PROM1 (CD133): A New Cell Surface Marker of Cancer Stem Cells (CSCs), A More Significant Target for Immunotherapy!

A study published in Science Translational Medicine recently made a big splash. The article entitled "Intracavity generation of glioma stem cell-specific CAR macrophages primes locoregional immunity for postoperative glioblastoma therapy" reports a cavity-injectable nanoporter-hydrogel superstructure that creates CD133-GSC-specific CAR-MΦ immunotherapy to target CD133+CSCs in human glioblastoma stem cells (GSCs), which exhibits high anti-tumor activity in vivo without inducing acute toxicity in normal cells that express CD133 [1].

CD133 (also termed as PROM1), it is noted for its specifically expressed on the surface of tumor stem cells (especially solid tumors). Notably, clinical studies have been conducted on PROM1 as markers of cancer stem cells (CSCs) or an excellent target for killing cancer cells. All in all, PROM1 can not only isolate stem cells but can also functions as a valuable therapeutic target.

1. What is the PROM Family?

The Prominin (PROM) family consists of two members, Prominin-1 and Prominin-2. Prominin is derived from the Greek word "prominere" as Prominin-1 and Prominin-2 are both located on the protrusions of the cell membrane. Prominin-1 is mainly expressed in epithelial cells such as pancreas, liver and prostate, and non-epithelial cells such as CSCs and glial cells. Prominin-2, the second member of the prominin family, it is mainly expressed in epithelial cells such as renal tubules [2-4].

Accumulating evidence has revealed that PROM has multiple functions in various organs, such as eye, tooth, peripheral nerve, and liver, associating with various molecular protein partners [2-4]. Notably, Prominin-1 as the first member of prominin family, researchers’ curiosity is growing rapidly as the clinical value of Prominin-1 increases with its ever-wider use as a stem cell marker in normal and cancer tissues.

2. What is Prominin-1?

2.1 Prominin-1 Structure

Prominin-1 (PROM1, also known as CD133) is a member of the prominin family of pentaspan membrane glycoproteins, which is specifically associated with plasma membrane protrusions. The human Prominin-1 is localized on chromosomes 4p15.32. It is composed of at least 37 exons that span more than 150 kb, and its structure is strikingly similar to that of the mouse Prominin-1 gene [5].

It contains five membrane-spanning domains, with the N-terminal domain exposed to the extracellular milieu followed by four, alternating short cytoplasmic and large glycosylated extracellular loops, and a cytoplasmic C-terminal domain (Figure 1) [5]. Many of them exhibit distinct cytoplasmic C-terminal domains, which might indicate various cytoplasmic protein-interacting partners.

2.2 Prominin-1 Function

The function of Prominin-1 remains unclear. Neither its ligand nor its interactions with intracellular molecules are known. PROM1 initially functioned as a specific marker for hematopoietic progenitor cells, but scientists subsequently discovered that PROM1 could also be used to isolate and identify cancer stem cells (CSCs). Existing studies have revealed that PROM1 not only serves as a marker for various stem cells, CSCs, and tumor cells, but also mediates PI3K/Akt, Akt, Src-FAK and other signaling pathways, which affect the behavior of PROM1 in cells. Typically, its expression exerts influence on cancer therapy. Therefore, the interest in this molecule has grown exponentially, since it appears to be an important cell surface marker, particularly cancer stem cells [6]!

3. PROM1 Related Mechanism of Action

Current studies on PROM1 suggested that it plays an important role in molecular regulation as a marker of stem cells or tumor cells. The findings revealed that PROM1 is associated with signaling pathway transduction in tumor cells, such as p38MAPK and P13k/Akt pathways, the NOTCH, Ras/MEK/ERK, mTOR, and other signals [7-9].

In glioma, PROM1 could be phosphorylated on tyrosine-828 in C-terminal cytoplasmic domain. Following this, PROM1 interacts with p85 depending on the tyrosine phosphorylation of PROM1 C-terminal cytoplasmic domain,which promotes the tumorigenic ability of glioma stem cells by activating the PI3K/Akt signaling pathway. As PROM1 knockdown, the activity of PI3K/Akt pathway was effectively inhibited, reducing the self-renewal and tumorigenicity of glioma stem cells. In addition, PTPRK has been identified as a novel binding partner of PROM1. As presented below, PTPRK could bind to the carboxy-terminal region of PROM1 and catalyzes the dephosphorylation of tyrosine positions 828 and 852 of PROM1, regulating PROM1-mediated Akt signaling pathway (Figure 2) [10-11].

Also in glioma, the self-renewal of PROM1/CD133 positive cells was associated with the Hedgehog-GLI (HH-GLI) pathway that involved in stem cell markers, such as OLIG2, BMI1, BCAN, BCAN, OCT.4, NANOG, PTEN, ABCG2, PDGFR-A, SOX2, and NRD1. In addition, PROM1/CD133+ cells express higher levels of neural precursor cell markers than PROM1/CD133- cells, like CD90, CD44, CXCR4, Nestin etc.; likewise, PROM1/CD133+ cells showed high levels of anti-apoptotic markers, such as Bcl-2, Bcl-xL, FLIP, c-IAP2, XIAP, NAIP [12].

4. The Roles of PROM1 in Diseases

PROM1 as a pentaspan transmembrane glycoprotein is currently found on multiple cancer stem cells. Tumor stem cells, postulated to be the source cells for malignancies, have been identified in several cancers. As of now, PROM1 has been identified as a cell surface marker of hematopoietic stem cells, prostatic epithelial stem cells, pancreatic stem cells, leukemic stem cells, liver cancer stem cells, and colorectal cancer stem cells [13]. To this end, PROM1 has been considered as a potential pan-cancer target!

4.1 PROM1 and Stem Cell-Based Diseases

In glioma cell lines, PROM1 promotes self-renewal and tumorigenicity of glioma stem cells. Another study suggested that the oxygen partial pressure level may affect the expression of PROM1 and even the transcriptional translation of PROM1 gene. Additionally, PROM1 expression involved in hypoxia induced related factor (HIF-α and SOX2). The two factors play a regulatory role in transcription [15]. Additional experiments suggested that in gastric tumor cell lines SOX17 expression correlates with PROM1, which is a key factor for maintaining self-renewal capacity and stem cell properties [16].

A close relationship between PROM1 and EMT markers was observed in patients with metastatic breast cancer. Besides, N-cadherin was positively correlated with PROM1, revealing a link between PROM1 positive tumor stem cells and EMT in breast cancer [17]. In hepatocellular carcinoma cells, the proliferation ability of PROM1/CD133+ cells in vitro and tumorigenic ability in immunodeficient mice were significantly stronger than PROM1/CD133- cells. Similarly, PROM1/CD133+ cells were lowly expressed in mature liver cells [18].

Both PROM1/CD133+ and PROM1/CD133- cells were found to enhance tumourigenic ability in human metastatic colon cancer in vitro and in vivo, but CD133+ cells exhibited more aggressive properties [19]. In melanoma, both CD133- and CD133+ cells were able to induce tumorigenic or differentiation. Other different reports suggested that in melanoma stem cells with enhanced tumourigenic potential, these cells can be identified by the expression of CD133, ABCG-2, notch 4 and prox-1 markers [20]. In ovarian cancer, the CSC-like properties of purified CD133+ cells from the ovarian cancer cell lines were demonstrated. Furthermore, CCL5 secreted by CD133+ ovarian carcinoma stem-like cells increases the invasive and metastatic properties of non-ovarian carcinoma stem-like cells (CD133-) and induces an EMT-like process [21].

4.2 PROM1 and Other Diseases

Early clinical trials indicated that PROM1/CD133+ cells improve transplantation compared to CD34+ cells; circulating PROM1/CD133+ cells can be related to the stem cell treatment of muscular dystrophy [22]; transplantation of PROM1+ bone marrow improves the function of infarcted heart muscle [23]. In addition, PROM1 is a key factor of retinal function. The mutation in the PROM1 gene causes autosomal-dominant macular degeneration. The PROM1-KO mice exhibited retinal degeneration phenotype even blindness [24]. PROM1 in human endothelial cell lines directly interacts with VEGF [25]. Knockout of PROM1 also leads to the development of intestinal inflammation. It is concluded that PROM1 likely plays a role in regulating intestinal homeostasis [26].

5. PROM1 Clinical Trials for Drug Development

Drug development focusing on elimination of CSC targeting PROM1(CD133) is still at an early stage. As of now, there are already several clinical drugs targeting PROM1 in development, mainly for cancer treatment, such as glioblastoma and ovarian epithelial cancer (Table 1). Among them, ICT-121, COR-3, and 293C3-SDIE targeting PROM1(CD133) are all anti-CD133 monoclonal antibody. In addition, CSCs have been defined as a crucial factor involved in tumor formation.

As mentioned early, a research team has developed a CAR-MΦ cell therapy based on a nanocarrier-hydrogel ultrastructural system targeting CD133-GSCs, which has made great achievements in the treatment of recurrent glioblastoma. Currently, PROM1 as well-characterized markers of CSCs in various tumor types, a number of papers strongly support the idea that targeting PROM1(CD133)+ cancer stem cells might offer new strategies in tumor therapy!

To fully assist researchers or pharmaceutical companies in their research on PROM1 in tumors or other diseases, CUSABIO presents PROM1 active protein product (Code: CSB-MP018751HU(A4)) to support their research on the mechanism of PROM1 or its potential clinical value.

• Recombinant Human Prominin-1(PROM1)-VLPs (Active)

References

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