RGS4 in gastric cancer: A multifaceted regulator of focal-adhesion-kinase -phosphatidyl-inositol-3-kinase - protein-kinase-B signaling and epithelial-mesenchymal transition beyond tumor progression

Abstract

Chen et al demonstrated that regulator of G protein signaling (RGS) 4 promotes gastric cancer (GC) progression by activating the focal adhesion kinase/phosphatidyl-inositol-3-kinase/protein kinase B pathway and inducing epithelial-mesenchymal transition. Although their multilevel approach integrating clinical data, functional assays, and xenograft models demonstrated a key role for RGS4 in GC pathogenesis, several limitations should be considered. The mechanism of the RGS4-focal adhesion kinase interaction remains unclear, specifically whether it involves direct binding or intermediaries. The clinical analysis of 90 patients lacks stratification by GC subtypes or immune features, potentially limiting generalizability. Furthermore, fully validating RGS4’s oncogenic role requires additional studies, including functional assays in chemotherapy-resistant and metastatic cell lines, metastasis models including orthotopic implantation and tail vein injection, and comparison with other RGS family members. Addressing these via targeted mechanistic studies and expanded clinical validation could strengthen RGS4’s potential as a therapeutic target in GC.

Key Words: RGS4; Focal adhesion kinase/phosphatidyl-inositol-3-kinase/protein kinase B; Epithelial-mesenchymal transition; Gastric cancer; Biomarker; Preclinical model

Core Tip: Chen et al provided compelling multilevel evidence that regulator of G-protein signaling 4 drives gastric cancer progression through focal adhesion kinase/phosphoinositide 3-kinase/protein kinase B activation and epithelial-mesenchymal transition, combining clinical correlation with functional validation. Although further mechanistic studies and expanded clinical cohorts would strengthen the findings, this work establishes G-protein signaling 4 as a promising therapeutic target worthy of deeper investigation.

TO THE EDITOR

Gastric cancer (GC), as the fifth most frequently diagnosed cancer and the fifth leading cause of cancer-related mortality, remains among the most clinically challenging malignancies globally[1]. The health burden of GC is particularly pronounced in East Asian populations[2], in which epidemiological studies have identified alarmingly high incidence rates coupled with characteristically late-stage diagnoses, collectively contributing to poor clinical outcomes[3]. The stark contrast in 5-year survival rates, exceeding 80% for early-stage disease but plummeting to lower than 35% for advanced disease[4], underscores the critical urgent need for novel molecular biomarkers and targeted therapeutic strategies that could enable earlier detection and more effective treatment[5,6].

The G protein-coupled receptor (GPCR) signaling axis has emerged as a central regulator of oncogenic processes, driving fundamental cellular behaviors including proliferative signaling, migratory capacity, and apoptotic resistance[7]. Within this signaling framework, the regulator of G protein signaling (RGS) protein family comprises more than 20 distinct members that serve as essential negative feedback modulators. With growing evidence implicating their dysregulation in various cancer types, these multifunctional proteins exhibit remarkable context-dependent behavior, functioning as either tumor suppressors or oncogenic drivers through their modulation of diverse signaling cascades (Figure 1)[8,9].

Figure 1 Regulation of phosphoinositide 3-kinase/protein kinase B, mitogen-activated protein kinase, and Wnt/β-catenin signaling pathways by regulator of G-protein signaling 4 and their crosstalk. GF: Growth factor; GPCR: G protein-coupled receptor; PIP: Prolactin-induced protein; PTEN: Phosphatase and tensin homolog deleted on chromosome ten; AKT: Protein kinase B; GSK-3β: Phosphorylate glycogen synthase kinase 3β; mTOR: Mammalian target of rapamycin; FAK: Focal adhesion kinase; FOXO: Forkhead box O; E-cadherin: Epithelial cadherin; FZD: Frizzled; LRP: Low-density lipoprotein receptor-related protein; DvI: Dishevelled; GBP: GSK3-binding protein; APC: Adenomatous polyposis coli; TCF: T cell factor; LEF: Lymphoid enhancer factor; cyc-D: Cyclin D; ZEB1: Zinc finger E-box-binding homeobox 1; CRH: Corticotropin-releasing hormone; PTH: Parathyroid hormone; RTK: Receptor tyrosine kinase; GRB2: Growth factor receptor-bound protein 2; SOS: Son of sevenless; Ras: Rat sarcoma virus oncogene; Raf: Rapidly accelerated fibrosarcoma; IRS1: Insulin receptor substrate 1; MLK: Mixed-lineage kinase; MKK: MAP kinase kinase; JNK: Jun N-terminal kinase; MKP: MAP kinase phosphatase; PTP: Protein tyrosine phosphatase; P38: P38 mitogen-activated protein kinase; ELK-1: ETS like kinase 1; C-Myc: Cellular myelocytomatosis oncogene; MEK: Mitogen-activated protein kinase kinase; ERK: Extracellular signal-regulated kinase; “+P”: Phosphorylation; “-P”: Dephosphorylation.

Among these pathways, the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) pathway is a critical intracellular signaling cascade that promotes cell survival, growth, and metabolism[10]. Upon activation by GPCRs or receptor tyrosine kinases, PI3K phosphorylates membrane lipids to generate phosphatidylinositol (3,4,5)-trisphosphate, which recruits AKT to the plasma membrane, where it is activated through phosphorylation[11]. Once active, AKT regulates numerous downstream effectors such as mammalian target of rapamycin, glycogen synthase kinase-3 beta, and fork head box O, thereby inhibiting apoptosis and enhancing cellular proliferation. Hyperactivation of this pathway is frequently observed in cancers, and it contributes to tumor progression and therapeutic resistance[12].

Similarly, the mitogen-activated protein kinase (MAPK) pathway, encompassing key subfamilies such as extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase, and p38 MAPK, plays a pivotal role in transmitting signals that regulate cell proliferation, differentiation, and stress responses[13]. The canonical rat sarcoma virus/rapidly accelerated fibrosarcoma/MAPK/ERK kinase/ERK cascade is often initiated by GPCR activation, and its activation results in the phosphorylation of transcription factors such as cellular myelocytomatosis oncogene and ETS domain-containing protein ETS like kinase 1, driving the expression of genes essential for cell cycle progression[14]. Dysregulation of MAPK signaling is a hallmark of many malignancies, facilitating uncontrolled growth and metastatic behavior[15].

Additionally, the Wnt/β-catenin signaling pathway represents another fundamental axis governing embryonic development and tissue homeostasis[16]. In the absence of Wnt ligands, cytoplasmic β-catenin is targeted for degradation by a destruction complex involving adenomatous polyposis coli, axis inhibition protein, and glycogen synthase kinase-3 beta. Wnt activation disrupts this complex, allowing β-catenin to accumulate and translocate to the nucleus, in which it partners with T-cell factor/Lymphoid enhancer factor to activate genes involved in cell proliferation, migration, and stemness[16]. Aberrant activation of Wnt/β-catenin signaling is implicated in various cancers, particularly those of the gastrointestinal tract, and is associated with tumor initiation and metastasis[17].

Through their influence on the PI3K/AKT, MAPK, and Wnt/β-catenin pathways, RGS proteins can exert profound and context-specific effects on oncogenesis[15,18-20], either curbing or promoting tumor development depending on cellular conditions and cancer type.

Emerging evidence has established that RGS proteins drive multiple oncogenic processes in GC through intricate signaling networks. RGS1 drives immunosuppression by polarizing macrophages toward the M2 phenotype[21], and RGS2 facilitates metabolic reprogramming to promote immune evasion[22]. Furthermore, RGS3 enhances tumor proliferation via Wnt/β-catenin activation, often under the regulation of tumor-suppressive miRNAs[23,24]. In parallel, RGS5 exerts dual roles in maintaining genomic stability while dynamically modulating programmed death ligand 1 expression to influence immune checkpoint responses[25].

RGS4’s role in cancer is context-dependent, varying across cancer types and tumor microenvironments. Notably, although RGS4 promotes invasion, migration, and anti-apoptotic activity in glioma cancer stem cells[26], it conversely manifests tumor-suppressive properties by inhibiting metastasis in breast cancer[27]. This striking tissue-specific functional duality highlights the molecular complexity of RGS4’s role in tumor biology and underscores the necessity for systematic investigation of its precise mechanisms in GC pathogenesis.

The current understanding of RGS4’s role in GC remains largely speculative, primarily derived from bioinformatic analyses, which revealed that RGS4 can serve as an independent prognostic biomarker, potentially through its regulation of cancer-associated fibroblasts and modulation of the tumor immune microenvironment[28]. Nevertheless, these findings require verification through robust fundamental experiments and validation with clinical samples.

Chen et al[29] provided valuable insights into the oncogenic role of RGS4 in GC through its regulation of the focal adhesion kinase (FAK)/PI3K/AKT pathway and epithelial-mesenchymal transition (EMT). The work provided several important contributions to the field. First, the study employed public databases to analyze RGS4 expression in GC tissues at the histological level, followed by validation using clinical samples. At the cellular level, cell biology experiments were conducted to investigate the biological functions of RGS4 in GC cells. Additionally, animal studies were utilized to confirm the role of RGS4 in promoting GC cell proliferation in vivo. Through these multilevel investigations and diverse methodological approaches, the study comprehensively demonstrated the tumor-promoting role of RGS4 in GC progression.

More importantly, this study is the first to demonstrate that RGS4 promotes GC proliferation and metastasis through activating the FAK/PI3K/AKT pathway and inducing EMT, offering significant mechanistic insight into GC pathogenesis and potential therapeutic targeting. The multilevel experimental validation, encompassing molecular analyses of signaling pathways and EMT markers, functional characterization of cancer cell behaviors, and in vivo confirmation, establishes a solid mechanistic framework for understanding RGS4’s role in GC pathogenesis.

AREAS FOR IMPROVEMENT AND FUTURE PERSPECTIVES

Nevertheless, some limitations of this study need to be addressed. From a mechanistic perspective, although this study demonstrated that RGS4 overexpression promotes FAK phosphorylation, potential indirect mechanisms, such as receptor tyrosine kinase activation or signaling crosstalk with other GPCRs, have not been investigated. In terms of clinical validation, the study included only 90 patients, a relatively limited cohort size, and the lack of subgroup analyses stratified by GC subtypes or prior treatment history restricts the broader applicability of RGS4 as a prognostic biomarker. From a methodological standpoint, although loss-of-function experiments were conducted, gain-of-function studies are needed to further confirm the oncogenic role of RGS4. Moreover, the study relied solely on subcutaneous xenograft models, which can only confirm the role of RGS4 in promoting GC cell proliferation in vivo. Further studies using orthotopic or metastatic models are necessary to validate the function of RGS4 in GC cell metastasis.

To address these limitations and enhance the translational value of our findings, we propose the following key research priorities. First, confirming the nature of the RGS4-FAK interaction, whether direct or mediated by other proteins, using co-immunoprecipitation and structural biology techniques would be highly valuable. Given that RGS4 can act as a signaling hub, exploring its crosstalk with pathways such as MAPK, Wnt/β-catenin, Hippo, and Janus kinase/signal transducer and activator of transcription could also reveal broader mechanisms underlying GC pathogenesis.

From a clinical perspective, expanding validation efforts to include larger and more diverse cohorts with stratification by GC subtype, treatment history, and programmed death ligand 1 expression would help strengthen the findings. Additionally, it would be useful to examine whether RGS4 expression is correlated with responses to targeted therapies and immune checkpoint inhibitors, which could support its potential role as a predictive biomarker.

Functionally, comprehensive studies using both loss- and gain-of-function approaches in models of chemotherapy resistance and metastasis are essential. Rescue experiments involving domain-specific RGS4 mutants and site-directed mutagenesis could help pinpoint critical functional regions. Meanwhile, genomic and epigenomic analyses, including promoter methylation and copy number variation, could help identify upstream factors driving RGS4 dysregulation.

For in vivo studies, employing orthotopic implantation and tail vein injection models would better mimic local invasion and systemic metastasis. Integrating these analyses with single-cell RNA sequencing and spatial transcriptomics could offer deeper insights into the influence of RGS4 on the tumor microenvironment, including stromal communication, immune infiltration, and spatial patterns of EMT.

Lastly, comparative studies across the RGS protein family could help clarify the unique role of RGS4 in GC and its specific connection to the FAK/PI3K/AKT axis. Structural modeling and an assessment of RGS4’s tractability as a drug target could additionally inform the development of targeted therapeutic strategies.

CONCLUSION

The study by Chen et al[29] makes an important contribution to GC research by systematically demonstrating RGS4’s functional role in tumor progression through multilevel experimental validation. The work provides valuable mechanistic insights into RGS4-mediated FAK/PI3K/AKT activation and EMT induction while also establishing its clinical relevance in patient cohorts. Further investigations addressing the mechanistic details and expanding clinical validations could enhance the translational potential of these findings.