RESUMO
Inappropriate activation of epidermal growth factor receptor (EGFR) plays a causal role in many cancers including colon cancer. The activation of EGFR by phosphorylation is balanced by receptor kinase and protein tyrosine phosphatase activities. However, the mechanisms of negative EGFR regulation by tyrosine phosphatases remain largely unexplored. Our previous results indicate that protein tyrosine phosphatase receptor type O (PTPRO) is down-regulated in a subset of colorectal cancer (CRC) patients with a poor prognosis. Here we identified PTPRO as a phosphatase that negatively regulates SRC by directly dephosphorylating Y416 phosphorylation site. SRC activation triggered by PTPRO down-regulation induces phosphorylation of both EGFR at Y845 and the c-CBL ubiquitin ligase at Y731. Increased EGFR phosphorylation at Y845 promotes its receptor activity, whereas enhanced phosphorylation of c-CBL triggers its degradation promoting EGFR stability. Importantly, hyperactivation of SRC/EGFR signaling triggered by loss of PTPRO leads to high resistance of colon cancer to EGFR inhibitors. Our results not only highlight the PTPRO contribution in negative regulation of SRC/EGFR signaling but also suggest that tumors with low PTPRO expression may be therapeutically targetable by anti-SRC therapies.
Assuntos
Neoplasias do Colo/enzimologia , Receptores ErbB/antagonistas & inibidores , Proteínas Tirosina Fosfatases Classe 3 Semelhantes a Receptores/metabolismo , Quinases da Família src/metabolismo , Células CACO-2 , Linhagem Celular Tumoral , Neoplasias do Colo/tratamento farmacológico , Neoplasias do Colo/genética , Neoplasias do Colo/patologia , Fator de Crescimento Epidérmico/farmacologia , Receptores ErbB/metabolismo , Gefitinibe , Células HCT116 , Células HEK293 , Células HT29 , Humanos , Sistema de Sinalização das MAP Quinases , Fosforilação , Inibidores de Proteínas Quinases/farmacologia , Proteínas Proto-Oncogênicas c-cbl/metabolismo , Quinazolinas/farmacologia , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Proteínas Tirosina Fosfatases Classe 3 Semelhantes a Receptores/biossíntese , Proteínas Tirosina Fosfatases Classe 3 Semelhantes a Receptores/genética , Transdução de SinaisRESUMO
The p16INK4a protein is a principal cyclin-dependent kinase inhibitor that decelerates the cell cycle. Abnormally high levels of p16INK4a are commonly observed in human papillomavirus (HPV)-positive head and neck squamous cell carcinomas (HNSCC). We and others found that p16INK4a overexpression is associated with improved therapy response and survival of patients with HNSCC treated with radiotherapy. However, the functional role of p16INK4a in HNSCC remains unexplored. Our results implicate p16INK4a in regulation of homologous recombination-mediated DNA damage response independently from its role in control of the cell cycle. We found that expression of p16INK4a dramatically affects radiation sensitivity of HNSCC cells. p16INK4a overexpression impairs the recruitment of RAD51 to the site of DNA damage in HPV-positive cells by downregulating of cyclin D1 protein expression. Consistent with the in vitro findings, immunostaining of HNSCC patient samples revealed that high levels p16INK4a expression significantly correlated with decreased cyclin D1 expression. In summary, these findings reveal an unexpected function of p16INK4a in homologous recombination-mediated DNA repair response and imply p16INK4a status as an independent marker to predict response of patients with HNSCC to radiotherapy.