[Selective 12-lipoxygenase inhibition potentiates the effect of radiation on human prostate cancer cells in vitro and in vivo]. / Humán prosztatarák sugárérzékenységének fokozása szelektív 12-lipoxigenáz-gátlással preklinikai modellben.

Prostate cancer is one of the leading cancer types in males in the developed world. Radiotherapy is a major method in the curative treatment of prostate cancer however, up to 30% of the patients experience local relapse. Arachidonic acid metabolites have been shown to have important role in cancer. 12-lipoxygenase (12-LOX) has been proven to significantly influence prostate cancer progression, by apoptosis regulation and by promoting cancer cell survival. In this study we examined whether 12-LOX inhibition may increase radiation sensitivity of prostate cancer cells in vitro and in vivo. Prostate cancer cell lines were treated with 12-LOX inhibitors, different doses of radiation and the combination of 12-LOX inhibitors and radiation. We measured the effect of these treatments through clonogenic survival and apoptosis in vitro and tumor growth in vivo in a tumor xenograft model. 12-LOX inhibition and radiation both increased apoptosis and decreased clonogenic survival of prostate cancer cell lines in vitro. Combined treatment resulted in a supra-additive effect in vitro. In vivo both 12-LOX inhibition and radiotherapy caused delay in growth of the xenograft tumors but the combined treatment resulted in the strongest growth inhibition. The presented data prove that 12-LOX and its metabolite 12(S)-HETE have a major role in prostate cancer cell progression and radiosensitivity. We have shown by different methods in vitro and in vivo that inhibition of 12-LOX activity significantly sensitizes prostate cancer cells to radiation. Therefore we can state that 12-LOX inhibitors are promising compounds to be developed to become a new class of clinical radiation sensitizers in prostate cancer.

Interaction of pregnane X receptor with hypoxia-inducible factor-1 regulates chemoresistance of prostate cancer cells.

Aim: The nuclear pregnane X receptor (PXR) is a pivotal regulator of steroid and xenobiotics metabolism and plays an important role in shaping tumor cell responses to chemotherapy. Hypoxia within tumor tissue has multifaceted effects, including multiple drug resistance. The goal of this study was to determine whether PXR contributes to hypoxia-induced drug resistance. Methods: Metastatic prostate cancer cells were used to study the interaction of PXR and hypoxia-inducible factor-1 (HIF-1 in drug resistance associated with hypoxia. The activities of PXR and HIF-1 were determined by assays for its reporter gene or target gene expression. Co-immunoprecipitation (Co-IP) was used to determine the interaction of PXR and HIF-1. Ablation or inhibition of PXR or HIF-1 was used to determine their roles in hypoxia-induced chemoresistance. Results: PXR was activated by hypoxia, leading to increased expression of multidrug resistance protein 1 (MDR1). Inhibition of PXR by pharmacological compounds or depletion by shRNAs reduced the hypoxic induction of MDR1 and sensitized prostate cancer cells to chemotherapy under hypoxia. HIF-1 was required for PXR activation under hypoxia. Co-immunoprecipitation results showed that HIF-1 and PXR could physically interact with each other, leading to crosstalk between these two transcription factors. Conclusion: PXR contributes to hypoxia-induced drug resistance in prostate cancer cells through its interaction with HIF-1.

Regulations of Tumor Microenvironment by Prostaglandins.

Prostaglandins, the bioactive lipids generated from the metabolism of arachidonic acid through cyclooxygenases, have potent effects on many constituents of tumor microenvironments. In this review, we will describe the formation and activities of prostaglandins in the context of the tumor microenvironment. We will discuss the regulation of cancer-associated fibroblasts and immune constituents by prostaglandins and their roles in immune escapes during tumor progression. The review concludes with future perspectives on improving the efficacy of immunotherapy through repurposing non-steroid anti-inflammatory drugs and other prostaglandin modulators.

TMEM97/Sigma 2 Receptor Increases Estrogen Receptor α Activity in Promoting Breast Cancer Cell Growth.

Aberrant estrogen receptor (ER) signaling is a major driver of breast tumor growth and progression. Sigma 2 receptor has long been implicated in breast carcinogenesis based on pharmacological studies, but its molecular identity had been elusive until TMEM97 was identified as the receptor. Herein, we report that the TMEM97/sigma 2 receptor is highly expressed in ER-positive breast tumors and its expression is strongly correlated with ERs and progesterone receptors (PRs) but not with HER2 status. High expression levels of TMEM97 are associated with reduced overall survival of patients. Breast cancer cells with increased expression of TMEM97 had a growth advantage over the control cells under both nutrition-limiting and sufficient conditions, while the knockdown of TMEM97 expression reduced tumor cell proliferations. When compared to their vector control cells, MCF7 and T47D cells with increased TMEM97 expression presented increased resistance to tamoxifen treatment and also grew better under estrogen-depleted conditions. The TMEM97/sigma 2 receptor enhanced the ERα transcriptional activities and increased the expression of genes responsive to estrogen treatment. Increased TMEM97 also stimulated the mTOR/S6K1 signaling pathways in the MCF7 and T47D cells. The increased level of active, phosphorylated ERα, and the enhanced resistance to tamoxifen treatment with increased TMEM97, could be blocked by an mTOR inhibitor. The knockdown of TMEM97 expression reduced the ERα and mTOR/S6K1 signaling activities, rendering the cells with an increased sensitivity to tamoxifen. The observations suggest that the TMEM97/sigma 2 receptor is a novel regulator of ERα activities in breast tumor cell growth.

G-protein-coupled receptor for short-chain fatty acids suppresses colon cancer.

GPR43 is a G-protein-coupled receptor for short-chain fatty acids (SCFAs). Expression of GPR43 is detected in hematopoietic tissues and the large intestine. SCFAs are derived from bacterial fermentation and metabolism of undigested dietary fibers and have been recognized for their cancer prevention activities in the colon. The role of SCFAs, particularly butyrate, in colon cancer therapy has been extensively studied, and its tumor suppressive functions are believed to be due to their intracellular actions, notably inhibition of histone deacetylase. In our study, we show that SCFAs also exert their antitumor effects via receptor GPR43 and that GPR43 is frequently lost in colon cancer cells. Immunohistostaining revealed that GPR43 immunoreactivity was high in normal colon tissues (N = 31) but was markedly reduced or completely lost in most colorectal adenocarcinoma tissues (N = 70) and their corresponding lymph node metastatic adenocarcinomas (N = 38). RT-PCR analysis detected the presence of full length GPR43 mRNA in only one (HT-29) of nine established human colon cancer cell lines. Restoration of GPR43 expression in HCT8 human colonic adenocarcinoma cells induced G0/G1 cell cycle arrest and activated caspases, leading to increased apoptotic cell death after propionate/butyrate treatment. Restored GPR43 expression, coupled with propionate treatment, induced an upregulation of p21 and a decrease in the levels of cyclin D3 and cyclin-dependent kinases (CDKs) 1 and 2, while the CDK4 and CDK6 levels remained unchanged. Our results suggest that GPR43 functions as a tumor suppressor by mediating SCFA-induced cell proliferation inhibition and apoptotic cell death in colon cancer.

12-Lipoxygenase and the regulation of hypoxia-inducible factor in prostate cancer cells.

12-Lipoxygenase, an arachidonic acid metabolizing enzyme of the lipoxygenase pathway, has been implicated as a major factor in promoting prostate cancer progression and metastasis. The ability of 12-LOX to aggravate the disease was linked to its proangiogenic role. Recent studies clearly demonstrated that 12-LOX enhances the expression and secretion of the angiogenic factor, vascular endothelial growth factor (VEGF) thus providing a direct link between this enzyme and its angiogenic properties. In the present study we have investigated the relationship between 12-LOX and hypoxia inducible factor-1alpha (HIF-1alpha), a transcription factor involved in the regulation of VEGF expression under hypoxic conditions in solid tumors. Our findings have revealed that HIF-1 is one of the target transcription factors regulated by 12-LOX and 12(S)-HETE, in hypoxic tumor cells of the prostate. Regulation of HIF-1alpha by 12-LOX adds to the complexity of pathways mediated by this enzyme in promoting prostate cancer angiogenesis and metastasis. We have evidence that 12-LOX increases the protein level, mRNA, and functional activity of HIF-1alpha under hypoxic conditions, one of the mechanisms by which it upregulates VEGF secretion and activity.

Megestrol acetate is a specific inducer of CYP3A4 mediated by human pregnane X receptor.

PURPOSE: Megestrol acetate is a synthetic progestogen used to treat some cancers and cancer-associated cachexia, but its potential interactions with other drugs are not well known. This study aims to determine the regulation of drug metabolizing enzymes by megestrol acetate. METHODS: Primary human hepatocytes were treated and analyzed by PCR array to identify genes involved in drug metabolism that are impacted by megestrol acetate. P450 3A4 (CYP3A4) reporter gene assay and HPLC analyses of nifedipine metabolites were used to determine CYP3A4 gene expression and activities. Competitive ligand binding assay was used to determine the affinity of megestrol acetate toward human pregnane x receptor (hPXR). Electrophoretic mobility shift assay and mammalian two hybrid assay were used to determine the mechanism of megestrol to activate hPXR. RESULTS: The levels and activities of CYP3A4 were significantly induced (> 4-folds) by megestrol acetate in human hepatocytes and HepG2 cells. Megestrol treatment induced CYP3A4 through the activation of hPXR, a ligand-activated transcription factor that plays a role in drug metabolism and transport. Other tested nuclear receptors showed no response. The mechanism studies showed that megestrol activated hPXR by binding to the ligand binding domain (LBD) of hPXR and increasing the recruitment of the cofactors such as steroid receptor cofactor (SRC-1). CONCLUSION: The results suggest that megestrol acetate is a specific inducer of CYP3A4 mediated by hPXR and therefore has the potential to cause drug interactions, especially in the co-administration with drugs that are substrates of CYP3A4.

Camptothecin attenuates cytochrome P450 3A4 induction by blocking the activation of human pregnane X receptor.

Differential regulation of drug-metabolizing enzymes (DMEs) is a common cause of adverse drug effects in cancer therapy. Due to the extremely important role of cytochrome P450 3A4 (CYP3A4) in drug metabolism and the dominant regulation of human pregnane X receptor (hPXR) on CYP3A4, finding inhibitors for hPXR could provide a unique tool to control drug efficacies in cancer therapy. Camptothecin (CPT) was demonstrated as a novel and potent inhibitor (IC(50) = 0.58 microM) of an hPXR-mediated transcriptional regulation on CYP3A4 in this study. In contrast, one of its analogs, irinotecan (CPT-11), was found to be an hPXR agonist in the same tests. CPT disrupted the interaction of hPXR with steroid receptor coactivator-1 but had effects on neither the competition of ligand binding nor the formation of the hPXR and retinoid X receptor alpha heterodimer, nor the interaction between the regulatory complex and DNA-responsive elements. CPT treatment resulted in delayed metabolism of nifedipine in human hepatocytes treated with rifampicin, suggesting a potential prevention of drug-drug interactions between CYP3A4 inducers and CYP3A4-metabolized drugs. Because CPT is the leading compound of topoisomerase I inhibitors, which comprise a quickly developing class of anticancer agents, the findings indicate the potential of a new class of compounds to modify hPXR activity as agonists/inhibitors and are important in the development of CPT analogs.

Promotion of tumor development in prostate cancer by progerin.

Progerin is a truncated form of lamin A. It is identified in patients with Hutchinson-Gilford progeria syndrome (HGPS), a disease characterized by accelerated aging. The contribution of progerin toward aging has been shown to be related to increased DNA damages. Since aging is one major risk factor for carcinogenesis, and genomic instability is a hallmark of malignant cancers, we investigated the expression of progerin in human cancer cells, and whether its expression contributes to carcinogenesis. Using RT-PCR and Western blotting, we detected the expression of progerin in prostate PC-3, DU145 and LNCaP cells at mRNA and protein levels. Ectopic progerin expression did not cause cellular senescence in PC-3 or MCF7 cells. PC-3 cells progerin transfectants were sensitized to DNA damage agent camptothecin (CPT); and persistent DNA damage responses were observed, which might be caused by progerin induced defective DNA damage repair. In addition, progerin transfectants were more tumorigenic in vivo than vector control cells. Our study for the first time describes the expression of progerin in a number of human cancer cell lines and its contributory role in tumorigenesis.

Downregulation of vascular endothelial growth factor and induction of tumor dormancy by 15-lipoxygenase-2 in prostate cancer.

The enzyme 15-lipoxygenase-2 (15-LOX-2) utilizes arachidonic acid, a polyunsaturated fatty acid, to synthesize 15(S)-hydroxyeicosatetraenoic acid. Abundantly expressed in normal prostate epithelium but frequently suppressed in the cancerous tissues, 15-LOX-2 has been suggested as a functional suppressor of prostate cancer, but the mechanism(s) involved remains unknown. To study the functional role of 15-LOX-2 in prostate cancer, we expressed 15-LOX-2 as a fusion protein with GFP in DU145 and PC-3 cells and found that 15-LOX-2 increased cell cycle arrest at G0/G1 phase. When injected into athymic nu/nu mice, prostate cancer cells with 15-LOX-2 expression could still form palpable tumors without significant changes in tumorigenicity. But, the tumors with 15-LOX-2 expression grew significantly slower than those derived from vector controls and were kept dormant for a long period of time. Histological evaluation revealed an increase in cell death in tumors derived from prostate cancer cells with 15-LOX-2 expression, while in vitro cell culture conditions, no such increase in apoptosis was observed. Further studies found that the expression of vascular endothelial growth factor A (VEGF-A) was significantly reduced in prostate cancer cells with 15-LOX-2 expression restored. Our studies suggest that 15-LOX-2 suppresses VEGF gene expression and sustains tumor dormancy in prostate cancer. Loss of 15-LOX-2 functionalities, therefore, represents a key step for prostate cancer cells to exit from dormancy and embark on malignant progression in vivo.

Impaired Recovery from Influenza A/X-31(H3N2) Infection in Mice with 8-Lipoxygenase Deficiency.

Lipoxygenase-derived lipid mediators can modulate inflammation and are stimulated in response to influenza infections. We report an effect of 8-lipoxygenase (ALOX8) on the recovery of mice after infection with Influenza virus X31. We compared the responses of 3- and 6-month-old mice with a deletion of ALOX8 (ALOX8-/-) to influenza infections with those of age-matched littermate wild-type mice (ALOX8+/+). The duration of illness was similar in 3-month-old ALOX8-/- and ALOX8+/+ mice. However, the 6-month-old ALOX8-/- mice showed a prolonged state of illness compared with ALOX8+/+ mice, as evidenced by reduced body temperatures, reduced locomotor activities, and delayed weight recovery. Although residual viral RNA in the lungs at day 10 post-inoculation was significantly influenced by the age of the ALOX8-/- mice, there were no significant differences between ALOX8-/- and ALOX8+/+ mice within the same age groups. The levels of cytokines interleukin 6 (IL-6) and keratinocyte chemoattractant (KC) differed significantly between 6-month-old ALOX8-/- and ALOX8+/+ mice 10 days after viral inoculation. Our data suggest that ALOX8 deficiency in mice leads to impaired recovery from influenza infection in an age-dependent manner.

Role of Rho GTPases in breast cancer.

Small GTPase Rho signaling pathways regulate the growth, motility, invasion and metastasis of breast cancer cells. Aberrant Rho signaling, as results from alterations in the levels of Rho GTPase proteins, the status of activation, and the abundance of effector proteins, is found in breast cancers. Alterations of Rho signaling particularly impact the cytoskeleton, whose organization and reorganization underpin the motility of breast cancer cells during the invasive growth and metastasis of breast cancer. Progress is being made to elucidate the underlying mechanisms by which Rho GTPases activate the downstream signaling effectors. Further investigations are required for development of novel tumor therapeutic strategies targeting the Rho GTPase signaling pathways to treat breast cancer.

Cyclooxygenases and lipoxygenases in prostate and breast cancers.

Oxidative metabolism of polyunsaturated fatty acids through cyclooxygenases or lipoxygenases can generate various lipid peroxides and bioactive lipids, and regulate cellular proliferation, apoptosis, differentiation and senescence. The role of the second cyclooxygenase isoform (COX-2) has been demonstrated in a number of studies and is regarded as a promising target for chemoprevention and treatment. The involvement for lipoxygenases in tumor initiation and progression has been implicated in several studies but remains controversial. Among the many members of lipoxygenase family, both tumor promoting and suppressing activities have been described. For example, 15-lipoxygenase-1 has been implicated as a tumor promoter in prostate cancer, but it suppresses colon cancer. In this review, the role of cyclooxygenases and lipoxygenases in cancer will be described, with the hope of attracting further research to define their functions in cancer.

Immune Characterization of the Programmed Death Receptor Pathway in High Risk Prostate Cancer.

BACKGROUND: Programmed cell death-1 (PD-1), a T-cell inhibitory receptor, and its ligand, PD-L1, have been reported to be expressed in many tumor types, and this expression has led to the development of many drugs targeting the PD-1 pathway. The objective of this study was to determine the expression of PD-1 and PD-L1 in high-grade prostate cancer tissues, and correlate the expression with disease and patient characteristics. MATERIALS AND METHODS: Immunohistochemistry for PD-1 (CD279), PD-L1 (B7-H1), and CD3 was performed and scored from 0 to 5 on prostatectomy/biopsy tissue samples taken from 25 men with high-grade prostate cancer. Charts were then retrospectively reviewed for numerous patient and disease characteristics. Statistical analyses were done to investigate the association of these patient and disease characteristics with PD-1, PD-L1, and CD3 expression. RESULTS: A score of 3 to 5 on the semiquantitative 0 to 5 score was deemed "high" expression whereas a score of 0 to 2 was deemed "low" expression. Of the 25 samples, 2 (8%) scored high for PD-1 expression, 2 (8%) scored high for PD-L1 expression, and 18 (72%) scored high for CD3 expression. There was no statistically significant difference between high and low expression groups of PD-1, PD-L1, or CD3 for any of the variables we collected. CONCLUSION: An overall low expression of PD-1 and PD-L1, and a concurrent high expression of CD3+ T cells was found in high-risk prostate cancer tissue. No significant association was found between expression of PD-1, PD-L1, or CD3, and patient or disease characteristics. Because of this, one might be able to question the role of PD-L1 in local immune suppression in prostate cancer.

CD24 Expression May Play a Role as a Predictive Indicator and a Modulator of Cisplatin Treatment Response in Head and Neck Squamous Cellular Carcinoma.

Platinum-based therapy is most often used to treat advanced cases of head and neck cancers, but only a small fraction of the patient population responds to cisplatin, with a median survival time of less than a year. Although gene signatures and molecular etiology of head and neck cancers have been previously described, none of them are predictive indicators of cisplatin treatment response in particular. Therefore, currently, there is a lack of clinically employable predictive indicators of the disease beyond HPV status to specifically predict patients' response to platinum-based therapy. It beckons a substantial effort to look for predictive indicators of cisplatin treatment response. In this regard, CD24 expression level appears to be a significant molecular phenotype of cisplatin-resistant residual cells in laryngeal carcinoma lines. CD24 expression level directly affects cisplatin sensitivity and affects the expression of critical apoptotic, stem and drug resistance genes. A relatively small retrospective patient tumor analysis suggests that CD24 high tumors go on to show an unfavorable response to cisplatin treatment. Overall, based on the strength of further analysis, CD24 presents a strong rationale to be utilized as a predictive indicator to stratify head and neck cancer patients for platinum-based therapy. It also provides a rationale for using CD24 as a therapeutic adjuvant target along with standard cisplatin therapy.

Increased metastatic potential in human prostate carcinoma cells by overexpression of arachidonate 12-lipoxygenase.

Arachidonate 12-lipoxygenase (LOX) converts arachidonic acid to 12(S)-hydroxyeicosatetraenoic acid (HETE), a bioactive lipid implicated in tumor angiogenesis, growth, and metastasis. Alteration in 12-LOX expression or activity has been reported in various carcinomas including prostate carcinoma. However, little is known about the impact of the altered expression or activity of 12-LOX on tumor metastasis. In the present study, we examined whether or not an increase in 12-LOX expression in human prostate carcinoma cells can modulate their metastatic potential. We report that increased expression of 12-LOX in PC-3 cells caused a significant change in cell adhesiveness, spreading, motility, and invasiveness. Specifically 12-LOX transfected PC-3 cells were more adhesive toward vitronectin, type I and IV collagen, but not to fibronectin or laminin, than cells transfected with control vector. Increased spreading on vitronectin, fibronectin, collagen type I and IV also was observed in 12-LOX transfected PC-3 cells when compared to control PC-3 cells. The increased spreading of 12-LOX transfected PC-3 cells was blocked by treatment with 12-LOX inhibitors, baicalein and CDC. 12-LOX transfected PC-3 cells were more invasive through Matrigel than cells transfected with control vector. In vivo, tumor cell invasion to surrounding muscle or fat tissues was more frequent in nude mice bearing s.c. tumors from 12-LOX transfected PC-3 cells than in those from control vector transfected cells. When injected via the tail vein into SCID mice with implanted human bone fragments, there was an increase in tumor metastasis to human bone by 12-LOX transfected PC-3 cells in comparison to control vector transfected cells. Taken together, our data suggest that an increase in 12-LOX expression enhances the metastatic potential of human prostate cancer cells.

Platelet-type 12-lipoxygenase activates NF-kappaB in prostate cancer cells.

Platelet-type arachidonate 12-lipoxygenase (12-LOX) is highly expressed in many types of cancers and plays an important role in cancer pathophysiology. Arachidonic acid metabolism by 12-LOX results in the stable end product 12(S)-hydroxy eicosatetraenoic acid (12(S)-HETE), which is a signaling molecule with effects on cell proliferation, motility, invasiveness, angiogenesis, and inhibition of apoptosis. The myriad biological activities manifested by 12(S)-HETE appear to be mediated, at least in part, by the activation of NF-kappaB. Overexpression of the 12-LOX in PC-3 prostate cancer cells resulted in the constitutive activation of the transcription factor. The enzymatic product of arachidonic acid metabolism, 12(S)-HETE, mediates the activation of NF-kappaB by the 12-LOX. 12(S)-HETE treatment of PC-3 cells induced the degradation of IkappaB by the S6 proteasomal pathway and the activated NF-kappaB translocated to the nucleus causing kappaB-induced transcription. Specificity of the NF-kappaB activation by 12(S)-HETE was established by the use of a 12-LOX-specific inhibitor and 13(S)-HODE, a known 12(S)-HETE antagonist. Considering the known involvement of MAP kinase pathway in NF-kappaB activation and that of 12(S)-HETE in MAP kinase pathway, 12-LOX present in prostate cancer tissues may contribute to the constitutive activation of NF-kappaB in prostate cancer cells.

Non-steroid anti-inflammatory drugs, prostaglandins, and cancer.

Fatty acids are involved in multiple pathways and play a pivotal role in health. Eicosanoids, derived from arachidonic acid, have received extensive attention in the field of cancer research. Following release from the phospholipid membrane, arachidonic acid can be metabolized into different classes of eicosanoids through cyclooxygenases, lipoxygenases, or p450 epoxygenase pathways. Non-steroid anti-inflammatory drugs (NSAIDs) are widely consumed as analgesics to relieve minor aches and pains, as antipyretics to reduce fever, and as anti-inflammatory medications. Most NSAIDs are nonselective inhibitors of cyclooxygenases, the rate limiting enzymes in the formation of prostaglandins. Long term use of some NSAIDs has been linked with reduced incidence and mortality in many cancers. In this review, we appraise the biological activities of prostanoids and their cognate receptors in the context of cancer biology. The existing literature supports that these lipid mediators are involved to a great extent in the occurrence and progression of cancer.

PPAR gamma, bioactive lipids, and cancer progression.

In this article we review the evolution of cancer research involving PPARgamma, including mechanisms, target genes, and clinical applications. For the last thirteen years, the effects of PPARgamma activity on tumor biology have been studied intensely. Most of this research has focused upon the potential for employing agonists of this nuclear receptor in cancer treatment. As a monotherapy such agonists have shown little success in clinical trials, while they have shown promise as components of combination treatments both in culture and in animal models. Other investigations have explored a possible role for PPARgamma as a tumor suppressor, and as an inducer of differentiation of cancer stem cells. Whereas early studies have yielded variable conclusions regarding the prevalence of PPARgamma mutations in cancer, the protein level of this receptor has been more recently identified as a significant prognostic marker. We predict that indicators of PPARgamma activity may also serve as predictive markers for tailoring treatments.

Cancer stem cells and resistance to chemo and radio therapy.

Cancer stem cells (CSCs, or tumor initiating cells) are responsible for tumor initiation. If cancer treatment kills most of cancer cells in the stage of transit amplifying and differentiation without killing the stem cells, the surviving CSCs will eventually lead to recurrence of tumors. Studies have suggested that CSCs may be the primary mediators of resistance to chemo- and radio-therapy, leading to failure in cancer therapy. Numerous targets are being investigated for their potential involvement in the self-renewal and chemo- and radio-resistance of cancer cells. However, despite the intensive efforts invested into characterizing the role of cancer stem cells, there is a sense of uncertainty regarding the identity and number of these cells as well as the implications in cancer treatment. In this review, we will discuss the identification of CSCs by cell surface markers, the biology of CSCs, and the role of CSCs in resistance to radio- and chemo-therapy. This review will discuss the advances in targeting CSCs to improve the efficacy of chemo- and radio-therapy.