Brain Cancer SCs: Surface Markers & Their Therapeutic Targets

Sugnyan Shivakumar, JSS Science and Technology University

Understanding Stem Cells

Stem cells are cells that can transform themselves into specialized cell types in the body that can be used as a replacement for injured cells and tissues. Potten and Loeffler, 1990 in their review came up with a functional definition for stem cells as undifferentiated cells with (i) an ability to proliferate, (ii) self-sustainment, (iii) propensity to produce differentiated forebears, (iv) capability of regenerating tissues after being injured, and (v) flexible allowance of the above abilities. Let us further understand what cancer stem cells and what the role surface markers are.

What is a Cancer Stem Cell?

The cancer stem cell (CSC) hypothesis states that there is a “small subpopulation” of cancer stem cells that are responsible for tumor relapse, tumor initiation, and metastasis also having intrinsic resistance to chemotherapy and radiotherapy. Contrary to this is the stochastic model, which says “Every cancerous cell can proliferate and regenerate a tumor”.

Identifying the CSCs is important and is the very first step taken towards targeted therapeutics. Also, deciphering the self-renewal pathways taken for CSC maintenance is necessary. Nevertheless, the aim is to get a therapeutic target that is not simple because the pathways in the CSCs are similar to that of normal cells. The CSCs are identified by markers and for every solid tumor; there exists a wide list of molecular markers.

Brain Cancer Stem Cells

The work done in breast cancer led to the identification of cells that were responsible for tumor initiation which further fueled the discovery of brain cancer stem cells. CD133 was the protein that was first identified in a study in the cancer stem-like population of both adults as well as pediatric brain tumors which formed neurospheres upon growing.

To find therapeutic targets, identification of the surface markers and understanding their biology becomes important. Different brain cancer stem cell surface markers identified are as follows – CD133, CD44, SoX2, Nestin, and L1CAM. This article will further discuss the above-mentioned surface markers and their existing/potential therapeutic targets.

CD133 as a surface marker

CD133 (also known as AC133/prominin-1) was discovered as a hematopoietic stem cell marker in 1997. It has two charges CD133⁺ and CD133^– both having different properties. To understand CSC as a stem cell marker, a brief understanding of its distribution is essential.

CD133 in hematopoietic cells has been associated with the earlier phases of human hematopoiesis. It is also involved in deciding the fate of stem cells and is a significant regulator in the maintenance of stem-like cells and proliferation.

It has been shown that CD133 was heterogeneously expressed in human neural stem cells (NSCs) cultures and clonogenicity was higher in CD133^–. This showed that potency was not exclusive to CD133⁺.

CD133 has been identified in various other solid tumors such as brain cancer, prostate cancer, colon cancer, lung cancer, and ovarian cancer. Additionally, there are limitations in using CD133 for identifying CSCs, one of which is that these cells are a marker of glandular epithelium. In these tissues, differentiating CSCs and non-cancer stem-like cells becomes problematic.

CD133 in brain cancer

The identification of solid tumors in humans and the cells responsible for initiating them was first demonstrated by Singh et al. In earlier studies, it was depicted that only CD133⁺ tissues were able to generate tumor mass in vitro. Underpinning this, another study involving xenotransplantation showed that as little as 100 CD133⁺ cells could suffice in forming tumors and produced similar phenocopy like that of patients’ tumors. In contrast, up to 10⁵ CD133^– cells were still not sufficient in forming tumors. But this is debatable as CD133^– showed higher clonogenicity in NSCs as mentioned earlier, and most studies argue that CD133⁺ has higher potential in forming tumor mass.

Further, a study strengthened that CD133 is an important stem cell marker also promoting the progression of glioma and is a significant marker in the prognosis of glioma. Moreover, they also proposed treatments when CD133 is expressed in abundance. CD133 has also been reported to have radioresistance by activating damage response of DNA and chemoresistance when temozolomide (a chemotherapeutic drug) was administered in cancer stem cell lines.

Therapeutic target for CD133

Since glioblastoma stem cells (GSCs) are known to show chemoresistance, as well as radioresistance and these, are responsible for tumor relapse. TMZ (temozolomide) is the chemotherapy drug used to treat glioblastoma multiforme (GBM). But, this is constrained by the blood-brain barrier (BBB) decreasing the efficiency of the drug. Therefore, in a study, a dual-targeting immunoliposome (Dual LP-TMZ) capsulizing temozolomide was prepared using angiopep-2 (An2) and anti-CD133 monoclonal antibody. The role of An2 was to pass through the BBB and the anti-CD133 was to target GBMSCs. It was demonstrated that dual targeting LP-TMZ was successful in reducing the size of tumor when compared to TMZ which showed the potential use of DUAL LP-TMZ as a therapeutic drug for GBM.

CD44 as a surface marker

It is a glycoprotein and the normal function of CD44 is to moderate stem cell homing. It plays a major role in tumor cell migration as well as adhesion. It has been demonstrated in a study that CD44 binds to a part of the extracellular matrix thereby acting as a linchpin for the cancer cell and the extracellular matrix. Another study further provides evidence that CD44 plays a role in the neurodegeneration of the brain by activating abnormal tau hyperphosphorylation.

Therapeutic target for CD44

MRK003 is a γ-secretase inhibitor which was first demonstrated in a study that showed that MRK003 possessed therapeutic targets for CD44⁺ and CD133^– expressed in glioblastoma stem cells.

SOX2 as a surface marker

SOX2plays a major role in embryonic development by forming tissues/organs and also contributes to the development of the eye. But, the SOX2 gene possesses a role in the maintenance of GSCs. It was proposed in a study that silencing of SOX2 reduces migration and also an invasion of epithelial-to-mesenchymal transition (EMT).

Also, an abnormal elevation of SOX2 can increase migration, invasion, and self-renewal activity in glioma cell lines.

Therapeutic target for SOX2

Zika virus causes a congenital condition by damaging the brain cells but has been shown to have oncolytic effects against glioblastoma. It was proposed that the Zika virus kills SOX2⁺ cells from malignant brain tumors. This was enabled by integrin ɑ_Vꞵ₅ (ITGAV) which is a glioma stem cell marker that promoted the Zika virus to attack GSCs. This recent study showed the potential therapy for glioma tumors and other brain tumors such as medulloblastoma and ependymoma.

Nestin as a surface marker

Nestin is a cytoskeletal filament. During mitosis, it is involved in the disassembly of intermediate filaments such as vimentin. It also helps in the self-renewal, differentiation, and migration of tumor cells. In a study, it was concluded that Nestin and CD133 were important stem cell markers that could predict the aggression level of gliomas. Nestin overexpression as a novel marker would mirror the degree of neoangiogenesis, along these lines target treatment against the EGFR pathway might be useful for glioblastoma.

Therapeutic target for Nestin

Nestin has proven to be a helpful marker in the diagnosis of glioma. It was also stated in a study that the expression of nestin corresponds to a poor prognosis in gliomas.

To investigate the proteins that are regulated by nestin in glioblastomas, two-dimensional electrophoresis using nestin shRNA-transfected glioblastoma cells was carried out. The outcome showed was that nestin shRNA-transfected glioblastoma cells expressed a decrease in the level of phosphorylation of heat shock cognate 71 kDa protein (HSPA8).

L1CAM as a surface marker (L1 – adhesion molecule)

It is usually found on the surface of neurons that make up the nervous system. These proteins help in brain development, assist in movement, and also the ability to think.

L1CAM has been shown in ovarian cancer as a biomarker that allows the use of this information in brain tumors as well.

In a recent study, it was first shown by using immunohistochemical staining that high levels of L1CAM were constituted in glioblastoma tumor cyst fluid. This shed light upon L1CAM being involved in tumor growth and cancer cell proliferation.

Therapeutic target for L1CAM

The cell cycle consists of various phases and has checkpoints to detect errors in the late S phase of the cell cycle. Checkpoints prevent damaged DNA to replicate into daughter cells or help by activating proteins that perform DNA repairing.

L1CAM was shown to be a radioresistant enhancer by identifying the L1CAM-mediated checkpoint which was activated by the NSB1-ATM axis as a regulator. It was proposed that by using anti-L1CAM antibodies, the enhanced DNA repairing capabilities and activation of checkpoints which was a response to radiotherapy can be reduced or avoided.

What can be derived from the potential of targeting CSCs and their surface markers?

It can be deciphered that understanding and targeting GSCs and surface markers can lead to potential therapeutics for the treatment of GBM.  However, there is a need to understand more about CD133 and other markers to avail better prognosis for this deadly disease. In light of the literature review, antibodies of the markers are demonstrated to have effectively suppressed tumors in cell lines and in vivo assays but for application of these in human trials must be met with further experiments with these antibodies.

Also read : IgA-virus Immune Complex: Neutrophil’s trap for SARS-CoV-2

References :                 

1. Potten, C., & Loeffler, M. (1990). Stem cells: attributes, cycles, spirals, pitfalls and uncertainties. Lessons for and from the crypt. Development, 110(4), 1001-1020. https://doi.org/10.1242/dev.110.4.1001
2. Li, B., McCrudden, C. M., Yuen, H. F., Xi, X., Lyu, P., Chan, K. W., Zhang, S. D., & Kwok, H. F. (2017). CD133 in brain tumor: the prognostic factor. Oncotarget, 8(7), 11144–11159. https://doi.org/10.18632/oncotarget.14406
3. Beier, D., Schriefer, B., Brawanski, K., Hau, P., Weis, J., Schulz, J. B., Beier C. P. (2012). Efficacy of clinically relevant temozolomide dosing schemes in glioblastoma cancer stem cell lines. J Neurooncol, 109, 45–52. https://doi.org/10.1007/s11060-012-0878-4
4. Kim, J., Shin, D., & Kim, J. (2018). Dual-targeting immunoliposomes using angiopep-2 and CD133 antibody for glioblastoma stem cells. Journal Of Controlled Release, 269, 245-257. https://doi.org/10.1016/j.jconrel.2017.11.026
5. Lim, S., Kim, D., Ju, S., Shin, S., Cho, I., Park, S. Grailbe, R., Lee, C., & Kim, Y. (2018). Glioblastoma-secreted soluble CD44 activates tau pathology in the brain. Experimental & Molecular Medicine, 50(4), 1-11. https://doi.org/10.1038/s12276-017-0008-7
6. Alonso, M. M., Diez-Valle, R., Manterola, L., Rubio, A., Liu, D., Cortes-Santiago, N., Urquiza, L., Jauregi, P., de Munain, A. P., Sampron, N.,  Aramburu, A., Tejada-Solı, S., Vicente, C., Odero, M. D., Bandre´ E., Garcia-Foncillas, J., Idoate, M. A., Lang, F. F., Fueyo, J., & Gomez-Manzano, C. (2011). Genetic and Epigenetic Modifications of Sox2 Contribute to the Invasive Phenotype of Malignant Gliomas. Plos ONE, 6(11), e26740. https://doi.org/10.1371/journal.pone.0026740
7. Wachowiak, R., Krause, M., Mayer, S., Peukert, N., Suttkus, A., Müller, W. C., Lacher, M., Meixensberger, J., Nestler, U. (2018). Increased L1CAM (CD171) levels are associated with glioblastoma and metastatic brain tumors. Medicine, 97(38), e12396. https://doi.org/10.1097/md.0000000000012396

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