CoCl2 Induces Apoptosis via the 18 kDa Translocator Protein in U118MG Human Glioblastoma Cells

The 18 kDa translocator protein (TSPO), formerly known as the peripheral-type benzodiazepine receptor, has been reported to be closely associated with the mitochondrial permeability transition pore (MPTP). TSPO is believed to exert pro-apoptotic functions via modulation of MPTP opening. Cobalt chloride (CoCl2), which is sometimes used as a hypoxia mimicking agent, is also known to be able to induce apoptosis. One of our questions was whether CoCl2 may induce apoptosis via the TSPO. To address this question, we used the U118MG human glioblastoma cell line. We applied the specific TSPO ligand, PK 11195, as well as TSPO knockdown with siRNA and studied their influence on the effects of CoCl2 on cell death, including activation of the mitochondrial apoptosis pathway. To assay TSPO expression, we applied binding assays and Western blotting to whole cell homogenates and mitochondrial fractions. To assay activation of the mitochondrial apoptosis pathway, including some of the cellular mechanisms involved, we determined the incidence of collapse of the mitochondrial membrane potential (Δψm) and cardiolipin oxidation and measured the level of DNA fragmentation to assay apoptotic rates. We found that the TSPO ligand, PK 11195, significantly counteracted induction of cell death by 0.4 mM CoCl2, including apoptosis, collapse of the Δψm, and cardiolipin oxidation. Moreover, we found that TSPO knockdown with siRNA fully protected against mentioned cell death mechanisms. Thus, we found that the TSPO is required for cell death induction by CoCl2, including apoptosis. In conclusion, our studies show that activation of TSPO by CoCl2 application is required for ROS generation, leading to cardiolipin oxidation, and collapse of the Δψm, as induced by CoCl2.