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EpCAM, also known as TACSTD1 (tumor-associated calcium signal transducer protein 1), is a single-transmembrane type I glycoprotein according to the leukocyte differentiation antigen as CD326. The EpCAM molecular structure consists of an extracellular domain, a single transmembrane domain and an intracellular domain, which encodes a tumor-associated antigen. EpCAM is expressed in normal epithelial cells and epithelial-derived malignant cells.
The Biological Function of EpCAM
EpCAM was first recognized in mouse fibroblasts. Its biological functions include cell adhesion, proliferation, maintenance of undifferentiated state, and regulation of differentiation. Studies have shown that EpCAM shows carcinogenic properties in certain in vitro cell models. EpCAM plays an important role in tumor development and invasion, and can increase the migration and proliferation of tumor cells. Overexpression of EpCAM may result in activation of the canonical signaling pathway of the Wnt-β-catenin signaling pathway, which activates the expression of the proto-oncogenes e-myc and cyclin A/E via the Wnt cascade to induce cell proliferation. EpCAM impairs E-cadherin-mediated adhesion and may result in an increase in non-membrane-bound β-catenin.
Figure 1. EpCAM signaling in cross-talk with E-Cadherin. (Schnell, et al. 2013)
AATF and Tumor
EpCAM is highly expressed in gastric cancer tissues and cell lines, and down-regulation of EpCAM by RNA interference leads to a decrease in cell proliferation and cell cycle arrest. Related studies have confirmed the high expression of EpCAM in gastric cancer and its prognosis. EpCAM is negative in normal esophageal epithelium, but in primary esophageal squamous cell carcinoma, approximately 80% have abnormal expression in varying degrees and are associated with prognosis. EpCAM was first discovered in colon cancer in 1979. A study of tissue microarrays by immunohistochemistry showed high levels of EpCAM expression in colon cancer.
It has been reported that EpCAM binds tightly to the protein claudin-7 to regulate the tight junctions between ovarian cancer cells, which promotes cell proliferation, apoptosis resistance and tumorigenicity. EpCAM and claudin-7 form the EpCAM-claudin-7 complex, which can increase the migration, proliferation and anti-cisplatin treatment of HEK293 cells. In vivo, the EpCAM-claudin-7 complex promotes the development of pancreatic tumors in rats and the dissemination of tumor cells. This suggests that EpCAM has a tumor promoting effect. Studies have shown that EpCAM directly mediates cell-cell adhesion, inhibits substrate and EGF-induced migration of human ovarian cancer cells, overexpresses EpCAM to reduce cell migration, and knocks out EpCAM to increase cell migration.
The high expression of EpCAM is important in the treatment of epithelial tumors. EpCAM antibodies or vaccines have been widely used in various clinical tumor immunotherapy trials, including monoclonal antibodies and antibody fragments. In 2009, the first anti-EpCAM antibody approved by the European Commission was named catumaxomab, which is effective in the treatment of malignant ascites in patients with EpCAM-positive tumors. The effect of rituximab is based on its binding to the surface EpCAM of tumor cells, CD3 antigen binding on the surface of T cells, and FC receptor binding on the surface of helper cells (such as natural killer cells, dendritic cells, macrophages). Through T cell-mediated cell lysis, antibody-dependent cell-mediated cytotoxicity, rituximab phagocytose and kill tumor cells.