Post-translational modifications of p53 tumor suppressor: determinants of its functional targets

Tumor suppressor p53 functions as a "guardian of the genome" to prevent cells from transformation. p53 is constitutively ubiquitinated and degradated in unstressed conditions, thereby suppressing the expression. However, cellular stimuli enable p53 to escape from the negative regulation, and then stably expressed p53 transactivates its target genes to induce cell cycle arrest, DNA repair, or apoptosis. Promoter preference of target genes is determined by modification status of p53. Because p53 has two critical roles in the decision of cell fate, stopping cell cycle to repair damaged DNA or induction of apoptotic cell death in response to DNA damage, elucidation of switching mechanisms on p53 functions is of particular importance. Here we review recent evidence how several post-translational modifications of p53 including methylation, phosphorylation, acetylation, and ubiquitination, affect the functions of p53 in response to cellular stress.     

DNA mismatch repair-dependent activation of c-Abl/p73alpha/GADD45alpha-mediated apoptosis

Cells with functional DNA mismatch repair (MMR) stimulate G(2) cell cycle checkpoint arrest and apoptosis in response to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). MMR-deficient cells fail to detect MNNG-induced DNA damage, resulting in the survival of "mutator" cells. The retrograde (nucleus-to-cytoplasm) signaling that initiates MMR-dependent G(2) arrest and cell death remains undefined. Since MMR-dependent phosphorylation and stabilization of p53 were noted, we investigated its role(s) in G(2) arrest and apoptosis. Loss of p53 function by E6 expression, dominant-negative p53, or stable p53 knockdown failed to prevent MMR-dependent G(2) arrest, apoptosis, or lethality. MMR-dependent c-Abl-mediated p73alpha and GADD45alpha protein up-regulation after MNNG exposure prompted us to examine c-Abl/p73alpha/GADD45alpha signaling in cell death responses. STI571 (Gleevec, a c-Abl tyrosine kinase inhibitor) and stable c-Abl, p73alpha, and GADD45alpha knockdown prevented MMR-dependent apoptosis. Interestingly, stable p73alpha knockdown blocked MMR-dependent apoptosis, but not G(2) arrest, thereby uncoupling G(2) arrest from lethality. Thus, MMR-dependent intrinsic apoptosis is p53-independent, but stimulated by hMLH1/c-Abl/p73alpha/GADD45alpha retrograde signaling.