**2.4 OSBPL1A (ORP1S) and OSBPL2 (ORP2)**

130 Dyslipidemia - From Prevention to Treatment

Insig-1 mRNA, were upregulated two-fold in the OSBP expressing livers. Silencing of OSBP in hepatocytes suppressed the induction of SREBP1-c by insulin and resulted in a reduction of TG synthesis. These results demonstrate that OSBP regulates hepatic TG metabolism and suggest the involvement of OSBP in the insulin signaling pathways that control hepatic

 Bowden et al. revealed that suppression of OSBP in Chinese hamster ovary cells by RNA interference resulted in increased ABCA1 protein expression and cholesterol efflux activity following induction with oxysterols or the synthetic LXR agonist TO901317. OSBP knockdown in J774 macrophages also increased ABCA1 expression in the presence and absence of LXR agonists. Their results demonstrate that OSBP opposes the activity of LXR by negatively regulating ABCA1 activity in the cytoplasm by sterol-binding domain-

Cephalostatin 1, OSW-1, ritterazine B and schweinfurthin A are natural products that potently, and in some cases selectively, inhibit the growth of cultured human cancer cell lines. Recently, Burgett et al. have discovered that these molecules target OSBP and its closest paralog, ORP4L, and have named these natural products ORPphilins (Burgett, Poulsen et al. 2011). By uncovering the cellular targets of the ORPphilins, they have revealed

They also show that ORPphilins perturb the cellular localization of OSBP and affect sphingomyelin biosynthesis. The ORPphilins are powerful probes of OSBP and ORP4L that will be useful in uncovering their cellular functions and their roles in human diseases.

**Structure, Tissue distribution and Intracellular localization:** ORP1L belongs to the subfamily II of OSBP/ORP homologues. ORP1L has an ORD, an FFAT motif, a PH domain

While macrophages, brain, and lung are the areas where ORP1L is expressed most predominantly, it is also found in colon, kidney, and liver (Johansson, Bocher et al. 2003).

**Molecular functions related to dyslipidemia:** Johansson et al. reported that ORP1L binds to Rab7, modifies its functional cycle, and can interfere with LE/lysosome organization and

They show that the GTPase Rab7, when bound to GTP, simultaneously binds to ORP1L and RILP to form a RILP-Rab7-ORP1L complex, which is required for the perinuclear localization of late endosomes/lysosomes (Johansson, Rocha et al. 2007). The later study of Rocha et al., went deeper in examining these processes more in detail. They found that the cholesterol levels in late endosomes are sensed by ORP1L and are lower in peripheral vesicles. Under low cholesterol conditions, ORP1L conformation induces the formation of endoplasmic reticulum (ER)- late endosome membrane contact sites. At these sites, the ER protein VAP (VAMP [vesicle-associated membrane protein]-associated ER protein) can interact in trans with the Rab7-RILP complex to remove p150 (Glued) and associated motors. late endosomes then move to the microtubule plus end. Under high cholesterol conditions, as in Niemann-Pick type C disease, this process is prevented, and late endosomes accumulate at the microtubule minus end as the result of dynein motor activity. These data explain how the ER and cholesterol control the association of late endosomes with motor proteins and their

**Epidemiological study:** Epidemiological study of OSBP is not reported yet.

dependent protein destabilization (Bowden and Ridgway 2008).

that OSBP and ORP4L are involved in cancer cell survival.

endocytic membrane trafficking (Johansson, Lehto et al. 2005).

lipogenesis (Yan, Lehto et al. 2007).

**2.3 OSBPL1B (ORP1L)** 

and three ankyrin repeats.

ORP1L localizes to late endosomes.

positioning in cells (Rocha, Kuijl et al. 2009).

**Structure, Tissue distribution and Intracellular localization:** ORP1S and ORP2 belong to the subfamily II of OSBP/ORP homologues. ORP1S and ORP2 have an ORD and an FFAT motif but lack PH domain.

ORP1S is expressed predominantly in skeletal muscle and heart (Johansson, Bocher et al. 2003). ORP2 is expressed ubiquitously in mammalian tissues. Highest mRNA levels of ORP2 are present in specific parts of the central nervous system (cerebellum, pituitary gland, pons, and putamen) as well as in leukocytes, placenta, and pancreas (Laitinen, Lehto et al. 2002).

ORP1S has been reported to be largely cytosolic (Johansson, Bocher et al. 2003) and ORP2 localizes, in addition to a cytosolic fraction, on the surface of lipid droplets (LDs) and also the plasma membrane (PM) (Hynynen, Suchanek et al. 2009).

**Molecular functions related to dyslipidemia:** In the earlier study of Hynynen et al., overexpression of ORP2 induces enhancement of [14C]cholesterol efflux to all extracellular acceptors, which results in a reduction of cellular free cholesterol. They also show that ORP2 binds PtdIns(3,4,5)P(3) and enhances endocytosis.

In their recent study, Hynynen et al. discover that ORP2 localizes not only cytosolic fraction but also on cytoplasmic LDs and reveal its function in neutral lipid metabolism. They show that the ORP2 LD association depends on sterol binding: Treatment with 5 mM 22(R)OHC inhibits the LD association, while a mutant defective in sterol binding is constitutively LD bound. Silencing of ORP2 using RNA interference slows down cellular TG hydrolysis. Furthermore, ORP2 silencing increases the amount of [14C]cholesteryl esters but only under conditions in which lipogenesis and LD formation are enhanced by treatment with oleic acid (Hynynen, Suchanek et al. 2009). These results identify ORP2 as a sterol receptor present on LD and provide an evidence for its role in the regulation of neutral lipid metabolism, possibly as a factor that integrates the cellular metabolism of TG with that of cholesterol.

By overexpressing all mammalian ORPs, Jansen et al. found that especially ORP1S and ORP2 enhanced PM-to-LD sterol transport. This reflected the stimulation of transport from the PM to the ER, rather than from the ER to LDs. Double knockdown of ORP1S and ORP2 inhibited sterol transport from the PM to the ER and LDs, suggesting a physiological role for these ORPs in the process (Jansen, Ohsaki et al. 2011). These findings suggest that ORP1S and ORP2 are essential in controlling cellular neutral lipid and cholesterol and has a strong association with the pathophysiology of dyslipidemia.

**Epidemiological study:** Epidemiological studies of ORP1S or ORP2 are not reported yet.

Functions of OSBP/ORP Family Proteins and Their Relation to Dyslipidemia 133

**Structure, Tissue distribution and Intracellular localization:** ORP5 belongs to the subfamily IV of OSBP/ORP homologues. ORP5 has an ORD, a PH domain and a

**Molecular functions related to dyslipidemia:** Knocking down ORP5 causes cholesterol accumulation in late endosomes and lysosomes, which is reminiscent of the cholesterol trafficking defect in Niemann Pick C (NPC) fibroblasts (Du, Kumar et al. 2011). Cholesterol appears to accumulate in the limiting membranes of endosomal compartments in ORP5 depleted cells, whereas depletion of NPC1 or both ORP5 and NPC1 results in luminal accumulation of cholesterol. Moreover, trans-Golgi resident proteins mislocalize to endosomal compartments upon ORP5 depletion, which depends on a functional NPC1. Niemann-Pick type C (NPC) disease is most often caused by mutations in the NPC1 gene, whose protein product is believed to facilitate the egress of cholesterol and other lipids from late endosomes and lysosomes to other cellular compartments (Boadu and Francis 2006). The results of the research by Du et al. establish the first link between NPC1 and a cytoplasmic sterol carrier, and suggest that ORP5 may cooperate with NPC1 to mediate the

**Structure, Tissue distribution and Intracellular localization:** ORP6 belongs to the subfamily III of OSBP/ORP homologues. ORP6 has an ORD, a PH domain and an FFAT motif. ORP6 shows the highest expression in brain and skeletal muscle (Lehto, Tienari et al. 2004). Endogenous ORP6 associated predominantly with the nuclear envelope. When expressed from the cDNA in cultured cells, ORP6 was distributed between the cytosol and

**Molecular functions related to dyslipidemia:** The direct evidence that ORP6 functions to

**Epidemiological study:** Using the Framingham Heart Study data set, a quantitative trait locus in the chromosome 2q was found to be significantly involved in variations of HDL cholesterol levels. ORP6 is identified as one of the candidate genes that are possibly involved in the

**Structure, Tissue distribution and Intracellular localization:** ORP7 belongs to the subfamily III of OSBP/ORP homologues. ORP7 has an ORD, a PH domain and a an FFAT motif. ORP7 shows the highest expression in the gastrointestinal tract (Lehto, Tienari et al. 2004). When expressed from the cDNA in cultured cells, ORP7 was distributed between the cytosol and ER membranes, with a minor portion found at the PM. The N-terminal portion of the proteins, containing a PH domain, has markedly strong PM targeting specificity, while the C-terminal half remains largely cytosolic. The dual targeting of the proteins

**Molecular functions related to dyslipidemia:** Recently, Zhong et al. identified by yeast twohybrid screening an interaction partner of ORP7, GATE-16, which (i) regulates the function and stability of Golgi SNARE of 28kDa (GS28), and (ii) plays a role in autophagosome

**2.7 OSBPL5 (ORP5)** 

**2.8 OSBPL6 (ORP6)** 

**2.9 OSBPL7 (ORP7)** 

transmembrane domain. ORP5 localizes to the ER.

exit of cholesterol from endosomes/lysosomes.

ER membranes, with a minor portion found at the PM.

regulate Dyslipidemia is yet to be reported.

biogenesis (Zhong, Zhou et al. 2011).

**Epidemiological study:** Epidemiological study of ORP5 is not reported yet.

regulation of HDL cholesterol levels in this region (North, Martin et al. 2003).

indicates a putative role in communication between the ER and the PM.
