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 (rho(0) 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 rho(0) epithelial cells. The focal
proteins in these networks include (i) FN1 (
fibronectin) and (ii) p53. Bioinformatic analyses of FN1 network identified
laminin,
integrin and 3 of 6 members of
peroxiredoxin whose expression were altered in rho(0) epithelial cells. In the p53 network, we identified SMC4 and WRN whose changes in expression suggest that this network may affect
chromosomal stability. Consistent with above finding our study revealed an increase in
DNA double strand breaks and unique chromosomal rearrangements in rho(0) 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 downregulated in rho(0) breast epithelial cells, (ii) downregulation of
claudin-1 or -7 led to neoplastic transformation of breast epithelial cells, and (iii)
claudin-1 and -7 were also downregulated 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.