Xuanqi Song

Glossary

Aromatase inhibitor, Astrocyte, Blastocyst, Chemotherapy, Ectoderm, Endoderm, Hematopoetic, Immunotherapy, in vitro, Lymphoma, Mesenchymal, Mesoderm, Molecular marker, Mutation, Pluripotent, Pluripotent, Quiescence, Somatic cell, Stem cell therapy

Abbreviations

Ais, Aromatase inhibitors; ASC, adult stem cell; CSC, cancer stem cell; ESC, embryonic stem cell; HER2, Human epidermal growth factor receptor 2; HR+, hormone receptor positive; HSCs, hematopoietic stem cells; iPSC, induced pluripotent stem cell; mTOR, mammalian Target of Rapamycin; NSCs, neural stem cells; PARP, Poly ADP-Ribose Polymerase; PSC Pluripotent stem cell; SSCs, skin stem cells

Introduction

There are over two hundred specialized cell types in the human body including skin, blood, and hair cells. These specialized cells carry out their functions in a localized manner in specific tissues or organs. Any damage to these cells can contribute to either minor or more serious health concerns (1). Stem cells are undifferentiated cells that have the ability to self-renew and differentiate into various specialized cell types. They arise from embryonic stem cells or through the process of cellular differentiation in which specialized cells arise from precursor cells. Stem cells can be found in various tissues and organs throughout the body, for example, in the bone marrow (such as hematopoietic stem cells), in the brain (such as neural stem cells) or in the liver (hepatic stem cells).

There are four possible stem cell fates. The most common fate for adult stem cells is to remain inactive until they receive a signal to differentiate. The second possible fate is symmetric self-renewal where the parent cell differentiates into two identical daughter stem cells. A third possibility is asymmetric self-renewal where one daughter cell is an identical stem cell to the parent cell, while the other daughter cell is a specialized cell  (somatic cell). The fourth possible fate for stem cells would be two daughter cells that are both different from the parent cell (2). PMM volunteer Lovisa Lindquist has written a detailed article about stem cell fates, titled “The Use of Stem Cells in Modern Medicine”, which you can read at https://www.personalizemymedicine.com/2022/08/10/the-use-of-stem-cells-in-modern-medicine/).

Figure 1. Four potential stem cell fates. 1) Quiescence is a common fate of adult multipotent stem cells. During this stage,  stem cells neither differentiate nor divide. 2) Stem cells can divide to produce daughter cells that are identical to the parent stem cells. This is known as symmetric self-renewal. 3) Stem cells can also divide into two daughter cells, where one of the daughter cells is identical to the parent stem cell, while the other is differentiated into a progenitor cell. This fate is known as asymmetric self-renewal. 4) Stem cells can also divide to produce two differentiated daughter cells, which are both progenitor cells. This is known as symmetric division without self-renewal. This figure was adapted from (4) and was created with BioRender.com (4).

There are some examples of other organisms in nature that use stem cells for their self-regeneration. An example of such an organism is the starfish. The unique cells of the starfish enable its cells to de-differentiate from specialized cells (i.e. skin cells) back to being stem cells (2). This surprising ability has intrigued researchers and encouraged them to investigate the potential of stem cell therapy (3).

Stem cell therapy

The first stem cell therapy was performed by French oncologist Georges Mathe in a stem cell transplantation of bone marrow in 1958. In the 1960s, Ernest McCulloch and James Till discovered the self-renewing ability of hematopoietic stem cells (HSBs) by carrying out experiments in mice. The results of these experiments have helped scientists to become aware of some of the main features of stem cells (4).

Currently, two main types of stem cells are used for therapeutic purposes. These are pluripotent stem cells (PSCs) and adult stem cells (ASCs). There is also another type of stem cell that is known as cancer stem cells (CSCs). However, these cells are used as a therapeutic target and are also known as tumor-initiating cells. Pluripotent stem cells include two types of cells. There are the embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). All PSCs can give rise to all cell types, whereas ASCs have more limited potential for differentiation. ASCs have the potential for self-renewal and differentiation, but those potentials are limited to specific tissue or organs. ASCs are also known as tissue-specific stem cells due to their limited potential. Hematopoietic stem cells (HSCs), skin stem cells (SSCs), neural stem cells (NSCs) and mesenchymal stem cells (MSCs) all belong to adult stem cells (5).

PSCs can differentiate into ESCs found in the inner blastocyst cell mass. The other type of PCS known as induced pluripotent stem cells (iPSCs) which is not included in Figure 1 is derived from the reprogramming of adult somatic cells in vitro.

As mentioned before, ASCs have a more restricted ability for self-renewal and regeneration as they can only give rise to cells that are specific to the tissue of origin. For instance, neural stem cells are found in the ectoderm (such as the brain and skin) and they can develop into nerve cells and astrocytes. Astrocytes are a type of cell that can regulate blood flow and transfer mitochondria to neurons to fuel the neuronal metabolism) (6).

Figure 2. Illustration showing two different types of stem cells. Stem cells for therapeutic use can be divided into two groups – pluripotent stem cells (PSCs) and adult stem cells (ASCs). PSCs derived from blastocytes, as shown on the left hand side of this figure. PSCs could give rise to embryonic stem cells. Using an in vitro approach, induced pluripotent stem cells (iPSCs) can be developed. The other type of stem cell, the ASC, has a more limited self-renewal potential as they can only differentiate into either tissue or organ-specific cells. Examples of different types of ASCs are shown on right hand side of this figure. This figure was created with BioRender.com

The development of stem cell therapy provides a new approach to addressing a range of clinical conditions. For example, bone marrow transplant can be an effective treatment for blood or immune system-related diseases (i.e. leukemia or lymphoma). Stem cell therapy may also prove to be a potential treatment for disorders such as Parkinson’s disease.

There are also some recent findings on the application of stem cell therapy in cancer treatment. For example, the new gene-drug delivery system designed by Shuyue Xu and her team may provide a new strategy for the clinical treatment of tumors (8). Research carried out by Stefan Barisic and Richard Childs in 2022, reviews the clinical results of allogeneic hematopoietic stem cell transplantation (HSCT). The research indicates that there may be potential for the development of tumor-specific immunotherapy using HSCT as the platform for identifying tumor antigens (21).

Challenges of using PSC and ASC as therapy

While PSCs have a greater potential, the use of PSCs is restricted due to ethical issues as extracting ESCs carries the risk of endangering the fetus. ASCs, which are mainly mesenchymal stem cells, are more ethically preferable. However, they have more limited potential as ASCs are limited to a specific tissue and require the employment of a complex isolation procedure (9).

Potentials of stem cell therapy

Stem cell therapy utilizing the characteristics of stem cells has provided various possibilities for alternative treatments of degenerative diseases, such as Parkinson’s disease with stem cell therapy. Some of the clinical trials have shown positive outcomes. However, it is important to be able to have a detailed procedure for creating the desired cell types in specific quantities. Also, the transplanted cells should have the ability to survive in the patient’s body. They could integrate into the body to perform their normal functions without over-production that may lead to tumor growth (10).

Potentials of stem cell therapy

Despite the challenges that might influence the range of effectiveness of stem cell therapy, there are great potential and space for the development of this type of therapy. The ability of stem cells to differentiate into specialized cell types provides possibilities for the treatments of diseases that is degenerative, and as mentioned before, even cancer. According to WHO, cancer is a leading cause of death around the world (11,500 breast cancer deaths in the UK every year). Breast cancer is one of the most common cancers in female in the UK (55,920 new cases each year and one in seven females in the UK will be diagnosed with breast cancer in their lifetime) (11) and stem cell therapy has provided a new perspective for the treatment of breast cancer.

For more statistics on breast cancer, please visit the WHO website:

https://www.cancerresearchuk.org/health-professional/cancer-statistics/statistics-by-cancer-type/breast-cancer#heading-Two
Breast cancer
Prevalence

There are around 55,920 new cases of breast cancer each year (2016-2018 average in the UK) and 76% of survival rates for 10 or more years (2013-2017 England) in the UK.

Causes of breast cancer

Breast cancer is likely to be multifactorial. A previous history of breast cancer, family history, genetic factors and environmental factors are the several most-known causes of breast cancer. Especially for genetic causes of breast cancer, BRCA1 and BRCA2 genes are the two genes that can be inherited and remarkably increase the risk of breast cancer. BRCA1 gene mutation tends to develop breast cancer at an early age, which is known as early onset breast cancer.

Breast cancer types

There are different ways to classify breast cancer types. The types can be classified as being invasive or non-invasive according to the site of the cancer. Breast cancer can also be classified into three subtypes based on the molecular markers which are 1) estrogen or progesterone receptor, 2) human epidermal growth factor 2 (HER2, or HERB-B2), 3) triple-negative, which means tumors that lacks all 3 standard molecular markers (12). According to Cancer Research UK, there are invasive, invasive lobular, triple negative, inflammatory, rare types, ductal carcinoma in situ (in the original place), lobular carcinoma in situ breast cancers and Paget’s disease of the nipple (which can be an indicator of the breast cancer.

(You can see various potential signs of breast cancer in figure 4 and find more information about the types of breast cancer at the following website: https://www.cancerresearchuk.org/about-cancer/breast-cancer/stages-types-grades/types).

Figure 3. Possible signs of breast cancer. This figure summarizes some of the common signs of breast cancer, including retracted nipple, pain of breast or nipple, swelling of breast and lump near underarm (13).

Treatment of breast cancer

Currently, the treatment for breast cancer depends on the stage of the breast cancer. According to WHO, treatment generally involves surgery and radiation therapy in order to control the spread of the cancer or to eradicate the tumor from the breast and/or lymph nodes.

For hormone receptor positive (HR+) type breast cancer, the main treatment is endocrine therapy, which blocks the effects of certain hormones. Drugs used for this therapy include tamoxifen, Fulvestrant (also known as Faslodex), and Aromatase inhibitors (AIs). For HER2+ type, treatment typically involves a combination of standard chemotherapy and molecular targeted agents, such as trastuzumab, pertuzumab, ado-trastuzumab emtansine, and lapatinib. These agents target specific pathways or inhibitors that impact the HER2 gene amplification or protein expression (more information about HER2+ breast cancer can be accessed via this link: https://www.cancerresearchuk.org/about-cancer/breast-cancer/treatment/hormone-therapy).

Triple negative breast cancer is more aggressive and challenging to treat than HR+ and HER2+ types. Mainstream treatment is still standard chemotherapy using taxanes, anthracyclines, and platinum drugs, with or without bevacizumab. However, there have been recent advances in treatment for this subtype of breast cancer, including the use of the PI3K-Akt-mTOR pathway to reverse hormonal resistance. Novel therapies for triple negative breast cancer include the Poly(ADP-ribose) Polymerase (PARP) inhibitor, which has shown significant clinical results in patients with BRCA1/BRCA2 gene mutations. Other treatments include anti-angiogenic agents, EGFR inhibitors, SRC inhibitors, and immunotherapies (15). To learn more about current approved drugs for breast cancer, please visit: https://www.cancer.gov/about-cancer/treatment/drugs/breast.

Stem cell therapy applied in the treatment of breast cancer

Related cell pathways of breast cancer stem cells (CSCs)

Researchers have identified various pathways that related with cancer cell growth and survival. There include the Wnt, NFκB, Notch, BMP2, STAT3, mTOR, and hedgehog (Hh) signaling pathways (16).

Phosphatidylinositol 3-kinase (PI3K)/Akt pathway

The phosphatidylinositol 3-kinase (PI3K)/Akt pathwayis an essential cellular cascade involving three key elements – PI3K, Akt and mTOR. The alteration of this pathway would lead to tumor growth in the context of breast cancer.

Figure 4. Overview of the PI3K signalling pathway

PI3K is a kinase that can convert the PIP2 molecule into a PIP3 molecule. PIP3 activates phosphoinositide-dependent kinase-1 (PDK1) and phosphorylated PDK1 activates AKT ( a type of protein kinase). AKT activation leads to a signalling cascade of mammalian target of rapamycin (mTOR). mTOR can upregulate cell growth. AKT and mTOR molecules are both potential targets for breast cancer treatment. AKT can be inhibited by Ipatasertib or Capivasertib. mTOR can be inhibited by Everolimus or Sapanisertib. This figure was adapted from (18) and was created with BioRender.com

PI3K is a type of protein that helps control many important processes in our cells, such as growth and survival. It works by converting a molecule called PIP2 into another molecule called PIP3, which can then activate other proteins that play a role in cell behavior. One of these proteins is called AKT, which can be activated through the help of PIP3. This activation triggers a chain reaction that leads to the activation of another protein called mTOR (Mammalian Target Of Rapamycin) (18).

mTOR is considered to be a key player in cell growth and survival, and is often targeted in cancer treatments. In the case of breast cancer, the most commonly mutated gene in the PI3K/AKT/mTOR pathway is PIK3CA. Research shows that these mutations occur in specific areas of the gene, and they can impact how this pathway functions (17).

The Wnt/b-catenin pathway

The Wnt/beta-catenin pathway plays an important role in many processes within our cells, such as in the development of an embryo and maintaining healthy tissues. When this pathway doesn’t work properly, it can result in the development of certain cancers. Normally, a protein called Wnt would bind to two other proteins, frizzled (FZD) and LRP5/6, and this would activate the pathway. However, when Wnt is not present, a group of proteins called the destruction complex takes over and keeps the pathway inactive. This complex includes proteins called AXIN, APC, CK1a, and glycogen synthase kinase. APC works to keep another protein, beta-catenin, in check, but when APC is not working properly (for example, because it has a mutation), beta-catenin can accumulate and activate the Wnt pathway even without the presence of Wnt, leading to the development of cancer.

(For more details about Wnt/beta-catenin pathway, please visit: https://www.sciencedirect.com/science/article/pii/S1359644621003172).

Another research study carried out by Samuel et al shows that the crosstalk between PI3K and Wnt pathway would promote cell proliferation and tumor growth (19). There are many other pathways related to the development of breast cancer such as the Hedgehog pathway, Notch signaling, TGF-beta signaling pathway.

Figure 5. An overview of the Wnt/b-catenin pathway

When the Wnt protein is activated, downstream signalling pathway leads to the recruitment of complex CBP/TCF/LEF/b-catenin, which induces gene transcription. Turning off the Wnt protein leads to CK1a phosphorylation of b-catenin resulting in its degradation (proteolysis). The proteolysis of b-catenin is mediated by ubiquitin. Without b-catenin bound to TCF/LEF complex, there is no induction of gene transcription. This figure was created with BioRender.com.

Challenges & limitations of stem cell therapy

As stem cell therapy is a therapy that is still under development, much research is still underway to find more efficient way of delivering this therapy in vivo. Some of the potential risks of stem cell therapy include tumorigenesis (the formation of a tumor), drug resistance, and increased immune responses (20). There is a risk of tumor formation and the spread of cancer cells during stem cell therapy, which can pose a challenge for safe and effective treatment. This is because normal stem cells and CSCs (Cancer Stem Cells) share the key biological pathways, which means that there is a risk that normal stem cells could differentiate into CSCs as a result of the therapy and subsequently lead to tumor formation. Also, determining the optimal dose and method of administration of stem cells for cancer remains a challenge and requires further research. Furthermore, carrying out clinical trials in this area is also a challenge as conducting clinical trials to evaluate the efficacy of stem cell therapy is costly and requires a significant investment of both time and other resources.

Conclusion

Stem cell therapy offers a promising solution for breast cancer treatment. Despite facing several challenges that still need to be addressed, significant progress has been made in this field. There have been more therapies developed apart from traditional surgical removal or radiation therapy, such as combination of chemotherapy and cell therapy, and chemotherapy that target various antibodies according to the type of breast cancer.

Researchers are conducting clinical trials to test the safety and effectiveness of different types of stem cell therapy such as using a patient’s own stem cells or those from donors. They are also exploring new technologies that may improve the quality and control of stem cell differentiation. In addition, the possibility combining stem cell therapy with other treatments like chemotherapy and immunotherapy is being investigated as this may increase the chances of successful treatment. These efforts are bringing us closer to making stem cell therapy a viable option for breast cancer patients, offering hope for better outcomes in the future.

Useful Links

            Cell therapy

Stem cell fates

https://www.cancer.gov/about-cancer/treatment/drugs/breast

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