Resistance of ρ0 Cells against Apoptosis

Mitochondrion is one of the master players in both apoptosis and necrosis. However, most previous articles report that mitochondrial DNA-depleted cells without oxidative phosphorylation underwent apoptosis by several apoptotic effectors as efficiently as their parental cells, suggesting that intact mitochondrial function is dispensable for the progression of apoptosis. We studied the role of mitochondrial function in several apoptosis models. TRAIL, a recently identified member of the TNF family with cytotoxicity on a wide variety of transformed cells, killed SK-Hep1 cells with characteristic features of apoptosis such as DNA fragmentation, sub-G1 ploidy peak, and cytochrome c translocation. In contrast with parental cells, mitochondrial DNA-deficient SK-Hepl ρ0 cells were resistant to TRAIL-induced apoptosis. Dissipation of mitochondrial potential or cytochrome c translocation did not occur in ρ0 cells after TRAIL treatment Bax translocation also was absent in ρ0 cells, accounting for the failure of cytochrome c release in ρ0 cells. SK-Hepl ρ0 cells were resistant to other death effectors such as staurosporine. Our results indicate that apoptosis of SK-Hepl hepatoma cells is dependent on intact mitochondrial function. Because aged cells or tumor cells have frequent mutations or deletions of mitochondrial DNA, they might acquire the ability to evade apoptosis or tumor surveillance imposed by TRAIL or other death effectors in vivo, accounting for the selection advantage of cancer cells and frequent development of cancer in aged individuals.