Heat shock protein 90α (Hsp90α) is phosphorylated in response to DNA damage and accumulates in repair foci

Regulatory Mechanisms of Hsp90

The ability of Hsp90 to activate a disparate clientele implicates this chaperone in diverse biological processes. To accommodate such varied roles, Hsp90 requires a variety of regulatory mechanisms that are coordinated in order to modulate its activity appropriately. Amongst these, the master-regulator heat shock factor 1 (HSF1) is critically important in upregulating Hsp90 during stress, but is also responsible, through interaction with specific transcription factors (such as STAT1 and Strap/p300) for the integration of a variety of biological signals that ultimately modulate Hsp90 expression. Additionally, transcription factors, such as STAT1, STAT3 (including STAT1-STAT3 oligomers), NF-IL6, and NF-kB, are known to influence Hsp90 expression directly. Co-chaperones offer another mechanism for Hsp90 regulation, and these can modulate the chaperone cycle appropriately for specific clientele. Co-chaperones include those that deliver specific clients to Hsp90, and others that regulate the chaperone cycle for specific Hsp90-client complexes by modulating Hsp90s ATPase activity. Finally, post-translational modification (PTM) of Hsp90 and its co-chaperones helps too further regulate the variety of different Hsp90 complexes found in cells.

Hyperthermia enhances 17-DMAG efficacy in hepatocellular carcinoma cells with aggravated DNA damage and impaired G2/M transition

Due to the lack of effective treatment, hepatocellular carcinoma (HCC) is one of the malignancies with low survival rates worldwide. Combination of hyperthermia and chemotherapy has shown promising results in several abdominal tumours, but high expression of HSP90 in tumours attenuated the efficacy of hyperthermia. Thus a combination of hyperthermia and inhibition of HSP90 might be a feasible therapeutic strategy for HCC. One hepatic cell line (L02) and two HCC cell lines (Huh7 and HepG2) were heated at 42 °C for 0, 0.5 or 4 h with or without 100 nM 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG). HCC cells of the combination group exhibited more G2/M arrest and higher apoptotic rates which might result from suffering from more reactive oxygen species and serious DNA damage. Heat shock/17-DMAG co-treatment of HCC cells also destabilized CDK1, Cyclin B1 and CDC25C with a concomitant decreased proportion of cells in the M phase. Furthermore, co-treatment impaired the interaction of HSP90α with CDC37 and with CDK1, accompanied with decreased soluble CDK1. Combination of 17-DMAG with a 1.5-h whole body hyperthermia treatment attenuated tumour growth in xenograft mice models. These results suggest hyperthermia sensitize HCC to 17-DMAG, and combination of hyperthermia with 17-DMAG might be a potential therapeutic strategy for HCC.

Increased extracellular heat shock protein 90α in severe sepsis and SIRS associated with multiple organ failure and related to acute inflammatory-metabolic stress response in children

Mammalian heat-shock-protein (HSP) 90α rapidly responses to environmental insults. We examined the hypothesis that not only serum HSP72 but also HSP90α is increased in the systemic inflammatory response syndrome (SIRS), severe-sepsis (SS), and/or sepsis (S) compared to healthy children (H); we assessed HSP90α relation to (a) multiple organ system failure (MOSF) and (b) inflammatory-metabolic response and severity of illness.A total of 65 children with S, SS, or SIRS and 25 H were included. ELISA was used to evaluate extracellular HSP90α and HSP72, chemiluminescence interleukins (ILs), flow-cytometry neutrophil-CD64 (nCD64)-expression.HSP90α, along with HSP72, were dramatically increased among MOSF patients. Patients in septic groups and SIRS had elevated HSP90α compared to H (P < 0.01). HSP90α was independently related to predicted death rate and severity of illness; positively to HSP72, nCD64, ILs, length of stay, days on ventilator, and fever; negatively to HDL and LDL (P < 0.05). The HSP72 was increased in SS/S and related negatively to HDL and LDL (P < 0.05).Serum HSP90α is markedly elevated in children with severe sepsis and is associated with MOSF. Better than the HSP72, also increased in SS, SIRS, and MOSF, HSP90α is related to the inflammatory stress, fever, outcome endpoints, and predicted mortality and inversely related to the low-LDL/low-HDL stress metabolic pattern.

Dual TORK/DNA-PK inhibition blocks critical signaling pathways in chronic lymphocytic leukemia

TORK/DNA-PK inhibition induces cytotoxicity and blocks signaling pathways important for CLL survival, proliferation, and drug resistance. Preliminary clinical effects of TORK/DNA-PK inhibition show 7 of 8 CLL patients with decreased lymphadenopathy.

Ovarian steroids regulate gene expression related to DNA repair and neurodegenerative diseases in serotonin neurons of macaques

Depression often accompanies the perimenopausal transition and it often precedes overt symptomology in common neurodegenerative diseases (NDDs, such as Alzheimer's, Parkinson's, Huntington, amyotrophic lateral sclerosis). Serotonin dysfunction is frequently found in the different etiologies of depression. We have shown that ovariectomized (Ovx) monkeys treated with estradiol (E) for 28 days supplemented with placebo or progesterone (P) on days 14-28 had reduced DNA fragmentation in serotonin neurons of the dorsal raphe nucleus, and long-term Ovx monkeys had fewer serotonin neurons than intact controls. We questioned the effect of E alone or E+P (estradiol supplemented with progesterone) on gene expression related to DNA repair, protein folding (chaperones), the ubiquitin-proteosome, axon transport and NDD-specific genes in serotonin neurons. Ovx macaques were treated with placebo, E or E+P (n=3 per group) for 1 month. Serotonin neurons were laser captured and subjected to microarray analysis and quantitative real-time PCR (qRT-PCR). Increases were confirmed with qRT-PCR in five genes that code for proteins involved in repair of strand breaks and nucleotide excision. NBN1, PCNA (proliferating nuclear antigen), GADD45A (DNA damage-inducible), RAD23A (DNA damage recognition) and GTF2H5 (gene transcription factor 2H5) significantly increased with E or E+P treatment (all analysis of variance (ANOVA), P<0.01). Chaperone genes HSP70 (heat-shock protein 70), HSP60 and HSP27 significantly increased with E or E+P treatment (all ANOVA, P<0.05). HSP90 showed a similar trend. Ubiquinase coding genes UBEA5, UBE2D3 and UBE3A (Parkin) increased with E or E+P (all ANOVA, P<0.003). Transport-related genes coding kinesin, dynein and dynactin increased with E or E+P treatment (all ANOVA, P<0.03). SCNA (α-synuclein) and ADAM10 (α-secretase) increased (both ANOVA, P<0.02) but PSEN1 (presenilin1) decreased (ANOVA, P<0.02) with treatment. APP decreased 10-fold with E or E+P administration. Newman-Keuls post hoc comparisons indicated variation in the response to E alone versus E+P across the different genes. In summary, E or E+P increased gene expression for DNA repair mechanisms in serotonin neurons, thereby rendering them less vulnerable to stress-induced DNA fragmentation. In addition, E or E+P regulated four genes encoding proteins that are often misfolded or malfunctioning in neuronal populations subserving overt NDD symptomology. The expression and regulation of these genes in serotonergic neurons invites speculation that they may mediate an underlying disease process in NDDs, which in turn may be ameliorated or delayed with timely hormone therapy in women.

Hsp90: A New Player in DNA Repair?

Heat shock protein 90 (Hsp90) is an evolutionary conserved molecular chaperone that, together with Hsp70 and co-chaperones makes up the Hsp90 chaperone machinery, stabilizing and activating more than 200 proteins, involved in protein homeostasis (i.e., proteostasis), transcriptional regulation, chromatin remodeling, and DNA repair. Cells respond to DNA damage by activating complex DNA damage response (DDR) pathways that include: (i) cell cycle arrest; (ii) transcriptional and post-translational activation of a subset of genes, including those associated with DNA repair; and (iii) triggering of programmed cell death. The efficacy of the DDR pathways is influenced by the nuclear levels of DNA repair proteins, which are regulated by balancing between protein synthesis and degradation as well as by nuclear import and export. The inability to respond properly to either DNA damage or to DNA repair leads to genetic instability, which in turn may enhance the rate of cancer development. Multiple components of the DNA double strand breaks repair machinery, including BRCA1, BRCA2, CHK1, DNA-PKcs, FANCA, and the MRE11/RAD50/NBN complex, have been described to be client proteins of Hsp90, which acts as a regulator of the diverse DDR pathways. Inhibition of Hsp90 actions leads to the altered localization and stabilization of DDR proteins after DNA damage and may represent a cell-specific and tumor-selective radiosensibilizer. Here, the role of Hsp90-dependent molecular mechanisms involved in cancer onset and in the maintenance of the genome integrity is discussed and highlighted.

Regulation and function of the human HSP90AA1 gene

Heat shock protein 90α (Hsp90α), encoded by the HSP90AA1 gene, is the stress inducible isoform of the molecular chaperone Hsp90. Hsp90α is regulated differently and has different functions when compared to the constitutively expressed Hsp90β isoform, despite high amino acid sequence identity between the two proteins. These differences are likely due to variations in nucleotide sequence within non-coding regions, which allows for specific regulation through interaction with particular transcription factors, and to subtle changes in amino acid sequence that allow for unique post-translational modifications. This article will specifically focus on the expression, function and regulation of Hsp90α.

The HSP90 Inhibitor Ganetespib Radiosensitizes Human Lung Adenocarcinoma Cells

The molecular chaperone HSP90 is involved in stabilization and function of multiple client proteins, many of which represent important oncogenic drivers in NSCLC. Utilization of HSP90 inhibitors as radiosensitizing agents is a promising approach. The antitumor activity of ganetespib, HSP90 inhibitor, was evaluated in human lung adenocarcinoma (AC) cells for its ability to potentiate the effects of IR treatment in both in vitro and in vivo. The cytotoxic effects of ganetespib included; G2/M cell cycle arrest, inhibition of DNA repair, apoptosis induction, and promotion of senescence. All of these antitumor effects were both concentration- and time-dependent. Both pretreatment and post-radiation treatment with ganetespib at low nanomolar concentrations induced radiosensitization in lung AC cells in vitro. Ganetespib may impart radiosensitization through multiple mechanisms: such as down regulation of the PI3K/Akt pathway; diminished DNA repair capacity and promotion of cellular senescence. In vivo, ganetespib reduced growth of T2821 tumor xenografts in mice and sensitized tumors to IR. Tumor irradiation led to dramatic upregulation of β-catenin expression in tumor tissues, an effect that was mitigated in T2821 xenografts when ganetespib was combined with IR treatments. These data highlight the promise of combining ganetespib with IR therapies in the treatment of AC lung tumors.

The HSP90 inhibitor ganetespib: A potential effective agent for Acute Myeloid Leukemia in combination with cytarabine

HSP90 is a multi-client chaperone involved in regulating a large array of cellular processes and is commonly overexpressed in many different cancer types including hematological malignancies. Inhibition of HSP90 holds promise for targeting multiple molecular abnormalities and is therefore an attractive target for heterogeneous malignancies such as Acute Myeloid Leukemia (AML). Ganetespib is a highly potent second generation HSP90 inhibitor which we show is significantly more effective against primary AML blasts at nanomolar concentrations when compared with cytarabine (p<0.001). Dose dependant cytotoxicity was observed with an apoptotic response coordinate with the loss of pro-survival signaling through the client protein AKT. Combination treatment of primary blasts with ganetespib and cytarabine showed good synergistic interaction (combination index (CI): 0.47) across a range of drug effects with associated reduction in HSP70 feedback and AKT signaling levels. In summary, we show ganetespib to have high activity in primary AMLs as a monotherapy and a synergistic relationship with cytarabine when combined. The combination of cytotoxic cell death, suppression of cytoprotective/drug resistance mechanisms such as AKT and reduced clinical toxicity compared to other HSP90 inhibitors provide strong rationale for the clinical assessment of ganetespib in AML.

HSP90 regulates DNA repair via the interaction between XRCC1 and DNA polymerase β

Cellular DNA repair processes are crucial to maintain genome stability and integrity. In DNA base excision repair, a tight heterodimer complex formed by DNA polymerase β (Polβ) and XRCC1 is thought to facilitate repair by recruiting Polβ to DNA damage sites. Here we show that disruption of the complex does not impact DNA damage response or DNA repair. Instead, the heterodimer formation is required to prevent ubiquitylation and degradation of Polβ. In contrast, the stability of the XRCC1 monomer is protected from CHIP-mediated ubiquitylation by interaction with the binding partner HSP90. In response to cellular proliferation and DNA damage, proteasome and HSP90-mediated regulation of Polβ and XRCC1 alters the DNA repair complex architecture. We propose that protein stability, mediated by DNA repair protein complex formation, functions as a regulatory mechanism for DNA repair pathway choice in the context of cell cycle progression and genome surveillance.

Both the charged linker region and ATPase domain of Hsp90 are essential for Rad51-dependent DNA repair

The inhibition of Hsp90 in cancerous cells has been correlated with the reduction in double-strand break (DSB repair) activity. However, the precise effect of Hsp90 on the DSB repair pathway in normal cells has remained enigmatic. Our results show that the Hsp82 chaperone, the ortholog of mammalian Hsp90, is indispensable for homologous-recombination (HR)-mediated DNA repair in the budding yeast Saccharomyces cerevisiae. A considerable reduction in cell viability is observed in an Hsp82-inactivated mutant upon methyl methanesulfonate (MMS) treatment as well as upon UV treatment. The loss of Hsp82 function results in a dramatic decrease in gene-targeting efficiency and a marked decrease in the endogenous levels of the key recombination proteins Rad51 and Rad52 without any notable change in the levels of RAD51 or RAD52 transcripts. Our results establish Rad51 as a client of Hsp82, since they interact physically in vivo, and also show that when Hsp82 is inhibited by 17-AAG, Rad51 undergoes proteasomal degradation. By analyzing a number of point mutants with mutations in different domains of Hsp82, we observe a strong association between the sensitivity of an ATPase mutant of Hsp82 to DNA damage and the decreases in the amounts of Rad51 and Rad52 proteins. The most significant observations include the dramatic abrogation of HR activity and the marked decrease in Rad51 focus formation in the charged linker deletion mutant of Hsp82 upon MMS treatment. The charged linker region of Hsp82 is evolutionarily conserved in all eukaryotes, but until now, no biological significance has been assigned to it. Our findings elucidate the importance of this region in DNA repair for the first time.

Quantitative proteomics of the yeast Hsp70/Hsp90 interactomes during DNA damage reveal chaperone-dependent regulation of ribonucleotide reductase

The highly conserved molecular chaperones Hsp90 and Hsp70 are indispensible for folding and maturation of a significant fraction of the proteome, including many proteins involved in signal transduction and stress response. To examine the dynamics of chaperone-client interactions after DNA damage, we applied quantitative affinity-purification mass spectrometry (AP-MS) proteomics to characterize interactomes of the yeast Hsp70 isoform Ssa1 and Hsp90 isoform Hsp82 before and after exposure to methyl methanesulfonate. Of 256 proteins identified and quantified via (16)O(/18)O labeling and LC-MS/MS, 142 are novel Hsp70/90 interactors. Nearly all interactions remained unchanged or decreased after DNA damage, but 5 proteins increased interactions with Ssa1 and/or Hsp82, including the ribonucleotide reductase (RNR) subunit Rnr4. Inhibiting Hsp70 or 90 chaperone activity destabilized Rnr4 in yeast and its vertebrate homolog hRMM2 in breast cancer cells. In turn, pre-treatment of cancer cells with chaperone inhibitors sensitized cells to the RNR inhibitor gemcitabine, suggesting a novel chemotherapy strategy. All MS data have been deposited in the ProteomeXchange with identifier PXD001284.

BIOLOGICAL SIGNIFICANCE

This study provides the dynamic interactome of the yeast Hsp70 and Hsp90 under DNA damage which suggest key roles for the chaperones in a variety of signaling cascades. Importantly, the cancer drug target ribonucleotide reductase was shown to be a client of Hsp70 and Hsp90 in both yeast and breast cancer cells. As such, this study highlights the potential of a novel cancer therapeutic strategy that exploits the synergy of chaperone and ribonucleotide reductase inhibitors.

Targeting Hsp90 and its co-chaperones to treat Alzheimer's disease

INTRODUCTION

Alzheimer's disease, characterized by the accumulation of hyperphosphorylated tau and β amyloid (Aβ), currently lacks effective treatment. Chaperone proteins, such as the heat shock protein (Hsp) 90, form macromolecular complexes with co-chaperones, which can regulate tau metabolism and Aβ processing. Although small molecule inhibitors of Hsp90 have been successful at ameliorating tau and Aβ burden, their development into drugs to treat disease has been slow due to the off- and on-target effects of this approach as well as challenges with the pharmacology of current scaffolds. Thus, other approaches are being developed to improve these compounds and to target co-chaperones of Hsp90 in an effort to limit these liabilities.

AREAS COVERED

This article discusses the most current developments in Hsp90 inhibitors including advances in blood-brain barrier permeability, decreased toxicity and homolog-specific small-molecule inhibitors. In addition, we discuss current strategies targeting Hsp90 co-chaperones rather than Hsp90 itself to reduce off-target effects.

EXPERT OPINION

Although Hsp90 inhibitors have proven their efficacy at reducing tau pathology, they have yet to meet with success in the clinic. The development of Hsp90/tau complex-specific inhibitors and further development of Hsp90 co-chaperone-specific drugs should yield more potent, less toxic therapeutics.

Genotypic characteristics of resistant tumors to pre-operative ionizing radiation in rectal cancer

Due to a wide range of clinical response in patients undergoing neo-adjuvant chemoradiation for rectal cancer it is essential to understand molecular factors that lead to the broad response observed in patients receiving the same form of treatment. Despite extensive research in this field, the exact mechanisms still remain elusive. Data raging from DNA-repair to specific molecules leading to cell survival as well as resistance to apoptosis have been investigated. Individually, or in combination, there is no single pathway that has become clinically applicable to date. In the following review, we describe the current status of various pathways that might lead to resistance to the therapeutic applications of ionizing radiation in rectal cancer.

RNF144A, an E3 ubiquitin ligase for DNA-PKcs, promotes apoptosis during DNA damage

Several ring between ring fingers (RBR) -domain proteins, such as Parkin and Parc, have been shown to be E3 ligases involved in important biological processes. Here, we identify a poorly characterized RBR protein, Ring Finger protein 144A (RNF144A), as the first, to our knowledge, mammalian E3 ubiquitin ligase for DNA-PKcs. We show that DNA damage induces RNF144A expression in a p53-dependent manner. RNF144A is mainly localized in the cytoplasmic vesicles and plasma membrane and interacts with cytoplasmic DNA-dependent protein kinase, catalytic subunit (DNA-PKcs). DNA-PKcs plays a critical role in the nonhomologous end-joining DNA repair pathway and provides prosurvival signaling during DNA damage. We show that RNF144A induces ubiquitination of DNA-PKcs in vitro and in vivo and promotes its degradation. Depletion of RNF144A leads to an increased level of DNA-PKcs and resistance to DNA damaging agents, which is reversed by a DNA-PK inhibitor. Taken together, our data suggest that RNF144A may be involved in p53-mediated apoptosis through down-regulation of DNA-PKcs when cells suffer from persistent or severe DNA damage insults.

Characterization of LGALS3 (galectin-3) as a player in DNA damage response

DNA damage repair (DDR) is an orchestrated process encompassing the injury detection to its complete resolution. DNA double-strand break lesions are repaired mainly by two distinct mechanisms: the error-free homologous recombination (HR) and the error-prone non-homologous end-joining. Galectin-3 (GAL3) is the unique member of the chimeric galectins subfamily and is reported to be involved in several cancer development and progression related events. Recently our group described a putative protein interaction between GAL3 and BARD1, the main partner of breast and ovarian cancer susceptibility gene product BRCA1, both involved in HR pathway. In this report we characterized GAL3/BARD1 protein interaction and evaluated the role of GAL3 in DDR pathways using GAL3 silenced human cells exposed to different DNA damage agents. In the absence of GAL3 we observed a delayed DDR response activation, as well as a decrease in the G 2/M cell cycle checkpoint arrest associated with HR pathway. Moreover, using a TAP-MS approach we also determined the protein interaction network of GAL3.

Centrosomes at M phase act as a scaffold for the accumulation of intracellular ubiquitinated proteins

Centrosome size varies considerably during the cell cycle; it is greatest during metaphase, partly because of pericentriolar matrix recruitment and an increase in microtubule-organizing activity. However, the mechanism of centrosome maturation during M phase is poorly defined. In the present study, we identified and quantified centrosomal proteins during S and M phases using stable isotope labeling by amino acids in cell culture (SILAC) coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS). We identified 991 proteins, of which 310 and 325 proteins were upregulated during S and M phases, respectively. Ubiquitinated proteins containing K48- and K63-linked polyubiquitin chains accumulated in the centrosomes during M phase, although 26S proteasome activity in the centrosomes did not markedly differ between S and M phases. Conversely, cytoplasmic dynein, which transports ubiquitinated proteins to the centrosomes, increased 2-fold in the centrosomes during M phase relative to S phase. Furthermore, PYR-41, a ubiquitin E1 inhibitor, reduced centrosome size during metaphase, causing increased aneuploidy. RNA interference suppression of Ecm29, which inhibits proteasome activity, decreased the accumulation of ubiquitinated proteins in the centrosomes. These results show that accumulation of ubiquitinated proteins promotes centrosome maturation during M phase and further suggest a novel function of centrosomes as a scaffold temporarily gathering intracellular ubiquitinated proteins.

DNA-PK: a dynamic enzyme in a versatile DSB repair pathway

DNA double stranded breaks (DSBs) are the most cytoxic DNA lesion as the inability to properly repair them can lead to genomic instability and tumorigenesis. The prominent DSB repair pathway in humans is non-homologous end-joining (NHEJ). In the simplest sense, NHEJ mediates the direct re-ligation of the broken DNA molecule. However, NHEJ is a complex and versatile process that can repair DSBs with a variety of damages and ends via the utilization of a significant number of proteins. In this review we will describe the important factors and mechanisms modulating NHEJ with emphasis given to the versatility of this repair process and the DNA-PK complex.

DNA-PK target identification reveals novel links between DNA repair signaling and cytoskeletal regulation

The DNA-dependent protein kinase (DNA-PK) may function as a key signaling kinase in various cellular pathways other than DNA repair. Using a two-dimensional gel electrophoresis approach and stable DNA double-strand break-mimicking molecules (Dbait32Hc) to activate DNA-PK in the nucleus and cytoplasm, we identified 26 proteins that were highly phosphorylated following DNA-PK activation. Most of these proteins are involved in protein stability and degradation, cell signaling and the cytoskeleton. We investigated the relationship between DNA-PK and the cytoskeleton and found that the intermediate filament (IF) vimentin was a target of DNA-PK in vitro and in cells. Vimentin was phosphorylated at Ser459, by DNA-PK, in cells transfected with Dbait32Hc. We produced specific antibodies and showed that Ser459-P-vimentin was mostly located at cell protrusions. In migratory cells, the vimentin phosphorylation induced by Dbait32Hc was associated with a lower cellular adhesion and migration capacity. Thus, this approach led to the identification of downstream cytoplasmic targets of DNA-PK and revealed a connection between DNA damage signaling and the cytoskeleton.

Regulation of stress-inducible phosphoprotein 1 nuclear retention by protein inhibitor of activated STAT PIAS1

Stress-inducible phosphoprotein 1 (STI1), a cochaperone for Hsp90, has been shown to regulate multiple pathways in astrocytes, but its contributions to cellular stress responses are not fully understood. We show that in response to irradiation-mediated DNA damage stress STI1 accumulates in the nucleus of astrocytes. Also, STI1 haploinsufficiency decreases astrocyte survival after irradiation. Using yeast two-hybrid screenings we identified several nuclear proteins as STI1 interactors. Overexpression of one of these interactors, PIAS1, seems to be specifically involved in STI1 nuclear retention and in directing STI1 and Hsp90 to specific sub-nuclear regions. PIAS1 and STI1 co-immunoprecipitate and PIAS1 can function as an E3 SUMO ligase for STI. Using mass spectrometry we identified five SUMOylation sites in STI1. A STI1 mutant lacking these five sites is not SUMOylated, but still accumulates in the nucleus in response to increased expression of PIAS1, suggesting the possibility that a direct interaction with PIAS1 could be responsible for STI1 nuclear retention. To test this possibility, we mapped the interaction sites between PIAS1 and STI1 using yeast-two hybrid assays and surface plasmon resonance and found that a large domain in the N-terminal region of STI1 interacts with high affinity with amino acids 450-480 of PIAS1. Knockdown of PIAS1 in astrocytes impairs the accumulation of nuclear STI1 in response to irradiation. Moreover, a PIAS1 mutant lacking the STI1 binding site is unable to increase STI1 nuclear retention. Interestingly, in human glioblastoma multiforme PIAS1 expression is increased and we found a significant correlation between increased PIAS1 expression and STI1 nuclear localization. These experiments provide evidence that direct interaction between STI1 and PIAS1 is involved in the accumulation of nuclear STI1. This retention mechanism could facilitate nuclear chaperone activity.

Stress-inducible phosphoprotein 1 has unique cochaperone activity during development and regulates cellular response to ischemia via the prion protein

Stress-inducible phosphoprotein 1 (STI1) is part of the chaperone machinery, but it also functions as an extracellular ligand for the prion protein. However, the physiological relevance of these STI1 activities in vivo is unknown. Here, we show that in the absence of embryonic STI1, several Hsp90 client proteins are decreased by 50%, although Hsp90 levels are unaffected. Mutant STI1 mice showed increased caspase-3 activation and 50% impairment in cellular proliferation. Moreover, placental disruption and lack of cellular viability were linked to embryonic death by E10.5 in STI1-mutant mice. Rescue of embryonic lethality in these mutants, by transgenic expression of the STI1 gene, supported a unique role for STI1 during embryonic development. The response of STI1 haploinsufficient mice to cellular stress seemed compromised, and mutant mice showed increased vulnerability to ischemic insult. At the cellular level, ischemia increased the secretion of STI1 from wild-type astrocytes by 3-fold, whereas STI1 haploinsufficient mice secreted half as much STI1. Interesting, extracellular STI1 prevented ischemia-mediated neuronal death in a prion protein-dependent way. Our study reveals essential roles for intracellular and extracellular STI1 in cellular resilience.

Kinesin KIFC1 actively transports bare double-stranded DNA

During the past years, exogenous DNA molecules have been used in gene and molecular therapy. At present, it is not known how these DNA molecules reach the cell nucleus. We used an in cell single-molecule approach to observe the motion of exogenous short DNA molecules in the cytoplasm of eukaryotic cells. Our observations suggest an active transport of the DNA along the cytoskeleton filaments. We used an in vitro motility assay, in which the motion of single-DNA molecules along cytoskeleton filaments in cell extracts is monitored; we demonstrate that microtubule-associated motors are involved in this transport. Precipitation of DNA-bound proteins and mass spectrometry analyses reveal the preferential binding of the kinesin KIFC1 on DNA. Cell extract depletion of kinesin KIFC1 significantly decreases DNA motion, confirming the active implication of this molecular motor in the intracellular DNA transport.

Protein phosphatase 2A and DNA-dependent protein kinase are involved in mediating rapamycin-induced Akt phosphorylation

BACKGROUND

The mechanisms underlying rapamycin-induced Akt phosphorylation have not been fully elucidated.

RESULTS

Inhibition of PP2A or DNA-PK attenuates or abrogates rapamycin-induced Akt phosphorylation and co-inhibition of mTOR and DNA-PK enhances anticancer activity.

CONCLUSION

PP2A-dependent and DNA-PK-mediated mechanism is involved in rapamycin-induced Akt phosphorylation.

SIGNIFICANCE

A previously unknown mechanism underlying rapamycin-induced Akt phosphorylation and a novel strategy to enhance mTOR-targeted cancer therapy may be suggested. Inhibition of mammalian target of rapamycin complex 1 (mTORC1), for example with rapamycin, increases Akt phosphorylation while inhibiting mTORC1 signaling. However, the underlying mechanisms have not been fully elucidated. The current study has uncovered a previously unknown mechanism underlying rapamycin-induced Akt phosphorylation involving protein phosphatase 2A (PP2A)-dependent DNA protein kinase (DNA-PK) activation. In several cancer cell lines, inhibition of PP2A with okadaic acid, fostriecin, small T antigen, or PP2A knockdown abrogated rapamycin-induced Akt phosphorylation, and rapamycin increased PP2A activity. Chemical inhibition of DNA-PK, knockdown or deficiency of DNA-PK catalytic subunit (DNA-PKcs), or knock-out of the DNA-PK component Ku86 inhibited rapamycin-induced Akt phosphorylation. Exposure of cancer cells to rapamycin increased DNA-PK activity, and gene silencing-mediated PP2A inhibition attenuated rapamycin-induced DNA-PK activity. Collectively these results suggest that rapamycin induces PP2A-dependent and DNA-PK-mediated Akt phosphorylation. Accordingly, simultaneous inhibition of mTOR and DNA-PK did not stimulate Akt activity and synergistically inhibited the growth of cancer cells both in vitro and in vivo. Thus, our findings also suggest a novel strategy to enhance mTOR-targeted cancer therapy by co-targeting DNA-PK.

Hsp90, an unlikely ally in the war on cancer

On the surface heat shock protein 90 (Hsp90) is an unlikely drug target for the treatment of any disease, let alone cancer. Hsp90 is highly conserved and ubiquitously expressed in all cells. There are two major isoforms α and β encoded by distinct genes and together they may constitute 1%-3% of the cellular protein. Deletion of the protein is embryonic lethal and there are no recognized polymorphisms suggesting an association or causal relationship with any human disease. With respect to cancer, the proteins absence from two recent high profile articles, 'Hallmarks of cancer: the next generation' [Hanahan & Weinberg (2011) Cell 144, 646-674] and 'Comprehensive molecular portraits of human breast tumours' [Koboldt et al. (2012) Nature] underlines the perception that it is an unlikely bona fide target to treat this disease. Yet, to date, there are 17 distinct Hsp90 inhibitors in clinical trials for multiple indications in cancer. The protein has been championed for over 20 years by the National Cancer Institute (Bethesda, MD, USA) as a cancer target since the discovery of the antitumor activity of the natural product geldanamycin. This review aims to look at the conundrum of why Hsp90 can even be considered a druggable target for the treatment of cancer. We propose that in contrast to the majority of chemotherapeutics our growing armamentarium of investigational Hsp90 drugs represents an elegant choice that offers real hope in the long-term treatment of certain cancers.

Detection and repair of ionizing radiation-induced DNA double strand breaks: new developments in nonhomologous end joining

DNA damage can occur as a result of endogenous metabolic reactions and replication stress or from exogenous sources such as radiation therapy and chemotherapy. DNA double strand breaks are the most cytotoxic form of DNA damage, and defects in their repair can result in genome instability, a hallmark of cancer. The major pathway for the repair of ionizing radiation-induced DSBs in human cells is nonhomologous end joining. Here we review recent advances on the mechanism of nonhomologous end joining, as well as new findings on its component proteins and regulation.

Heat shock protein 90α (HSP90α), a substrate and chaperone of DNA-PK necessary for the apoptotic response

The "apoptotic ring" is characterized by the phosphorylation of histone H2AX at serine 139 (γ-H2AX) by DNA-dependent protein kinase (DNA-PK). The γ-H2AX apoptotic ring differs from the nuclear foci patterns observed in response to DNA-damaging agents. It contains phosphorylated DNA damage response proteins including activated Chk2, activated ATM, and activated DNA-PK itself but lacks MDC1 and 53BP1, which are required to initiate DNA repair. Because DNA-PK can phosphorylate heat shock protein 90α (HSP90α) in biochemical assays, we investigated whether HSP90α is involved in the apoptotic ring. Here we show that HSP90α is phosphorylated by DNA-PK on threonines 5 and 7 early during apoptosis and that both phosphorylated HSP90α and DNA-PK colocalize in the apoptotic ring. We also show that DNA-PK is a client of HSP90α and that HSP90α is required for full DNA-PK activation, γ-H2AX formation, DNA fragmentation, and apoptotic body formation. In contrast, HSP90 inhibition by geldanamycin markedly enhances TRAIL-induced DNA-PK and H2AX activation. Together, our results reveal that HSP90α is a substrate and chaperone of DNA-PK in the apoptotic response. The response of phosphorylated HSP90α to TRAIL and its localization to the γ-H2AX ring represent epigenetic features of apoptosis that offer insights for studying and monitoring nuclear apoptosis.