RPS6KA6 Antibody

What is RPS6KA6 and what cellular functions does it perform?

RPS6KA6 (Ribosomal Protein S6 Kinase, 90kDa, Polypeptide 6), also known as RSK4, is a constitutively active serine/threonine-protein kinase that exhibits growth-factor-independent kinase activity. It participates in p53/TP53-dependent cell growth arrest signaling and plays an inhibitory role during embryogenesis . The protein belongs to the protein kinase superfamily, specifically the AGC Ser/Thr protein kinase family, and S6 kinase subfamily .

The wild-type RSK4 protein has a molecular weight of approximately 90 kDa, though research has shown that several isoforms at or smaller than 72-kD are expressed in human cancer cell lines . These smaller isoforms often appear as duplets or triplets on immunoblots, with levels varying greatly among different cell lines and culture conditions .

What is known about the structure and alternative splicing of RPS6KA6?

Research has identified a new RSK4 transcription initiation site and several alternative splice sites. The resulting mRNA variants encompass four possible first start codons . Specifically, the first 15 nucleotides of exon 22 in mouse and the penultimate exon in both human (exon 21) and mouse (exon 24) RSK4 undergo alternative splicing .

Interestingly, the penultimate exon deleted variant appears mainly in cell lines but not in most normal tissues. Treatment with demethylation agent 5-azacytidine inhibits the deletion of the penultimate exon, while indolocarbazole-derived inhibitors of cyclin dependent kinase 4 or 6 induce deletion of the first 39 nucleotides from exon 21 of human RSK4 . These findings suggest that the expression of RPS6KA6 splice variants is regulated by epigenetic mechanisms and cell cycle-related pathways.

How is RPS6KA6 regulated at the post-translational level?

RPS6KA6 is phosphorylated at several sites, specifically at Ser-232, Ser-372, and Ser-389 in serum-starved cells . This suggests that the protein maintains some level of activity even under growth factor-deprived conditions, which is consistent with its characterization as a constitutively active kinase.

Additionally, there appears to be a regulatory relationship between RPS6KA6, cyclin D1, and c-Myc. Research has shown that cyclin D1 inhibits RSK4 expression, and this inhibition is enhanced during serum starvation. Conversely, c-Myc and RSK4 inhibit cyclin D1 expression . This reciprocal regulation suggests that RPS6KA6 participates in complex signaling networks that control cell cycle progression and cellular growth.

What is the significance of RPS6KA6 in cancer research?

RPS6KA6 shows altered expression in various cancer types, with particularly significant implications in lung squamous cell carcinoma (LUSC). Immunohistochemical analysis has revealed that RPS6KA6 protein expression is significantly higher in LUSC tissues (35.4%, 62/175) compared to normal lung tissues (13.3%, 4/30, P=0.017) .

Furthermore, RPS6KA6 expression correlates with several clinicopathological parameters:

This data indicates that RPS6KA6 expression significantly correlates with disease progression and metastatic potential in LUSC . Additionally, mRNA expression analysis from TCGA data shows that RPS6KA6 is markedly higher in LUSC samples than in paired noncancerous samples (P=0.005), with a 1.145 fold change .

What are the best validated antibodies for detecting RPS6KA6 in human samples?

Several validated antibodies are available for detecting RPS6KA6 in human samples, with varying specificities and applications:

• Mouse monoclonal antibody (clone 3G12) targeting amino acids 636-745, suitable for ELISA, Western Blotting, and Immunofluorescence in human samples .

• Rabbit polyclonal antibody targeting the N-terminal region (amino acids 15-45), with reactivity to human and monkey samples and validated for Western Blot and IHC applications .

• Prestige Antibodies® rabbit polyclonal antibody targeting the sequence YTMLAGYTPFANGPNDTPEEILLRIGNGKFSLSGGNWDNISDGAKDLLSHMLHMDPHQRYTAEQILKHSWITHRDQLPNDQPKRNDVSHVVKGAMVATYSALTHKTF, recommended for immunofluorescence and immunohistochemistry .

For most research applications, antibodies validated through multiple methods (orthogonal validation, independent antibody validation, etc.) should be preferred as indicated by the Human Protein Atlas validation scores .

What are the optimal protocols for immunohistochemical detection of RPS6KA6?

For immunohistochemical detection of RPS6KA6, the following protocol has been successfully implemented in research studies:

• Fix tissue samples in 10% neutral-buffered formalin for 48 hours, then embed in paraffin .

• Perform antigen retrieval (method may vary based on the antibody used, but typically involves heat-induced epitope retrieval in citrate buffer at pH 6.0) .

• For antibody incubation, use:

  • Mouse monoclonal anti-human RPS6KA6 antibody (RPS6KA6 PL-68; Santa Cruz Biotech) diluted to 1:300 , or

  • For Prestige Antibodies®, use a dilution of 1:50-1:200

  • For immunofluorescence applications, use 0.25-2 μg/mL

• Count positive cells in 10 randomly chosen high magnification fields (400×) .

• Score results based on both percentage of positive cells and staining intensity:

  • Percentage scoring: 0 (0%), 1 (1-25%), 2 (26-50%), 3 (51-75%), and 4 (76-100%)

  • Intensity scoring: 0 (negative), 1 (weak), 2 (moderate), and 3 (strong)

  • Final score = percentage score × intensity score

  • Scores >2 are considered positive staining

Note that RPS6KA6-positive signaling has been observed in both the cytoplasm and nucleus of tumor cells .

What challenges might researchers encounter when detecting RPS6KA6 isoforms?

Detecting RPS6KA6 isoforms presents several challenges for researchers:

• Multiple isoforms: The wild-type RSK4 is 90-kD, but several isoforms at or smaller than 72-kD are expressed in human cancer cell lines. Each smaller isoform often appears as a duplet or triplet on immunoblots .

• Variable expression: The levels of these isoforms vary greatly among different cell lines and culture conditions .

• Alternative splicing: Various splice variants exist, which can affect antibody recognition. The first 15 nucleotides of exon 22 in mouse and the penultimate exon in both human and mouse RSK4 undergo alternative splicing .

• Multiple protein products from single cDNA: Research has observed that even a given cDNA might be expressed as multiple proteins. Therefore, when using a cDNA for experimental purposes, researchers need to exclude this possibility before attributing biological results to the anticipated protein .

• Contextual biological effects: The effects of RSK4 on cell growth, cell death, and chemoresponse depend on the mRNA variant or protein isoform expressed, cell line specificity, and growth conditions (anchorage-dependent or -independent) .

How do RPS6KA6 expression patterns differ between normal and cancerous tissues?

RPS6KA6 expression shows distinct patterns between normal and cancerous tissues:

This data indicates significantly higher expression of RPS6KA6 in lung squamous cell carcinoma compared to normal tissue .

Additionally, RPS6KA6 expression correlates with tumor progression markers:

• Larger tumor sizes (>7 cm) showed higher RPS6KA6 protein expression (70.0%, 14/20) compared to smaller tumors (≤7 cm) (31.0%, 48/115, P=0.001) .

• Samples with lymph node metastasis showed substantially higher positive rates (82.5%, 47/57) compared to those without (12.7%, 15/118, P<0.001) .

These findings suggest that RPS6KA6 may play an oncogenic role in LUSC, which is supported by observations that RSK4 is overexpressed in more than 50% of malignant lung cancers .

What controls should be included when performing Western blotting for RPS6KA6?

When performing Western blotting for RPS6KA6, researchers should include the following controls:

• Positive control: Use cell lines known to express RPS6KA6 (based on the literature, multiple human cancer cell lines express various isoforms) .

• Negative control: Include samples where RPS6KA6 expression is known to be low or absent, or use siRNA knockdown samples if available .

• Loading control: Use housekeeping proteins such as β-actin, GAPDH, or tubulin to ensure equal protein loading across lanes.

• Molecular weight markers: Include markers that span the range of 70-100 kDa to accurately identify the 90 kDa wild-type RPS6KA6 and the smaller isoforms (≤72 kDa) .

• Recombinant protein: When possible, include purified recombinant RPS6KA6 or a partial recombinant with GST tag (the MW of the GST tag alone is 26 kDa) .

• Treatment controls: If studying regulation, include samples from cells treated with relevant agents (e.g., 5-azacytidine or cyclin dependent kinase inhibitors, which affect alternative splicing) .

For enhanced validation, researchers should consider performing Western blotting with at least two independent antibodies targeting different epitopes of RPS6KA6 .

How can researchers optimize antigen retrieval for RPS6KA6 detection in FFPE tissues?

Optimizing antigen retrieval for RPS6KA6 detection in FFPE (Formalin-Fixed Paraffin-Embedded) tissues is critical for successful immunohistochemistry. Based on research protocols, the following approach is recommended:

• Fixation parameters: Fix tissue samples in 10% neutral-buffered formalin for 48 hours before paraffin embedding to standardize fixation effects .

• Antigen retrieval methods:

  • Heat-Induced Epitope Retrieval (HIER) using citrate buffer (pH 6.0) is generally effective

  • Alternatively, EDTA buffer (pH 9.0) can be tested if citrate buffer yields suboptimal results

  • Compare microwave, pressure cooker, and water bath methods to determine optimal conditions

• Duration and temperature: Test different combinations (e.g., 10-20 minutes at 95-121°C) to identify optimal conditions for your specific tissue and antibody.

• Protocol validation: Always include positive control tissues (e.g., LUSC samples known to express RPS6KA6) to verify the effectiveness of the antigen retrieval method .

• Antibody optimization: After establishing the optimal antigen retrieval method, titrate the primary antibody concentration to determine the optimal dilution (typically between 1:50-1:300 for most anti-RPS6KA6 antibodies) .

• Background reduction: If background staining is excessive, introduce additional blocking steps or adjust the retrieval conditions to milder parameters.

Remember that the effectiveness of antigen retrieval can vary depending on the specific epitope recognized by your antibody, so modifications may be necessary when switching between antibodies targeting different regions of RPS6KA6.

What is the relationship between RPS6KA6 and cell cycle regulation?

RPS6KA6 (RSK4) appears to have complex interactions with cell cycle regulatory proteins and pathways:

• Cyclin D1 interaction: Research has shown that cyclin D1 inhibits RSK4 expression, and this inhibition is enhanced during serum starvation conditions .

• Reciprocal regulation: Conversely, RSK4 and c-Myc both inhibit cyclin D1 expression, suggesting a feedback regulatory mechanism .

• CDK inhibitor response: Treatment with indolocarbazole-derived inhibitors of cyclin dependent kinase 4 or 6 induces alternative splicing of RSK4, specifically causing deletion of the first 39 nucleotides from exon 21 of human RSK4 .

• Growth arrest signaling: RPS6KA6 participates in p53/TP53-dependent cell growth arrest signaling pathways, indicating its role in mediating cell cycle arrest in response to stress signals or DNA damage .

• Constitutive activity: Unlike some other kinases that require activation by growth factors, RPS6KA6 exhibits growth-factor-independent kinase activity, which may allow it to maintain regulatory functions even under growth factor-deprived conditions .

• Phosphorylation state: RPS6KA6 is phosphorylated at Ser-232, Ser-372, and Ser-389 in serum-starved cells, suggesting that its activity may be regulated by phosphorylation in response to cellular stress or nutrient deprivation .

These interactions suggest that RPS6KA6 functions within a complex network of cell cycle regulators and may play different roles depending on the cellular context and expression of other regulatory proteins.

How can researchers differentiate between RPS6KA6 and other members of the RSK family?

Differentiating between RPS6KA6 (RSK4) and other members of the RSK family requires careful consideration of several factors:

• Antibody specificity: Select antibodies that have been validated for specificity against RPS6KA6 with minimal cross-reactivity to other RSK family members. Antibodies targeting unique regions of RPS6KA6 that have low sequence homology with other RSKs are preferable .

• Molecular weight discrimination: While most RSK family members have similar molecular weights around 90 kDa, subtle differences in migration patterns on SDS-PAGE can help differentiate them. RPS6KA6 wild-type is approximately 90 kDa, but also presents smaller isoforms (≤72 kDa) that might be distinctive .

• Expression pattern analysis: Different RSK family members have distinct tissue expression patterns. Comparing your results with published expression profiles can help confirm the identity of the detected protein .

• Isoform-specific detection: Use primers or antibodies that target unique splice variants or isoforms of RPS6KA6 that are not present in other RSK family members .

• Functional assays: Different RSK family members may have distinctive functional characteristics. For example, RPS6KA6 exhibits growth-factor-independent kinase activity, which may differ from other RSKs .

• Knockdown/overexpression controls: Include controls with specific knockdown or overexpression of RPS6KA6 versus other RSK family members to validate the specificity of your detection methods .

• Mass spectrometry validation: For definitive identification, consider using mass spectrometry to distinguish between closely related RSK family members based on unique peptide sequences.

What are the known correlations between RPS6KA6 expression and clinical outcomes in cancer?

Research has identified significant correlations between RPS6KA6 expression and clinical parameters in cancer, particularly in lung squamous cell carcinoma (LUSC):

• Tumor size correlation: RPS6KA6 expression positively correlates with tumor size (r=0.260, P=0.001), with significantly higher expression observed in tumors >7 cm (70.0% positive) compared to tumors ≤7 cm (31.0% positive) .

• Lymph node metastasis: A strong positive correlation exists between RPS6KA6 expression and lymph node metastasis (r=0.683, P<0.001). Samples with lymph node metastasis show substantially higher positive rates (82.5%) compared to those without (12.7%) .

• TNM staging: RPS6KA6 expression correlates with TNM stage (r=0.378, P<0.001), suggesting its potential utility as a marker for disease progression .

• Expression in cancer vs. normal tissue: RPS6KA6 protein expression is significantly higher in LUSC tissues (35.4%) compared to normal lung tissues (13.3%, P=0.017) .

• mRNA expression: Analysis of TCGA data shows that RPS6KA6 mRNA is markedly higher in LUSC samples than in paired noncancerous samples (P=0.005), with a 1.145 fold change .

What methodological approaches can be used to study RPS6KA6 phosphorylation status?

To study the phosphorylation status of RPS6KA6, researchers can employ several methodological approaches:

• Phospho-specific antibodies: Use antibodies that specifically recognize phosphorylated forms of RPS6KA6 at key sites (Ser-232, Ser-372, and Ser-389) . These can be used in Western blotting, immunohistochemistry, or immunofluorescence applications.

• Phos-tag SDS-PAGE: This technique uses a manganese-Phos-tag complex to specifically retard the migration of phosphorylated proteins in gels, allowing separation of differently phosphorylated forms of RPS6KA6.

• Mass spectrometry:

  • Phosphopeptide enrichment using titanium dioxide (TiO2) or immobilized metal affinity chromatography (IMAC)

  • Liquid chromatography-tandem mass spectrometry (LC-MS/MS) for identification and quantification of specific phosphorylation sites

  • Multiple reaction monitoring (MRM) for targeted analysis of known phosphorylation sites

• Kinase/phosphatase treatment assays:

  • Treatment of cell lysates with lambda phosphatase to remove phosphorylation

  • In vitro kinase assays to study site-specific phosphorylation

  • Comparison of migration patterns before and after phosphatase treatment

• Cellular manipulation:

  • Serum starvation experiments to study constitutive phosphorylation

  • Treatment with kinase inhibitors to identify upstream regulators

  • Site-directed mutagenesis of key phosphorylation sites (Ser to Ala mutations) to study functional consequences

• Bioluminescence resonance energy transfer (BRET) or Förster resonance energy transfer (FRET) assays to study phosphorylation in living cells using phospho-specific conformational sensors.

• Proximity ligation assay (PLA) to visualize interactions between RPS6KA6 and its phosphorylation machinery in situ.

These methods can be used complementarily to obtain a comprehensive understanding of RPS6KA6 phosphorylation patterns and their functional significance.

How does alternative splicing of RPS6KA6 affect antibody selection for research applications?

Alternative splicing of RPS6KA6 significantly impacts antibody selection and experimental design:

• Epitope location considerations: The choice of antibody should take into account known splice variants. For example, antibodies targeting the penultimate exon in human (exon 21) or mouse (exon 24) RSK4 may fail to detect variants where these exons are alternatively spliced .

• Multiple band pattern interpretation: When performing Western blotting, researchers should anticipate that alternative splicing may result in multiple bands. The first 15 nucleotides of exon 22 in mouse and the penultimate exon in both human and mouse RSK4 undergo alternative splicing, potentially generating multiple protein products .

• Tissue vs. cell line differences: The penultimate exon deleted variant appears mainly in cell lines but not in most normal tissues, suggesting that antibodies targeting this region may show different results depending on the sample type .

• Treatment-induced splicing changes: Demethylation agents like 5-azacytidine inhibit the deletion of the penultimate exon, while CDK4/6 inhibitors induce deletion of the first 39 nt from exon 21 of human RSK4. These treatment effects need to be considered when interpreting antibody-based detection results .

• Isoform-specific detection strategy: For specific detection of particular variants, researchers should consider:

  • Using antibodies that target junction-specific epitopes spanning exon-exon boundaries

  • Combining antibodies targeting different regions to differentiate between variants

  • Using RT-PCR to verify the presence of specific splice variants at the mRNA level before interpreting protein detection results

• Validation with multiple antibodies: Due to the complexity of RPS6KA6 splicing, using multiple antibodies targeting different epitopes is advisable for comprehensive detection and validation, particularly when studying novel splice variants .

What are the best practices for quantifying RPS6KA6 expression in tissue microarrays?

When quantifying RPS6KA6 expression in tissue microarrays (TMAs), researchers should follow these best practices:

• Scoring system implementation:

  • Use a dual parameter scoring system that accounts for both staining intensity and percentage of positive cells

  • Score staining intensity as: 0 (negative), 1 (weak), 2 (moderate), and 3 (strong)

  • Score percentage of positive cells as: 0 (0%), 1 (1-25%), 2 (26-50%), 3 (51-75%), and 4 (76-100%)

  • Calculate final score by multiplying these values, with scores >2 considered positive

• Standardized evaluation:

  • Count positive cells in 10 randomly chosen high magnification fields (400×) for each core

  • Have at least two independent pathologists score the samples blindly to reduce bias

  • Use digital image analysis software when possible to enhance objectivity and reproducibility

• Quality control measures:

  • Include positive and negative control tissues in each TMA

  • Use cores from multiple regions of each tumor to account for heterogeneity

  • Include duplicate or triplicate cores for each case to enhance reliability

• Subcellular localization assessment:

  • Record whether RPS6KA6 staining is cytoplasmic, nuclear, or both, as it has been observed in both locations in tumor cells

  • Consider separate scoring for nuclear and cytoplasmic staining if differential localization is observed

• Data analysis considerations:

  • Use appropriate statistical methods for correlation with clinicopathological parameters

  • Consider using Spearman's rank correlation test for assessing relationships with clinical parameters such as TNM stage, tumor size, and lymph node metastasis

  • Set clear thresholds for categorizing expression levels (e.g., negative/low/high) based on score distributions

• Validation and reproducibility:

  • Verify results with alternative detection methods when possible (e.g., qRT-PCR, Western blotting)

  • Document all antibody details, staining protocols, and scoring criteria thoroughly for reproducibility