DDR1: a Novel Member of the RTKs Family, a Potential Target for Tumor Therapy!

DDR1 has been newly identified as a member of the receptor tyrosine kinases (RTKs) family. It is widely recognized that RTKs control cell proliferation and differentiation, which are involved in tumorigenesis. Therefore, the discoidin domain receptors (DDRs) gained a considerable amount of attention. More recent research has highlighted the fact that DDR1 exerts key roles in the cell differentiation, proliferation, adhesion, and migration. More important, its functions has been revealed in malignant transformation of cells and tumor invasion and metastasis. In the earlier article published on Nature suggested that DDR1 could help to establish a physical barrier around tumors, therefore preventing T cells invasion. In other words, DDR1 inhibition would block the inhibitory effect of DDR1 on immune cells. All in all, DDR1, as a new member of RTKs, holds great clinical promise for cancer treatment!

1. The Discovery of DDR

In 1993, Johnson et al. identified the discoidin domain receptor (DDRs) while studying tyrosine phosphorylation in breast cancer cells, which are considered to be members of the transmembrane receptor tyrosine kinases (RTKs) superfamily. As of now, the DDR sub-family has two members: DDR1 and DDR2. [1]. DDRs consist of three parts: intracellular kinase region, transmembrane region and extracellular region. Unlike other members of the RTKs family, DDR has a discoidin region (DR) in the extracellular structural domain, so it is called a discoidin receptor type protein tyrosine kinase. DDR was discovered as a novel RTKs when researchers explored the relationship between tyrosine kinase protein expression and human malignancies. Thus, DDRs may be an important factor in the development of tumorigenesis. Successive studies have shown that DDRs are associated with tumor cell proliferation, invasion, and metastasis. Other studies indicated that DDRs are related to fibrosis and neurodegeneration. Further research on DDRs may provide new perspectives for cancer therapy or other diseases (Figure 1) [2].

2. What is DDR1?

DDR1 (Discoidin domain receptor 1), a member of DDRs, belongs to a novel class of receptor-type tyrosine kinases (RTKs). The DDR1 gene is located on the p-arm of human chromosome 6 (6p21.3). DDR1 consists of an extracellular structural domain that interacts with the ligand, a transmembrane region, and an intracytoplasmic structure that signals through the kinase structural domain structure (Figure 2) [3]. DDR1 has five isoforms: DDR1a, DDR1b, DDR1c, DDR1d, and DDR1e, generated by selective splicing or deletion of exons. Currently, the isoforms with proven biological functions are DDR1a and DDR1b. Studies have demonstrated that a number of proteins can bind to activated DDR1, NMHC-IIA, FAK, STAT3, Lyn etc. However, certain proteins binding to DDR1 without phosphorylation, such as dopamine and cAMP-regulated phosphoproteins, renal brain-expressed proteins, Notch1, CADH1 [4-8].

DDR1 is mainly founded in epithelial cells, smooth muscle cells, fibroblasts, oligodendrocytes, and macrophages. Its expression affects fibrotic lesions in kidney, liver, and lung tissues. Moreover, the abnormal expression of DDR1 has been observed in tumors, which lead to to poor tumor prognosis, such as lung cancer, breast cancer, ovarian cancer, esophageal cancer, liver cancer, and others. Given the link between DDR1 and tumor progression, DDR1 becomes a new target for cancer therapy [9-10]. Currently, several DDR1 based clinical trials in development, mainly for cancer treatment.

3. What are the Ligands for DDR1?

Generally, a soluble growth factor served as the ligand for RTKs, but the ligand for DDRs is collagen, a key component of the extracellular matrix. DDR1 binds to collagen I, II, III, and IV, and with a lower affinity for collagen X. DDRs recognizes a specific collagen on amino acid sequence GVMGFO. The binding of collagen to the DDRs eventually leads to a slow and sustained activation of tyrosine kinase (Figure 3) [3].

Technically, the activation of most RTKs is rapid activation and rapid inactivation, with activation of phosphorylation of tyrosine residues occurring within seconds of ligand binding. Back to DDR1, DDR1 activation does not occur in the same manner as most RTKs. the activation of DDR1 takes the form of slow activation and slow inactivation with a longer phosphorylation state. It has been shown that DDR1 is phosphorylated after about 30 minutes of exposure to collagen ligand stimulation and even can last for 16 hours [11]. In any case, abnormal tyrosine kinase activation is associated with tumorigenesis. Overall, DDR1/collagen activation could regulate the tumor cell proliferation, migration, and invasion.

4. How's the DDR1's Regulation Mechanism?

Current studies suggested that DDR1 mainly acts through these three mechanisms: i) classical collagen-DDR1 signaling pathway, dependent on DDR1 kinase activity, with its downstream molecules Shc, Nck2, Shp-2, etc.; ii) the collagen-dependent stimulation, but not DDR1 kinase activity, also known as the non-classical DDR1 signaling pathway; iii) the collagen-independent stimulation, and not DDR1 kinase activity.

In most cases, DDR1 exerts functions dependence on its kinase activity. For instance, it has been found that activated DDR1 recruits PSD4 to bind to ARF6, thereby activating ARF6 and its downstream MAPK signaling pathway, which in turn promotes lung metastasis in hepatocellular carcinoma cells [11]. Another study suggests that activated DDR1 could regulate Ras and Raf to induce MAPK activation, thereby promoting the expression of p19 ARF and subsequently inducing the expression of P53 [12]. In adipose tissue, DDR1 in the stromal vascular component (SVF) promotes the IL-6 secretion, which is important for SVF pro-tumor cell invasion, suggesting that extra-tumor DDR1 can also promote tumor progression [13]. As mentioned above, DDR1 can exert pro-tumor effects through a non-classical pathway. Such as, in breast cancer cells, TM4SF1 functions as a membrane bridging protein, recruiting syntenin 2 to interact with DDR1, thereby activating a series of molecules including PKC, JAK2, and STAT3. The non-classical pathway induces SOX-2 and NANOG expression, maintaining the identity of cancer stem cells [14-15]. Thus, DDR1 can act as a potential target for tumor therapy (Figure 4) [3].

5. What's the Role of DDR1 in Tumors?

5.1 DDR1 in Tumor Proliferation and Growth

DDR1 promotes cell proliferation and growth in both non-malignant and malignant tumor cells, non-malignant cells, such as in primary human bronchial epithelial cells, T cells, etc.; tumor cells, such as human glioma cells, colon cancer cells, liver cancer cells, etc [16-17]. Some researches pointed that DDR1 could inhibit the cell proliferation and growth in some non-malignant tumor cells, such as primary murine thylakoid cells. However, it has not been reported that DDR1 can inhibit the proliferation and growth of malignant tumor cells. Studies have shown that in human breast and colon cancers, DDR1 actives the Ras/Raf/ERK and PI3K/Akt pathways, which further upregulates the anti-apoptotic protein Bcl-xL and allowing cells to survive in a genotoxic stress state [5, 16-17].

5.2 DDR1 in Tumor Invasion and Metastasis

Studies have shown that DDRs promote a variety of human tumors invasion, such as breast, lung and testicular cancers [9-10]. In human pancreatic cancer cells AsPC-1, DDR1 plasmid was transfected with AsPC-1. The findings suggested that DDR1 expression was significantly elevated in AsPC-1 cells; Transwell assay results showed that AsPC-1/DDR1 mobility was upregulated and invasion as well. The expression levels of MMP2 and MMP9 were remarkably increased in AsPC-1 cells after overexpression of DDR1. It suggested that DDR1 could increase the MMP2 and MMP9 expression levels to promote the migration and invasion of pancreatic cancer cells [18].

5.3 DDR1 in Tumors' EMT

Epithelial-mesenchymal transition (EMT) plays an important role in embryonic development, chronic inflammatory responses, tissue reconstruction, fibrotic diseases, and cancer metastasis. Epithelial cells through EMT lose their epithelial phenotype, such as cell polarity and connection to the stromal membrane. As a result, epithelial cells acquire a mesenchymal phenotype such as higher migration, invasion, resistance to apoptosis, and degradation of the extracellular matrix [19-20]. It has been shown that DDR1 increases the invasive and migratory ability of non-small cell lung cancer cells by promoting EMT. Further studies revealed that DDR1 overexpression promotes epithelial mesenchymal transition in gastric cancer cells, which might be associated with the activation of snail signaling pathway [21]. Snail related signaling pathways are inextricably linked to epithelial mesenchymal transition. Snail accelerates tumor lymph node and distant metastasis by inhibiting E-cadherin-induced epithelial mesenchymal transition [22]; it has also been shown that snail/HDAC1/HDAC2 complex inhibits EMT, thereby promoting the metastatic ability of the in pancreatic cancer [23]. The role of DDR1 in the tumors’EMT still needs to be further explored.

5.4 DDR1 in the Tumor Microenvironment (TME)

It has been suggested that DDR1 could translate the extracellular matrix (ECM) into a highly ordered state during tumor development, hindering immune cell infiltration and its role in killing tumor cells. Further analysis revealed that knockdown of DDR1 in a TNBC mouse model promoted T-cell infiltration. Therefore, inhibiting DDR1 has the potential to block the ability of tumors to resist immune surveillance and allow immune cells to successfully enter the tumor to kill tumor cells [24]. Thus, DDR1 may be the key target for tumor immunotherapy.

6. DDR1 Clinical Prospects

In practice, several DDR1 based clinical trials have been performed for the treatment of tumors and other diseases, which are involved in global companies including Chinese’ InnoCare Pharma Ltd., South Korea’s Avixgen, Inc., United States’s KeifeRx LL, and so on. Generally speaking, tyrosine kinase inhibitors are designed to target tyrosine kinase activity and function, including chemical small molecules (designed to disrupt kinase activity inside cells) and monoclonal antibodies (designed to interfere with the function of extracellular structural domains). DDR1 antagonists in clinical trials are mainly small molecule chemotherapeutics, which are applied in the treatment of cancer, neurodegenerative diseases, Alzheimer's disease (AD), and other diseases (Table 1).

Some preclinical studies suggested that DDR1 inhibitors exhibit strong antitumor activity in tumor transplantation mouse models derived from patients with liver cancer, breast cancer, colorecta cancer, gastric cancer, esophageal cancer, and NSCLC. With a more in-depth understanding of DDR1, it is the notion that DDR1 might serve as an attractive target for tumor therapy or others.

To assist researchers and pharmaceutical companies in their research on DDR1 based drug research in tumors, neurodegenerative diseases, AD, and others. CUSABIO presents DDR1 active protein product (Code: CSB-MP006595HU) to assist you in your research on DDR1 mechanism or its potential clinical value.

● Recombinant Human Epithelial discoidin domain-containing receptor 1(DDR1), partial (Active)

References

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