Proteolytic Cleavage of the CD44 Adhesion Molecule in Multiple Human Tumors

Abstract

Cell surface adhesion molecules are crucial for the development and/or pathogenesis of various diseases including cancer. CD44 has received much interest as a major adhesion molecule that is involved in tumor progression. We have previously demonstrated that the ectodomain of CD44 undergoes proteolytic cleavage by membrane-associated metalloproteases in various tumor cell lines. The remaining membranebound CD44 cleavage product can be detected using antibodies against the cytoplasmic domain of CD44 (anti-CD44cyto antibody). However, the cleavage of CD44 in primary human tumors has not been investigated. Using Western blots with anti-CD44cyto antibody to assay human tumor tissues, we show that the CD44 cleavage product can be detected in 58% (42 of 72) of gliomas but not in normal brain. Enhanced CD44 cleavage was also found in 67% (28 of 42) of breast carcinomas, 45% (5 of 11) of non-small cell lung carcinomas, 90% (9 of 10) of colon carcinomas, and 25% (3 of 12) of ovarian carcinomas. Tumors expressing a CD44 splice variant showed a significantly higher incidence of enhanced CD44 cleavage. CD44 is a widely distributed cell-surface adhesion molecule that is implicated in a diverse range of physiological and pathological processes, including lymphocyte homing and activation, cell-matrix interactions, cell migration, and the regulation of tumor growth and metastasis.1 The gene encoding the CD44 protein contains 20 exons of which up to 10 variant exons encoding a portion of the ectodomain are alternatively spliced in various combinations, thereby generating numerous CD44 splice variant isoforms (CD44v).1,2 The standard CD44 (CD44s) lacks all variant exons. All forms of CD44 are heavily glycosylated to varying degrees. The diversity of CD44 functions is compounded by its variable structure.3-5 The expression of CD44 or its variants has been shown to be associated with tumor progression; however, the data concerning CD44v forms is controversial for some tumors.1,6 -13 Thus, the exact role of CD44 in the progression of human tumors remains obscure and increased interest has been directed at elucidating the possible mechanisms by which CD44 plays a role in human tumors.The extracellular domain of a number of membrane proteins can be proteolytically cleaved on the extracytoplasmic side.14 The proteolytic cleavage of membrane proteins has recently emerged as a key mechanism underlying their functional regulation. 15 We have previously demonstrated a proteolysis-based model as one mechanism involved in the regulation of CD44 function. 16 -18 Our studies showed that CD44 is proteolytically cleaved at the ectodomain through membrane-associated metalloproteases in various cancer cell lines to produce a membrane-bound cleavage product of ϳ25 kd.16 This CD44 ectodomain cleavage was found to play a critical role in CD44-mediated tumor cell migration by regulating the dynamic interaction between CD44 and the extracel-

CD44 as a stem cell marker in head and neck squamous cell carcinoma

“…this indicates that cD44 proteins can serve as 'platforms' for enzymes such as mmps, which are required for their effective functions. the inhibition of the reactions catalyzed by these mmps can block tumor cell migration (25,26). As tumor cells get in contact with the ecm via several cell surface adhesion molecules such as cD44, integrins and cadherins it is likely that these proteins are also needed for metastatic processes (27).…”

CD44 Proteolysis Increases CREB Phosphorylation and Sustains Proliferation of Thyroid Cancer Cells

“…5,6). Shedding of ecto-CD44 regulates cellextracellular matrix interaction (8), whereas CD44-ICD translocates to the nucleus and activates gene transcription (7).…”

Prostate cancer invasion is influenced more by expression of a CD44 isoform including variant 9 than by Muc18

“…Cleavage mediated by metalloproteinases occurs in the membraneproximal extracellular domain. Okamoto et al [6][7][8] have recently shown that, in several human tumors (exclusive of prostate cancer), cleavage products of 25-30 kDa are detectable by Western blot using antibody against the cytoplasmic portion of CD44. The soluble portion of CD44 has been detected in serum as a 100-160-kDa fragment using anti-CD44v monoclonal antibodies to extracellular portions of the molecule.…”

“…15,16 Inclusion of single or contiguous variant exons has been described by RT-PCR and sequencing in many benign and cancer tissues. 5,6,[14][15][16][17][18][19][20][21][22][23] We, 11 and subsequently others, 12,13 showed that PC loses immunohistochemical expression of CD44s and some CD44v isoforms. CD44v6 was absent by Figure 1 Splice variant CD44 exons are situated in the extracellular domain.…”

“…This form is detectable by competitive RT-PCR in bladder cancer tissue and cytologic specimens. 5,6,21,22 Clear cell renal cell carcinoma expresses this form in a minority of cases: 4/15 grade 1 and 2/15 grade 3. 19 Expression of this isoform has prognostic value in colon cancer.…”