Cytosolic-epithelial transforming factor (c-Met) belongs to a family of tyrosine kinase receptors that are overexpressed in various human cancers, and its ligand is hepatocyte growth factor (HGF). It has been found that the HGF/c-Met signaling pathway is involved in a wide range of cellular processes, including cell proliferation, migration and metastasis. Also, the HGF/c-Met signaling pathway is involved in mediating the development and progression of bladder cancer (BCa). Therefore, c-Met is a potential target for BCa therapy.
In addition, recent studies have shown that aberrant expression of non-coding RNAs was found to play an important role in tumor progression, and c-Met is closely associated with non-coding RNAs. In this paper we will present the biological function and prognostic value of c-Met in BCa, as well as its potential role as a drug target.
PART.01 Current status of bladder cancer
Bladder cancer (BCa) is one of the most common malignancies, and according to the World Health Organization International Agency for Research on Cancer, 54,939 new cases of BCa were diagnosed in 2018, ranking 12th among all types of cancers in men and women and 6th among men (Figure 1).BCa is usually divided into two categories: non-muscle invasive bladder cancer (NMIBC) and muscle invasive bladder cancer (MIBC). Currently, nearly 75% of cases are NMIBC and 25% are MIBC or metastatic disease. The 5-year survival rate for patients with aggressively treated MIBC is approximately 60%, and the 5-year recurrence rate for patients with NMIBC is 50% ~ 70%.
BCa is a complex disease that has long been a global burden. However, the current treatment for BCa is limited. More research is needed to gain more insight into the development of BCa and more effective therapeutic strategies. Currently, aberrant activity of cellular mesenchymal-epithelial transforming factor (c-Met) has been observed in a variety of human malignancies, including BCa, which predicts that c-Met has the potential to become a target for the treatment of bladder cancer.
PART.02c-Met structure and biological function
First identified in the mid-1980s, c-Met encodes a well-characterized oncogene located on chromosome 7q21-31. c-Met, also known as Met, HGFR, AUTS9, RCCP2 and DFNB97, belongs to the family of receptor tyrosine kinases and is mainly found in epithelial cells. Mature c-Met proteins are disulfide-linked heterodimers consisting of a highly glycosylated 45-kDa extracellular α-subunit and a 145-kDa transmembrane β-subunit.
The extracellular portion of c-Met includes three structural domains, an N-terminal SEMA structural domain, a cysteine-rich structural domain, and four IPT structural domains. The intracellular portion includes a tyrosine kinase catalytic structural domain, a paramembrane structural domain and a carboxy-terminal sequence.
Figure 2. c-Met structural features 1
Hepatocyte growth factor (HGF), the only high-affinity ligand for c-Met, was first identified in 1984.HGF is a representative of a family of cytokines associated with fibrinogen (called dispersing factors) and is expressed in cells of mesenchymal origin.The gene encoding HGF is located on chromosome 7q11.2-21.Mature HGF contains an n-terminal structural domain, four Kringle structural domains and a structural domain homologous to the serine protease and rennin families.
In general, dimerization is a potential regulatory mechanism for the activation of tyrosine kinase receptors. hGF contains two receptor binding sites: one with high affinity for the IPT3 and IPT4 structural domains of c-Met and one with low affinity for the SEMA structural domain.
Upon binding to biologically active HGF, c-Met dimerizes and Tyr residues Y1234 and Y1235 within the activation loop undergo trans-autophosphorylation. Then, the c-terminal segment Tyr residues Y1349 and Y1356 undergo autophosphorylation, providing binding sites for the recruitment of downstream signaling effector molecules.
These enzymatically active binding proteins, including phosphatidylinositol 3-kinase (PI3K), phospholipase C-γ1 (PLC-γ1), non-receptor tyrosine kinase Src, signal transducer and activator of transcription 3 (STA T3), and adapter proteins without enzymatic activity, such as growth factor binding protein 2 (GRB2) and GRB2-associated binding protein 1 (GAB1). Thus, these effectors play roles related to scattering effects, proliferative effects, morphogenesis and many other biological functions.
HGF/c-Met downstream signaling pathways include the Ras signaling pathway, PI3K signaling pathway, and Wnt/β-catenin signaling pathway (Figure 3). Under normal physiological conditions, HGF/c-Met plays a role in embryogenesis, tissue regeneration, wound healing, and nerve and muscle formation, the progression of which is partially mediated by the tumor suppressor p53. In addition, it has been shown that c-Met may activate signaling in a non-dependent manner with HGF. There is growing evidence that abnormal HGF/c-Met regulation and c-Met gene mutation, amplification and overexpression contribute to a large number of human diseases, especially cancers, such as lung, liver and BCa.
Figure 3. c-Met-mediated signaling pathw 1
PART.03c-Met in bladder cancer
Back in the 1990s, researchers demonstrated the involvement of the HGF/c-Met pathway in the progression of BCa in animal models and humans. And, increased expression of HGF/c-Met was observed during N-butyl-N-(4-hydroxybutyl)-nitrosamine induced bladder carcinogenesis in rats.
It was found that HGF promotes proliferation and growth in non-tumorigenic cell lines in rats, while in tumorigenic cell lines, HGF stimulates cell invasion and migration through paracrine or autocrine mechanisms. These results suggest that HGF acts as a mitogen in non-tumorigenic cell lines, but as an invasion and migration factor in tumorigenic cell lines. In addition, the researchers compared HGF levels in the urine of BCa patients and healthy patients and found that HGF was significantly elevated in BCa patients. Thus, there appears to be a positive correlation between BCa invasion and HGF levels.
In addition, researchers observed c-Met overexpression in 25% (31/123) of locally advanced or metastatic bladder uroepithelial carcinomas, compared to 8.3% (2/24) of non-bladder uroepithelial carcinomas. The same study suggests that autocrine HGF/c-Met signaling is rare in BCa, while paracrine signaling may predominate. Although c-Met and HGF showed different expression patterns in different types of BCa cells, these studies all showed enhanced expression of c-Met. c-Met was also observed to play a promotional role in BCa cell proliferation, invasion and migration, but an inhibitory role in apoptosis.
Since c-Met and HGF are considered to be characteristic molecules of BCa, current studies have focused on the role of HGF/c-Met in BCa and its mechanisms. In addition, c-Met expression correlates with the pathological stage, tumor grade and survival of BCa patients.
c-Met belongs to the family of receptor tyrosine kinases (RTKs) with sequence and structural homology, and other family members include RON, AXL and PDGFR. c-Met was found to:
c-Met co-expression with RON, AXL or PDGFR is commonly found in uroepithelial cells;
overexpression of c-Met and RON was positively correlated with histological grade, non-papillary profile, tumor size and muscle invasiveness of BCa;
Co-expression of c-Met and RON was positively correlated with progression of bladder metastatic cell carcinoma and poor overall survival of patients with superficial BCa.
A study showed that downregulation of c-Met inhibited BCa cell proliferation and induced apoptosis. In addition, c-Met silencing downregulated MMP2 and MMP9, which in turn inhibited cell proliferation.
Typically, there are multiple types of BCa, and studies have shown that the HGF/c-Met signaling pathway may be involved in multiple types of BCa, with neuroendocrine BCa being a rare type of BCa. c-Met was expressed in serum-free cultures of neuroendocrine BCa, animal models, and primary tissue sections. HGF was found to promote the proliferation, migration and wound healing of human neurogenic bladder smooth muscle cells and uroepithelial cells, and to mediate the growth of neuroendocrine BCa spheroids in vitro.
Another rare type of BCa, liver-like adenocarcinoma of the bladder, also expresses HGF and c-Met. The prognostic role of c-Met in BCa has now been analyzed and confirmed by several studies. A trial analysis including 8 studies with 1336 cases demonstrated that c-Met upregulation was associated with shorter overall survival (OS) in BCa patients. In addition, immunohistochemical analysis of 26 BCa specimens showed that matrix enzyme-mediated c-Met phosphorylation was strongly associated with poor outcome.
PART.04 c-Met inhibitors for the treatment of Bca
Generally speaking, c-Met inhibitors can be divided into three categories:
c-Met tyrosine kinase inhibitors (TKIs)
HGF antagonists that target c-Met
HGF-neutralizing antibodies
Small molecule TKIs block intracellular signaling pathways in tumor cells, most of which are ATP-binding competitive inhibitors. type I inhibitors bind to proteins via U-shaped structures, while type II inhibitors are multi-target agents that act in a broader spatial conformation; type III inhibitors are non-ATP-competitive drugs.
Currently, there are five drugs approved for marketing against c-Met: crizotinib, cabozantinib, camatinib, sevotinib, and terpotinib (Figure 4).
Figure 4. marketed c-Met small molecule 1
In addition, clinical trial results of c-Met targets have shown a promising treatment strategy for solid cancers with a good safety profile, both as monotherapy and in combination with other target agents (Figure 5).
Figure 5. Clinical trials of representat 1
In BCa, cabozantinib - a non-selective TKI that targets c-Met and others such as VEGFR2, FLT3, KIT, AXL and RET - is currently undergoing intensive clinical studies. As previously mentioned, researchers have found that cabozantinib blocks HGF/c-Met-mediated expression of MMP1.
In addition, a phase II clinical trial of cabozantinib and durvalumab in combination for the treatment of metastatic disease after platinum-based chemotherapy initially showed that the combination was safe, achieving an objective remission rate of 43.8%. Another multi-targeted TKI including c-Met, called crizotinib, is also promising for clinical use. A preclinical xenograft study showed that both crizotinib and cabozantinib significantly inhibited HGF/c-Met-induced proliferation and invasion in a variety of Bca tumor cell lines. Multiple clinical trials of cabozantinib and crizotinib for BCa are currently underway (Figure 6).
Figure 6. c-Met inhibitor clinical appli 1
PART.05 Summary
Currently, c-Met is considered a proto-oncogene that promotes a wide range of biological functions such as cell proliferation, growth, migration, invasion and angiogenesis by interacting with its ligand HGF. c-Met enhanced expression is positively correlated with the occurrence and poor prognosis of various types of BCa.
In addition, many studies have observed that c-Met overexpression is closely associated with the histological grading, non-papillary profile, tumor size and muscle infiltration of BCa. An increasing number of studies suggest that c-Met may be a novel and important biomarker for BCa. At present, c-Met inhibitors represented by cabozantinib have been widely used in anti-cancer therapy and have a broad prospect in clinical application of BCa. We look forward to more good news of c-Met inhibitors in the field of bladder cancer.
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doi:10.1038/nrc3205