CB2R downregulation drives trastuzumab resistance by reorganizing HER2 dimers to evade IFN-? and enhance EGFR signaling

Despite the remarkable advances that have been implemented in HER2+ breast cancer treatment, especially upon the development of monoclonal antibodies like trastuzumab, resistance to HER2-targeted therapies remains a significant clinical challenge. Here, we investigated the role of the endocannabinoid system, specifically the cannabinoid CB2 receptor (CB2R), in trastuzumab resistance mechanisms. Analysis of human breast cancer samples revealed that a decreased expression of HER2-CB2R heterodimers following neoadjuvant treatment, primarily due to CB2R downregulation, is linked to poor long-term patient outcomes. Using various cellular and animal models, we demonstrate that trastuzumab resistance is dependent on CB2R expression, as it occurs upon CB2R downregulation and is rescued by CB2R re-expression. Mechanistically, CB2R loss enables cancer cells to evade antitumor IFN-? signaling while promoting a shift from HER2-CB2R to HER2-EGFR heterodimers, thus reducing dependence on HER2 signaling and increasing reliance on EGFR-mediated pathways. Moreover, EGFR inhibition resensitized resistant cells to trastuzumab. In sum, our findings (a) identify a novel mechanism of trastuzumab resistance involving CB2R-mediated receptor heterodimerization and IFN-? signaling modulation, (b) establish HER2-CB2R/EGFR heterodimer status and CB2R/EGFR expression as potential predictive biomarkers, and (c) support dual HER2/EGFR targeting as a promising strategy to overcome trastuzumab resistance in patients with HER2+ breast cancer. Overall design: mRNA profile of the four BT474-derived cell lines (SCR, CB2 KO, trastuzumab sensible and trastuzumab resistance)