogeneously on the surface of adherent c ells, In fact, receptor concentration and activation appear to be high er on the ventral surface (adherent to the substrate) than on the apic al surface, Upon detachment, compartmentalization is lost, leading to a decrease in average receptor density on the plasma membrane and henc e to a lower activation.