Integrin α3β1 expressed by human colon cancer cells is a major carrier of oncodevelopmental carbohydrate epitopes

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. (Laferte S et al. [1995] 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, 7811, 8C7, 8H7, and 11D4) immunoprecipitated a 3A7-immunoreactive glycoprotein complex of 140 kDa which was subsequently identified by partial protein sequencing as α3β1 integrin. Flow cytometric analysis of Chinese hamster ovary (CHO) cells expressing different human integrin chains revealed that MAbs 7A8 and 7B11 detect the α3 integrin subunit whereas MAbs 8C7 and 8H7 detect the β1 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 α3 integrin subunit, suggesting that α3β1 integrin is a preferred target of the 3A7 epitope in colon cancer cells. Using lectins and glycosidases to examine further the carbohydrate structure of α3β1 integrin, we demonstrated that it is a sialoglycoprotein containing both N- and O-linked oligosaccharides. In addition, both α3 and β1 integrin subunits express α1-6 branched Asn- linked oligosaccharides and short poly-N-acetyllactosamine units (Galβ1- 4GlcNAc-R; n ≤ 3), glycans previously implicated in cancer metastasis. Thus, α3β1 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.