Phosphatidylinositol Induces Caspase-Independent Apoptosis of Malignant Pleural Mesothelioma Cells by Accumulating AIF in the Nucleus

Abstract

Background/Aims: Phosphatidylinositol (PI) regulates a variety of cell processes. The present study investigated the antitumor action of 1,2-dioleoyl-sn-glycero-3-phospho-(1'-myo-inositol)(DOPI) and 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-myo-inositol)(DPPI) on human malignant pleural mesothelioma (MPM) cell lines such as NCI-H28, NCI-H2052, NCI-H2452, and MSTO-211H cells. Methods: MTT assay, TUNEL staining, flow cytometry using propidium iodide (PI) and annexin V (AV), enzymatic caspase assay, and nuclear staining using DAPI were carried out, and mitochondrial membrane potentials and intracellular distribution of apoptosis-inducing factor (AIF) were monitored in cells with and without the siRNA silencing the Bid-targeted gene. Results: Both DOPI and DPPI reduced cell viability for all the investigated MPM cell lines in a concentration (0.01-100 µM)-dependent manner. DOPI and DPPI significantly increased TUNEL-positive cells and the population of PI-negative/AV-positive and PI-positive/AV-positive cells, corresponding to early apoptosis and late apoptosis/secondary necrosis, respectively. DOPI and DPPI perturbed mitochondrial membrane potentials in MSTO-211H cells, but no significant activation of caspase-3, -4, -8, and -9 was obtained. DOPI and DPPI upregulated expression of Bid in MSTO-211H cells. DOPI and DPPI significantly increased nuclear localization of AIF without affecting expression of the mRNAs and proteins in MSTO-211H cells, which was inhibited by knocking-down Bid. In the DAPI staining, nuclear fragmentation and condensation were found. Conclusion: The results of the present study indicate that DOPI and DPPI facilitate Bid-mediated AIF release from the mitochondria, to accumulate AIF in the nucleus and induce caspase-independent apoptosis of MPM cells.