Role of Oxysterol-Binding Protein Family in Cholesterol Metabolism and Cancer Progression: A Review

Introduction

The family of oxysterol-binding protein–related proteins (ORPs) represents a class of highly conserved lipid transport and signaling molecules, which are essential for various cellular processes such as lipid metabolism, lipid transport, and signal transduction [1–3]. Among these, the most extensively studied are human ORPs and yeast oxysterol-binding protein (OSBP) homologues (Osh) [1]. Human ORPs are systematically classified into 6 distinct classes (denoted by Roman numerals): Class I includes OSBP and ORP4, which enhance the invasiveness and metastasis of tumor cells [4–7]; Class II includes ORP1 and ORP2; Class III includes ORP3, which regulates cell polarity, adhesion, and migration [8], and ORP6 and ORP7, which also regulate cell adhesion and migration [9,10]; Class IV contains ORP5, which increases cell invasion and metastasis [11], and ORP8, which regulates apoptosis [12,13]; Class V includes ORP9, which maintains the functional integrity of the early secretory pathway [14]; and Class VI includes ORP10, which regulates lipoprotein B-100 secretion [15], and ORP11, which is associated with lipogenesis [16]. Osh4p (also known as Kes1p), the simplest and most extensively characterized member of the yeast Osh family, serves as a representative model for structural and functional studies [17]. Unlike other Osh proteins, Osh4p contains only the OSBP-related domain (ORD) and lacks the pleckstrin homology (PH) domain [18]. Im et al [19] reported that Osh4p can form complexes with 5 different sterols, including cholesterol and ergosterol. Its structural core features a β-barrel composed of 19 antiparallel β-strands, which forms a hydrophobic pocket to accommodate sterols [19]. This pocket is capped by a lid-like structure that remains closed upon sterol binding [19]. Antonny et al further discovered that the lid of Osh4p forms a membrane-bound α-helix, known as the ArfGAP1 lipid-packing sensor motif [20]. The opening of the lid reveals basic amino acid residues adjacent to the ligand-binding cavity, facilitating interactions with negatively charged lipid head groups on the membrane [17,21]. This mechanism facilitates the extraction of sterols from the lipid bilayer.

ORPs are a class of lipid-transferring proteins that play crucial roles in the intracellular distribution of sterols and phosphatidylserine [22]. These lipids are synthesized in the endoplasmic reticulum (ER) and are primarily distributed to the trans-Golgi network (TGN) and plasma membrane (PM) [23–26]. Sterols help maintain membrane rigidity and structural integrity [23,25], while phosphatidylserine – an anionic phospholipid – recruits signaling proteins to the inner leaflet of the PM through electrostatic interactions [25,26]. This asymmetric distribution of sterols and phosphatidylserine relies on the coupling of ORPs to the metabolism of phosphatidylinositol-4-phosphate (PI4P), one of the most abundant phosphatidylinositols in eukaryotes [22]. PI4P is generated by the phosphorylation of phosphatidylinositol at the 4-position of the inositol ring [27]. PI4P regulates cell growth, proliferation, lipid biosynthesis, and transport, and exacerbates these processes through oncogenic signaling pathways [28]. Studies show that ORPs facilitate the extraction of sterols or phosphatidylserine from the ER and exchange them with PI4P in the TGN and PM, where PI4P is catalytically generated by PI-4 kinase [22]. Subsequently, ORPs translocate PI4P back to the ER membrane, where it is hydrolyzed by the phosphatase Sac1 [22]. The continuous synthesis and hydrolysis of PI4P drive this exchange cycle, leading to the accumulation of sterols and phosphatidylserine in the TGN and/or PM, thereby creating lipid distribution asymmetry [29,30]. In addition to cholesterol transport, ORPs are implicated in regulating cell proliferation, survival, and migration [31]. ORPs play key roles in signaling by interacting with a variety of proteins. For example, ORP2 has been shown to modulate cell adhesion through its downregulation-mediated activation of focal adhesion kinase (FAK) [32], while ORP5 promotes cellular proliferation by activating the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway [33]. These functions highlight the multifaceted roles of ORPs in both cell physiology and pathology.

In mammals, cholesterol is the predominant sterol, accounting for about 20% of total cellular lipids [22]. Its metabolites, including oxysterols and steroid hormones, serve multiple functions in different physiological processes [34]. The liver, the primary organ for cholesterol synthesis, delivers endogenously synthesized and exogenously ingested cholesterol into the bloodstream in the form of low-density lipoproteins (LDL) for systemic cellular use [35]. Intracellular cholesterol homeostasis is maintained primarily through the uptake of LDL cholesterol via the LDL receptor (LDLR)-mediated endocytic pathway [36]. The LDL/LDLR complex is internalized and delivered to late endosomes/lysosomes, where LDL-derived cholesteryl esters are hydrolyzed to free cholesterol by acid lipases. Subsequently, cholesterol is transported to other membrane structures, including the ER, via Niemann-Pick type C1/2 proteins and ORPs [37]. Dysregulation of cholesterol homeostasis has been closely linked to various cancers [38,39]. Aberrations in cholesterol metabolism in tumor cells are primarily characterized by upregulation of cholesterol synthesis, increased uptake and accumulation of cholesterol metabolites, which in turn promote tumorigenesis and cancer progression [40–42]. On one hand, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), the rate-limiting enzyme for cholesterol synthesis, is overexpressed in several tumors, including gastric, ovarian, glioma, and prostate cancers [43–46]. Its inhibition has been explored as a therapeutic strategy for castration-resistant prostate cancer and hematological malignancies [47]. On the other hand, LDLR, which mediates cholesterol uptake, is frequently overexpressed in tumor cells [48,49], and LDL has been shown to promote tumor growth and invasion by regulating NF-κB signaling [50]. Sterol regulatory element-binding protein 2 (SREBP2) is a key regulator of cholesterol synthesis and uptake [51]. When cellular cholesterol levels are low, SREBP2 is activated, upregulating the expression of HMGCR and LDLR to enhance cholesterol synthesis and uptake [52]. Conversely, when cholesterol is in excess, the SREBP2 pathway is inhibited, and excess cholesterol is esterified by acyl–coenzyme A: cholesterol acyltransferase 1 and stored in lipid droplets [53]. Several ORPs have been shown to play important roles in cholesterol metabolism regulation [32,54–56]. For instance, in human epithelial cell carcinoma, ORP2 promotes LDL-derived cholesterol transport and activates FAK [32]; ORP8 downregulates SREBP2 to inhibit cholesterol synthesis and uptake [54]; ORP5 induces SREBP2 expression to enhance cholesterol metabolism [55]; and in the absence of ORP9 and GRAM domain-containing proteins (GRAMD1s), SREBP2 is even more active, further promoting cholesterol synthesis and uptake [56]. These findings suggest that ORPs influence tumor development by modulating cholesterol metabolism. Despite the considerable research on ORP2, ORP3, ORP4, ORP5, ORP8, and ORP9, their associations with cancer have not been extensively discussed. In this review, we aim to summarize the structural and functional characteristics of these ORPs, their contributions to tumor development, and their potential therapeutic value.

Major Structural Features of ORPs

Human ORPs share a conserved C-terminal ORD, which facilitates the transport of cholesterol from the ER to the Golgi apparatus [18]. In addition to the ORD, most ORPs possess upstream structural domains that enable them to target specific subcellular membrane organelles [57]. These include an N-terminal PH that recognizes PI4P and small G proteins (Arf1-GTP) on the Golgi complex [58,59]. ORPs containing an N-terminal PH domain are typically classified as long isoform proteins, while those lacking this domain are referred to as short isoform proteins [60]. Furthermore, certain ORPs feature an FFAT motif (2 phenylalanines [FF] within an acidic tract) located between the ORD and PH domains. This motif is recognized by the ER membrane proteins vesicle-associated membrane protein-associated protein A or B (VAPA or VAPB) through their major sperm protein domains (Table 1) [58,61,62]. This interaction targets ORPs to the ER, enabling their participation in cholesterol transport and signaling processes.

Functional Features of ORPs

ORP2 INHIBITS CELL ADHESION:

Takahashi et al demonstrated that the rapid degradation of ORP2 facilitates the transport of cholesterol from late endosomes to recycling endosomes, where it promotes the activation of FAK/integrin signaling [32]. This process is mediated by cholesterol enhancing the binding of FAK to lipid bilayers containing phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2) [32]. Activated FAK, in turn, increases the activity of endosomal phosphatidylinositol-4-phosphate 5-kinase type I gamma, leading to the elevated production of PI(4,5)P2 [70]. Deletion of ORP2 amplifies FAK activity and PI(4,5)P2 generation on endosomal membranes, thereby strengthening cell adhesion [71]. Furthermore, ORP2 knockdown redirects PI(4,5)P2 to late endosomes containing Niemann-Pick type C1 protein, influencing renal tubular dynamics [32,72]. The interplay between ORP2 deletion and FAK activation efficiently channels cholesterol to the plasma membrane, stimulating cell adhesion and highlighting the critical role of ORP2 in regulating cholesterol trafficking and cellular adhesion processes [32].

ORP3 INDUCES R-RAS ACTIVATION AND PROMOTES CELL MIGRATION AND INVASION:

Studies have revealed that the phosphorylation status of ORP3 plays a critical role in determining its subcellular localization and protein-protein interactions [73]. Hyperphosphorylated ORP3 specifically interacts with VAPA, and the PH domain of ORP3 facilitates the targeting of the ORP3-VAPA complex to the PM [73]. ORP3 exhibits dual targeting capabilities, localizing to both the ER and the PM, where it induces the activation of the small GTPase R-Ras [73]. This activation triggers downstream signaling pathways, including Akt phosphorylation and β1 integrin activation [8]. The co-expression of ORP3 and VAPA at ER-PM contact sites enhances R-Ras activation, a process that relies on the interaction of ORP3 with both VAPA and the PM [73]. Furthermore, the interaction between ORP3 and R-Ras has been shown to regulate cell adhesion and drive the migration and invasion of tumor cells [8], underscoring the multifaceted role of ORP3 in cellular signaling and cancer progression.

ORP5 AND ORP8:

ORP5 and ORP8, members of the class IV ORPs, are characterized by the presence of PH domains but lack the FFAT motif [74]. These proteins are directly anchored to the ER through a C-terminal transmembrane segment [74]. Functionally, ORP5 and ORP8 facilitate the enrichment of phosphatidylserine at the ER by exchanging it for PI4P, which is subsequently dephosphorylated by the ER-localized phosphatase Sac1 [75]. The recruitment of ORP5/8 to the PM is mediated by interactions between their PH and phosphoinositide-binding domains with PI4P and PI(4,5)P2 (Figure 1) [76]. Additionally, PI(4,5)P2 can undergo translocation during this process. By regulating the levels of phosphatidylserine and phosphoinositides at the PM, ORP5/8 play a critical role in modulating cellular processes such as proliferation and migration, potentially through the activation of key signaling kinases, including KRas and protein kinase B (AKT) [76].

DELETION OF ORP9 AND GRAMD1S LEADS TO DYSREGULATION OF CHOLESTEROL HOMEOSTASIS:

Oxysterol-binding protein–related protein 9 long isoform (ORP9L), containing a PH domain, is distributed between the ER and the Golgi apparatus in a VAP-dependent manner, and its forced overexpression disrupts the organization of the ER and ER-Golgi intermediate compartments [77]. The FFAT and sterol-binding domains of ORP9L facilitate its retention in the ER, while the PH domain promotes its association with the TGN [14]. Naito et al [56] demonstrated that ORP9 regulates Golgi cholesterol levels by modulating OSBP activity through the extraction of PI4P from the TGN. When cholesterol levels in the TGN membrane surpass a specific threshold, GRAMD1s (also known as Asters) transport excess cholesterol to the ER, thereby preventing cholesterol accumulation in the Golgi [78]. When ORP9 and GRAMD1s are simultaneously depleted, OSBP is overactivated, cholesterol transport to the ER is less efficient, and SREBP2 is more active under these conditions, which promotes cholesterol synthesis and uptake [56]. This dysregulation triggers aberrant cholesterol synthesis and accumulation in the Golgi and post-Golgi membranes. These findings underscore the critical role of ORP9 in maintaining cholesterol homeostasis.

ORPs and Cancer

ORP4 PROMOTES TUMOR CELL GROWTH AND PROLIFERATION:

ORP4 exhibits selective and limited expression in normal tissues, primarily localized to specific regions of the brain, retina, and testes [4,5]. However, it is notably overexpressed in ovarian cancer cell lines and patient-derived T-cell acute lymphoblastic leukemia cells, where it drives tumor cell proliferation [79,80]. ORP4, which is not present in healthy T cells but is highly expressed in T-cell acute lymphoblastic leukemia cells, plays a critical role in supporting a signaling complex composed of CD3ɛ, Gαq/11, and phospholipase Cβ3 (PLC-β3) [81]. Specifically, ORP4L extracts PI(4,5)P2 from the PM and delivers it to PLC-β3 for hydrolysis, facilitating downstream signaling [81]. Charman et al [5] demonstrated that knockdown of ORP4 in HEK293 (human embryonic kidney cells) or HeLa (cervical cancer cells) induces growth arrest, while depletion of ORP4 in untransformed intestinal epithelial cells triggers cell death. Bensen et al [80] further revealed that under conditions mimicking the ovarian cancer microenvironment, where exogenous cholesterol supply is limited, targeting ORP4 with the small molecule compound Ornithogalum saundersiae steroidal saponin-1 (OSW-1) significantly enhances its anti-proliferative activity. These findings underscore the role of ORP4 in promoting tumor cell growth and proliferation. Moreover, the anticancer effects of OSW-1 have been validated in a leukemia model [79,82] and in ovarian cancer 3-dimensional spheroids [80], where it demonstrates superior efficacy compared with conventional ovarian cancer treatments, such as cisplatin and paclitaxel. Collectively, these studies highlight ORP4 as a promising therapeutic target for precision cancer therapy.

ORP8 INHIBITS TUMOR CELL GROWTH AND PROLIFERATION AND PROMOTES APOPTOSIS:

Studies have demonstrated that ORP8 expression inhibits cell proliferation and tumor growth while promoting apoptosis [83,84]. Yan et al found that ORP8 reduces cholesterol efflux by suppressing the expression of ATP-binding cassette transporter A1, underscoring its essential role in the progression of atherosclerotic lesions [85]. Zhou et al also found that ORP8 suppresses the target genes of Sterol regulatory element binding protein 1 (SREBP1) and SREBP2, functioning as a negative modulator of intracellular cholesterol balance and consequently restraining cell growth and proliferation [54]. Zhong et al reported that ORP8 is significantly under-expressed in hepatocellular carcinoma cell lines, and its overexpression induces apoptosis [12]. In HepG2 cells (a hepatocellular carcinoma cell line), ORP8 overexpression triggers the ER stress response, leading to the upregulation of Fas ligand. This facilitates the transport of Fas ligand, expressed in T lymphocytes, to the cell membrane via P53-mediated trafficking, where it binds to Fas on the cell surface, ultimately inducing apoptosis [12]. Additionally, Guo et al [83] found that ORP8 expression is markedly reduced in gastric cancer tissues and cell lines. Elevated expression of ORP8 markedly curtails the proliferation of gastric cancer cells and restrains tumor growth in vivo. ORP8 overexpression affects cellular function through a dual mechanism: on the one hand, it regulates cellular activity by inhibiting the Wnt/β-catenin signaling pathway (manifested by the downregulation of Wnt3a and β-catenin expression); on the other hand, ORP8 overexpression induces ER stress, which is specifically reflected by the upregulation of the expression of ER stress markers, including C/EBP homologous protein (CHOP) and glucose-regulated protein 78, and the increased levels of protein kinase R-like endoplasmic reticulum kinase (PERK) phosphorylation [83]. Notably, they found that inhibition of endoplasmic reticulum stress reversed the downregulation of Wnt3a/β-catenin expression caused by ORP8 overexpression [83]. In summary, ORP8 exerts its function as a tumor suppressor by inducing apoptosis and inhibiting key signaling pathways.

ORP2 INHIBITS MIGRATION AND INVASION OF COLORECTAL CANCER CELLS THROUGH INHIBITION OF COLLAGEN I INHIBITION:

ORP2 facilitates the exchange of PI4P for cholesterol between adjacent membranes [86], enhancing cholesterol influx and synthesis while reducing efflux, thereby modulating the tumor microenvironment. The extracellular matrix of tumors, a critical component of the tumor microenvironment, consists of structural elements such as collagen and glycoproteins [87]. Notably, collagen type I is highly expressed in infiltrative colorectal cancer (CRC) [88]. ORP2 deficiency has been shown to promote collagen I-induced CRC cell growth, adhesion patch formation, migration, and invasion [89]. This suggests that collagen I could serve as a potential biomarker and therapeutic target within the tumor microenvironment for CRC treatment. Lin et al further demonstrated that ORP2 is associated with a favorable prognosis in metastatic CRC and significantly suppresses collagen I-induced focal adhesion, migration, and invasion [89]. Mechanistically, ORP2 reduces adhesion plaques by inhibiting the autophosphorylation of FAK, which interacts with the SRC family of kinases, thereby diminishing the formation of invasive pseudopods [89]. FAK, a scaffolding protein, mediates interactions between integrins and the actin cytoskeleton, regulating cell adhesion and cell-environment communication [90]. It is highly expressed in metastatic CRC and is influenced by lipid signaling [91,92]. Additionally, poly(ADP-ribose) polymerase 1 (PARP1), an ADP-ribosyltransferase essential for the transfer function of ORP2, plays a role in this process [89,93]. ORP2 inhibits CRC growth and distant metastasis, and these effects can be further enhanced by PARP1 inhibitors, such as AG14361 [89]. Collectively, these findings indicate that targeting ORP2 and FAK could represent a promising combination approach for CRC therapy. However, further studies are needed to validate these potential therapeutic approaches.

DUAL ROLE OF ORP3 IN TUMOR CELL MIGRATION AND INVASION: PROMOTION AND INHIBITION:

ORP3 is aberrantly upregulated in CRC and hepatocellular carcinoma and promotes tumor cell migration and invasion [94,95]. ORP3 expression is regulated by methylation, which is notably upregulated in CRC and correlates with poor prognosis in this malignancy [96]. Hyperphosphorylated ORP3 enhances the formation of ORP3-VAPA complexes by increasing the affinity of its FFAT motif for VAPA [73]. This interaction, in turn, activates R-Ras, a critical factor in multiple cancers, thereby promoting cell adhesion, migration, and invasion [73,97,98]. In CRC, high ORP3 expression is associated with poor tumor differentiation, advanced TNM stage, and reduced survival, while it also drives tumor proliferation, invasion, and metastasis [99,100]. Zhang et al reported that KRAS mutations are prevalent in CRC samples with high ORP3 expression, but the specific roles of these mutations in the context of ORP3 overexpression warrant further investigation [99].

Hu et al discovered that in gastric cancer cells, ORP3 activates the R-Ras/Akt signaling pathway through R-Ras phosphorylation, thereby promoting tumor growth [94]. The high expression of ORP3 was linked to promoter hypomethylation and increased DNA copy number, while mutations in ORP3 were rare in gastric cancer [94]. These findings indicate that ORP3 drives malignancy in gastric cancer primarily through DNA copy number amplification and promoter hypomethylation-induced overexpression. Su et al reported that in liver hepatocellular carcinoma, ORP3 expression was significantly elevated in patients with advanced-stage disease and correlated with TP53 mutations, highlighting its potential role in liver hepatocellular carcinoma progression [95]. Similarly, Tian et al observed that ORP3 was markedly overexpressed in hepatocellular carcinoma and associated with overall survival and disease-specific survival [101]. Knockdown of ORP3 induced G2/M–phase cell cycle arrest, promoting apoptosis and suppressing cell migration and proliferation [101]. These results suggest that ORP3 may serve as both a prognostic marker and a regulatory factor in hepatocellular carcinoma. However, further data and experimental studies are required to fully elucidate the role of ORP3 in liver cancer progression.

However, in bladder cancer, ORP3 exhibits a tumor-suppressive effect rather than promoting malignancy [102]. ORP3 exhibits high expression levels in normal bladder and ureteral epithelium but is markedly reduced in bladder cancer tissues from both humans and mice [102]. Experimental studies showed that ORP3 knockdown increased the migration and invasion ability of Rat bladder transitional cell carcinoma line 4 (bladder cancer cell known for its low migration and invasion potential), whereas overexpression of ORP3 instead impaired the migration and invasion of UMUC3 cells (a bladder cancer cell known for its high invasive potential) [102]. The regulation of actin dynamics is a key mechanism underlying cell migration, the formation of invasive pseudopods, and cell invasion [103]. Moreover, an intact actin cytoskeleton is vital for the proper localization of the spindle apparatus [104]. ORP3 affects cell migration and invasive pseudopod formation by regulating actin dynamics, and its downregulation leads to an increase in multipolar spindles that may trigger chromosome segregation defects [102]. Given that chromosomal abnormalities are positively associated with the invasive potential of bladder cancer, the downregulation of ORP3 promotes bladder cancer invasiveness and is associated with poor prognosis [102].

In summary, ORP3 plays a significant role in driving tumor progression in various solid cancers but exhibits a tumor-suppressive effect in bladder cancer. Targeting ORP3 may emerge as a promising therapeutic strategy for cancer treatment. However, its specific mechanisms in different cancer types still require further investigation.

ORP5 PROMOTES TUMOR CELL GROWTH AND PROLIFERATION AS WELL AS TUMOR CELL MIGRATION AND INVASION:

Koga et al [11] were the first to explore the role of ORP5 in hamster and human pancreatic cancer cells. Their findings revealed that ORP5 overexpression significantly enhanced the invasion rate of these cancer cells, while its deletion reduced invasiveness, suggesting that ORP5 serves as a key factor in promoting tumor cell invasion. Clinical sample analysis further supported these results, demonstrating that high levels of ORP5 expression are correlated with poor prognosis in human pancreatic cancer [11]. Building on this, Ishikawa et al investigated the underlying mechanisms and discovered that ORP5 induces the expression of SREBP2, a crucial transcriptional regulator of cholesterol biosynthesis [55]. This indicates that ORP5 facilitates cholesterol synthesis and uptake, which are processes essential for tumor cell proliferation and invasion. Elevated ORP5 expression is associated with poor prognosis in pancreatic cancer, and SREBP2-induced downstream targets, such as histone deacetylase 5, may further contribute to disease progression [11]. In lung cancer, studies have also identified high levels of ORP5 expression in metastasis-positive cases, with ORP5 overexpression enhancing the invasiveness of lung cancer cells [105]. The regulation of membrane lipid homeostasis by ORP5 and other lipid transfer proteins has significant implications for mTORC1 [33], a central regulator of lipid metabolism, protein synthesis, and cell proliferation [106–108]. In cancer, mTORC1 hyperactivation intersects with the PI3K/AKT and mitogen-activated protein kinase (MAPK)/MAPK ERK kinase (MEK)/extracellular signal–regulated kinase (ERK) signaling pathways, thereby driving oncogenic processes [109]. Thus, in some cancer cells, ORP5–mediated phosphatidylserine transport may facilitate mTORC1 activation, further promoting cancer cell proliferation [33]. These findings highlight ORP5 as a potential therapeutic target in cancer progression.

Wu et al found that in cervical cancer cells, ORP5 promoted migration and invasion in HeLa cells and C33A cells [110]. SREBP1 is a transmembrane protein present in the ER that activates ER stress by promoting the expression and phosphorylation of PERK [111]. In addition, overexpression of ORP5 reduced the protein level expression of key molecules in the unfolded protein response branch (activating transcription factor 6, activating transcription factor 4, binding immunoglobulin protein, and CHOP) and attenuated the fluorescence intensity of ER staining in live cells, suggesting that ER stress was alleviated [110]. Their findings suggest that ORP5 inhibits ER stress in cervical cancer cells by stimulating ubiquitination and proteasomal degradation of SREBP1 to reduce its expression, thereby promoting migration and invasion of cervical cancer cells [110]. Additionally, Li et al discovered that ORP5 was significantly overexpressed in renal cell carcinoma, and its elevated expression would promote migration and invasion of renal cell carcinoma cells [112]. ORP5 promotes the ubiquitination and degradation of C-Cbl, the E3 ligase responsible for negatively regulating mesenchymal-epithelial transition factor (c-Met), thereby suppressing the lysosomal degradation of c-Met [112]. This resulted in an abnormal elevation of c-Met and was able to activate mTORC1/AKT, a core signaling pathway of c-Met, thereby promoting tumor cell migration and invasion [112]. These results suggest that ORP5 may be a therapeutic target for the treatment of certain cancers together with mTORC1 inhibition.

ORP9 IS ASSOCIATED WITH TUMOR IMMUNITY:

Yan et al revealed that ORP9 is significantly associated with pancreatic adenocarcinoma prognosis and antigen-presenting cells [113]. They demonstrated that ORP9, a protein linked to ferroptosis, exhibits a positive correlation with various antigen-presenting cells. Moreover, elevated levels of ORP9 were linked to poorer overall survival and progression-free survival, particularly in advanced TNM stages [113]. These findings suggest that ORP9 may be processed by antigen-presenting cells, presented to T cells, recognized by B cells, and subsequently trigger an immune response. ORP9 induces ferroptosis in tumor cells, leading to their subsequent elimination. Zhou et al investigated the relationship between ORP family members and immune cell infiltration in pancreatic ductal adenocarcinoma [114]. Their results demonstrated that ORP9 is positively correlated with the infiltration of CD8+ T cells, macrophages, neutrophils, and dendritic cells [114]. These results indicate that ORP9 is not only associated with tumor metabolism and prognosis but also plays a key role in tumor immune regulation, which is of great clinical significance.

Moreover, ORP10, a hypoxia-inducible gene, is significantly upregulated in pancreatic cancer and correlates with unfavorable clinical outcomes [115]. This oncogenic protein directly interacts with cellular nucleic acid-binding protein, a key regulator implicated in tumor progression across multiple malignancies including CRC, melanoma, and gastric cancer [116–118]. Through this interaction, ORP10 modulates cellular nucleic acid-binding protein expression, thereby critically regulating the proliferative and metastatic potential of pancreatic cancer cells [115]. Furthermore, ORP10 mutations can drive breast cancer [119]. Despite these advances, the roles of ORP1, ORP6, ORP7, and ORP11 in cancer remain largely unexplored, with no studies to date elucidating their mechanisms of action in tumorigenesis. These findings indicate that targeting ORPs holds promise as a potential strategy for cancer therapy. However, additional research is needed to fully elucidate their mechanisms and therapeutic potential.

ORPs and Non-Cancer Diseases

OSBP MAY BE A POTENTIAL ANTIVIRAL TARGET:

OSBP has been recognized as a key broad-spectrum antiviral target that is crucial for the replication of multiple human RNA viruses, including enteroviruses, hepatitis C virus, dengue virus, and encephalomyocarditis viruses [120–125]. Its critical role in the formation of viral replication organelles underscores its importance in RNA virus replication, making it a highly attractive target for the treatment of a wide range of currently untreatable RNA viruses [126–128]. Recent studies have further highlighted the therapeutic potential of targeting OSBP [129]. Currently, several antiviral small molecules (OSW-1 [122], itraconazole [120], and T-00127-HEV2 [127]) have been demonstrated to target OSBP and exhibit significant inhibitory effects against enteroviruses and hepatitis C virus. These findings suggest that antiviral compounds targeting OSBP hold significant promise for the treatment of RNA viral infections, such as those caused by enteroviruses and hepatitis C virus. However, further research is needed to elucidate their precise mechanisms of action and to evaluate their potential for clinical application.

ORP2 IS ASSOCIATED WITH OBESITY-RELATED DISEASES:

ORP2 can bind to β-catenin to form a complex that restricts β-catenin to the cytoplasm, thereby preventing its translocation to the nucleus and subsequent regulation of downstream gene transcription [130]. As a key component of cell adhesion and a core effector in the canonical Wnt signaling pathway, β-catenin plays a critical role in cellular processes [131,132]. In the absence of ORP2, β-catenin undergoes increased ubiquitination, leading to its degradation and reduced cytoplasmic levels [130]. This disruption promotes the differentiation of preadipocytes into mature adipocytes, ultimately contributing to obesity-related characteristics and metabolic disorders [130]. Studies have demonstrated that ORP2−/− Bama minipigs exhibit significant obesity and hypercholesterolemic phenotypes, while adult OSBPL2b−/− zebrafish display abnormal lipid metabolism-related features [133]. However, the precise mechanistic role of ORP2 in the development of obesity-related diseases remains incompletely understood.

In addition to ORP2, other oxysterol-binding protein–related proteins, such as ORP8 and ORP11, have been implicated in obesity-related diseases. ORP8 has been identified as particularly significant in the pathogenesis of type II diabetes mellitus [134], while polymorphisms in the ORP11 gene have been linked to various cardiovascular risk factors in Canadian patients with obesity metabolic syndrome [16]. These findings underscore the broader involvement of ORP family members in metabolic and obesity-related disorders.

DEAFNESS CAUSED BY ORP2 DEFICIENCY IS ASSOCIATED WITH DYSREGULATION OF CHOLESTEROL HOMEOSTASIS:

Numerous studies have demonstrated that ORP2 deficiency in pigs leads to progressive hearing loss, characterized by cochlear hair cell degeneration and abnormal stereocilia morphology, alongside the development of hypercholesterolemia [133]. In auditory cells and zebrafish models lacking ORP2, elevated cholesterol biosynthesis has been observed, driven by the generation of reactive oxygen species due to the inhibition of AMPK activity [135]. Du et al further revealed that a long-term high-fat diet induced mitochondrial damage and apoptosis in the inner ear of hyperlipidemic mouse/rat models [136]. These findings indicate a potential link between ORP2 deficiency-induced lipid metabolism dysregulation and hearing impairment, although additional research is needed to elucidate the mechanisms involved.

In conclusion, these studies suggest that ORPs play an important role in cancer as well as in the development and progression of diseases by regulating lipid metabolism, cellular signaling pathways, and organelle function (Table 2).

Future Directions

Emerging evidence highlights ORPs and their associated signaling pathways as promising therapeutic targets in oncology, with their molecular mechanisms and pharmacological modulation becoming an active area of investigation [140]. Recent studies demonstrate that ORPs regulate tumor progression through multiple mechanisms, including cholesterol homeostasis maintenance [141] and modulation of key signaling pathways, such as mTORC1 [33].

Notably, Santos et al found that both the antifungal agent itraconazole and the novel synthetic compound PRR851 effectively suppress extracellular vesicle–dependent metastatic dissemination and cell motility by specifically disrupting the Rab7-ORP3-VAP-A tripartite complex [138]. Genetic ablation of ORP3-VAP-A similarly attenuates malignant progression in melanoma, cervical carcinoma, and CRC models [138]. Additional findings underscore the therapeutic potential of ORPs modulation: 1) The steroidal agent OSW-1 exerts antitumor effects in ovarian cancer by dual targeting of ORP4/OSBP and subsequent inhibition of cholesterol-driven mTORC1 activation [80]; 2) itraconazole demonstrates antiviral activity through OSBP/ORP4L inhibition [127]; and 3) rapamycin improves DFNA67-type hereditary hearing loss by reducing pathogenic ORP2 mutant accumulation [142]. While these discoveries position ORPs as versatile targets for multiple disease modalities, clinical translation requires further mechanistic validation. Future research directions should prioritize the following: 1) developing isoform-specific inhibitors with improved pharmacologic properties; 2) exploring therapeutic applications in neoplastic and metabolic disorders; and 3) advancing personalized treatment strategies through ORP pathway modulation.

Conclusions

The ORP family represents a crucial class of lipid regulatory proteins that orchestrate cholesterol distribution and lipid metabolism, playing pivotal roles in oncogenesis [143]. Current research demonstrates their dichotomous functions in cancer biology: ORP2 [89], ORP3 [102], ORP4 [80], and ORP5 [110] drive tumor progression, whereas ORP8 exhibits tumor-suppressive activity [144], and ORP9 modulates immune microenvironment dynamics in pancreatic cancer [113]. Additionally, ORP6/7 [9] and ORP10/11 [145] participate in cell migration and metabolic reprogramming, respectively. Notably, pharmacological targeting of ORPs using specific inhibitors, such as Orpinolide [146] (OSBP inhibitor) and OSW-1 [137] (dual OSBP/ORP4L inhibitor) has emerged as a promising therapeutic strategy. This comprehensive review delineates the multifaceted roles of ORP family members across solid and hematologic malignancies, establishing a molecular framework for developing innovative anticancer interventions targeting lipid metabolic pathways.