Background
Many members of the oxysterol binding protein related protein (ORP) family have been characterized in detail over the past decades, but the lipid transport and other functions of ORP7 still remain elusive. What is known about ORP7 points toward an endoplasmic reticulum and plasma membrane-localized protein, which also interacts with GABARAPL2 and unlipidated LC3B, suggesting a further autophagosomal/lysosomal association. Functional roles of ORP7 have been suggested in cholesterol efflux, hypercholesterolemia, and macroautophagy. We performed a hypothesis-free omics analysis of chemical ORP7 inhibition utilizing transcriptomics and lipidomics as well as proximity biotinylation interactomics to characterize ORP7 functions in a primary cell type, human umbilical vein endothelial cells (HUVECs). Moreover, assays on metrics such as angiogenesis, cholesterol efflux and lipid droplet quantification were conducted.
Results
Pharmacological inhibition of ORP7 lead to an increase in gene expression related to lipid metabolism and inflammation, while genes associated with cell cycle and cell division were downregulated. Lipidomic analysis revealed increases in ceramides, lysophosphaditylcholines, as well as saturated and monounsaturated triacylglycerols. Significant decreases were seen in all cholesteryl ester and in some unsaturated triacylglycerol species, compatible with the detected decrease of mean lipid droplet area. Along with the reduced lipid stores, ABCG1-mediated cholesterol efflux and angiogenesis decreased. Interactomics revealed an interaction of ORP7 with AKT1, a central metabolic regulator.
Conclusions
The transcriptomics results suggest an increase in prostanoid as well as oxysterol synthesis, which could be related to the observed upregulation of proinflammatory genes. We envision that the defective angiogenesis in HUVECs subjected to ORP7 inhibition could be the result of an unfavorable plasma membrane lipid composition and/or reduced potential for cell division. To conclude, the present study suggests multifaceted functions of ORP7 in lipid homeostasis, angiogenic tube formation and gene expression of lipid metabolism, inflammation and cell cycle in primary endothelial cells, possibly through AKT1 interaction.