Ligation of the β4 Integrin Triggers Adhesion Behavior of Human Keratinocytes by an “Inside-out” Mechanism

Carcinogenesis is considered as a multistep process involving functional changes in the hemidesmosomal organization. In normal skin keratinocytes, expression of the α 6 β 4 integrin is restricted to the proliferative basal layer and mediates stable adhesion to the underlying basement membrane. Observations in carcinoma cells show a functional and spatial dissociation of the α 6 β 4 integrin from the hemidesmosomal complex, which stimulates cell migration and, therefore, may contribute to carcinoma invasion. We now have evaluated the adhesion behavior of epithelial cells at different stages of transformation in response to activation of the β 4 integrin. It is demonstrated that ligation of the β 4 integrin augmented adhesion of carcinoma and pre-carcinoma cells to non-modified plastic. In contrast, adhesion behavior of normal human keratinocytes was not influenced by ligation of the β 4 integrin. In order to explain the mechanism of β 4 -mediated adhesion, the hypothesis of an "inside-out" activation of integrins was tested. Evidence is given that for cells expressing the α 6 β 4 integrin, ligation of the β 4 integrin increased β 1 integrin-mediated adhesion. Furthermore, ligation of the β 4 integrin led to phosphorylation of PKB/Akt at both phosphorylation sites. Functional blocking of PKB/Akt by dominant-negative overexpression decreased cell adhesion in response to β 4 integrin ligation. Taken together, the present data establish a link between the ligation of the β 4 integrin and β 1 integrin-mediated cell adhesion in carcinoma and pre-carcinoma cells. Hence, these findings provide further insight into the conversion processes during carcinogenesis and show the β 4 integrin to be a key regulator of cellular adhesion.