Wortmannin sensitizes human glioblastoma cell lines carrying mutant and wild type TP53 gene to radiation

The DNA Double-Strand Break Repair in Glioma: Molecular Players and Therapeutic Strategies

Gliomas are the most frequent type of tumor in the central nervous system, which exhibit properties that make their treatment difficult, such as cellular infiltration, heterogeneity, and the presence of stem-like cells responsible for tumor recurrence. The response of this type of tumor to chemoradiotherapy is poor, possibly due to a higher repair activity of the genetic material, among other causes. The DNA double-strand breaks are an important type of lesion to the genetic material, which have the potential to trigger processes of cell death or cause gene aberrations that could promote tumorigenesis. This review describes how the different cellular elements regulate the formation of DNA double-strand breaks and their repair in gliomas, discussing the therapeutic potential of the induction of this type of lesion and the suppression of its repair as a control mechanism of brain tumorigenesis.

Effect of hnRNPA2/B1 on the proliferation and apoptosis of glioma U251 cells via the regulation of AKT and STAT3 pathways

Glioma is the most common malignant tumor in the human central nervous system. Although heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNPA2/B1) was previously presumed to be a tumor-promoting gene, the relationship between hnRNPA2/B1 and glioma is unclear. Targeting hnRNPA2/B1 interference in glioma cells can significantly inhibit proliferation and increase apoptosis of human glioma cells in vitro. In a tumor xenograft model, knockdown of hnRNPA2/B1 suppressed tumor growth in glioma cells in vivo. In terms of a mechanism, the knockdown of hnRNPA2/B1 led to inactivation of the AKT and STAT3 signaling pathways, which ultimately reduced the expression of B-cell lymphoma-2 (Bcl-2), CyclinD1 and proliferating cell nuclear antigen (PCNA). Collectively, these data suggest that the inhibition of hnRNPA2/B1 can reduce the growth of gliomas through STAT3 and AKT signaling pathways, and this inhibition is expected to be a therapeutic target for gliomas.

The Role of Class IA Phosphatidylinositol-4,5-Bisphosphate 3-Kinase Catalytic Subunits in Glioblastoma

Phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) plays a critical role in the pathogenesis of cancer including glioblastoma, the most common and aggressive form of brain cancer. Targeting the PI3K pathway to treat glioblastoma has been tested in the clinic with modest effect. In light of the recent finding that PI3K catalytic subunits (PIK3CA/p110α, PIK3CB/p110β, PIK3CD/p110δ, and PIK3CG/p110γ) are not functionally redundant, it is imperative to determine whether these subunits play divergent roles in glioblastoma and whether selectively targeting PI3K catalytic subunits represents a novel and effective strategy to tackle PI3K signaling. This article summarizes recent advances in understanding the role of PI3K catalytic subunits in glioblastoma and discusses the possibility of selective blockade of one PI3K catalytic subunit as a treatment option for glioblastoma.

Phytochemicals and PI3K Inhibitors in Cancer-An Insight

In today's world of modern medicine and novel therapies, cancer still remains to be one of the prime contributor to the death of people worldwide. The modern therapies improve condition of cancer patients and are effective in early stages of cancer but the advanced metastasized stage of cancer remains untreatable. Also most of the cancer therapies are expensive and are associated with adverse side effects. Thus, considering the current status of cancer treatment there is scope to search for efficient therapies which are cost-effective and are associated with lesser and milder side effects. Phytochemicals have been utilized for many decades to prevent and cure various ailments and current evidences indicate use of phytochemicals as an effective treatment for cancer. Hyperactivation of phosphoinositide 3-kinase (PI3K) signaling cascades is a common phenomenon in most types of cancers. Thus, natural substances targeting PI3K pathway can be of great therapeutic potential in the treatment of cancer patients. This chapter summarizes the updated research on plant-derived substances targeting PI3K pathway and the current status of their preclinical studies and clinical trials.

Small Molecule and Monoclonal Antibody Therapies in Neurooncology

Background:

The prognosis for most patients with primary brain tumors remains poor. Recent advances in molecular and cell biology have led to a greater understanding of molecular alterations in brain tumors. These advances are being translated into new therapies that will hopefully improve the prognosis for patients with brain tumors.

Methods:

We reviewed the literature on small molecule targeted agents and monoclonal antibodies used in brain tumor research and brain tumor clinical trials for the past 20 years.

Results:

Brain tumors commonly express molecular abnormalities. These alterations can lead to the activation of cell pathways involved in cell proliferation. This knowledge has led to interest in novel anti-brain-tumor therapies targeting key components of these pathways. Many drugs and monoclonal antibodies have been developed that modulate these pathways and are in various stages of testing.

Conclusions:

The use of targeted therapies against brain tumors promises to improve the prognosis for patients with brain tumors. However, as the molecular pathogenesis of brain tumors has not been linked to a single genetic defect or target, molecular agents may need to be used in combinations or in tandem with cytotoxic agents. Further study of these agents in well-designed cooperative clinical trials is needed.

Current Understanding on EGFR and Wnt/β-Catenin Signaling in Glioma and Their Possible Crosstalk

Glioblastoma multiformes (GBMs) are extensively heterogeneous at both cellular and molecular levels. Current therapeutic strategies include targeting of key signaling molecules using pharmacological inhibitors in combination with genotoxic agents such as temozolomide. In spite of all efforts, the prognosis of glioma patients remains dismal. Therefore, a proper understanding of individual molecular pathways responsible for the progression of GBM is necessary. The epidermal growth factor receptor (EGFR) pathway is probably the most significant signaling pathway clinically implicated in glioma. Not surprisingly, anti-EGFR therapies mostly prevail for therapeutic purposes. The Wnt/β-catenin pathway is well implicated in multiple tumors; however, its role in glioma has only recently started to emerge. We give a concise account of the current understanding of the role of both these pathways in glioma. Last, taking evidences from a limited literature, we outline a number of points where these pathways intersect each other and put forward the possibility of combinatorially targeting them for treatment of glioma.

Molecular neuro-oncology and development of targeted therapeutic strategies for brain tumors. Part 2: PI3K/Akt/PTEN, mTOR, SHH/PTCH and angiogenesis

Brain tumors are a diverse group of malignancies that remain refractory to conventional treatment approaches. Molecular neuro-oncology has now begun to clarify the transformed phenotype of brain tumors and identify oncogenic pathways that might be amenable to targeted therapy. Activity of the phosphoinositide 3; kinase (PI3K)/Akt pathway is often upregulated in brain tumors due to excessive stimulation by growth factor receptors and Ras. Loss of function of the tumor suppressor gene PTEN also frequently contributes to upregulation of PI3K/Akt. Several compounds, such as wortmannin and LY-294002, can target PI3K and inhibit activity of this pathway. The mammalian target of rapamycin (mTOR) is an important regulator of cell growth and metabolism and is often upregulated by Akt. Clinical trials of CCI-779, an inhibitor of mTOR, are ongoing in recurrent malignant glioma patients. The sonic hedgehog/PTCH pathway is involved in the tumorigenesis of some familial and sporadic medulloblastomas. This pathway can be targeted by cyclopamine, which is under evaluation in preclinical studies. Angiogenesis is a critical process for development and progression of brain tumors. Targeted approaches to inhibit angiogenesis include monoclonal antibodies, receptor tyrosine kinase inhibitors, antisense oligonucleotides and gene therapy. Clinical trials are ongoing for numerous angiogenesis inhibitors, including thalidomide, CC-5103 and PTK 787/ZK 222584. Further development of targeted therapies and evaluation of these new agents in clinical trials will be needed to improve survival and quality of life of patients with brain tumors.

Novel therapies in glioblastoma

Conventional treatment of glioblastoma has advanced only incrementally in the last 30 years and still yields poor outcomes. The current strategy of surgery, radiation, and chemotherapy has increased median survival to approximately 15 months. With the advent of molecular biology and consequent improved understanding of basic tumor biology, targeted therapies have become cornerstones for cancer treatment. Many pathways (RTKs, PI3K/AKT/mTOR, angiogenesis, etc.) have been identified in GBM as playing major roles in tumorigenesis, treatment resistance, or natural history of disease. Despite the growing understanding of the complex networks regulating GBM tumors, many targeted therapies have fallen short of expectations. In this paper, we will discuss novel therapies and the successes and failures that have occurred. One clear message is that monotherapies yield minor results, likely due to functionally redundant pathways. A better understanding of underlying tumor biology may yield insights into optimal targeting strategies which could improve the overall therapeutic ratio of conventional treatments.

PI3Kinase signaling in glioblastoma

Glioblastoma (GBM) is the most common primary tumor of the CNS in the adult. It is characterized by exponential growth and diffuse invasiveness. Among many different genetic alterations in GBM, e.g., mutations of PTEN, EGFR, p16/p19 and p53 and their impact on aberrant signaling have been thoroughly characterized. A major barrier to develop a common therapeutic strategy is founded on the fact that each tumor has its individual genetic fingerprint. Nonetheless, the PI3K pathway may represent a common therapeutic target to most GBM due to its central position in the signaling cascade affecting proliferation, apoptosis and migration. The read-out of blocking PI3K alone or in combination with other cancer pathways should mainly focus, besides the cytostatic effect, on cell death induction since sublethal damage may induce selection of more malignant clones. Targeting more than one pathway instead of a single agent approach may be more promising to kill GBM cells.

Beryllium sulfate induces p21 CDKN1A expression and a senescence-like cell cycle arrest in susceptible cancer cell types

In fibroblasts, beryllium salt causes activation of the p53 transcription factor and induction of a senescence-like state. It is not known whether Be(2+) can affect the proliferation of cancer cells, which are generally unsusceptible to senescence. A172 glioblastoma and RKO colon carcinoma cell lines each have wildtype p53, so these cell types have the potential to be responsive to agents that activate p53. In A172 cells, BeSO(4) produced a G(0)/G(1)-phase cell cycle arrest and increased expression of senescence-associated β-galactosidase, an enzymatic marker of senescence. BeSO(4) caused phosphorylation of serine-15 of p53, accumulation of p53 protein, and expression of p21, the cyclin-dependent kinase inhibitor that is prominent during senescence. BeSO(4) inhibited A172 growth with an IC(50) = 4.7 μM in a 6-day proliferation assay. In contrast, BeSO(4) had no effect on RKO cells, even though Be(2+) uptake was similar for the two cell types. This differential responsiveness marks BeSO(4) as a reagent capable of activating a separable branch of the p53 signaling network. A172 and RKO cells are known to exhibit p53-dependent upregulation of p21 in response to DNA damage. The RKO cells produced high levels of p21 when exposed to DNA damaging agents, yet failed to express p21 when treated with BeSO(4). Conversely, BeSO(4) did not cause DNA damage in A172 cells, yet it was a potent inducer of p21 expression. These observations indicate that the growth control pathway affected by BeSO(4) is distinct from the DNA damage response pathway, even though both ultimately converge on p53 and p21.

Enhanced induction of apoptosis in a radio-resistant bladder tumor cell line by combined treatments with X-rays and wortmannin

The radiosensitizing effect of wortmannin (WM) treatment during and after irradiation was studied in radioresistant bladder tumor cell lines with normal (MGH-U1 cells) or defective p53 activity (RT112 cells). WM modulated G(2)/M cell cycle arrest induced by higher X-ray doses (10 Gy) in both cell lines, although the alteration was significant only in RT112 cells. The observation suggests that WM activity is independent of p53. Constitutive expression of DNA-PKcs was found to be higher in RT112 cells than in MGH-U1. Treatment with WM enhanced radiation-induced apoptosis significantly in RT112 cells while it had no effect on MGH-U1 cells. Although a variety of PI3-kinases and PI3-K like kinases (including ATM) could be inhibited by WM, our observation of increased early lethality by WM treatment in RT112 is in agreement with previous results. They suggest that the WM-dependent radiosensitization of RT112 is a direct consequence of the inhibition of DNA-PK, resulting in the inhibition of DSB repair in the fast component. This early effect in the p53 deficient cell line could also indicate that processes other than apoptosis may contribute to the increased radiosensitization. In our opinion, the expression level of DNA-PKcs in human tumor cells may be a good predictor for the success of DNA-PKcs inhibitors when used as radiosensitizers.

The Adenoviral E4orf6 Protein Induces Atypical Apoptosis in Response to DNA Damage

Adenoviral proteins interact with host-cell proteins to either exploit or inhibit cellular functions for the purpose of viral propagation. E4orf6, the 34-kDa gene product of the E4 gene, interacts with the double-strand break repair (DSBR) protein DNA-dependent protein kinase and cooperates with binding partner E1B-55K to degrade MRE11, preventing viral DNA concatemer formation. We previously demonstrated that E4orf6 radiosensitizes human tumor cells through the inhibition of DSBR, notably in the absence of E1B-55K. Here, we report that E4orf6 prolongs the signaling of DNA damage by inhibiting the activity of protein phosphatase 2A (PP2A), the phosphatase responsible for dephosphorylating gammaH2AX. The inhibition of PP2A occurs without significant disruption of the DNA re-ligation rate. Prolonged signaling of DNA damage in the presence of E4orf6 initiates caspase-dependent and independent cell death. This is accompanied by poly(ADP-ribose) polymerase (PARP) hyperactivation and the translocation of apoptosis-inducing factor (AIF) from the mitochondria to the nucleus. Knockdown of AIF by shRNA rescues the radiosensitization induced by E4orf6. Taken together, these data suggest that E4orf6 disrupts cellular DSBR signaling by inhibiting PP2A, leading to prolonged H2AX phosphorylation, hyperactivation of PARP, and AIF translocation to the nucleus. The function of E4orf6 as an inhibitor of PP2A and activator of PARP in the absence of other adenoviral gene products is of importance in delineating the adenovirus-host cell interplay.

Microarray analysis of temporal gene responses to ionizing radiation in two glioblastoma cell lines: up-regulation of DNA repair genes

To determine the patterns of gene expression responsible for the radiosensitivity of glioblastoma cells, we analyzed transcriptional changes after ionizing radiation in different cell lines. After completing clonogenic survival assays, we selected two glioblastoma cell lines with different radiosensitivities. Subsequently, they were investigated by using the technique of DNA microarray, and we then categorized the upregulated genes into 10 groups. Between the two cell lines, the difference in the percentage of DNA repair/replication category was the largest, and this category was present at a greater percentage with radioresistant cell line U87MG. Moreover, among the commonly upregulated genes, the DNA repair/replication category was present in the largest percentage. These genes included G22P1 (Ku70) and XRCC5 (Ku80) genes known as important members of the nonhomologous end-joining (NHEJ) pathway of DNA double strand break (DSB) repair. Furthermore, cell line that specifically upregulated genes included the members of major pathways of DNA DSB or single strand damage repair. These pathways were not only NHEJ, but also homologous recombination (HR) and postreplication repair (PRR). In conclusion, the distribution of genes involved in the DNA repair/replication category was most different between two human glioblastoma cell lines of different radiosensitivities. Among commonly upregulated genes, the DNA repair/replication category was present in the largest percentage.

Reduction of nitroxides and radioprotective ability in glioblastoma cells

Electron spin resonance (ESR) analyses were performed to clarify whether glioblastoma cells scavenge hydroxyl radicals (*OH) generated by x-ray irradiation. The rate of bioreduction of nitroxides by three human glioblastoma cells was also evaluated by the same technique and compared with their x-ray sensitivity. Aerated culture media containing 200mM of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) with or without U87MG cells were irradiated with x-rays at a dose of 20Gy. ESR was measured immediately after each irradiation. Continuous changes of the ESR spectra of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol) were analyzed in cell suspensions of TK1, U87MG, and A172 at a concentration of 1.0 x 10(7) cells/ml containing 5 microM Tempol. As a result, the signal of DMPO-OH in the U87MG cell suspension decayed faster than that in the control culture media without cells, and the rate of bioreduction of Tempol in each glioblastoma cell suspension was correlated with the x-ray sensitivity defined from the colony-forming assay in those cell lines. It was indicated that the resistance of glioblastoma cells to ionizing radiation could be closely related to their ability to scavenge radical species generated by ionizing radiation.