| Summary: | 博士 === 國立陽明大學 === 生物醫學影像暨放射科學系 === 102 === Cellular senescence is an irreversible process involving growth arrest, morphological enlargement and flattening, attrition of telomeres and expressive alterations of molecular profiles. Although senescence contributes to normal aging in a variety of organisms, it may be associated with diseases if the process was abnormally accelerated by environmental stresses. Morphological change of senescent cells is strongly associated with the appearance of tissues in aged organisms, but the underlying mechanisms remain unclear. This study demonstrated that decreased actin de-polymerization rate of actin filaments would contribute to increased actin cytoskeletal mass and shape change of senescent cells. Decreased actin de-polymerization rate was associated with up-regulated cofilin-1, a member of actin de-polymerization factor (ADF)/cofilin family, after a broad screening of different actin associated proteins in senescent human diploid fibroblast (HDF) cells. The up-regulated cofilin-1 was accompanied by the increased phosphorylated cofilin-1 representing an inactive form in depolymerizing actin filaments, suggesting that increased actin cytoskeleton in senescent cells is associated with regulation of cofilin-1 level and activity. Concomitantly, increased cofilin-1 level and actin cytoskeleton was found in several mammalian cell lines and in different tissues of mice and human. Furthermore, over-expression of cofilin-1 in young HDF promoted cellular senescence, and knockdown of cofilin-1 in old HDF compromised the senescent phenotypes. Oxidative stress induced senescence also caused cofilin-1 up-regulation, and knockdown of cofilin-1 could alleviate the induced senescence.
Cell spreading and actin remodeling are also associated with Yes-Associated Protein 1 (YAP1), a transcription coactivator regulated by Hippo signaling pathway is involved in diverse cell behaviors. Given that morphological enlargement is obvious in senescent cells, the relationship between cofilin-1 and YAP1 was investigated. Surprisingly, over-expressed cofilin-1 in senescent cells led to increase YAP1 mediated transcription activity, which is likely to be associated with nuclear entry of YAP1. Prohibition of YAP1 entering nucleus is mediated by binding to 14-3-3ξa cytoplasmic protein that also binds to cofilin-1. A competitive binding to 14-3-3ξ between cofilin-1 and YAP1 was found in senescent cells and favorable for nuclear entry of YAP1. On the contrary, YAP1 over-expression alone did not promote senescence, but co-expression of cofilin-1 and YAP1 could promote senescence. These results suggest that the cofilin-1- 14-3-3ξ-YAP1 regulatory axis may be a novel mechanism in cellular senescence.
The p27kip1 is one of the senescence related molecules that controls cell cycle progression. Knockdown of p27kip1 partially mitigated cofilin-1 over-expression induced senescence in normal HDF cells, but completely blocked senescence in cancerous cells lacking p53 and p16INK4 expression. Moreover, exogenous expression of cofilin-1 alone or YAP1 alone could increase p27kip1 mRNA, although co-expression of cofilin-1 and YAP1 further increased this phenomenon. The promoter assay showed that YAP1 could transactivate p27kip gene promoter, and the primary promoter region influenced by YAP1 may locate in 200bp upstream of the transcriptional initiation site. Taken together, up-regulation of cofilin-1 is likely to influence the actin cytoskeletal organization and the morphological change in senescent cells, and this event mediates the nuclear entry of YAP1 to transactivate p27kip1 gene for development of senescence. This is also a pathway independent to p53 and p16INK4. Therefore, this finding has extended contributions applied to induce cellular senescence for clinical cancer therapy even in cases of tumors with mutated or deleted p53 and p16INK4. Further, we also established a convenient tool to non-invasively monitor the inhibitory effect on tumor growth by cofilin-1 using biomedical imaging in vivo. My project may provide a novel pathway in both health care and clinical cancer therapy.
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