ESR1 (Ab-118) Antibody

What is ESR1-pS118 antibody and what epitope does it recognize?

ESR1-pS118 antibody specifically recognizes the phosphorylated serine residue at position 118 in the estrogen receptor alpha protein. This antibody binds to a post-translational modification that occurs in the N-terminal activation function 1 (AF-1) domain of ESR1. The phosphorylation at S118 is a crucial regulatory event that affects estrogen receptor transcriptional activity and is distinct from the ligand-binding domain. ESR1 is also known by several other names including estrogen receptor-1, estrogen receptor alpha, ERalpha, ER-alpha, estradiol receptor, and nuclear receptor subfamily 3 group A member 1 (NR3A1) . The antibody enables researchers to specifically detect this phosphorylation event, which has significant implications for estrogen-dependent signaling pathways in normal physiology and disease states.

What are the validated applications for ESR1-pS118 antibody?

ESR1-pS118 antibody has been validated for multiple research applications, with immunohistochemistry (IHC) and Western blot (WB) being the primary confirmed applications. In IHC applications, the antibody successfully stains nuclei in epithelial cells of human breast carcinoma sections, with recommended concentrations of 1-3μg/ml . For Western blot applications, the antibody effectively detects an approximately 56kDa band in lysates of MCF7 cells, particularly after overnight serum starvation and subsequent treatment with Calyculin A . Additionally, the antibody has been used in proximity ligation assays for detecting phosphorylated proteins with high specificity . These validated applications provide researchers with reliable tools for examining ESR1 phosphorylation status in various experimental contexts.

What is the biological significance of ESR1 phosphorylation at serine 118?

Phosphorylation of ESR1 at serine 118 (pS118) represents a key post-translational modification that regulates estrogen receptor function. Unlike the AF-2 domain which is regulated by ligand binding, the AF-1 domain where S118 resides is primarily regulated by phosphorylation . This phosphorylation event can occur through multiple signaling pathways, including estrogen-dependent mechanisms and growth factor-activated kinase cascades such as the MAPK pathway . Phosphorylation at S118 typically enhances transcriptional activity of ESR1, potentially leading to increased expression of estrogen-responsive genes like TFF1, GREB1, PGR, and CCND1 . In breast cancer research, altered phosphorylation patterns at this site have been observed in the context of endocrine resistance, making it an important marker for studying treatment response and resistance mechanisms in hormone-dependent cancers.

How should samples be prepared for optimal ESR1-pS118 detection in IHC?

For optimal detection of ESR1-pS118 in immunohistochemistry applications, proper sample preparation is critical. According to validated protocols, formaldehyde-fixed, paraffin-embedded tissue sections provide reliable results. For human breast carcinoma samples, the antibody has been successfully used at concentrations of 1-3μg/ml, with documented success at 10μg/ml for 30 minutes at room temperature . Epitope retrieval is a crucial step - microwave heating at pH7 to boiling, followed by 5 minutes at 95-98°C, 1 minute at 100°C, and then cooling to room temperature has proven effective . Detection systems utilizing horseradish peroxidase (HRP) polymer with DAB staining yield clear visualization of the phosphorylated receptor. Researchers should validate these conditions with appropriate positive controls (such as breast carcinoma tissue known to express phosphorylated ESR1) and negative controls to ensure specificity of the staining pattern.

What are the recommended protocols for Western blot analysis using ESR1-pS118 antibody?

For Western blot applications with ESR1-pS118 antibody, cell lysate preparation and treatment conditions significantly impact detection. The antibody successfully detects a ~56kDa band in MCF7 cell lysates, with optimal results observed after overnight serum starvation followed by treatment with 100nM Calyculin A (a phosphatase inhibitor) for 30 minutes at 37°C . A recommended antibody concentration of 1-3μg/ml applied for 1 hour at ambient temperature yields clear results. Standard ECL detection using HRP conjugates provides adequate sensitivity. When analyzing samples from different experimental conditions, researchers should consider loading 30μg of total protein per lane for consistent results . It's important to note that phosphorylation status may change rapidly during sample preparation, so phosphatase inhibitors should be included in lysis buffers to preserve the phosphorylation state. Comparing treated and untreated samples side by side (as with the Calyculin A treatment protocol) provides internal validation of antibody specificity.

How can ESR1-pS118 antibody be used in proximity ligation assays?

The ESR1-pS118 antibody can be effectively employed in proximity ligation assays (PLA) when paired with antibodies recognizing the total ESR1 protein. This dual recognition approach allows highly specific detection of phosphorylated ESR1 proteins with single-molecule resolution. In validated protocols, the phospho-specific rabbit polyclonal antibody is typically used at a 1:1200 dilution alongside a mouse monoclonal antibody against total ESR1 at 1:50 dilution . Each red dot visualized in this assay represents a single phosphorylated protein molecule, enabling accurate quantification. The results can be analyzed using specialized software such as BlobFinder from The Centre for Image Analysis at Uppsala University . This technique offers significant advantages over traditional immunofluorescence by providing enhanced specificity and sensitivity, allowing researchers to detect low-abundance phosphorylated ESR1 and quantify the proportion of phosphorylated versus total ESR1 protein in different cellular contexts or in response to various treatments.

What is the relationship between ESR1-pS118 levels and response to endocrine therapy?

The relationship between ESR1-pS118 levels and response to endocrine therapy is complex and context-dependent. Phosphorylation at S118 can occur through both estrogen-dependent and independent pathways, with the latter involving growth factor signaling through the MAPK pathway . In the context of endocrine therapy, altered phosphorylation patterns at S118 have been observed in resistant cells. Research with long-term estrogen-deprived (LTED) cell models, which mimic aromatase inhibitor resistance, has shown that pESR1 ser118 levels are generally greater in wild-type cells compared to LTED models harboring ESR1 mutations . This suggests that as resistance develops through ESR1 mutations, the reliance on phosphorylation at S118 for activation may decrease as the receptor becomes constitutively active through other mechanisms. Furthermore, when comparing different resistance mechanisms, studies have shown that ESR1 mutations and ER loss are mutually exclusive events, with each accounting for a portion of endocrine therapy resistance cases . Monitoring changes in pS118 levels during treatment may provide insights into the development of resistance and help guide therapeutic decisions.

How can ESR1-pS118 detection be integrated with genomic and transcriptomic analyses?

Integration of ESR1-pS118 detection with genomic and transcriptomic analyses offers a powerful approach to understanding estrogen receptor signaling dynamics. Researchers can correlate phosphorylation status with specific gene expression patterns by combining phospho-specific antibody studies with techniques like RNA-seq and ChIP-seq. Studies have demonstrated that ESR1 binding patterns and associated gene expression profiles differ significantly between wild-type cells and those harboring ESR1 mutations or exhibiting endocrine resistance . For instance, ChIP-seq analysis revealed that in SUM44-LTED cells with the Y537S mutation, there was enrichment for motifs representing transcription factors ESR1, RARA, PAX2, ANDR, and FOXA1 compared to wild-type cells . Gene set enrichment analysis (GSEA) further showed that increased ESR1 Y537S genomic binding correlated with increased transcription of target genes. K-means clustering identified distinct gene sets, including classical estrogen-regulated genes such as TFF1, GREB1, PGR, and CCND1, which exhibited specific expression patterns during the development of estrogen independence . By correlating ESR1-pS118 levels with these genomic and transcriptomic features, researchers can gain insights into how phosphorylation impacts receptor function in different cellular contexts and how this relates to treatment response or resistance.

What are common challenges when using ESR1-pS118 antibody in clinical samples?

Several challenges can arise when using ESR1-pS118 antibody in clinical samples. First, phosphorylation states are highly labile and can be lost during sample collection, fixation, and processing. The time from tissue removal to fixation (cold ischemia time) significantly impacts phosphoprotein preservation. Second, fixation conditions can affect epitope accessibility - overfixation may mask the phospho-epitope while underfixation may result in poor tissue morphology. Third, phosphorylation-specific antibodies like ESR1-pS118 may show variable performance across different patient samples due to heterogeneity in phosphatase activity, treatment history, and tumor biology. Fourth, interpreting results can be challenging without appropriate controls and standardization, particularly when comparing samples processed at different times or institutions. Finally, phosphorylation at S118 can occur through multiple signaling pathways, making it difficult to distinguish the mechanistic basis for observed phosphorylation patterns. To address these challenges, researchers should maintain strict protocols for sample handling, include appropriate positive controls (such as Calyculin A-treated samples), use phosphatase inhibitors during processing when possible, and consider analyzing total ESR1 expression in parallel to provide context for phosphorylation levels.

How can researchers ensure specificity when detecting ESR1-pS118?

Ensuring specificity when detecting ESR1-pS118 requires multiple validation approaches. First, include appropriate controls: positive controls such as MCF7 cells treated with Calyculin A, which enhances phosphorylation, and negative controls such as samples treated with lambda phosphatase to remove phosphorylation . Second, perform antibody validation using peptide competition assays, where a phosphorylated peptide corresponding to the S118 region should block antibody binding while the non-phosphorylated version should not. Third, validate results using complementary techniques - if a sample shows positive staining by IHC, confirm with Western blot or proximity ligation assay when possible . Fourth, compare the detection pattern with that of total ESR1 antibodies - pS118 should be a subset of total ESR1 staining and should maintain the expected subcellular localization (primarily nuclear). Fifth, when analyzing clinical samples, correlate pS118 detection with known clinical parameters and other molecular markers of ESR1 activity to ensure biological plausibility. Finally, consider using multiple antibodies targeting the same phospho-epitope from different vendors or clones to confirm findings, as each antibody may have slightly different binding characteristics and specificities.

What storage and handling precautions should be taken to maintain ESR1-pS118 antibody performance?

Maintaining optimal performance of ESR1-pS118 antibody requires careful attention to storage and handling procedures. The antibody's integrity is typically warranted for 24 months after purchase when handled according to instructions . To maximize longevity and performance, avoid repeated freeze/thaw cycles which can lead to protein denaturation and loss of epitope recognition. For long-term storage, divide the antibody into small aliquots and keep them at -20°C or -80°C . Keep one working aliquot at 4°C for daily experiments - this can be preserved with sodium azide for 6-12 months when kept away from direct sunlight . When preparing dilutions for experiments, use high-quality, sterile buffers and consider adding carrier proteins like BSA (0.1-1%) to prevent nonspecific adsorption to tubes. Always centrifuge antibody vials briefly before opening to collect liquid that may have gathered in the cap. For antibody pairs used in proximity ligation assays, store reagents at -20°C or lower and return them to cold storage immediately after use . Finally, maintain detailed records of antibody lot numbers, storage conditions, and performance to track any variations over time, as different lots may show slight variations in binding characteristics.

How can ESR1-pS118 antibody be used to study ESR1 mutations in breast cancer?

ESR1-pS118 antibody offers valuable tools for studying ESR1 mutations in breast cancer, particularly those associated with endocrine resistance. Multiple studies have identified activating ESR1 mutations in the ligand-binding domain, including Y537S, D538G, and L536H, which occur in approximately 13% of metastatic breast cancer samples and are associated with resistance to endocrine therapy . By combining ESR1-pS118 antibody detection with mutation analysis, researchers can investigate how these mutations alter the phosphorylation state of the receptor and subsequently its function. Studies have demonstrated that ESR1 mutations and ER loss are mutually exclusive mechanisms of resistance, together accounting for approximately 30% of endocrine resistance cases . The antibody can be used in various experimental models, including patient-derived xenografts and long-term estrogen-deprived (LTED) cell lines that spontaneously develop ESR1 mutations such as the SUM44-LTED model with the Y537S mutation . By analyzing changes in S118 phosphorylation patterns in these models, researchers can gain insights into how mutations affect receptor activation, potentially leading to novel therapeutic approaches for overcoming resistance. Additionally, monitoring pS118 levels in sequential patient samples may help track the emergence of ESR1 mutations during treatment.

What is the role of ESR1-pS118 in monitoring treatment response in metastatic breast cancer?

Table 1: Comparison of ER loss and ESR1 mutation as resistance mechanisms in metastatic breast cancer .

How do different fixation methods affect ESR1-pS118 detection in tissue samples?

Different fixation methods significantly impact ESR1-pS118 detection in tissue samples, affecting both sensitivity and specificity. Formalin fixation, the most common method, creates protein cross-links that can mask phospho-epitopes, necessitating careful optimization of epitope retrieval steps. For ESR1-pS118 detection in formalin-fixed, paraffin-embedded (FFPE) tissues, microwave-based epitope retrieval at pH7 has proven effective, with a specific protocol of heating to boiling, maintaining at 95-98°C for 5 minutes, followed by 1 minute at 100°C, and then cooling to room temperature . The duration of fixation is critical - overfixation (>24 hours) can irreversibly mask phospho-epitopes, while underfixation leads to poor tissue morphology and inconsistent staining. Alternative fixatives like Bouin's solution or zinc-based fixatives may preserve certain phospho-epitopes better than formalin but require protocol adjustments. Alcohol-based fixatives may preserve phosphorylation status but can cause protein extraction and altered morphology. Fresh-frozen tissues generally maintain phosphorylation states better than fixed tissues but present challenges in morphological assessment and long-term storage. Researchers should conduct comparative studies with different fixation methods when establishing protocols for new antibodies or tissue types, always including appropriate positive and negative controls to validate the specificity of the observed staining patterns.

What are the key differences in protocol optimization for ESR1-pS118 detection between cell lines and patient samples?

Protocol optimization for ESR1-pS118 detection differs significantly between cell lines and patient samples due to several key factors. First, sample heterogeneity: cell lines provide homogeneous populations with consistent ESR1 expression, while patient samples contain diverse cell types with variable receptor expression and phosphorylation states. Second, preservation conditions: cell lines can be processed immediately under controlled conditions, whereas patient samples often undergo variable ischemia times before fixation, affecting phosphoprotein preservation. Third, background interference: patient samples typically exhibit higher background staining due to endogenous peroxidases, biotin, and other factors requiring additional blocking steps not necessary for cell lines. Fourth, validation requirements: for cell lines, experimental manipulations (like Calyculin A treatment) can serve as internal controls, while patient samples require serial sections with positive and negative controls. Fifth, signal amplification needs: detection in patient samples often requires more sensitive methods due to lower or more variable target abundance compared to cell lines. For optimal results with patient samples, researchers should: use phosphatase inhibitors during collection when possible, minimize cold ischemia time, optimize antigen retrieval conditions specifically for the tissue type being studied, employ higher antibody concentrations than those used for cell lines (potentially 2-3× higher), and consider signal amplification systems like tyramide signal amplification for low-abundance phospho-proteins.

How can multiple phosphorylation sites on ESR1 be simultaneously assessed in research samples?

Simultaneous assessment of multiple ESR1 phosphorylation sites provides comprehensive insights into receptor activation status and signaling pathway engagement. Several advanced techniques facilitate this multi-site analysis. First, multiplex immunofluorescence allows visualization of different phosphorylation sites (such as pS118, pS167, and pS305) within the same tissue section by using primary antibodies from different species or isotypes, followed by species-specific secondary antibodies conjugated to distinct fluorophores. Second, proximity ligation assays can be adapted for multi-site analysis by combining antibodies against total ESR1 with antibodies against different phosphorylation sites in separate reactions on serial sections . Third, mass spectrometry-based phosphoproteomics offers the most comprehensive approach, enabling unbiased detection and quantification of all phosphorylation sites simultaneously, though it requires specialized equipment and expertise. Fourth, reverse phase protein arrays (RPPA) allow high-throughput analysis of multiple phosphorylation sites across many samples. Fifth, multiplex Western blotting systems permit detection of several phospho-epitopes on the same membrane through sequential stripping and reprobing or use of spectrally distinct fluorescent secondary antibodies. For accurate interpretation, researchers should consider the hierarchical and potentially interdependent nature of different phosphorylation events - some sites may influence the phosphorylation status of others. Additionally, correlation with functional assays such as reporter gene assays or gene expression analysis helps establish the biological significance of observed phosphorylation patterns in different experimental or clinical contexts.

What is the relationship between ESR1 mutations and phosphorylation at S118 in endocrine resistance?

The relationship between ESR1 mutations and phosphorylation at S118 in endocrine resistance reveals a complex interplay between different activation mechanisms of the estrogen receptor. ESR1 mutations, particularly those affecting the ligand-binding domain (LBD) such as Y537S, D538G, and L536H, enable the receptor to maintain an active conformation independently of estrogen binding . This constitutive activity reduces the reliance on phosphorylation-dependent activation mechanisms, including those involving S118. Research comparing wild-type and LTED cells harboring the Y537S mutation demonstrated lower levels of pESR1 ser118 in the mutant cells under estrogen-deprived conditions . This suggests that as cells develop ESR1 mutations under the selective pressure of endocrine therapy, the importance of S118 phosphorylation in receptor activation may diminish. Furthermore, studies have shown that ESR1 mutations and ER loss are mutually exclusive mechanisms of resistance (p = 0.042), with ESR1 mutations occurring in approximately 13% of metastatic samples and strongly associated with prior endocrine therapy (p = 0.002) . This mutual exclusivity indicates distinct evolutionary paths to resistance, with different implications for monitoring and treatment strategies. Understanding the changing relationship between phosphorylation and mutation status during disease progression may help identify optimal therapeutic approaches for different resistance mechanisms.

How can combined analysis of ESR1-pS118 and ESR1 mutations improve stratification of endocrine-resistant breast cancer?

Combined analysis of ESR1-pS118 and ESR1 mutations offers a more comprehensive approach to stratifying endocrine-resistant breast cancer, potentially leading to more personalized treatment strategies. Research has demonstrated that ER loss and ESR1 mutations together account for approximately 30% of endocrine resistance cases, with these mechanisms being mutually exclusive . By integrating phosphorylation analysis with mutation testing, researchers and clinicians can better categorize resistance mechanisms and predict treatment responses. For instance, tumors with ESR1 mutations but maintained pS118 levels might respond differently to selective estrogen receptor degraders (SERDs) compared to those with mutations and altered phosphorylation patterns. This stratification becomes particularly relevant as novel therapies targeting specific resistance mechanisms emerge. Recent clinical trials have shown that new selective estrogen receptor degraders can prolong progression-free survival specifically in patients with metastases harboring ESR1 mutations . The table below illustrates how combined analysis might inform treatment decisions:

How might ESR1-pS118 analysis contribute to liquid biopsy approaches for monitoring breast cancer?

ESR1-pS118 analysis could significantly enhance liquid biopsy approaches for monitoring breast cancer progression and treatment response. While current liquid biopsy techniques primarily focus on circulating tumor DNA (ctDNA) to detect ESR1 mutations, incorporating phosphoprotein analysis could provide complementary functional information about receptor activity. Emerging technologies for protein analysis in blood samples, such as proximity extension assays and highly sensitive immunoassays, could potentially detect phosphorylated ESR1 from circulating tumor cells or extracellular vesicles. This multi-analyte approach would offer several advantages: first, it could identify functional changes in ESR1 signaling before genetic alterations occur; second, it might detect resistance mechanisms not captured by mutation analysis alone; third, it could provide real-time monitoring of treatment effects on receptor activity. Research has shown that ESR1 mutations evolve under the selective pressure of endocrine therapy and are rarely found in primary tumors but appear in approximately 13% of metastatic samples . By monitoring both mutation status and phosphorylation patterns simultaneously through liquid biopsies, clinicians could potentially detect emerging resistance earlier and adjust treatment strategies accordingly. This approach aligns with current clinical guidelines recommending ESR1 mutation testing for metastatic breast cancer but extends beyond genetic analysis to incorporate functional protein assessment, potentially providing a more comprehensive view of disease evolution during treatment.

What role might artificial intelligence play in interpreting complex ESR1 phosphorylation patterns?

Artificial intelligence (AI) holds significant promise for interpreting complex ESR1 phosphorylation patterns in research and clinical settings. The dynamic nature of receptor phosphorylation, involving multiple sites (including S118, S167, S305, and others) influenced by various signaling pathways, creates intricate patterns that are challenging to interpret using traditional analytical approaches. AI algorithms, particularly deep learning models, could analyze these multi-dimensional data sets to identify subtle patterns associated with treatment response or resistance mechanisms. For image-based analyses, convolutional neural networks could be trained on immunohistochemistry or immunofluorescence images to quantify phosphorylation signals, distinguish subcellular localization patterns, and correlate these with clinical outcomes. Machine learning approaches could integrate phosphorylation data with other molecular features (mutations, gene expression profiles, chromatin binding patterns) to develop predictive models for treatment response. Studies have already demonstrated how integrated analysis of ChIP-seq and RNA-seq data can reveal distinct gene expression patterns associated with ESR1 mutations and endocrine resistance . AI could extend this approach by incorporating phosphorylation data to create more comprehensive predictive models. Furthermore, AI algorithms could help standardize phospho-protein analysis across laboratories by normalizing for technical variables and identifying optimal cut-points for categorizing phosphorylation levels. As the field moves toward more personalized treatment approaches based on molecular profiling, AI-assisted interpretation of complex phosphorylation patterns could become an important component of clinical decision support systems.