Cancer Stem Cell

(Scientific)

Introduction:

In general, stem cells are defined as cells that differentiate along distinct lineages through systemic differentiation steps generating progenitor to the final stage of differentiation. The following cartoon depicts this concept:

In most cases, differentiated cells do not proliferate. There are exceptions such as terminally differentiated T-cells that proliferate if they are activated. The stem cells retain their total numbers through self-renewal whereas the committed progenies replicate with limited divisions.

Self-Renewal:

The concept of self-renewal is crucial to understand cancer stem cells, and also to get insight on the mechanism by which current therapies might evade the available treatments. The following cartoon explains self-renewal of stem cells:

Self-renewal of a stem cell provides the cell with the ability to undergo infinite cellular divisions with only few of the stem cells dividing at a particular time. Furthermore, the doubling time of most stem cells is relatively long, as compared to their immediate progenitors, which replicate with shorter doubling times. In some stem cells, at division the `mother' cell retain the original chromosome while providing the daughter with the newly formed chromosome. This process is designated chromosomal preservation and the method minimizes mutation in the mother cell. In summary, at division, a stem cell provides two cells: one with the `blueprint' of the mother cell, through self-renewal, and the other committed to form specialized cells through terminal cell differentiation.

Microenvironmental Influence:

The rapidly growing body of literature indicates organ-specific stem cells that are capable of differentiating into tissues of the organ where it resides. Thus, an organ-specific stem cell might be crucial to the day-to-day repair caused by normal insults. The normal repair process by stem cells requires a supportive microenvironment to provide the appropriate cues to generate the appropriate cells. However, the function of the stem cells might become dysregulated when the microenvironmental cues are disrupted. At present, it is unclear what constitute threshold levels for a particular microenvironmental cue. As examples, epidermal stem cells generate epidermis; neural stem cells provide cells of the nervous system while cardiac stem cells generate cells of the heart.

As alluded to in the previous discussion, the control of the cellular divisions by stem cells and their differentiation are key to maintaining the appropriate balance of cells within an organ in a temporal manner. A healthy organ maintains a normal balance by controlling the different cells to replace basal insults to the organ. Replacing cells is necessary since the normal cells cease to divide after maturation into terminally differentiated specialized cells. Thus, a normal tissue has the capability of maintaining a balance between cell genesis and destruction. While much is known on the molecular pathways in the differentiation of a stem cell, the entire mechanism could take years. Regardless, the molecular pathways are complex indicating that dysregulation could occur at multiple sites to turn off the homeostatic balance and create abnormal cells, or cancer cells, also referred as malignant cells or transformed cells.

If left untreated, cancer cells replicate at a seemingly uncontrollable rate. Since stem cell biology has shown that progenitors have limited self-renewal, this type of proliferation suggests that the cancer cells might be derived from a pool of cancer stem cells that exhibit self-renew, similar to healthy stem cells. In this regard, parallels could be drawn between normal stem cells and cancer cells with the cancer stem cells undergoing lineage differentiation. However, the cancer progenitors have lost their ability to differentiate. Cancer progenitors would need chemical intervention, genetic engineer to differentiate and/or microenvironmental manipulation to revert the cancer cells to differentiated cells. For example, recent evidence suggests that melanoma could revert to neural crest cells when they are placed in embryonic microenvironment. Thus, it could be deduced that the principles of stem cell biology (self-renewal and differentiation) only partly apply to tumorigenesis since there is evidence for the former, but blunting of the latter remains the hallmark of cancer.

Parallels between normal and cancer stem cells:

If parallels are drawn with normal stem cells one would expect a developmental process of cancer stem cells to follow paths of organized, hierarchical structure of cells with different degrees of maturity. Each maturation step would be expected to comprise a heterogeneous population of cancer cells. Indeed, the latter is supported by the evidence in the literature on heterogeneity of cancer cells for a particular type of cancer.

Based on the discussion above, the proliferative potential of cancer cells could be limited to relatively few cancer stem cells. Thus, cancers, whether they are solid tumors (e.g., breast and lung) or hematological (e.g., leukemia), the cancers are comprised of two broad category of cells with respect to their cycling states: high proliferative potential, which would be consistent with cancer stem cells and those with limited proliferative potential, which could be consistent with the more differentiated cancer progenitors. High proliferative potential cells would therefore be able to form several lineages, and self-renew. Those with limited proliferative potential, on the other hand would exhibit opposing properties. Several normal stem cells, most notably the hematopoietic stem cells have high proliferative potential suggesting that if a cancer cells show similar property, then those cancer cells could be the cancer stem cells. The designations high proliferative versus low proliferative cancer cells could be experimentally tested by both in vitro and in vivo methods.

The existence of cancer stem cells was evident in earlier times with experimental studies in animals when relatively large number of tumor cells was injected to generate tumors. If all tumor cells have high proliferative potential, then few cancer cells would be necessary to produce evident tumors. This observation could be explained by the presence of relatively few cancer cells with high proliferative potential among the large number of cancer cells injected into the animal. If this is the case, then only few of the injected cells would be able to generate tumors. An understanding of the manner by which few cancer stem cells could form tumors is envisioned if one projects the image of a normal stem cell: Few cancer stem cells would form a daughter progenitor, which could proliferate rapidly to form a bulky tumor.

Old vs. New theories of Cancer Models: Relevance to Cancer Stem Cells:

Before discussing the revised model on cancer development, there is need to summarize the above points. A cancer stem cell is defined as those with the ability to self-renew while generating a daughter progenitor that is malignant and/or for the progenitor to undergo further differentiation into more mature progenitors into phenotypically diverse cancer cells. Following are two models on tumor development, referred as old and new cancer models (taken from the lecture by Dr. Kathryn Packman, Hoffman LaRoche, Nutley, NJ).

Old Cancer Model:

1. All tumor cells can form new tumors and are therefore equally tumorigenic.
2. Unregulated growth is due to serial acquisition of genetic events leading to the expression of genes that promote cell proliferation with concomitant silencing of growth inhibitory genes and blunting of cell death.
3. Cancer is a proliferative disease.

New Cancer Model

1. Only a minority of cells can form new tumors.
2. Unregulated cell growth is due to a disruption in the regulatory mechanism in stem cell renewal.
3. Cancer is a stem cell disorder and not a simple mechanism whereby cell proliferation is disrupted.

The new cancer model supports disruption of normal stem cell self-renewal pathways leading to cancers. Hence, cancer can be considered a disease of unregulated self-renewal. Examples of self renewal pathways include the Wnt and BMI-1 dependent pathways. These two pathways are implicated in both normal and cancer cells. They are tightly controlled in normal cells however in cancer cells the control is lost, thereby leading to dysegulated self-renewal. In the normal Wnt pathway the levels of the transcription factor ß-catenin mediates self-renewal. ?-catenin could be turned off by a destruction complex as a feedback mechanism. However, in cancer the control process is circumvented and ß-catenin levels constantly thrive, hence causing continual proliferation and self- renewal.

BMI-1 also promotes proliferation and self-renewal through a different pathway. In normal cells BMI-1 inhibits the transcription of CDNK2A which encodes two cyclin dependent kinase inhibitors, INK4A and ARF. Cell cycle progression is promoted in the absence of INK4A and pro-apoptotic genes are inhibited in the absence of ARF. Hence, BMI-1 promotes proliferation and inhibits apoptosis. In the case of cancer, BMI-1 is circumvented and CDNK2A is no longer inhibited, thereby resulting in unregulated proliferation and self-renewal.

The question of the mechanism whereby self-renewal genes become dysregulated has been hypothesized to occur at two points during the cell cycle. This has been best demonstrated in leukemia if one compares the self-renewal of hematopoietic stem cells and their transformation to leukemia. While hematopoietic stem cells self-renewal or differentiate along a particular lineage to mature cells, leukemic transformation has been thought to occur at the stem cell phase and/or perhaps in the progenitor phase when the cells adapt the properties of the stem cells.

The cancer stem cell hypothesis supports the new cancer model as well and states, "the cancer-initiating cell is a transformed tissue stem cell, which retains the essential property of self-protection through the activity of multiple drug resistance transporters. This resting constitutively drug-resistant cell remains at low frequency among a heterogeneous tumor mass. Hence the mutation allows for unbridled cell growth and resistance to chemotherapeutic efforts. Current cancer therapies focus on shrinking the tumor size, however that approach may not necessarily eradicate the cancer stem cell. The fact that cancer stem cells express genes for drug resistance and anti-apoptotic mechanism, they have a high probability of being resistant to chemotherapy. It should be noted that recent report showed no evidence of drug resistant gene in breast cancer stem cells, suggesting that not all cancer stem cells express this resistant gene.

Examples of Cancer Stem Cells:

The cancer stem cell was discovered in acute myeloid leukemia cells (AML). John Dick and colleagues demonstrated only a small subset of human AML cells were phenotypically similar to normal hematopoietic stem cells and had the ability to transfer AML when transplanted into immunodeficient mice. The investigators found that the other AML cells were incapable of inducing leukemia. These studies showed proof-of-principle regarding the existence of cancer stem cells within AML. Subsequently, cancer stem cells were discovered in other types of cancers. While some investigators have reported particular markers as identification of cancer stem cells, these markers require further research. For example, breast cancer stem cells have been thought to be CD44+/CD24 -/low. In addition, stem cells of central nervous system cancers have been associated with CD133.

To cure cancer it is imperative to devise therapies that effectively target the cancer stem cells. Chemotherapy targets the proliferating cancer cells, and has been used as adjuvant to surgery and radiation therapies. Due to normal high proliferating cells chemotherapy is toxic to normal cells. There is an obvious need for more effective cancer therapies; of which, the development is dependent on our understanding of the disease and its progression. Cancer stem cells may be able to provide some of these answers, with the hope to develop novel therapeutics to target the tumorigenic stem cells, ultimately eradicating cancer.

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Acknowledgements

This review was prepared by the following graduate students in the Stem Cell Biology Class, Graduate School of Biomedical Sciences, University of Medicine and Dentistry of New Jersey:

Brian Baker, Ioana Vlad, Jennifer Barbre, Michelle Moh, Ru Chen, Sarah Bliss

Teaching Assistant: Elaine Wong

The review was edited by two stem cell biologists.