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 epidermal growth factor (EGF)-induced phosphorylation of the beta 4 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 beta 4 integrin serine phosphorylation in SCC. We have found that around 60% of invasive cutaneous SCC show increased beta 4 phosphorylation on S(1356) when compared with carcinoma in situ or normal tissue. To assess the mechanisms by which SCC increases beta 4 phosphorylation, we performed in vitro analyses. Compared with 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. Epidermal growth factor receptor 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 beta 4 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 beta 4 serine phosphorylation may have an important role during SCC invasion by destabilizing HDs and facilitating migration. Laboratory Investigation (2011) 91, 1414-1426; doi:10.1038/labinvest.2011.104; published online 18 July 2011