Destabilization of Raf-1 by geldanamycin leads to disruption of the Raf-1-MEK-mitogen-activated protein kinase signalling pathway

Mechanisms of inhibition of the Ras-MAP kinase signaling pathway in 30.7b Ras 12 cells by tea polyphenols (-)-epigallocatechin-3-gallate and theaflavin-3,3'-digallate

Our previous study showed that tea polyphenols inhibited MAP kinase and AP-1 activities in mouse epidermal JB6 cells and the corresponding H-ras-transformed cell line 30.7b Ras 12. The present study investigated the mechanisms of this inhibition. The cells were incubated with (-)-epigallocatechin-3-gallate (EGCG) or theaflavin-3,3'-digallate (TFdiG) (20 mM) for different times, and the cell lysate was analyzed by immunoblotting. EGCG treatment decreased the levels of phospho-Erk1/2 and -MEK1/2 time-dependently (by 60% at 60 min). TFdiG lowered their levels by 38%-50% at 15 min. TFdiG effectively decreased total Raf-1 protein levels, most likely through lysosomal degradation. EGCG did not affect protein levels or the activity of Raf-1 significantly but decreased its association with MEK1 as determined by co-immunoprecipitation. In addition, EGCG and TFdiG (10 mM) inhibited the phosphorylation of Elk-1 by isolated phospho-Erk1/2 in vitro. This inhibition of Erk1/2 activity is Elk-1 concentration-dependent and ATP concentration-independent, which suggests that EGCG and TFdiG interfere with the binding of the protein substrate to the kinase. The presently demonstrated specific mechanisms of inhibition of MAP kinases by EGCG and TFdiG may help us to understand the effects of tea consumption on cancer, inflammatory diseases, and cardiovascular diseases.

Stressed to death: regulation of apoptotic signaling pathways by the heat shock proteins

Cellular damage can engage two fundamental cellular responses: apoptosis, a precisely regulated form of cell death; and the heat shock protein (Hsp), or stress response, which functions to protect cells and to mediate an accelerated recovery following damage. The coordinated balance between these two opposing pathways governs the ultimate fate of the cell--whether it lives or dies. The self-destruction of a cell is mediated by one of many signaling pathways culminating in the activation of the caspase proteases. The Hsps regulate the activity of multiple intracellular signaling intermediates, many of which are intimately involved in the execution of the apoptotic signaling pathways. This review addresses whether the antiapoptotic activities of several Hsps, including Hsp70, Hsp90, and Hsp27, can be attributed to their collective ability to regulate the activities, expression, or both of apoptotic signaling molecules. In summary, the functional interface between the ancient heat shock or stress protein response and the highly conserved biochemical pathways leading to the activation of apoptosis governs the susceptibility of a cell to damaging stimuli.

Microtubule integrity regulates Pak leading to Ras-independent activation of Raf-1. insights into mechanisms of Raf-1 activation

Growth factors activate Raf-1 by engaging a complex program, which requires Ras binding, membrane recruitment, and phosphorylation of Raf-1. The present study employs the microtubule-depolymerizing drug nocodazole as an alternative approach to explore the mechanisms of Raf activation. Incubation of cells with nocodazole leads to activation of Pak1/2, kinases downstream of small GTPases Rac/Cdc42, which have been previously indicated to phosphorylate Raf-1 Ser(338). Nocodazole-induced stimulation of Raf-1 is augmented by co-expression of small GTPases Rac/Cdc42 and Pak1/2. Dominant negative mutants of these proteins block activation of Raf-1 by nocodazole, but not by epidermal growth factor (EGF). Thus, our studies define Rac/Cdc42/Pak as a module upstream of Raf-1 during its activation by microtubule disruption. Although it is Ras-independent, nocodazole-induced activation of Raf-1 appears to involve the amino-terminal regulatory region in which the integrity of the Ras binding domain is required. Surprisingly, the Raf zinc finger mutation (C165S/C168S) causes a robust activation of Raf-1 by nocodazole, whereas it diminishes Ras-dependent activation of Raf-1. We also show that mutation of residues Ser(338) to Ala or Tyr(340)-Tyr(341) to Phe-Phe immediately amino-terminal to the catalytic domain abrogates activation of both the wild type and zinc finger mutant Raf by both EGF/4beta-12-O-tetradecanoylphorbol-13-acetate and nocodazole. Finally, an in vitro kinase assay demonstrates that the zinc finger mutant serves as a better substrate of Pak1 than the wild type Raf-1. Collectively, our results indicate that 1) the zinc finger exerts an inhibitory effect on Raf-1 activation, probably by preventing phosphorylation of (338)SSYY(341); 2) such inhibition is first overcome by an unknown factor binding in place of Ras-GTP to the amino-terminal regulatory region in response to nocodazole; and 3) EGF and nocodazole utilize different kinases to phosphorylate Ser(338), an event crucial for Raf activation.

Regulation of metamorphosis in ascidians involves NO/cGMP signaling and HSP90

Treatment of larvae of the ascidians Boltenia villosa (Family: Pyuridae) and Cnemidocarpa finmarkiensis (Family: Styelidae) with drugs that inhibit the function of the molecular chaperone HSP90 increased the frequency of tail resorption, the primary morphogenetic event of metamorphosis. If treatment was initiated at hatching, metamorphic events subsequent to tail resorption failed to occur, indicating an ongoing role for HSP90 during morphogenesis. Removal of tails from heads of mature, but not newly hatched larvae, induced metamorphosis of the head. Decapitation experiments indicate that the capacity of tails to shorten in response to inhibition of HSP90 function requires communication with heads. To identify candidate proteins with which HSP90 may interact to regulate metamorphosis, we noted that in mammalian cells, nitric oxide synthase (NOS) interacts with HSP90 and its activity is sensitive to drugs that inhibit HSP90 function. In addition, nitric oxide (NO) signaling in the marine snail Ilyanassa obsoleta is an important regulator of metamorphosis. Inhibition of NOS activity in these ascidian larvae with L-NAME increased the frequency of metamorphosis, consistent with a putative interaction of NOS and HSP90. NOS is present in tail muscle cells, implicating them as targets for the drug treatments, consistent with the decapitation experiments. Inhibition of soluble guanylyl cyclase, the most common effector of NO signaling, also increased the frequency of metamorphosis. In contrast to treatment with anti-HSP90 drugs, metamorphosis induced with L-NAME or ODQ was complete. The results presented suggest that an HSP90-dependent, NO-based regulatory mechanism localized in tails represses ascidian metamorphosis. We discuss these results in relation to the induction of ascidian metamorphosis by several unrelated agents.

Mitogen-activated protein (MAP) kinase pathways: regulation and physiological functions

Mitogen-activated protein (MAP) kinases comprise a family of ubiquitous proline-directed, protein-serine/threonine kinases, which participate in signal transduction pathways that control intracellular events including acute responses to hormones and major developmental changes in organisms. MAP kinases lie in protein kinase cascades. This review discusses the regulation and functions of mammalian MAP kinases. Nonenzymatic mechanisms that impact MAP kinase functions and findings from gene disruption studies are highlighted. Particular emphasis is on ERK1/2.

Regulation of Pim-1 by Hsp90

The protooncogene Pim-1 encodes serine/threonine protein kinases that are involved in cytokine-mediated cell proliferation and in lymphoma- and leukemogenesis. It is largely unknown how Pim-1 executes its biological effects. Here we show that Pim-1 physically interacts with heat shock protein 90 alpha and beta (Hsp90alpha and beta). The Hsp90-specific inhibitor geldanamycin (GA) induced a rapid degradation of Pim-1 and reduced its kinase activity. The expression of Hsp90alpha was regulated by a signal from the cytokine receptor gp130, as is Pim-1's expression. These results indicate that Hsp90 is coordinately regulated with Pim-1 and is involved in the stabilization and function of Pim-1.

Physiological functions of protein kinase inhibitors

Protein kinases are key regulatory enzymes involved in a multitude of biochemical pathways. This chapter will describe the current research on targeting specific protein kinases with inhibitors in attempts to disrupt flux through specific pathways. Targeting specific kinases presents a distinct challenge as there are hundreds of individual kinase enzymes that use ATP as a substrate to phosphorylate specific target molecules. The challenge clearly lies in obtaining specificity for a given kinase, thus allowing inhibition or activation of a specific pathway. This chapter will focus on two areas of kinase inhibitors, those that target the MAP kinase pathway and those directed against the phosphatidylinositol-3 kinase (PI-3K) related kinase family. The cellular and physiological effects of inhibition of the various pathways controlled by these kinases will be reviewed.

Opposite effects of the Hsp90 inhibitor Geldanamycin: induction of apoptosis in PC12, and differentiation in N2A cells

The inhibitor of the Hsp90 chaperone Geldanamycin has been reported to have several cellular effects, such as inhibition of v-src activity or destabilization of Raf-1 among others. We show now that Geldanamycin treatment induces different phenotypes in different cell lines. In PC12 cells, it triggers apoptosis, whereas in the murine neuroblastoma N2A, it induces differentiation with neurite outgrowth. Geldanamycin effects cannot be mimicked by inhibition of the c-src protein tyrosine kinases, and nerve growth factor does not protect PC12 cells from apoptosis. Mitogen-activated protein kinase activities ERK and JNK are activated differently according to cell type: in PC12 cells JNK is activated, and its inhibition abolishes apoptosis, but not ERK; in N2A cells, both ERK and JNK are activated, but with peak activities at different times.

Requirement of heat shock protein 90 in mesangial cell mitogenesis

BACKGROUND

Hyperplasia of mesangial cells (MCs) is a frequent finding in glomerulonephritis. Heat shock protein 90 (HSP90) is a major cellular chaperone that assists protein folding under physiological and stress conditions.

METHODS

To identify genes that are potentially involved in the pathogenesis of glomerulonephritis, we analyzed glomerular gene expression in mesangioproliferative rat anti-Thy1.1 nephritis by representational difference analysis (RDA). Expression of HSP90beta in anti-Thy1.1 nephritis was studied by Northern and Western blot analyses and immunohistochemistry. In cultured rat MCs, the requirement of HSP90 for mitogenic signaling steps and MC replication was studied by incubation with the specific HSP90 inhibitor geldanamycin.

RESULTS

By RDA, a cDNA fragment homologous to HSP90beta was identified. Glomerular mRNA and protein expression of HSP90beta was markedly and transiently up-regulated during the course of anti-Thy1.1 nephritis, with a maximum at day 6, coinciding with the peak of MC proliferation. By immunohistochemistry, HSP90beta expression in normal glomeruli was detected in podocytes. However, in anti-Thy1.1 nephritis, glomerular HSP90beta protein expression was strongly and transiently increased in mesangial localization. In vitro, mitogenic stimulation of rat MCs led to the induction of HSP90beta mRNA and protein. Incubation of MCs with geldanamycin dose-dependently inhibited DNA synthesis and replication. Moreover, geldanamycin interfered with mitogen-induced phosphorylation of extracellular signal-regulated kinase and transcription of c-fos and Egr-1, but not with transactivation of STAT1 transcription factor. Cell cycle analysis of serum-stimulated MCs revealed that geldanamycin inhibited kinase activity of cyclin D1/CDK4 complexes and blocked progression in the G0/G1 phase and at the S/G2 phase transition.

CONCLUSIONS

The up-regulation of HSP90beta in anti-Thy1.1 nephritis may reflect its functional involvement in phenotypical alterations of MCs in mesangioproliferative glomerulonephritis. Our in vitro studies indicate that HSP90 governs the capacity of MCs to respond to proliferative stimuli by regulating critical mitogenic signaling steps necessary for G1 entry and S-phase progression.

Novel oxime derivatives of radicicol induce erythroid differentiation associated with preferential G1 phase accumulation against chronic myelogenous leukemia cells through destabilization of Bcr-Abl with Hsp90 complex

Chronic myelogenous leukemia (CML) is a clonal disorder of a pluripotent hematopoietic stem cells characterized by a chimericbcr-abl gene giving rise to a p210Bcr-Ablprotein with dysregulated tyrosine kinase activity. Radicicol, a macrocyclic antifungal antibiotic, binds to the N-terminal of heat shock protein 90 (Hsp90) and destabilizes Hsp90-associated proteins such as Raf-1. This study investigated the effect of radicicol, novel oxime derivatives of radicicol (KF25706 and KF58333), and herbimycin A (HA), a benzoquinoid ansamycin antibiotic, on the growth and differentiation of human K562 CML cells. Although KF25706 and KF58333 induced the expression of glycophorin A in K562 cells, radicicol and HA caused erythroid differentiation transiently. Cell cycle analysis showed that G1 phase accumulation was observed in K562 cells treated with KF58333. KF58333 treatment depleted p210Bcr-Abl, Raf-1, and cellular tyrosine phosphorylated proteins in K562 cells, whereas radicicol and HA showed transient depletion of these proteins. KF58333 also down-regulated the level of cell cycle–dependent kinases 4 and 6 and up-regulated cell cycle–dependent kinase inhibitor p27Kip1protein without an effect on the level of Erk and Hsp90 proteins. Immunoprecipitation analysis showed that p210Bcr-Abl formed multiple complexes with Hsp90, some containing p23 and others Hsp70; KF58333 treatment dissociated p210Bcr-Abl from Hsp90/p23 chaperone complexes. Furthermore, KF58333 induced apoptosis in K562 cells and administration of KF58333 prolonged the survival time of SCID mice inoculated with K562 cells. These results suggest that KF58333 may have therapeutic potential for the treatment of CML that involves abnormal cellular proliferation induced by p210Bcr-Abl.

Novel oxime derivatives of radicicol induce erythroid differentiation associated with preferential G1 phase accumulation against chronic myelogenous leukemia cells through destabilization of Bcr-Abl with Hsp90 complex

Chronic myelogenous leukemia (CML) is a clonal disorder of a pluripotent hematopoietic stem cells characterized by a chimericbcr-abl gene giving rise to a p210Bcr-Ablprotein with dysregulated tyrosine kinase activity. Radicicol, a macrocyclic antifungal antibiotic, binds to the N-terminal of heat shock protein 90 (Hsp90) and destabilizes Hsp90-associated proteins such as Raf-1. This study investigated the effect of radicicol, novel oxime derivatives of radicicol (KF25706 and KF58333), and herbimycin A (HA), a benzoquinoid ansamycin antibiotic, on the growth and differentiation of human K562 CML cells. Although KF25706 and KF58333 induced the expression of glycophorin A in K562 cells, radicicol and HA caused erythroid differentiation transiently. Cell cycle analysis showed that G1 phase accumulation was observed in K562 cells treated with KF58333. KF58333 treatment depleted p210Bcr-Abl, Raf-1, and cellular tyrosine phosphorylated proteins in K562 cells, whereas radicicol and HA showed transient depletion of these proteins. KF58333 also down-regulated the level of cell cycle–dependent kinases 4 and 6 and up-regulated cell cycle–dependent kinase inhibitor p27Kip1protein without an effect on the level of Erk and Hsp90 proteins. Immunoprecipitation analysis showed that p210Bcr-Abl formed multiple complexes with Hsp90, some containing p23 and others Hsp70; KF58333 treatment dissociated p210Bcr-Abl from Hsp90/p23 chaperone complexes. Furthermore, KF58333 induced apoptosis in K562 cells and administration of KF58333 prolonged the survival time of SCID mice inoculated with K562 cells. These results suggest that KF58333 may have therapeutic potential for the treatment of CML that involves abnormal cellular proliferation induced by p210Bcr-Abl.

Novel strategies and therapeutics for the treatment of prostate carcinoma

BACKGROUND

An increased understanding of the biology of prostate carcinoma has led to the clinical evaluation of mechanism-based and targeted therapies. Modulating the immune system has been pursued through the use of both active and passive immunity as well as the ex vivo genetic manipulation of effector cells. A variety of gene therapies has been proposed not only to replace defective genes but to localize activation of prodrugs. Angiogenesis and tumor invasion also have been targeted, as have cell cycling and signal transduction. Strategies promoting apoptosis and augmenting differentiation are also under study.

METHODS

This study is a review of current clinical strategies using biologic, immunologic, and genetic approaches for the treatment of prostate carcinoma.

RESULTS

The clinical development of therapy targeting differentiation, apoptosis, cell signaling, angiogenesis, metastasis, immune surveillance, and others are in various stages of clinical development. A disease states model is used to discuss treatment groups, outcome measures, and other trial design elements in relation to specific therapeutic strategies.

CONCLUSIONS

Development of novel agents requires consideration of where in the natural history of the disease they should be applied. In addition, understanding the genetic and molecular alterations that occur as the disease progresses from a localized to a metastatic state, and from androgen dependence to independence, is necessary. Clinical trial design will require consideration of cytostatic and cytotoxic effects, the status of pathways not directly targeted, and potentially unexpected influences on prostate specific antigen expression by these agents.

Differential regulation of stress proteins by high hydrostatic pressure, heat shock, and unbalanced calcium homeostasis in chondrocytic cells

High hydrostatic pressure (HP) has recently been shown to increase cellular heat shock protein 70 (Hsp70) level in a specific way that does not involve transcriptional activation of the gene, but rather the stabilisation of the mRNA for Hsp70. In this study, we investigated whether there are other observable changes caused by HP stress, and compared them with those induced by certain other forms of stressors. A chondrocytic cell line T/C28a4 was exposed to 30 MPa continuous HP, heat shock at 43 degrees C, and increased cytosolic calcium concentration by the addition of sarco-endoplasmic reticulum Ca(2+) ATPase inhibitor thapsigargin (25 nM) or calcium ionophore A23187 (1 microM) in the cultures. The protein synthesis was studied by in vitro metabolic labelling followed by one- and two-dimensional polyacrylamide gel electrophoresis, and mass spectrometry was utilized to confirm the identity of the protein spots on two-dimensional gels. Continuous 30 MPa HP increased remarkably the relative labelling of Hsp70. Labelling of Hsp90 was also increased by 15-20%, although no clear change was evident at the protein level in Western blots. Elevated intracellular Ca(2+) concentration induced by thapsigargin and calcium ionophore A23187 increased mainly the synthesis of glucose-regulated protein 78 (Grp78/BiP), whereas Hsp70 and Hsp90 were decreased by the treatment. Heat shock was the strongest inducer of Hsp70 and Hsp90. This study further confirmed the induction of Hsp70 in chondrocytic cells exposed to high HP, but it also showed that calcium-mediated responses are unlikely to cause the stress response observed in the hydrostatically pressurized cells.

Targeting the Ras signaling pathway: a rational, mechanism-based treatment for hematologic malignancies?

A series of alterations in the cellular genome affecting the expression or function of genes controlling cell growth and differentiation is considered to be the main cause of cancer. These mutational events include activation of oncogenes and inactivation of tumor suppressor genes. The elucidation of human cancer at the molecular level allows the design of rational, mechanism-based therapeutic agents that antagonize the specific activity of biochemical processes that are essential to the malignant phenotype of cancer cells. Because the frequency of RAS mutations is among the highest for any gene in human cancers, development of inhibitors of the Ras–mitogen-activated protein kinase pathway as potential anticancer agents is a very promising pharmacologic strategy. Inhibitors of Ras signaling have been shown to revert Ras-dependent transformation and cause regression of Ras-dependent tumors in animal models. The most promising new class of these potential cancer therapeutics are the farnesyltransferase inhibitors. The development of these compounds has been driven by the observation that oncogenic Ras function is dependent upon posttranslational modification, which enables membrane binding. In contrast to many conventional chemotherapeutics, farnesyltransferase inhibitors are remarkably specific and have been demonstrated to cause no gross systemic toxicity in animals. Some orally bioavailable inhibitors are presently being evaluated in phase II clinical trials. This review presents an overview on some inhibitors of the Ras signaling pathway, including their specificity and effectiveness in vivo. Because Ras signaling plays a crucial role in the pathogenesis of some hematologic malignancies, the potential therapeutic usefulness of these inhibitors is discussed.

Targeting the Ras signaling pathway: a rational, mechanism-based treatment for hematologic malignancies?

A series of alterations in the cellular genome affecting the expression or function of genes controlling cell growth and differentiation is considered to be the main cause of cancer. These mutational events include activation of oncogenes and inactivation of tumor suppressor genes. The elucidation of human cancer at the molecular level allows the design of rational, mechanism-based therapeutic agents that antagonize the specific activity of biochemical processes that are essential to the malignant phenotype of cancer cells. Because the frequency of RAS mutations is among the highest for any gene in human cancers, development of inhibitors of the Ras–mitogen-activated protein kinase pathway as potential anticancer agents is a very promising pharmacologic strategy. Inhibitors of Ras signaling have been shown to revert Ras-dependent transformation and cause regression of Ras-dependent tumors in animal models. The most promising new class of these potential cancer therapeutics are the farnesyltransferase inhibitors. The development of these compounds has been driven by the observation that oncogenic Ras function is dependent upon posttranslational modification, which enables membrane binding. In contrast to many conventional chemotherapeutics, farnesyltransferase inhibitors are remarkably specific and have been demonstrated to cause no gross systemic toxicity in animals. Some orally bioavailable inhibitors are presently being evaluated in phase II clinical trials. This review presents an overview on some inhibitors of the Ras signaling pathway, including their specificity and effectiveness in vivo. Because Ras signaling plays a crucial role in the pathogenesis of some hematologic malignancies, the potential therapeutic usefulness of these inhibitors is discussed.

Depletion of Raf-1 protooncogene by geldanamycin causes apoptosis in human luteinized granulosa cells

OBJECTIVE

To investigate the hypothesis that epidermal growth factor (EGF) signaling in luteinized granulosa cells works through Raf-1 and mitogen-activated protein (MAP) kinases and that depletion of Raf-1 by geldanamycin will inhibit the signaling pathway and cause apoptosis.

DESIGN

Laboratory study.

SETTING

University of Minnesota.

PATIENT(S)

Human luteinized granulosa cells from IVF patients.

INTERVENTION(S)

The cells were treated with vehicle (DMSO), 0.5 microM of geldanamycin, 10 ng/mL of EGF, and geldanamycin + EGF.

MAIN OUTCOME MEASURE(S)

Radiochemical MAP kinase assay, Western blotting, confocal microscopy, and flow cytometry.

RESULT(S)

Geldanamycin treatment depleted Raf-1 and lowered MAP kinase activity in luteinized granulosa cells. EGF treatment increased MAP kinase phosphorylation and translocation of the phosphorylated MAP kinase to the nucleus. Geldanamycin blocked this effect. Cleavage of caspase-3, the executioner protein in apoptosis, into an active 17 kD fragment was observed by Western blotting in geldanamycin-treated cells. Finally, by flow cytometry we observed significantly increased percentages of subdiploid apoptotic nuclei in geldanamycin-treated cells.

CONCLUSION(S)

In human luteinized granulosa cells, EGF works through Raf-1, and MAP kinase and depletion of Raf-1 by geldanamycin resulted in decreased MAP kinase activity, increased activated caspase-3, and, ultimately, apoptosis.

Intracellular retention and degradation of the epidermal growth factor receptor, two distinct processes mediated by benzoquinone ansamycins

Epidermal growth factor (EGF) stimulates the growth of various types of cells via its cell surface tyrosine kinase receptor. The EGF receptor (EGF-R) has an oncogenic potential when overexpressed in a wide range of tumor cells. Geldanamycin (GA) and herbimycin (HA), specific inhibitors of the cytosolic chaperone HSP 90 and its endoplasmic reticulum homologue GRP 94, were shown to accelerate degradation of the EGF-R and of its homologue p185(c-)(erbB-2). Here we compared the effects of GA and HA on intracellular degradation and maturation of EGF-R. By using an inhibitor of proteasomal degradation, we learned that GA, but not HA, blocks processing of newly synthesized EGF-R. The effects of GA and HA on receptor degradation are mediated by the cytosolic portion of EGF-R and could be conferred to the erythropoietin receptor (EPO-R), by employing the respective chimera. Neither HA nor GA affected stability of newly synthesized EGF-R lacking the cytosolic domain (Ex EGF-R), but GA caused intracellular retention of this mutant. Taken together, our results imply that GA has two distinct targets of action on the EGF-R, one for promoting its degradation and another for mediating its intracellular retention. Apparently, degradation of the EGF-R mediated by GA or HA requires the presence of the EGF-R cytosolic domain, whereas intracellular retention in the presence of GA is coupled to the extracellular domain of the EGF-R.

Heat shock proteins--modulators of apoptosis in tumour cells

Apoptosis is a genetically programmed, physiological method of cell destruction. A variety of genes are now recognised as positive or negative regulators of this process. Expression of inducible heat shock proteins (hsp) is known to correlate with increased resistance to apoptosis induced by a range of diverse cytotoxic agents and has been implicated in chemotherapeutic resistance of tumours and carcinogenesis. Intensive research on apoptosis over the past number of years has provided significant insights into the mechanisms and molecular events that occur during this process. The modulatory effects of hsps on apoptosis are well documented, however, the mechanisms of hsp-mediated protection against apoptosis remain to be fully defined, although several hypotheses have been proposed. Elucidation of these mechanisms should reveal novel targets for manipulating the sensitivity of leukaemic cells to therapy. This review aims to explain the currently understood process of apoptosis and the effects of hsps on this process. Several proposed mechanisms for hsp protection against apoptosis and the therapeutic implications of hsps in leukaemia are also discussed.

Geldanamycin as a potential anti-cancer agent: its molecular target and biochemical activity

Heat shock protein 90 is one of the most abundant cellular proteins. Although its functions are still being characterized, it appears to serve as a chaperone for a growing list of cell signaling proteins, including many tyrosine and serine/threonine kinases, involved in proliferation and/or survival. The benzoquinone ansamycin geldanamycin has been shown to bind to Hsp90 and to specifically inhibit this chaperone's function, resulting in client protein destabilization. Its ability to simultaneously stimulate depletion of multiple oncogenic proteins suggests that geldanamycin, or other molecules capable of targeting Hsp90 in cancer cells, may be of clinical benefit.

Effects of geldanamycin, a heat-shock protein 90-binding agent, on T cell function and T cell nonreceptor protein tyrosine kinases

The benzoquinoid ansamycins geldanamycin (GA), herbimycin, and their derivatives are emerging as novel therapeutic agents that act by inhibiting the 90-kDa heat-shock protein hsp90. We report that GA inhibits the proliferation of mitogen-activated T cells. GA is actively toxic to both resting and activated T cells; activated T cells appear to be especially vulnerable. The mechanism by which GA acts is reflected by its effects on an essential hsp90-dependent protein, the T cell-specific nonreceptor tyrosine kinase lck. GA treatment depletes lck levels in cultured T cells by a kinetically slow dose-dependent process. Pulse-chase analyses indicate that GA induces the very rapid degradation of newly synthesized lck molecules. GA also induces a slower degradation of mature lck populations. These results correlate with global losses in protein tyrosine kinase activity and an inability to respond to TCR stimuli, but the activity of mature lck is not immediately compromised. Although the specific proteasome inhibitor lactacystin provides marginal protection against GA-induced lck depletion, proteasome inhibition also induces changes in lck detergent solubility independent of GA application. There is no other evidence for the involvement of the proteosome. Lysosome inhibition provides quantitatively superior protection against degradation. These results indicate that pharmacologic inhibition of hsp90 chaperone function may represent a novel immunosuppressant strategy, and elaborate on the appropriate context in which to interpret losses of lck as a reporter for the pharmacology of GA in whole organisms.

Disruption of Raf-1/heat shock protein 90 complex and Raf signaling by dexamethasone in mast cells

Antigen stimulation of mast cells via the IgE receptor, FcepsilonRI, results in the recruitment of the cytosolic tyrosine kinase, Syk, and the activation of various signaling cascades. One of these, the extracellular signal-regulated kinase (ERK2) cascade, is inhibited by low concentrations of the immunosuppressant drug, dexamethasone, probably at a step prior to the activation of Raf-1 (Rider, L. G., Hirasawa, N., Santini, F., and Beaven, M. A. (1996) J. Immunol. 157, 2374-2380). We now show that treatment of cultured RBL-2H3 mast cells with nanomolar concentrations of dexamethasone causes dissociation of the Raf-1.heat shock protein 90 (Hsp90) complex. Raf-1 bereft of this protein fails to associate with the membrane or Ras in antigen-stimulated cells. Upstream events such as the Syk-dependent phosphorylation of Shc, the engagement of Shc with the adapter protein, Grb2, and the activation of Ras itself are unaffected. Interestingly, the counterpart of Raf-1 in the c-Jun N-terminal kinase (JNK) cascade, MEKK-1 (mitogen-activated protein kinase/ERK kinase), is similarly associated with Hsp90, and this association as well as the activation of MEKK-1 are disrupted by dexamethasone treatment. Disruption of the ERK and JNK cascades at the level of Raf-1 and MEKK-1 could account for the inhibitory action of dexamethasone on the generation of inflammatory mediators in stimulated mast cells.

The hsp90-related protein TRAP1 is a mitochondrial protein with distinct functional properties

The hsp90 family of molecular chaperones was expanded recently due to the cloning of TRAP1 and hsp75 by yeast two-hybrid screens. Careful analysis of the human TRAP1 and hsp75 sequences revealed that they are identical, and we have cloned a similar protein from Drosophila. Immunofluorescence data show that human TRAP1 is localized to mitochondria. This mitochondrial localization is supported by the existence of mitochondrial localization sequences in the amino termini of both the human and Drosophila proteins. Due to the striking homology of TRAP1 to hsp90, we tested the ability of TRAP1 to function as an hsp90-like chaperone. TRAP1 did not form stable complexes with the classic hsp90 co-chaperones p23 and Hop (p60). Consistent with these observations, TRAP1 had no effect on the hsp90-dependent reconstitution of hormone binding to the progesterone receptor in vitro, nor could it substitute for hsp90 to promote maturation of the receptor to its hormone-binding state. However, TRAP1 is sufficiently conserved with hsp90 such that it bound ATP, and this binding was sensitive to the hsp90 inhibitor geldanamycin. In addition, TRAP1 exhibited ATPase activity that was inhibited by both geldanamycin and radicicol. Thus, TRAP1 has functions that are distinct from those of hsp90.

A Ras-dependent chloride current activated by adrenocorticotropin in rat adrenal zona glomerulosa cells

In the present study, we report that ACTH induces a transient chloride current. The lack of correlation between ACTH-induced cAMP production and amplitude of the Cl- current, as well as the absence of stimulation by forskolin or 8Br-cAMP indicated that the ACTH-induced current was not cAMP-dependent. We explored the possibility that one or several elements of the Ras/Raf MAPK cascade were involved. Indeed, we found that ACTH at 10(-10) M induced activation of Ras. Inhibition of the current by QEHA peptide, a Gbetagamma sequestrant, demonstrated that Gbetagamma subunits transduced the message. Blockage of the Ras activation using an inhibitor of farnesyl transferase (BZA-5B) or the monoclonal antibody H-Ras(259) abrogated the current. Moreover, the addition of Ras-GTPyS in the pipette medium gave rise to the Cl- current. Treatment of the cells with BZA decreased the aldosterone secretion induced by 10(-10) M ACTH but not that induced by 10(-8) M ACTH, confirming the involvement of Ras in steroid secretion. We conclude that ACTH triggers a Cl- current through the activation of the Ras protein by Gbetagamma subunits. This current, activated at physiological ACTH concentrations (1 to 100 pM) where cAMP production is very low, could play a significant role in aldosterone production.

The Hsp90-specific inhibitor geldanamycin selectively disrupts kinase-mediated signaling events of T-lymphocyte activation

The 90-kDa heat shock protein (Hsp90) is the most abundant molecular chaperone of eukaryotic cells. Its chaperone function in folding nascent proteins seems to be restricted to a subset of proteins including major components of signal transduction pathways (eg, nuclear hormone receptors, transcription factors, and protein kinases). Improper function of these proteins can be induced by selective disruption of their complexes with Hsp90 using the benzoquinonoid ansamycin geldanamycin. In this study, we demonstrate that geldanamycin treatment blocks interleukin (IL)-2 secretion, IL-2 receptor expression, and proliferation of stimulated T-lymphocytes. Moreover, geldanamycin decreases the amount and phosphorylation of Lck and Raf-1 kinases and prevents activation of the extracellular signal regulated kinase (ERK)-2 kinase. Geldanamycin also disrupts the T-cell receptor-mediated activation of nuclear factor of activated T-cells (NF-AT). Treatment with geldanamycin, however, does not affect the activation of lysophosphatide acyltransferase, which is a plasma membrane enzyme coupled to the T-cell receptor after T-cell stimulation. Through demonstrating the selective inhibition of kinase-related T-lymphocyte responses by geldanamycin, our results emphasize the substantial role of Hsp90-kinase complexes in T-cell activation.

The Hsp90-specific inhibitor geldanamycin selectively disrupts kinase-mediated signaling events of T-lymphocyte activation

The 90-kDa heat shock protein (Hsp90) is the most abundant molecular chaperone of eukaryotic cells. Its chaperone function in folding nascent proteins seems to be restricted to a subset of proteins including major components of signal transduction pathways (eg, nuclear hormone receptors, transcription factors, and protein kinases). Improper function of these proteins can be induced by selective disruption of their complexes with Hsp90 using the benzoquinonoid ansamycin geldanamycin. In this study, we demonstrate that geldanamycin treatment blocks interleukin (IL)-2 secretion, IL-2 receptor expression, and proliferation of stimulated T-lymphocytes. Moreover, geldanamycin decreases the amount and phosphorylation of Lck and Raf-1 kinases and prevents activation of the extracellular signal regulated kinase (ERK)-2 kinase. Geldanamycin also disrupts the T-cell receptor-mediated activation of nuclear factor of activated T-cells (NF-AT). Treatment with geldanamycin, however, does not affect the activation of lysophosphatide acyltransferase, which is a plasma membrane enzyme coupled to the T-cell receptor after T-cell stimulation. Through demonstrating the selective inhibition of kinase-related T-lymphocyte responses by geldanamycin, our results emphasize the substantial role of Hsp90-kinase complexes in T-cell activation.

Cyclic stretch up-regulates proliferation and heat shock protein 90 expression in human melanocytes

Human skin is repeatedly exposed to mechanical stretching in vivo, but in an ordinary culture of skin cells this prominent feature has been neglected. In order to study whether mechanical stretching plays a role for human melanocytes, we have established a culture technique to mimic this physical stretching: primary cultures of human melanocytes were plated on silicon supports, which undergo a stretching of about 10% of the initial length. After application of repeated stretching and relaxation for 4 days, cell count was significantly (about 40%) enhanced. In addition, we found approximately 2-fold increase in heat shock protein (HSP) 90, both at the protein and mRNA level. HSP 90 is known to bind to Raf-1 and, therefore, may contribute to the Raf-1-MEK (mitogen-activated protein-kinase kinase)-MAPK (mitogen-activated protein-kinase) signaling pathway. Disruption of the Raf-1-HSP 90 multimolecular complex by geldanamycin lead to a considerable decrease in melanocyte cell count. However, geldanamycin did not reverse the stretch-induced growth stimulation. Therefore, the stretch-mediated up-regulation of HSP 90 expression in melanocytes appears to be independent of stretch-mediated growth stimulation. These findings have strong implications for the in vitro cultivation of melanocytes for transplantation purposes.

Hsp90 as an anti-cancer target

Heat shock protein 90 is one of the most abundant cellular proteins. Although its functions are still being characterized, it appears to serve as a chaperone for a growing list of cell signaling proteins, including many tyrosine and serine/threonine kinases, involved in cell proliferation and/or survival. The recent discovery of natural products which are able to inhibit Hsp90 function have allowed for both identification of its client proteins and for a better understanding of its role in their activity. Accumulating data have suggested that targeting Hsp90 in cancer cells may be of clinical benefit. Copyright 1999 Harcourt Publishers Ltd.

Non-transcriptional action of oestradiol and progestin triggers DNA synthesis

The recent findings that oestradiol and progestins activate the Src/Ras/Erks signalling pathway raise the question of the role of this stimulation. Microinjection experiments of human mammary cancer-derived cells (MCF-7 and T47D) with cDNA of catalytically inactive Src or anti-Ras antibody prove that Src and Ras are required for oestradiol and progestin-dependent progression of cells through the cell cycle. The antitumoral ansamycin antibiotic, geldanamycin, disrupts the steroid-induced Ras-Raf-1 association and prevents Raf-1 activation and steroid-induced DNA synthesis. Furthermore, the selective MEK 1 inhibitor, PD 98059, inhibits oestradiol and progestin stimulation of Erk-2 and the steroid-dependent S-phase entry. The MDA-MB231 cells, which do not express oestradiol receptor, fail to respond to oestradiol in terms of Erk-2 activation and S-phase entry. Fibroblasts are made equally oestradiol-responsive in terms of DNA synthesis by transient transfection with either the wild-type or the transcriptionally inactive mutant oestradiol receptor (HE241G). Co-transfection of catalytically inactive Src as well as treatment with PD98059 inhibit the oestradiol-dependent S-phase entry of fibroblasts expressing either the wild-type oestrogen receptor or its transcriptionally inactive mutant. The data presented support the view that non-transcriptional action of the two steroids plays a major role in cell cycle progression.

Antisense oligonucleotide inhibition of serine/threonine kinases: an innovative approach to cancer treatment

The identification of genes that confer a growth advantage on neoplastic cells and the understanding of the genetic mechanism(s) responsible for their activation have made possible a direct genetic approach to cancer treatment using nucleic acid therapeutics. Moreover, the ability to block the expression of individual genes that promote carcinogenesis provides a powerful tool to explore the molecular basis of normal growth regulation, as well as the opportunity for therapeutic intervention. One technique for turning off a single activated gene is the use of antisense oligodeoxynucleotides and their analogs for inhibition of gene expression. The serine/threonine kinases are involved in mediating intracellular responses to external signals, such as growth factors, hormones, and neurotransmitters, and are involved in cell proliferation and oncogenesis. Described herein are recent studies supporting the potential use of oligonucleotides targeting these kinases as chemotherapeutic agents for cancer treatment. The serine/threonine kinases included here are protein kinase A, protein kinase C, and c-raf-1 kinase.

p50(cdc37) acting in concert with Hsp90 is required for Raf-1 function

Genetic screens in Drosophila have identified p50(cdc37) to be an essential component of the sevenless receptor/mitogen-activated kinase protein (MAPK) signaling pathway, but neither the function nor the target of p50(cdc37) in this pathway has been defined. In this study, we examined the role of p50(cdc37) and its Hsp90 chaperone partner in Raf/Mek/MAPK signaling biochemically. We found that coexpression of wild-type p50(cdc37) with Raf-1 resulted in robust and dose-dependent activation of Raf-1 in Sf9 cells. In addition, p50(cdc37) greatly potentiated v-Src-mediated Raf-1 activation. Moreover, we found that p50(cdc37) is the primary determinant of Hsp90 recruitment to Raf-1. Overexpression of a p50(cdc37) mutant which is unable to recruit Hsp90 into the Raf-1 complex inhibited Raf-1 and MAPK activation by growth factors. Similarly, pretreatment with geldanamycin (GA), an Hsp90-specific inhibitor, prevented both the association of Raf-1 with the p50(cdc37)-Hsp90 heterodimer and Raf-1 kinase activation by serum. Activation of Raf-1 via baculovirus coexpression with oncogenic Src or Ras in Sf9 cells was also strongly inhibited by dominant negative p50(cdc37) or by GA. Thus, formation of a ternary Raf-1-p50(cdc37)-Hsp90 complex is crucial for Raf-1 activity and MAPK pathway signaling. These results provide the first biochemical evidence for the requirement of the p50(cdc37)-Hsp90 complex in protein kinase regulation and for Raf-1 function in particular.

Regulation of Hsp90 ATPase activity by tetratricopeptide repeat (TPR)-domain co-chaperones

The in vivo function of the heat shock protein 90 (Hsp90) molecular chaperone is dependent on the binding and hydrolysis of ATP, and on interactions with a variety of co-chaperones containing tetratricopeptide repeat (TPR) domains. We have now analysed the interaction of the yeast TPR-domain co-chaperones Sti1 and Cpr6 with yeast Hsp90 by isothermal titration calorimetry, circular dichroism spectroscopy and analytical ultracentrifugation, and determined the effect of their binding on the inherent ATPase activity of Hsp90. Sti1 and Cpr6 both bind with sub-micromolar affinity, with Sti1 binding accompanied by a large conformational change. Two co-chaperone molecules bind per Hsp90 dimer, and Sti1 itself is found to be a dimer in free solution. The inherent ATPase activity of Hsp90 is completely inhibited by binding of Sti1, but is not affected by Cpr6, although Cpr6 can reactivate the ATPase activity by displacing Sti1 from Hsp90. Bound Sti1 makes direct contact with, and blocks access to the ATP-binding site in the N-terminal domain of Hsp90. These results reveal an important role for TPR-domain co-chaperones as regulators of the ATPase activity of Hsp90, showing that the ATP-dependent step in Hsp90-mediated protein folding occurs after the binding of the folding client protein, and suggesting that ATP hydrolysis triggers client-protein release.

Different early effets of tyrphostin AG957 and geldanamycins on mitogen-activated protein kinase and p120cbl phosphorylation in anti CD-3-stimulated T-lymphoblasts

AG957, a tyrphostin tyrosine kinase inhibitor, has been shown previously to inhibit p210(bcr-abl) phosphorylation with concurrent inhibition of p210(bcr-abl)-expressing K562 cell growth (Kaur G and Sausville EA, Anticancer Drugs 7: 815-824, 1996). To assess the specificity of the action of AG957, we have examined its effect in another tyrosine kinase-mediated system, anti CD-3-stimulated Jurkat T Acute Lymphoblastic Leukemia cells. We also compared the effects of AG957 with those of geldanamycin, which can disrupt tyrosine kinase signaling through binding to heat shock protein (hsp90), and two geldanamycin analogs, 17-amino-17-demethoxygeldanamycin (17AG) and 17-allylamino-17-demethoxygeldanamycin (17AAG). At concentrations found to produce 90% inhibition of Jurkat T-cell growth, AG957 within 4 hr of addition inhibited mitogen-activated protein (MAP) kinase activation and activity, as shown by a decreased anti CD-3-stimulated erk-2 mobility shift in lysates of treated cells and a decrease in the stimulated myelin basic protein peptide kinase activity in erk-2 immunoprecipitates, respectively. AG957 did not inhibit this activity when added directly to immunoprecipitates. Effects in cells were found to be accompanied by a decrease in the anti CD-3-stimulated phosphorylation of p120cbl. Under conditions of a similar degree of growth inhibition, geldanamycin initially did not inhibit MAP kinase activation. Geldanamycin analogs did not decrease anti CD-3-induced cbl phosphorylation, but did reduce basal p120cbl tyrosine phosphorylation. The action of AG957 occurred with an apparent shift of several tyrosine-phosphorylated proteins to apparent higher molecular weights, which also did not occur with the geldanamycins. These results suggest that growth inhibition by AG957 can alter tyrosine kinase signaling systems unrelated to p210(bcr-abl) with a prominent early effect on MAP kinase activation in T-lymphoblasts. AG957 and geldanamycin affect tyrosine kinase signaling by distinct mechanisms.

Structural basis for inhibition of the Hsp90 molecular chaperone by the antitumor antibiotics radicicol and geldanamycin

The cellular activity of several regulatory and signal transduction proteins, which depend on the Hsp90 molecular chaperone for folding, is markedly decreased by geldanamycin and by radicicol (monorden). We now show that these unrelated compounds both bind to the N-terminal ATP/ADP-binding domain of Hsp90, with radicicol displaying nanomolar affinity, and both inhibit the inherent ATPase activity of Hsp90 which is essential for its function in vivo. Crystal structure determinations of Hsp90 N-terminal domain complexes with geldanamycin and radicicol identify key aspects of their nucleotide mimicry and suggest a rational basis for the design of novel antichaperone drugs.

Strict regulation of c-Raf kinase levels is required for early organogenesis of the vertebrate inner ear

Regulation of organogenesis involves a dynamic balance of the mechanisms regulating cell division, differentiation and death. Here we have investigated the pattern of expression of c-Raf kinase in the inner ear during early developmental stages and the consequences of manipulating c-Raf levels by misexpression of c-raf viral vectors in organotypic cultures of otic vesicle explants. We found that otic vesicles expressed c-Raf and its level remained constant during embryonic days 2 and 3 (E2-E3). c-Raf activity was increased in response to insulin like growth factor-I (IGF-I) and the activation by IGF-I of the c-Raf kinase pathway was a requirement to turn on cell proliferation in the otic vesicle. Overexpression of c-raf in E2.5 explants increased the proliferative response to low serum and IGF-I and blocked differentiation induced by retinoic acid. The increase in c-Raf levels also prevented nerve growth factor (NGF)-dependent induction of programmed cell death. Consistent with these results, the expression of a dominant negative c-Raf mutant potentiated retinoic acid action and decreased the rate of cell proliferation. We conclude that a strict control of c-Raf levels is essential for the co-ordination of the biological processes that operate simultaneously during early inner ear development.

Association of MTG8 (ETO/CDR), a leukemia-related protein, with serine/threonine protein kinases and heat shock protein HSP90 in human hematopoietic cell lines

A proto-oncogene, MTG8 (ETO/CDR), is disrupted in the t(8;21) translocation associated with acute myeloid leukemia, and the gene product, MTG8, is a phosphoprotein capable of cell transformation in concert with v-H-ras. To obtain insight into functional regulation of MTG8 by phosphorylation, we studied protein kinases that interact with, and phosphorylate, MTG8 in vitro. Recombinant MTG8 protein was first found to be associated with two serine/threonine protein kinases in cell extracts from both HEL cells and a leukemic cell line carrying t(8;21). A cytoplasmic protein kinase of 61 kDa (MTG8N-kinase) phosphorylated the amino-terminal of MTG8, and another of 52 kDa (MTG8C-kinase) phosphorylated the carboxyl-terminal domain. In addition, we demonstrated that heat shock protein 90 (HSP90) specifically binds to the amino-terminal domain of MTG8 in vitro and in vivo. Thus, our results shed new light on post-translational regulation of MTG8, perturbation of which, in AML1-MTG8 protein, probably contributes to leukemogenesis.

The benzoquinone ansamycin geldanamycin stimulates proteolytic degradation of focal adhesion kinase

FAK is a nonreceptor tyrosine kinase involved in adhesion-mediated signal transduction whose level of expression is related to the invasiveness of malignant tumors. In seeking strategies to downregulate FAK, we treated various cell lines in vitro with the benzoquinone ansamycin geldanamycin (GA) which was previously described as a tyrosine kinase inhibitor, but recently has been shown to exert its effects by interfering with the chaperone function of members of the hsp90 family of heat-shock proteins. We evaluated the effects of benzoquinone ansamycins on FAK steady-state protein level and FAK half-life in breast and prostate carcinoma, Ewing's sarcoma, and 3T3 fibroblasts. Our data demonstrate that GA stimulates the proteolytic degradation of FAK in all cell lines examined and markedly reduces the half-life of newly synthesized FAK protein without significantly altering the level of FAK mRNA. These data demonstrate FAK to be another tyrosine kinase sensitive to the destabilizing effects of benzoquinone ansamycins and further show that small molecule-mediated pharmacologic modulation of FAK protein level is a feasible approach to the interdiction of FAK function.

Geldanamycin provides posttreatment protection against glutamate-induced oxidative toxicity in a mouse hippocampal cell line

The benzoquinoid ansamycin geldanamycin interferes with many cell signaling pathways and is currently being evaluated as an anticancer agent. The main intracellular target of geldanamycin is the 90-kDa heat shock protein, hsp90. In this report we demonstrate that geldanamycin is effective at preventing glutamate-induced oxidative toxicity in the HT22 mouse hippocampal cell line, even if given 4 h after glutamate treatment. Geldanamycin prevents glutamate-induced internucleosomal DNA cleavage in the HT22 cells but does not reverse the depletion of glutathione levels brought about by glutamate treatment. Both anabolic and catabolic effects are generated by geldanamycin treatment of HT22 cells, as evidenced by the induction of hsp70 expression and degradation of c-Raf-1 protein, respectively. Thus, geldanamycin may provide an effective strategy for manipulating signaling pathways in neuronal cells that use hsp90 as they proceed through a programmed cell death pathway in response to oxidative stress.

The mitogen-activated protein kinase pathway mediates growth arrest or E1A-dependent apoptosis in SKBR3 human breast cancer cells

Previously, we have shown that phorbol ester (PMA) induces p21(WAF1/CIP1)-dependent growth arrest in SKBr3 breast cancer and LNCaP prostate cancer cells. Here, I demonstrate that inhibition of Raf-1 kinase by dominant-negative Raf-1 or pharmacological depletion of Raf-1 prevented PMA-mediated induction of p21(WAF1/CIP1). Similarly, PD98059, a specific inhibitor of MEK, abolished p21(WAF1/CIP1) induction and PMA-induced growth arrest. Like PMA, the H-ras oncogene, another activator of the Raf-1/MEK/MAPK pathway, transactivated p21(WAF1/CIP1) in SKBr3 cells. I further investigated PMA-induced growth arrest following infection of SKBr3 cells with 12S E1A-expressing adenovirus. Although high levels of E1A oncoprotein prevented both PMA-induced p21(WAF1/CIP1) and growth arrest, smaller amounts of E1A abrogated growth arrest without down-regulation of p21(WAF1/CIP1). Therefore, E1A can stimulate proliferation downstream of p21(WAF1/CIP1). Albeit less effective than full activity, either Rb- or p300-binding activity of E1A was sufficient for the abrogation of PMA-mediated growth arrest. E1A-driven proliferation of PMA-treated SKBr3 cells was accompanied by apoptosis. New therapeutic approaches can be envisioned that would utilize stimulation of the Raf-1/MEK/MAPK pathway to inhibit growth of PMA-sensitive cancer cells.

Hsp90 is required for pheromone signaling in yeast

The heat-shock protein 90 (Hsp90) is a cytosolic molecular chaperone that is highly abundant even at normal temperature. Specific functions for Hsp90 have been proposed based on the characterization of its interactions with certain transcription factors and kinases including Raf in vertebrates and flies. We therefore decided to address the role of Hsp90 for MAP kinase pathways in the budding yeast, an organism amenable to both genetic and biochemical analyses. We found that both basal and induced activities of the pheromone-signaling pathway depend on Hsp90. Signaling is defective in strains expressing low levels or point mutants of yeast Hsp90 (Hsp82), or human Hsp90beta instead of the wild-type protein. Ste11, a yeast equivalent of Raf, forms complexes with wild-type Hsp90 and depends on Hsp90 function for accumulation. For budding yeast, Ste11 represents the first identified endogenous "substrate" of Hsp90. Moreover, Hsp90 functions in steroid receptor and pheromone signaling can be genetically separated as the Hsp82 point mutant T525I and the human Hsp90beta are specifically defective for the former and the latter, respectively. These findings further corroborate the view that molecular chaperones must also be considered as transient or stable components of signal transduction pathways.

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.

Transforming growth factor-beta1-induced activation of the Raf-MEK-MAPK signaling pathway in rat lung fibroblasts via a PKC-dependent mechanism

In fibroblasts transforming growth factor-beta1 (TGF-beta1) regulates cell proliferation and turnover of macromolecular components of the extracellular matrix. Here, intracellular signaling events in growth-inhibited embryonic rat lung fibroblasts (RFL-6) upon stimulation with TGF-beta1 were investigated. TGF-beta1 rapidly induced the activation of c-Raf-1, MEK-1, and MAPK p42 and p44. The activation of this pathway by TGF-beta1 did not depend on autocrine platelet-derived growth factor (PDGF) or basic fibroblast growth factor (bFGF). Inhibition of the binding of growth factors to their tyrosine kinase receptors did not affect MAPK activation by TGF-beta1. Ras activation by TGF-beta1 was significantly lower compared to the activation by PDGF or bFGF. The intracellular transduction of the TGF-beta1 signal was completely suppressed by depletion or inhibition of protein kinase C (PKC). It is shown that calcium-dependent isoforms of PKC are required for MAPK activation by TGF-beta1.

The 90-kDa molecular chaperone family: structure, function, and clinical applications. A comprehensive review

The 90-kDa molecular chaperone family (which comprises, among other proteins, the 90-kDa heat-shock protein, hsp90 and the 94-kDa glucose-regulated protein, grp94, major molecular chaperones of the cytosol and of the endoplasmic reticulum, respectively) has become an increasingly active subject of research in the past couple of years. These ubiquitous, well-conserved proteins account for 1-2% of all cellular proteins in most cells. However, their precise function is still far from being elucidated. Their involvement in the aetiology of several autoimmune diseases, in various infections, in recognition of malignant cells, and in antigen-presentation already demonstrates the essential role they likely will play in clinical practice of the next decade. The present review summarizes our current knowledge about the cellular functions, expression, and clinical implications of the 90-kDa molecular chaperone family and some approaches for future research.

Recent Advances in the Study of hsp90 Structure and Mechanism of Action

The 90kDa heat shock protein, hsp90, is a major molecular chaperone of the cell that appears to have particular significance to cellular regulatory processes. New tools and approaches have revealed a number of target proteins for hsp90, most of which are protein kinases or transcription factors. While the mechanism of action of hsp90 is not well understood, reasonable models have emerged describing some functional domains of this protein, the importance of conformational transitions for its activity and its role within a multi-component chaperoning pathway of the cell.