RSK4 knockdown promotes proliferation, migration and metastasis of human breast adenocarcinoma cells

Expression of RSK4 protein in non-small cell lung cancer tissues, adjacent tissues and its correlation with clinicopathological features

OBJECTIVE

To evaluate the expression of Ribosomal S6 kinase 4 (RSK4) protein in non-small cell lung cancer (NSCLC) tissues and adjacent non-tumor tissues, and to elucidate its correlation with clinicopathological features of NSCLC.

METHODS

We analyzed 100 NSCLC patients treated at the Second Affiliated Hospital of Zhejiang University School of Medicine from June 2020 to June 2022. Patient demographics and clinical data, including gender, age, history of diabetes, tumor location, degree of tumor differentiation, lymph node metastasis, and clinical stage, were collected. RSK4 protein expression was assessed in tissue samples via immunohistochemical staining.

RESULTS

RSK4 protein was positively expressed in 35.00% of cancerous tissues, significantly lower than the 69.00% observed in adjacent non-tumor tissues (P < 0.05). Patients with lower tumor differentiation, advanced Tumor Node Metastasis (TNM) stages, and lymph node metastases showed significantly reduced RSK4 expression compared to those with higher differentiation, earlier TNM stages, and no lymph node metastases (all P < 0.05). Cox regression analysis indicated that TNM stage, low differentiation, and lymph node metastases significantly influenced RSK4 expression (all P < 0.05). Survival analysis revealed a higher positive prognosis survival rate of 74.29% (26/35) among patients with positive RSK4 expression, versus 53.85% (35/65) in those with negative expression (P < 0.05). Spearman correlation analysis demonstrated a significant positive correlation of RSK4 expression with TNM stage, lymph node metastasis, and patient prognosis (all P < 0.05).

CONCLUSION

Positive RSK4 expression in NSCLC tissues is significantly correlated with advanced cancer stage, poor differentiation, and presence of lymph node metastasis, suggesting a potential tumor suppressor role for RSK4 in NSCLC. This association underscores its prognostic relevance in NSCLC patients.

RSK4 promotes the macrophage recruitment and M2 polarization in esophageal squamous cell carcinoma

High infiltration of tumor-associated macrophages (TAMs) participates in host immunity and tumor progression in patients with esophageal squamous cell carcinoma (ESCC). Ribosomal s6 kinase 4 (RSK4) has been shown to be aberrantly overexpressed in ESCC. The role of RSK4 in cytokine secretion and its impact on macrophage recruitment and M2 polarization remains unclear. Therefore, a thorough understanding of RSK4 is needed to expand our knowledge of its therapeutic potential. Herein, RSK4 expression in human ESCC tissues and a xenograft mouse model was positively correlated with high infiltration of M0 and M2 macrophages which is positively associated with unfavorable overall survival outcomes and treatment resistance in patients with ESCC. In vitro experiments revealed that RSK4 derived from ESCC cells promoted macrophage recruitment and M2 polarization by enhancingsoluble intercellular adhesion molecule-1 (sICAM-1) secretion via direct and indirect STAT3 phosphorylation. Furthermore, RSK4-induced macrophages enhanced tumor proliferation, migration, and invasion by secreting C-C motif chemokine ligand 22 (CCL22). We further showed that patients with elevated CD68 and CD206 expression had unfavorable overall survival. Collectively, these results demonstrate that RSK4 promotes the macrophage recruitment and M2 polarization by regulating the STAT3/ICAM-1 axis in ESCC, influencing tumor progression primarily in a CCL22-dependent manner. These data also offer valuable insights for developing novel agents for the treatment of ESCC.

RSK4 confers paclitaxel resistance to ovarian cancer cells, which is resensitized by its inhibitor BI-D1870

Many ovarian cancers initially respond well to chemotherapy, but often become drug-resistant after several years. Therefore, analysis of drug resistance mechanisms and overcoming resistance are urgently needed. Paclitaxel is one of the first-choice and widely-used drugs for ovarian cancer, but like most drugs, drug resistance is observed in subsequent use. RSK4 is known as a tumor-suppressor, however, it has increasingly been reported to lead to drug resistance. Here, we found that RSK4 expression was elevated in paclitaxel-resistant ovarian cancer cells using DNA microarray, quantitative real-time PCR, and western blotting analysis. We examined the contribution of RSK4 to paclitaxel resistance and found that paclitaxel sensitivity was restored by RSK inhibitor co-treatment. We analyzed the mechanism by which resistance is developed when RSK4 level is elevated, and accelerated phosphorylation of the downstream translation factor eIF4B was discovered. In the Kaplan-Meier plot, the overall survival time was longer with RSK4 high, supporting its role as a tumor suppressor, as in previous findings, but the tendency was reversed when focusing on paclitaxel treatment. In addition, RSK4 levels were higher in non-responders than in responders in the ROC plotter. Finally, external expression of RSK4 in ovarian cancer cells increased the cell viability under paclitaxel treatment. These findings suggest that RSK4 may contribute to paclitaxel resistance, and that co-treatment with RSK4 inhibitors is effective treatment of paclitaxel-resistant ovarian cancer in which RSK4 is elevated.

Ribosomal S6 kinase 4 (RSK4) tumor suppressor gene promoter methylation status in ovarian cancer

BACKGROUND

Previously, we reported lower RSK4 mRNA and protein levels in malignant ovarian tumors compared to normal and benign ovarian tissues. Also, we observed a significant inverse correlation between the advanced ovarian cancer stages and RSK4 mRNA levels. We did not investigate the mechanisms involved in RSK4-reduced expression in ovarian cancer. Thus, this study investigates whether RSK4 promoter methylation in ovarian cancer tissues is responsible for its low expression. Additionally, the reactivation of RSK4 expression and its effect was studied in ovarian cancer cell lines.

METHODS AND RESULTS

RSK4 promoter methylation percentage in malignant and benign ovarian tumors and normal ovary tissues was determined by combined bisulfite restriction analysis. The reactivation of RSK4 expression by decitabine treatment was studied in OVCAR3, SKOV3, TOV-112D, and TOV-21G cells by Western blotting. Cell proliferation was determined by XTT. A significantly high methylation percentage of the RSK4 promoter was observed among malignant and benign ovarian tumors but not in normal ovarian tissue. RSK4 promoter methylation was not associated with age, histological subtype, or stages of ovarian cancer. RSK4 promoter methylation correlates weakly but not significantly with RSK4 protein expression. No correlation was shown between RSK4 methylation and RSK4 mRNA expression. Decitabine induces RSK4 reactivation in all cell lines. However, cell proliferation was reduced only in TOV-112D cells.

CONCLUSION

These data indicate that although RSK4 promoter methylation is increased in malignant ovarian tumors, this mechanism is unlikely to regulate its expression in ovarian cancer. RSK4 reactivation reduced cell proliferation only in the endometroid histological subtype.

Differential Expression of RSK4 Transcript Isoforms in Cancer and Its Clinical Relevance

While we previously revealed RSK4 as a therapeutic target in lung and bladder cancers, the wider role of this kinase in other cancers remains controversial. Indeed, other reports instead proposed RSK4 as a tumour suppressor in colorectal and gastric cancers and are contradictory in breast malignancies. One explanation for these discrepancies may be the expression of different RSK4 isoforms across cancers. Four RNAs are produced from the RSK4 gene, with two being protein-coding. Here, we analysed the expression of the latter across 30 normal and 33 cancer tissue types from the combined GTEx/TCGA dataset and correlated it with clinical features. This revealed the expression of RSK4 isoforms 1 and 2 to be independent prognostic factors for patient survival, pathological stage, cancer metastasis, recurrence, and immune infiltration in brain, stomach, cervical, and kidney cancers. However, we found that upregulation of either isoform can equally be associated with good or bad prognosis depending on the cancer type, and changes in the expression ratio of isoforms fail to predict clinical outcome. Hence, differential isoform expression alone cannot explain the contradictory roles of RSK4 in cancers, and further research is needed to highlight the underlying mechanisms for the context-dependent function of this kinase.

Raltitrexed regulates proliferation and apoptosis of HGC-27 cells by upregulating RSK4

Background

Raltitrexed is a specific inhibitor of thymidylate synthase and a potential chemotherapeutic agent for the treatment of advanced gastric cancer. In this study, we investigated the effect of raltitrexed on the proliferation of HGC-27 human gastric cancer cells and its potential underlying molecular mechanism(s).

Methods

RT-qPCR and western blotting were used to quantify RSK4 levels. Colony formation and flow cytometry assays were used to assess HGC-27 cell proliferation, cell cycle progression, mitochondrial membrane potential, and apoptosis. The expression of cell cycle and apoptosis markers were determined by western blotting.

Results

Our results demonstrate that raltitrexed upregulated RSK4 mRNA and protein levels in HGC-27 cells. Moreover, raltitrexed significantly inhibited tumor cell colony formation, arrested the cell cycle, decreased the mitochondrial membrane potential, and induced apoptosis. We observed that raltitrexed was capable of upregulating the expression of Bax, cyclin A1, and CDK3, and downregulating the expression of Bcl-2 and cleaved caspase-3. Importantly, siRNA-mediated RSK4 knockdown significantly reduced the inhibitory effect of raltitrexed on cell proliferation and its promotion of cell apoptosis. Moreover, silencing of RSK4 inhibited the raltitrexed-induced upregulation of cytochrome C. In addition, the changes in molecular markers related to the cell cycle and apoptosis induced by raltitrexed were reduced upon RSK4 depletion.

Conclusion

Our study shows that RSK4 is a key target of raltitrexed in the regulation of gastric cancer cell proliferation, cell cycle progression, and apoptosis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40360-022-00605-2.

Significance of a PTEN Mutational Status-Associated Gene Signature in the Progression and Prognosis of Endometrial Carcinoma

BACKGROUND

PTEN mutations have been reported to be involved in the development and prognosis of endometrial carcinoma (EC). However, a prognostic gene signature associated with PTEN mutational status has not yet been developed. In this study, we generated a PTEN mutation-associated prognostic gene signature for EC.

METHODS

We obtained the single-nucleotide variation and transcriptomic profiling data from The Cancer Genome Atlas database as training data and implemented the least absolute shrinkage and selection operator (LASSO) Cox regression algorithm to establish a PTEN mutation-associated prognostic gene signature. The overall survival rates of the high-risk and low-risk groups were determined with the Kaplan-Meier (K-M) method, and the accuracy of risk score prediction was tested by using the receiver operating characteristic (ROC) curve.

RESULTS

The K-M curves revealed that the EC patients with PTEN mutations augured favorable survival outcomes. Differential expression analysis between the EC patients with PTEN mutation and wild-type PTEN identified 224 differentially expressed genes (DEGs). Eighty-four DEGs that manifested prognostic value were fitted into the LASSO-Cox analysis, and a PTEN gene signature with seven mutation-associated prognostic genes that showed robust prognostic ability was constructed; this signature was then successfully validated in the other two datasets from the cBioPortal database as well as with 60 clinical specimens. Furthermore, the PTEN mutation-associated prognostic gene signature proved to be an independent prognostic predictor of EC. Remarkably, the EC patients in the high-risk group were characterized by higher tumor stages and grades as well as lower tumor mutation burden with respect to EC, with a poor survival outcome. Collectively, the PTEN mutation-associated prognostic gene signature that we developed could now be used as a favorable prognostic biomarker for EC.

CONCLUSION

In summary, we developed and validated a prognostic predictor for EC associated with PTEN mutational status that may be used as a favorable prognostic biomarker and therapeutic target for EC.

Repurposed floxacins targeting RSK4 prevent chemoresistance and metastasis in lung and bladder cancer

Lung and bladder cancers are mostly incurable because of the early development of drug resistance and metastatic dissemination. Hence, improved therapies that tackle these two processes are urgently needed to improve clinical outcome. We have identified RSK4 as a promoter of drug resistance and metastasis in lung and bladder cancer cells. Silencing this kinase, through either RNA interference or CRISPR, sensitized tumor cells to chemotherapy and hindered metastasis in vitro and in vivo in a tail vein injection model. Drug screening revealed several floxacin antibiotics as potent RSK4 activation inhibitors, and trovafloxacin reproduced all effects of RSK4 silencing in vitro and in/ex vivo using lung cancer xenograft and genetically engineered mouse models and bladder tumor explants. Through x-ray structure determination and Markov transient and Deuterium exchange analyses, we identified the allosteric binding site and revealed how this compound blocks RSK4 kinase activation through binding to an allosteric site and mimicking a kinase autoinhibitory mechanism involving the RSK4's hydrophobic motif. Last, we show that patients undergoing chemotherapy and adhering to prophylactic levofloxacin in the large placebo-controlled randomized phase 3 SIGNIFICANT trial had significantly increased (P = 0.048) long-term overall survival times. Hence, we suggest that RSK4 inhibition may represent an effective therapeutic strategy for treating lung and bladder cancer.

Prominent roles of ribosomal S6 kinase 4 (RSK4) in cancer

RSK4 refers to one Ser/Thr protein kinase functioning downstream pertaining to the signaling channel of protein kinase (MAPK) stimulated by Ras/mitogen. RSK4 can regulate numerous substrates impacting cells' surviving state, growing processes and proliferating process. Thus, dysregulated RSK4 active state display a relationship to several carcinoma categories, covering breast carcinoma, esophageal squamous cell carcinoma, glioma, colorectal carcinoma, lung carcinoma, ovarian carcinoma, leukemia, endometrial carcinoma, and kidney carcinoma. Whether RSK4 is a tumor suppressor gene or one oncogene remains controversial. No specific inhibiting elements for RSK4 have been found. This review briefs the existing information regarding RSK4 activating process, the function and mechanism of RSK4 in different tumors, and the research progress and limitations of existing RSK inhibitors. RSK4 may be a potential target of molecular therapy medicine in the future.

MiR-548d-3p Promotes Gastric Cancer by Targeting RSK4

Purpose

Previous studies have demonstrated that RSK4 inhibits the proliferation of gastric cancer cells and the occurrence of tumors. However, to date, studies involving microRNAs (miRNAs) that target RSK4 have rarely been reported. Thus, this study aimed to investigate the miRNAs that target RSK4.

Materials and Methods

We screened miRNAs related to RSK4 in miRDB, microT-CDS, TargetScan, and mirDIP databases and found 18 miRNAs. We chose miR-548d-3p for follow-up research, identified the interaction site in RSK4 by comparing the sequence, and mutated it. Thereafter, we used the dual-luciferase reporter system, real-time PCR (RT-PCR), and Western blotting to assess the effect of miR-548d-3p on RSK4. The proliferation, apoptosis, migration, and invasion of gastric cancer cells were evaluated using MTT assay, propidium iodide (PI), EdU, annexin V-FITC/PI apoptosis detection kit, wound healing assay, and transwell assay after overexpression of miR-548d-3p and RSK4. Finally, a nude mouse tumorigenesis experiment was conducted to explore the role of RSK4-targeting miR-548d-3p in tumorigenesis.

Results

miR-548d-3p negatively regulated the expression of RSK4, resulting in suppressed apoptosis, enhanced proliferation, migration, and invasion of gastric cancer cells, and accelerated tumor growth. In addition, an increase in miR-548d-3p expression enhanced the mRNA levels of CDK2, cyclin A1, cyclin D1, Bax, Bcl-2, N-cadherin, and Vimentin, and decreased E-cadherin mRNA levels by targeting RSK4.

Conclusion

miR-548d-3p promotes gastric cancer by lowering the expression of RSK4.

Wide-ranging analysis of survival-related alternative splicing events in invasive breast carcinoma

Invasive breast carcinoma (BRCA) is a serious disease that threatens the survival time of those affected. Alternative splicing (AS) involved in BRCA pathogenesis may be a potential therapeutic target. However, to the best of our knowledge, a systematic analysis of survival-related alternative splicing events (SREs) has not yet been reported. The aim of the present study was to identify SREs and analyze their potential biological functions as BRCA prognostic biomarkers. An UpSet plot demonstrated AS global characteristics. Cox's proportional hazards regression model quantitatively demonstrated the prognostic relevance of AS events. Functional enrichment analysis investigated the potential pathways through which AS events affect BRCA progression. The receiver operating characteristic curve model determined the clinical significance of AS events represented using percent-spliced-in (PSI) values. The regulatory network of splicing factors (SFs) and AS events laid the foundation for studying the role of SFs in BRCA. The present study identified 1,215 SREs and their distribution characteristics, suggesting that AS events in exon skipping (ES) primarily exerted normal physiological functions, while AS events in alternative terminator sites had the most significant prognostic effect. The present study demonstrated that survival-associated genes are involved primarily in certain biological processes of ribosomal proteins. In the diagnostic model, the alternative acceptor site, alternative donor site, alternative promoter site and ES performed well. ELAVL4 was the key gene associated with prognosis and SREs. In conclusion, a number of AS events affect BRCA initiation, progression and prognosis. The PSI value of AS events has the potential to diagnose BRCA and predict a prognosis; however, this must be confirmed in additional studies.

RSK4: a new prognostic factor in glioma

Glioma is the most common and fatal brain tumour and has a poor prognosis. Ribosomal S6 protein kinase 4 (RSK4) has been found to be involved in multiple tumour types; however, the role of RSK4 in gliomas and its clinical relevance remain unclear. In the present study, RSK4 expression was found to be significantly increased in glioma tissues compared with matched adjunct non-noncancerous tissues. Moreover, the expression of RSK4 was significantly higher in high-grade (III and IV) glioma tissues than in low-grade (I and II) glioma tissues. The data showed that the expression of RSK4 was significantly correlated with WHO grade, three-year survival rate and five-year survival rate. Kaplan-Meier analyses showed that patients with high RSK4 expression had poor overall survival. In addition, multivariate Cox regression analysis showed that RSK4 might be an independent prognostic factor in glioma patients. Collectively, these results suggest that RSK4 may be a new prognostic factor in glioma patients, and RSK4 is expected to be a potential biomarker and a potential target for glioma therapy.

Effect of RSK4 on Biological Characteristics of Gastric Cancer

Purpose

Gastric cancer is one of the most common cancers with high mortality. Emerging evidences show that ribosomal s6 kinase4 (RSK4) may be an anti-oncogene in several types of cancers, while its function in GC is still unclear. In the present study, we investigated the role of RSK4 in GC progression using MGC-803 and HGC-27 cell lines in vitro and in vivo.

Methods

The expression of RSK4 in gastric cancer cells was evaluated using RT-qPCR and Western blot analysis. We transfected cells with RSK4 siRNA to reduce the expression of RSK4 and then evaluated the effect of RSK4 on cellular function. MTT and cell cycle assays were used to study its effect on cell growth. Flow cytometry was used to evaluate cell apoptosis. Wound healing and Transwell assays were performed to investigate metastasis. Stable cell lines with or without RSK4 knockdown were constructed with lentivirus and tumor-bearing mice were used to investigate the effect of RSK4 on cancer progression.

Results

The results revealed that reduction of RSK4 expression inhibited cell apoptosis and promoted cell proliferation, migration, and invasion. Additionally, RSK4 knockdown promoted tumorigenesis in vivo.

Conclusion

Our study demonstrated that RSK4 serves as a tumor suppressor in GC.

Estradiol promotes the progression of ER+ breast cancer through methylation-mediated RSK4 inactivation

Background: It’s well documented that estrogen plays a vital role in breast cancer progression, but the molecular mechanisms underlying it still remains incompletely clear. This study aimed to explore whether ribosomal protein S6 kinase 4 (RSK4) was involved in estrogen-induced breast cancer development and its underlying mechanism.

Methods: The expressions of estrogen receptor (ER) and RSK4 were assessed by immunohistochemistry, RT-PCR and Western blotting. The methylation of RSK4 promoter was evaluated by bisulfite genomic sequencing. MTT, clone formation, flow cytometry and Transwell chamber assays were performed to detect cell proliferation, clone formation, apoptosis and invasion abilities. Luciferase gene reporter assay was used to detect the transcriptional activity of RSK4. The expressions of methyltransferases, such as DNMT1, DNMT3A and DNMT3B were tested by Western blotting.

Results: ER expression and RSK4 methylation were significantly elevated in ER positive (ER+) breast cancer tissues and MCF-7 cells, whereas RSK4 expression was reduced. 17β-estradiol (E2) treatment obviously decreased RSK4 expression and transcriptional activity, as well as promoted cell proliferation, clone formation and invasion and reduced cell apoptosis in ER+ MCF-7 cells. Furthermore, E2 reduced RSK4 expression through promoting DNMT3B-induced RSK4 methylation.

Conclusion:  In summary, the present study reveals that estrogen promotes the progression of breast cancer through methylation-mediated RSK4 inactivation in ER+ breast cancer. Our study might provide a novel target for the treatment of ER+ breast cancer.

Effect of RSK4 on biological characteristics of colorectal cancer

Background

This study aimed to investigate the expression of P90 Ribosomal Protein S6 kinase 4 (RSK4) in colorectal cancer cells and its biological function.

Methods

We selected early SW480 and HCT116 colorectal cancer cell lines, using Lipofectamine™ 2000 transfection reagent carrying RSK4 gene transfected into cells to establish the colorectal cancer cell lines with high expression of RSK4. RT-PCR and western blot (WB) analysis confirmed RSK4 expression in SW480 and HCT116 cancer cell lines. We used methylthiazoltetrazolium (MTT) assay and flow cytometry to detect the proliferation of colorectal cancer cells. After transfection of RSK4, the effect of RSK4 on the RNA levels associated with epithelial–mesenchymal transition (EMT) of colorectal cancer cells was analyzed by real-time fluorescence quantitative PCR and the expression of EMT-related protein was detected by WB analysis.

Results

After transfection of RSK4 overexpression, the MTT assay detected that RSK4 could significantly inhibit the growth of colorectal cancer cells in vitro; flow cytometry detected that S-phase cells decreased significantly, and G0/1 cells increased significantly (P < 0.05). The invasion ability of SW480 and HCT116 cells transfected with RSK4 was markedly lower than that in the control group, and the difference was statistically significant (P < 0.05). Fluorescent quantitative PCR and WB analysis showed that the expression of EMT-associated molecular E-cadherin was remarkably increased and the expression of Snail was significantly decreased (P < 0.01).

Conclusion

RSK4 gene in colorectal cancer cell lines with low expression of RSK4 after transfection can inhibit the growth and invasion of tumor cells. RSK4 gene may inhibit EMT and inhibit metastasis of colorectal cancer cells, may be a potential tumor suppressor gene and inhibit tumor distant metastasis, and may provide the biological basis for new therapeutic targets.

microRNA from brush biopsy to characterize oral squamous cell carcinoma epithelium

Few cancers are diagnosed based on RNA expression signatures. Oral squamous cell carcinoma (OSCC) is no exception; it is currently diagnosed by scalpel biopsy followed by histopathology. This study sought to identify oral tumor epithelial microRNA (miRNA) expression changes to determine if these changes could be used to diagnose the disease noninvasively. Analysis of miRNA profiles from surgically obtained OSCC tissue, collected under highly standardized conditions for The Cancer Genome Atlas, was done to determine the potential accuracy in differentiating tumor from normal mucosal tissue. Even when using small 20 subject datasets, classification based on miRNA was 90 to 100% accurate. To develop a noninvasive classifier for OSSC, analysis of brush biopsy miRNA was done and showed 87% accuracy in differentiating tumor from normal epithelium when using RT‐qPCR or miRNAseq to measure miRNAs. An extensive overlap was seen in differentially expressed miRNAs in oral squamous cell carcinoma epithelium obtained using brush biopsy and those reported in saliva and serum of oral squamous cell carcinoma patients in several studies. This suggested that nonselective release of these miRNAs into body fluids from tumor epithelium was largely responsible for the changes in levels in these fluids seen with this disease. Using a variation in mirRPath we identified the KEGG pathway of neurotrophin signaling as a target of these miRNAs disregulated in tumor epithelium. This highlights the utility of brush biopsy of oral mucosa to allow simple acquisition of cancer relevant miRNA information from tumor epithelium.