Mitochondrial inner membrane protein (mitofilin) knockdown induces cell death by apoptosis via an AIF-PARP-dependent mechanism and cell cycle arrest

Mitofilin is an inner membrane protein that has been defined as a mitochondria-shaping protein in controlling and maintaining mitochondrial cristae structure and remodeling. We determined the role of mitofilin in cell survival by investigating the mechanism underlying mitofilin knockdown-induced cell death by apoptosis. Cultured H9c2 myoblasts and HEK 293 cells were treated with mitofilin siRNA or scrambled siRNA for 24 h. Cell death (apoptosis), caspase 3 activity and cell cycle phases were assessed by flow cytometry, while cytochrome c release and intracellular ATP production were measured by ELISA. Mitofilin, apoptosis-inducing factor (AIF) and poly(ADP-ribose) polymerase (PARP) expression were measured by Western blot analysis and calpain activity was assessed using a calpain activity kit. Mitochondrial images were taken using electron microscopy. We found that mitofilin knockdown increases apoptosis mainly via activation of the AIF-PARP pathway leading to nuclear fragmentation that is correlated with S phase arrest of the cell cycle. Knockdown of mitofilin also led to mitochondrial swelling and damage of cristae that is associated with the increase in reactive oxygen species production and mitochondrial calpain activity, as well as a marked decrease in intracellular ATP production and mitochondrial membrane potential. Together, these results indicate that mitofilin knockdown by siRNA increases calpain activity that presumably leads to mitochondrial structural degradation resulting in a critical reduction of mitochondrial function that is responsible for the increase in cell death by apoptosis via an AIF-PARP mechanism and associated with nuclear fragmentation, and S phase arrest of the cell cycle.

Synthesis and electronic properties of ester substituted 1,4-dicyanodibenzodioxins and evaluation of anti-proliferative activity of all isomeric 1,2-, 2,3- and 1,4-dicyanodibenzodioxins against HeLa cell line

1,4-Dicyanodibenzodioxins bearing carboxy methyl ester groups were synthesized using our established one-step SNAr coupling reaction between ortho- and meta-ester substituted catechols and perfluorinated terephthalonitrile. These are the first examples of 1,4-dicyanodibenzodioxins substituted at both the benzene moieties. Optical spectra were similar to the earlier examples reported, with a marginal blue shift for the ester dibenzodioxins. Theoretical analysis of the molecular orbitals reveals modest destabilization of the frontier molecular orbitals of one carboxy methyl ester isomer over the other and overall higher HOMO-LUMO gap for both isomers when compared to the earlier published 1,4-dicyanodibenzodioxins. In vitro cytotoxicity against human cervical cancer HeLa cell line was evaluated for these two compounds and all other previously published dibenzodioxins from our laboratory (1,4-dicyano, 1,2-dicyano and 2,3-dicyano variants). A number of derivatives showed anti-tumor activity in μM ranges and also exhibited no cytotoxicity against normal HEK 293 cell line. Mechanistic investigation of cell death pathways indicated high levels of reactive oxygen species (ROS) in the dibenzodioxin treated tumor cell lines along with cellular nuclear fragmentation, both of which are markers of the apoptotic cell death pathway.