siRNA Therapeutics in Ocular Diseases.

The field of RNAi therapeutics has quickly adapted to the treatment of ocular diseases. Although the eye provides a unique system for the delivery of siRNAs, its complex structure and composition fostered the development of novel strategies for efficient gene silencing in the target compartment. Moreover, anterior and posterior segments differ in their multiple drug barriers and clearance mechanisms. This chapter summarizes the recent achievements in terms of routes of administration, chemical modifications, and delivery systems for siRNAs that specifically apply to eye disorders. Methods employed for siRNA detection/quantitation in ocular tissues are also described, together with safety concerns that need to be addressed to fulfill regulatory requirements of new drug approval. Even though RNAi therapies for ocular diseases have not yet translated into patient care, we document herein the rising number of candidate drugs currently under preclinical or clinical development.

Ocular surface analysis and automatic non-invasive assessment of tear film breakup location, extension and progression in patients with glaucoma.

BACKGROUND: Tear film stability is the key event in ocular surface diseases. The purpose of this study is to evaluate spatial and temporal progression of the tear film breakup using an automatic non-invasive device. METHODS: Non-invasive tear breakup time (NITBUT) parameters, such as First NITBUT (F-NITBUT) and Average NITBUT (A-NITBUT), were evaluated in 132 glaucoma and 87 control eyes with the Keratograph 5 M device. Further analysis of this data was used to determine size, location and progression of tear film breakup with automatically identified breakup areas (BUA). The progression from First BUA (F-BUA) to total BUA (T-BUA) was expressed as Dry Area Growth Rate (DAGR). Differences between both groups were analysed using Student t-test for parametric data and Mann-Whitney U test for non-parametric data. Pearson's correlation coefficient was used to assess the relationship between parametric variables and Spearman in the case of non-parametric variables. RESULTS: F-NITBUT was 11.43 ± 7.83 s in the control group and 8.17 ± 5.73 in the glaucoma group (P = 0.010). A-NITBUT was 14.04 ± 7.21 and 11.82 ± 6.09 s in control and glaucoma groups, respectively (P = 0.028). F-BUA was higher in the glaucoma group than in the control group (2.73 and 2.28; P = 0.022) and was more frequently located at the centre of the cornea in the glaucoma group (P = 0.039). T-BUA was also higher in the glaucoma group than in the control group (13.24 and 9.76%; P = 0.012) and the DAGR was steeper in the glaucoma group than in the control group (34.38° and 27.15°; P = 0.009). CONCLUSIONS: Shorter NITBUT values and bigger, more central tear film breakup locations were observed in the glaucoma group than in the control group. The DAGR indicates that tear film rupture is bigger and increases faster in glaucomatous eyes than in normal eyes.

Relationship between OSDI questionnaire and ocular surface changes in glaucomatous patients.

PURPOSE: To evaluate the quality of life of glaucoma patients using the Ocular Surface Disease Index (OSDI) questionnaire and their association with dry eye clinical signs. METHODS: The study included patients into three groups. The treated group diagnosed with bilateral open-angle glaucoma and treated with one or more topical medication at least 1 year. The operated group underwent glaucoma surgery without the need for topical medications. The control group entered subjects without ocular diseases or previous surgeries. Dry eye clinical signs were evaluated; noninvasive tear break-up time, Meibomian gland depletion (MGD), and conjunctival hyperemia were measured using the Keratograph 5 M. The total-OSDI (T-OSDI) score was divided into the visual field-OSDI and discomfort-OSDI scores. RESULTS: Two hundred and nine subjects participated in this cross-sectional study, 147 using glaucoma medications, 21 patients underwent glaucoma surgery and 41 were controls. The T-OSDI and subscores were higher in glaucoma patients compared with controls (p < 0.05); we found no differences between treated and surgically groups. Correlations were observed between the T-OSDI values and Schirmer test (p = 0.016), ocular surface staining (p < 0.001) and the MGD (p = 0.006). The subscores were associated with the ocular surface staining (VF p = 0.013 and D p = 0.003). In treated patients, the number of drops per day correlates with T-OSDI and subscores (p = 0.017 and p = 0.005). CONCLUSION: OSDI scores increased in the glaucoma patients compared to controls without significant changes between treated and surgical patients. OSDI scores were associated to dry eye signs and medication in glaucoma patients.

Tivanisiran, a novel siRNA for the treatment of dry eye disease.

INTRODUCTION: Dry eye disease (DED) is characterized by an alteration of the tear film with ocular inflammation and neurosensory abnormalities. The main clinical signs of this condition are tear instability and ocular damage. Although DED has gained significant attention in the past few years, limited prescription treatment options are available for patients. Areas covered: The current manuscript summarizes the pre-clinical and clinical development of tivanisiran, a novel small interfering oligonucleotide of RNA (siRNA) used for the treatment of DED. Tivanisiran was designed to silence Transient Receptor Potential Vanilloid 1 (TRPV1); herein the chemistry and mechanism of action of this new compound is also described. Expert opinion: Drugs currently on the market mostly target the inflammatory component of the disease and show only partial efficacy. New compounds addressing other aspects of the disease would provide significant advantages and contribute to a more personalized treatment of the disease. Tivanisiran has been designed to reduce ocular discomfort and pain, and was shown to improve ocular hyperemia and tear quality in human and animal models. Consequently, if the results of the ongoing and future clinical trials meet their study endpoints, tivanisiran could be submitted to obtain approval for the treatment of DED.

miR-146a targets c-met and abolishes colorectal cancer liver metastasis.

A major complication of colorectal cancer (CRC), one of the most frequent and deadly types of cancer, is disease progression via liver metastases. At this stage, very few treatment options are available for patients, and the disease remains incurable. Herein, we used a well-established mouse model of CRC liver metastasis (CLM) to identify new regulators of this process. Using serial transplantation of murine MC38 adenocarcinoma cells, we obtained liver metastatic variants that displayed extremely strong colonization abilities. Using these newly established cell lines, we performed gene expression arrays and microRNA (miR) profiling. Comparative and predictive analyses between the two arrays showed higher expression of c-met and concomitant reduction of miR-146a in the mestastatic variants. In CRC patients, expression levels of both c-met and miR-146a were similar between primary tumors and liver metastases. Interestingly, we identified c-met as a new target for miR-146a, as miR-146a was able to impede c-met translation. Of relevance, overexpression of miR-146a in metastatic clones showed reduced in vitro malignancy and abolished the development of primary tumor and liver metastases. Our results document a new mechanism for c-met regulation in CLM and highlight the crucial role of miR-146a in suppressing tumorigenesis.

Safety and Efficacy Clinical Trials for SYL1001, a Novel Short Interfering RNA for the Treatment of Dry Eye Disease.

Purpose: To evaluate the efficacy and safety of SYL1001, a short interfering (si) RNA targeting the transient receptor potential cation channel subfamily V member 1 (TRPV1), for the treatment of dry eye disease (DED). Methods: This study combines a phase I and two phase II clinical trials to test different doses of SYL1001 in a total of 156 healthy subjects and patients with DED. After 10 days of treatment, the primary efficacy endpoints were the effect on (1) the scoring in the Visual Analogue Scale (VAS) and Ocular Surface Disease Index (OSDI) questionnaires, and (2) ocular tolerance evaluated by corneal fluorescein staining and conjunctival hyperemia. Secondary endpoints included the assessment of systemic and local tolerance. Results: Topical administration of SYL1001 1.125% once daily produced a significant decrease in VAS scores compared with placebo from day 4 until the end of treatment (change from baseline at day 10: -1.73 ± 0.32 vs. -0.91 ± 0.34; P = 0.013). For all treatments, OSDI scores were significantly reduced compared to their respective baseline values (P < 0.01), although no significant changes were detected between groups. Conjunctival hyperemia (quantified as normal or abnormal) significantly improved after instillation of SYL1001 1.125% compared with placebo (50% vs. 20%; P < 0.05). Excellent tolerability was reported, with no differences in the rates of occurrence of adverse events between groups. Conclusion: These trials achieved their primary endpoints of identifying the most effective dose of SYL1001 (1.125%). SYL1001 showed a large safety margin and may provide novel therapeutic opportunity for the relief of dry eye. (ClinicalTrials.gov numbers, NCT01438281, NCT01776658, and NCT02455999.).

Sphere-derived tumor cells exhibit impaired metastasis by a host-mediated quiescent phenotype.

The spread of lung cancer cells to distant sites represents a common event associated with poor prognosis. A fraction of tumor cells named cancer stem cells (CSCs) have the ability to overcome therapeutic stress and remain quiescent. However, whether these CSCs have also the capacity to initiate and sustain metastasis remains unclear. Here, we used tumor sphere cultures (TSC) isolated from mouse and human lung cancer models to enrich for CSCs, and assessed their metastatic potential as compared to non-CSCs. As expected, TSC overexpressed a variety of stem cell markers and displayed chemoresistance. The CSC phenotype of TSC was confirmed by their higher growth ability in soft agar and tumorigenic potential in vivo, despite their reduced in vitro cell growth kinetics. Surprisingly, the appearance of spontaneous lung metastases was strongly delayed in mice injected with TSC as compared to non-TSC cells. Similarly, this finding was confirmed in several other models of metastasis, an effect associated with a retarded colonization activity. Interestingly, such delay correlated with a quiescent phenotype whose underlined mechanisms included an increase in p27 protein and lower phospho-ERK1/2 levels. Thus, these data suggest that cells enriched for CSC properties display an impaired metastatic activity, a finding with potential clinical implications.

Co-migration of colon cancer cells and CAFs induced by TGFß1 enhances liver metastasis.

Colorectal cancer (CRC) cells often metastatize to the liver. Cancer-associated fibroblasts (CAFs) enhance metastasis by providing cytokines that create a favorable microenvironment and by inducing co-dissemination with tumor cells. However, the mechanisms of co-metastatization remain elusive. The aim of this study is to assess the role of TGFß1 in CRC cell-CAFs attachment and its impact on liver metastasis. CAFs were obtained after xenotransplantation of Mc38 cells into EGFP-C57BL/6 mice. Attachment experiments with CRC cells and CAFs (with or without TGFß1 and the inhibitory peptide P17) were carried out, as well as in vivo liver metastasis assays. TGFß1 induced adhesion of CRC cells to CAFs, whereas exposure to P17 abrogated this effect. Co-injection of Mc38 cells with CAFs intrasplenically increased liver metastasis, as compared to injection of tumor cells alone. Pretreatment of Mc38 cells with TGFß1 enhanced the metastatic burden, in comparison to untreated Mc38 + CAFs. TGFß1-pretreated Mc38 cells co-metastatized with CAFs to the liver in a highly efficient way. Importantly, the metastatic burden was significantly reduced (p < 0.001) when P17 was administered in mice. The number of PCNA+ and CD-31+ cells was also reduced by P17 in these animals, indicating a decrease in proliferation and angiogenesis upon TGFß1 signaling blockade. Through microarray analysis, we identified potential TGFß1-regulated genes that may mediate cancer cell-stroma interactions to increase metastasis. In conclusion, TGFß1 promotes co-travelling of CRC cells and CAFs to the liver to enhance metastasis. Our results strongly support the use of TGFß1 targeted drugs as a novel strategy to reduce liver metastasis in CRC patients.

The inhibitor of differentiation isoform Id1b, generated by alternative splicing, maintains cell quiescence and confers self-renewal and cancer stem cell-like properties.

Id1 has been shown to play a critical role in tumorigenesis and angiogenesis. Moreover, recent reports have involved Id1 in the maintenance of cancer stem cell features in some tumor types. The Id1 gene generates two isoforms through alternative splicing: Id1a and Id1b. We have investigated the role of each isoform in cancer development. Using lentiviral systems we modified the endogenous expression of each of these isoforms in cancer cells and analyzed their biological effect both in vitro and in vivo. Overexpression of Id1b in murine CT26 and 3LL cells caused a G0/G1 cell cycle arrest and reduced proliferation, clonogenicity and phospho-ERK1/2 levels, while increasing p27 levels. High levels of Id1a had an opposite effect and the proportion of cells in the S phase increased significantly. In vivo models confirmed the inhibitory role of Id1b in primary tumor growth and metastasis. Through microarray analysis we found that the cancer stem cell (CSC) markers ALDH1A1 and Notch-1 were up-regulated specifically in Id1b-overexpressing cells. By using qPCR we also found overexpression of Sca-1, Tert, Sox-2 and Oct-4 in these cells. Increased levels of Id1b promoted self-renewal and CSC-like properties, as shown by their high capacity for developing secondary tumorspheres and retaining the PKH26 dye. The acquisition of CSC phenotype was confirmed in human PC-3 cells that overexpressed Id1b. Our results show that Id1b maintains cells in a quiescent state and promotes self-renewal and CSC-like features. On the contrary, Id1a promotes cell proliferation.

Metastatic dormancy: a complex network between cancer stem cells and their microenvironment.

Metastasis represents the major threat of cancer progression and generally emerges years after the detection of the primary tumor. An important rate-limiting step resides in cellular dormancy, where a disseminated tumor cell remains in a quiescent state at a remote organ. Herein we review the molecular mechanisms leading to tumor dormancy, mainly in regards to cellular quiescence and the tumor microenvironment. Based on the current published literature, we provide evidence that links the cancer stem cell (CSC) theory with dormancy and metastasis. Once a disseminated tumor cell reaches a target tissue, a tight regulation imposed by the foreign microenvironment will dictate the fate of these cells, which implies a balance in the secretion of soluble factors, modulation of the extracellular matrix and the angiogenic switch. We investigate thoroughly whether the CSC theory could also apply to metastasis initiation. In fact, the resistance of CSCs to therapy, leading to the minimal residual disease and cellular quiescence phenotypes, predisposes for the development of metastases. Finally, we describe the new technologies available for the identification of circulating tumor cells (CTCs), as well as their clinical relevance in dormancy of metastatic cancer patients.

New syngeneic inflammatory-related lung cancer metastatic model harboring double KRAS/WWOX alterations.

New mouse models with specific drivers of genetic alterations are needed for preclinical studies. Herein, we created and characterized at the genetic level a new syngeneic model for lung cancer and metastasis in Balb-c mice. Tumor cell lines were obtained from a silica-mediated airway chronic inflammation that promotes tumorigenesis when combined with low doses of N-nitrosodimethylamine, a tobacco smoke carcinogen. Orthotopic transplantation of these cells induced lung adenocarcinomas, and their intracardiac injection led to prominent colonization of various organs (bone, lung, liver and brain). Driver gene alterations included a mutation in the codon 12 of KRAS (G-A transition), accompanied by a homozygous deletion of the WW domain-containing oxidoreductase (WWOX) gene. The mutant form of WWOX lacked exons 5-8 and displayed reduced protein expression level and activity. WWOX gene restoration decreased the in vitro and in vivo tumorigenicity, confirming the tumor suppressor function of this gene in this particular model. Interestingly, we found that cells displayed remarkable sphere formation ability with expression of specific lung cancer stem cell markers. Study of non-small-cell lung cancer patient cohorts demonstrated a deletion of WWOX in 30% of cases, with significant reduction in protein levels as compared to normal tissues. Overall, our new syngeneic mouse model provides a most valuable tool to study lung cancer metastasis in balb-c mice background and highlights the importance of WWOX deletion in lung carcinogenesis.

Silica-induced chronic inflammation promotes lung carcinogenesis in the context of an immunosuppressive microenvironment.

The association between inflammation and lung tumor development has been clearly demonstrated. However, little is known concerning the molecular events preceding the development of lung cancer. In this study, we characterize a chemically induced lung cancer mouse model in which lung cancer developed in the presence of silicotic chronic inflammation. Silica-induced lung inflammation increased the incidence and multiplicity of lung cancer in mice treated with N-nitrosodimethylamine, a carcinogen found in tobacco smoke. Histologic and molecular analysis revealed that concomitant chronic inflammation contributed to lung tumorigenesis through induction of preneoplastic changes in lung epithelial cells. In addition, silica-mediated inflammation generated an immunosuppressive microenvironment in which we observed increased expression of programmed cell death protein 1 (PD-1), transforming growth factor-ß1, monocyte chemotactic protein 1 (MCP-1), lymphocyte-activation gene 3 (LAG3), and forkhead box P3 (FOXP3), as well as the presence of regulatory T cells. Finally, the K-RAS mutational profile of the tumors changed from Q61R to G12D mutations in the inflammatory milieu. In summary, we describe some of the early molecular changes associated to lung carcinogenesis in a chronic inflammatory microenvironment and provide novel information concerning the mechanisms underlying the formation and the fate of preneoplastic lesions in the silicotic lung.

Epithelial to mesenchymal transition and cancer stem cell phenotypes leading to liver metastasis are abrogated by the novel TGFß1-targeting peptides P17 and P144.

Colorectal cancer (CRC) frequently metastasizes to the liver, a phenomenon that involves the participation of transforming-growth-factor-ß(1) (TGFß(1)). Blockade of the protumorigenic effects elicited by TGFß(1) in advanced CRC could attenuate liver metastasis. We aimed in the present study to assess the antimetastatic effect of TGFß(1)-blocking peptides P17 and P144, and to study mechanisms responsible for this activity in a mouse model. Colon adenocarcinoma cells expressing luciferase were pretreated with TGFß(1) (Mc38-luc(TGFß1) cells), injected into the spleen of mice and monitored for tumor development. TGFß(1) increased primary tumor growth and liver metastasis, whereas systemic treatment of mice with either P17 or P144 significantly reduced tumor burden (p<0.01). In metastatic nodules, mitotic/apoptotic ratio, mesenchymal traits and angiogenesis (evaluated by CD-31, as well as circulating endothelial and progenitor cells) induced by TGFß(1) were consistently reduced following injection of peptides. In vitro experiments revealed a direct effect of TGFß(1) in Mc38 cells, which resulted in activation of Smad2, Smad3 and Smad1/5/8, and increased invasion and transendothelial migration, whereas blockade of TGFß(1)-signaling reverted these features. Because TGFß(1)-mediated epithelial-mesenchymal transition (EMT) has been suggested to induce a cancer stem cell (CSC) phenotype, we analyzed the ability of this cytokine to induce tumorsphere formation and the expression of CSC markers. In TGFß(1)-treated cells, tumorspheres were enriched in CD44 and SOX2, which were diminished in the presence of P17. Our data provide a preclinical rationale to evaluate P17 and P144 as potential therapeutic options for the treatment of metastatic CRC.

Residual dormant cancer stem-cell foci are responsible for tumor relapse after antiangiogenic metronomic therapy in hepatocellular carcinoma xenografts.

Hepatocellular carcinoma (HCC) is the fifth most common solid tumor and the third leading cause of cancer-related deaths. Currently available chemotherapeutic options are not curative due in part to tumor resistance to conventional therapies. We generated orthotopic HCC mouse models in immunodeficient NOD/SCID/IL2rγ null mice by injection of human alpha-feto protein (hAFP)- and/or luciferase-expressing HCC cell lines and primary cells from patients, where tumor growth and spread can be accurately monitored in a non-invasive way. In this model, low-dose metronomic administration of cyclophosphamide (LDM-CTX) caused complete regression of the tumor mass. A significant increase in survival (P<0.0001), reduced aberrant angiogenesis and hyperproliferation, and decrease in the number of circulating tumor cells were found in LDM-CTX-treated animals, in comparison with untreated mice. Co-administration of LDM-CTX with anti-VEGF therapy further improved the therapeutic efficacy. However, the presence of residual circulating hAFP levels suggested that some tumor cells were still present in livers of treated mice. Immunohistochemistry revealed that those cells had a hAFP+/CD13+/PCNA- phenotype, suggesting that they were dormant cancer stem cells (CSC). Indeed, discontinuation of therapy resulted in tumor regrowth. Moreover, in-vitro LDM-CTX treatment reduced hepatosphere formation in both number and size, and the resulting spheres were enriched in CD13+ cells indicating that these cells were particularly resistant to therapy. Co-treatment of the CD13-targeting drug, bestatin, with LDM-CTX leads to slower tumor growth and a decreased tumor volume. Therefore, combining a CD13 inhibitor, which targets the CSC-like population, with LDM-CTX chemotherapy may be used to eradicate minimal residual disease and improve the treatment of liver cancer.

Inhibition of telomerase activity preferentially targets aldehyde dehydrogenase-positive cancer stem-like cells in lung cancer.

BACKGROUND: Mortality rates for advanced lung cancer have not declined for decades, even with the implementation of novel chemotherapeutic regimens or the use of tyrosine kinase inhibitors. Cancer Stem Cells (CSCs) are thought to be responsible for resistance to chemo/radiotherapy. Therefore, targeting CSCs with novel compounds may be an effective approach to reduce lung tumor growth and metastasis. We have isolated and characterized CSCs from non-small cell lung cancer (NSCLC) cell lines and measured their telomerase activity, telomere length, and sensitivity to the novel telomerase inhibitor MST312. RESULTS: The aldehyde dehydrogenase (ALDH) positive lung cancer cell fraction is enriched in markers of stemness and endowed with stem cell properties. ALDH+ CSCs display longer telomeres than the non-CSC population. Interestingly, MST312 has a strong antiproliferative effect on lung CSCs and induces p21, p27 and apoptosis in the whole tumor population. MST312 acts through activation of the ATM/pH2AX DNA damage pathway (short-term effect) and through decrease in telomere length (long-term effect). Administration of this telomerase inhibitor (40 mg/kg) in the H460 xenograft model results in significant tumor shrinkage (70% reduction, compared to controls). Combination therapy consisting of irradiation (10Gy) plus administration of MST312 did not improve the therapeutic efficacy of the telomerase inhibitor alone. Treatment with MST312 reduces significantly the number of ALDH+ CSCs and their telomeric length in vivo. CONCLUSIONS: We conclude that antitelomeric therapy using MST312 mainly targets lung CSCs and may represent a novel approach for effective treatment of lung cancer.

EGFR signaling is differentially activated in patient-derived glioblastoma stem cells.

Evidence suggests that stem-like cells are responsible for initiation, maintenance and recurrence of solid tumors, including Glioblastoma Multiforme (GBM). GBM is an intractable, highly lethal tumor of the central nervous system. Although epidermal growth factor receptor (EGFR) is highly expressed in many GBMs, anti-EGFR therapies have been unsuccessful as treatment. Few studies have examined EGFR activation in GBM stem cells (GSCs) to determine if patient-specific GSCs are amenable to anti-EGFR therapy pre-clinically. We hypothesized that EGFR activation in GSCs varied between patients and was an important determinant of responsiveness to anti-EGFR therapy. Cell cycle and apoptosis analysis was performed on tumor-spheres by immuncytochemistry in the presence and absence of the AG1478. Second messenger pathways operative in these processes were elucidated by immunoblotting. EGFR activated AKT and inactivated GSK3beta in EGFR+/PTEN+ GSCs. AG1478 and erlotinib significantly decreased the total number of tumor-spheres that EGFR+/ PTEN+ GSCs generated and the rate of sphere formation. Inhibition of EGFR signaling by AG1478 increased GSC senescence and apoptosis, likely via inhibition of AKT and activation of GSK3beta. Sphere formation by EGFR-/ PTEN- GSCs was independent of EGF stimulation, but dependant on B27 growth supplement. Our data suggest that EGFR+/PTEN+ GSCs are susceptible to anti-EGFR therapy in vitro.

Perivascular nitric oxide activates notch signaling and promotes stem-like character in PDGF-induced glioma cells.

eNOS expression is elevated in human glioblastomas and correlated with increased tumor growth and aggressive character. We investigated the potential role of nitric oxide (NO) activity in the perivascular niche (PVN) using a genetic engineered mouse model of PDGF-induced gliomas. eNOS expression is highly elevated in tumor vascular endothelium adjacent to perivascular glioma cells expressing Nestin, Notch, and the NO receptor, sGC. In addition, the NO/cGMP/PKG pathway drives Notch signaling in PDGF-induced gliomas in vitro, and induces the side population phenotype in primary glioma cell cultures. NO also increases neurosphere forming capacity of PDGF-driven glioma primary cultures, and enhances their tumorigenic capacity in vivo. Loss of NO activity in these tumors suppresses Notch signaling in vivo and prolongs survival of mice. This mechanism is conserved in human PDGFR amplified gliomas. The NO/cGMP/PKG pathway's promotion of stem cell-like character in the tumor PVN may identify therapeutic targets for this subset of gliomas.

The ABCG2 resistance network of glioblastoma.

Glioblastoma multiforme (GBM) is the most common and lethal primary brain tumor variant. It exhibits heterogeneity at both the morphologic and genetic levels, with a complex combination of somatic alterations rendering the tumor class difficult to adequately treat. The role of so-called "cancer stem-cells" (CSCs) in the resistance of high-grade gliomas like GBM to conventional treatment regimens has received much recent attention, especially with regard to the enhanced ability of stem-like cells to activate survival pathways, repair DNA damage, and expel cytotoxic drugs. Furthermore, the biology of GBM is actually even more convoluted, characterized by a constantly changing microenvironment that greatly influences tumor growth and response to therapy. Herein we review the most recently reported genetic events in human glioma patients and their influence on treatment response, particularly in relation to O6-methylguanine-DNA methyltransferase methylation status. We aim to present evidence for a role for cancer stem-like cells and their unique microenvironment in therapy resistance and forward our views on glioma initiation and origin. Finally, given the recent interest in "side population" (SP) cells as a model of CSCs in gliomagenesis, we further describe the function of ABCG2 transporters, the mediators of the SP phenotype, at both the blood-brain barrier and in stem-like tumor cells.

PTEN/PI3K/Akt pathway regulates the side population phenotype and ABCG2 activity in glioma tumor stem-like cells.

In normal brain, the side population (SP) phenotype is generated by ABC transporter activity and identifies stem cell and endothelial cell subpopulations by dye exclusion. By drug efflux, the ABCG2 transporter provides chemoresistance in stem cells and contributes to the blood brain barrier (BBB) when active in endothelial cells. We investigated the SP phenotype of mouse and human gliomas. In glioma endothelial cells, ABC transporter function is impaired, corresponding to disruption of the BBB in these tumors. By contrast, the SP phenotype is increased in nonendothelial cells that form neurospheres and are highly tumorigenic. In this cell population, Akt, but not its downstream target mTOR, regulates ABCG2 activity, and loss of PTEN increases the SP. This Akt-induced ABCG2 activation results from its transport to the plasma membrane. Temozolomide, the standard treatment of gliomas, although not an ABCG2 substrate, increases the SP in glioma cells, especially in cells missing PTEN.