Formation of focal adhesion and spreading of polarized human colon cancer cells in association with tyrosine phosphorylation of paxillin in response to phorbol ester

Regulation of cell adhesion systems is involved in both normal development and the invasive behavior of carcinomas. We examined alterations of cell morphology and adhesion molecules in response to phorbol ester treatment of the SW1116 colon cancer cell line, which forms well-organized dome-like tubular structures in culture. 12-O-Tetradecanoylphorbol-13 acetate (TPA) induced rapid spreading of cancer-cell colonies through formation of focal adhesion and disappearance of adherens junctions. Immunologic analyses demonstrated that tyrosine-phosphorylated proteins were concentrated at focal adhesions, and that tyrosine phosphorylation of two proteins, paxillin and an unidentified 130-kd protein, was significantly increased. Tyrosine phosphorylation of paxillin was detectable within 15 min after TPA treatment, when only lamellipodia had extended from the colony, and in cells treated with blocking antibodies against integrins β1 and β5, which strongly inhibited spreading and disorganization while preserving adherens junctions. The level of paxillin phosphorylation correlated well with the degree of morphologic change induced by low-dose TPA, and the dephosphorylation occurred before reversion of morphology upon removal of TPA. These findings suggest that the TPA signal was transduced to the tyrosine phosphorylation of paxillin strongly associated with formation of focal adhesion, and that this in turn induced dysfunction of the cadherin system and caused spreading and disorganization of the tubular structure. The mechanism responsible for disruption of the cadherin system at adherens junctions was not clear, but the transition of β-catenin into nuclei corresponded to the disappearance of its signal along areas of cell-cell contact. This SW1116 model provides a good system for studying the molecules involved in transient regulation and crosstalk between the cell-cell and cell-substratum adhesion systems, which may explain the mechanism of invasion of well-differentiated human carcinomas.