Investigating the immunological function of alpha-2-glycoprotein 1, zinc-binding in regulating tumor response in the breast cancer microenvironment.

BACKGROUND: Alpha-2-glycoprotein 1, zinc-binding (ZAG), a secreted protein encoded by the AZGP1 gene, is structurally similar to HLA class I. Despite its presumed immunological function, little is known about its role in tumor immunity. In this study, we thus aimed to determine the relationship between the expression of AZGP1/ZAG and the immunological profiles of breast cancer tissues at both the gene and protein level. METHODS: Using a publicly available gene expression dataset from a large-scale breast cancer cohort, we conducted gene set enrichment analysis (GSEA) to screen the biological processes associated with AZGP1. We analyzed the correlation between AZGP1 expression and immune cell composition in breast cancer tissues, estimated using CIBERSORTx. Previously, we evaluated the infiltration of 11 types of immune cells for 45 breast cancer tissues using flow cytometry (FCM). ZAG expression was evaluated by immunohistochemistry on these specimens and analyzed for its relationship with immune cell infiltration. The action of ZAG in M1/M2 polarization models using primary cultures of human peripheral blood mononuclear cells (PBMC)-derived macrophage (Mφ) was analyzed based on the expression of M1/M2 markers (CD86, CD80/CD163, MRC1) and HLA class I/II by FCM. RESULTS: AZGP1 expression was negatively correlated with multiple immunological processes and specific immune cell infiltration including Mφ M1 using GSEA and CIBERSORTx. ZAG expression was associated with decreased infiltration of monocytes/macrophages, non-classical monocytes, and myeloid-derived suppressor cells in tumor tissues assessed using FCM. In in vitro analyses, ZAG decreased the expression of CD80, CD163, MRC1, and HLA classes I/II in the M1 polarization model and the expression of CD163 and MRC1 in the M2 polarization model. CONCLUSION: ZAG is suggested to be a novel immunoregulatory factor affecting the Mφ phenotype in breast cancer tissues.

Expression of hormone receptors is associated with specific immunological profiles of the breast cancer microenvironment.

BACKGROUND: Elucidating the unique immunoregulatory mechanisms in breast cancer microenvironment may help develop new therapeutic strategies. Some studies have suggested that hormone receptors also have immune regulatory functions, but their mechanisms are not fully understood. In this study, we have comprehensively analyzed the relationship between the expressions of estrogen (ER), progesterone (PgR), and androgen receptors (AR), and the immunological profile in breast cancer. METHODS: Using publicly available gene expression profile datasets, METABRIC and SCAN-B, the associations between the expressions of hormone receptors and the immune cell compositions in breast cancer tissue, estimated by CIBERSORTx algorithm, were analyzed. We histologically evaluated tumor-infiltrating lymphocytes (hTIL), PD-L1 (hPD-L1) expression, and the infiltration of 11 types of immune cells by flow cytometry (FCM) for 45 breast cancer tissue samples. The relationships between them and the expressions of ER, PgR, and AR of tumor tissues, evaluated immunohistochemically, were analyzed. RESULTS: Expressions of ESR1, PGR, and AR were negatively correlated with overall immune composition. Expressions of ER and AR, but not that of PgR, were inversely associated with hTIL and hPD-L1 expression. FCM analysis showed that the expressions of ER and AR, but not that of PgR, were associated with decreased total leukocyte infiltration. Both CIBERSORTx and FCM analysis showed that ER expression was associated with reduced infiltration of macrophages and CD4+ T cells and that of AR with reduced macrophage infiltration. CONCLUSION: Hormone receptor expression correlates with specific immunological profiles in the breast cancer microenvironment both at the gene and protein expression levels.

Clinicopathological features, genetic alterations, and BRCA1 promoter methylation in Japanese male patients with breast cancer.

PURPOSE: Male breast cancer (MBC) is a rare cancer accounting for only 1% of all male cancers and is, therefore, poorly studied. We aimed to characterize the subtypes of MBC in Japanese patients based on genetic profiling, the presence of tumor-infiltrating cells, and the expression of immunohistochemical markers. METHODS: This retrospective study included 103 patients with MBC diagnosed between January 2009 and December 2019 at various hospitals in Japan. Clinicopathological patient characteristics were obtained from medical records, and formalin-fixed paraffin-embedded tissue specimens were analyzed for histological markers, mutations of 126 genes, BRCA1 methylation, and stromal tumor-infiltrating lymphocytes. RESULTS: The median patient age was 71 (range 31-92) years. T1-stage tumors were the most frequent (47.6%), and most were node negative (77.7%). The majority of tumors were positive for estrogen receptor (98.1%), progesterone receptor (95.1%), and androgen receptor (96.1%), and BRCA2 was the most frequently mutated gene (12.6%). The most common treatment was surgery (99.0%), either total mastectomy (91.1%) or partial mastectomy (7.0%). Survival analysis showed a 5-year recurrence-free survival rate of 64.4% (95% confidence interval [CI] 46.7-88.8) and a 5-year overall survival rate of 54.3% (95% CI 24.1-100.0). CONCLUSION: Japanese MBC is characterized by a high rate of hormonal receptor positivity and BRCA2 somatic mutation. Due to the observed clinicopathological differences in MBC between the Western countries and Japan, further prospective studies are needed to evaluate the most suitable treatment strategies.

Secreted indicators of androgen receptor activity in breast cancer pre-clinical models.

PURPOSE: Accumulating evidence has attracted attention to the androgen receptor (AR) as a biomarker and therapeutic target in breast cancer. We hypothesized that AR activity within the tumor has clinical implications and investigated whether androgen responsive serum factors might serve as a minimally invasive indicator of tumor AR activity. METHODS: Based on a comprehensive gene expression analysis of an AR-positive, triple negative breast cancer patient-derived xenograft (PDX) model, 163 dihydrotestosterone (DHT)-responsive genes were defined as an androgen responsive gene set. Among them, we focused on genes that were DHT-responsive that encode secreted proteins, namely KLK3, AZGP1 and PIP, that encode the secreted factors prostate specific antigen (PSA), zinc-alpha-2-glycoprotein (ZAG) and prolactin induced protein (PIP), respectively. Using AR-positive breast cancer cell lines representing all breast cancer subtypes, expression of candidate factors was assessed in response to agonist DHT and antagonist enzalutamide. Gene set enrichment analysis (GSEA) was performed on publically available gene expression datasets from breast cancer patients to analyze the relationship between genes encoding the secreted factors and other androgen responsive gene sets in each breast cancer subtype. RESULTS: Anti-androgen treatment decreased proliferation in all cell lines tested representing various tumor subtypes. Expression of the secreted factors was regulated by AR activation in the majority of breast cancer cell lines. In GSEA, the candidate genes were positively correlated with an androgen responsive gene set across breast cancer subtypes. CONCLUSION: KLK3, AZGP1 and PIP are AR regulated and reflect tumor AR activity. Further investigations are needed to examine the potential efficacy of these factors as serum biomarkers.

Clinical significance of serum PSA in breast cancer patients.

BACKGROUND: Recent preclinical data suggest that androgen receptor (AR) signaling plays a significant role in subsets of breast cancer. Clinical trials testing AR-targeting therapies in breast cancer have been conducted. Assessment of AR-signal in breast cancer tissue maybe useful for treatment selections. Prostate specific antigen (PSA) is the product of an androgen-responsive gene. Serum PSA (sPSA) can be detected in women by a highly sensitive assay although the concentration is much lower than that observed in males. We investigated if sPSA reflects tumor biology, including AR signaling in breast cancer patients. METHODS: In this study, 132 healthy controls and 144 breast cancer patients were enrolled. sPSA was evaluated by the chemiluminescent enzyme immunoassay (CLEIA) method. Correlations between sPSA and the various clinicopathological factors were analyzed. RESULTS: In post-menopausal state, sPSA detection rate was significantly higher in breast cancer patients compared with controls (27.4% vs 11.3%: p = 0.0090), but not in the whole cohort (29.2% vs 25.8%: p = 0.5265) or pre-menopausal subgroup (37.0% vs 42.6%: p = 0.6231). In post-menopausal breast cancer cases, higher sPSA value was associated with clinic-pathological factors including the expression of AR protein in primary legion. In a correlation analysis of quantitative data limited to post-menopausal metastatic breast cancer (MBC), sPSA was positively, albeit weakly correlated with clinic-pathological features including serum testosterone levels and AR positivity. CONCLUSIONS: Our data suggest that sPSA may reflect tumor biological properties including AR activity in post-menopausal breast cancer.

Possible role of the aromatase-independent steroid metabolism pathways in hormone responsive primary breast cancers.

Aromatase inhibitors (AIs) exert antiproliferative effects by reducing local estrogen production from androgens in postmenopausal women with hormone-responsive breast cancer. Previous reports have shown that androgen metabolites generated by the aromatase-independent enzymes, 5α-androstane-3ß, 17ß-diol (3ß-diol), androst-5-ene-3ß, and 17ß-diol (A-diol), also activate estrogen receptor (ER) α. Estradiol (E2) can also reportedly be generated from estrone sulfate (E1S) pooled in the plasma. Estrogenic steroid-producing aromatase-independent pathways have thus been proposed as a mechanism of AI resistance. However, it is unclear whether these pathways are functional in clinical breast cancer. To investigate this issue, we assessed the transcriptional activities of ER in 45 ER-positive human breast cancers using the adenovirus estrogen-response element-green fluorescent protein assay and mRNA expression levels of the ER target gene, progesterone receptor, as indicators of ex vivo and in vivo ER activity, respectively. We also determined mRNA expression levels of 5α-reductase type 1 (SRD5A1) and 3ß-hydroxysteroid dehydrogenase type 1 (3ß-HSD type 1; HSD3B1), which produce 3ß-diol from androgens, and of steroid sulfatase (STS) and 17ß-hydroxysteroid dehydrogenase type 1 (17ß-HSD type 1; HSD17B1), which produce E2 or A-diol from E1S or dehydroepiandrosterone sulfate. SRD5A1 and HSD3B1 expression levels were positively correlated with ex vivo and in vivo ER activities. STS and HSD17B1 expression levels were positively correlated with in vivo ER activity alone. Elevated expression levels of these steroid-metabolizing enzymes in association with high in vivo ER activity were particularly notable in postmenopausal patients. Analysis of the expression levels of steroid-metabolizing enzymes revealed positive correlations between SRD5A1 and HSD3B1, and STS and HSD17B1. These findings suggest that the SRD5A1-HSD3B1 as well as the STS-HSD17B pathways, could contributes to ER activation, especially postmenopause. These pathways might function as an alternative estrogenic steroid-producing, aromatase-independent pathways.

A newly identified missense mutation in RET codon 666 is associated with the development of medullary thyroid carcinoma.

A 38-year-old woman with a thyroid nodule measuring approximately 2 cm was suspected to have medullary thyroid carcinoma (MTC) because of markedly elevated serum calcitonin and carcinoembryonic antigen levels. There were no signs of pheochromocytoma, whereas primary hyperparathyroidism was suspected based on the findings of inappropriate hypersecretion of parathyroid hormone although no parathyroid tumor was detected with imaging studies. RET mutation analysis revealed a novel germline missense mutation in codon 666, c.1997A>G (p.K666R). She underwent total thyroidectomy with lymphadenectomy and simultaneous total parathyroidectomy with autotransplantation of parathyroid tissue. She was given calcium lactate and alfacalcidol to prevent postoperative hypocalcemia. Pathological findings of the thyroid tumor were compatible with MTC, but the resected parathyroid glands were intact. To our knowledge, c.1997A>G (p.K666R) is a new RET mutation. This is a minor variant, but it is significant because of the possible pathogenicity in tumor formation. It is often difficult to determine whether MTC is generated as part of MEN2-related disease or familial MTC when it is a unique manifestation. In addition, it is still unclear whether all missense mutations in this codon reported previously will lead to the same clinical course and prognosis. Further careful observations of clinical presentation are required to determine the clinical features associated with this variant.

Efficacy of Nursing Interventions Using Motivational Interviewing Aimed at Weight Loss in Overweight/Obese Breast Cancer Patients Undergoing Endocrine Therapy.

OBJECTIVE: Obesity adversely impacts breast cancer treatment and outcomes. This study assessed the efficacy of nurses' motivational interviews (MI) in promoting weight loss among breast cancer patients. METHODS: Motivational Interviewing was performed at 4, 8, and 12 weeks from baseline in 27 overweight/ obese breast cancer patients receiving adjuvant endocrine therapy. An average weight loss rate of 5% at week 12 was the threshold for determining whether MI intervention was clinically meaningful. Clinical and sociodemographic variables were gathered from medical records and self-administered questionnaires. Body weight, body mass index (BMI), physical activity time, sedentary time, self-efficacy for weight loss, and mood scores were evaluated at baseline, 4, 8, 12, and 24 weeks. RESULTS: Significant reductions in body weight were observed throughout compared with baseline; 51.9% of participants attained the 5% weight loss target, but the average weight loss rate was 3.9% at week 12. BMI notably decreased at 8, 12, and 24 weeks compared with baseline. Physical activity increased significantly at 12 weeks, while sedentary time decreased at 8 and 24 weeks. CONCLUSIONS: Nursing-administered MI did not achieve the goal of 5% weight loss at week 12. However, it increased physical activity and reduced sedentary time, showing potential for promoting healthier habits.

Androgen metabolite-dependent growth of hormone receptor-positive breast cancer as a possible aromatase inhibitor-resistance mechanism.

Aromatase inhibitors (AIs) have been reported to exert their antiproliferative effects in postmenopausal women with hormone receptor-positive breast cancer not only by reducing estrogen production but also by unmasking the inhibitory effects of androgens such as testosterone (TS) and dihydrotestosterone (DHT). However, the role of androgens in AI-resistance mechanisms is not sufficiently understood. 5α-Androstane-3ß,17ß-diol (3ß-diol) generated from DHT by 3ß-hydroxysteroid dehydrogenase type 1 (HSD3B1) shows androgenic and substantial estrogenic activities, representing a potential mechanism of AI resistance. Estrogen response element (ERE)-green fluorescent protein (GFP)-transfected MCF-7 breast cancer cells (E10 cells) were cultured for 3 months under steroid-depleted, TS-supplemented conditions. Among the surviving cells, two stable variants showing androgen metabolite-dependent ER activity were selected by monitoring GFP expression. We investigated the process of adaptation to androgen-abundant conditions and the role of androgens in AI-resistance mechanisms in these variant cell lines. The variant cell lines showed increased growth and induction of estrogen-responsive genes rather than androgen-responsive genes after stimulation with androgens or 3ß-diol. Further analysis suggested that increased expression of HSD3B1 and reduced expression of androgen receptor (AR) promoted adaptation to androgen-abundant conditions, as indicated by the increased conversion of DHT into 3ß-diol by HSD3B1 and AR signal reduction. Furthermore, in parental E10 cells, ectopic expression of HSD3B1 or inhibition of AR resulted in adaptation to androgen-abundant conditions. Coculture with stromal cells to mimic local estrogen production from androgens reduced cell sensitivity to AIs compared with parental E10 cells. These results suggest that increased expression of HSD3B1 and reduced expression of AR might reduce the sensitivity to AIs as demonstrated by enhanced androgen metabolite-induced ER activation and growth mechanisms. Androgen metabolite-dependent growth of breast cancer cells may therefore play a role in AI-resistance.

Ataxia Telangiectasia in a Patient with Breast Cancer: A Case Report.

Ataxia telangiectasia (AT) is a rare autosomal recessive disorder caused by the pathological variants of the ATM gene. Owing to i ts r arity a nd n ature, complications of AT, such a s malignant tumors, a re often difficult to manage with standard imaging studies and treatments, and there are no established management strategies. We report the case of a woman who had AT in childhood and developed breast cancer in her 20s; the disease was successfully managed by the decision-making of multidisciplinary physicians professionals with ethics support. She was immunocompromised, ataxic, and mentally impaired. The patient's mother noticed a tumor in her right breast and subsequently brought her to our department. Although preoperative testing and surgical procedures were limited as AT is extremely radiosensitive, the patient was diagnosed with cT2N0M0 breast cancer and underwent right mastectomy and axillary lymph node sampling. The final diagnosis was pT2N0M0 pStage IIA mucinous carcinoma, and immunohistochemistry of the tumor specimen was estrogen receptor-positive, progesterone receptor-positive, and HER2-negative. Tamoxifen was administered as postoperative adjuvant therapy, and the patient has survived to date without recurrence. Here, we report our experience with breast cancer treatment for AT, along with a review of the literature.

A case of breast cancer with extensive colon metastasis.

Breast cancer is one of the most common malignancies in women worldwide. Although most breast cancers are curable, in cases of metastasis, many are often found in the lungs, bones, liver, and central nervous system; however, metastasis to the gastrointestinal tract is rare. Invasive lobular carcinoma, which represents only 5%-10% of breast cancers, has a higher risk of metastasis to the gastrointestinal tract than invasive ductal carcinoma. Here, we report a rare case of gastrointestinal metastasis of invasive lobular carcinoma that spread extensively to the colonic mucosa. Given the improved survival rates of breast cancer patients with current treatments, many rarer metastatic diseases, including gastrointestinal metastases, are likely to be increased in the future.

One DNA Methylation Regulates CHIP Gene Expression of Human Breast Cancer and Predicts Recurrence.

BACKGROUND/AIM: Carboxyl terminus of Hsc70-interacting protein (CHIP) is a ubiquitin ligase that induces ubiquitination and degradation of its target proteins including oncoproteins. We reported that its down-regulation is associated with tumor progression and metastasis of breast cancer. However, the mechanism through which CHIP gene affects cancer cells is unclear. MATERIALS AND METHODS: We extracted RNA from 45 primary breast cancer samples and compared CHIP mRNA expression profiles, promoter DNA methylation status, and clinicopathological information. RESULTS: CHIP mRNA expression was significantly correlated with the tumor progression status. In several samples, a pinpoint CpG methylation in the CHIP gene promoter region was significantly correlated with CHIP mRNA expression. When this specific CpG was methylated in estrogen receptor (ER)-positive cases, a significant difference in 5-year recurrence was not found compared with ER-negative cases. CONCLUSION: CpG methylation contributes to the long-term prognosis of ER-positive breast cancer.

Immunological profiles of the breast cancer microenvironment represented by tumor-infiltrating lymphocytes and PD-L1 expression.

Tumor-infiltrating lymphocytes (TILs) and programmed cell death 1 ligand 1 (PD-L1) are established prognostic and predictive biomarkers for certain breast cancer subsets. However, their association with the immune response complexity is not fully understood. Therefore, we analyzed the association between the immune cell fractions in breast cancer tissues and histologically assessed TIL (hTIL) and PD-L1 (hPD-L1). Forty-five tumor and eighteen blood samples were collected from patients with breast cancer. Total leukocyte counts, frequency of 11 immune cell populations, and PD-L1 expression in each cell fraction were evaluated by flow cytometry. TILs and PD-L1 were assessed by hematoxylin and eosin staining and immunohistochemistry, respectively. A higher hTIL score showed association with increased leukocyte infiltration, higher CD4+ and CD8+ T cell proportions, and lower natural killer and natural killer T cell proportions. PD-L1 was highly expressed in nonclassical monocytes, monocyte/macrophages, myeloid-derived suppressor cells, myeloid dendritic cells, dendritic cells, and other lineages in tumors. hPD-L1 positivity reflected PD-L1 expression accurately in these fractions, as well as increased leukocyte infiltration in tumors. These results indicate that hTILs reflect differences in the immune responses in the tumor microenvironment, and certain immune cell fractions are favorably expressed in the PD-L1 pathway in breast cancer microenvironments.

CSLEX (Sialyl Lewis X) is a useful tumor marker for monitoring of breast cancer patients.

BACKGROUND: CSLEX is a type II carbohydrate antigen that interacts with the CSLEX-1 monoclonal antibody. CSLEX in combination with carbohydrate antigen 15-3 may be more useful than Carcinoembryonic Antigen with carbohydrate antigen 15-3 as tumor markers for monitoring of breast cancer. METHODS: The serum levels of tumor markers, including CSLEX, were measured in 480 consecutive breast cancer patients with or without metastasis who visited the outpatient clinic of the Division of Breast and Endocrine Surgery, Shinshu University Hospital, between April 2007 and September 2007. RESULTS: Serum levels of each of the tumor markers correlated significantly with the status of metastasis (P < 0.01). Combinations of Carcinoembryonic Antigen and carbohydrate antigen 15-3, Carcinoembryonic Antigen and Nation Cancer Center-Stomach-439, Carcinoembryonic Antigen and CSLEX, carbohydrate antigen 15-3 and Nation Cancer Center-Stomach-439, and carbohydrate antigen 15-3 and CSLEX levels also correlated significantly with the status of metastasis (P < 0.01). The sensitivity, specificity, positive predictive value, negative predictive value and accuracy were almost the same for CSLEX and Nation Cancer Center-Stomach-439, which are both type II carbohydrate antigens. The cutoff indexes of serum CSLEX and Nation Cancer Center-Stomach-439 for detection of breast cancer metastasis were 38.8 ± 52.7-fold and 22.1 ± 27.8-fold, respectively (P = 0.16). CONCLUSIONS: These data suggest that the diagnostic values of CSLEX and Nation Cancer Center-Stomach-439 are similar in single or combined use. However, the cutoff index of serum CSLEX tended to be higher than that of Nation Cancer Center-Stomach-439, which may make CSLEX more useful for detection of breast cancer metastasis.

Full-thickness chest-wall resection followed by thorax reconstruction for recurrent malignant phyllodes tumor.

We present a case of a 39-year-old woman with a giant recurrent malignant phyllodes tumor accompanied with bleeding and infection. She underwent full-thickness chest-wall resection. Bony thorax reconstruction and stabilization was accomplished using a Composix mesh™, and soft tissue reconstruction was performed with a musculocutaneous flap of latissimus dorsi muscle. The patient had a good postoperative outcome, and the surgical treatment remarkably improved her quality of life. Because chemotherapy and radiation are not established for treating malignant phyllodes tumors, an aggressive surgical approach should be considered for patients with a locally advanced malignant phyllodes tumor.

[Encounter of cancer cells with bone. Development of bone metastasis-specific targeting therapy].

Bone, as well as liver and lungs, is one of the most preferential metastatic targets for solid cancers including breast, prostate, and lung cancers. Control of the bone metastasis is clinically important ; because they are frequently associated with bone pain and pathological fracture which greatly diminish the quality of life for patients. Interaction between cancer cells and microenvironment of metastatic site play an important role for establishment of cancer metastasis. The understanding of their mechanisms has been improved in recent years, and they will be new therapeutic strategies to cure the bone metastasis. Bisphosphonates has been clinically used for the bone metastasis. In addition, RANKL-RANK targeted drugs and cathepsin K inhibitors are also expected to be fruitful drugs for the bone metastasis. In this review paper, we summarize the molecular mechanism of these drugs, especially with regarding to the breast cancer bone metastasis. Furthermore, we touch upon the ubiquitin ligase CHIP which recently found as a master gene in the progression and metastasis of breast cancer.

A Positive Feedback Loop Between TGFß and Androgen Receptor Supports Triple-negative Breast Cancer Anoikis Resistance.

Triple-negative breast cancer (TNBC) is an aggressive subtype with peak recurrence as metastatic disease within the first few years of diagnosis. Androgen receptor (AR) expression is increased in anchorage-independent cells in TNBC preclinical models. Both AR knockdown and inhibition lead to reduced TNBC invasion in vitro, reduced tumorigenicity, and less recurrence in vivo in preclinical models. Transforming growth factor ß (TGFß) pathway gene signatures also increased during anchorage-independent survival both in vitro and in vivo in preclinical models and in circulating tumor cells (CTCs) from patients during emergence of chemo resistant disease. We hypothesized that a positive loop between AR and TGFß signaling facilitates TNBC anchorage-independent survival. We find that multiple components of the TGFß pathway, including TGFß1 and 3, as well as pathway activity measured by nuclear localization and transcriptional activity of phosphorylated Smad3, are enhanced in anchorage-independent conditions. Further, exogenous TGFß increased AR protein while TGFß inhibition decreased AR and TNBC viability, particularly under anchorage-independent culture conditions. ChIP-seq experiments revealed AR binding to TGFB1 and SMAD3 regulatory regions in MDA-MB-453 cells. In clinical datasets, TGFB3 and AR positively correlate and high expression of both genes together corresponded to significantly worse recurrence-free and overall survival in both ER-negative and basal-like breast cancer. Finally, inhibiting both AR and TGFß decreased cell survival, particularly under anchorage-independent conditions. These findings warrant further investigations into whether combined inhibition of AR and TGFß pathways might decrease metastatic recurrence rates and mortality from TNBC.

Steroid Hormone Receptor and Infiltrating Immune Cell Status Reveals Therapeutic Vulnerabilities of ESR1-Mutant Breast Cancer.

Mutations in ESR1 that confer constitutive estrogen receptor alpha (ER) activity in the absence of ligand are acquired by ≥40% of metastatic breast cancers (MBC) resistant to adjuvant aromatase inhibitor (AI) therapy. To identify targetable vulnerabilities in MBC, we examined steroid hormone receptors and tumor-infiltrating immune cells in metastatic lesions with or without ER mutations. ER and progesterone receptor (PR) were significantly lower in metastases with wild-type (WT) ER compared with those with mutant ER, suggesting that metastases that evade AI therapy by mechanism(s) other than acquiring ER mutations lose dependency on ER and PR. Metastases with mutant ER had significantly higher T regulatory and Th cells, total macrophages, and programmed death ligand-1 (PD-L1)-positive immune-suppressive macrophages than those with WT ER. Breast cancer cells with CRISPR-Cas9-edited ER (D538G, Y537S, or WT) and patient-derived xenografts harboring mutant or WT ER revealed genes and proteins elevated in mutant ER cells, including androgen receptor (AR), chitinase-3-like protein 1 (CHI3L1), and IFN-stimulated genes (ISG). Targeting these proteins blunted the selective advantage of ER-mutant tumor cells to survive estrogen deprivation, anchorage independence, and invasion. Thus, patients with mutant ER MBC might respond to standard-of-care fulvestrant or other selective ER degraders when combined with AR or CHI3L1 inhibition, perhaps with the addition of immunotherapy. SIGNIFICANCE: Targetable alterations in MBC, including AR, CHI3L1, and ISG, arise following estrogen-deprivation, and ER-mutant metastases may respond to immunotherapies due to elevated PD-L1+ macrophages.See related article by Arnesen et al., p. 539.