Oncodevelopmental
carbohydrate epitopes are a common feature of human
colorectal carcinoma, yet their
biological significance remains unclear. We have shown previously that
monoclonal antibody (MAb) 3A7, which recognizes a determinant on type 2 chain
blood group A and B
oligosaccharides, detects oncodevelopmental changes in
azoxymethane-induced rat colon
tumors and some human
colon cancer cell lines. (Laferté S et al. [19951 J. Cell. Biochem. 57:101-119). In this study, we set out to purify
gp140, the major
glycoprotein carrier of the 3A7
epitope expressed by human
colon cancer cells, as a first step towards elucidating the contribution of the 3A7
epitope and its major
glycoprotein carrier to
colon cancer progression. To this end,
gp140 was purified from HT29 cells and used for the preparation of
polypeptide-specific
monoclonal antibodies. Five
monoclonal antibodies (7A8, 7B11, 8C7, 8H7, and 11D4) immunoprecipitated a 3A7-immunoreactive
glycoprotein complex of 140 kDa which was subsequently identified by partial
protein sequencing as
alpha3beta1 integrin. Flow cytometric analysis of Chinese hamster ovary (CHO) cells expressing different human
integrin chains revealed that MAbs 7A8 and 7B11 detect the
alpha3 integrin subunit whereas MAbs 8C7 and 8H7 detect the
beta1 integrin subunit. MAb 11D4, which did not bind to any of the CHO transfectants, detected type 2 chain
blood group A determinant. Flow cytometric analysis of a panel of human colon
carcinoma cell lines obtained from
blood group A, AB, or B individuals revealed a direct correlation between cell-surface expression of the 3A7
epitope and
alpha3 integrin subunit, suggesting that
alpha3beta1 integrin is a preferred target of the 3A7
epitope in
colon cancer cells. Using
lectins and
glycosidases to examine further the
carbohydrate structure of
alpha3beta1 integrin, we demonstrated that it is a
sialoglycoprotein containing both N- and O-linked
oligosaccharides. In addition, both alpha3 and
beta1 integrin subunits express beta1-6 branched Asn-linked
oligosaccharides and short
poly-N-acetyllactosamine units (Galbeta1-4GlcNAc-R; n < or = 3),
glycans previously implicated in
cancer metastasis.Thus,
alpha3beta1 integrin expressed by human colon
carcinoma cells is a major carrier of oncodevelopmental
carbohydrate epitopes whose presence may modulate
tumor cell adhesion, migration, and/or invasion.