Severe hypoxia increases expression of ATM and DNA-PKcs and it increases their activities through Src and AMPK signaling pathways

Glucose-oxygen deprivation constrains HMGCR function and Rac1 prenylation and activates the NLRP3 inflammasome in human monocytes

Hypoxia and low glucose abundance often occur simultaneously at sites of inflammation. In monocytes and macrophages, glucose-oxygen deprivation stimulates the assembly of the NLRP3 inflammasome to generate the proinflammatory cytokine IL-1β. We found that concomitant glucose deprivation and hypoxia activated the NLRP3 inflammasome by constraining the function of HMG-CoA reductase (HMGCR), the rate-limiting enzyme of the mevalonate kinase pathway. HMGCR is involved in the synthesis of geranylgeranyl pyrophosphate (GGPP), which is required for the prenylation and lipid membrane integration of proteins. Under glucose-oxygen deprivation, GGPP synthesis was decreased, leading to reduced prenylation of the small GTPase Rac1, increased binding of nonprenylated Rac1 to the scaffolding protein IQGAP1, and enhanced activation of the NLRP3 inflammasome. In response to restricted oxygen and glucose supply, patient monocytes with a compromised mevalonate pathway due to mevalonate kinase deficiency or Muckle-Wells syndrome released more IL-1β than did control monocytes. Thus, reduced GGPP synthesis due to inhibition of HMGCR under glucose-oxygen deprivation results in proinflammatory innate responses, which are normally kept in check by the prenylation of Rac1. We suggest that this mechanism is also active in inflammatory autoimmune conditions.

Feedback loop between hypoxia and energy metabolic reprogramming aggravates the radioresistance of cancer cells

Radiotherapy is one of the mainstream approaches for cancer treatment, although the clinical outcomes are limited due to the radioresistance of tumor cells. Hypoxia and metabolic reprogramming are the hallmarks of tumor initiation and progression and are closely linked to radioresistance. Inside a tumor, the rate of angiogenesis lags behind cell proliferation, and the underdevelopment and abnormal functions of blood vessels in some loci result in oxygen deficiency in cancer cells, i.e., hypoxia. This prevents radiation from effectively eliminating the hypoxic cancer cells. Cancer cells switch to glycolysis as the main source of energy, a phenomenon known as the Warburg effect, to sustain their rapid proliferation rates. Therefore, pathways involved in metabolic reprogramming and hypoxia-induced radioresistance are promising intervention targets for cancer treatment. In this review, we discussed the mechanisms and pathways underlying radioresistance due to hypoxia and metabolic reprogramming in detail, including DNA repair, role of cancer stem cells, oxidative stress relief, autophagy regulation, angiogenesis and immune escape. In addition, we proposed the existence of a feedback loop between energy metabolic reprogramming and hypoxia, which is associated with the development and exacerbation of radioresistance in tumors. Simultaneous blockade of this feedback loop and other tumor-specific targets can be an effective approach to overcome radioresistance of cancer cells. This comprehensive overview provides new insights into the mechanisms underlying tumor radiosensitivity and progression.

AMPK/FOXO3a Pathway Increases Activity and/or Expression of ATM, DNA-PKcs, Src, EGFR, PDK1, and SOD2 and Induces Radioresistance under Nutrient Starvation

Most solid tumors contain hypoxic and nutrient-deprived microenvironments. The cancer cells in these microenvironments have been reported to exhibit radioresistance. We have previously reported that nutrient starvation increases the expression and/or activity of ATM and DNA-PKcs, which are involved in the repair of DNA double-strand breaks induced by ionizing radiation. In the present study, to elucidate the molecular mechanisms underlying these phenomena, we investigated the roles of AMPK and FOXO3a, which play key roles in the cellular response to nutrient starvation. Nutrient starvation increased clonogenic cell survival after irradiation and increased the activity and/or expression of AMPKα, FOXO3a, ATM, DNA-PKcs, Src, EGFR, PDK1, and SOD2 in MDA-MB-231 cells. Knockdown of AMPKα using siRNA suppressed the activity and/or expression of FOXO3a, ATM, DNA-PKcs, Src, EGFR, PDK1, and SOD2 under nutrient starvation. Knockdown of FOXO3a using siRNA suppressed the activity and/or expression of AMPKα, ATM, DNA-PKcs, FOXO3a, Src, EGFR, PDK1, and SOD2 under nutrient starvation. Nutrient starvation decreased the incidence of apoptosis after 8 Gy irradiation. Knockdown of FOXO3a increased the incidence of apoptosis after irradiation under nutrient starvation. AMPK and FOXO3a appear to be key molecules that induce radioresistance under nutrient starvation and may serve as targets for radiosensitization.

Interaction Among Noncoding RNAs, DNA Damage Reactions, and Genomic Instability in the Hypoxic Tumor: Is it Therapeutically Exploitable Practice?

Hypoxia is a classical function of the tumor's microenvironment with a substantial effect on the development and therapeutic response of cancer. When put in hypoxic environments, cells undergo several biological reactions, including activation of signaling pathways that control proliferation, angiogenesis, and death. These pathways have been adapted by cancer cells to allow tumors to survive and even develop in hypoxic conditions, and poor prognosis is associated with tumor hypoxia. The most relevant transcriptional regulator in response to hypoxia, Hypoxia-inducible factor-1 alpha (HIF-1α), has been shown to modulate hypoxic gene expression and signaling transduction networks significantly. The significance of non-coding RNAs in hypoxic tumor regions has been revealed in an increasing number of studies over the past few decades. In regulating hypoxic gene expression, these hypoxia-responsive ncRNAs play pivotal roles. Hypoxia, a general characteristic of the tumor's microenvironment, significantly affects the expression of genes and is closely associated with the development of cancer. Indeed, the number of known hypoxia-associated lncRNAs has increased dramatically, demonstrating the growing role of lncRNAs in cascades and responses to hypoxia signaling. Decades of research have helped us create an image of the shift in hypoxic cancer cells' DNA repair capabilities. Emerging evidence suggests that hypoxia can trigger genetic instability in cancer cells because of microenvironmental tumor stress. Researchers have found that critical genes' expression is coordinately repressed by hypoxia within the DNA damage and repair pathways. In this study, we include an update of current knowledge on the presentation, participation, and potential clinical effect of ncRNAs in tumor hypoxia, DNA damage reactions, and genomic instability, with a specific emphasis on their unusual cascade of molecular regulation and malignant progression induced by hypoxia.

Overcoming the Impact of Hypoxia in Driving Radiotherapy Resistance in Head and Neck Squamous Cell Carcinoma

Hypoxia is very common in most solid tumours and is a driving force for malignant progression as well as radiotherapy and chemotherapy resistance. Incidences of head and neck squamous cell carcinoma (HNSCC) have increased in the last decade and radiotherapy is a major therapeutic technique utilised in the treatment of the tumours. However, effectiveness of radiotherapy is hindered by resistance mechanisms and most notably by hypoxia, leading to poor patient prognosis of HNSCC patients. The phenomenon of hypoxia-induced radioresistance was identified nearly half a century ago, yet despite this, little progress has been made in overcoming the physical lack of oxygen. Therefore, a more detailed understanding of the molecular mechanisms of hypoxia and the underpinning radiobiological response of tumours to this phenotype is much needed. In this review, we will provide an up-to-date overview of how hypoxia alters molecular and cellular processes contributing to radioresistance, particularly in the context of HNSCC, and what strategies have and could be explored to overcome hypoxia-induced radioresistance.

DNA-PKcs: A Targetable Protumorigenic Protein Kinase

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is a pleiotropic protein kinase that plays critical roles in cellular processes fundamental to cancer. DNA-PKcs expression and activity are frequently deregulated in multiple hematologic and solid tumors and have been tightly linked to poor outcome. Given the potentially influential role of DNA-PKcs in cancer development and progression, therapeutic targeting of this kinase is being tested in preclinical and clinical settings. This review summarizes the latest advances in the field, providing a comprehensive discussion of DNA-PKcs functions in cancer and an update on the clinical assessment of DNA-PK inhibitors in cancer therapy.

AMPK increases expression of ATM through transcriptional factor Sp1 and induces radioresistance under severe hypoxia in glioblastoma cell lines

We have previously reported that severe hypoxia increases expression and activity of the DNA damage sensor ATM by activation of the key energy sensor AMPK. Here, to elucidate molecular mechanisms underlying increased expression and activity of ATM by AMPK under severe hypoxia, we investigated roles of transcriptional factors Sp1 and FoxO3a using human glioblastoma cell lines T98G and A172. Severe hypoxia increased expression of ATM, AMPKα and Sp1 but not that of FoxO3a. Knockdown of AMPKα suppressed expression of ATM and Sp1 and suppressed cellular radioresistance under severe hypoxia without affecting cell cycle distribution. Knockdown of Sp1 suppressed expression of ATM. These results suggest that increased expression and activity of AMPK under severe hypoxia induce cellular radioresistance through AMPK/Sp1/ATM pathway.

Involvement of HIF-1α in the Detection, Signaling, and Repair of DNA Double-Strand Breaks after Photon and Carbon-Ion Irradiation

Simple Summary

Hypoxia-Inducible Factor 1α (HIF-1α), the main regulator of the oxygen homeostasis, promotes cancer cell survival through proliferation, angiogenesis, metastasis and radioresistance. Previously, our group demonstrated that silencing HIF-1α under hypoxia leads to a substantial radiosensitization of Head-and-Neck Squamous Cell Carcinoma (HNSCC) cells after both photons and carbon-ions, probably resulting from an accumulation of deleterious complex DNA damage. In this study, we aimed at determining the potential role of HIF-1α in the detection, signaling, and repair of DNA Double-Strand-Breaks (DSBs) in response to both irradiations, under hypoxia, in two HNSCC cell lines and their subpopulations of Cancer-Stem Cells (CSCs). Silencing HIF-1α under hypoxia led us to demonstrate the involvement of this transcriptional regulator in DSB repair in non-CSCS and CSC, thus highlighting its targeting together with radiation as a promising therapeutic strategy against radioresistant tumor cells in hypoxic niches.

Abstract

Hypoxia-Inducible Factor 1α (HIF-1α), which promotes cancer cell survival, is the main regulator of oxygen homeostasis. Hypoxia combined with photon and carbon ion irradiation (C-ions) stabilizes HIF-1α. Silencing HIF-1α under hypoxia leads to substantial radiosensitization of Head-and-Neck Squamous Cell Carcinoma (HNSCC) cells after both photons and C-ions. Thus, this study aimed to clarify a potential involvement of HIF-1α in the detection, signaling, and repair of DNA Double-Strand-Breaks (DSBs) in response to both irradiations, in two HNSCC cell lines and their subpopulations of Cancer-Stem Cells (CSCs). After confirming the nucleoshuttling of HIF-1α in response to both exposure under hypoxia, we showed that silencing HIF-1α in non-CSCs and CSCs decreased the initiation of the DSB detection (P-ATM), and increased the residual phosphorylated H2AX (γH2AX) foci. While HIF-1α silencing did not modulate 53BP1 expression, P-DNA-PKcs (NHEJ-c) and RAD51 (HR) signals decreased. Altogether, our experiments demonstrate the involvement of HIF-1α in the detection and signaling of DSBs, but also in the main repair pathways (NHEJ-c and HR), without favoring one of them. Combining HIF-1α silencing with both types of radiation could therefore present a potential therapeutic benefit of targeting CSCs mostly present in tumor hypoxic niches.

Tumor microenvironment and radioresistance

Metastasis is not the result of a random event, as cancer cells can sustain and proliferate actively only in a suitable tissue microenvironment and then form metastases. Since Dr. Stephen Paget in the United Kingdom proposed the seed and soil hypothesis of cancer metastasis based on the analogy that plant seeds germinate and grow only in appropriate soil, considerable attention has focused on both extracellular environmental factors that affect the growth of cancer cells and the tissue structure that influences the microenvironment. Malignant tumor tissues consist of not only cancer cells but also a wide variety of other cells responsible for the inflammatory response, formation of blood vessels, immune response, and support of the tumor tissue architecture, forming a complex cellular society. It is also known that the amounts of oxygen and nutrients supplied to each cell differ depending on the distance from tumor blood vessels in tumor tissue. Here, we provide an overview of the tumor microenvironment and characteristics of tumor tissues, both of which affect the malignant phenotypes and radioresistance of cancer cells, focusing on the following keywords: diversity of oxygen and nutrient microenvironment in tumor tissue, inflammation, immunity, and tumor vasculature.

Hypoxia in Solid Tumors: How Low Oxygenation Impacts the "Six Rs" of Radiotherapy

Radiotherapy is an important component of cancer treatment, with approximately 50% of all cancer patients receiving radiation therapy during their course of illness. Nevertheless, solid tumors frequently exhibit hypoxic areas, which can hinder therapies efficacy, especially radiotherapy one. Indeed, hypoxia impacts the six parameters governing the radiotherapy response, called the « six Rs of radiation biology » (for Radiosensitivity, Repair, Repopulation, Redistribution, Reoxygenation, and Reactivation of anti-tumor immune response), by inducing pleiotropic cellular adaptions, such as cell metabolism rewiring, epigenetic landscape remodeling, and cell death weakening, with significant clinical repercussions. In this review, according to the six Rs, we detail how hypoxia, and associated mechanisms and pathways, impact the radiotherapy response of solid tumors and the resulting clinical implications. We finally illustrate it in hypoxic endocrine cancers through a focus on anaplastic thyroid carcinomas.

Impact of hypoxia on the double-strand break repair after photon and carbon ion irradiation of radioresistant HNSCC cells

DNA double-strand breaks (DSBs) induced by photon irradiation are the most deleterious damage for cancer cells and their efficient repair may contribute to radioresistance, particularly in hypoxic conditions. Carbon ions (C-ions) act independently of the oxygen concentration and trigger complex- and clustered-DSBs difficult to repair. Understanding the interrelation between hypoxia, radiation-type, and DNA-repair is therefore essential for overcoming radioresistance. The DSBs signaling and the contribution of the canonical non-homologous end-joining (NHEJ-c) and homologous-recombination (HR) repair pathways were assessed by immunostaining in two cancer-stem-cell (CSCs) and non-CSCs HNSCC cell lines. Detection and signaling of DSBs were lower in response to C-ions than photons. Hypoxia increased the decay-rate of the detected DSBs (γH2AX) in CSCs after photons and the initiation of DSB repair signaling (P-ATM) in CSCs and non-CSCs after both radiations, but not the choice of DSB repair pathway (53BP1). Additionally, hypoxia increased the NHEJ-c (DNA-PK) and the HR pathway (RAD51) activation only after photons. Furthermore, the involvement of the HR seemed to be higher in CSCs after photons and in non-CSCs after C-ions. Taken together, our results show that C-ions may overcome the radioresistance of HNSCC associated with DNA repair, particularly in CSCs, and independently of a hypoxic microenvironment.

Roles of AMP-Activated Protein Kinase (AMPK) in Mammalian Reproduction

Reproduction is an energy demanding function and only take place in case of sufficient available energy status in mammals. Metabolic diseases such as anorexia nervosa are clinically associated with reduced fertility. AMP-activated protein kinase (AMPK), as a major regulator of cellular energy homeostasis, is activated in limited energy reserves to ensure the orderly progress of various physiological activities. In recent years, mounting evidence shows that AMPK is involved in the regulation of reproductive function through multiple mechanisms. AMPK is likely to be a metabolic sensor integrating central and peripheral signals. In this review, we aim to explore the preclinical studies published in the last decade that investigate the role of AMP-activated protein kinase in the reproductive field, and its role as a target for drug therapy of reproductive system-related diseases. We also emphasized the emerging roles of AMPK in transcriptional regulation of reproduction processes and metabolisms, which are tightly related to the energy state and fertility of an organism.

Inside the hypoxic tumour: reprogramming of the DDR and radioresistance

The hypoxic tumour is a chaotic landscape of struggle and adaption. Against the adversity of oxygen starvation, hypoxic cancer cells initiate a reprogramming of transcriptional activities, allowing for survival, metastasis and treatment failure. This makes hypoxia a crucial feature of aggressive tumours. Its importance, to cancer and other diseases, was recognised by the award of the 2019 Nobel Prize in Physiology or Medicine for research contributing to our understanding of the cellular response to oxygen deprivation. For cancers with limited treatment options, for example those that rely heavily on radiotherapy, the results of hypoxic adaption are particularly restrictive to treatment success. A fundamental aspect of this hypoxic reprogramming with direct relevance to radioresistance, is the alteration to the DNA damage response, a complex set of intermingling processes that guide the cell (for good or for bad) towards DNA repair or cell death. These alterations, compounded by the fact that oxygen is required to induce damage to DNA during radiotherapy, means that hypoxia represents a persistent obstacle in the treatment of many solid tumours. Considerable research has been done to reverse, correct or diminish hypoxia's power over successful treatment. Though many clinical trials have been performed or are ongoing, particularly in the context of imaging studies and biomarker discovery, this research has yet to inform clinical practice. Indeed, the only hypoxia intervention incorporated into standard of care is the use of the hypoxia-activated prodrug Nimorazole, for head and neck cancer patients in Denmark. Decades of research have allowed us to build a picture of the shift in the DNA repair capabilities of hypoxic cancer cells. A literature consensus tells us that key signal transducers of this response are upregulated, where repair proteins are downregulated. However, a complete understanding of how these alterations lead to radioresistance is yet to come.

MiRNAs in Radiotherapy Resistance of Nasopharyngeal Carcinoma

Nasopharyngeal carcinoma (NPC) is one of the most common malignant tumors of the head and neck in Southeast Asia and southern China. Although the comprehensive treatment based on intensity-modulated radiation therapy improves outcomes, the five-year survival rate of NPC patients is low, and the recurrence remains high. Radiotherapy resistance is the main cause of poor prognosis in NPC patients. MicroRNAs (miRNAs) are a class of endogenous non-coding RNAs regulating various biological functions in eukaryotes. These miRNAs can regulate the development and progression of nasopharyngeal carcinoma by affecting the proliferation, apoptosis, movement, invasion and metastasis of NPC cells. The abnormal expression of miRNAs is closely related to radiotherapy sensitivity and prognosis of NPC patients, which can affect the transmission of related signaling pathways by regulating the expression of tumor suppressor genes and / or oncogenes, and therefore participate in radiotherapy resistance in nasopharyngeal carcinoma. Here, we review the mechanisms by which miRNAs may be involved in the radiotherapy resistance of nasopharyngeal carcinoma.

Activation of AMPK under Hypoxia: Many Roads Leading to Rome

AMP-activated protein kinase (AMPK) is known as a pivotal cellular energy sensor, mediating the adaptation to low energy levels by deactivating anabolic processes and activating catabolic processes in order to restore the cellular ATP supply when the cellular AMP/ATP ratio is increased. Besides this well-known role, it has also been shown to exert protective effects under hypoxia. While an insufficient supply with oxygen might easily deplete cellular energy levels, i.e., ATP concentration, manifold other mechanisms have been suggested and are heavily disputed regarding the activation of AMPK under hypoxia independently from cellular AMP concentrations. However, an activation of AMPK preceding energy depletion could induce a timely adaptation reaction preventing more serious damage. A connection between AMPK and the master regulator of hypoxic adaptation via gene transcription, hypoxia-inducible factor (HIF), has also been taken into account, orchestrating their concerted protective action. This review will summarize the current knowledge on mechanisms of AMPK activation under hypoxia and its interrelationship with HIF.

DNA-PKcs is activated under nutrient starvation and activates Akt, MST1, FoxO3a, and NDR1

BACKGROUND

Presence of unperfused regions containing cells under hypoxia and nutrient starvation; contributes to radioresistance in solid human tumors. We have previously reported that cultured cells; under nutrient starvation show resistance to ionizing radiation compare with cells under normal; condition, and that nutrient starvation increases ATM activity, which causes cellular resistance to; ionizing radiation (Murata et al., BBRC2018). For further investigation of molecular mechanisms; underlying radioresistance of cells under nutrient starvation, effects of nutrient starvation on activity; of DNA-PKcs have been investigated because both DNA-PKcs and ATM belong to the PIKK family; and are required for DNA DSBs repair. In addition to DNA-PKcs, effects of nutrient starvation on; activities of FoxO3a and its regulators Akt, MST1 and AMPK have been investigated because FoxO3a; mediates cellular responses to stress and is activated under nutrient starvation.

METHODS

A human glioblastoma cell line, T98G was used to examine the effects of nutrient starvation on activities and expression of DNA-PKcs, Akt, MST1, FoxO3a, NDR1, and AMPK. To elucidate; signal transduction pathways for FoxO3a activation under nutrient starvation, we examined effects of; specific inhibitors or siRNA for DNA-PKcs or Akt on activities and expression of MST1, FoxO3, NDR1, andAMPK.

RESULTS

Under nutrient starvation, phosphorylations of DNA-PKcs at Ser2056, Akt at Ser473, MST at Thr183, FoxO3a at Ser413, NDR1 at Ser281 and Thr282, and AMPK at Thr172 were increased, which suggests their activation. Nutrient starvation did not affect expression of DNA-PKcs, Akt, MST1, or NDR1, with decreased expression of FoxO3a and increased expression of AMPK. Inhibition; of DNA-PK suppressed phosphorylation of Akt under nutrient starvation. Inhibition of DNA-PK or; Akt suppressed phosphorylations of MST1, FoxO3a, and NDR1 under nutrient starvation, which; suggests DNA-PKcs and Akt activate MST1, FoxO3a, and NDR1. Inhibition of DNA-PK did not; suppress phosphorylation ofAMPK under nutrient starvation.

CONCLUSION

Our data suggest that DN-PKcs is activated under nutrient starvation and activates AktMST1, FoxO3a, and NDR1.

The Emerging Role of Glucose Metabolism in Cartilage Development

PURPOSE OF REVIEW

Proper cartilage development is critical to bone formation during endochondral ossification. This review highlights the current understanding of various aspects of glucose metabolism in chondrocytes during cartilage development.

RECENT FINDINGS

Recent studies indicate that chondrocytes transdifferentiate into osteoblasts and bone marrow stromal cells during endochondral ossification. In cartilage development, signaling molecules, including IGF2 and BMP2, tightly control glucose uptake and utilization in a stage-specific manner. Perturbation of glucose metabolism alters the course of chondrocyte maturation, suggesting a key role for glucose metabolism during endochondral ossification. During prenatal and postnatal growth, chondrocytes experience bursts of nutrient availability and energy expenditure, which demand sophisticated control of the glucose-dependent processes of cartilage matrix production, cell proliferation, and hypertrophy. Investigating the regulation of glucose metabolism may therefore lead to a unifying mechanism for signaling events in cartilage development and provide insight into causes of skeletal growth abnormalities.