TROP2: A New and Important Molecular Target for Cancer Treatment

What Is TROP2?

TROP2 is a cell-surface glycoprotein belonging to the TACSTD family and encoded by the TACSTD2 (tumor-associated calcium signal transducer 2) gene. It was first identified and isolated from the gastrointestinal tumor-associated antigen (GA733-1). It is therefore also known as GA733-1. Epidermal glycoprotein 1 (EGP-1) and surface marker 1 (M1S1) are other names used to describe TROP2.

TROP2 mRNA can be detected in normal tissues such as keratinocytes, the kidneys, lungs, ovaries and testes. It is mainly expressed in epithelial cells, which play an important role in the development of embryonic organs. TROP2, which is present at low levels in normal tissues, is overexpressed on the surfaces of various cancer cells.

TROP2 expression depends on various transcription (the production of RNA from DNA) factors. The transcription factor HNF4A is an important factor affecting the transcription of TACSTD2 (TROP2). Other factors associated with cancer development, such as TP63/TP53L and Wilm’s tumor 1 (WT1), are also involved in TACSTD2 transcription. In addition, factors involved in TACSTD2 transcription include ERG, HNF1A/TCF-1, autoimmune regulatory factors, FOXP3 and more.

What Is the Function of TROP2?

TROP2 is a single-copy gene in human cells that encodes a type 1 transmembrane glycoprotein overexpressed in various malignant/cancer cells. In recent years, a close relationship between tumor development and the signaling of molecules that can regulate tumor-cell growth has been identified. TROP2, which has a signal-transduction function, is homologous to the protein-binding IGF-II. TROP2 also acts as a glycoprotein on the epithelial membrane surface and as a transmembrane transducer for intracellular calcium.

In 2020, when the TROP2-targeting drug sacituzumab govitecan received FDA approval for advanced triple-negative breast cancer, this molecular target began to occupy a lasting and important place on the oncology agenda.

TROP2 plays a regulatory role in cellular self-renewal, proliferation and transformation. Studies show that TROP2 can promote tumor growth. When this gene is silenced, the growth of these tumor cells can also be inhibited.

TROP2 plays an important role in embryonic development. While contributing to embryo implantation and placental tissue formation, it also has an important role in embryonic stem-cell proliferation and organ development.

TROP2 Signal Transduction

TROP2 is a tumor-associated gene. It promotes tumor growth, proliferation and metastasis through the calcium signaling pathway, regulation of cyclin expression and reduced fibronectin adhesion. TROP2 can also interact with beta-catenin in the Wnt signaling cascade and plays a role in nuclear oncogene transcription and cell proliferation.

The types of TROP2 signal transduction are as follows:

The intracellular tail serine residues (S303) of the TROP2 protein are phosphorylated by protein kinase C (PKC), which enables the hydrolysis of phosphatidylinositol 4,5-diphosphate (PIP2) into IP3 and DAG.

IP3 interacts with IP3 receptors on the surface of the endoplasmic reticulum and releases calcium ions stored in the endoplasmic reticulum, which activates the mitogen-activated protein kinase pathway (MAPK) and promotes the cell-cycle process. In addition, increased intracellular calcium activates cyclin D1 and the ERK/MEK pathways, increasing the expression of CREB1, c-Jun, NF-KB, Rb, STAT1 and STAT3. This suppresses tumor-cell apoptosis (programmed cell death).

DAG regulates TROP2 phosphorylation by activating PKC.

Figure 1: Cell cycle mediated by the PIP2 domain of TROP2.

TROP2 is hydrolyzed into the intracellular domain (ICD) by TACE, y-secretase and presenilin 1/2 (PS-1, PS-2) and is found together with the beta-catenin protein. Here, they upregulate the downstream expression of cyclin D1 and the proto-oncogene c-myc, promoting cell growth and proliferation.

Figure 2: TROP2 regulates the cell cycle through intracellular hydrolysis.

TROP2 also increases ERK1/2 phosphorylation and further increases the downstream activity of the target AP-1 transcription factors, downregulates Bd-2 expression and inhibits apoptosis. AP-1 increases cyclin D1 and cyclin E expression and reduces p27 expression, accelerating the cell cycle. At the same time, TROP2 regulates the expression of the proliferation marker Ki-67, thereby promoting tumor proliferation.

Figure 3: TROP2-mediated apoptosis and proliferation signals

TROP2 reduces the binding of fibronectin to integrin β1 and keeps cytoplasmic RACK1 close to integrin β1, enabling its accumulation on the cell surface.

TROP2 forms a complex with integrin β-1 and Talin proteins, and this complex activates Src and FAK. Cell adhesion is regulated by the integrin β-1-RACK1-Src and FAK signaling axes. Tumor-cell adhesion therefore decreases and metastasis occurs.

Figure 4: TROP2 promotes tumor invasion and metastasis

Which Cancers Are Associated with TROP2?

TROP2 is highly expressed in various tumors:

• Reproductive system tumors
• Digestive system tumors
• Head and neck cancer
• Nervous system tumors

Reproductive System Tumors

Current studies focus mainly on the relationship between the female reproductive system, particularly endometrial cancer, and TROP2.

In endometrial cancer tissue, there is a positive correlation between TROP2 expression and the pathological level; that is, the TROP2 expression level is associated with the degree of malignancy of the disease. The survival rate is significantly reduced in patients whose TROP2 protein is overexpressed, so TROP2 expression can be used as an independent prognostic indicator for patients with shortened survival. TROP2 protein overexpression is also associated with highly invasive ovarian cancer (cancer of the female ovary). In conclusion, increased TROP2 expression is closely associated with malignant tumors of the female reproductive system and the degree of malignancy of the disease.

TROP2 is also closely associated with tumors of the male reproductive system. Prostate cancer cell gene profiling, using cells isolated from solid tumor tissue, has shown abnormal expression of the TROP2 gene. This indicates that TROP2 is associated with the occurrence and development of prostate cancer, and abnormal TROP2 expression also plays an important role in the biological behavior of cancer cells.

Digestive System Tumors

TROP2 overexpression, also seen in pancreatic adenocarcinoma, is associated with tumor stage and poor patient survival, particularly lymph-node metastasis. This raises the idea that, for pancreatic cancer, TROP2 could serve as a unique prognostic biomarker.

In colon cancer, TROP2 is expressed at a particularly higher rate in cancer tissues than in normal tissues. High TROP2 expression increases the incidence of liver metastasis and mortality. The extent of metastasis is deeper, accompanied by metastasis to more lymph nodes and a poor prognosis.

TROP2 expression is necessary for tumor formation and the invasion of colon cancer cells. Following the use of RNA interference, tumor formation and the invasive capacity of cancer cells were inhibited. Accordingly, exogenous TROP2 expression can increase the proliferative capacity of colon cancer cells.

TROP2 is also highly expressed in gastric cancer. Treating gastric adenocarcinoma cell lines with apoptotic activator 2 against the TROP2 antigen particularly promotes cell apoptosis.

Finally, esophageal cancer tissues and cholangiocarcinoma also show high TROP2 expression.

Head and Neck Cancer

TROP2 overexpression is observed in the tumor tissues of patients with oral cancer. TROP2 expression also increases in squamous cell carcinoma of the larynx and is associated with the degree of tissue differentiation and lymph-node metastasis. TROP2 expression can be used as an independent prognostic factor for lymph-node metastasis, differentiation, tumor size and T level.

Nervous System Tumors

TROP2 is closely associated with the degree of malignancy, proliferation and angiogenesis of glioma and may help with targeted treatment of glioma.

- Related topic: Brain Tumor Symptoms, Stages and Treatment

TROP2 as an Antitumor Drug Target

TROP2 is a transmembrane protein expressed in various tumors and is a natural candidate for the development of targeted therapeutics. The limitations of TROP2 tissue expression reduce treatment toxicity, which is an advantage of TROP2-targeting therapy.

Immunotherapy targeting TROP2 is being tested. Various drug forms, such as antibodies, antibody conjugates and combinations of drugs targeting TROP2, are also being investigated.

Sacituzumab govitekan was evaluated for treating epithelial malignancies caused by TROP2 overexpression.

The Antibody Conjugate Sacituzumab Govitecan

Sacituzumab govitecan, previously approved by the FDA for triple-negative breast cancer and metastatic urothelial cancer, has also shown activity in hormone receptor (HR)-positive breast cancer, small-cell lung cancer and endometrial cancer. Toxicity results were consistent with those in triple-negative breast cancer and metastatic urothelial cancer.

Patients with previously treated metastatic triple-negative breast cancer have a poor prognosis, and cytotoxic chemotherapy remains the mainstay of systemic treatment for this subtype. Sacituzumab govitekan is an antibody-drug conjugate containing the topoisomerase I inhibitor irinotecan’s active metabolite, the cytotoxic agent SN-38, linked to an anti-trophoblast cell-surface antigen 2 (Trop-2) antibody. Trop 2 is a transmembrane calcium signal transducer highly expressed in many tumor types, including breast cancer. Trop-2 upregulation has been shown to stimulate tumor growth in several cell lines, and retrospective studies have associated Trop-2 membrane expression with a poorer prognosis in breast cancer.

In April 2020, the FDA granted accelerated approval to sacituzumab govitekan for patients with metastatic triple-negative breast cancer who had previously received at least 2 lines of therapy for metastatic disease. Subsequently, in April 2021, the FDA granted regular approval to sacituzumab govitekan in patients who had previously received ≥2 systemic therapies (at least one in the metastatic setting), based on the positive results of the confirmatory phase 3 ASCENT trial. The ASCENT clinical trial is a global, open-label, randomized, phase 3 study evaluating the efficacy of sacituzumab govitekan at 10 mg/kg on days 1 and 8 of every 21-day cycle versus chemotherapy. No biomarker selection was required for enrollment in the nonworking study, and Trop-2 expression was assessed only for correlative analyses. Patients receiving sacituzumab govitecan had a better objective response rate (35% versus 5%), a longer response duration (median, 6.3 versus 3.6 months), longer PFS among those without brain metastases (primary endpoint; median PFS, 5.6 versus 1.7 months; HR, 0.41), and a doubling of overall survival (median, 12.1 versus 6.7 months; HR, 0.48) compared with those receiving chemotherapy. The most common grade 3 treatment-related AEs with sacituzumab govitekan were neutropenia (51%), leukopenia (10%), diarrhea (10%), anemia (8%) and febrile neutropenia (6%).

The Latest Position Reached for TROP2

Virus-like particles (VLPs) are highly immunogenic and versatile immunostimulants that can be used as cancer vaccines by modifying exogenous proteins on cell membranes. A virus-like particle combined with TROP2 has been found to be a potential immunotherapeutic approach for pancreatic cancer.

In 2019, in Biochemical and Biophysical Research Communications, Jinrong Liu and colleagues produced a TRPO2-IgG antibody using a eukaryotic expression system, based on previously developed TROP2-Fab antibodies. This new monoclonal TROP2-IgG antibody was shown to have an inhibitory effect on tumors in in vitro and in vivo experiments.

In 2018, in International Journal of Oncology, another study by Lianhua Z and colleagues demonstrated that curcumin inhibited the proliferation and movement of bladder cancer cells through TROP2.

In 2019, in Oncology Report, another study by Hou and colleagues showed that the JAK2/STAT3 pathway promoted the growth and metastasis of glioblastoma cells. This study provides a basis for a better understanding of the role of TROP2 in glioblastoma. It points to a promising biomarker and therapeutic target for patients with glioblastoma.