| Summary: | Aberrant communication between survival signalling- and cell death pathways enables tumour cells to resist anoikis - the subset of apoptosis triggered by loss of cell-extracellular matrix (ECM) contact. Focal adhesion kinase (FAK) has been implicated as the key intermediate in the acquisition of anoikis resistance, as constitutively active FAK rescues epithelial cells from anoikis. Pertinent to anoikis-related regulation, enhanced FAK-dependent signalling has been demonstrated to repress p53- and Fas-mediated apoptosis. Elevated expression of FAK correlates with increased metastasis of human oesophageal squamous cell carcinoma (HOSCC). Furthermore, downregulation of Fas as well as a loss of p53 tumour suppressor function are early events in HOSCC progression. In this study, staurosporine (STS) was used to experimentally induce apoptosis in HOSCC cell lines harbouring either wild type (wt) or mutant (mt) p53-R175H. Dephosphorylation of FAK accompanied STS-mediated FAK cleavage and caspase-3 activation in the wt p53 cell lines. Consistent with the lack of FAK cleavage observed post STS treatment, the mt p53-R175H cell line displayed sustained FAK Tyr397 phosphorylation and persistent integrin β1-activated FAK. However, although the survival signals transduced by integrin-activated FAK are attributed to protein kinase B (PKB) activation, fibronectin-mediated protection to STS-mediated detachment demonstrated in the wt p53 cells lines was independent of pPKB Ser473 phosphorylation. Moreover, the altered regulation of FAK in the mt p53-R175H cell line is not due to the inability of mt p53-R175H to associate with FAK. Constitutive activation of FAK has been previously shown to protect cells from anoikis-mediated activation of caspase-8. Correspondingly, in the mt p53-R175H cell line, delayed caspase-8 activation was accompanied by the maintenance of FAK Tyr397 phosphorylation, integrin β1-associated FAK and a Fas/FAK complex. Thus, the data presented highlight that, by interacting with both major apoptotic regulators, FAK is central to anoikis-related regulation. Moreover, through maintained FAK phosphorylation, the “hot spot” mt p53-R175H may have a significant impact on the survival of tumor cells post cell detachment, by opposing the induction of anoikis.
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