The potential role of the anti-diabetic drug metformin that has been shown to have effects on CSCs, and known function as an anti-tumor agent, provides an example of this new class of chemotherapeutics. While cancer stem cells CSCs were recognized several decades ago, it is only in the the last 15 years that they have been identified and characterized in hematological malignancies, such as leukemia Bonnet and Dick, and other tumors.
This led to an increased interest in the potential role of CSCs in tumor aggressiveness, treatment resistance, and tumor recurrence relapse and metastasis Rasheed and Matsui, ; Sarkar et al. Like normal pluripotent stem cells, CSCs are long-lived, and display quiescent potentials in a dormant state, and are responsible for angiogenic induction, apoptotic resistance, self-renewal and differentiation.
These characteristics suggest that CSCs themselves contribute to tumor development and progression. While the pathogenic effects of CSCs remains to be elucidated, it is widely believed that intrinsic and extrinsic alterations in the stem cell tumor microenvironment, together with mutations and epigenetic regulations, are mainly responsible for the development of CSCs that are involved in tumor initiation and progression Figure 1 Bao et al.
The implication of stem cells in the development and progression of tumor. It is known that CSCs constitute only a small percentage 0. These CSCs have the capacity for self-renewal, giving rise to uncontrolled amplification of differentiated cell populations with altered molecular and cellular phenotypes. These eventually lead to the heterogeneous primary and metastatic tumor cells within a tumor mass that may be resistant to therapeutics and contribute to tumor recurrence Li et al.
It is interesting to note that CSCs are important in the prognosis of many malignant diseases which has been demonstrated by the finding that they are present in the majority of malignant tumor tissues, and appear to be resistant to chemo-radiation therapy as compared to their differentiated progenies Creighton et al. This may explain why tumor regression alone does not correlate with overall survival rate in cancer patients Creighton et al.
The identification of CSC-specific markers, the isolation and characterization of CSCs from malignant tissues, and the development of strategies for targeted eradication of CSCs represent an important opportunity in cancer research. Described in the present report are the implications of several common CSC markers and their relevance to common malignant diseases such as leukemia, breast, prostate, pancreatic and lung cancers.
As an example of the possible success of this approach, the anti-tumor activity of metformin, an anti-diabetic agent and potential CSC regulator, is discussed. Cancer stem cells were first identified and characterized in the bone marrow of AML patients in Subsequent clinical and laboratory studies provided additional evidence supporting the role of CSCs in drug resistance and cancer metastasis, thereby contributing to the poor outcomes experienced by patients with pancreatic, prostate, liver, breast, and brain tumors Bauerschmitz et al.
Moreover, the CSC-like glioma cells also contribute to resistance to radio-therapy via preferential activation of DNA damage response pathways, and by increasing DNA repair capacity Bao et al.
It has been reported that mouse mammary tumor CSCs are responsible for cisplatin resistance Shafee et al. This suggests that CSCs are also involved in the regulation of breast cancer metastasis.
The involvement of CSCs in drug resistance has been demonstrated in pancreatic, colon, breast, and brain tumors. Moreover, CSC-containing tumors display greater tumorigenic and metastatic potential in vitro and in vivo than non-CSC cancer cells. Elimination of these CSC populations suppresses the metastatic phenotype of pancreatic tumors without modulating their tumorigenic potential Hermann et al.
Similarly, CSCs in pancreatic tumor tissues are associated with drug resistance and metastatic potential with pancreatic tumor cells having a CD44 positive CSC phenotype, which correlates with tumor histological grade and poor clinical outcomes Hong et al. These findings suggest that the CSCs, promote tumor aggressiveness. Given these findings CSCs appear to be an excellent target for treating malignancies.
The identification and characterization of CSCs in malignant diseases provides insights as to ways in which to selectively inhibit or eradicate CSCs as a treatment for tumor aggressive phenotypes. Leukemia is one of the most commonly diagnosed malignant diseases in children and adults Siegel et al. Within each of these groups there is significant patient-to-patient heterogeneity in leukemic blast cell morphology.
For instance, AML is classified into seven French-American-English subtypes, according to the maturation stage of the acute leukemias, the preferential expression of multi-lineage cell markers, and morphology among individual patients Bennett et al. Targeting these different leukemic blasts, especially leukemia CSCs, while avoiding normal hematopoietic stem cells, provides an opportunity for the treatment or eradication of these conditions.
The injection of 5, leukemic CSCs developed human leukemia in immune compromised mice Bhatia et al. These small subpopulations of leukemia CSC cells also display a greater capacity for stem cell self-renewal as compared to normal adult bone marrow cells.
This seminal discovery led to the identification and characterization of CSCs in other cancers. This suggests that CD90 may be a useful marker for distinguishing leukemic CSCs from normal hematopoietic stem cell sub-populations Blair et al. These findings suggest that there are cell markers unique for leukemia CSCs that are not found in normal hematopoietic stem cells. Targeting these leukemic CSC related markers or proteins could provide an effective strategy for the eradication of CSC populations in leukemia.
The identification and characterization of leukemic CSC-specific markers that are not shared with normal hematopoietic stem cells makes it possible to identify and develop novel therapeutic agents for the management of human leukemias. Breast cancer is the second most deadly malignancy for females, with one in eight women expected to develop the condition in their lifetime Siegel et al. Breast cancer affects per , people, with a greater incidence among African Americans than other ethnic groups.
Nonetheless, breast cancer remains a major cause of cancer-related death because of treatment resistance and metastatic disease, especially for those diagnosed at advanced stages of the disease. Breast CSCs are a small subpopulation 0. Expression of mucin 1 MUC-1 , a mediator of the growth of undifferentiated human embryonic stem cells Hikita et al. This suggests that it is a potential CSC marker for breast cancer. Prostate cancer is the most commonly diagnosed malignant disease in males, being the second leading cause of cancer-related death for men in the United States Siegel et al.
As most prostate cancer patients are treatable, the survival rate has increased significantly over the years, although fatalities still occur, especially with aggressive phenotypes that are resistant to chemotherapy.
Considerable effort has been expended to identify and characterize prostate CSC populations so as to target them in treating this condition. However, different CSC subpopulations in prostate cancer are dependent on the enrichment of CSCs in different cellular compartments Kasper, Subpopulations of CSCs in prostate cancer have high proliferative capacity, increased clonogenic potential, and a greater capacity for tumorigenesis and metastasis in xenograft models in vitro and in vivo.
It has been reported that Sca-1, a cell surface protein, is a potential CSC marker that is unique to prostate cancer. It is found primarily in the proximal regions of the organ Zhou et al. The expression of these proteins is down-regulated in prostate cancer cells, especially prostate CSCs. Pancreatic ductal adenocarcinoma is currently the fourth leading cause of cancer-related deaths in the United States Siegel et al.
Due to the absence of specific symptoms, the lack of early sensitive detection tests, and its rapid and insidious growth, pancreatic cancer is typically diagnosed at an advanced and incurable stage. Thus, the overall survival of patients is approximately 5—6 months, even with conventional therapy for locally advanced and metastatic disease. They have the capacity for self-renewal and uncontrolled potential of differentiated progeny.
The CSC marker-positive cells display a fold increased capacity for the development of tumors and exhibit tumor morphology similar to primary pancreatic cancer. Moreover, these CSCs maintain their cell surface marker phenotype after repeated passages as xenografts in immunocompromised mice Li et al. Unlike normal pancreatic epithelial cells and non-CSC-like cancer cells, these pancreatic cancer CSC phenotypic cells also display a strong transcriptional up-regulation of sonic hedgehog SHH and the polycomb group PCG gene family member Bmi All of these are known mediators for maintaining CSC characteristics Li et al.
Accordingly, it remains possible that there is more than one type of CSC sub-population in pancreatic cancer tissues, which would be consistent with the known heterogeneity of most human tumors Klonisch et al. Expression of EZH2 increases in various tumors, including pancreatic cancer Bao et al. Injection of as few as 5, of these CSC-like sphere cells into SCID mice produces tumors within 2—3 weeks, whereas it is necessary to inject 10 7 non-CSC parental cancer cells to achieve comparable tumor incidence.
As these pancreatic CSC-like cells display an increased capacity for migration and invasion, aggressiveness, and self-renewal capacity Bao et al. In the United States, , people are diagnosed with lung cancer each year, with , individuals dying annually from this malignancy Siegel et al. Although there has been significant progress over the past decade in the diagnosis and the treatment of this condition, prognosis remains poor due to treatment resistance, rapid tumor growth, and metastatic capacity.
A small subpopulation of lung CSC cells appears to be responsible for the aggressive phenotypes of lung cancer. Because Rac-1 is a signal transducer for several oncogenic pathways involved in cell survival, proliferation, migration and invasion in tumors, it appears to be essential for the K-ras mediated tumor growth of lung tumors Kissil et al. This makes it necessary to identify and use multiple markers for characterizing the lung CSC subpopulations Wu et al.
This attachment results in either mRNA degradation or inhibition of protein synthesis Liu and Tang, , making miRNAs useful tools in characterizing the role of select proteins in cell function. The members of the Let-7 and miR family participate in the development and progression of tumors by targeting multiple cell signaling pathways involved in cell survival.
Moreover, it has been demonstrated that the expression levels of let-7 and miR are related to clinical outcomes in cancer patients Olson et al. As the expression of both of these miRNA family members is either lost or significantly reduced in leukemia, breast, prostate, pancreatic, and lung cancers, it is possible that they may be tumor suppressors.
Indeed, let-7 family members are negative regulators of the epithelial-to-mesenchymal transition EMT , a developmental event associated with treatment resistance, metastasis, and tumor recurrence. The let-7 family members also inhibit the expression of EZH2, a major epigenetic component of the polycomb repressive complex 2 PRC2 that inhibits the expression of developmental genes in embryonic and adult stem cells Kong et al. Down-regulation of let-7 and miR family members occurs in CSCs of various tumors, including breast cancer Golestaneh et al.
It appears that miR acts as an oncogenic molecule by targeting multiple survival signaling pathways Pang et al. Increased expression of miR in pancreatic, prostate, lung, and breast cancer tumors, as well as in leukemia, is associated with poor clinical outcomes Dillhoff et al. In addition, miRdisplays anti-apoptotic activity and enhances the proliferative, invasive and angiogenic potentials in a wide variety of tumor cells Moriyama et al. Expression of miR is increased significantly in CSC populations as compared to non-CSC cancer cells while forced overexpression of miR by its mimic cells enhances survival of bone marrow mesenchymal stem cells Golestaneh et al.
Conversely, functional loss of miR increased apoptosis of mesenchymal stem cells Nie et al.
It has been reported that miRa is under-expressed in a variety of tumors, including those associated with pancreatic, prostate, breast and lung cancers, and with leukemia Kent et al. Decreased levels of miRa are also thought to be associated with poor clinical outcomes Kent et al. It has been shown that miRa inhibits the expression of CSC signature genes such as CD44, CD, and Notch-1, which is consistent with its ability to attenuate the self-renewal capacity of many tumor cells Kong et al. This suggests the loss of miRa acts as a tumor suppressor in the regulation of the CSC function, and points to the possibility of employing miRa as a CSC marker and therapeutic target.
A strategy to up-regulate miRa in tumors would be a novel approach for the treatment of cancer. Summarized on Table 1 are several common methods or techniques that have been used to better understand the biology of CSCs. Metformin, a biguanide, oral hypoglycemic agent, is the most commonly used drug for the treatment of type 2 diabetes mellitus DM. Metformin reduces blood glucose levels by the down-regulation of hepatic gluconeogenesis and up-regulation of glucose uptake in peripheral tissues, such as skeletal muscle and fat Shaw et al.
It also enhances the insulin sensitivity of tissues, thereby reducing insulin levels overall. Epidemiological and clinical studies indicate a reduced incidence of breast and pancreatic cancers in DM patients taking metformin, and that its consumption improves the clinical outcome of cancer patients Bowker et al.
While these findings suggest that metformin has anti-tumor effects, the mechanism of this action remains undefined. Moreover, metformin can directly inhibit tumor cell growth and proliferation by regulating the cyclin D1-mediated cell cycle, p53 expression, and phosphorylation in breast and pancreatic cancers Ben, I et al.
These findings indicate that the anti-tumor effects of metformin may involve the targeting of CSC subpopulations, providing additional proof in support of the importance of CSC cells in cancer. A considerable body of evidence supports the hypothesis that a very small population of CSCs is associated with an aggressive tumor phenotype characterized by increased cell survival, migration, invasion, metastatic capacity, treatment resistance, and tumor recurrence, all of which ultimately contribute to poor prognosis.
Although there have been efforts to characterize CSCs, their pathogenesis and molecular interactions in the tumor microenvironment are not well defined. The identification of CSC-specific markers, and the isolation and characterization of CSCs in malignant tissues will provide insights that will be of value in designing strategies for the development of chemotherapeutics that is expected to reduce tumor aggressiveness by targeting CSCs.
Drug-induced modification of CSC-associated markers will modulate the phenotype and consequent function of these cells. For example, miRNAs such as, let-7, miR, miR and miRa are possible targets as they play key roles in CSC regulation via multiple signaling pathways that regulate cell growth and survival. The validity of this approach is suggested by the finding that metformin, an anti-diabetic drug, displays anti-tumor effects that may be due to the targeted elimination of CSCs.
Additional clinical and preclinical work is required to demonstrate conclusively the therapeutic benefit of metformin, and CSC-targeting drugs in general, for the management of particular cancers. We thank the Puschelberg and Guido foundations for their generous financial support, and Ms. Ahmedi Bee Fnu, Mr. Anthony Badie Oraha, and Mr. Evan Bao for their technical assistance. National Center for Biotechnology Information , U. Author manuscript; available in PMC Jun 1.
Azmi , 1 Shadan Ali , 2 and Fazlul H. The publisher's final edited version of this article is available at Curr Protoc Pharmacol. See other articles in PMC that cite the published article. Abstract The identification of small subpopulations of cancer stem cells CSCs from blood mononuclear cells in human acute myeloid leukemia AML in was the landmark observation for recognizing the potential role of CSCs in tumor aggressiveness.
Introduction While cancer stem cells CSCs were recognized several decades ago, it is only in the the last 15 years that they have been identified and characterized in hematological malignancies, such as leukemia Bonnet and Dick, and other tumors. Open in a separate window. CSCs and tumor aggressiveness Cancer stem cells were first identified and characterized in the bone marrow of AML patients in Identification of CSC markers in common malignant diseases The identification and characterization of CSCs in malignant diseases provides insights as to ways in which to selectively inhibit or eradicate CSCs as a treatment for tumor aggressive phenotypes.
Leukemia Leukemia is one of the most commonly diagnosed malignant diseases in children and adults Siegel et al. Breast cancer Breast cancer is the second most deadly malignancy for females, with one in eight women expected to develop the condition in their lifetime Siegel et al. Prostate cancer Prostate cancer is the most commonly diagnosed malignant disease in males, being the second leading cause of cancer-related death for men in the United States Siegel et al.
Pancreatic cancer Pancreatic ductal adenocarcinoma is currently the fourth leading cause of cancer-related deaths in the United States Siegel et al. This type of stem cells represent a small proportion of the types of cell found in a tumour, and can replicate tumour cells, causing tumours to grow and spread. Cancer stem cells are considered resistant to drug and radiotherapy treatments, and therefore can be left behind after the course of cancer treatment comes to an end, allowing the tumour to regrow and spread around the body.
Cancer stem cells offer a new avenue to treat cancer, if we can understand these cells, we can target them to effectively prevent cancer metastasis and relapse.
Cancer stem cells (CSCs) are cancer cells that possess characteristics associated with normal stem cells, specifically the ability to give rise to all cell types found. The identification of small subpopulations of cancer stem cells (CSCs) from blood mononuclear cells in human acute myeloid leukemia (AML) in was the.
Evidence shows that cancer stem cells are crucial in the formation of tumours. Not only can they renew themselves, but they can generate all of the other types of cells that are found in a tumour. If we can treat cancer by eliminating the cancer stem cells in the tumour, we will be able to treat cancer in a specialised and targeted manner. At the European Cancer Stem Cell Research Institute, we aim to understand the processes of these cells, and beat their mechanisms of drug resistance.
Through this, we can develop treatments that target these cancer stem cells, to create more effective cancer therapies. About cancer stem cells Understanding cancer stem cells offers the potential to transform the way we tackle this disease, giving us the knowledge to investigate novel and targeted cancer therapies. Types of stem cells Stem cells are cells that are required for the maintenance of healthy tissue, especially in tissues where cells continually need to be replaced.
Bile duct cancer cells. Embryonic stem cells Embryonic stem cells and stem cells in adults differ in the number of cell types they can form. Somatic stem cells An adult stem cell, called a somatic stem cell, has a limited range of cells that it can differentiate into. Induced pluripotent stem cells Induced Pluripotent Stem Cells are adult stem cells that have been grown in special conditions to reprogramme them in a laboratory, so that they can form a wider range of new cell types.
Cancer stem cells Cancer stem cells, like all stem cells, are unspecialised and can divide and renew themselves, as well as giving rise to specialised cells.