RESUMO
The HMGA2 protein belongs to the HMGA family of architectural transcription factors, which play an important role in chromatin organization. HMGA proteins are overexpressed in several experimental and human tumors and have been implicated in the process of neoplastic transformation. Hmga2 knockout results in the pygmy phenotype in mice and in a decreased growth rate of embryonic fibroblasts, thus indicating a role for HMGA2 in cell proliferation. Here we show that HMGA2 associates with the E1A-regulated transcriptional repressor p120(E4F), interfering with p120(E4F) binding to the cyclin A promoter. Ectopic expression of HMGA2 results in the activation of the cyclin A promoter and induction of the endogenous cyclin A gene. In addition, chromatin immunoprecipitation experiments show that HMGA2 associates with the cyclin A promoter only when the gene is transcriptionally activated. These data identify the cyclin A gene as a cellular target for HMGA2 and, for the first time, suggest a mechanism for HMGA2-dependent cell cycle regulation.
Assuntos
Ciclina A/genética , Proteína HMGA2/metabolismo , Proteínas E4 de Adenovirus/química , Proteínas E4 de Adenovirus/genética , Proteínas E4 de Adenovirus/metabolismo , Animais , Sequência de Bases , Sítios de Ligação , Células CHO , Ciclo Celular , Linhagem Celular , Transformação Celular Neoplásica , Cricetinae , DNA Complementar/genética , Proteína HMGA2/genética , Humanos , Camundongos , Modelos Biológicos , Células NIH 3T3 , Regiões Promotoras Genéticas , Proteínas Repressoras/química , Proteínas Repressoras/genética , Proteínas Repressoras/metabolismo , Ativação Transcricional , Dedos de ZincoRESUMO
Programmed cell death is characterized by posttranslational modifications of a limited and specific set of nuclear proteins. We demonstrate that during apoptosis of different types of tumor cells there is a monomethylation of the nuclear protein HMGA1a that is associated to its previously described hyperphosphorylation/dephosphorylation process. HMGA1a methylation is strictly related to the execution of programmed cell death and is a massive event that involves large amounts of the protein. In some tumor cells, HMGA1a protein is already methylated to an extent that depends on cell type. The degree of methylation in any case definitely increases during apoptosis. In the studied cell systems (human leukaemia, human prostate tumor, and rat thyroid transformed cells) among the low-molecular-mass HMG proteins, only HMGA1a was found to be methylated. A tryptic digestion map of HPLC-purified HMGA1a protein showed that methylation occurs at arginine 25 in the consensus G(24)R(25)G(26) that belongs to one of the DNA-binding AT-hooks of the protein. An increase of HMGA1a methylation could be related to heterochromatin and chromatin remodeling of apoptotic cells.