Hsf1-mediated stress response can transiently enhance cellular radioresistance

Advances in the study of HSP70 inhibitors to enhance the sensitivity of tumor cells to radiotherapy

The 70 kDa heat shock protein (HSP70) is one of the most conserved proteins and a ubiquitous molecular chaperone that plays a role in the folding, remodeling, and degradation of various proteins to maintain proteostasis. It has been shown that HSP70 is abundantly expressed in cancer and enhances tumor resistance to radiotherapy by inhibiting multiple apoptotic pathways, such as interfering with the cellular senescence program, promoting angiogenesis, and supporting metastasis. Thus, HSP70 provides an effective target for enhancing the effects of radiation therapy in the clinical management of cancer patients. Inhibition of HSP70 enhances the radiation-induced tumor-killing effect and thus improves the efficacy of radiotherapy. This article reviews the sensitivity of Hsp70 and its related inhibitors to radiotherapy of tumor cells.

Hypoxia-Induced Cancer Cell Responses Driving Radioresistance of Hypoxic Tumors: Approaches to Targeting and Radiosensitizing

Simple Summary

Some regions of aggressive malignancies experience hypoxia due to inadequate blood supply. Cancer cells adapting to hypoxic conditions somehow become more resistant to radiation exposure and this decreases the efficacy of radiotherapy toward hypoxic tumors. The present review article helps clarify two intriguing points: why hypoxia-adapted cancer cells turn out radioresistant and how they can be rendered more radiosensitive. The critical molecular targets associated with intratumoral hypoxia and various approaches are here discussed which may be used for sensitizing hypoxic tumors to radiotherapy.

Abstract

Within aggressive malignancies, there usually are the “hypoxic zones”—poorly vascularized regions where tumor cells undergo oxygen deficiency through inadequate blood supply. Besides, hypoxia may arise in tumors as a result of antiangiogenic therapy or transarterial embolization. Adapting to hypoxia, tumor cells acquire a hypoxia-resistant phenotype with the characteristic alterations in signaling, gene expression and metabolism. Both the lack of oxygen by itself and the hypoxia-responsive phenotypic modulations render tumor cells more radioresistant, so that hypoxic tumors are a serious challenge for radiotherapy. An understanding of causes of the radioresistance of hypoxic tumors would help to develop novel ways for overcoming this challenge. Molecular targets for and various approaches to radiosensitizing hypoxic tumors are considered in the present review. It is here analyzed how the hypoxia-induced cellular responses involving hypoxia-inducible factor-1, heat shock transcription factor 1, heat shock proteins, glucose-regulated proteins, epigenetic regulators, autophagy, energy metabolism reprogramming, epithelial–mesenchymal transition and exosome generation contribute to the radioresistance of hypoxic tumors or may be inhibited for attenuating this radioresistance. The pretreatments with a multitarget inhibition of the cancer cell adaptation to hypoxia seem to be a promising approach to sensitizing hypoxic carcinomas, gliomas, lymphomas, sarcomas to radiotherapy and, also, liver tumors to radioembolization.

HSF1: Primary Factor in Molecular Chaperone Expression and a Major Contributor to Cancer Morbidity

Heat shock factor 1 (HSF1) is the primary component for initiation of the powerful heat shock response (HSR) in eukaryotes. The HSR is an evolutionarily conserved mechanism for responding to proteotoxic stress and involves the rapid expression of heat shock protein (HSP) molecular chaperones that promote cell viability by facilitating proteostasis. HSF1 activity is amplified in many tumor contexts in a manner that resembles a chronic state of stress, characterized by high levels of HSP gene expression as well as HSF1-mediated non-HSP gene regulation. HSF1 and its gene targets are essential for tumorigenesis across several experimental tumor models, and facilitate metastatic and resistant properties within cancer cells. Recent studies have suggested the significant potential of HSF1 as a therapeutic target and have motivated research efforts to understand the mechanisms of HSF1 regulation and develop methods for pharmacological intervention. We review what is currently known regarding the contribution of HSF1 activity to cancer pathology, its regulation and expression across human cancers, and strategies to target HSF1 for cancer therapy.

Induction of Hsp70 in tumor cells treated with inhibitors of the Hsp90 activity: A predictive marker and promising target for radiosensitization

We studied a role of the inducible heat shock protein 70 (Hsp70) in cellular response to radiosensitizing treatments with inhibitors of the heat shock protein 90 (Hsp90) chaperone activity. Cell lines derived from solid tumors of different origin were treated with the Hsp90 inhibitors (17AAG, geldanamycin, radicicol, NVP-AUY922) or/and γ-photon radiation. For comparison, human cells of the non-cancerous origin were subjected to the same treatments. We found that the Hsp90 inhibitors yielded considerable radiosensitization only when they cause early and pronounced Hsp70 induction; moreover, a magnitude of radiosensitization was positively correlated with the level of Hsp70 induction. The quantification of Hsp70 levels in Hsp90 inhibitor-treated normal and cancer cells enabled to predict which of them will be susceptible to any Hsp90-inhibiting radiosensitizer as well as what concentrations of the inhibitors ensure the preferential cytotoxicity in the irradiated tumors without aggravating radiation damage to adjacent normal tissues. Importantly, the Hsp70 induction in the Hsp90 inhibitor-treated cancer cells appears to be their protective response that alleviates the tumor-sensitizing effects of the Hsp90 inactivation. Combination of the Hsp70-inducing inhibitors of Hsp90 with known inhibitors of the Hsp induction such as quercetin, triptolide, KNK437, NZ28 prevented up-regulation of Hsp70 in the cancer cells thereby increasing their post-radiation apoptotic/necrotic death and decreasing their post-radiation viability/clonogenicity. Similarly, co-treatment with the two inhibitors conferred the enhanced radiosensitization of proliferating rather than quiescent human vascular endothelial cells which may be used for suppressing the tumor-stimulated angiogenesis. Thus, the easily immunodetectable Hsp70 induction can be a useful marker for predicting effects of Hsp90-inhibiting radiosensitizers on tumors and normal tissues exposed to ionizing radiation. Moreover, targeting the Hsp70 induction in Hsp90 inhibitor-treated cancer cells and tumor vasculature cells may beneficially enhance the radiosensitizing effect.

Coniferyl aldehyde reduces radiation damage through increased protein stability of heat shock transcriptional factor 1 by phosphorylation

PURPOSE

We previously screened natural compounds and found that coniferyl aldehyde (CA) was identified as an inducer of HSF1. In this study, we further examined the protective effects of CA against ionizing radiation (IR) in normal cell system.

METHODS AND MATERIALS

Western blotting and reverse transcription-polymerase chain reaction tests were performed to evaluate expression of HSF1, HSP27, and HSP70 in response to CA. Cell death and cleavage of PARP and caspase-3 were analyzed to determine the protective effects of CA in the presence of IR or taxol. The protective effects of CA were also evaluated using animal models.

RESULTS

CA increased stability of the HSF1 protein by phosphorylation at Ser326, which was accompanied by increased expression of HSP27 and HSP70. HSF1 phosphorylation at Ser326 by CA was mediated by EKR1/2 activation. Cotreatment of CA with IR or taxol in normal cells induced protective effects with phosphorylation- dependent patterns at Ser326 of HSF1. The decrease in bone marrow (BM) cellularity and increase of terminal deoxynucleotidyl transferase dUTP nick end labeling-positive BM cells by IR were also significantly inhibited by CA in mice (30.6% and 56.0%, respectively). A549 lung orthotopic lung tumor model indicated that CA did not affect the IR-mediated reduction of lung tumor nodules, whereas CA protected normal lung tissues from the therapeutic irradiation.

CONCLUSIONS

These results suggest that CA may be useful for inducing HSF1 to protect against normal cell damage after IR or chemotherapeutic agents.

Active heat shock transcription factor 1 supports migration of the melanoma cells via vinculin down-regulation

Heat shock transcription factor 1 (HSF1), the major regulator of stress response, is frequently activated in cancer and has an apparent role in malignant transformation. Here we analyzed the influence of the over-expression of a constitutively active transcriptionally-competent HSF1 mutant form on phenotypes of mouse and human melanoma cells. We observed that the expression of active HSF1 supported anchorage-independent growth in vitro, and metastatic spread in the animal model in vivo, although the proliferation rate of cancer cells was not affected. Furthermore, active HSF1 enhanced cell motility, reduced the adherence of cells to a fibronectin-coated surface, and affected the actin cytoskeleton. We found that although the expression of active HSF1 did not affect levels of epithelial-to-mesenchymal transition markers, it caused transcriptional down-regulation of vinculin, protein involved in cell motility, and adherence. Functional HSF1-binding sites were found in mouse and human Vcl/VCL genes, indicating a direct role of HSF1 in the regulation of this gene. An apparent association between HSF1-induced down-regulation of vinculin, increased motility, and a reduced adherence of cells suggests a possible mechanism of HSF1-mediated enhancement of the metastatic potential of cancer cells.

Apoptosis and molecular targeting therapy in cancer

Apoptosis is the programmed cell death which maintains the healthy survival/death balance in metazoan cells. Defect in apoptosis can cause cancer or autoimmunity, while enhanced apoptosis may cause degenerative diseases. The apoptotic signals contribute into safeguarding the genomic integrity while defective apoptosis may promote carcinogenesis. The apoptotic signals are complicated and they are regulated at several levels. The signals of carcinogenesis modulate the central control points of the apoptotic pathways, including inhibitor of apoptosis (IAP) proteins and FLICE-inhibitory protein (c-FLIP). The tumor cells may use some of several molecular mechanisms to suppress apoptosis and acquire resistance to apoptotic agents, for example, by the expression of antiapoptotic proteins such as Bcl-2 or by the downregulation or mutation of proapoptotic proteins such as BAX. In this review, we provide the main regulatory molecules that govern the main basic mechanisms, extrinsic and intrinsic, of apoptosis in normal cells. We discuss how carcinogenesis could be developed via defective apoptotic pathways or their convergence. We listed some molecules which could be targeted to stimulate apoptosis in different cancers. Together, we briefly discuss the development of some promising cancer treatment strategies which target apoptotic inhibitors including Bcl-2 family proteins, IAPs, and c-FLIP for apoptosis induction.

Phosphorylated Hsp27 activates ATM-dependent p53 signaling and mediates the resistance of MCF-7 cells to doxorubicin-induced apoptosis

DNA damage activates p53 and its downstream target genes, which further leads to apoptosis or survival either by the cell cycle arrest or by DNA repair. In many tumors, the heat shock protein 27 (Hsp27) is expressed at high levels to provide protection against anticancer drugs. However, the roles of Hsp27 in p53-mediated cellular responses to DNA damage are controversial. Here, we investigated the interplay between the phosphorylation status of Hsp27 and p53 in kidney 293A (HEK293A) cells and found that over-expressing phosphorylated Hsp27 mimics (Hsp27-3D) activated p53/p21 in an ATM-dependent manner. In addition, incubation with doxorubicin (Dox), an anticancer drug, induced Hsp27 phosphorylation in human adenocarcinoma cells (MCF-7). In contrast, inhibition of Hsp27 phosphorylation retarded both p53 induction and p21 accumulation, and led to cell apoptosis. Furthermore, phosphorylated Hsp27 increased p53 nuclear importing and its downstream target gene expression such as p21 and MDM2, while de-phosphorylated Hsp27 impeded this procession. Taken together, our data suggest that Hsp27, in its phosphorylated or de-phosphorylated status, plays different roles in regulating p53 pathway and cell survival.

Silencing of heat shock protein 70 expression enhances radiotherapy efficacy and inhibits cell invasion in endometrial cancer cell line

AIM

To investigate the role of heat shock proteins 70 (HSP70) in radiosensitivity and invasiveness of endometrial cancer in vitro.

METHODS

HSP70 expression was silenced in relatively radioresistant, well-differentiated human endometrial cancer cell line ISK, using small interference RNA method, or by HSP70 overexpression after transfecting a HSP70-expressing vector. The effect of HSP70 on ISK cell line response to irradiation was evaluated. The surviving fraction was measured using colony-formation assay. Apoptosis was detected by flow cytometry and HSP70 expression was determined by quantitative real-time polymerase chain reaction, western-blot, and/or immunocytochemistry. Cell invasiveness was measured using transwell invasion assay.

RESULTS

HSP70 silencing caused a significant increase in irradiation-induced cell killing in comparison with control cells, with an enhancement factor of 1.27, and in the percentage of apoptotic cells (14.22% vs 6.74%, P = 0.021). After 4 Gy irradiation, mean +/- standard deviation survival fraction in ISK cells was reduced to 0.32 +/- 0.04 in comparison with control values but in ISK/siRNA-HSP70 cells the survival fraction was higher and amounted to 0.51 +/- 0.08 (P = 0.026). Silencing HSP70 significantly inhibited cell invasion before and after irradiation (106 +/- 19 vs 219 +/- 18 and 119 +/- 16 vs 256 +/- 31, P = 0.007). On the contrary, ectopic overexpression of HSP70 attenuated irradiation-induced apoptosis (7.15% vs 4.08%, P = 0.043) and induced more ISK/HSP70 cells invaded through the filters than mock-infected cells before and after irradiation (274 +/- 21 vs 194 +/- 16 before irradiation, and 298 +/- 24 vs 227 +/- 19 after irradiation, respectively, P = 0.032).

CONCLUSION

Disruption of HSP70-induced cytoprotection during irradiation enhances therapeutic effect of irradiation, which makes HSP70 a promising target in the research of endometrial cancer.

Hsf-1 and POB1 induce drug sensitivity and apoptosis by inhibiting Ralbp1

Hsf-1 (heat shock factor-1) is a transcription factor that is known to regulate cellular heat shock response through its binding with the multispecific transporter protein, Ralbp1. Results of present studies demonstrate that Hsf-1 causes specific and saturable inhibition of the transport activity of Ralbp1 and that the combination of Hsf-1 and POB1 causes nearly complete inhibition through specific bindings with Ralbp1. Augmentation of cellular levels of Hsf-1 and POB1 caused dramatic apoptosis in non-small cell lung cancer cell line H358 through Ralbp1 inhibition. These findings indicate a novel model for mutual regulation of Hsf-1 and Ralbp1 through Ralbp1-mediated sequestration of Hsf-1 in the cellular cytoskeleton and Hsf-1-mediated inhibition of the transport activity of membrane-bound Ralbp1.

Life span alteration after irradiation in Drosophila melanogaster strains with mutations of Hsf and Hsps

The life span alteration after gamma-irradiation and/or paraquat treatment in Drosophila in wild type strain Canton-S and strains with mutations of heat shock factor (1-4 alleles) and heat shock proteins (Hsp70Ba ( 304 ), Hsp83 ( e6A ), Hsp22 ( EY09909 ), Hsp67Bb ( EY099099 )) was investigated. Chronic low-dose rate gamma-irradiation (0.017 and 0.17 cGy/h) on pre-imago stages was used as a priming dose (absorbed doses were 4 and 40 cGy). Paraquat, a free radical inducing agent, was a challenging factor (20 mM for 1 day). It was shown that chronic irradiation led to adaptive response in both sexes except homozygous males and females with mutations of Hsf ( 4 ) and Hsp70Ba ( 304 ). The gender-specific differences in stress response were discovered in wild type strain Canton-S, Hsp22 ( EY09909 ) Hsp67Bb ( EY09909 ) homozygotes and Hsp83 ( e6A ) heterozygotes: the adaptive response persisted in males, but not in females. Thus, Drosophila Hsp and Hsf mutation homozygotes did not demonstrate the adaptive response in the majority of cases, implying an important role of those genes in radiation hormesis and adaptation to stresses.

Triggering senescence programs suppresses Chk1 kinase and sensitizes cells to genotoxic stresses

Depletion of the major heat shock protein Hsp72 leads to activation of the senescence program in a variety of tumor cell lines via both p53-dependent and p53-independent pathways. Here, we found that the Hsp72-depleted cells show defect in phosphorylation and activation of the protein kinase Chk1 by genotoxic stresses, such as UVC irradiation or camptothecin. Under these conditions, phosphorylation of Rad17 was also suppressed, whereas phosphorylation of p53 at Ser(15) was not affected, indicating a specific defect in phosphorylation of a subset of the ATR kinase substrates. Similarly, suppression of Chk1 activation was seen when senescence signaling was triggered by direct stimulation of p53, depletion of Cdc2, or overexpression of the cell cycle inhibitors p21 or p16. Thus, defect in Chk1 activation was not a consequence of the chaperone imbalance, but rather a downstream effect of activation of the senescence signaling. Inhibition of Chk1 was associated with inefficient inter-S phase checkpoint, as Hsp72 depleted cells failed to halt cell cycle progression upon UVC irradiation. Accordingly, sensitivity of cells to genotoxic stimuli after Hsp72 depletion was significantly enhanced. Thus, activation of the senescence signaling causes a defect in the DNA damage response manifested in increased sensitivity to genotoxic stresses.

Hyperosmotic stress response: comparison with other cellular stresses

Cellular responses induced by stress are essential for the survival of cells under adverse conditions. These responses, resulting in cell adaptation to the stress, are accomplished by a variety of processes at the molecular level. After an alteration in homeostatic conditions, intracellular signalling processes link the sensing mechanism to adaptive or compensatory changes in gene expression. The ability of cells to adapt to hyperosmotic stress involves early responses in which ions move across cell membranes and late responses characterized by increased synthesis of either membrane transporters essential for uptake of organic osmolytes or of enzymes involved in their synthesis. The goal of these responses is to return the cell to its normal size and maintain cellular homeostasis. The enhanced synthesis of molecular chaperones, such as heat shock proteins, is another important component of the adaptive process that contributes to cell survival. Some responses are common to different stresses, whereas others are specific. In the first part of the review, we illustrate the characteristic and specific features of adaptive response to hypertonicity; we then describe similarities to and differences from other cellular stresses, such as genotoxic agents, nutrient starvation and heat shock.

Heat shock protein 70 enhanced deoxyribonucleic acid base excision repair in human leukemic cells after ionizing radiation

Base excision repair (BER) of DNA damage in irradiated THP1 human leukemic cells was stimulated by pretreating the cells with exogenous recombinant Hsp70. The treatment of THP1 cells with recombinant Hsp70 in cell culture promoted repair by reducing the frequency of apurinic, apyrimidinic (AP) sites in DNA before and after 1.3 Gy of radiation. However, by 30 minutes after 2.6 Gy, accelerated repair of abasic sites supervened, which may contribute to the loss of the very-low-dose cell hypersensitivity seen in clonogenic studies of other laboratories. After irradiation with 2.6 Gy, the crucial initial glycosylase step was markedly incomplete when cells had been transfected 24 hours before with a small interfering RNA (siRNA) designed to inhibit synthesis of Hsp70. In confirmation, lysates from irradiated siRNA-treated cells after 2.6 Gy were deficient in uracil glycosylase activity (UDG). Transfection with a scrambled RNA of the same size did not interfere with the glycosylase step, ie, the prompt conversion of damaged pyrimidine sites to abasic sites as well as the subsequent repair of those sites. BER measured by reduction of DNA AP sites before and after low-dose radiation was also deficient in THP1 cells that had been transfected with the siRNA designed to inhibit synthesis of Hsp70. These results implicate BER and the participation of Hsp70 in the repair of DNA in human leukemic cells with the doses of ionizing radiation used in clinical regimens.