Novel Hsp90 inhibitor platycodin D disrupts Hsp90/Cdc37 complex and enhances the anticancer effect of mTOR inhibitor.

Heat shock protein 90 (Hsp90) is a critically conserved molecular chaperone protein and promising therapeutic target for cancer treatment. In this study, platycodin D (PD), a saponin isolated from traditional Chinese herb Platycodonis Radix, was identified as a novel Hsp90 inhibitor. We verified that PD did not affect the ATPase activity of Hsp90. However, PD disrupted the co-chaperone interaction of Hsp90/cell division cycle protein 37 (Cdc37) and subsequently degraded multiple Hsp90 client proteins without the feedback increase of Hsp70. In different genotypes of non-small cell lung cancer cells, co-treatment with the mTOR inhibitor Everolimus and PD enhanced antiproliferation activity and apoptotic effect. The feedback survival signal upon mTOR inhibition was fully terminated by the co-administration with PD through reduced epidermal growth factor receptor (EGFR) and insulin growth factor 1 receptor (IGF1R) expression, suppressed AKT activity, and reinforced 4E-BP1 inhibition. Our results not only identified PD as a novel Hsp90 inhibitor by disrupting the protein-protein interaction of Hsp90/Cdc37 complex, but also provided mechanistic insights into the ineffectiveness of mTOR inhibitors and identified therapeutic strategy for cancer treatment.

The Hsp90 complex--a super-chaperone machine as a novel drug target.

Cells respond to sudden changes in the environmental temperature with increased synthesis of a distinct number of heat shock proteins (Hsps). Analysis of the function of these proteins in recent years has shown that all the major classes of conserved Hsps are molecular chaperones involved in assisting cellular protein folding and preventing irreversible side-reactions, such as unspecific aggregation. In addition to their function under stress conditions, molecular chaperones also play a critical role under physiological conditions. Hsp90 is one of the most abundant chaperones in the cytosol of eukaryotic cells. It is part of the cell's powerful network of chaperones to fight the deleterious consequences of protein unfolding caused by nonphysiological conditions. In the absence of stress, however, Hsp90 is an obligate component of fundamental cellular processes such as hormone signaling and cell cycle control. In this context, several key regulatory proteins, such as steroid receptors, cell cycle kinases, and p53, have been identified as substrates of Hsp90. Recently, Hsp90 was shown to be the unique target for geldanamycin, a potent new anti-tumor drug that blocks cell proliferation. Interestingly, under physiological conditions, Hsp90 seems to perform its chaperone function in a complex with a set of partner proteins, suggesting that the Hsp90 complex is a multi-chaperone machine specialized in guiding the maturation of conformationally labile proteins. The regulation of key signaling molecules of the cell by the Hsp90 machinery is a stimulating new concept emerging from these studies, and Hsp90 has become a promising new drug target.

Targeting the oncogene and kinome chaperone CDC37.

CDC37 is a molecular chaperone that physically stabilizes the catalytic domains found in protein kinases and is therefore a wide-spectrum regulator of protein phosphorylation. It is also an overexpressed oncoprotein that mediates carcinogenesis by stabilizing the compromised structures of mutant and/or overexpressed oncogenic kinases. Recent work shows that such dependency of malignant cells on increased CDC37 expression is a vulnerability that can be targeted in cancer by agents that deplete or inhibit CDC37. CDC37 is thus a candidate for broad-spectrum molecular cancer therapy.

Caveolin-1 interacts with the chaperone complex TCP-1 and modulates its protein folding activity.

We report that caveolin-1, one of the major structural protein of caveolae, interacts with TCP-1, a hetero-oligomeric chaperone complex present in all eukaryotic cells that contributes mainly to the folding of actin and tubulin. The caveolin-TCP-1 interaction entails the first 32 amino acids of the N-terminal segment of caveolin. Our data show that caveolin-1 expression is needed for the induction of TCP-1 actin folding function in response to insulin stimulation. Caveolin-1 phosphorylation at tyrosine residue 14 induces the dissociation of caveolin-1 from TCP-1 and activates actin folding. We show that the mechanism by which caveolin-1 modulates TCP-1 activity is indirect and involves the cytoskeleton linker filamin. Filamin is known to bind caveolin-1 and to function as a negative regulator of insulin-mediated signaling. Our data support the notion that the caveolin-filamin interaction contributes to restore insulin-mediated phosphorylation of caveolin, thus allowing the release of active TCP-1.

Review: allostery in chaperonins.

Chaperonins mediate protein folding in an ATP-dependent manner. ATP binding and hydrolysis by chaperonins are subject to both homotropic and heterotropic allosteric regulation. In the case of GroEL and CCT, homotropic regulation by ATP is manifested in nested cooperativity, which involves positive intra-ring cooperativity and negative inter-ring cooperativity in ATP binding. Both types of cooperativity are modulated by various heterotropic allosteric effectors, which include nonfolded proteins, ADP, Mg2+, monovalent ions such as K+, and cochaperonins in the case of type I chaperonins such as GroEL. Here, the allosteric properties of chaperonins are reviewed and new results of ours are presented with regard to allosteric effects of ADP. The role of allostery in the reaction cycle and folding function of chaperonins is discussed.

Small heat shock protein Hsp16.3 modulates its chaperone activity by adjusting the rate of oligomeric dissociation.

Small heat shock proteins usually exist as oligomers and appear to undergo dynamic dissociation/reassociation, with oligomeric dissociation being a prerequisite for their chaperone activities. However, contradictory cases were also reported that chaperone activities could be enhanced with no change or even increase in oligomeric sizes. Using Hsp16.3 as a model system, our studies show the following: (1) Although a preheat (over 60 degrees C) treatment or the presence of low concentrations of urea (around 0.8M) hardly caused any change in the oligomeric size of Hsp16.3 proteins when examined by size exclusion chromatography, its chaperone activities were increased significantly. (2) Further analysis using the unique pore-gradient polyacrylamide gel electrophoresis revealed a dramatic increase in the tendency of oligomeric dissociation for both the preheated and urea-containing Hsp16.3. (3) Meanwhile, for both cases, an apparent increase in the rate constants of oligomeric dissociation was also observed, as determined by utilizing conjugated fluorescence probes whose quantum yield increases accompanying oligomeric dissociation. (4) Moreover, the fluorescence anisotropy analysis also demonstrated that the oligomeric structures for the preheated or urea-containing Hsp16.3 proteins seem to be more dynamic and variable. In light of these observations, we propose that the small heat shock proteins like Hsp16.3 can modulate their chaperone activities by adjusting the rate of oligomeric dissociation in responding to environmental changes. Results obtained here also suggest that small heat shock proteins might be able to "remember" their stress experiences via certain structural alterations which will allow them to act as better chaperones when the stress conditions reappear.

Review: nucleotide-dependent conformational changes of the chaperonin containing TCP-1.

Current biochemical and structural studies on the conformational changes induced by the nature of nucleotide bound to the chaperonin containing testis complex polypeptide 1 (CCT) are examined to see how consistent the data are. This exercise suggests that the biochemical and structural data are in good agreement. CCT clearly appears as a folding nano-machine fueled by ATP. A careful comparison of the biochemical and structural data, however, highlights a number of points that remain to be carefully documented in order to better understand the nature of the conformational changes in CCT that yield folded target proteins. Special effort should be made to clearly answer the points listed at the end of this review in order to obtain the dynamic sequence of events yielding folded proteins in the eukaryotic cytoplasm similar to what has been obtained for prokaryotes.

Chaperonin filaments: the archaeal cytoskeleton?

Chaperonins are high molecular mass double-ring structures composed of 60-kDa protein subunits. In the hyperthermophilic archaeon Sulfolobus shibatae the two chaperonin proteins represent approximately 4% of its total protein and have a combined intracellular concentration of >30 mg/ml. At concentrations >/= 0.5 mg/ml purified chaperonins form filaments in the presence of Mg2+ and nucleotides. Filament formation requires nucleotide binding (not hydrolysis), and occurs at physiological temperatures in biologically relevant buffers, including a buffer made from cell extracts. These observations suggest that chaperonin filaments may exist in vivo and the estimated 4600 chaperonins per cell suggest that such filaments could form an extensive cytostructure. We observed filamentous structures in unfixed, uranyl-acetate-stained S. shibatae cells, which resemble the chaperonin filaments in size and appearance. ImmunoGold (Janssen) labeling using chaperonin antibodies indicated that many chaperonins are associated with insoluble cellular structures and these structures appear to be filamentous in some areas, although they could not be uranyl-acetate-stained. The existence of chaperonin filaments in vivo suggests a mechanism whereby their protein-folding activities can be regulated. More generally, the filaments themselves may play a cytoskeletal role in Archaea.

Alterations of chaperone protein expression in presenilin mutant neurons in response to glutamate excitotoxicity.

Mutations in the presenilin-1 (PS1) gene underlie the most common form of familial dementia of the Alzheimer type (DAT). We demonstrated previously that the expression of PS1 with a M146V mutation in transgenic mice potentiates glutamate toxicity to neurons, due to an altered calcium homeostasis. Here, using a subtractive cDNA library approach, we report the identification of several genes, the altered expression of which may be associated with this unique PS1-related vulnerability to glutamate. The identified genes, including chaperonin subunit 2 and nucleophosmin 1/B23, are involved in the intracellular trafficking of proteins and ions. Northern blot analysis revealed that the effect of glutamate on calcium-binding proteins was augmented in neurons from PS1 mutation mice, compared with neurons from mice lacking other genes relevant to the pathogenesis of DAT (FE65 and APOE) or neurons from control wild-type mice. Interestingly, mRNA for two chaperone proteins were expressed at lower levels specifically in neurons from PS1 mutant mice. These findings suggest that PS1 mutations may, in part, contribute to the development of DAT via altered expression of chaperone proteins.

Mechanisms of HSP65 expression induced by gamma delta T cells in murine Toxoplasma gondii infection.

We have previously reported that the expression of an endogenous 65-kD heat shock protein (HSP65) in macrophages is closely correlated with the protection against infection by Toxoplasma gondii in mice, and gamma delta T cells play a critical role in the expression of this protein. In this study, we investigated how gamma delta T cells contribute to the protection and HSP65 expression. After intraperitoneal infection with bradyzoites of the Beverley strain of T. gondii, mRNA encoding IFN-gamma and TNF-alpha was detected in the peritoneal gamma delta T cells by RT-PCR technique, and macrophages that produced nitric oxide (NO) and expressed HSP65 were also detected. Depletion of gamma delta T cells resulted in suppression of NO production by macrophages, and it also inhibited HSP65 expression. HSP65 expression, however, does not appear to be induced by stimulation with NO, since treatment with NG-monomethylarginine, an inhibitor of NO synthesis, did not attenuate the expression of HSP65. This expression was completely suppressed when mice were simultaneously treated with anti-IFN-gamma and anti-TNF-alpha although either antibody alone was less effective. The synergistic effect of these cytokines was also demonstrated by an in vitro experiment, in which peritoneal macrophages were cultured with recombinant IFN-gamma and TNF-alpha. These results indicate that gamma delta T cells, which protect against infection with T. gondii induce the expression of HSP65 by secreting IFN-gamma and TNF-alpha and the production of NO, and that the expression of HSP65 is independent of inflammatory chemical compounds like NO and H2O2.

The thermosome from Methanopyrus kandleri possesses an NH4+-dependent ATPase activity.

The ATPase activity of the thermosome from a methanogen, Methanopyrus kandleri, was characterized in detail. In contrast to all other known chaperonins, enzymatic ATP hydrolysis was found to be strictly dependent on high levels of ammonium salts in vitro. The ths gene encoding the thermosome subunit from the hyperthermophilic M. kandleri was functionally expressed in Escherichia coli and the overproduced polypeptide was assembled into intact thermosome complexes in the mesophilic host. The recombinant particles could be purified by a simple two-step procedure including only one chromatographic step. Structural and biochemical properties of the recombinant protein were closely similar to those of the natural complex. Western blot analysis with an antiserum against the M. kandleri thermosome indicated the presence of at least two subfamilies of archaeal chaperonins.