RESUMEN
Muscle growth results from a set of complex processes including myogenic transcription factor's expression and activity, cell cycle withdrawal, myoblast fusion in myotubes, and acquisition of an apoptosis-resistant phenotype. Myostatin, a member of the TGFbeta family, described as a strong regulator of myogenesis in vivo Nature 387 (1997), 83; FEBS Lett. 474 (2000), 71 is upregulated during in vitro differentiation Biochem. Biophys. Res. Commun. 280 (2001), 561. To improve characterization of myostatin's myogenic influence, we stably transfected vectors expressing myostatin and myostatin antisense in C2C12 myoblasts. Here, we found that myostatin inhibits cell proliferation and differentiation. Our results also indicate that myogenin is an important target of myostatin. In addition, overexpressed but not endogenous myostatin decreases MyoD protein levels and induces changes in its phosphorylation pattern. We also established that myostatin overexpression reduces the frequency of G0/G1-arrested cells during differentiation. Conversely, inhibition of myostatin synthesis leads to enhanced cell cycle withdrawal and consequently stimulates myoblast differentiation. We examined the expression patterns of the pRb, E2F1, p53, and p21 proteins involved in cell cycle withdrawal. We found that myostatin overexpression increases p21 and p53 expression, as it does accumulation of hypophosphorylated Rb. Interestingly, myostatin overexpression strongly reduced low-mitogen-induced apoptosis, whereas antisense expression induced contrary changes. In conclusion, these data show the influence of overexpressed myostatin on myoblast proliferation, differentiation, and apoptosis is extended to endogenous myostatin. Though some differences in overexpression or inhibition of endogenous myostatin were observed, it appears that myogenin and p21 are essential targets of this growth factor.
Asunto(s)
Proteínas de Ciclo Celular , Diferenciación Celular/genética , División Celular/genética , Proteínas de Unión al ADN , Fibras Musculares Esqueléticas/metabolismo , Músculo Esquelético/crecimiento & desarrollo , Mioblastos Esqueléticos/metabolismo , Factor de Crecimiento Transformador beta/deficiencia , Animales , Apoptosis/genética , Línea Celular , Inhibidor p21 de las Quinasas Dependientes de la Ciclina , Ciclinas/genética , Ciclinas/metabolismo , Factores de Transcripción E2F , Factor de Transcripción E2F1 , Regulación del Desarrollo de la Expresión Génica/genética , Genes cdc/fisiología , Interfase/genética , Ratones , Fibras Musculares Esqueléticas/citología , Músculo Esquelético/citología , Músculo Esquelético/metabolismo , Proteína MioD/genética , Proteína MioD/metabolismo , Mioblastos Esqueléticos/citología , Miogenina/genética , Miogenina/metabolismo , Miostatina , Oligodesoxirribonucleótidos Antisentido , Fosforilación , Proteína de Retinoblastoma/genética , Proteína de Retinoblastoma/metabolismo , Factores de Transcripción/genética , Factores de Transcripción/metabolismo , Factor de Crecimiento Transformador beta/genética , Proteína p53 Supresora de Tumor/genética , Proteína p53 Supresora de Tumor/metabolismoRESUMEN
Resistance to the antiestrogen tamoxifen is the main stumbling block for the success of breast cancer therapy. We focused our study on cellular alterations induced by a prolonged treatment with the active tamoxifen metabolite hydroxytamoxifen (OHT). We show that a prolonged OHT treatment (for up to 7 days) led to a progressive increase in the level of phosphorylated p44/42 mitogen activated kinase (MAP kinase) induced by 10(-7) M TPA stimulation, without any significant change in the protein level. This effect was also observed in MCF-7 cells grown first in medium containing dextran-coated charcoal-treated FCS (DCC medium) for 20 days prior to OHT treatment, indicating a specific effect of the antiestrogen and not an effect of estrogen deprivation. It was prevented by cotreatment with estradiol and not observed in the estrogen receptor negative HeLa cell line, suggesting that it was mediated by the estrogen receptor. TPA induced phosphorylation of MEK1/2 was also raised by OHT treatment, without any change in their protein level or Raf-1 and H-Ras levels. When the MCF-7R OHT resistant cell line was grown in antiestrogen containing medium, the level of phosphorylated p44/42 MAP kinase was also high but reversed when the antiestrogen was removed. The 2 other MAP kinase, JNK and P38 pathways were not affected in the same way by OHT treatment. In conclusion, our data reveal that a prolonged OHT treatment, by increasing p44/42 MAPK activity, affects a key step in the growth control of MCF-7 cells, although not sufficiently to overcome the growth inhibitory effect of the drug.