TI - p53 activation contributes to the SS -dependent endothelial cell death . AB - Elevated oxidative stress , genotoxic damage and protein nitration represent conditions that turn p53 toward a pro-apoptotic pathway[17] . The presence of DNA damage is , in fact , the stimulus to activate a signalling cascade controlled in part by the ataxia-telangectasia mutated ( ATM ) kinase which phosphorylaTEs the histone isoform 2Ax ( gammaH2Ax ) and stabilizes the homeodomain-interacting protein kinase ( HIPK2 ) determining p53 PHOSphorylation on serine 15 and 46 and enhancing the expression of proapoptotic molecules , including BCL2 family members such as BAX instead of p21waf1 , cip1 , sdi1 . Notably , p21waf1 , cip1 , sdi1 is one of the transcriptional targets of p53 which is important for cell cycle arrest and DNA repair . Situations in which p21waf1 , cip1 , sdi1 function results compromised , in fact , often lead to caspase -dependent apoptosis[26] . To investigate the effect of Bleo alone or combined to SS in our experimental setting a series of western blotting analyses were performed on ATM , HIPK2 , p53 and p21waf1 , cip1 , sdi1 . In both conditions , ATM was PHOSphorylated , however this activator modification was significantly increased in the presence of SS ( figure 3a , panel 1 , left ) , reaching the highest peak of about 8 fold above static control ( figure 3a , panel 1 , right ) . Activated ATM kinase stabilizes HIPK2 protein expression[18] . To regulation of HIPK2 in our system was examined in the presence or absence of Bleo , SS or both in combination . Figure 3a , panel 2 , left , shows that the coincident presence of Bleo and SS determined the highest increase in the intracellular content HIPK2 which raised about four fold above control ( Figure 3a , panel 2 , right ) . This effect was paralleled by p53 acetylation on Lysine 382 and PHOSphorylation on Serine 15 and 46[27] , as indicated by western blotting and densitometric analyses ( Figure 3a , panel 3 , left and right ) . In normal condition , the cell cycle inhibitor p21waf1 , cip1 , sdi1 is elevated by laminar flow and contributes to the SS -dependent growth arrest and endothelial survival [3] , [28] . However , figure 3a , panel 4 , left and right , shows that in the presence of Bleo the SS -dependent increase in p21waf1 , cip1 , sdi1 expression is abrogated . Altogether , these results suggest that the ATM-HIPK2-p53 signalling pathway activation leads to alteration in the SS -dependent effect on p21waf1 , cip1 , sdi1 .