Regulation of cellular senescence and p16(INK4a) expression by Id1 and E47 proteins in human diploid fibroblast.

Id1, a member of Id family of helix-loop-helix transcriptional regulatory proteins, is implicated in cellular senescence by repressing p16(INK4a) expression, but the mechanisms and cellular effects in human diploid fibroblasts remain unknown. Here we analyzed the patterns of p16(INK4a) and Id1 expression during the lifespan of 2BS cells and presented the inverse correlation between these two proteins. Immunoprecipitation assays demonstrated the presence of endogenous interaction of Id1 and E47 proteins that was strong in young 2BS cells and weakened during replicative senescence and, thereby, influenced the transcription activation of p16(INK4a) by E47. Furthermore, we found that E47 protein could bind to the E-box-containing region in p16(INK4a) promoter in senescent cells by chromatin immunoprecipitation analyses, suggesting that E47 is indeed ultimately involved in the regulation of p16(INK4a) transcription in vivo. Silencing Id1 expression in young cells by RNA interference induced an increased p16(INK4a) level and premature cellular senescence, whereas silencing E47 expression inhibited the expression of p16(INK4a) and delayed the onset of senescent phenotype. The present study demonstrated not only the capacity of Id1 to regulate p16(INK4a) gene expression by E47, but also the phenotypic consequence of the regulation on cellular senescence, moreover, raised the possibility of Id1-specific gene silencing for human cancer therapy.