OSBP-related protein-2 (ORP2): a novel Akt effector that controls cellular energy metabolism

Abstract

ORP2 is a ubiquitously expressed OSBP-related protein previously implicated in endoplasmic reticulum (ER)-lipid droplet (LD) contacts, triacylglycerol (TG) metabolism, cholesterol transport, adrenocortical steroidogenesis, and actin-dependent cell dynamics. Here, we characterize the role of ORP2 in carbohydrate and lipid metabolism by employing ORP2-knockout (KO) hepatoma cells (HuH7) generated by CRISPR-Cas9 gene editing. The ORP2-KO and control HuH7 cells were subjected to RNA sequencing, analyses of Akt signaling, carbohydrate and TG metabolism, the extracellular acidification rate, and the lipidome, as well as to transmission electron microscopy. The loss of ORP2 resulted in a marked reduction of active phosphorylated Akt(Ser473) and its target Glycogen synthase kinase 3β(Ser9), consistent with defective Akt signaling. ORP2 was found to form a physical complex with the key controllers of Akt activity, Cdc37, and Hsp90, and to co-localize with Cdc37 and active Akt(Ser473) at lamellipodial plasma membrane regions, in addition to the previously reported ER-LD localization. ORP2-KO reduced glucose uptake, glycogen synthesis, glycolysis, mRNA-encoding glycolytic enzymes, and SREBP-1 target gene expression, and led to defective TG synthesis and storage. ORP2-KO did not reduce but rather increased ER-LD contacts under basal culture conditions and interfered with their expansion upon fatty acid loading. Together with our recently published work (Kentala et al. in FASEB J 32:1281-1295, 2018), this study identifies ORP2 as a new regulatory nexus of Akt signaling, cellular energy metabolism, actin cytoskeletal function, cell migration, and proliferation.

OSBP-related protein 2 (ORP2): Unraveling its functions in cellular lipid/carbohydrate metabolism, signaling and F-actin regulation

“…These observations suggested that a major function of ORP2 could involve the ER-LD interfaces and the TG metabolic processes occurring therein. However, the study of Kentala et al [27] revealed that the loss of ORP2 did not reduce ER-LD contacts, but under basal culture conditions rather increased them. However, this work indicated that the dynamic expansion of the ER-LD MCSs upon fatty acid loading may be disturbed in the ORP2 KO cells.…”

“…Cell cortex localization of ORP2 was also observed in human umbilical vein endothelial cells (HUVECs) that seldom have large LDs. In these cells, the protein was frequently found co-localizing with the actin regulator cortactin in lamellipodia [27].…”

“…Kentala et al [27,35] generated HuH7 hepatoma cells entirely devoid of ORP2 by using the CRISPR-Cas9 technology. Analysis of the lipidome of these cells revealed no change in free cholesterol (FC), cholesterol ester (CE) or oxysterol (25OHC, 27OHC, 7αOHC, 7βOHC, 7KC) content [27].…”

“…Kentala et al [27,35] generated HuH7 hepatoma cells entirely devoid of ORP2 by using the CRISPR-Cas9 technology. Analysis of the lipidome of these cells revealed no change in free cholesterol (FC), cholesterol ester (CE) or oxysterol (25OHC, 27OHC, 7αOHC, 7βOHC, 7KC) content [27]. Even challenge of the cells by sterol depletion by employing lipoprotein-deficient serum plus statin treatment, or by LDL loading, did not change the situation, suggesting that a chronic lack of ORP2 did not distort cholesterol homeostatic regulation in the hepatoma cells.…”

Bis(monoacylglycero)phosphate regulates oxysterol binding protein-related protein 11 dependent sterol trafficking

“…More interestingly, three ORP genes were upregulated upon incubation of the cells with oxLDL: ORP2, ORP9 and ORP11. ORP2 regulates the membrane contact between ER and lipid droplets, providing a plausible explanation for its up-regulation under oxLDL exposure [42][43][44][45]. ORP9 localizes at the Golgi/ER interface and regulates phosphatidylinositol-4-phosphate (PI4P) metabolism in Golgi membranes, competitively with ORP4S [46].…”