The Claudin family, encoded by the single ancestral gene CLDN, comprises at least 27 members. These membrane proteins are crucial components of tight junctions, playing a vital role in maintaining cell-cell barrier function. Research indicates that the expression levels of claudins are often reduced or lost in many human tumors, which aligns with the disruption or loss of functional tight junctions during carcinogenesis. Conversely, increasing evidence suggests that claudins may be overexpressed or abnormally regulated in various cancers, indicating their specific roles in tumorigenesis. Importantly, the dysregulation of claudins exerts oncogenic or tumor-suppressive effects depending on the target tissue or cell type, impacting tumor development and progression. Although tight junctions are primarily cellular structures in epithelial cells, the specific roles of claudins in cancer suggest that tight junctions are not only static components but also dynamic receptors and signal transmitters. Given the association between claudin expression profiles and patient prognosis in various cancers, understanding the expression patterns and subcellular localization of claudins in pathological states will aid in their use as biomarkers for cancer detection and diagnosis.

Figure 1. Claudins are 20–27 kDa proteins with four transmembrane domains, two extracellular loops, a longer third transmembrane domain, an extracellular helix, and two variable regions. (Hashimoto Y, et al., 2019)

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Tight Junctions

Tight junctions (TJs) are the closest cell-cell adhesion structures found in epithelial and endothelial cells, forming the most intimate contact at the apical edge of the cell membrane. These specialized structures are crucial for creating a tight seal between cells, serving as the main barrier to fluid and solute diffusion. In multicellular organisms, the separation and compartmentalization of internal environments through tight junctions are essential for maintaining tissue homeostasis during embryonic development and throughout life. Tight junctions also play a key role in establishing cell polarity by forming a fence that prevents the lateral diffusion of membrane proteins and lipids, creating boundaries between apical and basolateral membrane domains.

Data have accumulated showing that tight junctions directly participate in regulating cellular functions, including proliferation, differentiation, and apoptosis. These functions are based on the ability of tight junction proteins to recruit various types of cytoskeletal and signaling molecules, which regulate proliferation, differentiation, and apoptosis, including transcription factors, lipid phosphatases, and cell cycle regulators. Additionally, some tight junction proteins directly interact with cytoskeletal proteins, such as zonula occludens-1 (ZO-1), providing a direct link to the actin cytoskeleton. Consequently, tight junction proteins, including claudins, have a role as signaling molecules that receive and transmit environmental signals within cells, suggesting that tight junctions are not only static components but also dynamic participants in intracellular signaling.

Figure 2. TJ proteins act as both signal receivers and transmitters, interacting with various intracellular signaling pathways and scaffolding proteins to regulate gene expression. (Osanai M, et al., 2016)

Claudins as Key Components of Tight Junctions

The Claudin family comprises at least 27 members, encoded by the single ancestral gene CLDN, and is a major component of tight junctions. Claudins are critical for forming tight junction strands between adjacent cells through homophilic or heterophilic interactions. The CLDN gene family has relatively small and short introns, spanning only a few thousand base pairs. The human genome includes several CLDN gene pairs, which are closely related and located in proximity, including CLDN3 and CLDN4 on chromosome 7, CLDN8 and CLDN17 on chromosome 21, and CLDN22 and CLDN24 on chromosome 4. Sequence analysis also shows that many human CLDNs have sequence similarities, such as CLDN6 and CLDN9, CLDN3 and CLDN4, and CLDN22 and CLDN24. These findings suggest that certain claudins are co-regulated in a tissue-specific manner.

Other tight junction membrane components include proteins with four transmembrane domains, such as occludin, and proteins with a single transmembrane domain, such as junctional adhesion molecules (JAMs). These molecules are involved in the formation of tight junction strands, and the structural integrity of advanced tissue structures is achieved through membrane-anchored scaffold proteins like ZO-1. Notably, tight junctions were observed to reassemble into highly coordinated strands when specific types of claudins were expressed in claudin-deficient fibroblasts lacking endogenous claudins. These observations support the notion that claudins are major components of tight junctions and primarily determine their structure and function.

Signaling Functions of Claudins

Most claudin proteins range in size from 20 to 34 kDa and have widespread impacts on cellular physiology and pathology. In the structural model of tight junctions, claudins have four transmembrane domains, two extracellular loops, and a long cytoplasmic tail. The extracellular loops are crucial for maintaining tight junction function and epithelial barrier integrity. The first extracellular loop of claudins is identified as a key component in regulating cell permeability to ions, with differences observed between claudin family members in ion selectivity. For example, overexpression of certain claudins in vitro affects epithelial transmembrane resistance and permeability to water and solutes, with claudin-specific effects. Further data suggest that claudins serve as receptors for environmental signals due to their extracellular domains. Notably, CLDN3 and CLDN4 have been identified as receptors for Clostridium perfringens enterotoxin (CPE), while CLDN1, CLDN6, and CLDN9 are co-receptors for the hepatitis C virus.

Claudins possess amino and carboxy-terminal tails that extend into the cytoplasm, showing various differences in size and sequence among different claudins. The C-terminal region contains a PDZ-binding motif, allowing claudins to interact directly with cytoplasmic tight junction-associated proteins such as ZO-1. Importantly, the C-terminal regions of claudins have multiple phosphorylation sites, which are involved in interactions with various signaling molecules. Indeed, claudins function as signaling proteins in intracellular signaling processes, interacting with various signaling pathways, including oncogenic tyrosine-protein kinase c-Yes, protein kinase A (PKA), protein kinase C (PKC), Rho, Akt protein, phosphatidylinositol-3-kinase (PI3K), and mitogen-activated protein kinase (MAPK) pathways. The Y-box transcription factor ZONAB, which interacts with the SH-3 domain of ZO-1, functions in regulating erbB-2 promoter activity, indicating that tight junction proteins, including claudins, directly participate in regulating gene expression. Additionally, other tight junction proteins like ZO-2 interact with Fos, Jun, CCAAT/enhancer-binding protein (C/EBP), and activator protein-1 (AP-1), suggesting functional connections between tight junctions and various signaling pathways.

Claudins and Various Signaling Pathways

Previous studies have shown that claudin expression and function are regulated by oncogenes and tumor-promoting growth factors. For example, in Ha-Ras-transformed MDCK cells, claudin-1, occludin, and ZO-1 are lost at cell-cell contacts but reassemble at the cell membrane upon blocking the MAPK pathway, accompanied by tight junction formation. Claudin-1 is also identified as a potential target of β-catenin/Tcf signaling, supporting a possible role of claudin-1 in carcinogenesis (e.g., colorectal cancer). Downregulation of claudin-1 in salivary gland epithelial cells transfected with the oncogene raf-1 leads to tight junction disruption. This downregulation may be mediated by Rho signaling pathways and PTEN/Akt pathways, resulting in the downregulation of claudin-3. Overall, claudins impact various cellular biological functions and tumorigenesis through interactions with signaling pathways.

Changes in Claudin's Expression and Tumors

Research indicates that claudin expression changes significantly in many cancer types. For example, upregulation of CLDN1 is associated with the progression of basal cell carcinoma (BCC); upregulation of CLDN3 and CLDN4 is linked with the invasiveness and metastasis of ovarian cancer. Upregulation of CLDN1 in breast cancer is associated with poor prognosis. Conversely, downregulation of CLDN7 in pancreatic cancer correlates with poor prognosis, suggesting that changes in claudin expression are closely related to tumor onset, development, and prognosis. The expression changes of claudins in various cancer types may be related to their role in epithelial-mesenchymal transition (EMT).

Potential Biomarker Role of Claudins

Given the correlation between claudin expression patterns and tumor progression and prognosis, there is growing interest in their potential as cancer biomarkers. For instance, the combined detection of CLDN3 and CLDN4 shows promise for early screening and prognosis assessment in ovarian cancer. CLDN1 expression levels may predict prognosis in breast cancer patients and guide personalized treatment. Comprehensive analysis of claudin expression profiles may provide new biomarkers and therapeutic targets for early cancer diagnosis, treatment monitoring, and prognosis evaluation.

The Claudin family, as key components of tight junctions, plays an essential role in maintaining cell-cell barriers and participating in intracellular signaling. Changes in claudin expression are closely related to the disruption or abnormal regulation of tight junctions during carcinogenesis. Understanding the mechanisms and expression patterns of claudins in cancer will aid in developing new diagnostic tools and therapeutic strategies, ultimately improving patient prognosis and quality of life.