OPN is encoded by the SPP1 gene located on chromosome 4q22.1 (19). The SPP1 gene contains seven exons, which undergo selective splicing to generate distinct variants. At present, seven exons of SPP1 have been reported. Exon 1 remains untranslated, whereas exons 2–7 contain coding sequences. Exon 2 encodes a signal peptide along with two amino acids constituting the mature protein; exon 3 encodes a serine phosphorylation motif; exon 4 encodes a proline-rich region and transglutaminase site; and exon 5 encodes an additional protein phosphorylation site. Notably, exons 6 and 7 account for >80% of the expressed OPN protein (19). Five isoforms of OPN have been reported, including OPN-a (comprising all exons and consisting of 314 amino acids), OPN-b (lacking exon 5 and comprising 300 amino acids), OPN-c (lacking exon 4 and comprising 287 amino acids), OPN4 (lacking exons 4 and 5, and consisting of 273 amino acids) and OPN5 (containing an additional exon, with a length of 327 amino acids) (10,20). Distinct splicing of the SPP1 gene has been implicated in cancer occurrence, progression and prognosis (19).

Human OPN contains ~314 amino acid residues, with a molecular weight ranging between 41 and 75 kDa, depending on different post-translational modifications (8,21). OPN contains several highly conserved domains, including the aspartate domain, arginine-glycine-aspartic acid (RGD) domain, serine-valine-valine-tyrosine-glutamate-leucine-arginine (SVVYGLR) domain, thrombin cleavage domain, calcium-binding domain and C-terminal heparin-binding domain (22). Given that OPN is a secreted protein, it is present not only in various tissues but also in biological fluids. The secreted OPN is subjected to various post-translational modifications, including glycosylation, phosphorylation and sulfuration, which endows OPN with diversified biological functions (8). Fig. 1 schematically illustrates the structure and domains of OPN.

OPN needs to bind with its receptors to exert its function. The well-characterized receptors of OPN include integrins and CD44 (23,24). Notably, under some conditions, OPN is subjected to protease cleavage to expose its specific domains, which exhibit binding affinity for potential receptors (25).

Integrins are transmembrane heterodimers that recognize various ligands, including OPN and other extracellular matrix proteins and cell surface proteins (26). The RGD sequence of OPN exhibits a strong binding affinity for integrins, including αvβ1, αvβ3, αvβ5, αvβ6, αvβ8, α8β1, α5β1 and α_IIbβ3 (27,28). Another integrin-binding domain (SVVYGLR) can only be exposed after thrombin cleavage at Arg¹⁶⁸-Ser¹⁶⁹ (29). The exposed SVVYGLR interacts with integrin subtypes such as α9β1, α4β1 and α4β7 (25,28,30). Similarly, plasma proteases cleave OPN at Lys¹⁵⁴-Ser¹⁵⁵, producing an N-terminal OPN fragment that binds more strongly to α5β1 and αvβ3 integrins than the full-length OPN (25). MMP could cleave OPN at various sites such as Gly¹⁶⁶-Leu¹⁶⁷, Ala²⁰¹-Tyr²⁰² and Asp²¹⁰-Leu²¹¹, producing a fragment that preferably binds with the α4β1 isoform of integrin (29).

CD44, also known as hyaluronic acid receptor, is expressed in various cell types such as osteocytes, endothelial cells, fibroblasts, epithelial cells and smooth muscle cells (31). The C-terminal fragment heparin-binding site of OPN can directly interact with CD44 variants CD44v3 and CD44v6 (30).

Therefore, OPN, with or without cleavage, interacts with integrins or CD44 to regulate multiple cellular processes, including cell attachment, migration, chemotaxis and immune modulation in diverse types of cells (24).

OPN is expressed in various types of cells and tissues. With the deepening of research, diverse functions of OPN in both physiological and pathological conditions have been revealed (32,33).

OPN is also known as Eta-1 and can trigger the immune response of macrophages, T cells and B cells (34). For example, OPN binds with integrin (αvβ3) of macrophages and reduces the production of anti-inflammatory IL-10, while its binding with CD44 promotes the secretion of proinflammatory IL-12 (23). OPN also influences the migration of immune cells (27,35). Wang and Denhardt (27) reported that OPN served as a type 1 T helper cell cytokine, contributing to mucosal defense against viral pathogens. OPN may regulate inflammatory reactions by stimulating the natural immune response of macrophages and neutrophils (27). OPN could also be secreted by activated T cells to aid in the recruitment and migration of immune cells, particularly macrophages, toward the site of inflammation, resulting in persistence and exacerbation of the inflammatory response (36).

OPN exerts pro-inflammatory effects by activating inflammatory signaling pathways, such as the NF-κB and STAT3 pathways, which leads to the secretion of cytokines, including TNF-α and CD16 (37). Additionally, OPN modulates immune cell functions by enhancing macrophage chemotaxis and phagocytic activity, promoting their polarization toward the pro-inflammatory M1 phenotype, and activating the immune responses of T cells, natural killer cells and dendritic cells (38,39). Furthermore, OPN stimulates tumor cells to secrete IL-17, thereby activating the JNK/c-Jun signaling pathway and contributing to immunosuppressive effects (40). OPN also serves a critical role in mediating immune escape mechanisms, such as enabling tumor cells to evade immune surveillance through the suppression of T cell activity (41). Development of single-cell RNA sequencing (scRNA-seq) has provided a better way to define the context-dependent functions of OPN. Ding et al (42) utilized scRNA-seq and revealed that CD163⁺ macrophages were enriched in lung tissues of patients with coronavirus disease 2019, and these cells highly expressed OPN. Wang et al (43) used scRNA-seq analysis and revealed that OPN expression was upregulated in menstrual blood cells from patients with endometriosis, which in synergy with inflammatory factors, such as IL-10 and IL-6, promoted local fibrotic processes.

Aberrant OPN expression may induce dysregulation of the immune response, thereby impairing the normal recognition and response to self-antigens and foreign antigens, which contributes to the initiation and progression of autoimmune disorders (44). Improper expression of OPN is associated with the pathogenesis of various autoimmune diseases (such as multiple sclerosis, systemic lupus erythematosus, rheumatoid arthritis and atherosclerosis) and other inflammatory diseases (including cardiovascular diseases, chronic obstructive pulmonary disease, inflammatory bowel disease, liver diseases and asthma) (27).

OPN also participates in osteogenesis, regulating bone cell adhesion and osteoclast function, and promoting matrix mineralization. The strong affinity of OPN for hydroxyapatite leads to its accumulation in the bone (45–47). OPN exerts an effect on skeletal cell proliferation, differentiation and migration, while facilitating calcium phosphate mineralization (48). Given its crucial role in bone metabolism, aberrant OPN expression is associated with various types of bone diseases. In patient with osteolysis, OPN expression is downregulated, whereas in patients with fractures, OPN expression is increased (49).

OPN exhibits an important role in tissue repair and regeneration by facilitating cell migration, proliferation, neoangiogenesis and the formation of fibrous connective tissue (50). For example, OPN has been found to promote fibroblast proliferation during wound healing (50). Furthermore, Wen et al (51) found that, in a model of regenerating liver after partial hepatectomy, OPN promoted the inflammatory response and activated the IL-6/STAT3 pathway to facilitate the proliferation of hepatocytes and the repair of the liver. A previous study has demonstrated that OPN expression could induce vascular intimal thickening and promote neointimal formation following injury, suggesting a role in vascular remodeling (52). Upregulation of OPN leads to tissue fibrosis, including liver and kidney fibrosis, by facilitating the synthesis and deposition of extracellular matrix proteins (53).

OPN also participates in the regulation of cardiovascular function. OPN is upregulated in atherosclerosis and present in atherosclerotic plaques, suggesting its potential relevance (54). Furthermore, it has been demonstrated that OPN induces vascular calcification by facilitating mineral uptake and formation of apatite crystals (55). Speer et al (56) further revealed that phosphorylation of OPN could induce vascular calcification.

In summary, OPN serves diverse and important pathophysiological roles in a context-dependent manner. Elucidating the intricate structure and multifaceted functionality of OPN will consequently facilitate the development of OPN-based diagnostic and therapeutic strategies.

Tumor cells exhibit anti-apoptosis capacities (68). Sun et al (69) demonstrated that OPN regulated Bcl-2 and Bax expression, thus enhancing the viability of tumor cells. Another group found that OPN could induce Bcl-2 expression and attenuate apoptosis (70). In addition, OPN facilitates tumor progression by regulating tumor surveillance mechanisms and inhibiting tumor cell apoptosis (71). Upregulated OPN interacts with integrin αvβ3 or CD44, facilitating complement factor H cell surface translocalization, and thus, complement-mediated cell death is suppressed (72). Zhou et al (73) found that OPN knockdown enhanced apoptosis and caused cell cycle arrest in leukemia stem cells.

Tumor metastasis is closely associated with the capacity of cell migration and invasion (72,74). Irby et al (75) reported that overexpressed OPN in SW480, HT29 and HCT116 cells bound with its receptors, including integrins (αvβ1, αvβ3 and αvβ5) and CD44, and enhanced colon cancer invasion. OPN has the ability to mediate the adhesion and interaction between tumor cells and the surrounding matrix, thereby augmenting the metastatic potential of tumor cells (76). Kale et al (77) found that OPN, after binding with integrin (α9β1), triggered cyclooxygenase-2 (COX-2) and prostaglandin E2 expression, and thus, promoted tumor growth and metastasis. OPN induces the expression and activation of MMP-2, MMP-3 and urokinase-type plasminogen activator in an integrin-dependent manner, which promotes extracellular matrix degradation and consequent metastasis (78). The capacity of OPN to induce cellular migration and extracellular matrix degradation contributes to the invasive and metastatic potential of cancer (78).

Angiogenesis is a feature of tumors, particularly solid tumors. Neovascularization enhances the supply of nutrients and oxygen, thus promoting tumor growth and metastasis (71,79). OPN is involved in tumor angiogenesis by facilitating the migration and proliferation of vascular endothelial cells, which promotes neovascularization (79). Gupta et al (80) reported that OPN activated MMP-9, which resulted in increased VEGF secretion and promoted the angiogenesis of prostate cancer, while OPN knockdown or αvβ3 mutation reversed these effects. Fukusada et al (81) reported that OPN induced VEGF expression and angiogenesis, which promoted pancreatic ductal adenocarcinoma progression.

Tumor cells are surrounded by the TME, which consists of extracellular matrix, stromal cells, blood vessels, immune cells, fibroblasts and secretory factors (82). A recent study has demonstrated that the interplay between tumor cells and the TME governs tumorigenesis, invasion, metastasis, chemotherapy resistance and the immune response, thereby driving tumor progression and invasion (83). OPN, as a component of the TME, could derive from tumor cells, endothelial cells, fibroblasts and immune cells. OPN exerts an influence on the formation and regulation of the TME (9,84,85). Wei et al (36) found that OPN promoted tumor progression by attracting macrophages and activating T cells in the glioblastoma TME. Butti et al (86) reported that OPN could remodel the TME immune suppression and promoted tumor progression by modulating the polarization of T helper cells, T-regulatory cells and tumor-associated macrophages. Rogers et al (87) reported that myofibroblastic cancer-associated fibroblasts in the TME promoted cancer stem cell proliferation and metastasis, whereas inhibition of OPN in a mouse breast cancer model resulted in diminished tumor stemness. Therefore, destruction of the TME by targeting OPN may effectively inhibit tumor progression.

OPN exhibits distinct effects across various cancer types. For example, OPN inhibits autophagy in colorectal cancer cells, induces breast cancer cell migration and increases angiogenesis in breast cancer, and leads to cell migration and invasion in lung adenocarcinoma (88,89). Further studies elucidating the precise involvement of OPN in cancer pathogenesis are required.

The essential role of OPN in tumorigenesis and development is well-established, and OPN exerts its effects by activating multiple intertwined signaling pathways such as the JNK, Ras/Raf/MEK/ERK, PI3K/Akt, Janus kinase (JAK)/STAT, NF-κB and TIAM Rac1 associated GEF 1/Rac1 pathways (14,57). These pathways regulate cell proliferation, adhesion, diffusion, migration, invasion and epithelial-mesenchymal transition (EMT) of tumor cells, as well as immunosuppression and drug resistance (1). The main pathways regulated by OPN that are involved in tumorigenesis and development are summarized and illustrated in Fig. 2 (90–99).

The JNK signaling pathway has been demonstrated to be activated in various types of cancer such as lymphoma, pancreatic cancer, hepatocellular carcinoma and childhood sarcoma (91). Messex et al (100) found that extraneously added OPN activated the JNK pathway, which accelerated cell proliferation and prostate cancer development. Insua-Rodríguez et al (101) demonstrated that JNK was activated in breast cancer xenograft tumor mouse models, which enhanced OPN expression, resulting in chemotherapy resistance and metastasis. Li et al (102) demonstrated that OPN bound to MB231 breast cancer cells and activated the JNK signaling pathway, which resulted in EMT initiation and cellular migration.

The Ras/Raf/MEK/ERK signaling pathway serves a vital role in promoting cell proliferation, survival and metastasis in various cancer types (103). Yang et al (104) reported that knockdown of OPN in squalene synthase-overexpressing lung cancer cells inhibited their migration and invasion. Chernaya et al (105) found that, in thyroid cancer, activation of the RAS-RAF-MEK pathway led to upregulation of OPN and its receptors (integrin), which in turn increased the metastatic potential of tumor cells. Sun et al (106) adopted RNA interference (RNAi) to reduce OPN expression, which resulted in a decrease in the expression levels of MMP-2 and inhibition of the MEK/ERK pathway, eventually retarding hepatic carcinoma growth and metastasis.

The PI3K/AKT signaling pathway is aberrantly activated in various types of cancer (91). OPN has been found to activate the PI3K/AKT pathway by binding with CD44, consequently promoting cell proliferation and suppressing apoptosis (107). Zhang et al (108) reported that OPN bound with integrin αvβ3 and activated the PI3K/AKT pathway in breast cancer, while knockdown of OPN suppressed breast cancer metastasis. Furthermore, Fu et al (109) revealed that upregulation of OPN facilitated chemoresistance of non-small cell lung cancer (NSCLC), while silencing of OPN inhibited the phosphorylation of AKT and ERK, weakening gefitinib resistance in NSCLC cells.

The JAK/STAT signaling pathway is overactivated in cancer (110). OPN interacts with CD44 and triggers the activation of JAK protein kinase, which induces STAT phosphorylation and consequent enhanced hepatic carcinoma cell proliferation (111). Behera et al (112) demonstrated that OPN activated JAK2/STAT3 signaling and stimulated breast tumor growth in mice. JAK2 inhibitor (AG 490) administration could suppress OPN-induced STAT3 phosphorylation and promote tumor cell apoptosis (112).

The NF-κB signaling pathway has been found to be activated in cancer (104). OPN binding to integrin αvβ3 results in the translocation of NF-κB from the cytoplasm to the nucleus, and promotes tumorigenesis, proliferation, invasion and angiogenesis of HCC (113). Chen et al (114) demonstrated that specific small interfering RNA (siRNA) inhibition of OPN could effectively attenuate NF-κB levels, thereby inhibiting proliferation of esophageal squamous cell carcinoma cells and suppressing tumor formation and metastasis. Saurav et al (115) reported that inhibition of OPN and/or NF-κB signaling could enhance the efficacy of irinotecan, and the combination of OPN and/or an NF-κB inhibitor with irinotecan may enhance the therapeutic efficacy while reducing the development of drug resistance.

The p38 MAPK signaling pathway is a key signal transduction cascade that cells possess in response to environmental stimuli, and has attracted much attention as a promising target for cancer therapy (116,117). Yu et al (118) reported that OPN was highly expressed in head and neck squamous cell carcinoma (HNSCC), and OPN promoted the proliferation and invasion of HNSCC cells by activating the p38-MAPK signaling pathway. Huang et al (66) stated that OPN participated in the activation of the p38 MAPK signaling pathway, thereby promoting cell migration and invasion of colorectal cancer. OPN-conferred chemoresistance is also associated with the p38 MAPK signaling pathway. Pang et al (119) demonstrated that OPN mediated cyclophosphamide resistance in MDA-MB-231 tumor cells by activating p38 MAPK.

siRNAs and short hairpin RNAs (shRNAs) can efficiently downregulate the expression of specific genes. Knockdown of OPN with siRNA represents a promising intervention for various types of cancer such as liver tumors, chronic myeloid leukemia, familial adenomatous polyposis and metastatic melanoma, and it is currently undergoing clinical trial (121). Ben-David-Naim et al (122) demonstrated that administration of nanoparticle-encapsulated OPN siRNA effectively reduced OPN expression and suppressed tumor growth in a mouse breast cancer model. Yang et al (123) reported that knockdown of OPN with specific siRNA or shRNA could inhibit breast tumor progression. These findings suggest that downregulation of OPN expression could inhibit tumor cell proliferation, migration, invasion, angiogenesis and other processes. Using siRNA technology to silence OPN offers a plethora of strategic opportunities to develop effective treatment modalities against cancer.

Given their compact size and efficient cell membrane permeability, small-molecule inhibitors targeting specific signaling pathways have attracted increasing attention in disease intervention (23,124). Small-molecule inhibitors toward OPN have been applied for cancer treatment. Sharma et al (125) demonstrated that inhibition of OPN expression with trichostatin A effectively prevented tumor growth and metastasis in a mouse xenograft model. Androstenedione (Andro), a diterpenoid compound isolated from androsacetum, inhibits OPN expression, resulting in cell cycle arrest and apoptosis of breast cancer cells, as well as tumor-endothelial cell interactions (126). Kumar et al (126) further demonstrated that Andro suppressed breast cancer cell proliferation by downregulating c-jun expression and inhibiting PI3K/Akt signaling activation. Parecoxib, a traditional COX-2 inhibitor, has been reported to retard the progression of colorectal cancer by suppressing OPN expression via blockade of the nuclear receptor subfamily 4 group A member 2 and Wnt signaling pathways (17). Matsuura et al (127) demonstrated that simvastatin, a high mobility group-CoA inhibitor, downregulated the expression levels of OPN and αvβ1, αvβ3, α5β1 and α5β3 integrin, accompanied by an enhancement in the invasion of ovarian cancer. Lv et al (128) found that combined administration of curcumin and basilic polysaccharides could effectively suppress cell proliferation and invasion by downregulating OPN and its receptor in ovarian cancer.

Aptamers, as short single-stranded deoxyribonucleic acid or ribonucleic acid molecules, have unique biometric capabilities. They are able to fold to form a three-dimensional conformation and precisely bind to a protein target (129). This process enables the aptamers to bind to specific targets with high selectivity, including proteins, peptides, small carbohydrate molecules and even living cells, showing great potential in biomedical research and diagnostic applications (130). Aptamers exhibit advantages such as high affinity for the target, biochemical stability and low immunogenicity, and could potentially be used as cancer therapeutics (130). Mi et al (15) demonstrated that the treatment of MDA-MB-231 breast cancer cells with OPN aptamers, OPN-R3, resulted in inhibition of cell adhesion, migration and invasion. The authors further demonstrated that OPN-R3 inactivated OPN-associated signaling pathways, including the PI3K, JNK1/2, Src and Akt pathways (15). Furthermore, in vivo, OPN-R3 administration prevented tumor growth in breast cancer xenotransplantation mouse models (131).

OPN needs to bind with its receptors to exert its roles in tumor genesis and progression. Therefore, disrupting the binding of OPN and its receptors represents an alternative strategy for tumor treatment. Rangaswami et al (90) reported that blocking antibodies against αvβ3-integrin effectively suppressed OPN-induced tumor growth and angiogenesis. Furthermore, αvβ3-blocking antibodies have been found to effectively inhibit OPN-mediated activator protein 1 activation in breast cancer cells (132). Ahmed et al (133) showed that administration of CD44 antibody disrupted the interaction between OPN and CD44, consequently restricting OPN-CD44 mediated tumor invasion and metastasis. Blocking the binding of OPN with its receptors offers a novel therapeutic strategy.

OPN needs to be cleaved by specific proteases to expose its receptor binding domain in order to exert its function. Therefore, blocking enzyme cleavage could render OPN inactive. Chiou et al (134) showed that follistatin-like protein 1 directly bound to the uncleaved form of OPN and inhibited the proteolytic activation of OPN, which blocked OPN-integrin/CD44-related signaling pathways and consequent lung cancer metastasis. Thrombin has been demonstrated to participate in OPN hydrolysis, which is essential for effective binding of OPN with its receptors (135). Peraramelli et al (136) reported that a thrombin inhibitor, dabigatran etexilate, retarded B16 melanoma growth by blocking OPN activation. Therefore, inhibiting OPN activation by blocking OPN signaling and proteolytic processing may be a feasible therapeutic approach to stop tumor progression. Current OPN-based interventions are summarized in Table I.

Although OPN is a promising target for cancer therapy, there are still some limitations in practical applications. Gene interference technologies such as RNAi have poor stability and low delivery efficiency, making it difficult to continuously and specifically inhibit OPN expression in vivo (137). Small-molecule inhibitors have several drawbacks, including a short half-life, poor oral bioavailability, drug resistance, multiple side effects and high production costs. Off-target effects occur when a molecule in the body binds non-specifically to molecules other than its intended target, which may lead to unexpected biological effects or adverse reactions (138). Blocking the interaction between OPN and receptors is also challenging because OPN can bind to multiple receptors, and single blocking has limited effects, and may also trigger unknown biological effects. Complete inactivation of OPN may affect its normal physiological functions and cause side effects. Despite the limitations of current strategies, each approach has its unique advantages and applicable scenarios (23). In practical applications, appropriate strategies should be selected based on specific diseases and treatment requirements, and their limitations should be overcome through technological innovation and optimization. For example, the therapeutic efficacy and safety can be effectively improved by optimizing aptamer design, improving delivery systems and enhancing the bioavailability of small-molecule inhibitors (89).

The combination of multiple therapeutic strategies could provide more comprehensive and effective treatments, and a representative combination is OPN inhibitors together with immune checkpoint intervention. Lu et al (139) found that administration of the WD repeat domain 5 protein (WDR5), an inhibitor of OPN, suppressed the growth of orthotopic pancreatic tumors in mice. Furthermore, WDR5 enhanced the efficacy of anti-programmed cell death protein 1 immunotherapy in inhibiting pancreatic tumor growth in mice (139). Zhu et al (140) reported that OPN expression in HCC was positively associated with the programmed death-ligand 1 levels, whereas OPN knockout enhanced the antitumor function of T-helper 1 cells. Future research should focus on mechanism verification and optimization of multi-target combination strategies to achieve more precise antitumor treatment.