Modification of radiation sensitivity: the oxygen effect

Rerouting the drug response: Overcoming metabolic adaptation in KRAS-mutant cancers

Mutations in guanosine triphosphatase KRAS are common in lung, colorectal, and pancreatic cancers. The constitutive activity of mutant KRAS and its downstream signaling pathways induces metabolic rewiring in tumor cells that can promote resistance to existing therapeutics. In this review, we discuss the metabolic pathways that are altered in response to treatment and those that can, in turn, alter treatment efficacy, as well as the role of metabolism in the tumor microenvironment (TME) in dictating the therapeutic response in KRAS-driven cancers. We highlight metabolic targets that may provide clinical opportunities to overcome therapeutic resistance and improve survival in patients with these aggressive cancers.

In silico simulation of the effect of hypoxia on MCF-7 cell cycle kinetics under fractionated radiotherapy

The treatment outcome of a given fractionated radiotherapy scheme is affected by oxygen tension and cell cycle kinetics of the tumor population. Numerous experimental studies have supported the variability of radiosensitivity with cell cycle phase. Oxygen modulates the radiosensitivity through hypoxia-inducible factor (HIF) stabilization and oxygen fixation hypothesis (OFH) mechanism. In this study, an existing mathematical model describing cell cycle kinetics was modified to include the oxygen-dependent G1/S transition rate and radiation inactivation rate. The radiation inactivation rate used was derived from the linear-quadratic (LQ) model with dependence on oxygen enhancement ratio (OER), while the oxygen-dependent correction for the G1/S phase transition was obtained from numerically solving the ODE system of cyclin D-HIF dynamics at different oxygen tensions. The corresponding cell cycle phase fractions of aerated MCF-7 tumor population, and the resulting growth curve obtained from numerically solving the developed mathematical model were found to be comparable to experimental data. Two breast radiotherapy fractionation schemes were investigated using the mathematical model. Results show that hypoxia causes the tumor to be more predominated by the tumor subpopulation in the G1 phase and decrease the fractional contribution of the more radioresistant tumor cells in the S phase. However, the advantage provided by hypoxia in terms of cell cycle phase distribution is largely offset by the radioresistance developed through OFH. The delayed proliferation caused by severe hypoxia slightly improves the radiotherapy efficacy compared to that with mild hypoxia for a high overall treatment duration as demonstrated in the 40-Gy fractionation scheme.

KRAS, A Prime Mediator in Pancreatic Lipid Synthesis through Extra Mitochondrial Glutamine and Citrate Metabolism

Kirsten rat sarcoma viral oncogene homolog (KRAS)-driven pancreatic cancer is very lethal, with a five-year survival rate of <9%, irrespective of therapeutic advances. Different treatment modalities including chemotherapy, radiotherapy, and immunotherapy demonstrated only marginal efficacies because of pancreatic tumor specificities. Surgery at the early stage of the disease remains the only curative option, although only in 20% of patients with early stage disease. Clinical trials targeting the main oncogenic driver, KRAS, have largely been unsuccessful. Recently, global metabolic reprogramming has been identified in patients with pancreatic cancer and oncogenic KRAS mouse models. The newly reprogrammed metabolic pathways and oncometabolites affect the tumorigenic environment. The development of methods modulating metabolic reprogramming in pancreatic cancer cells might constitute a new approach to its therapy. In this review, we describe the major metabolic pathways providing acetyl-CoA and NADPH essential to sustain lipid synthesis and cell proliferation in pancreatic cancer cells.

IκBα targeting promotes oxidative stress-dependent cell death

BACKGROUND

Oxidative stress is a hallmark of many cancers. The increment in reactive oxygen species (ROS), resulting from an increased mitochondrial respiration, is the major cause of oxidative stress. Cell fate is known to be intricately linked to the amount of ROS produced. The direct generation of ROS is also one of the mechanisms exploited by common anticancer therapies, such as chemotherapy.

METHODS

We assessed the role of NFKBIA with various approaches, including in silico analyses, RNA-silencing and xenotransplantation. Western blot analyses, immunohistochemistry and RT-qPCR were used to detect the expression of specific proteins and genes. Immunoprecipitation and pull-down experiments were used to evaluate protein-protein interactions.

RESULTS

Here, by using an in silico approach, following the identification of NFKBIA (the gene encoding IκBα) amplification in various cancers, we described an inverse correlation between IκBα, oxidative metabolism, and ROS production in lung cancer. Furthermore, we showed that novel IκBα targeting compounds combined with cisplatin treatment promote an increase in ROS beyond the tolerated threshold, thus causing death by oxytosis.

CONCLUSIONS

NFKBIA amplification and IκBα overexpression identify a unique cancer subtype associated with specific expression profile and metabolic signatures. Through p65-NFKB regulation, IκBα overexpression favors metabolic rewiring of cancer cells and distinct susceptibility to cisplatin. Lastly, we have developed a novel approach to disrupt IκBα/p65 interaction, restoring p65-mediated apoptotic responses to cisplatin due to mitochondria deregulation and ROS-production.

ROS-Mediated Therapeutic Strategy in Chemo-/Radiotherapy of Head and Neck Cancer

Head and neck cancer is a highly genetic and metabolic heterogeneous collection of malignancies of the lip, oral cavity, salivary glands, pharynx, esophagus, paranasal sinuses, and larynx with five-year survival rates ranging from 12% to 93%. Patients with head and neck cancer typically present with advanced stage III, IVa, or IVb disease and are treated with comprehensive modality including chemotherapy, radiotherapy, and surgery. Despite advancements in treatment modality and technique, noisome recurrence, invasiveness, and resistance as well as posttreatment complications severely influence survival rate and quality of life. Thus, new therapeutic strategies are urgently needed that offer enhanced efficacy with less toxicity. ROS in cancer cells plays a vital role in regulating cell death, DNA repair, stemness maintenance, metabolic reprogramming, and tumor microenvironment, all of which have been implicated in resistance to chemo-/radiotherapy of head and neck cancer. Adjusting ROS generation and elimination to reverse the resistance of cancer cells without impairing normal cells show great hope in improving the therapeutic efficacy of chemo-/radiotherapy of head and neck cancer. In the current review, we discuss the pivotal and targetable redox-regulating system including superoxide dismutases (SODs), tripeptide glutathione (GSH), thioredoxin (Trxs), peroxiredoxins (PRXs), nuclear factor erythroid 2-related factor 2/Kelch-like ECH-associated protein 1 (Nrf2/keap1), and mitochondria electron transporter chain (ETC) complexes and their roles in regulating ROS levels and their clinical significance implicated in chemo-/radiotherapy of head and neck cancer. We also summarize several old drugs (referred to as the non-anti-cancer drugs used in other diseases for a long time) and small molecular compounds as well as natural herbs which effectively modulate cellular ROS of head and neck cancer to synergize the efficacy of conventional chemo-/radiotherapy. Emerging interdisciplinary techniques including photodynamic, nanoparticle system, and Bio-Electro-Magnetic-Energy-Regulation (BEMER) therapy are promising measures to broaden the potency of ROS modulation for the benefit of chemo-/radiotherapy in head and neck cancer.

Physicochemical Considerations of Tumor Selective Drug Delivery and Activity Confinement with Particular Reference to 1,2-Bis(Sulfonyl)-1- Alkylhydrazines Delivery

Hypoxic tumor cell sub-populations are highly resistant to radiotherapy and their presence frequently causes disease recurrence and death. Here, we described the physicochemical properties required to develop superior tumor-targeted hypoxia-activated modular prodrugs that liberate extremely short-lived bis(sulfonyl)hydrazines (BSHs) as reactive cytotoxins, thereby precisely focusing cytotoxic stress on these radio-resistant hypoxic sub-populations. Therefore, cytotoxic stress will be focused on radiation resistant areas and thus strongly synergizing with radiotherapy.

Pancreatic ductal adenocarcinoma: biological hallmarks, current status, and future perspectives of combined modality treatment approaches

Pancreatic ductal adenocarcinoma (PDAC) is a highly devastating disease with poor prognosis and rising incidence. Late detection and a particularly aggressive biology are the major challenges which determine therapeutic failure. In this review, we present the current status and the recent advances in PDAC treatment together with the biological and immunological hallmarks of this cancer entity. On this basis, we discuss new concepts combining distinct treatment modalities in order to improve therapeutic efficacy and clinical outcome - with a specific focus on protocols involving radio(chemo)therapeutic approaches.

Effects of oxygen challenging to tissue redox and pO2 status

Nitroxide free radicals can serve as redox-sensitive MRI contrast agents useful to image the redox status of tissue of interest. In this study, the effect of oxygen content in the inspired gas on the kinetics of metabolism of three nitroxides has been evaluated in the muscle and tumor in mice. SCC tumors (approximate size of 1.0 cm³) on the right hind leg of female C3H/Hen MTV^- mice were prepared. Three nitroxides, 3-carboxy-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CxP), 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CmP), and 4-hydroxy-tetramethylpiperidine-N-oxyl (TEMPOL), having different lipophilicities were compared using MR redox imaging. T₁-mapping of the tissues was obtained using a multi-slice multi-echo (MSME) sequence with several TRs. The three nitroxides showed differences in accumulation and metabolism/clearance in muscle and tumor. The cell impermeable nitroxide CxP displayed kinetic patterns of slow enhancement followed by a slow decline typical of clearance rather than metabolism. The cell permeable CmP on the other hand showed a relatively faster uptake and metabolism with a modestly higher rate of metabolism in the tumor than muscle. The TEMPOL on the other hand displayed a rapid uptake and reduction with a trend of significantly rapid decay rate in tumor tissue, while slightly higher maximum signal intensity and slower decay rate was observed in normal muscle. The reduction rate of TEMPOL in the tumor was significantly enhanced when the breathing gas had 100%-oxygen while it was not significantly different in the muscle. EPR oximetry studies monitoring the oxygen dependent linewidth of TEMPOL showed that the pO₂ in the healthy tissue during carbogen breathing significantly increased normal tissue pO₂ compared to air breathing whereas breathing 100%-oxygen made normal tissue slight hypoxic. Since TEMPOL is a radioprotector, our studies show that a combination of 100%-oxygen breathing and TEMPOL has a potential to enhance radioprotective effects to normal tissue.

The anti-EGFR antibody cetuximab sensitizes human head and neck squamous cell carcinoma cells to radiation in part through inhibiting radiation-induced upregulation of HIF-1α

In this study, we investigated the mechanisms underlying cetuximab-mediated radiosensitization of HNSCC. Irradiation of HNSCC cells upregulated hypoxia-inducible factor-1 alpha (HIF-1α) via a mechanism involving de novo synthesis of HIF-1α protein. Radiation-induced upregulation of HIF-1α was completely abolished by concurrent treatment of HNSCC cells with cetuximab. Experimental elevation of constitutively expressed HIF-1α abolished cetuximab-mediated radiosensitization in HNSCC cells, whereas downregulation of HIF-1α by siRNA or a small molecule inhibitor enhanced responses of cetuximab-resistant HNSCC cells to cetuximab plus radiation. Our data suggest that cetuximab sensitizes cancer cells to ionizing radiation in part through inhibition of radiation-induced upregulation of HIF-1α.

Activity of psoralen-functionalized nanoscintillators against cancer cells upon X-ray excitation

We report development of a nanoparticle-based, X-ray-activated anticancer "nanodrug" composed of yttrium oxide (Y(2)O(3)) nanoscintillators, a fragment of the HIV-1 TAT peptide, and psoralen. In this formulation, X-ray radiation is absorbed by the Y(2)O(3) nanoscintillators, which then emit UVA light. Absorption of UVA photons by nanoparticle-tethered psoralen has the potential to cross-link adenine and thymine residues in DNA. UVA-induced cross-linking by free psoralen upon activation with UVA light has previously been shown to cause apoptosis in vitro and an immunogenic response in vivo. Studies using the PC-3 human prostate cancer cell line demonstrate that X-ray excitation of these psoralen-functionalized Y(2)O(3) nanoscintillators yields concentration-dependent reductions in cell number when compared to control cultures containing psoralen-free Y(2)O(3) nanoscintillators.

The selective hypoxia inducible factor-1 inhibitor PX-478 provides in vivo radiosensitization through tumor stromal effects

Hypoxia inducible factor-1 (HIF-1) promotes tumor cell adaptation to microenvironmental stress. HIF-1 is up-regulated in irradiated tumors and serves as a promising target for radiosensitization. We initially confirmed that the orally bioavailable HIF-1 inhibitor PX-478 reduces HIF-1 protein levels and signaling in vitro in a dose-dependent manner and provides direct radiosensitization of hypoxic cancer cells in clonogenic survival assays using C6 glioma, HN5 and UMSCCa10 squamous cells, and Panc-1 pancreatic adenocarcinoma cell lines. However, PX-478 yields striking in vivo tumor sensitization to single-dose irradiation, which cannot be explained by incremental improvement in direct tumor cell killing. We show that PX-478 prevents postradiation HIF-1 signaling and abrogates downstream stromal adaptation in C6 and HN5 reporter xenografts as measured by serial ultrasound, vascular magnetic resonance imaging, and hypoxia response element-specific micro-positron emission tomography imaging. The primacy of indirect PX-478 in vivo effects was corroborated by our findings that (a) either concurrent or early postradiation sequencing of PX-478 provides roughly equivalent sensitization and (b) constitutive vascular endothelial growth factor expression maintains refractory tumor vessel function and progression following combined radiation and PX-478. These results confirm that disruption of postradiation adaptive HIF-1 signaling by PX-478 imparts increased therapeutic efficacy through blockade of HIF-1-dependent reconstitution of tumor stromal function. Successful translation of targeted HIF-1 radiosensitization to the clinical setting will require specific consideration of tumor microenvironmental effects and mechanisms.

Oxygen enhances lethal effect of high-intensity, ultrashort electrical pulses

The study explored the effect of ambient oxygen on mammalian cell survival after exposure to 10 ns duration, high voltage electrical pulses (nsEP, 80-90 or 120-130 kV/cm; 200-400 pulses per exposure). Cell samples were equilibrated with pure nitrogen, atmospheric air, or pure oxygen prior to the nsEP treatment and were returned to the incubator (air + 5% CO2) shortly after the exposure. The experiments established that survival of hypoxic Jurkat and U937 cells exceeded that of air-equilibrated controls about twofold (P < .01). Conversely, saturation of the medium with oxygen prior to exposure decreased Jurkat cell survival about 1.5 times, P < .01. Attenuation of the cytotoxic effect under hypoxic conditions resembled a well-known effect of oxygen on cell killing by sparsely ionizing radiations and may be indicative of the similarity of underlying cell damage mechanisms.

Relationships between cell killing, mutation induction and DNA damage in X-irradiated V79 cells: the influence of oxygen and DMSO

The relationships between cell killing, mutation induction and DNA double (dsb) and single (ssb) strand breaks have been studied in V79 cells irradiated with X-rays under oxic and anoxic conditions in the presence and in the absence of dimethylsulphoxide (DMSO). Curvilinear relationships were found between all pairs of endpoints, except for dsb versus ssb. Statistical analysis of experimental data has shown that in the absence of DMSO there is evidence of good correlations between cell killing, mutation induction and dsb in oxic and anoxic conditions. However, when DMSO was present, no significant correlation was found. In the presence of oxygen DMSO always exerts a protective effect while in anoxia it is generally much less protective and induces a strong sensitization with respect to mutation induction. Possibly DMSO acts not only as a radical scavenger but also as an agent inducing chromatin relaxation and/or under anoxia, forming highly mutagenic short-term radicals. The present data suggest that lethal and mutational events are at least partially independent and not proportional to the initial number of DNA breaks. This may imply that either other kinds of lesions are involved in cell lethality and mutability, or dose-dependent repair mechanisms of dsb have to be considered.

Respiration-induced oxygen gradients in cultured mammalian cells

The effect of cell respiration on the availability of intracellular oxygen was investigated by comparing the radiosensitivities of respiring and non-respiring cells over a range of oxygen tensions. Monolayers of Chinese hamster V79 fibroblasts on glass Petri dishes were irradiated at respiration-inhibiting (4 degrees C) and normal (37 degrees C) cell-culturing temperatures. Desired extracellular oxygen concentrations were achieved by aspirating the culture medium above the cells prior to irradiation, leaving a residual thin film which prevented drying of the cells while allowing rapid equilibration with the overlying gas. Measurement of clonogenic survival revealed that, at equivalent extracellular oxygen concentrations, the cells irradiated at 37 degrees C were less radiosensitive than those irradiated at 4 degrees C. The difference in respiring and non-respiring cell radiosensitivity was dependent on cell shape, and decreased when the cells attached to the Petri dish surface were allowed to assume a flatter configuration. These results imply that at low extracellular oxygen tensions the oxygenation of critical cellular radiation target(s) is dependent on respiration and diffusion distance, as would be expected if oxygen gradients induced by respiration exist within and immediately around actively metabolizing cells.

The importance of NPSH on the radiosensitizing effect of oxygen in Chinese hamster V-79 cells

The radiosensitivity of Chinese hamster V-79-171B fibroblasts increased more rapidly with increasing partial pressure of oxygen when the cell cultures had low endogenous levels of non-protein sulphydryl (NPSH), about 5 mumol per cell compared with about 15 mumol per cell. There was a good correlation between initial NPSH content and sensitization by oxygen concentrations between 0.06 and 0.7 per cent.

Radiation injury in mouse lung: dependence on oxygen levels in the inspired gas

The relationship between radiosensitivity and the partial pressure of oxygen (PO2) in the inspired gas has been established for radiation pneumonitis as a measure of lung damage following irradiation of the mouse thorax. The radiosensitivity at low PO2 (0-1 per cent) fitted the linear transformation of the Alper, Howard-Flanders relationship giving a K value for lung tissue of 1.35 per cent oxygen with an oxygen enhancement ratio, m, of 2.13. The radiosensitivity at higher PO2 (5-21 per cent) did not fit the Alper, Howard-Flanders relationship probably because the PO2 of the inspired gas was greater than the PO2 in the alveolus. At the low PO2 levels in the inspired gas, back diffusion of oxygen from blood into the alveolus may lead to errors in the estimated value of K. If the low value of m is due to this 'contaminating' oxygen from blood then by taking a higher value for m, the amount of contaminating oxygen can be calculated (0.23 per cent) and a 'true' value for K(1.1 per cent) determined. Other uncertainties in this estimate of K due to the radiolytic consumption of oxygen and possible inadequacies in equilibration are discussed. Allowing for the uncertainties, it is concluded that the K value for lung damage lies towards the upper end of the range of K values measured for cells in vitro.

The inhibitory action of dithiocarbamates and carbon disulphide on malondialdehyde formation resulting from lipid peroxidation in rat liver microsomes

The dithiocarbamates (DTCs) disulfiram, thiram, diethyldithiocarbamate and dimethyldithiocarbamate on the equimolar base inhibited to the same extent both the lipid peroxidation (LPO) induced by ascorbic acid (non-enzymatic) and that stimulated by an NADPH-regenerating system with CCl4 admixture (enzymatic). Lipid peroxidation measurements were made in terms of malondialdehyde (MDA) formation in rat liver microsomes, or in the 9000 X g supernatant. The inhibitory action of tetramethylthiuram monosulphide was considerably weaker. Carbon disulphide (CS2) inhibited the enzymatic and non-enzymatic stimulated microsomal LPO by 4 orders less than the DTCs. In parallel with the inhibition of MDA formation, oxidative destruction of microsomal cytochrome P-450 was delayed with increasing concentrations of the DTCs. A well-correlated, non-linear, semi-logarithmic relation was found for the concentration-activity relationship for all DTCs and CS2. As the DTCs inhibited LPO both in heat-denatured and freshly prepared microsomes, it can be deduced that the DTCs intervene in a non-enzymatic oxidation phase of the LPO. The DTC inhibitory action is attributed to a radical-trap mechanism since LPO that has already been initiated was inhibited with the DTCs. However, more inhibitor is required to trap the radicals the later the DTC administration takes place.

Oxygen-effect as an inhibition of repair: radiation studies on excision repair deficient mei-9L1-embryos of Drosophila

The excision repair deficient mei-9L1-embryos of Drosophila melanogaster are up to four times more radiosensitive than normal +/+ embryos. The lack of oxygen-effect in the repair deficient 4-h-embryos and the reduced O2-effect in the 13/4-h embryos suggest an interpretation of the oxygen effect as a modification of the ability to repair. The conversion of the early death (heavy damage) to late death (slight damage) by irradiation of normal embryos in N2 supports this interpretation. This theory can also explain the dependence of O2-effect on LET. The spontaneous lethality and the increase in radiosensitivity depend in heterozygous mei-9L1-embryos strictly upon the genotype of the mother, thus representing a maternal effect.