Geldanamycin-induced degradation of Chk1 is mediated by proteasome

The Role of the Ubiquitin Proteasome System in Glioma: Analysis Emphasizing the Main Molecular Players and Therapeutic Strategies Identified in Glioblastoma Multiforme

Gliomas constitute the most frequent tumors of the brain. High-grade gliomas are characterized by a poor prognosis caused by a set of attributes making treatment difficult, such as heterogeneity and cell infiltration. Additionally, there is a subgroup of glioma cells with properties similar to those of stem cells responsible for tumor recurrence after treatment. Since proteasomal degradation regulates multiple cellular processes, any mutation causing disturbances in the function or expression of its elements can lead to various disorders such as cancer. Several studies have focused on protein degradation modulation as a mechanism of glioma control. The ubiquitin proteasome system is the main mechanism of cellular proteolysis that regulates different events, intervening in pathological processes with exacerbating or suppressive effects on diseases. This review analyzes the role of proteasomal degradation in gliomas, emphasizing the elements of this system that modulate different cellular mechanisms in tumors and discussing the potential of distinct compounds controlling brain tumorigenesis through the proteasomal pathway.

Heat shock proteins and DNA repair mechanisms: an updated overview

Heat shock proteins (HSPs), also known as molecular chaperones, participate in important cellular processes, such as protein aggregation, disaggregation, folding, and unfolding. HSPs have cytoprotective functions that are commonly explained by their antiapoptotic role. Their involvement in anticancer drug resistance has been the focus of intense research efforts, and the relationship between HSP induction and DNA repair mechanisms has been in the spotlight during the past decades. Because DNA is permanently subject to damage, many DNA repair pathways are involved in the recognition and removal of a diverse array of DNA lesions. Hence, DNA repair mechanisms are key to maintain genome stability. In addition, the interactome network of HSPs with DNA repair proteins has become an exciting research field and so their use as emerging targets for cancer therapy. This article provides a historical overview of the participation of HSPs in DNA repair mechanisms as part of their molecular chaperone capabilities.

Understanding the Physical and Molecular Basis of Stability of Arabidopsis DNA Pol λ under UV-B and High NaCl Stress

Here, we have investigated the physical and molecular basis of stability of Arabidopsis DNA Pol λ, the sole X family DNA polymerase member in plant genome, under UV-B and salinity stress in connection with the function of the N-terminal BRCT (breast cancer-associated C terminus) domain and Ser-Pro rich region in the regulation of the overall structure of this protein. Tryptophan fluorescence studies, fluorescence quenching and Bis-ANS binding experiments using purified recombinant full length Pol λ and its N-terminal deletion forms have revealed UV-B induced conformational change in BRCT domain deficient Pol λ. On the other hand, the highly conserved C-terminal catalytic core PolX domain maintained its tertiary folds under similar condition. Circular dichroism (CD) and fourier transform infrared (FT-IR) spectral studies have indicated appreciable change in the secondary structural elements in UV-B exposed BRCT domain deficient Pol λ. Increased thermodynamic stability of the C-terminal catalytic core domain suggested destabilizing effect of the N-terminal Ser-Pro rich region on the protein structure. Urea-induced equilibrium unfolding studies have revealed increased stability of Pol λ and its N-terminal deletion mutants at high NaCl concentration. In vivo aggregation studies using transient expression systems in Arabidopsis and tobacco indicated possible aggregation of Pol λ lacking the BRCT domain. Immunoprecipitation assays revealed interaction of Pol λ with the eukaryotic molecular chaperone HSP90, suggesting the possibility of regulation of Pol λ stability by HSP90 in plant cell. Overall, our results have provided one of the first comprehensive information on the biophysical characteristics of Pol λ and indicated the importance of both BRCT and Ser-Pro rich modules in regulating the stability of this protein under genotoxic stress in plants.

Canonical and kinase activity-independent mechanisms for extracellular signal-regulated kinase 5 (ERK5) nuclear translocation require dissociation of Hsp90 from the ERK5-Cdc37 complex

The mitogen-activated protein (MAP) kinase extracellular signal-regulated kinase 5 (ERK5) plays a crucial role in cell proliferation, regulating gene transcription. ERK5 has a unique C-terminal tail which contains a transcriptional activation domain, and activates transcription by phosphorylating transcription factors and acting itself as a transcriptional coactivator. However, the molecular mechanisms that regulate its nucleocytoplasmatic traffic are unknown. We have used tandem affinity purification to identify proteins that interact with ERK5. We show that ERK5 interacts with the Hsp90-Cdc37 chaperone in resting cells, and that inhibition of Hsp90 or Cdc37 results in ERK5 ubiquitylation and proteasomal degradation. Interestingly, activation of cellular ERK5 induces Hsp90 dissociation from the ERK5-Cdc37 complex, leading to ERK5 nuclear translocation and activation of transcription, by a mechanism which requires the autophosphorylation at its C-terminal tail. Consequently, active ERK5 is no longer sensitive to Hsp90 or Cdc37 inhibitors. Cdc37 overexpression also induces Hsp90 dissociation and the nuclear translocation of a kinase-inactive form of ERK5 which retains transcriptional activity. This is the first example showing that ERK5 transcriptional activity does not require kinase activity. Since Cdc37 cooperates with ERK5 to promote cell proliferation, Cdc37 overexpression (as happens in some cancers) might represent a new, noncanonical mechanism by which ERK5 regulates tumor proliferation.

The ubiquitin-proteasome system in glioma cell cycle control

A major determinant of cell fate is regulation of cell cycle. Tight regulation of this process is lost during the course of development and progression of various tumors. The ubiquitin-proteasome system (UPS) constitutes a universal protein degradation pathway, essential for the consistent recycling of a plethora of proteins with distinct structural and functional roles within the cell, including cell cycle regulation. High grade tumors, such as glioblastomas have an inherent potential of escaping cell cycle control mechanisms and are often refractory to conventional treatment. Here, we review the association of UPS with several UPS-targeted proteins and pathways involved in regulation of the cell cycle in malignant gliomas, and discuss the potential role of UPS inhibitors in reinstitution of cell cycle control.

Cooperative enhancement of radiosensitivity after combined treatment of 17-(allylamino)-17-demethoxygeldanamycin and celecoxib in human lung and colon cancer cell lines

We investigated whether the combined treatment of 17-(allylamino)-17-demethoxygeldanamycin (17-AAG), an inhibitor of heat-shock protein 90 (hsp90), and celecoxib, an inhibitor of cyclooxygenase-2, can cooperatively enhance the radiosensitivity of various human cancer cells. Combined treatment with 17-AAG and celecoxib, at clinically relevant concentrations, cooperatively induced radiosensitization in all tested cancer cells, but not in normal cells. Cooperative radiosensitization by the drug combination was also shown in a human tumor xenograft system. We found that ataxia-telangiectasia and rad3-related (ATR) and ataxia-telangiectasia mutated (ATM) are novel client proteins of hsp90. Combined treatment with 17-AAG and celecoxib cooperatively induced downregulation of ATR and ATM. In conclusion, combined treatment with 17-AAG and celecoxib at clinically relevant concentrations may significantly enhance the therapeutic efficacy of ionizing radiation.

Stabilization of phosphatidylinositol 4-kinase type IIbeta by interaction with Hsp90

Mammalian cells express two isoforms of type II phosphatidylinositol 4-kinase: PI4KIIα and PI4KIIβ. PI4KIIα exists almost exclusively as a constitutively active integral membrane protein because of its palmitoylation (Barylko, B., Gerber, S. H., Binns, D. D., Grichine, N., Khvotchev, M., Südhof, T. C., and Albanesi, J. P. (2001) J. Biol. Chem. 276, 7705-7708). In contrast, PI4KIIβ is distributed almost evenly between membranes and cytosol. Whereas the palmitoylated membrane-bound pool is catalytically active, the cytosolic kinase is inactive (Wei, Y. J., Sun, H. Q., Yamamoto, M., Wlodarski, P., Kunii, K., Martinez, M., Barylko, B., Albanesi, J. P., and Yin, H. L. (2002) J. Biol. Chem. 277, 46586-46593; Jung, G., Wang, J., Wlodarski, P., Barylko, B., Binns, D. D., Shu, H., Yin, H. L., and Albanesi, J. P. (2008) Biochem. J. 409, 501-509). In this study, we identify the molecular chaperone Hsp90 as a binding partner of PI4KIIβ, but not of PI4KIIα. Geldanamycin (GA), a specific Hsp90 inhibitor, disrupts the Hsp90-PI4KIIβ interaction and destabilizes PI4KIIβ, reducing its half-life by 40% and increasing its susceptibility to ubiquitylation and proteasomal degradation. Cytosolic PI4KIIβ is much more sensitive to GA treatment than is the integrally membrane-associated species. Exposure to GA induces a partial redistribution of PI4KIIβ from the cytosol to membranes and, with brief GA treatments, a corresponding increase in cellular phosphatidylinositol 4-kinase activity. Stimuli such as PDGF receptor activation that also induce recruitment of the kinase to membranes disrupt the Hsp90-PI4KIIβ interaction to a similar extent as GA treatment. These results support a model wherein Hsp90 interacts predominantly with the cytosolic, inactive pool of PI4KIIβ, shielding it from proteolytic degradation but also sequestering it to the cytosol until an extracellular stimulus triggers its translocation to the Golgi or plasma membrane and subsequent activation.

Mechanistic evaluation of the novel HSP90 inhibitor NVP-AUY922 in adult and pediatric glioblastoma

The dismal prognosis of glioblastoma (GB) indicates the urgent need for new therapies for these tumors. Heat shock protein 90 (HSP90) inhibitors induce the proteasome-mediated degradation of many oncogenic client proteins involved in all of the hallmark characteristics of cancer. Here, we explored the mechanistic potential of the potent synthetic diarylisoxazole amide resorcinol HSP90 inhibitor, NVP-AUY922, in adult and pediatric GB. In vitro antiproliferative potency (nanomolar range) was seen in both adult and pediatric human GB cell lines with different molecular pathologies. A cytostatic effect was observed in all GB lines; more apoptosis was observed at lower concentrations in the SF188 pediatric GB line and at 144 hours in the slower growing KNS42 pediatric GB line, as compared with the adult GB lines U87MG and SF268. In vitro combination studies with inhibitors of phosphoinositide 3-kinase/mammalian target of rapamycin (PI-103) or mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase (PD-0325901) supported the hypothesis that sustained inhibition of ERK up to 72 hours and at least temporary inhibition of AKT were necessary to induce apoptosis in GB lines. In athymic mice bearing established s.c U87MG GB xenografts, NVP-AUY922 (50 mg/kg i.p x 3 days) caused the inhibition of ERK1/2 and AKT phosphorylation and induced apoptosis, whereas 17-AAG used at maximum tolerated dose was less effective. NVP-AUY922 antitumor activity with objective tumor regression resulted from antiproliferative, proapoptotic, and antiangiogenic effects, the latter shown by decreased microvessel density and HIF1alpha levels. Our results have established a mechanistic proof of concept for the potential of novel synthetic HSP90 inhibitors in adult and pediatric GB, alone or in combination with phosphoinositide 3-kinase/mammalian target of rapamycin and mitogen-activated protein/ERK kinase inhibitors.

DCB-3503, a tylophorine analog, inhibits protein synthesis through a novel mechanism

Background

DCB-3503, a tylophorine analog, inhibits the growth of PANC-1 (human pancreatic ductal cancer cell line) and HepG2 (human hepatocellular cancer cell line) tumor xenografts in nude mice. The inhibition of growth leads to cancer cell differentiation instead of cell death. However, the mechanisms of action of tylophorine analogs is unknown.

Methodology/Principal Findings

In this study, we show that DCB-3503 suppresses the expression of pro-oncogenic or pro-survival proteins with short half-lives, including cyclin D1, survivin, β-catenin, p53, and p21, without decreasing their mRNA levels. Proteasome inhibitor reversed the inhibitory effect of DCB-3503 on expression of these proteins. DCB-3503 inhibited the incorporation of radiolabeled amino acid and thymidine, and to a much lesser degree of uridine, in a panel of cell lines. The mechanism of inhibition of protein synthesis is different from that of cycloheximide (CHX) as assayed in cell culture and HeLa in vitro translation system. Furthermore, in contrast to rapamycin, DCB-3503 does not affect protein synthesis through the mTOR pathway. DCB-3503 treatment shifts the sedimentation profiles of ribosomes and mRNAs towards the polysomal fractions while diminishing monosome abundance, indicative of the inhibition of the elongation step of protein synthesis. Preferential down regulation of several studied proteins under these conditions is likely due to the relative short half-lives of these proteins.

Conclusion/Significance

The inhibitory effect of DCB-3503 on translation is apparently distinct from any of the current anticancer compounds targeting protein synthesis. Translation inhibitors with novel mechanism could complement current chemotherapeutic agents for the treatment of human cancers and suppress the occurrence of drug resistance.

ITZ-1, a client-selective Hsp90 inhibitor, efficiently induces heat shock factor 1 activation

ITZ-1 is a chondroprotective agent that inhibits interleukin-1beta-induced matrix metalloproteinase-13 (MMP-13) production and suppresses nitric oxide-induced chondrocyte death. Here we describe its mechanisms of action. Heat shock protein 90 (Hsp90) was identified as a specific ITZ-1-binding protein. Almost all known Hsp90 inhibitors have been reported to bind to the Hsp90 N-terminal ATP-binding site and to simultaneously induce degradation and activation of its multiple client proteins. However, within the Hsp90 client proteins, ITZ-1 strongly induces heat shock factor-1 (HSF1) activation and causes mild Raf-1 degradation, but scarcely induces degradation of a broad range of Hsp90 client proteins by binding to the Hsp90 C terminus. These results may explain ITZ-1's inhibition of MMP-13 production, its cytoprotective effect, and its lower cytotoxicity. These results suggest that ITZ-1 is a client-selective Hsp90 inhibitor.

Roles of human AND-1 in chromosome transactions in S phase

Coordinated execution of DNA replication, checkpoint activation, and postreplicative chromatid cohesion is intimately related to the replication fork machinery. Human AND-1/chromosome transmission fidelity 4 is localized adjacent to replication foci and is required for efficient DNA synthesis. In S phase, AND-1 is phosphorylated in response to replication arrest in a manner dependent on checkpoint kinase, ataxia telangiectasia-mutated, ataxia telangiectasia-mutated and Rad3-related protein, and Cdc7 kinase but not on Chk1. Depletion of AND-1 increases DNA damage, delays progression of S phase, leads to accumulation of late S and/or G2 phase cells, and induces cell death in cancer cells. It also elevated UV-radioresistant DNA synthesis and caused premature recovery of replication after hydroxyurea arrest, indicating that lack of AND-1 compromises checkpoint activation. This may be partly due to the decreased levels of Chk1 protein in AND-1-depleted cells. Furthermore, AND-1 interacts with cohesin proteins Smc1, Smc3, and Rad21/Scc1, consistent with proposed roles of yeast counterparts of AND-1 in sister chromatid cohesion. Depletion of AND-1 leads to significant inhibition of homologous recombination repair of an I-SceI-driven double strand break. Based on these data, we propose that AND-1 coordinates multiple cellular events in S phase and G2 phase, such as DNA replication, checkpoint activation, sister chromatid cohesion, and DNA damage repair, thus playing a pivotal role in maintenance of genome integrity.

DDB1 targets Chk1 to the Cul4 E3 ligase complex in normal cycling cells and in cells experiencing replication stress

The Chk1 protein kinase preserves genome integrity in normal proliferating cells and in cells experiencing replicative and genotoxic stress. Chk1 is currently being targeted in anticancer regimens. Here, we identify damaged DNA-binding protein 1 (DDB1) as a novel Chk1-interacting protein. DDB1 is part of an E3 ligase complex that includes the cullin proteins Cul4A and Cul4B. We report that Cul4A/DDB1 negatively regulates Chk1 stability in vivo. Chk1 associates with Cul4A/DDB1 during an unperturbed cell division cycle and both Chk1 phosphorylation and replication stress enhanced these interactions. Cul4A/DDB1 regulates Chk1 ubiquitination in vivo and Chk1 is directly ubiquitinated in vitro in a Cul4A/DDB1-dependent manner. Furthermore, Chk1 is stabilized in cells deficient for Cul4A/DDB1. This study shows that Chk1 abundance is regulated by the Cul4A/DDB1 ubiquitin ligase during an unperturbed cell division cycle, in response to replicative stress and on heat shock protein 90 inhibition, and that deregulation of the Chk1/Cul4A/DDB1 pathway perturbs the ionizing radiation-induced G(2) checkpoint.

Indirect participation of Hsp90 in the regulation of the cyclin E turnover

Cyclin E is the Cdk2-regulatory subunit required for the initiation of DNA replication at the G1/S transition. It accumulates in late G1 phase and gets rapidly degraded by the ubiquitin/proteasome pathway during S phase. The degradation of cyclin E is a consequence of its phosphorylation and subsequent isomerization by the peptidyl-prolyl isomerase Pin1. We show that in the colon cancer cells HT-29 the inhibition of the chaperone function of Hsp90 by geldanamycin (GA) enhances the ubiquitinylation of cyclin E and triggers active degradation via the proteasome pathway. As Hsp90 forms multiprotein complexes with and regulates the function and cell contents of numerous signaling proteins, this observation suggests a direct interaction between Hsp90 and cyclin E. However, experiments using cell lysate fractionation did not reveal the presence of complexes containing both Hsp90 and cyclin E. Coupled transcription/translation experiments also failed to detect the formation of complexes between newly synthesized cyclin E and Hsp90. We conclude that Hsp90 can regulate the degradation of cellular proteins without binding to them, by an indirect mechanism. This conclusion postulates a new category of proteins that are affected by the inactivation of Hsp90. Our observations do not support the possible involvement of a PPIase in this indirect mechanism. Besides, we did not observe active geldanamycin-dependent degradation of cyclin E in the prostate cancer-derived cell line DU-145, indicating that the Hsp90-dependent stabilization of cyclin E requires specific regulatory mechanism which may be lost in certain types of cancer cells.

Phorbol 12-myristate 13-acetate (PMA)-induced migration of glioblastoma cells is mediated via p38MAPK/Hsp27 pathway

We investigated the mechanism of phorbol 12-myristate 13-acetate (PMA)-induced migration of glioblastoma cells focusing on the p38 mitogen-activated protein kinase (MAPK)/heat shock protein 27 (Hsp27) pathway. PMA-induced cell migration and activation of p38MAPK in A172 glioblastoma cells. PMA-induced formation of lamellipodia and focal complexes was blocked by inhibiting p38MAPK with SB203580 or small interfering RNA (siRNA). Furthermore, activation of p38MAPK resulted in phosphorylation of an F-actin polymerization regulator, Hsp27. Immunohistochemical analysis showed that upon PMA stimulation, both unphosphorylated and phosphorylated Hsp27 were translocated to the lamellipodia. SB203580 or p38MAPK siRNA blocked these phenomena, indicating that Hsp27 phosphorylation and translocation from cytosol to membrane were mediated by p38MAPK. To address the question of whether endogenous Hsp27 participates in PMA-induced migration, we inhibited the expression of Hsp27 using Hsp27 siRNA. Although knockdown of Hsp27 by siRNA had little effect on p38MAPK activation, lamellipodia and focal complex formation was markedly inhibited. Migration was also abolished in Hsp27 siRNA-transfected cells. In conclusion, p38MAPK activation followed by Hsp27 phosphorylation was required for PMA-induced migration. Furthermore, Hsp27 itself played critical roles in PMA-induced migration. Our data provide substantial evidence for a model elucidating the molecular mechanisms of regulation of actin dynamics and migration by PMA-activated protein kinase C in glioblastoma cells.

Geldanamycin induces G2 arrest in U87MG glioblastoma cells through downregulation of Cdc2 and cyclin B1

Cell cycle progression requires precise expression and activation of several cyclins and cyclin-dependent kinases. Geldanamycin (GA) affects cell cycle progression in various kinds of cells. We analyzed GA-induced cell cycle regulation in glioblastoma cells. GA-induced G2 or M arrest in glioblastoma cells in a cell line-dependent manner. GA decreased the expression of Cdc2 and cyclin B1 in U87MG cells. And phosphorylated Cdc2 decreased along with Cdc2 in the GA-treated cells. This cell line showed G2 arrest after GA treatment. In contrast, GA failed to down-regulate these cell cycle regulators in U251MG cells. In U251MG cells, the cell cycle was arrested at M phase in addition to G2 by GA. Next, we analyzed the mechanism of the GA-induced regulation of Cdc2 and cyclin B1 in U87MG cells. Cdc2 and cyclin B1 were ubiquitinated by GA. MG132 abrogated the GA-induced decrease of Cdc2 and cyclin B1 indicating that these proteins were degraded by proteasomes. In conclusion, GA controls the stability of Cdc2 and cyclin B1 in glioblastomas cell species-dependently. Cdc2 and cyclin B1 might be responsible for the different responses of glioblastoma cell lines to GA.

Differential expression of Hsp90 isoforms in geldanamycin-treated 9L cells

In mammals, two major Hsp90 isoforms (Hsp90alpha and Hsp90beta) have been identified and found to be highly conserved among different species. However, the expression control of Hsp90 isoforms at both transcriptional and translational levels is largely unknown. Herein, we quantitatively investigate the changes in the total mRNA and inductive protein levels of Hsp90alpha and Hsp90beta in rat gliosarcoma cells treated with geldanamycin (GA). The stability of mRNA and protein was estimated. The translational efficiency of Hsp90 isoforms was measured employing in vitro translation techniques. It was found that Hsp90alpha was more inducible than Hsp90beta after GA treatment, whereas the hsp90alpha mRNA level was lower than that of hsp90beta. In addition, higher translational efficiency of hsp90alpha mRNA was observed, suggesting that translational control played an important role. Taken together, our results indicate that differential expression between Hsp90alpha and Hsp90beta is a consequence of both distinct mRNA profiles and differential translation processes.

Repair of topoisomerase I-mediated DNA damage

Topoisomerase I (Top1) is an abundant and essential enzyme. Top1 is the selective target of camptothecins, which are effective anticancer agents. Top1-DNA cleavage complexes can also be trapped by various endogenous and exogenous DNA lesions including mismatches, abasic sites and carcinogenic adducts. Tyrosyl-DNA phosphodiesterase (Tdp1) is one of the repair enzymes for Top1-DNA covalent complexes. Tdp1 forms a multiprotein complex that includes poly(ADP) ribose polymerase (PARP). PARP-deficient cells are hypersensitive to camptothecins and functionally deficient for Tdp1. We will review recent developments in several pathways involved in the repair of Top1 cleavage complexes and the role of Chk1 and Chk2 checkpoint kinases in the cellular responses to Top1 inhibitors. The genes conferring camptothecin hypersensitivity are compiled for humans, budding yeast and fission yeast.