Fibronectin (FN) and its receptors are important regulatory components in tumor cell survival. We have determined that the carboxyl-terminal heparin-binding domain and alternatively spliced V region of FN are important to this process, since a FN miniprotein (V+H-) containing a mutated heparin-binding domain and the V region of FN induces apoptosis of squamous cell carcinoma (SCC) cells. In contrast, the counterpart wildtype protein (V+H+) or other FN miniproteins not containing the V region promote survival of these cells. Furthermore, in SCC cells, the rate of V+H--mediated apoptosis is delayed compared to normal primary keratinocytes. These data suggest that tumorigenicity has enabled the SCC cells to delay their onset of apoptosis in response to an altered matrix, and that the V region and heparin-binding domain of FN regulate survival of these cells. Our preliminary data suggest that the mechanism by which the V+H- protein induces delayed SCC cell apoptosis may involve chondroitin sulfate proteoglycan and integrin receptors, and p53 and c-myc mediated signals. In primary fibroblasts, this apoptotic mechanism is mediated by a chondroitin sulfate proteoglycan, the alpha4 integrin, and by an intriguing new pathway that requires downregulation of p53 and c-myc. In addition, since p53 relocalizes from the nucleus to the cell membrane in this mechanism, and since the integrin-associated signaling molecule focal adhesion kinase (pp125FAK) and c-Jun N-terminal kinase (JNK) are depressed in this pathway, this suggests that p53 may communicate with integrin/FAK generated signals. We posit that SCC cells resist apoptosis in response to an altered FN matrix (V+H-) via cell surface proteoglycan and integrin receptors. This initiates a signal transduction pathway that leads to downregulation of FAK, p53 and c-myc. Secondarily, because of its important role in regulating SCC cell invasion, migration, and apoptosis, we posit that the V region of FN may be important to SCC cell pathogenesis. We will test these hypotheses in the following Specific Aims: (1) Identify the SCC cell-surface receptors involved in delaying apoptosis induced by the V+H- FN protein in these cells and compare these receptors to those present in primary keratinocytes. (2) Examine the SCC cell signaling response involved in delaying apoptosis induced by the V+H- FN protein. (3) Examine the expression of the alternatively spliced V region of FN in low grade and high-grade oral dysplasia and oral cancer. These studies will help explain some of the cell-matrix interactions and signaling mechanisms that regulate tumor cell biology, and may provide insights into the pathogenesis of oral SCC.
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