α3β1 integrin promotes keratinocyte cell survival through activation of a MEK/ERK signaling pathway

Inadequate or inappropriate adhesion of epithelial cells to extracellular matrix leads to a form of apoptosis known as anoikis. During various tissue remodelling events, such as wound healing or carcinoma invasion, changes in the physical properties, and/or composition of the extracellular matrix, can lead to anoikis of epithelial cells that lack appropriate receptor-matrix interactions. Laminin-5 is the major ligand for keratinocyte adhesion in the epidermis, and it also promotes keratinocyte survival in vivo and in vitro. Integrins alpha3beta1 and alpha6beta4 are the major receptors for laminin-5; however, specific roles for these integrins in keratinocyte survival have not been determined. In the current study, we exploited keratinocyte cell lines derived from wild-type or alpha3 integrin knockout mice to reveal a critical role for alpha3beta1 in protecting keratinocytes from apoptosis upon serum withdrawal. We show that alpha3beta1-mediated adhesion to laminin-5 extracellular matrix inhibits proteolytic activation of caspase-3 and TUNEL-staining, both hallmarks of apoptosis. We also show that alpha3beta1-mediated adhesion activates focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK), and that inhibition of either FAK or ERK signaling leads to apoptosis of keratinocytes attached to laminin-5. alpha6beta4-mediated adhesion to laminin-5 only partially protects cells from apoptosis in the absence of alpha3beta1, and alpha6beta4 is not necessary for cell survival in the presence of alpha3beta1. These results suggest that alpha3beta1 is necessary and sufficient for maximal keratinocyte survival on laminin-5. We propose a model to address the potential importance of alpha3beta1-mediated survival for migrating keratinocytes at the leading edge of a cutaneous wound.