OSBP is a cholesterol-regulated scaffolding protein in control of ERK 1/2 activation

Lipid- and protein-directed photosensitizer proximity labeling captures the cholesterol interactome

The physical properties of cellular membranes, including fluidity and function, are influenced by protein and lipid interactions. In situ labeling chemistries, most notably proximity-labeling interactomics are well suited to characterize these dynamic and often fleeting interactions. Established methods require distinct chemistries for proteins and lipids, which limits the scope of such studies. Here we establish a singlet-oxygen-based photocatalytic proximity labeling platform (POCA) that reports intracellular interactomes for both proteins and lipids with tight spatiotemporal resolution using cell-penetrant photosensitizer reagents. Using both physiologically relevant lipoprotein-complexed probe delivery and genetic manipulation of cellular cholesterol handling machinery, cholesterol-directed POCA captured established and unprecedented cholesterol binding proteins, including protein complexes sensitive to intracellular cholesterol levels and proteins uniquely captured by lipoprotein uptake. Protein-directed POCA accurately mapped known intracellular membrane complexes, defined sterol-dependent changes to the non-vesicular cholesterol transport protein interactome, and captured state-dependent changes in the interactome of the cholesterol transport protein Aster-B. More broadly, we find that POCA is a versatile interactomics platform that is straightforward to implement, using the readily available HaloTag system, and fulfills unmet needs in intracellular singlet oxygen-based proximity labeling proteomics. Thus, we expect widespread utility for POCA across a range of interactome applications, spanning imaging to proteomics.

Oxysterol binding protein regulates the resolution of TLR-induced cytokine production in macrophages

Toll-like receptors (TLRs) on macrophages sense microbial components and trigger the production of numerous cytokines and chemokines that mediate the inflammatory response to infection. Although many of the components required for the activation of the TLR pathway have been identified, the mechanisms that appropriately regulate the magnitude and duration of the response and ultimately restore homeostasis are less well understood. Furthermore, a growing body of work indicates that TLR signaling reciprocally interacts with other fundamental cellular processes, including lipid metabolism but only a few specific molecular links between immune signaling and the macrophage lipidome have been studied in detail. Oxysterol-binding protein (Osbp) is the founding member of a family of lipid-binding proteins with diverse functions in lipid sensing, lipid transport, and cell signaling but its role in TLR responses is not well defined. Here, we demonstrate that altering the state of Osbp with its natural ligand, 25-hydroxycholesterol (25HC), or pharmacologically, sustains and thereby amplifies Tlr4-induced cytokine production in vitro and in vivo. CRISPR-induced knockdown of Osbp abrogates the ability of these ligands to sustain TLR responses. Lipidomic analysis suggested that the effect of Osbp on TLR signaling may be mediated by alterations in triglyceride production and treating cells with a Dgat1 inhibitor, which blocks triglyceride production and completely abrogates the effect of Osbp on TLR signaling. Thus, Osbp is a sterol sensor that transduces perturbations of the lipidome to modulate the resolution of macrophage inflammatory responses.

Coordination of oxysterol binding protein 1 and VAP-A/B modulates the generation of cholesterol and viral inclusion bodies to promote grass carp reovirus replication

Similar to other RNA viruses, grass carp reovirus, the causative agent of the hemorrhagic disease, replicates in cytoplasmic viral inclusion bodies (VIBs), orchestrated by host proteins and lipids. The host pathways that facilitate the formation and function of GCRV VIBs are poorly understood. This work demonstrates that GCRV manipulates grass carp oxysterol binding protein 1 (named as gcOSBP1) and vesicle-associated membrane protein-associated protein A/B (named as gcVAP-A/B), 3 components of cholesterol transport pathway, to generate VIBs. By siRNA-mediated knockdown, we demonstrate that gcOSBP1 is an essential host factor for GCRV replication. We reveal that the nonstructural proteins NS80 and NS38 of GCRV interact with gcOSBP1, and that the gcOSBP1 is recruited by NS38 and NS80 for promoting the generation of VIBs. gcOSBP1 increases the expression of gcVAP-A/B and promotes the accumulation of intracellular cholesterol. gcOSBP1 also interacts with gcVAP-A/B for forming gcOSBP1-gcVAP-A/B complexes, which contribute to enhance the accumulation of intracellular cholesterol and gcOSBP1-mediated generation of VIBs. Inhibiting cholesterol accumulation by lovastatin can completely abolish the effects of gcOSBP1 and/or gcVAP-A/B in promoting GCRV infection, suggesting that cholesterol accumulation is vital for gcOSBP1- and/or gcVAP-A/B-mediated GCRV replication. Thus, our results, which highlight that gcOSBP1 functions in the replication of GCRV via its interaction with essential viral proteins for forming VIBs and with host gcVAP-A/B, provide key molecular targets for obtaining anti-hemorrhagic disease grass carp via gene editing technology.

Untargeted lipidomics reveals lipid metabolism disorders induced by oxathiapiprolin in Phytophthora sojae

BACKGROUND

Oxathiapiprolin, an oxysterol-binding protein inhibitor (OSBPI), shows unexceptionable inhibitory activity against plant pathogenic oomycetes. FRAC (Fungicide Resistance Action Committee) classifies it into the mode of action group F9 (lipid homeostasis and transfer/storage), but very little is known about the lipid metabolism of oomycete pathogens when subjected to oxathiapiprolin.

RESULTS

In this study, seven lipid categories and 1435 lipid molecules were identified in Phytophthora sojae, among which glycerolipids, glycerophospholipids, and sphingolipids account for 30.10%, 50.59%, and 7.28%, respectively. These lipids were categorized into 31 subclasses, which varied to different extents when treated with oxathiapiprolin. A total of 11 lipid subclasses showed significant changes. Among them, 10 lipid subclasses, lysophosphatidylcholine (LPC), lysophosphatidylethanolamine (LPE), phosphatidylcholine (PC), phosphatidylserine (PS), ceramide (Cer), triglyceride (TG), (o-acyl)-1-hydroxy fatty acid, diglycosylceramide, sphingoshine (So), and sitosterol ester, were significantly up-regulated, while digalactosyldiacylglycerol was the only lipid that was significantly down-regulated by a factor of almost three. These lipid molecules were further analyzed at the lipid species level. A total of 542 species were significantly altered when treated with oxathiapiprolin, including 212 glycerolipids [186 TG and 26 diglycerides (DG)], 167 glycerophospholipids (38 PC, 15 LPC, 19 LPE, seven PS, etc.), 156 sphingolipids (146 Cer, four So, etc.), and some other lipid molecules. Finally, from the orthogonal partial least-squares discrimination analysis model, variable importance for the projection score analysis showed that Cer, TG, and some glycerophospholipids contribute to the metabolic disorder when subjected to oxathiapiprolin.

CONCLUSION

Glycerolipids, glycerophospholipids, and sphingolipids in P. sojae undergo significant changes with oxathiapiprolin treatment. These results provided valuable information for further understanding the function of the target protein and the mode of action of OSBPIs in oomycetes. © 2022 Society of Chemical Industry.

Structure-Activity Relationships of Ligand Binding to Oxysterol-Binding Protein (OSBP) and OSBP-Related Protein 4

Oxysterol-binding protein (OSBP) and OSBP-related protein 4 (ORP4) have emerged as potentially druggable targets in antiviral and precision cancer drug development. Multiple structurally diverse small molecules function through targeting the OSBP/ORP family of proteins, including the antiviral steroidal compounds OSW-1 and T-00127-HEV2. Here, the structure-activity relationships of oxysterols and related compound binding to human OSBP and ORP4 are characterized. Oxysterols with hydroxylation at various side chain positions (i.e., C-20, C-24, C-25, and C-27)─but not C-22─confer high affinity interactions with OSBP and ORP4. A library of 20(S)-hydroxycholesterol analogues with varying sterol side chains reveal that side chain length modifications are not well tolerated for OSBP and ORP4 interactions. This side chain requirement is contradicted by the high affinity binding of T-00127-HEV2, a steroidal compound lacking the side chain. The binding results, in combination with docking studies using homology models of OSBP and ORP4, suggest multiple modes of steroidal ligand binding to OSBP and ORP4.

Long Non-Coding RNA TUG1 Attenuates Insulin Resistance in Mice with Gestational Diabetes Mellitus via Regulation of the MicroRNA-328-3p/SREBP-2/ERK Axis

BACKGROUND

Long non-coding RNAs (lncRNAs) have been illustrated to contribute to the development of gestational diabetes mellitus (GDM). In the present study, we aimed to elucidate how lncRNA taurine upregulated gene 1 (TUG1) influences insulin resistance (IR) in a high-fat diet (HFD)-induced mouse model of GDM.

METHODS

We initially developed a mouse model of HFD-induced GDM, from which islet tissues were collected for RNA and protein extraction. Interactions among lncRNA TUG1/microRNA (miR)-328-3p/sterol regulatory element binding protein 2 (SREBP-2) were assessed by dual-luciferase reporter assay. Fasting blood glucose (FBG), fasting insulin (FINS), homeostasis model assessment of insulin resistance (HOMA-IR), HOMA pancreatic β-cell function (HOMA-β), insulin sensitivity index for oral glucose tolerance tests (ISOGTT) and insulinogenic index (IGI) levels in mouse serum were measured through conducting gain- and loss-of-function experiments.

RESULTS

Abundant expression of miR-328 and deficient expression of lncRNA TUG1 and SREBP-2 were characterized in the islet tissues of mice with HFD-induced GDM. LncRNA TUG1 competitively bound to miR-328-3p, which specifically targeted SREBP-2. Either depletion of miR-328-3p or restoration of lncRNA TUG1 and SREBP-2 reduced the FBG, FINS, HOMA-β, and HOMA-IR levels while increasing ISOGTT and IGI levels, promoting the expression of the extracellular signal-regulated kinase (ERK) signaling pathway-related genes, and inhibiting apoptosis of islet cells in GDM mice. Upregulation miR-328-3p reversed the alleviative effects of SREBP-2 and lncRNA TUG1 on IR.

CONCLUSION

Our study provides evidence that the lncRNA TUG1 may prevent IR following GDM through competitively binding to miR-328-3p and promoting the SREBP-2-mediated ERK signaling pathway inactivation.

Alpha lipoic acid treatment in late middle age improves cognitive function: Proteomic analysis of the protective mechanisms in the hippocampus

Alpha lipoic acid (ALA), a powerful antioxidant, has the potential to relieve age-related cognitive impairment and neurodegenerative disease. Clinical randomized controlled studies have demonstrated the cognitive improvement effects of lipoic acid in Alzheimer's disease (AD). In the present study, we examined the effects of ALA on cognitive function in ageing mice and its protective mechanisms. Eighteen-month-old male C57BL6/J mice received ALA or normal saline for 2 months. The Morris water maze test revealed improved cognitive function in animals that received ALA. Furthermore, tandem Mass Tags (TMT) based liquid chromotography with mass spectrometry/mass spectrometry (LC-MS/MS) was established to identify the target proteins. The results showed that 10 proteins were changed significantly. Gene Ontology (GO) analysis indicated that the upregulated proteins were enriched in terminal bouton, synaptic transmission and lipid transporter activity while the down-regulated proteins were involved in nuclear transcription factor-κB binding, apoptosis and mitogen-activated protein kinase binding. Based on the GO results, two upregulated proteins oxysterol-binding protein-related protein 10 (OSBPL10) and oligophrenin 1 (OPHN1), and one downregulated protein, CDK5 regulatory subunit-associated protein 3 (CDK5rap3), were validated through Western blotting. The results were consistent with the proteomic results. Modulation of synaptic transmission, lipid transporter activity and neuroinflammation appears to be the mechanisms of ALA in the aged brain.

Unregulated LDL cholesterol uptake is detrimental to breast cancer cells

Tumor uptake of exogenous cholesterol has been associated with the proliferation of various cancers. Previously, we and others have shown that hypercholesterolemia promotes tumor growth and silencing of the LDL receptor (LDLR) in high LDLR-expressing tumors reduces growth. To advance understanding of how LDL uptake promotes tumor growth, LDLR expression was amplified in breast cancer cell lines with endogenously low LDLR expression. Murine (Mvt1) and human (MDA-MB-468) breast cancer cell lines were transduced to overexpress human LDLR (LDLROE). Successful transduction was confirmed by RNA and protein analysis. Fluorescence-labeled LDL uptake was increased in both Mvt1 and MDA-MD-468 LDLROE cells. The expression of the cholesterol-metabolizing genes, ABCA1 and ABCG1, was increased, while HMGCR was decreased in the MDA-MB-468 LDLROE cells. In contrast, Mvt1 LDLROE cells showed no differences in Abca1 and Abcg1 expression and increased Hmgcr expression. Using a Seahorse analyzer, Mvt1 LDLROE cells showed increased respiration (ATP-linked and maximal) relative to controls, while no statistically significant changes in respiration in MDA-MB-468 LDLROE cells were observed. Growth of LDLROE cells was reduced in culture and in hypercholesterolemic mice by two-fold. However, the expression of proliferation-associated markers (Ki67, PCNA and BrdU-label incorporation) was not decreased in the Mvt1 LDLROE tumors and cells. Caspase-3 cleavage, which is associated with apoptosis, was increased in both the Mvt1 and MDA-MB-468 LDLROE cells relative to controls, with the Mvt1 LDLROE cells also showing decreased phosphorylation of p44/42MAPK. Taken together, our work suggests that while additional LDL can promote tumor growth, unregulated and prolonged LDL uptake is detrimental.

Association of longitudinal changes in serum lipids with the natural history of subclinical hypothyroidism: A retrospective cohort study using data from the REACTION study

BACKGROUND

Subclinical hypothyroidism (SCH) often leads to alterations in lipid profile, which may negatively impact humans health. Whether lipids in turn affect the natural history of SCH is unknown. We aimed to assess the association between longitudinal changes in serum lipid levels and the natural history of SCH.

METHODS

This retrospective cohort study using data from the REACTION study included 581 patients with SCH who were enrolled between July 1, 2011, and December 19, 2014, with a median follow-up of three [IQR, 2·86-3·21] years. Patients with missing data or conditions that can affect thyroid function were excluded. Changes in serum lipid levels were calculated from serum lipid measurements 3 years apart and classified in two ways: 1) the first, second, and third tertiles of the difference between baseline and follow-up and 2) the percent change from baseline, namely, serum lipid decrease ≥ 25%, minor change, and serum lipid increase ≥ 25%. The natural history of SCH includes regression to euthyroidism, SCH persistence, or progression to overt hypothyroidism (OH). Odds ratios (ORs) were estimated by multivariable logistic regression. Validation was performed on data from a health management cohort study conducted from January 1, 2012, to December 31, 2016, with a median follow-up of two [IQR, 1·92-2·08] years. After using the same inclusion and exclusion criteria as the REACTION cohort study, 412 patients with SCH were eligible for the validation analysis.

FINDINGS

There were 132 (22·7%) men and 449 (77·3%) women in the study, with a median age of 56 [IQR,49-62] years. During follow-up, 270 (46·5%), 266 (45·8%), and 27 (4·6%) patients had regression to euthyroidism, persistent SCH, and progression to OH, respectively. Both grouping manners showed a significant association between changes in lipid levels and the natural history of SCH. A total cholesterol (TC)-level increase was independently associated with a greater risk of progression to OH (OR for ≥ 25% TC increase vs. minor change: 5·40; 95% CI 1·46-21·65), whereas TC-level declines increased the likelihood of regressing to euthyroidism (OR for ≥ 25% TC decrease vs. minor change: 3·45; 95% CI 1·09-12·43). Similarly, the likelihood of regression according to changes in triglyceride (TG) levels exhibited a consistent trend with that according to TC-level changes. A similar pattern of association was observed in the validation cohort.

INTERPRETATION

Changes in serum lipid levels in SCH are associated with future progression or regression risk, suggesting that the changes in serum lipid levels may affect the natural history of SCH. Clinicians should pay attention to the long-term control of serum lipids levels in populations with SCH, which may benefit thyroid function.

FUNDING

This work was supported by grants from the National Key Research and Development Program of China (2017YFC1309800), the National Natural Science Foundation (81430020, 82070818), and the "Outstanding University Driven by Talents" Program and Academic Promotion Program of Shandong First Medical University (2019LJ007).

YM750, an ACAT Inhibitor, Acts on Adrenocortical Cells to Inhibit Aldosterone Secretion Due to Depolarization

Primary aldosteronism (PA) is considered the most common form of secondary hypertension, which is associated with excessive aldosterone secretion in the adrenal cortex. The cause of excessive aldosterone secretion is the induction of aldosterone synthase gene (CYP11B2) expression by depolarization of adrenocortical cells. In this study, we found that YM750, an Acyl-coenzyme A: cholesterol acyltransferase (ACAT) inhibitor, acts on adrenocortical cells to suppress CYP11B2 gene expression and aldosterone secretion. YM750 inhibited the induction of CYP11B2 gene expression by KCl stimulation, but not by angiotensin II and forskolin stimulation. Interestingly, YM750 did not inhibit KCl-stimulated depolarization via an increase in intracellular calcium ion concentration. Moreover, ACAT1 expression was relatively abundant in the zona glomerulosa (ZG) including these CYP11B2-positive cells. Thus, YM750 suppresses CYP11B2 gene expression by suppressing intracellular signaling activated by depolarization. In addition, ACAT1 was suggested to play an important role in steroidogenesis in the ZG. YM750 suppresses CYP11B2 gene expression and aldosterone secretion in the adrenal cortex, suggesting that it may be a potential therapeutic agent for PA.

The GARP complex prevents sterol accumulation at the trans-Golgi network during dendrite remodeling

Membrane trafficking is essential for sculpting neuronal morphology. The GARP and EARP complexes are conserved tethers that regulate vesicle trafficking in the secretory and endolysosomal pathways, respectively. Both complexes contain the Vps51, Vps52, and Vps53 proteins, and a complex-specific protein: Vps54 in GARP and Vps50 in EARP. In Drosophila, we find that both complexes are required for dendrite morphogenesis during developmental remodeling of multidendritic class IV da (c4da) neurons. Having found that sterol accumulates at the trans-Golgi network (TGN) in Vps54KO/KO neurons, we investigated genes that regulate sterols and related lipids at the TGN. Overexpression of oxysterol binding protein (Osbp) or knockdown of the PI4K four wheel drive (fwd) exacerbates the Vps54KO/KO phenotype, whereas eliminating one allele of Osbp rescues it, suggesting that excess sterol accumulation at the TGN is, in part, responsible for inhibiting dendrite regrowth. These findings distinguish the GARP and EARP complexes in neurodevelopment and implicate vesicle trafficking and lipid transfer pathways in dendrite morphogenesis.

Loss of the Fbw7 tumor suppressor rewires cholesterol metabolism in cancer cells leading to activation of the PI3K-AKT signalling axis

The sterol regulatory-element binding proteins (SREBPs) are transcription factors controlling cholesterol and fatty acid synthesis and metabolism. There are three SREBP proteins, SREBP1a, SREBP1c and SREBP2, with SREBP1a being the strongest transcription factor. The expression of SREBP1a is restricted to rapidly proliferating cells, including cancer cells. The SREBP proteins are translated as large, inactive precursors bound to the endoplasmic reticulum (ER) membranes. These precursors undergo a two-step cleavage process that releases the amino terminal domains of the proteins, which translocate to the nucleus and function as transcription factors. The nuclear forms of the SREBPs are rapidly degraded by the ubiquitin-proteasome system in a manner dependent on the Fbw7 ubiquitin ligase. Consequently, inactivation of Fbw7 results in the stabilization of active SREBP1 and SREBP2 and enhanced expression of target genes. We report that the inactivation of Fbw7 in cancer cells blocks the proteolytic maturation of SREBP2. The same is true in cells expressing a cancer-specific loss-of-function Fbw7 protein. Interestingly, the activation of SREBP2 is restored in response to cholesterol depletion, suggesting that Fbw7-deficient cells accumulate cholesterol. Importantly, inactivation of SREBP1 in Fbw7-deficient cells also restores the cholesterol-dependent regulation of SREBP2, suggesting that the stabilization of active SREBP1 molecules could be responsible for the blunted activation of SREBP2 in Fbw7-deficient cancer cells. We suggest that this could be an important negative feedback loop in cancer cells with Fbw7 loss-of-function mutations to protect these cells from the accumulation of toxic levels of cholesterol and/or cholesterol metabolites. Surprisingly, we also found that the inactivation of Fbw7 resulted in the activation of AKT. Importantly, the activation of AKT was dependent on SREBP1 and on the accumulation of cholesterol. Thus, we suggest that the loss of Fbw7 rewires lipid metabolism in cancer cells to support cell proliferation and survival.

Development of a novel spatiotemporal depletion system for cellular cholesterol

Cholesterol is an essential component of mammalian cell membranes whose subcellular concentration and function are tightly regulated by de novo biosynthesis, transport, and storage. Although recent reports have suggested diverse functions of cellular cholesterol in different subcellular membranes, systematic investigation of its site-specific roles has been hampered by the lack of a methodology for spatiotemporal manipulation of cellular cholesterol levels. Here, we report the development of a new cholesterol depletion system that allows for spatiotemporal manipulation of intracellular cholesterol levels. This system utilizes a genetically encoded cholesterol oxidase whose intrinsic membrane binding activity is engineered in such a way that its membrane targeting can be controlled in a spatiotemporally specific manner via chemically induced dimerization. In combination with in situ quantitative imaging of cholesterol and signaling activity measurements, this system allows for unambiguous determination of site-specific functions of cholesterol in different membranes, including the plasma membrane and the lysosomal membrane.

Our evolving understanding of how 27-hydroxycholesterol influences cancer

Cholesterol has been implicated in the pathophysiology and progression of several cancers now, although the mechanisms by which it influences cancer biology are just emerging. Two likely contributing mechanisms are the ability for cholesterol to directly regulate signaling molecules within the membrane, and certain metabolites acting as signaling molecules. One such metabolite is the oxysterol 27-hydroxycholesterol (27HC), which is a primary metabolite of cholesterol synthesized by the enzyme Cytochrome P450 27A1 (CYP27A1). Physiologically, 27HC is involved in the regulation of cholesterol homeostasis and contributes to cholesterol efflux through liver X receptor (LXR) and inhibition of de novo cholesterol synthesis through the insulin-induced proteins (INSIGs). 27HC is also a selective modulator of the estrogen receptors. An increasing number of studies have identified its importance in cancer progression of various origins, especially in breast cancer. In this review, we discuss the physiological roles of 27HC targeting these two nuclear receptors and the subsequent contribution to cancer progression. We describe how 27HC promotes tumor growth directly through cancer-intrinsic factors, and indirectly through its immunomodulatory roles which lead to decreased immune surveillance and increased tumor invasion. This review underscores the importance of the cholesterol metabolic pathway in cancer progression and the potential therapeutic utility of targeting this metabolic pathway.

Coordination of inter-organelle communication and lipid fluxes by OSBP-related proteins

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute one of the largest families of lipid-binding/transfer proteins (LTPs) in eukaryotes. The current view is that many of them mediate inter-organelle lipid transfer over membrane contact sites (MCS). The transfer occurs in several cases in a 'counter-current' fashion: A lipid such as cholesterol or phosphatidylserine (PS) is transferred against its concentration gradient driven by transport of a phosphoinositide in the opposite direction. In this way ORPs are envisioned to maintain the distinct organelle lipid compositions, with impacts on multiple organelle functions. However, the functions of ORPs extend beyond lipid homeostasis to regulation of processes such as cell survival, proliferation and migration. Important expanding areas of mammalian ORP research include their roles in viral and bacterial infections, cancers, and neuronal function. The yeast OSBP homologue (Osh) proteins execute multifaceted functions in sterol and glycerophospholipid homeostasis, post-Golgi vesicle transport, phosphatidylinositol-4-phosphate, sphingolipid and target of rapamycin (TOR) signalling, and cell cycle control. These observations identify ORPs as lipid transporters and coordinators of signals with an unforeseen variety of cellular processes. Understanding their activities not only enlightens the biology of the living cell but also allows their employment as targets of new therapeutic approaches for disease.

How Viruses Use the VCP/p97 ATPase Molecular Machine

Viruses are obligate intracellular parasites that are dependent on host factors for their replication. One such host protein, p97 or the valosin-containing protein (VCP), is a highly conserved AAA ATPase that facilitates replication of diverse RNA- and DNA-containing viruses. The wide range of cellular functions attributed to this ATPase is consistent with its participation in multiple steps of the virus life cycle from entry and uncoating to viral egress. Studies of VCP/p97 interactions with viruses will provide important information about host processes and cell biology, but also viral strategies that take advantage of these host functions. The critical role of p97 in viral replication might be exploited as a target for development of pan-antiviral drugs that exceed the capability of virus-specific vaccines or therapeutics.

The emerging roles of OSBP-related proteins in cancer: Impacts through phosphoinositide metabolism and protein-protein interactions

Oxysterol-binding protein -related proteins (ORPs) form a large family of intracellular lipid binding/transfer proteins. A number of ORPs are implicated in inter-organelle lipid transfer over membrane contacts sites, their mode of action involving in several cases the transfer of two lipids in opposite directions, termed countercurrent lipid transfer. A unifying feature appears to be the capacity to bind phosphatidylinositol polyphosphates (PIPs). These lipids are in some cases transported by ORPs from one organelle to another to drive the transfer of another lipid against its concentration gradient, while they in other cases may act as allosteric regulators of ORPs, or an ORP may introduce a PIP to an enzyme for catalysis. Dysregulation of several ORP family members is implicated in cancers, ORP3, -4, -5 and -8 being thus far the most studied examples. The most likely mechanisms underlying their associations with malignant growth are (i) impacts on PIP-mediated signaling events resulting in altered Ca²⁺ homeostasis, bioenergetics, cell survival, proliferation, and migration, (ii) protein-protein interactions affecting the activity of signaling factors, and (iii) modification of cellular lipid transport in a way that facilitates the proliferation of malignant cells. In this review I discuss the existing functional evidence for the involvement of ORPs in cancerous growth, discuss the findings in the light of the putative mechanisms outlined above and the possibility of employing ORPs as targets of anti-cancer therapy.

Downregulation of ORP3 Correlates with Reduced Survival of Colon Cancer Patients with Advanced Nodal Metastasis and of Female Patients with Grade 3 Colon Cancer

Genome instability is an essential hallmark in tumor development, including colorectal cancer. We have recently identified the oxysterol binding protein-related protein 3 (ORP3), also known as oxysterol binding protein-like 3 (OSBPL3), as a novel ploidy-control gene, whose knock-out leads to aneuploidy induction and promotes tumor formation, indicating that ORP3 is a bona fide tumor suppressor protein. Here we analyzed expression of ORP3 in a cohort (n = 206) of colon cancer patients in relation to patient survival. We show that low ORP3 mRNA levels correlate with reduced survival of patients with advanced nodal metastasis (N2). While patient survival does not associate with grading when the whole cohort is evaluated, importantly, low ORP3 mRNA levels associate with worse survival of female patients with grade 3 colon cancer. Similarly, low ORP3 mRNA levels associate with worse survival of grade 3 colon cancer patients 70 years of age and younger while low ORP3 mRNA levels seem to be beneficial for colon cancer patients with a T2 tumor size. Together, the data show that ORP3 expression is downregulated during colon cancer progression, which correlates with reduced patient survival. Thus, ORP3 mRNA levels may be a prognostic marker for better stratification of colon cancer patients.

ORP/Osh mediate cross-talk between ER-plasma membrane contact site components and plasma membrane SNAREs

OSBP-homologous proteins (ORPs, Oshp) are lipid binding/transfer proteins. Several ORP/Oshp localize to membrane contacts between the endoplasmic reticulum (ER) and the plasma membrane, where they mediate lipid transfer or regulate lipid-modifying enzymes. A common way in which they target contacts is by binding to the ER proteins, VAP/Scs2p, while the second membrane is targeted by other interactions with lipids or proteins.We have studied the cross-talk of secretory SNARE proteins and their regulators with ORP/Oshp and VAPA/Scs2p at ER-plasma membrane contact sites in yeast and murine primary neurons. We show that Oshp-Scs2p interactions depend on intact secretory SNARE proteins, especially Sec9p. SNAP-25/Sec9p directly interact with ORP/Osh proteins and their disruption destabilized the ORP/Osh proteins, associated with dysfunction of VAPA/Scs2p. Deleting OSH1-3 in yeast or knocking down ORP2 in primary neurons reduced the oligomerization of VAPA/Scs2p and affected their multiple interactions with SNAREs. These observations reveal a novel cross-talk between the machineries of ER-plasma membrane contact sites and those driving exocytosis.

Proteomics-based identification of different training adaptations of aged skeletal muscle following long-term high-intensity interval and moderate-intensity continuous training in aged rats

Aging-associated loss of skeletal muscle mass and force increases the risk of falls, impairs mobility, and leads to a reduced quality of life. High-intensity interval training (HIIT) is superior to moderate-intensity continuous training (MICT) for improving morphological and metabolic adaptations of skeletal muscle in older adults, but the underlying mechanism is unknown. Aged female rats underwent HIIT and MICT for 8 months, and their differential impacts on skeletal muscle proteome were investigated. HIIT resulted in a larger improvement in grip strength and fiber cross-sectional area, with similar increases in inclined plane performance and time to exhaustion. Proteomic analysis showed that common training adaptations of both protocols included changes to muscle contraction, focal adhesion signaling, mitochondrial function, apoptosis and regeneration, and anti-oxidation, whereas protein processing in the endoplasmic reticulum and adipocytokine signaling were specifically altered in the MICT and HIIT groups, respectively. Immunoblotting showed that upregulation of the adiponectin/AMPK signaling pathway may be associated with improvements in autophagy, oxidative stress, mitochondrial function, and apoptosis in aged skeletal muscle following HIIT. Thus, understanding the molecular differences in training adaptations from these two exercise modalities may aid in combatting sarcopenia.

Urolithin A attenuated ox-LDL-induced cholesterol accumulation in macrophages partly through regulating miR-33a and ERK/AMPK/SREBP1 signaling pathways

Promoting cholesterol efflux from foam cells represents one of the therapeutic strategies for ameliorating atherosclerosis. Urolithin A (UA) has been shown before to attenuate ox-LDL induced endothelial dysfunction in endothelial cells with its anti-inflammatory properties. The aim of this study was to investigate whether UA could promote cholesterol efflux via modulating related microRNA (miR) and signaling pathways. RAW264.7 cells were treated with 50 μg mL^-1 ox-LDL to induce foam cell formation. After treatment with UA at different concentrations, intercellular and extracellular cholesterol levels were determined. Expression of Erk1/2, AMPKα and their phosphorylation forms, and SREBP1, was analyzed by western-blotting. The effect of UA on miR-33a expression and the involvement of miR-33a in cholesterol efflux regulation were also investigated. UA reduced ox-LDL induced cholesterol accumulation in macrophage cells and promoted cholesterol efflux from cells. Compared with ox-LDL treated cells, UA treatment reduced the level of phosphorylated ERK1/2, increased the expression of phosphorylated AMPKα and decreased the SREBP1 expression. Moreover, UA decreased the miR-33a expression at the transcriptional level but increased the transcriptional expression of ATP-binding cassette transporter A1 (ABCA1) and ABCG1, two genes contributing to reverse cholesterol transport. Furthermore, pre-miR-33a attenuated cholesterol efflux induced by UA. Collectively, UA promoted the reverse cholesterol transport in macrophage-derived foam cells and interfered with cholesterol metabolism possibly through regulating the miRNA-33 expression and interaction with the ERK/AMPKα/SREBP1 signaling pathway.

[A lipid exchange market : vectorial cholesterol transport by the protein OSBP]

Cholesterol is synthesized in the endoplasmic reticulum (RE) and then transported to cellular compartments whose functions require high cholesterol levels. Here, we describe the mechanism by which cholesterol is transported from the RE to the trans-Golgi network (TGN) by the protein OSBP (Oxysterol-Binding Protein). OSBP has two complementary activities. First, it tethers the RE to the TGN by forming a contact site where the two membranes are about twenty nanometers away. Then, it exchanges RE cholesterol for a TGN lipid, phosphatidylinositol 4-phosphate (PI4P). Eventually, PI4P is hydrolyzed at the RE, making the exchange cycle irreversible. Thus, OSBP is at the center of a lipid exchange market where a transported cholesterol "costs" a PI4P. Antiviral or anti-cancer molecules target OSBP, suggesting the importance of the OSBP cycle in different physiopathological contexts. The general principles of this cycle are shared by other lipid-transfer proteins.

Endoplasmic reticulum-plasma membrane contacts: Principals of phosphoinositide and calcium signaling

The endoplasmic reticulum (ER) forms an extensive network of membrane contact sites with intra-cellular organelles and the plasma membrane (PM). Interorganelle contacts have vital roles in membrane lipid and ion dynamics. In particular, ER-PM contacts are integral to numerous inter-cellular and intra-cellular signaling pathways including phosphoinositide lipid and calcium signaling, mechanotransduction, metabolic regulation, and cell stress responses. Accordingly, ER-PM contacts serve important signaling functions in excitable cells including neurons and muscle and endocrine cells. This review highlights recent advances in our understanding of the vital roles for ER-PM contacts in phosphoinositide and calcium signaling and how signaling pathways in turn regulate proteins that form and function at ER-PM contacts.

Fine Mapping of the High-pH Tolerance and Growth Trait-Related Quantitative Trait Loci (QTLs) and Identification of the Candidate Genes in Pacific White Shrimp (Litopenaeus vannamei)

High-pH tolerance and growth are important traits for the shrimp culture industry in areas with saline-alkali water. In the present study, an F1 full-sib family of Pacific white shrimp (Litopenaeus vannamei) was generated with a new "semidirectional cross" method, and double-digest restriction site-associated DNA sequencing (ddRAD-Seq) technology was applied to genotype the 2 parents and 148 progenies. A total of 3567 high-quality markers were constructed for the genetic linkage map, and the total map length was 4161.555 centimorgans (cM), showing 48 linkage groups (LGs) with an average interlocus length of 1.167 cM. With a constrained logarithm of odds (LOD) score ≥ 2.50, 12 high-pH tolerance and 2 growth (body weight) QTLs were located. L. vannamei genomic scaffolds were used to assist with the detection of 21 stress- and 5 growth-related scaffold genes. According to the high-pH transcriptome data of our previous study, 6 candidate high-pH response genes were discovered, and 5 of these 6 genes were consistently expressed with the high-pH transcriptome data, validating the locations of the high-pH tolerance trait-related QTLs in this study. This paper is the first report of fine-mapping high-pH tolerance and growth (body weight) trait QTLs in one L. vannamei genetic map. Our results will further benefit marker-assisted selection work and might be useful for promoting genomic research on the shrimp L. vannamei.

Role of oxysterol-binding protein-related proteins in malignant human tumours

The oxysterol-binding protein-related protein (ORP) family is a group of proteins that mediate oxysterol metabolism and bioactivity in cells. ORPs constitute a large family of lipid transfer proteins. Much of the current evidence indicates that certain members of the family of oxysterol-binding proteins (OSBPs) can lead to cancer. Many studies have revealed the putative roles of OSBPs in various cancer types. However, the exact effects and mechanisms of action of members of the OSBP/ORP family in cancer initiation and progression are currently unclear. This review focuses on ORP family members that can accelerate human tumour cell proliferation, migration, and invasion. The mechanisms and functions of various ORPs are introduced in detail. We also attempt to identify the roles of these proteins in malignant tumours with the ultimate aim of determining the exact role of the OSBP/ORP family in human tumour cells.

Lipid transfer proteins: the lipid commute via shuttles, bridges and tubes

Lipids are distributed in a highly heterogeneous fashion in different cellular membranes. Only a minority of lipids achieve their final intracellular distribution through transport by vesicles. Instead, the bulk of lipid traffic is mediated by a large group of lipid transfer proteins (LTPs), which move small numbers of lipids at a time using hydrophobic cavities that stabilize lipid molecules outside membranes. Although the first LTPs were discovered almost 50 years ago, most progress in understanding these proteins has been made in the past few years, leading to considerable temporal and spatial refinement of our understanding of the function of these lipid transporters. The number of known LTPs has increased, with exciting discoveries of their multimeric assembly. Structural studies of LTPs have progressed from static crystal structures to dynamic structural approaches that show how conformational changes contribute to lipid handling at a sub-millisecond timescale. A major development has been the finding that many intracellular LTPs localize to two organelles at the same time, forming a shuttle, bridge or tube that links donor and acceptor compartments. The understanding of how different lipids achieve their final destination at the molecular level allows a better explanation of the range of defects that occur in diseases associated with lipid transport and distribution, opening up the possibility of developing therapies that specifically target lipid transfer.

Aneuploidy-inducing gene knockdowns overlap with cancer mutations and identify Orp3 as a B-cell lymphoma suppressor

Aneuploidy can instigate tumorigenesis. However, mutations in genes that control chromosome segregation are rare in human tumors as these mutations reduce cell fitness. Screening experiments indicate that the knockdown of multiple classes of genes that are not directly involved in chromosome segregation can lead to aneuploidy induction. The possible contribution of these genes to cancer formation remains yet to be defined. Here we identified gene knockdowns that lead to an increase in aneuploidy in checkpoint-deficient human cancer cells. Computational analysis revealed that the identified genes overlap with recurrent mutations in human cancers. The knockdown of the three strongest selected candidate genes (ORP3, GJB3, and RXFP1) enhances the malignant transformation of human fibroblasts in culture. Furthermore, the knockout of Orp3 results in an aberrant expansion of lymphoid progenitor cells and a high penetrance formation of chromosomal instable, pauci-clonal B-cell lymphoma in aging mice. At pre-tumorous stages, lymphoid cells from the animals exhibit deregulated phospholipid metabolism and an aberrant induction of proliferation regulating pathways associating with increased aneuploidy in hematopoietic progenitor cells. Together, these results support the concept that aneuploidy-inducing gene deficiencies contribute to cellular transformation and carcinogenesis involving the deregulation of various molecular processes such as lipid metabolism, proliferation, and cell survival.

The Great Escape: how phosphatidylinositol 4-kinases and PI4P promote vesicle exit from the Golgi (and drive cancer)

Phosphatidylinositol 4-phosphate (PI4P) is a membrane glycerophospholipid and a major regulator of the characteristic appearance of the Golgi complex as well as its vesicular trafficking, signalling and metabolic functions. Phosphatidylinositol 4-kinases, and in particular the PI4KIIIβ isoform, act in concert with PI4P to recruit macromolecular complexes to initiate the biogenesis of trafficking vesicles for several Golgi exit routes. Dysregulation of Golgi PI4P metabolism and the PI4P protein interactome features in many cancers and is often associated with tumour progression and a poor prognosis. Increased expression of PI4P-binding proteins, such as GOLPH3 or PITPNC1, induces a malignant secretory phenotype and the release of proteins that can remodel the extracellular matrix, promote angiogenesis and enhance cell motility. Aberrant Golgi PI4P metabolism can also result in the impaired post-translational modification of proteins required for focal adhesion formation and cell–matrix interactions, thereby potentiating the development of aggressive metastatic and invasive tumours. Altered expression of the Golgi-targeted PI 4-kinases, PI4KIIIβ, PI4KIIα and PI4KIIβ, or the PI4P phosphate Sac1, can also modulate oncogenic signalling through effects on TGN-endosomal trafficking. A Golgi trafficking role for a PIP 5-kinase has been recently described, which indicates that PI4P is not the only functionally important phosphoinositide at this subcellular location. This review charts new developments in our understanding of phosphatidylinositol 4-kinase function at the Golgi and how PI4P-dependent trafficking can be deregulated in malignant disease.

Crystallographic Studies of Steroid-Protein Interactions

Steroid molecules have a wide range of function in eukaryotes, including the control and maintenance of membranes, hormonal control of transcription, and intracellular signaling. X-ray crystallography has served as a successful tool for gaining understanding of the structural and mechanistic aspects of these functions by providing snapshots of steroids in complex with various types of proteins. These proteins include nuclear receptors activated by steroid hormones, several families of enzymes involved in steroid synthesis and metabolism, and proteins involved in signaling and trafficking pathways. Proteins found in some bacteria that bind and metabolize steroids have been investigated as well. A survey of the steroid-protein complexes that have been studied using crystallography and the insight learned from them is presented.

Heterologous expression and functional characterization of the ligand-binding domain of oxysterol-binding protein from Aspergillus oryzae

Oxysterol-binding proteins (OSBPs) comprise a family of sterol-binding proteins. In this study, we focused on AoOSBP1, one of the five OSBP proteins identified from the industrial fungus Aspergillus oryzae. The temporal expression pattern analysis showed that the expression of AoOSBP1, in both gene and protein levels, was stably expressed throughout the developmental stages, while was upregulated during the accelerated growth stage. The immunofluorescence observation revealed that AoOSBP1 protein was mainly distributed in the conidiophore, indicating its underlying role in spore formation. The ligand-binding domain of AoOSBP1, namely OSBP-related domain (ORD), was heterologously expressed in Escherichia coli and purified. The binding assay carried out using microscale thermophoresis showed that the recombinant AoORD protein exhibited binding affinity for ergosterol, and exhibited much higher affinity to oxysterols (25-hydroxycholesterol and 7-ketocholesterol) and phytosterols (β-sitosterol and stigmasterol). By contrast, MBP tag as the negative control showed no binding affinity for sterols. The present work demonstrates that AoORD domain in AoOSBP1 is capable of binding sterols, plays an underlying role in sterols transportation, and may participate in spore formation.

ORP2 interacts with phosphoinositides and controls the subcellular distribution of cholesterol

ORP2 is a sterol-binding protein with documented functions in lipid and glucose metabolism, Akt signaling, steroidogenesis, cell adhesion, migration and proliferation. Here we investigate the interactions of ORP2 with phosphoinositides (PIPs) by surface plasmon resonance (SPR), its affinity for cholesterol with a pull-down assay, and its capacity to transfer sterol in vitro. Moreover, we determine the effects of wild-type (wt) ORP2 and a mutant with attenuated PIP binding, ORP2(mHHK), on the subcellular distribution of cholesterol, and analyze the interaction of ORP2 with the related cholesterol transporter ORP1L. ORP2 showed specific affinity for PI(4,5)P₂, PI(3,4,5)P₃ and PI(4)P, with suggestive K_d values in the μM range. Also binding of cholesterol by ORP2 was detectable, but a K_d could not be determined. Wt ORP2 was in HeLa cells mainly detected in the cytosol, ER, late endosomes, and occasionally on lipid droplets (LDs), while ORP2(mHHK) displayed an enhanced LD localization. Overexpression of wt ORP2 shifted the D4H cholesterol probe away from endosomes, while ORP2(mHHK) caused endosomal accumulation of the probe. Although ORP2 failed to transfer dehydroergosterol in an in vitro assay where OSBP is active, its knock-down resulted in the accumulation of cholesterol in late endocytic compartments, as detected by both D4H and filipin probes. Interestingly, ORP2 was shown to interact and partially co-localize on late endosomes with ORP1L, a cholesterol transporter/sensor at ER-late endosome junctions. Our data demonstrates that ORP2 binds several phosphoinositides, both PI(4)P and multiply phosphorylated species. ORP2 regulates the subcellular distribution of cholesterol dependent on its PIP-binding capacity. The interaction of ORP2 with ORP1L suggests a concerted action of the two ORPs.

OSBP-related protein 4L promotes phospholipase Cβ3 translocation from the nucleus to the plasma membrane in Jurkat T-cells

Phosphoinositide phospholipases C (PLCs) are a family of eukaryotic intracellular enzymes with important roles in signal transduction. In addition to their location at the plasma membrane, PLCs also exist within the cell nucleus where they are stored. We previously demonstrated that OSBP-related protein 4L (ORP4L) anchors cluster of differentiation 3ϵ (CD3ϵ) to the heterotrimeric G protein subunit (Gα_q/11) to control PLCβ3 relocation and activation. However, the underlying mechanism by which ORP4L facilitates PLCβ3 translocation remains unknown. Here, using confocal immunofluorescence microscopy and coimmunoprecipitation assays, we report that ORP4L stimulates PLCβ3 translocation from the nucleus to the plasma membrane in Jurkat T-cells in two steps. First, we found that ORP4L is required for the activation of Ras-related nuclear protein (RAN), a GTP-binding nuclear protein that binds to exportin 1 and eventually promotes the nuclear export of PLCβ3. Second, we also observed that ORP4L interacts with vesicle-associated membrane protein-associated protein A (VAPA) through its two phenylalanines in an acidic tract (FFAT) motif. This complex enabled PLCβ3 movement to the plasma membrane, indicating that PLCβ3 translocation occurs in a VAPA-dependent manner. This study reveals detailed mechanistic insight into the role of ORP4L in PLCβ3 redistribution from storage within the nucleus to the plasma membrane via RAN activation and interaction with VAPA in Jurkat T-cells.

Lipid transfer proteins rectify inter-organelle flux and accurately deliver lipids at membrane contact sites

The endoplasmic reticulum (ER) is the main center for the synthesis of various lipid types in cells, and newly synthesized lipids are delivered from the ER to other organelles. In the past decade, various lipid transfer proteins (LTPs) have been recognized as mediators of lipid transport from the ER to other organelles; inter-organelle transport occurs at membrane contact sites (MCSs) and in a nonvesicular manner. Although the intermembrane transfer reaction catalyzed by LTPs is an equilibrium reaction, various types of newly synthesized lipids are transported unidirectionally in cells. This review provides a brief history of the inter-organelle trafficking of lipids and summarizes the structural and biochemical characteristics of the ceramide transport protein (CERT) as a typical LTP acting at MCSs. In addition, this review compares several LTP-mediated inter-organelle lipid trafficking systems and proposes that LTPs generate unidirectional fluxes of specific lipids between different organelles by indirect coupling with the metabolic reactions that occur in specific organelles. Moreover, the available data also suggest that the major advantage of LTP-mediated lipid transport at MCSs may be the accuracy of delivery. Finally, how cholesterol is enriched in the plasma membrane is discussed from a thermodynamic perspective.

The Oxysterol-Binding Protein Cycle: Burning Off PI(4)P to Transport Cholesterol

To maintain an asymmetric distribution of ions across membranes, protein pumps displace ions against their concentration gradient by using chemical energy. Here, we describe a functionally analogous but topologically opposite process that applies to the lipid transfer protein (LTP) oxysterol-binding protein (OSBP). This multidomain protein exchanges cholesterol for the phosphoinositide phosphatidylinositol 4-phosphate [PI(4)P] between two apposed membranes. Because of the subsequent hydrolysis of PI(4)P, this counterexchange is irreversible and contributes to the establishment of a cholesterol gradient along organelles of the secretory pathway. The facts that some natural anti-cancer molecules block OSBP and that many viruses hijack the OSBP cycle for the formation of intracellular replication organelles highlight the importance and potency of OSBP-mediated lipid exchange. The architecture of some LTPs is similar to that of OSBP, suggesting that the principles of the OSBP cycle-burning PI(4)P for the vectorial transfer of another lipid-might be general.

The Rab11-binding protein RELCH/KIAA1468 controls intracellular cholesterol distribution

Sobajima et al. identify the novel protein RELCH/KIAA1468 as a Rab11-binding protein and show that RELCH/KIAA1468 and Rab11 regulate OSBP-dependent nonvesicular cholesterol transport from recycling endosomes to the trans-Golgi network.

Cholesterol, which is endocytosed to the late endosome (LE)/lysosome, is delivered to other organelles through vesicular and nonvesicular transport mechanisms. In this study, we discuss a novel mechanism of cholesterol transport from recycling endosomes (REs) to the trans-Golgi network (TGN) through RELCH/KIAA1468, which is newly identified in this study as a Rab11-GTP– and OSBP-binding protein. After treating cells with 25-hydroxycholesterol to induce OSBP relocation from the cytoplasm to the TGN, REs accumulated around the TGN area, but this accumulation was diminished in RELCH- or OSBP-depleted cells. Cholesterol content in the TGN was decreased in Rab11-, RELCH-, and OSBP-depleted cells and increased in the LE/lysosome. According to in vitro reconstitution experiments, RELCH tethers Rab11-bound RE-like and OSBP-bound TGN-like liposomes and promotes OSBP-dependent cholesterol transfer from RE-like to TGN-like liposomes. These data suggest that RELCH promotes nonvesicular cholesterol transport from REs to the TGN through membrane tethering.

Bobbing of Oxysterols: Molecular Mechanism for Translocation of Tail-Oxidized Sterols through Biological Membranes

Translocation of sterols between cellular membrane leaflets is of key importance in membrane organization, dynamics, and signaling. We present a novel translocation mechanism that differs in a unique manner from the established ones. The bobbing mechanism identified here is demonstrated for tail-oxidized sterols, but is expected to be viable for any molecule containing two polar centers at the opposite sides of the molecule. The mechanism renders translocation across a lipid membrane possible without a change in molecular orientation. For tail-oxidized sterols, the bobbing mechanism provides an exceptionally facile means to translocate these signaling molecules across membrane structures and may thus represent an important pathway in the course of their biological action.

Oxysterol-binding protein homologs mediate sterol transport from the endoplasmic reticulum to mitochondria in yeast

Sterols are present in eukaryotic membranes and significantly affect membrane fluidity, permeability, and microdomain formation. They are synthesized in the endoplasmic reticulum (ER) and transported to other organelles and the plasma membrane. Sterols play important roles in the biogenesis and maintenance of mitochondrial membranes. However, the mechanisms underlying ER-to-mitochondrion sterol transport remain to be identified. Here, using purified yeast membrane fractions enriched in ER and mitochondria, we show that the oxysterol-binding protein homologs encoded by the OSH genes in the yeast Saccharomyces cerevisiae mediate sterol transport from the ER to mitochondria. Combined depletion of all seven Osh proteins impaired sterol transport from the ER to mitochondria in vitro; however, sterol transport was recovered at different levels upon adding one of the Osh proteins. Of note, the sterol content in the mitochondrial fraction was significantly decreased in vivo after Osh4 inactivation in a genetic background in which all the other OSH genes were deleted. We also found that Osh5-Osh7 bind cholesterol in vitro We propose a model in which Osh proteins share a common function to transport sterols between membranes, with varying contributions by these proteins, depending on the target membranes. In summary, we have developed an in vitro system to examine intracellular sterol transport and provide evidence for involvement of Osh proteins in sterol transport from the ER to mitochondria in yeast.

Oxysterol-binding protein-related protein 5 (ORP5) promotes cell proliferation by activation of mTORC1 signaling

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute a large family of proteins that mainly function in lipid transport and sensing. ORP5 is an endoplasmic reticulum (ER)-anchored protein implicated in lipid transfer at the contact sites between the ER and other membranes. Recent studies indicate that ORP5 is also involved in cancer cell invasion and tumor progression. However, the molecular mechanism underlying ORP5's involvement in cancer is unclear. Here, we report that ORP5 promotes cell proliferation and motility of HeLa cells, an effect that depends on its functional OSBP-related domain (ORD). We also found that ORP5 depletion or substitutions of key residues located within ORP5-ORD and responsible for interactions with lipids interfered with cell proliferation, migration, and invasion. ORP5 interacted with the protein mechanistic target of rapamycin (mTOR), and this interaction also required ORP5-ORD. Of note, whereas ORP5 overexpression induced mTOR complex 1 (mTORC1) activity, ORP5 down-regulation had the opposite effect. Finally, ORP5-depleted cells exhibited impaired mTOR localization to lysosomes, which may have accounted for the blunted mTORC1 activation. Together, our results suggest that ORP5 expression is positively correlated with mTORC1 signaling and that ORP5 stimulates cell proliferation, at least in part, by activating mTORC1.

Systematic Analysis of Cotton Non-specific Lipid Transfer Protein Family Revealed a Special Group That Is Involved in Fiber Elongation

Non-specific lipid transfer proteins (nsLTPs) had been previously isolated from cotton fiber but their functions were unclear so far. Bioinformatic analysis of the tetraploid cotton genome database identified 138 nsLTP genes, falling into the 11 groups as reported previously. Different from Arabidopsis, cacao, and other crops, cotton type XI genes were considerably expanded and diverged earlier on chromosome At11, Dt11, and Dt08. Corresponding to the type XI genes, the type XI proteins (GhLtpXIs) all contained an extra N-terminal cap resulting in larger molecular weight. The research revealed that the expression of type XI genes was dramatically increased in fibers of tetraploid cotton compared with the two diploid progenitors. High-level of GhLtpXIs expression was observed in long-fibered cotton cultivars during fiber elongation. Ectopic expression of GhLtpXIs in Arabidopsis significantly enhanced trichome length, suggesting that GhLtpXIs promoted fiber elongation. Overall, the findings of this research provide insights into phenotypic evolution of Gossypium species and regulatory mechanism of nsLTPs during fiber development. HIGHLIGHT A specific group, type XI nsLTPs, was identified with predominant expression in elongating fibers of Gossypium hirsutum based on evolutionary, transcriptional, and functional analyses.

Cholesterol and Its Metabolites in Tumor Growth: Therapeutic Potential of Statins in Cancer Treatment

Cholesterol is essential for cell function and viability. It is a component of the plasma membrane and lipid rafts and is a precursor for bile acids, steroid hormones, and Vitamin D. As a ligand for estrogen-related receptor alpha (ESRRA), cholesterol becomes a signaling molecule. Furthermore, cholesterol-derived oxysterols activate liver X receptors (LXRs) or estrogen receptors (ERs). Several studies performed in cancer cells reveal that cholesterol synthesis is enhanced compared to normal cells. Additionally, high serum cholesterol levels are associated with increased risk for many cancers, but thus far, clinical trials with 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) have had mixed results. Statins inhibit cholesterol synthesis within cells through the inhibition of HMG-CoA reductase, the rate-limiting enzyme in the mevalonate and cholesterol synthetic pathway. Many downstream products of mevalonate have a role in cell proliferation, since they are required for maintenance of membrane integrity; signaling, as some proteins to be active must undergo prenylation; protein synthesis, as isopentenyladenine is an essential substrate for the modification of certain tRNAs; and cell-cycle progression. In this review starting from recent acquired findings on the role that cholesterol and its metabolites fulfill in the contest of cancer cells, we discuss the results of studies focused to investigate the use of statins in order to prevent cancer growth and metastasis.

Sterol transfer, PI4P consumption, and control of membrane lipid order by endogenous OSBP

The network of proteins that orchestrate the distribution of cholesterol among cellular organelles is not fully characterized. We previously proposed that oxysterol-binding protein (OSBP) drives cholesterol/PI4P exchange at contact sites between the endoplasmic reticulum (ER) and the trans-Golgi network (TGN). Using the inhibitor OSW-1, we report here that the sole activity of endogenous OSBP makes a major contribution to cholesterol distribution, lipid order, and PI4P turnover in living cells. Blocking OSBP causes accumulation of sterols at ER/lipid droplets at the expense of TGN, thereby reducing the gradient of lipid order along the secretory pathway. OSBP consumes about half of the total cellular pool of PI4P, a consumption that depends on the amount of cholesterol to be transported. Inhibiting the spatially restricted PI4-kinase PI4KIIIβ triggers large periodic traveling waves of PI4P across the TGN These waves are cadenced by long-range PI4P production by PI4KIIα and PI4P consumption by OSBP Collectively, these data indicate a massive spatiotemporal coupling between cholesterol transport and PI4P turnover via OSBP and PI4-kinases to control the lipid composition of subcellular membranes.

Synthesis and bioactivity of bis-steroidal pyrazine 23-deoxy-25-epi ritterostatin GN1N

Cephalostatins, ritterazines and their hybrid bis-steroidal pyrazine analogs such as 25-epi-rittereostatin G_N1_N show unusually high potency against a wide range of cancer cell lines. Herein, we report the synthesis and bioactivity of 23-deoxy-25-epi ritterostatin G_N1_N, which lacks the 23-hydroxyl group of 25-epi rittereostatin G_N1_N. The less oxygenated bis-steroidal pyrazine was ∼50- to 1000-fold less potent than 25-epi ritterostatin G_N1_N, highlighting the importance of the 23-hydroxyl group for the antiproliferative activity of the cephalostatin/ritterazine class of drugs.

Apolipoprotein E Promotes Invasion in Oral Squamous Cell Carcinoma

Oral squamous cell carcinoma (OSCC) patients generally have a poor prognosis, because of the invasive nature of these tumors. In comparing transcription profiles between OSCC tumors with a more invasive (worst pattern of tumor invasion 5) versus a less invasive (worst pattern of tumor invasion 3) pattern of invasion, we identified a total of 97 genes that were overexpressed at least 1.5-fold in the more invasive tumor subtype. The most functionally relevant genes were assessed using in vitro invasion assays with an OSCC cell line (UM-SCC-1). Individual siRNA knockdown of 15 of these 45 genes resulted in significant reductions in tumor cell invasion compared to a nontargeting siRNA control. One gene whose knockdown had a strong effect on invasion corresponded to apolipoprotein E (APOE). Both matrix degradation and the number of mature invadopodia were significantly decreased with APOE knockdown. APOE knockdown also resulted in increased cellular cholesterol, consistent with APOE's role in regulating cholesterol efflux. APOE knockdown resulted in decreased levels of phospho-extracellular signal-regulated kinase 1/2, phospho-c-Jun N-terminal kinase, and phospho-cJun, as well as decreased activator protein 1 (AP-1) activity. Expression of matrix metalloproteinase 7 (MMP7), an AP-1 target, was also significantly decreased. Our findings suggest that APOE protein plays a significant role in OSCC tumor invasion because of its effects on cellular cholesterol and subsequent effects on cell signaling and AP-1 activity, leading to changes in the expression of invasion-related proteins, including MMP7.

The role of oxysterol-binding protein and its related proteins in cancer

Oxysterol-binding protein (OSBP) and its related proteins (ORPs) constitute a large, evolutionarily conserved family of lipid-binding proteins that are associated with a wide range of cellular activities. The core function of OSBP/ORPs appears to be moving lipids between cellular membranes in a non-vesicular manner. Recent studies have unveiled a novel, counter-transport mechanism of cellular lipid transfer mediated by OSBP/ORPs at the membrane contact sites that involves phosphatidylinositol 4-phosphate. Importantly, the OSBP/ORPs family has also been implicated in cell signalling pathways and cancer development. Here, we summarize recent progress in understanding the role of OSBP/ORPs in cancer development, and discuss how the lipid transfer function of OSBP/ORPs may underpin their role in tumorigenesis.