β1 integrins regulate keratinocyte adhesion and differentiation by distinct mechanisms

In keratinocytes, the pi integrins mediate adhesion to the extracellular matrix and also regulate the initiation of terminal differentiation. To explore the relationship between these functions, we stably infected primary human epidermal keratinocytes and an undifferentiated squamous cell carcinoma line, SCC4, with retroviruses encoding wild-type and mutant chick pi integrin subunits. We examined the ability of adhesion-blocking chick pi-specific antibodies to inhibit suspension-induced terminal differentiation of primary human keratinocytes and the ability of the chick pi subunit to promote spontaneous differentiation of SCC4. A D154A point mutant clustered in focal adhesions but was inactive in the differentiation assays, showing that differentiation regulation required a functional ligand-binding domain. The signal transduced by pi integrins in normal keratinocytes was "do not differentiate" (transduced by Ligand-occupied receptors) as opposed to "do differentiate" (transduced by unoccupied receptors), and the signal depended on the absolute number, rather than on the proportion, of occupied receptors. Single and double point mutations in cyto-2 and -3, the NPXY motifs, prevented focal adhesion targeting without inhibiting differentiation control. However, deletions in the proximal part of the cytoplasmic domain, affecting cyto-l, abolished the differentiation-regulatory ability of the pi subunit. We conclude that distinct signaling pathways are involved in pi integrin-mediated adhesion and differentiation control in keratinocytes.