RESUMO
Inhibitor of growth (ING) proteins have multiple functions in the control of cell proliferation, mainly by regulating processes associated with chromatin regulation and gene expression. ING5 has been described to regulate aspects of gene transcription and replication. Moreover deregulation of ING5 is observed in different tumors, potentially functioning as a tumor suppressor. Gene transcription in late G1 and in S phase and replication is regulated by cyclin-dependent kinase 2 (CDK2) in complex with cyclin E or cyclin A. CDK2 complexes phosphorylate and regulate several substrate proteins relevant for overcoming the restriction point and promoting S phase. We have identified ING5 as a novel CDK2 substrate. ING5 is phosphorylated at a single site, threonine 152, by cyclin E/CDK2 and cyclin A/CDK2 in vitro. This site is also phosphorylated in cells in a cell cycle dependent manner, consistent with it being a CDK2 substrate. Furthermore overexpression of cyclin E/CDK2 stimulates while the CDK2 inhibitor p27KIP1 represses phosphorylation at threonine 152. This site is located in a bipartite nuclear localization sequence but its phosphorylation was not sufficient to deregulate the subcellular localization of ING5. Although ING5 interacts with the tumor suppressor p53, we could not establish p53-dependent regulation of cell proliferation by ING5 and by phospho-site mutants. Instead we observed that the knockdown of ING5 resulted in a strong reduction of proliferation in different tumor cell lines, irrespective of the p53 status. This inhibition of proliferation was at least in part due to the induction of apoptosis. In summary we identified a phosphorylation site at threonine 152 of ING5 that is cell cycle regulated and we observed that ING5 is necessary for tumor cell proliferation, without any apparent dependency on the tumor suppressor p53.
Assuntos
Quinase 2 Dependente de Ciclina/metabolismo , Fatores de Transcrição/metabolismo , Proteína Supressora de Tumor p53/metabolismo , Proteínas Supressoras de Tumor/metabolismo , Substituição de Aminoácidos , Sítios de Ligação , Linhagem Celular , Proliferação de Células/fisiologia , Ciclina A/antagonistas & inibidores , Ciclina A/genética , Ciclina A/metabolismo , Ciclina E/antagonistas & inibidores , Ciclina E/genética , Ciclina E/metabolismo , Técnicas de Silenciamento de Genes , Células HEK293 , Células HeLa , Humanos , Mutagênese Sítio-Dirigida , Fosforilação , Proteínas Recombinantes/química , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Especificidade por Substrato , Treonina/química , Fatores de Transcrição/química , Fatores de Transcrição/genética , Transfecção , Proteínas Supressoras de Tumor/química , Proteínas Supressoras de Tumor/genéticaRESUMO
Cyclin-dependent kinases (Cdks) fulfill key functions in many cellular processes, including cell cycle progression and cytoskeletal dynamics. A limited number of Cdk substrates have been identified with few demonstrated to be regulated by Cdk-dependent phosphorylation. We identify on protein expression arrays novel cyclin E-Cdk2 substrates, including SIRT2, a member of the Sirtuin family of NAD(+)-dependent deacetylases that targets alpha-tubulin. We define Ser-331 as the site phosphorylated by cyclin E-Cdk2, cyclin A-Cdk2, and p35-Cdk5 both in vitro and in cells. Importantly, phosphorylation at Ser-331 inhibits the catalytic activity of SIRT2. Gain- and loss-of-function studies demonstrate that SIRT2 interfered with cell adhesion and cell migration. In postmitotic hippocampal neurons, neurite outgrowth and growth cone collapse are inhibited by SIRT2. The effects provoked by SIRT2, but not those of a nonphosphorylatable mutant, are antagonized by Cdk-dependent phosphorylation. Collectively, our findings identify a posttranslational mechanism that controls SIRT2 function, and they provide evidence for a novel regulatory circuitry involving Cdks, SIRT2, and microtubules.