TROP2: A Promising Target for Anti-Tumor Therapy

Molecular Structure and Biological Characteristics of TROP2

Trophoblast cell surface antigen 2 (TROP2), also known as tumor-associated calcium signal transducer 2 (TACSTD2), is an important cell surface glycoprotein. As a member of the TACSTD family, TROP2 has been referred to in various studies as epithelial glycoprotein-1 (EGP-1), membrane component chromosome 1 surface marker 1 (M1S1), or gastric antigen 733-1 (GA733-1). Initially identified in human placental trophoblasts, subsequent research has demonstrated its high expression in many human malignant tumor cells.

From a molecular perspective, TROP2 is composed of 323 amino acids and exhibits the typical structural features of a type I transmembrane protein. Its domains include:

A hydrophobic leader peptide (amino acids 1–26)

A large extracellular domain (ECD, amino acids 27–274)

A transmembrane domain (TM, amino acids 275–297)

A short cytoplasmic tail (amino acids 298–323)

Notably, the cytoplasmic tail of TROP2 contains a highly conserved phosphatidylinositol 4,5-bisphosphate (PIP2)-binding sequence, as well as multiple phosphorylatable tyrosine and serine sites. These structural features suggest that PIP2 plays a significant regulatory role in TROP2-mediated signal transduction.

Figure 1 Structure of TROP2

Expression Profile and Genetic Variations of TROP2 in Malignant Tumors

Extensive research has shown that TROP2 exhibits abnormal overexpression in many human malignant tumors, while its expression in normal tissues displays significant tissue-specific differences. Systematic analysis of tumor genome atlas data reveals the following:

Expression levels:

The highest TROP2 gene expression is observed in bladder urothelial carcinoma (BLCA)

High expression is also seen in head and neck squamous cell carcinoma (HNSC) and lung squamous cell carcinoma (LUSC)

Upregulation occurs in bladder urothelial carcinoma, cholangiocarcinoma, colon adenocarcinoma, esophageal cancer, lung adenocarcinoma, prostate cancer, and other malignancies

Downregulation is observed in head and neck squamous cell carcinoma, renal chromophobe cell carcinoma, and clear cell renal cell carcinoma

Genetic variations:

Sarcomas exhibit the highest mutation rate in the TROP2 gene

High-frequency mutations are also observed in various neuroepithelial tumors and ovarian epithelial cancers

The mutation spectrum of the TROP2 gene varies significantly across different tumor types

Current research indicates that TROP2 overexpression in tumors is primarily due to dysregulation at the transcriptional and post-transcriptional levels rather than structural alterations in the gene itself. This dysregulation may involve epigenetic modifications, transcription factor regulation, and microRNA-mediated mechanisms.

Figure 2 Expression of TROP2 in malignant tumors
BLCA: Bladder and Urinary Tract Epithelial Cancer; BRCA: breast cancer; CHOL: cholangiocarcinoma; COAD: Adenocarcinoma of the colon; ESCA: Esophageal cancer; GBM: Glioblastoma; HNSC: Head and neck squamous cell carcinoma; KICH: renal suspected cell carcinoma; KIRC: renal clear cell carcinoma; KIRP: renal papillary cell carcinoma; LGG: low-grade glioma; LIHC: Hepatocellular carcinoma; LUAD: lung adenocarcinoma; LUSC: Lung squamous cell carcinoma; PAAD: pancreatic cancer; PRAD: Prostate cancer; READ: Rectal adenocarcinoma; STAD: Gastric adenocarcinoma; THCA: thyroid cancer; UCEC: Endometrial cancer

TROP2-Mediated Signal Transduction Networks

As a critical receptor protein on the cell membrane, TROP2 primarily regulates calcium ion signaling pathways and the expression of cell cycle-related proteins while influencing extracellular matrix adhesion properties, thereby contributing to tumor cell growth, proliferation, and metastasis. Its signal transduction mechanisms include the following:

Calcium ion signaling pathway activation:

Phosphorylation of the serine residue (S303) in the cytoplasmic tail of TROP2 by protein kinase C (PKC)

Promotion of PIP2 hydrolysis to generate inositol trisphosphate (IP3) and diacylglycerol (DAG)

IP3-induced calcium ion release from the endoplasmic reticulum, activating the MAPK signaling pathway

DAG further activates PKC through a positive feedback mechanism

Intracellular hydrolysis regulation:

Cleavage of TROP2 into extracellular (ECD) and intracellular (ICD) domains by TACE, γ-secretase, and other enzymes

Translocation of ICD to the nucleus, where it colocalizes with β-catenin

Upregulation of cell cycle regulators such as cyclin D1 and c-myc

Apoptosis and proliferation regulation:

Activation of ERK1/2 phosphorylation, enhancing the activity of transcription factor AP-1

AP-1-mediated regulation of multiple tumor-related target genes

Induction of angiogenesis via VEGF

Regulation of apoptosis through Bcl-2 family proteins

Promotion of cell cycle progression via cyclinD1/cyclinE-CDK complexes

Tumor metastasis promotion mechanisms:

Enrichment of RACK1 protein on the cell membrane

Reduction of fibronectin binding to integrin β-1

Activation of downstream signaling molecules such as Src and FAK

Decreased tumor cell adhesion capacity, facilitating metastasis

Figure 3 TROP2 signaling pathway
EC: extracellular; ECD: Extracellular Domain; IC: intracellular; ICD: intracellular structural domain; TM: transmembrane helix; PIP2: Phosphatidylinositol 4,5-diphosphate; IP3：1， 4,5-triphosphate inositol; DAG: diacylglycerol; PKC: protein kinase C; MAPK: mitogen activated protein kinase

Clinical Prognostic Value of TROP2

Substantial clinical research data indicate that TROP2 expression levels significantly correlate with clinical outcomes in various malignancies, making it a potential prognostic biomarker. Analyses of public databases and clinical samples have revealed the following:

Correlation with overall survival (OS):

High TROP2 levels are associated with better OS in diffuse large B-cell lymphoma and renal chromophobe cell carcinoma

Low TROP2 levels are linked to improved OS in pancreatic cancer, melanoma, and lung adenocarcinoma

Correlation with disease-free survival (DFS):

High TROP2 levels correlate with better DFS in endometrial cancer and renal chromophobe cell carcinoma

Low TROP2 levels are associated with improved DFS in ovarian cancer and pancreatic cancer

Relationship with other clinicopathological features:

Serves as an independent prognostic indicator for lymph node metastasis, tumor differentiation, and tumor size

Correlates with tissue differentiation and lymph node metastasis in head and neck squamous cell carcinoma

Associated with more aggressive clinical progression in prostate cancer

It is worth noting that the prognostic value of TROP2 may be influenced by its subcellular localization in tumor tissues. Additionally, hypermethylation of the TACSTD2 gene promoter in certain tumor types may lead to its downregulation, which must be considered in clinical interpretation.

Development Prospects for TROP2-Targeted Therapies

Given the critical role of TROP2 in tumorigenesis and its tumor-specific expression pattern, TROP2-targeted therapeutic strategies have garnered significant attention in recent years. Current research directions include:

Antibody-based drugs:

Monoclonal antibodies

Bispecific antibodies

Antibody-drug conjugates (ADCs)

Novel targeted delivery systems:

Virus-like particles

Targeted nanoparticles

Combination therapy strategies:

Combination with traditional chemotherapy

Combination with immunotherapy

Combination with radiotherapy

Among these, the development of TROP2-targeted ADCs has shown the most progress, offering new treatment options for advanced metastatic malignancies. These drugs achieve specific tumor cell killing by conjugating TROP2-targeting antibodies with potent cytotoxic agents while minimizing toxicity to normal tissues.

Summary and Future Perspectives

TROP2, as a transmembrane protein highly expressed in many epithelial-derived tumors, plays multiple roles in tumor development. It promotes malignant progression by regulating calcium signaling, cell cycle, apoptosis, metastasis, and other pathways. Clinical studies have demonstrated that TROP2 expression levels are closely associated with patient prognosis, highlighting its value as a biomarker.

With deepening understanding of TROP2's biological functions, significant progress has been made in drug development targeting this molecule. The successful development of ADCs, in particular, provides new therapeutic options for TROP2-overexpressing tumors. Future research should further elucidate the precise mechanisms of TROP2 in different tumors, optimize targeted therapy strategies, and explore more effective combination regimens to maximize clinical benefits for cancer patients.