Resistance of mtDNA-depleted cells to apoptosis

Cells lacking mitochondrial genome (defined as ρ⁰) are useful models in studies on cancer, aging, mitochondrial diseases and apoptosis, but several of their functional aspects have been poorly characterized. Using different clones of ρ⁰ cells derived from the human osteosarcoma line 143B, we have tested the effects of different apoptogenic molecules such as staurosporine (STS), doxorubicin, daunomycin and quercetin, and have analyzed apoptosis, mitochondrial membrane potential (MMP), levels of oxygen free radicals, reduced glutathione (GSH) content, and expression of P-glycoprotein (P-gp). When compared to parental cells, ρ⁰ cells resulted much less sensitive to apoptosis. MMP was well maintained in ρ⁰ cells, and remained unchanged after adding apoptogenic agents, and did not change after treatment with molecules able to depolarize mitochondria such as valinomycin. After adding STS, the production of reactive oxygen species was similar in both cell types, but ρ⁰ cells maintained higher levels of GSH. In ρ⁰ cells, P-gp was strongly over-expressed both at mRNA and protein level, and its functionality was higher. The resistance to apoptosis of ρ⁰ cells could be not only due to an increased scavenger capacity of GSH, but also due to a selection of multidrug resistant cells that hyperexpress P-gp.