Reversibility of inhibition of DNA double strand break repair by 2-deoxy-D-glucose in Ehrlich ascites tumour cells

Auxiliary ATP binding sites support DNA unwinding by RecBCD

The RecBCD helicase initiates double-stranded break repair in bacteria by processively unwinding DNA with a rate approaching ∼1,600 bp·s⁻¹, but the mechanism enabling such a fast rate is unknown. Employing a wide range of methodologies — including equilibrium and time-resolved binding experiments, ensemble and single-molecule unwinding assays, and crosslinking followed by mass spectrometry — we reveal the existence of auxiliary binding sites in the RecC subunit, where ATP binds with lower affinity and distinct chemical interactions as compared to the known catalytic sites. The essentiality and functionality of these sites are demonstrated by their impact on the survival of E.coli after exposure to damage-inducing radiation. We propose a model by which RecBCD achieves its optimized unwinding rate, even when ATP is scarce, by using the auxiliary binding sites to increase the flux of ATP to its catalytic sites.

RecBCD is a remarkably fast DNA helicase. Using a battery of biophysical methods, Zananiri et. al reveal additional, non-catalytic ATP binding sites that increase the ATP flux to the catalytic sites that allows fast unwinding when ATP is scarce.

Radiosensitization of Murine Ehrlich Ascites Tumor by a Combination of 2-deoxy-D-Glucose and 6-Aminonicotinamide

Enhanced radiosensitizing effects of a combination of 2-deoxy-D-glucose (2-DG), a glycolytic inhibitor and 6-aminonicotinamide (6-AN) an analogue of nicotinamide, which inhibits hexose monophosphate shunt (HMP) have been demonstrated in vitro. The purpose of the present studies is to investigate in vivo effects of this combination in Ehrlich ascites tumor (EAT) bearing mice. EAT tumor was grown in Swiss albino strain A mice. Treatment induced growth delay and tumor free animal survival were evaluated as parameters of radiation response. Focal irradiation of the tumor with a single fraction of 10 Gy induced a moderate delay in tumor growth but did not lead to complete regression of the tumor. Intravenous administration of either 6-AN or 2-DG immediately before irradiation enhanced radiation-induced growth delay with a cure rate of 45%. However, administration of a combination of 2-DG (2 g/kg b.wt.) and 6-AN (2 mg/kg b.wt.) immediately before irradiation led to complete regression of tumor in 80% animals resulting in survival of more than 300 days. A similar response (≈80%) was observed when 2-DG dose was reduced to 1 g/kg in combination with 6-AN. It is concluded that 6-AN enhances the radiosensitizing effects of 2-DG and the combination may have potential application in improving radiotherapy of tumors.

A combination of 2-deoxy-D-glucose and 6-aminonicotinamide induces cell cycle arrest and apoptosis selectively in irradiated human malignant cells

Previously, we have shown that a combination of metabolic modifiers 2-deoxy-D-glucose (2-DG) and 6-aminonicotinamide (6-AN) results in oxidative stress mediated radiosensitization of malignant cells via mitochondrial dysfunction and non-coordinated expression of antioxidant defense, besides inhibition of repair and recovery. In the present study, our objective was to study, in a panel of human malignant cells of various origins (lung carcinoma, squamous carcinoma, oral carcinoma, and glioblastoma), if the inhibitory activity of combination (2-DG+6-AN+2 Gy) against tumor growth could be considered a general phenomenon and to determine its effect on the cell cycle. The results revealed that combination (2-DG+6-AN+2 Gy) treatment result in significant cell growth inhibition and induced ROS generation in all cancer cells studied. The anti-proliferative effect was related to the ability of combination (2-DG+6-AN+2 Gy) to provoke growth inhibition at the G2/M arrest and apoptosis. Furthermore, combination (2-DG+6-AN+2 Gy) induced G2/M arrest is closely correlated to decreased cyclin A, cyclin B1, and cdc2 levels.

p53 is an important factor for the radiosensitization effect of 2-deoxy-D-glucose

Metabolic change in cancer cells by preferential production of energy through glycolysis is a well-documented characteristic of cancer. However, whether inhibition of glycolysis will enhance the efficacy of radiation therapy is a matter of debate. In this study which uses lung cancer as the model, we demonstrate that the improvement of radiotherapy by 2-deoxy-D-glucose (2DG) is p53-dependent. Based on clonogenic survival data, we show that p53-deficient lung cancer cells (H358) are more sensitive to 2DG treatment when compared to p53 wild-type lung cancer cells (A549). The effective doses of 2DG at 0.5-surviving fraction of A549 and H358 are 17.25 and 4.61 mM, respectively. Importantly, 2DG exhibits a significant radiosensitization effect in A549 cells but not in H358 cells. Treatment with 2DG increases radiation-induced p53 protein levels in A549 cells. siRNA inhibition of p53 in A549 cells reduces the radiosensitization effect of 2DG. Furthermore, ectopic expression of wild-type p53 in H358 cells significantly enhances the radiosensitization effect of 2DG as determined by colony formation assay. In nude mice injected with A549 cells, treatment of 2DG enhances the efficacy of radiation therapy. Together, these results suggest that inhibition of glycolysis may only be beneficial for radiation therapy of cancer expressing wild-type p53.

Radiosensitization by 6-aminonicotinamide and 2-deoxy-D-glucose in human cancer cells

The aim was to exploit simultaneous inhibition of glycolytic and pentose phosphate pathways of energy production for radiosensitization using 2-deoxy-D-glucose (2-DG) and 6-aminonicotinamide (6-AN) in transformed mammalian cells. Two human tumour cell lines (cerebral glioma, BMG-1 and squamous carcinoma cells 4197) were investigated. 2-DG and/or 6-AN added at the time of irradiation were present for 4 h after radiation. Radiation-induced cell death (macrocolony assay), cytogenetic damage (micronuclei formation), cell cycle delay (bromodeoxyuridne (BrdU) pulse chase), apoptosis (externalization of phosphotidylserine (PS) by annexin V), chromatin-bound proliferation cell nuclear antigen (PCNA) and cellular glutathione (GSH) levels were investigated as parameters of radiation response. The presence of 2-DG (5 mM) during and for 4 h after irradiation increased the radiation-induced micronuclei formation and cell death, and caused a time-dependent decrease in GSH levels in BMG-1 cells while no significant effects could be observed in 4197 cells. 6-AN (5 microM) enhanced the radiosensitivity of both cell lines and reduced the GSH content by nearly 50% in gamma-irradiated 4197 cells. Combining 2-DG and 6-AN caused a profound decrease in the GSH content and enhanced the radiation damage in both the cell lines by increasing mitotic and apoptotic cell death. Further, the combination (2-DG + 6-AN) enhanced the radiation-induced G2 block, besides arresting cells in S phase and inhibited the recruitment of PCNA. The combination of 2-DG and 6-AN enhances radiation damage by modifying damage response pathways and has the potential for improving radiotherapy of cancer.

Hypoxia, HIF1 and glucose metabolism in the solid tumour

It has been known for many years that cellular metabolism within the solid tumour is markedly different from that of the corresponding normal tissue. The transcription factor hypoxia-inducible factor 1 (HIF1) has been implicated in regulating many of the genes that are responsible for the metabolic difference. However, it remains unclear how this 'aerobic glycolysis', originally described by Otto Warburg, offers tumour cells a growth advantage. As discussed in this Perspective, new data suggests that this metabolic switch may provide a benefit to the tumour not by increasing glycolysis but by decreasing mitochondrial activity.

Establishment and characterization of multicellular spheroids from a human glioma cell line; Implications for tumor therapy

Background

Multicellular spheroids, an appropriate in vitro system for simulating 3-D tumor micro-milieu can be used for evaluating and predicting tumor response to therapeutic agents including metabolic inhibitors. However, detailed understanding of the nature, distribution and sensitivity/responses of cellular sub-populations to potential therapeutic agents/strategies is required for using this unique model with optimal precision. Spheroid characteristics may also vary considerably with the origin and type of cell line used, and thorough characterization of viable and dissociated glioma cell spheroids is not yet completely known. In order to evaluate in vivo responses of gliomas to various therapeutic strategies, especially the metabolic inhibitors capable of penetrating the blood brain barrier, we have characterized continuously growing spheroids of a human glioma cell line (BMG-1) with respect to organization, growth, viability, cell survival, cell death, metabolic and mitochondrial status, oxidative stress and radiation response using microscopy, flow cytometry and enzymatic assays. Spheroids were fed daily with fresh medium in order to maintain nutrient supply to outer cellular layers while hypoxia/necrosis developed in the innermost cells of enlarging spheroids.

Results

Volume of spheroids, fed daily with fresh medium, increased exponentially during 7–28 days of growth through three population doublings. Proportion of G₁-phase cells was higher (~60%) than exponentially growing monolayer cells (~48%). A significant fraction of S-phase cells turned metabolically inactive (disengaged in DNA synthesis) with increasing age of the spheroids, unlike in quiescent monolayer cultures, where the fraction of S-phase cells was less than 5%. With increasing spheroid size, increasing sub-populations of cells became non-viable and entered apoptosis or necrosis revealed by Annexin-V-FITC/PI staining. PI positive (necrotic) cells were not confined to the centre of the spheroid, but distributed at certain discrete foci. Average glucose consumption and lactate production were 2–3 folds higher in viable spheroid cells compared to monolayer cells, implying a compensatory increase in glycolysis possibly due to hypoxic environment. HIF-1α was expressed only in spheroids and increased in an age-dependent manner, whereas c-Myc (known to induce apoptosis in glucose-deprived cells) levels were three times higher than monolayer cells. Mitochondrial mass and activity decreased significantly during first 14 days of growth but increased with age, and were not associated with increase in ROS levels. Bcl-2 and Bax levels were higher (~2 folds) than monolayers, while the ratio (Bcl/Bax) remained unaltered. Radiation-induced oxidative stress was considerably less in spheroids as compared to monolayers, and corresponded well with increase in radioresistance demonstrated by the clonogenic assay, similar to hypoxia induced radioresistance observed in tumors.

Conclusion

Development of S-negative cells and reduced endogenous and radiation-induced ROS coupled with higher levels of anti (Bcl2) as well as pro (Bax) apoptotic regulators observed in spheroids suggest the intricate/complex nature of endogenous as well as induced stress resistance that could exist in tumors, which contribute to the treatment resistance.

Heterogeneity in 2-deoxy-D-glucose-induced modifications in energetics and radiation responses of human tumor cell lines

PURPOSE

The glucose analog and glycolytic inhibitor, 2-deoxy-D-glucose (2-DG), has been shown to differentially enhance the radiation damage in tumor cells by inhibiting the postirradiation repair processes. The present study was undertaken to examine the relationship between 2-DG-induced modification of energy metabolism and cellular radioresponses and to identify the most relevant parameter(s) for predicting the tumor response to the combined treatment of radiation + 2-DG.

METHODS AND MATERIALS

Six human tumor cell lines (glioma: BMG-1 and U-87, squamous cell carcinoma: 4451 and 4197, and melanoma: MeWo and Be-11) were investigated. Cells were exposed to 2 Gy of Co-60 gamma-rays or 250 kVP X-rays and maintained under liquid-holding conditions 2-4 h to facilitate repair. 2-DG (5 mM, equimolar with glucose) that was added at the time of irradiation was present during the liquid holding. Glucose utilization, lactate production (enzymatic assays), and adenine nucleotides (high performance liquid chromatography and capillary isotachophoresis) were investigated as parameters of energy metabolism. Induction and repair of DNA damage (comet assay), cytogenetic damage (micronuclei formation), and cell death (macrocolony assay) were analyzed as parameters of radiation response.

RESULTS

The glucose consumption and lactate production of glioma cell lines (BMG-1 and U-87) were nearly 2-fold higher than the squamous carcinoma cell lines (4197 and 4451). The ATP content varied from 3.0 to 6.5 femto moles/cell among these lines, whereas the energy charge (0.86-0.90) did not show much variation. Presence of 2-DG inhibited the rate of glucose usage and glycolysis by 30-40% in glioma cell lines and by 15-20% in squamous carcinoma lines, while ATP levels reduced by nearly 40% in all the four cell lines. ATP:ADP ratios decreased to a greater extent ( approximately 40%) in glioma cells than in squamous carcinoma 4451 and MeWo cells; in contrast, presence of 2-DG reduced ADP:AMP ratios by 3-fold in the squamous carcinoma 4451, whereas an increase was noted in the glioma cell line BMG-1. 2-DG significantly reduced the initial rates of DNA repair in all cells, resulting in an excess residual damage after 2 h of repair in BMG-1, U-87, and 4451 cell lines, whereas no significant differences could be observed in the other cell lines. Recovery from potentially lethal damage was also significantly inhibited in BMG-1 cells. 2-DG increased the radiation-induced micronuclei formation in the melanoma line (MeWo) by nearly 60%, while a moderate (25-40%) increase was observed in the glioma cell lines (BMG-1 and U-87). Presence of 2-DG during liquid holding (4 h) enhanced the radiation-induced cell death by nearly 40% in both the glioma cell lines, while significant effects were not observed in others.

CONCLUSIONS

The modifications in energetics and radiation responses by 2-DG vary considerably among different human tumor cell lines, and the relationships between energy metabolism and various radiobiologic parameters are complex in nature. The 2-DG-induced modification of radiation response does not strictly correlate with changes in the levels of ATP. However, a significant enhancement of the radiation damage by 2-DG was observed in cells with high rates of glucose usage and glycolysis, which appear to be the two most important factors determining the tumor response to the combined treatment of 2-DG + radiation therapy.

Abrogation of G(2)/M-phase block enhances the cytotoxicity of daunorubicin, melphalan and cisplatin in TP53 mutant human tumor cells

Irradiation of human melanoma (MeWo, Be11) and squamous cell carcinoma (4451, 4197) cells induces cell cycle blocks from which the cells recover to re-enter mitosis after 40-60 h. In the TP53 mutant cell lines, MeWo and 4451, irradiation induces a G(2)-phase block, where the fraction of cells in G(2) phase reaches a maximum after 18-20 h. In the TP53 wild-type cell lines, 4197 and Be11, a G(1)- and G(2)-phase block is reached 12 and 16 h postirradiation, respectively. Addition of pentoxifylline after irradiation at the time when the number of cells in G(2) phase has reached a maximum shortens the normal recovery from G(2)-phase block to approximately 7 h. Addition of daunorubicin, melphalan and cisplatin under these conditions markedly enhanced drug toxicity. In the TP53-mutated cell lines MeWo and 4451, the survival ratio at 7 Gy measured by colony formation was 2.3-2.8, 8.6-85 and 52-74 for daunorubicin, melphalan and cisplatin, respectively. In the TP53 wild-type cell lines, the corresponding survival ratios were found to be 1.3-1.4, 2.3-3.0 and 1.2-2.6, respectively. The survival ratios are for clonogenic survival after 7 Gy and 2 mM pentoxifylline and measure the influence of drug doses that ensure 95% survival in nonirradiated controls. The results indicate that the G(2)-phase block is a crucial event in the damage response that can be manipulated to achieve a significant enhancement of drug toxicity. These effects are particularly pronounced in TP53 mutant cells and are observed at drug doses well below the clinical range.

Hematoporphyrin derivatives potentiate the radiosensitizing effects of 2-deoxy-D-glucose in cancer cells

PURPOSE

Two deoxy-D-glucose (2-DG), an inhibitor of glucose transport and glycolysis, has been shown to differentially inhibit the repair of radiation damage in cancer cells by reducing the flow of metabolic energy. Since hematoporphyrin derivatives (Hpd) inhibit certain enzymes of the respiratory metabolism, resulting in an increase in the glucose usage and glycolysis, Hpd could possibly enhance the energy-linked radiosensitizing effects of 2-DG in cancer cells. The purpose of the present work was to verify this suggestion.

METHODS AND MATERIALS

Two human tumor cell lines (cerebral glioma, BMG-1 and squamous cell carcinoma, 4197) and a murine tumor cell line (Ehrlich ascites tumor [EAT], F-15) in vitro were investigated. A commercially available preparation of Hpd, Photosan-3 (PS-3) was used in the present studies. Cells incubated with 0-10 microg/ml PS-3 for 0-24 h before irradiation were exposed to 2.5 Gy of Co-60 gamma rays and maintained under liquid holding conditions for 1-4 h to facilitate repair. 2-DG (0-5 mM) added at the time of irradiation was present during the liquid holding. Radiation-induced cytogenetic damage (micronuclei formation) and cell death (macrocolony assay) were analyzed as parameters of radiation response. Effects of these radiosensitizers on glucose usage and glycolysis were also studied by measuring the glucose consumption and lactate production using enzymatic assays.

RESULTS

The glucose consumption and lactate production of BMG-1 cells (0.83 and 1.43 pmole/cell/h) were twofold higher than in the 4197 cells (0.38 and 0.63 pmole/cell/h). Presence of PS-3 (10 microg/ml) enhanced the rate of glycolysis (glucose consumption and lactate production) in these cells by 35% to 65%, which was reduced by 20% to 40% in the presence of 5 mM 2-DG. In exponentially growing BMG-1 and EAT cells, presence of 2-DG (5 mM; equimolar with glucose) for 4 hours after irradiation increased the radiation-induced micronuclei formation and cell death by nearly 40%, whereas no significant effects could be observed in 4197 cells. In EAT cells, radiation was also observed to induce apoptotic death, which was significantly increased in the presence of the combination (PS-3 + 2-DG). The combination (PS-3 + 2-DG) enhanced the radiation damage in all three cell systems by 60-100%. Furthermore, the radiosensitizing effects of the combination (PS-3 + 2-DG) were higher at pH 6.7 as compared to pH 7. 4. In the plateau phase, presence of 2-DG alone did not significantly influence the radiation response of either BMG-1 or of 4197 cells, whereas in combination with PS-3, 2-DG enhanced the radiation damage in both these cell lines by 40% to 50%. Furthermore, in BMG-1 cells, the effects of 2-DG were observed to be reversible to a very great extent, while that of the combination were mostly irreversible.

CONCLUSION

The hematoporphyrin derivative PS-3 enhances the radiosensitizing effects of 2-DG in cancer cells, possibly by further reducing the energy supply leading to an irreversible inhibition of DNA repair, and increased cytogenetic damage and cell death. Since both these compounds have been used in clinical practice, further studies to investigate their use in improving radiotherapy of tumors are warranted.

Comparative study of the repair kinetics of chromosomal aberrations and DNA strand breaks in proliferating and quiescent CHO cells

Repair kinetics observable at the level of exchange-type chromosomal aberrations (dicentric chromosomes), using fractionation and delayed-plating techniques, have been compared with repair kinetics of radiation-induced DNA double-strand breaks, measured with PFGE, and with repair kinetics of all strand breaks, measured with the alkali-unwinding technique. Only data from quiescent or proliferating CHO K1 cells obtained in the same laboratory were used. We determined repair kinetics in terms of the time constant tau (equal to half-time/log(e)2). The repair kinetics (tau approximately 11-14 min) observed in the split-dose formation of dicentric chromosomes agrees with fast repair kinetics of double-strand breaks (tau approximately 11-13 min), thus permitting us to identify the latter as the 'primary lesions' whose pairwise interaction leads to the beta D2 yield term of the aberrations. The repair kinetics observed for dicentric chromosomes formed under delayed-plating conditions (tau approximately 75 min), which mainly affects the alpha D yield term, is attributed to an intermediate interchromosomal product temporarily existing in the course of aberration formation; it is suggested that this product is mechanistically correlated with the slow repair kinetics of 'clustered damage' to DNA seen with the applied molecular methods (tau approximately 90 min).

Improving cancer radiotherapy with 2-deoxy-D-glucose: phase I/II clinical trials on human cerebral gliomas

PURPOSE

Evaluation of tolerance, toxicity, and feasibility of combining large fraction (5 Gy) radiotherapy with 2-deoxy-D-glucose (2DG), an inhibitor of glucose transport and glycolysis, which has been shown to differentially inhibit repair of radiation damage in cancer cells.

METHODS AND MATERIALS

Twenty patients with supratentorial glioma (Grade 3/4), following surgery were treated with four weekly fractions of oral 2DG (200 mg/kg body weight) followed by whole brain irradiation (5 Gy). Two weeks later, supplement focal radiation to the tumor (14 Gy/7 fractions) was given. Routine clinical evaluation, x-ray computerized tomography (CT), and magnetic resonance (MR) imaging were carried out to study the acute and late radiation effects.

RESULTS

All the 20 patients completed the treatment without any interruption. The vital parameters were within normal limits during the treatment. None reported headache during the treatment. Mild to moderate nausea and vomiting were observed during the days of combined therapy (2DG + RT) in 10 patients. No significant deterioration of the neurological status was observed during the treatment period. Seven patients were alive at 63, 43, 36, 28, 27, 19, and 18 months of follow-up. In these patients, the clinical and MR imaging studies did not reveal any late radiation effects.

CONCLUSIONS

Feasibility of administering the treatment (2DG + 5 Gy) is demonstrated by the excellent tolerance observed in all 20 patients. Further, the clinical and MR studies also show the absence of any brain parenchymal damage.