Inhibition of transforming activity of tyrosine kinase oncogenes by herbimycin A

Geldanamycin, an hsp90/GRP94-binding drug, induces increased transcription of endoplasmic reticulum (ER) chaperones via the ER stress pathway

Geldanamycin, a benzoquinone ansamycin, binds specifically to hsp90 and GRP94 in vitro and in vivo. Treatment of cells with geldanamycin alters the molecular chaperone function of hsp90, and as a result, blocks certain cytosolic proteins from reaching their mature form, inhibits their activity, and/or affects their stability. In contrast, little is known about either the effects of geldanamycin on GRP94, the endoplasmic reticulum (ER) homologue of hsp90, or the role of GRP94 in protein folding. In this study, we demonstrate in a variety of cell lines that geldanamycin is a potent inducer of the cellular response to stress in the ER, resulting in the transcriptional up-regulation of ER chaperones and expression of the gadd153/CHOP transcription factor. Their induction occurs through the unfolded protein response pathway originating in the ER and is not due to effects of the drug on hsp90. Geldanamycin increases the association of nascent proteins with BiP, which indicates that their folding and/or assembly has been altered. These data suggest that GRP94 may play an essential role in the maturation of a number of secretory pathway proteins.

The Ah receptor is a sensitive target of geldanamycin-induced protein turnover

Geldanamycin (GA) binds directly to hsp90 and apparently disrupts certain hsp90 heterocomplexes. We have investigated the GA-hsp90 interaction and its effect on other associated proteins. Incubation of 2-[125I]-iodo-3-azido-7,8-dibromo-p-dioxin-labeled Hepa 1c1c7 cytosol with GA-coupled beads revealed a stable association of Ah receptor (AhR)/hsp90 complex with GA. In addition, sucrose gradient sedimentation analysis demonstrated that GA does not disrupt the 9S Ah receptor complex in vitro. HeLa and Hepa 1c1c7 cells were subjected to a dose-response and time-course treatment with GA and the level of the AhR was determined. A 75% depletion in AhR levels was observed within an hour of exposure to 100 nM GA. The relative stability of other proteins that associate with hsp90 was determined with the following rank order of sensitivity to GA exposure: AhR >> c-Raf-1 > glucocorticoid receptor > CDK4 >> p50. A series of hsp90 deletion mutants were used to map the domain that interacts with GA. Deletion of the first 221 amino acids in NH2-terminal domain resulted in loss of binding to solid-phase GA. Epitopes of monoclonal antibodies specific for hsp90 were also determined by direct immunoprecipitation with hsp90 mutants. Results indicated that monoclonal antibodies 8D3 and 3G3 interact with hsp90 via the first 221 amino acids in NH2-terminal region, whereas AC88 requires a COOH-terminal region between amino acids 661-677.

Geldanamycin-induced destabilization of Raf-1 involves the proteasome

The Raf-1-MEK-MAPK pathway plays an important role in transducing extracellular growth factor signaling into altered nuclear transcription factor function. The benzoquinone ansamycin Geldanamycin (GA) specifically binds to the heat shock protein HSP90 and alters its complex with Raf-1. This leads to a decrease in Raf-1 levels and to disruption of the Raf-1-MEK-MAPK signaling pathway. The enhanced degradation of Raf-1 protein was prevented by inhibitors of the proteasome, while inhibition of lysosomal or other proteases was ineffective. Raf-1 that was protected from GA-induced degradation was of higher molecular weight and showed a laddering pattern consistent with its polyubiquitination. Unlike Raf-1 in untreated cells, the protein was insoluble in Triton X100- or NP40-based buffers. Signaling through this pathway was inhibited by GA, concomitant with loss of Raf-1 protein, but was restored if Raf-1 was protected from GA-induced degradation by proteasome inhibitors.

Transcriptional regulation of N-acetylglucosaminyltransferase V by the src oncogene

Transformation of baby hamster kidney fibroblasts by the Rous sarcoma virus causes a significant increase in the GlcNAcbeta(1, 6)Man-branched oligosaccharides by elevating the activity and mRNA transcript levels encoding N-acetylglucosaminyltransferase V (GlcNAc-T V). Elevated activity and mRNA levels could be inhibited by blocking cell proliferation with herbimycin A, demonstrating that Src kinase activity can regulate GlcNAc-T V expression. 5' RACE analysis was used to identify a 3-kilobase 5'-untranslated region from GlcNAc-T V mRNA and locate a transcriptional start site in a 25-kilobase pair GlcNAc-T V human genomic clone. A 6-kilobase pair fragment of the 5' region of the gene contained AP-1 and PEA3/Ets binding elements and, when co-transfected with a src expression plasmid into HepG2 cells, conferred src-stimulated transcriptional enhancement upon a luciferase reporter gene. This stimulation by src could be antagonized by co-transfection with a dominant-negative mutant of the Raf kinase, suggesting the involvement of Ets transcription factors in the regulation of GlcNAc-T V gene expression. The src-responsive element was localized by 5' deletion analysis to a 250-base pair region containing two overlapping Ets sites. src stimulation of transcription from this region was inhibited by co-transfection with a dominant-negative mutant of Ets-2, demonstrating that the effects of the src kinase on GlcNAc-T V expression are dependent on Ets.

Identification of an ubiquitin-ligation system for the epidermal-growth-factor receptor--herbimycin A induces in vitro ubiquitination in rabbit-reticulocyte lysate

Some receptor tyrosine kinases such as the receptors for epidermal-growth factor (EGF) and platelet-derived growth factor undergo polyubiquitination as a consequence of ligand binding. The EGF receptor is also ubiquitinated by treatment with herbimycin A, an ansamycin antibiotic widely used as a tyrosine kinase inhibitor. To investigate the mechanism of the receptor ubiquitination, we have established an assay system in which herbimycin-A-induced ubiquitination processes can be analyzed in vitro. We now show that herbimycin A treatment of the purified EGF receptor induces polyubiquitination of the receptor in rabbit-reticulocyte lysate. Both DEAE unadsorbed material (fraction I) and high salt eluate (fraction II) of the reticulocyte lysate are involved cooperatively in the ubiquitination process, where the ubiquitin-conjugating enzyme UBC4 can functionally substitute for fraction I. A ubiquitin-protein ligase-like activity, partially purified from fraction II by DEAE anion-exchange chromatography, also functions in concert with UBC4. The precise mechanism of herbimycin A-induced ubiquitination of the EGF receptor is not fully understood, however, our present findings suggest that direct interaction with herbimycin A results in some modification of the receptor which is recognized by the ubiquitin-conjugating system in rabbit-reticulocyte lysate.

Mitochondrial proliferation and paradoxical membrane depolarization during terminal differentiation and apoptosis in a human colon carcinoma cell line

Herbimycin A, a tyrosine kinase inhibitor, induces cellular differentiation and delayed apoptosis in Colo-205 cells, a poorly differentiated human colon carcinoma cell line. Cell cycle analysis in conjunction with end labeling of DNA fragments revealed that G2 arrest preceded apoptotic cell death. Ultrastructural examination of herbimycin-treated cells demonstrated morphologic features of epithelial differentiation, including formation of a microvillar apical membrane and lateral desmosome adhesions. A marked accumulation of mitochondria was also observed. Fluorometric analysis using the mitochondrial probes nonyl-acridine orange and JC-1 confirmed a progressive increase in mitochondrial mass. However these cells also demonstrated a progressive decline in unit mitochondrial transmembrane potential (DeltaPsim) as determined by the DeltaPsim-sensitive fluorescent probes rhodamine 123 and JC-1 analyzed for red fluorescence. In concert with these mitochondrial changes, Colo-205 cells treated with herbimycin A produced increased levels of reactive oxygen species as evidenced by oxidation of both dichlorodihydrofluorescein diacetate and dihydroethidium. Cell-free assays for apoptosis using rat-liver nuclei and extracts of Colo-205 cells at 24 h showed that apoptotic activity of Colo-205 lysates requires the early action of mitochondria. Morphological and functional mitochondrial changes were observed at early time points, preceding cleavage of poly (ADP-ribose) polymerase. These results suggest that apoptosis in differentiated Colo-205 cells involves unrestrained mitochondrial proliferation and progressive membrane dysfunction, a novel mechanism in apoptosis.

The function of steroid hormone receptors is inhibited by the hsp90-specific compound geldanamycin

The ansamycin antibiotic geldanamycin, which specifically interacts with the heat shock protein hsp90, was used to study the function of hsp90 in steroid hormone receptors. We observed inhibition of glucocorticoid-specific gene induction in several responsive cell systems. Hormone binding abilities of receptors for glucocorticoid, progestin, androgen, and estrogen were inhibited upon exposing intact cells to geldanamycin. Inhibition was only seen when geldanamycin was applied to cell cultures under growth conditions or was present during in vitro synthesis; presynthesized receptors in cell extracts were not affected. Upon withdrawal of geldanamycin, glucocorticoid binding ability was regained; this was partially independent of de novo protein synthesis. Geldanamycin caused decreased levels of immunoreactive glucocorticoid receptors in wild-type cells with enhanced degradation occurring through the ubiquitin-proteasome pathway. Analysis of receptors from treated cells revealed a heteromeric structure of normal size in which the receptor polypeptide is complexed with normal amounts of hsp90 and the immunophilin p59. These data support the view that hsp90 actively participates in steroid-induced signal transduction, and they suggest that geldanamycin affects receptor action without disrupting hsp90-containing heterocomplexes per se. Nevertheless, complexes synthesized and assembled in vitro in the presence of geldanamycin differ from receptors of cellular origin.

Role of Tyr518 and Tyr519 in the regulation of catalytic activity and substrate phosphorylation by Syk protein-tyrosine kinase

The Syk protein-tyrosine kinase is expressed in many hematopoietic cells and is involved in signaling from various receptors for antigen and Fc portions of IgG and IgE. After cross-linking of these receptors, Syk is rapidly phosphorylated on tyrosine residues. We have previously reported that Syk expressed in COS cells is predominantly phosphorylated at both Tyr518 and Tyr519 at its putative autophosphorylation site. In this study, we have examined the role of each of these two residues for the catalytic activity of Syk in vitro and for the Syk-induced phosphorylation of cellular proteins in intact cells. Mutation of either residue had minor effects on the catalytic activity of Syk, and even the double mutant [F518, F519]Syk was about 60% as active as the wild-type enzyme. In intact cells, however, all three mutants consistently failed to induce the extensive tyrosine phosphorylation of cellular proteins typically observed with wild-type Syk. We have recently shown that the doubly phosphorylated Y518/Y519 site is also the site for association of Syk with the SH2 domain of the Lck kinase, which suggests that although phosphates at Y518/Y519 may enhance the catalytic activity of Syk, its interaction with Src family protein-tyrosine kinases is at least equally important for the induction of downstream substrate phosphorylation.

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

Differential effects of the tyrosine kinase inhibitors on collagen type 1-induced platelet aggregation and adhesion to this protein

Herbimycin A, lavendustin A, and methyl 2,5-dihydroxycinnamate were used to study the role of protein tyrosine kinases in collagen-platelet interaction. All three compounds produced a concentration dependent inhibition of platelet aggregation induced by collagen type I, characterized by values of IC50 equaled to 0.9, 10.0, and 5.0 microM, respectively. This effect was accompanied by strong inhibition of phosphorylation of p125FAK, p90, p72syk, p60c-arc, and p56lyn. In the absence of the inhibitors, phosphorylation of these proteins is evoked by aggregation of platelets. In addition to the antiaggregatory effect, the tyrosine kinase inhibitors reduced adhesion of platelets to collagen although to much lower extent than aggregation. Platelets which adhered to collagen showed also the presence of phosphorylated p125FAK, p90, p72syk, p60c-arc, and p56lyn. Of these proteins, the extent of phosphorylation of p90 was particularly high. Adhesion of platelets was associated with inhibition of phosphorylation of p125FAK, p60c-arc, and p56lyn only when high concentration of lavendustin A and methyl 2,5-dihydroxycinnamate were used. Herbimycin A did not affect adhesion-evoked protein tyrosine phosphorylation. Phosphorylation of p90 and p72syk was not affected by inhibitors. This study indicates that collagen type I can induce different transmembrane signalling dependent upon whether platelet aggregates formation or adhesion of platelets to this protein occurs.

Modulation of cyclic AMP levels in a clonal neural cell line by inhibitors of tyrosine phosphorylation

The convergence of tyrosine kinase and cyclic AMP (cAMP) signal transduction pathways was investigated in the HT4.7 neural cell line with inhibitors of tyrosine kinases and tyrosine phosphatases. The protein tyrosine kinase inhibitor genistein inhibited isoproterenol-stimulated cAMP production by 40-60% in whole cells, with no effect on basal cAMP levels. In both whole cells and membranes, genistein also inhibited cAMP produced in response to direct stimulation of adenylyl cyclase with forskolin. However, in the absence of phosphodiesterase inhibitors, genistein presentation resulted in an increase in cAMP levels. Genistein inhibited phosphodiesterase activity by 80-85%, indicating that tyrosine phosphorylation stimulates both cAMP synthesis and degradation. The decrease in cAMP levels by genistein was not merely competitive inhibition of adenylyl cyclase with respect to ATP, since the Km of adenylyl cyclase for ATP remained essentially the same in either the presence or the absence of genistein. Another tyrosine kinase inhibitor, herbimycin A, which inhibits by a different mechanism than genistein, also decreased forskolin-stimulated cAMP in whole cells. As would be expected for the involvement of tyrosine phosphorylation in the control of cAMP production, inhibition of tyrosine phosphatases by vandate increased forskolin-stimulated cAMP production. These results suggest that cAMP production can be regulated by tyrosine phosphorylation, and the simultaneous activation of both cAMP synthesis and degradation may serve to alter the duration of cAMP elevation.

Regulation of glucocorticoid receptor by the tyrosine kinase inhibitor herbimycin A in the cytosolic fraction of primary cultured rat hepatocytes

The participation of tyrosine kinase in the regulation of the glucocorticoid receptor (GR) was studied in primary cultured rat hepatocytes using the tyrosine kinase inhibitor herbimycin A. Herbimycin A decreased the number of high-affinity binding sites of glucocorticoids in the cytosolic fraction and increased the equilibrium dissociation constant (Kd). Western blot analysis revealed that it also decreased the amount of GR protein. On the other hand, cycloheximide did not affect the GR protein level. Although herbimycin A slightly increased the amount of GR protein in the nuclear fraction, the increase was much lower than that of its decrease in the cytosolic fraction. Therefore, the decrease of GR protein in the cytosolic fraction was not caused by the inhibition of GR protein synthesis nor the translocation of GR from cytosol to nuclei. As herbimycin A also suppressed the dexamethasone (Dex)-dependent induction of tyrosine aminotransferase (TAT) activity, the decrease of GR protein was followed by the suppression of the GR-mediated biological response. These findings indicate that tyrosine kinase is necessary for the maintenance of the level of GR protein and its affinity of binding sites in the cytosolic fraction. Our results also suggested that the increase of GR protein stability is the most probable explanation for the maintenance of its level.

The hsp90-binding antibiotic geldanamycin decreases Raf levels and epidermal growth factor signaling without disrupting formation of signaling complexes or reducing the specific enzymatic activity of Raf kinase

We have expressed the mitogenic signaling proteins Src, Ras, Raf-1, Mek (MAP kinase kinase), and Erk (MAP kinase) in baculovirus-infected Sf9 insect cells in order to study a potential role for the chaperone hsp90 in formation of multiprotein complexes. One such complex obtained by immunoadsorption with anti-Ras antibody of cytosol prepared from cells simultaneously expressing Ras, Raf, Mek, and Erk contained Ras, Raf, and Erk. To detect directly the protein-protein interactions involved in forming multiprotein complexes, we combined cytosols from single infections in vitro in all possible combinations of protein pairs. We detected complexes between Ras.Raf, Ras.Src, Raf.Mek, and Raf.Src, but no complex containing Erk was obtained by mixing cytosols. Thus, cellular factors appear to be required for assembly of the Erk-containing multiprotein complex. One cellular factor thought to be involved in signaling protein complex formation is the chaperone hsp90, and we show that Src, Raf, and Mek are each complexed with insect hsp90. Treatment of Sf9 cells with geldanamycin, a benzoquinone ansamycin that binds to hsp90 and disrupts its function, did not decrease coadsorption of either Raf or Erk with Ras, although it did decrease the level of cytosolic Raf. To study geldanamycin action, we treated rat 3Y1 fibroblasts expressing v-Raf and showed that the antibiotic blocked assembly of Raf.hsp90 complexes at an intermediate stage of assembly where Raf is still bound to the p60 and hsp70 components of the assembly mechanism. As in Sf9 cells, Raf levels decline with geldanamycin treatment of 3Y1 cells. To determine if geldanamycin affects mitogenic response, we treated HeLa cells with epidermal growth factor (EGF) and showed that geldanamycin treatment decreased EGF signaling and decreased the level of Raf protein without affecting the EGF-mediated increase in Raf kinase activity. We conclude that hsp90 is not required for forming complexes between the mitogenic signaling proteins or for Raf kinase activity and that EGF signaling is decreased indirectly by geldanamycin because the antibiotic increases degradation of Raf and perhaps other components of the signaling pathway.

Src tyrosine kinase activity in rat thecal-interstitial cells and mouse TM3 Leydig cells is positively associated with cAMP-specific phosphodiesterase activity

Phosphodiesterases (PDEs) play a critical role in the regulation of intracellular cyclic nucleotide concentration and, consequently, regulate the state of cellular differentiation. We have reported that the Src-selective tyrosine kinase inhibitor, herbimycin A, potentiates luteinizing hormone (LH)-stimulated cAMP accumulation in culture media by ovarian thecal-interstitial cells (TIC; see Taylor, C and Terranova, P.F. (1995) Lipopolysaccharide inhibits rat ovarian thecal-interstitial cell steroid secretion in vitro. Endocrinology 136, 5527-5532). The present study was conducted to investigate the effects of herbimycin, and changes in Src tyrosine kinase activity, on PDE activity in rat TIC an in the mouse TM3 Leydig cell line. Treatment of TIC with herbimycin (1 microM) for 24 h inhibited basal and LH-stimulated PDE activity (approximately 50 and 70%, respectively) and was associated with an increase in cAMP and progesterone accumulation in culture media. Treatment of TM3 cells with herbimycin inhibited PDE activity and increased cAMP accumulation in a dose- and time-dependent manner. TM3 cell cultures challenged with herbimycin had lower Src tyrosine kinase activity than controls (approximately 50%); however, protein kinase A activity was unaffected. TM3 cells stably transfected with a dominant negative Src tyrosine kinase (TM3Srck-) had lower PDE activity than cells transfected with a G418 resistance gene alone (TM3pSV2neo) which served as control cells. Conversely, TM3 cells expressing a temperature-sensitive Src kinase had significantly greater PDE activity at the Src active temperature (35 degrees C; the temperature at which the enzyme is active) than TM3pSV2neo control cells grown at the same temperature. TM3 cell lysates hydrolyzed minimal amounts of cGMP, indicating a cAMP-specific PDE. Phosphodiesterase activity in both TM3 and rat TIC was sensitive to the PDE4-selective inhibitor RO20-1724, indicating the predominant active enzyme is probably a member of the cAMP-specific PDE4 family. From the present data, we conclude that a tyrosine kinase of the Src family may play an important role in regulating phosphodiesterase activity in thecal and Leydig cells, and thus regulate intracellular cAMP and the state of cellular differentiation.

The role of the hsp90-based chaperone system in signal transduction by nuclear receptors and receptors signaling via MAP kinase

The multicomponent heat-shock protein (hsp) 90-based chaperone system is an ubiquitous protein-folding system in the cytoplasm of eukaryotes. Several signal transduction systems utilize an interaction with hsp90 as an essential component of the signaling pathway. The steroid and dioxin receptors are bound to hsp90 through their hormone-binding domains, and several of them must be bound to hsp90 in order to have a ligand-binding site. The binding of ligands to these receptors promotes their dissociation from hsp90, an event that is the first step in their signaling pathways. Several protein kinases, including the Src and Raf components of the MAP kinase system, are also bound to hsp90. Genetic studies in yeast have demonstrated that hsp90 is required for normal signaling via steroid and dioxin receptors and for the activity of Src in vivo. The hsp90-based chaperone system has been reconstituted from purified components, permitting detailed analysis of the molecular basis of the chaperone's role in signal transduction.

Pharmacologic shifting of a balance between protein refolding and degradation mediated by Hsp90

The role of the abundant stress protein Hsp90 in protecting cells against stress-induced damage is not well understood. The recent discovery that a class of ansamycin antibiotics bind specifically to Hsp90 allowed us to address this problem from a new angle. We find that mammalian Hsp90, in cooperation with Hsp70, p60, and other factors, mediates the ATP-dependent refolding of heat-denatured proteins, such as firefly luciferase. Failure to refold results in proteolysis. The ansamycins inhibit refolding, both in vivo and in a cell extract, by preventing normal dissociation of Hsp90 from luciferase, causing its enhanced degradation. This mechanism also explains the ansamycin-induced proteolysis of several protooncogenic protein kinases, such as Raf-1, which interact with Hsp90. We propose that Hsp90 is part of a quality control system that facilitates protein refolding or degradation during recovery from stress. This function is used by a limited set of signal transduction molecules for their folding and regulation under nonstress conditions. The ansamycins shift the mode of Hsp90 from refolding to degradation, and this effect is probably amplified for specific Hsp90 substrates.

Tyrosine kinase inhibitors alter composition of nicotinic receptors on neurons

Protein tyrosine kinase (PTK) inhibitors were used to examine the roles of tyrosine phosphorylation in synaptic function. We show here that two different PTK inhibitors, herbimycin A and lavendustin A, both selectively downregulate a subpopulation of nicotinic acetylcholine receptors (AChRs) on chick ciliary ganglion neurons in culture. The downregulation requires a number of hours to occur and involves only those receptors containing the alpha 3, alpha 5, and beta 4 gene products. Not affected are AchRs that additionally contain the beta 2 gene product or AchRs that are made up of the alpha 7 gene product. The downregulation preferentially targets receptors destined for the cell surface and has little effect on the large pool of intracellular receptors. The receptor loss is not additive with that seen in the presence of either cycloheximide or tunicamycin, two compounds that the block appearance of new receptors. The downregulation induced by herbimycin A in surface receptors is accompanied by a specific decrement in the amount of alpha 3 protein in the cells. The results indicate that PTKs, either by phosphorylating AChR gene products directly or by acting through intermediary proteins, regulate the size and composition of the AChR pool maintained on the cell surface. Receptor regulation by PTKs may provide a mechanism for long-term control of synaptic signaling between neurons.

Tyrosine kinase and cAMP-dependent protein kinase activities in CD40-activated human B lymphocytes

In vitro, human B lymphocytes undergo long-term proliferation when activated through CD40, a protein expressed on their cell surface. The nature of CD40-dependent signals in proliferating fresh human Epstein-Barr virus-negative B lymphocytes is currently unknown. In this study, a CD40-dependent B cell culture system was used to examine the role of different signal transduction elements. Protein kinase C (PKC) depletion generated by a long-term phorbol 12 myristate 13-acetate treatment had weak effects on proliferation. Rather, tyrosine phosphorylation was shown to be directly involved in mediating CD40-dependent signals. The use of the protein tyrosine kinase (PTK)-specific inhibitor herbimycin A dramatically decreased cellular proliferation without altering the activity of the human immunodeficiency virus-1 long terminal repeat (HIV-1 LTR), a promoter largely dependent on the binding of nuclear factor kappa B (NF- kappa B). In contrast, the cAMP-dependent protein kinase specific inhibitor H-89 totally inhibited HIV-1 LTR activity at a concentration as low as 100 nM without affecting cellular proliferation. Electrophoretic mobility shift assay (EMSA) and supershift assay using an NF-kappa B binding sequence from the kappa light chain as a probe, revealed that both p65 (RelA) and c-Rel were present in CD40-stimulated B cells. While PKC depletion did not alter the NF-kappa B level, treatment of B lymphocytes with H-89 or herbimycin A provoked a decrease in the NF-kappa B level. These observations establish the importance of different signal transducing pathways leading to CD40 activation of B lymphocytes.

Polyubiquitination and proteasomal degradation of the p185c-erbB-2 receptor protein-tyrosine kinase induced by geldanamycin

Treatment of SKBr3 human breast carcinoma cells with the benzoquinoid ansamycin, geldanamycin, rapidly depletes p185c-erbB-2 protein-tyrosine kinase. Loss of p185c-erbB-2 is initiated by disruption of a heteromeric complex between p185c-erbB-2 and the 94-kDa glucose-regulated protein, GRP94, to which geldanamycin binds avidly. Here we report that within minutes of exposure to geldanamycin, mature p185c-erbB-2 becomes polyubiquitinated. Treatment of cells with the specific proteasome proteolytic inhibitor, lactacystin, blocked geldanamycin-induced degradation of p185c-erbB-2 and enhanced the accumulation of polyubiquitinated p185c-erbB-2. Following geldanamycin and lactacystin treatment, a higher molecular weight form of p185c-erbB-2, which likely represents ubiquitin-p185c-erbB-2 conjugates, was detected by anti-p185c-erbB-2 immunoblotting. Nascent p185c-erbB-2 synthesized in the presence of geldanamycin is incompletely glycosylated and remains sequestered in the endoplasmic reticulum. While this immature form of the protein is not ubiquitinated in the presence of geldanamycin, its marked, drug-induced instability is nonetheless antagonized by lactacystin. Thus, the rapid depletion of mature p185c-erbB-2 caused by geldanamycin and the marked, drug-stimulated decrease in half-life of the newly synthesized protein are both mediated by the proteasome, although only the former phenomenon involves polyubiquitination.

C-Met signalling in an HGF/SF-insensitive variant MDCK cell line with constitutive motile/invasive behaviour

The Met protein is a receptor tyrosine kinase for hepatocyte growth factor/scatter factor (HGF/SF), a multifunctional growth factor with mitogenic, motogenic and morphogenic properties. A morphologically altered variant of the MDCK cell line, MDCK-1, spontaneously exhibits a number of features associated with a partial HGF/SF-Met induced phenotype (less adhesive colonies in culture, enhanced invasion and motility, nascent tubule formation), but paradoxically does not respond to HGF/SF treatment. Although the overall cell surface expression and distribution of Met were found to be similar in parental MDCK cells and the MDCK-1 cell line, p145met autophosphorylation (+/ HGF/SF) was significantly reduced in MDCK-1 cells in vitro and in vivo when compared with parental MDCK cells. In contrast, EGF induced cell proliferation and EGF receptor autophosphorylation to similar levels in both cell lines. The basal levels of protein tyrosine phosphorylation were higher in MDCK-1 cells when compared with parental MDCK cells, including that of two prominent proteins with molecular masses of approximately 185 kDa and 220 kDa. Moreover, both p185 and p220 are present and tyrosine phosphorylated in Met immunoprecipitates from MDCK-1 cells (+/-HGF/SF), but not parental MDCK cells. In addition, Met immunocomplexes from MDCK-1 cells exhibited an approximately 3-fold increased tyrosine kinase activity in vitro when compared with MDCK cells, correlating with the higher basal levels of total phosphotyrosine. Treatment of MDCK-1 cells with the tyrosine kinase inhibitor herbimycin A reverted the cell phenotype to a more MDCK-like morphology in culture, with a concomitant reduction in the tyrosine phosphorylation predominantly of p220. Taken together these data suggest that aberrations in Met activity and associated signalling render MDCK-1 cells insensitive to HGF/SF, and may also mediate alterations in MDCK-1 cell behaviour.

Retro-retinoids in regulated cell growth and death

Vitamin A serves as a prohormone from which three classes of active metabolites are derived: the aldehydes, the carboxylic acids, and the retro-retinoids. Although these three classes are united under the rubric of signal transduction, they act by different molecular mechanisms: the 11-cis-retinaldehydes combine with opsin to form the universal visual pigments and the retinoic acids form ligands for transcription factors, whereas the retro-retinoids, as shown here, intersect with signal transduction at a cytoplasmic or membrane site. The retro-retinoid, anhydroretinol (AR), has long been known to act as a growth inhibitor in lymphocytes, whereas 14-hydroxy-4,14-retro-retinol (14-HRR) is required for normal lymphocyte proliferation. A mutually reversible relationship exists between these two retro-retinoids as one can reverse the effects of the other when given in pharmacological doses. The common explanation for reversible inhibition is competition for a shared receptor. We now provide evidence that when AR is given to T cells unmitigated by 14-HRR, rapid cell death can occur. The circumstances are closely related to nonclassical forms of apoptosis: within 2 h of AR administration the T cells undergo widespread morphological changes, notably surface blebbing and ballooning and, inevitably, bursting. In contrast, nuclear changes are comparatively mild, as indicated by absence of chromatin condensation and overt DNA cleavage to discrete nucleosomal fragments, although DNA nicks are readily discernible by terminal deoxynucleotidyl transferase assay. What further distinguishes the AR-induced form of apoptosis from classical ones is a lack of requirements of messenger RNA and protein synthesis, suggesting that the events leading to cell death are primarily initiated and play themselves out in the cytoplasm. This view is further reinforced by the finding that herbimycin A can prevent the onset of programmed cell death. The importance of our findings is that they strongly suggest a second messenger role for vitamin A metabolites in the cytoplasmic realm that has not been seen previously. These findings are entirely compatible with a general notion that in a cell requiring multiple coordinated signals for survival, the provision of an unbalanced signal can initiate programmed cell death. Collectively, our data also challenge the paradigm that retinoids (outside vision) solely mediate their function via the steroid/ retinoic acid receptor family of nuclear transcription factors. Instead, a mode of action in the cytoplasmic realm akin to one attributed to other small lipophilic second messenger molecules, such as diacyl glycerol or ceramide, may apply to retro-retinoids.

Induction of c-jun protooncogene expression by hydrogen peroxide through hydroxyl radical generation and p60SRC tyrosine kinase activation

The mechanisms of signal transduction of c-jun induction by hydrogen peroxide are elucidated in NIH3T3 cells by using trapping agents of hydroxyl free radical or inhibitors of various protein kinases. Pre-treatment of the cell with hydroxyl radical scavenger dimethyl sulfoxide (DMSO) abolishes the H2O2-induced c-jun expression. Hydroxyl radical generation can be detected and quantified in cells treated sequentially with DMSO and H2O2 for 30 min respectively by methane sulfinic acid (MSA) production, especially that from particulate fraction. Induction of c-jun by H2O2 is also dramatically reduced by pretreating the cells with biological antioxidant vit. E. Protein tyrosine kinase activity of membrane fraction is induced by H2O2 within 5 to 10 min, which can be prevented by DMSO pre-treatment. Inhibitor of non-receptor type tyrosine kinase, herbimycin A, has inhibitory effect on H2O2-induced c-jun expression while the inhibitor of receptor type tyrosine kinase, tyrphostin 23 or inhibitor of cyclic AMP dependent protein kinase, KT 5720, has not. TPA pre-treatment that depletes protein kinase C (PKC) has no influence on the c-jun induction by H2O2. Our results suggest that the highly reactive species HO is generated after H2O2 enter cells and mediate the signal responses of H2O2 including c-jun induction and the activation of p60src tyrosine kinase might be one of the molecular events associated with the c-jun induction pathway.

Ornithine decarboxylase- and ras-induced cell transformations: reversal by protein tyrosine kinase inhibitors and role of pp130CAS

We have found that overexpression of human ornithine decarboxylase (ODC) induces cell transformation in NIH 3T3 and Rat-1 cells (M. Auvinen, A. Paasinen, L. C. Andersson, and E. Hölttä, Nature (London) 360:355-358, 1992). The ODC-transformed cells display increased levels of tyrosine phosphorylation, in particular of a cluster of 130-kDa proteins. Here we show that one of the proteins with enhanced levels of tyrosine phosphorylation in ODC-overexpressing cells is the previously described p130 substrate of pp60v-src, known to associate also with v-Crk and designated p130CAS. We also studied the role of protein tyrosine phosphorylation in the ODC-induced cell transformation by exposing the cells to herbimycin A, a potent inhibitor of Src-family kinases, and to other inhibitors of protein tyrosine kinases. Treatment with the inhibitors reversed the phenotype of ODC-transformed cells to normal, with an organized, filamentous actin cytoskeleton. Coincidentally, the tyrosine hyperphosphorylation of p130 was markedly reduced, while the level of activity of ODC remained highly elevated. A similar reduction in pp130 phosphorylation and reversion of morphology by herbimycin A were observed in v-src- and c-Ha-ras-transformed cells. In addition, we show that expression of antisense mRNA for p130CAS resulted in reversion of the transformed phenotype of all these cell lines. An increased level of tyrosine kinase activity, not caused by c-Src or c-Abl, was further detected in the cytoplasmic fraction of ODC-transformed cells. Preliminary characteristics of this kinase are shown. These data indicate that p130CAS is involved in cell transformation by ODC, c-ras, and v-src oncogenes, raise the intriguing possibility that p130CAS may be generally required for transformation, and imply that there is at least one protein tyrosine kinase downstream of ODC that is instrumental for cell transformation.

Progesterone receptor structure and function altered by geldanamycin, an hsp90-binding agent

The assembly of progesterone receptor (PR) heterocomplexes in vitro involves at least eight components of the molecular chaperone machinery, and as earlier reports have shown, these proteins exhibit complex, dynamic, but ordered, interactions with one another and PR. Using the selective hsp90 binding agent geldanamycin (GA), we have found that PR assembly in vitro can be arrested at a previously observed intermediate assembly step. Like mature PR complexes, the intermediate complexes contain hsp90, but they differ from mature complexes by the presence of hsp70, p60, and p48 and the absence of immunophilins and p23. Arrest of PR assembly is likely due to GA's ability to directly block binding of p23 to hsp90. An important functional consequence of GA-mediated assembly arrest in vitro is the inability of the resulting PR complexes to bind progesterone, despite the presence of hsp90 in the receptor complexes. The biological significance of the in vitro observations is demonstrated by GA's ability to (i) rapidly block PR's hormone binding capacity in intact cells and (ii) alter the composition of COS cell PR complexes in a manner similar to that observed during in vitro reconstitutions. An updated model for the cyclic assembly pathway of PR complexes that incorporates the present findings with earlier results is presented.

Mycotrienins. A new class of potent inhibitors of osteoclastic bone resorption

Pharmacological intervention using selective tyrosine kinase inhibitors has been shown to be an effective approach to inhibit osteoclast function. Here, we report on the structure-activity relations of benzoquinone ansamycins isolated from Streptomyces rishirensis, which form a new class of potent inhibitors of osteoclast-mediated bone resorption. Parathyroid hormone-stimulated bone resorption was inhibited concentration dependently by both mycotrienin I and mycotrienin II, showing half-maximal inhibition in the low nanomolar range in fetal rat long bones in vitro. Structure-activity relation studies indicate that position 19 contained within the quinone/hydroquinone element and the double bonds in position 4, 6, and 8 are crucial for full bioactivity. In contrast, substitutions in position 22 are well tolerated. The lack of a similar effect of 2,6-dimethyl-p-benzoquinone and vitamin K signifies that the mechanism of action is not solely due to the oxygen scavenger capacity of the quinone/hydroquinone moiety. The inhibition of osteoclastic bone resorption is in line with the diminished activity of immunopurified pp60c-src from bone suggesting that pp60c-src is a possible target of mycotrienins in the organ culture. Thus, mycotrienins may be useful as pharmacologic inhibitors of osteoclastic bone resorption.

Herbimycin A induces the 20 S proteasome- and ubiquitin-dependent degradation of receptor tyrosine kinases

Herbimycin A is an ansamycin antibiotic isolated as an agent that reverses morphological transformation induced by v-src. Although herbimycin A is widely used as a tool for inhibiting multiple tyrosine protein kinases and tyrosine kinase-activated signal transduction, its mechanism of action is not well defined and includes a decrease in both tyrosine kinase protein levels and activity (Uehara, Y., Murakami, Y., Sugimoto, Y., and Mizuno, S. (1989) Cancer Res. 49, 780-785). We now show that herbimycin A induces a profound decrease in the total cellular activity of transmembrane tyrosine kinase receptors, such as insulin-like growth factor, insulin, and epidermal growth factor receptors. A substantial proportion of the in vivo inhibition could be explained by an increase in the rate of degradation. The enhanced degradation of insulin-like growth factor-insulin receptor was prevented by inhibitors of the 20S proteasome, whereas neither lysosomotropic agents nor general serine- and cysteine-protease inhibitors were active in preventing receptor degradation induced by herbimycin A. Moreover, in a temperature-sensitive mutant cell line defective in the E1-catalyzed activation of ubiquitin, herbimycin A treatment at the restrictive temperature did not result in the degradation of insulin receptor. These results suggest that herbimycin A represents a novel class of drug that targets the degradation of tyrosine kinases by the 20S proteasome. The ubiquitin dependence of this process indicates that this degradation of tyrosine kinases might involve the 20S proteasome as the proteolytic core of the ubiquitin-dependent 26S protease.

Thrombin induces the preproendothelin-1 gene in endothelial cells by a protein tyrosine kinase-linked mechanism

Thrombin stimulates synthesis and secretion of endothelin-1 (ET-1), a vasoactive peptide that triggers responses in the vascular endothelium and smooth muscle. We investigated the signal transduction pathways by which thrombin stimulates preproET-1 gene expression and ET-1 peptide secretion in macrovascular cells (human umbilical vein endothelial cells [HUVECs] and bovine pulmonary artery endothelial cells [BPAECs]) and microvascular cells (human microvascular endothelial cell line [HMEC-1]). Thrombin (4 U/mL) stimulated maximal induction of ET-1 peptide secretion and preproET-1 mRNA after 2 hours in HUVECs and BPAECs and after 1 hour in HMEC-1. A synthetic thrombin receptor activator peptide confirmed ligand-specific receptor actions to induce preproET-1 mRNA. Protein kinase C (PKC) activation by phorbol ester transiently induced preproET-1 mRNA but had no effect on ET-1 peptide synthesis. PKC inhibitors sangivamycin and calphostin C and PKC depletion failed to suppress thrombin-stimulated preproET-1 mRNA. Adenylate cyclase and cAMP-dependent protein kinase did not participate in thrombin-induced preproET-1 gene activation. Thrombin stimulated a rapid increase in phosphotyrosine-containing proteins, suggesting a role for tyrosine phosphorylation in thrombin signaling. These data demonstrate that thrombin induces the preproET-1 gene and ET-1 peptide synthesis by a PKC-independent PTK-dependent pathway in macrovascular and microvascular endothelial cells. Protein tyrosine kinase inhibitors herbimycin A and genistein blocked thrombin-stimulated preproET-1 mRNA and peptide secretion, whereas daidzein, which lacks inhibitory activity, did not suppress thrombin-induced ET-1.

Effect of epidermal growth factor on cadherin-mediated adhesion in a human oesophageal cancer cell line

Epidermal growth factor (EGF) mediates many pleiotrophic biological effects, one of which is alteration of cellular morphology. In the present study, we examine the possibility that this alteration in cell morphology is caused in part by the dysfunction of cadherin-mediated cell-cell adhesion using the human oesophageal cancer cell line TE-2R, which expresses E-cadherin and EGF receptor. In the presence of EGF, TE-2R changed its shape from round to fibroblastic and its colony formation from compact to sparse. Vanadate, a tyrosine phosphatase inhibitor, further potentiated the EGF response, whereas herbimycin A, a tyrosine kinase inhibitor, interfered with it. Moreover, EGF enabled the cells to invade in organotypic raft culture. These phenomena were accompanied not by decreased expression of the E-cadherin molecule but by a change in its localisation from the lateral adhesion site to the whole cell surface. Both alpha- and beta-catenin, cadherin-binding proteins, were also expressed at the same level throughout these morphological changes. Finally, we examined tyrosine phosphorylation of E-cadherin and alpha- and beta-catenin, and observed tyrosine phosphorylation of beta-catenin induced by EGF. These results suggest that EGF counteracts E-cadherin-mediated junctional assembly through phosphorylation of beta-catenin and modulates tumour cell behaviour to a more aggressive phenotype.

In vivo antitumor activity of herbimycin A, a tyrosine kinase inhibitor, targeted against BCR/ABL oncoprotein in mice bearing BCR/ABL-transfected cells

Herbimycin A, a benzoquinoid ansamycin antibiotic, has been shown to reverse the oncogenic phenotype of p60v-src transformed cells because of the inhibition of src protein tyrosine kinase. We previously demonstrated that herbimycin A displayed antitumor activity on the in vitro growth of Philadelphia chromosome-positive leukemia cells and BCR/ABL-transfected murine hematopoietic FDC-P2 cells through the inhibition of BCR/ABL protein tyrosine kinase. In this study, the transformed FDC-P2 cells were demonstrated to be tumorigenic in syngeneic DBA/2 mice. The intraperitoneal (i.p.) injection of the transformed tumor cells into DBA/2 mice induced infiltrations of abdominal organs, and then all of the mice died within time periods proportional to the cell numbers of inoculation. In mice that received an i.p. inoculation with greater than 1 x 10(5) cells, in vivo administration of herbimycin A by i.p. injection inhibited tumor formation and significantly prolonged survival time, and further, in mice inoculated with 1 x 10(4) cells, herbimycin A completely suppressed the in vivo growth of transformant FDC-P2 cells and brought about a complete remission. The present study revealed the in vivo efficacy of herbimycin A in mice bearing BCR/ABL-transfected cells.

Tyrosine protein kinase inhibition and cancer

The various aspects of the research on tyrosine protein kinase inhibition and its connections with cancer are presented. The emphasis was made on the theoretical low toxic side effects of specific tyrosine protein kinase inhibitors. Particularly, the strategy of finding peptidic substrate-derived inhibitors or modulators is discussed, with an almost complete compendium of the tyrosine protein kinase peptidic substrates published so far. A series of data has been gathered that may serve as a basis for the discovery of selective and specific tyrosine protein kinase inhibitors by screening on molecular and cellular models. The potential of SH2 domain-interfering agents are also presented as a promising route to new anticancer compounds.

Inhibition by two lavendustins of the tyrosine kinase activity of pp60F527 in vitro and in intact cells

The mutant pp60F527 protein possesses an activated protein-tyrosine kinase (PTK) activity correlated with a transforming activity. We have studied the inhibition of the pp60F527 PTK activity by two EGF-R tyrosine kinase inhibitors, lavendustin A and one of its derivatives, lavendustin C6. In vitro, both molecules were non-competitive inhibitors for the ATP binding site and uncompetitive inhibitors for the peptide binding site. The determined IC50S of the inhibition of pp60F527 kinase activity were 18 microM for lavendustin A and 5 microM for lavendustin C6, as determined on the exogenous substrate enolase, showing that lavendustin C6 was more potent than lavendustin A. Lavendustin C6, but not lavendustin A, inhibited the tyrosine phosphorylation of pp60F527 cellular substrates (the GAP-associated p190, pp125FAK and cortactin) in intact cells. However, this in situ inhibitory effect did not result in a reversion of the morphological changes induced by pp60F527 in cells. On the other hand, lavendustin C6 and lavendustin A exerted antiproliferative effects on cells, suggesting that inhibition of cellular targets related or not to the kinase was also possible.

Free radical formation by ansamycin benzoquinone in human breast tumor cells: implications for cytotoxicity and resistance

The benzoquinonoid ansamycin antibiotics, geldanamycin and herbimycin A, are potent cytotoxins against tumor cells in vitro. We have examined the mechanism of their in vitro cytotoxicity against human breast adenocarcinoma (MCF-7) cells and we have found that multidrug-resistant MCF-7/ADRR cells that exhibit the MDR phenotype and the overexpression of P-170-glycoprotein, were cross-resistant to geldanamycin and herbimycin A. Verapamil, which binds competitively with P-170-glycoprotein, enhanced geldanamycin cytotoxicity 12-fold only in resistant cells, suggesting that geldanamycin may interact with the drug efflux protein. Geldanamycin and herbimycin A, like adriamycin, were reductively activated by the NADPH-cytochrome P450-reductase and formed reactive .OH. The formation of .OH was significantly lower in resistant cells. In contrast to adriamycin, the formation of .OH was unaffected by the addition of DNA, indicating that a DNA-complexed drug was redoxactive and may, therefore, may be more effective in killing tumor cells at the DNA level. These observations indicate that both the decreased free radical formation and interactions with P170 glycoprotein may be important in geldanamycin and herbimycin A resistance in multidrug resistant human breast tumor cells.

Inhibition of heat shock protein HSP90-pp60v-src heteroprotein complex formation by benzoquinone ansamycins: essential role for stress proteins in oncogenic transformation

The molecular mechanisms by which oncogenic tyrosine kinases induce cellular transformation are unclear. Herbimycin A, geldanamycin, and certain other benzoquinone ansamycins display an unusual capacity to revert tyrosine kinase-induced oncogenic transformation. As an approach to the study of v-src-mediated transformation, we examined ansamycin action in transformed cells and found that drug-induced reversion could be achieved without direct inhibition of src phosphorylating activity. To identify mechanisms other than kinase inhibition for drug-mediated reversion, we prepared a solid phase-immobilized geldanamycin derivative and affinity precipitated the molecular targets with which the drug interacted. In a range of cell lines, immobilized geldanamycin bound elements of a major class of heat shock protein (HSP90) in a stable and pharmacologically specific manner. Consistent with these binding data, we found that soluble geldanamycin and herbimycin A inhibited specifically the formation of a previously described src-HSP90 heteroprotein complex. A related benzoquinone ansamycin that failed to revert transformed cells did not inhibit the formation of this complex. These results demonstrate that HSP participation in multimolecular complex formation is required for src-mediated transformation and can provide a target for drug modulation.

Conversion of epidermal growth factor (EGF) into a stimulatory ligand for A431-cell growth by herbimycin A by decreasing the level of expression of EGF receptor

We examined effect of the tyrosine kinase inhibitor herbimycin A on A431 human epidermoid carcinoma cells which over-express epidermal growth factor (EGF) receptor. Herbimycin A inhibited the autophosphorylation of EGF-stimulated receptors in intact cells both time- and dose-dependently. The inhibition was found to be due to a decrease in the level of expression of the receptor amount, because herbimycin A equally decreased the receptor quantity and EGF-stimulated receptor kinase activity in intact cells, but did not exhibit a direct inhibitory effect on EGF receptor kinase activity in vitro. The decrease of the level of EGF receptor was also confirmed by 125I-EGF binding to herbimycin A-treated cells, and Scatchard analysis showed that the decrease in the receptor number occurred in the major population of the low-affinity binding ones, whereas the number with high-affinity binding was unaffected. Interestingly, although the proliferation of A431 cells was inhibited by EGF, herbimycin A converted EGF into a stimulatory ligand for cell growth, as determined by both cell number and DNA synthesis. These findings indicated that herbimycin A decreased the level of expression of EGF receptor by a mechanism other than inactivation of the receptor kinase and reversed the transformed phenotype of A431 cells to a normal one in the proliferative response to EGF.

Accelerated degradation of 160 kDa epidermal growth factor (EGF) receptor precursor by the tyrosine kinase inhibitor herbimycin A in the endoplasmic reticulum of A431 human epidermoid carcinoma cells

The effect of herbimycin A on the biosynthesis of epidermal growth factor (EGF) receptor was examined in human epidermoid carcinoma A431 cells. Cells were pulse-labelled with [35S]methionine, and EGF receptor biosynthesis was quantified by immunoprecipitation using a monoclonal anti-(EGF receptor) antibody. In the presence of herbimycin A, an immature 160 kDa EGF receptor precursor accumulated in 1 h and disappeared completely in 4 h. Pulse-labelled 160 kDa receptor precursor in the absence of herbimycin A, however, was converted normally into a 170 kDa one by chase with herbimycin A. Herbimycin A affected neither the synthesis of the secreted form of EGF receptor devoid of cytoplasmic domain, nor that of the transferrin receptor in A431 cells. The herbimycin A-induced degradation of 160 kDa EGF receptor precursor was not inhibited by an inhibitor of lysosomal enzymes, NH4Cl. Endoglycosidase H digestion of the 160 kDa precursor converted it into the deglycosylated 130 kDa precursor peptide. These results suggested that herbimycin A selectively acted on the EGF receptor precursor during the synthesis of the 160 kDa form, probably on the cytoplasmic domain, to form an aberrant molecule which was subjected to rapid degradation in the endoplasmic reticulum.

Tyrosine kinase inhibitor-induced differentiation of K-562 cells: alterations of cell cycle and cell surface phenotype

Protein tyrosine kinase (PTK) inhibitor herbimycin A inhibited proliferation, induced accumulation of cells in the G0/G1 phase of the cell cycle and a marked increae of hemoglobin-producing human leukemic K-562 cells in vitro. The isoflavonoid PTK- and topoisomerase II inhibitor genistein produced a similar effect with the accumulation of cells in the G2/M phase of cell cycle. Genistein potentiated the effect of herbimycin A on the cell cycle (i.e. decreased the proportion of S-phase cells) and induced an increased proportion of hemoglobin-producing cells. Genistein, but not herbimycin A induced a marked increase in cell surface expression of CD15 (LewisX) antigen. Both of these agents down-regulated CD45 (leukocyte common antigen) and monocyte-associated CD14 antigen on K-562 cells. Neither genistein nor herbimycin A induced increased cell surface expression of glycophorin.

Signal transduction pathways regulating Rho-mediated stress fibre formation: requirement for a tyrosine kinase

Lysophosphatidic acid (LPA) and bombesin rapidly stimulate the formation of focal adhesions and actin stress fibres in serum-starved Swiss 3T3 fibroblasts, a process regulated by the small GTP binding protein Rho. To investigate further the signalling pathways leading to these responses, we have tested the roles of three intracellular signals known to be induced by LPA: activation of protein kinase C (PK-C), Ca2+ mobilization and decreased cAMP levels. Neither PK-C activation nor increased [Ca2+]i, alone or in combination, induced stress fibre formation, and in fact activators of PK-C inhibited this response to LPA and bombesin. The G(i)-mediated decrease in cAMP was not required for the response to LPA, and increased cAMP levels did not prevent stress fibre formation. In contrast, the tyrosine kinase inhibitor genistein inhibited the formation of stress fibres induced by both extracellular factors and microinjected Rho protein. Genistein also inhibited the Rho-dependent clustering of phosphotyrosine-containing proteins at focal adhesions, and the increased tyrosine phosphorylation of several proteins including pp125FAK, induced by LPA and bombesin. This suggests a model where Rho-induced activation of a tyrosine kinase is required for the formation of stress fibres.

Staurosporine inhibits agrin-induced acetylcholine receptor phosphorylation and aggregation

Agrin, a protein that mediates nerve-induced acetylcholine receptor (AChR) aggregation at developing neuromuscular junctions, has been shown to cause an increase in phosphorylation of the beta, gamma, and delta subunits of AChRs in cultured myotubes. As a step toward understanding the mechanism of agrin-induced AChR aggregation, we examined the effects of inhibitors of protein kinases on AChR aggregation and phosphorylation in chick myotubes in culture. Staurosporine, an antagonist of both protein serine and tyrosine kinases, blocked agrin-induced AChR aggregation in a dose-dependent manner; 50% inhibition occurred at approximately 2 nM. The extent of inhibition was independent of agrin concentration, suggesting an effect downstream of the interaction of agrin with its receptor. Staurosporine blocked agrin-induced phosphorylation of the AChR beta subunit, which occurs at least in part on tyrosine residues, but did not reduce phosphorylation of the gamma and delta subunits, which occurs on serine/threonine residues. Staurosporine also prevented the agrin-induced decrease in the rate at which AChRs are extracted from intact myotubes by mild detergents. H-7, an antagonist of protein serine kinases, inhibited agrin-induced phosphorylation of the gamma and delta subunits but did not block agrin-induced phosphorylation of the AChR beta subunit, AChR aggregation, or the decrease in AChR extractability. The results provide support for the hypothesis that tyrosine phosphorylation of the beta subunit plays a role in agrin-induced AChR aggregation.

Cell proliferation status, cytokine action and protein tyrosine phosphorylation modulate leukotriene biosynthesis in a basophil leukaemia and a mastocytoma cell line

Mast cells, mastocytoma cells and basophil leukaemia cells are well-established producers of leukotrienes when grown and stimulated appropriately. I report that the cells' ability to produce leukotrienes is dependent on the cells' proliferative status or their provision with growth factors. Proliferating MC/9 and subconfluent RBL2H3 cells respond maximally to stimulation by 1 microM ionomycin with the production of 56 and 32 pmol of cysteinyl-leukotrienes/10(6) cells respectively. In contrast, confluent RBL2H3 or growth-arrested MC/9 cells lose their ability to generate leukotrienes in response to ionomycin treatment. This rapid down-regulation of leukotriene synthesis is also observed when proliferating RBL2H3 cells are transferred to growth-factor-free medium, wherein cellular leukotriene-synthesis capacity has an apparent half-lifetime of 60 min. Transfer back into growth medium results in the regeneration of leukotriene synthesis capacity within 6 h. In growth-arrested MC/9 cells, leukotriene production ability can at least partially be restored by priming the cells with interleukin 3, but not with interleukin 4. In RBL2H3 cells, pretreatment with protein tyrosine kinase inhibitors such as genistein (5 min, 37 microM), herbimycin A (6 h, 3 microM) or tyrphostin 25 (16 h, 100 microM) completely inhibits leukotriene generation, whereas okadaic acid (15 min, 0.5 microM) has no effect. Under these conditions, both genistein and herbimycin A strongly impair ionomycin-induced protein tyrosine phosphorylation. Our study indicates that leukotriene generation in these tumour cells is tightly regulated by their proliferation status and supply with growth factors, and cell stimulation towards leukotriene synthesis appears to involve protein tyrosine kinase activity.

Labeling of v-Src and BCR-ABL tyrosine kinases with [14C]herbimycin A and its use in the elucidation of the kinase inactivation mechanism

The ansamycin antibiotic, herbimycin A, selectively inactivates cytoplasmic tyrosine kinases, most likely by binding irreversibly to the reactive SH group(s) of kinases. To further investigate the mechanism of herbimycin A action, we attempted to label tyrosine kinases with [14C]herbimycin A. p60v-src and p210BCR-ABL in immune complexes were labeled with [14C]herbimycin A, demonstrating that the antibiotic binds directly to tyrosine kinases. Digestion of [14C]herbimycin A-labeled p60v-src with Staphylococcus aureus V8 protease revealed that the herbimycin A binding site is within the C-terminal 26-kDa fragment of p60v-src, which contains the tyrosine kinase domain. Herbimycin A treatment inhibited labeling of p60v-src by [14C]fluorosulfonylbenzoyl adenosine, an affinity labeling reagent of nucleotide binding sites, indicating that herbimycin A-modified p60v-src cannot interact with ATP. The results suggest that herbimycin A inactivates tyrosine kinases by binding directly to the kinase domain, thereby inhibiting access to ATP.

Effects of herbimycin A and its derivatives on growth and differentiation of Ph1-positive acute lymphoid leukemia cell lines

The molecular basis of the Philadelphia chromosome (Ph1) is a structurally altered c-abl (bcr/abl) gene which encodes an abnormally large protein with protein tyrosine kinase activity. Herbimycin A, an inhibitor of tyrosine kinase, preferentially inhibited the growth of Ph1-positive acute lymphoid leukemia (ALL) cell lines, as well as Ph1-positive chronic myeloid leukemia (CML) cell lines. Although noncytotoxic concentrations of herbimycin A induced erythroid differentiation of two CML-derived cell lines, K562 and KU812, in a previous study, the differentiation-inducing effect of herbimycin A on Ph1-positive ALL cell lines was less strong. Herbimycin A enhanced some differentiation-associated properties of one Ph1-positive ALL cell line, L2, but the effect of herbimycin A on the other Ph1-positive ALL cell lines was cytotoxic rather than cytostatic (differentiation-inducing). Several derivatives of herbimycin A were synthesized and their effects on the cell proliferation of Ph1-positive CML and ALL cell lines were examined. The sensitivities of the Ph1-positive cell lines to herbimycin A derivatives were different from the data on the rat kidney cell line infected with Rous sarcoma virus (v-src) derived from a previous study, suggesting bcr/abl kinase may differ in sensitivity from other tyrosine kinases. Moreover, the sensitivities of the ALL cell lines were not the same as those of the CML cell lines. These results suggest that a specific inhibitor of bcr/abl kinase could be an effective antileukemic agent against Ph1-positive CML or ALL.

Effect of herbimycin A, an inhibitor of tyrosine kinase, on protein tyrosine kinase activity and phosphotyrosyl proteins of Ph1-positive leukemia cells

Herbimycin A, a benzoquinonoid anasamycin antibiotic, preferentially inhibited the in vitro growth of Ph1-positive leukemia cell lines. On the other hand, genistein, which was developed as an inhibitor of receptor-type tyrosine kinase, and other protein kinase inhibitors showed no selective inhibition of Ph1-positive leukemia cell growth. Herbimycin A also displayed an abrogative effect on the transformation of murine hematopoietic cells by transfection with a bcr/abl oncoprotein-expressing retroviral vector. The antitumor action of herbimycin A on Ph1-positive leukemia cells is related to an inhibition of activity of bcr/abl protein tyrosine kinase and a subsequent reduction of the constitutive phosphotyrosyl proteins, however, the antibiotic has no effect on the expression of bcr/abl mRNA and oncoprotein. Therefore, herbimycin A may provide an important insight into the oncogenic action of bcr/abl oncoprotein and the future development of oncoprotein-targeted therapeutic agents.

New insight into oncoprotein-targeted antitumor effect: herbimycin A as an antagonist of protein tyrosine kinase against Ph1-positive leukemia cells

Herbimycin A, a benzoquinonoid anasamycin antibiotic, has been shown to reserve the oncogenic phenotypes of p60v-src transformed cells by the virtue of the inhibition of src protein tyrosine kinase. Furthermore, we previously demonstrated that herbimycin A displayed the antitumor activity on Ph1-positive leukemia cells and bcr/abl oncoprotein-associated transformed murine hematopoietic cells with the transfection of a retroviral vector expressing bcr/abl. Herbimycin A showed preferential inhibition on the in vitro growth of Ph1-positive leukemia cells and bcr/abl oncoprotein-associated murine hematopoietic cells through the inhibition of bcr/abl tyrosine kinase activity and the reduction of subsequent phosphotyrosyl proteins. Recently, from the view of investigating the oncogenic significance or of developing a future clinical application in malignancies, several developing agents targeted against oncoprotein have been tried. We reviewed the present progress in the mechanism of oncoprotein-targeted antitumor effects and focused on herbimycin A-induced antitumor activity on Ph1-positive leukemia cells.

v-Src activates a unique phospholipase D activity that can be distinguished from the phospholipase D activity activated by phorbol esters

Phospholipase D (PLD) activity, as measured by the transphosphatidylation of cellular phospholipids, is elevated in BALB/c 3T3 cells transformed by v-Src. Phorbol esters that activate protein kinase C (PKC) also increase PLC activity in BALB/c 3T3 cells. v-Src-induced PLD activity could be distinguished from phorbol ester-induced PLD activity by differential radiolabelling of phospholipids, which are the substrates of PLD. Both v-Src- and phorbol ester-induced PLD activity could be detected when phospholipids were prelabelled with either radiolabelled myristate or palmitate; however, only phorbol ester-induced PLD activity could be detected when either arachidonate or 1-O-alkyl-sn-glyceryl-3-phosphorylcholine (alkyl-lysoPC) was used to prelabel the phospholipids. The increased PLD activity in v-Src-transformed cells was not detected when the cells were prelabelled with either arachidonic acid or alkyl-lysoPC, which contains an ether linkage at sn-1 of the glycerol backbone. As both arachidonic acid and alkyl-lysoPC are incorporated into phosphatidylcholine (PC), the substrate for v-Src-induced PLD activity, these data suggest that the PLD activated by v-Src can distinguish PCs lacking arachidonic acid and ether linkages. Consistent with v-Src activating a PLD activity that is distinct from that activated by phorbol esters that activate PKC directly, neither depleting cells of PKC nor treatment with the protein kinase inhibitor, staurosporine, had any effect on v-Src-induced PLD activity, whereas both PKC depletion and staurosporine inhibited phorbol ester-induced PLD activity. Taken together, these data suggest that v-Src activates a PKC-independent PLD activity that is specific for a subpopulation of PC and distinct from the PLD activity induced by PKC activity induced by phorbol esters. The diacylglycerol produced from PC by the action of the v-Src-induced PLD may therefore be responsible for the activation of PKC by v-Src.

Integrated system for the screening of the specificity of protein kinase inhibitors

Tyrosine protein kinases (TPKs) play a major role in the transformation of cells. They are currently used as molecular targets for new generations of anticancer compounds. Numerous TPKs have been described from various tissues using either classical molecular biochemical techniques or cloning strategies. As a natural extension of these discoveries, a large number of "specific" inhibitors have been described in the literature. The major problem with these inhibitors is that there is no simple way to compare their specificity and/or selectivity from one report to another. We have set up a simple, straightforward technique to compare the inhibitory potency of 14 classical inhibitors towards six well-described and at least partially purified protein kinases. This technique is based on a new assay, easy to carry out and non-restrictive in terms of the type of protein substrate used. It permits direct comparisons between the results obtained from various sources. Data obtained showed that, when assessed in this integrated system, specificity and selectivity of many "classical" inhibitors are often weak, thus demonstrating that a universal technique such as ours is essential for the molecular screening of new protein kinase inhibitors. Compounds showing specificity for this panel of protein kinases will be more easily targeted to some defined types of oncogene and of transformed cells.

BCR/ABL oncoprotein-targeted antitumor activity of antisense oligodeoxynucleotides complementary to bcr/abl mRNA and herbimycin A, an antagonist of protein tyrosine kinase: inhibitory effects on in vitro growth of Ph1-positive leukemia cells and BCR/ABL oncoprotein-associated transformed cells

We investigated whether antisense oligodeoxynucleotides complementary to bcr/abl mRNA or protein kinase antagonists display antitumor activity on Ph1-positive leukemia cell lines. bcr/abl antisense oligomers showed inhibitory effects on the in vitro growth of Ph1-positive leukemia cell lines in liquid culture, and further displayed an inhibitory effect on transformed murine hematopoietic cells using transfection with a retroviral vector expressing P210bcr/abl oncoprotein. However, in vitro treatment with a bcr/abl antisense oligomer did not completely abolish the expression of bcr/abl mRNA and did not display the desired "killing effect" on Ph1-positive leukemia cells. On the other hand, investigation of the effect on Ph1-positive leukemia cells by various types of protein kinase antagonists revealed that herbimycin A, a protein tyrosine kinase antagonist, displays preferential and remarkable suppression of the growth of Ph1-positive leukemia cells and P210bcr/abl associated transformed cells by virtue of suppressing bcr/abl protein tyrosine kinase activity. These results may provide important future insights in developing a new category of antitumor therapy by targeting oncogene products.