Myc Down-Regulation Sensitizes Melanoma Cells to Radiotherapy by Inhibiting MLH1 and MSH2 Mismatch Repair Proteins

Abstract

Purpose: Melanoma patients have a very poor prognosis with a response rate of <1% due to advanced diagnosis. This type of tumor is particularly resistant to conventional chemotherapy and radiotherapy, and the surgery remains the principal treatment for patients with localized melanoma. For this reason, there is particular interest in the melanoma biological therapy. Experimental Design: Using two p53 mutant melanoma models stably expressing an inducible c-myc antisense RNA, we have investigated whether Myc protein down-regulation could render melanoma cells more susceptible to radiotherapy, reestablishing apoptotic p53-independent pathway. In addition to address the role of p53 in the activation of apoptosis, we studied the effect of Myc down-regulation on radiotherapy sensitivity also in a p53 wild-type melanoma cell line. Results: Myc down-regulation is able per se to induce apoptosis in a fraction of the cell population (f40% at 72 hours) and in combination with g radiation efficiently enhances the death process. In fact, f80% of apoptotic cells are evident in Myc down-regulated cells exposed to g radiation for 72 hours compared with f13% observed after only g radiation treatment. Consistent with the enhanced apoptosis is the inhibition of the MLH1 and MSH2 mismatch repair proteins, which, preventing the correction of ionizing radiation mismatches occurring during DNA replication, renders the cells more prone to radiation-induced apoptosis. Conclusions: Data herein reported show that Myc down-regulation lowers the apoptotic threshold in melanoma cells by inhibiting MLH1and MSH2 proteins, thus increasing cell sensitivity to g radiation in a p53-independent fashion. Our results indicate the basis for developing new antitumoral therapeutic strategy, improving the management of melanoma patients.

Tumor Cell Kill by c-MYC Depletion: Role of MYC-Regulated Genes that Control DNA Double-Strand Break Repair

“…S2). MYC downregulation has previously been shown to promote apoptosis (3,12), and given that siRNA to MYC was more toxic in H1299 cells, we determined whether apoptosis was the mode of cell death in this cell system (Fig. 4C).…”

“…Previous studies have indicated the association of MYC with the promoters of DNA double-strand break (DSB) repair genes Nbs1, Ku70, Rad51, BRCA2, Rad50, Rad54L, and DNAPKcs (8)(9)(10)(11) and the expression of mismatch repair genes (12)(13)(14). DSBs are repaired by two major pathways: homologous recombination (HR) and nonhomologous end joining (NHEJ).…”

c-MYC-Induced Genomic Instability

“…MYC also regulates mismatch repair genes. For example, MYC down-regulation correlated with decreased levels of MSH2 and MLH1 in a gamma radiation-treated melanoma cell line (Bucci et al 2005). It was also observed in hypoxic conditions in multiple tumor cell lines that the promoters of MSH2 and MLH1 changed from being occupied by MYC/MAX to MAX/MAD1 and MAX/MNT complexes (Bindra and Glazer 2007;see Hurlin 2013 andConacci-Sorrell et al 2014 for a description of the MXD/MNT proteins).…”

Phosphorylation and stabilization of c-Myc by NEMO renders cells resistant to ionizing radiation through up-regulation of γ-GCS

“…c-Myc induces apoptosis and sensitizes cells to various apoptotic stimuli (12,15), but also has been reported to protect cells from apoptosis (6,8,10). In this study, we report that c-Myc, when it is phosphorylated and stabilized by NEMO, protects cells from IR-induced cell death.…”

“…Sensitization of non-Hodgkin's lymphoma cells to ionizing radiation by rituximab is achieved through the inhibition of c-Myc expression (9). Another study has suggested that c-Myc down-regulation sensitizes melanoma cells to radiotherapy in a p53-independent fashion by inhibiting MLH1 and MSH2 mismatch repair proteins (10). On the other hand, there have also been many reports evidencing proapoptotic function of c-Myc.…”