From signalling pathways to targeted therapies: unravelling glioblastoma's secrets and harnessing two decades of progress

Glioblastoma, a rare, and highly lethal form of brain cancer, poses significant challenges in terms of therapeutic resistance, and poor survival rates for both adult and paediatric patients alike. Despite advancements in brain cancer research driven by a technological revolution, translating our understanding of glioblastoma pathogenesis into improved clinical outcomes remains a critical unmet need. This review emphasises the intricate role of receptor tyrosine kinase signalling pathways, epigenetic mechanisms, and metabolic functions in glioblastoma tumourigenesis and therapeutic resistance. We also discuss the extensive efforts over the past two decades that have explored targeted therapies against these pathways. Emerging therapeutic approaches, such as antibody-toxin conjugates or CAR T cell therapies, offer potential by specifically targeting proteins on the glioblastoma cell surface. Combination strategies incorporating protein-targeted therapy and immune-based therapies demonstrate great promise for future clinical research. Moreover, gaining insights into the role of cell-of-origin in glioblastoma treatment response holds the potential to advance precision medicine approaches. Addressing these challenges is crucial to improving outcomes for glioblastoma patients and moving towards more effective precision therapies.

Study protocol: PreOperative Brain Irradiation in Glioblastoma (POBIG) - A phase I trial

Background

Glioblastoma is a high-grade aggressive neoplasm whose outcomes have not changed in decades. In the current treatment pathway, tumour growth continues and remains untreated for several weeks post-diagnosis. Intensified upfront therapy could target otherwise untreated tumour cells and improve the treatment outcome. POBIG will evaluate the safety and feasibility of single-fraction preoperative radiotherapy for newly diagnosed glioblastoma, assessed by the maximum tolerated dose (MTD) and maximum tolerated irradiation volume (MTIV).

Methods

POBIG is an open-label, dual-centre phase I dose and volume escalation trial that has received ethical approval. Patients with a new radiological diagnosis of glioblastoma will be screened for eligibility. This is deemed sufficient due to the high accuracy of imaging and to avoid treatment delay. Eligible patients will receive a single fraction of preoperative radiotherapy ranging from 6 to 14 Gy followed by their standard of care treatment comprising maximal safe resection and postoperative chemoradiotherapy (60 Gy/30 fr) with concurrent and adjuvant temozolomide). Preoperative radiotherapy will be directed to the part of the tumour that is highest risk for remaining as postoperative residual disease (hot spot). Part of the tumour will remain unirradiated (cold spot) and sampled separately for diagnostic purposes. Dose/volume escalation will be guided by a Continual Reassessment Method (CRM) model. Translational opportunities will be afforded through comparison of irradiated and unirradiated primary glioblastoma tissue.

Discussion

POBIG will help establish the role of radiotherapy in preoperative modalities for glioblastoma.

Trial registration

NCT03582514 (clinicaltrials.gov).

Effects of Fractionated Radiation Exposure on Vimentin Expression in Cervical Cancers: Analysis of Association with Cancer Stem Cell Response and Short-Term Prognosis

Elucidation of the mechanisms for the response of cancer stem cells (CSCs) to radiation exposure is of considerable interest for further improvement of radio- and chemoradiotherapy of cervical cancer (CC). The aim of this work is to evaluate the effects of fractionated radiation exposure on the expression of vimentin, which is one of the end-stage markers of epithelial-mesenchymal transition (EMT), and analyze its association with CSC radiation response and short-term prognosis of CC patients. The level of vimentin expression was determined in HeLa, SiHa cell lines, and scrapings from the cervix of 46 CC patients before treatment and after irradiation at a total dose of 10 Gy using real-time polymerase chain reaction (PCR) assay, flow cytometry, and fluorescence microscopy. The number of CSCs was assessed using flow cytometry. Significant correlations were shown between vimentin expression and postradiation changes in CSC numbers in both cell lines (R = 0.88, p = 0.04 for HeLa and R = 0.91, p = 0.01 for SiHa) and cervical scrapings (R = 0.45, p = 0.008). Associations were found at the level of tendency between postradiation increase in vimentin expression and unfavorable clinical outcome 3-6 months after treatment. The results clarify some of the relationships between EMT, CSCs, and therapeutic resistance that are needed to develop new strategies for cancer treatment.

Integrative multi-omics networks identify PKCδ and DNA-PK as master kinases of glioblastoma subtypes and guide targeted cancer therapy

Despite producing a panoply of potential cancer-specific targets, the proteogenomic characterization of human tumors has yet to demonstrate value for precision cancer medicine. Integrative multi-omics using a machine-learning network identified master kinases responsible for effecting phenotypic hallmarks of functional glioblastoma subtypes. In subtype-matched patient-derived models, we validated PKCδ and DNA-PK as master kinases of glycolytic/plurimetabolic and proliferative/progenitor subtypes, respectively, and qualified the kinases as potent and actionable glioblastoma subtype-specific therapeutic targets. Glioblastoma subtypes were associated with clinical and radiomics features, orthogonally validated by proteomics, phospho-proteomics, metabolomics, lipidomics and acetylomics analyses, and recapitulated in pediatric glioma, breast and lung squamous cell carcinoma, including subtype specificity of PKCδ and DNA-PK activity. We developed a probabilistic classification tool that performs optimally with RNA from frozen and paraffin-embedded tissues, which can be used to evaluate the association of therapeutic response with glioblastoma subtypes and to inform patient selection in prospective clinical trials.

PNJ scaffolds promote microenvironmental regulation of glioblastoma stem-like cell enrichment and radioresistance

Glioblastoma (GBM) brain tumors contain a subpopulation of self-renewing multipotent Glioblastoma stem-like cells (GSCs) that are believed to drive the near inevitable recurrence of GBM. We previously engineered temperature responsive scaffolds based on the polymer poly(N-isopropylacrylamide-co-Jeffamine M-1000 acrylamide) (PNJ) for the purpose of enriching GSCs in vitro from patient-derived samples. Here, we used PNJ scaffolds to study microenvironmental regulation of self-renewal and radiation response in patient-derived GSCs representing classical and proneural subtypes. GSC self-renewal was regulated by the composition of PNJ scaffolds and varied with cell type. PNJ scaffolds protected against radiation-induced cell death, particularly in conditions that also promoted GSC self-renewal. Additionally, cells cultured in PNJ scaffolds exhibited increased expression of the transcription factor HIF2α, which was not observed in neurosphere culture, providing a potential mechanistic basis for differences in radio-resistance. Differences in PNJ regulation of HIF2α in irradiated and untreated conditions also offered evidence of stem plasticity. These data show PNJ scaffolds provide a unique biomaterial for evaluating dynamic microenvironmental regulation of GSC self-renewal, radioresistance, and stem plasticity.

Anticancer Activities of 9-chloro-6-(piperazin-1-yl)-11H-indeno[1,2-c] quinolin-11-one (SJ10) in Glioblastoma Multiforme (GBM) Chemoradioresistant Cell Cycle-Related Oncogenic Signatures

Simple Summary

Glioblastoma multiforme (GBM) remains to be the most frequent malignant tumor of the central nervous system (CNS), which accounts for approximately 54% of all primary brain gliomas. Current treatment modalities for GBM include surgical resection, followed by radiotherapy and chemotherapy with temozolomide (TMZ). However, due to its genetic heterogeneity, GBM tumors always recur, due to treatment reasistance. The aim of this study was to identify molecular gene signatures, responsible for cancer initiation, progression, resistances and to treatment, metastasis, and also evaluate the potency of our novel compounds SJ10 as potential target for CCNB1/CDC42/MAPK7/CD44 oncogenic signatures. Accordingly, we used computational simulation and identify these signatures as regulators of the cell cycle in GBM, which leads to cancer development and metastasis. We also showed the antiproliferative and cytotoxic effects of SJ10 compound against a panel of NCI-60 cancer cell lines. This suggests the potential of the compounds to inhibit CCNB1/CDC42/MAPK7/CD44 in GBM.

Abstract

Current anticancer treatments are inefficient against glioblastoma multiforme (GBM), which remains one of the most aggressive and lethal cancers. Evidence has shown the presence of glioblastoma stem cells (GSCs), which are chemoradioresistant and associated with high invasive capabilities in normal brain tissues. Moreover, accumulating studies have indicated that radiotherapy contributes to abnormalities in cell cycle checkpoints, including the G₁/S and S phases, which may potentially lead to resistance to radiation. Through computational simulations using bioinformatics, we identified several GBM oncogenes that are involved in regulating the cell cycle. Cyclin B1 (CCNB1) is one of the cell cycle-related genes that was found to be upregulated in GBM. Overexpression of CCNB1 was demonstrated to be associated with higher grades, proliferation, and metastasis of GBM. Additionally, increased expression levels of CCNB1 were reported to regulate activation of mitogen-activated protein kinase 7 (MAPK7) in the G₂/M phase, which consequently modulates mitosis; additionally, in clinical settings, MAPK7 was demonstrated to promote resistance to temozolomide (TMZ) and poor patient survival. Therefore, MAPK7 is a potential novel drug target due to its dysregulation and association with TMZ resistance in GBM. Herein, we identified MAPK7/extracellular regulated kinase 5 (ERK5) genes as being overexpressed in GBM tumors compared to normal tissues. Moreover, our analysis revealed increased levels of the cell division control protein homolog (CDC42), a protein which is also involved in regulating the cell cycle through the G₁ phase in GBM tissues. This therefore suggests crosstalk among CCNB1/CDC42/MAPK7/cluster of differentiation 44 (CD44) oncogenic signatures in GBM through the cell cycle. We further evaluated a newly synthesized small molecule, SJ10, as a potential target agent of the CCNB1/CDC42/MAPK7/CD44 genes through target prediction tools and found that SJ10 was indeed a target compound for the above-mentioned genes; in addition, it displayed inhibitory activities against these oncogenes as observed from molecular docking analysis.

Ionizing Radiation Induces Resistant Glioblastoma Stem-Like Cells by Promoting Autophagy via the Wnt/β-Catenin Pathway

Therapeutic resistance in recurrent glioblastoma multiforme (GBM) after concurrent chemoradiotherapy (CCRT) is a challenging issue. Although standard fractionated radiation is essential to treat GBM, it has led to local recurrence along with therapy-resistant cells in the ionizing radiation (IR) field. Lines of evidence showed cancer stem cells (CSCs) play a vital role in therapy resistance in many cancer types, including GBM. However, the molecular mechanism is poorly understood. Here, we proposed that autophagy could be involved in GSC induction for radioresistance. In a clinical setting, patients who received radiation/chemotherapy had higher LC3II expression and showed poor overall survival compared with those with low LC3 II. In a cell model, U87MG and GBM8401 expressed high level of stemness markers CD133, CD44, Nestin, and autophagy marker P62/LC3II after receiving standard fractionated IR. Furthermore, Wnt/β-catenin proved to be a potential pathway and related to P62 by using proteasome inhibitor (MG132). Moreover, pharmacological inhibition of autophagy with BAF and CQ inhibit GSC cell growth by impairing autophagy flux as demonstrated by decrease Nestin, CD133, and SOX-2 levels. In conclusion, we demonstrated that fractionated IR could induce GSCs with the stemness phenotype by P62-mediated autophagy through the Wnt/β-catenin for radioresistance. This study offers a new therapeutic strategy for targeting GBM in the future.

Targeting Protein Kinase C in Glioblastoma Treatment

Glioblastoma (GBM) is the most frequent and aggressive primary brain tumor and is associated with a poor prognosis. Despite the use of combined treatment approaches, recurrence is almost inevitable and survival longer than 14 or 15 months after diagnosis is low. It is therefore necessary to identify new therapeutic targets to fight GBM progression and recurrence. Some publications have pointed out the role of glioma stem cells (GSCs) as the origin of GBM. These cells, with characteristics of neural stem cells (NSC) present in physiological neurogenic niches, have been proposed as being responsible for the high resistance of GBM to current treatments such as temozolomide (TMZ). The protein Kinase C (PKC) family members play an essential role in transducing signals related with cell cycle entrance, differentiation and apoptosis in NSC and participate in distinct signaling cascades that determine NSC and GSC dynamics. Thus, PKC could be a suitable druggable target to treat recurrent GBM. Clinical trials have tested the efficacy of PKCβ inhibitors, and preclinical studies have focused on other PKC isozymes. Here, we discuss the idea that other PKC isozymes may also be involved in GBM progression and that the development of a new generation of effective drugs should consider the balance between the activation of different PKC subtypes.

Radiation Response of Cervical Cancer Stem Cells Is Associated with Pretreatment Proportion of These Cells and Physical Status of HPV DNA

Radio- and chemoresistance of cancer stem cells (CSCs) is considered as one of the possible causes of adverse results of chemoradiotherapy for various malignancies, including cervical cancer. However, little is known about quantitative changes in the CSC subpopulation in the course of treatment and mechanisms for individual response of CSCs to therapy. The purpose of the study was to evaluate the association of radiation response of cervical CSCs with clinical and morphological parameters of disease and features of human papillomavirus (HPV) infection. The proportion of CD44⁺CD24^low CSCs was determined by flow cytometry in cervical scrapings from 55 patients with squamous cell carcinoma of uterine cervix before treatment and after fractionated irradiation at a total dose of 10 Gy. Real-time PCR assay was used to evaluate molecular parameters of HPV DNA. Post-radiation increase in the CSC proportion was found in 47.3% of patients. Clinical and morphological parameters (stage, status of lymph node involvement, and histological type) were not significantly correlated with radiation changes in the CSC proportion. Single- and multifactor analyses revealed two independent indicators affecting the radiation response of CSCs: initial proportion of CSCs and physical status of HPV DNA (R = 0.86, p = 0.001 for the multiple regression model in the whole).

CD44-associated radioresistance of glioblastoma in irradiated brain areas with optimal tumor coverage

Glioblastoma multiforme (GBM) requires radiotherapy (RT) as its definitive management. However, GBM still has a high local recurrence rate even after RT. Cancer stem-like cells (CSCs) might enable GBM to evade irradiation damage and cause therapeutic failure. The optimal RT plan should achieve a planning target volume (PTV) coverage of more than 95% but cannot always meet the requirements. Here, we demonstrate that irradiation with different tumor coverage rates to different brain areas has similar effects on GBM. To retrospectively analyze the relationship between PTV coverage and the survival rate in 26 malignant glioblastoma patients, we established primary cell lines from patient-derived malignant glioblastoma cells with the PTV95 (PTV coverage of more than 95%) program (GBM-MG1 cells) and the Non-PTV95 (poor PTV coverage of less than 95%) program (GBM-MG2 cells). The clinical results of PTV95 and Non-PTV95 showed no difference in the overall survival (OS) rate (P = .390) between the two different levels of PTV coverage. GBM-MG1 (PTV95 program) cells exhibited higher radioresistance than GBM-MG2 (Non-PTV95 program) cells. CD44 promotes radioresistance, CSC properties, angiogenesis and cell proliferation in GBM-MG1 (PTV95 program) cells. GBM patients receiving RT with the PTV95 program exhibited higher radioresistance, CSC properties, angiogenesis and cell proliferation than GBM patients receiving RT with the Non-PTV95 program. Moreover, CD44 plays a crucial role in these properties of GBM patients with the PTV95 program.

Cancer stem cells: A review from origin to therapeutic implications

Cancer stem cells (CSCs), also known as tumor-initiating cells (TICs), are elucidated as cells that can perpetuate themselves via autorestoration. These cells are highly resistant to current therapeutic approaches and are the main reason for cancer recurrence. Radiotherapy has made a lot of contributions to cancer treatment. However, despite continuous achievements, therapy resistance and tumor recurrence are still prevalent in most patients. This resistance might be partly related to the existence of CSCs. In the present study, recent advances in the investigation of different biological properties of CSCs, such as their origin, markers, characteristics, and targeting have been reviewed. We have also focused our discussion on radioresistance and adaptive responses of CSCs and their related extrinsic and intrinsic influential factors. In summary, we suggest CSCs as the prime therapeutic target for cancer treatment.

The Caspase-3/PKCδ/Akt/VEGF-A Signaling Pathway Mediates Tumor Repopulation during Radiotherapy

PURPOSE

Tumor repopulation is known as a major cause of treatment failure and/or tumor recurrence after radiotherapy. The underlying mechanism remains unclear. Our previous study demonstrated that irradiated apoptotic cells mediated tumor repopulation, in which caspase-3 played an important role. Herein, we investigated downstream effectors of caspase-3 involved in this process.

EXPERIMENTAL DESIGN

A dominant-negative protein kinase Cδ (DN_PKCδ) mutant that could not be cleaved by caspase-3 and therefore could not be activated by irradiation-induced apoptosis was constructed. DN_PKCδ stably transduced tumor cells were compared with wild-type tumor cells for their growth stimulation effects in in vitro and in vivo tumor repopulation models. Downstream effectors of caspase-3 and PKCδ were investigated. The role of PKCδ was further verified in human colorectal tumor specimens.

RESULTS

Inactivation of caspase-3 or caspase-7 attenuated tumor repopulation and weakened PKCδ cleavage. Both DN_PKCδ and PKCδ inhibitors restrained tumor repopulation both in vitro and in vivo. Phosphorylated Akt was attenuated in caspase-3-, caspase-7-, or PKCδ-inactivated tumor cells. Furthermore, expression of vascular endothelial growth factor (VEGF)-A but not hypoxia-inducible factor 1α (HIF1α) was decreased in PKCδ- or Akt-inactivated tumor cells. In addition, inhibition of p-Akt, HIF1α, VEGF-A, or VEGF-A receptor reduced tumor repopulation significantly. Finally, increased nuclear translocation of PKCδ in colorectal tumor specimens was associated with worse patient prognosis.

CONCLUSIONS

The caspase-3/PKCδ/Akt/VEGF-A axis is involved in tumor repopulation and could be exploited as a potential target to enhance the efficacy of radiotherapy.

The STAT3/Slug Axis Enhances Radiation-Induced Tumor Invasion and Cancer Stem-like Properties in Radioresistant Glioblastoma

Glioblastoma multiforme (GBM) requires radiotherapy (RT) as a part of definitive management strategy. RT is highly effective, destroying cancer cells that may exist around the surgical tumor bed. However, GBM still has a poor prognosis and a high local recurrence rate after RT. Accumulating research indicates that GBM contains cancer stem-like cells (CSCs), which are radioresistant and result in therapeutic failure. Additionally, GBM cells can aggressively invade normal brain tissue, inducing therapeutic failure. Using clinical observations, we evaluated the effect of radiation on tumor control. We also explored the biomolecular pathways that connect radioresistance and CSC- and epithelial-mesenchymal transition (EMT)-associated phenotypes in patient-derived GBM cells. Transwell and microarray assay demonstrated that radioresistant GBM cells (GBM-R2I2) exhibit increased invasion and self-renewal abilities compared with parental GBM cells. Finally, to identify potential mechanisms underlying these observations, we used a PCR array to search for molecular markers of cell motility. Signal transducer and activator of transcription 3 (STAT3) directly bound to the Slug promoter in a chromatin immunoprecipitation assay. Reduced STAT3 decreased Slug expression and suppressed cell invasion in GBM-R2I2 cells while increasing Slug reversed these effects. In addition, STAT3 knockdown significantly inhibited CSC properties, synergistically increased the radiotherapeutic effect, and effectively increased the survival rate in vivo. We deciphered a new pathway of GBM radioresistance, invasion, and recurrence via the STAT3/Slug axis that could be a new target of GBM therapy.

Hypoxia-inducible transgelin 2 selects epithelial-to-mesenchymal transition and γ-radiation-resistant subtypes by focal adhesion kinase-associated insulin-like growth factor 1 receptor activation in non-small-cell lung cancer cells

Microenvironment, such as hypoxia common to cancer, plays a critical role in the epithelial‐to‐mesenchymal transition (EMT) program, which is a major route of cancer metastasis and confers γ‐radiation resistance to cells. Herein, we showed that transgelin 2 (TAGLN2), an actin‐binding protein, is significantly induced in hypoxic lung cancer cells and that Snail1 is simultaneously increased, which induces EMT by downregulating E‐cadherin expression. Forced TAGLN2 expression induced severe cell death; however, a small population of cells surviving after forced TAGLN2 overexpression showed γ‐radiation resistance, which might promote tumor relapse and recurrence. These surviving cells showed high metastatic activity with an increase of EMT markers including Snail1. In these cells, TAGLN2 activated the insulin‐like growth factor 1 receptor β (IGF1Rβ)/PI3K/AKT pathway by recruitment of focal adhesion kinase to the IGF1R signaling complex. Activation of the IGF1Rβ/PI3K/AKT pathway also induced inactivation of glycogen synthase kinase 3β (GSK3β), which is involved in Snail1 stabilization. Therefore, both the IGF1Rβ inhibitor (AG1024) and the PI3K inhibitor (LY294002) or AKT inactivation with MK2206 lower the cellular level of Snail1. Involvement of GSK3β was also confirmed by treatment with lithium chloride, the inducer of GSK3β phosphorylation, or MG132, the 26S proteasomal inhibitor, which also stabilized Snail1. In conclusion, the present study provides important evidence that hypoxia‐inducible TAGLN2 is involved in the selection of cancer cells with enhanced EMT properties to overcome the detrimental environment of cancer cells.

Cancer stem cells in progression of colorectal cancer

Colorectal cancer is one of the most common cancers worldwide with high mortality. Distant metastasis and relapse are major causes of patient death. Cancer stem cells (CSCs) play a critical role in the metastasis and relapse of colorectal cancer. CSCs are a subpopulation of cancer cells with unique properties of self-renewal, infinite division and multi-directional differentiation potential. Colorectal CSCs are defined with a group of cell surface markers, such as CD44, CD133, CD24, EpCAM, LGR5 and ALDH. They are highly tumorigenic, chemoresistant and radioresistant and thus are critical in the metastasis and recurrence of colorectal cancer and disease-free survival. This review article updates the colorectal CSCs with a focus on their role in tumor initiation, progression, drug resistance and tumor relapse.

In vitro investigation of head and neck cancer stem cell proportions and their changes following X-ray irradiation as a function of HPV status

INTRODUCTION

Some head and neck squamous cell carcinomas (HNSCC) have a distinct aetiology, which depends on the presence of oncogenic human papilloma virus (HPV). Also, HNSCC contains cancer stem cells (CSCs) that have greater radioresistance and capacity to change replication dynamics in response to irradiation compared to non-clonogenic cells. Since there is limited data on CSCs in HNSCC as a function of HPV status, better understanding of their radiobiology may enable improved treatment outcome.

METHODS

Baseline and post-irradiation changes in CSC proportions were investigated by flow cytometry in a HPV-negative (UM-SCC-1) and a HPV-positive (UM-SCC-47) HNSCC cell line, using fluorescent staining with CD44/ALDH markers. CSC proportions in both irradiated and unirradiated cultures were compared for the two cell lines at various times post-irradiation. To assess repopulation of CSCs, untreated cultures were depleted of CD44+/ALDH+ cells and re-cultured for 3 weeks before flow cytometry analysis.

RESULTS

CSC proportions in untreated cell lines were 0.57% (UM-SCC-1) and 2.87% (UM-SCC-47). Untreated cell lines depleted of CD44+/ALDH+ repopulated this phenotype to a mean of 0.15% (UM-SCC-1) and 6.76% (UM-SCC-47). All UM-SCC-47 generations showed elevated CSC proportions after irradiation, with the most significant increase at 2 days post-irradiation. The highest elevation in UM-SCC-1 CSCs was observed at 1 day post-irradiation in the 2nd generation and at 3 days after irradiation in the 3rd generation. When measured after 10 days, only the 3rd generation of UM-SCC-1 showed elevated CSCs.

CONCLUSIONS

CSC proportions in both cell lines were elevated after exposure and varied with time post irradiation. UM-SCC-47 displayed significant plasticity in repopulating the CSC phenotype in depleted cultures, which was not seen in UM-SCC-1.

Targeting therapy-resistant cancer stem cells by hyperthermia

Eradication of all malignant cells is the ultimate but challenging goal of anti-cancer treatment; most traditional clinically-available approaches fail because there are cells in a tumour that either escape therapy or become therapy-resistant. A subpopulation of cancer cells, the cancer stem cells (CSCs), is considered to be of particular significance for tumour initiation, progression and metastasis. CSCs are considered in particular to be therapy-resistant and may drive disease recurrence, which positions CSCs in the focus of anti-cancer research, but successful CSC-targeting therapies are limited. Here, we argue that hyperthermia - a therapeutic approach based on local heating of a tumour - is potentially beneficial for targeting CSCs in solid tumours. First, hyperthermia has been described to target cells in hypoxic and nutrient-deprived tumour areas where CSCs reside and ionising radiation and chemotherapy are least effective. Second, hyperthermia can modify factors that are essential for tumour survival and growth, such as the microenvironment, immune responses, vascularisation and oxygen supply. Third, hyperthermia targets multiple DNA repair pathways, which are generally upregulated in CSCs and protect them from DNA-damaging agents. Addition of hyperthermia to the therapeutic armamentarium of oncologists may thus be a promising strategy to eliminate therapy-escaping and -resistant CSCs.

Radiation induces the generation of cancer stem cells: A novel mechanism for cancer radioresistance

Radioresistance remains a major obstacle for the radiotherapy treatment of cancer. Previous studies have demonstrated that the radioresistance of cancer is due to the existence of intrinsic cancer stem cells (CSCs), which represent a small, but radioresistant cell subpopulation that exist in heterogeneous tumors. By contrast, non-stem cancer cells are considered to be radiosensitive and thus, easy to kill. However, recent studies have revealed that under conditions of radiation-induced stress, theoretically radiosensitive non-stem cancer cells may undergo dedifferentiation subsequently obtaining the phenotypes and functions of CSCs, including high resistance to radiotherapy, which indicates that radiation may directly result in the generation of novel CSCs from non-stem cancer cells. These findings suggest that in addition to intrinsic CSCs, non-stem cancer cells may also contribute to the relapse and metastasis of cancer following transformation into CSCs. This review aims to investigate the radiation-induced generation of CSCs, its association with epithelial-mesenchymal transition and its significance with regard to the radioresistance of cancer.

Arsenic trioxide disrupts glioma stem cells via promoting PML degradation to inhibit tumor growth

Glioblastoma multiforme (GBM) is the most lethal brain tumor. Tumor relapse in GBM is inevitable despite maximal therapeutic interventions. Glioma stem cells (GSCs) have been found to be critical players in therapeutic resistance and tumor recurrence. Therapeutic drugs targeting GSCs may significantly improve GBM treatment. In this study, we demonstrated that arsenic trioxide (As₂O₃) effectively disrupted GSCs and inhibited tumor growth in the GSC-derived orthotopic xenografts by targeting the promyelocytic leukaemia (PML). As₂O₃ treatment induced rapid degradation of PML protein along with severe apoptosis in GSCs. Disruption of the endogenous PML recapitulated the inhibitory effects of As₂O₃ treatment on GSCs both in vitro and in orthotopic tumors. Importantly, As₂O₃ treatment dramatically reduced GSC population in the intracranial GBM xenografts and increased the survival of mice bearing the tumors. In addition, As₂O₃ treatment preferentially inhibited cell growth of GSCs but not matched non-stem tumor cells (NSTCs). Furthermore, As₂O₃ treatment or PML disruption potently diminished c-Myc protein levels through increased poly-ubiquitination and proteasome degradation of c-Myc. Our study indicated a potential implication of As₂O₃ in GBM treatment and highlighted the important role of PML/c-Myc axis in the maintenance of GSCs.

K+ channel signaling in irradiated tumor cells

K⁺ channels crosstalk with biochemical signaling cascades and regulate virtually all cellular processes by adjusting the intracellular K⁺ concentration, generating the membrane potential, mediating cell volume changes, contributing to Ca²⁺ signaling, and directly interacting within molecular complexes with membrane receptors and downstream effectors. Tumor cells exhibit aberrant expression and activity patterns of K⁺ channels. The upregulation of highly "oncogenic" K⁺ channels such as the Ca²⁺-activated IK channel may drive the neoplastic transformation, malignant progression, metastasis, or therapy resistance of tumor cells. In particular, ionizing radiation in doses used for fractionated radiotherapy in the clinic has been shown to activate K⁺ channels. Radiogenic K⁺ channel activity, in turn, contributes to the DNA damage response and promotes survival of the irradiated tumor cells. Tumor-specific overexpression of certain K⁺ channel types together with the fact that pharmacological K⁺ channel modulators are already in clinical use or well tolerated in clinical trials suggests that K⁺ channel targeting alone or in combination with radiotherapy might become a promising new strategy of anti-cancer therapy. The present article aims to review our current knowledge on K⁺ channel signaling in irradiated tumor cells. Moreover, it provides new data on molecular mechanisms of radiogenic K⁺ channel activation and downstream signaling events.

Design of a PKCδ-specific small peptide as a theragnostic agent for glioblastoma

Glioblastoma is an aggressive malignant brain tumor that starts in the brain or spine and frequently recurs after anticancer treatment. The development of an accurate diagnostic system combined with effective cancer therapy is essential to improve prognosis of glioma patients. Peptides, produced from phage display, are attractive biomolecules for glioma treatment because of their biostability, nontoxicity, and small size. In this study, we employed phage display methodology to screen for peptides that specifically recognize the target PKCδ as a novel biomarker for glioma. The phage library screening yielded four different peptides displayed on phages with a 20- to 200-pM Kd value for the recombinant PKCδ catalytic domain. Among these four phage peptides, we selected one to synthesize and tagged it with fluorescein isothiocyanate (FITC) based on the sequence of the PKCδ-binding phage clone. The synthetic peptide showed a relative binding affinity for antibody and localization in the U373 glioma cell. The kinase activity of PKCδ was inhibited by FITC-labeled peptide with an IC50 of 1.4 μM in vitro. Consequently, the peptide found in this study might be a promising therapeutic agent against malignant brain tumor.

Stem cell-like gene expression signature identified in ionizing radiation-treated cancer cells

Recent studies have reported that embryonic stem (ES) cell-associated gene expression signatures have been identified in poorly differentiated tumors, revealing a link between ES cell identity and cancer cells. Cancer cells originate from cancer stem cells (CSCs). Both types of cells share common properties such as self-renewal and heterogeneity. CSCs are also resistant to conventional chemotherapy and radiotherapy. Here, we show similar gene expression patterns between ES cells and ionizing radiation (IR)-treated cancer cells. Using genome-wide transcriptome analysis, we compared the gene expression profiles among ES cells, cancer cells, and irradiated cancer cells, and identified a subset of similar gene expression patterns between ES cells and irradiated cancer cells, indicated by hierarchical clustering. These gene expression patterns were then confirmed by quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) analyses. Using bioinformatic analyses, these candidate genes are also associated with various biological pathways related to stemness in cancer. Taken together, our data suggest that identification of common molecular characteristics between ES cells and irradiated cancer cells is important to understand the properties of cancer stem cells and their resistance to radiotherapy.

Cancer stem cells: small subpopulation or evolving fraction?

This review discusses quantitative modeling studies of stem and non-stem cancer cell interactions and the fraction of cancer stem cells.

Ionizing radiations sustain glioblastoma cell dedifferentiation to a stem-like phenotype through survivin: possible involvement in radioresistance

Glioblastomas (GBM) are some bad prognosis brain tumors despite a conventional treatment associating surgical resection and subsequent radio-chemotherapy. Among these heterogeneous tumors, a subpopulation of chemo- and radioresistant GBM stem-like cells appears to be involved in the systematic GBM recurrence. Moreover, recent studies showed that differentiated tumor cells may have the ability to dedifferentiate and acquire a stem-like phenotype, a phenomenon also called plasticity, in response to microenvironment stresses such as hypoxia. We hypothesized that GBM cells could be subjected to a similar dedifferentiation process after ionizing radiations (IRs), then supporting the GBM rapid recurrence after radiotherapy. In the present study we demonstrated that subtoxic IR exposure of differentiated GBM cells isolated from patient resections potentiated the long-term reacquisition of stem-associated properties such as the ability to generate primary and secondary neurospheres, the expression of stemness markers and an increased tumorigenicity. We also identified during this process an upregulation of the anti-apoptotic protein survivin and we showed that its specific downregulation led to the blockade of the IR-induced plasticity. Altogether, these results demonstrated that irradiation could regulate GBM cell dedifferentiation via a survivin-dependent pathway. Targeting the mechanisms associated with IR-induced plasticity will likely contribute to the development of some innovating pharmacological strategies for an improved radiosensitization of these aggressive brain cancers.

Dying tumor cells stimulate proliferation of living tumor cells via caspase-dependent protein kinase Cδ activation in pancreatic ductal adenocarcinoma

Pancreatic cancer is one of the most lethal human cancers, and radiotherapy is often implemented for locally advanced pancreatic ductal adenocarcinoma. Tumor cell repopulation is a major challenge in treating cancers after radiotherapy. In order to address the problem of tumor repopulation, our previous studies have demonstrated that dying cells stimulate the proliferation of living tumor cells after radiotherapy. In particular, dying cells undergoing apoptosis also activate survival or proliferation signals and release growth factors to surrounding living cells. In the present study, we used an in vitro model to examine the possible mechanisms for dying cell stimulated tumor repopulation in pancreatic cancer. In this model, a small number of living, luciferase-labeled pancreatic cancer cells (reporter) were seeded onto a layer of a much larger number of irradiated, unlabeled pancreatic cancer cells and the growth of the living cells was measured over time as a gage of tumor repopulation. Our results indicate that irradiated, dying Panc1 feeder cells significantly stimulated the proliferation of living Panc1 reporter cells. Importantly, we identified that the percentage of apoptotic cells and the cleavage of caspases 3 and 7 and protein kinase Cδ (PKCδ) were increased in irradiated Panc1 cells. We presumed that caspases 3 and 7 and PKCδ as integral mediators in the process of dying pancreatic cancer cell stimulation of living tumor cell growth. In order to demonstrate the importance of caspases 3, 7 and PKCδ, we introduced dominant-negative mutants of caspase 3 (DN_C3), caspase 7 (DN_C7), or PKCδ (DN_PKCδ) into Panc1 cells using lentiviral vectors. The stably transduced Panc1 cells were irradiated and used as feeders and we found a significant decrease in the growth of living Panc1 reporter cells when compared with irradiated wild-type Panc1 cells as feeders. Moreover, the role of PKCδ in the growth stimulation of living tumor cells was further confirmed using a pan PKC inhibitor GF109203x and a specific PKCδ inhibitor, rottlerin. Additionally, we found significantly increased phosphorylation of Akt, p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase/stress-activated protein kinase (JNK1/2) in the irradiated Panc1 cells. Mechanistically, PKCδ cleavage was attenuated in both DN_C3 and DN_C7 transduced Panc1 cells, and both Akt and p38 MAPK phosphorylation were attenuated in DN_PKCδ transduced Panc1 cells following radiation. Thus, this report suggests a novel finding that cellular signaling caspase 3/7-PKCδ-Akt/p38 MAPK is crucial to the repopulation in Panc1 cells after radiotherapy.

A proposed quantitative index for assessing the potential contribution of reprogramming to cancer stem cell kinetics

Enrichment of cancer stem cells (CSCs) is thought to be responsible for glioblastoma multiforme (GBM) recurrence after radiation therapy. Simulation results from our agent-based cellular automata model reveal that the enrichment of CSCs may result either from an increased symmetric self-renewal division rate of CSCs or a reprogramming of non-stem cancer cells (CCs) to a stem cell state. Based on plateau-to-peak ratio of the CSC fraction in the tumor following radiation, a downward trend from peak to subsequent plateau (i.e., a plateau-to-peak ratio exceeding 1.0) was found to be inconsistent with increased symmetric division alone and favors instead a strong reprogramming component. The two contributions together are seen to be the product of a dynamic equilibrium between CSCs and CCs that is highly regulated by the kinetics of single cells, including the potential for CCs to reacquire a stem cell state and confer phenotypic plasticity to the population as a whole. We conclude that tumor malignancy can be gauged by a degree of cancer cell plasticity.

Protein Kinase C (PKC) Isozymes and Cancer

Protein kinase C (PKC) is a family of phospholipid-dependent serine/threonine kinases, which can be further classified into three PKC isozymes subfamilies: conventional or classic, novel or nonclassic, and atypical. PKC isozymes are known to be involved in cell proliferation, survival, invasion, migration, apoptosis, angiogenesis, and drug resistance. Because of their key roles in cell signaling, PKC isozymes also have the potential to be promising therapeutic targets for several diseases, such as cardiovascular diseases, immune and inflammatory diseases, neurological diseases, metabolic disorders, and multiple types of cancer. This review primarily focuses on the activation, mechanism, and function of PKC isozymes during cancer development and progression.

Cancer stem cells and radioresistance

PURPOSE

Radiation therapy has made significant contributions to cancer treatment. However, despite continuous improvements, tumor recurrence and therapy resistance still occur in a high proportion of patients. One underlying reason for this radioresistance might be attributable to the presence of cancer stem cells (CSC). The purpose of this review is to discuss CSC-specific mechanisms that confer radiation resistance.

CONCLUSIONS

We focus our discussions on breast cancer and glioblastoma multiforme (GBM) and conclude that both CSC-intrinsic and CSC-extrinsic factors as well as adaptive responses in CSC caused by irradiation and microenvironmental changes all make contributions to CSC-mediated radioresistance. Our discussions emphasize CSC as novel therapeutic targets in order to potentiate radiotherapy efficacy.

IGF1 receptor signaling regulates adaptive radioprotection in glioma stem cells

Cancer stem cells (CSCs) play an important role in disease recurrence after radiation treatment as a result of intrinsic properties such as high DNA repair capability and antioxidative capacity. It is unclear, however, how CSCs further adapt to escape the toxicity of the repeated irradiation regimens used in clinical practice. Here, we have exposed a population of murine glioma stem cells (GSCs) to fractionated radiation in order to investigate the associated adaptive changes, with the ultimate goal of identifying a targetable factor that regulates acquired radioresistance. We have shown that fractionated radiation induces an increase in IGF1 secretion and a gradual upregulation of the IGF type 1 receptor (IGF1R) in GSCs. Interestingly, IGF1R upregulation exerts a dual radioprotective effect. In the resting state, continuous IGF1 stimulation ultimately induces downregulation of Akt/extracellular-signal-regulated kinases (ERK) and FoxO3a activation, which results in slower proliferation and enhanced self-renewal. In contrast, after acute radiation, the abundance of IGF1R and increased secretion of IGF1 promote a rapid shift from a latent state toward activation of Akt survival signaling, protecting GSCs from radiation toxicity. Treatment of tumors formed by the radioresistant GSCs with an IGF1R inhibitor resulted in a marked increase in radiosensitivity, suggesting that blockade of IGF1R signaling is an effective strategy to reverse radioresistance. Together, our results show that GSCs evade the damage of repeated radiation not only through innate properties but also through gradual inducement of resistance pathways and identify the dynamic regulation of GSCs by IGF1R signaling as a novel mechanism of adaptive radioprotection.

Acute and fractionated irradiation differentially modulate glioma stem cell division kinetics

Glioblastoma multiforme (GBM) is one of the most aggressive human malignancies with a poor patient prognosis. Ionizing radiation either alone or adjuvant after surgery is part of standard treatment for GBM but remains primarily noncurative. The mechanisms underlying tumor radioresistance are manifold and, in part, accredited to a special subpopulation of tumorigenic cells. The so-called glioma stem cells (GSC) are bestowed with the exclusive ability to self-renew and repopulate the tumor and have been reported to be less sensitive to radiation-induced damage through preferential activation of DNA damage checkpoint responses and increased capacity for DNA damage repair. During each fraction of radiation, non-stem cancer cells (CC) die and GSCs become enriched and potentially increase in number, which may lead to accelerated repopulation. We propose a cellular Potts model that simulates the kinetics of GSCs and CCs in glioblastoma growth and radiation response. We parameterize and validate this model with experimental data of the U87-MG human glioblastoma cell line. Simulations are conducted to estimate GSC symmetric and asymmetric division rates and explore potential mechanisms for increased GSC fractions after irradiation. Simulations reveal that in addition to their higher radioresistance, a shift from asymmetric to symmetric division or a fast cycle of GSCs following fractionated radiation treatment is required to yield results that match experimental observations. We hypothesize a constitutive activation of stem cell division kinetics signaling pathways during fractionated treatment, which contributes to the frequently observed accelerated repopulation after therapeutic irradiation.

uPAR and cathepsin B knockdown inhibits radiation-induced PKC integrated integrin signaling to the cytoskeleton of glioma-initiating cells

Despite advances in radiotherapeutic and chemotherapeutic techniques and aggressive surgical resection, the prognosis of glioblastoma patients is dismal. Accumulation of evidence indicates that some cancer cells survive even the most aggressive treatments, and these surviving cells, which are resistant to therapy and are perhaps essential for the malignancy, may be cancer stem cells. The CD133 surface marker is commonly used to isolate these extremely resistant glioma-initiating cells (GICs). In the present study, GICs which tested positive for the CD133 marker (CD133⁺) were isolated from both the established U251 cell line and the 5310 xenograft glioma cell line to study the events related to the molecular pathogenesis of these cells. Simultaneous down-regulation of uPAR and cathepsin B by shRNA (pUC) treatment caused the disruption of radiation-induced complex formation of pPKC θ/δ, integrin β1 and PKC ζ, integrin β1 in glioma cells. Further, pUC treatment inhibited PKC/integrin signaling via FAK by causing disassociation of FAK and the cytoskeletal molecules vinculin and α-actinin. Also, we observed the inhibition of ERK phosphorylation. This inhibition was mediated by pUC and directed a negative feedback mechanism over the FAK signaling molecules, which led to an extensive reduction in the signal for cytoskeletal organization generating migratory arrest. Altogether, it can be hypothesized that knockdown of uPAR and cathepsin B using shRNA is an effective strategy for controlling highly invasive glioma cells and extremely resistant glioma-initiating cells.