APA-1 Induces Genes Associated with Fibroblast Senescence

Abstract

Benanti et al. identified a novel zinc-finger protein that regulates genes associated with fibroblast senescence independently of telomere length, suggesting that induction of the senescent phenotype is distinct from telomere-induced cell-cycle arrest. Unlike tumor cells, which proliferate indefinitely in culture, most normal mammalian cells undergo cellular senescence, an irreversible cell-cycle arrest, after a finite number of divisions. Human fibroblasts cease dividing in response to progressive shortening of telomeres and can be immortalized through telomere elongation by telomerase. Senescent fibroblasts show phenotypic changes involving expression of genes associated with wound healing. Using a yeast two-hybrid screen to identify proteins that interact with the tumor suppressor p14 ARF , the authors identified a zinc-finger protein that they named APA-1. Western blot analysis indicated that amounts of APA-1 protein increased in human fibroblasts as they approached senescence and in response to induction of premature senescence. Fibroblasts immortalized with telomerase catalytic subunit showed decreased APA-1 at equivalent passage number, but, after extended passaging, accumulated APA-1 to levels similar to those in senescent cells. Western analysis of immunoprecipitates demonstrated that a large form of APA-1 was modified by the ubiquitin-like protein SUMO-1, which increased APA-1 half-life after protein synthesis inhibition. APA-1 overexpression did not induce premature senescence, but did induce expression of MMP1 and PAI2 , genes associated with the senescent phenotype. Expression of MMP1 and PAI2 was also increased after extended passage of telomerase-immortalized fibroblasts. APA-1 transactivated reporter genes containing the MMP1 promoter and bound to the MMPI promoter as indicated by gel-shift analysis. These data suggest that elements of the senescent phenotype may occur independently of telomere reduction and cell-cycle arrest. J. A. Benanti, D. K. Williams, K. L. Robinson, H. L. Ozer, D. A. Galloway, Induction of extracellular matrix-remodeling genes by the senescence-associated protein APA-1. Mol. Cell. Biol. 22 , 7385-7397 (2002). [Abstract] [Full Text]