xCT inhibition depletes CD44v-expressing tumor cells that are resistant to EGFR-targeted therapy in head and neck squamous cell carcinoma

The CD44 standard isoform contributes to radioresistance of pancreatic cancer cells

Resistance to chemoradiotherapy is one reason for the increased recurrence rate of pancreatic cancer after these therapies. These cells change the expression levels of several proteins, such as epithelial-mesenchymal transition (EMT), while acquiring the chemo- or radio-resistance. In this study, we focused on CD44, a pancreatic cancer stem cell marker. CD44 has isoforms with different functions: standard isoform (CD44s) and several variant isoforms (CD44v). However, little is known about the roles of these isoforms after ionizing irradiation. The purpose of this study was to investigate the role of CD44 isoforms in radioresistance of pancreatic cancer cells. AsPC-1 (a human pancreatic cancer cell line) was irradiated with 4 MV X-rays. The mRNA and protein levels of CD44s were strongly upregulated, dose dependently, compared with CD44v after irradiation. Thus, we further investigated CD44s at the point of cell proliferation. We evaluated cell proliferation and survival, using CD44s knockdown cells. CD44s knockdown did not change the proliferation rate for up to 72 h after the irradiation, but decreased cell viability in the colony formation assay. As one of the reasons for these effects, we found downregulation of phosphorylated extracellular signal-regulated kinase (Erk; which is involved with cell proliferation) by CD44s knockdown, time dependently. Moreover, radiation-induced EMT-like expression changes were detected and suppressed by CD44s knockdown. In conclusion, our work demonstrated that CD44 standard isoform was especially upregulated after high-dose X-ray irradiation in several isoforms of CD44 and contributed to longer-term cell survival after the irradiation through the maintenance of Erk phosphorylation and radiation-induced EMT.

CD44 variant-dependent regulation of redox balance in EGFR mutation-positive non-small cell lung cancer: A target for treatment

OBJECTIVES

The regulation of redox balance in cancer cells is an important factor in tumor development and chemoresistance, with oncogene activation having been shown to induce the generation of reactive oxygen species (ROS). Activating mutations of the epidermal growth factor receptor gene (EGFR) are oncogenic drivers in non-small cell lung cancer (NSCLC), but it has remained unknown whether ligand-independent EGFR signaling conferred by EGFR mutation triggers ROS generation in NSCLC cells.

MATERIALS AND METHODS

HEK293T cells were transfected with an expression vector for mutant EGFR. The expression of CD44 variant (CD44v) isoforms in NSCLC cell lines was evaluated by flow cytometry. Cells were depleted of CD44v by RNA interference and assayed for ROS and glutathione (GSH) levels. The effect of CD44v on cisplatin sensitivity was evaluated in vitro with the MTS assay.

RESULTS

EGFR signaling due to EGFR mutation increased ROS levels in transfected HEK293T cells. The expression of CD44v isoforms was found to be inversely correlated with basal ROS levels in EGFR mutation-positive NSCLC cell lines. Knockdown of CD44v induced depletion of intracellular GSH and increased ROS levels in EGFR-mutated NSCLC cells that express CD44v at a high level (CD44v^high). In addition, depletion of GSH by treatment with buthionine-[S, R]-sulfoximine induced marked accumulation of ROS and enhanced the cytotoxicity of cisplatin in CD44v^highEGFR-mutated NSCLC cells but not in corresponding CD44v^low cells. This enhancement of cisplatin cytotoxicity by GSH depletion was prevented by treatment with the antioxidant N-acetyl-l-cysteine. Knockdown of CD44v also enhanced cisplatin cytotoxicity in CD44v^highEGFR mutation-positive NSCLC cells but not in CD44v^low cells.

CONCLUSION

Our results thus implicate CD44v in redox adaptation and as a potential target for treatment in CD44v^highEGFR-mutated NSCLC cells.

Cystine addiction of triple-negative breast cancer associated with EMT augmented death signaling

Despite the advances in the diagnosis and treatment of breast cancer, breast cancers still cause significant mortality. For some patients, especially those with triple-negative breast cancer, current treatments continue to be limited and ineffective. Therefore, there remains an unmet need for a novel therapeutic approach. One potential strategy is to target the altered metabolic state that is rewired by oncogenic transformation. Specifically, this rewiring may render certain outside nutrients indispensable. To identify such a nutrient, we performed a nutrigenetic screen by removing individual amino acids to identify possible addictions across a panel of breast cancer cells. This screen revealed that cystine deprivation triggered rapid programmed necrosis, but not apoptosis, in the basal-type breast cancer cells mostly seen in TNBC tumors. In contrast, luminal-type breast cancer cells are cystine-independent and exhibit little death during cystine deprivation. The cystine addiction phenotype is associated with a higher level of cystine-deprivation signatures noted in the basal type breast cancer cells and tumors. We found that the cystine-addicted breast cancer cells and tumors have strong activation of TNFα and MEKK4-p38-Noxa pathways that render them susceptible to cystine deprivation-induced necrosis. Consistent with this model, silencing of TNFα and MEKK4 dramatically reduces cystine-deprived death. In addition, the cystine addiction phenotype can be abrogated in the cystine-addictive cells by miR-200c, which converts the mesenchymal-like cells to adopt epithelial features. Conversely, the introduction of inducers of epithelial-mesenchymal transition (EMT) in cystine-independent breast cancer cells conferred the cystine-addiction phenotype by modulating the signaling components of cystine addiction. Together, our data reveal that cystine-addiction is associated with EMT in breast cancer during tumor progression. These findings provide the genetic and mechanistic basis to explain how cystine deprivation triggers necrosis by activating pre-existing oncogenic pathways in cystine-addicted TNBC with prominent mesenchymal features.

Phase I study of salazosulfapyridine in combination with cisplatin and pemetrexed for advanced non-small-cell lung cancer

Spliced variant isoforms of CD44 (CD44v) are a marker of cancer stem cells in solid tumors. They stabilize the xCT subunit of the transporter system xc(–) and thereby promote synthesis of the antioxidant glutathione. Salazosulfapyridine (SASP) is an inhibitor of xCT and suppresses the proliferation of CD44v‐positive cancer cells. Chemotherapy‐naïve patients with advanced non‐squamous non‐small‐cell lung cancer were enrolled in a dose‐escalation study (standard 3 + 3 design) of SASP in combination with cisplatin and pemetrexed. The primary end‐point was the percentage of patients who experience dose‐limiting toxicity. Fifteen patients were enrolled in the study. Dose‐limiting toxicity was observed in one of six patients at a SASP dose of 1.5 g/day (elevation of aspartate and alanine aminotransferase levels, each of grade 3), two of five patients at 3 g/day (hypotension or pneumonitis, each of grade 3), and two of three patients at 4.5 g/day (anorexia of grade 3). The maximum tolerated dose was thus 3 g/day, and the recommended dose was 1.5 g/day. The overall response rate was 26.7% and median progression‐free survival was 11.7 months, much longer than that for cisplatin–pemetrexed alone in previous studies. Exposure to SASP varied markedly among individuals according to ABCG2 and NAT2 genotypes. The serum concentration of free CD44v protein was increased after the first cycle of treatment, possibly reflecting death of cancer stem cells. Salazosulfapyridine was thus given safely in combination with cisplatin–pemetrexed, with the addition of SASP tending to prolong progression‐free survival. This trial is registered in the UMIN Clinical Trials Registry as UMIN000017854.

Iron and thiol redox signaling in cancer: An exquisite balance to escape ferroptosis

Epidemiological data indicate a constant worldwide increase in cancer mortality, although the age of onset is increasing. Recent accumulation of genomic data on human cancer via next-generation sequencing confirmed that cancer is a disease of genome alteration. In many cancers, the Nrf2 transcription system is activated via mutations either in Nrf2 or Keap1 ubiquitin ligase, leading to persistent activation of the genes with antioxidative functions. Furthermore, deep sequencing of passenger mutations is clarifying responsible cancer causative agent(s) in each case, including aging, APOBEC activation, smoking and UV. Therefore, it is most likely that oxidative stress is the principal initiating factor in carcinogenesis, with the involvement of two essential molecules for life, iron and oxygen. There is evidence based on epidemiological and animal studies that excess iron is a major risk for carcinogenesis, suggesting the importance of ferroptosis-resistance. Microscopic visualization of catalytic Fe(II) has recently become available. Although catalytic Fe(II) is largely present in lysosomes, proliferating cells harbor catalytic Fe(II) also in the cytosol and mitochondria. Oxidative stress catalyzed by Fe(II) is counteracted by thiol systems at different functional levels. Nitric oxide, carbon monoxide and hydrogen (per)sulfide modulate these reactions. Mitochondria generate not only energy but also heme/iron sulfur cluster cofactors and remain mostly dysfunctional in cancer cells, leading to Warburg effects. Cancer cells are under persistent oxidative stress with a delicate balance between catalytic iron and thiols, thereby escaping ferroptosis. Thus, high-dose L-ascorbate and non-thermal plasma as well as glucose/glutamine deprivation may provide additional benefits as cancer therapies over preexisting therapeutics.

Variant CD44 expression is enriching for a cell population with cancer stem cell-like characteristics in human lung adenocarcinoma

Background: Preliminary studies have identified cancer stem cells (CSCs) in various cancers and there are several ongoing clinical studies targeting these cells. CD44 (standard or variant isoforms) and/or aldehyde dehydrogenase (ALDH) expression is the most commonly used markers for the identification of CSCs. The goal of the current study was to examine the ability of CD44v, either alone or in combination with ALDH, to identify CSCs within human lung cancer cells lines.

Methods: We examined several lung adenocarcinoma cell lines for the ability of CD44v and/or ALDH expression to enrich for cells with CSC characteristics such as in vitro differential proliferation rate, chemotherapeutic-resistance, tumorsphere formation, and in vivo tumorigenicity. We also compared their in vivo secondary tumor formation, and histological characteristics of their xenograft tumors, and examined their expression of PD-L1, EGFR, xCT, and reactive oxygen species (ROS).

Results: Both CD44v^high/ALDH^high and CD44v^high/ALDH^low cells were enriched in cells with CSC characteristics, with the CD44v^high/ALDH^low cells being more proliferative and more resistant to chemotherapeutics, whereas CD44v^high/ALDH^high cells were more efficient in forming tumorspheres in vitro, in making primary xenograft tumors, and in propagating secondary tumors in vivo. Applying stricter sorting gates to select for cells with the highest CD44v/ALDH expression caused the CD44v^high/ALDH^low cells to lose their high proliferation rates and chemotherapeutic resistance ability, but enriched for the tumorsphere-forming cells among the CD44v^high/ALDH^high and CD44v^high/ALDH^low cells. CD44v^high expression was associated with PD-L1 and xCT expression in both H1650 and HCC827 cells. This association was not modified by ALDH expression in the H1650 cell line. However, in the HCC827 cell line, ALDH expression was negatively associated with PD-L1 and positively associated with xCT expression.

Conclusion: Lung adenocarcinoma cells with high CD44v expression are enriched for CSCs. Addition of ALDH as an enrichment marker bestowed some CSCs characteristics to CD44v^high/ALDH^low cells and others to CD44v^high/ALDH^high cells. We propose that lung adenocarcinoma contains different CSCs, each of them endowed with different CSC characteristics.

cRGD peptide installation on cisplatin-loaded nanomedicines enhances efficacy against locally advanced head and neck squamous cell carcinoma bearing cancer stem-like cells

Recalcitrant head and neck squamous cell carcinoma (HNSCC) usually relapses after therapy due to the enrichment of drug resistant cancer stem-like cells (CSCs). Nanomedicines have shown potential for eradicating both cancer cells and CSCs by effective intratumoral navigation for reaching particular cell populations and controlling drug delivery. The installation of ligands on nanomedicines is an attractive approach for improving the delivery to CSCs within tumors, though the development of CSC-selective ligand-receptor systems has been challenging. Herein, we found that the CSC subpopulation in HNSCC cells overexpresses α_vβ₅ integrins, which is preferentially expressed in tumor neovasculature and cancer cells, and can be effectively targeted by using cyclic Arg-Gly-Asp (cRGD) peptide. Thus, in this study, we propose installing cRGD peptide on micellar nanomedicines incorporating cisplatin for improving their activity against CSCs and enhancing survival. Both cisplatin-loaded micelles (CDDP/m) and cRGD-installed CDDP/m (cRGD-CDDP/m) were effective against HNSCC SAS-L1-Luc cells in vitro, though cRGD-installed CDDP/m was more potent than CDDP/m against the CSC fraction. In vivo, the cRGD-CDDP/m also showed significant antitumor activity against HNSCC orthotopic tumors, i.e. SAS-L1 and HSC-2. Moreover, cRGD-CDDP/m rapidly accumulated into the lymph node metastasis of SAS-L1 tumors, effectively inhibiting their growth, and prolonging mice survival. These findings indicate cRGD-installed nanomedicines as an advantageous strategy for targeting CSCs in HNSCC, and particularly, cRGD-CDDP/m as a significant therapeutic strategy against regionally advanced HNSCC.

CD44 Variant 6 as a Predictive Biomarker for Distant Metastasis in Patients With Epithelial Ovarian Cancer

OBJECTIVE

To investigate the role of cancer stem cell marker CD44 variant 6 in the development of distant metastasis in patients with epithelial ovarian cancer.

METHODS

A retrospective cohort study was performed in 186 patients who underwent surgery for ovarian cancer from 2005 to 2013 at the Kumamoto University Hospital. The association between the expression of CD44 variant 6 and distant metastasis was evaluated based on a large-scale immunohistochemical analysis. Primary ovarian tumors that contained at least 10% CD44 variant 6-positive cancer cells were categorized as CD44v6-high (n=53), and the tumors that contained less than 10% CD44 variant 6-positive cells were categorized as CD44v6-low (n=133). Distant metastasis-free survival was compared between the CD44v6-high and -low groups. Multivariate analysis was performed to estimate the influence of various clinicopathologic factors on the development of distant metastasis.

RESULTS

At the time of ovarian cancer diagnosis, distant metastasis occurred in 13 of 53 patients (24.5%) in the CD44v6-high group and 17 of 133 patients (12.8%) in the CD44v6-low group (P=.049). The median metastasis-free survival after stage I-III ovarian cancer diagnosis was 19.5 months (range 11-55 months) in the CD44v6-high group (n=40) and 39.5 months (range 22-57 months) in the CD44v6-low group (n=116) (P=.071). Multivariate analysis demonstrated that CD44 variant 6 expression was an independent risk factor for distant metastatic recurrence (hazard ratio 4.09, 95% confidence interval 1.29-12.98; P=.017).

CONCLUSION

CD44 variant 6 represents an important predictor of distant metastasis and CD44 variant 6-positive ovarian cancer cells play a crucial role in the formation of distant metastases in patients with ovarian cancer.

Rheostatic CD44 isoform expression and its association with oxidative stress in human malignant mesothelioma

CD44 exists as a standard (CD44s) isoform and different variant isoforms (CD44v) due to alternative splicing. While the complex nature of these different isoforms has not been fully elucidated, CD44v expression has been shown to exert oncogenic effects by promoting tumor progression, metastasis and resistance of tumor cells to chemotherapy. One of the CD44v isoforms, CD44v8-10, was recently shown to protect cancer cells from oxidative stress by increasing the synthesis of glutathione (GSH). However, data regarding CD44 isoform expression in malignant mesothelioma (MM) are still lacking. Here, we show that most of the MM cell lines express both the CD44s and CD44v isoforms, in contrast to non-tumorigenic mesothelial cells, which express only CD44s. Moreover, we show here that these MM cell lines are positive for CD44 variable exon 9, with CD44v8-10 among the variant isoforms expressed. The expression of CD44 variable exon 9 was found to be statistically associated with NF2 inactivation, a common occurrence in MM. Knockdown of CD44 reduced the protein level of xCT, a cystine transporter, and increased oxidative stress. However, an increase in GSH was also observed and was associated with enhanced chemoresistance in CD44-knockdown cells. Increased GSH was mediated by the Nrf2/AP-1-induced upregulation of GCLC, a subunit of the enzyme catalyzing GSH synthesis. Our results thus suggest that the response to CD44 depletion is cell type-dependent and, in cases such as MM cells, compensatory pathway(s) might be activated rheostatically to account for the loss of CD44 and counteract enhanced oxidative stress.

CD44 variant 9 expression as a predictor for gastric cancer recurrence: immunohistochemical and metabolomic analysis of surgically resected tissues

CD44 variant 9 (CD44v9) and the heavy chain of 4F2 cell-surface antigen (CD98hc) appear important for regulation of reactive oxygen species defence and tumor growth in gastric cancer. This study examined the roles of CD44v9 and CD98hc as markers of gastric cancer recurrence, and investigated associations with energy metabolism. We applied capillary electrophoresis time-of-flight mass spectrometry to metabolome profiling of gastric cancer specimens from 103 patients who underwent resection with no residual tumor or microscopic residual tumor, and compared metabolite levels to immunohistochemical staining for CD44v9 and CD98hc. Positive expression rates were 40.7% for CD44v9 and 42.7% for CD98hc. Various tumor characteristics were significantly associated with CD44v9 expression. Five-year recurrence-free survival rate was significantly lower for CD44v9-positive tumors (39.1%) than for CD44v9-negative tumors (73.5%; P < 0.0001), but no significant differences in recurrence-free survival were seen according to CD98hc expression. Uni- and multivariate analyses identified positive CD44v9 expression as an independent predictor of poorer recurrence-free survival. Metabolome analysis of 110 metabolites found that levels of glutathione disulfide were significantly lower and reduced glutathione (GSH)/ glutathione disulfide (GSSG) ratio was significantly higher in CD44v9-positive tumors than in CD44v9-negative tumors, suggesting that CD44v9 may enhance pentose phosphate pathway flux and maintain GSH levels in cancer cells.

Therapeutic strategies of drug repositioning targeting autophagy to induce cancer cell death: from pathophysiology to treatment

The 2016 Nobel Prize in Physiology or Medicine was awarded to the researcher that discovered autophagy, which is an evolutionally conserved catabolic process which degrades cytoplasmic constituents and organelles in the lysosome. Autophagy plays a crucial role in both normal tissue homeostasis and tumor development and is necessary for cancer cells to adapt efficiently to an unfavorable tumor microenvironment characterized by hypo-nutrient conditions. This protein degradation process leads to amino acid recycling, which provides sufficient amino acid substrates for cellular survival and proliferation. Autophagy is constitutively activated in cancer cells due to the deregulation of PI3K/Akt/mTOR signaling pathway, which enables them to adapt to hypo-nutrient microenvironment and exhibit the robust proliferation at the pre-metastatic niche. That is why just the activation of autophagy with mTOR inhibitor often fails in vain. In contrast, disturbance of autophagy–lysosome flux leads to endoplasmic reticulum (ER) stress and an unfolded protein response (UPR), which finally leads to increased apoptotic cell death in the tumor tissue. Accumulating evidence suggests that autophagy has a close relationship with programmed cell death, while uncontrolled autophagy itself often induces autophagic cell death in tumor cells. Autophagic cell death was originally defined as cell death accompanied by large-scale autophagic vacuolization of the cytoplasm. However, autophagy is a “double-edged sword” for cancer cells as it can either promote or suppress the survival and proliferation in the tumor microenvironment. Furthermore, several studies of drug re-positioning suggest that “conventional” agents used to treat diseases other than cancer can have antitumor therapeutic effects by activating/suppressing autophagy. Because of ever increasing failure rates and high cost associated with anticancer drug development, this therapeutic development strategy has attracted increasing attention because the safety profiles of these medicines are well known. Antimalarial agents such as artemisinin and disease-modifying antirheumatic drug (DMARD) are the typical examples of drug re-positioning which affect the autophagy regulation for the therapeutic use. This review article focuses on recent advances in some of the novel therapeutic strategies that target autophagy with a view to treating/preventing malignant neoplasms.

Dose-escalation study for the targeting of CD44v+ cancer stem cells by sulfasalazine in patients with advanced gastric cancer (EPOC1205)

BACKGROUND

Cancer stem cells (CSCs) have enhanced mechanisms of protection from oxidative stress. A variant form of CD44 (CD44v), a major CSC marker, was shown to interact with xCT, a subunit of cystine-glutamate transporter, which maintains high levels of intracellular reduced glutathione (GSH) which defend the cell against oxidative stress. Sulfasalazine (SSZ) is an inhibitor of xCT and was shown to suppress the survival of CD44v-positive stem-like cancer cells both in vitro and in vivo. To find the dose of SSZ which can safely reduce the population of CD44v-positive cells in tumors, a dose-escalation study in patients with advanced gastric cancer was conducted.

METHODS

SSZ was given four times daily by oral administration with 2 weeks as one cycle. Tumor biopsies were obtained before and after 14 days of administration of SSZ to evaluate expression of CD44v and the intratumoral level of GSH.

RESULTS

Eleven patients were enrolled and received a dosage from 8 to 12 g/day. Safety was confirmed up to a dosage of 12 g/day, which was considered the maximum tolerated dose. Among the eight patients with CD44v-positive cells in their pretreatment biopsy samples, the CD44v-positive cancer cell population appeared to be reduced in the posttreatment biopsy tissues of four patients. Intratumoral GSH levels were also decreased in two patients, suggesting biological effectiveness of SSZ at 8 g/day or greater.

CONCLUSIONS

This is the first study of SSZ as an xCT inhibitor for targeting CSCs. Reduction of the levels of CD44v-positive cells and GSH was observed in some patients, consistent with the mode of action of SSZ in CSCs.

Regulation of stem-like cancer cells by glutamine through β-catenin pathway mediated by redox signaling

Background

Cancer stem cells (CSCs) are thought to play an important role in tumor recurrence and drug resistance, and present a major challenge in cancer therapy. The tumor microenvironment such as growth factors, nutrients and oxygen affect CSC generation and proliferation by providing the necessary energy sources and growth signals. The side population (SP) analysis has been used to detect the stem-like cancer cell populations based on their high expression of ABCG2 that exports Hoechst-33342 and certain cytotoxic drugs from the cells. The purpose of this research is to investigate the effect of a main nutrient molecule, glutamine, on SP cells and the possible underlying mechanism(s).

Methods

Biochemical assays and flow cytometric analysis were used to evaluate the effect of glutamine on stem-like side population cells in vitro. Molecular analyses including RNAi interfering, qRT-PCR, and immunoblotting were employed to investigate the molecular signaling in response to glutamine deprivation and its influence on tumor formation capacity in vivo.

Results

We show that glutamine supports the maintenance of the stem cell phenotype by promoting glutathione synthesis and thus maintaining redox balance for SP cells. A deprivation of glutamine in the culture medium significantly reduced the proportion of SP cells. L-asparaginase, an enzyme that catalyzes the hydrolysis of asparagine and glutamine to aspartic acid and glutamate, respectively, mimics the effect of glutamine withdrawal and also diminished the proportion of SP cells. Mechanistically, glutamine deprivation increases intracellular ROS levels, leading to down-regulation of the β-catenin pathway.

Conclusion

Glutamine plays a significant role in maintaining the stemness of cancer cells by a redox-mediated mechanism mediated by β-catenin. Inhibition of glutamine metabolism or deprivation of glutamine by L-asparaginase may be a new strategy to eliminate CSCs and overcome drug resistance.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-017-0623-x) contains supplementary material, which is available to authorized users.

Compounds From Celastraceae Targeting Cancer Pathways and Their Potential Application in Head and Neck Squamous Cell Carcinoma: A Review

Squamous cell carcinoma of the head and neck is one of the most common cancer types worldwide. It initiates on the epithelial lining of the upper aerodigestive tract, at most instances as a consequence of tobacco and alcohol consumption. Treatment options based on conventional therapies or targeted therapies under development have limited efficacy due to multiple genetic alterations typically found in this cancer type. Natural products derived from plants often possess biological activities that may be valuable in the development of new therapeutic agents for cancer treatment. Several genera from the family Celastraceae have been studied in this context. This review reports studies on chemical constituents isolated from species from the Celastraceae family targeting cancer mechanisms studied to date. These results are then correlated with molecular characteristics of head and neck squamous cell carcinoma in an attempt to identify constituents with potential application in the treatment of this complex disease at the molecular level.

18F-5-Fluoroaminosuberic Acid as a Potential Tracer to Gauge Oxidative Stress in Breast Cancer Models

The cystine transporter (system x_C^-) is an antiporter of cystine and glutamate. It has relatively low basal expression in most tissues and becomes upregulated in cells under oxidative stress (OS) as one of the genes expressed in response to the antioxidant response element promoter. We have developed ¹⁸F-5-fluoroaminosuberic acid (FASu), a PET tracer that targets system x_C^- The goal of this study was to evaluate ¹⁸F-FASu as a specific gauge for system x_C^- activity in vivo and its potential for breast cancer imaging. Methods:¹⁸F-FASu specificity toward system x_C^- was studied by cell inhibition assay, cellular uptake after OS induction with diethyl maleate, with and without anti-xCT small interfering RNA knockdown, in vitro uptake studies, and in vivo uptake in a system x_C^--transduced xenograft model. In addition, radiotracer uptake was evaluated in 3 breast cancer models: MDA-MB-231, MCF-7, and ZR-75-1. Results: Reactive oxygen species-inducing diethyl maleate increased glutathione levels and ¹⁸F-FASu uptake, whereas gene knockdown with anti-xCT small interfering RNA led to decreased tracer uptake. ¹⁸F-FASu uptake was robustly inhibited by system x_C^- inhibitors or substrates, whereas uptake was significantly higher in transduced cells and tumors expressing xCT than in wild-type HEK293T cells and tumors (P < 0.0001 for cells, P = 0.0086 for tumors). ¹⁸F-FASu demonstrated tumor uptake in all 3 breast cancer cell lines studied. Among them, triple-negative breast cancer MDA-MB-231, which has the highest xCT messenger RNA level, had the highest tracer uptake (P = 0.0058 when compared with MCF-7; P < 0.0001 when compared with ZR-75-1). Conclusion:¹⁸F-FASu as a system x_C^- substrate is a specific PET tracer for functional monitoring of system x_C^- and OS imaging. By enabling noninvasive analysis of x_C^- responses in vivo, this biomarker may serve as a valuable target for the diagnosis and treatment monitoring of certain breast cancers.

Role of KEAP1/NRF2 and TP53 Mutations in Lung Squamous Cell Carcinoma Development and Radiation Resistance

Lung squamous cell carcinoma (LSCC) pathogenesis remains incompletely understood, and biomarkers predicting treatment response remain lacking. Here, we describe novel murine LSCC models driven by loss of Trp53 and Keap1, both of which are frequently mutated in human LSCCs. Homozygous inactivation of Keap1 or Trp53 promoted airway basal stem cell (ABSC) self-renewal, suggesting that mutations in these genes lead to expansion of mutant stem cell clones. Deletion of Trp53 and Keap1 in ABSCs, but not more differentiated tracheal cells, produced tumors recapitulating histologic and molecular features of human LSCCs, indicating that they represent the likely cell of origin in this model. Deletion of Keap1 promoted tumor aggressiveness, metastasis, and resistance to oxidative stress and radiotherapy (RT). KEAP1/NRF2 mutation status predicted risk of local recurrence after RT in patients with non-small lung cancer (NSCLC) and could be noninvasively identified in circulating tumor DNA. Thus, KEAP1/NRF2 mutations could serve as predictive biomarkers for personalization of therapeutic strategies for NSCLCs.

SIGNIFICANCE

We developed an LSCC mouse model involving Trp53 and Keap1, which are frequently mutated in human LSCCs. In this model, ABSCs are the cell of origin of these tumors. KEAP1/NRF2 mutations increase radioresistance and predict local tumor recurrence in radiotherapy patients. Our findings are of potential clinical relevance and could lead to personalized treatment strategies for tumors with KEAP1/NRF2 mutations. Cancer Discov; 7(1); 86-101. ©2016 AACR.This article is highlighted in the In This Issue feature, p. 1.

Sulfasalazine, a therapeutic agent for ulcerative colitis, inhibits the growth of CD44v9(+) cancer stem cells in ulcerative colitis-related cancer

BACKGROUND AND OBJECTIVE

Sulfasalazine reduces the risk of ulcerative colitis (UC)-related cancer through its anti-inflammatory effect and induction of oxidative stress in cancer cells by inhibiting the glutamate-cystine transporter, which is closely associated with the cancer stem cell surface marker CD44v9. This study aimed to quantify the effects of sulfasalazine on CD44v9 expression and pathological factors in colorectal cancers (CRCs) arising from UC.

METHODS

Twenty-six patients with UC-related cancer were classified into groups according to the length of sulfasalazine treatment as follows: (1) long-term (LT) (≥ 5 years) and (2) short-term (ST) (< 5 years). Using immunohistochemistry, we compared CD44v9 and Ki-67 expression and pathological characteristics of each group's tumors. In vitro assay was performed to investigate the effect of sulfasalazine on epithelial differentiation and proliferation of CD44(+) cancer cells.

RESULTS

Immunohistochemical analysis revealed that CD44v9 expression tended to be lower in the LT group (LT:ST=15.4%:46.2%, P=0.20), and Ki-67/CD44v9 double-stained cells were significantly lower in the LT group (LT:ST=0%:6.9%, P=0.01). Pathologically, the frequency of well-differentiated adenocarcinomas was higher in the LT group (LT:ST=84.6%:38.5%, P=0.04). In vitro assay revealed that sulfasalazine promoted the expression of epithelial differentiation markers (E-cadherin and CDX2) and inhibited the proliferation of CD44(+) cancer cells.

CONCLUSIONS

Long-term sulfasalazine administration reduced proliferative CD44v9(+) cells and increased the degree of differentiation of adenocarcinomas. These findings indicate the importance of CD44v9(+) cells in UC-related cancer progression and suggest that sulfasalazine may serve as a novel therapeutic agent that targets CD44v9(+) cells.

Clinical significance of CD44 variant 9 expression as a prognostic indicator in bladder cancer

CD44, a major surface receptor for hyaluronic acid, has multiple isoforms and represents a major cancer stem cell marker for various epithelial tumors. CD44 variant 9 (CD44v9) was correlated with recurrence and metastasis in gastric and colon cancer. We examined its role in invasion and as a biomarker for the basal muscle invasive molecular subtype showing worse prognosis, and for tumor progression in high risk (pT1/high grade) non‑muscle invasive bladder cancers (NMIBCs). CD44v9, cytokeratin 5/6 (CK5/6), and cytokeratin 20 (CK20) expression was evaluated by immunohistochemistry in 98 pathologically confirmed specimens (36 muscle and 62 high‑risk non‑muscle) and correlated to clinical outcome. In vitro analysis was performed using two human bladder cancer cell lines (HT1376 and 5637). The CD44v9 high‑expressing group exhibited significantly lower progression‑free and cancer‑specific survival rates in both muscle (P=0.0349 and 0.0382, respectively) and non‑muscle (P=0.0002 and 0.0079) invasive patients. CD44v9 expression was significantly correlated with CK5/6 (P<0.001), colocalizing at the muscle invasion front but distinctly separated from CK20 in non‑muscle invasion. CD44 and CD44v9 siRNA knockdown demonstrated significantly lower Matrigel invasion ability and significantly shorter migration distance (all P<0.01). CD44 and CD44v9 knockdown increased E‑cadherin and decreased N‑cadherin, snail, and slug epithelial‑mesenchymal transition marker protein expression. Thus, higher CD44v9 expression was associated with worse prognosis, likely impacting invasion and migration via the epithelial‑mesenchymal transition. Together, these findings suggest that CD44v9 expression might be a useful predictive biomarker in basal‑type muscle and high-risk NMIBC.

Induction of ferroptotic cell death for overcoming cisplatin resistance of head and neck cancer

Inhibition of key molecules related to ferroptosis, cystine/glutamate antiporter and glutathione peroxidase, may induce eradication of chemotherapy/radiotherapy-resistant cancer cells. The present study investigated whether ferroptosis could overcome head and neck cancer (HNC) resistance to cisplatin treatment. Three cisplatin-resistant HNC cell lines (AMC-HN3R, -HN4R, and -HN9R) and their parental lines were used. The effects of cystine and glutamate alteration and pharmacological and genetic inhibition of cystine/glutamate antiporter were assessed by measuring viability, death, reactive oxygen species production, protein expression, and preclinical mouse tumor xenograft models. Conditioned media with no cystine or glutamine excess induced ferroptosis of both cisplatin-sensitive and -resistant HNC cells without any apparent changes to necrosis and apoptosis markers. The cystine/glutamate antiporter inhibitors erastin and sulfasalazine inhibited HNC cell growth and accumulated lipid reactive oxygen species, thereby inducing ferroptosis. Genetic silencing of cystine/glutamate antiporter with siRNA or shRNA treatment also induced effective ferroptotic cell death of resistant HNC cells and enhanced the cisplatin cytotoxicity of resistant HNC cells. Pharmacological and genetic inhibition of cystine/glutamate antiporter significantly sensitized resistant HNC cells to cisplatin in vitro and in vivo. Pharmacological and genetic inhibition of cystine/glutamate antiporter overcomes the cisplatin resistance of HNC cells by inducing ferroptosis.

Neurodegeneration and the Brain Tumor Microenvironment. [corrected]

Malignant brain tumors are characterized by destructive growth and neuronal cell death making them one of the most devastating diseases. Neurodegenerative actions of malignant gliomas resemble mechanisms also found in many neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and amyotrophic lateral sclerosis. Recent data demonstrate that gliomas seize neuronal glutamate signaling for their own growth advantage. Excessive glutamate release via the glutamate/cystine antiporter xCT (system xc-, SLC7a11) renders cancer cells resistant to chemotherapeutics and create the tumor microenvironment toxic for neurons. In particular the glutamate/cystine antiporter xCT takes center stage in neurodegenerative processes and sets this transporter a potential prime target for cancer therapy. Noteworthy is the finding, that reactive oxygen species (ROS) activate transient receptor potential (TRP) channels and thereby TRP channels can potentiate glutamate release. Yet another important biological feature of the xCT/glutamate system is its modulatory effect on the tumor microenvironment with impact on host cells and the cancer stem cell niche. The EMA and FDA-approved drug sulfasalazine (SAS) presents a lead compound for xCT inhibition, although so far clinical trials on glioblastomas with SAS were ambiguous. Here, we critically analyze the mechanisms of action of xCT antiporter on malignant gliomas and in the tumor microenvironment. Deciphering the impact of xCT and glutamate and its relation to TRP channels in brain tumors pave the way for developing important cancer microenvironmental modulators and drugable lead targets.

CD44 variant-dependent redox status regulation in liver fluke-associated cholangiocarcinoma: A target for cholangiocarcinoma treatment

Expression of CD44, especially the variant isoforms (CD44v) of this major cancer stem cell marker, contributes to reactive oxygen species (ROS) defense through stabilizing xCT (a cystine–glutamate transporter) and promoting glutathione synthesis. This enhances cancer development and increases chemotherapy resistance. We investigate the role of CD44v in the regulation of the ROS defense system in cholangiocarcinoma (CCA). Immunohistochemical staining of CD44v and p38^MAPK (a major ROS target) expression in Opisthorchis viverrini‐induced hamster CCA tissues (at 60, 90, 120, and 180 days) reveals a decreased phospho‐p38^MAPK signal, whereas the CD44v signal was increased during bile duct transformation. Patients with CCA showed CD44v overexpression and negative‐phospho‐p38^MAPK patients a significantly shorter survival rate than the low CD44v signal and positive‐phospho‐p38^MAPK patients (P = 0.030). Knockdown of CD44 showed that xCT and glutathione levels were decreased, leading to a high level of ROS. We examined xCT‐targeted CD44v cancer stem cell therapy using sulfasalazine. Glutathione decreased and ROS increased after the treatment, leading to inhibition of cell proliferation and induction of cell death. Thus, the accumulation of CD44v leads to the suppression of p38^MAPK in transforming bile duct cells. The redox status regulation of CCA cells depends on the expression of CD44v to contribute the xCT function and is a link to the poor prognosis of patients. Thus, an xCT inhibitor could inhibit cell growth and activate cell death. This suggests that an xCT‐targeting drug may improve CCA therapy by sensitization to the available drug (e.g. gemcitabine) by blocking the mechanism of the cell's ROS defensive system.

Variant isoforms of CD44 expression in upper tract urothelial cancer as a predictive marker for recurrence and mortality

PURPOSE

The variant isoforms of CD44 (CD44v), which has been associated with treatment resistance and tumor recurrence, are contributed to the feature of some of the new cell surface markers for cancer stem cells. The aim of this study is to address the prognostic significant of CD44v expression in upper tract urothelial cancer (UTUC).

MATERIALS AND METHODS

We retrospectively analyzed the medical records of 110 patients treated surgically for≥pT2 UTUC. To determine the biological significance of CD44v in UTUC, we examined the immunohistochemical expression of CD44v9 and its association with clinical features and oncological outcomes.

RESULTS

During the mean follow-up of 4.8 years, tumor recurrence, including local and distant metastasis, was observed in 57 patients (51.8%), and 42 patients (38.2%) died of the disease. The incidence of ureter cancers was significantly higher in patients expressing CD44v9. In this series, 37 patients in the CD44v9-positive group (45.1%) and 5 (17.9%) in the CD44v9-negative group died of the disease. Kaplan-Meier curves revealed that cancer-specific survival and recurrence-free survival rates were significantly lower in the CD44v9-positive group than in the CD44v9-negative group (P = 0.032 and P = 0.038, respectively). A multivariate analysis identified the expression of CD44v9 as one of the independent risk factors for cancer-specific survival (P = 0.040, hazard ratio = 2.67) in addition to tumor grade G3, and also for recurrence-free survival (P = 0.028, hazard ratio = 2.33), in addition to tumor grade G3 and lymphovascular invasion.

CONCLUSIONS

The expression of CD44v9 may be a new biomarker of malignant potential in muscle invasive UTUC and provide additional prognostic information in patients with UTUC.

The Importance of CD44 as a Stem Cell Biomarker and Therapeutic Target in Cancer

CD44 is a cell surface HA-binding glycoprotein that is overexpressed to some extent by almost all tumors of epithelial origin and plays an important role in tumor initiation and metastasis. CD44 is a compelling marker for cancer stem cells of many solid malignancies. In addition, interaction of HA and CD44 promotes EGFR-mediated pathways, consequently leading to tumor cell growth, tumor cell migration, and chemotherapy resistance in solid cancers. Accumulating evidence indicates that major HA-CD44 signaling pathways involve a specific variant of CD44 isoforms; however, the particular variant almost certainly depends on the type of tumor cell and the stage of the cancer progression. Research to date suggests use of monoclonal antibodies against different CD44 variant isoforms and targeted inhibition of HA/CD44-mediated signaling combined with conventional radio/chemotherapy may be the most favorable therapeutic strategy for future treatments of advanced stage malignancies. Thus, this paper briefly focuses on the association of the major CD44 variant isoforms in cancer progression, the role of HA-CD44 interaction in oncogenic pathways, and strategies to target CD44-overexpressed tumor cells.

Eradication of CD44-variant positive population in head and neck tumors through controlled intracellular navigation of cisplatin-loaded nanomedicines

Eventual relapse of tumor growth is commonly observed in head and neck cancer patients, following treatment with platinum-based chemotherapies. This occurrence is believed to be related to the failure to eradicate drug resistant, cancer stem cell (CSC) niches, thereby enriching their population in tumors after treatment. In this study, we show that in contrast to free cisplatin (CDDP), the polymer micelle-based nanomedicine incorporating cisplatin (CDDP/m), can eradicate both the undifferentiated cell and the differentiated cancer cell populations within a head and neck tumor model. Immunohistochemistry of treated tumors showed that opposing to CDDP treatment, CDDP/m could reduce tumor growth without concentrating the CSC-like population. We further showed that CDDP/m, but not CDDP, can localize into hypoxic regions, possibly CSC-rich areas, in the tumors, and can overcome their detoxification mechanism based-on high cellular expression of glutathione to successfully deliver Pt to nuclear DNA. Our data suggests CDDP/m to be a replacement for current platinum therapies, for its ability to eradicate both bulk and CSC-like populations, and in turn to prevent recurrence of tumor growth.

CD44 variant 9 is a potential biomarker of tumor initiating cells predicting survival outcome in hepatitis C virus-positive patients with resected hepatocellular carcinoma

This study investigated whether the expression of CD44 variant 9 (CD44v9) might be a functional marker of tumor‐initiating stem‐like cells in primary hepatocellular carcinomas (HCCs) of hepatitis C virus (HCV)⁺ patients and provide an indicator of patient survival, as well as associated mechanisms. A total of 90 HCV⁺HCC patients who underwent surgery from 2006 to 2011 were enrolled and monitored for 2–8 years. Expression of CD44v9 was validated immunohistochemically in all HCCs, followed by comparative proteome, survival, and clinicopathological analyses. CD44 variant 8–‐10 was further evaluated in diethylnitrosamine‐induced HCCs of C57Bl/6J mice. Focally localized CD44v⁺ cells with a membranous staining pattern were detected in human HCV⁺ and mouse HCCs. CD44v9⁺ cells of HCCs were predominantly negative for Ki67 and P‐p38, indicating decrease of cell proliferation in the CD44v9⁺ tumor cell population, likely to be related to suppression of intracellular oxidative stress due to activation of Nrf2‐mediated signaling, DNA repair, and inhibition of xenobiotic metabolism. CD44v9 IHC evaluation in 90 HCV⁺HCC cases revealed that positive expression was significantly associated with poor overall and recurrence‐free survival, a younger age, poor histological differentiation of HCCs, and high alkaline phosphatase levels compared with patients with negative expression. CD44v9 is concluded to be a potential biomarker of tumor‐initiating stem‐like cells and a prognostic marker in HCV⁺HCC patients associated with Nrf2‐mediated resistance to oxidative stress.

The EGF Receptor Promotes the Malignant Potential of Glioma by Regulating Amino Acid Transport System xc(-)

Extracellular free amino acids contribute to the interaction between a tumor and its microenvironment through effects on cellular metabolism and malignant behavior. System xc(-) is composed of xCT and CD98hc subunits and functions as a plasma membrane antiporter for the uptake of extracellular cystine in exchange for intracellular glutamate. Here, we show that the EGFR interacts with xCT and thereby promotes its cell surface expression and function in human glioma cells. EGFR-expressing glioma cells manifested both enhanced antioxidant capacity as a result of increased cystine uptake, as well as increased glutamate, which promotes matrix invasion. Imaging mass spectrometry also revealed that brain tumors formed in mice by human glioma cells stably overexpressing EGFR contained higher levels of reduced glutathione compared with those formed by parental cells. Targeted inhibition of xCT suppressed the EGFR-dependent enhancement of antioxidant capacity in glioma cells, as well as tumor growth and invasiveness. Our findings establish a new functional role for EGFR in promoting the malignant potential of glioma cells through interaction with xCT at the cell surface. Cancer Res; 76(10); 2954-63. ©2016 AACR.

Cystine Deprivation Triggers Programmed Necrosis in VHL-Deficient Renal Cell Carcinomas

Oncogenic transformation may reprogram tumor metabolism and render cancer cells addicted to extracellular nutrients. Deprivation of these nutrients may therefore represent a therapeutic opportunity, but predicting which nutrients cancer cells become addicted remains difficult. Here, we performed a nutrigenetic screen to determine the phenotypes of isogenic pairs of clear cell renal cancer cells (ccRCC), with or without VHL, upon the deprivation of individual amino acids. We found that cystine deprivation triggered rapid programmed necrosis in VHL-deficient cell lines and primary ccRCC tumor cells, but not in VHL-restored counterparts. Blocking cystine uptake significantly delayed xenograft growth of ccRCC. Importantly, cystine deprivation triggered similar metabolic changes regardless of VHL status, suggesting that metabolic responses alone are not sufficient to explain the observed distinct fates of VHL-deficient and restored cells. Instead, we found that increased levels of TNFα associated with VHL loss forced VHL-deficient cells to rely on intact RIPK1 to inhibit apoptosis. However, the preexisting elevation in TNFα expression rendered VHL-deficient cells susceptible to necrosis triggered by cystine deprivation. We further determined that reciprocal amplification of the Src-p38 (MAPK14)-Noxa (PMAIP1) signaling and TNFα-RIP1/3 (RIPK1/RIPK3)-MLKL necrosis pathways potentiated cystine-deprived necrosis. Together, our findings reveal that cystine deprivation in VHL-deficient RCCs presents an attractive therapeutic opportunity that may bypass the apoptosis-evading mechanisms characteristic of drug-resistant tumor cells. Cancer Res; 76(7); 1892-903. ©2016 AACR.

Cancer stem cells in hepatocellular carcinoma: Therapeutic implications based on stem cell biology

Hepatocellular carcinoma (HCC) is the sixth most common cancer and the third most frequent cause of cancer-related death worldwide. Despite advances in its diagnosis and treatment, the prognosis of patients with advanced HCC remains unfavorable. Recent advances in stem cell biology and associated technologies have enabled the identification of minor components of tumorigenic cells, termed cancer stem cells (CSC) or tumor-initiating cells, in cancers such as HCC. Furthermore, because CSC play a central role in tumor development, metastasis and recurrence, they are considered to be a therapeutic target in cancer treatment. Hepatic CSC have been successfully identified using functional and cell surface markers. The analysis of purified hepatic CSC has revealed the molecular machinery and signaling pathways involved in their maintenance. In addition, epigenetic transcriptional regulation has been shown to be important in the development and maintenance of CSC. Although inhibitors of CSC show promise as CSC-targeting drugs, novel therapeutic approaches for the eradication of CSC are yet to be established. In this review, we describe recent progress in hepatic CSC research and provide a perspective on the available therapeutic approaches based on stem cell biology.

Targeting CD44, ABCG2 and CD133 markers using aptamers: in silico analysis of CD133 extracellular domain 2 and its aptamer

Truncated CSC marker aptamers penetrate tumor spheres and inhibits cell proliferation; a bioinformatics approach to decipher their structural interactions.

Redox Modulating NRF2: A Potential Mediator of Cancer Stem Cell Resistance

Tumors contain a distinct small subpopulation of cells that possess stem cell-like characteristics. These cells have been called cancer stem cells (CSCs) and are thought to be responsible for anticancer drug resistance and tumor relapse after therapy. Emerging evidence indicates that CSCs share many properties, such as self-renewal and quiescence, with normal stem cells. In particular, CSCs and normal stem cells retain low levels of reactive oxygen species (ROS), which can contribute to stem cell maintenance and resistance to stressful tumor environments. Current literatures demonstrate that the activation of ataxia telangiectasia mutated (ATM) and forkhead box O3 (FoxO3) is associated with the maintenance of low ROS levels in normal stem cells such as hematopoietic stem cells. However, the importance of ROS signaling in CSC biology remains poorly understood. Recent studies demonstrate that nuclear factor-erythroid 2-related factor 2 (NRF2), a master regulator of the cellular antioxidant defense system, is involved in the maintenance of quiescence, survival, and stress resistance of CSCs. Here, we review the recent findings on the roles of NRF2 in maintenance of the redox state and multidrug resistance in CSCs, focusing on how NRF2-mediated ROS modulation influences the growth and resistance of CSCs.

Immunotargeting of Antigen xCT Attenuates Stem-like Cell Behavior and Metastatic Progression in Breast Cancer

Resistance to therapy and lack of curative treatments for metastatic breast cancer suggest that current therapies may be missing the subpopulation of chemoresistant and radioresistant cancer stem cells (CSC). The ultimate success of any treatment may well rest on CSC eradication, but specific anti-CSC therapies are still limited. A comparison of the transcriptional profiles of murine Her2(+) breast tumor TUBO cells and their derived CSC-enriched tumorspheres has identified xCT, the functional subunit of the cystine/glutamate antiporter system xc(-), as a surface protein that is upregulated specifically in tumorspheres. We validated this finding by cytofluorimetric analysis and immunofluorescence in TUBO-derived tumorspheres and in a panel of mouse and human triple negative breast cancer cell-derived tumorspheres. We further show that downregulation of xCT impaired tumorsphere generation and altered CSC intracellular redox balance in vitro, suggesting that xCT plays a functional role in CSC biology. DNA vaccination based immunotargeting of xCT in mice challenged with syngeneic tumorsphere-derived cells delayed established subcutaneous tumor growth and strongly impaired pulmonary metastasis formation by generating anti-xCT antibodies able to alter CSC self-renewal and redox balance. Finally, anti-xCT vaccination increased CSC chemosensitivity to doxorubicin in vivo, indicating that xCT immunotargeting may be an effective adjuvant to chemotherapy.

Snail-induced EMT promotes cancer stem cell-like properties in head and neck cancer cells

Epithelial-mesenchymal transition (EMT) is a key process involved in the invasion and metastasis of cancer cells. Furthermore, EMT can induce a cancer stem cell (CSC)-like phenotype in a number of tumor types. We demonstrated that Snail is one of the master regulators that promotes EMT and mediates cancer cell migration and invasion in many types of malignancies including head and neck squamous cell carcinoma (HNSCC). In the present study, we investigated the role of Snail in inducing and maintaining CSC-like properties through EMT in HNSCC. We established HNSCC cell lines transfected with Snail. Stem cell markers were evaluated with real-time RT-PCR and western blot analysis. CSC properties were assessed using sphere formation and WST-8 assays as well as chemosensitivity and chick chorioallantoic membrane in vivo invasion assays. Introduction of Snail induced EMT properties in HNSCC cells. Moreover, Snail-induced EMT maintained the CSC-like phenotype, and enhanced sphere formation capability, chemoresistance and invasive ability. These data suggest that Snail could be one of the critical molecular targets for the development of therapeutic strategies for HNSCC.

Therapeutic strategies targeting cancer stem cells

Cancer stem cells (CSCs) are undifferentiated cancer cells with a high tumorigenic activity, the ability to undergo self-renewal, and a multilineage differentiation potential. Cancer stem cells are responsible for the development of tumor cell heterogeneity, a key feature for resistance to anticancer treatments including conventional chemotherapy, radiation therapy, and molecularly targeted therapy. Furthermore, minimal residual disease, the major cause of cancer recurrence and metastasis, is enriched in CSCs. Cancer stem cells also possess the property of "robustness", which encompasses several characteristics including a slow cell cycle, the ability to detoxify or mediate the efflux of cytotoxic agents, resistance to oxidative stress, and a rapid response to DNA damage, all of which contribute to the development of therapeutic resistance. The identification of mechanisms underlying such characteristics and the development of novel approaches to target them will be required for the therapeutic elimination of CSCs and the complete eradication of tumors. In this review, we focus on two prospective therapeutic approaches that target CSCs with the aim of disrupting their quiescence or redox defense capability.

Metabolic reprogramming: the emerging concept and associated therapeutic strategies

Tumor tissue is composed of cancer cells and surrounding stromal cells with diverse genetic/epigenetic backgrounds, a situation known as intra-tumoral heterogeneity. Cancer cells are surrounded by a totally different microenvironment than that of normal cells; consequently, tumor cells must exhibit rapidly adaptive responses to hypoxia and hypo-nutrient conditions. This phenomenon of changes of tumor cellular bioenergetics, called “metabolic reprogramming”, has been recognized as one of 10 hallmarks of cancer. Metabolic reprogramming is required for both malignant transformation and tumor development, including invasion and metastasis. Although the Warburg effect has been widely accepted as a common feature of metabolic reprogramming, accumulating evidence has revealed that tumor cells depend on mitochondrial metabolism as well as aerobic glycolysis. Remarkably, cancer-associated fibroblasts in tumor stroma tend to activate both glycolysis and autophagy in contrast to neighboring cancer cells, which leads to a reverse Warburg effect. Heterogeneity of monocarboxylate transporter expression reflects cellular metabolic heterogeneity with respect to the production and uptake of lactate. In tumor tissue, metabolic heterogeneity induces metabolic symbiosis, which is responsible for adaptation to drastic changes in the nutrient microenvironment resulting from chemotherapy. In addition, metabolic heterogeneity is responsible for the failure to induce the same therapeutic effect against cancer cells as a whole. In particular, cancer stem cells exhibit several biological features responsible for resistance to conventional anti-tumor therapies. Consequently, cancer stem cells tend to form minimal residual disease after chemotherapy and exhibit metastatic potential with additional metabolic reprogramming. This type of altered metabolic reprogramming leads to adaptive/acquired resistance to anti-tumor therapy. Collectively, complex and dynamic metabolic reprogramming should be regarded as a reflection of the “robustness” of tumor cells against unfavorable conditions. This review focuses on the concept of metabolic reprogramming in heterogeneous tumor tissue, and further emphasizes the importance of developing novel therapeutic strategies based on drug repositioning.

CD44 variant 6 is correlated with peritoneal dissemination and poor prognosis in patients with advanced epithelial ovarian cancer

Cancer stem cells (CSCs) drive tumor initiation and metastasis in several types of human cancer. However, the contribution of ovarian CSCs to peritoneal metastasis remains unresolved. The cell adhesion molecule CD44 has been identified as a major marker for CSCs in solid tumors, including epithelial ovarian cancer. CD44 exists as a standard form (CD44s) and also as numerous variant isoforms (CD44v) generated by alternative mRNA splicing. Here we show that disseminated ovarian tumors in the pelvic peritoneum contain highly enriched CD44v6-positive cancer cells, which drive tumor metastasis and are responsible for tumor resistance to chemotherapy. Clinically, an increased number of CD44v6-positive cancer cells in primary tumors was associated with a shortened overall survival in stage III–IV ovarian cancer patients. Furthermore, a subpopulation of CD44v6-positive cancer cells manifested the ability to initiate tumor metastasis in the pelvic peritoneum in an in vivo mouse model, suggesting that CD44v6-positive cells show the potential to serve as metastasis-initiating cells. Thus, the peritoneal disseminated metastasis of epithelial ovarian cancer is initiated by the CD44v6-positive subpopulation, and CD44v6 expression is a biomarker for the clinical outcome of advanced ovarian cancer patients. Given that a distinct subpopulation of CD44v6-positive cancer cells plays a critical role in peritoneal metastasis, definitive treatment should target this subpopulation of CD44v6-positive cells in epithelial ovarian cancer.

Concise Review: Emerging Role of CD44 in Cancer Stem Cells: A Promising Biomarker and Therapeutic Target

The reception and integration of the plethora of signals a cell receives from its microenvironment determines the cell's fate. CD44 functions as a receptor for hyaluronan and many other extracellular matrix components, as well as a cofactor for growth factors and cytokines, and thus, CD44 is a signaling platform that integrates cellular microenvironmental cues with growth factor and cytokine signals and transduces signals to membrane-associated cytoskeletal proteins or to the nucleus to regulate a variety of gene expression levels related to cell-matrix adhesion, cell migration, proliferation, differentiation, and survival. Accumulating evidence indicates that CD44, especially CD44v isoforms, are cancer stem cell (CSC) markers and critical players in regulating the properties of CSCs, including self-renewal, tumor initiation, metastasis, and chemoradioresistance. Furthermore, there is ample evidence that CD44, especially CD44v isoforms, are valuable prognostic markers in various types of tumors. Therefore, therapies that target CD44 may destroy the CSC population, and this holds great promise for the cure of life-threatening cancers. However, many challenges remain to determining how best to use CD44 as a biomarker and therapeutic target. Here we summarize the current findings concerning the critical role of CD44/CD44v in the regulation of cancer stemness and the research status of CD44/CD44v as biomarkers and therapeutic targets in cancer. We also discuss the current challenges and future directions that may lead to the best use of CD44/CD44v for clinical applications.

SIGNIFICANCE

Mounting evidence indicates that cancer stem cells (CSCs) are mainly responsible for cancer aggressiveness, drug resistance, and tumor relapse. CD44, especially CD44v isoforms, have been identified as CSC surface markers for isolating and enriching CSCs in different types of cancers. The current findings concerning the critical role of CD44/CD44v in regulation of cancer stemness and the research status of CD44/CD44v as biomarkers and therapeutic targets in cancer are summarized. The current challenges and future directions that may lead to best use of CD44/CD44v for clinical applications are also discussed.

Opportunities and challenges of radiotherapy for treating cancer

The past 20 years have seen dramatic changes in the delivery of radiation therapy, but the impact of radiobiology on the clinic has been far less substantial. A major consideration in the use of radiotherapy has been on how best to exploit differences between the tumour and host tissue characteristics, which in the past has been achieved empirically by radiation-dose fractionation. New advances are uncovering some of the mechanistic processes that underlie this success story. In this Review, we focus on how these processes might be targeted to improve the outcome of radiotherapy at the individual patient level. This approach would seem a more productive avenue of treatment than simply trying to increase the radiation dose delivered to the tumour.

The Antitumor Activity of Plant-Derived Non-Psychoactive Cannabinoids

As a therapeutic agent, most people are familiar with the palliative effects of the primary psychoactive constituent of Cannabis sativa (CS), Δ(9)-tetrahydrocannabinol (THC), a molecule active at both the cannabinoid 1 (CB1) and cannabinoid 2 (CB2) receptor subtypes. Through the activation primarily of CB1 receptors in the central nervous system, THC can reduce nausea, emesis and pain in cancer patients undergoing chemotherapy. During the last decade, however, several studies have now shown that CB1 and CB2 receptor agonists can act as direct antitumor agents in a variety of aggressive cancers. In addition to THC, there are many other cannabinoids found in CS, and a majority produces little to no psychoactivity due to the inability to activate cannabinoid receptors. For example, the second most abundant cannabinoid in CS is the non-psychoactive cannabidiol (CBD). Using animal models, CBD has been shown to inhibit the progression of many types of cancer including glioblastoma (GBM), breast, lung, prostate and colon cancer. This review will center on mechanisms by which CBD, and other plant-derived cannabinoids inefficient at activating cannabinoid receptors, inhibit tumor cell viability, invasion, metastasis, angiogenesis, and the stem-like potential of cancer cells. We will also discuss the ability of non-psychoactive cannabinoids to induce autophagy and apoptotic-mediated cancer cell death, and enhance the activity of first-line agents commonly used in cancer treatment.

Induction of CD44 variant 9-expressing cancer stem cells might attenuate the efficacy of chemoradioselection and Worsens the prognosis of patients with advanced head and neck cancer

Background

At our institute, a chemoradioselection strategy has been used to select patients for organ preservation on the basis of response to an initial 30–40 Gy concurrent chemoradiotherapy (CCRT). Patients with a favorable response (i.e., chemoradioselected; CRS) have demonstrated better outcomes than those with an unfavorable response (i.e., nonchemoradioselected; N-CRS). Successful targeting of molecules that attenuate the efficacy of chmoradioselection may improve results. Thus, the aim of this study was to evaluate the association of a novel cancer stem cell (CSC) marker, CD44 variant 9 (CD44v9), with cellular refractoriness to chemoradioselection in advanced head and neck squamous cell carcinoma (HNSCC).

Materials and Methods

Through a medical chart search, 102 patients with advanced HNSCC treated with chemoradioselection from 1997 to 2008 were enrolled. According to our algorithm, 30 patients were CRC following induction CCRT and 72 patients were N-CRS. Using the conventional immunohistochemical technique, biopsy specimens and surgically removed tumor specimens were immunostained with the anti-CD44v9 specific antibodies.

Results

The intrinsic expression levels of CD44v9 in the biopsy specimens did not correlate with the chemoradioselection and patient survival. However, in N-CRS patients, the CD44v9-positive group demonstrated significantly (P = 0.008) worse prognosis, than the CD44v9-negative group. Multivariate analyses demonstrated that among four candidate factors (T, N, response to CCRT, and CD44v9), CD44v9 positivity (HR: 3.145, 95% CI: 1.235–8.008, P = 0.0163) was significantly correlated with the poor prognosis, along with advanced N stage (HR: 3.525, 95% CI: 1.054–9.060, P = 0.0228). Furthermore, the survival rate of the CD44v9-induced group was significantly (P = 0.04) worse than the CD44v9-non-induced group.

Conclusions

CCRT-induced CD44v9-expressing CSCs appear to be a major hurdle to chemoradioselection. CD44v9-targeting seems to be a promising strategy to enhance the efficacy of chemoradioselection and consequent organ preservation and survival.

Gamma-glutamyl transpeptidase: redox regulation and drug resistance

The expression of gamma-glutamyl transpeptidase (GGT) is essential to maintaining cysteine levels in the body. GGT is a cell surface enzyme that hydrolyzes the gamma-glutamyl bond of extracellular reduced and oxidized glutathione, initiating their cleavage into glutamate, cysteine (cystine), and glycine. GGT is normally expressed on the apical surface of ducts and glands, salvaging the amino acids from glutathione in the ductal fluids. GGT in tumors is expressed over the entire cell membrane and provides tumors with access to additional cysteine and cystine from reduced and oxidized glutathione in the blood and interstitial fluid. Cysteine is rate-limiting for glutathione synthesis in cells under oxidative stress. The induction of GGT is observed in tumors with elevated levels of intracellular glutathione. Studies in models of hepatocarcinogenesis show that GGT expression in foci of preneoplastic hepatocytes provides a selective advantage to the cells during tumor promotion with agents that deplete intracellular glutathione. Similarly, expression of GGT in tumors enables cells to maintain elevated levels of intracellular glutathione and to rapidly replenish glutathione during treatment with prooxidant anticancer therapy. In the clinic, the expression of GGT in tumors is correlated with drug resistance. The inhibitors of GGT block GGT-positive tumors from accessing the cysteine in extracellular glutathione. They also inhibit GGT activity in the kidney, which results in the excretion of GSH in the urine and a rapid decrease in blood cysteine levels, leading to depletion of intracellular GSH in both GGT-positive and GGT-negative tumors. GGT inhibitors are being developed for clinical use to sensitize tumors to chemotherapy.

Novel treatments for head and neck squamous cell carcinoma: preclinical identification and clinical investigation

Head and neck squamous cell carcinoma (HNSCC) is a common cancer worldwide. Classically, it is a disease related to tobacco and alcohol use; an increasing number of patients are being diagnosed with HNSCC caused by infection with the human papillomavirus. New deep-sequencing techniques have confirmed the importance of p53 and EGF receptor in HNSCC development, and have identified pathways of critical importance, such as PI3K/mTOR and NOTCH. Increasing knowledge of key molecular features has lead to new therapeutic avenues for HNSCC. Novel therapies under investigation in HNSCC include antibody and small molecule inhibitors of EGF receptor and its family members, PI3K inhibitors, antiangiogenic agents, immunotherapies and agents interacting with early developmental pathways such as Hedgehog.

Transcription factor Nrf1 negatively regulates the cystine/glutamate transporter and lipid-metabolizing enzymes

Liver-specific Nrf1 (NF-E2-p45-related factor 1) knockout mice develop nonalcoholic steatohepatitis. To identify postnatal mechanisms responsible for this phenotype, we generated an inducible liver-specific Nrf1 knockout mouse line using animals harboring an Nrf1(flox) allele and a rat CYP1A1-Cre transgene (Nrf1(flox/flox)::CYP1A1-Cre mice). Administration of 3-methylcholanthrene (3-MC) to these mice (Nrf1(flox/flox)::CYP1A1-Cre+3MC mice) resulted in loss of hepatic Nrf1 expression. The livers of mice lacking Nrf1 accumulated lipid, and the hepatic fatty acid (FA) composition in such animals differed significantly from that in the Nrf1(flox/flox)::CYP1A1-Cre control. This change was provoked by upregulation of several FA metabolism genes. Unexpectedly, we also found that the level of glutathione was increased dramatically in livers of Nrf1(flox/flox)::CYP1A1-Cre+3MC mice. While expression of glutathione biosynthetic enzymes was unchanged, xCT, a component of the cystine/glutamate antiporter system x(c)(-), was significantly upregulated in livers of Nrf1(flox/flox)::CYP1A1-Cre+3MC mice, suggesting that Nrf1 normally suppresses xCT. Thus, stress-inducible expression of xCT is a two-step process: under homeostatic conditions, Nrf1 effectively suppresses nonspecific transactivation of xCT, but when cells encounter severe oxidative/electrophilic stress, Nrf1 is displaced from an antioxidant response element (ARE) in the gene promoter while Nrf2 is recruited to the ARE. Thus, Nrf1 controls both the FA and the cystine/cysteine content of hepatocytes by participating in an elaborate regulatory network.

Suppression of the xCT-CD44v antiporter system sensitizes triple-negative breast cancer cells to doxorubicin

The xCT antiporter is known to be upregulated in 30 % of triple-negative breast cancer (TNBC) cell lines. The xCT-CD44 variant (CD44v) system regulates the levels of reactive oxygen species (ROS) in cancer cells and promotes tumor growth. Here, the role of this antiporter system in relation to chemotherapy was evaluated. MDA-MB-231 and MDA-MB-436 cells were transfected with lentiviral vectors expressing short hairpin RNA against xCT or CD44v. Following doxorubicin treatment, cellular proliferation was monitored, ROS were measured, and intracellular levels of cysteine and glutathione (GSH) were determined using liquid chromatography-mass spectrometry. A TNBC orthotopic tumor model was used to evaluate the impact of xCT-CD44v inhibition on doxorubicin efficacy in vivo. Doxorubicin treatment of TNBC cells caused increased expression of xCT through upregulation of CD44v. Consequently, the intracellular uptake of cystine increased, enabling rapid synthesis of GSH, and neutralization of doxorubicin-induced ROS. Suppression of xCT or CD44v impaired the defense against drug-induced oxidative stress, thereby sensitizing cells to doxorubicin. The importance of the xCT-CD44v in supporting tumor growth during doxorubicin treatment was also demonstrated in an in vivo tumor model of TNBC. These findings suggest that the antiporter system could serve as a target for increasing the anticancer efficacy of conventional therapy in patients with TNBC.

A cisplatin-resistant head and neck cancer cell line with cytoplasmic p53(mut) exhibits ATP-binding cassette transporter upregulation and high glutathione levels

PURPOSE

Head and neck squamous cell carcinoma (HNSCC) cell lines with cytoplasmically sequestered mutant p53 (p53(mut_c)) are frequently more resistant to cisplatin (CDDP) than cells with mutant but nuclear p53 (p53(mut_n)). The aim of the study was to identify underlying mechanisms implicated in CDDP resistance of HNSCC cells carrying cytoplasmic p53(mut).

METHODS

Microarray analysis, quantitative reverse transcription polymerase chain reaction, Western blot analysis and immunocytochemistry were used to identify and evaluate candidate genes involved in CDDP resistance of p53(mut_c) cells. RNAi knockdown or treatment with inhibitors together with flow cytometry-based methods was used for functional assessment of the identified candidate genes. Cellular metabolic activity was assessed with the XTT assay, and the redox capacity of cells was evaluated by measuring cellular glutathione (GSH) levels.

RESULTS

Upregulation of ABCC2 and ABCG2 transporters was observed in CDDP-resistant p53(mut_c) HNSCC cells. Furthermore, p53(mut_c) cells exhibited a pronounced side population that could be suppressed by RNAi knockdown of ABCG2 as well as treatment with the ATP-binding-cassette transporter inhibitors imatinib, MK571 and tariquidar. Metabolic activity and cellular GSH levels were higher in CDDP-resistant p53(mut_c) cells, consistent with a higher capacity to fend off cytotoxic oxidative effects such as those caused by CDDP treatment. Finally, ABCC2/G2 inhibition of HNSCC cells with MK571 markedly enhanced CDDP sensitivity of HNSCC cells.

CONCLUSIONS

The observations in this study point to a major role of p53(mut_c) in conferring a stem cell like phenotype to HNSCC cells that is associated with ABCC2/G2 overexpression, high GSH and metabolic activity levels as well as CDDP resistance.

Functional imaging of oxidative stress with a novel PET imaging agent, 18F-5-fluoro-L-aminosuberic acid

Glutathione is the predominant endogenous cellular antioxidant, playing a critical role in the cellular defensive response to oxidative stress by neutralizing free radicals and reactive oxygen species. With cysteine as the rate-limiting substrate in glutathione biosynthesis, the cystine/glutamate transporter (system xc(-)) represents a potentially attractive PET biomarker to enable in vivo quantification of xc(-) activity in response to oxidative stress associated with disease. We have developed a system xc(-) substrate that incorporates characteristics of both natural substrates, L-cystine and L-glutamate (L-Glu). L-aminosuberic acid (L-ASu) has been identified as a more efficient system xc(-) substrate than L-Glu, leading to an assessment of a series of anionic amino acids as prospective PET tracers. Herein, we report the synthesis and in vitro and in vivo validation of a lead candidate, (18)F-5-fluoro-aminosuberic acid ((18)F-FASu), as a PET tracer for functional imaging of a cellular response to oxidative stress with remarkable tumor uptake and retention.

METHODS

(18)F-FASu was identified as a potential PET tracer based on an in vitro screening of compounds similar to L-cystine and L-Glu. Affinity toward system xc(-) was determined via in vitro uptake and inhibition studies using oxidative stress-induced EL4 and SKOV-3 cells. In vivo biodistribution and PET imaging studies were performed in mice bearing xenograft tumors (EL4 and SKOV-3).

RESULTS

In vitro assay results determined that L-ASu inhibited system xc(-) as well as or better than L-Glu. The direct comparison of uptake of tritiated compounds demonstrated more efficient system xc(-) uptake of L-ASu than L-Glu. Radiosynthesis of (18)F-FASu allowed the validation of uptake for the fluorine-bearing derivative in vitro. Evaluation in vivo demonstrated primarily renal clearance and uptake of approximately 8 percentage injected dose per gram in SKOV-3 tumors, with tumor-to-blood and tumor-to-muscle ratios of approximately 12 and approximately 28, respectively. (18)F-FASu uptake was approximately 5 times greater than (18)F-FDG uptake in SKOV-3 tumors. Dynamic PET imaging demonstrated uptake in EL4 tumor xenografts of approximately 6 percentage injected dose per gram and good tumor retention for at least 2 h after injection.

CONCLUSION

(18)F-FASu is a potentially useful metabolic tracer for PET imaging of a functional cellular response to oxidative stress. (18)F-FASu may provide more sensitive detection than (18)F-FDG in certain tumors.

Expression of the cancer stem cell markers CD44v6 and ABCG2 in tongue cancer: effect of neoadjuvant chemotherapy on local recurrence

The efficacy of neoadjuvant chemotherapy (NAC) is controversial, and no report supports NAC with a high evidence level. Recently, we showed that a deep surgical margin was resected very close to the tumor site in many NAC-treated oral squamous cell carcinoma patients, suggesting that NAC may lead to local recurrence and poor outcomes. The purpose of this study was to evaluate the effect of NAC on tumor local recurrence using cancer stem cell marker immunohistochemistry. We retrospectively analyzed 89 patients who underwent radical surgery for tongue cancer, and examined the effect of NAC on tumor local recurrence. Cancer stem cell marker (CD44v6 and ABCG2) expression was detected by immunohistochemistry. In our study, the local recurrence rate was 12.4%. CD44v6 and ABCG2 expression was significantly associated with regional lymph node metastasis, pattern of invasion, depth of invasion, perineural invasion and local recurrence, respectively. Tumor local recurrence was a significant independent predictive factor of the 5-year disease specific survival. CD44v6 or ABCG2 positivity in NAC-treated patients was significantly associated with tumor local recurrence. It was suggested that local recurrence in NAC-treated cases is associated with cancer stem-like cells. We propose that NAC leads to the selection and/or residue of more aggressive cancer stem-like cells.