Heat induces gammaH2AX foci formation in mammalian cells

Hyperthermia in Combination with Emerging Targeted and Immunotherapies as a New Approach in Cancer Treatment

Despite significant advancements in the development of novel therapies, cancer continues to stand as a prominent global cause of death. In many cases, the cornerstone of standard-of-care therapy consists of chemotherapy (CT), radiotherapy (RT), or a combination of both. Notably, hyperthermia (HT), which has been in clinical use in the last four decades, has proven to enhance the effectiveness of CT and RT, owing to its recognized potency as a sensitizer. Furthermore, HT exerts effects on all steps of the cancer-immunity cycle and exerts a significant impact on key oncogenic pathways. Most recently, there has been a noticeable expansion of cancer research related to treatment options involving immunotherapy (IT) and targeted therapy (TT), a trend also visible in the research and development pipelines of pharmaceutical companies. However, the potential results arising from the combination of these innovative therapeutic approaches with HT remain largely unexplored. Therefore, this review aims to explore the oncology pipelines of major pharmaceutical companies, with the primary objective of identifying the principal targets of forthcoming therapies that have the potential to be advantageous for patients by specifically targeting molecular pathways involved in HT. The ultimate goal of this review is to pave the way for future research initiatives and clinical trials that harness the synergy between emerging IT and TT medications when used in conjunction with HT.

Cooperatively enhanced photothermal-chemotherapy via simultaneously downregulating HSPs and promoting DNA alkylation in cancer cells

Photothermal therapy (PTT) has emerged as one of the important strategies for cancer treatment due to its precision and no drug resistance. However, upregulation of heat shock protein (HSP) expression during PTT severely limits its overall therapeutic effect. Accordingly, in this study, we developed a new anticancer strategy based on an l-glutathione (GSH)-activated prodrug (Cy-S-S-Cbl), which consisted of an alkylating reagent (Cbl) covalently linked to a photothermal photosensitizer (Cy7), to achieve cooperatively enhanced photothermal-chemotherapy. In the presence of overexpressed GSH in cancer cells, Cy-S-S-Cbl was converted into Cy-NH2 to achieve photothermal effect enhancement by the photo-induced electron transfer (PET) effect and release the alkylation reagent. Meanwhile, the photothermal effect of Cy-NH2 enhanced the DNA alkylation of chemotherapy drugs. Surprisingly, we first found that the therapeutic efficacy of PTT was improved owing to the down-regulation of heat shock protein 70 (HSP70) by chemotherapy. The two treatments had a synergistic promotion effect achieving higher cancer cell killing efficiency. Under 808 nm light irradiation, Cy-S-S-Cbl could effectively realize selective killing of cancer cells and tumor growth inhibition. Therefore, we strongly believe that this efficient cooperative design strategy will provide a new idea to improve the treatment efficiency of prodrugs.

Stress Responses as Master Keys to Epigenomic Changes in Transcriptome and Metabolome for Cancer Etiology and Therapeutics

From initiation through progression, cancer cells are subjected to a magnitude of endogenous and exogenous stresses, which aid in their neoplastic transformation. Exposure to these classes of stress induces imbalance in cellular homeostasis and, in response, cancer cells employ informative adaptive mechanisms to rebalance biochemical processes that facilitate survival and maintain their existence. Different kinds of stress stimuli trigger epigenetic alterations in cancer cells, which leads to changes in their transcriptome and metabolome, ultimately resulting in suppression of growth inhibition or induction of apoptosis. Whether cancer cells show a protective response to stress or succumb to cell death depends on the type of stress and duration of exposure. A thorough understanding of epigenetic and molecular architecture of cancer cell stress response pathways can unveil a plethora of information required to develop novel anticancer therapeutics. The present view highlights current knowledge about alterations in epigenome and transcriptome of cancer cells as a consequence of exposure to different physicochemical stressful stimuli such as reactive oxygen species (ROS), hypoxia, radiation, hyperthermia, genotoxic agents, and nutrient deprivation. Currently, an anticancer treatment scenario involving the imposition of stress to target cancer cells is gaining traction to augment or even replace conventional therapeutic regimens. Therefore, a comprehensive understanding of stress response pathways is crucial for devising and implementing novel therapeutic strategies.

High Temperature Drives Topoisomerase Mediated Chromosomal Break Repair Pathway Choice

Simple Summary

Targeting topoisomerases has been widely used as anticancer therapeutics. Exposure to high temperature (hyperthermia) protects cells from the cytotoxic effect of topoisomerase-targeting therapeutics, yet the mechanism remains unknown. Here, we report that hyperthermia inhibits the nucleolytic processing of topoisomerase-induced DNA damage and drives repair to a more faithful pathway mediated by TDP1 and TDP2. We further show that hyperthermia suppresses topoisomerase-induced chromosomal translocation and hallmarks of inflammation, which has broad implications in cancer development and therapy.

Abstract

Cancer-causing mutations often arise from inappropriate DNA repair, yet acute exposure to DNA damage is widely used to treat cancer. The challenge remains in how to specifically induce excessive DNA damage in cancer cells while minimizing the undesirable effects of genomic instability in noncancerous cells. One approach is the acute exposure to hyperthermia, which suppresses DNA repair and synergizes with radiotherapy and chemotherapy. An exception, however, is the protective effect of hyperthermia on topoisomerase targeting therapeutics. The molecular explanation for this conundrum remains unclear. Here, we show that hyperthermia suppresses the level of topoisomerase mediated single- and double-strand breaks induced by exposure to topoisomerase poisons. We further uncover that, hyperthermia suppresses hallmarks of genomic instability induced by topoisomerase targeting therapeutics by inhibiting nuclease activities, thereby channeling repair to error-free pathways driven by tyrosyl-DNA phosphodiesterases. These findings provide an explanation for the protective effect of hyperthermia from topoisomerase-induced DNA damage and may help to explain the inverse relationship between cancer incidence and temperature. They also pave the way for the use of controlled heat as a therapeutic adjunct to topoisomerase targeting therapeutics.

DNA damage response and apoptosis induced by hyperthermia in human umbilical cord blood lymphocytes

While hyperthermia (HT) is a promising modality for cancer treatment, the knowledge on mechanisms of its effect on cells is still limited. We have investigated DNA double-strand break (DSB) and apoptosis induced by HT. Umbilical cord blood lymphocytes (UCBL) were subjected to HT at 43 °C. We have treated cells for 1 h (1 h HT), 2 h (2 h HT) and by combined HT and ice treatment (both lasting 1 h). Enumeration of DSB by 53BP1/γH2AX DNA repair focus formation and early apoptosis by γH2AX pan-staining was conducted by automated fluorescent microscopy. Apoptotic stages and viability were assessed by the annexin/propidium iodide (PI) assay using flow cytometry 0, 18, and 42 h post-treatment. HT induced either immediate (2 h HT) or postponed (1 h HT) DNA damage. The levels of 53BP1 and γH2AX foci differed under the same treatment conditions, suggesting that the ratio of co-localized γH2AX/53BP1 foci to all γH2AX and also to all 53BP1 foci could be a valuable marker. The ratio of co-localized foci increased immediately after 2 h HT regardless the way of assessment. For the first time we show, by both annexin/PI and γH2AX pan-staining assay that apoptosis can be induced during or immediately after the 2 h HT treatment. Our results suggest that HT may induce DSB in dependence on treatment duration and post-treatment time due to inhibition of DNA repair pathways and that HT-induced apoptosis might be dependent or associated with DSB formation in human lymphocytes. Assessment of γH2AX pan-staining in lymphocytes affected by HT may represent a valuable marker of HT treatment side effects.

Triterpenoid CDDO-Me induces ROS generation and up-regulates cellular levels of antioxidative enzymes without induction of DSBs in human peripheral blood mononuclear cells

Ionizing radiation produces reactive oxygen species (ROS) leading to cellular DNA damage. Therefore, patients undergoing radiation therapy or first responders in radiological accident scenarios could both benefit from the identification of specifically acting pharmacological radiomitigators. The synthetic triterpenoid bardoxolone-methyl (CDDO-Me) has previously been shown to exert antioxidant, anti-inflammatory and anticancer activities in several cell lines, in part by enhancing the DNA damage response. In our study, we examined the effect of nanomolar concentrations of CDDO-Me in human peripheral blood mononuclear cells (PBMC). We observed increased cellular levels of the antioxidative enzymes heme oxygenase-1 (HO-1), NAD(P)H dehydrogenase (quinone1) and mitochondrial superoxide dismutase 2 by immunoblotting. Surprisingly, we found increased intracellular ROS-levels using imaging flow-cytometry. However, the radiation-induced DNA double-strand break (DSB) formation using the γ-H2AX + 53BP1 DSB focus assay and the cytokinesis-block micronucleus assay both revealed, that nanomolar CDDO-Me pre-treatment of PBMC for 2 h or 6 h ahead of X irradiation with 2 Gy did neither significantly affect γ-H2AX + 53BP1 DSB foci formation nor the frequency of micronuclei. CDDO-Me treatment also failed to alter the nuclear division index and the frequency of IR-induced PBMC apoptosis as investigated by Annexin V-labeled live-cell imaging. Our results indicate that pharmacologically increased cellular concentrations of antioxidative enzymes might not necessarily exert radiomitigating short-term effects in IR-exposed PBMC. However, the increase of antioxidative enzymes could also be a result of a defensive cellular mechanism towards elevated ROS levels.

Inhibition of the ATR kinase enhances 5-FU sensitivity independently of nonhomologous end-joining and homologous recombination repair pathways

The anticancer agent 5-fluorouracil (5-FU) is cytotoxic and often used to treat various cancers. 5-FU is thought to inhibit the enzyme thymidylate synthase, which plays a role in nucleotide synthesis and has been found to induce single- and double-strand DNA breaks. ATR Ser/Thr kinase (ATR) is a principal kinase in the DNA damage response and is activated in response to UV- and chemotherapeutic drug-induced DNA replication stress, but its role in cellular responses to 5-FU is unclear. In this study, we examined the effect of ATR inhibition on 5-FU sensitivity of mammalian cells. Using immunoblotting, we found that 5-FU treatment dose-dependently induced the phosphorylation of ATR at the autophosphorylation site Thr-1989 and thereby activated its kinase. Administration of 5-FU with a specific ATR inhibitor remarkably decreased cell survival, compared with 5-FU treatment combined with other major DNA repair kinase inhibitors. Of note, the ATR inhibition enhanced induction of DNA double-strand breaks and apoptosis in 5-FU-treated cells. Using gene expression analysis, we found that 5-FU induced the activation of the intra-S cell-cycle checkpoint. Cells lacking BRCA2 were sensitive to 5-FU in the presence of ATR inhibitor. Moreover, ATR inhibition enhanced the efficacy of the 5-FU treatment, independently of the nonhomologous end-joining and homologous recombination repair pathways. These findings suggest that ATR could be a potential therapeutic target in 5-FU-based chemotherapy.

Cytogenetic and Transcriptomic Analysis of Human Endometrial MSC Retaining Proliferative Activity after Sublethal Heat Shock

Temperature is an important exogenous factor capable of leading to irreversible processes in the vital activity of cells. However, the long-term effects of heat shock (HS) on mesenchymal stromal cells (MSC) remain unstudied. We investigated the karyotype and DNA repair drivers and pathways in the human endometrium MSC (eMSC) survived progeny at passage 6 after sublethal heat stress (sublethal heat stress survived progeny (SHS-SP)). G-banding revealed an outbreak of random karyotype instability caused by chromosome breakages and aneuploidy. Molecular karyotyping confirmed the random nature of this instability. Transcriptome analysis found homologous recombination (HR) deficiency that most likely originated from the low thermostability of the AT-rich HR driving genes. SHS-SP protection from transformation is provided presumably by low oncogene expression maintained by tight co-regulation between thermosensitive HR drivers BRCA, ATM, ATR, and RAD51 (decreasing expression after SHS), and oncogenes mTOR, MDM2, KRAS, and EGFR. The cancer-related transcriptomic features previously identified in hTERT transformed MSC in culture were not found in SHS-SP, suggesting no traits of malignancy in them. The entrance of SHS-SP into replicative senescence after 25 passages confirms their mortality and absence of transformation features. Overall, our data indicate that SHS may trigger non-tumorigenic karyotypic instability due to HR deficiency and decrease of oncogene expression in progeny of SHS-survived MSC. These data can be helpful for the development of new therapeutic approaches in personalized medicine.

Monoubiquitinated γ-H2AX: Abundant product and specific biomarker for non-apoptotic DNA double-strand breaks

DNA double-strand breaks (DSBs) are a highly toxic form of DNA damage produced by a number of carcinogens, drugs, and metabolic abnormalities. Involvement of DSBs in many pathologies has led to frequent measurements of these lesions, primarily via biodosimetry of S139-phosphorylated histone H2AX (γ-H2AX). However, γ-H2AX is also induced by some non-DSB conditions and abundantly formed in apoptosis, raising concerns about the overestimation of potential genotoxic agents and accuracy of DSB assessments. DSB-triggered γ-H2AX undergoes RNF168-mediated K13/K15 monoubiquitination, which is rarely analyzed in DSB/genotoxicity studies. Here we identified critical methodological factors that are necessary for the efficient detection of mono- (ub1) and diubiquitinated (ub2) γ-H2AX. Using optimized technical conditions, we found that γ-H2AX-ub1 was a predominant form of γ-H2AX in three primary human cell lines containing mechanistically distinct types of DSBs. Replication stress-associated DSBs also triggered extensive formation of γ-H2AX-ub1. For DSBs induced by oxidative damage or topoisomerase II, both γ-H2AX and γ-H2AX-ub1 showed dose-dependent increases whereas γ-H2AX-ub2 plateaued at low levels of breaks. Despite abundance of γ-H2AX, γ-H2AX-ub1,2 formation was blocked in apoptosis, which was associated with proteolytic cleavage of RNF168. Chromatin damage also caused only the production of γ-H2AX but not its ub1,2 forms. Our results revealed a major contribution of ubiquitinated forms to the overall γ-H2AX response and demonstrated the specificity of monoubiquitinated γ-H2AX as a biodosimeter of non-apoptotic DSBs.

Gas-Phase Enrichment of Multiply Charged Peptide Ions by Differential Ion Mobility Extend the Comprehensiveness of SUMO Proteome Analyses

The small ubiquitin-like modifier (SUMO) is a member of the family of ubiquitin-like modifiers (UBLs) and is involved in important cellular processes, including DNA damage response, meiosis and cellular trafficking. The large-scale identification of SUMO peptides in a site-specific manner is challenging not only because of the low abundance and dynamic nature of this modification, but also due to the branched structure of the corresponding peptides that further complicate their identification using conventional search engines. Here, we exploited the unusual structure of SUMO peptides to facilitate their separation by high-field asymmetric waveform ion mobility spectrometry (FAIMS) and increase the coverage of SUMO proteome analysis. Upon trypsin digestion, branched peptides contain a SUMO remnant side chain and predominantly form triply protonated ions that facilitate their gas-phase separation using FAIMS. We evaluated the mobility characteristics of synthetic SUMO peptides and further demonstrated the application of FAIMS to profile the changes in protein SUMOylation of HEK293 cells following heat shock, a condition known to affect this modification. FAIMS typically provided a 10-fold improvement of detection limit of SUMO peptides, and enabled a 36% increase in SUMO proteome coverage compared to the same LC-MS/MS analyses performed without FAIMS. Graphical Abstract ᅟ.

Microcirculation-mediated preconditioning and intracellular hypothermia

Microcirculation is a network of perfused capillaries that connects macrocirculation with the cells. Although research has provided insight into microcirculatory blood flow, our knowledge remains limited. In this article, we propose a new role of microcirculation in physiological and shock states. In healthy individuals, microcirculation maintains cellular homeostasis via preconditioning. When blood volume decreases, the ensuing microcirculatory changes result in heterogeneity of perfusion and tissue oxygenation. Initially, this is partly compensated by the preserved autoregulation and the increase in the metabolism rate of cells, but at later stages, the loss of autoregulation activates the cascade of intracellular hypothermia.

Recurrent ubiquitin B silencing in gynecological cancers establishes dependence on ubiquitin C

Transcriptional repression of ubiquitin B (UBB) is a cancer-subtype-specific alteration that occurs in a substantial population of patients with cancers of the female reproductive tract. UBB is 1 of 2 genes encoding for ubiquitin as a polyprotein consisting of multiple copies of ubiquitin monomers. Silencing of UBB reduces cellular UBB levels and results in an exquisite dependence on ubiquitin C (UBC), the second polyubiquitin gene. UBB is repressed in approximately 30% of high-grade serous ovarian cancer (HGSOC) patients and is a recurrent lesion in uterine carcinosarcoma and endometrial carcinoma. We identified ovarian tumor cell lines that retain UBB in a repressed state, used these cell lines to establish orthotopic ovarian tumors, and found that inducible expression of a UBC-targeting shRNA led to tumor regression, and substantial long-term survival benefit. Thus, we describe a recurrent cancer-specific lesion at the level of ubiquitin production. Moreover, these observations reveal the prognostic value of UBB repression and establish UBC as a promising therapeutic target for ovarian cancer patients with recurrent UBB silencing.

BRCA2 protects mammalian cells from heat shock

PURPOSE

Heat shock induces DNA double-strand breaks (DSBs) in mammalian cells. Mammalian cells are capable of repairing DSBs by utilising the homologous recombination (HR) pathway. Breast cancer susceptibility gene 2 (BRCA2) is known to regulate the HR pathway. Here, we investigate the role of BRCA2 in repairing DNA damage induced by heat shock.

MATERIALS AND METHODS

Chinese hamster lung fibroblast cell lines and human tongue squamous cell carcinoma SAS cells were used. RAD51 foci formation assay was used as an HR indicator. Heat sensitivity was analysed with colony forming assays. Phosphorylated histone H2AX (γH2AX) intensity, which correlates with the number of DSBs, was analysed with flow cytometry.

RESULTS

RAD51 foci appeared with heat shock, and the number of cells with RAD51 foci was maximal at about 4 h after heat shock. Heat-induced RAD51 foci co-localised with γH2AX foci. BRCA2-deficient cells were sensitive to heat when compared to their parental wild-type cells. Heat-induced γH2AX was higher in BRCA2-deficient cells compared to parental cells. In SAS cells, cells transfected with BRCA2-siRNA were more sensitive to heat than cells transfected with negative control siRNA. Apoptotic bodies increased in number more rapidly in BRCA2-siRNA transfected cells than in cells transfected with negative control siRNA when cells were observed at 48 h after a heat treatment. In addition, cells deficient in BRCA2 were incapable of activating heat-induced G₂/M arrest.

CONCLUSION

BRCA2 has a protecting role against heat-induced cell death. BRCA2 might be a potential molecular target for hyperthermic cancer therapy.

Homologous recombination preferentially repairs heat-induced DNA double-strand breaks in mammalian cells

PURPOSE

Heat shock induces DNA double-strand breaks (DSBs), but the precise mechanism of repairing heat-induced damage is unclear. Here, we investigated the DNA repair pathways involved in cell death induced by heat shock.

MATERIALS AND METHODS

B02, a specific inhibitor of human RAD51 (homologous recombination; HR), and NU7026, a specific inhibitor of DNA-PK (non-homologous end-joining; NHEJ), were used for survival assays of human cancer cell lines with different p53-gene status. Mouse embryonic fibroblasts (MEFs) lacking Lig4 (NHEJ) and/or Rad54 (HR) were used for survival assays and a phosphorylated histone H2AX at Ser139 (γH2AX) assay. MEFs lacking Rad51d (HR) were used for survival assays. SPD8 cells were used to measure HR frequency after heat shock.

RESULTS

Human cancer cells were more sensitive to heat shock in the presence of B02 despite their p53-gene status, and the effect of B02 on heat sensitivity was specific to the G₂ phase. Rad54-deficient MEFs were sensitive to heat shock and showed prolonged γH2AX signals following heat shock. Rad51d-deficient MEFs were also sensitive to heat shock. Moreover, heat shock-stimulated cells had increased HR.

CONCLUSIONS

The HR pathway plays an important role in the survival of mammalian cells against death induced by heat shock via the repair of heat-induced DNA DSBs.

Effect of mild temperature shift on poly(ADP-ribose) and γH2AX levels in cultured cells

Poly (ADP-ribose) (PAR) is rapidly synthesized by PAR polymerases (PARPs) upon activation by DNA single- and double-strand breaks. In this study, we examined the quantitative amount of PAR in HeLa cells cultured within the physiological temperatures below 41 °C for verification of the effect of shifting-up or -down the temperature from 37.0 °C on the DNA breaks, whether the temperature-shift caused breaks that could be monitored by the level of PAR. While PAR level did not change significantly when HeLa cells were cultured at 33.5 °C or 37.0 °C, it was significantly increased 2- and 3-fold when cells were cultured for 12 h and 24 h, respectively, at 40.5 °C as compared to 37.0 °C. Similar to the results with HeLa cells, PAR level was increased 2-fold in CHO-K1 cells cultured at 40.5 °C for 24 h as compared to 37.0 °C. As the cellular levels of PAR polymerase1 (PARP1) and PAR glycohydrolase (PARG), a major degradation enzyme for PAR, did not seem to change significantly, this increase could be caused by activation of PARP1 by DNA strand breaks. In fact, γH2AX, claimed to be a marker of DNA double-strand breaks, was found in cell extracts of HeLa cells and CHO-K1 cells at elevated temperature vs. 37.0 °C, and these γH2AX signals were intensified in the presence of 3-aminobenzamide, a PARP inhibitor. The γH2AX immunohistochemistry results in HeLa cells were consistent with Western blot analyses. In HeLa cells, proliferation was significantly suppressed at 40.5 °C in 72 h-continuous cultures and decreased viabilities were also observed after 24-72 h at 40.5 °C. Flow cytometric analyses showed that the HeLa cells were arrested at G2/M after temperature shift-up to 40.5 °C. These physiological changes were potentiated in the presence of 3-aminobenzamide. Decrease in growth rates, increased cytotoxicity and G2/M arrest, were associated with the temperature-shift to 40.5 °C and are indirect evidence of DNA breaks. In addition to γH2AX, PAR could be a sensitive marker for DNA single- and double-strand breaks. These two molecular markers provide evidence of physiological changes occurring within cells.

Improving efficacy of hyperthermia in oncology by exploiting biological mechanisms

It has long been established that hyperthermia increases the therapeutic benefit of radiation and chemotherapy in cancer treatment. During the last few years there have been substantial technical improvements in the sources used to apply and measure heat, which greatly increases enthusiasm for the clinical use of hyperthermia. These advances are converging with a better understanding of the physiological and molecular effects of hyperthermia. Therefore, we are now at a juncture where the parameters that will influence the efficacy of hyperthermia in cancer treatment can be optimised in a more systematic and rational manner. In addition, the novel insights in hyperthermia's many biological effects on tumour cells will ultimately result in new treatment regimes. For example, the molecular effects of hyperthermia on the essential cellular process of DNA repair suggest novel combination therapies, with DNA damage response targeting drugs that should now be clinically explored. Here, we provide an overview of recent studies on the various macroscopic and microscopic biological effects of hyperthermia. We indicate the significance of these effects on current treatments and suggest how they will help design novel future treatments.

Effects of hyperthermia as a mitigation strategy in DNA damage-based cancer therapies

Utilization of thermal therapy (hyperthermia) is defined as the application of exogenous heat induction and represents a concept that is far from new as it goes back to ancient times when heat was used for treating various diseases, including malignancies. Such therapeutic strategy has gained even more popularity (over the last few decades) since various studies have shed light into understanding hyperthermia's underlying molecular mechanism(s) of action. In general, hyperthermia is applied as complementary (adjuvant) means in therapeutic protocols combining chemotherapy and/or irradiation both of which can induce irreversible cellular DNA damage. Furthermore, according to a number of in vitro, in vivo and clinical studies, hyperthermia has been shown to enhance the beneficial effects of DNA targeting therapeutic strategies by interfering with DNA repair response cascades. Therefore, the continuously growing evidence supporting hyperthermia's beneficial role in cancer treatment can also encourage its application as a DNA repair mitigation strategy. In this review article, we aim to provide detailed information on how hyperthermia acts on DNA damage and repair pathways and thus potentially contributing to various adjuvant therapeutic protocols relevant to more efficient cancer treatment strategies.

Effects of hyperthermia on DNA repair pathways: one treatment to inhibit them all

The currently available arsenal of anticancer modalities includes many DNA damaging agents that can kill malignant cells. However, efficient DNA repair mechanisms protect both healthy and cancer cells against the effects of treatment and contribute to the development of drug resistance. Therefore, anti-cancer treatments based on inflicting DNA damage can benefit from inhibition of DNA repair. Hyperthermia – treatment at elevated temperature – considerably affects DNA repair, among other cellular processes, and can thus sensitize (cancer) cells to DNA damaging agents. This effect has been known and clinically applied for many decades, but how heat inhibits DNA repair and which pathways are targeted has not been fully elucidated. In this review we attempt to summarize the known effects of hyperthermia on DNA repair pathways relevant in clinical treatment of cancer. Furthermore, we outline the relationships between the effects of heat on DNA repair and sensitization of cells to various DNA damaging agents.

Mechanism of heat stress-induced cellular senescence elucidates the exclusive vulnerability of early S-phase cells to mild genotoxic stress

Heat stress is one of the best-studied cellular stress factors; however, little is known about its delayed effects. Here, we demonstrate that heat stress induces p21-dependent cellular senescence-like cell cycle arrest. Notably, only early S-phase cells undergo such an arrest in response to heat stress. The encounter of DNA replication forks with topoisomerase I-generated single-stranded DNA breaks resulted in the generation of persistent double-stranded DNA breaks was found to be a primary cause of heat stress-induced cellular senescence in these cells. This investigation of heat stress-induced cellular senescence elucidates the mechanisms underlying the exclusive sensitivity of early S-phase cells to ultra-low doses of agents that induce single-stranded DNA breaks.

Hyperthermia Induces Apoptosis of 786-O Cells through Suppressing Ku80 Expression

Hyperthermia as an anticancer method has been paid increasing attention in recent years. Several studies have shown that hyperthermia can kill tumor cells by inducing apoptosis. However, the underlying molecular mechanisms of hyperthermia-induced apoptosis are largely unknown. To investigate the effects and molecular mechanism of hyperthermia on the apoptosis in renal carcinoma 786-O cells, we firstly examined apoptosis and Ku expression in 786-O cell line treated with heat exposure (42°C for 0-4 h). The results showed that hyperthermia induced apoptosis of 786-O cells, and suppressed significantly Ku80 expression, but not Ku70 expression. Next, we knock-down Ku80 in 786-O cells, generating stable cell line 786-O-shKu80, and detected apoptosis, cell survival and cell cycle distribution. Our data showed higher apoptotic rate and lower surviving fraction in the stable cell line 786-O-shKu80 compared with those in control cells, exposed to the same heat stress (42°C for 0-4 h). Moreover, the results also showed suppression of Ku80 led to G2/M phase arrest in the stable cell line 786-O-shKu80 following heat treatment. Together, these findings indicate that Ku80 may play an important role in hyperthermia-induced apoptosis and heat-sensitivity of renal carcinoma cells through influencing the cell cycle distribution.

When heat casts a spell on the DNA damage checkpoints

Peregrine Laziosi (1265–1345), an Italian priest, became the patron saint of cancer patients when the tumour in his left leg miraculously disappeared after he developed a fever. Elevated body temperature can cause tumours to regress and sensitizes cancer cells to agents that break DNA. Why hyperthermia blocks the repair of broken chromosomes by changing the way that the DNA damage checkpoint kinases ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) are activated is an unanswered question. This review discusses the current knowledge of how heat affects the ATR–Chk1 and ATM–Chk2 kinase networks, and provides a possible explanation of why homeothermal organisms such as humans still possess this ancient heat response.

Heat shock-induced dissociation of TRF2 from telomeres does not initiate a telomere-dependent DNA damage response

Telomeric repeat binding factor 2 (TRF2) is a well-studied shelterin complex subunit that plays a major role in the protection of chomosome ends and the prevention of the telomere-associated DNA damage response. We show that heat shock induces the dissociation of TRF2 from telomeres in human primary and cancer cell cultures. TRF2 is not simply degraded in response to heat shock, but redistributed thoughout the nucleoplasm. This TRF2 depletion/redistribution does not initiate the DNA damage response at chomosome termini.

Mechanisms of heat shock response in mammals

Heat shock (HS) is one of the best-studied exogenous cellular stresses. The cellular response to HS utilizes ancient molecular networks that are based primarily on the action of stress-induced heat shock proteins and HS factors. However, in one way or another, all cellular compartments and metabolic processes are involved in such a response. In this review, we aimed to summarize the experimental data concerning all aspects of the HS response in mammalian cells, such as HS-induced structural and functional alterations of cell membranes, the cytoskeleton and cellular organelles; the associated pathways that result in different modes of cell death and cell cycle arrest; and the effects of HS on transcription, splicing, translation, DNA repair, and replication.

Dual effect of heat shock on DNA replication and genome integrity

The data presented here suggest that in an asynchronous cell culture, heat shock might affect DNA integrity both directly and via arrest of replication fork progression and that the phosphorylation of histone H2AX has a protective effect on the arrested replication forks in addition to its known DNA damage signaling function.

Heat shock (HS) is one of the better-studied exogenous stress factors. However, little is known about its effects on DNA integrity and the DNA replication process. In this study, we show that in G1 and G2 cells, HS induces a countable number of double-stranded breaks (DSBs) in the DNA that are marked by γH2AX. In contrast, in S-phase cells, HS does not induce DSBs but instead causes an arrest or deceleration of the progression of the replication forks in a temperature-dependent manner. This response also provoked phosphorylation of H2AX, which appeared at the sites of replication. Moreover, the phosphorylation of H2AX at or close to the replication fork rescued the fork from total collapse. Collectively our data suggest that in an asynchronous cell culture, HS might affect DNA integrity both directly and via arrest of replication fork progression and that the phosphorylation of H2AX has a protective effect on the arrested replication forks in addition to its known DNA damage signaling function.

Quantification of gamma-H2AX foci in human lymphocytes: a method for biological dosimetry after ionizing radiation exposure

Recent studies have suggested that visualization of gamma-H2AX nuclear foci can be used to estimate exposure to very low doses of ionizing radiation. Although this approach is widely used for various purposes, its suitability for individual human biodosimetry has not yet been assessed. We therefore conducted such an assessment with the help of available software for observing and automatically scoring gamma-H2AX foci. The presence of gamma-H2AX foci was evaluated in human peripheral blood lymphocytes exposed ex vivo to gamma rays in a dose range of 0.02 to 2 Gy. We analyzed the response of gamma-H2AX to ionizing radiation in relation to dose, time after exposure, and individual variability. We constructed dose-effect calibration curves at 0.5, 8 and 16 h after exposure and evaluated the threshold of detection of the technique. The results show the promise of automatic gamma-H2AX scoring for a reliable assessment of radiation doses in a dose range of 0.6 Gy to 2 Gy up to 16 h after exposure. This gamma-H2AX-based assay may be useful for biodosimetry, especially for triage to distinguish promptly among individuals the ones who have received negligible doses from those with significantly exposures who are in need of immediate medical attention. However, additional in vivo experiments are needed for validation.

Medium-mediated DNA repair response after ionizing radiation is correlated with the increase of specific cytokines in human fibroblasts

Radiation induced bystander effects, either protective or adverse, have been identified in a variety of cells and for different endpoints. They are thought to arise from communication between cells through direct cell-cell contacts and via transmissible molecules secreted into the medium by targeted cells. We have investigated medium-mediated damage response in human dermal fibroblasts (HDF) after exposure to ionizing irradiation. We show that HDF experience an elevated level of double stranded DNA damage repair response when incubated with conditioned growth medium of irradiated cells. The magnitude of this response is much lower than observed for directly irradiated cells and is proportional to the radiation dose, as is its persistence across time. Since secretion of cytokines is one of the possible pathways linking targeted and non-targeted cells a multiplex analysis was performed. Four cytokines - IL6, IL8, MCP-1 and RANTES - were identified in the growth medium of irradiated cells after exposure to X-rays (2Gy). These cytokines were significantly upregulated and each cytokine showed differential upregulation kinetics. Finally we performed a functional analysis to see if IL6 and MCP-1 could induce gammaH2AX foci formation. IL6 caused a significant increase in spot occupancy compared to controls. Although only indicative MCP-1 appears to have the opposite effect as it caused a drop in spot occupancy. The combined addition of these two cytokines produced no significant response was observed. Both IL6 and MCP-1 have an effect on the gammaH2AX spot occupancy possibly linking these cytokines to the bystander response.

DNA double-strand breaks: their production, recognition, and repair in eukaryotes

Human cells accumulate at least 10,000 DNA lesions every day. Failure to repair such lesions can lead to mutations, genomic instability, or cell death. Among the various types of damage which can be expressed in a cell, DNA double-strand breaks (DSBs) represent the most serious threat. Different kinds of physical, chemical, and biological factors have been reported to induce DNA lesions, including DSBs. The aim of this review is to provide a basic understanding and overview of how DSBs are produced, recognized and repaired, and to describe the role of some of the genes and proteins involved in DSB repair.