Protein phosphatase-2A maintains focal adhesion complexes in keratinocytes and the loss of this regulation in squamous cell carcinomas.

Studies assessed if the serine/threonine protein phosphatase-2A (PP-2A) maintains cytoskeletal integrity of normal keratinocytes and if this differs in malignant cells. Murine and human keratinocyte cell lines contained more PP-2A activity than did the murine SCC VII/SF squamous cell carcinoma cells or primary cultures of human head and neck squamous cell carcinoma (HNSCC) cells. Since tyrosine phosphorylation of the focal adhesion proteins paxillin and FAK is indicative of more stable focal adhesions, cells were immunostained for phosphotyrosine plus either paxillin or FAK, and then examined by confocal microscopy. In non-malignant keratinocytes, phosphotyrosine staining co-localized with paxillin and FAK. This co-localization occurred at the cell periphery in a pattern resembling focal adhesions. In contrast, the co-localization of phosphotyrosine with either paxillin or FAK along the cell periphery was almost absent in the SCC cells or in keratinocytes that were treated with okadaic acid to inhibit PP-2A activity. Consistent with this was a rounded cellular morphology with less extended processes as compared to control keratinocytes. These studies indicate PP-2A maintains the organization and tyrosine-phosphorylated state of the focal adhesion proteins FAK and paxillin, and that the loss of PP-2A activity results in a loss of cytoskeletal organization, as is seen in SCC.