The Multifaceted Role of Epithelial Membrane Protein 2 in Cancer: from Biomarker to Therapeutic Target

Abstract

Tetraspanin superfamily proteins not only facilitate the trafficking of specific proteins to distinct plasma membrane domains but also influence cell-to-cell and cell-extracellular matrix interactions. Among these proteins, Epithelial Membrane Protein 2 (EMP2), a member of the growth arrest-specific gene 3/peripheral myelin protein 22 (GAS3/PMP22) family, is known to affect key cellular processes. Recent studies have revealed that EMP2 modulates critical signaling pathways and interacts with adhesion molecules and growth factor receptors, underscoring its potential as a biomarker for cancer diagnosis and prognosis. These findings suggest that EMP2 expression patterns could provide valuable insights into tumorigenesis and metastasis. Moreover, EMP2 has emerged as a promising therapeutic target, with approaches aimed at inhibiting or modulating its activity showing potential to disrupt tumor growth and metastasis. This review provides a comprehensive overview of recent advances in understanding the multifaceted roles of EMP2 in cancer, with a focus on its underlying mechanisms and clinical significance.

INTRODUCTION

Despite significant advancements in cancer therapies and prevention that have led to a decline in mortality rates since the early 1990s, this progress is increasingly threatened by the rising incidence and mortality of several major cancers, including breast, prostate, and colorectal cancers. This has intensified the need to develop effective new therapeutic strategies to address the increasing challenges in cancer treatment (Siegel et al., 2024). Among epithelial markers, particularly membrane proteins, the tetraspanin family has garnered significant attention, not only as cancer markers but also as therapeutic targets. Notably, this group includes members of the Epithelial Membrane Protein (EMP) family, with EMP2 being of particular interest. EMP2 belongs to the growth arrest-specific 3 / peripheral myelin protein 22 (GAS3/PMP22) family and plays a crucial role in the development and metastasis of various types of cancer (Wang et al., 2017b). EMP2 is located in the lipid raft domains and is believed to be involved in the trafficking of integrins to the plasma membrane (Wadehra et al., 2002). Through this process, EMP2 promotes integrin binding to the extracellular matrix, contributing to intracellular signaling (Morales et al., 2009). Here, we have explored the potential reasons why EMP2 has diverse roles depending on the cellular context and cancer type. Why does EMP2 act as an oncogene in some cancers and as a tumor suppressor in others? This discrepancy may be due to variations in EMP2 expression levels across different organs or tissues, leading to distinct effects on cancer cells and the surrounding microenvironment. Therefore, further study into the role of EMP2 within the cancer microenvironment should be actively pursued.

According to TNMplot.com, a tool for comparing gene expression in normal, tumor, and metastatic tissues, EMP2 expression varies across different organs (Bartha and Gyorffy, 2021). EMP2 is highly expressed in normal tissues of the esophagus, lung, and skin. It is upregulated in tumor tissues of the adrenal gland, breast, colon, liver, pancreas, rectum, stomach, testis, thyroid, and endometrium (https://tnmplot.com/). Recent studies suggest a close relationship between EMP2 and angiogenesis. Conversely, reduced expression of EMP2 has been reported to cause changes in keratin reorganization, which is involved in cancer metastasis, and to decrease the viscoelasticity of cancer cells, thereby promoting cancer migration and invasion (Lee et al., 2016). Continuous loss of EMP2 has also been linked to enhanced metastasis by inducing epithelial-mesenchymal transition (EMT). This suggests that EMP2 can be an oncogene in some cancers and a tumor suppressor in others. Through this review, we aim to summarize recent findings on EMP2 and offer new insights for further research directions.

BIOLOGICAL AND PHYSIOLOGICAL FUNCTIONS OF EMP2

Epithelial membrane protein 2 (EMP2), a member of the tetraspanin superfamily (TM4SF), functions similarly to other members of this superfamily, serving as a molecular adapter involved in the trafficking within specific cellular compartments (Wadehra et al., 2003b). The TM4SF is composed of three primary families: GAS3/PMP22, which includes EMP2, connexins, and tetraspanins (Maecker et al., 1997). A key emerging function of many TM4SF proteins is their role in facilitating the trafficking of specific proteins to distinct plasma membrane domains (Berditchevski, 2001). This activity allows numerous TM4SF members to directly or indirectly influence cell-cell and cell-extracellular matrix interactions, and they are known to modulate the surface expression of integrin heterodimers (Evans and Martin, 2002). The GAS3/ PMP22 family members include PMP22, EMP1, EMP2, EMP3, PERP, BCM1, and lens fiber membrane intrinsic protein MP20 (Wadehra et al., 2003b). EMP2 was originally identified based on its sequence homology to PMP22 (Taylor and Suter, 1996). PMP22 is predominantly expressed in peripheral nerves, where it is localized to the compact regions of myelin and plays a vital role in both normal physiological functions and pathological processes within the peripheral nervous system (Jetten and Suter, 2000). PMP22 expression is associated with apoptosis, cell spreading, and protein trafficking (Dickson et al., 2002; Hou et al., 2021). PMP22 and EMPs (EMP1, EMP2, and EMP3) form a subfamily of small hydrophobic membrane proteins (Jetten and Suter, 2000). EMP2 also regulates the trafficking of different proteins to specific cell surface lipid raft microdomains (Lin et al., 2020), which play a role in the regulation of Toll-like receptor (TLR) signaling pathways (Fessler and Parks, 2011).

REGULATORY MECHANISMS OF EMP2 IN CANCERS

EMP2 expression and its structural information

The human EMP2 gene is mapped to chromosome 16 (16p13.13) (Street et al., 2002). EMP2 mRNAs are present as multiple transcripts in various tissues, with high expression in the lung, moderate expression in the eye, heart, and uterus, low expression in the brain, liver, ovary, and prostate, and no detectable expression in the kidney, pancreas, thymus, and spleen (Wang et al., 2001). Additionally, both murine and human EMP2 show moderate expression in the cornea, sclera, and iris, but undetectable expression in the lens or retina (Wadehra et al., 2003b). These mRNAs may result from alternative splicing or the use of different polyadenylation signals (Taylor and Suter, 1996).

EMP2 expression is regulated by several upstream pathways, including signaling cascades such as hormones, microRNAs, and external stimuli, all of which play a crucial role in modulating its function in cancer progression (Fig. 1). For example, genistein has been shown to increase the expression and phosphorylation of CREB in a cell line derived from urinary bladder urothelial carcinoma, leading to increased EMP2 expression (Li et al., 2015). Similarly, progesterone upregulated EMP2 expression in endometrial cancer cell lines, suggesting the involvement of the progesterone receptor (Wadehra et al., 2008). Additionally, circRNA-0002109 upregulated EMP2 by competitively binding to microRNA-129-5P (miR-129-5P), alleviating the inhibitory effect of miR-129-5P on EMP2 (Xia et al., 2022). In contrast, certain microRNAs act to suppress EMP2 expression. miR-133b inhibited EMP2 expression in glioma (Zhang et al., 2018), and miR-101-3p reduced EMP2 levels in nasopharyngeal carcinoma (Wang et al., 2017a). miR-340-5p also inhibited EMP2 expression in corneal epithelial cells (Pan et al., 2022). Moreover, EMP2 expression was found to be decreased in smokers (Powell et al., 2003). These pathways influence various cellular processes, contributing to the aggressive behavior of tumors and presenting potential targets for therapeutic intervention.

Fig. 1.

Pathways regulating EMP2 expression in cancer. EMP2 expression is regulated by various pathways, including signaling networks involving hormones, microRNAs, and external stimuli. EMP2, Epithelial membrane protein 2; CREB, cAMP response element binding protein.

EMP2 is highly expressed in cancer tissue compared to normal tissue in patients with the following types of cancer: acute lymphoblastic leukemia, adrenocortical carcinoma, breast invasive carcinoma (Fu et al., 2014), colon adenocarcinoma, neuroblastoma, osteosarcoma, pancreatic adenocarcinoma, central nervous system tumor (Qin et al., 2014), kidney renal clear cell carcinoma, liver hepatocellular carcinoma, rectum adenocarcinoma, stomach adenocarcinoma, testicular germ cell tumors, and thyroid carcinoma (only in anaplastic carcinoma) (Kim and Koo, 2022), as well as uterine corpus endometrial carcinoma.

Conversely, EMP2 is expressed at lower levels in cancer tissue compared to normal tissue in patients with the following types of cancer: esophageal carcinoma, head and neck squamous cell carcinoma (Chen et al., 2012), kidney chromophobe cancer, kidney renal papillary cell carcinoma, lung adenocarcinoma (Powell et al., 2003), lung squamous cell carcinoma, skin cutaneous melanoma, and uterine carcinosarcoma (Wang et al., 2019) (https://tnmplot.com/) (Fig. 2). This suggests that EMP2 plays a significant role in the development and metastasis of tumors (Mozaffari et al., 2023). The biological functions and therapeutic strategies involving EMP2 in various cancers are summarized in Table 1.

Fig. 2.

Expression levels of EMP2 across various cancer types. Significant differences determined by the Mann–Whitney U test are marked in black color (*p<0.05, **p<0.01, and ***p<0.001). The analysis was performed using the TNM plotter (https://tnmplot.com/) (Bartha and Gyorffy, 2021). ALL, Acute lymphoblastic leukemia; Adrenal, Adrenocortical carcinoma; AML, Acute myeloid leukemia; Breast, Breast invasive carcinoma; Colon. Colon adenocarcinoma; CNS, Central nervous system tumor; Esoph, Esophageal carcinoma; HNSCC, Head and neck squamous cell carcinoma; Kidney_ch, Kidney chromophobe cancer; Kidney_cc, Kidney renal clear cell carcinoma; Kidney_PR, Kidney renal papillary cell carcinoma; Liver, Liver hepatocellular carcinoma; LUAD, Lung adenocarcinoma; LUSC, Lung squamous cell carcinoma; NB, Neuroblastoma; Osteo, Osteosarcoma; PDAC, Pancreatic adenocarcinoma; Rectum, Rectum adenocarcinoma; Skin, Skin cutaneous melanoma; Stomach, Stomach adenocarcinoma; Testicular, Testicular germ cell tumors; Thyroid, Thyroid carcinoma; UCS, Uterine carcinosarcoma; UCEC, Uterine corpus endometrial carcinoma.

Table 1.

Biological functions and therapeutic approaches of EMP2 in diverse cancers

Cancer type	Tumor suppressive/ oncogenic role	Biological functions/ Therapeutics	Ref.
B-cell lymphoma	Tumor suppressive	Overexpression of EMP2 reduced the tumorigenic potential of MV cells by promoting cell death.	Wang et al., 2001
Lung cancer	Tumor suppressive	Downregulation of EMP2 was involved in ERK and JNK activation and a decrease in PP2A expression, suggesting that EMP2 plays a pivotal role in modifying cellular characteristics.	Lee et al., 2016
	Tumor suppressive	Overexpression of EMP2 reduced tumor cell growth, proliferation, migration, and invasion by suppressing the MAPK pathway.	Ma et al., 2021
Nasopharyngeal carcinoma	Tumor suppressive	Loss of EMP2 expression was prevalent and linked to unfavorable DSS and LRFS, potentially contributing to increased tumor aggressiveness.	Chen et al., 2012
Urothelial carcinoma	Tumor suppressive	Overexpression of EMP2 stimulated apoptosis and reduced migration by altering the expression of integrin β3 and αV.	Wang et al., 2013
Gallbladder cancer	Tumor suppressive	Downregulation of EMP2 was observed in advanced tumors and was linked to poor survival in GBC.	Li et al., 2013
Melanoma	Tumor suppressive	EMP2 expression was negatively associated with mTOR-mediated autophagy, as determined by GSEA.	Wang et al., 2019
	Oncogenic	EMP2 overexpression promoted the activation of melanogenesis, as well as increase invasion and migration.	Enkhtaivan et al., 2022
Endometrial cancer	Oncogenic	EMP2 expression was associate with tumor progression and poor prognosis.	Wadehra et al., 2006
	Oncogenic	EMP2 diabodies induced apoptosis in a dose-dependent manner and demonstrated a synergistic effect in combination with progesterone.	Shimazaki et al., 2008
	Oncogenic	Co-localization of EMP2 and FAK facilitated tumor progression and migration.	Fu et al., 2011
	Oncogenic	High levels of EMP2 promoted tumor migration and angiogenesis by modulating VEGF. Furthermore, treatment of EMP2 IgG improved survival rates in mouse models.	Gordon et al., 2013
Ovarian cancer	Oncogenic	Tissue microarray analysis of 129 ovarian samples revealed elevated EMP2 levels in malignant tissues, and EMP2 diabodies effectively reduced tumor growth in a xenograft mouse model, suggesting that EMP2 could be a promising therapeutic target for ovarian cancer.	Fu et al., 2010
Glioblastoma	Oncogenic	Increased EMP2 expression enhanced migration by regulating αVβ3 integrin on the cell surface. Besides, treatment with anti-EMP2 IgG1 resulted in a dose-dependent reduction in cell invasion.	Qin et al., 2014
	Oncogenic	Ant-EMP2 IgG1 blocked EMP2-mediated migration and angiogenesis in GBM by suppressing VEGF-A expression.	Qin et al., 2017
	Oncogenic	High EMP2 expression was associated with Ki-67 positivity and poor survival outcomes in patient samples, suggesting that EMP2 may be a valuable diagnostic and prognostic marker for GBM.	Chung et al., 2018
	Oncogenic	An immunohistological study of 110 patients with GBM demonstrated a further increase in EMP2 expression in patient samples following bevacizumab therapy.	Patel et al., 2020b
Meningioma	Oncogenic	Elevated EMP2 expression enhanced tumor angiogenesis in meningioma.	Patel et al., 2020a
Glioma	Oncogenic	miR-129-5P directly targets EMP2, promoting the migration and invasion of glioma cells by regulating EMT.	Xia et al., 2022
Breast cancer	Oncogenic	In HER2-positive breast tumors, high expression of EMP2 was identified and increased carcinoma invasiveness and lymph node metastasis. Treatment with anti-EMP2 antibodies significantly reduced tumor growth by inhibiting Src phosphorylation.	Fu et al., 2014
	Oncogenic	The correlation between EMP2 and β1 integrin was increased in breast cancer, particularly ER+ PR+ luminal types.	El-Ghlban et al., 2020
	Oncogenic	EMP2 regulated cancer stem cell (CSC) markers, such as CD44, and was correlated with increased ALDH1 in metastatic tumors. Treatment with an anti-EMP2 monoclonal antibody reduced the proportion of BCSCs, consequently inhibiting tumor initiation, growth, and metastasis.	Dillard et al., 2020
	Oncogenic	High levels of EMP2 expression were detected in breast cancer patients, especially following chemotherapy with taxane. An anti-EMP2 monoclonal antibody was shown to improve taxane-resistance in an orthotopic mouse model.	Chan et al., 2024

The structural domain of EMP2 consists of two extracellular domains and a small cytoplasmic tail (Ashki et al., 2015) (Fig. 3). Although little specific research has been conducted on the post-translational modifications (PTMs) of EMP2, it is known to have three potential N-glycosylation sites (N44, N47, and N52) and three possible phosphorylation sites (Y116, Y141, and S142). Among these, the phosphorylation of Y116 has been confirmed through high-throughput methods. Additionally, it has been reported that acetylation and ubiquitination may occur at lysine 129 of EMP2 (Zhang et al., 2022) (Fig. 3A). However, research on how these modifications alter biological function and their role in cancer remains limited.

Fig. 3.

Structure, downstream signaling pathways, and biological functions of EMP2 protein. (A) Domain architecture of the EMP2 protein, three-dimensional structure (source: https://www.genecards.org), and post-translational modifications of EMP2 protein. (B) Mechanisms of EMP2 in cancer progression and metastasis. Interaction between EMP2 and integrins or cell surface receptors triggers activation or inactivation of downstream signaling pathways, including PI3K/AKT/mTOR/NFκB, FAK/Src/MAPK, and JNK/STAT3. This modulation influences key biological processes such as migration, invasion, proliferation, angiogenesis, metastasis, and apoptosis. Additionally, the inhibition of EMP2 by diabodies or IgG can lead to alterations in these molecular signaling pathways. RTK, Receptor tyrosine kinase; VEGFR, vascular endothelial growth factor receptor.

Integrin-mediated signaling and cancer cell invasion

Integrins function as the heterodimeric transmembrane receptors that mediate the communication between the extracellular matrix (ECM) and the cytoskeleton, transmitting biochemical signaling (Desgrosellier and Cheresh, 2010). Integrins play a crucial role in cancer by mediating cell adhesion, migration, survival of circulating cells, resistance to therapy, and proliferation (Oh et al., 2015; Pang et al., 2023). Wadehra et al. observed that EMP2 co-localized with β1 integrin, leading to heterodimer modification, upregulation of α5β1 integrin, and downregulation of α6β1 integrin on the plasma membrane (Wadehra et al., 2002). Overexpression of EMP2 was linked to increased surface expression of αVβ3 integrin and altered ECM components, such as fibronectin, in endometrial cells (Wadehra et al., 2005). High EMP2 expression significantly increased αVβ3 integrin levels and enhanced fibronectin-mediated cell invasion in U373/EMP2 and T98/EMP2 glioblastoma multiforme (GBM) cells (Qin et al., 2014). Additionally, the upregulation of EMP2 was correlated with integrin β1 in malignant breast tissues (El-Ghlban et al., 2020). These findings suggest that EMP2 potentially facilitates integrin recruitment (α5β1, α6β1, and αVβ3) and alters cellular adhesion and activation.

Conformation changes in integrins trigger ligand affinity and activate diverse downstream signaling pathways, such as focal adhesion kinase (FAK), steroid receptor coactivator (Src), mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinases (PI3K)/ protein kinase B (AKT), and nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) pathways (Pang et al., 2023). In the ARPE-19 cell line, a spontaneously arising retinal pigment epithelial (RPE) cell line, EMP2 was correlated with increased collagen gel contraction by activating FAK (Morales et al., 2009). Fu et al. (2011) observed that cellular migration was reduced in HEC-1A/EMP2 cells treated with FAK-Src small molecule inhibitor (PP2) and Src family inhibitor (dasatinib). Additionally, EMP2 co-localized with FAK in endometrial cancer cells (Fu et al., 2011). The correlation with EMP2 and FAK/Src activation also promoted HIF-1α expression under hypoxic conditions in endometrial cancer (Gordon et al., 2013). Similarly, the phosphorylation of FAK-Src was regulated by EMP2 expression in the MDA-MB-231 breast cancer cells (Fu et al., 2014). Aberrant EMP2 expression also induced signal transducer and activator of transcription 3 (STAT3) in some GBM cell lines (Qin et al., 2017). These results suggest that EMP2 modulates migration and invasion by activating FAK (phosphorylation at Y-576/577) and Src (phosphorylation at Y-416) in glioblastoma, endometrial, and breast cancers.

Elevation of EMP2 expression led to a reduction of caveolin-1 and -2, along with an increase of glycosylphosphatidylinositol-anchored proteins (GPI-APs) on the cell surface (Wadehra et al., 2004). In NIH3T3 cells, the interaction between EMP2 and major histocompatibility complex class 1 proteins (MHC I) or ICAM-1 (CD54) was discovered and leading to increased susceptibility to cytotoxic T lymphocytes (CTLs) (Wadehra et al., 2003a). Loss of EMP2 contributed to changes in AT1 membrane organization, including caveolin-2-dependent reductions of neutrophil transendothelial migration (TEM). Additionally, EMP2 knockout mice showed decreased levels of serum interleukin 6 (IL-6) and tumor necrosis factor-α (TNF-α), as well as reduced neutrophilic pulmonary consolidation, indicating that EMP2 deficiency may lead to systemic inflammation. Lin et al. (2020) also confirmed an increase in CD47 and a decrease in ICAM-1 and β3 integrin in the AT1 plasma membranes of EMP2 knockout mice.

In lung cancer studies, depletion of EMP2 was implicated in sphingosylphosphoryl-choline (SPC)-induced keratin 8 phosphorylation and reorganization (Lee et al., 2016). Ma et al. (2021) found decreased phosphorylation of AKT, p38, mTOR, and extracellular signal-regulated kinase1/2 (ERK1/2) in EMP2-overexpressing H1299 cells. Gene Set Enrichment Analysis (GESA) results showed that EMP2 regulates mTOR-mediated autophagy, particularly the autolysosome in melanoma (Wang et al., 2019).

Taken together, these findings indicate that EMP2 plays a critical role in cancer progression by modulating integrin-mediated signaling and influencing cellular adhesion, migration, and invasion through its interactions with key signaling pathways (Fig. 3).

Apoptosis and cell cycle regulation

EMP2 plays a crucial role in regulating apoptosis and the cell cycle, influencing these processes through various mechanisms across several cancer types. In HEK293 cells, EMP2 has been shown to contribute to the upregulation of apoptosis through interaction with the C-terminal domain of P2RX7 protein (Wilson et al., 2002). Overexpression of EMP2 significantly induced cell cycle arrest at the G2-M phase and increased annexin V-positive cells in urothelial cancer (Wang et al., 2013). Similarly, EMP2 overexpression modulated the MAPK pathway, leading to reduced cell proliferation and enhanced G2/M cell cycle arrest in lung cancer. The mRNA expression of cell cycle-related genes, including p53, p21, and cyclin D1, was increased in H1299/EMP2 and H2170/EMP2 cells (Ma et al., 2021). Collectively, these findings indicate that EMP2 modulates apoptosis and cell cycle regulation in various cancers, highlighting its potential as a therapeutic target for manipulating cell survival and proliferation.

Angiogenic functions

EMP2 is essential for controlling angiogenesis and vascular formation, influencing both tumor-associated blood vessel development and retinal neovascularization by modulating key signaling pathways. Since higher EMP2 expression has been identified in fetal retina and retinal pigment epithelium (RPE) compared to adult tissues, Sun et al. (2022) suggested that EMP2 is involved in early eye development. In endometrial and GBM tumors, EMP2 promotes the formation of tumor-associated vascular structures and cell migration by regulating vascular endothelial growth factor (VEGF), a critical signaling molecule in angiogenesis (Gordon et al., 2013; Qin et al., 2017). Additionally, EMP2 has been implicated in mediating aberrant ocular neovascularization in patients with proliferative vitreoretinopathy (PVR).

Several functional pathway alterations have been observed in EMP2 knockout mice under hypoxic conditions, including changes in proliferation, angiogenesis, oxidation, and apoptosis. In the knockout mice, hypoxia response genes (Lif and ll1b) and ocular angiogenesis genes (Ptger4) were upregulated, while genes involved in VEGF expression (Hif3α, Adra2b, and Lpar140) were downregulated, suggesting that EMP2 plays a role in oxygen-induced retinopathy (Sun et al., 2020). A recent RNA sequencing analysis in the oxygen-induced retinopathy (OIR) model showed that EMP2 blockade inhibits the expression of genes related to eye development and angiogenesis pathway (Wnt5aa, Col1al, Nrp1, and Epas1), while upregulating genes involved in biosynthesis, fatty acid oxidation, and mitochondrial pathways (Acat1, Ech1, and Pparg) (Aguirre et al., 2024). In conclusion, EMP2’s role in angiogenesis and vasculogenesis highlights its crucial impact on cancer progression and retinal diseases, making it a potential therapeutic target.

BIOMARKER IN CANCER DIAGNOSIS AND PROGNOSIS

EMP2 as a tumor suppressor

EMP2 transcript was considerably decreased in MV cells. However, the induction of EMP2 attenuated the tumorigenic potential of B-cell lymphoma in BALB/c mice. Overexpression of EMP2 increased apoptosis and cell death under the low-serum conditions in NIH3T3 cells, suggesting that EMP2 functions as a tumor suppressor in the context of B-cell lymphoma (Wang et al., 2001). EMP2 is expressed in human alveolar type I (AT1) cells, but not in alveolar type II (AT2) cells and alveolar macrophages (AMs) in the lung (Dahlin et al., 2004; Lin et al., 2020). Importantly, there were no significant histological changes in the lungs of EMP2 knockout mice at early (8-12 weeks) time points, indicating that EMP2 is not essential for lung development (Lin et al., 2020).

A microarray assay revealed that upregulation of EMP2 is detected in cells transfected with HOPX, a known tumor suppressor gene. In lung cancer, real-time PCR analysis showed that EMP2 mRNA expression is downregulated in six out of ten non-small cell lung cancer (NSCLC) cell lines (H2170, H1299, H226, H157, H2030, and H23) compared to human bronchial epithelial cells (HBEC). EMP2 transfectants attenuated cell growth, colony-forming ability, migration, and invasion in NSCLC cells (Ma et al., 2021). Additionally, low expression of EMP2 was observed in lung adenocarcinoma patients by gene expression array (Powell et al., 2003). Lee et al. (2016) confirmed that loss of EMP2 enhances the interaction of caveolin-1 with PP2A, leading to its ubiquitination. This degradation of PP2A increased migration via ERK and c-Jun N-terminal kinases (JNK) activation in SPC-mediated lung cancer (Lee et al., 2016).

A broad range of immunohistologically stained EMP2 was detected in the cytoplasmic and plasma membranes of nasopharyngeal tumors. Loss of EMP2 was significantly linked with poor disease-specific survival (DSS) and local recurrence-free survival (LRFS). Thus, the downregulation of EMP2 serves as an adverse prognostic marker for nasopharyngeal carcinoma (Chen et al., 2012).

Histologically, high-grade uroepithelial cell lines showed low expression of EMP2. Multivariate Cox regression analysis suggested that patients with upper urinary tract urothelial carcinoma (UUT-UC) who exhibited downregulation of EMP2 are at risk for tumor progression and worsened clinical outcomes. In a xenograft model, overexpression of EMP2 led to a decrease in urothelial tumor volume and weight, with EMP2 suppressing the progression of urothelial carcinoma by modulating αV and β3 integrins expression (Wang et al., 2013). EMP2 expression steadily declined during the progression of gallbladder cancer (GBC), from adenoma to adenocarcinoma. Li et al. (2013) identified that the loss of EMP2 expression was associated with clinicopathological features, such as advanced primary tumors, vascular, perineural invasion, and a high Ki-67 index, suggesting its potential as a prognostic marker in GBC. The role of EMP2 as a tumor suppressor is evident through its downregulation or absence being associated with tumor progression and adverse outcomes, underscoring its promise as a key therapeutic target for enhancing cancer treatment and patient prognosis.

EMP2 as an oncogene

High expression of EMP2 is observed in advanced stages of disease, including myometrial invasive, proliferative, and hyperplastic endometrial cancer (Wadehra et al., 2006). Emp2 expression gradually increases as hyperplasia transitions to malignant endometrial tumors, making it a potential predictor of endometrial cancer progression (Habeeb et al., 2010). Tissue microarray (TMA), immunohistochemistry (IHC), and western blot analyses have shown strong EMP2 expression in ovarian tumors, including endometrioid carcinoma and serous carcinoma, compared to normal ovarian tissues. In a xenograft model, treatment with EMP2 diabodies reduced tumor progression and induced necrosis in tumor lesions (Fu et al., 2010).

In a human GBM study, EMP2 mRNA levels were higher in tumors than in normal brain tissues. High EMP2 expression was associated with early mortality in GBM patients (Qin et al., 2014; Chung et al., 2018). As low-grade glioma progresses to high-grade glioblastoma, EMP2 expression gradually increases in aggressive tumors (Chung et al., 2018). In vitro and in vivo studies using U373 and U87MG GBM cell lines showed that EMP2 enhances cell migration and fibronectin-mediated invasion, indicating that EMP2 is a novel target and worse prognostic marker for GBM (Qin et al., 2014).

Meningioma accounts for about 30% of primary central nervous system (CNS) tumors, and high mRNA expression of EMP2 was detected in meningiomas compared to non-pathologic meninges, linking to increased new blood vessel formation (Patel et al., 2020a). Following bevacizumab therapy, EMP2 expression was elevated in GBM, suggesting that EMP2 is closely related to the response and resistance of bevacizumab (Patel et al., 2020b). Among the molecular subtypes of glioblastoma, the highest EMP2 expression was observed in the mesenchymal subtype. In a glioma study, Xia et al. (2022) identified that EMP2 is a direct target of miR-129-5P, altering epithelial-mesenchymal transition (EMT) marker expression, including vimentin, slug, E-cadherin, N-cadherin, and snail.

High expression of EMP2 has also been observed in human invasive ductal carcinoma, as confirmed by western blot and IHC (Fu et al., 2014). Breast cancer is classified into three subtypes: hormone receptor-positive (estrogen receptor [ER] or progesterone receptor [PR]), ERBB2-amplified (HER2 positive), and triple-negative breast cancer (TNBC; ER-negative, PR-negative, HER2-negative) (Waks and Winer, 2019). Among these subtypes, EMP2 expression was particularly associated with TNBC (Fu et al., 2014). Although fewer clinicopathological studies exist, EMP2 expression has also been correlated with HER2-positive breast cancer (Cha and Koo, 2020). Reduction of EMP2 by anti-EMP2 IgG decreased tumor burden and metastasis in a xenograft model (Fu et al., 2014). Univariate analysis indicated that aberrant EMP2 and integrin β1 expression is observed in ER-positive, PR-positive, luminal-type breast tumor tissues (El-Ghlban et al., 2020). Additionally, knockdown or overexpression of EMP2 influenced the expression of cancer stem cell markers, such as CD44, CD24, and ALDH isoforms. The positive correlation of EMP2 and ALDH1 was linked to enhanced lymph node metastasis in breast cancer cells (Dillard et al., 2020). These findings suggest that EMP2 may be a valuable biomarker and therapeutic target for breast cancer.

In melanoma cell lines, protein and mRNA levels of EMP2 were decreased. Wang et al. (2019) performed gene set enrichment analysis (GSEA) and confirmed that EMP2 is involved in mTOR-mediated autophagy. Enkhtaivan et al. (2022) suggested that the loss of EMP2 reduces melanin production by downregulating the melanogenesis-related gene TRP-2 while also reducing cell migration and invasion. In a TMA study, high EMP2 expression was detected in anaplastic carcinoma (AC), while lower levels were observed in papillary thyroid carcinoma (PTC), follicular carcinoma (FC), and poorly differentiated carcinoma (PDC) (Kim and Koo, 2022).

Collectively, the dual role of EMP2 as both a tumor suppressor and oncogene highlights its significant potential as a biomarker and therapeutic target in various cancer types, offering new opportunities for cancer diagnosis, prognosis, and treatment.

THERAPEUTIC POTENTIAL OF TARGETING EMP2 IN CANCER TREATMENT

Anti-EMP2 diabodies

Diabodies, a type of bispecific antibody, were developed to have a distinct affinity for two target antigens (Holliger et al., 1993). The structure of diabodies consists of the heavy-chain variable domain (VH) and the light-chain variable domain (VL), which originate from different parent IgG molecules. Diabodies are used in clinical applications, including cancer treatment and diagnosis (Todorovska et al., 2001). Anti-EMP2 recombinant bivalent antibody fragments, known as diabodies, are composed of a second extracellular domain (ECD) of human EMP2 (hEMP2) and murine EMP2 (mEMP2). Shimazaki et al. (2008) confirmed that treatment with KS49 (a diabody against hEMP2) or K83 (a diabody against mEMP2) inhibited endometrial tumor growth by regulating apoptosis in vitro and in vivo studies. Furthermore, anti-EMP2 diabodies demonstrated a synergetic effect with progesterone in the treatment of endometrial cancer (Shimazaki et al., 2008). In ovarian cancer studies, anti-EMP2 diabodies significantly suppressed tumor cell growth in the OVACA5 xenograft model. Moreover, FACS analysis using KS83 and KS49 diabodies revealed enhanced cell death via an apoptotic pathway in ovarian cancer cells (Fu et al., 2010). These findings demonstrate that anti-EMP2 diabodies exhibit promise as a targeted therapeutic strategy by effectively regulating tumor cell apoptosis, potentially enhancing treatment outcomes in endometrial and ovarian cancers.

Anti-EMP2 IgG antibody

An anti-EMP2 IgG1 antibody, a fully human recombinant immunoglobulin-based reagent with variable heavy and light chains containing functional single peptides for proper secretion, was designed by Fu et al. (2014). Treatment with EMP2 IgG1 antibody inhibited the growth of HEC1/A EMP2 tumors (Gordon et al., 2013). In GBM cells and xenograft mouse models, EMP2-specific IgG antibodies reduced tumor growth, cell migration, and invasion (Qin et al., 2014). In addition, anti-EMP2 IgG induced tumor regression and necrosis in breast tumors (Fu et al., 2014). Blocking EMP2 using a specific antibody diminished tumor growth and angiogenesis in U87MG/EMP2 (Qin et al., 2017). Anti-EMP2 monoclonal Ab (mAb) treatment also reduced the proportion of breast cancer stem cells (BCSC), inhibiting the growth and metastasis of breast cancer cells (Dillard et al., 2020).

Upregulation of EMP2 mRNA levels has been observed in taxane-resistant tumors, and high EMP2 expression following taxane therapy was associated with poor overall and recurrence-free survival. Chan et al. (2024) discovered a synergistic effect of anti-EMP2 mAb and taxane in an orthotopic TNBC model. Furthermore, an ex vivo choroid sprouting assay demonstrated that targeting EMP2 with an antibody reduced vessel sprouting, while in the oxygen-induced retinopathy (OIR) mouse model, blocking of EMP2 decreased pathologic neovascularization (Aguirre et al., 2024).

These studies indicate that anti-EMP2 IgG antibodies have significant potential as a therapeutic strategy for various cancers, including endometrial, ovarian, glioblastoma, and breast cancer. These antibodies effectively inhibit tumor growth, cell migration, and angiogenesis, and they improve treatment outcomes, particularly in combination with other therapies.

Challenges and opportunities in the development of EMP2 inhibitors for cancer treatment

Numerous preclinical trials involving anti-EMP2 treatment are underway. However, further studies are required to evaluate both the acute and long-term effects of these therapies. For enhanced clinical application, it is crucial to focus on advancing preclinical implementation and exploring novel delivery techniques (Ahmat Amin et al., 2019). Additionally, the molecular mechanisms underlying the action of EMP2 inhibitors are not fully understood. Therefore, further studies are required to gain a better understanding of how EMP2 inhibitors mediate tumor growth, migration, invasion, apoptosis, angiogenesis, and resistance in various cancers (Chan et al., 2024). In conclusion, while significant progress has been made in preclinical trials of anti-EMP2 treatments, more comprehensive studies are needed to assess both the acute and long-term effects of these therapies. Advancements in preclinical development and innovative delivery methods are essential to improve clinical applications. Moreover, additional research is imperative to elucidate the molecular mechanisms of EMP2 inhibitors and their impact on cancer processes.

CONCLUSION

EMP2 is emerging as a critical player in cancer biology, significantly influencing processes such as cellular adhesion, migration, proliferation, and apoptosis. The diverse roles of EMP2, which are dependent on the cellular context, underscore its complexity across different cancer types. In some cases, the loss or downregulation of EMP2 is associated with poor prognosis and enhanced tumor progression, as seen in lung adenocarcinoma, nasopharyngeal carcinoma, and urothelial carcinoma, suggesting its potential as a tumor suppressor. Conversely, EMP2 overexpression is linked to the progression and aggressiveness of cancers such as endometrial cancer, ovarian tumors, and glioblastomas, emphasizing its role as an oncogene.

The therapeutic implications of targeting EMP2 are promising. Anti-EMP2 diabodies, engineered antibodies specially designed to bind to and inhibit EMP2, have shown potential in preclinical models by reducing tumor growth and metastasis in cancers where EMP2 is overexpressed. This therapeutic strategy leverages EMP2’s oncogenic role by neutralizing its function, thereby inhibiting tumor progression. However, fully realizing the potential of these therapies requires a deeper understanding of the precision molecular mechanisms that dictate EMP2’s roles in cancer and optimizing these treatments for clinical use.

In summary, EMP2 represents a promising target in the ongoing battle against cancer, with significant potential to improve patient outcomes through innovative therapeutic strategies.