A potential role for VPARP in multi-drug resistant GLC4 small cell lung carcinoma cells as determined by immunoprecipitation and mass spectrometry
Only discovered about 20 years ago, the structure of the eukaryotic vault particle has been studied extensively, but the function has yet to be determined. Vault numbers are up regulated in many types of cancer cells that are treated with chemotherapy agents and it is thought that they may act to transport chemotherapy drugs out of such cells, leading to multi-drug resistance (MDR). To determine a possible role of the vault particle in MDR, the goal of this research was to examine one of the functional vault proteins, vault poly(ADP)ribose (VPARP) for interactions with other proteins. Two forms of small cell lung cancer cells were used; GLC4/S which do not exhibit MDR and the MDR cells GLC4/ADR, which are cultured with the chemotherapy drug doxorubicin. Both cell cultures were subjected to a subcellular fractionation followed by gentle immunoprecipitation with an antibody to VPARP. Immunoprecipitated proteins interacting with VPARP were only observed in GLC4/ADR cells, as seen on a PAGE gel. This sample was taken to Monarch Life Sciences and analyzed by mass spectrometry. One interacting protein was found to be NALP1 pyrin domain (PYD), a member of the death domain family of proteins which is involved in inflammation and apoptosis. The interaction of VPARP with NALP1, which only occurred in MDR cells, suggests an exciting, previously unreported possibility – that VPARP binding may inhibit NALP 1-stimulated apoptosis when MDR is occurring. Future studies are needed to examine if levels of NALP1 vary in GLC4 cells with and without treatment with doxorubicin and in normal lung cells. The cellular location (nucleus or cytoplasm) of the interactions should also be identified. Furthermore, immunoprecipitation of proteins interacting with NALP1 should include VPARP and perhaps identify other proteins interacting in the signaling pathways under MDR and normal culture conditions. This information may contribute insight into the function of VPARP and vaults within the cell.