Objective:To investigate the effect of the stress-induced protein sestrin 3(SESN3)on tumor proliferation and the sensitivity to endocrine therapy in hormone receptor-positive(HR+)breast cancer(BC)and its mechanism.Methods:The cancer genome atlas(TCGA)database was used to analyze the expression level of SESN3 in breast cancer tissue and paracancerous tissue.The clinical samples of breast cancer tissue and paracancerous tissue were collected,and immunohistochemistry(IHC)microarray was used to vali-date the expression of SESN3.Western blot was used to measure the protein expression level of SESN3 in BC cell lines with different pathological subtypes.HR+BC cell lines were used to establish the models of SESN3 overexpression and knockdown,and cell counting kit-8(CCK-8)assay and colony formation assay were used to assess the proliferative capacity of cells;Western blot was used to ana-lyze cell cycle alterations;a subcutaneous xenograft mouse model was used to validate the effect of SESN3 on proliferation in vivo.The bioinformatics analysis and Western blot were used to investigate the downstream mechanisms of SESN3 in regulating proliferation.Results:The analysis of TCGA database and clinical samples showed that the expression of the SESN3 gene in BC tissue was signifi-cantly lower than that in paracancerous tissue,and the patients with high SESN3 expression had a longer progression-free survival time.Compared with the other subtypes of BC cell lines,the HR+BC cells lines MCF-7 and T-47D had a relatively high expression level of SESN3.MCF-7 and T-47D cell lines were used to establish the cell models of knockdown(MCF-7KD and T-47DKD)and overexpres-sion(MCF-7OE and T-47DOE),and compared with the control group,MCF-7KD and T-47DKD cells showed increases in proliferative capacity and the proportion of cells arrested in the G2 and M phases.In vivo tumorigenesis experiments showed a significant increase in tumor growth in mice in the MCF-7KD group.The bioinfor-matics analyses showed a significant negative correlation between the mammalian Target of Rapamycin(mTOR)signaling pathway and SESN3 expression,and Western blot validation showed a significant increase in the expression of mTOR in MCF-7KD and T-47DKD cells,as well as reductions in the protein expression levels of autophagy-related genes.Subsequently,we further confirmed that the knockdown of MCF-7 and T-47D cells led to a decreased sensitivity to endocrine therapy.In addition,MCF-7、T-47D、MCF-7KD、and T-47DKD cell lines were treated with the mTOR inhibitor everolimus(EVE),and there was a significant reduction in proliferative capac-ity in the MCF-7KD group and the T-47DKD group;since the mTOR pathway reduced the sensitivity to endocrine therapy by reducing cell apopto-sis,there was a significant increase in the sensitivity to endocrine therapy with tamoxifen(TAM).Conclusion:The inte-grated database analysis,clinical sample validation,and in vivo/in vitro experimental models show that SESN3 inhibits the proliferation of HR+BC cells via the mTOR pathway and enhances the sensitivity to endocrine therapy.