RESUMO
Cell signaling pathways and protein translation are crucial for understanding malignant transformation. 4E-BP1 and the eIF4F complex regulate cap-dependent translation. We investigated how 4E-BP1 and eIF4E phosphorylation status affects in vitro and in vivo cell proliferation in a breast cancer model. Cells from 2 breast carcinoma lines (MDA-MB 231 and MDA-MB 468) and human fibroblasts (IMR90 cells) were infected in vitro with a retrovirus carrying a wild-type 4E-BP1 or a mutant 4E-BP1 unable to hyperphosphorylate. Overexpression of the mutant 4E-BP1 induced a significant decrease in cell proliferation in IMR90 and MDA-MB 468 cells, but not in MDA-MB 231 cells. A correlation was observed between baseline-phosphorylated eIF4E (p-eIF4E) levels and sensitivity to 4E-BP1 transduction. By co-immunoprecipitation, p-eIF4E seemed to present lower affinity for 4E-BP1 than total eIF4E in MDA-MB 468 cells. After treatment with CGP57380, the MAP kinase-interacting kinase (MNK) inhibitor, downregulation of p-eIF4E levels was associated with an increase of E-cadherin and ß-catenin protein expression. These results provide evidence that 4E-BP1 transduction leads to a decrease in cell proliferation, and that high p-eIF4E levels may counteract the suppressor effect of 4E-BP1. We propose that high p-4E-BP1 and p-eIF4E levels are central factors in cell signaling and reflect the oncogenic potential of cell signaling pathways in breast cancer.
Assuntos
Proteínas Adaptadoras de Transdução de Sinal/metabolismo , Neoplasias da Mama/metabolismo , Fator de Iniciação 4E em Eucariotos/metabolismo , Fosfoproteínas/metabolismo , Proteínas Adaptadoras de Transdução de Sinal/genética , Animais , Neoplasias da Mama/genética , Proteínas de Ciclo Celular , Linhagem Celular Tumoral , Proliferação de Células , Modelos Animais de Doenças , Fator de Iniciação 4E em Eucariotos/genética , Feminino , Expressão Gênica , Humanos , Camundongos , Camundongos Nus , Complexos Multiproteicos/metabolismo , Proteínas Mutantes/genética , Proteínas Mutantes/metabolismo , Fosfoproteínas/genética , Fosforilação , Ligação Proteica , Transdução de Sinais , Carga Tumoral/genética , Ensaios Antitumorais Modelo de XenoenxertoRESUMO
p90 Ribosomal S6 kinase (RSK) 4 is a serine-threonine kinase that belongs to the p90RSK family. RSK4 has been proposed as a tumor suppressor gene, related with anti-invasive activity, inhibition of the RAS-mitogen-activated protein kinase (MAPK) pathway and induction of senescence. Despite the related findings, little is known about RSK4 effectors. In human tumors, RSK4 is downregulated even in some benign lesions, such as colon adenomas and breast papillomas, indicating that RSK4 inhibition could be an early event in cellular transformation. For cells to achieve immortality and transformation, it is believed that they must override senescence. In the present study, we found that when RSK4 is inhibited in vitro using short hairpin RNA technology, cells can bypass stress-induced senescence and oncogene-induced senescence: normal human fibroblasts grew following oxidative stress, induction of DNA damage and KRAS(V12) or BRAF(E600) overexpression. To investigate the RSK4 effectors, we used short hairpin RNA or inhibitor molecules against major senescence mediators. We found that RSK4-induced senescence is mediated through p21, but is independent of p16, p38MAPKs and induction of reactive oxygen species, delimiting RSK4 signaling. These data support the importance of RSK4 for regulating senescence and indicate that downregulation of this kinase could be an important element in facilitating cell transformation.
Assuntos
Senescência Celular , Oncogenes , Proteínas Quinases S6 Ribossômicas 90-kDa/antagonistas & inibidores , Estresse Fisiológico , Células Cultivadas , Inibidor de Quinase Dependente de Ciclina p21/antagonistas & inibidores , Inibidor de Quinase Dependente de Ciclina p21/fisiologia , Humanos , Proteínas Proto-Oncogênicas B-raf/fisiologia , Proteínas Quinases S6 Ribossômicas 90-kDa/fisiologia , Proteína Supressora de Tumor p53/fisiologia , Proteínas Quinases p38 Ativadas por Mitógeno/fisiologiaRESUMO
PURPOSE: The control of senescence and its biochemical pathways is a crucial factor for understanding cell transformation. In a large RNA interference screen, the RSK4 gene was found to be related to p53-dependent arrest. The purpose of the present study was to investigate the potential role of RSK4 as a tumor suppressor gene. EXPERIMENTAL DESIGN: RSK4 expression was determined by quantitative real-time PCR and immunoblot in 30 colon and 20 renal carcinomas, and in 7 colon adenomas. Two HCT116 colon carcinoma cell lines (p53 wt and p53 null), IMR90 human fibroblasts, and E1A-expressing IMR90 cells were infected with RSK4 cDNA and/or shRNA. RSK4 expression levels were analyzed in HCT116 p53 wt or p53 null and IMR90 after senescence induction by quantitative real-time PCR and Western blot. RESULTS: The RSK4 gene was down-regulated in 27 of 30 colon carcinomas (P < 0.001), 16 of 20 renal cell carcinomas (P < 0.01), and 6 of 7 colon adenomas (P < 0.01). In vitro overexpression of RSK4 induced cell arrest and senescence features in normal fibroblasts and malignant colon carcinoma cell lines. Interestingly, in these cell lines RSK4 mRNA levels were increased both in replicative and stress-induced senescence. Moreover, IMR90 partially immortalized by RSK4 shRNA and HCT116 with this short hairpin RNA were more resistant to cisplatin treatment. Finally, cells expressing E1A or Rb short interfering RNA were resistant to RSK4-mediated senescence. CONCLUSION: These results support the concept that RSK4 may be an important tumor suppressor gene by modulating senescence induction and contributing to cell proliferation control in colon carcinogenesis and renal cell carcinomas.
