Low GPR81 in ER + breast cancer cells drives tamoxifen resistance through inducing PPAR α-mediated fatty acid oxidation

Aims: The intricate molecular mechanisms underlying estrogen receptor -positive (ER + ) breast carcinogenesis and resistance to endocrine therapy remain elusive. In this study, we elucidate the pivotal role of GPR81, a G proteincoupled receptor, in ER + breast cancer (BC) by demonstrating low expression of GPR81 in tamoxifen (TAM)resistant ER + BC cell lines and tumor samples, along with the underlying molecular mechanisms. Main methods: Fatty acid oxidation (FAO) levels and lipid accumulation were explored using MDA and FA beta O assay, BODIPY 493/503 staining, and Lipid TOX staining. Autophagy levels were assayed using CYTO-ID detection and Western blotting. The impact of GPR81 on TAM resistance in BC was investigated through CCK8 assay, colony formation assay and a xenograft mice model. Results: Aberrantly low GPR81 expression in TAM -resistant BC cells disrupts the Rap1 pathway, leading to the upregulation of PPAR alpha and CPT1. This elevation in PPAR alpha/CPT1 enhances FAO, impedes lipid accumulation and lipid droplet (LD) formation, and subsequently inhibits cell autophagy, ultimately promoting TAM -resistant BC cell growth. Moreover, targeting GPR81 and FAO emerges as a promising therapeutic strategy, as the GPR81 agonist and the CPT1 inhibitor etomoxir effectively inhibit ER + BC cell and tumor growth in vivo, re -sensitizing TAM -resistant ER + cells to TAM treatment. Conclusion: Our data highlight the critical and functionally significant role of GPR81 in promoting ER + breast tumorigenesis and resistance to endocrine therapy. GPR81 and FAO levels show potential as diagnostic biomarkers and therapeutic targets in clinical settings for TAM -resistant ER + BC.