Involvement of tyrosine phosphorylation in HMG-CoA reductase inhibitor-induced cell death in L6 myoblasts.
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Our previous studies have shown that the HMG-CoA reductase (HCR) inhibitor (HCRI), simvastatin, causes myopathy in rabbits and kills L6 myoblasts. The present study was designed to elucidate the molecular mechanism of HCRI-induced cell death. We have demonstrated that simvastatin induces the tyrosine phosphorylation of several cellular proteins within 10 min. These phosphorylations were followed by apoptosis, as evidenced by the occurrence of internucleosomal DNA fragmentation and by morphological changes detected with Nomarski optics. Simvastatin-induced cell death was prevented by tyrosine kinase inhibitors. The MTT assay revealed that the addition of mevalonic acid into the culture medium partially inhibited simvastatin-induced cell death. Thus, these results suggested that protein tyrosine phosphorylation might play an important role in the intracellular signal transduction pathway mediating the HCRI-induced death of myoblasts. (+info)
Effect of inhibition of cholesterol synthetic pathway on the activation of Ras and MAP kinase in mesangial cells.
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Intermediary metabolites of cholesterol synthetic pathway are involved in cell proliferation. Lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, blocks mevalonate synthesis, and has been shown to inhibit mesangial cell proliferation associated with diverse glomerular diseases. Since inhibition of farnesylation and plasma membrane anchorage of the Ras proteins is one suggested mechanism by which lovastatin prevents cellular proliferation, we investigated the effect of lovastatin and key mevalonate metabolites on the activation of mitogen-activated protein kinase (MAP kinase) and Ras in murine glomerular mesangial cells. The preincubation of mesangial cells with lovastatin inhibited the activation of MAP kinase stimulated by either FBS, PDGF, or EGF. Mevalonic acid and farnesyl-pyrophosphate, but not cholesterol or LDL, significantly prevented lovastatin-induced inhibition of agonist-stimulated MAP kinase. Lovastatin inhibited agonist-induced activation of Ras, and mevalonic acid and farnesylpyrophosphate antagonized this effect. Parallel to the MAP kinase and Ras data, lovastatin suppressed cell growth stimulated by serum, and mevalonic acid and farnesylpyrophosphate prevented lovastatin-mediated inhibition of cellular growth. These results suggest that lovastatin, by inhibiting the synthesis of farnesol, a key isoprenoid metabolite of mevalonate, modulates Ras-mediated cell signaling events associated with mesangial cell proliferation. (+info)
Lovastatin-induced proliferation inhibition and apoptosis in C6 glial cells.
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3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase is the rate-limiting enzyme in cholesterol biosynthesis. HMG-CoA reductase converts HMG-CoA to mevalonate, which is then converted into cholesterol or various isoprenoids through multiple enzymatic steps. In this study, we examined the cytotoxic effects of lovastatin, an HMG-CoA reductase inhibitor, in C6 glial cells. Lovastatin at concentrations higher than 10 microM suppressed cell proliferation and induced cell death, which were prevented completely by mevalonate (300 microM). The data from lactate dehydrogenase assay and fluorescence microscopic assay using Hoechst 33342 and propidium iodide showed that mevalonate at a concentration of 100 microM could prevent lovastatin-induced cell death, whereas it could not prevent lovastatin-induced inhibition of cell proliferation. These data suggest that the lovastatin-induced interruption of cell cycle transition was not sufficient to induce cell death in C6 glial cells. In the presence of lovastatin at concentrations higher than 10 microM, DNA laddering, the typical finding of apoptosis, was identified. Lovastatin-induced apoptosis was prevented by mevalonate (100 microM). Both cycloheximide (0.5 microgram/ml) and actinomycin D (0.1 microgram/ml) prevented lovastatin-induced DNA laddering. In this study, we demonstrated that the cytotoxic effects of lovastatin fall into two categories: suppression of cell growth and induction of apoptosis in C6 glial cells. (+info)
Lens epithelia contain a high-affinity, membrane steroid hormone-binding protein.
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PURPOSE: To describe the serendipitous discovery of a high-affinity, membrane steroid-binding protein (MSBP) in lens epithelial cells and to examine the binding of progesterone to epithelial cell membranes. METHODS: Bovine lens epithelial cells (BLECs) were cultured in media containing 3H-mevalonolactone to examine protein prenylation by mevalonate-derived isoprenes. Cell proteins were divided into insoluble and soluble fractions, separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and label detected by fluorography. Insoluble proteins were then fractionated on a C18 reversed-phase column. A high-performance liquid chromatography fraction containing a 28kDa 3H-labeled hydrophobic protein was collected, lyophilized, and subjected to SDS-PAGE and the separated proteins transferred to membrane. Protein in the recovered 28-kDa band was submitted for identification by N-terminal sequence analysis. Microsomal membranes prepared from fresh epithelia of intact bovine, rat, and human lens and cultured BLECs were tested for the presence of MSBP by western blot analysis using an antiserum to porcine liver microsomal MSBP. Radiolabeling of MSBP from 3H-mevalonate was confirmed by immunoprecipitation using the same antiserum. 3H-Progesterone was incubated with microsomal membrane from bovine lens epithelia to measure high-affinity binding. Radiolabeled progesterone-protein complexes were trapped on glass filters and radioactivity measured and the binding data subjected to Scatchard analysis. RESULTS: Membrane recovered from BLECs incubated with 3H-mevalonolactone contained a 3H-labeled 28-kDa protein fraction. The N-terminal sequence of the principal protein in this fraction was very similar to that of the recently discovered MSBP. Western blot analysis with antiserum to MSBP indicated the presence of the 28-kDa protein in the microsomal fraction from BLECs and epithelia of bovine, rat, and young human lenses but not in lens fiber cell membrane. Microsomal membrane from intact bovine lens epithelium bound progesterone with high affinity, with disso ciation constant (Kd) at approximately 75 nM and a receptor concentration of approximately 3 picomoles/mg protein. CONCLUSIONS: The lens epithelium contains a 28-kDa membrane protein that can bind progesterone and perhaps other steroid hormones with high affinity. The protein appears to be microsomal and prenylated. The MBSP may mediate rapid nongenomic steroid effects that contribute to steroid-induced cataracts. (+info)
Evidence for covalent attachment of diphytanylglyceryl phosphate to the cell-surface glycoprotein of Halobacterium halobium.
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In a previous study, we demonstrated the occurrence of novel proteins modified with a diphytanylglyceryl group in thioether linkage in Halobacterium halobium (Sagami, H., Kikuchi, A., and Ogura, K. (1995) J. Biol. Chem. 270, 14851-14854). In this study, we further investigated protein isoprenoid modification in this halobacterium using several radioactive tracers such as [3H]geranylgeranyl diphosphate. One of the radioactive bands observed on SDS-polyacrylamide gel electrophoresis corresponded to a periodic acid-Schiff stain-positive protein (200 kDa). Radioactive and periodic acid-Schiff stain-positive peptides (28 kDa) were obtained by trypsin digestion of the labeled proteins. The radioactive materials released by acid treatment of the peptides showed a similar mobility to dolichyl (C55) phosphate on a normal-phase thin-layer plate. However, radioactive hydrolysates obtained by acid phosphatase treatment co-migrated not with dolichol (C55-65), but with diphytanylglycerol on both reverse- and normal-phase thin-layer plates. The mass spectrum of the hydrolysate was also coincident with that of diphytanylglycerol. The partial amino acid sequences of the 28-kDa peptides were found in a fragment (amino acids 731-816) obtainable by trypsin cleavage of the known cell-surface glycoprotein of this halobacterium. These results indicate that the cell-surface glycoprotein (200 kDa) is modified with diphytanylglyceryl phosphate. (+info)
Lovastatin-mediated G1 arrest is through inhibition of the proteasome, independent of hydroxymethyl glutaryl-CoA reductase.
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In this paper we present the finding that lovastatin arrests cells by inhibiting the proteasome, which results in the accumulation of p21 and p27, leading to G1 arrest. Lovastatin is an inhibitor of hydroxymethyl glutaryl (HMG)-CoA reductase, the rate-limiting enzyme in cholesterol synthesis. Previously, we reported that lovastatin can be used to arrest cultured cells in the G1 phase of the cell cycle, resulting in the stabilization of the cyclin-dependent kinase inhibitors (CKIs) p21 and p27. In this report we show that this stabilization of p21 and p27 may be the result of a previously unknown function of the pro-drug, beta-lactone ring form of lovastatin to inhibit the proteasome degradation of these CKIs. The lovastatin mixture used in this study is 80% open-ring form and 20% pro-drug, beta-lactone form. We show that while the lovastatin open-ring form and pravastatin (a lovastatin analogue, 100% open ring) inhibit the HMG-CoA reductase enzyme, lovastatin pro-drug inhibits the proteasome but does not inhibit HMG-CoA reductase. In addition, many of the properties of proteasome inhibition by the pro-drug are the same as the specific proteasome inhibitor lactacystin. Lastly, mevalonate (used to rescue cells from lovastatin arrest) unexpectedly abrogates the lactacystin and lovastatin pro-drug inhibition of the proteasome. Mevalonate increases the activity of the proteasome, which results in degradation of the CKIs, allowing lovastatin- and lactacystin-arrested cells to resume cell division. The lovastatin-mediated inhibition of the proteasome suggests a unique mechanism for the chemopreventative effects of this agent seen in human cancer. (+info)
Identification and characterization of three novel missense mutations in mevalonate kinase cDNA causing mevalonic aciduria, a disorder of isoprene biosynthesis.
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Mevalonic aciduria is a rare autosomal recessive metabolic disorder, characterized by psychomotor retardation, failure to thrive, hepatosplenomegaly, anemia and recurrent febrile crises. The disorder is caused by a deficient activity of mevalonate kinase due to mutations in the encoding gene. Thus far, only two disease-causing mutations have been identified. We now report four different missense mutations including three novel ones, which were identified by sequence analysis of mevalonate kinase cDNA from three mevalonic aciduria patients. All mutations affect conserved amino acids. Heterologous expression of the corresponding mutant mevalonate kinases as fusion proteins with glutathione S -transferase in Escherichia coli showed a profound effect of each of the mutations on enzyme activity. In addition, immunoblot analysis of fibroblast lysates from patients using specific antibodies against mevalonate kinase identified virtually no protein. These results demonstrate that the mutations affect not only the activity but also the stability of the mutant proteins. (+info)
3-Hydroxy-3-methylglutaryl-CoA reductase inhibitors attenuate vascular smooth muscle proliferation by preventing rho GTPase-induced down-regulation of p27(Kip1).
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The mechanism by which platelet-derived growth factor (PDGF) regulates vascular smooth muscle cell (SMC) DNA synthesis is unknown, but may involve isoprenoid intermediates of the cholesterol biosynthetic pathway. Inhibition of isoprenoid synthesis with the 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor, simvastatin (Sim, 1-10 microM), inhibited PDGF-induced SMC DNA synthesis by >95%, retinoblastoma gene product hyperphosphorylation by 90%, and cyclin-dependent kinases (cdk)-2, -4, and -6 activity by 80 +/- 5, 50 +/- 3, and 48 +/- 3%, respectively. This correlated with a 20-fold increase in p27(Kip1) without changes in p16, p21(Waf1), or p53 levels compared with PDGF alone. Since Ras and Rho require isoprenoid modification for membrane localization and are implicated in cell cycle regulation, we investigated the effects of Sim on Ras and Rho. Up-regulation of p27(Kip1) and inhibition of Rho but not Ras membrane translocation by Sim were reversed by geranylgeranylpyrophosphate, but not farnesylpyrophosphate. Indeed, inhibition of Rho by Clostridium botulinum C3 transferase or overexpression of dominant-negative N19RhoA mutant increased p27(Kip1) and inhibited retinoblastoma hyperphosphorylation. In contrast, activation of Rho by Escherichia coli cytotoxic necrotizing factor-1 decreased p27(Kip1) and increased SMC DNA synthesis. These findings indicate that the down-regulation of p27(Kip1) by Rho GTPase mediates PDGF-induced SMC DNA synthesis and suggest a novel direct effect of 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors on the vascular wall. (+info)