Silencing of type II phosphatidylinositol 4-kinase β stabilizes prostate apoptosis response-4 and induces apoptosis in cancer cells

Type II Phosphatidylinositol 4-kinase β (PtdIns 4-kinase II β) is an enigma among the phosphatidylinositol 4-kinase family. The role of PtdIns 4-kinase II β in MCF-7 cells was addressed with the help of short hairpin RNA (shRNA). PtdIns 4-kinase II β shRNA transfection increased pan caspase activity and induced apoptosis in cancerous MCF-7 cells.  Non-cancerous MCF-10A cells were resistant to PtdIns 4-kinase II β shRNA induced apoptosis. Caspase 8 and 9 inhibitors rescued MCF-7 cells from apoptosis. Shotgun proteomic studies with Flag-tagged PtdIns 4-kinase II β immunoprecipitates showed tumor suppressor prostate apoptosis response-4 (Par-4) as one of the interacting proteins in HEK293 cells. In reciprocal experiments, Par-4 antibodies co-precipitated PtdIns 4-kinase II β from MCF-7 cells. Deletion of membrane localization motif (DCCPCC) or a mutation in ATP binding region (D304A) of PtdIns 4-kinase II β did not affect its interaction with Par-4. Pull down assays with GST-PtdIns 4-kinase II β truncated mutants showed the region between 101 to 215 amino acid residues is essential for interaction with Par-4. At molecular level, PtdIns 4-kinase II β shRNA transfection increased Par-4 stability, its nuclear localization and inhibition of NF-kappa B binding to target DNA. Knocking down of Par-4 with siRNA rescued MCF-7 cells from PtdIns 4-kinase II β shRNA induced apoptosis. These results suggest that PtdIns 4-kinase II β may be a novel regulator of Par-4 through protein-protein interactions. These studies have potential implications in cancer therapy.