Increased mucosal expression of TF, the Thomsen-Friedenreich oncofetal
blood group antigen (galactose beta1-3 N-acetylgalactosamine alpha-) occurs in
colon cancer and
colitis. This allows binding of TF-specific
lectins, such as
peanut agglutinin (PNA), which is mitogenic to the colorectal epithelium. To identify the cell surface TF-expressing
glycoprotein(s), HT29 and Caco2
colon cancer cells were surface-labeled with Na[(125)I] and subjected to PNA-
agarose affinity purification and electrophoresis.
Proteins, approximately 110-180 kDa, present in HT29 but not Caco2 were identified by Western blotting as high molecular weight splice variants of CD44 (CD44v). Selective removal of
TF antigen by Streptococcus pneumoniae
endo-alpha-N-acetylgalactosaminidase substantially reduced PNA binding to CD44v. Immunoprecipitated CD44v from HT29 cell extracts also expressed sialyl-Tn (sialyl 2-6 N-acetylgalactosaminealpha-). Incubation of PNA 15 microg/ml with HT29 cells caused no additional proliferative effect in the presence of anti-CD44v6 mAb. In
colon cancer tissue extracts (N = 3) PNA bound to CD44v but not to standard CD44. These data show that CD44v is a major PNA-binding
glycoprotein in
colon cancer cells. Because CD44 high molecular weight splice variants are present in
colon cancer and
inflammatory bowel disease tissue but are absent from normal mucosa, these results may also explain the increased PNA reactivity in
colon cancer and
inflammatory bowel disease. The coexpression of oncofetal
carbohydrate antigens TF and sialyl-Tn on CD44 splice variants provides a link between
cancer-associated changes in glycosylation and CD44 splicing, both of which correlate with increased metastatic potential.