Protein-Tyrosine Phosphatase H1 Increases Breast Cancer Sensitivity to Antiestrogens by Dephosphorylating Estrogen Receptor at Tyr537

Protein tyrosine phosphatase PTPH1 potentiates receptor tyrosine kinase HER2 oncogenesis via a PDZ-coupled and phosphorylation-driven scaffold

Cancer cell overexpresses numerus proteins, however, how these up-regulated proteins, especially those enzymatically opposite kinases and phosphatases, act together to promote oncogenesis is unknown. Here, we reported that protein tyrosine phosphatase H1 (PTPH1) is a scaffold protein for receptor tyrosine kinase (HER2) to potentiate breast tumorigenesis. PTPH1 utilizes its PDZ domain to bind HER2, p38γ, PBK, and YAP1 and to increase HER2 nuclear translocation, stemness, and oncogenesis. PTPH1 de-phosphorylates HER2 and reciprocally increases HER2 protein expression dependent on cellular content. PTPH1 itself can be phosphorylated at S459 by redundant kinases p38γ and/or PBK, thereby distinctively regulating expression and/or turnover of scaffold proteins. Moreover, PTPH1 and HER2 cooperate to increase PBK and Yap1 transcription thus acting as an additional mechanism to activate the scaffold. PTPH1 protein levels are higher in HER2+ breast cancer in which their phosphorylated forms are inversely correlated, indicating an integrated oncogenic activity through coordinated PTPH1 phosphorylation and HER2 de-phosphorylation. Combinational, but not individual, application of scaffold-kinases' inhibitors suppresses xenograft growth in mice. Thus, a PDZ-coupled and phosphorylation-driven scaffold can integrate proliferative signaling of enzymatically distinct proteins as a super-oncogene and as a target for combination therapy.

Identification and verification of PTPN3 as a novel biomarker in predicting cancer prognosis, immunity, and immunotherapeutic efficacy

BACKGROUND

The importance of protein tyrosine phosphatase non-receptor type 3 (PTPN3) in controlling multifaceted tumor cell behaviors throughout cancer development has received widespread attention. Nevertheless, little is known about the biological roles of PTPN3 in drug sensitivity, immunotherapeutic effectiveness, tumor immune microenvironment, and cancer prognosis.

METHODS

The Cancer Genome Atlas (TCGA) database's RNAseq data were used to examine the expression of PTPN3 in 33 different cancer types. In addition, immunohistochemistry (IHC) was performed to validate the expression of PTPN3 across various cancer types within our clinical cohorts. The features of PTPN3 alterations were demonstrated throughout the cBioPortal database. This study focused on examining the prognostic and clinicopathological importance of PTPN3 through the acquisition of clinical data from the TCGA database. The investigation of PTPN3's probable role in the tumor immune microenvironment was demonstrated by the application of CIBERSORT, ESTIMATE algorithms, and the TISIDB database. Using Spearman's rank correlation coefficient, the relationships between PTPN3 expression and tumor mutation burden (TMB) and microsatellite instability (MSI) were evaluated. To further investigate the putative biological activities and downstream pathways of PTPN3 in various cancers in humans, Gene Set Enrichment Analysis (GSEA) was carried out. In addition, an examination was conducted to explore the associations between PTPN3 and the effectiveness of PD-1/PD-L1 inhibitors, utilizing data extracted from the GEO database.

RESULTS

PTPN3 was abnormally expressed in multiple cancer types and was also strictly associated with the prognosis of cancer patients. IHC was used to investigate and confirm the various expression levels of PTPN3 in various malignancies, including breast cancer, lung cancer, sarcoma, and kidney renal clear cell carcinoma in our clinical cohorts. There is a high correlation between the levels of PTPN3 expression in different cancers and infiltrating immune cells, including mast cells, B cells, regulatory T cells, CD8 + T cells, macrophages, and dendritic cells. Infiltrating immune cells, such as regulatory T cells, CD8 + T cells, macrophages, B cells, dendritic cells, and mast cells, are strongly correlated with PTPN3 expression levels in various tumors. The expression of PTPN3 exhibited a substantial correlation with many immune-related biomolecules and the expression of TMB and MSI in multiple types of cancer. In addition, PTPN3 has demonstrated promise in predicting the therapeutic benefits of PD-1/PD-L1 inhibitors and the susceptibility to anti-cancer medications in the treatment of clinical cancer.

CONCLUSIONS

Our findings highlight the importance of PTPN3 as a prognostic biomarker and predictor of immunotherapy success in various forms of cancer. Furthermore, PTPN3 appears to have an important role in modifying the tumor immune microenvironment, highlighting its potential as a promising biomarker for prognosis prediction, immunotherapeutic efficacy evaluation, and identification of immune-related characteristics in diverse cancer types.

p38γ MAPK Inflammatory and Metabolic Signaling in Physiology and Disease

p38γ MAPK (also called ERK6 or SAPK3) is a family member of stress-activated MAPKs and has common and specific roles as compared to other p38 proteins in signal transduction. Recent studies showed that, in addition to inflammation, p38γ metabolic signaling is involved in physiological exercise and in pathogenesis of cancer, diabetes, and Alzheimer's disease, indicating its potential as a therapeutic target. p38γphosphorylates at least 19 substrates through which p38γ activity is further modified to regulate life-important cellular processes such as proliferation, differentiation, cell death, and transformation, thereby impacting biological outcomes of p38γ-driven pathogenesis. P38γ signaling is characterized by its unique reciprocal regulation with its specific phosphatase PTPH1 and by its direct binding to promoter DNAs, leading to transcriptional activation of targets including cancer-like stem cell drivers. This paper will review recent findings about p38γ inflammation and metabolic signaling in physiology and diseases. Moreover, we will discuss the progress in the development of p38γ-specific pharmacological inhibitors for therapeutic intervention in disease prevention and treatment by targeting the p38γ signaling network.

Decoding the Therapeutic Implications of the ERα Stability and Subcellular Distribution in Breast Cancer

Approximately 70% of all breast cancer cases are estrogen receptor-alpha positive (ERα+) and any ERα signaling pathways deregulation is critical for the progression of malignant mammary neoplasia. ERα acts as a transcription factor that promotes the expression of estrogen target genes associated with pro-tumor activity in breast cancer cells. Furthermore, ERα is also part of extranuclear signaling pathways related to endocrine resistance. The regulation of ERα subcellular distribution and protein stability is critical to regulate its functions and, consequently, influence the response to endocrine therapies and progression of this pathology. This minireview highlights studies that have deciphered the molecular mechanisms implicated in controlling ERα stability and nucleo-cytoplasmic transport. These mechanisms offer information about novel biomarkers, therapeutic targets, and promising strategies for breast cancer treatment.

Protein Tyrosine Phosphatases: Mechanisms in Cancer

Protein tyrosine kinases, especially receptor tyrosine kinases, have dominated the cancer therapeutics sphere as proteins that can be inhibited to selectively target cancer. However, protein tyrosine phosphatases (PTPs) are also an emerging target. Though historically known as negative regulators of the oncogenic tyrosine kinases, PTPs are now known to be both tumor-suppressive and oncogenic. This review will highlight key protein tyrosine phosphatases that have been thoroughly investigated in various cancers. Furthermore, the different mechanisms underlying pro-cancerous and anti-cancerous PTPs will also be explored.

The Role of p38γ in Cancer: From review to outlook

p38γ is a member of the p38 Mitogen Activated Protein Kinases (p38 MAPKs). It contains four subtypes in mammalian cells encoded by different genes including p38α (MAPK14), p38β (MAPK11), p38γ (MAPK12), and p38δ (MAPK13). Recent studies revealed that p38γ may exhibit a crucial role in tumorigenesis and cancer aggressiveness. Despite the large number of published literatures, further researches are demanded to clarify its role in cancer development, the tissue-specific function and associated novel treatment strategies. In this article, we provide the latest view on the connection between p38γ and malignant tumors, highlighting the function of p38γ. The clinical value of p38γ is also discussed, helping the translation into the remarkable therapeutic strategy in tumor diseases.

miR-210 Participates in Hepatic Ischemia Reperfusion Injury by Forming a Negative Feedback Loop With SMAD4

BACKGROUND AND AIMS

Hepatic ischemia-reperfusion (IR) injury is a major complication of liver transplantation, resection, and hemorrhagic shock. Hypoxia is a key pathological event associated with IR injury. MicroRNA-210 (miR-210) has been characterized as a micromanager of hypoxia pathway. However, its function and mechanism in hepatic IR injury is unknown.

APPROACH AND RESULTS

In this study, we found miR-210 was induced in liver tissues from patients subjected to IR-related surgeries. In a murine model of hepatic IR, the level of miR-210 was increased in hepatocytes but not in nonparenchymal cells. miR-210 deficiency remarkably alleviated liver injury, cell inflammatory responses, and cell death in a mouse hepatic IR model. In vitro, inhibition of miR-210 decreased hypoxia/reoxygenation (HR)-induced cell apoptosis of primary hepatocytes and LO2 cells, whereas overexpression of miR-210 increased cells apoptosis during HR. Mechanistically, miR-210 directly suppressed mothers against decapentaplegic homolog 4 (SMAD4) expression under normoxia and hypoxia condition by directly binding to the 3' UTR of SMAD4. The pro-apoptotic effect of miR-210 was alleviated by SMAD4, whereas short hairpin SMAD4 abrogated the anti-apoptotic role of miR-210 inhibition in primary hepatocytes. Further studies demonstrated that hypoxia-induced SMAD4 transported into nucleus, in which SMAD4 directly bound to the promoter of miR-210 and transcriptionally induced miR-210, thus forming a negative feedback loop with miR-210.

CONCLUSIONS

Our study implicates a crucial role of miR-210-SMAD4 interaction in hepatic IR-induced cell death and provides a promising therapeutic approach for liver IR injury.

Prognostic Significance of Metastatic Lymph Nodes Ratio (MLNR) Combined with Protein-Tyrosine Phosphatase H1 (PTPH1) Expression in Operable Breast Invasive Ductal Carcinoma

Purpose

The metastatic lymph node ratio (MLNR) is one of the most important factors in prognostic analysis of breast cancer. The objective of this study was to determine if MLNR combined with protein-tyrosine phosphatase H1 (PTPH1) pathological expression can be used to predict the prognosis of patients with breast invasive ductal carcinoma (IDC) better than one factor only.

Patients and Methods

A total of 136 patients with invasive ductal carcinoma (IDC) of breast who underwent modified radical mastectomy and were treated with chemotherapy after operation at Qilu Hospital of Shandong University from December 2008 to October 2011 were included. PTPH1 expression was evaluated by immunohistochemistry in surgical specimens retrospectively collected from patients with histologically proven invasive ductal breast cancer. Kaplan–Meier survival analysis and Cox regression analysis were performed to assess the prognostic significance of PTPH1 expression. A prognostic factor for disease-free survival (DFS) was identified by univariate and multivariate analyses. ROC analysis was used to evaluate the performance of single factors and combined feature.

Results

One hundred and thirty-six patients were included in the analysis. By cut-point survival analysis, MLNR cut-off was designed as 0.2. On multivariate analysis, a MLNR>0.2 was associated with a worse DFS (HR=2.581, 95% CI=1.303–5.113, P=0.007). PTPH1 overexpression is correlated with a better DFS (HR=0.391, 95% CI=0.162–0.945, P=0.037). In addition, MLNR and PTPH1 combined feature had better performance in predicting clinical outcomes after surgery long before recurrence had occurred (Area under the curve=0.795 [95% CI=0.694–0.896], P<0.001).

Conclusion

These findings indicate that both PTPH1 and MLNR are accurate independent prognostic parameters in patients with IDC of the breast. Better information on IDC prognosis could be obtained from the combined feature.

Gene expression changes and promoter methylation with the combined effects of estradiol and leptin in uterine tissue of the ovariectomized mice model of menopause

Substantial epidemiological studies have shown an association of obesity with the common gynecological malignancy, endometrial cancer. The relevant interactions and contribution of estradiol and the adipose cytokine, leptin, in endometrial lesions are not completely understood. Suitable animal models to understand the physiological response of uterine tissue to the combined effects of estradiol-leptin are lacking. To investigate the effect of estradiol-leptin crosstalk on gene expression and associated altered pathways, we established an ovariectomized mouse model, treated with 17-β estradiol (0.1 µg/mouse subcutaenously., for every 12 h) and/or recombinant mouse leptin (1 μg/g Bwt intraperitoneally., for every 12 h) for 4 h, 20 h, and 40 h. Gene expressions by semi-quantitative RT-PCR, uterine tissue protein phosphorylation status by western blotting and promoter methylation were analyzed in estradiol, progesterone insufficient animals. Semi-quantitative RT-PCR demonstrated significantly increased expression of Esr, Igf1, Igfbp3, Vegfr1, and Vegf, and significantly decreased expression of Mmp9 after co-treatment with estradiol and leptin, indicating a common transcriptional network regulated by the treatments. Ovariectomy-induced histomorphological changes were only reversed by estradiol. Methylation-specific PCR, analyzing methylation of CpG sites of Vegfa, Pgr, and Igf1, revealed that transcriptional regulation after hormonal treatments is independent of methylation at the examined CpG sites. Western blot confirmed the increased expression of PSTAT-3 (Ser-727) and PERK1/2 proteins after estradiol + leptin treatment, confirming the estradiol + leptin cross-talk hypothesis. In conclusion, our in vivo studies determined specific gene expression and signaling protein changes, and further unraveled the molecular targets of estradiol + leptin that may perturb endometrial homeostasis and lead to endometrial hyperplasia development in the chronic stimulated state.

Up-regulation of microRNA-497-5p inhibits colorectal cancer cell proliferation and invasion via targeting PTPN3

To investigate the role of microRNA-497-5p (miR-497-5p) in the tumorigenesis of colorectal cancer (CRC), the present study applied qRT-PCR to detect the expression level of miR-497-5p in both clinical samples and CRC cell lines. Furthermore, to specifically evaluate the carcinogenic role of miR-497-5p in CRC, the expression of miR-497-5p was monitored by transfecting with the mimics or inhibitors of miR-497-5p. Transwell assay as well as CCK-8 assay were used to determine the functions of miR-497-5p on cell invasion, migration and proliferation, respectively. miR-497-5p expression was remarkably down-regulated in clinical samples with cancer development as well as in CRC cell lines. Additionally, low miR-497-5p expression was remarkably correlated with higher TNM stage and lymph node metastasis of CRC patients. Up-regulation of miR-497-5p significantly inhibited proliferation, migration, and invasion of LOVO CRC cell line. Conversely, antagonizing miR-497-5p significantly promoted cell proliferation, migration and invasion. Mechanistic analysis revealed that miR-497-5p directly bound to its downstream target, protein tyrosine phosphatase non-receptor type 3 (PTPN3), whose aberrant expression partially reversed inhibition of cell proliferation and migration. Taken together, the present study elucidated the inhibitory role of miR-497-5p in CRC via targeting PTPN3, which potentiated miR-497-5p as a potential therapeutic target for combating CRC.

The Protein Tyrosine Phosphatase H1 PTPH1 Supports Proliferation of Keratinocytes and is a Target of the Human Papillomavirus Type 8 E6 Oncogene

Human papillomaviruses (HPV) replicate their DNA in the suprabasal layer of the infected mucosa or skin. In order to create a suitable environment for vegetative viral DNA replication HPV delay differentiation and sustain keratinocyte proliferation that can lead to hyperplasia. The mechanism underlying cell growth stimulation is not well characterized. Here, we show that the E6 oncoprotein of the βHPV type 8 (HPV8), which infects the cutaneous skin and is associated with skin cancer in Epidermodysplasia verruciformis patients and immunosuppressed organ transplant recipients, binds to the protein tyrosine phosphatase H1 (PTPH1), which resulted in increased protein expression and phosphatase activity of PTPH1. Suppression of PTPH1 in immortalized keratinocytes reduced cell proliferation as well as the level of epidermal growth factor receptor (EGFR). Furthermore, we report that HPV8E6 expressing keratinocytes have increased level of active, GTP-bound Ras. This effect was independent of PTPH1. Therefore, HPV8E6-mediated targeting of PTPH1 might result in higher level of EGFR and enhanced keratinocyte proliferation. The HPV8E6-mediated stimulation of Ras may be an additional step to induce cell growth. Our results provide novel insights into the mechanism how βHPVE6 proteins support proliferation of infected keratinocytes, thus creating an environment with increased risk of development of skin cancer particularly upon UV-induced DNA mutations.

Targeting an oncogenic kinase/phosphatase signaling network for cancer therapy

Protein kinases and phosphatases signal by phosphorylation and dephosphorylation to precisely control the activities of their individual and common substrates for a coordinated cellular outcome. In many situations, a kinase/phosphatase complex signals dynamically in time and space through their reciprocal regulations and their cooperative actions on a substrate. This complex may be essential for malignant transformation and progression and can therefore be considered as a target for therapeutic intervention. p38γ is a unique MAPK family member that contains a PDZ motif at its C-terminus and interacts with a PDZ domain-containing protein tyrosine phosphatase PTPH1. This PDZ-coupled binding is required for both PTPH1 dephosphorylation and inactivation of p38γ and for p38γ phosphorylation and activation of PTPH1. Moreover, the p38γ/PTPH1 complex can further regulate their substrates phosphorylation and dephosphorylation, which impacts Ras transformation, malignant growth and progression, and therapeutic response. This review will use the p38γ/PTPH1 signaling network as an example to discuss the potential of targeting the kinase/phosphatase signaling complex for development of novel targeted cancer therapy.

Molecular pathways involved in the transport of nuclear receptors from the nucleus to cytoplasm

Nuclear receptors (NRs) are transcription regulators that direct the expression of many genes linked to cellular processes, such as proliferation, differentiation, and apoptosis. Additionally, some cellular events are also modulated by signaling pathways induced by NRs outside of the nucleus. Hence, the subcellular transport of NRs is dynamic and is modulated by several signals, protein-protein interactions, and posttranslational modifications. Particularly, the exit of NRs from the nucleus to cytoplasm and/or other compartments is transcendental, as it is this export event, which determines their abundance in the cells' compartments, the activation or attenuation of nuclear or extranuclear pathways, and the magnitude and duration of their effects inside or outside of the nucleus. Consequently, an adequate control of the distribution of NRs is critical for homeostasis, because a deregulation in the nucleo-cytoplasmic transport of NRs could be involved in diseases including cancer as well as metabolic and vascular alterations. In this review, we investigated the pathways and molecular and biological aspects that have been described for the nuclear export of NRs so far and their functional relevance in some diseases. This information suggests that the transport of NRs out of the nucleus is a key mechanism for the identification of new therapeutic targets for alterations associated with the deregulation of the function of NRs.

The K-Ras effector p38γ MAPK confers intrinsic resistance to tyrosine kinase inhibitors by stimulating EGFR transcription and EGFR dephosphorylation

Mutations in K-Ras and epidermal growth factor receptor (EGFR) are mutually exclusive, but it is not known how K-Ras activation inactivates EGFR, leading to resistance of cancer cells to anti-EGFR therapy. Here, we report that the K-Ras effector p38γ MAPK confers intrinsic resistance to small molecular tyrosine kinase inhibitors (TKIs) by concurrently stimulating EGFR gene transcription and protein dephosphorylation. We found that p38γ increases EGFR transcription by c-Jun-mediated promoter binding and stimulates EGFR dephosphorylation via activation of protein-tyrosine phosphatase H1 (PTPH1). Silencing the p38γ/c-Jun/PTPH1 signaling network increased sensitivities to TKIs in K-Ras mutant cells in which EGFR knockdown inhibited growth. Similar results were obtained with the p38γ-specific pharmacological inhibitor pirfenidone. These results indicate that in K-Ras mutant cancers, EGFR activity is regulated by the p38γ/c-Jun/PTPH1 signaling network, whose disruption may be a novel strategy to restore the sensitivity to TKIs.

Human colorectal cancer cells induce vascular smooth muscle cell apoptosis in an exocrine manner

Tumor vessels often lack the smooth muscle layer, and the instability is conducive to tumor invasion and metastasis. The effect of tumor microenvironment on vascular smooth muscle cells needs to be explored. In the present study, we examined the density of the tumor vessels in human colorectal cancer tissues, and used the tumor conditioned medium of human colorectal cancer HT29 cells to mimic the tumor microenvironment. We showed that the vessel density in colorectal cancer tissues increased, which displayed a decreased expression of smooth muscle α-actin, a specific marker of vascular smooth muscle cells and an attenuated or a discontinuous layer of vascular smooth muscle cells compared with the matched normal tissues. We also showed that the tumor conditioned medium decreased the cell viability, and induced the apoptosis in vascular smooth muscle cells in a concentration-dependent manner. The expression of pro-Caspase-3 was down-regulated, accompanied by increasing of cleaved-Caspase-3 in the cells treated with the tumor conditioned medium, suggesting that Caspase-3 was activated. Moreover, the expression of Bax was increased, and the ratio of Bcl-2/Bax was decreased under the same conditions. Furthermore, the treatment with the tumor conditioned medium resulted in loss of mitochondrial membrane potential in vascular smooth muscle cells. These findings suggest that HT29 cells induce apoptosis of vascular smooth muscle cells in an exocrine manner, associated with activating caspase-3 via mitochondrial apoptotic pathway. This may be one of the mechanisms underlying tumor vascular structural abnormalities.

Nucleo-cytoplasmic transport of estrogen receptor alpha in breast cancer cells

Approximately 70% cases of breast cancers exhibit high expression and activity levels of estrogen receptor alpha (ERα), a transcription regulator that induces the expression of genes associated with cellular proliferation and survival. These nuclear functions of the receptor are associated with the development of breast cancer. However, ERα localization is not static, but rather, dynamic with continuous shuttling between the nucleus and the cytoplasm. Interestingly, both the nuclear import and export of ERα are modulated by several stimuli that include estradiol, antiestrogens, and growth factors. As ERα nuclear accumulation is critical to the regulation of gene expression, nuclear export of this receptor modulates the intensity and duration of its transcriptional activity. Thus, the subcellular spatial distribution of ERα ensures tight modulation of its concentration in cellular compartments, as well as of its nuclear and extranuclear functions. In this review, we will discuss current findings regarding the biological importance of molecular mechanisms of, and proteins responsible for, the nuclear import and export of ERα in breast cancer cells.

Antiestrogens: structure-activity relationships and use in breast cancer treatment

About 70% of breast tumors express estrogen receptor alpha (ERα), which mediates the proliferative effects of estrogens on breast epithelial cells, and are candidates for treatment with antiestrogens, steroidal or non-steroidal molecules designed to compete with estrogens and antagonize ERs. The variable patterns of activity of antiestrogens (AEs) in estrogen target tissues and the lack of systematic cross-resistance between different types of molecules have provided evidence for different mechanisms of action. AEs are typically classified as selective estrogen receptor modulators (SERMs), which display tissue-specific partial agonist activity (e.g. tamoxifen and raloxifene), or as pure AEs (e.g. fulvestrant), which enhance ERα post-translational modification by ubiquitin-like molecules and accelerate its proteasomal degradation. Characterization of second- and third-generation AEs, however, suggests the induction of diverse ERα structural conformations, resulting in variable degrees of receptor downregulation and different patterns of systemic properties in animal models and in the clinic.

Tyrosine dephosphorylation enhances the therapeutic target activity of epidermal growth factor receptor (EGFR) by disrupting its interaction with estrogen receptor (ER)

Protein-protein interactions can increase or decrease its therapeutic target activity and the determining factors involved, however, are largely unknown. Here, we report that tyrosine-dephosphorylation of epidermal growth factor receptor (EGFR) increases its therapeutic target activity by disrupting its interaction with estrogen receptor (ER). Protein tyrosine phosphatase H1 (PTPH1) dephosphorylates the tyrosine kinase EGFR, disrupts its interaction with the nuclear receptor ER, and increases breast cancer sensitivity to small molecule tyrosine kinase inhibitors (TKIs). These effects require PTPH1 catalytic activity and its interaction with EGFR, suggesting that the phosphatase may increase the sensitivity by dephosphorylating EGFR leading to its dissociation with ER. Consistent with this notion, a nuclear-localization defective ER has a higher EGFR-binding activity and confers the resistance to TKI-induced growth inhibition. Additional analysis show that PTPH1 stabilizes EGFR, stimulates the membranous EGFR accumulation, and enhances the growth-inhibitory activity of a combination therapy of TKIs with an anti-estrogen. Since EGFR and ER both are substrates for PTPH1 in vitro and in intact cells, these results indicate that an inhibitory EGFR-ER protein complex can be switched off through a competitive enzyme-substrate binding. Our results would have important implications for the treatment of breast cancer with targeted therapeutics.

Estrogen receptor mutations found in breast cancer metastases integrated with the molecular pharmacology of selective ER modulators

The consistent reports of mutations at Asp538 and Tyr537 in helix 12 of the ligand-binding domain (LBD) of estrogen receptors (ERs) from antihormone-resistant breast cancer metastases constitute an important advance. The mutant amino acids interact with an anchor amino acid, Asp351, to close the LBD, thereby creating a ligand-free constitutively activated ER. Amino acids Asp 538, Tyr 537, and Asp 351 are known to play a role in either the turnover of ER, the antiestrogenic activity of the ER complex, or the estrogen-like actions of selective ER modulators. A unifying mechanism of action for these amino acids to enhance ER gene activation and growth response is presented. There is a range of mutations described in metastases vs low to zero in primary disease, so the new knowledge is of clinical relevance, thereby confirming an additional mechanism of acquired resistance to antihormone therapy through cell population selection pressure and enrichment during treatment. Circulating tumor cells containing ER mutations can be cultured ex vivo, and tumor tissues can be grown as patient-derived xenografts to add a new dimension for testing drug susceptibility for future drug discovery.