PROTEIN-TYROSINE-PHOSPHATASE-ALPHA REGULATES SRC FAMILY KINASES AND ALTERS CELL-SUBSTRATUM ADHESION

The roles of protein-tyrosine phosphatases (PTPs) in processes such as cell growth and adhesion are poorly understood. To explore the abilit y of specific PTPs to regulate cell signaling pathways initiated by st imulation of growth factor receptors, we expressed the receptor-like P TP, PTP alpha, in A431 epidermoid carcinoma cells. These cells express high levels of the epidermal growth factor (EGF) receptor and prolife rate in response to the autocrine production of transforming growth fa ctor-alpha. Conversely, EGF stimulation of A431 cells in vitro leads t o growth inhibit-ion and triggers the rapid detachment of these cells from the substratum. Although PTP alpha expression did not alter the g rowth characteristics of either unstimulated or EGF-stimulated cells, this phosphatase was associated with increased cell-substratum adhesio n. Furthermore, PTP alpha-expressing A431 cells were strikingly resist ant to EGF-induced cell rounding. Overexpression of PTP alpha in A431 cells was associated with the dephosphorylation/activation of specific Src family kinases, suggesting a potential mechanism for the observed alteration in A431 cell-substratum adhesion. Src kinase activation wa s dependent on the D1 catalytic subunit of PTP alpha, and there was ev idence of association between PTP alpha and Src kinase(s). PTP alpha e xpression also led to increased association of Src kinase with the int egrin-associated focal adhesion kinase, pp125(FAK). I,addition, paxill in, a Src and/or pp125(FAK) substrate, displayed increased levels of t yrosine phosphorylation in PTP alpha-expressing cells and was associat ed with elevated amounts of Csk. In view of these alterations in focal adhesion-associated molecules in PTP alpha-expressing A431 cells, as well as the changes in adhesion demonstrated by these cells, we propos e that PTP alpha may have a role in regulating cell-substratum adhesio n.