OSBP-Related Protein 8 (ORP8) Regulates Plasma and Liver Tissue Lipid Levels and Interacts with the Nucleoporin Nup62

Abstract

We earlier identified OSBP-related protein 8 (ORP8) as an endoplasmic reticulum oxysterol-binding protein implicated in cellular lipid homeostasis. We now investigated its action in hepatic cells in vivo and in vitro. Adenoviral overexpression of ORP8 in mouse liver induced a decrease of cholesterol, phospholipids, and triglycerides in serum (−34%, −26%, −37%, respectively) and liver tissue (−40%, −12%, −24%), coinciding with reduction of nuclear (n)SREBP-1 and -2 and mRNA levels of their target genes. Consistently, excess ORP8 reduced nSREBPs in HuH7 cells, and ORP8 overexpression or silencing by RNA interference moderately suppressed or induced the expression of SREBP-1 and SREBP-2 target genes, respectively. In accordance, cholesterol biosynthesis was reduced by ORP8 overexpression and enhanced by ORP8 silencing in [3H]acetate pulse-labeling experiments. ORP8, previously shown to bind 25-hydroxycholesterol, was now shown to bind also cholesterol in vitro. Yeast two-hybrid, bimolecular fluorescence complementation (BiFC), and co-immunoprecipitation analyses revealed the nuclear pore component Nup62 as an interaction partner of ORP8. Co-localization of ORP8 and Nup62 at the nuclear envelope was demonstrated by BiFC and confocal immunofluorescence microscopy. Furthermore, the impact of overexpressed ORP8 on nSREBPs and their target mRNAs was inhibited in cells depleted of Nup62. Our results reveal that ORP8 has the capacity to modulate lipid homeostasis and SREBP activity, probably through an indirect mechanism, and provide clues of an entirely new mode of ORP action.

Oxysterol-binding Protein-related Protein 8 (ORP8) Increases Sensitivity of Hepatocellular Carcinoma Cells to Fas-Mediated Apoptosis

“…ORP8 was previously shown to localize to the ER and nuclear envelope (11,12). We found that ORP8 was also distributed to the hepatocyte plasma membrane.…”

“…Cell Lines-Our previous study demonstrated that ORP8 overexpression in mouse liver significantly reduces the plasma total cholesterol level in vivo (12). Because aberrant elevation of the cholesterol level is associated with various types of cancers (16), a total of 67 clinical HCC samples were analyzed for ORP8 expression for both mRNA and protein expression by qRT-PCR and Western blot.…”

“…Our previous study indicated that ORP8 decreases cholesterol efflux in macrophages by suppressing ABCA1 expression, implying that it may play a role in the development of atherosclerotic lesions (11). In hepatic cells ORP8 functions as a negative regulator of intracellular cholesterol (12). Other roles have been suggested for ORP8, including the inhibition of cell migration through interaction with nucleoporin Nup62 (13) and mediation of oxysterol interference of the cell cycle through interaction with Astrin/SPAG5 in HepG2 cells (14).…”

Cholesterol metabolism and the pathogenesis of non-alcoholic steatohepatitis

“…Hepatic StAR expression was increased 7-and 15-fold in steatosis and NASH patients, respectively 11 , thereby enhancing mitochondrial free cholesterol accumulation and toxicity Heterozygous NPC1/2 deficiency leads to fatty liver, obesity and metabolic syndrome 82 NPC1 gene is linked to to early-onset and morbid adult obesity in Europeans 83 ORP8 ↓ hepatic cholesterol and triglyceride accumulation by inhibiting activation of SREBP-2 and SREBP-1c ORP8 ↑ hepatic insulin sensitivity through an AKTmediated mechanism 86,87 Cholesterol absorption and secretion [39][40][41] 25OHC3S had opposite effects than 25OHC on lipid metabolism and inflammation and improved NAFLD in cellular and mouse models( [39][40][41] Enhancement of on of SULT2B1b activity may counteract effects of toxic oxysterols …”

Functions of OSBP/ORP Family Proteins and Their Relation to Dyslipidemia

“…ORP8 negatively regulates ABCA1 expression and macrophage cholesterol efflux (Yan, Mayranpaa et al 2008). ORP8 has the capacity to modulate lipid homeostasis and SREBP activity, probably through an indirect mechanism required Nup62 (Zhou, Li et al 2011). The OSBPL8-ZDHHC17 region (chr12) is detected for HDL cholesterol identified by one new SNP with genome-wide significance (Ma, Yang et al 2010).…”