The Hsp90 inhibitor IPI-504 rapidly lowers EML4-ALK levels and induces tumor regression in ALK-driven NSCLC models.

Heat shock protein 90 (Hsp90) is an emerging target for cancer therapy due to its important role in maintaining the activity and stability of key oncogenic signaling proteins. We show here that the echinoderm microtubule-associated protein-like 4 (EML4)-anaplastic lymphoma kinase (ALK) fusion protein, presumed to be the oncogenic driver in about 5% of patients with non-small cell lung cancer (NSCLC), is associated with Hsp90 in cells and is rapidly degraded upon exposure of cells to IPI-504. We find EML4-ALK to be more sensitive to Hsp90 inhibition than either HER2 or mutant epidermal growth factor receptor (EGFR) with an inhibitory concentration (IC)(50) for protein degradation in the low nanomolar range. This degradation leads to a potent inhibition of downstream signaling pathways and to the induction of growth arrest and apoptosis in cells carrying the EML4-ALK fusion. To generate a causative link between the expression of EML4-ALK and sensitivity to IPI-504, we introduced an EML4-ALK cDNA into HEK293 cells and show that the expression of the fusion protein sensitizes cells to IPI-504 both in vitro and in vivo. In a xenograft model of a human NSCLC cell line containing the ALK rearrangement, we observe tumor regression at clinically relevant doses of IPI-504. Finally, cells that have been selected for resistance to ALK kinase inhibitors retain their sensitivity to IPI-504. We have recently observed partial responses to administration of IPI-504 as a single agent in a phase 2 clinical trial in patients with NSCLC, specifically in patients that carry an ALK rearrangement. This study provides a molecular explanation for these clinical observations.

The proteasome inhibitor PS-341 inhibits growth, induces apoptosis, and overcomes drug resistance in human multiple myeloma cells.

Human multiple myeloma (MM) is a presently incurable hematological malignancy, and novel biologically based therapies are urgently needed. Proteasome inhibitors represent a novel potential anticancer therapy. In this study, we demonstrate that the proteasome inhibitor PS-341 directly inhibits proliferation and induces apoptosis of human MM cell lines and freshly isolated patient MM cells; inhibits mitogen-activated protein kinase growth signaling in MM cells; induces apoptosis despite induction of p21 and p27 in both p53 wild-type and p53 mutant MM cells; overcomes drug resistance; adds to the anti-MM activity of dexamethasone; and overcomes the resistance to apoptosis in MM cells conferred by interleukin-6. PS-341 also inhibits the paracrine growth of human MM cells by decreasing their adherence to bone marrow stromal cells (BMSCs) and related nuclear factor kappaB-dependent induction of interleukin-6 secretion in BMSCs, as well as inhibiting proliferation and growth signaling of residual adherent MM cells. These data, therefore, demonstrate that PS-341 both acts directly on MM cells and alters cellular interactions and cytokine secretion in the BM millieu to inhibit tumor cell growth, induce apoptosis, and overcome drug resistance. Given the acceptable animal and human toxicity profile of PS-341, these studies provide the framework for clinical evaluation of PS-341 to improve outcome for patients with this universally fatal hematological malignancy.

Proteasome inhibition: a new strategy in cancer treatment.

The ubiquitin proteasome pathway is a highly conserved intracellular pathway for the degradation of proteins. Many of the short-lived regulatory proteins which govern cell division, growth, activation, signaling and transcription are substrates that are temporally degraded by the proteasome. In recent years, new and selective inhibitors of the proteasome have been employed in cell culture systems to examine the anti-tumor potential of these agents. This review covers the chemistry of selected proteasome inhibitors, possible mechanisms of action in cell culture and the in vivo examination of proteasome inhibitors in murine and human xenograft tumor models in mice. One inhibitor, PS-341, has recently entered Phase I clinical trials in cancer patients with advanced disease to further test the potential of this approach.

Nedd8 modification of cul-1 activates SCF(beta(TrCP))-dependent ubiquitination of IkappaBalpha.

Regulation of NF-kappaB occurs through phosphorylation-dependent ubiquitination of IkappaBalpha, which is degraded by the 26S proteasome. Recent studies have shown that ubiquitination of IkappaBalpha is carried out by a ubiquitin-ligase enzyme complex called SCF(beta(TrCP)). Here we show that Nedd8 modification of the Cul-1 component of SCF(beta(TrCP)) is important for function of SCF(beta(TrCP)) in ubiquitination of IkappaBalpha. In cells, Nedd8-conjugated Cul-1 was complexed with two substrates of SCF(beta(TrCP)), phosphorylated IkappaBalpha and beta-catenin, indicating that Nedd8-Cul-1 conjugates are part of SCF(beta(TrCP)) in vivo. Although only a minute fraction of total cellular Cul-1 is modified by Nedd8, the Cul-1 associated with ectopically expressed betaTrCP was highly enriched for the Nedd8-conjugated form. Moreover, optimal ubiquitination of IkappaBalpha required Nedd8 and the Nedd8-conjugating enzyme, Ubc12. The site of Nedd8 ligation to Cul-1 is essential, as SCF(beta(TrCP)) containing a K720R mutant of Cul-1 only weakly supported IkappaBalpha ubiquitination compared to SCF(beta(TrCP)) containing WT Cul-1, suggesting that the Nedd8 ligation of Cul-1 affects the ubiquitination activity of SCF(beta(TrCP)). These observations provide a functional link between the highly related ubiquitin and Nedd8 pathways of protein modification and show how they operate together to selectively target the signal-dependent degradation of IkappaBalpha.

The proteasome inhibitor PS-341 in cancer therapy.

The anticancer activity of the boronic acid dipeptide proteasome inhibitor PS-341 was examined in vitro and in vivo. PS-341 was a potent cytotoxic agent toward MCF-7 human breast carcinoma cells in culture, producing an IC90 of 0.05 microM on 24 h of exposure to the drug. In the EMT-6 tumor cell survival assay, PS-341 was equally cytotoxic administered p.o. or by i.p. injection up to a dose of 2 mg/kg. PS-341 was also toxic to the bone marrow colony-forming unit-granulocyte macrophage. PS-341 increased the tumor cell killing of radiation therapy, cyclophosphamide, and cisplatin in the EMT-6/Parent tumor, but was not able to overcome the in vivo resistance of the EMT-6/CTX and EMT-6/CDDP tumors. In the tumor growth delay assay, PS-341 administered p.o. had antitumor activity against the Lewis lung carcinoma, both primary and metastatic disease. In combination, regimens with 5-fluorouracil, cisplatin, Taxol and adriamycin, PS-341 seemed to produce primarily additive tumor growth delays against the s.c. tumor and was highly effective against disease metastatic to the lungs. The proteasome is an interesting new target for cancer therapy, and the proteasome inhibitor PS-341 warrants continued investigation in cancer therapy.

Proteasome inhibitors: a novel class of potent and effective antitumor agents.

The ubiquitin-proteasome pathway plays a critical role in the regulated degradation of proteins involved in cell cycle control and tumor growth. Dysregulating the degradation of such proteins should have profound effects on tumor growth and cause cells to undergo apoptosis. To test this hypothesis, we developed a novel series of proteasome inhibitors, exemplified by PS-341, which we describe here. As determined by the National Cancer Institute in vitro screen, PS-341 has substantial cytotoxicity against a broad range of human tumor cells, including prostate cancer cell lines. The PC-3 prostate cell line was, therefore, chosen to further examine the antitumor activity of PS-341. In vitro, PS-341 elicits proteasome inhibition, leading to an increase in the intracellular levels of specific proteins, including the cyclin-dependent kinase inhibitor, p21. Moreover, exposure of such cells to PS-341 caused them to accumulate in the G2-M phase of the cell cycle and subsequently undergo apoptosis, as indicated by nuclear condensation and poly(ADP-ribose) polymerase cleavage. Following weekly i.v. treatment of PS-341 to mice bearing the PC-3 tumor, a significant decrease (60%) in tumor burden was observed in vivo. Direct injection of PS-341 into the tumor also caused a substantial (70%) decrease in tumor volume with 40% of the drug-treated mice having no detectable tumors at the end of the study. Studies also revealed that i.v. administration of PS-341 resulted in a rapid and widespread distribution of PS-341, with highest levels identified in the liver and gastrointestinal tract and lowest levels in the skin and muscle. Modest levels were found in the prostate, whereas there was no apparent penetration of the central nervous system. An assay to follow the biological activity of the PS-341 was established and used to determine temporal drug activity as well as its ability to penetrate tissues. As such, PS-341 was shown to penetrate PC-3 tumors and inhibit intracellular proteasome activity 1.0 h after i.v. dosing. These data illustrate that PS-341 not only reaches its biological target but has a direct effect on its biochemical target, the proteasome. Importantly, the data show that inhibition of this target site by PS-341 results in reduced tumor growth in murine tumor models. Together, the results highlight that the proteasome is a novel biochemical target and that inhibitors such as PS-341 represent a unique class of antitumor agents. PS-341 is currently under clinical evaluation for advanced cancers.

Inhibition of NF-kappa B activation in vitro and in vivo: role of 26S proteasome.

It is becoming increasingly apparent that NF-kappa B plays a critical role in regulating the inflammatory response. Data obtained from studies in our laboratories demonstrate that the proteasome plays an important role in the inflammatory cascade by regulating the activation of NF-kappa B. Indeed, the availability of selective and orally active proteasome inhibitors should prove useful in delineating the roles of the proteasome and NF-kappa B in other pathophysiological conditions such as cancer and heart disease.

Role of the proteasome and NF-kappaB in streptococcal cell wall-induced polyarthritis.

The transcription factor NF-kappaB activates a number of genes whose protein products are proinflammatory. In quiescent cells, NF-kappaB exists in a latent form and is activated via a signal-dependent proteolytic mechanism in which the inhibitory protein IkappaB is degraded by the ubiquitin-proteasome pathway. Consequently, inhibition of the proteasome suppresses activation of NF-kappaB. This suppression should therefore decrease transcription of many genes encoding proinflammatory proteins and should ultimately have an anti-inflammatory effect. To this end, a series of peptide boronic acid inhibitors of the proteasome, exemplified herein by PS-341, were developed. The proteasome is the large multimeric protease that catalyzes the final proteolytic step of the ubiquitin-proteasome pathway. PS-341, a potent, competitive inhibitor of the proteasome, readily entered cells and inhibited the activation of NF-kappaB and the subsequent transcription of genes that are regulated by NF-kappaB. Significantly, PS-341 displayed similar effects in vivo. Oral administration of PS-341 had anti-inflammatory effects in a model of Streptococcal cell wall-induced polyarthritis and liver inflammation in rats. The attenuation of inflammation in this model was associated with an inhibition of IkappaBalpha degradation and NF-kappaB-dependent gene expression. These experiments clearly demonstrate that the ubiquitin-proteasome pathway and NF-kappaB play important roles in regulating chronic inflammation and that, as predicted, proteasome inhibition has an anti-inflammatory effect.

Proteasome inhibition attenuates nitric oxide synthase expression, VCAM-1 transcription and the development of chronic colitis.

The objectives of this study were to (1) assess the role of the 26S proteasome complex in regulating the expression of the inducible isoform of nitric oxide synthase (iNOS) and vascular cell adhesion molecule-1 (VCAM-1) in a model of chronic granulomatous colitis in vivo and (2) determine the role of the proteasome in regulating the inflammatory response observed in this model of chronic gut inflammation. The selective proteasome inhibitor MG-341 (0.3 mg/kg) was administered by gavage beginning immediately before the induction of colitis and continuing daily thereafter for the entire 14-day experimental period. We found that chronic proteasome inhibition using MG-341 significantly attenuated the peptidoglycan/polysaccharide (PG/PS)-induced up-regulation of iNOS in the colon and spleen and the consequent increase in plasma levels of nitrate and nitrite. Furthermore, we found that the proteasome inhibitor suppressed the up-regulation of the adhesion molecule VCAM-1 in the colon. We also found that MG-341 attenuated PG/PS-induced increases in macroscopic colonic inflammation, bowel wall thickness, colonic dry weight and colonic MPO activity. Treatment with MG-341 also significantly reduced PG/PS-induced increases in macroscopic spleen inflammation, spleen weight and spleen MPO activity. We conclude that the 26S proteasome complex plays an important role in regulating the PG/PS-induced up-regulation of iNOS and VCAM-1 in vivo and appears to be important in regulating colonic and splenic inflammation.

Mechanistic studies on the inactivation of the proteasome by lactacystin in cultured cells.

The natural product lactacystin exerts its cellular antiproliferative effects through a mechanism involving acylation and inhibition of the proteasome, a cytosolic proteinase complex that is an essential component of the ubiquitin-proteasome pathway for intracellular protein degradation. In vitro, lactacystin does not react with the proteasome; rather, it undergoes a spontaneous conversion (lactonization) to the active proteasome inhibitor, clasto-lactacystin beta-lactone. We show here that when the beta-lactone is added to mammalian cells in culture, it rapidly enters the cells, where it can react with the sulfhydryl of glutathione to form a thioester adduct that is both structurally and functionally analogous to lactacystin. We call this adduct lactathione, and like lactacystin, it does not react with the proteasome, but can undergo lactonization to yield back the active beta-lactone. We have studied the kinetics of this reaction under appropriate in vitro conditions as well as the kinetics of lactathione accumulation and proteasome inhibition in cells treated with lactacystin or beta-lactone. The results indicate that only the beta-lactone (not lactacystin) can enter cells and suggest that the formation of lactathione serves to concentrate the inhibitor inside cells, providing a reservoir for prolonged release of the active beta-lactone.

Signal-induced site-specific phosphorylation targets I kappa B alpha to the ubiquitin-proteasome pathway.

The transcription factor NF-kappa B is sequestered in the cytoplasm by the inhibitor protein I kappa B alpha. Extracellular inducers of NF-kappa B activate signal transduction pathways that result in the phosphorylation and subsequent degradation of I kappa B alpha. At present, the link between phosphorylation of I kappa B alpha and its degradation is not understood. In this report we provide evidence that phosphorylation of serine residues 32 and 36 of I kappa B alpha targets the protein to the ubiquitin-proteasome pathway. I kappa B alpha is ubiquitinated in vivo and in vitro following phosphorylation, and mutations that abolish phosphorylation and degradation of I kappa B alpha in vivo prevent ubiquitination in vitro. Ubiquitinated I kappa B alpha remains associated with NF-kappa B, and the bound I kappa B alpha is degraded by the 26S proteasome. Thus, ubiquitination provides a mechanistic link between phosphorylation and degradation of I kappa B alpha.

The proteasome pathway is required for cytokine-induced endothelial-leukocyte adhesion molecule expression.

Multiple cell adhesion proteins are up-regulated in vascular endothelial cells in response to TNF alpha and other inflammatory cytokines. This increase in cell adhesion gene expression is thought to require the transcription factor NF-kappa B. Here, we show that peptide aldehyde inhibitors of the proteasome, a multicatalytic protease recently shown to be required for the activation of NF-kappa B, block TNF alpha induction of the leukocyte adhesion molecules E-selectin, VCAM-1, and ICAM-1. Striking functional consequences of this inhibition were observed in analyses of leukocyte-endothelial interactions under defined flow conditions. Lymphocyte attachment to TNF alpha-treated endothelial monolayers was totally blocked, while neutrophil attachment was partially reduced but transmigration was essentially prevented.

The ubiquitin-proteasome pathway is required for processing the NF-kappa B1 precursor protein and the activation of NF-kappa B.

We demonstrate an essential role for the proteasome complex in two proteolytic processes required for activation of the transcription factor NF-kappa B. The p105 precursor of the p50 subunit of NF-kappa B is processed in vitro by an ATP-dependent process that requires proteasomes and ubiquitin conjugation. The C-terminal region of p105 is rapidly degraded, leaving the N-terminal p50 domain. p105 processing can be blocked in intact cells with inhibitors of the proteasome or in yeast with proteasome mutants. These inhibitors also block the activation of NF-kappa B and the rapid degradation of I kappa B alpha induced by tumor necrosis factor alpha. Thus, the ubiquitin-proteasome pathway functions not only in the complete degradation of polypeptides, but also in the regulated processing of precursors into active proteins.

Inducible processing of interferon regulatory factor-2.

PRDI-BFc and PRDI-BFi are proteins that bind specifically to a regulatory element required for virus induction of the human beta interferon (IFN-beta). PRDI-BFc is a constitutive binding activity, while the PRDI-BFi binding activity is observed only after cells are treated with inducers such as virus or poly(I).poly(C) plus cycloheximide or in some cells by cycloheximide alone. In this paper we report that PRDI-BFc is interferon regulatory factor-2 (IRF-2), a known transcriptional repressor. In addition, we find that PRDI-BFi is a truncated form of IRF-2, lacking approximately 185 C-terminal amino acids. Thus, PRDI-BFi appears to be generated by inducible proteolysis. Although the affinity of PRDI-BFc/IRF-2 for the IFN-beta promoter does not appear to be affected by the removal of C-terminal amino acids, the ability of PRDI-BFi to function as a repressor in cotransfection experiments is significantly less than that of intact IRF-2. Studies have shown that IRF-2 can block the activity of the transcriptional activator IRF-1, which also binds specifically to the IFN-beta gene promoter. Thus, the inducible proteolysis of IRF-2 may be involved in the regulation of the IFN-beta gene or of other genes in which the ratio of IRF-1 to IRF-2 can affect the level of transcription.

Mitogenic and cytotoxic actions of tumor necrosis factor in BALB/c 3T3 cells. Role of phospholipase activation.

In addition to its cytotoxic/cytostatic action on many tumor cells in vitro, tumor necrosis factor (TNF) was recently shown to stimulate the growth of some types of cells in culture. We examined the action of TNF in BALB/c 3T3 cells which have been used extensively to study growth regulation. In subconfluent, rapidly dividing 3T3 cultures, murine (Mu) TNF was cytotoxic, while human (Hu) TNF had virtually no antiproliferative action on the cells. In contrast, in density-arrested BALB/c 3T3 cells maintained in a chemically defined, serum-free medium, MuTNF produced a dose-dependent stimulation of DNA synthesis. In stimulating DNA synthesis, MuTNF acted synergistically with both epidermal growth factor or platelet-derived growth factor. While stimulating DNA synthesis in quiescent 3T3 cultures, high doses of MuTNF (100 or 10 ng/ml) were also cytotoxic for a portion of the cells in the same cultures. Cytotoxicity was apparent 2 h after the addition of MuTNF, well before the onset of DNA synthesis. BALB/c 3T3 cell variants selected for their resistance to the cytotoxic action of MuTNF retained the capacity to respond to the mitogenic action of MuTNF, indicating that the stimulation of DNA synthesis by TNF is not a consequence of a TNF "wounding effect." Addition of TNF to density-arrested 3T3 cells resulted in the release of free arachidonic acid and palmitic acid from the cells. Quinacrine, a phospholipase inhibitor, inhibited both cytotoxicity and DNA synthesis in response to TNF, and melittin, a phospholipase activator, mimicked both the cytotoxic and mitogenic actions of TNF in quiescent BALB/c 3T3 cells. These results suggest that phospholipid breakdown is part of the essential early signal transduction events required both for the cytotoxic and mitogenic response to TNF action.

Mitogenic action of tumor necrosis factor in human fibroblasts: interaction with epidermal growth factor and platelet-derived growth factor.

We have previously shown that tumor necrosis factor (TNF) can increase the number of epidermal growth factor (EGF) receptors on human FS-4 fibroblasts and that this increase may be related to the mitogenic action of TNF in these cells. Here we show that TNF stimulated the growth of FS-4 fibroblasts in a chemically defined, serum-free medium in the absence of EGF. Anti-EGF receptor antibody, which blocked the mitogenic effects of EGF in FS-4 cells, did not inhibit the mitogenic action of TNF in serum-free or serum-containing medium, indicating that EGF or an EGF-like molecule was not responsible for the mitogenic effects of TNF. However, the simultaneous addition of TNF and EGF to cells grown in serum-free medium resulted in a synergistic stimulation of DNA synthesis and cell growth. The actions of TNF and EGF were also examined in growth-arrested FS-4 cells and were compared with the action of platelet-derived growth factor (PDGF). In the absence of other growth factors, TNF was a relatively weak mitogen in growth-arrested cells, compared with EGF or PDGF. Nevertheless, TNF synergized with EGF or high doses of PDGF in stimulating DNA synthesis. Furthermore, antibodies specific for TNF or the EGF receptor were used to selectively inhibit the actions of these two factors, after specific incubation periods, in growth-arrested cells treated concurrently with EGF and TNF. To produce an optimal stimulation of DNA synthesis, EGF had to be present for a longer period of time than TNF. We conclude that in their synergistic action on growth-arrested FS-4 cells, EGF was responsible for driving the majority of the cells into S phase, while TNF appeared to make the cells more responsive to the mitogenic action of EGF. The findings indicate that TNF can cooperate with, and enhance the actions of, EGF in promoting DNA synthesis and cell division.

Tumor necrosis factor increases the number of epidermal growth factor receptors on human fibroblasts.

Tumor necrosis factor (TNF) was shown previously to stimulate the growth of human FS-4 fibroblasts. Here we show that human recombinant TNF can increase the binding of epidermal growth factor (EGF) to these cells. Incubation with TNF resulted in a 40-80% increase in the number of EGF-binding sites with no apparent change in receptor binding affinity. The increase in EGF binding was apparent 8-12 h after the addition of TNF. TNF also increased the amount of EGF receptor protein immunoprecipitated from cells labeled with [35S]methionine. Stimulation of EGF receptor protein synthesis was demonstrable 2-4 h following TNF treatment. TNF increased EGF binding with a dose-response relationship similar to that reported earlier for the mitogenic action. Increased expression of EGF receptors, due to enhanced synthesis of the EGF receptor protein, may be functionally related to the mitogenic action of TNF in human fibroblasts.

Tumor necrosis factor: receptor binding and mitogenic action in fibroblasts.

Until about two years ago, the only known function of tumor necrosis factor (TNF) was inhibition of tumor growth. Since then it has become apparent that many types of normal and transformed cells express specific high-affinity TNF receptors (Kd 200 pM) and that the presence of receptors does not correlate with susceptibility to the cytotoxic/cytostatic action of TNF. Recent evidence shows that TNF exerts a variety of other important biological activities on cells in culture and in the intact organism. Among the newly recognized activities is a potent mitogenic effect in fibroblasts. Many of the activities of TNF overlap the actions of interleukin-1 (IL-1).