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.

Components of the phosphatidylserine endoplasmic reticulum to plasma membrane transport mechanism as targets for KRAS inhibition in pancreatic cancer

KRAS is mutated in 90% of human pancreatic ductal adenocarcinomas (PDACs). To function, KRAS must localize to the plasma membrane (PM) via a C-terminal membrane anchor that specifically engages phosphatidylserine (PtdSer). This anchor-binding specificity renders KRAS-PM localization and signaling capacity critically dependent on PM PtdSer content. We now show that the PtdSer lipid transport proteins, ORP5 and ORP8, which are essential for maintaining PM PtdSer levels and hence KRAS PM localization, are required for KRAS oncogenesis. Knockdown of either protein, separately or simultaneously, abrogated growth of KRAS-mutant but not KRAS-wild-type pancreatic cancer cell xenografts. ORP5 or ORP8 knockout also abrogated tumor growth in an immune-competent orthotopic pancreatic cancer mouse model. Analysis of human datasets revealed that all components of this PtdSer transport mechanism, including the PM-localized EFR3A-PI4KIIIα complex that generates phosphatidylinositol-4-phosphate (PI4P), and endoplasmic reticulum (ER)-localized SAC1 phosphatase that hydrolyzes counter transported PI4P, are significantly up-regulated in pancreatic tumors compared to normal tissue. Taken together, these results support targeting PI4KIIIα in KRAS-mutant cancers to deplete the PM-to-ER PI4P gradient, reducing PM PtdSer content. We therefore repurposed the US Food and Drug Administration-approved hepatitis C antiviral agent, simeprevir, as a PI4KIIIα inhibitor In a PDAC setting. Simeprevir potently mislocalized KRAS from the PM, reduced the clonogenic potential of pancreatic cancer cell lines in vitro, and abrogated the growth of KRAS-dependent tumors in vivo with enhanced efficacy when combined with MAPK and PI3K inhibitors. We conclude that the cellular ER-to-PM PtdSer transport mechanism is essential for KRAS PM localization and oncogenesis and is accessible to therapeutic intervention.

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.

Oxysterol-binding protein related-proteins (ORPs) 5 and 8 regulate calcium signaling at specific cell compartments

Oxysterol-binding protein related-protein 5 and 8 (ORP5/8) localize to the membrane contact sites (MCS) of the endoplasmic reticulum (ER) and the mitochondria, as well as to the ER-plasma membrane (PM) MCS. The MCS are emerging as important regulators of cell signaling events, including calcium (Ca²⁺) signaling. ORP5/8 have been shown to interact with phosphatidylinositol-4,5-bisphosphate (PIP₂) in the PM, and to modulate mitochondrial respiration and morphology. PIP₂ is the direct precursor of inositol trisphosphate (IP₃), a key second messenger responsible for Ca²⁺-release from the intracellular Ca²⁺ stores. Further, mitochondrial respiration is linked to Ca²⁺ transfer from the ER to the mitochondria. Hence, we asked whether ORP5/8 would affect Ca²⁺ signaling in these cell compartments, and employed genetically engineered aequorin Ca²⁺ probes to investigate the effect of ORP5/8 in the regulation of mitochondrial and caveolar Ca²⁺. Our results show that ORP5/8 overexpression leads to increased mitochondrial matrix Ca²⁺ as well as to increased Ca²⁺ concentration at the caveolar subdomains of the PM during histamine stimulation, while having no effect on the cytoplasmic Ca²⁺. Also, we found that ORP5/8 overexpression increases cell proliferation. Our results show that ORP5/8 regulate Ca²⁺ signaling at specific MCS foci. These local ORP5/8-mediated Ca²⁺ signaling events are likely to play roles in processes such as mitochondrial respiration and cell proliferation.

Monitoring Non-vesicular Transport of Phosphatidylserine and Phosphatidylinositol 4-Phosphate in Intact Cells by BRET Analysis

Non-vesicular lipid transport via lipid transfer proteins (LTPs) at membrane contact sites (MCSs) is critical for the maintenance of the lipid composition of biological membranes. The ability to measure lipid transfer activity of diverse LTPs in live cells without interrupting the fine structural organization is essential to better understand the contribution of non-vesicular lipid transport to membrane organization. Here, we describe a semiquantitative method to analyze phosphatidylinositol 4-phosphate (PI4P) and phosphatidylserine (PS) changes at the plasma membrane (PM) as they relate to LTP functions. This live cell method is based on bioluminescence resonance energy transfer (BRET) measurements between a luciferase-tagged lipid-recognizing module and a PM-targeted fluorescent acceptor. Oxysterol-binding protein-related protein (ORP) 5 is a PI4P/PS lipid transfer protein which is stably tethered to the ER and also dynamically interacts with PM PI4P/PI(4,5)P2 via its pleckstrin homology (PH) domain. We show that this method is able to detect PI4P and PS changes in the PM upon acute recruitment of an ORP5 construct to the PM. This method is convenient and robust, utilizing population of cells in 96-well plates analyzed in a plate reader. We will also highlight potential further applications extending the method for other LTPs and other lipid cargoes.

ORP5 promotes migration and invasion of cervical cancer cells by inhibiting endoplasmic reticulum stress

ORP5 is a transmembrane protein anchored to the endoplasmic reticulum, which mainly functions as a lipid transporter and has reportedly been linked to cancer. However, the specific mechanism of ORP5 action in cervical cancer (CC) is unclear. In this study, we found that ORP5 promotes the migration and invasive ability of CC cells in vitro and in vivo. In addition, ORP5 expression was linked to endoplasmic reticulum stress, and ORP5 encouraged CC metastasis by inhibiting endoplasmic reticulum stress. Mechanistically, ORP5 inhibited endoplasmic reticulum stress in CC cells by stimulating ubiquitination and proteasomal degradation of SREBP1 to reduce its expression. In conclusion, ORP5 promotes the malignant progression of CC by inhibiting endoplasmic reticulum stress, providing a therapeutic target and strategy for CC treatment.

Stigmasterol Activates the mTOR Signaling Pathway by Inhibiting ORP5 Ubiquitination to Promote Milk Synthesis in Bovine Mammary Epithelial Cells

Stigmasterol (ST), a phytosterol found in food, has various biological activities. However, the effect of ST on milk synthesis in dairy cows remains unclear. Therefore, bovine primary mammary epithelial cells (BMECs) were isolated, cultured, and treated with ST to determine the effect of ST on milk synthesis. The study revealed that 10 μM ST significantly increased milk synthesis in BMECs by activating the mammalian target of rapamycin (mTOR) signaling pathway. Further investigation revealed that this activation depends on the regulatory role of oxysterol binding protein 5 (ORP5). ST induces the translocation of ORP5 from the cytoplasm to the lysosome, interacts with the mTOR, recruits mTOR to target the lysosomal surface, and promotes the activation of the mTOR signaling pathway. Moreover, ST was found to increase ORP5 protein levels by inhibiting its degradation via the ubiquitin-proteasome pathway. Specifically, the E3 ubiquitin ligase membrane-associated cycle-CH-type finger 4 (MARCH4) promotes the ubiquitination and subsequent degradation of ORP5. ST mitigates the interaction between MARCH4 and ORP5, thereby enhancing the structural stability of ORP5 and reducing its ubiquitination. In summary, ST stabilizes ORP5 by inhibiting the interaction between MARCH4 and ORP5, thereby activating mTOR signaling pathway and enhancing milk synthesis.

ORP5 promotes cardiac hypertrophy by regulating the activation of mTORC1 on lysosome

INTRODUCTION

Oxysterol binding protein (OSBP)-related protein 5 (ORP5) mainly functions as a lipid transfer protein at membrane contact sites (MCS). ORP5 facilitates cell proliferation through the activation of mTORC1 signaling. While the pro-hypertrophic effects of mTORC1 are well-documented, the specific role of ORP5 in the context of pathological cardiac hypertrophy remains inadequately understood.

METHODS

To investigate the role of ORP5 in pathological cardiac hypertrophy, AAV9-treated mice and neonatal rat ventricular myocytes (NRVMs) were utilized. Cardiac function, morphology, and mTORC1 signaling alterations induced by pro-hypertrophic stimuli were assessed in both myocardium and NRVMs. Additionally, a range of molecular techniques were employed to elucidate the regulatory mechanisms of ORP5 on mTORC1 in hypertrophied hearts.

RESULTS

Increased expression of ORP5 was observed in the hearts of patients with hypertrophic cardiomyopathy (HCM), in mice subjected to transverse aortic constriction (TAC), and in NRVMs treated with angiotensin II (AngII). We found that ORP5 binds to mTOR in cardiomyocytes. Upon exposure to TAC surgery, ORP5-deficient hearts exhibited enhanced cardiac function, reduced cardiomyocyte hypertrophy, and diminished collagen deposition than wild type. Conversely, overexpression of ORP5 significantly aggravated hypertrophic responses in both hearts and NRVMs. Notably, the promotion of cardiac hypertrophy induced by ORP5 overexpression was reversed by rapamycin, an inhibitor of mTORC1. Mechanistically, our study elucidated that the ORD domain of ORP5 interacts with mTORC1, facilitating its translocation to the outer membrane of the lysosome for subsequent activation. This activation triggers the downstream signaling pathways involving S6K1 and 4E-BP1, which initiate protein synthesis, thereby promoting pathological cardiac hypertrophy.

CONCLUSIONS

Our findings provide the inaugural evidence that ORP5 facilitates pathological ventricular hypertrophy through the translocation of mTORC1 to the lysosome for subsequent activation. Consequently, ORP5 has the potential to serve as a diagnostic biomarker or therapeutic target for pathological cardiac hypertrophy in the future.