DNA mismatch repair-dependent activation of c-Abl/p73α/GADD45α- mediated apoptosis

Cells with functional DNA mismatch repair (MMR) stimulate G₂ 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-tocytoplasm) 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 G₂ arrest and apoptosis. Loss of p53 function by E6 expression, dominant-negative p53, or stable p53 knockdown failed to prevent MMR-dependent G₂ arrest, apoptosis, or lethality. MMR-dependent c-Abl-mediated p73α and GADD45α protein up-regulation after MNNG exposure prompted us to examine c-Abl/p73α/GADD45α signaling in cell death responses. STI571 (Gleevec™, a c-Abl tyrosine kinase inhibitor) and stable c-Abl, p73α, and GADD45α knockdown prevented MMR-dependent apoptosis. Interestingly, stable p73α knockdown blocked MMR-dependent apoptosis, but not G₂ arrest, thereby uncoupling G₂ arrest from lethality. Thus, MMR-dependent intrinsic apoptosis is p53-independent, but stimulated by hMLH1/c-Abl/p73α/GADD45α retrograde signaling.