STAT3 signaling is induced by intercellular adhesion in squamous cell carcinoma cells

The signal transducer and activator of transcription-3 (STAT3) frequently activated during tumor progression has been linked to enhanced cell growth. In squamous cell carcinoma of the head and neck (HNSCC), STAT3 signaling has been shown to inhibit apoptosis and induce a more aggressive phenotype through the activation of specific signaling pathways. In the present study, we have examined the potential mechanism by which cell-cell contact initiates STAT3 activation. Using a panel of HNSCC cell lines, Ca(+2)-dependent cell-cell adhesion and adherens junction formation in multicellular aggregates triggered phosphorylation of STAT3-Y705 and STAT1-Y701. This intercellular adhesion-induced STAT3 activation was mediated by JAK and Src signaling and partially by EGFR signaling. In addition, immunolocalization studies revealed initial formation of phosphorylated STAT3-Y705 at nascent E-cadherin cell junctions with eventual translocation to the nucleus in cell aggregates. Adhesion-mediated STAT activation in monolayer and cell aggregate cultures required functional E-cadherin. These results indicate that, in HNSCC cells, cadherin-mediated intercellular adhesion induces STAT signaling that may modulate cell survival and resistance to apoptosis during tumor progression.