TM4SF1: a Member of the Transmembrane-4-Superfamily (L6), a New Vascular Therapeutic Target?

1. What is the TM4SF Family?

Transmembrane-4 superfamily (TM4SF) includes at least 30 members that are variably expressed in leukocytes, like CD9 (MRP1), CD82 (KAI1), CD63 (LAMP-3), CD81 (TAPA1), CD151 (PETA3), CD37, CD53, C0-029, and Sm23, etc ^[2-4]. Among these, TM4SF1 (L6) distinguishes itself by lacking the highly conserved CCG (Cys-Cys-Gly) sequence found in other TM4SF family members, despite sharing a similar topology. Therefore, it stands apart from the TM4SF proteins family and is classified within the L6 superfamily. Other members in L6 superfamily include TM4SF4, TM4SF5, TM4SF18, TM4SF19, and TM4SF20 ^[5-6]. Some TM4SF family members exhibit a negative regulatory effect on tumor cell growth, movement, and metastasis, inhibiting the migration and invasion of tumor cells. However, certain others within this family promote the development and metastatic infiltration of tumors ^{[1, 2-4]}.

2. What is TM4SF1?

2.1 TM4SF1 Structure

TM4SF1 (Transmembrane-4-L-six-family-1) is also recognized as L6 or TAL6. It belongs to one of the members of the transmembrane-4 superfamily (TM4SF) because of its structural and functional similarity to the TM4SF family. TM4SF members are structurally similar and mostly consist of 200-350 amino acids with four transmembrane regions, characterized by their short C- and N-termini, potentially involved in cytoskeletal interactions and signaling pathways. Outside the cell, they interact with spatial proteins such as specific receptors ^[7-8].

TM4SF1 structure has not yet been fully elucidated. It has been identified as a cysteine-rich and hydrophobic polypeptide composed of 202 amino acids. Its peptide chain contains four hydrophobic domains, with three proximal to the amino terminus and one near the hydroxyl terminus. These hydrophobic segments are interspersed with a 29-amino acid hydrophilic structural domain, housing two potential glycosylation sites. This domain is crucial for its association with cell surface proteins, displaying a topoenzyme-like configuration ^[10].

Figure 1. TM4SF1 structure ^[10]

2.2 TM4SF1 Expression and Functions

TM4SF1 typically exhibits low expression levels in normal tissues. However, in various epithelial malignant tumors such as pancreatic, ovarian, liver, prostate, colorectal, and breast cancers, TM4SF1 shows markedly high expression levels. Its involvement in tumor cell growth, adhesion, invasion, and metastasis is well-documented. Additionally, TM4SF1 plays a crucial role in regulating vascular endothelial cell function and pathological angiogenesis ^[11-12].

Studies indicate that TM4SF1 antibody has shown promise in targeting tumor cells. It does so by affecting human monocytes' antibody-dependent cytotoxicity and human complement factor's complement-dependent cytotoxicity ^[13]. In Phase I clinical trials with recurrent breast, colon, lung, and ovarian cancer patients, this antibody displayed excellent tolerance and significant effectiveness against tumor cells. Some patients even achieved complete remission. Therefore, TM4SF1 antibody is expected to become a new target for future tumor research ^[14-16].

3. The Mechanisms of TM4SF1-related Anti-Tumor Angiogenesis

Tumor growth depends on tumor blood vessel formation for metabolic support. TM4SF1 plays a critical role in tumor cell activities like growth, adhesion, invasion, and metastasis by forming complexes with integrin family members, contributing to tumor blood vessel development. It also triggers the Rho signaling pathway, thereby mediating epithelial-mesenchymal transition (EMT), which is involved in tumor cell movement and spread. Ongoing research aims to uncover more about its precise mechanisms ^[17].

Research findings highlight that TM4SF1 knockdown leads to several significant effects: it prevents the formation of filamentous pseudopods, inhibits cell migration, halts cell division, and encourages endothelial cell senescence. Upon binding, TM4SF1 interacts with integrin α5 and integrin beta 1 (CD29), but this interaction occurs only after vascular endothelial growth factor A (VEGF-A) or thrombin stimulation. Consequently, TM4SF1 knockdown inhibits angiogenic endothelial growth factor maturation ^[17].

TM4SF1 is found in tumor cell membranes and organelles like lysosomes. Its binding to proteins like LAMP-1, LAMP-2, and lactoferrin-binding protein (LTF) affects cellular function upon co-localization. TM4SF1 plays a role in promoting angiogenesis and influencing cell migration through recruitment to the region of TEMs (tetraspanin-enriched microdomains) with associated ubiquitination modifications. Additionally, TM4SF1 interacts with CD13, CD63, CD82, and other transmembrane proteins, regulating cell motility and invasive ability. Overall, TM4SF1 holds significant importance in endothelial cells in vitro and pathological angiogenesis in vivo, potentially serving as a promising target for anti-tumor angiogenesis ^[17].

Figure 2. The mechanisms of TM4SF1-related anti-tumor angiogenesis ^[17]

4. The Roles of TM4SF1 in Tumor Research

4.1 TM4SF1 and Pancreatic Cancer Research

TM4SF1 expression exhibited a significant increase in pancreatic cancer compared to benign tumors and normal tissues. Silencing TM4SF1 using siRNA showed inhibited invasiveness and migration abilities in pancreatic cancer cells, while the cells' inherent proliferation ability remained unaffected. This suggests that TM4SF1 primarily impacts tumor cell migration and metastasis pathways, displaying no direct link with tumorigenesis. Silencing TM4SF1 with siRNA increased tumor cell sensitivity to the chemotherapeutic drug capecitabine, yet the specific mechanism behind this remains unclear ^[18-21].

Studies suggest that TM4SF1 influences tumor development by regulating MMP-2 and MMP-9 gene expression in pancreatic cancer cells, affecting cell migration and invasion. Furthermore, various TM4SF family members, such as CD151, CD9, CD81, and others, demonstrate high expression in pancreatic cancer tissues. These proteins are broadly expressed in the cell membrane and cytoplasm of pancreatic ductal adenocarcinoma ^[18-21].

4.2 TM4SF1 and Breast Cancer Research

TM4SF1 demonstrates high expression levels in breast cancer tissues, suggesting its involvement in breast cancer development and metastasis, thereby potentially facilitating tumor cell metastasis. Notably, within the HER-2 high expression subtype of breast cancer, TM4SF1 protein expression peaks, indicating a potential correlation between the two ^[22]. In a separate investigation, TM4SF1 exhibited high expression levels in breast tumor tissues compared to adjacent normal breast tissues.

The synthesis of HLA-A2, derived from an antigenic epitope, was found to hinder tumor cell growth, offering significant implications for future breast cancer drug development ^[23]. Moreover, studies revealed that TM4SF1 might impede tumor growth by impacting the PI3K/AKT/mTOR signaling pathway in TNBC (Triple-Negative Breast Cancer) cell invasion and apoptosis ^[24].

4.3 TM4SF1 and Prostate Cancer Research

TM4SF1 is directly regulated by the androgen receptor (AR), a pivotal transcription factor in prostate cancer. The androgen receptor significantly influences the internal environment of prostate cancer, driving its progression. Excessive proliferation of tumor cells may occur due to the loss of control over androgen activity upon ligand binding ^[25-27].

Notably, the TM4SF1 gene contains an androgen response element (ARE) in its promoter region, showcasing regulation by androgens. In prostate cancer, TM4SF1 expression levels surpass those found in benign prostatic hyperplasia (BPH). Further functional analysis indicates TM4SF1's involvement in regulating tumor cell migration, potentially contributing to tumor metastasis. Delving deeper into TM4SF1's role as an androgen receptor (AR) target holds promise for novel therapeutic approaches in prostate cancer research ^[25-27].

4.4 TM4SF1 and Ovarian Cancer Research

The SEREX (Serological analysis of recombinant cDNA expression libraries) technique was utilized to screen TM4SF1 from a human ovarian cancer ascites cDNA library. Subsequently, the related antigen was isolated and purified for further analysis. TM4SF1 gene expression varied across different ovarian tissues, exhibiting a progressive increase in conjunction with ovarian cancer development and progression ^[28-29].

Its expression profile displayed characteristics linked to ovarian cancer advancement ^[28]. Additionally, gene chip database analysis and protein interaction network studies revealed direct interactions between TM4SF1 and DDR1. These interactions are associated with signaling pathways involving the extracellular matrix, collagen, and integrins, shedding light on their potential roles in ovarian cancer ^[29].

4.5 TM4SF1 and Other Cancer Research

It is currently known that TM4SF1 plays an important role in the migration and invasion of various cancers. For instance, in hepatocellular carcinoma, TM4SF1 regulates crucial genes like caspase-3, caspase-9, MMP-2, MMP-9, and VEG, promoting cell proliferation, invasion, and metastasis ^[30]. Microarray-based findings in cervical cancer identified EFNB2 and TM4SF1 as highly expressed in primary SP cells, strongly associated with tumor metastasis and invasion ^{[28, 31]}. In gastric cancer, TM4SF1 facilitates cell proliferation, migration, and invasion via regulation of apoptosis-linked proteins like caspase-3 and modulation of the PPARγ-SIRT1 feedback loop, impacting bladder cancer cell apoptosis and the cell cycle dynamics ^[32].

5. TM4SF1 Clinical Research Perspectives

Presently, four drugs targeting TM4SF1 are in development, including Anti-TM4SF1 ADC, CART-TM4SF1 cells, KQ-L6, Anti-TM4SF1-CAR-T-cell therapy. These medications function by inhibiting or eliminating TM4SF1-expressing tumor cells, employing diverse mechanisms. Primarily designed for solid tumors and digestive system tumors, these drugs are at varying stages of development. Anti-TM4SF1 ADC remains in the preclinical phase, while the other three drugs have progressed to clinical Phase I trials. TM4SF1, as a highly promising target in tumor research, holds significant potential in clinical drug development. Yet, further data collection from clinical studies and comprehensive safety assessments are imperative to validate its effectiveness and benefits.

In Conclusion:

TM4SF1, a member of the Transmembrane-4-L-six-family-1, is a crucial protein implicated in various cancers. Its high expression levels in these cancers indicate involvement in tumor progression, metastasis, and cell migration. TM4SF1 exhibits multifaceted roles: regulating cellular signaling pathways, impacting angiogenesis, and interacting with other proteins such as integrins, DDR1, and androgen receptors. Ongoing research focuses on developing targeted therapies like ADCs and CAR-T cell therapies to inhibit or eliminate TM4SF1-expressing tumor cells. However, further clinical studies are needed to confirm its effectiveness and safety for potential therapeutic applications.

6. CUSABIO TM4SF1 Recombinant Proteins & Antibodies for Research Use

To fully support researchers and pharmaceutical companies in their research on TM4SF1 in tumors and other diseases, CUSABIO presents TM4SF1 active proteins & antibodies to support your research on the mechanism of TM4SF1 or its potential clinical value.

CUSABIO TM4SF1 Protein

● Recombinant Human Transmembrane 4 L6 family member 1(TM4SF1)-VLPs (Active) Code: CSB-MP023615HU

High Specifity Validated by SDS-PAGE

CSB-MP023615HU is detected by Mouse anti-6*His monoclonal antibody.The two bands respectively correspond to monomer, Homodimer.

Excellent Bioactivity Validated by Functional ELISA

Immobilized Human TM4SF1 at 5μg/mL can bind Anti-TM4SF1 recombinant antibody (CSB-RA023615MA1HU). The EC₅₀ is 4.079-4.472 ng/mL. VLPs (CSB-MP3838) is negative control.

● Recombinant Macaca fascicularis Transmembrane 4 L6 family member 1 (TM4SF1)-VLPs (Active) Code: CSB-MP5031MOV

High Specifity Validated by SDS-PAGE

CSB-MP5031MOV is detected by Mouse anti-6*His monoclonal antibody.The two bands respectively correspond to monomer, Homodimer.

Excellent Bioactivity Validated by Functional ELISA

Immobilized Cynomolgus TM4SF1 at 5μg/mL can bind Anti-TM4SF1 recombinant antibody (CSB-RA023615MA1HU). The EC₅₀ is 4.480-4.930 ng/mL. VLPs (CSB-MP3838) is negative control.