RB1 (Ab-795) Antibody

What is the biological significance of RB1 phosphorylation at Serine 795?

Rb1 is a central regulator of the G1/S transition of the cell cycle, primarily functioning in its hypophosphorylated form to bind and inhibit E2F transcription factors. Phosphorylation at Serine 795 is one of several key phosphorylation events that collectively regulate Rb1 activity. When Rb1 remains hypophosphorylated, it physically blocks the E2F transactivation domain and recruits chromatin-modifying enzymes that actively repress transcription of cell cycle genes . Upon phosphorylation by cyclin-dependent kinases, particularly at sites including Ser795, Rb1 undergoes conformational changes that disrupt its interaction with E2F factors, thereby enabling E2F-dependent transcription and cell cycle progression . The phosphorylation of Ser795 specifically occurs in late G1 phase and is primarily mediated by CDK4/6-cyclin D complexes, making it an early phosphorylation event in the sequential inactivation of Rb1 .

What experimental applications is the RB1 (Ab-795) antibody validated for?

The RB1 (Ab-795) antibody has been validated for multiple experimental applications across various research settings:

• Western Blotting (WB): Detects phosphorylated Rb1 in protein lysates at approximately 110 kDa

• Immunohistochemistry on paraffin-embedded sections (IHC-P): Visualizes phospho-Rb1 distribution in tissue samples with cellular resolution

• Immunohistochemistry on frozen sections (IHC-fro): Allows detection in frozen tissue preparations

• Immunofluorescence (IF): For both cultured cells and tissue sections, enabling subcellular localization studies

• Enzyme-Linked Immunosorbent Assay (ELISA): For quantitative detection in solution-based assays

• Immunocytochemistry (ICC): For detailed cellular localization studies

The antibody has been tested and confirmed to work with human, mouse, and rat samples across multiple suppliers, making it versatile for various model systems in cancer and cell cycle research .

How does phosphorylation at Ser795 relate to the structural domains of RB1?

Serine 795 is located within the C-terminal domain of the Rb1 protein, which plays a critical role in mediating protein-protein interactions. The Rb1 protein consists of three major structural domains: the N-terminal domain, the central pocket domain (consisting of A and B cyclin folds connected by a flexible linker), and the C-terminal domain . While the central pocket domain (residues 379-792) is responsible for the primary interaction with E2F1, the C-terminal domain provides additional structural and functional interactions . Specifically, two regions in the C-terminal domain (residues 786-800 and 829-846) interact with the E2F1 coiled-coil marked box domain and the transcription factor DP1, forming a stable complex . Phosphorylation at Ser795, which falls near these interaction regions, disrupts these protein-protein interactions, contributing to the release of E2F factors and subsequent activation of E2F-dependent transcription .

What is the recommended protocol for Western blotting with RB1 (Ab-795) antibody?

For optimal Western blotting results with the RB1 (Ab-795) antibody, the following protocol is recommended:

Sample Preparation:

• Prepare cell/tissue lysates by boiling in 1X SDS sample buffer containing 1% SDS and 1.25% β-mercaptoethanol at 95°C for 5 minutes

• Include phosphatase inhibitors in all extraction buffers to prevent dephosphorylation

• Load 20-50 μg of total protein per lane (Rb1 is approximately 110 kDa)

Western Blotting Protocol:

• Separate proteins on an 8-10% SDS-PAGE gel

• Transfer to PVDF or nitrocellulose membrane

• Block with 5% non-fat dry milk or BSA in TBST for 1 hour at room temperature

• Dilute the RB1 (Ab-795) antibody to 1:500-1:1000 in blocking buffer

• Incubate membrane with primary antibody solution overnight at 4°C with gentle agitation

• Wash 3-5 times with TBST, 5 minutes each

• Incubate with HRP-conjugated anti-rabbit secondary antibody (1:2000-1:5000)

• Wash thoroughly and develop using ECL substrate

Expected Results:
A specific band should be detected at approximately 110 kDa, corresponding to phosphorylated Rb1 at Ser795. Signal intensity typically correlates with the proliferative status of the cells, with higher signals in proliferating cells and reduced signals in quiescent cells or after CDK inhibitor treatment .

How should I optimize immunohistochemistry protocols for the RB1 (Ab-795) antibody?

For effective immunohistochemistry using the RB1 (Ab-795) antibody:

Antigen Retrieval:
Heat-induced epitope retrieval is critical for optimal detection of phospho-epitopes in fixed tissues. For phospho-Ser795 Rb1:

• Citrate buffer (pH 6.0) heating for 15-20 minutes is generally effective

• In cases of weak signal, try EDTA buffer (pH 8.0)

Staining Protocol:

• Deparaffinize and rehydrate sections through xylene and graded alcohols

• Perform antigen retrieval as described above

• Block endogenous peroxidase with 3% hydrogen peroxide

• Block non-specific binding with 5-10% normal serum

• Apply RB1 (Ab-795) antibody at 1:100-1:200 dilution

• Incubate overnight at 4°C in a humidified chamber

• Wash thoroughly with PBS

• Apply appropriate HRP-conjugated secondary antibody

• Develop using DAB substrate and counterstain

Critical Controls:

• Include proliferating tissues known to express phosphorylated Rb1 as positive controls

• Use phosphatase-treated serial sections as negative controls to verify phospho-specificity

• Include isotype controls to assess non-specific binding

When optimized, phospho-Ser795 Rb1 staining should show predominantly nuclear localization, with intensity varying based on proliferative status of the tissues .

What are the key considerations for quantitative analysis of phospho-RB1 (Ser795) levels?

For accurate quantitative analysis of phospho-Rb1 (Ser795) levels:

Normalization Strategies:

Quantification Methods:

• For Western blots: Use densitometry software with linear range capture of signal

• For immunohistochemistry: Apply digital image analysis with appropriate thresholding

• For flow cytometry: Measure on a per-cell basis to account for population heterogeneity

Data Representation:
Present data as:

• Phospho-Rb1/Total Rb1 ratio

• Fold change relative to control conditions

• Percentage of positive cells (for imaging or flow cytometry)

Experimental Design Considerations:

• Include biological replicates (minimum n=3) for statistical validity

• Add technical controls like lambda phosphatase-treated samples

• Consider cell cycle synchronization to reduce heterogeneity in proliferating cultures

This quantitative approach enables reliable comparison of phospho-Ser795 levels between experimental conditions or across different cell types .

How can I distinguish between phospho-specific binding and total RB1 detection in my experiments?

Confirming phospho-specificity of the RB1 (Ab-795) antibody is crucial for experimental validity:

Phosphatase Treatment Control:

• Split your protein sample into two equal portions

• Treat one portion with lambda protein phosphatase for 30-60 minutes at 30°C

• Run both treated and untreated samples side by side on Western blot

• Probe with RB1 (Ab-795) antibody

• The signal should be substantially reduced or eliminated in the phosphatase-treated sample

Functional Validation Approaches:

• Treat cells with CDK inhibitors (palbociclib, ribociclib) which should reduce Ser795 phosphorylation

• Compare serum-starved (G0/G1 arrested) versus serum-stimulated cells

• Analyze synchronized cell populations at different cell cycle phases

Complementary Antibody Strategy:

• Run parallel analyses with antibodies against:

  • Total Rb1 (phosphorylation-independent)

  • Other phospho-specific Rb1 antibodies (pSer780, pSer807/811)

  • Downstream targets (E2F-regulated genes)

These validation steps ensure that the observed signals specifically represent the phosphorylated form of Rb1 at Ser795 rather than total protein levels or non-specific binding .

How does phosphorylation at Ser795 interact with other post-translational modifications of RB1?

Rb1 undergoes multiple post-translational modifications that function in concert to regulate its activity:

Phosphorylation Interdependence:
Ser795 phosphorylation occurs within a network of multiple phosphorylation sites:

• Early phosphorylation sites (CDK4/6-dependent): Ser780, Ser795

• Later phosphorylation sites (CDK2-dependent): Thr821, Thr826

• These modifications occur in a semi-sequential manner where initial phosphorylation events promote subsequent modifications

Cross-talk with Other Modifications:

• Acetylation: Acetylation at lysine residues can prevent phosphorylation at nearby sites

• Methylation: Rb1 methylation affects its interaction with chromatin-modifying enzymes

• Ubiquitination: Phosphorylated Rb1 can be targeted for proteasomal degradation

Functional Consequences:
Different combinations of modifications produce distinct functional outcomes:

• Partial phosphorylation (including at Ser795) may allow some E2F-binding while disrupting other interactions

• Hyperphosphorylation at multiple sites is required for complete E2F release

• Specific phosphorylation patterns can redirect Rb1 to different protein complexes or cellular compartments

Understanding this complex modification landscape is essential for interpreting the significance of Ser795 phosphorylation in different cellular contexts.

What is the relationship between RB1 phosphorylation at Ser795 and its E2F-independent functions?

Beyond E2F regulation, phosphorylated Rb1 participates in several other cellular processes:

Epigenetic Regulation:
Rb1 contributes to the silencing of repetitive DNA sequences such as endogenous retroviruses and LINE-1 elements through interactions with E2F1 and recruitment of EZH2 . This silencing is dependent on H3K27 trimethylation and is affected by Rb1 phosphorylation status . Even when Rb1 is phosphorylated at some sites, it may retain the ability to associate with chromatin modifiers but show altered targeting specificity.

Cell Cycle Regulation via Ubiquitin Ligase Complexes:
Rb1 interacts with ubiquitin ligase complexes like SCF and APC/C, which regulate cell cycle progression . Specifically, Rb1 binds to the SCF complex component SKP2 through its C-terminal domain, and this interaction is modulated by phosphorylation . The sequestration of SKP2 by Rb1 leads to stabilization of the CDK inhibitor p27, providing an E2F-independent mechanism of cell cycle regulation.

Experimental Approaches to Study E2F-Independent Functions:

• Chromatin immunoprecipitation to assess binding to repetitive elements

• Co-immunoprecipitation with components of ubiquitin ligase complexes

• Analysis of p27 and other cell cycle regulator stability

• Use of Rb1 mutants that specifically disrupt E2F binding while maintaining other functions

These E2F-independent functions highlight the need to consider broader cellular contexts when interpreting results from phospho-Ser795 antibody experiments .

What are common issues when using RB1 (Ab-795) antibody and how can they be resolved?

When working with the RB1 (Ab-795) antibody, researchers may encounter several technical challenges:

Weak or No Signal in Western Blots:

• Potential causes: Low phosphorylation levels, protein degradation, insufficient blocking

• Solutions: Ensure cells are proliferating (not quiescent), add phosphatase inhibitors to all buffers, optimize antibody concentration (try 1:200-1:500), increase protein load (50-75 μg), extend primary antibody incubation time to overnight at 4°C

Multiple Bands or Non-specific Signals:

• Potential causes: Protein degradation, cross-reactivity, high antibody concentration

• Solutions: Use fresh samples with protease inhibitors, increase washing steps (5 washes of 10 minutes each), use higher dilution of antibody (1:1000), increase blocking time or concentration

High Background in Immunohistochemistry:

• Potential causes: Insufficient blocking, high antibody concentration, inadequate washing

• Solutions: Extended blocking (2 hours at room temperature), further dilute antibody (1:200-1:500), add 0.1% Triton X-100 to washing buffer, use a different blocking agent (try 5% BSA instead of serum)

Inconsistent Results Between Experiments:

• Potential causes: Variation in cell cycle status, phosphatase activity, antibody degradation

• Solutions: Synchronize cells before experiments, standardize sample preparation protocols, aliquot antibody to avoid freeze-thaw cycles, include consistent positive controls across experiments

Following these troubleshooting strategies can significantly improve the reliability and reproducibility of results when using the RB1 (Ab-795) antibody.

How should I interpret phospho-RB1 (Ser795) patterns in cancer models?

Interpreting phospho-Rb1 patterns in cancer research requires careful consideration of several factors:

Cell Cycle Context:
Cancer cells often have dysregulated cell cycles, making interpretation challenging. Expected patterns in normal cells include:

• G0/G1: Minimal Ser795 phosphorylation

• Late G1 → S: Progressive increase in phosphorylation

• G2/M: Maintained high phosphorylation levels

In cancer models, these patterns may be altered due to:

• Constitutive CDK activation (leading to persistent phosphorylation)

• Defects in phosphatases (preventing dephosphorylation)

• Mutations in Rb1 or upstream regulators

Heterogeneity Considerations:
Cancer tissues and some cell lines show considerable heterogeneity:

• Use single-cell techniques (immunofluorescence, flow cytometry) rather than bulk analyses

• Correlate with proliferation markers (Ki-67, PCNA)

• Consider spatial distribution in tissue samples

Pathway Context:
Interpret phospho-Ser795 data in relation to the entire Rb pathway:

• Examine E2F target gene expression

• Assess CDK activity and cyclin levels

• Evaluate p16INK4a and other CDK inhibitor status

When analyzing cancer models, elevated phospho-Ser795 generally indicates pathway activation and ongoing proliferation, but must be interpreted in the context of other Rb1 modifications and potential mutations in pathway components .

What controls should be included when publishing research using the RB1 (Ab-795) antibody?

For rigorous publication-quality research using the RB1 (Ab-795) antibody, include these essential controls:

Antibody Validation Controls:

• Phosphatase treatment: Include lambda phosphatase-treated samples to confirm phospho-specificity

• Competing peptide: When available, pre-incubate antibody with phospho-peptide immunogen to demonstrate specific binding

• RB1-null cells: If possible, include RB1-knockout or -mutant cells as negative controls

Experimental Treatment Controls:

• Cell cycle manipulation: Include serum-starved (low phosphorylation) and serum-stimulated (high phosphorylation) samples

• Kinase inhibition: Treatment with CDK4/6 inhibitors should reduce Ser795 phosphorylation

• Phosphatase inhibition: Treatment with phosphatase inhibitors like okadaic acid can enhance phosphorylation signals

Technical Controls:

• Loading controls: Include both total Rb1 detection and invariant proteins (GAPDH, β-actin)

• Antibody dilution series: Demonstrate signal linearity across antibody concentrations

• Secondary-only controls: Exclude non-specific binding from secondary antibodies

Reproducibility Controls:

• Biological replicates: Minimum three independent experiments

• Technical replicates: Multiple samples within each experiment

• Alternative detection methods: Confirm key findings with a second method (e.g., both Western blot and immunofluorescence)

Including these controls will significantly strengthen the validity and reliability of research findings when publishing work using the RB1 (Ab-795) antibody .

How can phospho-RB1 (Ser795) detection be integrated with advanced molecular techniques?

Integration of RB1 (Ab-795) antibody with cutting-edge technologies opens new research possibilities:

Proximity Ligation Assay (PLA):

• Detect interactions between phospho-Rb1 and specific binding partners in situ

• Visualize complexes with E2F factors, chromatin modifiers, or other cell cycle regulators

• Quantify interaction dynamics during cell cycle progression or in response to treatments

Chromatin Immunoprecipitation (ChIP):

• Map genomic binding sites of phosphorylated Rb1

• Compare binding patterns of different phosphorylated forms

• Integrate with sequencing (ChIP-seq) or mass spectrometry (ChIP-MS) for comprehensive analysis

Live Cell Imaging:

• Combine with phospho-sensors or FRET-based approaches

• Monitor real-time changes in Rb1 phosphorylation

• Correlate with cell cycle events using dual-labeling strategies

Single-Cell Analysis Platforms:

• Apply in flow cytometry or mass cytometry (CyTOF)

• Integrate with single-cell Western blotting technologies

• Combine with single-cell RNA-seq to correlate phosphorylation with transcriptional changes

These integrated approaches can provide deeper insights into phospho-Rb1 function in complex biological systems and disease models while overcoming limitations of traditional bulk analyses .

What is the potential of phospho-RB1 (Ser795) as a biomarker in cancer research and therapeutics?

Phosphorylated Rb1 at Ser795 has emerging value as a biomarker in oncology:

Diagnostic Applications:

• Distinguish proliferating from quiescent tissues in tumor biopsies

• Identify dysregulation of the CDK-Rb pathway in specific cancer types

• Serve as a complementary marker to standard proliferation indicators

Therapeutic Response Prediction:

• Predict sensitivity to CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib)

• Baseline phosphorylation levels may correlate with treatment response

• Dynamic changes during treatment may indicate developing resistance

Combination Therapy Rationale:

• Guide rational combinations of cell cycle inhibitors with other targeted therapies

• Identify compensatory mechanisms when Rb phosphorylation persists despite treatment

• Monitor pathway reactivation during emergence of resistance

Methodological Considerations for Biomarker Development:

• Standardize tissue collection, fixation, and processing protocols

• Develop quantitative scoring systems (H-score, Allred score) for clinical application

• Validate cutoff values in large patient cohorts

• Combine with other markers for improved predictive power

As targeted therapies focusing on cell cycle regulation continue to develop, accurate assessment of Rb1 phosphorylation status using well-validated antibodies like RB1 (Ab-795) will become increasingly important in clinical research and precision oncology approaches .