BLOC1S2 interacts with the HIPPI protein and sensitizes NCH89 glioblastoma cells to apoptosis

The HIPPI (HIP-1 protein interactor) protein is a multifunctional protein that is involved in the regulation of apoptosis. The interaction partners of HIPPI include HIP-1 (Huntingtin-interacting protein-1), Apoptin, Homer1c, Rybp/DEDAF, and BAR (bifunctional apoptosis regulator). In search for other binding partners of HIPPI, we performed a yeast two hybrid screen and identified BLOC1S2 (Biogenesis of lysosome-related organelles complex-1 subunit 2) as a novel HIPPI-interacting protein. In co-immunoprecipitation assays, BLOC1S2 specifically associates with HIPPI, but not with HIP-1. To study the expression of BLOC1S2 on the protein level, we generated a mouse monoclonal antibody specific for BLOC1S2 and a multiple tissue array comprising 70 normal and cancer tissue samples of diverse origin. BLOC1S2 protein is widely expressed in normal tissue as well as in malignant tumors with a tendency towards lower expression levels in certain subtypes of tumors. On the subcellular level, BLOC1S2 is expressed in an organellar-like pattern and co-localizes with mitochondria. Over-expression of BLOC1S2 in the presence or absence of HIPPI does not induce apoptosis. However, BLOC1S2 and HIPPI sensitize NCH89 glioblastoma cells to the pro-apoptotic actions of staurosporine and the death ligand TRAIL by enhancing caspase activation, cytochrome c release, and disruption of the mitochondrial membrane potential. Given its interaction with HIPPI and its pro-apoptotic activity, BLOC1S2 might play an important functional role in cancer and neurodegenerative diseases.