ORP2 - a lipid-binding protein controlling cellular cholesterol transport and signal transduction

Lipids are distributed in a highly heterogeneous manner between different cellular membranes. It is becoming evident that while the bulk lipid transport occurs via vesicular transport, a major portion of lipids are trafficked by lipid transfer proteins (LTPs). LTPs are known to be of central importance in the maintenance of organelle-specific lipid compositions and thus to have a role in numerous physiological and pathological processes. In addition, many of them take part in integral cell signaling processes. Oxysterol binding protein-related protein 2 (ORP2) belongs to a ubiquitously expressed and conserved family of LTPs, the oxysterol binding protein (OSBP)-relate proteins. ORP2 has been implicated to participate e.g. in triacylglycerol (TG) and cholesterol homeostasis, and in cell adhesion, migration and proliferation. Here, we characterize its functions in carbohydrate and TG metabolism in hepatocytes (HCs), examine its involvement on subcellular transport of cholesterol in endothelial cells (ECs) and assess the role of ORP2 in key cell signaling processes in both HCs and ECs. Finally, we investigate the involvement of ORP2 in EC angiogenesis in vitro and in vivo, a function that has not been previously studies for any intracellular LTP. The depletion of ORP2 led to dysregulated transcriptional regulation of lipogenesis in both of the studied cell types. In addition, it reduced glucose uptake, glycogen synthesis and glycolysis, and led to defective TG synthesis and storage in HCs. ORP2 was also found to facilitate cholesterol transport between the late endosomes/lysosomes (LE/LY) and plasma membrane (PM) and to interact with the related LE/LY cholesterol transporter ORP1L. Suppression of ORP2 led to reduced cholesterol content in the PM raft domains, while increasing the PM PI(4,5)P2 content, identifying ORP2 as a novel regulator of PM lipid organization. ORP2 was found to regulate the activity of a central cell homeostasis effector Akt, and to physically interact with a complex known to sustain its activity. We also demonstrated that the depletion of ORP2 inhibited EC angiogenic signaling by reducing the activating phosphorylation of the principal angiogenic receptor, VEGFR2, and several of its downstream targets. These defects coincided with reduced angiogenic cell functions including migration, proliferation, viability and tube formation. Retinal analysis of ORP2 knock-out mice generated during this study revealed abnormal vascular morphology validating our results in vivo. Taken together, this study identifies ORP2 as a novel nexus interconnecting cellular bioenergetics and signaling to cell anabolic reactions. Among only few known LTPs, ORP2 was shown to regulate PM lipid organization, and to impact vascular development. The data generated promote the detailed understanding of the molecular basis of common diseases such as cardiovascular diseases and cancers, as well as the development of new treatment modalities.