Cancer stem cells have been identified in a growing number of hematopoietic cancer and solid tumors and are typically recognized by virtue of the expression of cell surface markers. These cells have been isolated from the bulk-tumor population by the expression pattern of cell surface proteins (e.g., CD24, CD44, CD133) and cellular activities, such as the efflux of Hoechst dye or aldehyde dehydrogenase activity by flow cytometry and/or fluorescence activated cell sorting (FACS). The identification of markers that allow the prospective isolation of CSCs from whole tumor tissues will lead to the understanding of important biological properties of CSCs and provide the possibility to target them.
CD133 is a glycosylated, 120KD protein with five transmembrane domains and two large extracellular loops. CD133+
phenotype was first used to identify and isolate brain tumor stem cells in malignant tumors and now it has recently been used to define the CSC populations in lung, pancreatic, liver, prostate, gastric, colorectal, and head and neck cancers. The expression of genes known to play important roles in the maintenance of cancer stem cells have been investigated in putative CD133+
CSC populations of multiple tissues. These CD133+ cells undergo multi-lineage differentiation to neurons, astrocytes, and oligodendrocytes in vitro, and can recapitulate the original tumor phenotype in vivo, unlike the CD133−. Some genes associated with cancer stem cell like Nestin, BMI1, Olig2, and Nanog are also found upregulated in CD133+
populations of brain, lung, liver and prostate cancers [17
]. CSCs is often associated with resistance to traditional chemotherapies, CD133+ cells have had increased survival in vitro and have been enriched in vivo after treatment with cisplatin, etoposide, doxorubicin, and paclitaxel, as the expression of genes known to be markers of stemness, ABC transporters and the DNA repair pathway [21
CD44 is reported as at least one characteristic of CSCs across tissues, including breast, pancreas, gastric, head and neck, ovarian and colon, whereas other markers (e.g., CD24) are not. Early results showed that invasive CD44+ prostate cells also had increased expression of Nanog, BMI1 and SHH, which is similar to CD133+
]. The standard CD44 (CD44s) molecule is an 85- to 90-kDa transmembrane glycoprotein containing 10 standard exons and four major domains, including the hyaluronan-binding and variably spliced regions, the transmembrane sequence and the intracellular cytoskeletal-signaling domain. Interactions between CD44 and the extracellular matrix glycosaminoglycan hyaluronan (HA) are currently an exciting area of investigation. Several studies show that the binding of CD44 with HA protein is crucial for tumor progression [24
] and also some other research suggest that CD44 variants play an important role in metastasis, especially the CD44v6 isoforms.
Aldehyde dehydrogenase is a polymorphic enzyme responsible for the oxidation of aldehydes to carboxylic acids, which leave the liver and are metabolized by the body’s muscle and heart. This cellular function is likely crucial for CSC longevity and probably a key explanation for the reported resistance of CSCs to chemotherapies, especially those that generate toxic aldehyde intermediates [25
is also investigated as a marker and leukemia CSCs are determined to be highly ALDH positive. ALDH activity is usually measured by using BIODIPY aminoacetaldehyde (BAAA), commonly known as Aldefuor [26
]. In the past few years, ALDH has been used to characterize CSCs in breast, lung, head and neck, colon and liver tumors. And several groups have found that shRNA and siRNA knockdown of ALDH1 in colorectal xenografts and lung cells, respectively, sensitized ALDH+ CSCs to CPA and 4-hydroperoxycyclophosphamide treatment. Although CSCs are enriched in ALDH+ populations in several tissues, it is important to acknowledge possible limitations, especially when used as a single marker.
ABCG2 is a member of the ATP binding cassette (ABC) transporters, which can pump a wide variety of endogenous and exogenous compounds out of cells. It was first cloned from doxorubicin-resistant human MCF-7 breast cancer cells and named as breast cancer resistance protein (BCRP) [27
]. The side population phenotype, which is characterized by the ability to transport Hoechst 33342 out of cells, has been identified as a characteristic feature of stem cells. ABCG2 plays an important role in promoting stem cell proliferation and the maintenance of the stem cell phenotype. Based on a RNA interference approach, the suppression of ABCG2 could significantly inhibit cancer cell proliferation. Furthermore, the blocking of ABCG2 function by fumitremorgin C, a chemical inhibitor, also inhibited cell proliferation via the prolonged G0/G1 interval [28
]. These data suggest that ABCG2 may contribute to cancer cell proliferation. A research learned about diffuse large B-cell lymphoma suggest high levels of ABCG2 is correlated with shorter overall survival and ABCG2 protein levels is correlated with the expression of SHH protein levels which plays a critical role in growth and differentiation during embryonic development [29
]. ABCG2 is found to confer the side population (SP) phenotype and is recognized as a universal marker of stem cells [30
]. Evidence is growing that the SP is enriched in CSCs because of the upregulation of “stemness” genes when compared with the non-SP cells. Recent studies have shown that ABCG2 is expressed in stem cell populations derived from a wide range of tissues, including pancreas, lung, limbal epithelium, brain and prostate. Since ABCG2 functions as a high capacity transporter with a wide range of substrates, various chemotherapy drugs included, it has been shown to participate in the multidrug resistance of tumors and lead to the cancer relapse. Side population and chemo-resistance suggest a close link between ABCG2 and CSCs.
Besides the CD133, CD44, ALDH, and ABCG2, there have been many other cell surface proteins been identified as the marker of many tumors in the recent years (). Although CD133, CD44, ALDH, ABCG2 and other proteins have been used to identify cancer stem cells, it is still important to clarify cellular and signaling functions of markers themselves. Biomarkers of the cancer stem cells not only the tools to which were used to identify the cancer stem cells, but also might become the target of many drugs which were developed to cure cancer. In the series research of those proteins, they are proved to be associated with tumor progression, maintenance and metastasis. At present, CSC markers must be clearly defined for each tissue, and investigation is needed to determine whether variations between tumors can be used as indicators of sensitivity to therapy. However, no specified drugs have been developed to target those cell markers, monoclonal antibodies and siRNA and /or shRNA could sensitize cells to traditional therapy or even cure cancer. Finally, clarifying cellular and signaling functions of markers themselves will lead to more therapeutic options to destroy tumor cells that have the ability to repopulate.
Cancer stem marker and their distribution