CD44 is a transmembrane glycoprotein and also major hyaluronan receptor, mediating cell responses to the extracellular microenvironment. In 1980s, it was first described as a surface molecule restricted to few immune cells like lymphocytes, thymocytes, and granulocytes (Dalchau et al. 1980), but later it was recognized as a novel human erythrocyte cell surface antigen, lymphocyte homing receptor, and leukocyte surface glycoprotein (Stefanová et al. 1989). Today, CD44 is being described as an adhesion molecule expressed in various cell types which includes both epithelial cells (even in keratinocytes) and connective tissue cells (endothelial cells, fibroblasts and leukocytes). It participates in a number of signaling pathway mediating proliferation, migration, hematopoiesis, lymphocyte activation and lymphnode homing (Jordan et al. 2015).
Gene Structure and Its Transcription
Structure and Its Distribution
CD44 protein consists of three functional domains, including an extracellular domain (or ectodomain), a transmembrane (TM) domain, and an intracellular domain (ICD). The extracellular domain contains a highly conserved amino-terminal globular protein domain (~180 aa) that functions as ligand-binding receptor primarily for hyaluronic acid (HA), a negatively charged, nonsulfated glycosaminoglycan. This also contains six cysteine residues which are thought to form three disulfide bonds and sites for five conserved N-glycosylation located in the amino-terminal 120 aa of CD44. The nonconserved region of extracellular domain is mainly concerned with carbohydrate modification (O-glycosylation) and alternative splicing which allows for the insertion of extra aa sequence from variable exons of the CD44 gene. The transmembrane domain, which is 100% conserved in several mammals, is thought to be involved in posttranscriptional modification like palmitoylation as it contains one of two cysteine residue substrate Cys286 and helpful in mediating HA binding. The cytoplasmic tail (72 aa) serves diverse functions like site for palmitoylation (Cys295), phosphorylation, and binding to cytoskeletal proteins (Lesley and Hyman 1998).
CD44 in Embryogenesis
During embryogenesis, CD44 plays a general role through interaction. At early stages, the embryo contains large amount of HA as a pericellular coat, and thus increase in intercellular spaces prevents cell cohesion. During limb bud formation and reduction of HA concentration occurs via various mechanisms, i.e, its decreased synthesis, increased receptor-mediated degradation, and HA-mediated cross-bridging of mesonchymal cells. All the mechanisms are controlled by CD44. The cell surface CD44 expression mediates contrast mechanisms like both binding and degradation of HA, thus playing a pivotal role in embryogenesis.
CD44 in Adult Tissue
Since its discovery, it has been found that a wide range of tissues both epithelial and mesenchymal cells showed CD44 expression whereas its isoforms like CD44v appear to have a much more restricted distribution as it is expressed only in a few cells (keratinocytes, activated lymphocytes, macrophages, and some epithelial cells of bladder, stomach, and uterine cervix) suggesting that the process of alternative splicing is normally tightly regulated. In keratinocytes, varied and controversial expression of CD44 has been reported. Keratinocytes express CD44s and at least five isoforms containing variable exons including v2–v10, v3–v10, v4–v10, v6–v10, and v8–v10, among which the isoform v3–v10 is predominant in normal human keratinocytes. Intraepithelial variation in CD44 expression also is observed as it is reported that CD44v6 expression was limited to the stratum basale and the stratum spinosum of normal uterine cervical squamous epithelium (Sneath and Mangham 1998).
Cell-Cell and Cell-Extracellular Interaction
Lymphnode Homing, Lymphocyte Maturation and Activation
Lymphnode homing , an another important function of CD44, is mainly orchestrated by its middle domain and a protein called mucosal addressin mainly present in endothelial cells of Peyer’s patches and lymphnodes. Whereas CD44v6 plays a crucial role in activation of B and T lymphocytes and maturation of stem cell progenitors by its cross-linking and adhesion molecules. Though the exact mechanism is not clearly elucidated, some authors also suggested that the costimulatory signal of CD44 is largely dependent on signaling through CD2. Transient expression of variants CD44v3 to CD44v6 in early event on the blast cells of lymphnodes and the spleen after activation of lymphocytes support contributory role of CD44 in immune-related function.
Contribution of CD44 in hemoposis mainly concentrated around myelopoiesis and lymphopoiesis as its isoform CD44v6 is required for maturation of the stem cell population. CD44v6 behaves as a transducer of signal exchange between stromal cells and stem cells, and thereby initiates differentiation. Upon ligand interaction, CD44s is thought to provide proliferation signals for early progenitors of hematopoietic lineages.
Hyaluronic Acid Degradation
Hyaluronate degradation is an important functional role of CD44 as this mechanism is essential for some tissue-specific function like in alveoli of lungs (surface clearance for gas exchange), cell migration, and embryological development events. Hyaluronate degradation occurs in three steps: first, it binds the cell surface to form hyaluronate-CD44 complex; second, endocytosis of the complex; and third, degradation by acid hydrolases.
CD44 in Tumor Biology
CD44-mediated signaling has been much explored in context with tumorigenesis and can be understood by three primary domains like (1) hyluronate-independent signaling (interactions between intracellular domain of CD44 and cytoskeletal proteins or membrane-associated kinases), (2) HA-dependent signaling, and (3) its diverse role in cancer stem cell.
Both tumor promoting and tumor inhibiting role of CD44 can be appreciated in observations of experimental studies of benign and malignant tumors by its ligation through various cofactors. By binding with LRP6 (low-density lipoprotein receptor-related protein 6) with its extracellular domain, CD44 can trigger Wnt-βcatenin-mediated cell proliferation pathway and also triggers the interaction of cytoplasmic domain with ERM protein to modulate the cytoskeletal rearrangement and hence the cell motility. Furthermore, through interaction with hepatocyte growth factor (HGF), CD44 has been observed to enhance cell proliferation and migration via c-Met activation-mediated MAPK-ERK pathway. In contrast, in association with PP2A (a phosphatase known to dephosphorylate Raf, MEK, and Akt) CD44 favors intrinsic apoptosis by decreasing ERK phosphorylation.
Role in Cancer Stem Cell
Multivalent role of CD44 can be attributed to its complex structure, varied isoforms, ligand-binding capacity, extensive posttranslational modifications, and ability interaction with various signaling molecules. Beyond its vital physiological roles, cumulative evidences support its contrast role in tumorigenesis. In addition, CD44 also gained much attention as an established cancer stem cell marker. Hence its multipotency could be utilized not only in the exploration of etiopathogenesis but also in interventional targeted therapy of various human cancers.
- Sneath RJS, Mangham DC. The normal structure and function of CD44 and its role in neoplasia. J Clin Pathol: Mol Pathol. 1998;51:191–200.Google Scholar