Phospho-AR (S650) Antibody

What is the biological significance of AR S650 phosphorylation?

Serine 650 (S650) is a key phosphorylation site in the androgen receptor (AR) hinge region that regulates nuclear localization, DNA binding, and co-activator recruitment. Based on research findings, S650 appears to be constitutively phosphorylated, meaning it occurs without hormone stimulation. This phosphorylation was identified in the smallest AR isoform (110 kDa) in studies using reversed-phase HPLC and mass spectrometry techniques . AR phosphorylation at S650 likely plays a regulatory role in AR trafficking and nuclear-cytoplasmic shuttling, which differentiates it from other phosphorylation sites that may be more directly involved in transcriptional activation .

Methodology for verification: To confirm the biological relevance of S650 phosphorylation, researchers typically employ site-directed mutagenesis to create S650A (serine to alanine) mutants that prevent phosphorylation at this site, followed by functional assays comparing wild-type and mutant AR activity in different cellular contexts .

How does S650 phosphorylation differ from other AR phosphorylation sites?

AR contains multiple phosphorylation sites, predominantly in the N-terminal domain, including S81, S94, S213, and S515, while S650 is located in the hinge region. Research has shown that:

• S650 is constitutively phosphorylated, whereas sites like S81 show hormone-dependent phosphorylation

• S81 phosphorylation is associated with chromatin binding and transcriptional activation

• S515 phosphorylation relates to nuclear-cytoplasmic shuttling

• S213 phosphorylation affects AR stability and can be regulated by different kinases including Akt and PIM-1

Unlike S81, which has been shown to influence AR stability and transcriptional activity, mutation of S650 to alanine (S650A) did not significantly alter AR transcriptional activity in several cell lines when tested in transactivation, N- and C-terminal-domain interaction, and co-activation assays .

What experimental applications is the Phospho-AR (S650) Antibody validated for?

The Phospho-AR (S650) antibody has been validated primarily for:

• Immunohistochemistry (IHC): Recommended dilutions range from 1:50-1:300, with most suppliers suggesting 1:100 for optimal results in paraffin-embedded tissues

• ELISA: Typically used at dilutions of 1:10,000-1:20,000

When using this antibody for IHC, researchers have successfully applied it to various tissue types, including human breast carcinoma and prostate carcinoma tissues . The antibody specifically detects endogenous levels of AR protein only when phosphorylated at Serine 650, making it valuable for studying this specific post-translational modification in tissue samples .

What controls should be included when using Phospho-AR (S650) Antibody?

To ensure experimental rigor when using Phospho-AR (S650) antibody, the following controls are recommended:

Positive controls:

• Paraffin-embedded human breast carcinoma or prostate carcinoma tissues known to express phosphorylated AR at S650

• Cell lysates from androgen-treated LNCaP cells, which express endogenous AR that becomes phosphorylated at S650

Negative controls:

• Pre-incubation of the antibody with the immunizing phosphopeptide (blocking peptide), which should abolish specific staining as demonstrated in validation studies

• Samples treated with lambda phosphatase to remove phosphate groups

• Tissues from AR knockout models or cell lines with CRISPR-mediated deletion of AR

• S650A AR mutant-expressing cells where the serine is replaced with alanine to prevent phosphorylation

Successful validation is indicated by loss of signal in negative controls while maintaining specific staining in positive controls.

What kinases are responsible for AR S650 phosphorylation and how can this be experimentally determined?

While the search results don't explicitly identify the kinases responsible for S650 phosphorylation, researchers can determine this through:

Kinase Prediction Analysis:

• Bioinformatic tools like Scansite (mentioned in reference ) can predict potential kinases based on consensus sequences surrounding the phosphorylation site

Kinase Assays:

• In vitro kinase assays: Incubate purified candidate kinases with AR peptides containing the S650 site and measure phosphorylation using radiolabeled ATP or phospho-specific antibodies

• Cellular kinase inhibition: Treat cells expressing AR with specific kinase inhibitors (similar to how roscovitine was used to study S81 phosphorylation ) and assess changes in S650 phosphorylation levels

• Kinase overexpression or knockdown: Manipulate expression levels of candidate kinases and observe effects on S650 phosphorylation status

MS/MS Analysis:

• Use tandem mass spectrometry following AR immunoprecipitation to confirm S650 phosphorylation status after various treatments, as was done in the study by Doesburg et al.

How does AR S650 phosphorylation relate to the different AR isoform patterns observed on SDS-PAGE?

Research has established a direct link between AR phosphorylation status and its isoform pattern on SDS/polyacrylamide gels:

• AR appears as three distinct isoforms on SDS-PAGE: 110 kDa, 112 kDa, and 114 kDa

• The smallest 110 kDa isoform contains phosphorylation only at S650

• The 112 kDa isoform contains phosphorylation at both S650 and S94

• The hormone-induced 114 kDa isoform shows increased phosphorylation at multiple sites

This pattern indicates that S650 is the first site to be phosphorylated during AR synthesis, followed by sequential phosphorylation of additional sites.

Experimental approach to verify this relationship:

• Immunoprecipitate AR from cells at different time points after synthesis

• Separate proteins by SDS-PAGE

• Analyze isoforms using phospho-specific antibodies or mass spectrometry

• Compare wild-type AR with phosphorylation site mutants (e.g., S650A)

How can researchers investigate the functional consequences of S650 phosphorylation in disease models?

To investigate functional consequences of S650 phosphorylation in disease models, researchers should consider:

Genetic Approaches:

• Generate cell lines or animal models expressing S650A (phospho-deficient) or S650E/D (phospho-mimetic) AR mutants

• Compare phenotypes related to:

  • Cell proliferation and survival

  • AR nuclear localization and chromatin binding (ChIP assays)

  • Gene expression profiles (RNA-seq)

  • Response to hormonal treatments or AR antagonists

Clinical Sample Analysis:

• Use Phospho-AR (S650) antibody for IHC on tissue microarrays from patient samples

• Correlate S650 phosphorylation status with:

  • Clinical parameters (disease stage, recurrence, survival)

  • Response to therapies

  • Expression of AR target genes

Integration with Other Signaling Pathways:

• Study potential cross-talk between S650 phosphorylation and other pathways implicated in disease, such as mTORC1 signaling which has been linked to AR phosphorylation in hepatocellular carcinoma

The study by Willder et al. provides a methodological framework where they investigated the prognostic significance of AR phosphorylation in prostate cancer .

What factors may affect the specificity and sensitivity of Phospho-AR (S650) antibody detection?

Several factors can influence the detection of phosphorylated AR at S650:

Sample Preparation:

• Phosphorylation status can be lost during tissue fixation or protein extraction

• Rapid tissue processing is critical as phosphatases may remain active

• Use of phosphatase inhibitors (e.g., sodium fluoride, sodium orthovanadate) is essential during all extraction steps

Antibody Cross-Reactivity:

• Potential cross-reactivity with similar phosphorylated epitopes in other proteins

• Prior validation with peptide competition assays is recommended to ensure specificity

Signal Amplification Issues:

• For low-abundance phospho-proteins, consider using signal amplification systems like tyramide signal amplification

• Titrate antibody concentration to optimize signal-to-noise ratio

Technical Variables:

• Antigen retrieval methods can significantly impact phospho-epitope detection

• Buffer pH and composition may affect antibody binding efficiency

• Detection method (fluorescent vs. chromogenic) may have different sensitivity thresholds

How can phosphorylation dynamics of AR S650 be monitored in living systems?

Studying phosphorylation dynamics requires specialized approaches:

Time-Course Experiments:

• Treat cells with hormones, kinase inhibitors, or other stimuli

• Collect samples at multiple time points

• Analyze S650 phosphorylation by western blotting with the phospho-specific antibody

• Normalize to total AR levels to account for changes in protein expression

Phosphorylation-Specific Biosensors:

• Design FRET-based biosensors containing the AR sequence around S650

• Changes in phosphorylation alter protein conformation and FRET signal

• Real-time monitoring in living cells is possible with this approach

Pulse-Chase Analysis:

• Label newly synthesized proteins with radioactive amino acids

• Immunoprecipitate AR at different chase times

• Analyze phosphorylation status using phospho-specific antibodies or mass spectrometry

• This approach was used to show that S650 is constitutively phosphorylated shortly after AR synthesis

Mass Spectrometry-Based Quantification:

• Use SILAC (Stable Isotope Labeling with Amino acids in Cell culture) or TMT (Tandem Mass Tag) labeling

• Quantify changes in phosphopeptide abundance across conditions or time points

• Provides site-specific phosphorylation dynamics

How does S650 phosphorylation interact with other post-translational modifications of AR?

The interplay between S650 phosphorylation and other AR modifications represents an important area for investigation:

Known Interactions:

• Research has suggested potential cross-talk between phosphorylation sites, as mutation of S515 to alanine (S515A) resulted in an unphosphorylated form of the peptide containing S650, indicating that S515 may modulate phosphorylation at S650

Methodological Approaches:

• Sequential immunoprecipitation: Use antibodies against different modifications sequentially to isolate AR populations with specific modification patterns

• Mass spectrometry: Analyze combinations of modifications that occur simultaneously on the same AR molecule

• Mutational studies: Create single and combination phosphosite mutants to assess hierarchical relationships between modifications

• Proximity ligation assays: Detect co-occurrence of multiple modifications on the same AR molecule in situ

What is the relationship between AR S650 phosphorylation and disease progression in hormone-dependent cancers?

Tissue Microarray Analysis:

• Using phospho-specific S650 antibody on large cohorts of patient samples with long-term follow-up data

• Correlating expression with clinicopathological parameters and survival outcomes

• Similar approaches were used for other AR phosphorylation sites like S515, which was found to predict biochemical relapse in prostate cancer

Multi-parameter Analysis:

• Combining S650 phosphorylation status with other biomarkers

• Creating predictive models for disease progression

• For example, high expression of phosphorylated AR S515 in patients with PSA ≤20 ng/ml at diagnosis was associated with shorter time to biochemical relapse and reduced disease-specific survival

Functional Studies in Disease Models:

• Testing the effects of S650 phosphorylation status on cancer cell behavior

• Assessing impact on therapeutic response, particularly to androgen deprivation therapy and next-generation AR-targeting agents

The relationship between mTORC1 signaling and AR phosphorylation in hepatocellular carcinoma represents a novel direction where similar methodologies could be applied to study S650 phosphorylation .

What approaches can be used to determine the structural consequences of S650 phosphorylation?

Understanding how S650 phosphorylation affects AR structure requires specialized structural biology techniques:

X-ray Crystallography:

• Crystallize AR fragments containing phosphorylated S650 or phosphomimetic mutations

• Compare with non-phosphorylated structures to identify conformational changes

• Challenges include obtaining crystals of flexible protein regions

Nuclear Magnetic Resonance (NMR) Spectroscopy:

• Particularly useful for studying dynamic regions like the AR hinge domain containing S650

• Can detect subtle structural changes induced by phosphorylation

• Allows study of protein dynamics in solution

Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS):

• Compare exchange rates between phosphorylated and non-phosphorylated AR

• Identifies regions with altered solvent accessibility or structural stability

• Provides information about conformational changes without requiring protein crystallization

Molecular Dynamics Simulations:

• In silico modeling of AR with and without S650 phosphorylation

• Predictions of structural consequences that can guide experimental design

• Integration with experimental data for validation