Figure 7.

Cross-resistance of T-DM1-resistant cells to doxorubicin and paclitaxel. (A, B) Cell growth profiles of JIMT1 parental or T-DM1R cells treated with either 50 nM doxorubicin or left untreated. T-DM1R cells were cultured in the media containing 4 μg/ml of T-DM1. (C, D) Cell growth profiles of JIMT1 parental or T-DM1R cells treated with either 5 nM paclitaxel or left untreated. T-DM1R cells were cultured in the media containing 4 μg/ml of T-DM1. (E and F) The levels of MDR1 were evaluated in the WCL of parental and T-DM1R cells by Western blot analysis using anti-MDR1 antibody. HeLa cells were used as a positive control. The experimental procedure for Figure 7F is essentially the same as described in Figure 7E, except that anti-MRP1 antibody was used to detect the protein levels of MRP1 in WCL. (G) Working model: Loss of HER2 confers T-DM1 resistance in HER2-positive breast cancer cells, which in turn upregulates EGFR via a compensatory mechanism. Increased EGFR activity changes the expression of the major RGD integrins, α5β1 and αVβ1, leading to the enhanced cell invasion compared with parental JIMT1 breast cancer cells. Importantly, when β1 integrin function is blocked by either specific siRNA or inhibitory monoclonal antibody, MAB 13, this leads to significant increase in cell invasion of T-DM1R cells.