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.