Reduced mtDNA copy number increases the sensitivity of tumor cells to chemotherapeutic drugs.

H Mei,Y Bai,R Chai,S Sun,H Li,Y Chen

doi:10.1038/cddis.2015.78

Abstract

Many cancer drugs are toxic to cells by activating apoptotic pathways. Previous studies have shown that mitochondria have key roles in apoptosis in mammalian cells, but the role of mitochondrial DNA (mtDNA) copy number variation in the pathogenesis of tumor cell apoptosis remains largely unknown. We used the HEp-2, HNE2, and A549 tumor cell lines to explore the relationship between mtDNA copy number variation and cell apoptosis. We first induced apoptosis in three tumor cell lines and one normal adult human skin fibroblast cell line (HSF) with cisplatin (DDP) or doxorubicin (DOX) treatment and found that the mtDNA copy number significantly increased in apoptotic tumor cells, but not in HSF cells. We then downregulated the mtDNA copy number by transfection with shRNA-TFAM plasmids or treatment with ethidium bromide and found that the sensitivity of tumor cells to DDP or DOX was significantly increased. Furthermore, we observed that levels of reactive oxygen species (ROS) increased significantly in tumor cells with lower mtDNA copy numbers, and this might be related to a low level of antioxidant gene expression. Finally, we rescued the increase of ROS in tumor cells with lipoic acid or N-acetyl-L-cysteine and found that the apoptosis rate decreased. Our studies suggest that the increase of mtDNA copy number is a self-protective mechanism of tumor cells to prevent apoptosis and that reduced mtDNA copy number increases ROS levels in tumor cells, increases the tumor cells' sensitivity to chemotherapeutic drugs, and increases the rate of apoptosis. This research provides evidence that mtDNA copy number variation might be a promising new therapeutic target for the clinical treatment of tumors.

Highlights

The regulation of intracellular mitochondrial DNA (mtDNA) copy number is complicated and precise, but the exact mechanism behind this regulation remains unclear
We found that the mRNA level of TFAM in apoptotic HNE2 cells increased, that TFAM mRNA expression was downregulated by transfection with shRNA-TFAM, and that the mtDNA copy number decreased and the apoptosis rate increased after DDP treatment (Supplementary Figures S2C–E)
Different signal transduction pathways interact with the mitochondria to influence apoptosis, but the role of mtDNA copy number in apoptosis has remained largely unexplored

Summary

Introduction

The regulation of intracellular mtDNA copy number is complicated and precise, but the exact mechanism behind this regulation remains unclear. MtDNA copy number variation has been shown to be associated with tumor development.[8,9] These variations cannot be explained by the abnormal proliferation of cells, which has significant tissue specificity. Transgenic mice with mtDNA deletions cannot survive the embryonic period, and a large number of apoptotic cells are found in these embryos.[10,11] In tissue-specific mtDNA knockout mice, a large number of apoptotic cells are found in the myocardium.[11] This suggests that mtDNA copy number and apoptosis are related. Exploring the relationship between mtDNA copy mtDNA CNVs in cancer cell apoptosis H Mei et al number and tumor cell apoptosis can provide novel insights into these mechanisms and has significant research value. We downregulated the mtDNA copy number to determine the impact on apoptosis and related mechanisms

Methods

Results

Conclusion