Post-translational modification of low molecular mass GTP-binding proteins by isoprenoid

Metabolic labeling with an alkyne probe reveals similarities and differences in the prenylomes of several brain-derived cell lines and primary cells

Protein prenylation involves the attachment of one or two isoprenoid group(s) onto cysteine residues positioned near the C-terminus. This modification is essential for many signal transduction processes. In this work, the use of the probe C15AlkOPP for metabolic labeling and identification of prenylated proteins in a variety of cell lines and primary cells is explored. Using a single isoprenoid analogue, 78 prenylated protein groups from the three classes of prenylation substrates were identified including three novel prenylation substrates in a single experiment. Applying this method to three brain-related cell lines including neurons, microglia, and astrocytes showed substantial overlap (25%) in the prenylated proteins identified. In addition, some unique prenylated proteins were identified in each type. Eight proteins were observed exclusively in neurons, five were observed exclusively in astrocytes and three were observed exclusively in microglia, suggesting their unique roles in these cells. Furthermore, inhibition of farnesylation in primary astrocytes revealed the differential responses of farnesylated proteins to an FTI. Importantly, these results provide a list of 19 prenylated proteins common to all the cell lines studied here that can be monitored using the C15AlkOPP probe as well as a number of proteins that were observed in only certain cell lines. Taken together, these results suggest that this chemical proteomic approach should be useful in monitoring the levels and exploring the underlying role(s) of prenylated proteins in various diseases.

Activating Transcription Factor 4 (ATF4) modulates Rho GTPase levels and function via regulation of RhoGDIα

In earlier studies, we showed that ATF4 down-regulation affects post-synaptic development and dendritic spine morphology in neurons through increased turnover of the Rho GTPase Cell Division Cycle 42 (Cdc42) protein. Here, we find that ATF4 down-regulation in both hippocampal and cortical neuron cultures reduces protein and message levels of RhoGDIα, a stabilizer of the Rho GTPases including Cdc42. This effect is rescued by an shATF4-resistant active form of ATF4, but not by a mutant that lacks transcriptional activity. This is, at least in part, due to the fact that Arhgdia, the gene encoding RhoGDIα, is a direct transcriptional target of ATF4 as is shown in ChIP assays. This pathway is not restricted to neurons. This is seen in an impairment of cell migration on ATF4 reduction in non-neuronal cells. In conclusion, we have identified a new cellular pathway in which ATF4 regulates the expression of RhoGDIα that in turn affects Rho GTPase protein levels, and thereby, controls cellular functions as diverse as memory and cell motility.

Statins induce lethal effects in acute myeloblastic leukemia [corrected] cells within 72 hours

HMG-CoA Reductase inhibitors (statins) induce apoptosis in acute myeloid leukemia (AML) cells in vitro; however, the concentrations associated with cell death in AML cells are higher than those clinically tolerated during prolonged therapy. We therefore wished to determine whether short exposures to lovastatin might induce cell death in AML cells at clinically attainable concentrations. The time and concentration dependence of HL60 and U937 cells was determined and showed that cell death was delayed. IC₅₀ values and IC₉₀ values determined on day 6 suggested that the sensitivity of AML cells to statins may occur at lower concentrations than previously reported. After 72 h, mevalonate did not rescue AML cells from cytotoxic concentrations of statins, suggesting that, although cell death was delayed, lovastatin induced lethal effects within 72 h. In conjunction with previously reported Phase I studies, the data presented here suggest that the high-dose, short course statins may be useful for the treatment of patients with AML.

ROCK and Rho: biochemistry and neuronal functions of Rho-associated protein kinases

Rho-associated protein kinases (ROCKs) play key roles in mediating the control of the actin cytoskeleton by Rho family GTPases in response to extracellular signals. Such signaling pathways contribute to diverse neuronal functions from cell migration to axonal guidance to dendritic spine morphology to axonal regeneration to cell survival. In this review, the authors summarize biochemical knowledge of ROCK function and categorize neuronal ROCK-dependent signaling pathways. Further study of ROCK signal transduction mechanisms and specificities will enhance our understanding of brain development, plasticity, and repair. The ROCK pathway also provides a potential site for therapeutic intervention to promote neuronal regeneration and to limit degeneration.

Thematic review series: lipid posttranslational modifications. Fighting parasitic disease by blocking protein farnesylation

Protein farnesylation is a form of posttranslational modification that occurs in most, if not all, eukaryotic cells. Inhibitors of protein farnesyltransferase (PFTIs) have been developed as anticancer chemotherapeutic agents. Using the knowledge gained from the development of PFTIs for the treatment of cancer, researchers are currently investigating the use of PFTIs for the treatment of eukaryotic pathogens. This "piggy-back" approach not only accelerates the development of a chemotherapeutic agent for protozoan pathogens but is also a means of mitigating the costs associated with de novo drug design. PFTIs have already been shown to be efficacious in the treatment of eukaryotic pathogens in animal models, including both Trypanosoma brucei, the causative agent of African sleeping sickness, and Plasmodium falciparum, one of the causative agents of malaria. Here, current evidence and progress are summarized that support the targeting of protein farnesyltransferase for the treatment of parasitic diseases.

Regulation of Cystic Fibrosis Transmembrane Regulator Trafficking and Protein Expression by a Rho Family Small GTPase TC10

The cystic fibrosis transmembrane conductance regulator (CFTR)-interacting protein, CFTR-associated ligand (CAL) down-regulates total and cell surface CFTR by targeting CFTR for degradation in the lysosome. Here, we report that a Rho family small GTPase TC10 interacts with CAL. This interaction specifically up-regulates CFTR protein expression. Co-expression of the constitutively active form, TC10Q75L, increases total and cell surface CFTR in a dose-dependent fashion. Moreover, co-expression of the dominant-negative mutant TC10T31N causes a dose-dependent reduction in mature CFTR. The effect of TC10 is independent of the level of CFTR expression, because a similar effect was observed in a stable cell line that expresses one-tenth of CFTR. Co-expression of TC10Q75L did not have a similar effect on the expression of plasma membrane proteins such as Frizzled-3 and Pr-cadherin or cytosolic proteins such as tubulin and green fluorescent protein. TC10Q75L also did not have a similar effect on the vesicular stomatitis virus glycoprotein. Co-expression of constitutively active and dominant-negative forms of Cdc42 or RhoA did not affect CFTR expression in a manner similar to TC10, indicating that the effect of TC10 is unique within the Rho family. Metabolic pulse-chase experiments show that TC10 did not affect CFTR maturation, suggesting that it exerts its effects on the mature CFTR. Importantly, TC10Q75L reverses CAL-mediated CFTR degradation, suggesting that TC10Q75L inhibits CAL-mediated degradation of CFTR. TC10Q75L does not operate by reducing CAL protein expression or its ability to form dimers or interact with CFTR. Interestingly, the expression of TC10Q75L causes a dramatic redistribution of CAL from the juxtanuclear region to the plasma membrane where the two molecules overlap. These data suggest that TC10 regulates both total and plasma membrane CFTR expression by interacting with CAL. The GTP-bound form of TC10 directs the trafficking of CFTR from the juxtanuclear region to the secretory pathway toward the plasma membrane, away from CAL-mediated degradation of CFTR in the lysosome.

ExoS Rho GTPase-activating protein activity stimulates reorganization of the actin cytoskeleton through Rho GTPase guanine nucleotide disassociation inhibitor

ExoS is a bifunctional Type III cytotoxin of Pseudomonas aeruginosa with N-terminal Rho GTPase-activating protein (RhoGAP) and C-terminal ADP-ribosyltransferase domains. Although the ExoS RhoGAP inactivates Cdc42, Rac, and RhoA in vivo, the relationship between ExoS RhoGAP and the eukaryotic regulators of Rho GTPases is not clear. The present study investigated the roles of Rho GTPase guanine nucleotide disassociation inhibitor (RhoGDI) in the reorganization of actin cytoskeleton mediated by ExoS RhoGAP. A green fluorescent protein-RhoGDI fusion protein was engineered and found to elicit actin reorganization through the inactivation of Rho GTPases. Green fluorescent protein-RhoGDI and ExoS RhoGAP cooperatively stimulated actin reorganization and translocation of Cdc42 from membrane to cytosol, and a RhoGDI mutant, RhoGDI(I177D), that is defective in extracting Rho GTPases off the membrane inhibited the actions of RhoGDI and ExoS RhoGAP on the translocation of Cdc42 from membrane to cytosol. A human RhoGDI small interfering RNA was transfected into HeLa cells to knock down 90% of the endogenous RhoGDI expression. HeLa cells with knockdown RhoGDI were resistant to the reorganization of the actin cytoskeleton elicited by type III-delivered ExoS RhoGAP. This indicates that ExoS RhoGAP and RhoGDI function in series to inactivate Rho GTPases, in which RhoGDI extracting GDP-bound Rho GTPases off the membrane and sequestering them in cytosol is the rate-limiting step in Rho GTPase inactivation. A eukaryotic GTPase-activating protein, p50RhoGAP, showed a similar cooperativity with RhoGDI on actin reorganization, suggesting that ExoS RhoGAP functions as a molecular mimic of eukaryotic RhoGAPs to inactivate Rho GTPases through RhoGDI.

Statins as Immunomodulatory Agents

Statins have long been thought to exert their benefits by reducing cholesterol synthesis. However, the fact that mevalonate is the precursor of isoprenoids that regulate diverse cellular functions has led investigators to examine pleiotropic effects for these agents. Major histocompatibility complex class II (MHC-II) molecules, which affect the immune response and organ rejection after transplantation, may be induced by the proinflammatory cytokine interferon gamma (IFN-γ). An experiment was conducted to determine whether statins affect the regulation of MHC-II expression by IFN-γ in cultured human endothelial cells and monocyte/macrophages. Statins were found to repress the induction of MHC-II by IFN-γ. This may explain the immunosuppressive effects of statins seen in two clinical trials of organ transplantation and suggest a potential role for statins as immunosuppressive agents.

Statins as immunomodulators

HMG-CoA reductase inhibitors, or statins, are effective lipid lowering agents, extensively used in medical practice. Statins have never been shown to be involved in the immune response, although few clinical reports have suggested a better outcome of cardiac transplantation in patients under pravastatin therapy. Major histocompatibility complex class II (MHC-II) molecules are directly involved in the activation of T lymphocytes and in the control of the immune response. Whereas only a limited number of specialized cell types express MHC-II constitutively, numerous other cells become MHC-II positive upon induction by interferon gamma (IFN-gamma). We and others recently demonstrated that statins act as direct inhibitors of induction of MHC-II expression by IFN-gamma and thus as repressors of MHC-II-mediated T cell activation. This effect was observed in several cell types, including primary human endothelial cells and macrophages. Interestingly, this inhibition is specific for inducible MHC-II expression and does not concern either constitutive expression of MHC-II or expression of MHC-I. In repressing induction of MHC-II, and subsequent T lymphocyte activation, statins therefore behave as a novel type of immunomodulator. This unexpected effect provides a scientific rationale for suggesting the use of statins as novel immunosuppressors, not only in organ transplantation but in numerous other pathologies as well.

Immunosuppressive effects of statins

3-Hydroxy-3-methhylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, or statins, are effective lipid lowering agents, extensively used in medical practice [Circulation 2000;101:207; J Am Med Assoc 2000;283:2935]. Statins have never been shown to be involved in the immune response, although reports have suggested a better outcome of cardiac transplantation in patients under pravastatin therapy [New Engl J Med 1995;333:621; Circulation 1997;96:1398]. Major histocompatibility complex class II (MHC class II) molecules are directly involved in the activation of T lymphocytes and in the control of the immune response. Whereas only a limited number of specialized cell types express MHC-II constitutively, numerous other cells become MHC-II positive upon induction by interferon-gamma (IFN-gamma) [Annu Rev Immunol 1996;14:301]. This complex regulation is under the control of the transactivator CIITA [Science 1994;265:106]. Recently, we demonstrated that statins act as direct inhibitors of induction of MHC-II expression by IFN-gamma and thus as repressors of MHC-II-mediated T cell activation [Nat Med 2000;6:1399; Swiss Med Wkly 2001;131:41]. This effect of statins is due to inhibition of the inducible promoter IV of the transactivator CIITA. Interestingly, this inhibition is specific for inducible MHC-II expression and does not concern constitutive expression of CIITA and MHC-II. In repressing induction of MHC-II, and subsequent T lymphocyte activation, statins therefore behave as a novel type of immunomodulator. This unexpected effect provides a scientific rationale for suggesting the use of statins as novel immunosuppressors, not only in organ transplantation but in numerous other pathologies as well.

Identification and characterisation of Toxoplasma gondii protein farnesyltransferase

Prenylated proteins are involved in the regulation of DNA replication and cell cycling and have important roles in the regulation of cell proliferation. Protein farnesyltransferase and protein geranylgeranyltransferase are the two enzymes responsible for catalysing isoprene lipid modifications. Recently these enzymes have been targets for the development of cancer chemotherapeutics. Using metabolic labelling we identified isoprenylated proteins which suggests the presence of protein farnesyltransferase in Toxoplasma gondii. T. gondii protein farnesyltransferase is heat-labile and requires Mg(2+) and Zn(2+) ions for full activity. Peptidomimetic analogues as well as short synthetic peptides were tested in vitro as possible competitors for farnesyltransferase substrates. We found that the synthetic peptide (KTSCVIA) specifically inhibited T. gondiiprotein farnesyltransferase but not mammalian (HeLa cells) farnesyltransferase. Therefore this study suggests the possible development of specific inhibitors of T. gondiiprotein farnesyltransferase as an approach to parasitic protozoa therapy.

Effect of simvastatin alone and in combination with cytosine arabinoside on the proliferation of myeloid leukemia cell lines

BACKGROUND

Cholesterol biosynthesis is regulated by the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Cholesterol and its derivatives are required in high concentrations by neoplastic proliferating cells for both DNA synthesis and cell growth. Thus, inhibition of HMG-CoA reductase could effect cell cycle progression and proliferation. Therefore, we examined the effect of an HMG-CoA reductase inhibitor (simvastatin) alone and in combination with cytosine arabinoside (ARA-C) on the proliferation of two AML cell lines.

METHODS

AML blasts derived from two cell lines (HL-60 and AML-2) were incubated with increasing concentrations of either simvastatin alone or simvastatin alone for 24 hours with ARA-C added thereafter. The effect of the drugs on cell proliferation in liquid culture (3H thymidine uptake) and on clonogenic assay was analyzed.

RESULTS

We found that the number of proliferating AML blasts (suspension cultures) and colony formations (agar cultures) of both cell lines declined significantly after incubation with simvastatin. Preincubation of both cell lines with simvastatin by the addition of increasing concentrations of ARA-C produced a degree of growth inhibition that was significantly greater than that of the individual compounds. This antigrowth interaction was additive rather than synergistic.

CONCLUSIONS

We conclude that simvastatin has a major antiproliferative effect on AML blasts in vitro. Also, the combination of simvastatin and ARA-C significantly enhanced the antiproliferative effect of each drug. These findings may open new avenues in both the laboratory and clinical research of the treatment of leukemia.

Retention of the Alzheimer's amyloid precursor fragment C99 in the endoplasmic reticulum prevents formation of amyloid beta-peptide

gamma-Secretase is a membrane-associated endoprotease that catalyzes the final step in the processing of Alzheimer's beta-amyloid precursor protein (APP), resulting in the release of amyloid beta-peptide (Abeta). The molecular identity of gamma-secretase remains in question, although recent studies have implicated the presenilins, which are membrane-spanning proteins localized predominantly in the endoplasmic reticulum (ER). Based on these observations, we have tested the hypothesis that gamma-secretase cleavage of the membrane-anchored C-terminal stump of APP (i.e. C99) occurs in the ER compartment. When recombinant C99 was expressed in 293 cells, it was localized mainly in the Golgi apparatus and gave rise to abundant amounts of Abeta. Co-expression of C99 with mutant forms of presenilin-1 (PS1) found in familial Alzheimer's disease resulted in a characteristic elevation of the Abeta(42)/Abeta(40) ratio, indicating that the N-terminal exodomain of APP is not required for mutant PS1 to influence the site of gamma-secretase cleavage. Biogenesis of both Abeta(40) and Abeta(42) was almost completely eliminated when C99 was prevented from leaving the ER by addition of a di-lysine retention motif (KKQN) or by co-expression with a dominant-negative mutant of the Rab1B GTPase. These findings indicate that the ER is not a major intracellular site for gamma-secretase cleavage of C99. Thus, by inference, PS1 localized in this compartment does not appear to be active as gamma-secretase. The results suggest that presenilins may acquire the characteristics of gamma-secretase after leaving the ER, possibly by assembling with other proteins in peripheral membranes.

Antiviral activity of lovastatin against respiratory syncytial virus in vivo and in vitro

Respiratory syncytial virus (RSV) is an important human pathogen that can cause severe and life-threatening respiratory infections in infants and immunocompromised adults. We have recently shown that the RSV F glycoprotein, which mediates viral fusion, binds to RhoA. One of the steps in RhoA activation involves isoprenylation at the carboxy terminus of the protein by geranylgeranyltransferase. This modification allows RhoA to be attached to phosphatidyl serine on the inner leaflet of the plasma membrane. Treatment of mice with lovastatin, a drug that inhibits prenylation pathways in the cell by directly inhibiting hydroxymethylglutaryl coenzyme A reductase, diminishes RSV but not vaccinia virus replication when administered up to 24 h after RSV infection and decreases virus-induced weight loss and illness in mice. The inhibition of replication is not likely due to the inhibition of cholesterol biosynthesis, since gemfibrozil, another cholesterol-lowering agent, did not affect virus replication and serum cholesterol levels were not significantly lowered by lovastatin within the time frame of the experiment. Lovastatin also reduces cell-to-cell fusion in cell culture and eliminates RSV replication in HEp-2 cells. These data indicate that lovastatin, more specific isoprenylation inhibitors, or other pharmacological approaches for preventing RhoA membrane localization should be considered for evaluation as a preventive antiviral therapy for selected groups of patients at high risk for severe RSV disease, such as the institutionalized elderly and bone marrow or lung transplant recipients.

Protein geranylgeranylation is required for osteoclast formation, function, and survival: inhibition by bisphosphonates and GGTI-298

Bisphosphonates are the important class of antiresorptive drugs used in the treatment of metabolic bone diseases. Although their molecular mechanism of action has not been fully elucidated, recent studies have shown that the nitrogen-containing bisphosphonates can inhibit protein prenylation in macrophages in vitro. In this study, we show that the nitrogen-containing bisphosphonates risedronate, zoledronate, ibandronate, alendronate, and pamidronate (but not the non nitrogen-containing bisphosphonates clodronate, etidronate, and tiludronate) prevent the incorporation of [14C]mevalonate into prenylated (farnesylated and geranylgeranylated) proteins in purified rabbit osteoclasts. The inhibitory effect of nitrogen-containing bisphosphonates on bone resorption is likely to result largely from the loss of geranylgeranylated proteins rather than loss of farnesylated proteins in osteoclasts, because concentrations of GGTI-298 (a specific inhibitor of geranylgeranyl transferase I) that inhibited protein geranylgeranylation in purified rabbit osteoclasts prevented osteoclast formation in murine bone marrow cultures, disrupted the osteoclast cytoskeleton, inhibited bone resorption, and induced apoptosis in isolated chick and rabbit osteoclasts in vitro. By contrast, concentrations of FTI-277 (a specific inhibitor of farnesyl transferase) that prevented protein farnesylation in purified rabbit osteoclasts had little effect on osteoclast morphology or apoptosis and did not inhibit bone resorption. These results therefore show the molecular mechanism of action of nitrogen-containing bisphosphonate drugs in osteoclasts and highlight the fundamental importance of geranylgeranylated proteins in osteoclast formation and function.

Inhibition of cholesterol production but not of nonsterol isoprenoid products induces neuronal cell death

Deficiency of nonsterol isoprenoids, intermediate metabolites of the cholesterol biosynthetic pathway, has been known to cause an inhibition of DNA synthesis and cell growth, and to induce apoptosis in nonneuronal cells. To investigate whether this is also the case in neurons, we examined the effect of a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor on the viability of neuronal cultures prepared from fetal rat brains. Treatment with compactin, a competitive inhibitor of HMG-CoA reductase, induced neuronal death in a dose-dependent manner. Concurrent treatment with cholesterol, beta-migrating very low density lipoprotein, mevalonate, or squalene substantially inhibited the induction of neuronal death by compactin. Cell death was also induced by treatment with squalestatin, which specifically inhibits cholesterol biosynthesis at a site downstream from the generation of nonsterol metabolites. Furthermore, squalestatin-induced neuronal death was inhibited by concurrent incubation with squalene but not mevalonate. In contrast, cell growth of proliferating cells such as NIH 3T3 and PC12 cells was exclusively dependent on the level of nonsterol isoprenoid products and not that of cholesterol. The results of this study clearly indicate that the viability of neurons, different from that of nonneuronal cells, depends on the intracellular cholesterol content and not on the intermediate nonsterol isoprenoid products.

Enhancement of sterol synthesis by the monoterpene perillyl alcohol is unaffected by competitive 3-hydroxy-3-methylglutaryl-CoA reductase inhibition

Monoterpenes such as limonene and perillyl alcohol (PA) are currently under investigation for their chemotherapeutic properties which have been tied to their ability to affect protein isoprenylation. Because PA affects the synthesis of isoprenoids, such as ubiquinone, and cholesterol is the end product of the synthetic pathway from which this isoprenoid pathway branches, we investigated the effects of this compound upon cholesterol metabolism in the colonic adenocarcinoma cell line SW480. PA (1 mM) inhibited incorporation of 14C-mevalonate into 21-26 kDa proteins by 25% in SW480 cells. Cholesterol (CH) biosynthesis was assessed by measuring the incorporation of 14C-acetate and 14C-mevalonate into 27-carbon-sterols. Cells treated with PA (1 mM) exhibited a fourfold increase in the incorporation of 14C-acetate but not 14C-mevalonate into cholesterol. Mevinolin (lovastatin), an inhibitor of 3-hydroxy-3-methylglutaryl-CoA(HMG-CoA) reductase, at 2 microM concentration, inhibited CH synthesis from 14C-acetate by 80%. Surprisingly, concurrent addition of mevinolin and PA did not significantly alter the stimulatory effects of PA. As observed differences in 14C-acetate and 14C-mevalonate precursor labeling could indicate PA affects early pathway events, the effects of this monoterpene on HMG-CoA reductase activity were evaluated. Unexpectedly, 1 mM PA did not stimulate activity of this enzyme. Consistent with its action as a reversibly bound inhibitor, in washed microsomes, 2 microM mevinolin pretreatment increased reductase protein expression causing a 12.7 (+/- 2.4)-fold compensatory HMG-CoA reductase activity increase; concurrent treatment with 1 mM PA attenuated this to a 5.3 (+/- 0.03)-fold increase. Gas chromatographic analysis confirmed CH was the major lipid present in the measured thin-layer chromatography spot. Since 14C-acetate incorporation into free fatty acid and phospholipid pools was not significantly affected by PA treatment, nonspecific changes in whole acetate pool sizes were not indicated. Because increases in endogenous CH synthesis should result in compensatory changes in exogenous sterol utilization, the effects of PA upon low density lipoprotein (LDL) receptor activity were evaluated. Consistent with the observed increases in CH synthesis, 1 mM PA decreased 125I-LDL internalization to 50% of the fetal bovine serum control; concurrent addition of 2 microM mevinolin attenuated this effect to a reduction of 80% of the control value. Data suggest that in certain colonic tumor cells PA strongly affects cholesterol metabolism via a mechanism of action that is insensitive to the HMG-CoA reductase inhibitor mevinolin.

Application of two-dimensional protein gels in biotechnology

The optimal use of biological systems for technologically developed products will not be achieved until biological systems are completely defined in biochemical terms. Two-dimensional polyacrylamide gel electrophoresis, 2-D gels, are contributing to this goal. These gels separate complex mixtures of proteins into individual polypeptide species. The ultimate use of 2-D gels is the construction of cellular 2-D gel databases which identify the proteins on the gels and catalog their responses to different environmental conditions. In addition to these global analyses, many applications for 2-D gels in basic, applied and clinical research have been shown.

Protein prenyl transferase activities of Plasmodium falciparum

Prenylated proteins have been shown to function in important cellular regulatory processes including signal transduction. The enzymes involved in protein prenylation, farnesyl transferase and geranylgeranyl transferase, have been recent targets for development of cancer chemotherapeutics. We have initiated a systematic study of protein prenyl transferases of the malaria parasite, Plasmodium falciparum, to determine whether these enzymes can be developed as targets for antimalarial chemotherapy. We report here the identification of protein farnesyl transferase and protein geranylgeranyl transferase-I in the malaria parasite, P. falciparum. The farnesyl transferase has been partially purified from the cytosolic fraction through ammonium sulfate precipitation and Mono-Q chromatography. Farnesyl and geranylgeranyl transferase-I activities are present at all stages of P. falciparum intraerythrocytic development with maximum specific activity in the ring stage. Geranylgeranyl transferase-I specific activity is two times that of farnesyl transferase in the ring stage. Peptidomimetics and prenyl analogues of protein farnesyl transferase substrates were tested as in vitro inhibitors of partially purified P. falciparum prenyl transferase and of malaria parasite growth. The peptidomimetics were significantly more potent inhibitors than lipid substrate analogues of both the activity of Mono-Q purified enzyme and parasite growth in intraerythrocytic cultures. Exposure of the parasite to the peptidomimetic L-745,631 also showed significant inhibition of morphological development beyond the trophozoite stage. These studies suggest the potential of designing or identifying differential inhibitors of P. falciparum and mammalian prenyl transferases as an approach to novel malaria therapy.

Association of Rab1B with GDP-dissociation inhibitor (GDI) is required for recycling but not initial membrane targeting of the Rab protein

We have identified the Rab1B effector-domain mutant (D44N) that, when geranylgeranylated by Rab:geranylgeranyltransferase (GGTase II) in cell-free systems or intact cells, fails to form detectable complexes with GDP-dissociation inhibitors (GDIs). GDI-Rab complexes were collected on anti-FLAG affinity beads after incubating recombinant geranylgeranylated Rab1B with FLAG epitope-tagged GDI in vitro, or transiently coexpressing Myc-tagged Rab1B with FLAG-GDI-alpha or FLAG-GDI-2 in human embryonal kidney 293 cells. [3H]Mevalonate labeling and immunoprecipitation studies confirmed that the inability of Myc-Rab1BD44N to associate with GDI in vivo was not due to failure of the mutant to undergo geranylgeranylation. Immunofluorescence localization and immunoblot analysis of subcellular fractions indicated that expressed Myc-Rab1BD44N was efficiently delivered to intracellular membranes in 293 cells. This was confirmed when the fate of the prenylated pool of Rab1BD44N in 293 cells was traced by labeling the geranylgeranyl groups attached to the nascent protein with [3H]meval onate. However, in contrast to the prenylated Rab1BWT, which was distributed in both the membrane and soluble fractions, the prenylated Rab1BD44N was completely absent from the cytosol. Overexpression of Myc-Rab1BD44N did not impair ER --> Golgi glycoprotein trafficking in 293 cells, which was assessed by monitoring the Golgi-dependent processing of coexpressed beta-amyloid precursor protein. The current findings suggest that nascent prenylated Rab1B can be delivered to intracellular membranes in intact cells without forming a stable complex with GDI, but that recycling of prenylated Rab1B to the cytosolic compartment is absolutely dependent on GDI interaction.

Differential effects of a Rab6 mutant on secretory versus amyloidogenic processing of Alzheimer's beta-amyloid precursor protein

The Ras-related GTP-binding protein, Rab6, is localized in late Golgi compartments where it mediates intra-Golgi vesicular trafficking. Herein we report that coexpression of Alzheimer's beta-amyloid precursor protein (beta APP751) with a dominant-negative Rab6 mutant (Rab6N126I) in human embryonal kidney 293 cells causes an increase in secretion of the soluble amino-terminal exodomain (s-APP alpha) derived from non-amyloidogenic processing of beta-APP751 by alpha-secretase. The effect was specific to Rab6N126I, since the corresponding mutation in Rab8 (i.e. Rab8N121I), which has been implicated in protein transport to the plasma membrane, caused a modest reduction in s-APP alpha secretion. While Rab6N126I stimulated secretion of APP alpha, the accumulation of amyloid beta peptide (A beta) in the medium was either moderately reduced or unaffected. Similar differential effects of Rab6N126I on secretion of s-APP alpha versus A beta were observed in cell cultures that were overproducing A beta after transfection with a plasmid encoding Swedish variant of beta APP751. Moreover, assays of medium from the latter cultures revealed a marked increase in secretion of s-APP alpha relative to s-APP beta (the immediate product derived from cleavage of beta APP by beta-secretase). The results indicate that vesicular transport events controlled by Rab6 occur at or near a critical juncture in the trans-Golgi network where beta APP is sorted into either the constitutive alpha-secretase pathway or the amyloidogenic beta-secretase pathway.

Mammary tumorigenesis and chemoprevention studies in carcinogen-treated rats

The objectives of this review are to describe the induction of mammary gland tumors by chemical carcinogens and to discuss their application to mammary cancer chemoprevention research. Special emphasis will be placed on the dimethylbenz[a]anthracene (DMBA) and methylnitrosourea (MNU) models because of the extensive information available about the pathogenesis of tumor growth associated with these two compounds. Both models have been widely used in the investigation of novel cancer chemopreventive agents. The current status of a number of different approaches will be summarized briefly here to provide an overview of research opportunities. Despite the popularity of the DMBA and MNU models in laboratory studies of mammary cancer biology and prevention, neither of these carcinogens has ever been implicated in the etiology of human breast cancer. This shortcoming has prompted a growing interest in other relevant environmental chemicals which are capable of producing mammary tumors in experimental animals. The new models have yet to be fully characterized, but they may be more appropriate than the DMBA and MNU models as paradigms for assessing cancer risk in humans and for developing suitable cancer prevention strategies.

Selective inhibition of Ras-dependent cell growth by farnesylthiosalisylic acid

S-trans,trans-Farnesylthiosalicylic acid (FTS) is a novel farnesylated rigid carboxylic acid derivative. In cell-free systems, it acts as a potent competitive inhibitor (Ki = 2.6 microM) of the enzyme prenylated protein methyltransferase (PPMTase), which methylates the carboxyl-terminal S-prenylcysteine in a large number of prenylated proteins including Ras. In such systems, FTS inhibits Ras methylation but not Ras farnesylation. Inhibition of the PPMTase by FTS in homogenates or membranes of a variety of tissues and cell lines is inferred from a block in the methylation of exogenously added substrates such as N-acetyl-S-trans,trans-farnesyl-L-cysteine and of endogenous substrates including small GTP-binding proteins. FTS can also inhibit methylation of these proteins in intact cells (e.g. in Rat-1 fibroblasts, Ras-transformed Rat-1, and B16 melanoma cells). Unlike in cell-free systems, however, relatively high concentrations of FTS (50-100 microM) are required for partial blocking (10-40%) of protein methylation in the intact cells. Thus, FTS is a weak inhibitor of methylation in intact cells. Because methylation is the last step in the processing of Ras and related proteins, FTS is not likely to affect steps that precede it, e.g. protein prenylation. This may explain why the growth and gross morphology of a variety of cultured cell types (including Chinese hamster ovary, NIH3T3, Rat1, B16 melanoma, and PC12) is not affected by up to 25 microM FTS and is consistent with the observed lack of FTS-induced cytotoxicity. Nevertheless, FTS reduces the levels of Ras in cell membranes and can inhibit Ras-dependent cell growth in vitro, independently of methylation. It inhibits the growth of human Ha-ras-transformed cells (EJ cells) and reverses their transformed morphology in a dose-dependent manner (0.1-10 microM). The drug does not interfere with the growth of cells transformed by v-Raf or T-antigen but inhibits the growth of ErbB2-transformed cells and blocks the mitogenic effects of epidermal and basic fibroblast growth factors, thus implying its selectivity toward Ras growth signaling, possibly via modulation of Ras-Raf communication. Taken together, the results raise the possibility that FTS may specifically interfere with the interaction of Ras with a farnesylcysteine recognition domain in the cell membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular transport and maturation of nascent low density lipoprotein receptor is blocked by mutation in the Ras-related GTP-binding protein, RAB1B

The relationship between the Ras-related GTP-binding protein, Rab1B, and intracellular transport of nascent low density lipoprotein (LDL) receptor was studied in cultured human embryonic kidney cells (line 293) cotransfected with plasmids encoding the LDL-receptor and either wild-type Rab1B or a Rab1B mutant (N121I) known to act as a dominant suppressor of endogenous Rab1B function. [35S]Methionine pulse-chase analysis of immunoprecipitated LDL-receptor indicated that coexpression with Rab1BN121I, but not Rab1BWT, impaired its conversion from the Endo-H-sensitive 120-125 kDa form to the O-glycosylated 160-170 kDa form, consistent with a block in ER-->Golgi trafficking of the nascent receptor. In cells expressing Rab1BN121I, the newly synthesized LDL-receptor was unable to reach the cell surface as evidenced by its inaccessibility to sulfo-NHS-biotin added to the cultures. These observations provide a direct demonstration of Rab protein involvement in LDL receptor trafficking and lend support to the concept of Rab1B as a universal mediator of ER-->Golgi transport of membrane glycoproteins in mammalian cells.

Effects of pravastatin, a hydroxymethylglutaryl-CoA reductase inhibitor, on two human tumour cell lines

Competitive inhibitors of 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) reductase are currently used to treat patients with hypercholesterolaemia. These inhibitors affect not only cholesterol biosynthesis, but also the production of non-steroidal mevalonate derivatives, that are involved in a number of growth-regulatory processes. As a consequence, their potential use as anticancer drugs has been suggested. In order to examine long-term effects of this potential therapeutic approach, we cultivated the gastric carcinoma cell line, EPG85-257, and the breast tumour cell line, MDA-MB231, in the presence of increasing concentrations of the HMG-CoA reductase inhibitor, pravastatin. For both cell lines, this procedure led to the selection of resistant variants able to proliferate in more than 1000 microM inhibitor. By competitive reverse transcriptase/polymerase chain reaction assay (cRT-PCR), the expression of the mRNA for two key proteins of cellular cholesterol metabolism, HMG-CoA reductase and low-density lipoprotein (LDL) receptor, were analysed in sensitive and resistant cells. Despite similar growth rates, MDA-MB231 cells expressed approximately four times more HMG-CoA reductase mRNA than EPG85-257 cells and over 30 times more LDL receptor mRNA. Both mRNA species were coordinately regulated in the parental and in the pravastatin-resistant variant cells. Expression was highly stimulated (3- to 4-fold for the HMG-CoA reductase and 2- to 3-fold for the LDL receptor) in the resistant variants when cultured in lipoprotein-deficient medium in the presence of 1000 microM pravastatin. Immunocytological analysis of the expression of the HMG-CoA reductase and LDL receptor protein were in accordance with the data on specific mRNA expression obtained by cRT-PCR. Southern blot analysis revealed a 1.5-fold amplification of the HMG-CoA reductase gene in resistant MDA-MB231 cells, but not in the resistant EPG85-257 variant. Our data provide evidence for resistance mechanisms to pravastatin that are independent of the amplification of the HMG-CoA reductase gene. By analogy to the cell-culture models employed in this study, it is conceivable that similar mechanisms might occur in human tumour cells in vivo during long-term treatment with HMG-CoA reductase inhibitors. This might limit their application as chemotherapeutic anticancer agents.

The Ras-related GTP-binding protein, Rab1B, regulates early steps in exocytic transport and processing of beta-amyloid precursor protein

The role of the Ras-related GTP-binding protein, Rab1B, in intracellular trafficking of beta-amyloid precursor protein (beta APP) was studied in cultured 293 cells. beta APP is processed via one of two alternative routes. In the major secretory pathway, beta APP is cleaved by alpha-secretase within the region comprising the beta-amyloid peptide (A beta), resulting in release of a soluble NH2-terminal exodomain (APP alpha) and a 3-kDa peptide (p3) derived from the carboxyl-terminal tail. In the alternative amyloidogenic pathway, beta APP is cleaved by beta-secretase, with the release of a truncated exodomain (APP beta) and an intact A beta peptide. When beta APP751 was coexpressed with Rab1B(wt) or dominant-negative Rab1B mutants (Rab1BN121I or Rab1BS22N) there was a marked decrease in conversion of the immature Endo-H sensitive form of beta APP751 (108 kDa) to the mature O-glycosylated form of beta APP751 (130 kDa) in cells expressing the mutant forms of Rab1B. The block in Golgi-dependent processing of beta APP was accompanied by inhibition of secretion of APPS (APP alpha). A similar decrease in secretion of APPS (APP alpha+APP beta) was observed in cells that were coexpressing Rab1BN121I with the "Swedish" variant of beta APP751 (i.e. beta APPSW751), which undergoes increased amyloidogenic processing. Coincident with the decline in APPS secretion, the cells coexpressing beta APPSW751 with Rab1BN121I showed a 90% decrease in A beta secretion. The data indicate that Rab1B plays a key role in endoplasmic reticulum-->Golgi transport of beta APP, and that beta APP must pass through a late Golgi compartment before entering either the alpha-secretase or the amyloidogenic beta-secretase pathway. The results also suggest that mutant versions of other Rab proteins that function in different parts of the exocytic and endocytic pathways may be useful in defining the specific routes of beta APP transport involved in the biogenesis of A beta.

Characterization of the prenylated protein methyltransferase in human endometrial carcinoma

The processing of ras and of other GTP-binding proteins includes a final reversible step in which the carboxy terminal prenylated cysteine is methylated by the enzyme prenylated protein methyltransferase (PPMTase). The significance of this modification and of the role of PPMTase in human tumors has yet to be fully elucidated. Here we characterize the PPMTase of human endometrial carcinomas (tumors in which the frequency of ras gene mutations is relatively high) and compare it to the PPMTase of the normal endometrium. Our results show that in both types of tissues the enzyme is bound to the membranes. It can utilize synthetic substrates such as N-acetyl-S-farnesyl-L-cysteine (Km = 18-20 microM) and is blocked by the PPMTase inhibitor S-farnesylthioacetic acid (Ki = 2 microM). In vitro methylation assays and [alpha-32P]GTP blot-overlay assays showed that the major endogenous PPMTase substrates are small GTP-binding proteins. Methylation of these proteins in vitro is blocked by farnesylthioacetic acid. The kinetic properties of PPMTase from the carcinomas and the normal tissues are very similar. However, levels of PPMTase activity (but not of its endogenous substrates) are higher in the carcinomatous endometrium than in the normal one. The elevated enzyme activity is restricted to the crude mitochondrial fraction (8.0 +/- 0.4 vs. 5.4 +/- 0.1 pmol N-acetyl farnesylcysteine methyl ester formed/min/mg protein by the carcinoma and by the normal endometrial preparations, respectively). As this fraction is enriched in plasma membranes, it appears that the elevated enzyme activity could be related to ras protein methylation; if so, selective methylation blockers might inhibit the growth of endometrial carcinomas.

Structure-activity relationships among monoterpene inhibitors of protein isoprenylation and cell proliferation

The monoterpene d-limonene inhibits the post-translational isoprenylation of p21ras and other small G proteins, a mechanism that may contribute to its efficacy in the chemoprevention and therapy of chemically induced rodent cancers. In the present study, the relative abilities of 26 limonene-like monoterpenes to inhibit protein isoprenylation and cell proliferation were determined. Many monoterpenes were found to be more potent than limonene as inhibitors of small G protein isoprenylation and cell proliferation. The relative potency of limonene-derived monoterpenes was found to be: monohydroxyl = ester = aldehyde > thiol > acid = diol = epoxide > triol = unsubstituted. All monoterpenes that inhibited protein isoprenylation did so in a selective manner, such that 21-26 kDa proteins were preferentially affected. Perillyl alcohol, one of the most potent terpenes, reduced 21-26 kDa protein isoprenylation to 50% of the control level at a concentration of 1 mM, but had no effect on the isoprenylation of 67, 47 or 17 kDa proteins. In particular, p21ras farnesylation was inhibited 40% by 1 mM perillyl alcohol. At the same concentration, perillyl alcohol completely inhibited the proliferation of human HT-29 colon carcinoma cells. The structure-activity relationships observed among the monoterpene isoprenylation inhibitors support a role for small G proteins in cell proliferation, and suggest that many limonene-derived monoterpenes warrant further investigation as antitumor agents.

Prenylated proteins and lymphocyte proliferation: inhibition by d-limonene related monoterpenes

The aim of the present study was to explore the role of post-translational isoprenoid modification of cellular proteins in the proliferation of human lymphocytes. We here report that treatment of phytohemagglutinin-stimulated peripheral blood mononuclear cells with monoterpenes including d-limonene, perillic acid and perillyl alcohol (0.5-5 mM) which selectively inhibit the isoprenylation of 21-26-kDa proteins resulted in a dose-dependent inhibition of DNA synthesis. Cell cycle analysis revealed that perillic acid arrested cells in G1 and prevented cells from entering S phase in a manner similar to that induced by the specific 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor, compactin. However, unlike compactin, the perillic acid-induced effects on lymphocyte proliferation were not prevented by addition of mevalonate. We also examined the incorporation of [3H]mevalonate into proteins in resting and phytohemagglutinin-stimulated lymphocytes during the first 30 h of culture. While in unstimulated lymphocytes radioactivity was predominantly incorporated into a cluster of 21-26-kDa proteins, mitogenic stimulation was associated with a striking increase in [3H]mevalonate incorporation into a protein (approximately 68 kDa) with migration characteristics similar to that of nuclear lamin B. Treatment of phytohemagglutinin-stimulated lymphocytes with 5 mM d-limonene, 2.5 mM perillic acid or 1.25 mM perillyl alcohol strongly suppressed [3H]mevalonate-labeling of proteins to a degree that correlated with the level of DNA synthesis inhibition. These findings suggest that those mevalonate-derived products required for lymphocyte proliferation may include one or more isoprenylated proteins and that the isoprenylation of these proteins is required for cell cycle progression.

Relative steady-state expression of the different post-translational products of p21-H-ras from normal rat tissues. A 2D-western immunoblot study

Proteins extracts from rat cell lines or tissues expressing normal or activated c-H-ras genes, or normal N-ras gene were submitted to westernblot analysis with an anti-H, K, N p21-ras antibody. This showed that p21-H-ras products resolved into four spots (a, b, c, d) that are readily distinguishable from the normal p21-N-ras products (spots e, f, g), and also from two other products (spots a', b') present in extracts from cells which overexpress a Val12-mutated H-ras gene. Considering metabolic isotopic labeling and cell fractionation, we were able to establish the correspondance of spots a, b, c, d with the known steps of the sequential post-translational processing (farnesylation, further carboxymethylation and ultimate palmitoylation) of p21-H-ras. The palmitoylated product predominates in normal brain and still more in normal adult liver tissues, whereas its relative level decreases in proliferating liver cells.

Farnesylamine: an inhibitor of farnesylation and growth of ras-transformed cells

Farnesylamine, an analogue of farnesol, was shown to inhibit growth of PAP2 cells (ras-transformed NIH 3T3 cells) in a dose-dependent manner. This inhibition was overcome by adding farnesol to the culture medium, but not by adding geranylgeraniol, squalene, cholesterol, dolichol, myristic acid or palmitic acid. Farnesylamine inhibited both farnesyl/protein transferase and geranylgeranyl/protein transferase in whole cell extracts and also inhibited the prenylation of proteins, particularly ras p21, in PAP2 cells. Inhibition of prenylation was associated with increased biosynthesis of other products of the mevalonate biosynthetic pathway. These observations suggest that inhibition of the growth of PAP2 cells by farnesylamine may be due to blocking of ras-mediated signal transduction. This offers a means of investigating mechanisms involved in ras action and raises the possibility of developing novel strategies for anticancer therapy.

Nerve growth factor induces a succession of increases in isoprenylated methylated small GTP-binding proteins of PC-12 pheochromocytoma cells

Pheochromocytoma (PC-12) cells exposed to nerve growth factor (NGF) acquire a sympathetic neuron-like phenotype. This NGF-response is blocked by methylation inhibitors and can be mimicked by the farnesylated methylated small GTP-binding protein p21ras. The implicated involvement of prenylation, methylation and a small GTP-binding protein in the NGF-response has been studied by directly measuring 3H-mevalonic acid (MVA)-metabolites incorporated into proteins, protein carboxy [methyl-3H]ester formation and levels of [alpha-32P]GTP-binding proteins in NGF-induced PC-12 cells. We demonstrate that NGF induces a 2-3-fold increase in 21-24 kDa methylated membrane proteins that incorporate 3H-MVA-metabolites, and bind GTP. Levels of [alpha-32P]GTP-binding in these proteins were increased by 2-3-fold. Methylation and membrane association of the small GTP-binding proteins were blocked by lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, which also enhanced their labeling by 3H-MVA-metabolites. Cycloheximide reduced the levels of [methyl-3H] labeled 21-24 kDa proteins and of the overlapping [alpha-32P]GTP binding-proteins. About 70% of the [methyl-3H]-groups found in these proteins were recovered from two dimensional gel blots in nine distinct spots of [alpha-32P]GTP-binding proteins. Taken together these results strongly suggest that in PC-12 cells, NGF induces an increase in the synthesis of prenylated methylated small GTP-binding proteins. The efficacy of lovastatin blockage of protein methylation and enhancement of 3H-MVA-metabolites incorporation into GTP-binding proteins was lower in NGF-induced cells than in controls. This suggests that NGF also induces an increase in HMG-CoA reductase activity. At the early phase of the NGF response in PC-12 cells (15 min-1 h), the levels of two small GTP-binding proteins (molecular mass of 21-22 kDa and 23-24 kDa) were increased. Thus, at least two proteins, of which one but not the other may be p21ras, appear to be involved in the early response. After a lag period of 24 h with NGF, a second more robust phase of increase in methylated small GTP-binding proteins was apparent. This relatively late response, which was almost completed within 24 h, may reflect involvement of small GTP-binding proteins in neurite-outgrowth and in the functional activity of the differentiated cells. Many small GTP-binding proteins were increased during the second phase, precluding electrophoretic separation of all of them. 3 proteins, however, were well separated (one 23-24 kDa protein and two 21-22 kDa proteins).(ABSTRACT TRUNCATED AT 400 WORDS)

Reversal of ras-induced inhibition of gap-junctional intercellular communication, transformation, and tumorigenesis by lovastatin

The plasma-membrane association and transforming activity of the ras oncoprotein p21 are dependent upon posttranslational farnesylation. Farnesyl synthesis and p21 ras farnesylation are inhibited by hydroxymethylglutaryl-CoA reductase inhibitors such as lovastatin. In this study, we examined whether lovastatin could reverse the transformed phenotype of a v-Ha-ras-transformed rat liver epithelial cell line (WB-ras cells) and if changes were associated with the enhancement of gap-junctional intercellular communication (GJIC). WB-ras cells grow in soft agar, have reduced GJIC, and are highly tumorigenic. Membrane association of p21 ras in these cells was inhibited after in vitro treatment with lovastatin (0.1-0.5 microM) for 48 h. Concomitantly, the cells displayed a more normal morphology, decreased growth in soft agar, and enhanced GJIC. These changes were prevented by cotreatment with mevalonic acid. The morphology and GJIC of rat liver epithelial cells transformed with other oncogenes (src, neu, and raf/myc) were not affected by lovastatin. Intrahepatic WB-ras tumors were induced in male rats by intraportal-vein injection of WB-ras cells. The size and DNA labeling index of these tumors were decreased approximately 75% by administration of lovastatin (5 mg/kg orally twice daily for 2 wk). These results suggest that lovastatin reversed the transformed phenotype of WB-ras cells by inhibiting p21 ras plasma membrane association. Furthermore, the concomitant enhancement of GJIC in lovastatin-treated cells suggests a role for reduced GJIC in the expression of the transformed phenotype.

Isoprenylation and carboxylmethylation in small GTP-binding proteins of pheochromocytoma (PC-12) cells

1. A group of 21 to 24-kDa proteins of pheochromocytoma (PC-12) cells was found in blot overlay assays to bind specifically [alpha-32P]GTP. Binding was inhibited by GTP analogues but not by ATP. Such small GTP-binding proteins were found in the cytosolic and in the particulate fraction of the cells, but they were unevenly distributed: about 75% of the small GTP-binding proteins were localized within the particulate fraction of the cells. Separation of these proteins by two-dimensional gel electrophoresis revealed the existence of seven distinct [alpha-32P]GTP-binding proteins. 2. Targeting of the small GTP-binding proteins to the particulate fraction of PC-12 cells requires modification by isoprenoids, since depleting the cells of the isoprenoid precursor mevalonic acid (MVA) by the use of lovastatin resulted in a 50% decrease in membrane-bound small GTP-binding proteins, with a proportionate increase in the cytosolic form. This blocking effect of lovastatin was reversed by exogenously added MVA. 3. In addition, metabolic labeling of PC-12 cells with [3H]MVA revealed incorporation of [3H]MVA metabolites into the cluster of 21 to 24-kDa proteins in a form typical of isoprenoids; the label was not removed from the proteins by hydroxylamine, and labeling was enhanced in cells incubated with lovastatin. The latter effect reflects a decrease in the isotopic dilution of the exogenously added [3H]MVA, as the addition of exogenous MVA reversed the effect of lovastatin on [3H]MVA-metabolite incorporation into the 21 to 24-kDa proteins. 4. Additional experiments demonstrated that isoprenylation is required not only for membrane association of small GTP-binding proteins, but also for their further modification by a methylation enzyme. This was evident in experiments in which the cells were metabolically labeled with [methyl-3H]methionine, a methylation precursor. The group of 21 to 24-kDa proteins was labeled with a methyl-3H group in a form typical of C-terminal-cysteinyl carboxylmethyl esters. Their methylation was blocked by the methylation inhibitors methylthioadenosine (MTA), 3-deazadenosine and homocysteine thiolactone as well as by lovastatin. MVA reversed the lovastatin block of methylation. 5. Two-dimensional gel analysis of the [3H]methylated proteins detected seven methylated small GTP-binding proteins that correspond to the isoprenylated proteins. Levels of the small GTP-binding proteins as well as isoprenylation and methylation were reduced by cycloheximide. 6. Distribution of the methylated proteins between particulate and cytosolic fractions was found to be similar to that of the small GTP-binding proteins (i.e., a 4:1 ratio).(ABSTRACT TRUNCATED AT 400 WORDS)

Polymyxin B inhibits insulin-induced glucose transporter and IGF II receptor translocation in isolated adipocytes

In isolated adipocytes, polymyxin B inhibited insulin-induced glucose incorporation into lipids in a dose-dependent manner, while polymyxin E, a structurally related antibiotic, was ineffective. To approach the mechanism of this effect, the subcellular distribution of the glucose transporter Glut 4 was investigated. Adipocytes were pretreated without or with polymyxin B before insulin stimulation, subcellular fractionation was performed and Glut 4 was detected by immunodetection. Incubation of adipocytes with polymyxin B prevented the insulin-induced appearance of Glut 4 in the plasma membranes, but did not prevent their decrease from the low-density microsomal fraction. A lower purity of the plasma membrane fractions, a detergent effect of polymyxin B on the membranes or an interference of the substance with the immunodetection of the Glut 4 molecules were excluded. These results suggest that polymyxin B was interfering with the Glut 4 translocation process stimulated by insulin in adipocytes. In a similar fashion, polymyxin B inhibited the insulin-induced increase in IGF II binding to adipocytes. This resulted from a blockade of the appearance of IGF II receptors in the plasma membranes. Since low-molecular-mass GTP-binding proteins have been implicated in the regulation of vesicular trafficking, we have used [alpha-32P]GTP binding to analyze such proteins in adipocyte fractions, after SDS/PAGE and transfer to nitrocellulose. Specific and distinct subsets of GTP-binding proteins were revealed in plasma membrane and low-density microsomal fractions of control adipocytes, whether they were stimulated or not with insulin. Polymyxin B treatment of adipocytes markedly modified the profile of the low-molecular-mass GTP-binding proteins in plasma membranes, but not in low-density microsomal fractions. Our results suggest that polymyxin B was interfering with the exocytotic process of the Glut 4 and IGF II receptor-containing vesicles, perhaps at the fusion step between vesicles and plasma membranes.

Protein prenylation in Schizosaccharomyces pombe

S. pombe is shown to be a powerful system for studies concerning attachment of polyisoprenoid moieties to proteins, due to its ability to take up exogenous mevalonic acid efficiently. The fission yeast can take up about 5% of the exogenously added mevalonic acid and incorporate approximately 10% of this into protein. By contrast, the uptake obtained with the budding yeast S. cerevisiae is less than 0.5%. HPLC analysis of total S. pombe protein-bound isoprenoids revealed that approximately 55% of the counts co-migrated with the geranylgeraniol standard, while approximately 45% of the counts co-migrated with farnesol. We could not detect any effects of mevinolin or other HMG-CoA reductase inhibitors in S. pombe.

Low molecular mass GTP-binding proteins in subcellular fractions of the pancreas: regulated phosphoryl G proteins

Low molecular mass guanine nucleotide-binding proteins [small guanosine 5'-triphosphate (GTP)-binding proteins] and phosphoproteins of the pancreatic acinar cell were compared by two-dimensional gel electrophoresis. [35S]GTP alpha S blotting analysis of the total cell protein revealed 20 GTP-binding proteins ranging in molecular mass from 20 to 28 kDa and pI of 4.8-6.4. Analysis of 32P-labeled total cell protein revealed over 300 phosphoproteins. The subcellular distribution of the small GTP-binding proteins was examined: 17 were located in the rough endoplasmic reticulum (RER) fraction, 19 in the smooth microsome fraction, 14 in the zymogen granule membrane fraction, and 11 in the cytosolic fraction, with overlap between fractions. Of the GTP-binding proteins, two were also found to be phosphoproteins, one located on the RER and one on the zymogen granule membrane. The phosphorylation of both small GTP-binding proteins was increased by secretagogue stimulation of the cells but with different time courses. The RER small GTP-binding protein demonstrated a rapid and transient increase in 32P labeling, whereas the granule membrane small GTP-binding protein showed an increase at longer times (30 min). Two of the cytosolic small GTP-binding proteins were also seen in particulate fractions, especially in the zymogen granule membrane fraction, suggesting the possibility of cycling between cytosolic and membrane-associated forms.

Inhibition of farnesyl transferases from malignant and non-malignant cultured human lymphocytes by prenyl substrate analogues

Cytosolic prenyl transferases from two human lymphoid tissue-derived cell lines, IM-9 and Molt-4 cells, are shown to isoprenylate recombinant p21H-ras. Isoprenylation was inhibited by an N-acetylated pentapeptide (N-Ac-Lys-Cys-Val-Leu-Ser), c,t-farnesyl diphosphate, c,t,t-geranylgeranyl diphosphate, t,t,t-geranylgeranyl diphosphate and a photolabile farnesyl diphosphate analogue. c,t-Farnesyl and t,t,t-geranylgeranyl monophosphates were also effective inhibitors of the Molt-4 enzyme but not the IM-9 enzyme.

Isoprenoids and astroglial cell cycling: diminished mevalonate availability and inhibition of dolichol-linked glycoprotein synthesis arrest cycling through distinct mechanisms

Primary astroglial cultures were used to compare the relationships to cell cycling of dolichol-linked glycoprotein synthesis, and of availability of mevalonate, the precursor of dolichol and other isoprenoid lipids. With shift-up to 10% serum (time 0) after 48 h of serum depletion, the proportion of cells in S phase (bromodeoxyuridine immunofluorescence) remained under 15% for 12 h, then increased by 20 h to 72 +/- 10%; DNA synthetic rates (thymidine incorporation) increased 5-fold. S phase transition was prevented by addition at 10-12 h of tunicamycin, an inhibitor of transfer of saccharide moieties to dolichol. Mevinolin, an inhibitor of mevalonate biosynthesis, also blocked cycle progression when added at this time. However, mevinolin markedly inhibited the isoprenoid pathway, as reflected by over 90% reduction of sterol synthesis, without inhibiting net glycoprotein synthesis. Removal of mevinolin after a 24 h exposure delayed S phase until 48 h, following recovery of sterol synthesis, even though kinetics of glycoprotein synthesis were unaffected. Tunicamycin removal after 24 h spared sterol synthesis, but caused delay of S phase until 72 h, following recovery of glycoprotein synthesis. In mevinolin-treated cultures, S phase transition was restored by 1 h of exposure to mevalonate at 10 h, although cycling was thereby rendered sensitive to inhibition by cycloheximide and by tunicamycin. Cell cycle progression following hydroxyurea exposure and release was unaffected by mevinolin, tunicamycin, or cycloheximide. Thus, in these developing astroglia, mevalonate and its isoprenoid derivatives have at least two cell cycle-specific roles: dolichol-linked glycoprotein synthesis is required at or before the G1/S transition, while a distinct mevalonate requirement is apparent also in late G1.