Tamoxifen is a widely used anti‑estrogen drug in the endocrine therapy of breast cancer (BC). It blocks estrogen signaling by competitively binding to estrogen receptor α (ERα), thereby inhibiting the growth of BC cells. However, with the long‑term application of tamoxifen, a subset of patients with BC have shown resistance to tamoxifen, which leads to low overall survival and progression‑free survival. The molecular mechanism of resistance is mainly due to downregulation of ERα expression and abnormal activation of the PI3K/AKT/mTOR signaling pathway. Moreover, the downregulation of targeted gene expression mediated by DNA methylation is an important regulatory mode to control protein expression. In the present review, methylation and tamoxifen are briefly introduced, followed by a focus on the effect of methylation on tamoxifen resistance and sensitivity. Finally, the clinical application of methylation for tamoxifen is described, including its use as a prognostic indicator. Finally, it is hypothesized that when methylation is used in combination with tamoxifen, it could recover the resistance of tamoxifen.

OBJECTIVE: As an antagonist of bone morphogenetic protein (BMP), Noggin facilitates osteolytic bone metastases from breast cancer. The present study aimed to further dissect its role in oestrogen receptor (ER) positive breast cancer.
METHODS: Noggin expression in ER positive breast cancer cell lines (MCF-7 and T-47D) was determined under conditions of oestrogen deprivation and treatment with 17-β-oestradiol (E2). Activation of Smad1/5/8 in the oestrogen-regulated Noggin was examined using recombinant human BMP7 (rhBMP7) and a BMP receptor inhibitor (LDN-193189). The influence of Noggin on cellular functions was evaluated in MCF-7 and T-47D cell lines. Responses to tamoxifen and chemotherapy drugs were determined in MCF-7 and T-47D cells with Noggin over-expression using MTT assay.
RESULTS: Noggin expression was negatively correlated with ERα in breast cancers. Noggin was up-regulated upon oestrogen deprivation, an effect that was eliminated by E2 Furthermore, increased levels of phosphorylated Smad1/5/8 were observed in the oestrogen-deprived MCF-7 and T-47D cells, which was prevented by E2 and LDN-193189, respectively. BMP7-induced Noggin expression and activation of Smad1/5/8 was also prevented by E2 and LDN-193189. Noggin over-expression resulted in an increase in the proliferation of both MCF-7 and T-47D cells. MCF-7 and T-47D cells over-expressing Noggin exhibited a good tolerance to tamoxifen (TAM), DTX, and 5-FU, but the percentage of viable cells was higher compared with the controls.
CONCLUSIONS: Noggin expression can be repressed by oestrogen through inference with the BMP/Smad signalling. Over-expression of Noggin promotes the proliferation of MCF-7 and T-47D cells, contributing to drug resistance.

Estrogen receptor-positive (ER+) breast cancer seriously endangers the women\'s physical and mental health worldwide and ER targeting therapy is vital. Here, we found that a citrus polymethoxyflavones (PMFs)-rich hydrolysate (C-H) and its major components (nobiletin and 3-methoxynobiletin) potently degrade ERα protein via the ubiquitin-proteasome pathway, thereby impairing the proliferation of ER+ breast cancer cells. Moreover, our study exhibited that C-H combined with tamoxifen (TAM) inhibited the cell proliferation of ER+ breast cancer in vitro. It was further confirmed that C-H decreased tumor growth of ER+ breast cancer in tumor-bearing 129 mice in vivo and improved the efficacy of tamoxifen. Our study revealed that the citrus PMFs have potential applications as pharmaceutical and healthcare products in breast cancer treatment by targeting ERα protein degradation.

Endocrine resistance poses a significant clinical challenge for patients with hormone receptor-positive and human epithelial growth factor receptor 2-negative (HR + HER2-) breast cancer. Dysregulation of estrogen receptor (ER) and ERBB signaling pathways is implicated in resistance development; however, the integration of these pathways remains unclear. While SMAD4 is known to play diverse roles in tumorigenesis, its involvement in endocrine resistance is poorly understood. Here, we investigate the role of SMAD4 in acquired endocrine resistance in HR + HER2- breast cancer. Genome-wide CRISPR screening identifies SMAD4 as a regulator of 4-hydroxytamoxifen (OHT) sensitivity in T47D cells. Clinical data analysis reveals downregulated SMAD4 expression in breast cancer tissues, correlating with poor prognosis. Following endocrine therapy, SMAD4 expression is further suppressed. Functional studies demonstrate that SMAD4 depletion induces endocrine resistance in vitro and in vivo by enhancing ER and ERBB signaling. Concomitant inhibition of ER and ERBB signaling leads to aberrant autophagy activation. Simultaneous inhibition of ER, ERBB, and autophagy pathways synergistically impacts SMAD4-depleted cells. Our findings unveil a mechanism whereby endocrine therapy-induced SMAD4 downregulation drives acquired resistance by integrating ER and ERBB signaling and suggest a rational treatment strategy for endocrine-resistant HR + HER2- breast cancer patients.

BACKGROUND: Though tamoxifen achieves success in treating estrogen receptor α (ERα)-positive breast cancer, the followed development of tamoxifen resistance is a common challenge in clinic. Signals downstream of prolactin receptor (PRLR) could synergize with ERα in breast cancer progression. However, the potential effect of targeting PRL-PRLR axis combined with tamoxifen has not been thoroughly investigated.
METHODS: High-throughput RNA-seq data obtained from TCGA, Metabric and GEO datasets were analyzed to explore PRLR expression in breast cancer cell and the association of PRLR expression with tamoxifen treatment. Exogenous or PRL overexpression cell models were employed to investigate the role of activated PRLR pathway in mediating tamoxifen insensitivity. Immunotoxin targeting PRLR (N8-PE24) was constructed with splicing-intein technique, and the efficacy of N8-PE24 against breast cancer was evaluated using in vitro and in vivo methods, including analysis of cells growth or apoptosis, 3D spheroids culture, and animal xenografts.
RESULTS: PRLR pathway activated by PRL could significantly decrease sensitivity of ERα-positive breast cancer cells to tamoxifen. Tamoxifen treatment upregulated transcription of PRLR and could induce significant accumulation of PRLR protein in breast cancer cells by alkalizing lysosomes. Meanwhile, tamoxifen-resistant MCF7 achieved by long-term tamoxifen pressure exhibited both upregulated transcription and protein level of PRLR. Immunotoxin N8-PE24 enhanced sensitivity of breast cancer cells to tamoxifen both in vitro and in vivo. In xenograft models, N8-PE24 significantly enhanced the efficacy of tamoxifen and paclitaxel when treating PRLR-positive triple-negative breast cancer.
CONCLUSIONS: PRL-PRLR axis potentially associates with tamoxifen insensitivity in ERα-positive breast cancer cells. N8-PE24 could inhibit cell growth of the breast cancers and promote drug sensitivity of PRLR-positive breast cancer cells to tamoxifen and paclitaxel. Our study provides a new perspective for targeting PRLR to treat breast cancer.

BACKGROUND: Tamoxifen (TAM) is recommended as the first-line strategy for men with estrogen receptor (ER)-positive early breast cancer who are candidates for adjuvant endocrine therapy in ASCO guideline. Our study aims to analyze the cost-effectiveness of receiving adjuvant endocrine therapy with TAM compared to no TAM, and to assess the cost-effectiveness of using TAM with high adherence over low adherence for ER-positive early male breast cancer in the USA.
METHODS: Two Markov models comprising three mutually exclusive health states were constructed: (1) the first Markov model compared the cost-effectiveness of adding TAM with not using TAM (TAM versus Not-TAM); (2) the second model compared the cost-effectiveness of receiving TAM with high adherence and low adherence (High-adherence-TAM versus Low-adherence-TAM). The simulation time horizon for both models was the lifetime of patients. The efficacy and safety data of two models were elicited from the real-world studies. Model inputs were derived from the US website and published literature. The main outcomes of two models both included the total cost, quality-adjusted life-years (QALYs), and incremental cost-effectiveness ratios (ICERs).
RESULTS: In the first model, TAM yielded an ICER of $5707.29 per QALY compared to Not-TAM, which was substantially below the WTP threshold of $50,000.00 per QALY in the USA. Probabilistic sensitivity analysis results demonstrated a 100.00% probability of cost-effectiveness for this strategy. In the second model, High-adherence-TAM was dominated absolutely compared to Low-adherence-TAM. The High-adherence-TAM was cost-effective with a 99.70% probability over Low-adherence-TAM when WTP was set as $50,000.00/QALY. All of these parameters within their plausible ranges did not reversely change the results of our models.
CONCLUSIONS: Our study will offer valuable guidance for physicians or patients when making treatment decisions and provide an effective reference for decision-making to consider the appropriate allocation of funds to this special group.

Breast cancer is the leading cause of cancer-related deaths in women worldwide, with Hormone Receptor (HR)+ being the predominant subtype. Tamoxifen (TAM) serves as the primary treatment for HR+ breast cancer. However, drug resistance often leads to recurrence, underscoring the need to develop new therapies to enhance patient quality of life and reduce recurrence rates. Artemisinin (ART) has demonstrated efficacy in inhibiting the growth of drug-resistant cells, positioning art as a viable option for counteracting endocrine resistance. This study explored the interaction between artemisinin and tamoxifen through a combined approach of bioinformatics analysis and experimental validation. Five characterized genes (ar, cdkn1a, erbb2, esr1, hsp90aa1) and seven drug-disease crossover genes (cyp2e1, rorc, mapk10, glp1r, egfr, pgr, mgll) were identified using WGCNA crossover analysis. Subsequent functional enrichment analyses were conducted. Our findings confirm a significant correlation between key cluster gene expression and immune cell infiltration in tamoxifen-resistant and -sensitized patients. scRNA-seq analysis revealed high expression of key cluster genes in epithelial cells, suggesting artemisinin\'s specific impact on tumor cells in estrogen receptor (ER)-positive BC tissues. Molecular target docking and in vitro experiments with artemisinin on LCC9 cells demonstrated a reversal effect in reducing migratory and drug resistance of drug-resistant cells by modulating relevant drug resistance genes. These results indicate that artemisinin could potentially reverse tamoxifen resistance in ER-positive breast cancer.

OBJECTIVE: 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 protein-coupled 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.
METHODS: Fatty acid oxidation (FAO) levels and lipid accumulation were explored using MDA and FAβO 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 PPARα and CPT1. This elevation in PPARα/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.
CONCLUSIONS: 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.

Tamoxifen (TAM) is a classical anti-estrogenic drug that antagonizes estrogen by competitively binding to estrogen receptor α (ERα). However, drug resistance to TAM remains a significant challenge in breast cancer treatment. In this study, we aimed to design an actively targeted drug delivery system to enhance the proliferation inhibitory effects of TAM on ER positive breast cancer cells. Herein, chitosan (CS) was modified with genistein (GEN) to obtain the actively targeted GEN-CS. The TAM-loaded nanoparticles (TAM-GEN-CS-NPs) were constructed using an ionic-crosslinking method, with GEN-CS as the carrier material and sodium tripolyphosphate (TPP) as the crosslinking agent. As a result, TAM-GEN-CS-NPs exhibited a spherical morphology with an average size of 299.8 nm. The encapsulation efficiency and drug loading content were 85.77% and 14.13 µg/mg, respectively. Compared with free TAM, TAM-GEN-CS-NPs displayed obvious slow-release performance. In vitro cellular assays demonstrated that TAM-GEN-CS-NPs had active targeting and proliferation inhibitory effects on MCF-7 cells. The IC50 of TAM and TAM-GEN-CS-NPs were 10.25 µg/mL and 7.22 µg/mL, respectively. More importantly, the combination index (CI) value of TAM and GEN was less than 1, indicating synergistic effects. Therefore, TAM-GEN-CS-NPs hold the potential to enhance TAM therapy for breast cancer through active targeting and synergistic treatment strategies.

Tamoxifen is an estrogen receptor modulator that has been reported to alleviate hepatic lipid accumulation in mice, but the mechanism is still unclear. Peroxisome fatty acid β-oxidation is the main metabolic pathway for the overload of long-chain fatty acids. As long-chain fatty acids are a cause of hepatic lipid accumulation, the activation of peroxisome fatty acid β-oxidation might be a novel therapeutic strategy for metabolic associated fatty liver disease. In this study, we investigated the mechanism of tamoxifen against hepatic lipid accumulation based on the activation of peroxisome fatty acid β-oxidation. Tamoxifen reduced liver long-chain fatty acids and relieved hepatic lipid accumulation in high fat diet mice without sex difference. In vitro, tamoxifen protected primary hepatocytes against palmitic acid-induced lipotoxicity. Mechanistically, the RNA-sequence of hepatocytes isolated from the liver revealed that peroxisome fatty acid β-oxidation was activated by tamoxifen. Protein and mRNA expression of enoyl CoA hydratase and 3-hydroxyacyl CoA hydratase were significantly increased in vivo and in vitro. Small interfering RNA enoyl CoA hydratase and 3-hydroxyacyl CoA hydratase in primary hepatocytes abolished the therapeutic effects of tamoxifen in lipid accumulation. In conclusion, our results indicated that tamoxifen could relieve hepatic lipid accumulation in high fat diet mice based on the activation of enoyl CoA hydratase and 3-hydroxyacyl CoA hydratase-mediated peroxisome fatty acids β-oxidation.