Contribution of ₄ and α₅integrin subunits to FN adhesion. Drug-sensitive (8226/S), melphalan-resistant (8226/LR5), and doxorubicin-resistant (8226/DOX6) were adhered to FN-coated wells (horizontal striped bars) for 1 hour. In order to determine percentage binding due to ₄ and ₅, some cells were pre-incubated with ₄function blocking Ab P4G9 (hatched bars) or ₅ function blocking Ab P1D6 (vertical striped bars) for 15 minutes before application to wells. FN adhesion by 8226/S was found to be mediated equally by ₄ and ₅, while FN adhesion for both drug-resistant cell lines was mediated only by ₄(P < .05), as determined by complete inhibition of adherence using the ₄ blocking Ab. Total FN adhesion mediated by ₄ was also higher in drug-resistant lines compared with drug-sensitive 8226/S (P < .05). Values shown are the percentage of total cells applied to each well corrected for nonspecific adhesion to BSA-coated wells. Bars are the SD of n = 6 from three different experiments.