Research progress on the development of hepatocyte growth factor/c-Met signaling pathway in gastric cancer: A review

Abstract

Hepatocyte growth factor (HGF) and its receptor, c-Met, play important roles in the occurrence, development, and treatment of gastric cancer (GC). This review explored the function of the HGF/c-Met signaling pathway in GC and its potential targeted therapeutic mechanisms. As one of the most common malignant tumors worldwide, GC has a complex pathogenesis and limited therapeutic options. Therefore, a thorough understanding of the molecular mechanism of GC is very important for the development of new therapeutic methods. The HGF/c-Met signaling pathway plays an important role in the proliferation, migration, and invasion of GC cells and has become a new therapeutic target. This review summarizes the current research progress on the role of HGF/c-Met in GC and discusses targeted therapeutic strategies targeting this signaling pathway, providing new ideas and directions for the treatment of GC.

Key Words: Targeted therapy, Hepatocyte growth factor, c-Met, Gastric cancer, Prognosis assessment, Review

Core Tip: Hepatocyte growth factor (HGF) and its receptor, c-Met, play important roles in the occurrence, development, and treatment of gastric cancer (GC). This review explored the function of the HGF/c-Met signaling pathway in GC and its potential targeted therapeutic mechanisms. As one of the most common malignant tumors worldwide, GC has a complex pathogenesis and limited therapeutic options. Therefore, a thorough understanding of the molecular mechanism of GC is important for the development of new therapeutic methods. The HGF/c-Met signaling pathway plays an important role in the proliferation, migration, and invasion of GC cells and has become a novel therapeutic target.

INTRODUCTION

Gastric cancer (GC) is the second most common cause of cancer death worldwide, with an annual incidence of approximately 1 million people and an annual death of approximately 700000[1-3]. Therefore, seeking effective treatment has become a research hotspot internationally. Current studies[4-6] have shown that hepatocyte growth factor (HGF) and its receptor (c-Met) signaling pathway are closely related to the genesis, development, metastasis, and prognosis of GC, and targeted drugs targeting this signaling pathway will become a new breakthrough point for the treatment of GC[7].

In the field of oncology, the incidence and mortality of stomach cancer remain high, making it a serious health problem worldwide[8]. Despite significant advances in the diagnosis and treatment of GC in recent years, the therapeutic effect on GC is still limited due to its complexity and heterogeneity[9-11]. Therefore, in-depth study of the molecular mechanism of GC and the search for new therapeutic targets and strategies are highly important for improving the therapeutic efficacy of treatments for GC[12]. The HGF/c-Met signaling pathway regulates cell proliferation, migration, and differentiation in normal physiological processes, but in tumors, it is closely related to tumor cell proliferation, survival, migration, invasion, and drug resistance[13-16].

Studies internationally have shown that the HGF/c-Met signaling pathway plays an important role in the occurrence and development of GC[17-20]. Some studies have shown that the expression level of c-Met in GC tissues is significantly greater than that in normal gastric tissues, and its overexpression is closely related to the poor prognosis of patients with GC[21]. In addition, activation of the HGF/c-Met signaling pathway not only promotes the proliferation and migration of GC cells but also enhances their resistance to chemotherapy drugs, increasing the difficulty of treatment[22].

In view of the important role of the HGF/c-Met signaling pathway in GC, this review further explored the specific mechanism of this signaling pathway in the occurrence and development of GC and evaluated targeted therapeutic strategies targeting the HGF/c-Met signaling pathway to provide new ideas for the clinical treatment of GC[23-30].

HGF/C-MET STRUCTURE AND FUNCTION

HGF structure and function

HGF is a multifunctional active factor that was first isolated and purified from the platelets and plasma of rats subjected to partial hepatectomy by the Japanese scholar Fushida et al[23] and is mainly produced by mesenchymal cells. The HGF gene is located at 7q21 in humans, and the protein is composed of 728 amino acids and is a heterodimer of 1 α chain and 1 β chain connected by a disulfide bond[31-33]. There is a hairpin structure at the N end of the α chain, which is the main component of the biological effect of HGF[34]. The β chain has a serine proteinase-like structure, which is the binding site of HGF to the interstitial epidermal transgene (or c-Met)[35]. HGF is a biological factor that can stimulate a variety of endothelial and epithelial cells to undergo mitosis and tissue formation and induce epithelial cell migration, invasion, vascular endothelial regeneration, and other functions.

c-Met structure and function

The c-Met protein, a member of the receptor tyrosine kinase (RTK) superfamily, is a transmembrane receptor with phosphorylated activity that is encoded by the MET proto-oncogene and is mainly produced by epithelial cells[36-38]. c-Met is a fragment with transformational activity that was first cloned. It fused with the translocation promoter region (TPR) in the osteosarcoma cell line to form a TPR-Met fusion body[39,40]. The MET gene is located on human chromosomes 7q21-q31, and the protein is connected by α and β subunits via disulfide bonds[41]. The α subunit contains only extracellular regions, while the β subunit has extracellular, transmembrane, and intracellular regions, with the former attached to the latter by disulfide bonds to form heterodimers. The extracellular region of the two subunits is the ligand recognition site, while intracellular division has tyrosine kinase activity[42]. c-Met is related to a variety of oncogene products and regulatory proteins and has a strong role in promoting cell proliferation, inducing the dispersal and motility of epithelial cells and fibroblasts[43-45]. It is involved in a variety of in vivo processes, such as signal transduction and cytoskeleton reconstruction, and is an important factor regulating cell proliferation, differentiation, and repair processes (Figure 1).

Figure 1 c-Met structure and function. A: Structures of MET; B: Hepatocyte growth factor (HGF); C: PSI-MET. Created with BioRender.com (Supplementary material).

HGF/c-Met signal transduction pathway

Under normal circumstances, HGF itself has no serine protease activity, and only after binding to the c-Met receptor can it exert its physiological effects[46-48]. Similar to other RTKs, the c-Met transmembrane region consists of a single alpha helix with two phosphorylation sites in most subdomains of the cytoplasmic near-membrane region: S985 and Y1003. After phosphorylation of S985, kinase activity is inhibited[49]. The phosphorylation of Y1003 can degrade internalized c-Met[50]. When c-Met binds to HGF, the tyrosine residue Tyr1234/Tyr1235 in the intracellular region is self-phosphorylated, and RTK in the protein kinase domain is activated. Activated RTK self-phosphorylates tyrosine Tyr1349/Tyr1356 at the carboxyl terminus of c-Met[51-54]. Finally, a variety of substrate proteins in the cytoplasm, such as phosphoinositide 3- kinase (PI3K) and phospholipase C, are recruited here and rapidly phosphorylated for use, thus activating a variety of intracellular signaling pathways such as the Ras/Raf, PI3K/AKT/mammalian target of rapamycin, Janus kinase/signal transducer and activator of transcription (STAT), and Crk/C3 glomerulopathy/Ras-associated protein-1 pathways. These pathways promote cell proliferation, regeneration, differentiation, angiogenesis, invasion, and metastasis; regulate cytoskeletal rearrangement; and promote cell movement. HGF and c-Met exist widely in a variety of cells and participate in a variety of physiological processes[55-57]. For example, it functions in repair, regeneration, and angiogenesis in damaged tissues and controls cell movement during embryonic development. However, when the HGF/c-Met signaling pathway is dysfunctional, the growth and movement of cells are disrupted, which may eventually lead to the occurrence, development, and metastasis of tumors (Figure 2).

Figure 2 Timeline of the terms used and hallmarks in hepatocyte growth factor/c-MET pathway research. HGF: Hepatocyte growth factor. Created with BioRender.com (Supplementary material).

RELATIONSHIP BETWEEN C-MET AND GC

Relationship between c-Met and tumors

In a physiological state, the HGF/c-Met signaling pathway is strictly regulated by both positive and negative factors so that it is in a state of equilibrium[58-60]. Specific gene damage, transcriptional upregulation, and ligand-dependent autosecretory or paracrine mechanisms can cause abnormal activation of c-Met, resulting in cell proliferation, migration, invasion, and growth[61]. The c-Met gene mutation was originally reported to occur only in the tyrosine kinase region and is associated with inherited and sporadic renal cell carcinoma[62-64]. Since then, a large number of studies have reported that c-Met overexpression and amplification also occur in many tumors, such as lung, gastric, colorectal, liver, pancreatic, prostate, ovarian, lymphatic, and head and neck cancers (Figure 3).

Figure 3 Role of the hepatocyte growth factor/c-MET pathway in signet ring cell carcinoma. HGF: Hepatocyte growth factor. Created with BioRender.com (Supplementary material).

Relationship between c-Met and GC

The pathogenesis of GC is a multifactor, multistep process caused by the precursors of the disease, such as Helicobacter pylori infection, and genetic and epigenetic abnormalities that lead to cell proliferation and metastasis[65]. As early as 1995, researchers noted that the expansion or mutation of oncogenes encoding tyrosine kinase receptors plays an important role in the development of GC. c-Met is a member of the tyrosine kinase receptor family, which is also closely related to GC[66-68]. Under normal circumstances, the HGF/c-Met signaling pathway can be temporarily activated; for example, HGF and c-Met can be detected in the injured kidney, liver, and heart, indicating that the signaling pathway has a role in protecting the body from injury and promoting repair and regeneration under physiological conditions[69]. Abnormal regulation of this signaling pathway plays an important role in tumor survival, growth, angiogenesis, and metastasis. In recent years, domestic and foreign scholars have shown that high c-Met expression in GC tissues is related to GC invasion, metastasis, and poor prognosis but not to patient sex or age, tumor size, tumor location, or degree of differentiation. In this review, c-Met expression in 182 patients with GC, chronic atrophic gastritis, intestinal metaplasia, abnormal hyperplasia, and normal gastric mucosa was detected by immunohistochemistry. The results showed that the expression rate of c-Met in GC tissues was significantly greater than that in other tissues, and the expression rate in lymph node metastatic GC tissues was significantly greater than that in nonmetastatic GC tissues[70]. Another study used immunohistochemistry to detect c-Met expression in the tissues of 124 GC patients and found that the overexpression of c-Met was associated with lymph node metastasis and vascular invasion[71]. A retrospective immunohistochemistry study of tissue samples from 495 patients with GC suggested that the overall survival (OS) and disease-free survival of patients with c-Met overexpression were shorter than those of patients without c-Met overexpression (Figure 4).

Figure 4 Pharmacochemical structure targeting hepatocyte growth factor/c-Met in gastric cancer. A: Related gene mutation sites; B: Pharmacochemical structure of MET mutations. Created with BioRender.com (Supplementary material).

RESEARCH STATUS OF GC DRUGS TARGETING HGF/C-MET SIGNALING PATHWAY

Anti-HGF monoclonal antibody

HGF monoclonal antibodies prevent tumor growth mainly by interfering with the binding of HGF to c-Met. At present, many growth factor inhibitors are in the clinical trial stage, and the main HGF inhibitors under study are rilotumumab, ficlatuzumab, and TAK-701[72-74]. All three medicines are humanized immunoglobulin G monoclonal antibodies that bind only to HGF. They stop HGF from binding to c-Met and stop c-Met kinase from activating. Rilotumumab is currently in phase II clinical trials and is associated with fluorouracil, leucovorin, and oxaliplatin (FOLFOX). (fluorouracil + calcium folinate + oxaliplatin) and (epirubicin + cisplatin + xeloda, ECX) are combined for the treatment of advanced gastrointestinal tumors. A phase II randomized controlled trial with 121 people who had gastric or gastroesophageal junction cancer revealed that compared with ECX plus a placebo, ECX plus rilotumumab improved their chance of not having their cancer worse. Progression-free survival (PFS) increased from 4.2 mo to 5.6 mo (hazard ratio [HR] = 0.64), and OS increased from 5.7 mo to 11.1 mo (HR = 0.29)[75-78]. However, an independent data committee safety review revealed that patients treated with rilotumumab combined with chemotherapy had a greater mortality rate than patients treated with chemotherapy alone. Amgen announced the termination of the clinical study on November 26, 2014.

Anti-c-MET monoclonal antibody

Onartuzumab/MetMAb is a c-Met monoclonal antibody that can inhibit dimerization of the c-Met receptor and further block the binding of HGF and c-Met, resulting in downstream signal transduction obstruction and tumor suppression[79]. More in-depth clinical research on non-small cell lung cancer (NSCLC) has focused on onartuzumab. The results of a phase II clinical trial revealed that after patients with NSCLC with mutations or amplifications of the c-Met gene were treated with the drug, PFS and OS were significantly longer than those in the placebo group. However, another phase III clinical trial ended early after failing to produce meaningful clinical results, and the trial ended in failure[80]. Researchers are still investigating the causes of the failure of phase III clinical trials. In a phase I clinical trial for GC, a chemotherapy-refractory patient with liver metastasis of GC with pathological stage PT3N1M1 achieved a sustained complete response (> 2 years) after treatment with onartuzumab[81-83]. The MetGastric study is a phase III randomized clinical trial of onartuzumab for GC that used onartuzumab combined with FOLFOX to treat human epidermal growth factor receptor (HER) (-)/c-Met (+) metastatic gastric and esophageal cancer patients (Figure 5).

Figure 5 c-Met may exhibit synergistic therapeutic effects on gastric cancer. HGF: Hepatocyte growth factor. Created with BioRender.com (Supplementary material).

Small molecule tyrosine kinase inhibitors

Small molecule tyrosine kinase inhibitors inhibit downstream signal transduction by blocking c-Met kinase activation, thereby inhibiting tumor proliferation, invasion, and metastasis. According to the different binding sites of c-Met, small-molecule kinase inhibitors can be divided into class I (SU-11274) and class II (AM 7)[84-86]. In addition, there is a third class of small molecule kinases, namely, ARQ197 and tivantinib, which selectively bind to unphosphorylated kinases and cut off key catalytic sites, thereby inhibiting c-Met phosphorylation.

Tivantinib: Tivantinib (ARQ197) is a non-ATP-competitive small-molecule c-Met kinase inhibitor that inhibits ligand-dependent or ligand-independent c-MET phosphorylation, thereby preventing downstream signal transduction[87]. A phase II clinical trial of tivantinib and erlotinib in the treatment of non-small cell lung cancer showed that tivantinib is effective in inhibiting tumor invasion, metastasis, and proliferation and inducing apoptosis[88]. However, tivantinib monotherapy did not show significant efficacy in patients with advanced metastatic GC.

Crizotinib: Crizotinib, a dual inhibitor of c-Met and anaplastic lymphoma kinase, has been approved by the United States Food and Drug Administration for the treatment of NSCLC[89]. However, in patients with GC with c-Met amplification, overexpression, or mutation, there is no consensus on the efficacy of treatment[90]. Kawakami and Okamoto[7] generated a mouse model of GC with crizotinib, and found that mice with positive c-Met amplification showed tumor inhibition after being administered this drug. However, c-Met amplification did not have a significant curative effect on these mice. The clinical symptoms of the other 2 patients improved after medication, mainly manifesting as better gastric, pain relief, and mass reduction, and the curative effect was maintained for approximately 4 mo.

Foretinib: Foretinib is a multimolecule tyrosine kinase inhibitor with high affinity for c-Met, macrophage-stimulating protein receptor, angiogenin receptor, and vascular endothelial growth factor receptor (VEGFR)[91-93]. It changes its structure and inhibits kinase activity by binding to ATP. The results showed that 20.2% of patients had stable disease after taking foretinib, with a median time of 3.2 mo for stabilization, and none of the patients achieved complete or partial remission (Table 1).

Table 1 List of c-Met inhibitors.

Drug (alternative name)	Study	Phase
Crizotinib (PF0234I066)	NCT00585I95	I
Cabozantinib (XL184)	NCT0I8664I0	II
	NCT0I708954	II
	NCT00940225	II
Foretinib (GSKI363089)	NCT0I068587	I/II
Tepotinib (EMDI2I4063)	NCT02864992	II
Rilotumumab (AMG102)	NCT02137343	III
Ficlatuzumab	NCT03422536	II
Tivantinib (ARQ197)	NCT01244191	III
Capmatinib (INC280, INCB28060)	NCT0I324479	I
	NCT0I60336	II

In the past, although the potential of the HGF/c-Met signaling pathway for the treatment of GC has received much attention, the results of many clinical trials have been unsatisfactory. For example, a phase III clinical trial using the c-Met inhibitor rilotumumab showed no significant improvement in OS in patients with advanced GC, despite showing some antitumor activity in earlier trials. Similarly, another phase III trial using onartuzumab failed to achieve the expected efficacy, with no significant difference in PFS and OS compared with placebo. In addition, a phase II clinical trial of foretinib as a multitarget tyrosine kinase inhibitor in the treatment of GC showed limited efficacy of its monotherapy and failed to meet the expected clinical endpoint. These negative trial results suggest that the HGF/c-Met signaling pathway in the treatment of GC needs further research to explore its complex biological mechanisms and possible combination therapy strategies to improve patient outcomes.

ACTIVATION STATE AND REGULATORY MECHANISM OF THE HGF/C-MET SIGNALING PATHWAY

The expression and activation status of the HGF/c-Met signaling pathway in GC tissues and cells were analyzed

This review focused on the expression and activation status of the HGF/c-Met signaling pathway in GC tissues and cells and revealed that this pathway plays a crucial role in the occurrence and development of GC. Studies have shown that abnormal activation of the HGF/c-Met signaling pathway is closely related to the proliferation, invasion, and metastasis of GC cells[94-96]. In particular, overexpression of the c-Met protein in GC tissues is negatively correlated with tumor malignancy and patient prognosis[97]. In addition, activation of the HGF/c-Met signaling pathway not only promotes the growth of tumor cells but may also affect the tumor microenvironment, thus participating in tumor immune escape. Therefore, targeted therapeutic strategies targeting the HGF/c-Met signaling pathway show broad application prospects for the treatment of GC[98]. Current research has also further explored the mechanism of action of this signaling pathway in the treatment of GC, hoping to provide more effective treatment options for patients with GC (Figure 6).

Figure 6 Effect of menin loss on maternally expressed 3 and c-MET expression. MEG3: Maternally expressed 3. Created with BioRender.com (Supplementary material).

Exploration of the downstream effects of signaling pathways and their roles in the occurrence and development of GC

This review explored the downstream effects and biological functions of the HGF/c-Met signaling pathway in GC. When HGF binds to its receptor, c-Met, it initiates a chain of events that include the activation of signaling pathways such as the Ras/mitogen-activated protein kinase (MAPK), PI3K/AKT, and STAT pathways. The activation of these signaling pathways not only promotes the proliferation and survival of GC cells but also promotes tumor cell invasion and metastasis. The HGF/c-Met signaling pathway is a key part of the epithelial-mesenchymal transition of GC cells and is a necessary step for tumor cells to become more aggressive and able to move around. In addition, the activation of HGF/c-Met signaling is also associated with the stem cell properties of GC cells and may be involved in tumor chemotherapy resistance[99].

During the development of GC, abnormal activation of the HGF/c-Met signaling pathway is closely related to the biological behavior of the tumor. Clinical studies have shown that high expression of c-Met is closely associated with poor prognosis in patients with GC, suggesting its importance in tumor development[100-102]. The HGF/c-Met signaling pathway also affects different cells in the area around the tumor, such as tumor-associated macrophages and cancer-associated fibroblasts, which further impacts tumor growth and progression.

Based on these findings, the HGF/c-Met signaling pathway has become an important target for targeted therapy in GC. Several small molecule inhibitors and antibody drugs targeting c-Met are being evaluated in clinical trials, some of which have shown potential therapeutic efficacy in patients with GC. However, due to tumor heterogeneity and complex signaling networks, therapeutic strategies targeting the HGF/c-Met signaling pathway need to be more personalized when combined with other therapies. Future studies will focus on a more in-depth analysis of the mechanism of action of the HGF/c-Met signaling pathway in GC and search for more effective treatment combinations to improve treatment outcomes and quality of life for patients with GC.

Evaluation of targeted therapy strategies

Through their application in GC cell lines and animal models, these drugs have shown significant antitumor effects. The primary targeted drugs used in the study included small-molecule inhibitors of c-Met and monoclonal antibodies against HGF or c-Met. These drugs can effectively inhibit the proliferation, migration, and invasion of GC cells, indicating their potential in the treatment of GC[103].

DISCUSSION

In particular, these agents inhibit MAPK, PI3K/AKT, and STAT signaling, which are activated downstream of the HGF/c-Met pathway. They do this by stopping HGF from binding to c-Met or stopping c-Met from working as a kinase. This intervention reduces the proliferation and viability of tumor cells while inhibiting cell invasion and migration, which is directly related to the tumor's ability to metastasize. In addition, targeted drugs can also affect the tumor microenvironment, reducing the interaction between tumor cells and the surrounding microenvironment and thereby limiting the further development of tumors. The HGF/c-Met signaling pathway is thought to be one of the main reasons why chemotherapy and targeted therapy do not affect GC patients who are drug resistant. Therefore, drugs that target this signaling pathway show potential for overcoming drug resistance in GC. Studies have shown that the application of inhibitors of the HGF/c-Met signaling pathway can effectively reduce the drug resistance of tumor cells and improve the sensitivity of chemotherapy in GC cells and animal models treated with traditional chemotherapy drugs[104].

However, therapeutic strategies targeting the HGF/c-Met signaling pathway also face challenges. The heterogeneity and complex signaling network of tumors make it difficult for single-target therapy to completely inhibit the growth and spread of tumors. Therefore, future research needs to focus on how to combine these targeted drugs with other therapies, such as chemotherapy, radiation, and other types of targeted therapy, to improve treatment effectiveness[105]. At the same time, an in-depth understanding of the mechanism of the HGF/c-Met signaling pathway in different types of GC will help to more accurately select treatment strategies, develop personalized medicine, and improve the survival rate and quality of life of GC patients.

This study had some limitations. First, despite our best efforts to collect data from the latest and most comprehensive literature, there may still be recent advances that are not included in time due to the rapid pace of development in the field of research. Second, the complexity of the HGF/c-Met signaling pathway and heterogeneity of GC lead to different results in different studies, which increases the difficulty of summary and analysis.

CONCLUSION

In recent years, molecular targeted therapy has become a hot topic in the treatment of GC. Mutations in HER2, epidermal growth factor receptor, VEGFR, and other genes are all associated with GC, and c-Met is a potential target. Therefore, the use of HGF/c-Met signaling pathway inhibitors is a potential breakthrough in the treatment of GC. At present, some drugs targeting this signaling pathway are in clinical trials, including onartuzumab, crizotinib, and rilotumumab, have been shown to have fixed therapeutic effects in early-stage clinical trials for the treatment of late-stage GC. A phase III clinical trial of onartuzumab combined with chemotherapy, which is expected to become an effective means for the treatment of GC, is ongoing. However, the efficacy of most drugs has not yet been determined, so much research needs to be done by researchers. In addition, researchers can expand the research target to the downstream genes of the HGF/c-Met signaling pathway, increasing the scope of research to identify appropriate treatments.