Highlights
• By inhibiting the t-DARPP/p95-HER2 complex, Quercetin can act as an adjuvant drug of Trastuzumab to alleviate the drug resistance in HER2+ breast cancer cells. The novelty and significance of this study are summarized as follows:
• This study confirmed that higher DARPP-32 and t-DARPP mRNA and protein levels are detected in HER2+ breast cancer tissues.
• This study demonstrated that the HER2 intracellular domain (HER2-ICD, p95-HER2) detected in breast cancer cells positively correlates with t-DARPP protein expression.
• This PDX mice model study demonstrated that the t-DARPP/p95-HER2 interaction is a potential diagnostic marker that could help clinicians identify patients resistant to Trastuzumab therapy (Non-responder).
• This study confirmed that Quercetin attenuated Trastuzumab resistance by inhibiting the t-DARPP expression and downregulating p95-HER2 -mediated signal activation (Responder).
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Abstract
Trastuzumab resistance is one of the causes of poor prognosis in patients with human epidermal growth factor receptor 2 (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 polymerase chain reaction and confirmed higher DARPP-32 kDa family mRNA expression in HER2+ BC tumor tissues. We established 2 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.