Tumor-Associated Fibroblasts Promote HER2-Targeted Therapy Resistance through FGFR2 Activation

Purpose: Despite the therapeutic success of existing HER2-targeted therapies, tumors invariably relapse. This study aimed at identifying new mechanisms responsible for HER2-targeted therapy resistance. Experimental Design: We have used a platform of HER2-targeted therapy-resistant cell lines and primary cultures of healthy and tumor-associated fibroblasts (TAF) to identify new potential targets related to tumor escape from anti-HER2 therapies. Results: We have shown that TAFs promote resistance to HER2-targeted therapies. TAFs produce and secrete high levels of FGF5, which induces FGFR2 activation in the surrounding breast cancer cells. FGFR2 transactivates HER2 via c-Src, leading to resistance to HER2-targeted therapies. In vivo, coinoculating nonresistant cell lines with TAFs results in more aggressive and resistant tumors. Resistant cells activate fibroblasts and secrete FGFR ligands, creating a positive feedback loop that fuels resistance. FGFR2 inhibition not only inhibits HER2 activation, but also induces apoptosis in cells resistant to HER2-targeted therapies. In vivo, inhibitors of FGFR2 reverse resistance and resensitize resistant cells to HER2-targeted therapies. In HER2 patients' samples, alpha-SMA, FGF5, and FGFR2 contribute to poor outcome and correlate with c-Src activation. Importantly, expression of FGF5 and phospho-HER2 correlated with a reduced pathologic complete response rate in patients with HER2-positive breast cancer treated with neoadjuvant trastuzumab, which highlights the significant role of TAFs/FGF5 in HER2 breast cancer progression and resistance. Conclusions: We have identified the TAF/FGF5/FGFR2/c-Src/HER2 axis as an escape pathway responsible for HER2-targeted therapy resistance in breast cancer, which can be reversed by FGFR inhibitors.