Distinct radiation responses after in vitro mtDNA depletion are potentially related to oxidative stress

Tamoxifen induces radioresistance through NRF2-mediated metabolic reprogramming in breast cancer

BACKGROUND

Recently, we reported that tamoxifen-resistant (TAM-R) breast cancer cells are cross-resistant to irradiation. Here, we investigated the mechanisms associated with tamoxifen-induced radioresistance, aiming to prevent or reverse resistance and improve breast cancer treatment.

METHODS

Wild-type ERα-positive MCF7 and ERα-negative MDA-MB-231 breast cancer cells and their TAM-R counterparts were analyzed for cellular metabolism using the Seahorse metabolic analyzer. Real-time ROS production, toxicity, and antioxidant capacity in response to H2O2, tamoxifen, and irradiation were determined. Tumor material from 28 breast cancer patients before and after short-term presurgical tamoxifen (ClinicalTrials.gov Identifier: NCT00738777, August 19, 2008) and cellular material was analyzed for NRF2 gene expression and immunohistochemistry. Re-sensitization of TAM-R cells to irradiation was established using pharmacological inhibition.

RESULTS

TAM-R cells exhibited decreased oxygen consumption and increased glycolysis, suggesting mitochondrial dysfunction. However, this did not explain radioresistance, as cells without mitochondria (Rho-0) were actually more radiosensitive. Real-time measurement of ROS after tamoxifen and H2O2 exposure indicated lower ROS levels and toxicity in TAM-R cells. Consistently, higher antioxidant levels were found in TAM-R cells, providing protection from irradiation-induced ROS. NRF2, a main activator of the antioxidant response, was increased in TAM-R cells and in tumor tissue of patients treated with short-term presurgical tamoxifen. NRF2 inhibition re-sensitized TAM-R cells to irradiation.

CONCLUSION

Mechanisms underlying tamoxifen-induced radioresistance are linked to cellular adaptations to persistently increased ROS levels, leading to cells with chronically upregulated antioxidant capacity and glycolysis. Pharmacological inhibition of antioxidant responses re-sensitizes breast cancer cells to irradiation.

Atherogenic potential of microgravity hemodynamics in the carotid bifurcation: a numerical investigation

Long-duration spaceflight poses multiple hazards to human health, including physiological changes associated with microgravity. The hemodynamic adaptations occurring upon entry into weightlessness have been associated with retrograde stagnant flow conditions and thromboembolic events in the venous vasculature but the impact of microgravity on cerebral arterial hemodynamics and function remains poorly understood. The objective of this study was to quantify the effects of microgravity on hemodynamics and wall shear stress (WSS) characteristics in 16 carotid bifurcation geometries reconstructed from ultrasonography images using computational fluid dynamics modeling. Microgravity resulted in a significant 21% increase in flow stasis index, a 22-23% decrease in WSS magnitude and a 16-26% increase in relative residence time in all bifurcation branches, while preserving WSS unidirectionality. In two anatomies, however, microgravity not only promoted flow stasis but also subjected the convex region of the external carotid arterial wall to a moderate increase in WSS bidirectionality, which contrasted with the population average trend. This study suggests that long-term exposure to microgravity has the potential to subject the vasculature to atheroprone hemodynamics and this effect is modulated by subject-specific anatomical features. The exploration of the biological impact of those microgravity-induced WSS aberrations is needed to better define the risk posed by long spaceflights on cardiovascular health.

Upregulating sirtuin 6 ameliorates glycolysis, EMT and distant metastasis of pancreatic adenocarcinoma with krüppel-like factor 10 deficiency

Krüppel-like factor 10 (KLF10) is a tumor suppressor in multiple cancers. In a murine model of spontaneous pancreatic adenocarcinoma (PDAC), additional KLF10 depletion accelerated distant metastasis. However, Klf10 knockout mice, which suffer from metabolic disorders, do not develop malignancy. The mechanisms of KLF10 in PDAC progression deserve further exploration. KLF10-depleted and KLF10-overexpressing PDAC cells were established to measure epithelial-mesenchymal transition (EMT), glycolysis, and migration ability. A murine model was established to evaluate the benefit of genetic or pharmacological manipulation in KLF10-depleted PDAC cells (PDACshKLF10). Correlations of KLF10 deficiency with rapid metastasis, elevated EMT, and glycolysis were demonstrated in resected PDAC tissues, in vitro assays, and murine models. We identified sirtuin 6 (SIRT6) as an essential mediator of KLF10 that modulates EMT and glucose homeostasis. Overexpressing SIRT6 reversed the migratory and glycolytic phenotypes of PDACshKLF10 cells. Linoleic acid, a polyunsaturated essential fatty acid, upregulated SIRT6 and prolonged the survival of mice injected with PDACshKLF10. Modulating HIF1α and NFκB revealed that EMT and glycolysis in PDAC cells were coordinately regulated upstream by KLF10/SIRT6 signaling. Our study demonstrated a novel KLF10/SIRT6 pathway that modulated EMT and glycolysis coordinately via NFκB and HIF1α. Activation of KLF10/SIRT6 signaling ameliorated the distant progression of PDAC.Clinical Trial Registration: ClinicalTrials.gov. identifier: NCT01666184.

Metabolic Rewiring in Radiation Oncology Toward Improving the Therapeutic Ratio

To meet the anabolic demands of the proliferative potential of tumor cells, malignant cells tend to rewire their metabolic pathways. Although different types of malignant cells share this phenomenon, there is a large intracellular variability how these metabolic patterns are altered. Fortunately, differences in metabolic patterns between normal tissue and malignant cells can be exploited to increase the therapeutic ratio. Modulation of cellular metabolism to improve treatment outcome is an emerging field proposing a variety of promising strategies in primary tumor and metastatic lesion treatment. These strategies, capable of either sensitizing or protecting tissues, target either tumor or normal tissue and are often focused on modulating of tissue oxygenation, hypoxia-inducible factor (HIF) stabilization, glucose metabolism, mitochondrial function and the redox balance. Several compounds or therapies are still in under (pre-)clinical development, while others are already used in clinical practice. Here, we describe different strategies from bench to bedside to optimize the therapeutic ratio through modulation of the cellular metabolism. This review gives an overview of the current state on development and the mechanism of action of modulators affecting cellular metabolism with the aim to improve the radiotherapy response on tumors or to protect the normal tissue and therefore contribute to an improved therapeutic ratio.

The hypoxia-sensor carbonic anhydrase IX affects macrophage metabolism, but is not a suitable biomarker for human cardiovascular disease

Hypoxia is prevalent in atherosclerotic plaques, promoting plaque aggravation and subsequent cardiovascular disease (CVD). Transmembrane protein carbonic anhydrase IX (CAIX) is hypoxia-induced and can be shed into the circulation as soluble CAIX (sCAIX). As plaque macrophages are hypoxic, we hypothesized a role for CAIX in macrophage function, and as biomarker of hypoxic plaque burden and CVD. As tumor patients with probable CVD are treated with CAIX inhibitors, this study will shed light on their safety profile. CAIX co-localized with macrophages (CD68) and hypoxia (pimonidazole), and correlated with lipid core size and pro-inflammatory iNOS+ macrophages in unstable human carotid artery plaques. Although elevated pH and reduced lactate levels in culture medium of CAIX knock-out (CAIXko) macrophages confirmed its role as pH-regulator, only spare respiratory capacity of CAIXko macrophages was reduced. Proliferation, apoptosis, lipid uptake and expression of pro- and anti-inflammatory genes were not altered. Plasma sCAIX levels and plaque-resident CAIX were below the detection threshold in 50 and 90% of asymptomatic and symptomatic cases, respectively, while detectable levels did not associate with primary or secondary events, or intraplaque hemorrhage. Initial findings show that CAIX deficiency interferes with macrophage metabolism. Despite a correlation with inflammatory macrophages, plaque-resident and sCAIX expression levels are too low to serve as biomarkers of future CVD.

Mitochondrial Dysfunction Inhibits Hypoxia-Induced HIF-1α Stabilization and Expression of Its Downstream Targets

mtDNA variations often result in bioenergetic dysfunction inducing a metabolic switch toward glycolysis resulting in an unbalanced pH homeostasis. In hypoxic cells, expression of the tumor-associated carbonic anhydrase IX (CAIX) is enhanced to maintain cellular pH homeostasis. We hypothesized that cells with a dysfunctional oxidative phosphorylation machinery display elevated CAIX expression levels. Increased glycolysis was observed for cytoplasmic 143B mutant hybrid (m.3243A>G, >94.5%) cells (p < 0.05) and 143B mitochondrial DNA (mtDNA) depleted cells (p < 0.05). Upon hypoxia (0.2%, 16 h), genetic or pharmacological oxidative phosphorylation (OXPHOS) inhibition resulted in decreased CAIX (p < 0.05), vascular endothelial growth factor (VEGF) and hypoxia-inducible factor 1-alpha (HIF-1α) expression levels. Reactive oxygen species (ROS) and prolyl-hydroxylase 2 (PHD2) levels could not explain these observations. In vivo, tumor take (>500 mm³) took longer for mutant hybrid xenografts, but growth rates were comparable with control tumors upon establishment. Previously, it has been shown that HIF-1α is responsible for tumor establishment. In agreement, we found that HIF-1α expression levels and the pimonidazole-positive hypoxic fraction were reduced for the mutant hybrid xenografts. Our results demonstrate that OXPHOS dysfunction leads to a decreased HIF-1α stabilization and subsequently to a reduced expression of its downstream targets and hypoxic fraction in vivo. In contrast, hypoxia-inducible factor 2-alpha (HIF-2α) expression levels in these xenografts were enhanced. Inhibition of mitochondrial function is therefore an interesting approach to increase therapeutic efficacy in hypoxic tumors.

Evaluation of the role of mitochondria in the non-targeted effects of ionizing radiation using cybrid cellular models

Radiobiology is moving towards a better understanding of the intercellular signaling that occurs upon radiation and how its effects relate to the dose applied. The mitochondrial role in orchestrating this biological response needs to be further explored. Cybrids (cytoplasmic hybrids) are useful cell models for studying the involvement of mitochondria in cellular processes. In the present study we used cybrid cell lines to investigate the role of mitochondria in the response to radiation exposure. Cybrid cell lines, derived from the osteosarcoma human cell line 143B, harboring, either wild-type mitochondrial DNA (Cy143Bwt), cells with mitochondria with mutated DNA that causes mitochondrial dysfunction (Cy143Bmut), as well as cells without mitochondrial DNA (mtDNA) (143B-Rho0), were irradiated with 0.2 Gy and 2.0 Gy. Evaluation of the non-targeted (or bystander) effects in non-irradiated cells were assessed by using conditioned media from the irradiated cells. DNA double stranded breaks were assessed with the γH2AX assay. Both directly irradiated cells and cells treated with the conditioned media, showed increased DNA damage. The effect of the irradiated cells media was different according to the cell line it derived from: from Cy143Bwt cells irradiated with 0.2 Gy (low dose) and from Cy143Bmut irradiated with 2.0 Gy (high dose) induced highest DNA damage. Notably, media obtained from cells without mtDNA, the143B-Rho0 cell line, produced no effect in DNA damage. These results point to a possible role of mitochondria in the radiation-induced non-targeted effects. Furthermore, it indicates that cybrid models are valuable tools for radiobiological studies.

Mitochondrial dysfunction increases pro-inflammatory cytokine production and impairs repair and corticosteroid responsiveness in lung epithelium

COPD is characterized by chronic lung inflammation and irreversible lung tissue damage. Inhaled noxious gases, including cigarette smoke, are the major risk factor for COPD. Inhaled smoke first encounters the epithelial lining of the lungs, causing oxidative stress and mitochondrial dysfunction. We investigated whether a mitochondrial defect may contribute to increased lung epithelial pro-inflammatory responses, impaired epithelial repair and reduced corticosteroid sensitivity as observed in COPD. We used wild-type alveolar epithelial cells A549 and mitochondrial DNA-depleted A549 cells (A549 Rho-0) and studied pro-inflammatory responses using (multiplex) ELISA as well as epithelial barrier function and repair (real-time impedance measurements), in the presence and absence of the inhaled corticosteroid budesonide. We observed that A549 Rho-0 cells secrete higher levels of pro-inflammatory cytokines than wild-type A549 cells and display impaired repair upon wounding. Budesonide strongly suppressed the production of neutrophil attractant CXCL8, and promoted epithelial integrity in A549 wild-type cells, while A549 Rho-0 cells displayed reduced corticosteroid sensitivity compared to wild-type cells. The reduced corticosteroid responsiveness may be mediated by glycolytic reprogramming, specifically glycolysis-associated PI3K signaling, as PI3K inhibitor LY294002 restored the sensitivity of CXCL8 secretion to corticosteroids in A549 Rho-0 cells. In conclusion, mitochondrial defects may lead to increased lung epithelial pro-inflammatory responses, reduced epithelial repair and reduced corticosteroid responsiveness in lung epithelium, thus potentially contributing to the pathogenesis of COPD.

Lipid peroxidation increases hydrogen peroxide permeability leading to cell death in cancer cell lines that lack mtDNA

4-Hydroxynonenal (HNE) is an important product of plasma membrane lipid peroxidation, which is a cause of cell and tissue injury. Mitochondrial DNA (mtDNA)-depleted ρ⁰ cells were established using human cervical cancer and oral squamous cell carcinoma cell lines. We investigated the effect of reactive oxygen species in ρ⁰ cells, especially the mechanism of hydrogen peroxide (H₂ O₂ )-mediated cell death. These cell were subjected to high oxidative stress and, compared with their parental cells, showed greater sensitivity to H₂ O₂ and high lipid peroxidation. Upregulation of HNE in the plasma membrane was observed prior to the increase in intracellular H₂ O₂ . The amount of oxidized lipid present changed H₂ O₂ permeability and administration of oxidized lipid led to further cell death after treatment with H₂ O₂ . Expression levels of lipoxygenase ALOX genes (ie ALOX5, ALOX12, and ALOX15) were upregulated in ρ⁰ cells, as were expression levels of ALOX12 and ALOX15 proteins. ALOX5 protein was mainly distributed in the nucleus, while ALOX12 and ALOX15 proteins were distributed in the nucleus and the cytoplasm. Although expression of COX2 gene was upregulated, its protein expression did not increase. ALOX (especially ALOX15) may be involved in the sensitivity of cancer cells to treatment. These data offer promise for the development of novel anticancer agents by altering the oxidation state of the plasma membrane. Our results showed that lipid peroxidation status is important for H₂ O₂ sensitivity and that ALOX15 is involved in lipid peroxidation status.

Targeting glucose and glutamine metabolism combined with radiation therapy in non-small cell lung cancer

PURPOSE

Metabolic inhibition might sensitize tumors to irradiation. Here, we examined the effect of lonidamine (several metabolic effects, inhibiting hexokinase amongst others) and/or 968 (glutaminase inhibitor) on tumor cell metabolism, cell growth, cytotoxicity and radiosensitivity in NSCLC cell lines in vitro in relation to histology.

MATERIALS AND METHODS

Adeno- (H23, HCC827, H1975) and squamous cell carcinoma (H520, H292, SW900) NSCLC cells were treated with lonidamine and/or 968 for 72 h under physiological levels of glucose (1.5 mM). Cells were irradiated with 0, 4 or 8 Gy. Cell growth of H2B-mCherry transduced cells and cytotoxicity (CellTox™ Green Cytotoxicity Assay) were measured using live cell imaging (IncuCyte). Inhibitory effects on metabolic profiles was determined using the Seahorse XF96 extracellular Flux analyzer.

RESULTS

NSCLC cell lines responded differently to glycolysis (lonidamine) and/or glutaminase (968) inhibition, largely corresponding with changes in glycolytic and mitochondrial metabolism upon treatment. Response patterns were not related to histology. 968 was cytotoxic in cell lines with high glutaminase C expression (H1975 and H520), whereas combination treatment was cytotoxic in KRAS mutated cell lines SW900 and H23. H292 and HCC827 were resistant to combination treatment. Treatment with 968 and especially lonidamine resulted in radiosensitization of H292 and HCC827 in terms of decreased relative cell growth and increased cytotoxicity.

CONCLUSION

NSCLC is a heterogeneous disease, which is reflected in the response of different cell lines to the treatment (combinations) reported here. Only a part of NSCLC patients may benefit from the combination of radiation therapy and metabolic inhibition, making stratification necessary.

Melatonin and cancer: From the promotion of genomic stability to use in cancer treatment

Cancer remains among the most challenging human diseases. Several lines of evidence suggest that carcinogenesis is a complex process that is initiated by DNA damage. Exposure to clastogenic agents such as heavy metals, ionizing radiation (IR), and chemotherapy drugs may cause chronic mutations in the genomic material, leading to a phenomenon named genomic instability. Evidence suggests that genomic instability is responsible for cancer incidence after exposure to carcinogenic agents, and increases the risk of secondary cancers following treatment with radiotherapy or chemotherapy. Melatonin as the main product of the pineal gland is a promising hormone for preventing cancer and improving cancer treatment. Melatonin can directly neutralize toxic free radicals more efficiently compared with other classical antioxidants. In addition, melatonin is able to regulate the reduction/oxidation (redox) system in stress conditions. Through regulation of mitochondrial nction and inhibition of pro-oxidant enzymes, melatonin suppresses chronic oxidative stress. Moreover, melatonin potently stimulates DNA damage responses that increase the tolerance of normal tissues to toxic effect of IR and may reduce the risk of genomic instability in patients who undergo radiotherapy. Through these mechanisms, melatonin attenuates several side effects of radiotherapy and chemotherapy. Interestingly, melatonin has shown some synergistic properties with IR and chemotherapy, which is distinct from classical antioxidants that are mainly used for the alleviation of adverse events of radiotherapy and chemotherapy. In this review, we describe the anticarcinogenic effects of melatonin and also its possible application in clinical oncology.

Mitochondrial Dysfunctions Regulated Radioresistance through Mitochondria-to-Nucleus Retrograde Signaling Pathway of NF-κB/PI3K/AKT2/mTOR

We investigated the relationship between significantly different genes of the mitochondria-to-nucleus retrograde signaling pathway (RTG) in H1299 ρ⁰ cells (mtDNA depleted cell) and compared their radiosensitivity to that of parental ρ⁺ cells, to determine the possible intervention targets of radiosensitization. ρ⁰ cells were depleted of mitochondrial DNA by chronic culturing in ethidium bromide at low concentration. Radiosensitivity was analyzed using clonogenic assay. Western blot was used to analyze the cell cycle-related proteins, serine/threonine kinase ataxia telangiectasia mutant (ATM), ataxia telangiectasia and Rad3-related protein (ATR) and cyclin B1 (CCNB1). The γ-H2AX foci were detected using confocal fluorescence microscopy. RNA samples were hybridized using the Agilent human genome expression microarray. The Kyoto Encyclopedia of Genes and Genomes (KEGG) database was used for Gene Ontology (GO) Consortium and pathway annotations of differentially expressed genes, respectively. The H1299 ρ⁰ cells were found to be more radioresistant than ρ⁺ cells. The ATP production of H1299 ρ⁰ cells was lower than that of the ρ⁺ cells before or after irradiation. Both H1299 ρ⁰ and ρ⁺ cells had higher ROS levels after irradiation, however, the radiation-induced ROS production in ρ⁰ cells was significantly lower than in ρ⁺ cells. In addition, the percentage of apoptosis in H1299 ρ⁰ cells was lower than in ρ⁺ cells after 6 Gy irradiation. As for the cell cycle and DNA damage response-related proteins ATM, ATR and CCNB1, the expression levels in ρ⁰ cells were significantly higher than in ρ⁺ cells, and there were less γ-H2AX foci in the ρ⁰ than ρ⁺ cells after irradiation. Furthermore, the results of the human genome expression microarray demonstrated that the phosphorylated protein levels of the NF-κB/PI3K/AKT2/mTOR signaling pathway were increased after 6 Gy irradiation and were decreased after treatment with the AKT2-specific inhibitor MK-2206 combined with radiation in H1299 ρ⁰ cells. MK-2206 treatment also led to an increase in pro-apoptotic proteins. In conclusion, these results demonstrate that mtDNA depletion might activate the mitochondria-to-nucleus retrograde signaling pathway of NF-κB/PI3K/AKT2/mTOR and induce radioresistance in H1299 ρ⁰ cells by evoking mitochondrial dysfunctions.