First printed in R&D Systems' 1997 Catalog.
Many of the activities of leukocytes are dependent on adhesive events and the molecules involved include integrins, immunoglobulin superfamily members, selectins and the mucin-like molecules.1, 2 Another molecule apparently involved in white cell adhesive events is CD44, a molecule ubiquitously expressed on both hematopoietic and non-hematopoietic cells. CD44 is remarkable for its ability to generate alternatively spliced forms, many of which differ in their activities.3 This remarkable flexibility has led to speculation that CD44, via its changing nature, plays a role in some of the methods that tumor cells use to progress successfully through growth and metastasis.
The metastatic process has been described as a series of steps: 1) developing tumors proliferate, migrate and disperse from their primary focus; 2) disseminated tumor cells move from their site of formation into either the lymphatics or the vasculature; 3) metastasizing tumor cells survive in the circulation by interacting with platelets and leukocytes; 4) at some critical point in time or location, tumor cells stop their circulation or migration via interactions with vascular or lymphatic endothelium and/or endothelial basement membrane; 5) from these points of adhesion, tumor cells migrate out of lymphatic or blood circulation; 6) under favorable conditions, tumors again return to proliferation.4, 5 This activity is reminiscent of the requirements of white blood cells during their passage through, and extravasation from, both the blood and lymphatic circulations. Thus it has been proposed that successful metastasis requires both an oncogenic transformation and an acquisition of typical leukocyte properties or behaviors.6
|Figure 1. Mouse CD44 gene organization. Sherman, L. et al. (1996) Ciba Found. Symp. 213:249; Yu, Q. & B.P. Toole (1996) J. Biol. Chem. 271:20603; Screaton, G.R. et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89:12160; Screaton, G.R. et al. (1993) J. Biol. Chem. 268:12235; Gunthert, V. et al. (1991) Cell 65:13; Zhou, D.F.H. (1989) J. Immunol. 143:3390.|
CD44 is a 80-250 kDa type I (extracellular N-terminus) transmembrane glycoprotein.4 The gene for CD44 has been identified in mouse, human and rat.7-9 In mice the CD44 gene is known to be composed of 21 exons,10 only 19 or 20 exons have been identified in the human.8, 11 Based on the mouse gene structure of 21 exons, the extracellular domain is coded for by the first 18 exons while the transmembrane segment is assigned to the 19th exon and the cytoplasmic tail to the 20th or 21st exon.8, 10 In the common form of human CD44, called CD44H (hematopoietic), exons 2-5 plus 16 and 17 code for the extracellular domain, while exon 18 codes for the transmembrane segment (plus 3 amino acid residues of the cytoplasmic region), and either exon 19 or 20 codes for the cytoplasmic tail.7, 9 For CD44H, this translates into an 80 kDa molecule with an extracellular region of 248 amino acid (aa) residues, a transmembrane segment of 21 aa residues and a cytoplasmic tail (long form) of 72 aa residues.12, 13 At least 45 alternatively spliced variants also exist.14 These alternate isoforms are created by varying combinations of exons 6 through 16 (based on the mouse gene nomenclature), exons also referred to as v1 through v11.10, 11 Some of the better known isoforms are CD44M, which incorporates exons v4, v5, v6 and v7 into the extracellular segment,15, 16 and CD44E, which incorporates alternate exons v8, v9 and v10 into the extracellular segment.16, 17 Cells known to express CD44H include T cells, B cells, monocytes, fibroblasts and keratinocytes,18, 19 vascular endothelial cells,20 columnar epithelium of the GI tract and urinary tract transitional epithelium,21 NK cells,22 granulocytes,23, 24 macrophages and type II pneumocytes,25 osteoclasts, osteocytes and chrondroclasts,26 chrondrocytes27 and neutrophils.28 Although variant forms of CD44 are usually associated with abnormal cell populations, variant exons are also expressed in normal cells. CD44v6 is temporarily expressed by T cells during activation,29 v5 and v6 are expressed together in simple columnar epithelium of the stomach,30 v6 and v7 are expressed on activated B cells31 and keratinocytes plus transitional epithelium both exhibit v3, v4/5, v6 and v8/9.32
Finally, soluble forms of CD44 have been detected in both human synovial fluid and serum.33, 34 Soluble CD44 has also been detected in mouse serum.35 It appears that both proteolytic processing35 and alternative splicing10 can generate soluble forms.
There are multiple ligands for CD44, including osteopontin,36 fibronectin,37 collagen types I and IV23, 37 and hyaluronate.38 Binding to fibronectin is reported to be limited to CD44 variants expressing chrondroitin sulfate,37 with the chrondroitin sulfate attachment site localized to exons v8-v11.4, 10 Hyaluronate binding is suggested to be possible for virtually all CD44 isoforms.17, 39 One of the principal binding sites is proposed to be centered in exon 2 and to involve lysine and arginine residues.9, 40, 41 Factors other than the simple expression of a known hyaluronate-binding motif also appear to be necessary for hyaluronate binding. Successful hyaluronate binding is facilitated by the combination of exons expressed, a distinctive cytoplasmic tail, glycosylation patterns, and the activity state of the cell.42-47 Thus, in terms of its hyaluronate-binding function, a great deal of "potential" flexibility exists within each CD44-expressing cell.
Interest in CD44 has centered on its potential ability to impart white cell activities to dysplastic cells. The varying degree to which CD44 isoforms bind ECM suggest a possibility for fine-tuning of adhesive activities. CD44 activities are not limited to ECM-interactions. Like CD31/PECAM-1, CD44v8-10 is reported to engage in homotypic binding,48 an activity that contributes to leukocyte extravasation in the CD31 system. In addition, CD44, via its glycosaminoglycan chains, is proposed to present select cytokines to neighboring cells.49 Such presentation could influence adhesion molecule expression and/or activation of both vascular endothelial cells and tumor cells. In the six step process for metastasis formation, adhesive events are followed by dissociative events, etc. While CD44 is not the only cell-surface molecule involved in adhesion, it seems reasonable to assume that it may make a contribution.3, 5, 6
Although it is tempting to speculate that CD44 plays a role in metastasis formation, clinical studies are somewhat equivocal in their conclusions.50 On one hand, CD44v8-10 is reported to confer metastasizing capability on gastric carcinomas;51 CD44v6 is increasingly expressed during colorectal cancer progression;52 and the appearance of v5 and v6 reportedly indicates the presence of pancreatic adenocarcinoma cells.53 On the other hand, while CD44v6 is preferentially expressed on pancreatic endocrine tumors, there is no apparent correlation between the presence of CD44 and metastatic behavior.54 Also, CD44v6 plus v9 expression seems only to be a marker for cell differentiation rather than tumor progression.55 The issue is further complicated by reports that variant CD44 isotypes are lost during the development of oral squamous cell carcinoma.56 Finally, early upregulation of CD44 variants in bladder carcinoma is followed by a marked downregulation of CD44 proteins as the tumor acquires an aggressive phenotype.57 In all, there does not appear to be a simple correlation between the expression of CD44 isotypes and tumors.