Resumen de Papel de c-abl, mkk3 y mkk6 en la activación de la ruta de p38mapk en respuesta a cisplatino: implicación en terapía de cáncer
Although chemotherapy has revolutionized the treatment of haematological tumours, in many common solid tumours the success has been limited. The main reason for the limitations is resistance to the drug. The present thesis has been focus on the role that proteins such as p38MAPK, MKK3/6 and c-Abl have in the cellular response to Cisplatin, which is among the most effective cytotoxic agents used in solid tumours treatments. Activation of p38MAPK is a critical requisite for the therapeutic activity of this antitumor agent. According to this, a growing body of evidences supports the role of c-Abl as a major determinant of p38MAPK activation, especially in response to genotoxic stress when triggered by Cisplatin. For this reason, we decided to study the molecular basis for the link between c-Abl tyrosine kinase activity and the p38MAPK signalling pathway and the implications in the cellular response to Cisplatin. The results we have obtained demonstrate that p38MAPK activation in response to Cisplatin does not require the tyrosine kinase activity of c-Abl. The effect of c-Abl on the p38MAPK pathway is not mediated by conventional mechanisms, such as the direct phosphorylation of upstream components of the MKK6-p38MAPK kinase cascade. Indeed, c-Abl can activate the p38MAPK signalling pathway by a mechanism that is independent of its tyrosine kinase activity, but that instead involves the ability of c-Abl to increase the stability of MKK6. Similar results were obtained in Chronic Myeloid Leukaemia-derived cell lines, in which a chimeric Bcr/Abl protein mimics the effects of c-Abl overexpression on p38MAPK activation. These findings may explain why a clinically used c-Abl inhibitor, Imatinib Mesylate, fails to inhibit the p38MAPK pathway alone or in combination with Cisplatin, and provide evidence of a novel signalling mechanism in which these antitumor agents act. Therefore, our data provides evidences for a new model in which c-Abl can exert biochemical effects due to its expression level rather than its tyrosine kinase activity, which has direct implications in cancer therapy. Although the over-expression of c-Abl in a cell line is able to regulate MKK6 levels and trigger to the activation of p38MAPK pathway, in Cisplatin-mediated p38MAPK activation, c-Abl/MKK6 relationship seems not to be critical. Therefore, in the second part of this work, we have gone into this chemotherapeutic effect on the p38MAPK pathway in depth. We chose two pathologies in which Cisplatin was the most used treatment and in which no alteration of c-Abl had been described, as non-small lung cancer (NSCLC) and head and neck squamous cell carcinoma (HNSCC). In this regard, we evaluated the p38MAPK activation status in response to Cisplatin in a wide number of NSCLC and HNSCC cell lines and we correlated this status with the response to Cisplatin treatment.
Only sensitive cell lines were able to induce a marked phosphorylation on p38MAPK, although little difference was observed regarding resistant cell lines. This may be attributed to the fact that resistant cell lines already have a high basal activation of the protein. The results obtained in this second part also suggest that MKK3 may be the responsible of that high basal activation of p38MAPK and that MKK6 seems not to be necessary in this case. Nevertheless, MKK6 may be used as a universal prognosis marker to the response to Cisplatin, since sensitive cell lines show high levels of this MAPKK and cell lines with intrinsic resistant to the drug show very low expression levels.
