RESUMO
Although the small GTPase Ran is best known for its roles in nucleocytoplasmic transport, mitotic spindle assembly, and nuclear envelope formation, recent studies have demonstrated the overexpression of Ran in multiple tumor types and that its expression is correlated with a poor patient prognosis, providing evidence for the importance of this GTPase in cell growth regulation. Here we show that Ran is subject to growth factor regulation by demonstrating that it is activated in a serum-dependent manner in human breast cancer cells and, in particular, in response to heregulin, a growth factor that activates the Neu/ErbB2 tyrosine kinase. The heregulin-dependent activation of Ran requires mTOR (mammalian target of rapamycin) and stimulates the capped RNA binding capability of the cap-binding complex in the nucleus, thus influencing gene expression at the level of mRNA processing. We further demonstrate that the excessive activation of Ran has important consequences for cell growth by showing that a novel, activated Ran mutant is sufficient to transform NIH-3T3 cells in an mTOR- and epidermal growth factor receptor-dependent manner and that Ran-transformed cells form tumors in mice.
Assuntos
Neoplasias da Mama/enzimologia , Núcleo Celular/enzimologia , Transformação Celular Neoplásica/metabolismo , Neuregulina-1/metabolismo , Proteína ran de Ligação ao GTP/metabolismo , Animais , Neoplasias da Mama/genética , Linhagem Celular Tumoral , Núcleo Celular/genética , Transformação Celular Neoplásica/genética , Ativação Enzimática/efeitos dos fármacos , Feminino , Humanos , Peptídeos e Proteínas de Sinalização Intracelular/genética , Peptídeos e Proteínas de Sinalização Intracelular/metabolismo , Camundongos , Mutação , Células NIH 3T3 , Neuregulina-1/farmacologia , Proteínas Serina-Treonina Quinases/genética , Proteínas Serina-Treonina Quinases/metabolismo , Proteínas de Ligação ao Cap de RNA/genética , Proteínas de Ligação ao Cap de RNA/metabolismo , Processamento Pós-Transcricional do RNA/efeitos dos fármacos , Processamento Pós-Transcricional do RNA/genética , RNA Mensageiro/biossíntese , RNA Mensageiro/genética , Receptor ErbB-2/genética , Receptor ErbB-2/metabolismo , Serina-Treonina Quinases TOR , Proteína ran de Ligação ao GTP/genéticaRESUMO
Opposing cellular responses are typically regulated by distinct sets of genes. However, tissue transglutaminase (TGase) provides an interesting example of a single gene product that has been implicated both in affording protection against cellular insults as well as in promoting cell death. Here, we shed some light on how these conflicting activities might be manifested by demonstrating that alternative transcripts of TGase differentially affect cell viability. We show that although the full-length TGase protein affords strong protection against cell death signals, a shorter version of TGase that is truncated at the 3' end, and thus called TGase-short (TGase-S), is cytotoxic. The apoptotic activity of TGase-S is not dependent on its transamidation activity because the mutation of a cysteine residue that is essential for catalyzing this reaction does not compromise the ability of TGase-S to induce cell death. Intriguingly, TGase-S undergoes inappropriate oligomer formation in cells before cell death, suggesting a novel mechanism for the apoptotic effects of this protein.