RESUMEN
The autosomal dominant disease tuberous sclerosis (TSC) is caused by mutations in either TSC1 on chromosome 9q34, encoding hamartin, or TSC2 on chromosome 16p13.3, encoding tuberin. TSC is characterized by hamartomas that occur in many organs of affected patients and these have been considered to likely result from defects in proliferation control. Although the true biochemical functions of the two TSC proteins have not been clarified, a series of independent investigations demonstrated that modulated hamartin or tuberin expression cause deregulation of proliferation/cell cycle in human, rodent and Drosophila cells. In support of tuberin acting as a tumor suppressor, ectopic overexpression of TSC2 has been shown to decrease proliferation rates of mammalian cells. Furthermore, overexpression of TSC2 has been demonstrated to trigger upregulation of the cyclin-dependent kinase inhibitor p27. We report that three different naturally occurring and TSC causing mutations within the TSC2 gene eliminate neither the anti-proliferative capacity of tuberin nor tuberin's effects on p27 expression. For the first time these data provide strong evidence that deregulation of proliferation and/or upregulation of p27 are not likely to be the primary/only mechanisms of hamartoma development in TSC. These results demand reassessment of previous hypotheses of the pathogenesis of TSC.
Asunto(s)
Proteínas de Microfilamentos/biosíntesis , Proteínas Musculares , Mutación Missense , Proteínas Represoras/genética , Esclerosis Tuberosa/etiología , Animales , División Celular , ADN Complementario/análisis , Humanos , Ratas , Esclerosis Tuberosa/genética , Proteína 2 del Complejo de la Esclerosis Tuberosa , Proteínas Supresoras de TumorRESUMEN
Two genes, TSC1 and TSC2, have been shown to be responsible for tuberous sclerosis (TSC). The detection of loss of heterozygosity of TSC1 or TSC2 in hamartomas, the growths characteristically occurring in TSC patients, suggested a tumor suppressor function for their gene products hamartin and tuberin. Studies analyzing ectopically modulated expression of TSC2 in human and rodent cells together with the finding that a homolog of TSC2 regulates the Drosophila cell cycle suggest that TSC is a disease of proliferation/cell cycle control. We discuss this question including very recent data obtained from analyzing mice expressing a modulated TSC2 transgene, and from studying the effects of deregulated TSC1 expression. Elucidation of the cellular functions of these proteins will form the basis of a better understanding of how mutations in these genes cause the disease and for the development of new therapeutic strategies.
Asunto(s)
Ciclo Celular/genética , División Celular/genética , Proteínas de Drosophila , Genes Supresores de Tumor , Proteínas/fisiología , Proteínas Represoras/fisiología , Esclerosis Tuberosa/genética , Proteínas Supresoras de Tumor , Transporte Activo de Núcleo Celular , Animales , Carcinoma de Células Renales/genética , Compartimento Celular , Proteínas de Ciclo Celular/genética , Proteínas de Ciclo Celular/metabolismo , Proteínas de Ciclo Celular/fisiología , Tamaño de la Célula/genética , Cromosomas Humanos Par 16/genética , Cromosomas Humanos Par 9/genética , Inhibidor p27 de las Quinasas Dependientes de la Ciclina , Drosophila melanogaster/citología , Drosophila melanogaster/genética , Genes Dominantes , Hamartoma/genética , Humanos , Proteínas de Insectos/genética , Proteínas de Insectos/fisiología , Neoplasias Renales/genética , Pérdida de Heterocigocidad , Sustancias Macromoleculares , Proteínas/genética , Ratas , Ratas Mutantes , Proteínas Represoras/genética , Proteína 1 del Complejo de la Esclerosis Tuberosa , Proteína 2 del Complejo de la Esclerosis TuberosaRESUMEN
Tuberous sclerosis is an autosomal dominant hereditary disease caused by mutations in either the TSC1 or the TSC2 tumor suppressor gene. The TSC1 gene on chromosome 9q34 encodes a 130 kDa protein named hamartin, and the TSC2 gene on chromosome 16p13.3 codes for tuberin, a 200 kDa protein. Here we show that expression of hamartin, assayed by immunoblot analyses, is high in G(0)-arrested cells and hamartin is expressed throughout the entire ongoing cell cycle. An interaction of hamartin and tuberin can be detected in every phase of the cell cycle. Ectopic expression of high levels of hamartin attenuates cellular proliferation. We provide evidence that this effect could depend on a coiled-coil region earlier proposed to be involved in binding of hamartin to tuberin. Further investigations revealed that hamartin affects cell proliferation via deregulation of G(1) phase. Our data have a clear impact on understanding the role of hamartin during development of this disease.
Asunto(s)
Proteínas de Ciclo Celular , Genes Supresores de Tumor , Proteínas/fisiología , Proteínas Supresoras de Tumor , Animales , Ciclo Celular , División Celular , Línea Celular , Inhibidor p27 de las Quinasas Dependientes de la Ciclina , Células HeLa , Humanos , Proteínas Asociadas a Microtúbulos/biosíntesis , Pruebas de Precipitina , Biosíntesis de Proteínas , Proteínas/genética , Proteínas/metabolismo , Ratas , Proteínas Represoras/metabolismo , Proteína 1 del Complejo de la Esclerosis Tuberosa , Proteína 2 del Complejo de la Esclerosis TuberosaRESUMEN
In the mammalian cell cycle, the transition from the G1 phase to S phase, in which DNA replication occurs, is dependent on tight cell size control and has been shown to be regulated by the cyclin-dependent kinases (Cdks) 2, 3, 4 and 6. Activities of Cdks are controlled by association with cyclins and reversible phosphorylation reactions. An additional level of regulation is provided by inhibitors of Cdks. G1-S and S phase substrates of these enzymes include proteins implicated in replication and transcription. Whereas the regulation and role of Cdk2, 4 and 6 has intensively been studied, less is known about Cdk3. Recent data provide first insights into the regulation of Cdk3-associate kinase activity and suggest a model how Cdk3 participates in the regulation of the G1-S transition. Although it has been shown that these G1-Cdks are absolutely essential for a proper transition into S phase, their physiological activation is not sufficient to directly initiate replication independently of cell size. Evidence obtained from yeast and Xenopus indicate the initiation of DNA replication to be a two-step process: the origin recognition complex, Cdc6 and Mcm proteins are required for establishing the prereplicative complex and the activities of Cdks and of Cdc7 kinase then trigger the G1-S transition. Recent findings provide evidence that the overall mechanism of initiation of replication is conserved in mammalian cells.