Determining the Role of Oxysterol Binding Protein-Related Protein 4 (ORP4) in Cell Proliferation and Survival

Oxysterol binding protein (OSBP) and related proteins (ORPs) constitute a 12-member family of mammalian lipid binding proteins with diverse cellular expression, localization and ligands. ORP4 is closely related to OSBP and expressed as short (ORP4S), medium (ORP4M) and long (ORP4L) isoforms as a result of alternate transcription start sites. ORP4 was initially discovered in disseminated tumor cells and considered a potential marker of metastasis. We recently showed that lentiviral short-hairpin RNA silencing of ORP4 induced cell-specific growth arrest or apoptosis. To determine how ORP4 promotes cell proliferation and survival, we identified relevant signaling pathways that are activated upon ORP4 silencing. In HeLa cells, ORP4 silencing was accompanied by elevated expression of p21 and phospho-p53 isoforms as well as cell cycle arrest but not apoptosis. ORP4 silencing also induced growth arrest in HEK 293 cells but triggered apoptosis in non-transformed rat intestinal epithelial cells. A potential role f...