Cellular senescence is a tumor-suppressive process instigated by proliferation in the absence of telomere replication, by cellular stresses such as oncogene activation, or by activation of the tumor suppressor proteins, such as Rb or p53. This process is characterized by an irreversible cell cycle exit, a unique morphology, and expression of senescence-associated-beta-galactosidase (SA-beta-gal). Despite the potential biological importance of cellular senescence, little is known of the mechanisms leading to the senescent phenotype. p41-Arc has been known to be a putative regulatory component of the mammalian Arp2/3 complex, which is required for the formation of branched networks of actin filaments at the cell cortex. In this study, we demonstrate that p41-Arc can induce senescent phenotypes when it is overexpressed in human tumor cell line, SaOs-2, which is deficient in p53 and Rb tumor suppressor genes, implying that p41 can induce senescence in a p53-independent way. p41-Arc overexpression causes a change in actin filaments, accumulating actin filaments in nuclei. Therefore, these results imply that a change in actin filament can trigger an intrinsic senescence program in the absence of p53 and Rb tumor suppressor genes.
Actin Cytoskeleton/metabolism ; Actin-Related Protein 2-3 Complex/*metabolism ; *Cell Aging ; Cell Cycle Proteins/metabolism ; Cell Line, Tumor ; Cell Nucleus/metabolism ; Fibroblasts/physiology ; Humans ; Recombinant Proteins/genetics/*metabolism ; Retinoblastoma Protein/*deficiency/genetics ; Tumor Suppressor Protein p53/*deficiency/genetics

Cellular senescence is an irreversible cell cycle arrest triggered by the activation of oncogenes or mitogenic signaling as well as the enforced expression of tumor suppressors such as p53, p16(INK4A) and promyelocytic leukemia protein (PML) in normal cells. E2F-binding protein 1 (E2FBP1), a transcription regulator for E2F, induces PML reduction and suppresses the formation of PML-nuclear bodies, whereas the down-regulation of E2FBP1 provokes the PML-dependent premature senescence in human normal fibroblasts. Here we report that the depletion of E2FBP1 induces the accumulation of PML through the Ras-dependent activation of MAP kinase signaling. The cellular levels of p16(INK4A) and p53 are elevated during premature senescence induced by depletion of E2FBP1, and the depletion of p16(INK4A), but not p53 rescued senescent cells from growth arrest. Therefore, the premature senescence induced by E2FBP1 depletion is achieved through the p16(INK4A)-Rb pathway. Similar to human normal fibroblasts, the growth inhibition induced by E2FBP1 depletion is also observed in human tumor cells with intact p16(INK4A) and Rb. These results suggest that E2FBP1 functions as a critical antagonist to the p16(INK4A)-Rb tumor suppressor machinery by regulating PML stability.
Cell Line, Tumor ; Cells, Cultured ; Cellular Senescence ; genetics ; physiology ; Cyclin-Dependent Kinase Inhibitor p16 ; antagonists & inhibitors ; genetics ; physiology ; DNA-Binding Proteins ; deficiency ; genetics ; physiology ; Down-Regulation ; Fibroblasts ; Gene Expression Regulation ; Humans ; Intranuclear Inclusion Bodies ; metabolism ; MAP Kinase Signaling System ; Nuclear Proteins ; genetics ; metabolism ; physiology ; Promyelocytic Leukemia Protein ; Protein Isoforms ; Protein Stability ; RNA Interference ; Retinoblastoma Protein ; antagonists & inhibitors ; genetics ; physiology ; Transcription Factors ; deficiency ; genetics ; metabolism ; physiology ; Transfection ; Tumor Suppressor Protein p53 ; physiology ; Tumor Suppressor Proteins ; genetics ; metabolism ; physiology ; Ubiquitination ; ras Proteins ; metabolism