CD326 in Cancer: Expression, Function, and Immune Roles

CD326, also known as EpCAM, is a transmembrane glycoprotein widely recognized for its role in cancer biology. Frequently overexpressed in carcinomas, it has been implicated in tumor progression, metastasis, and immune modulation. Its clinical relevance has made it a target for diagnostic and therapeutic strategies, particularly in epithelial-derived malignancies.

Understanding CD326’s expression patterns, functional contributions to cancer, and interactions with the immune system provides valuable insights into its potential as a biomarker and treatment target.

Structural And Molecular Features

CD326 is a type I transmembrane glycoprotein with an extracellular domain, a single-pass transmembrane region, and a short intracellular tail. The extracellular portion contains two epidermal growth factor (EGF)-like repeats and a thyroglobulin type-1 domain, which contribute to its adhesive and signaling functions. These structural elements facilitate homophilic cell-cell interactions, critical for epithelial tissue integrity. The intracellular domain participates in signaling cascades that influence proliferation and differentiation.

Post-translational modifications, including glycosylation and phosphorylation, regulate CD326’s stability and function. Glycosylation enhances protein folding and trafficking, while phosphorylation modulates downstream signaling. Proteolytic cleavage by tumor necrosis factor-α-converting enzyme (TACE) and γ-secretase releases the extracellular domain and generates an intracellular fragment capable of nuclear translocation, influencing transcriptional regulation of genes linked to cell cycle progression and survival.

CD326 also co-regulates Wnt/β-catenin signaling by stabilizing β-catenin at the plasma membrane, affecting gene expression patterns that govern epithelial behavior. Additionally, it interacts with cytoskeletal components like actin-binding proteins, modulating cellular morphology and motility. These interactions highlight its dual role as both an adhesion molecule and a signaling mediator.

Expression In Epithelial Tissues

CD326 is predominantly expressed in epithelial tissues, contributing to cellular organization and barrier function. It is found on the basolateral membrane of epithelial cells in the gastrointestinal tract, liver, pancreas, and respiratory epithelium. Its presence in rapidly proliferating epithelial compartments, such as intestinal crypts and basal layers of stratified epithelia, underscores its role in cellular renewal and differentiation.

In the gastrointestinal tract, CD326 is abundant in the small intestine and colon, particularly in progenitor cells within the crypts of Lieberkühn, where it supports epithelial regeneration. In the liver, it is present on cholangiocytes lining the bile ducts, maintaining ductal integrity. Similarly, in the pancreas, it is expressed on ductal epithelial cells, ensuring cellular cohesion and exocrine function.

The respiratory epithelium also exhibits significant CD326 expression, particularly in bronchial and alveolar linings. In the lungs, it is localized to alveolar epithelial cells, including type II pneumocytes, which contribute to surfactant production and alveolar repair. This suggests a role in epithelial homeostasis, as type II pneumocytes can differentiate into type I alveolar cells following injury.

CD326 expression is regulated by developmental and pathological cues. During embryogenesis, it is transiently expressed in epithelial progenitor populations, guiding tissue morphogenesis and differentiation. Dysregulation has been observed in pathological conditions such as epithelial hyperplasia and metaplasia, where aberrant levels correlate with altered cellular behavior. Its expression is also influenced by extracellular matrix components and mechanical stimuli, indicating microenvironmental factors contribute to its regulation.

Functional Roles In Malignancy

CD326 influences tumor initiation, progression, and metastasis. Its overexpression in carcinomas enhances proliferative capacity by modulating intracellular signaling pathways that drive unchecked cell division. By stabilizing β-catenin and facilitating its nuclear translocation, CD326 activates Wnt/β-catenin signaling, promoting transcription of genes associated with cell cycle progression, such as cyclin D1 and MYC, fostering rapid expansion and resistance to apoptosis.

Beyond proliferation, CD326 affects tumor cell plasticity by contributing to epithelial-to-mesenchymal transition (EMT), a process in which epithelial cells lose adhesion properties and acquire a migratory phenotype. This transition, marked by E-cadherin downregulation and vimentin upregulation, enables tumor cells to detach and invade surrounding tissues. EMT is a precursor to metastasis, and CD326 expression correlates with increased invasiveness in cancers such as colorectal, breast, and pancreatic carcinomas.

CD326 also maintains cancer stem cell (CSC) populations, a subpopulation of tumor cells with self-renewing capabilities and heightened resistance to therapies. It serves as a marker for CSCs in multiple malignancies, including hepatocellular carcinoma and ovarian cancer, where its expression is enriched in tumor-initiating cells. CD326-positive CSCs exhibit enhanced tumorigenicity in xenograft models, driving tumor relapse and progression. This has led to efforts targeting CD326 in CSCs, with therapeutic strategies such as monoclonal antibodies and CAR-T cell therapies being explored.

Interactions With Immune Cells

CD326 plays a role in tumor immune interactions, influencing immune recognition and evasion. Its expression on malignant cells affects antigen presentation and cytokine signaling, reducing susceptibility to cytotoxic T lymphocytes (CTLs). Tumors overexpressing CD326 often exhibit altered major histocompatibility complex (MHC) molecule expression, impairing immune surveillance.

Regulatory T cells (Tregs) are also affected, particularly in immunosuppressive tumor environments. CD326-expressing cancer cells engage with Tregs through cytokine secretion, including transforming growth factor-beta (TGF-β) and interleukin-10 (IL-10), promoting immune suppression. This fosters Treg expansion and weakens effector T cell responses, shielding tumor cells from immune-mediated destruction. Additionally, CD326 impacts dendritic cell (DC) maturation, with evidence suggesting tumor-derived extracellular vesicles expressing CD326 interfere with antigen presentation, diminishing naïve T cell activation.

Laboratory Detection And Analysis

CD326 expression is analyzed using molecular, histological, and flow cytometric techniques in clinical and research settings. These methods help identify CD326-positive cells in tumor samples, circulating tumor cells (CTCs), and tissue biopsies, aiding in diagnosis and prognosis.

Immunohistochemistry (IHC) is widely used to assess CD326 expression in formalin-fixed, paraffin-embedded tissue sections. Monoclonal antibodies targeting its extracellular domain help visualize staining patterns that indicate tumor extent and localization. High expression levels correlate with aggressive tumor phenotypes in cancers such as breast, colorectal, and pancreatic carcinoma.

Flow cytometry quantifies CD326 expression in single-cell suspensions, particularly in CTC analysis. Fluorescently conjugated anti-CD326 antibodies allow researchers to isolate and characterize epithelial-derived tumor cells from blood samples, aiding in monitoring minimal residual disease and metastasis.

Molecular approaches such as quantitative PCR and RNA sequencing provide further insight into CD326’s transcriptional regulation, detecting alternative splicing and expression variations. Advanced imaging techniques, including multiplex immunofluorescence and mass cytometry, allow for spatial analysis of CD326-positive cells within the tumor microenvironment. These high-resolution methods help study CD326’s co-expression with other biomarkers, shedding light on tumor heterogeneity and therapeutic resistance.