Induction of two DNA mismatch repair proteins, MSH2 and MSH6, in differentiated human neuroblastoma SH-SY5Y cells exposed to doxorubicin.

The MutS homologues MSH2 and MSH6 form a heterodimeric protein complex that is involved in the recognition of base/base mismatches and insertion/deletion loops, as well as some other types of DNA damage. We investigated the expression of these proteins in undifferentiated and retinoic acid-differentiated human neuroblastoma SH-SY5Y cells by immunocytochemistry, western blot analysis, and RT-PCR. Nuclei from undifferentiated SH-SY5Y cells were found to be immunoreactive to anti-MSH2 and anti-MSH6 antibodies. Following differentiation, the cells stop dividing and change morphology to acquire a neuron-like phenotype. Under these conditions, both anti-MSH2 and anti-MSH6 immunoreactivities were still detectable, although the signals were somewhat less intense. When these cells were exposed for 2 h to neurotoxic concentrations of doxorubicin (50 nM), they exhibited a marked and homogeneous increase of both anti-MSH2 and anti-MSH6 immunoreactivities. As revealed by western blot analysis, these effects were associated with increased protein content and were dose-dependent. Using RT-PCR technology, we also found that doxorubicin treatment did not change MSH2 or MSH6 mRNA levels. Our data indicate that human postmitotic, neuron-like cells constitutively express the molecular machinery devoted to recognition of DNA mismatches and that this system is activated by specific treatment leading to cell death. These findings might help clarify the molecular mechanisms underlying various human neurological diseases that are associated with deficiencies in DNA repair and/or a high rate of DNA damage acquisition.