Showing papers on "MG132 published in 1998"

Skeletal muscle myocytes undergo protein loss and reactive oxygen-mediated NF-κB activation in response to tumor necrosis factor α

Abstract: Skeletal muscle atrophy and weakness are thought to be stimulated by tumor necrosis factor alpha (TNF-alpha) in a variety of chronic diseases. However, little is known about the direct effects of TNF-alpha on differentiated skeletal muscle cells or the signaling mechanisms involved. We have tested the effects of TNF-alpha on the mouse-derived C2C12 muscle cell line and on primary cultures from rat skeletal muscle. TNF-alpha treatment of differentiated myotubes stimulated time- and concentration-dependent reductions in total protein content and loss of adult myosin heavy chain (MHCf) content; these changes were evident at low TNF-alpha concentrations (1-3 ng/ml) that did not alter muscle DNA content and were not associated with a decrease in MHCf synthesis. TNF-alpha activated binding of nuclear factor kappaB (NF-kappaB) to its targeted DNA sequence and stimulated degradation of I-kappaBalpha, an NF-kappaB inhibitory protein. TNF-alpha stimulated total ubiquitin conjugation whereas a 26S proteasome inhibitor (MG132 10-40 microM) blocked TNF-alpha activation of NF-kappaB. Catalase 1 kU/ml inhibited NF-kappaB activation by TNF-alpha; exogenous hydrogen peroxide 200 microM activated NF-kappaB and stimulated I-kappaBalpha degradation. These data demonstrate that TNF-alpha directly induces skeletal muscle protein loss, that NF-kappaB is rapidly activated by TNF-alpha in differentiated skeletal muscle cells, and that TNF-alpha/NF-kappaB signaling in skeletal muscle is regulated by endogenous reactive oxygen species.

Basal and human papillomavirus E6 oncoprotein-induced degradation of Myc proteins by the ubiquitin pathway.

Abstract: We have previously shown that the degradation of c-myc and N-myc in vitro is mediated by the ubiquitin system. However, the role of the system in targeting the myc proteins in vivo and the identity of the conjugating enzymes and possible ancillary proteins involved has remained obscure. Here we report that the degradation of the myc proteins in cells is inhibited by lactacystin and MG132, two inhibitors of the 20S proteasome. Inhibition is accompanied by accumulation of myc-ubiquitin conjugates. Dissection of the ancillary proteins involved revealed that the high-risk human papillomavirus oncoprotein E6-16 stimulates conjugation and subsequent degradation of the myc proteins in vitro. Expression of E6-16 in cells results in significant shortening of the t1/2 of the myc proteins with subsequent decrease in their cellular level. Analysis of the conjugating enzymes revealed that under basal conditions the proteins can be conjugated by two pairs of E2s and E3s—E2-14 kDa and E3α involved in the “N-end rule” pathway, and E2-F1 (UbcH7) and E3-Fos involved also in conjugation of c-Fos. In the presence of E6-16, a third pair, E2-F1 and E6-AP mediate conjugation of myc by means of a mechanism that appears to be similar to that involved in the targeting of p53, formation of a myc⋅E6⋅E6–AP targeting complex. It is possible that in certain cells E6-mediated targeting of myc prevents myc-induced apoptosis and thus ensures maintenance of viral infection.

Proteasome inhibitors cause induction of heat shock proteins and trehalose, which together confer thermotolerance in saccharomyces cerevisiae

Abstract: An accumulation in cells of unfolded proteins is believed to be the common signal triggering the induction of heat shock proteins (hsps). Accordingly, in Saccharomyces cerevisiae, inhibition of protein breakdown at 30°C with the proteasome inhibitor MG132 caused a coordinate induction of many heat shock proteins within 1 to 2 h. Concomitantly, MG132, at concentrations that had little or no effect on growth rate, caused a dramatic increase in the cells’ resistance to very high temperature. The magnitude of this effect depended on the extent and duration of the inhibition of proteolysis. A similar induction of hsps and thermotolerance was seen with another proteasome inhibitor, clasto-lactacystin β-lactone, but not with an inhibitor of vacuolar proteases. Surprisingly, when the reversible inhibitor MG132 was removed, thermotolerance decreased rapidly, while synthesis of hsps continued to increase. In addition, exposure to MG132 and 37°C together had synergistic effects in promoting thermotolerance but did not increase hsp expression beyond that seen with either stimulus alone. Although thermotolerance did not correlate with hsp content, another thermoprotectant trehalose accumulated upon exposure of cells to MG132, and the cellular content of this disaccharide, unlike that of hsps, quickly decreased upon removal of MG132. Also, MG132 and 37°C had additive effects in causing trehalose accumulation. Thus, the resistance to heat induced by proteasome inhibitors is not just due to induction of hsps but also requires a short-lived metabolite, probably trehalose, which accumulates when proteolysis is reduced.

Differential regulation of endothelial cell adhesion molecule expression by nitric oxide donors and antioxidants

Abstract: Although nitric oxide (NO) and antioxidants inhibit adhesion molecule expression, their inhibitory effects on nuclear factor kappaB (NF-kappaB) activation may differ. The NO donors, but not 8-bromo-cGMP, decreased tumor necrosis factor alpha (TNF-alpha)-induced VCAM-1, ICAM-1, and E-selectin expression by 11-70%. In contrast, NAC completely abolished VCAM-1 and E-selectin expression and decreased ICAM-1 expression by 56%. Gel shift assays demonstrate that NF-kappaB activation was inhibited by both NO and antioxidants. The activation of NF-kappaB involves the phosphorylation and degradation of its cytoplasmic inhibitor IkappaB-alpha by 26S proteasomes. The 26S proteasome inhibitor MG132 prevented the degradation of phosphorylated IkappaB-alpha. NAC inhibited IkappaB kinase (IKK) activity and prevented IkappaB-alpha phosphorylation and degradation. In contrast, NO did not inhibit IKK activity, IkappaB-alpha phosphorylation, or IkappaB-alpha degradation. However, NO, but not antioxidants, induced IkappaB-alpha promoter activity. The inhibitory effects of NO on adhesion molecule expression, therefore, differs from that of antioxidants in terms of the mechanism by which NF-kappaB is inactivated.

Antiviral Activity of the Proteasome on Incoming Human Immunodeficiency Virus Type 1

Abstract: Following cell surface receptor binding and membrane fusion, human immunodeficiency virus (HIV) virion cores are released in the cytoplasm. Incoming viral proteins represent potential targets for cytosolic proteases. We show that treatment of target cells with the proteasome inhibitors MG132 and lactacystin increased the efficiency of HIV infection. Proteasome inhibitors were active at the early steps of the viral cycle. Incoming p24Gag proteins accumulated in the cytosol, and larger amounts of proviral DNA were synthesized. In vitro, purified 20S proteasome degraded HIV virion components. Thus, degradation of incoming viral proteins by the proteasome represents an early intracellular defense against infection.

mdm2 and bax, downstream mediators of the p53 response, are degraded by the ubiquitin-proteasome pathway.

Abstract: Upon activation in response to cellular stress or DNA damage, the p53 tumor suppressor induces the expression of gene products involved in cell cycle arrest and apoptosis. Using the proteasome-specific inhibitors, MG132 (N-acetyl-L-leucinyl-L-leucinal-L-leucinal) and lactacystin, here we show that the p53-response proteins, bax and mdm2 as well as p21, are degraded by the ubiquitin-proteasome pathway in HeLa cells. MG132 also increased expression of the three proteins in cells that lack p53, showing that stabilization of the p53 response proteins is not due to increased levels of p53 itself. Increases in mdm2 protein levels by MG132 was accompanied by increases in polyubiquitinated forms of the proteins. Our results indicate that ubiquitin-dependent protein degradation influences the turnover of downstream targets of p53, therefore suggesting that the proteasome plays a role in regulating apoptosis and cell cycle arrest in response to p53.

Epidermal growth factor activation of NF-κb is mediated through IκBα degradation and intracellular free calcium

Abstract: The transcription factor NF-kappa-B is normally sequestered in the cytoplasm by its inhibitory subunit IkappaB. Most extracellular signals activate NF-kappa-B through a mechanism involving the phosphorylation and proteasome-dependent degradation of IkappaB. EGF activates NF-kappaB in A-431 carcinoma cells, which overexpress EGF receptors and in mouse embryo fibroblasts, which have a normal complement of receptors. Supershift experiments indicate that the NF-kappa-B complexes induced by EGF are composed of p50/p50 homodimers and p65/p50 heterodimers, but not c-rel. EGF stimulation enhances the degradation of IkappaBalpha, but not IkappaBbeta nor an N-terminal deletion mutant of IkappaBalpha. Treatment of cells with a proteasome inhibitor, such as ALLN or MG132, blocks EGF-mediated NF-kappaB activation, indicating that EGF-induced NF-kappa-B activation requires proteasome-dependent IkappaB degradation. Also, Bapta A/M (a cell-permeable chelator of intracellular calcium) blocks EGF-induced NF-kappa-B activation and IkappaBalpha degradation, suggesting a requirement of intracellular free Ca2+ for this growth factor response. Protein kinase C inhibition, in contrast, did not influence EGF activation of NF-kappaB.

Ste6p Mutants Defective in Exit from the Endoplasmic Reticulum (ER) Reveal Aspects of an ER Quality Control Pathway in Saccharomyces cerevisiae

Abstract: We are studying the intracellular trafficking of the multispanning membrane protein Ste6p, the a-factor transporter in Saccharomyces cerevisiae and a member of the ATP-binding cassette superfamily of proteins. In the present study, we have used Ste6p as model for studying the process of endoplasmic reticulum (ER) quality control, about which relatively little is known in yeast. We have identified three mutant forms of Ste6p that are aberrantly ER retained, as determined by immunofluorescence and subcellular fractionation. By pulse-chase metabolic labeling, we demonstrate that these mutants define two distinct classes. The single member of Class I, Ste6–166p, is highly unstable. We show that its degradation involves the ubiquitin–proteasome system, as indicated by its in vivo stabilization in certain ubiquitin–proteasome mutants or when cells are treated with the proteasome inhibitor drug MG132. The two Class II mutant proteins, Ste6–13p and Ste6–90p, are hyperstable relative to wild-type Ste6p and accumulate in the ER membrane. This represents the first report of a single protein in yeast for which distinct mutant forms can be channeled to different outcomes by the ER quality control system. We propose that these two classes of ER-retained Ste6p mutants may define distinct checkpoint steps in a linear pathway of ER quality control in yeast. In addition, a screen for high-copy suppressors of the mating defect of one of the ER-retained ste6 mutants has identified a proteasome subunit, Hrd2p/p97, previously implicated in the regulated degradation of wild-type hydroxymethylglutaryl-CoA reductase in the ER membrane.

Proteasome-mediated degradation of the vitamin D receptor (VDR) and a putative role for SUG1 interaction with the AF-2 domain of VDR

Abstract: The AF-2 helix of nuclear receptors is essential for ligand-activated transcription, and it may function to couple the receptor to transcriptional coactivator proteins. This domain also contacts components of the proteasome machinery, suggesting that nuclear receptors may be targets for proteasome-mediated proteolysis. In the present study, we demonstrate that mSUG1 (P45), a component of the 26S proteasome, interacts in a 1,25-(OH)2D3–dependent manner with the AF-2 domain of the vitamin D receptor (VDR). Furthermore, treatment of ROS 17/ 2.8 osteosarcoma cells with the proteasome inhibitors MG132 or β-lactone increased steady-state levels of the VDR protein. In the presence cycloheximide (10 μg/ ml), the liganded VDR protein was degraded with a half-life of approximately 8 h, and this rate of degradation was completely blocked by 0.05 mM MG132. The role of SUG1-VDR interaction in this process was investigated in transient expression studies. Overexpression of wild-type mSUG1 in ROS17/ 2.8 cells generated a novel proteolytic VDR fragment of approximately 50 kDa, and its production was blocked by proteasome inhibitors or by a nonhydrolyzable ATP analog. Parallel studies with SUG1(K196H), a mutant that does not interact with the VDR, did not produce the 50 kDa VDR fragment. Functionally, expression of SUG1 in a VDR-responsive reporter gene assay resulted in a profound inhibition of 1,25-(OH)2D3–activated transcription, while expression of SUG1(K196H) had no significant effect in this system. These data show that the AF-2 domain of VDR interacts with SUG1 in a 1,25-(OH)2D3–dependent fashion and that this interaction may target VDR to proteasome-mediated degradation as a means to downregulate the 1,25-(OH)2D3–activated transcriptional response. J. Cell. Biochem. 71:429–440, 1998. © 1998 Wiley-Liss, Inc.

Type 2 Iodothyronine Deiodinase in Rat Pituitary Tumor Cells Is Inactivated in Proteasomes

Abstract: The goal of these studies was to define the rate-limiting steps in the inactivation of type 2 iodothyronine deiodinase (D2). We examined the effects of ATP depletion, a lysosomal protease inhibitor, and an inhibitor of actin polymerization on D2 activity in the presence or absence of cycloheximide or 3,3', 5'-triiodothyronine (reverse T3, rT3) in rat pituitary tumor cells (GH4C1). We also analyzed the effects of the proteasomal proteolysis inhibitor carbobenzoxy- L-leucyl-L-leucyl-L-leucinal (MG132). The half-life of D2 activity in hypothyroid cells was 47 min after cycloheximide and 60 min with rT3 (3 nM). rT3 and cycloheximide were additive, reducing D2 half-life to 20 min. D2 degradation was partially inhibited by ATP depletion, but not by cytochalasin B or chloroquine. Incubation with MG132 alone increased D2 activity by 30-40% for several hours, and completely blocked the cycloheximide- or rT3-induced decrease in D2 activity. These results suggest that D2 is inactivated by proteasomal uptake and that substrate reduces D2 activity by accelerating degradation through this pathway. This is the first demonstration of a critical role for proteasomes in the post-translational regulation of D2 activity.

Proteasome Activation Occurs at an Early, Premitochondrial Step of Thymocyte Apoptosis

Abstract: Proteasomes and mitochondrial membrane changes are involved in thymocyte apoptosis. The hierarchical relationship between protease activation and mitochondrial alterations has been elusive. Here we show that inhibition of proteasomes by two specific agents, lactacystin or MG132, prevents all manifestations of thymocyte apoptosis induced by the glucocorticoid receptor agonist dexamethasone or by the topoisomerase II inhibitor etoposide. Lactacystin and MG132 prevent the early disruption of the mitochondrial transmembrane potential (ΔΨ m ), which precedes caspase activation, exposure of phosphatidylserine, and nuclear DNA fragmentation. In contrast, stabilization of the ΔΨ m using the permeability transition pore inhibitor bongkrekic acid or inhibition of caspases by N -benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone does not prevent the activation of proteasomes, as determined with the fluorogenic substrate N -succinyl-l-leucyl-l-leucyl-l-valyl-l-tyrosine-7-amido-4-methylcoumarin. Thus, proteasome activation occurs upstream from mitochondrial changes and caspase activation. Whereas the proteasome-specific agents lactacystin and MG132 truly maintain thymocyte viability, a number of protease inhibitors that inhibit nuclear DNA fragmentation (acetyl-Asp-Glu-Val-Asp-fluoromethylketone; N- Boc-Asp(OMe)-fluoromethylketone; N- tosyl-l-Phe-chloromethylketone) do not prevent the cytolysis induced by DEX or etoposide. These latter agents fail to interfere with the preapoptotic ΔΨ m disruption. Altogether, our data indicate that different proteases may be involved in the pre- or postmitochondrial phase of apoptosis. Only those protease inhibitors that interrupt the apoptotic process at the premitochondrial stage can actually preserve cell viability.

Inhibition versus induction of apoptosis by proteasome inhibitors depends on concentration.

Abstract: We previously established that NF-kappaB DNA binding activity is required for Sindbis Virus (SV)-induced apoptosis. To investigate whether SV induces nuclear translocation of NF-kappaB via the proteasomal degradation pathway, we utilized MG132, a peptide aldehyde inhibitor of the catalytic subunit of the proteasome. 20 microM MG132 completely abrogated SV-induced NF-kappaB nuclear activity at early time points after infection. Parallel measures of cell viability 48 h after SV infection revealed that 20 microM MG132 induced apoptosis in uninfected cells. In contrast, a lower concentration of MG132 (200 nM) resulted in partial inhibition of SV-induced nuclear NF-kappaB activity and inhibition of SV-induced apoptosis without inducing toxicity in uninfected cells. The specific proteasomal inhibitor, lactacystin, also inhibited SV-induced death. Taken together, these results suggest that the pro-apoptotic and anti-apoptotic functions of peptide aldehyde proteasome inhibitors such as MG-132 depend on the concentration of inhibitor utilized and expand the list of stimuli requiring proteasomal activation to induce apoptosis to include viruses.

Reduced thermotolerance in aged cells results from a loss of an hsp72- mediated control of JNK signaling pathway

Abstract: Aged organisms exhibit a greatly decreased ability to induce the major heat shock protein, Hsp72, in response to stresses, a phenomenon that can also be observed in cell cultures (Heydari AR, Takahashi R, Gutsmann A, You S and Richardson A (1994) Hsp70 and aging. Experientia 50: 1092-1098). Hsp72 was shown to protect cells from a variety of stresses. The protective function of Hsp72 has been commonly ascribed to its chaperoning ability. However, recently we showed that Hsp72 protects cells from heat shock by suppression of a stress-kinase JNK, an essential component of the heat-induced apoptotic pathway (Gabai VL, Meriin AB, Mosser DD, Caron AW, Rits S, Shifrin VI and Sherman MY (1997) Hsp70 prevents activation of stress kinases. A novel pathway of cellular thermotolerance. J Biol Chem 272: 18033-18037). Here we demonstrate that because of the diminished inducibility of Hsp72 in aged cells, Hsp72-mediated control of JNK signaling pathway is compromised. This results in increased rate of apoptotic cell death following heat shock. We show that forced expression of Hsp72 in aged cells from an adenovirus-based vector completely suppresses activation of JNK by heat shock and consequently protects from heat-induced apoptosis. We also demonstrate for the first time that it is possible to restore endogenous expression of Hsp72 in aged cells. This can be achieved by treatment with the proteasome inhibitor MG132. Induction of Hsp72 in aged cells under these conditions leads to suppression of JNK activation by a heat shock and restoration of thermotolerance manifested in a lower rate of apoptosis.

Dyclonine and alverine citrate enhance the cytotoxic effects of proteasome inhibitor MG132 on breast cancer cells

Abstract: Proteasome is an important target in cancer therapy. To enhance the efficacy of proteasome inhibitors is a challenging task due to the paucity of understanding the functional interactions between proteasome and other cellular pathways in mammalian cells. Taking advantage of the knowledge gained from Saccharomyces cerevisiae, we show that dyclonine and alverine citrate, the major components of two over-the-counter medicines, can substantially enhance the cytotoxic effects of proteasome inhibitor MG132 on breast cancer cells. This study also highlights an important yeast genetic approach to identification of potential therapeutics that can be used for combination therapy with proteasome inhibitors.