Study on the mechanism of p53 family-mediated senescence

Abstract

Replicative senescence is a terminal differentiation state in which metabolically active cells are permanently and irreversibly growth-arrested, with enlarged and flattened morphology and expression of the senescence-associated β-galactosidase activity at pH 6.0 in normal mammalian cells. It has been shown that expression of several tumor suppressor genes such as p53, Rb, p21 and p16 can trigger replicative senescence program, which is inactivated in human tumor cells. However, a detailed mechanism by which tumor suppressor genes trigger replicative senescence is largely unknown. The present study investigated a mechanism by p53 family proteins including two p53 homologues, p63 and p73 mediates senescence in tumor cells. p63 and p73, like p53, induced replicative senescence when overexpressed in a tetracycline-regulated manner in EJ cells lacking a functional p53. In addition to transcription activation of p53-responsive genes, p63 and p73 repressed transcription of the cdk1 and cyclin B genes, both of which were irreversibly repressed in senescent human fibroblast. Furthermore, DNA binding activity of NF-Y transcription factor, which is essential for transcription of the cdk1 and cyclin B genes and inactivated in senescent fibroblast, was significantly decreased by expression of either of p53, p63, or p73. Through these results, it is implicated that p53 family induce replicative senescence through inhibition of NF-Y activity. Based on above results, it was further examined whether anti-apoptotic Bcl-2 family proteins affects to p53-induced replicative senescence. Anti-apoptotic function of Bcl-2 family has been thought to contribute its oncogenic potential. Although the detailed mechanisms of its anti-apoptosis function remain unknown, Bcl-2 family has been shown to affect a variety of biochemical changes during apoptosis. To address whether Bcl-2 family proteins affect to senescence in tumor cells, human bladder cancer cells, EJ, expressing Bcl-2, $Bcl-x_L$...