Gemcitabine treatment enhances HER2 expression in low HER2-expressing breast cancer cells and enhances the antitumor effects of trastuzumab emtansine.

Trastuzumab emtansine (T-DM1), trastuzumab-conjugated with a cytotoxic agent, has shown promising antitumor effects in breast cancer. Since a good therapeutic response using T-DM1 treatment requires high human epidermal growth factor receptor 2 (HER2) expression, breast cancers with low or no HER2 expression have not been used for T-DM1 treatment. The aim of the present study was to show that treatment of low HER2-expressing breast cancer cells with gemcitabine (GEM) enhanced HER2 expression using RT-qPCR, immunoblot and flow cytometric analysis. The results showed that GEM treatment significantly enhanced HER2 expression in MDA-MB-231, MCF7 and BT-20 breast cancer cells, while paclitaxel (PTX) treatment induced lower or no enhancement in HER2 expression. The expression of HER2 mRNA was also enhanced in GEM-treated MCF7 cells. Treatment with an inhibitor for nuclear factor-(NF)-κB suppressed GEM-induced HER2 upregulation, indicating that NF-κB activation by GEM may be associated with HER2 upregulation. T-DM1 binding to HER2 on MCF-7 cells was enhanced by GEM pretreatment and the combined treatment of GEM and T-DM1 synergistically inhibited the proliferation of MCF7 cells. Thus, the combined treatment with GEM and T-DM1 may be a promising therapeutic modality for low HER2-expressing breast cancers, which was facilitated by the unique HER2-upregulating effect of GEM.