Inhibition of MDM2 by hsp90 contributes to mutant p53 stabilization

CHIP deletion reveals functional redundancy of E3 ligases in promoting degradation of both signaling proteins and expanded glutamine proteins

CHIP (carboxy terminus of Hsc70-interacting protein) an E3 ubiquitin ligase that binds to Hsp70 and Hsp90, promotes degradation of several Hsp90-regulated signaling proteins and disease-causing proteins containing expanded glutamine tracts. In polyglutamine disease models, CHIP has been considered a primary protection factor by promoting degradation of these misfolded proteins. Here, we show that two CHIP substrates, the glucocorticoid receptor (GR), a classic Hsp90-regulated signaling protein, and the expanded glutamine androgen receptor (AR112Q), are degraded at the same rate in CHIP(-/-) and CHIP(+/+) mouse embryonic fibroblasts after treatment with the Hsp90 inhibitor geldanamycin. CHIP(-/-) cytosol has the same ability as CHIP(+/+) cytosol to ubiquitinate purified neuronal nitric oxide synthase (nNOS), another established CHIP substrate. To determine whether other E3 ubiquitin ligases that bind to Hsp70 (Parkin) or Hsp90 (Mdm2) act on CHIP substrates, each E3 ligase was co-expressed with the GR, nNOS, AR112Q or Q78 ataxin-3. CHIP lowered the levels of all four proteins, Parkin acted on nNOS and Q78 ataxin-3 but not on the steroid receptors, and Mdm2 did not affect any of the co-expressed proteins. Moreover, both CHIP and Parkin co-localized to aggregates of the expanded glutamine AR formed in cell culture and in a knock-in mouse model of spinal and bulbar muscular atrophy. These observations establish that CHIP does not play an exclusive role in regulating the turnover of Hsp90 client signaling proteins or expanded glutamine tract proteins, and show that the Hsp70-dependent E3 ligase Parkin acts redundantly to CHIP on some substrates.

Heat shock proteins in animal neoplasms and human tumours--a comparison

Heat shock proteins (HSPs) are implicated in all phases of cancer from proliferation, impaired apoptosis and sustained angiogenesis to invasion and metastasis. The presence of abnormal HSP levels in several human tumours suggests that these proteins could be used as diagnostic and/or prognostic markers, whilst the direct correlation between HSP expression and drug resistance in neoplastic tissues means they could also be used to predict cancer response to specific treatment. HSPs have also been successfully targeted in clinical trials modifying their expression or chaperone activity. Preliminary studies in veterinary medicine have also demonstrated the presence of altered HSP expression in neoplasms, and the study of carcinogenesis and the role of HSPs in animal models will surely be an additional source of information for clinical cancer research.

ATP stimulates MDM2-mediated inhibition of the DNA-binding function of E2F1

Murine double minute 2 (MDM2) protein exhibits many diverse biochemical functions on the tumour suppressor protein p53, including transcriptional suppression and E3 ubiquitin ligase activity. However, more recent data have shown that MDM2 can exhibit ATP-dependent molecular chaperone activity and directly mediate folding of the p53 tetramer. Analysing the ATP-dependent function of MDM2 will provide novel insights into the evolution and function of the protein. We have established a system to analyse the molecular chaperone function of MDM2 on another of its target proteins, the transcription factor E2F1. In the absence of ATP, MDM2 was able to catalyse inhibition of the DNA-binding function of E2F1. However, the inhibition of E2F1 by MDM2 was stimulated by ATP, and mutation of the ATP-binding domain of MDM2 (K454A) prevented the ATP-stimulated inhibition of E2F1. Further, ATP stabilized the binding of E2F1 to MDM2 using conditions under which ATP destabilized the MDM2:p53 complex. However, the ATP-binding mutant of MDM2 was as active as an E3 ubiquitin ligase on E2F1 and p53, highlighting a specific function for the ATP-binding domain of MDM2 in altering substrate protein folding. Antibodies to three distinct domains of MDM2 neutralized its activity, showing that inhibition of E2F1 is MDM2-dependent and that multiple domains of MDM2 are involved in E2F1 inhibition. Dimethylsulfoxide, which reduces protein unfolding, also prevented E2F1 inhibition by MDM2. These data support a role for the ATP-binding domain in altering the protein-protein interaction function of MDM2.

Chaperone-dependent stabilization and degradation of p53 mutants

p53 missense mutant proteins commonly show increased stability compared to wild-type p53, which is thought to depend largely on the inability of mutant p53 to induce the ubiquitin ligase MDM2. However, recent work using mouse models has shown that the accumulation of mutant p53 occurs only in tumour cells, indicating that stabilization requires additional factors. To clarify the stabilization of p53 mutants in tumours, we analysed factors that affect their folding and degradation. Although all missense mutants that we studied are more stable than wild-type p53, the levels correlate with individual structural characteristics, which may be reflected in different gain-of-function properties. In the absence of Hsp90 activity, the less stable unfolded p53 mutants preferentially associate in a complex with Hsp70 and CHIP (carboxy terminus of Hsp70-interacting protein), and we show that CHIP is responsible for ubiquitination and degradation of these mutants. The demonstration of a complex interplay between Hsp90, Hsp70 and CHIP that regulate the stability of different p53 mutant proteins improves our understanding of the pro-tumorigenic effects of increased Hsp90 activity during multi-stage carcinogenesis. Understanding the roles of Hsp90, Hsp70 and CHIP in cancers may also provide an important avenue through which to target p53 to enhance treatment of human cancers.

Human cancer cells harbor T-stumps, a distinct class of extremely short telomeres

Using a modified single telomere length analysis protocol (STELA) to clone and examine the sequence composition of individual human XpYp telomeres, we discovered a distinct class of extremely short telomeres in human cancer cells with active telomerase. We name them "t-stumps," to distinguish them from the well-regulated longer bulk telomeres. T-stumps contained arrangements of telomeric repeat variants and a minimal run of seven canonical telomeric TTAGGG repeats, but all could bind at least one TRF1 or TRF2 in vitro. The abundance of t-stumps was unaffected by ATM alteration but could be changed by manipulating telomerase catalytic subunit (hTERT) levels in cancer cells. We propose that in the setting of active telomerase and compromised checkpoints characteristic of human cancer cells, t-stumps define the minimal telomeric unit that can still be protected by a TRF1/TRF2-capping complex and, further, that hTERT (or telomerase) may have a role in protecting t-stumps.

Ubiquitination and degradation of mutant p53

While wild-type p53 is normally a rapidly degraded protein, mutant forms of p53 are stabilized and accumulate to high levels in tumor cells. In this study, we show that mutant and wild-type p53 proteins are ubiquitinated and degraded through overlapping but distinct pathways. While Mdm2 can drive the degradation of both mutant and wild-type p53, our data suggest that the ability of Mdm2 to function as a ubiquitin ligase is less important in the degradation of mutant p53, which is heavily ubiquitinated in an Mdm2-independent manner. Our initial attempts to identify ubiquitin ligases that are responsible for the ubiquitination of mutant p53 have suggested a role for the chaperone-associated ubiquitin ligase CHIP (C terminus of Hsc70-interacting protein), although other unidentified ubiquitin ligases also appear to contribute. The contribution of Mdm2 to the degradation of mutant p53 may reflect the ability of Mdm2 to deliver the ubiquitinated mutant p53 to the proteasome.

MDM2 chaperones the p53 tumor suppressor

The murine double minute (mdm2) gene encodes an E3 ubiquitin ligase that plays a key role in the degradation of p53 tumor suppressor protein. Nevertheless recent data highlight other p53-independent functions of MDM2. Given that MDM2 protein binds ATP, can interact with the Hsp90 chaperone, plays a role in the modulation of transcription factors and protection and activation of DNA polymerases, and is involved in ribosome assembly and nascent p53 protein biosynthesis, we have evaluated and found MDM2 protein to possess an intrinsic molecular chaperone activity. MDM2 can substitute for the Hsp90 molecular chaperone in promoting binding of p53 to the p21-derived promoter sequence. This reaction is driven by recycling of MDM2 from the p53 complex, triggered by binding of ATP to MDM2. The ATP binding mutant MDM2 protein (K454A) lacks the chaperone activity both in vivo and in vitro. Mdm2 cotransfected in the H1299 cell line with wild-type p53 stimulates efficient p53 folding in vivo but at the same time accelerates the degradation of p53. MDM2 in which one of the Zn(2+) coordinating residues is mutated (C478S or C464A) blocks degradation but enhances folding of p53. This is the first demonstration that MDM2 possesses an intrinsic molecular chaperone activity, indicating that the ATP binding function of MDM2 can mediate its chaperone function toward the p53 tumor suppressor.

Establishment of immortalized rat Kupffer cell lines

BACKGROUND

Kupffer cells have been implicated in the pathogenesis of various liver diseases. Primary cultures of Kupffer cells have a very limited life span, tend to de-differentiate and become senescent, and therefore are not suitable for cell signaling studies.

AIM

To establish immortalized rat Kupffer cell lines that facilitate mechanistic studies of cell signaling and signal transduction.

METHODS

Rat Kupffer cells were sub-cultured with EGF to obtain rat Kupffer Cell line 1 (RKC1), and subsequently transfected with Simian Virus 40 Large T-antigen expression vector to obtain rat Kupffer Cell line 2 (RKC2).

RESULTS

RKC1 and RKC2 are similar to primary Kupffer cells as they express the molecular markers ED1, ED2, ED3, and F4/80, and upregulate TNF-alpha, IL-6, IL-1beta, Fas /FasL, and NF-kappaB, as well as TLR4 in response to LPS or pancreatic elastase. Additionally, RKC1 and RKC2 maintain phagocytic properties of latex beads and exhibit increased telomerase and stabilized p53 activity.

CONCLUSION

Immortalized RKC1 and RKC2 cells maintain properties of primary Kupffer cells and can be valuable tools in evaluating the role of Kupffer cells in immune diseases and in liver-cell based drug discovery.

Reactivation of mutant p53: molecular mechanisms and therapeutic potential

The p53 tumor suppressor gene is the most frequently mutated gene in cancer. Most p53 mutations are missense point mutations that cluster in the DNA-binding core domain. This results in distortion of core domain folding and disruption of DNA binding and transcriptional transactivation of p53 target genes. Structural studies have demonstrated that mutant p53 core domain unfolding is not irreversible. Mutant p53 is expressed at high levels in many tumors. Therefore, mutant p53 is a promising target for novel cancer therapy. Mutant p53 reactivation will restore p53-dependent apoptosis, resulting in efficient removal of tumor cells. A number of strategies for targeting mutant p53 have been designed, including peptides and small molecules that restore the active conformation and DNA binding to mutant p53 and induce p53-dependent suppression of tumor cell growth in vitro and in vivo. This opens possibilities for the clinical application of mutant p53 reactivation in the treatment of cancer.

Regulation of p53 nuclear export through sequential changes in conformation and ubiquitination

Wild-type p53 is a conformationally labile protein that undergoes nuclear-cytoplasmic shuttling. MDM2-mediated ubiquitination promotes p53 nuclear export by exposing or activating a nuclear export signal (NES) in the C terminus of p53. We observed that cancer-derived p53s with a mutant (primary antibody 1620-/pAb240+) conformation localized in the cytoplasm to a greater extent and displayed increased susceptibility to ubiquitination than p53s with a more wild-type (primary antibody 1620+/pAb240-) conformation. The cytoplasmic localization of mutant p53s required the C-terminal NES and an intact ubiquitination pathway. Mutant p53 ubiquitination occurred at lysines in both the DNA-binding domain (DBD) and C terminus. Interestingly, Lys to Arg mutations that inhibited ubiquitination restored nuclear localization to mutant p53 but had no apparent effect on p53 conformation. Further studies revealed that wild-type p53, like mutant p53, is ubiquitinated by MDM2 in both the DBD and C terminus and that ubiquitination in both regions contributes to its nuclear export. MDM2 binding can induce a conformational change in wild-type p53, but this conformational change is insufficient to promote p53 nuclear export in the absence of MDM2 ubiquitination activity. Taken together, these results support a stepwise model for mutant and wild-type p53 nuclear export. In this model, the conformational change induced by either the cancer-derived mutation or MDM2 binding precedes p53 ubiquitination. The addition of ubiquitin to DBD and C-terminal lysines then promotes nuclear export via the C-terminal NES.

p53 mediates the negative regulation of MDM2 by orphan receptor TR3

MDM2 is an oncoprotein whose transforming potential is activated by overexpression. The expression level of MDM2 is negatively regulated by orphan receptor TR3 that mainly acts as a transcriptional factor to regulate gene expression. However, the underlying mechanism is largely unclear. Here, we present the first evidence that inhibition of TR3 on MDM2 is mediated by p53. We found that TR3 directly interacts with p53 but not MDM2, and such interaction is critical for TR3 to inhibit MDM2 expression. TR3 downregulates p53 transcriptional activity by blocking its acetylation, leading to a decrease on the transcription level of MDM2. Furthermore, TR3 binding to p53 obstructs its ubiquitination and degradation induced by MDM2, resulting in the MDM2 ubiquitination and degradation. In addition, TR3 could enhance p53-mediated apoptosis induced by UV irradiation. Taken together, our findings demonstrate that p53 mediates the suppression of TR3 on MDM2 at both transcriptional and post-transcriptional level and suggest TR3 as a potential target to develop new anticancer agents that restrict MDM2-induced tumor progression.

p53—A Natural Cancer Killer: Structural Insights and Therapeutic Concepts

Every single day, the DNA of each cell in the human body is mutated thousands of times, even in absence of oncogenes or extreme radiation. Many of these mutations could lead to cancer and, finally, death. To fight this, multicellular organisms have evolved an efficient control system with the tumor-suppressor protein p53 as the central element. An intact p53 network ensures that DNA damage is detected early on. The importance of p53 for preventing cancer is highlighted by the fact that p53 is inactivated in more than 50 % of all human tumors. Thus, for good reason, p53 is one of the most intensively studied proteins. Despite the great effort that has been made to characterize this protein, the complex function and the structural properties of p53 are still only partially known. This review highlights basic concepts and recent progress in understanding the structure and regulation of p53, focusing on emerging new mechanistic and therapeutic concepts.

Destabilizing missense mutations in the tumour suppressor protein p53 enhance its ubiquitination in vitro and in vivo

p53 ubiquitination catalysed by MDM2 (murine double minute clone 2 oncoprotein) provides a biochemical assay to dissect stages in E3-ubiquitin-ligase-catalysed ubiquitination of a conformationally flexible protein. A mutant form of p53 (p53(F270A)) containing a mutation in the second MDM2-docking site in the DNA-binding domain of p53 (F270A) is susceptible to modification of long-lived and high-molecular-mass covalent adducts in vivo. Mutant F270A is hyperubiquitinated in cells as defined by immunoprecipitation and immunoblotting with an anti-ubiquitin antibody. Transfection of His-tagged ubiquitin along with p53(R175H) or p53(F270A) also results in selective hyperubiquitination in cells under conditions where wild-type p53 is refractory to covalent modification. The extent of mutant p53(R175H) or p53(F270A) unfolding in cells as defined by exposure of the DO-12 epitope correlates with the extent of hyperubiquitination, suggesting a link between substrate conformation and E3 ligase function. The p53(F270A:6KR) chimaeric mutant (where 6KR refers to the simultaneous mutation of lysine residues at positions 370, 372, 373, 381, 382 and 386 to arginine) maintains the high-molecular-mass covalent adducts and is modified in an MDM2-dependent manner. Using an in vitro ubiquitination system, mutant p53(F270A) and the p53(F270A:6KR) chimaeric mutant is also subject to hyperubiquitination outwith the C-terminal domain, indicating direct recognition of the mutant p53 conformation by (a) factor(s) in the cell-free ubiquitination system. These data identify an in vitro and in vivo assay with which to dissect how oligomeric protein conformational alterations are linked to substrate ubiquitination in cells. This has implications for understanding the recognition of misfolded proteins during aging and in human diseases such as cancer.

Mutant p53 in MDA-MB-231 breast cancer cells is stabilized by elevated phospholipase D activity and contributes to survival signals generated by phospholipase D

p53 is the most commonly mutated gene in human cancer. Although the loss of tumor suppressor functions for p53 in tumorigenesis is well characterized, gain-of-function p53 mutations observed in most cancers are not as widely appreciated. The human breast cancer cell line MDA-MB-231, which has high levels of a mutant p53, has high levels of phospholipase D (PLD) activity, which provides a survival signal in these cells when deprived of serum growth factors. We report here that the mutant p53 in MDA-MB-231 cells is stabilized by the elevated PLD activity in these cells. Surprisingly, the survival of MDA-MB-231 cells deprived of serum was dependent on the mutant p53. These data indicate that a mutant p53, stabilized by elevated PLD activity, can contribute to the suppression of apoptosis in a human breast cancer cell line and suggest a rationale for the selection of p53 mutations early in tumorigenesis to suppress apoptosis in an emerging tumor.

A New IκB Kinase β Inhibitor Prevents Human Breast Cancer Progression through Negative Regulation of Cell Cycle Transition

Constitutive nuclear factor-kappaB (NF-kappaB) activity plays a crucial role in the development and progression of lymphoma, leukemia, and some epithelial cancers. Given the contribution of NF-kappaB in carcinogenesis, a novel approach that interferes with its activity might have therapeutic potential against cancers that respond poorly to conventional treatments. Here, we have shown that a new IkappaB kinase beta inhibitor, IMD-0354, suppressed the growth of human breast cancer cells, MDA-MB-231, HMC1-8, and MCF-7, by arresting cell cycle and inducing apoptosis. In an electrophoretic mobility shift assay and a reporter assay, IMD-0354 abolished the NF-kappaB activity in MDA-MB-231 cells in a dose-dependent manner. In the cells incubated with IMD-0354, cell cycle arrested at the G0-G1 phase and apoptotic cells were increased. The expression of some cell cycle regulatory molecules and antiapoptotic molecules was suppressed in cells treated with IMD-0354. On the other hand, cyclin-dependent kinase suppressor p27Kip1 was up-regulated by the addition of IMD-0354. Daily administration of IMD-0354 inhibited tumor expansion in immunodeficient mice into which MDA-MB-231 cells were transplanted. These results indicate that NF-kappaB may contribute to cell proliferation through up-regulation of cell cycle progression; accordingly, inhibition of NF-kappaB activity might have a therapeutic ability in the treatment of human breast cancers.

The chaperone-associated ubiquitin ligase CHIP is able to target p53 for proteasomal degradation

The cellular level of the tumor suppressor p53 is tightly regulated through induced degradation via the ubiquitin/proteasome system. The ubiquitin ligase Mdm2 plays a pivotal role in stimulating p53 turnover. However, recently additional ubiquitin ligases have been identified that participate in the degradation of the tumor suppressor. Apparently, multiple degradation pathways are employed to ensure proper destruction of p53. Here we show that the chaperone-associated ubiquitin ligase CHIP is able to induce the proteasomal degradation of p53. CHIP-induced degradation was observed for mutant p53, which was previously shown to associate with the chaperones Hsc70 and Hsp90, and for the wild-type form of the tumor suppressor. Our data reveal that mutant and wild-type p53 transiently associate with molecular chaperones and can be diverted onto a degradation pathway through this association.

Independent ATPase activity of Hsp90 subunits creates a flexible assembly platform

The ATPase activity of the molecular chaperone Hsp90 is essential for its function in the assembly of client proteins. To understand the mechanism of human Hsp90, we have carried out a detailed kinetic analysis of ATP binding, hydrolysis and product release. ATP binds rapidly in a two-step process involving the formation of a diffusion-collision complex followed by a conformational change. The rate-determining step was shown to be ATP hydrolysis and not subsequent ADP dissociation. There was no evidence from any of the biophysical measurements for cooperativity in either nucleotide binding or hydrolysis for the dimeric protein. A monomeric fragment, lacking the C-terminal dimerisation domain, showed no dependence on protein concentration and, therefore, subunit association for activity. The thermodynamic linkage between client protein binding and nucleotide affinity revealed ATP bound Hsp90 has a higher affinity for client proteins than the ADP bound form. The kinetics are consistent with independent Michaelis-Menten catalysis in each subunit of the Hsp90 dimer. We propose that Hsp90 functions in an open-ring configuration for client protein activation.

Hsp90 Is Essential for Restoring Cellular Functions of Temperature-sensitive p53 Mutant Protein but Not for Stabilization and Activation of Wild-type p53

Several signaling pathways that monitor the dynamic state of the cell converge on the tumor suppressor p53. The ability of p53 to process these signals and exert a dynamic downstream response in the form of cell cycle arrest and/or apoptosis is crucial for preventing tumor development. This p53 function is abrogated by p53 gene mutations leading to alteration of protein conformation. Hsp90 has been implicated in regulating both wild-type and mutant p53 conformations, and Hsp90 antagonists are effective for the therapy of some human tumors. Using cell lines that contain human tumor-derived temperature-sensitive p53 mutants we show that Hsp90 is required for both stabilization and reactivation of mutated p53 at the permissive temperature. A temperature decrease to 32 degrees C causes conversion to a protein conformation that is capable of inducing expression of MDM2, leading to reduction of reactivated p53 levels by negative feedback. Mutant reactivation is enhanced by simultaneous treatment with agents that stabilize the reactivated protein and is blocked by geldanamycin, a specific inhibitor of Hsp90 activity, indicating that Hsp90 antagonist therapy and therapies that act to reactivate mutant p53 will be incompatible. In contrast, Hsp90 is not required for maintaining wild-type p53 or for stabilizing wild-type p53 after treatment with chemotherapeutic agents, indicating that Hsp90 therapy might synergize with conventional therapies in patients with wild-type p53. Our data demonstrate the importance of the precise characterization of the interaction between p53 mutants and stress proteins, which may shed valuable information for fighting cancer via the p53 tumor suppressor pathway.

Hsp90 Chaperones Wild-type p53 Tumor Suppressor Protein

Immortalized human fibroblasts were used to investigate the putative interactions of the Hsp90 molecular chaperone with the wild-type p53 tumor suppressor protein. We show that geldanamycin or radicicol, specific inhibitors of Hsp90, diminish specific wild-type p53 binding to the p21 promoter sequence. Consequently, these inhibitors decrease p21 mRNA levels, which lead to a reduction in cellular p21/Waf1 protein, known to induce cell cycle arrest. In control experiments, we show that neither geldanamycin nor radicicol affect p53 mRNA levels. A minor decrease in p53 protein level following the treatment of human fibroblasts with the inhibitors suggests the potential involvement of Hsp90 in the stabilization of wild-type p53. To support our in vivo findings, we used a reconstituted system with highly purified recombinant proteins to examine the effects of Hsp90 on wild-type p53 binding to the p21 promoter sequence. The human recombinant Hsp90 alpha-isoform as well as bovine brain Hsp90 were purified to homogeneity. Both of these molecular chaperones displayed ATPase activity and the ability to refold heat-inactivated luciferase in a geldanamycin- and radicicol-sensitive manner, suggesting that post-translational modifications are not involved in the modulation of Hsp90alpha activity. We show that the incubation of recombinant p53 at 37 degrees C decreases the level of its wild-type conformation and strongly inhibits the in vitro binding of p53 to the p21 promoter sequence. Interestingly, Hsp90 in an ATP-dependent manner can positively modulate p53 DNA binding after incubation at physiological temperature of 37 degrees C. Other recombinant human chaperones from Hsp70 and Hsp40 families were not able to efficiently substitute Hsp90 in this reaction. Consistent with our in vivo results, geldanamycin can suppress Hsp90 ability to regulate in vitro p53 DNA binding to the promoter sequence. In summary, the results presented in this article state that chaperone activity of Hsp90 is important for the transcriptional activity of genotypically wild-type p53.

Hsp90 regulates the activity of wild type p53 under physiological and elevated temperatures

The activity and structural integrity of the tumor suppressor protein p53 is of crucial importance for the prevention of cancer. p53 is a conformational flexible and labile protein, in which structured and unstructured regions function in a synergistic manner. The molecular chaperone Hsp90 is known to bind to mutant and wild type p53 in vivo. Using highly purified proteins we analyzed the interaction and the binding sites between both proteins in detail. Our results demonstrate that Hsp90 binds to a folded, native-like conformation of p53 in vitro with micromolar affinity. Specifically, the DNA-binding domain of p53 and the middle and carboxy-terminal domains of Hsp90 are responsible for this interaction, which is essential to stabilize p53 at physiological temperatures and to prevent it from irreversible thermal inactivation. Our results are in agreement with a model in which Hsp90 is required to maintain the folded, active state of p53 by a reversible interaction, thus introducing an additional level of regulation.

Pifithrin-alpha inhibits p53 signaling after interaction of the tumor suppressor protein with hsp90 and its nuclear translocation

Pifithrin-alpha (PFTalpha) was originally thought to be a specific inhibitor of signaling by the tumor suppressor protein p53. However, the laboratory that discovered pifithrin recently reported that the compound also inhibits heat shock and glucocorticoid receptor (GR) signaling, and they suggested that PFTalpha targets a factor common to all three signal transduction pathways, such as the hsp90/hsp70-based chaperone machinery (Komarova, E. A., Neznanov, N., Komarov, P. G., Chernov, M. V., Wang, K., and Gudkov, A. V. (2003) J. Biol. Chem. 278, 15465-15468). Because it is important for the mechanistic study of this machinery to identify unique inhibitors of chaperone action, we have examined the effect of PFTalpha on transcriptional activation, the hsp90 heterocomplex assembly, and hsp90-dependent nuclear translocation for both p53 and the GR. At concentrations where PFTalpha blocks p53-mediated induction of p21/Waf-1 in human embryonic kidney cells, we observed no inhibition of GR-mediated induction of a chloramphenicol acetyl transferase reporter in LMCAT cells. PFTalpha did, however, cause a left shift in the dexamethasone dose response curve by increasing intracellular dexamethasone concentration, apparently by competing for dexamethasone efflux from the cell. The assembly of p53 or GR heterocomplexes with hsp90 and immunophilins was not affected by PFTalpha either in vivo or in vitro and did not affect the nuclear translocation of either transcription factor. Thus, we conclude that PFTalpha does not inhibit GR-mediated induction or the function of the chaperone machinery, and, as originally thought, it may specifically inhibit p53 signaling by acting at a stage after p53 translocation to the nucleus.

Hsp90-binding Immunophilins Link p53 to Dynein During p53 Transport to the Nucleus

The tumor suppressor protein p53 is known to be transported to the nucleus along microtubular tracks by cytoplasmic dynein. However, the connection between p53 and the dynein motor protein complex has not been established. Here, we show that hsp90.binding immunophilins link p53.hsp90 complexes to dynein and that prevention of that linkage in vivo inhibits the nuclear movement of p53. First, we show that p53.hsp90 heterocomplexes from DLD-1 human colon cancer cells contain an immunophilin (FKBP52, CyP-40, or PP5) as well as dynein. p53.hsp90.immunophilin.dynein complexes can be formed by incubating immunopurified p53 with rabbit reticulocyte lysate, and we show by peptide competition that the immunophilins link via their tetratricopeptide repeat domains to p53-bound hsp90 and by means of their PPIase domains to the dynein complex. The linkage of immunophilins to the dynein motor is indirect by means of the dynamitin component of the dynein-associated dynactin complex, and we show that purified FKBP52 binds directly by means of its PPIase domain to purified dynamitin. By using a temperature-sensitive mutant of p53 where cytoplasmic-nuclear movement occurs by shift to permissive temperature, we show that p53 movement is impeded when p53 binding to hsp90 is inhibited by the hsp90 inhibitor radicicol. Also, nuclear movement of p53 is inhibited when immunophilin binding to dynein is competed for by expression of a PPIase domain fragment in the same manner as when dynein linkage to cargo is dissociated by expression of dynamitin. This is the first demonstration of the linkage between an hsp90-chaperoned transcription factor and the system for its retrograde movement to the nucleus both in vitro and in vivo.

Expansion of Protein Interaction Maps by Phage Peptide Display Using MDM2 as a Prototypical Conformationally Flexible Target Protein

Expanding on the possible protein interaction partners in a biochemical pathway is one key molecular goal in the post-genomic era. Phage peptide display is a versatile in vitro tool for mapping novel protein-protein interfaces and the advantage of this technique in expanding protein interaction maps is that in vitro manipulation of the bait protein conformational integrity can be controlled carefully. Phage peptide display was used to expand on the possible types of binding proteins for the conformationally responsive protein MDM2. Peptides enriched differ depending upon whether MDM2 is ligand-free, zinc-bound, or RNA-bound, suggesting that MDM2 conformational changes alter the type of peptide ligands enriched. Classes of putative/established MDM2-binding proteins identified by this technique included ubiquitin-modifying enzymes (F-box proteins, UB-ligases, UBC-E1) and apoptotic modifiers (HSP90, GAS1, APAF1, p53). Of the many putative MDM2 proteins that could be examined, the impact of HSP90 on MDM2 activity was studied, since HSP90 has been linked with p53 protein unfolding in human cancers. Zinc ions were required to reconstitute a stable MDM2-HSP90 protein complex. Zinc binding converted MDM2 from a monomer to an oligomer, and activated MDM2 binding to its internal RING finger domain, providing evidence for a conformational change in MDM2 protein when it binds zinc. Reconstitution of an HSP90-MDM2 protein complex in vitro stimulated the unfolding of the p53 tetramer. A p53 DNA-binding inhibitor purified from human cells that is capable of unfolding p53 at ambient temperature in vitro contains co-purifying pools of HSP90 and MDM2. These data highlight the utility of phage peptide display as a powerful in vitro method to identify regulatory proteins that bind to a conformationally flexible protein like MDM2.

Hsp70 and Hsp90--a relay team for protein folding

Molecular chaperones are a functionally defined set of proteins which assist the structure formation of proteins in vivo. Without certain protective mechanisms, such as binding nascent polypeptide chains by molecular chaperones, cellular protein concentrations would lead to misfolding and aggregation. In the mammalian system, the molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins, like steroid hormone receptors, transcription factors, and kinases, some of which are involved in cancer progression. Hsp70 and Hsp90 form a multichaperone complex, in which both are connected by a third protein called Hop. The connection of and the interplay between the two chaperone machineries is of crucial importance for cell viability. This review provides a detailed view of the Hsp70 and Hsp90 machineries, their cofactors and their mode of regulation. It summarizes the current knowledge in the field, including the ATP-dependent regulation of the Hsp70/Hsp90 multichaperone cycle and elucidates the complex interplay and their synergistic interaction.

P53 hot-spot mutants are resistant to ubiquitin-independent degradation by increased binding to NAD(P)H:quinone oxidoreductase 1

Proteasomal degradation of p53 is mediated by two alternative pathways that are either dependent or independent of both Mdm2 and ubiquitin. The ubiquitin-independent pathway is regulated by NAD(P)H: quinone oxidoreductase 1 (NQO1) that stabilizes p53. The NQO1 inhibitor dicoumarol induces ubiquitin-independent p53 degradation. We now show that, like dicoumarol, several other coumarin and flavone inhibitors of NQO1 activity, which compete with NAD(P)H for binding to NQO1, induced ubiquitin-independent p53 degradation and inhibited wild-type p53-mediated apoptosis. Although wild-type p53 and several p53 mutants were sensitive to dicoumarol-induced degradation, the most frequent "hot-spot" p53 mutants in human cancer, R175H, R248H, and R273H, were resistant to dicoumarol-induced degradation, but remained sensitive to Mdm2-ubiquitin-mediated degradation. The two alternative pathways for p53 degradation thus have different p53 structural requirements. Further mutational analysis showed that arginines at positions 175 and 248 were essential for dicoumarol-induced p53 degradation. NQO1 bound to wild-type p53 and dicoumarol, which induced a conformational change in NQO1, inhibited this binding. Compared with wild-type p53, the hot-spot p53 mutants showed increased binding to NQO1, which can explain their resistance to dicoumarol-induced degradation. NQO1 thus has an important role in stabilizing hot-spot p53 mutant proteins in human cancer.

Rescue of mutant p53 transcription function by ellipticine

The p53 tumor suppressor is frequently inactivated in tumors by point mutations in the DNA-binding domain, resulting in loss of sequence-specific DNA binding and transcription function. We present evidence that ellipticine can restore the transactivation function of several transfected p53 mutants (175 H, 248W, 249S, 273 H, 281G), resulting in the induction of p53-responsive genes (p21(WAF1),MDM2) and activation of a p53-responsive luciferase reporter. Ellipticine also activates mutant p53 function in tumor cells expressing endogenous 194F, 233L, 241F, and 273C mutants. Treatment with ellipticine alters mutant p53 reactivity to conformation-sensitive Pab1620 and Pab240 antibodies and increases its sequence-specific DNA-binding activity in vivo. Finally, ellipticine activates mutant p53 and induces p21(WAF1) and MDM2 expression in nude mouse tumor xenografts. These results demonstrate that ellipticine can restore transcription function to mutant p53. This property may contribute to the selectivity of ellipticine-derived compounds against tumor cell lines expressing mutant p53.

Apoptosis and melanoma chemoresistance

Melanoma is the most aggressive form of skin cancer and is notoriously resistant to all current modalities of cancer therapy. A large set of genetic, functional and biochemical studies suggest that melanoma cells become 'bullet proof' against a variety of chemotherapeutic drugs by exploiting their intrinsic resistance to apoptosis and by reprogramming their proliferation and survival pathways during melanoma progression. In recent years, the identification of molecules involved in the regulation and execution of apoptosis, and their alteration in melanoma, have provided new insights into the molecular basis for melanoma chemoresistance. With this knowledge in hand, the challenge is now to devise strategies potent enough to compensate or bypass these cell death defects and improve the actual poor prognosis of patients at late stages of the disease.

Between genotype and phenotype: protein chaperones and evolvability

Protein chaperones direct the folding of polypeptides into functional proteins, facilitate developmental signalling and, as heat-shock proteins (HSPs), can be indispensable for survival in unpredictable environments. Recent work shows that the main HSP chaperone families also buffer phenotypic variation. Chaperones can do this either directly through masking the phenotypic effects of mutant polypeptides by allowing their correct folding, or indirectly through buffering the expression of morphogenic variation in threshold traits by regulating signal transduction. Environmentally sensitive chaperone functions in protein folding and signal transduction have different potential consequences for the evolution of populations and lineages under selection in changing environments.

Phosphorylation and hsp90 binding mediate heat shock stabilization of p53

The p53 tumor suppressor is stabilized and activated by diverse stress signals. In this study, we investigated the mechanism of p53 activation by heat shock. We found that heat shock inhibited p53 ubiquitination and caused accumulation of p53 at the post-transcriptional level. Heat shock induced phosphorylation of p53 at serine 15 in an ATM kinase-dependent fashion, which may contribute partially to heat-induced p53 accumulation. However, p53 accumulation also occurred after heat shock in ATM-deficient cells. Heat shock induced conformational change of wild type p53 and binding to hsp90. Inhibition of hsp90-p53 interaction by geldanamycin prevented p53 accumulation partially in ATM-wild type cells and completely in ATM-deficient cells. Therefore, phosphorylation and interaction with hsp90 both contribute to stabilization of p53 after heat shock.

Mdm-2 and ubiquitin-independent p53 proteasomal degradation regulated by NQO1

The tumor suppressor p53 is a labile protein whose level is known to be regulated by the Mdm-2-ubiquitin-proteasome degradation pathway. We have found another pathway for p53 proteasomal degradation regulated by NAD(P)H quinone oxidoreductase 1 (NQO1). Inhibition of NQO1 activity by dicoumarol induces p53 and p73 proteasomal degradation. A mutant p53 (p53([22,23])), which is resistant to Mdm-2-mediated degradation, was susceptible to dicoumarol-induced degradation. This finding indicates that the NQO1-regulated proteasomal p53 degradation is Mdm-2-independent. The tumor suppressor p14(ARF) and the viral oncogenes SV40 LT and adenovirus E1A that are known to stabilize p53 inhibited dicoumarol-induced p53 degradation. Unlike Mdm-2-mediated degradation, the NQO1-regulated p53 degradation pathway was not associated with accumulation of ubiquitin-conjugated p53. In vitro studies indicate that dicoumarol-induced p53 degradation was ubiquitin-independent and ATP-dependent. Inhibition of NQO1 activity in cells with a temperature-sensitive E1 ubiquitin-activating enzyme induced p53 degradation and inhibited apoptosis at the restrictive temperature without ubiquitination. Mdm-2 failed to induce p53 degradation under these conditions. Our results establish a Mdm-2- and ubiquitin-independent mechanism for proteasomal degradation of p53 that is regulated by NQO1. The lack of NQO1 activity that stabilizes a tumor suppressor such as p53 can explain why humans carrying a polymorphic inactive NQO1 are more susceptible to tumor development.

Estradiol stabilizes p53 protein in breast cancer cell line, MCF-7

Overexpression of the oncoprotein MDM2, an important regulator of the p53 tumor suppressor protein, is often observed in breast cancer tissues and cell lines, particularly in those which express estrogen receptor alpha (ERalpha). In MCF-7 breast cancer cell line possessing wild-type p53, ERalpha, and overexpressing MDM2, p53 accumulation was stimulated by 17beta-estradiol (E2) in a concentration-dependent manner. On the other hand, E2 caused no change of the expression of p53 mRNA, indicating that E2 affects p53 at the post-transcriptional level. To analyze the mechanism of p53 accumulation by E2, the stability of p53, ERalpha and MDM2 proteins was analyzed in the presence of cycloheximide under an E2-supplemented or -depleted condition. E2 significantly extended the half-life of p53 protein, but shortened that of ERalpha in MCF-7 cells. E2 significantly decreased the stability of p90(MDM2) and p60(MDM2) in MCF-7. Interestingly, E2 increased the ratio p60(MDM2)/p90(MDM2) inversely proportionally to the degradation of p53. These results suggest that the ratio of the two MDM2 proteins, p90(MDM2) and p60(MDM2), may affect the accumulation of wild-type p53 protein in response to E2.

NQO1 stabilizes p53 through a distinct pathway

Wild-type p53 is a tumor-suppressor gene that encodes a short-lived protein that, upon accumulation, induces growth arrest or apoptosis. Accumulation of p53 occurs mainly by posttranslational events that inhibit its proteosomal degradation. We have reported previously that inhibition of NAD(P)H: quinone oxidoreductase 1 (NQO1) activity by dicoumarol induces degradation of p53, indicating that NQO1 plays a role in p53 stabilization. We now have found that wild-type NQO1, but not the inactive polymorphic NQO1, can stabilize endogenous as well as transfected wild-type p53. NQO1-mediated p53 stabilization was especially prominent under induction of oxidative stress. NQO1 also partially inhibited p53 degradation mediated by the human papilloma virus E6 protein, but not when mediated by Mdm-2. Inhibitors of heat shock protein 90 (hsp90), radicicol and geldanamycin, induced degradation of p53 and suppressed p53-induced apoptosis in normal thymocytes and myeloid leukemic cells. Differences in the effectiveness of dicoumarol and hsp90 inhibitors to induce p53 degradation and suppress apoptosis in these cell types indicate that NQO1 and hsp90 stabilize p53 through different mechanisms. Our results indicate that NQO1 has a distinct role in the regulation of p53 stability, especially in response to oxidative stress. The present data on the genetic and pharmacologic regulation of the level of p53 have clinical implications for tumor development and therapy.

Beta-arrestin and Mdm2, unsuspected partners in signaling from the cell surface

Mdm2 is a ubiquitin-protein ligase known to ubiquitinate p53, promoting its degradation by the ubiquitin-proteasome system. Shenoy and co-workers showed that Mdm2 can act as a key factor in the sequestration of the cell surface beta(2)-adrenergic receptor (beta-AR) through interactions with beta-arrestin. Strous and Schantl discuss how Mdm2 may be a switch connecting extracellular signals mediated through G protein-coupled receptors (GPCRs) to p53 and its functions in apoptosis and cell cycle progression.