RESUMEN
A truncated isoform of C/EBPbeta, C/EBPbeta-LIP, is required for liver proliferation. This isoform is expressed at high levels in proliferating liver and in liver tumors. However, high levels of C/EBPbeta-LIP are also observed in non-proliferating livers during acute phase response (APR). In this paper we present mechanisms by which liver regulates activities of C/EBPbeta-LIP. We found that calmodulin (CaM) inhibits the ability of C/EBPbeta-LIP to promote liver proliferation during APR through direct interactions. This activity of CaM is under negative control of Ca(2+), which is reduced in nuclei of livers with APR, whereas it is increased in nuclei of proliferating livers. A mutant CaM, which does not interact with C/EBPbeta-LIP, also fails to inhibit the growth promotion activity of C/EBPbeta-LIP. Down-regulation of CaM in livers of LPS-treated mice causes liver proliferation via activation of C/EBPbeta-LIP. Overexpression of C/EBPbeta-LIP above levels of CaM also initiates liver proliferation in LPS-treated mice. In addition, CaM regulates transcriptional activity of another isoform of C/EBPbeta, C/EBPbeta-LAP, and might control liver biology through the regulation of both isoforms of C/EBPbeta. In searching for molecular mechanisms by which C/EBPbeta-LIP promotes cell proliferation, we found that C/EBPbeta-LIP releases E2F.Rb-dependent repression of cell cycle genes by a disruption of E2F1.Rb complexes and by a direct interaction with E2F-dependent promoters. CaM inhibits these growth promotion activities of C/EBPbeta-LIP and, therefore, supports liver quiescence. Thus, our findings discover a new pathway of the regulation of liver proliferation that involves calcium-CaM signaling.
Asunto(s)
Proteína beta Potenciadora de Unión a CCAAT/metabolismo , Calmodulina/metabolismo , Hígado/citología , Hígado/metabolismo , Reacción de Fase Aguda/metabolismo , Reacción de Fase Aguda/patología , Animales , Calcio/metabolismo , Proteínas de Ciclo Celular/genética , Línea Celular , Núcleo Celular/efectos de los fármacos , Núcleo Celular/metabolismo , Proliferación Celular/efectos de los fármacos , Regulación hacia Abajo/efectos de los fármacos , Factores de Transcripción E2F/metabolismo , Humanos , Lipopolisacáridos/farmacología , Hígado/efectos de los fármacos , Ratones , Regiones Promotoras Genéticas/genética , Unión Proteica , Proteína de Retinoblastoma/metabolismo , Transactivadores/genéticaRESUMEN
Differentiation of myocytes is impaired in patients with myotonic dystrophy type 1, DM1. CUG repeat binding protein, CUGBP1, is a key regulator of translation of proteins that are involved in muscle development and differentiation. In this paper, we present evidence that RNA-binding activity of CUGBP1 and its interactions with initiation translation complex eIF2 are differentially regulated during myogenesis by specific phosphorylation and that this regulation is altered in DM1. In normal myoblasts, Akt kinase phosphorylates CUGBP1 at Ser28 and increases interactions of CUGBP1 with cyclin D1 mRNA. During differentiation, CUGBP1 is phosphorylated by cyclinD3-cdk4/6 at Ser302, which increases CUGBP1 binding with p21 and C/EBPbeta mRNAs. While cyclin D3 and cdk4 are elevated in normal myotubes; DM1 differentiating cells do not increase these proteins. In normal myotubes, CUGBP1 interacts with cyclin D3/cdk4/6 and eIF2; however, interactions of CUGBP1 with eIF2 are reduced in DM1 differentiating cells and correlate with impaired muscle differentiation in DM1. Ectopic expression of cyclin D3 in DM1 cells increases the CUGBP1-eIF2 complex, corrects expression of differentiation markers, myogenin and desmin, and enhances fusion of DM1 myoblasts. Thus, normalization of cyclin D3 might be a therapeutic approach to correct differentiation of skeletal muscle in DM1 patients.
Asunto(s)
Diferenciación Celular/fisiología , Ciclinas/metabolismo , Desarrollo de Músculos/fisiología , Músculo Esquelético , Mioblastos/fisiología , ARN Mensajero/metabolismo , Proteínas de Unión al ARN/metabolismo , Animales , Proteína beta Potenciadora de Unión a CCAAT/genética , Proteína beta Potenciadora de Unión a CCAAT/metabolismo , Proteínas CELF1 , Fusión Celular , Línea Celular , Ciclina D1/genética , Ciclina D1/metabolismo , Ciclina D3 , Quinasa 4 Dependiente de la Ciclina/genética , Quinasa 4 Dependiente de la Ciclina/metabolismo , Inhibidor p21 de las Quinasas Dependientes de la Ciclina/genética , Inhibidor p21 de las Quinasas Dependientes de la Ciclina/metabolismo , Ciclinas/genética , Humanos , Ratones , Músculo Esquelético/citología , Músculo Esquelético/fisiología , Mioblastos/citología , Fosforilación , Proteínas Proto-Oncogénicas c-akt/genética , Proteínas Proto-Oncogénicas c-akt/metabolismo , Proteínas de Unión al ARN/genética , Serina/metabolismo , Transducción de Señal/fisiologíaRESUMEN
Expression of a dominant negative 20-kDa isoform of CCAAT/enhancer-binding protein (C/EBPbeta), LIP, is increased in proliferating livers and in tumor cells. Two RNA-binding proteins, CUGBP1 and calreticulin, have been implicated in the translational regulation of C/EBPbeta. In this paper, we present evidence showing several critical steps by which liver increases translation of LIP after partial hepatectomy. At early stages after partial hepatectomy, liver activates CUGBP1 by a hyperphosphorylation. The activated CUGBP1 binds to the 5' region of C/EBPbeta mRNA and replaces calreticulin, which partially represses translation of C/EBPbeta in quiescent livers. The hyperphosphorylated CUGBP1 also interacts with the alpha and beta subunits of initiation factor eIF2. Our data demonstrate that the interaction of CUGBP1 with the eIF2alpha enhances the association of CUGBP1 with ribosomes and correlates with increased translation of LIP in the liver after partial hepatectomy. Our data support the hypothesis that CUGBP1 increases translation of LIP by the interaction with the eIF2alpha subunit. This facilitates subsequent recruitment of larger numbers of ribosomes to initiate translation of LIP.
