The Monoclonal Antibody BQ16 Identifies the α6β4 Integrin on Bladder Cancer

Monoclonal antibody BQ16, raised against UM-UC-9, a human bladder cancer cell line, exhibited strong reactivity with most bladder carcinoma tissue samples and cell lines. In normal urothelium, BQ16 stained only the basal surface of urothelial cells at the junction with the lamina propria. BQ16 immunoprecipitated two protein bands of ̃140 and 180 kDa (under non-reducing conditions), while on Western blots, BQ16 identified only the 140 kDa protein indicating that BQ16 binds to one chain of a dimeric protein complex. The dimeric structure, molecular size, and basal orientation of the BQ16 antigen prompted a comparison with the α6β4 integrin identified by monoclonal antibody UM-A9. In most tissues BQ16 and UM-A9 produced identical staining patterns. However, normal lymphocytes and certain bladder cancer cell lines were BQ16 positive but failed to react with UM-A9, indicating that the BQ16 and UM-A9 epitopes can be expressed independently. Pulse-chase immunoprecipitation experiments showed that the α6 subunit was more prominent in early BQ16 precipitates and the β4 subunit was more prominent in early UM-A9 precipitates. Furthermore, preclearing cell extracts with the anti-α6 antibody GoH3 removed all BQ16 reactivity and m UM-A9-negative, BQ16-positive cells, BQ16 precipitated the α6β1 complex. We conclude that BQ16 identifies the α6 integrin subunit and that α6β4 integrin is strongly expressed in most bladder cancers.