Ligation of β₄ integrins activates PKB/Akt and ERK1/2 by distinct pathways-relevance of the keratin filament

In normal epithelial cells hemidesmosomes mediate stable adhesion to the underlying basement membrane. In carcinoma cells a functional and spatial dissociation of the hemidesmosomal complex is observed stimulating the hypothesis that the β₄ integrin may trigger essential signalling cascades determining cell fate. In the present study we dissected the signalling pathways giving rise to PKB/Akt and ERK1/2 activation in response to β₄ ligation by 3E1. It was found that the activation of PKB/Akt is sensitive towards alterations of the keratin filament as demonstrated by using KEB-7 cells that carry a keratin mutation typical for epidermolysis bullosa simplex. Similar results were achieved by chemically induced keratin aggregations. Of note, the signalling to ERK1/2 was not affected. ERK1/2 activation utilizes an EGF-R transactivation mechanism as shown by dominant-negative expression experiments and also by treatment with a specific inhibitor (AG1478). Downstream from the EGF-R the activation of ERK1/2 takes the prototypical signalling cascade via Shc, Ras and Raf-1 as demonstrated by dominant-negative expression experiments. Taken together our data define a new model of β₄-dependent PKB/Akt and ERK1/2 activation demonstrating the keratin filament as a structure necessary in signal transmission.