TheCD44 antigen is acell-surfaceglycoprotein involved in cell–cell interactions, cell adhesion and migration. In humans, the CD44 antigen is encoded by theCD44gene on chromosome 11.[5] CD44 has been referred to as HCAM (homingcell adhesion molecule), Pgp-1 (phagocyticglycoprotein-1), Hermes antigen, lymphocyte homing receptor, ECM-III, and HUTCH-1.
CD44 is expressed in a large number of mammalian cell types. The standardisoform, designated CD44s, comprisingexons 1–5 and 16–20 is expressed in most cell types. CD44 splice variants containing variable exons are designated CD44v. Someepithelial cells also express a larger isoform (CD44E), which includes exons v8–10.[6]
CD44 is a receptor forhyaluronic acid[7] and internalizes metals bound to hyaluronic acid[8][9] and can also interact with otherligands, such asosteopontin,collagens, andmatrix metalloproteinases (MMPs). CD44 function is controlled by its posttranslational modifications. One critical modification involves discrete sialofucosylations rendering the selectin-binding glycoform of CD44 called HCELL (for Hematopoietic Cell E-selectin/L-selectin Ligand).[10] (see below)
Transcripts for thisgene undergo complexalternative splicing that results in many functionally distinctisoforms; however, the full length nature of some of these variants has not been determined. Alternative splicing is the basis for the structural and functional diversity of this protein, and may be related to tumor metastasis. Splice variants of CD44 on colon cancer cells display sialofucosylated HCELL glycoforms that serve as P-, L-, and E-selectin ligands and fibrin, but not fibrinogen, receptors under hemodynamic flow conditions pertinent to the process of cancer metastasis.[11][12]
HCELL (Hematopoietic Cell E-/L-selectin Ligand) is a specialized glycoform of the CD44 protein that functions as a criticaladhesion molecule inhematopoietic[13] and immune processes.[14] It is distinguished by unique post-translational modifications that enable potent binding toE-selectin andL-selectin, guiding cell migration and homing.[15]
The HCELL glycoform was originally discovered on human hematopoietic stem cells and leukemic blasts,[10][16][17][18] and was subsequently identified on cancer cells.[12][19][20][21][22] HCELL functions as a "bonehoming receptor", directing migration of human hematopoietic stem cells andmesenchymal stem cells to bone marrow.[17] Ex vivo glycan engineering of the surface of live cells has been used to enforce HCELL expression on any cell that expresses CD44.[23] CD44 glycosylation also directly controls its binding capacity to fibrin and immobilized fibrinogen.[11][24]
CD44, along withCD25, is used to track earlyT cell development in thethymus.
CD44 expression is an indicative marker for effector-memory T-cells. Memory cell proliferation (activation) can also be assayedin vitro withCFSE chemical tagging.
CD44 is a biomarker ofcancer stem cells,[25] and these cells can potentially be targeted with novel therapies.[26]
In addition, variations in CD44 are reported as cell surface markers for some breast and prostatecancer stem cells. In breast cancer research CD44+/CD24- expression is commonly used as a marker for breast CSCs and is used to sort breast cancer cells into a population enriched in cells with stem-like characteristics[27] and has been seen as an indicator of increased survival time inepithelialovarian cancer patients.[28]
Endometrial cells in women withendometriosis demonstrate increased expression of splice variants of CD44, and increased adherence to peritoneal cells.[29]
High levels of the adhesion molecule CD44 on leukemic cells are essential to generate leukemia.[32] Furthermore, becausealternative splicing andposttranslational modifications generate many different CD44 sequences, including, perhaps, tumor-specific sequences, the production of anti-CD44 tumor-specific agents may be a realistic therapeutic approach.[33]
In many cancers (renal cancer andnon-Hodgkin's lymphomas are exceptions), a high level of CD44 expression is not always associated with an unfavorable outcome.[34] On the contrary, in some neoplasms CD44upregulation is associated with a favorable outcome. This is true ofprostate cancer, where thetranscript variant CD44v5 (includes the fifth 'v5' exon) is associated with better prognosis (increased time to recurrence following surgery).[35][34] In prostate cancer, the exclusion of the v5 exon through alternative splicing was associated with the presence of RNA binding proteinKHDRBS1 and became included in the presence of increasedYTHDC1 ormetadherin expression.[35]
In other cases different research groups analyzing the same neoplastic disease reached contradictory conclusions regarding the correlation between CD44 expression and disease prognosis, possibly due to differences in methodology. These problems must be resolved before applying anti-CD44 therapy to human cancers.[34]
^abcAlves CS, Burdick MM, Thomas SN, Pawar P, Konstantopoulos K (April 2008). "The dual role of CD44 as a functional P-selectin ligand and fibrin receptor in colon carcinoma cell adhesion".American Journal of Physiology. Cell Physiology.294 (4):C907 –C916.doi:10.1152/ajpcell.00463.2007.PMID18234849.S2CID31373618.
^Sackstein R, Dimitroff CJ (October 2000). "A hematopoietic cell L-selectin ligand that is distinct from PSGL-1 and displays N-glycan-dependent binding activity".Blood.96 (8):2765–2774.doi:10.1182/blood.V96.8.2765.PMID11023510.
^abSackstein R, Merzaban JS, Cain DW, Dagia NM, Spencer JA, Lin CP, et al. (February 2008). "Ex vivo glycan engineering of CD44 programs human multipotent mesenchymal stromal cell trafficking to bone".Nature Medicine.14 (2):181–187.doi:10.1038/nm1703.PMID18193058.S2CID5095656.
^Assimakopoulos D, Kolettas E, Patrikakos G, Evangelou A (October 2002). "The role of CD44 in the development and prognosis of head and neck squamous cell carcinomas".Histology and Histopathology.17 (4):1269–1281.doi:10.14670/HH-17.1269.PMID12371152.
^Eibl RH, Pietsch T, Moll J, Skroch-Angel P, Heider KH, von Ammon K, et al. (December 1995). "Expression of variant CD44 epitopes in human astrocytic brain tumors".Journal of Neuro-Oncology.26 (3):165–170.doi:10.1007/bf01052619.PMID8750182.S2CID21251441.
Yasuda M, Nakano K, Yasumoto K, Tanaka Y (2003). "CD44: functional relevance to inflammation and malignancy".Histology and Histopathology.17 (3):945–950.doi:10.14670/HH-17.945.PMID12168806.
Sun CX, Robb VA, Gutmann DH (November 2002). "Protein 4.1 tumor suppressors: getting a FERM grip on growth regulation".Journal of Cell Science.115 (Pt 21):3991–4000.doi:10.1242/jcs.00094.PMID12356905.S2CID14243743.
Ponta H, Sherman L, Herrlich PA (January 2003). "CD44: from adhesion molecules to signalling regulators".Nature Reviews. Molecular Cell Biology.4 (1):33–45.doi:10.1038/nrm1004.PMID12511867.S2CID19495756.
Martin TA, Harrison G, Mansel RE, Jiang WG (May 2003). "The role of the CD44/ezrin complex in cancer metastasis".Critical Reviews in Oncology/Hematology.46 (2):165–186.doi:10.1016/S1040-8428(02)00172-5.PMID12711360.
