This study presents novel findings on the potential of phloretin, an apple polyphenol, to enhance the effectiveness of anti-HER2 antibody therapy in HER2-positive breast cancer patients. Our research reveals that phloretin inhibits type II glucose transporter (GLUT2) activity, significantly reducing cancer cell glucose uptake. We confirmed the overexpression of GLUT1 and GLUT2 mRNA in paired human breast tumor tissues, with GLUT2 overexpression associated explicitly with poorer survival rates in breast cancer patients. Treatment with phloretin was observed to increase the interaction between GLUT2 and HER2 proteins, attenuate glycolysis, and enhance the binding affinity of anti-HER2 antibody drugs to target human breast cancer cells. Furthermore, the efficacy of the combination therapy involving phloretin and antibody drugs was reaffirmed in a cell-derived xenograft (CDX) tumor animal model, particularly in suppressing the growth of Trastuzumab-resistant HER2+ breast cancer. These significant findings suggest that targeting GLUT2 activity with phloretin in combination with anti-HER2 antibody drugs may help mitigate the development of drug-resistant breast cancer, offering valuable insights for enhancing tumor treatment strategies and contributing to developing more effective therapies.

Trastuzumab resistance is one of the causes of poor prognosis in patients with HER2-positive (HER2+) breast cancer (BC). The truncated isoform of dopamine- and cAMP-regulated phosphoprotein (t-DARPP) has been reported to be involved in Trastuzumab therapy resistance and promoting tumor progression. To evaluate the t-DARPP expression in BC, paired tumors and surrounding normal tissues were analyzed by real-time PCR and confirmed higher DARPP-32 family mRNA expression in HER2+ BC tumor tissues. We established two patient-derived xenografts (PDX) mice models to test the efficacy of Trastuzumab, named model 1 (non-responder) and model 2 (responder). t-DARPP and p95-HER2 protein-protein interactions were detected in PDX tumor tissue from non-responders using Förster resonance energy transfer assays. Instead, there is no response from the responder. Furthermore, mechanistic studies using transwell and western blot assays demonstrated that t-DARPP could upregulate epithelial-mesenchymal transition signaling proteins, enhance p95-HER2 expression and promote cell migration. We found that Quercetin effectively reduced t-DARPP expression in HER2+ BC cells. In t-DARPP ShRNA-suppressed cells, Quercetin synergistically enhanced Trastuzumab-induced apoptotic cell death and G2/M phase arrest. In conclusion, the combination of Quercetin and Trastuzumab treatment by targeting t-DARPP in HER2+ BC patients has the potential as a biomarker for mitigating drug resistance.