Tumorigenic transformation of human breast epithelial cells induced by mitochondrial DNA depletion

Abstract

Human mitochondrial DNA (mtDNA) encodes 13 proteins involved in oxidative phosphorylation (OXPHOS). In order to investigate the role of mitochondrial OXPHOS genes in breast tumorigenesis, we have developed a breast epithelial cell line devoid of mtDNA (ρ0 cells). Our analysis revealed that depletion of mtDNA in breast epithelial cells results in in vitro tumorigenic phenotype as well as breast tumorigenesis in a xenograft model. We identified two major gene networks which were differentially regulated between parental and ρ0 epithelial cells. The focal proteins in these networks include i) FN1 (fibronectin) and ii) p53. Bioinformatic analyses of FN1 network identified laminin, integrin and 5 of 6 members of peroxiredoxin whose expression were altered in ρ0 epithelial cells. In the p53 network, we identified SMC4 and WRN whose changes in expression suggest that this network may affect the chromosomal stability. Consistent with above finding our study revealed an increase DNA double strand breaks, and unique chromosomal rearrangements in ρ0 breast epithelial cells. Additionally, we identified tight junction proteins claudin-1 and claudin-7 in p53 network. To determine the functional relevance of altered gene expression, we focused on detailed analyses of claudin-1 and -7 proteins in breast tumorigenesis. Our study determined that i) claudin-1 and 7 were indeed down regulated in ρ0 breast epithelial cells, ii) down regulation of claudin-1 or -7 led to neoplastic transformation of breast epithelial cells, and iii) claudin-1 and -7 were also down regulated in primary breast tumors. Together, our study suggest that mtDNA encoded OXPHOS genes play a key role in transformation of breast epithelial cells and that multiple pathway involved in mitochondria-to-nucleus retrograde regulation contribute to transformation of breast epithelial cells