The A9 antigen associated with aggressive human squamous carcinoma is structurally and functionally similar to the newly defined integrin alpha 6 beta 4.
We previously reported that altered expression of the A9 antigen (defined by monoclonal antibody UM-A9) is a predictive marker of early recurrence and progression of squamous cell carcinoma (SCC). In normal squamous cells A9 expression is limited to the site of contact with the basement membrane in vivo and the culture surface in vitro, whereas aggressive SCCs exhibit loss of polarity and increased intensity of A9 expression. The potential relationship of the A9 antigen to structures known to be involved in cell adhesion was analyzed by immunobiochemical and cell adhesion assays. UM-A9 precipitates a complex of protein chains reminiscent of the alpha and beta heterodimer glycoproteins that characterize the integrin family of extracellular matrix receptors. Proteins were isolated from A9-positive cells using UM-A9 and well-defined antibodies specific for integrin alpha and beta chains. UM-A9, anti-alpha 6, and anti-beta 4 monoclonal antibodies (mAbs) all precipitated proteins with comparable electrophoretic mobilities. Furthermore, UM-A9 mAb precleared the SCC alpha 6 beta 4 integrin complex isolated with anti-alpha 6 or anti-beta 4 mAbs but not that isolated by anti-beta 1 mAb. The isoelectric points of the A9 complex chains were consistent with those reported for alpha 6 and beta 4. Three of the polypeptide chains (140, 175, and 205 kDa) precipitated by UM-A9 showed peptide homology to one another and to the beta 4 chain precipitated by mAb 439-9B. The A9/alpha 6 subunit is composed of 125- and 30-kDa chains and was distinguished from beta 4 and beta 1 chains by its peptide map and isoelectric point. UM-A9 binds to an epitope common to the beta 4 subunits since in pulse-chase analysis the beta 4 species are precipitated at an early time point, whereas detection of alpha-subunit synthesis is detected during assembly of the mature complex. Immunoprecipitation and preclearing experiments demonstrated that in SCC the alpha 6 subunit is associated primarily with the beta 4 species and not with the 130-kDa beta 1 subunit. In cell adhesion assays on extracellular matrix proteins, the alpha 6-specific GoH3 mAb inhibited binding of SCC to laminin, suggesting that alpha 6 beta 4 may function as a laminin receptor in SCC. These data and our prior observations showing an association between altered A9 expression and early recurrence in SCC provide the first evidence that altered expression of alpha 6 beta 4 integrin is associated with the clinical behavior of human squamous cell carcinomas.