G protein-coupled receptors as novel therapeutic targets in non-small-cell lung cancer

GPR78 promotes migration and metastasis

Some GPCRs have a structure similar to that of other identified receptors, but their endogenous selective and exclusive ligand(s) are currently unknown; therefore they are called orphan GPCRs. GPR78 is an orphan G-protein coupled receptor containing 363 amino acids and belongs to the class A family. GPR78 shares 51 % amino acid sequence identity with GPR26. In the analysis of The Cancer Genome Atlas (TCGA) dataset, GPR78 was identified as a potential prognostic marker for oral squamous cell carcinoma and gastric carcinoma [25].

Recently, it has been reported that GPR78 is highly expressed in lung cancer cells. Knockdown of GPR78 significantly suppressed cell migration and metastasis. In this analysis, GPR78 was found to be highly expressed in lung squamous cell carcinoma (LUSC) tissues obtained from TCGA database. Similarly, it is highly expressed in LUAD cell lines [26]. GPR78 promotes cell migration and metastasis of lung cancer cells in a Gq-Rho GTPase-dependent manner. The results suggest that GPR78 is a potential regulator of lung cancer metastasis and may serve as a potential drug target for lung cancer metastasis (Figure 2). Although these findings reveal an emerging oncogenic role for GPR78 in NSCLC, the pathological roles and mechanisms of GPR78 in NSCLC require further investigation. Notably, the ability to interfere with GPR78 may provide unique opportunities for the prevention and treatment of NSCLC. Studies have shown that the use of miRNA-936 can reduce the expression of GPR78 and regulate the proliferation, invasion and migration of laryngeal squamous cell carcinoma [27].

Figure 2:

GPR78 promotes the migration and metastasis of lung cancer cells in a Gq-Rho GTPase-dependent manner. GPR78, G Protein-Coupled Receptor 78; NSCLC, Non-small cell lung cancer. (Created with BioRender.com).

Future screening efforts to identify surrogate or natural ligands are required to delineate the exact signaling pathways in which these receptors participate. Using the AlphaFold prediction model of GPR78, a large-scale virtual screening was conducted on the endogenous metabolite library, peptide library, or known drug library to identify a group of molecules with potential binding capabilities. A recent study that employed a very similar workflow – AlphaFold modeling of GPR78 followed by structure-based virtual screening of the ChemDiv and Enamine REAL libraries – has been published [28]. Then, combined with parallelized high-throughput screening (HTS), activity verification, and pharmacological characterization, an innovative hypothesis regarding the GPR78 signaling pathway network was proposed based on existing evidence and the general laws of tumor biology: (1) GPR78 serves as a key driver node in epithelial-mesenchymal transition (EMT); (2) there may be functional crosstalk between GPR78 on the cell membrane and the key protein GRP78 of endoplasmic reticulum stress; (3) GPR78 may exhibit ligand-dependent biased agonism (Figure 3). These analyses and hypotheses aim to provide new directions and theoretical foundations for future research, with the expectation of accelerating the translational process of GPR78 as a therapeutic target for lung cancer.

Figure 3:

Analysis and hypotheses on the transformation of GPR78 as a target for lung cancer Treatment. GPR78, by binding to specific ligands, can regulate the transcription factors related to EMT, causing epithelial cells to lose polarity and intercellular connections, thereby transforming into mesenchymal cells. The interaction between GPR78 and GRP78 within the endoplasmic reticulum is crucial for the survival and migration capabilities of cells in stressful environments. At present, there are no experimental evidence of ligand-dependent biased agonism, but based on the general mechanism of GPCR biased agonism, it is entirely possible to achieve Gs-biased or β-arrestin-biased effects on GPR78. GPR78, G Protein-Coupled Receptor 78; EMT, Epithelial-mesenchymal transition; GPCR, G protein-coupled receptor. (Created with BioRender.com).

PAR2 drives proliferation, migration and inhibits apoptosis

PAR2 is a member of the PAR family, which belongs to the class A family and includes four members: PAR1, PAR2, PAR3, and PAR4. PAR2 is also known as coagulation factor II (thrombin) receptor-like 1 (F2RL1) or G protein-coupled receptor 11 (GPR11). PAR2 can be activated after cleavage of the N-terminus by many serine proteases, such as trypsin, tryptase, coagulation factor Xa, tissue factor (TF)-factor VIIa (FVIIa) complex, and testisin [29]. However, it is insensitive to thrombin, unlike PAR1, PAR3, and PAR4. To date, activated PAR2 has been shown to modulate several downstream signaling pathways. PAR2 transduces signals primarily through G proteins, including Gαq, Gαi, and Gα12/13, but not directly through Gαs, which further induces various signaling cascades through different downstream effectors, such as calcium ions, cyclic adenosine monophosphate (cAMP), c-Jun N-terminuskinases (JNK), and RhoA [30], [31], [32]. Additionally, PAR2 couples with β-arrestins to orchestrate activation of distinct effectors, including Extracellular Signal-Regulated Kinase (ERK)1/2 and PI3K pathways [33]. Furthermore, PAR2 can modulate transactivation of certain cell surface receptors, such as tyrosine kinase receptors [34], 35].

The contribution of PAR2 to tumorigenesis has gained interest over the past few years. PAR2 expression is significantly enhanced during NSCLC progression, and there is a strong correlation between PAR2 expression and the survival of patients with NSCLC [36]. Mounting evidence shows that PAR2 exerts oncogenic potential in NSCLC, although its roles and signaling mechanisms in NSCLC are not fully understood [37]. PAR2 has been reported to the promote proliferation of lung adenocarcinoma A549 cells via an epidermal growth factor receptor (EGFR)-dependent signaling pathway and to induce the migration of these cells by inhibiting the expression of microRNA-125b [38], 39]. Kallikrein-related peptidase 6 (KLK6) promotes proliferation and restrains apoptosis in NSCLC cells through PAR2 [38]. In addition, Ma et al. [40] found that Bcl2-like protein-12 mediates the effects of PAR2 on suppressing the expression of p53 in lung cancer cells. PAR2-induced migration is mediated by β-arrestin 1-dependent activation of the Src/p38 MAPK signaling pathway, whereas PAR2-induced EMT is regulated by ERK2-mediated Slug stabilization [41]. In the majority of studies to date, PAR2 plays a central role in epithelial tumor advancement and is a potent inducer of the canonical Wnt/β-catenin stabilization pathway, a core process in both developmental and tumor progression pathways [42]. PAR-mediated MCP-1 expression also depends on PAR2 activation [43]. Taken together, PAR2 contributes to the proliferation and migration of NSCLC cells and inhibits their apoptosis. Furthermore, PAR2 methylation is associated with malignant progression in NSCLC, affecting cell proliferation, migration, and invasion [44]. Therefore, PAR2 has emerged as a promising therapeutic target for inhibiting rapidly metastasizing lung cancer cells (Figure 4).

Figure 4:

PAR2 promotes the proliferation and migration of NSCLC cells by activating the G protein-β-arrestin signaling pathway, which further induces EGFR transactivation and activates the MEK/ERK, PI3K/AKT and ROCK pathways. PAR2, Protease-activated receptor 2; NSCLC, Non-small cell lung cancer; EGFR, epidermal growth factor receptor; PI3K, Phosphatidylinositol 3 kinase; AKT, Protein Kinase B. (Created with BioRender.com).

However, the elusive molecular mechanism of its activation and signaling has made it a difficult target for drug development. Understanding PAR2 signaling has been challenging compared to other GPCRs because of its unique mechanism of activation. Many researchers have identified unique ligands that bind to PAR2 and induce biased signaling. For example, DF253 (2f-LAAAAI-NH2) triggered PAR2-mediated calcium release but not ERK1/2 phosphorylation. AY77 (Isox-Cha-Chg-NH2) is a more potent calcium-biased agonist, whereas its analog AY254 (Isox-Cha-Chg-A-R-NH2) is an ERK-biased agonist [45]. FSLLRY-NH2, a PAR2 inhibitor, reduced apoptosis resistance in A549 and H1299 cells [37]. Data on putatively biased agonists and antagonists would be useful for probing their physiological roles and designing novel therapeutic applications. IK187 exhibits PAR2-dependent G protein activation and β-arrestin2 recruitment [46]. However, DF253, AY77 and AY254 showed a preference for Gαq/11 activation rather than for β-arrestin2 recruitment (Figure 5) [47]. Therefore, PAR2-tailored drug discovery is a significant challenge. While designing specific PAR2 inhibitors, we should also consider means to reduce adverse effects. Clinical trials are still limited and have been directed toward patients with diseases other than cancer.

Figure 5:

The mechanism of action of PAR2-related ligands. Gαq/11 - calcium pathway bias: DF253, AY77; ERK pathway bias: AY254; Balanced activation: IK187; pathway inhibition: FSLLRY-NH2. PAR2, protease-activated receptor 2; ERK, extracellular signal-regulated kinase. (Created with BioRender.com).

CXCR4 triggers invasion, metastasis and promotes therapeutic resistance

CXCR4 is a 352-amino acid rhodopsin-like GPCR known as an alpha-chemokine receptor specific for stromal cell-derived factor 1 (SDF-1; also called CXCL12). Indeed, the biological characteristics of CXCR4 have been extensively studied. CXCR4 is overexpressed in 23 types of cancer and has been studied in clinical trials of hematological malignancies and solid tumors, such as lung cancer [48], [49], [50]. The CXCL12/CXCR4 pathway plays a crucial role in the migration and transport of different cell populations and in the progression and metastasis of various tumors [51], 52].

Recently, a number of studies have shown that CXCR4 is overexpressed in NSCLC and may account for the progression, metastasis, and prognosis of NSCLC [52], 53]. In a meta-analysis [54] of 736 NSCLC patients from eight studies, CXCR4 expression was significantly higher in NSCLC than in normal lung tissue. Another meta-study of 1872 NSCLC patients reported that CXCR4 protein expression is associated with clinical stage, metastasis status, increased risk, and decreased overall survival in patients [55]. CXCR4 regulates the migration of alveolar epithelial cells by activating Rac1, MMP-2, and MMP-14 [56]. CXCR4 overexpression enhances the motility and invasion of NSCLC cells by upregulating ERK, EGFR/HER2-neu, MMP-9, and NF-κB-dependent pathways [57], 58]. Conversely, decreasing the expression of CXCR4 by genetic knockdown of CXCR4 or neutralizing antibodies significantly inhibits the invasion of NSCLC cells [59]. As the EGFR-L858R mutant has been reported to promote the expression of CXCR4 and increased NSCLC cell invasion, targeting CXCR4 significantly inhibited EGFR-L858R-dependent invasion of NSCLC cell [60]. Moreover, CXCR4 promotes EMT processes as evidenced by involvement in CD133-induced as well as miR-1246-induced EMT processes in NSCLC [61].

The CXCR4/CXCL12 axis is closely related to cancer metastasis, which involves adhesion, invasion, cell proliferation, and survival. It has been discovered that CXCL12/CXCR4 axis plays a critical role in determining the metastatic destination of NSCLC cells [23], 62]. CXCR4 promotes cancer metastasis to organs where CXCL12 is generated in large quantities. The interaction between CXCL12 and CXCR4 causes cancer cells to escape from the circulation and enter organs that contain large amounts of chemokines, thus forming metastatic tumors [63]. CXCR4 expression levels were significantly correlated with lymph node metastasis in NSCLC. CXCR4 and VEGF-C synergistically promote lymphatic metastasis in NSCLC patients [64]. CXCR4 mRNA, negatively regulated by breast cancer metastasis suppressor 1 (BRMS1), was significantly higher in the NSCLC group with distant metastasis than in that without distant metastasis [65]. In addition, Li H et al. chose to block the CXCL12/CXCR4 axis with AMD3100 (a small-molecule competitive antagonist of CXCL12), and the process of lung cancer metastasis to the brain was significantly slowed down [66]_, providing a potential drug candidate for inhibiting lung cancer metastasis to the brain.

Researchers have reported that the development of intrinsic or acquired drug resistance is a major challenge associated with chemotherapeutic resistance in cancer cells [67]. Cisplatin-based chemotherapy is frequently used to treat NSCLC; however, the exact mechanism of cisplatin resistance in lung cancer remains unknown. Previous studies have revealed a link between CXCR4 overexpression and chemoresistance in cancer. Cisplatin treatment increases the secretion of CXCL12 and recruitment of metastasis-initiating cells and pro-invasive CXCR4⁺ macrophages, thereby promoting spontaneous metastasis. Combined treatment with cisplatin and CXCR4 antagonists can prevent this metastasis and emphasizes the approach of CXCR4-targeted therapy for NSCLC [68]. Li J et al. also showed that elevated CXCR4 expression in lung cancer cell lines A549 and 95D promoted cell proliferation and drug resistance to cisplatin [69]. A population of CD133⁺/CXCR4⁺ cells still survived in vivo, although lung cancer xenografts established from primary tumors with cisplatin were undertreated, indicating the role of CXCR4 in the anti-chemotherapy of lung cancer [70]. Mechanistically, Nakamura et al. observed that CXCR4 overexpression promoted drug resistance by upregulating transforming growth factor-β2 [71]. Xie et al. [72]observed that CXCR4 mediated cisplatin resistance via regulating CYP1B1 expression. Additionally, it has been shown that CXCR4 is correlated with EGFR tyrosine kinase inhibitors (EGFR-TKIs) resistance. Despite the initial high response rate of EGFR-TKIs in NSCLC patients with EGFR mutations, acquired resistance inevitably occurs in patients undergoing EGFR-TKI therapy. CXCR4 overexpression in NSCLC promotes gefitinib resistance associated with EMT [73], 74]. Zhu et al. [75] showed that gefitinib induces an enhanced invasive phenotype through CXCR4, which is dependent on TGF-β1/Smad2 signaling. Furthermore, blocking CXCR4 was shown to reverse the EMT process and reduce invasiveness following gefitinib stimulation. These data suggest that the combination of EGFR-TKIs, CXCR4 antagonists, and TGFβR inhibitors may provide a novel strategy to prevent the progression of lung cancer bearing EGFR-activating mutations. Further investigation is required to address the underlying mechanisms and evaluate the clinical efficacy of this combinatorial approach.

In further support of its oncogenic role, CXCR4 has been reported to mediate the pro-tumorigenic effects of tumor-associated macrophages (TAMs) through pro-inflammatory molecules such as NF-κB and HIF-1α, which represent key components of the tumor microenvironment and play a key role in cellular carcinogenesis, invasion, and metastasis of NSCLC [76]. The CXCL12/CXCR4 axis also plays an important role in the efficacy of anti-PD-1 immunotherapy in NSCLC [77]. Collectively, these findings highlight the pro-tumorigenic function of the CXCL12/CXCR4 axis in NSCLC, which acts locally to promote tumor growth, enhance cell motility and invasion, facilitate metastasis, induce chemoresistance, and establish an immune-permissive tumor microenvironment.

The accumulating knowledge regarding the fundamental roles that CXCL12/CXCR4 plays in solid malignancies has driven multiple pharmaceutical companies to develop specific CXCR4 antagonists. Several promising anti-cancer drugs targeting CXCR4 are currently under evaluation in preclinical and clinical studies. AMD3100 is the most potent non-peptide, small-molecule CXCR4 antagonist. It inhibits the CXCL12/CXCR4 axis by binding to the negatively charged region within the second extracellular loop of CXCR4, thereby blocking downstream signal transduction. This inhibition results in elevated levels of tight junction proteins, reduced blood-brain barrier (BBB) damage, and decreased BBB permeability [66]. In the fifth week, although three mice in the AMD3100-treated group developed brain metastases, the size and number of metastatic foci were significantly smaller than those in the control group. The process of lung cancer metastasis to the brain was substantially slowed [66]. Another small-peptide CXCR4 antagonists, TN14003, is utilized as an adjuvant in traditional anticancer therapies against NSCLC by inhibiting tumor cell proliferation [78], 79], although it is currently not undergoing further clinical development. BKT140 (BL-8040), a small CXCR-4 inhibitor more potent than AMD3100, was shown to increase the effects of standard chemotherapy and radiotherapy in NSCLC cells [80]. LY2510924 is a small cyclic peptide antagonist of CXCR4 that shows antitumor effects in NSCLC [81]. Based on these findings, phase I and II trials using LY2510924 are currently ongoing in patients with advanced tumors. Thus, these findings provide useful information for further study of these effects and a guide for further research on the role of CXCR4 and its prognostic significance in NSCLC (Figure 6).

Figure 6:

CXCR4 triggers invasion, metastasis and promotes chemoresistance in NSCLC. CXCR4 drives tumor cell invasion and metastatic spread by activating the CXCL12-CXCR4 axis and downstream MAPK and PI3K/AKT signaling. High CXCR4 have been shown to confer resistance to platinum-based chemotherapy. Targeting CXCR4 (AMD3100, TN14003, BKT140 and LY2510924) can simultaneously suppress invasion, chemoresistance, and metastasis making it a promising therapeutic target in NSCLC. CXCR4, C-X-C motif chemokine receptor 4; NSCLC, Non-small cell lung cancer; MAPK, Mitogen-activated protein kinase; PI3K, Phosphatidylinositol 3 kinase; AKT, Protein Kinase B (Created with BioRender.com)

GPBAR regulates the malignant phenotype of NSCLC

GPBAR, also known as TGR5 or GPR131, was first discovered in 2002 as a membrane receptor activated by bile acids [82]. GPBAR can interact with endogenous bile acids such as cholic acid (CA), chenodeoxycholic acid (CDCA), deoxycholic acid (DCA), and lithocholic acid (LCA), as well as various ligands including synthetically derived bile acid analogs such as INT-777, and small-molecule compounds R399 and P395. These ligands recognize and bind to GPBAR, thereby activating multiple signaling pathways and inducing the transcription and expression of downstream target genes. They participate in regulating various physiological and pathological processes, including energy metabolism, inflammatory responses, cell proliferation, and apoptosis [83], [84], [85], [86], [87].

GPBAR is widely expressed in various tissues, including the lungs. However, at present, there is limited research on GPBAR in lung-related diseases, and studies investigating its role in NSCLC remain scarce. Existing studies have reported that in NSCLC cells, GPBAR promotes cell proliferation, migration, and invasion by activating the JAK2/STAT3 signaling pathway; Further analysis has indicated that GPBAR influences NSCLC cell proliferation primarily by modulating cell cycle progression, whereas its effects on migration and invasion involve the regulation of matrix metalloproteinases (MMPs) and the RhoA/Rho kinase signaling pathways [88]. The net effect produced by the activation of GPBAR is complex and highly dependent on the specific context. GPBAR would exhibit a biased agonistic effect, that is, depending on the specific ligand, it preferentially activates Gαs or other pathways (for example, β-arrestin). This flexibility of the signal, known as biased agonism, is increasingly regarded as a key factor for its possible dual role in cancer. For instance, the activation of LCA on GPBAR in breast cancer cells induces cell-inhibitory oxidative stress [89], while the activation of SBA on GPBAR in colon cancer cells promotes the recruitment of regulatory T cells through the β-catenin/CCL28 signaling pathway, thereby facilitating immune escape [90]. The Gαs-preferring GPBAR activation induced by INT-777 or DCA inhibited YAP activity and suppressed proliferation. In contrast, the β-arrestin 1-preferring agonist R399 activated YAP and promoted the growth of NSCLC, indicating that a Gs-biased GPBAR agonist has a therapeutic window. That is to say, two specific GPBAR agonists, R399 and INT 777, regulate YAP activity via the Gs/β-arrestin pathway, which may affect the malignant phenotype of NSCLC cells [91]. Moreover, studies have shown that GPBAR promotes the differentiation of TAMs into M2 type by activating the cAMP-STAT3/STAT6 signaling pathway, thereby promoting tumor growth in NSCLC. It was also observed that CDCA-induced phenotypic changes in macrophages are dependent on GPBAR, resulting in attenuated M1 polarization, enhanced M2 polarization, and upregulated expression of anti-inflammatory genes in response to conditioned medium collected from Lewis lung carcinoma cell [92]. Yi-Fei-San-Jie Formula (YFSJF), a patented drug from Guangzhou University of Chinese Medicine’s First Affiliated Hospital, has been widely used in clinical practice in China for several years. The unique role of YFSJF in inhibiting immune escape lies in blocking the STAT3/PD-L1 pathway mediated by GPBAR, weakening the binding of PD-L1 and PD-1, and restoring the immune activity of T cells to inhibit immune escape, thereby combating immune escape in lung cancer and exerting anti-tumor effects [93]. In conclusion, GPBAR interacts with various bile acids and plays a significant role in the progression of multiple tumor types [85]. In the “biased agonism” of the GPBAR, how can different receptor conformations be preferentially stabilized to selectively activate specific downstream signaling pathways (such as the Gαs-cAMP and β-arrestin pathways, which leads to the multi-effect and sometimes contradictory results of cellular outcomes; for example, the activation of GPBAR may be anti-proliferative or pro-tumorigenic, depending on the specific ligand and the involved pathways). This complexity not only makes therapeutic regulation difficult, but also provides a complex opportunity for drug design: developing new, biased GPBAR ligands that can selectively promote beneficial anti-inflammatory signaling while avoiding the activation of pro-tumorigenic pathways.

GPRC5A exhibits tumor suppressive role

GPRC5A, located on chromosome 12p13.1, belongs to the family of orphan GPCRs. GPRC5A is predominantly expressed in the lungs and exhibits weak or negligible expression in other tissues, suggesting a potential role in maintaining lung tissue homeostasis [94], [95], [96]. Some studies have indicated that GPRC5A may function as an oncogene in breast, colorectal, and pancreatic cancers, whereas it has been identified as a tumor suppressor in NSCLC. Notably, p53-null human NSCLC H1299 cells showed increased GPRC5A expression, while p53 wild-type A549 cells showed decreased expression of GPRC5A, suggesting its involvement in the antitumor activity of p53 in NSCLC cells [97]. These findings suggest that GPRC5A may exert context-dependent roles across different cancer types, highlighting its biological complexity and making it a compelling candidate for further investigation.

Previous studies have provided valuable insights into the role of GPRC5A in the development of NSCLC. Both mRNA and protein expression levels of GPRC5A are significantly reduced in lung cancer tissues compared to those in healthy lung tissues [98]. Reduced GPRC5A expression has been linked to poorly differentiated NSCLC cells [99]. Consistently, downregulation of GPRC5A in NSCLC is closely correlated with poor tumor differentiation and advanced TNM stage [100]. Furthermore, studies have shown that the low expression of GPRC5A is related to the early pathological stage of NSCLC, and it has been observed that the low expression of GPRC5A is related to the increased infiltration of immune cells, elevated IPS, and spatial distribution of PD-L1-positive tumor cells [101]. Studies using mouse models have demonstrated that homozygous GPRC5A knockout mice are more susceptible to developing lung tumors at 1–2 years of age compared to heterozygous or wild-type mice. Simultaneously, nicotine-derived nitrosamine ketone (NNK) enhances lung tumorigenesis in GPRC5A^−/− mice [102], suggesting that GPRC5A functions as a lung-specific tumor suppressor. Mechanistically, recent advances have shown that GPRC5A is involved in various tumor-associated signaling pathways, including cAMP, NF-κB, signal transducer and activator of transcription (STAT) 3, and focal adhesion kinase (FAK)/Src signaling. Biologically, knockout of GPRC5A promotes lung inflammation and tumorigenesis through activation of NF-κB, and enhances the malignant transformation of normal lung epithelial cells via activation of STAT3 and downstream signals [103], 104]. A recent study revealed the potential role of GPRC5A in inhibiting EGFR expression and activation through direct translational suppression in combination with the eIF4F complex [98], 105]. Deletion of GPRC5A significantly enhanced the expression of ionizing radiation (IR)-stimulated EGFR, increasing IR-induced lung tumor incidence. In addition, GPRC5A deficiency contributes to dysregulated MDM2 via activated EGFR signaling, which promotes lung tumor development [106]. In summary, GPRC5A deficiency may lead to dysregulation of oncogenic pathways that are crucial for lung tumorigenesis.

GPRC5A contains multiple phosphorylation sites that have been implicated in various biological processes. The biological functions of GPRC5A seem to be differentially regulated by tyrosine phosphorylation. Although GPRC5A does not affect cell proliferation and migration regardless of tyrosine phosphorylation status [107], tyrosine phosphorylation decreases or abolishes GPRC5A-mediated inhibition of EGF-enhanced anchoring-independent growth of NSCLC cells [108]. EGFR-mediated phosphorylation of GPRC5A at TYR-317/TYR-320 and TYR-347/TYR-350, two conserved double-tyrosine (TYR) motifs, leads to weakened tumor-suppressive activities of GPRC5A [108], 109]. Thus, targeting EGFR can restore the tumor-suppressive function of GPRC5A in lung cancer. As previously discussed, GPRC5A is dysregulated in NSCLC and plays a critical role in suppressing tumorigenesis, suggesting its potential as both a diagnostic biomarker and a therapeutic target. Notably, GPRC5A holds significant promise in guiding the optimization of clinical medication strategies. EGFR inhibitors have been shown to be more effective in GPRC5A knockout lung cancer cells compared to GPRC5A wildtype lung cancer cells [98]. Some studies have demonstrated that chromosome deletion at 12p12.3 is rare in NSCLC and that epigenetic alterations may contribute to GPRC5A downregulation [110]. As a complex signaling node, GPRC5A’s regulatory network may extend beyond the EGFR/STAT3 and NF-κB pathways. The way GPRC5A inhibits EGFR may not be simply through direct binding. GPRC5A may play the role of a “signal scaffold protein” on the cell membrane. Its intracellular domain may recruit protein tyrosine phosphatases (PTPs, such as PTPN1/2) to specifically dephosphorylate the specific sites of EGFR, thereby “blocking” the activation of the EGFR signaling pathway at multiple levels. Furthermore, the absence of GPRC5A leads to the continuous activation of the EGFR/STAT3 and NF-κB pathways, which drives significant metabolic reprogramming.

Despite biochemical evidence supporting its critical role in NSCLC, further studies are required to elucidate the specific mechanisms through which GPRC5A contributes to NSCLC development and metastasis. Although previous investigations have sought to identify a potential ligand for GPRC5A [103], 111], these efforts have not yet been successful; therefore, additional research is necessary to characterize its possible ligands. Moreover, further analyses are warranted to confirm the potential utility of GPRC5A as an independent prognostic marker for NSCLC. Therefore, the successful “orphanicization” of GPRC5A is not only expected to offer new treatment options for lung cancer patients, but also to provide a model for understanding the complex role of GPCRs in cancer.

Somatostatin receptor type 2 behaves as a tumor suppressor

The somatostatin receptor family comprises five distinct members, each encoded by a separate gene located on a different chromosome. Somatostatin receptors (SSTRs) are highly expressed in a variety of tumors [98], 99]. The endogenous high-affinity ligand for SSTRs is somatostatin, a polypeptide naturally present in the CNS as well as various peripheral tissues and organs. Somatostatin is an inherently pleiotropic inhibitory neuropeptide with anti-secretory, anti-proliferative, and anti-angiogenic properties [100]. However, its clinical application is limited due to an extremely short half-life of approximately 2–3 min, which has led to the development of numerous synthetic somatostatin analogs (SSAs). Somatostatin and its analogs or agonists inhibit tumor growth and metastatic progression by activating SSTRs in both cancer cells and components of the tumor microenvironment. Among the five receptor subtypes, SSTR2 is the most frequently overexpressed subtype in small cell lung cancer (SCLC) [112]. However, accumulating evidence indicates that SSTR2 functions as a tumor suppressor in NSCLC. Upon activation, SSTR2 induces cell cycle arrest [113], promotes cancer cell apoptosis via a p53-dependent or p53-independent mechanism [114], suppresses cell invasion by modulating the PI3K pathway, and restores gap junction communication, which plays a critical role in contact inhibition and the maintenance of cellular differentiation (Figure 7).

Figure 7:

The mechanism of SSTR2 in NSCLC. Activation of SSTR2 induces cell cycle arrest, promotes cancer cell apoptosis through p53-dependent or -independent mechanisms, inhibits cell invasion by modulating the PI3K pathway, and restores gap junctional communication. SSTR2, Somatostatin receptor type 2; NSCLC, Non-small cell lung cancer; PI3K, Phosphatidylinositol 3 kinase. (Created with BioRender.com).

Furthermore, SSTR2 is also associated with other molecules and pathways. The co-expression of SSTR2A and EGFR suggests the existence of a potential synergistic signaling pathway [115], which is worthy of further exploration for the potential of combined targeting; the expression of SSTR2 is positively correlated with neuroendocrine genes such as NEUROD1 and ASCL1, suggesting that some NSCLC may have a neuroendocrine phenotype [116]. Although mainly reported in SCLC, the downregulation of SSTR2 can lead to a decrease in AMPKα phosphorylation and an increase in oxidative metabolism. This mechanism has yet to be verified in NSCLC [112].

Currently, various strategies such as gene transfection [117], radioactive ligands, and traditional somatostatin analogs have been validated for their inhibitory effects in experimental models, providing a theoretical basis for future clinical translation. Shen and colleagues evaluated the antitumor effects of a conjugate formed by linking two molecules of paclitaxel to octreotide (SMS 201–995, Sandostatin), an octapeptide analog of endogenous somatostatin with high affinity for SSTR2, in a xenograft model using A549 human NSCLC cells implanted in nude mice [118]. The conjugate significantly inhibited tumor growth and increased necrosis. These findings suggest that the paclitaxel-octreotide conjugate may serve as a potential targeted therapeutic agent for NSCLC. Similarly, Sun et al. developed paclitaxel-octreotide conjugates that exhibited dose- and time-dependent inhibition of A549 and Calu-6 NSCLC cell proliferation. Paclitaxel and its conjugated derivatives were also shown to increase the G2/M phase population in A549 cells [119]. Preclinical studies have demonstrated that targeting SSTR2 effectively inhibits lung tumor growth both in vitro and in vivo. These findings suggest that SSTR2 and associated signaling pathways may represent promising biomarkers for personalized therapeutic strategies in NSCLC.