Stem Cell Utilization for Potential Reduction in Glioblastoma Multiforme (GBM): A Focused Narrative Review

1. Introduction

Glioblastoma Multiforme is one of the most aggressive tumors that can form primarily within adults, located in the cerebral hemispheres, temporal, and frontal lobes [1,2]. One of the main issues regarding GBM is that, usually, in most adults, it becomes detectable once it reaches an aggressive state, and by then, the cancer is untreatable and results in a quick death [1]. Most other cancers can be identified during early stages, for which chemotherapy, radiation therapy, and various oral medications can be prescribed to bring the malignant growth under control. However, because GBM is localized in the brain, normal cancer treatments are often not effective, due to its fragile and sensitive area [2]. Since the current methods of cancer treatment are followed by the death of healthy cells and tissue, applying this to the brain could be highly life-threatening. Of those who do get diagnosed with GBM, they have approximately 15 months of survival time, and only 5.5% of the patients survive after diagnosis and treatment [1]. Although there is no scientific evidence for the exact etiology of GBM, some studies postulate it’s due to exposure to high ionizing radiation, which may be one of the probable causes of the various genetic and epigenetic mutations found in GBM [1]. There are two forms of GBM, each with its own genetic and epigenetic makeup and each targeting specific age groups. Primary GBM is predominantly seen in elderly patients without the presence of any prior tumor precursors, and it develops quickly [3]. These tumorigenic cells arise from neural stem cell precursors [1]. Secondary GBM, on the other hand, develops from low-grade glioma [3]. They are found mostly in younger patients, predominantly localized in the frontal lobe, with a better prognosis [3].

2. Methods

A narrative review was conducted using two main databases to obtain articles and perform a literature review: PubMed and Google Scholar. The following key terms were used to find relevant articles: glioblastoma multiforme, stem cells, cancer stem cells, and Wnt pathway. No date ranges or journal exclusions were applied.

3. GBM Pathophysiology and CSC

3.1. Stem Cells

Stem cells are unspecialized cells in the body that can differentiate into various types of cells, as well as repair the body from disease-stricken states [4]. Stem cells have different properties than other cells in the body because they can differentiate and become any form of specialized cells, such as blood, brain, muscle, etc [4]. There are two main categories of stem cells: pluripotent and somatic [5]. Pluripotent stem cells consist of embryonic and induced pluripotent stem cells, whereas somatic stem cells are known as adult stem cells [5]. Pluripotent stem cells can differentiate into any type of cell or tissue in the body, regardless of the organ [5]. Adult stem cells, on the other hand, are found in specific tissues and organs of the body and can only differentiate into that specific organ’s tissue [5]. In order to utilize stem cells as a probable GBM therapeutic target, the usage of pluripotent stem cells for investigation of what would be the most practical target, because they can differentiate into brain cells from the ectoderm of the embryonic stem cells.

3.2. Cancer Stem Cells

Cancer stem cells (CSC) exist in any form of cancerous cells and are responsible for the growth, proliferation, metastasis, and regeneration of those cancer cells. For some cancers, CSC is a precursor to the pathophysiology of the disease, such as GBM [6]. Cancer stem cells are termed “tumor-initiating” cells because they not only embody the characteristics of normal stem cells, but they can amplify the growth of the tumorigenic cells within the cellular environment and result in aggressive malignancy [7]. The rise of these CSCs can be from the mutation of normal stem cells, or it can also be a result of progenitor cells [7]. For stem cells to arise from progenitor cells, some form of mutation must be induced first for these progenitor cells to become cancer stem cells, because progenitor cells do not have the automatic ability to self-renew like normal stem cells, so that characteristic must be gained [7]. Some studies have indicated that the CSCs that arise from normal stem cells constitute similar machinery, as well as certain pathways that regulate cancer [8]. One of the main regulatory factors of CSCs is the miRNA, which is responsible for self-renewal, tumorigenesis, and differentiation [9]. In addition to the miRNA, the microenvironment, or niche, of the cancer stem cell plays a vital role in its regulatory mechanisms [9]. Niches, the residence of stem cells, are cell-specific environments where they thrive and have all the cellular necessities to flourish and sustain themselves [9]. The overall dynamic and composition of the niche contribute to the onset of tumorigenesis, because the miRNAs, cytokines, growth factors, etc are vital components to the cell’s nutrition, intercellular communication, signal transduction, and cell fate [9]. Cancer stem cells found within GBM are also known as glioblastoma stem cells, and they can transform into astrocytes, oligodendrocytes, and neurons to be part of the CNS [10]. The niche of the GSCs is located within the perivascular environment of the brain tumor, and normal cells are also captured by this niche [10].

3.3. Cancer Stem Cell Markers of GBM

Certain protein molecular markers on the surface of the cell help distinguish the presence of GBM or GSC in the brain. In a study conducted by some neuroscientists, discovered that one such brain tumor marker was the CD133+ [11]. To begin their experimentation in vivo, they compared the abilities of CD133+ and CD133- to induce tumor initiation in mouse brains [11]. After injecting the mice with the above markers, they discovered that there were indeed large, solid masses of tumor cells present in the mice’s brains, post-surgically [11]. Their results determined that the percentage of the CD133+ fraction was very high in aggressive glioblastoma multiforme (GBM), ranging from 19% to 29% [11]. Furthermore, after performing extensive research and engraftment, they discovered that as few as 100 CD133+ cells are enough to produce cancer in the mice [11]. Also, when they used the CD133- as a control to test the presence of cancer cells in the absence of CD133+, their findings were confirmed [11]. When approximately 50,0000 to 100,000 CD133- glias were injected into the mice, their results concluded that there was no presence of tumor development [11]. This data further strengthens the understanding that CD133+ indeed is a cancer-initiating marker in brain cancer [11]. One of the key components of stem cells in general is the ability to proliferate and self-renew, so to test this on the cancer marker CD133+, researchers performed serial retransplantation experiments derived from xenografts of pediatric and adult GBM [11]. After the CD133+ cells were isolated and reinjected into the mice, they discovered that all the mice that were injected with each of the pediatric and adult GBM had displayed the same cancer cells in the host mice from the original patients. This indicated the self-renewal and proliferating capabilities of the CD133+ tumor-initiating cells [11]. Thus, this study demonstrated that the cancer stem cell hypothesis is valid and the origination of cancer cells in the brain can be due to the presence of CD133+ marker, derived from stem cells.

3.4. Wnt/Beta-Catenin Pathway

Amongst other cellular issues that arise from GSC, cell proliferation, invasiveness, and promotion of tumorigenesis are the main concerns. The Wnt pathway is responsible for this in GBM [12]. The activation of the Wnt pathway due to mutations is one of the etiologies of CSCs [13]. When the Wnt pathway was activated, there were significant expressions of OCT-4, SOX2, NANOG, NESTIN, and CD133, which are considered markers for GBM, indicating that the mutations in the Wnt pathway play an immense role in the pathology of GSCs [13]. Of this Wnt pathway, one specific pathway that has been and is currently being heavily studied is the Wnt canonical signaling, which regulates the amount of transcriptional co-activator beta-catenin, which is involved in gene expression [14]. To understand the Wnt/beta-catenin pathway in depth, it is important to know that the Wnt signaling pathway can be divided into two main components: Wnt proteins and beta-catenin [14]. Wnt proteins are involved in embryonic development, cell proliferation, and cell determination and beta-catenin is a protein involved with Wnt signaling in the nucleus, development, and homeostasis [15,16]. There are two forms of this pathway, an active state and an inactive state, resulting in different outcomes in the cell [14].

Inactive state:

In this state, the beta-catenin is bound and regulated by a large protein complex that contains proteins such as Axin, CKI, GSK3, APC, Dvl (Disheveled), and beta-TrCP [14]. These proteins comprise a whole complex termed the Destruction Complex because this large multisubunit complex ultimately leads to the phosphorylation, ubiquitination, and degradation of the beta-catenin [14]. The mechanism by which this process occurs is embedded within the beta-TrCP protein because it’s an E3 ubiquitous ligase, and once the beta-catenin has been phosphorylated, it signals beta-TrCP to ubiquitinate the beta-catenin [14]. This ubiquitination results in the proteasomal degradation of beta-catenin, thus naturally resulting in low levels of beta-catenin [14].

Activated state:

In this state, the Wnt is involved. It is transported from somewhere in the body, and it acts as an extracellular signaling molecule to help activate its receptor Frizzled, located on the cellular membrane [14]. When the receptor Frizzled is activated, it results in the phosphorylation of LRP, which is located within the same vicinity of Frizzled on the membrane [14]. This phosphorylation is very critical because it induces the translocation of the Destruction Complex to the region of the membrane located between the Frizzled and LRP receptors [14]. When Dvl of the Destruction Complex binds to the LRP, the Dvl becomes activated, which leads to the inhibition of the Destruction Complex, preventing beta-catenin from being phosphorylated and also inhibiting beta-TrCP from being incorporated into the Destruction Complex, thus avoiding the ubiquitination of the beta-catenin [14]. As a result of this, the beta-catenin does not undergo degradation but rather increases the beta-catenin levels [14]. The transcription factor TCF mediates the genetic mechanisms of the Wnt signaling pathways, leading to the induction of Wnt target genes [14]. However, with the inactivation of the Wnt signaling pathway, TCF is inhibited and bound to Groucho, which inhibits TCF’s ability to bind to the DNA and its overall function as a whole [14]. But when there is a rise of beta-catenin levels in the cytosol due to the activation of the Wnt signaling pathway, the beta-catenin translocates into the mitochondria, overriding and dislodging the Groucho molecule from TCF as beta-catenin binds to the TCF, resulting in the transcription of Wnt target genes [14]. These Wnt target genes play a role in growth and proliferation (such as migration or motility) of the cells they target [17].

Based on the understanding from the Wnt/beta-catenin model, cancer emerges within the APC mutations [17]. Even if Wnt is not bound to Frizzled and the pathway is inactivated, with a plausible mutation in the APC protein, this will result in increases in the beta-catenin levels and activation of the pathway, resulting in an overabundance and uncontrolled rate of growth and proliferation of the cells. This occurs because the loss of function of APC has a component of the Destruction Complex, which leads to the stabilization of beta-catenin, preventing the protein from becoming ubiquitinated and degraded by the proteasome. Furthermore, Wnt targets genes that encode for the EMT activators [17]. The EMT gene is responsible for the motility of cancer cells, contributing to their metastasis [17]. Given the outcome from the activation of the Wnt/beta-catenin signaling pathway with the addition of a mutation in the APC protein, which results in overgrowth and proliferation, coupled with the activation of EMT, causing increased motility, one would suggest that the recurrence of tumor formation becomes an imminent possibility [17]. However, a fascinating finding from their study regarding GBM specifically, the activation of the Wnt/beta-catenin levels did not impact its aggressive rate of proliferation but rather has a direct association with its motility [17]. This suggests that perhaps the EMT activators are a prominent feature in GBM, causing it to be more hostile than other forms of cancer [17]. Moreover, an interesting correlation this study found, which supported the experiment performed by Singh and colleagues is that Kahlert and colleagues discovered that this extrinsic activation of the Wnt/beta-catenin pathway produces an increase in mRNA in the cell and also the CD133 brain tumor stem cell markers [11,17].

4. Neural Stem Cells

Neural stem cells are a type of progenitor cells that arise from the ectoderm, and they can differentiate into any neuron-related cells in the CNS, such as astrocytes, neurons, and oligodendrocytes [18]. These are located in the SVZ of the lateral ventricles as well as in the SGZ of the hippocampal dentate gyrus [19]. Each of these areas has different goals for the NSCs. Dormant neural stem cells in the SVZ respond to unique cell populations that are located only within the SVZ, while contacting the ventricles through the apical surface [19]. These cells also share common features with GFAP+ astrocytes and CD133+ cells [19]. Dormant NSCs in the SGZ are activated in response to radial glial-like cells, giving rise to neuroblasts and new neurons [19].

5. Mesenchymal Stem Cells

Mesenchymal stem cells are derived from adult stem cells, specifically stromal cells, and they have the ability to undergo self-renewal and experience versatile cell differentiation [20]. In humans, these MSCs can be collected from areas of the body that require less invasive accessibility, such as menstrual blood, bone marrow, adipose tissue, etc [20]. These multipotent cells can differentiate into various lineages, such as the mesodermal lineage, the ectodermal lineage, and the endodermal lineage [21]. These three lineages can produce osteocytes, neurocytes, and hepatocytes, respectively [21].

6. Discussion and Therapeutic Advances

After researching GBM, it can be concluded that stem cells hold the potential to be utilized in future therapeutic advancements for it. As studied, neural stem cells and mesenchymal stem cells may be plausible stem cells to utilize in targeting GSC in the brain and potentially reducing the tumorigenic cells within the environment [22]. To begin with, neural stem cells would be a successful target for GBM therapy because GSCs and NSCs have a great deal of similar characteristics [22]. One of these is that NSCs contain CD133+ proteins, one of the markers for GBM [11]. If NSCs were injected into the niche of the GSCs, they would not recognize it as a foreign substance; rather, it would allow for it to enter also due to NSCs’ proliferative abilities. Furthermore, a study comparing the mutations that are present in a tumor versus tumor-free SVZ discovered that the tumor-free SVZ fostered similar mutations that were found in GBM, further confirming the correlation of NSCs lineage and GBM [22]. Utilization of the NSCs will allow for the tumorigenic environment to not feel as if it is being invaded and welcome the NSCs to proliferate and differentiate into new neurons, oligodendrocytes, and glial cells. Moreover, a study uncovered that NSCs have the potential to inhibit the Wnt/beta-catenin pathway, thus decreasing the mutation plausibility and high proliferation seen in GBM [23]. Glioma cells that were treated with human NSC-condition medium displayed a significant decrease in the expression of various Wnt/beta-catenin pathway proteins [23]. According to the activated state of the Wnt/beta-catenin pathway mentioned above, this indicates that the NSCs may also play a role in beta-catenin degradation and decreasing its levels [14].

Mesenchymal stem cells are another plausible form of treatment for GBM due to their less complex and easy accessibility and their ability to differentiate into any type of tissue. This would be a positive factor because the brain does not consist of neurons and glial cells; there are adipose tissues there as well, which NSCs cannot differentiate into, so these MSCs are able to provide the necessities plus more to the cellular niche. Unfortunately, MSCs also have the ability to induce tumor growth in other cancerous cells [23]. Although this has not yet been seen in any clinical studies, the risk still persists, which is why scientists believe NSCs provide a more promising therapeutic front toward brain tumors [23].

It is also important to keep in mind that other natural biological pathways in the body will resist stem cell therapy if initiated [23]. Moreover, if stem cell therapy is solidified to be utilized in GBM, it will require undergoing multiple clinical trials to understand the side effects and consequences it has on humans.

7. Conclusions

Glioblastoma multiforme is one of the most aggressive forms of brain cancer due to its therapeutic inaccessibility in the brain. Stem cells have the ability to differentiate into any type of tissue in the body, providing an advantage to utilize them as a potential therapeutic measure for life-threatening diseases, such as cancer. There are cancer stem cells, known as GSCs, which cause the malignant and aggressive characteristics of GBM. The Wnt/beta-catenin pathway was investigated because mutations and activation in this pathway caused the highly proliferative and tumorigenic nature of GBM. Through further investigation, primarily neural stem cells hold the ability to provide a plausible reduction in the GSCs in the brain because they mimic the cancerous cells, so the brain does not interpret them as a foreign substance. Given that this is a very novel area of research, there are various limitations that could arise, such as preventing the NSCs from affecting the healthy brain tissues. Also, in the zones NSCs are localized in, GBM is usually not present within those cortical areas, and NSCs are very region-specific, so confirming that the NSCs will reach their proper GSC target will be challenging. Some future considerations would be to ensure that the NSCs will not affect the healthy brain tissues and cause other consequences. Also, it is important to understand what other mechanisms NSCs mimic and how they can be used against the GSCs. The future implications of utilizing stem cells against aggressive tumors in the brain are versatile, but the primary goal is to ensure that a healthy environment in the brain is preserved while eradicating the malignant cells.