TYROSINE PHOSPHORYLATION AND SRC FAMILY KINASES CONTROL KERATINOCYTE CELL-CELL ADHESION

In their progression from the basal to upper differentiated layers of the epidermis, keratinocytes undergo significant structural changes, i ncluding establishment of close intercellular contacts. An important b ut so far unexplored question is how these early structural events are related to the biochemical pathways that trigger differentiation. We show here that beta-catenin, gamma-catenin/plakoglobin, and p120-Cas a re all significantly tyrosine phosphorylated in primary mouse keratino cytes induced to differentiate by calcium, with a time course similar to that of cell junction formation. Together with these changes, there is an increased association of alpha-catenin and p120-Cas with E-cadh erin, which is prevented by tyrosine kinase inhibition. Treatment of E -cadherin complexes with tyrosine-specific phosphatase reveals that th e strength of alpha-catenin association is directly dependent on tyros ine phosphorylation. In parallel with the biochemical effects, tyrosin e kinase inhibition suppresses formation of cell adhesive structures, and causes a significant reduction in adhesive strength of differentia ting keratinocytes, The Fyn tyrosine kinase colocalizes with E-cadheri n at the cell membrane in calcium-treated keratinocytes, Consistent wi th an involvement of this kinase, fyn-deficient keratinocytes have str ongly decreased tyrosine phosphorylation levels of beta- and gamma-cat enins and p120-Cas, and structural and functional abnormalities in cel l adhesion similar to those caused by tyrosine kinase inhibitors. Wher eas skin of fyn -/- mice appears normal, skin of mice with a disruptio n in both the fyn and sr e genes shows intrinsically reduced tyrosine phosphorylation of beta-catenin, strongly decreased p120-Cas levels, a nd important structural changes consistent with impaired keratinocyte cell adhesion. Thus, unlike what has been proposed for oncogene-transf ormed or mitogenically stimulated cells, in differentiating keratinocy tes tyrosine phosphorylation plays a positive role in control of cell adhesion, and this regulatory function appears to be important both in vitro and in vivo.