Bcl-2/Bax protein ratio predicts 5-fluorouracil sensitivity independently of p53 status

p53 tumour-suppressor gene is involved in cell growth control, arrest and a poptosis. Nevertheless cell cycle arrest and apoptosis induction can be obs erved in p53-defective cells after exposure to DNA-damaging agents such as 5-fluorouracil (5-FU) suggesting the importance of alternative pathways via p53-independent mechanisms. In order to establish relationship between p53 status, cell cycle arrest, Bcl-2/Bax regulation and 5-FU sensitivity, we e xamined p53 mRNA and protein expression and p53 protein functionality in wi ld-type (wt) and mutant (mt) p53 cell lines. p53 mRNA and p53 protein expre ssion were determined before and after exposure to equitoxic 5-FU concentra tion in six human carcinoma cell lines differing in p53 status and displayi ng marked differences in 5-FU sensitivity, with IC,, values ranging from 0. 2-22.6 mM. 5-FU induced a rise in p53 mRNA expression in mt p53 cell lines and in human papilloma virus positive wt p53 cell line, whereas significant decrease in p53 mRNA expression was found in wt p53 cell line. Whatever p5 3 status, 5-FU altered p53 transcriptional and translational regulation lea ding to up-regulation of p53 protein. In relation with p53 functionality, b ut independently of p53 mutational status, after exposure to 5-FU equitoxic concentration, all cell lines were able to arrest in G1. No relationship w as evidenced between G1 accumulation ability and 5-FU sensitivity. Moreover , after 5-FU exposure, Bar and Bcl-2 proteins regulation was under p53 prot ein control and a statistically significant relationship (r= 0.880, P = 0.0 097) was observed between Bcl-2/Bax ratio and 5-FU sensitivity. In conclusi on, whatever p53 status, Bcl-2 or Bar induction and Bcl-2/Bax protein ratio were correlated to 5-FU sensitivity. (C) 2000 Cancer Research Campaign.