DNA mismatch repair-dependent activation of c-Abl/p73α/GADD45α-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 p73 alpha and GADD45 alpha protein up-regulation after MNNG exposure prompted us to examine c-Abl/p73/alpha GADD45 alpha signaling in cell death responses. STI571 (Gleevec, a c-Abl tyrosine kinase inhibitor) and stable c-Abl, p73 alpha, and GADD45 alpha knockdown prevented MMR-dependent apoptosis. Interestingly, stable p73 alpha 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/p73 alpha/GADD45 alpha retrograde signaling.