Targeting the Osteopontin-regulated PI3K/AKT signaling pathway: A molecular approach to overcome drug resistance and metastasis in gastrointestinal tumors

Abstract

Osteopontin (OPN), a key extracellular matrix protein, promotes gastrointestinal tumor progression by activating the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) pathway. OPN enhances tumor proliferation and survival through mechanistic target of rapamycin and B-cell lymphoma 2 upregulation (e.g., via denticleless E3 ubiquitin protein ligase homolog in hepatocellular carcinoma) and drives metastasis via PI3K/AKT-mediated epithelial-mesenchymal transition and androgen receptor (AR) activation (e.g., via the OPN-RAN-AR axis in pancreatic cancer). Additionally, OPN induces chemoresistance by activating anti-apoptotic proteins (e.g., XIAP via CXCR3/PI3K/AKT in colorectal cancer) and remodels the tumor microenvironment through VEGF-dependent angiogenesis and cluster of differentiation 44-PI3K/AKT-mediated immune evasion. Its interaction with TLR4, WNT, and other pathways amplifies oncogenic effects. Therapies targeting the OPN-PI3K/AKT axis (e.g., PI3K inhibitors like LY294002) or combination treatments (e.g., with EGFR-TKIs) show promise for reversing drug resistance. Future research should focus on OPN isoform specificity, clinical translation, and interactions with autophagy and long non-coding RNAs to refine precision therapies. This review summarizes recent advances in understanding the molecular mechanisms, therapeutic targets, and clinical challenges of the OPN-PI3K/AKT axis in gastrointestinal tumors, providing a foundation for overcoming resistance and developing precision therapies.

Key Words: Osteopontin; Gastrointestinal cancers; Chemoresistance; Targeted therapy; Tumor microenvironment remodeling

Core Tip: Osteopontin (OPN) drives gastrointestinal tumor progression by activating the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) pathway, promoting proliferation, metastasis, chemoresistance, and immune evasion. Key mechanisms involve upregulating anti-apoptotic proteins (e.g., B-cell lymphoma 2), inducing epithelial-mesenchymal transition via Snail/Twist, and remodeling the tumor microenvironment through VEGF-driven angiogenesis and cluster of differentiation 44-mediated immune suppression. Targeting this axis with PI3K/AKT inhibitors (e.g., Alpelisib) or OPN-neutralizing antibodies may reverse drug resistance and suppress metastasis. Challenges include pathway redundancy and the need for biomarker-guided precision strategies. Future research should focus on OPN isoform specificity, combination therapies (e.g., with immune checkpoint inhibitors), and multi-omics approaches to optimize outcomes in refractory cancers.

INTRODUCTION

Digestive tract tumors [including esophageal cancer, gastric cancer (GC), liver cancer and colorectal cancer (CRC)] are among the malignant tumors with the highest incidence and mortality rates worldwide. They are characterized by high metastasis rates, recurrence rates and treatment resistance. Despite the continuous advancement of multimodal treatment approaches, the 5-year survival rate of advanced patients remains unsatisfactory, highlighting the urgent need to reveal the molecular mechanisms of tumor invasiveness and treatment resistance[1-3]. Against this background, Osteopontin (OPN) - a multifunctional extracellular matrix protein encoded by the SPP1 gene - has become a core regulatory factor for oncogenic signaling and therapeutic resistance in digestive tract tumors[4-6].

The elevated expression of OPN in digestive tract tumors is strongly associated with poor prognosis[7,8], It facilitates tumor cell proliferation, epithelial-mesenchymal transition (EMT), angiogenesis, and immune evasion via the interaction of integrins (αvβ3, αvβ5) and cluster of differentiation 44 (CD44) receptors[9-12]. Notably, the oncogenic effects of OPN are predominantly mediated through the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) pathway[4], which serves as a central hub for regulating cell survival, metabolism, and drug resistance[13-15]. Upon activation of the PI3K/AKT signaling cascade by OPN, it can reprogram glucose metabolism, stabilize anti-apoptotic proteins [e.g., B-cell lymphoma 2 (Bcl-2), surviving], and upregulate DNA repair mechanisms, thereby enhancing resistance to chemotherapy and radiotherapy (Figure 1)[13,16]. For instance, in hepatocellular carcinoma, OPN activates the PI3K/AKT pathway by upregulating Galectin-3 Binding Protein, driving tumor cell migration[17,18]; In GC, OPN enhances AKT activity by suppressing the phosphatase and tensin homolog (PTEN) gene, thereby promoting metastasis and therapeutic resistance[19,20].

Figure 1 Molecular regulatory network of Osteopontin in gastrointestinal tumors. OPN: Osteopontin; CD44: Cluster of differentiation 44; PI3K: Phosphoinositide 3-kinases; PIP2: Phosphatidylinositol-4:5-bisphosphate; PIP3: Phosphatidylinositol-3:4:5-trisphosphate; PTEN: Phosphatase and tensin homolog; AKT: Protein kinase B; mTOR: Mammalian target of rapamycin; NF-κB: Nuclear factor kappa-light-chain-enhancer of activated B cells; EMT: Epithelial-mesenchymal transition.

However, the translational potential of interventions targeting the OPN-PI3K/AKT axis is limited by pathway redundancy, feedback loops, and the complex interactions within the tumor microenvironment. For instance, distinct OPN subtypes (e.g., OPNa and OPNb) exert differential effects on immune cell polarization and matrix remodeling[8], Additionally, their post-translational modifications, such as phosphorylation and glycosylation, further enhance the diversity of signal transduction mechanisms[21]. Moreover, current PI3K/AKT inhibitors (e.g., alpelisib and capivasertib) face challenges including off-target effects and adaptive drug resistance, underscoring the critical need for biomarker-guided personalized treatment strategies[22]. Recent investigations have highlighted that the combination of OPN-neutralizing antibodies with immune checkpoint inhibitors can elicit a synergistic effect[23], offering a novel approach to overcome therapeutic resistance mediated by the tumor microenvironment.

This article provides an in-depth review of the latest advancements in OPN-driven PI3K/AKT signaling within digestive tract tumors, with particular emphasis on elucidating its molecular mechanisms, identifying therapeutic targets, and addressing clinical transformation challenges. By synthesizing insights from preclinical models, multi-omics data, and emerging therapeutic strategies, we strive to bridge the gap between fundamental research and clinical application, thereby establishing a robust theoretical foundation for the precise treatment of refractory digestive tract tumors.

BIOLOGICAL FUNCTIONS AND REGULATORY MECHANISMS OF OPN

OPN is a multifunctional protein that exhibits extensive biological functions and is regulated by complex expression mechanisms, thereby participating in both physiological and pathological processes (Figure 2).

Figure 2 An illustrative map of the biological functions of Osteopontin. OPN: Osteopontin; TGF-β: Transforming growth factor-beta; MMPs: Matrix metalloproteinases; CAFs: Cancer-associated fibroblasts; CD: Cluster of differentiation.

Biological function

Bone metabolism and biomineralization: The expression of OPN is significantly upregulated during the differentiation of osteoblasts, thereby playing a critical role in regulating the bone remodeling process and maintaining bone mass and strength[24,25]. OPN modulates the activities of osteoclasts and osteoblasts via integrin and CD44 receptors, influencing calcium and phosphorus metabolism[26]. In atherosclerosis, OPN interacts with extracellular matrix proteins in the aortic wall, contributing to pathological mineralization[27].

Immune regulation and inflammation: OPN is secreted by various immune cells, including macrophages, T cells, and natural killer cells. It plays a critical role in immune regulation by recruiting immune cells and stimulating pro-inflammatory cytokines, such as interleukin-1β (IL-1β), thereby modulating both acute and chronic inflammatory responses[28]. In sepsis, OPN contributes to the pathological process through its dual effects on excessive inflammation and immunosuppression[29]; In allergic diseases, such as allergic rhinitis, OPN exacerbates the inflammatory response by regulating type 2 innate lymphoid cells and Th2-mediated inflammation[30].

Tumor progression and metastasis: OPN isoforms (e.g., OPNa, OPNb, OPNc) play distinct roles in gastrointestinal malignancies. OPN is significantly overexpressed in various types of cancer, including CRC, head and neck squamous cell carcinoma, and lung cancer. Specifically, OPNa promotes immune evasion by polarizing macrophages toward an M2 phenotype and suppressing CD8+ T-cell activity through integrin αvβ3/PI3K/AKT signaling, thereby contributing to the establishment of an immunosuppressive tumor microenvironment (TME)[8]. In contrast, OPNb enhances matrix remodeling by activating cancer-associated fibroblasts (CAFs) to secrete matrix metalloproteinases (MMPs) and interleukin-8 (IL-8), which facilitates invasion and metastasis in GC[4,8]. The OPNc variant (e.g., OPN-5) has been associated with metastatic potential, as evidenced in pancreatic and breast cancer models where it induces EMT via the PI3K/AKT/Snail signaling pathway[31]. Clinically, elevated levels of OPNa and OPNc predict poor prognosis in CRC, underscoring their potential as biomarkers for PI3K/AKT-targeted therapies[8,31]. Overall, OPN drives malignant progression by promoting tumor proliferation, metastasis, angiogenesis, chemotherapy resistance, and immune evasion[10,31,32]. Its splicing variants, such as OPNa, OPNb, and OPNc, exhibit isoform-specific functions in tumorigenesis. For example, the expression of OPN5 in mouse cancer models has been linked to enhanced metastatic potential[33].

Tissue repair and fibrosis: OPN plays a pivotal role in tissue repair and fibrosis during wound healing and tissue remodeling by activating transforming growth factor-beta and MMPs. Notably, it exacerbates organ damage in chronic conditions such as chronic kidney disease and idiopathic pulmonary fibrosis[26,34]. In acute lung injury (acute respiratory distress syndrome), elevated levels of OPN in the lungs correlate with disease severity[35].

Nervous system and vascular regulation: In neurological injuries, such as stroke, OPN exhibits neuroprotective effects by upregulating scavenger receptors and facilitating blood-brain barrier reconstruction[36]. Its full-length form (FL-OPN) and N-terminal fragment (OPN N-half) demonstrate distinct functional roles in neurovascular events[37].

Express the regulatory mechanism

Cell type-specific expression: OPN is secreted by various cells, including osteoblasts, immune cells, tumor cells and endothelial cells. For example, tumor-associated macrophages (TAMs) promote immunosuppression of the tumor microenvironment through OPN[38].

Transcription and post-translational modification: The glycosylation and phosphorylation modifications of OPN (molecular weight 44-70 kDa) affect its functional diversity. The expression of splicing variants is tissue-specific, such as the differential regulation of OPN-SVs in CRC[8].

Microenvironment and signal pathways: Integrins (such as αvβ3) and CD44 receptors are the key targets of OPN, activating the downstream PI3K/AKT, extracellular signal-regulated kinase (ERK) and signal transducer and activator of transcription 3 (STAT3) pathways[39]. In tumors, autophagy deficiencies promote angiogenesis through the OPN-Janus Kinase/Stat3 axis[40].

Pathological stimulation induction: Infection, hypoxia, tissue injury, etc., upregulate the expression of OPN. For example, pseudorabies virus (PRV) infection regulates inflammation through the OPN-ERK-IL-1β axis[39]; Titanium nanomaterials enhance the osteogenic expression of OPN through bone morphogenetic protein-7[41].

Secretion and intracellular forms: Secretory OPN (sOPN) and intracellular OPN (iOPN) have different functions: SOPN mediates cell migration and adhesion, while iOPN regulates signal transduction within immune cells and tumor cells[24].

KEY FUNCTIONS OF THE PI3K/AKT SIGNALING AXIS IN CANCER PATHOBIOLOGY

The PI3K/AKT pathway, which drives cell proliferation, drug resistance, and metabolic reprogramming, has emerged as a critical molecular hub in the treatment of gastrointestinal tumors. Its core significance is reflected in the implementation of multi-level targeted therapeutic strategies (Table 1).

Table 1 Targeted therapeutic strategies against the phosphatidylinositol 3-kinase/protein kinase B pathway in gastrointestinal cancers.

Target type	Representative agents	Mechanism of action	Applicable tumor types
Genetic vulnerability targeting	ARID1A-deficient subtype (GC)	Synthetic lethality via PI3K/AKT hyperactivation	GC (30% mutation prevalence)
Small-molecule inhibitors	LY294002	Suppression of ZDHHC20-mediated HCC progression by blocking PI3K catalytic activity	HCC
Combination therapy	EGFR inhibitor + PI3K inhibitor	Synergistic inhibition of autophagy-dependent drug resistance	Colorectal cancer/head and neck squamous cell carcinoma
Natural compound synergy	Astragalus polysaccharide + radiotherapy	Radiosensitization through AKT inhibition and enhanced DNA damage response	Gastrointestinal tumors (adjuvant therapy)

ARID1A: AT-rich interaction domain 1A; GC: Gastric cancer; PI3K: Phosphatidylinositol 3-kinase; LY294002: PI3K inhibitors; AKT: Protein kinase B; ZDHHC20: Zinc finger DHHC-type containing 20; HCC: Hepatocellular carcinoma; EGFR: Epidermal growth factor receptor.

Regulation of tumor biological behaviors

Abnormal activation directly drives the proliferation, survival, metastasis, and angiogenesis of tumor cells. In various digestive tract malignancies, including CRC, GC, and HCC, the incidence of gene mutations or overexpression in this pathway is relatively high[42,43]. Furthermore, it regulates the characteristics of cancer stem cells (CSCs) and mediates chemotherapy resistance. For instance, variations in the Rictor gene have been associated with chemotherapy resistance in gastrointestinal tumors[44].

Key molecular interaction network

As emphasized in recent seminal work, long non-coding RNAs (lncRNAs) constitute essential regulators of PI3K/AKT signaling in gastrointestinal tumors, modulating metastasis and therapy response[45]. Formation of Regulatory Axes with lncRNAs: Multiple lncRNAs influence the chemosensitivity of tumor cells by abnormally activating the PI3K/AKT pathway[45,46]. Cross-Dialogue with the EGFR pathway: Both pathways are jointly involved in autophagy regulation, and combined inhibition can overcome resistance to EGFR inhibitors[47]. Regulation of protein synthesis and cellular metabolism via the PI3K/AKT/mechanistic target of rapamycin (mTOR) Branch: This branch plays a critical role in modulating protein synthesis and cellular metabolism[48].

Therapeutic target value

For AT-rich interaction domain 1A (ARID1A)-deficient GC: Approximately 30% of GC patients harbor ARID1A mutations. Specifically targeting the PI3K/AKT pathway activated by this defect holds therapeutic potential[49]. Combination Therapy Enhances Efficacy: For example, the PI3K inhibitor LY294002 and the AKT inhibitor MK2206 can block zinc finger DHHC-type containing 20-mediated progression of liver cancer[50]. Synergistic Effects of Traditional Chinese Medicine: Certain components of traditional Chinese medicine enhance radiotherapy effects by inhibiting this pathway[48].

MOLECULAR MECHANISMS UNDERLYING OPN-PI3K/AKT AXIS-MEDIATED MALIGNANT PROGRESSION

The OPN-PI3K/AKT axis establishes a vicious cycle of "proliferation-metastasis-immune escape" by activating the BAD/Bcl-2 apoptosis regulatory network, inducing Snail/Twist-mediated EMT, and promoting the secretion of pro-inflammatory factors such as VEGF and IL-8 by CAFs. This drives the aggressive progression of gastrointestinal tumors, including gastric and liver cancers. Furthermore, this pathway exerts multidimensional tumorigenic effects through metabolic reprogramming (e.g., MTOR-mediated protein synthesis) and microenvironment remodeling [e.g., chemokine ligand 11 (CXCL11)-promoted enrichment of CSCs] (Table 2).

Table 2 Core mechanisms of Osteopontin-phosphatidylinositol 3-kinase/protein kinase B axis in malignant progression.

Mechanistic category	Key molecules/pathways	Functional outcomes	Associated cancers
Promotion of proliferation & survival	Integrin receptors, BAD/Bcl-2, PI3K/AKT/mTOR	Inhibition of apoptosis; Upregulation of MMP-2/VEGF; Enhanced cell survival	GC, pancreatic cancer
Induction of EMT & metastasis	E-cadherin, Snail, Twist, EGFR/PI3K/AKT	Loss of epithelial markers; Activation of mesenchymal programs; Enhanced invasiveness	HCC, NSCLC
Remodeling tumor microenvironment	CAFs, VEGF, IL-8, CXCL11/POSTN	Angiogenesis; Immune suppression; CSC enrichment; Stromal activation	GC, colorectal cancer

PI3K: Phosphatidylinositol 3-kinase; AKT: Protein kinase B; mTOR: Mechanistic target of rapamycin; EGFR: Epidermal growth factor receptor; CAFs: Cancer-associated fibroblasts; VEGF: Vascular endothelial growth factor; CXCL11: C-X-C motif chemokine ligand 11; POSTN: Periostin; EMT: Epithelial-mesenchymal transition; MMP-2: Matrix metalloproteinase 2; CSC: Cancer stem cells; NSCLC: Non-small cell lung cancer; GC: Gastric cancer; HCC: Hepatocellular carcinoma.

Promotion of proliferation and survival

OPN regulates apoptosis-related proteins, such as BAD and Bcl-2, by binding to integrin receptors and activating the PI3K/AKT signaling pathway. This represents a critical mechanism for enhancing tumor cell survival. In GC, OPN upregulates the expression of MMP-2 and VEGF via the PI3K/AKT/mTOR signaling axis, thereby augmenting tumor invasion and metastasis capabilities[4]. For instance, in pancreatic cancer research, it has been demonstrated that inhibiting BAD protein phosphorylation (mediated by Adaptor Protein Family molecules) significantly reduces tumor cell survival rates[51].

Induction of EMT and enhancement of invasion and metastasis

The OPN-PI3K/AKT signaling axis drives the EMT process by suppressing E-cadherin expression, activating Snail and Twist, and enhancing tumor cell invasiveness. In nude mouse experiments, silencing OPN or treating with PI3K/AKT inhibitors (e.g., LY294002) significantly reduces the number of lung metastases. Mechanistically, this is attributed to the restoration of E-cadherin expression, downregulation of Snail/Twist, and reversal of the EMT process[52]. The compound Triptonide inhibits EMT and metastasis by suppressing the EGFR/PI3K/AKT pathway, reducing Snail expression, and upregulating E-cadherin[53]. Similarly, Fenofibrate suppresses liver cancer progression by inhibiting the OPN-PI3K/AKT/Twist axis[14]. In GC, lncRNA AC010457.1 binds PI3K p110δ subunit to activate AKT, accelerating EMT and invasion[54].

Remodeling of the tumor microenvironment

OPN activates the PI3K/AKT pathway in CAFs, promoting the secretion of pro-inflammatory factors such as VEGF and IL-8, thereby accelerating angiogenesis and the formation of an immunosuppressive microenvironment. In GC, the OPN-PI3K/AKT axis accelerates tumor progression through VEGF-mediated angiogenesis[4]. In GC and CRC, CAFs secrete factors such as C-X-C CXCL11 and POSTN (periostin) via the PI3K/AKT pathway, facilitating the enrichment and metastasis of CSCs[55-57].

OPN-MEDIATED THERAPEUTIC RESISTANCE: MECHANISMS AND MOLECULAR CROSSROADS

OPN mediates therapeutic resistance by activating PI3K/AKT/mTOR (enhancing DNA repair in chemoresistance) and PI3K/AKT-EMT pathways (suppressing apoptosis and promoting survival in targeted therapy resistance); inhibition of OPN or its downstream pathways effectively reverses drug resistance (Table 3).

Table 3 Osteopontin-mediated therapeutic resistance mechanisms and interventions.

Mechanism category	Key molecules/pathways	Functional effects	Intervention strategies	Related cancers
Chemoresistance	PI3K/AKT/mTOR, NEMO-ATM/IKKα, DNA repair pathways	Enhances DNA damage repair; Reduces sensitivity to 5-FU/radiotherapy	Wortmannin (PI3K inhibitor), PRDM15 knockout	Colorectal cancer
Targeted therapy resistance	HER2/EGFR, PI3K/AKT-EMT, DTL	Inhibits trastuzumab efficacy; Activates EMT; Upregulates pro-survival genes (e.g., Bcl-2)	OPN-neutralizing antibodies, LY294002	GC, HCC, pancreatic cancer
Autophagy-mediated resistance	OPN/NF-κB, DRP1, LC3-II/p62	Sustains chemoresistance via autophagic flux blockade	Liensinine (autophagy inhibitor), DRP1 activation	Lung adenocarcinoma
Immune evasion	CD44-PI3K/AKT, VEGF/IL-8	Reprograms TME to suppress CD8+ T cell infiltration	αRANKL blockade, Anti-VEGF/IL-8 antibodies	NSCLC, bone metastatic cancers

OPN: Osteopontin; PI3K: Phosphatidylinositol 3-kinase; AKT: Protein kinase B; mTOR: Mechanistic target of rapamycin; NEMO: NF-κB essential modulator; ATM: Ataxia-telangiectasia mutated; IKKα: IκB kinase alpha; HER2: Human epidermal growth factor receptor 2; EGFR: Epidermal growth factor receptor; EMT: Epithelial-mesenchymal transition; Bcl-2: B-cell lymphoma 2; DTL: Denticleless e3 ubiquitin protein ligase homolog; NF-κB: Nuclear Factor kappa-light-chain-enhancer of activated B Cells; DRP1: Dynamin-related protein 1; LC3-II: Microtubule-associated protein 1A/1B-light chain 3-II; p62: Sequestosome 1; CD44: Cluster of differentiation 44; VEGF: Vascular endothelial growth factor; IL-8: Interleukin-8; PRDM15: PR/SET Domain 15; LY294002: PI3K inhibitors; αRANKL: Anti-receptor activator of nuclear factor kappa-B ligand antibody; VEGF: Vascular endothelial growth factor; GC: Gastric cancer; HCC: Hepatocellular carcinoma; NSCLC: Non-small cell lung cancer.

Chemotherapy resistance

OPN plays a pivotal regulatory role in the development of drug resistance in CRC via the PI3K/AKT/mTOR signaling pathway. Its activation promotes DNA damage repair and reduces the sensitivity of tumor cells to radiotherapy and chemotherapy[58]. For instance, Nuclear Factor kappa-light-chain-enhancer of activated B Cells (NF-κB) essential modulator significantly enhances DNA repair capacity by targeting the ataxia-telangiectasia mutated (ATM)/IκB kinase alpha complex to DNA damage sites, thereby contributing to chemotherapy resistance[59]. When this pathway is inhibited (e.g., through Wortmannin treatment or PR domain zinc finger protein 15 knockout), the sensitivity of CRC cells to DNA-damaging treatments (such as 5-fluorouracil or radiotherapy) can be markedly increased[60]. Clinical research indicates that the combination of 5-fluorouracil and oxaliplatin exhibits sub-additive cytotoxicity, partly due to compensatory activation of DNA repair-related pathways[61]. Exosomal lncRNA HOTAIR transfers from CAFs to tumor cells, activating PI3K/AKT/NF-κB to suppress autophagy and induce chemoresistance[62].

Targeted therapy resistance

OPN promotes tumor cell survival and induces drug resistance by upregulating the phosphorylation level of the PI3K/AKT signaling pathway, thereby weakening the anti-proliferative effect of trastuzumab[63]. In pancreatic cancer research, OPN has been shown to drive tumor metastasis by activating the PI3K/AKT pathway, suggesting its potential role in enhancing tumor invasiveness through a similar mechanism in human epidermal growth factor receptor 2 (HER2)-positive GC[15]. Additionally, in the EGFR-TKI resistance model, OPN overexpression inhibits apoptosis by activating the PI3K/AKT-EMT pathway, while inhibiting OPN or blocking the PI3K/AKT signaling pathway can significantly restore drug sensitivity. This mechanism may also apply to HER2-targeted therapy resistance[13]. Experimental data further demonstrate that suppressing OPN expression or blocking PI3K/AKT signal transduction effectively reverses trastuzumab resistance. For example, in liver cancer, OPN upregulates the expression of the downstream target gene denticleless E3 ubiquitin protein ligase homolog via the PI3K/AKT pathway, thereby promoting tumor progression[17]; In pancreatic cancer, PI3K/AKT inhibitors can efficiently block the tumor metastasis process mediated by OPN[15].

Immune evasion

OPN-CD44/PI3K/AKT signaling induces PD-L1 upregulation in tumor cells and recruits immunosuppressive myeloid-derived suppressor cells, thereby inhibiting CD8+ T-cell activity[12,23]. Tan et al[38] demonstrated that OPN secreted by TAMs polarizes TAMs toward an M2 phenotype via the PI3K/AKT/STAT3 axis, further suppressing T-cell function[38]. Targeting this axis with OPN-neutralizing antibodies (e.g., 2C5) synergizes with PD-1 inhibitors by blocking the CD47-SIRPα "don't eat me" signal, enhancing macrophage phagocytosis and antitumor immunity[23,38]. Concurrently, PI3Kα inhibitors (e.g., Alpelisib) reverse VEGF/IL-10-mediated Treg expansion and increase intratumoral CD8+ T-cell infiltration in Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA)-mutant models, providing a rationale for combining OPN-targeted therapies with immune checkpoint blockade[22,64,65].

CLINICAL TRANSFORMATION FOCUSED ON THE OPN-PI3K/AKT AXIS

Targeting the OPN-PI3K/AKT axis involves small-molecule inhibitors (e.g., PI3K/AKT inhibitors and OPN-neutralizing antibodies) to block metastasis-promoting pathways, and biomarker development (e.g., PIK3CA mutations and OPN expression) to guide precision therapy with PI3Kα inhibitors (e.g., Alpelisib), while exploring OPN's role in immune microenvironment remodeling and drug resistance mechanisms (Table 4).

Table 4 Key mechanisms and interventions in targeted therapy of the Osteopontin-phosphatidylinositol 3-kinase/protein kinase B axis.

Category	Key mechanisms	Intervention strategies	Related cancers
Small-molecule inhibitors	OPN activates PI3K/AKT to drive metastasis; PIK3CA mutations enhance pathway dependency	PI3Kα inhibitors (Alpelisib) combined with OPN-neutralizing antibodies	Pancreatic cancer, GC
Biomarker development	PIK3CA mutations (E542K/E545K) and OPN overexpression indicate PI3K activation; OPN regulates autophagy and immune microenvironment (IL-6/IL-8)	Alpelisib guided by PIK3CA mutation subtypes; Targeting OPN-immune interactions	GC, pancreatic cancer

OPN: Osteopontin; PI3K: Phosphatidylinositol 3-kinase; AKT: Protein kinase B; PIK3CA: Phosphatidylinositol-4:5-bisphosphate 3-kinase catalytic subunit alpha; E542K/E545K: Glutamic acid 542 to lysine/glutamic acid 545 to lysine; IL-6: Interleukin-6; IL-8: Interleukin-8; PI3Kα: Phosphatidylinositol 3-kinase catalytic subunit alpha; GC: Gastric Cancer.

Small molecule inhibitors

OPN promotes the metastasis of pancreatic cancer by activating the PI3K/AKT signaling pathway[15]. Experimental evidence demonstrates that OPN knockout significantly reduces the expression of the androgen receptor (AR) and inhibits PI3K/AKT pathway activation, thereby attenuating the migratory and invasive capabilities of cancer cells[15]. Moreover, OPN overexpression can reverse the suppressive effect of RAN gene silencing on AR, while AKT inhibitors effectively block the metastasis-promoting effects of OPN or RAN. These findings suggest that OPN-neutralizing antibodies may enhance the efficacy of AKT inhibitors by disrupting this pathway[15]. In pancreatic cancer cells silenced for RAN/OPN, PI3K/AKT inhibitors completely abrogate residual metastatic activity[15]. When neutralizing antibodies target OPN or other pathway proteins (e.g., ephrin type-A receptor 2), a reduction in phosphorylated AKT levels is observed, accompanied by inhibition of cancer cell proliferation and migration[66].

Biomarker development

PIK3CA mutations are detected in approximately 4% to 25% of GC patients, with particularly high mutation rates (up to 80%) in Epstein-Barr virus-associated GCs[67]. Studies indicate that PI3Kα inhibitors (e.g., Alpelisib) significantly suppress the proliferative activity of certain GC cell lines[68]. Different PIK3CA mutation subtypes may influence treatment responses; for instance, E542K (approximately 15.8%) and E545K mutations are relatively common in GC[64]. Furthermore, the immune microenvironment in PIK3CA-mutated tumors may undergo reprogramming, characterized by the enrichment of cytokine signaling pathways. This phenomenon is closely linked to the immunomodulatory function of OPN[64]. High OPN expression may reflect underlying PI3K pathway activation, while PIK3CA mutations further intensify pathway dependence, enhancing tumor cell sensitivity to Alpelisib. In the context of PIK3CA mutations, OPN may modulate therapeutic responses by regulating autophagy processes or reshaping the immune microenvironment (e.g., through the secretion of cytokines such as IL-6 and IL-8)[69]. Future research should focus on the role of OPN in PI3K inhibitor resistance mechanisms, including potential pathways involved in resistance formation via autophagy regulation or immunomodulation.

CLINICAL TRIAL LANDSCAPE OF PI3K/AKT INHIBITORS

The current clinical trial landscape of PI3K/AKT inhibitors in gastrointestinal tumors highlights significant advancements and challenges. In the Phase I clinical trial of M2698 (NCT01971515), while the study primarily focused on a broad spectrum of tumor types, including breast cancer, lung cancer, and CRC, the exploratory analysis of gastrointestinal tumors was particularly noteworthy. Among the 62 patients treated with monotherapy, 5 (8.1%) had colon cancer and 1 (1.6%) had rectal cancer. However, efficacy data for the gastrointestinal tumor subgroup were not reported separately. The overall results demonstrated that the disease control rate (DCR) of M2698 monotherapy was 27.4%, with the DCR among patients harboring PAM pathway alterations reaching 40.7%. Nonetheless, specific responses in patients with gastrointestinal tumors were not documented[70]. Additionally, in a Phase II study for GC/gastroesophageal junction cancer (NCT01896531), the combination of mFOLFOX6 chemotherapy and Akt inhibitor ipatasertib did not significantly improve progression-free survival (PFS) or overall survival (OS). The incidence of grade 3 or higher toxicities in the combined treatment group was 79%, compared to 74% in the control group. Although the predefined subgroups with low PTEN expression or activated PI3K/Akt pathway did not show clear benefits, the study underscored the necessity for more precise biomarker screening strategies and optimization of combination treatment regimens. Currently, the treatment of GC remains predominantly reliant on traditional chemotherapy and anti-HER2 targeted therapy[71]. In another Phase II study for advanced esophageal squamous cell carcinoma (UMIN 000011217), the efficacy of PI3K inhibitor BKM120 monotherapy was evaluated. The primary endpoint of DCR was 51.2% (95%CI: 35.1-67.1), and the objective response rate (ORR) was only 4.8% (2/42 partial responses), with median PFS and OS being 2.3 months and 9.0 months, respectively. Common grade 3 or higher toxicities included rash (9.5%), anorexia (7.1%), and abnormal liver function. Although some patients exhibited PI3K pathway gene alterations (e.g., PIK3CA amplification or PTEN deletion) and showed tumor shrinkage, the limited sample size precluded a definitive association between biomarkers and efficacy[72]. In a Phase II study for advanced biliary tract cancer (NCT02631590), the efficacy of PI3K inhibitor copanlisib combined with gemcitabine and cisplatin was assessed. The 6-month PFS rate as the primary endpoint was 51%, consistent with historical data (57%-59%). The ORR was 31.6%, and the median OS was 13.7 months[73]. Finally, in a Phase I study for KRAS wild-type advanced CRC (NCT01591421), the efficacy of EGFR inhibitor panitumumab combined with PI3K inhibitor BKM120 was investigated. The primary endpoint was to determine the recommended phase II dose, and ultimately, BKM120 60mg (5/7 days) combined with panitumumab 6 mg/kg (q2w) was established as the safe dose. In terms of efficacy, only 1 patient (5.9%) achieved partial response, with a median PFS of 2.0 months, and the median OS was not reached (6-month OS rate was 52.6%)[74].

CONCLUSION

Targeting the OPN-PI3K/AKT pathway offers a promising strategy for overcoming therapeutic resistance in digestive tract tumors. Future efforts should focus on developing specific OPN inhibitors and investigating their synergistic effects with immune checkpoint blockade therapies. Additionally, biomarker research integrating multi-omics approaches will facilitate the advancement of precision medicine in this field.