Role of β4 integrin phosphorylation in human invasive squamous cell carcinoma: regulation of hemidesmosome stability modulates cell migration

Hemidesmosomes (HDs) are multiprotein structures that anchor epithelia to the basement membrane. During squamous cell carcinoma (SCC) invasion there is a reduction in the number of HDs, which may facilitate dissemination. Mechanisms of HD disassembly are incompletely understood. Previous work has shown that EGF-induced phosphorylation of the β4 integrin on three of its serines, S(1356)S(1360)S(1364), can induce HD disassembly in normal cells. Here we examine the role of β4 integrin serine phosphorylation in SCC. We have found that around 60% of invasive cutaneous SCC show increased β4 phosphorylation on S(1356) when compared to carcinoma in situ or normal tissue. To assess mechanisms by which SCC increases β4 phosphorylation we performed in vitro analyses. Compared to keratinocytes, SCC cells showed increased levels of S(1356) phosphorylation in the absence of EGF, correlating with reduced HD-like structures. In addition, phospho-S(1356) signal was largely segregated from other HD components. EGFR and PKC inhibitors inhibited basal levels of S(1356) phosphorylation in SCC, suggesting that cells use intrinsic mechanisms to activate the EGF signaling pathway to induce β4 phosphorylation. Moreover, these inhibitors stabilized HD-like structures in SCC cells and reduced their migratory ability. Mutation of S(1356)S(1360)S(1364) in SCC cells to non-phosphorylatable alanines stabilized HD-like structures and substantially reduced migration, while mutation into phosphorylation mimicking aspartate reduced HD-like structures but had no effect on migration, suggesting that serine phosphorylation function is releasing anchorage rather than promoting migration. Altogether these results suggest that β4 serine phosphorylation may have an important role during SCC invasion by destabilizing HDs and facilitating migration.