Alterations in focal adhesion kinase activity and associated proteins during malignant conversion of mouse keratinocytes.

Focal adhesion kinase (pp125FAK) has well-established functions in the attachment and growth of cells in culture and has been implicated as a marker of malignant progression in human tumors. To evaluate its role in the metastatic conversion of mouse skin tumors, pp125FAK activity and protein expression were examined in normal and transformed keratinocyte cell lines. Malignant mouse keratinocyte lines exhibited a reproducible increase in the specific activity of pp125FAK compared with that of nontransformed control cells. An increase in pp125FAK activity was not observed in papilloma-derived keratinocytes, indicating that this response correlated with malignant progression of cells and not cell transformation per se. Immune complex kinase assays and metabolic labeling with [32P]orthophosphate also revealed the specific loss of pp125FAK-associated proteins in the metastatic keratinocytes. Furthermore, immunocytochemical examination revealed an altered distribution of pp125FAK in the cells with malignant potential compared with normal and papilloma-inducing keratinocytes. The cells with malignant potential also exhibited reduced levels of paxillin and integrin beta1 as well as altered distribution of paxillin, reinforcing the notion that specific changes in the composition of focal adhesions contribute to the malignant conversion of mouse keratinocytes.