ERBB2 and breast cancer: Moreover, exposure of HER2+ (AU565, BT474, and MCF7-HER2), but not HER2-negative [MCF7, ZR75-1 and MCF7-empty vector (MCF7-EV)], breast cancer cell lines to the anti-HER2 mAb trastuzumab for 72 h induced parallel downregulation of HER2 and upregulation of HSP90 proteins, respectively, as assessed by WB and immunofluorescence (Figure 1C,D).