Effects of farnesylcysteine analogs on protein carboxyl methylation and signal transduction

N-Succinyl-S-Farnesyl-L-Cysteine (SFC): A Novel Isoprenylcysteine Analog with In Vitro Anti-Inflammatory Activity and Clinical Skin Protecting Properties

Over the past 15 years, small molecule isoprenylcysteine (IPC) analogs have been identified as a potential new class of topical anti-inflammatories. Clinical studies have demonstrated that IPCs are both safe and effective in promoting healthy skin when applied topically. This work aims to demonstrate N-Succinyl-S-farnesyl-L-cysteine (SFC) as a novel IPC molecule that provides a broad spectrum of benefits for skin. Human promyelocytic cell line HL-60, human dermal microvascular endothelial cells (HDMECs), human dermal fibroblasts (HDFs), and normal human epidermal keratinocytes (NHEKs) were exposed in culture to various inducers to trigger reactive oxygen species, cytokines, or collagenase production. A 49-subject randomized double-blind, vehicle-controlled, split face trial was performed with 1% SFC gel, or 5% niacinamide and vehicle applied for 12 weeks to evaluate anti-wrinkle and anti-aging endpoints. We demonstrated that SFC inhibited GPCR and TLR-induced pro-inflammatory cytokine release in NHEKs and HDMECs from several inflammatory inducers such as UVB, chemicals, cathelicidin, and bacteria. SFC successfully reduced GPCR-induced oxidation in differentiated neutrophils. Moreover, photoaging studies showed that SFC reduced UVA-induced collagenase (pro-MMP-1) production in HDFs. Clinical assessment of 1% SFC gel demonstrated improvement above the vehicle for wrinkle reduction, hydration, texture, and overall appearance of skin. N-Succinyl-S-farnesyl-L-cysteine (SFC) is a novel anti-inflammatory small molecule and is the first farnesyl-cysteine IPC shown to clinically improve appearance and signs of aging, while also having the potential to ameliorate inflammatory skin disorders.

Biology, pathology, and therapeutic targeting of RAS

RAS was identified as a human oncogene in the early 1980s and subsequently found to be mutated in nearly 30% of all human cancers. More importantly, RAS plays a central role in driving tumor development and maintenance. Despite decades of effort, there remain no FDA approved drugs that directly inhibit RAS. The prevalence of RAS mutations in cancer and the lack of effective anti-RAS therapies stem from RAS' core role in growth factor signaling, unique structural features, and biochemistry. However, recent advances have brought promising new drugs to clinical trials and shone a ray of hope in the field. Here, we will exposit the details of RAS biology that illustrate its key role in cell signaling and shed light on the difficulties in therapeutically targeting RAS. Furthermore, past and current efforts to develop RAS inhibitors will be discussed in depth.

Metabolite Cross-Feeding between Rhodococcus ruber YYL and Bacillus cereus MLY1 in the Biodegradation of Tetrahydrofuran under pH Stress

Bacterial consortia are among the most basic units in the biodegradation of environmental pollutants. Pollutant-degrading strains frequently encounter different types of environmental stresses and must be able to survive with other bacteria present in the polluted environments. In this study, we proposed a noncontact interaction mode between a tetrahydrofuran (THF)-degrading strain, Rhodococcus ruber YYL, and a non-THF-degrading strain, Bacillus cereus MLY1. The metabolic interaction mechanism between strains YYL and MLY1 was explored through physiological and molecular studies and was further supported by the metabolic response profile of strain YYL, both monocultured and cocultured with strain MLY1 at the optimal pH (pH 8.3) and under pH stress (pH 7.0), through a liquid chromatography-mass spectrometry-based metabolomic analysis. The results suggested that the coculture system resists pH stress in three ways: (i) strain MLY1 utilized acid metabolites and impacted the proportion of glutamine, resulting in an elevated intracellular pH of the system; (ii) strain MLY1 had the ability to degrade intermediates, thus alleviating the product inhibition of strain YYL; and (iii) strain MLY1 produced some essential micronutrients for strain YYL to aid the growth of this strain under pH stress, while strain YYL produced THF degradation intermediates for strain MLY1 as major nutrients. In addition, a metabolite cross-feeding interaction with respect to pollutant biodegradation is discussed.IMPORTANCE Rhodococcus species have been discovered in diverse environmental niches and can degrade numerous recalcitrant toxic pollutants. However, the pollutant degradation efficiency of these strains is severely reduced due to the complexity of environmental conditions and limitations in the growth of the pollutant-degrading microorganism. In our study, Bacillus cereus strain MLY1 exhibited strong stress resistance to adapt to various environments and improved the THF degradation efficiency of Rhodococcus ruber YYL by a metabolic cross-feeding interaction style to relieve the pH stress. These findings suggest that metabolite cross-feeding occurred in a complementary manner, allowing a pollutant-degrading strain to collaborate with a nondegrading strain in the biodegradation of various recalcitrant compounds. The study of metabolic exchanges is crucial to elucidate mechanisms by which degrading and symbiotic bacteria interact to survive environmental stress.

Prominent role of histone lysine demethylases in cancer epigenetics and therapy

Protein methylation has an important role in the regulation of chromatin, gene expression and regulation. The protein methyl transferases are genetically altered in various human cancers. The enzymes that remove histone methylation have led to increased awareness of protein interactions as potential drug targets. Specifically, Lysine Specific Demethylases (LSD) removes methylated histone H3 lysine 4 (H3K4) and H3 lysine 9 (H3K9) through formaldehyde-generating oxidation. It has been reported that LSD1 and its downstream targets are involved in tumor-cell growth and metastasis. Functional studies of LSD1 indicate that it regulates activation and inhibition of gene transcription in the nucleus. Here we made a discussion about the summary of histone lysine demethylase and their functions in various human cancers.

N-Acetylglutaminoyl-S-farnesyl-L-cysteine (SIG-1191): an anti-inflammatory molecule that increases the expression of the aquaglyceroporin, aquaporin-3, in human keratinocytes

Isoprenylcysteine (IPC) small molecules were discovered as signal transduction modulating compounds ~25 years ago. More recently, IPC molecules have demonstrated antioxidant and anti-inflammatory properties in a variety of dermal cells as well as antimicrobial activity, representing a novel class of compounds to ameliorate skin conditions and disease. Here, we demonstrate a new IPC compound, N-acetylglutaminoyl-S-farnesyl-L-cysteine (SIG-1191), which inhibits UVB-induced inflammation blocking pro-inflammatory cytokine interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) production. To investigate further the previously reported hydrating potential of IPC compounds, SIG-1191 was tested for its ability to modulate aquaporin expression. Specifically, aquaporin 3 (AQP3) the most abundant aquaporin found in skin has been reported to play a key role in skin hydration, elasticity and barrier repair. Results show here for the first time that SIG-1191 increases AQP3 expression in both cultured normal human epidermal keratinocytes as well as when applied topically in a three-dimensional (3D) reconstructed human skin equivalent. Additionally, SIG-1191 dose dependently increased AQP3 protein levels, as determined by specific antibody staining, in the epidermis of the 3D skin equivalents. To begin to elucidate which signaling pathways SIG-1191 may be modulating to increase AQP3 levels, we used several pharmacological pathway inhibitors and determined that AQP3 expression is mediated by the Mitogen-activated protein kinase/Extracellular signal-regulated kinase kinase (MEK) pathway. Altogether, these data suggest SIG-1191 represents a new IPC derivative with anti-inflammatory activity that may also promote increased skin hydration based on its ability to increase AQP3 levels.

In vitro and clinical evaluation of SIG1273: a cosmetic functional ingredient with a broad spectrum of anti-aging and antioxidant activities

BACKGROUND

Isoprenylcysteine (IPC) small molecules were identified as a new class of anti-inflammatory compounds over 20 years ago. Since then, they have been developed as novel cosmetic functional ingredients (CFI) and topical drug candidates. SIG1273 is a second generation CFI that has previously been shown to provide a broad spectrum of benefits for the skin through its anti-inflammatory and antimicrobial properties.

OBJECTIVE

To determine whether SIG1273 possesses anti-aging properties in vitro and evaluate the tolerability and activity of SIG1273 when applied topically to human subjects.

METHODS

To model photoaging in vitro, human dermal fibroblasts (HDFs) were exposed in culture to UVA to induce collagenase (MMP-1) production. An in vitro wound-healing model was based on the activation of HDF migration into cell-free tissue culture surface. Hydrogen peroxide-induced oxidative stress was performed using HDFs to measure intracellular ROS activity. Radical scavenging capacity was determined using a colorimetric antioxidant assay kit (ABTS method). Lastly, a 4-week, 29-subject study was performed in which SIG1273 was applied topically as a cream to assess its tolerance and activity in reducing the appearance of aging.

RESULTS

In vitro studies demonstrate SIG1273 inhibits UVA-induced MMP-1 production, hydrogen peroxide-induced oxidative stress and promotes wound healing. Moreover, SIG1273 was shown to be a radical scavenging antioxidant. Clinical assessment of SIG1273 cream (0.25%) showed it was well tolerated with significant improvement in the appearance of fine lines, coarse wrinkles, radiance/luminosity, pore size, texture/smoothness, hydration and increased firmness.

CONCLUSIONS

SIG1273 represents a novel CFI with antioxidant, anti-aging, and anti-inflammatory properties that when applied topically is well tolerated and provides benefits to individuals with aging skin.

N-acetyl-S-farnesyl-l-cysteine suppresses chemokine production by human dermal microvascular endothelial cells

Isoprenylcysteine (IPC) molecules modulate G-protein-coupled receptor signalling. The archetype of this class is N-acetyl-S-farnesyl-l-cysteine (AFC). Topical application of AFC locally inhibits skin inflammation and elicitation of contact hypersensitivity in vivo. However, the mechanism of these anti-inflammatory effects is not well understood. Dermal microvascular endothelial cells (ECs) are involved in inflammation, in part, by secreting cytokines that recruit inflammatory cells. We have previously shown that the sympathetic nerve cotransmitter adenosine-5'-triphosphate (ATP) and adenosine-5'-O-(3-thio) triphosphate (ATPγS), an ATP analogue that is resistant to hydrolysis, increase secretion of the chemokines CXCL8 (interleukin-8), CCL2 (monocyte chemotactic protein-1) and CXCL1 (growth-regulated oncogene α) by dermal microvascular ECs. Production of these chemokines can also be induced by the exposure to the proinflammatory cytokine TNFα. We have now demonstrated that AFC dose-dependently inhibits ATP-, ATPγS- and TNFα-induced production of CXCL1, CXCL8 and CCL2 by a human dermal microvascular EC line (HMEC-1) in vitro under conditions that do not affect cell viability. Inhibition of ATPγS- or TNFα-stimulated release of these chemokines was associated with reduced mRNA levels. N-acetyl-S-geranyl-l-cysteine, an IPC analogue that is inactive in inhibiting G-protein-coupled signalling, had greatly reduced ability to suppress stimulated chemokine production. AFC may exert its anti-inflammatory effects through the inhibition of chemokine production by stimulated ECs.

Evaluation of substrate and inhibitor binding to yeast and human isoprenylcysteine carboxyl methyltransferases (Icmts) using biotinylated benzophenone-containing photoaffinity probes

Isoprenylcysteine carboxyl methyltransferases (Icmts) are a class of integral membrane protein methyltransferases localized to the endoplasmic reticulum (ER) membrane in eukaryotes. The Icmts from human (hIcmt) and Saccharomyces cerevisiae (Ste14p) catalyze the α-carboxyl methyl esterification step in the post-translational processing of CaaX proteins, including the yeast a-factor mating pheromones and both human and yeast Ras proteins. Herein, we evaluated synthetic analogs of two well-characterized Icmt substrates, N-acetyl-S-farnesyl-L-cysteine (AFC) and the yeast a-factor peptide mating pheromone, that contain photoactive benzophenone moieties in either the lipid or peptide portion of the molecule. The AFC based-compounds were substrates for both hIcmt and Ste14p, whereas the a-factor analogs were only substrates for Ste14p. However, the a-factor analogs were found to be micromolar inhibitors of hIcmt. Together, these data suggest that the Icmt substrate binding site is dependent upon features in both the isoprenyl moiety and upstream amino acid composition. Furthermore, these data suggest that hIcmt and Ste14p have overlapping, yet distinct, substrate specificities. Photocrosslinking and neutravidin-agarose capture experiments with these analogs revealed that both hIcmt and Ste14p were specifically photolabeled to varying degrees with all of the compounds tested. Our data suggest that these analogs will be useful for the future identification of the Icmt substrate binding sites.

Amide-modified prenylcysteine based Icmt inhibitors: Structure-activity relationships, kinetic analysis and cellular characterization

Human protein isoprenylcysteine carboxyl methyltransferase (hIcmt) is the enzyme responsible for the α-carboxyl methylation of the C-terminal isoprenylated cysteine of CaaX proteins, including Ras proteins. This specific posttranslational methylation event has been shown to be important for cellular transformation by oncogenic Ras isoforms. This finding led to interest in hIcmt inhibitors as potential anti-cancer agents. Previous analog studies based on N-acetyl-S-farnesylcysteine identified two prenylcysteine-based low micromolar inhibitors (1a and 1b) of hIcmt, each bearing a phenoxyphenyl amide modification. In this study, a focused library of analogs of 1a and 1b was synthesized and screened versus hIcmt, delineating structural features important for inhibition. Kinetic characterization of the most potent analogs 1a and 1b established that both inhibitors exhibited mixed-mode inhibition and that the competitive component predominated. Using the Cheng-Prusoff method, the K(i) values were determined from the IC(50) values. Analog 1a has a K(IC) of 1.4±0.2μM and a K(IU) of 4.8±0.5μM while 1b has a K(IC) of 0.5±0.07μM and a K(IU) of 1.9±0.2μM. Cellular evaluation of 1b revealed that it alters the subcellular localization of GFP-KRas, and also inhibits both Ras activation and Erk phosphorylation in Jurkat cells.

N-Acetylfarnesylcysteine is a novel class of peroxisome proliferator-activated receptor γ ligand with partial and full agonist activity in vitro and in vivo

The thiazolidedione (TZD) class of drugs is clinically approved for the treatment of type 2 diabetes. The therapeutic actions of TZDs are mediated via activation of peroxisome proliferator-activated receptor γ (PPARγ). Despite their widespread use, concern exists regarding the safety of currently used TZDs. This has prompted the development of selective PPARγ modulators (SPPARMs), compounds that promote glucose homeostasis but with reduced side effects due to partial PPARγ agonism. However, this also results in partial agonism with respect to PPARγ target genes promoting glucose homeostasis. Using a gene expression-based screening approach we identified N-acetylfarnesylcysteine (AFC) as both a full and partial agonist depending on the PPARγ target gene (differential SPPARM). AFC activated PPARγ as effectively as rosiglitazone with regard to Adrp, Angptl4, and AdipoQ, but was a partial agonist of aP2, a PPARγ target gene associated with increased adiposity. Induction of adipogenesis by AFC was also attenuated compared with rosiglitazone. Reporter, ligand binding assays, and dynamic modeling demonstrate that AFC binds and activates PPARγ in a unique manner compared with other PPARγ ligands. Importantly, treatment of mice with AFC improved glucose tolerance similar to rosiglitazone, but AFC did not promote weight gain to the same extent. Finally, AFC had effects on adipose tissue remodeling similar to those of rosiglitazone and had enhanced antiinflammatory effects. In conclusion, we describe a new approach for the identification of differential SPPARMs and have identified AFC as a novel class of PPARγ ligand with both full and partial agonist activity in vitro and in vivo.

Communication in fungi

We will discuss fungal communication in the context of fundamental biological functions including mating, growth, morphogenesis, and the regulation of fungal virulence determinants. We will address intraspecies but also interkingdom signaling by systematically discussing the sender of the message, the molecular message, and receiver. Analyzing communication shows the close coevolution of fungi with organisms present in their environment giving insights into multispecies communication. A better understanding of the molecular mechanisms underlying microbial communication will promote our understanding of the “fungal communicome.”

Topical N-acetyl-S-farnesyl-L-cysteine inhibits mouse skin inflammation, and unlike dexamethasone, its effects are restricted to the application site

N-acetyl-S-farnesyl-L-cysteine (AFC), a modulator of G protein and G-protein coupled receptor signaling, inhibits neutrophil chemotaxis and other inflammatory responses in cell-based assays. Here, we show topical AFC inhibits in vivo acute inflammation induced by 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and arachidonic acid using the mouse ear model of inflammation. AFC inhibits edema, as measured by ear weight, and also inhibits neutrophil infiltration as assayed by direct counting in histological sections and by measuring myeloperoxidase (MPO) activity as a neutrophil marker. In addition, AFC inhibits in vivo allergic contact dermatitis in a mouse model utilizing sensitization followed by a subsequent challenge with 2,4-dinitrofluorobenzene. Unlike the established anti-inflammatories dexamethasone and indomethacin, AFC's action was restricted to the site of application. In this mouse model, both dexamethasone and indomethacin inhibited TPA-induced edema and MPO activity in the vehicle-treated, contralateral ear. AFC showed no contralateral ear inhibition for either of these end points. A marginally significant decrease due to AFC treatment was seen in TPA-induced epidermal hyperplasia at 24 hours. This was much less than the 90% inhibition of neutrophil infiltration, suggesting that AFC does not act by directly inhibiting protein kinase C.

Isoprenylcysteine carboxy methylation is essential for development in Dictyostelium discoideum

Members of the Ras superfamily of small GTPases and the heterotrimeric G protein gamma subunit are methylated on their carboxy-terminal cysteine residues by isoprenylcysteine methyltransferase. In Dictyostelium discoideum, small GTPase methylation occurs seconds after stimulation of starving cells by cAMP and returns quickly to basal levels, suggesting an important role in cAMP-dependent signaling. Deleting the isoprenylcysteine methyltransferase-encoding gene causes dramatic defects. Starving mutant cells do not propagate cAMP waves in a sustained manner, and they do not aggregate. Motility is rescued when cells are pulsed with exogenous cAMP, or coplated with wild-type cells, but the rescued cells exhibit altered polarity. cAMP-pulsed methyltransferase-deficient cells that have aggregated fail to differentiate, but mutant cells plated in a wild-type background are able to do so. Localization of and signaling by RasG is altered in the mutant. Localization of the heterotrimeric Ggamma protein subunit was normal, but signaling was altered in mutant cells. These data indicate that isoprenylcysteine methylation is required for intercellular signaling and development in Dictyostelium.

Liver prenylated methylated protein methyl esterase is an organophosphate-sensitive enzyme

Prenylation and subsequent methylation are essential modifications on a significant proportion of eucaryotic proteins. Proteins such as the G-gamma subunits of G-protein coupled receptors, nuclear lamins, and guanine nucleotide-binding proteins such as Ras are prenylated and undergo methylation. Prenylated methylated protein methyl esterase (PMPMEase) readily hydrolyses the prenylated protein methyl esters, thus making this step reversible and possibly regulatory. Benzoyl-glycyl-farnesyl-cysteine methyl ester (BzGFCM) was developed as a specific PMPMEase substrate and characterized by electron spray ionization mass spectrometry (ESI-MS) to be of the calculated molecular mass. Rat liver and brain PMPMEase hydrolyzed BzGFCM, forming benzoyl-glycyl-farnesyl-cysteine (BzGFC) in a time- and concentration-dependent manner. Both enzymes cleaved BzGFCM with K(m) values of 4.58 +/- 0.30 and 25.57 +/- 2.36 microM and V(max) values of 2.21 +/- 0.03 and 0.17 +/- 0.003 nmol/min/mg, respectively. The liver enzyme eluted from a gel-filtration column as a single peak of apparent size, 89 kDa. The brain enzyme eluted as two main peaks of 53 and 890 kDa. Organophosphorus pesticides (OPs), which are suspected to be involved in human disorders such as parkinsonism, neuronal, and retinal degeneration, inhibited the liver enzyme with IC(50) values from 4.77 muM for parathion to 0.04 microM for paraoxon, respectively. Only about 25% of the brain enzyme was inhibited by 0.5-1 mM solutions of mipafox, while 0.1 and 1 mM paraoxon inhibited over 50% and 95% of the enzyme, respectively. Paraoxon is thus about 2,250 times less potent against the brain than the liver PMPMEase. BzGFCM was not hydrolyzed by various cholinesterases, indicating its specificity for PMPMEase. Perturbations in prenylated protein metabolism might play a role in noncholinergic OPs-induced toxicity, since prenylated proteins play such important roles in cell signaling, proliferation, differentiation, and apoptosis.

Genetic and pharmacologic analyses of the role of Icmt in Ras membrane association and function

After isoprenylation, the Ras proteins and other proteins terminating with a so-called CAAX motif undergo two additional modifications: (1) endoproteolytic cleavage of the -AAX by Ras converting enzyme 1 (Rce1) and (2) carboxyl methylation of the isoprenylated cysteine residue by isoprenylcysteine carboxyl methyltransferase (Icmt). Although CAAX protein isoprenylation has been studied in great detail, until recently, very little was known about the biological role and functional importance of Icmt in mammalian cells. Studies over the past few years, however, have begun to fill in the blanks. Genetic experiments showed that Icmt-deficient embryos die at mid-gestation, whereas conditional inactivation of Icmt in the liver, spleen, and bone marrow is not associated with obvious pathology. One potential explanation for the embryonic lethality is that Icmt is the only enzyme in mouse cells capable of methylating isoprenylated CAAX proteins--including the Ras proteins. Furthermore, in addition to the CAAX proteins, Icmt methylates the CXC class of isoprenylated Rab proteins. In the absence of carboxyl methylation, the Ras proteins are mislocalized away from the plasma membrane and exhibit a shift in electrophoretic mobility. Given the important role of oncogenic Ras proteins in human tumorigenesis and the mislocalization of Ras proteins in Icmt-deficient cells, it has been hypothesized that inhibition of Icmt could be a strategy to block Ras-induced oncogenic transformation. Recent data provide strong support to that hypothesis: conditional inactivation of Icmt in mouse embryonic fibroblasts and treatment of cells with a novel selective inhibitor of Icmt, termed cysmethynil, results in a striking inhibition of Ras-induced oncogenic transformation.

Reactive oxygen species-mediated signaling pathways in angiotensin II-induced MCP-1 expression of proximal tubular cells

Angiotensin II (AngII) has pleiotropic effects, the most well known of which is the generation of reactive oxygen species (ROS) and chemokines in inflammatory lesions. Monocyte chemoattractant protein-1 (MCP-1) is considered a major chemokine in the pathogenesis of kidney diseases. We examined signaling pathways of AngII-induced MCP-1 expression and the role of ROS in the murine proximal tubular cells (mProx) using various inhibitors. Furthermore, we compared the signaling pathways between mProx and mesangial cells (MC). AngII-induced MCP-1 protein expression in mProx at 6 h was largely blocked by ROS (N-acetylcysteine; 82 +/- 14%), Ras (N-acetyl-S-trans,trans-farnesyl-L-cysteine; 82 +/- 13%), and nuclear factor-kappaB (NF-kappaB) (parthenolide; 89 +/- 7.9%) inhibitors. Both AT1 receptor (AT1R) (Olmesartan; 41 +/- 12%) and the AT2R (PD123319; 24 +/- 11%) antagonists partially blocked the MCP-1 expression. Furthermore, mitogen-activated protein kinase (MAPK) pathways were also implicated in this protein expression, but it is less dependent on ROS/Ras pathways. In MC, protein kinase (calphostin C; 84 +/- 2.8%) and NF-kappaB (89 +/- 1.4%) inhibitors attenuated acute AngII-induced MCP-1 expression stronger than ROS/Ras inhibitors (1.0 +/- 0.9/29 +/- 9.5%). MAPK pathways, especially p38 MAPK, were involved in MC more than in mProx. AT1R (69 +/- 8.6%) and AT2R (57 +/- 21%) antagonists also were blocked. We suggested that, although NF-kappaB activation has a critical role, signaling pathways are different between mProx and MC. ROS-mediated signaling in mProx may have more contribution to AngII-induced inflammatory responses than to those in MC.

Ras inhibition leads to transcriptional activation of p53 and down-regulation of Mdm2: two mechanisms that cooperatively increase p53 function in colon cancer cells

Activated Ras, operating through the Raf/MEK/ERK pathway, is known to regulate transcription of both Mdm2 and its inhibitor p19ARF, resulting in opposing effects on the tumor suppressor protein p53. We show here that a decrease in Ras in SW480 cells induced either by the Ras inhibitor farnesylthiosalicylic acid (FTS) or by K-Ras antisense oligonucleotides, resulted in a similar increase in p53 protein. The increase in p53 was accompanied by an increase in p21(waf1/cip1) mRNA transcripts and protein. Consistent with the Ras/Raf/MEK/ERK-mediated control of Mdm2, treatment of SW480 cells with the Ras inhibitor FTS caused a marked (80%) decrease in Mdm2, which itself would account for the increase in p53. However, FTS also caused a 1.6-fold increase in p53 mRNA, indicative of a Ras-dependent mechanism that regulates p53 transcription. Thus, oncogenic Ras appears to attenuate p53 in SW480 cells by two independent regulatory mechanisms, the one leading to increased Mdm2-dependent p53 degradation and the other leading to a decrease in p53 transcription.

Isoprenylcysteine Carboxyl Methyltransferase Modulates Endothelial Monolayer Permeability

RhoA and Rac1 regulate formation of stress fibers and intercellular junctions, thus modulating endothelial monolayer permeability. Posttranslational modifications of RhoA and Rac1 regulate enzyme activity and subcellular localization, resulting in altered cellular function. The role of RhoA and Rac1 carboxyl methylation in modulating endothelial monolayer permeability is not known. In this study, we found that inhibition of isoprenylcysteine-O-carboxyl methyltransferase (ICMT) with adenosine plus homocysteine or N-acetyl-S-geranylgeranyl-l-cysteine decreased RhoA carboxyl methylation, RhoA activity, and endothelial monolayer permeability, suggesting that RhoA carboxyl methylation may play a role in the ICMT-modulated monolayer permeability. Similar studies showed no effect of ICMT inhibition on Rac1 carboxyl methylation or localization. Bovine pulmonary artery endothelial cells (PAECs) stably overexpressing ICMT-GFP cDNA were established to determine if increased ICMT expression could alter RhoA or Rac1 carboxyl methylation, activation, and endothelial monolayer permeability. PAECs stably overexpressing ICMT demonstrated increased RhoA carboxyl methylation, membrane-bound RhoA, and RhoA activity. Additionally, PAECs stably overexpressing ICMT had diminished VE-cadherin and beta-catenin at intercellular junctions, with resultant intercellular gap formation, as well as enhanced monolayer permeability. These effects were blunted by adenosine plus homocysteine and by inhibition of RhoA, but not by inhibition of Rac1. These results indicate that ICMT modulates endothelial monolayer permeability by altering RhoA carboxyl methylation and activation, thus changing the organization of intercellular junctions. Therefore, carboxyl methylation of RhoA may modulate endothelial barrier function.

Carboxyl methylation of Ras regulates membrane targeting and effector engagement

Post-translational modification of Ras proteins includes prenylcysteine-directed carboxyl methylation. Because Ras participates in Erk activation by epidermal growth factor (EGF), we tested whether Ras methylation regulates Erk activation. EGF stimulation of Erk was inhibited by AFC (N-acetyl-S-farnesyl-L-cysteine), an inhibitor of methylation, but not AGC (N-acetyl-S-geranyl-L-cysteine), an inactive analog of AFC. AFC inhibited Ras methylation as well as the activation of pathway enzymes between Ras and Erk but did not inhibit EGF receptor phosphorylation, confirming action at the level of Ras. Transient transfection of human prenylcysteine-directed carboxyl methyltransferase increased EGF-stimulated Erk activation. AFC but not AGC inhibited movement of transiently transfected green fluorescent protein-Ras from the cytosol to the plasma membrane of COS-1 cells and depleted green fluorescent protein-Ras from the plasma membrane in stably transfected Madin-Darby canine kidney cells, suggesting that methylation regulates Erk by ensuring proper membrane localization of Ras. However, when COS-1 cells were transfected with Ras complexed to CD8, plasma membrane localization of Ras was unaffected by AFC, yet EGF-stimulated Erk activation was inhibited by AFC. Thus, Ras methylation appears to regulate Erk activation both through the localization of Ras as well as the propagation of Ras-dependent signals.

Activation of Rac2 and Cdc42 on Fc and complement receptor ligation in human neutrophils

Phagocytosis is a complex process engaging a concerted action of signal-transduction cascades that leads to ingestion, subsequent phagolysosome fusion, and oxidative activation. We have previously shown that in human neutrophils, C3bi-mediated phagocytosis elicits a significant oxidative response, suggesting that activation of the small GTPase Rac is involved in this process. This is contradictory to macrophages, where only Fc receptor for immunoglobulin G (FcgammaR)-mediated activation is Rac-dependent. The present study shows that engagement of the complement receptor 3 (CR3) and FcgammaR and CR3- and FcgammaR-mediated phagocytosis activates Rac, as well as Cdc42. Furthermore, following receptor-engagement of the CR3 or FcgammaRs, a downstream target of these small GTPases, p21-activated kinase, becomes phosphorylated, and Rac2 is translocated to the membrane fraction. Using the methyltransferase inhibitors N-acetyl-S-farnesyl-L-cysteine and N-acetyl-S-geranylgeranyl-L-cysteine, we found that the phagocytic uptake of bacteria was not Rac2- or Cdc42-dependent, whereas the oxidative activation was decreased. In conclusion, our results indicate that in neutrophils, Rac2 and Cdc42 are involved in FcR- and CR3-induced activation and for properly functioning signal transduction involved in the generation of oxygen radicals.

Isoprenylcysteine carboxyl methyltransferase activity modulates endothelial cell apoptosis

Extracellular ATP, adenosine (Ado), and adenosine plus homocysteine (Ado/HC) cause apoptosis of cultured pulmonary artery endothelial cells through the enhanced formation of intracellular S-adenosylhomocysteine and disruption of focal adhesion complexes. Because an increased intracellular ratio of S-adenosylhomocysteine/S-adenosylmethionine favors inhibition of methylation, we hypothesized that Ado/HC might act by inhibition of isoprenylcysteine-O-carboxyl methyltransferase (ICMT). We found that N-acetyl-S-geranylgeranyl-L-cysteine (AGGC) and N-acetyl-S-farnesyl-L-cysteine (AFC), which inhibit ICMT by competing with endogenous substrates for methylation, caused apoptosis. Transient overexpression of ICMT inhibited apoptosis caused by Ado/HC, UV light exposure, or tumor necrosis factor-alpha. Because the small GTPase, Ras, is a substrate for ICMT and may modulate apoptosis, we also hypothesized that inhibition of ICMT with Ado/HC or AGGC might cause endothelial apoptosis by altering Ras activation. We found that ICMT inhibition decreased Ras methylation and activity and the activation of the downstream signaling molecules Akt, ERK-1, and ERK-2. Furthermore, overexpression of wild-type or dominant active H-Ras blocked Ado/HC-induced apoptosis. These findings suggest that inhibition of ICMT causes endothelial cell apoptosis by attenuation of Ras GTPase methylation and activation and its downstream antiapoptotic signaling pathway.

Prenylcysteine alpha-carboxyl methyltransferase expression and function in Arabidopsis thaliana

Farnesylated proteins undergo a series of post-translational modifications, including carboxyl terminal isoprenylation, proteolysis, and methylation. In Arabidopsis thaliana, protein farnesylation has been shown to be necessary for negative regulation of ABA signaling. However, the role of post-isoprenylation protein processing in ABA signal transduction has not been described. Here, we show that the A. thaliana genome contains two distinct genes on chromosome V, AtSTE14A and AtSTE14B, which encode functional prenylcysteine alpha-carboxyl methyltransferases. AtSTE14B encodes a methyltransferase with lower apparent Kms for prenylcysteine substrates and higher specific activities than the previously described AtSTE14A-encoded methyltransferase. Furthermore, whereas AtSTE14A transcription is restricted to root and shoot tips, young leaves, and vascular tissue, AtSTE14B transcription is observed in all organs except hypocotyls and petioles. Pharmacological inhibitors of prenylcysteine alpha-carboxyl methyltransferase activity cause increased ABA sensitivity, seed dormancy, and stomatal closure, consistent with the hypothesis that prenylcysteine alpha-carboxyl methylation is necessary for negative regulation of ABA signaling. These results suggest that carboxyl methylation, which is a reversible and potentially regulated step in the processing, targeting, and function of isoprenylated plant proteins, may be an important biochemical target for introducing altered ABA sensitivity and drought tolerance into plants.

Prenylcysteine lyase deficiency in mice results in the accumulation of farnesylcysteine and geranylgeranylcysteine in brain and liver

In in vitro experiments, prenylcysteine lyase (Pcly) cleaves the thioether bond of prenylcysteines to yield free cysteine and the aldehyde of the isoprenoid lipid. However, the importance of this enzyme has not yet been fully defined at the biochemical or physiologic level. In this study, we show that Pcly is expressed at high levels in mouse liver, kidney, heart, and brain. To test whether Pcly deficiency would cause prenylcysteines to accumulate in tissues and result in pathologic consequences, we produced Pcly-deficient cell lines and Pcly-deficient mice (Pcly-/-). Pcly activity levels were markedly reduced in Pcly-/- cells and tissues. Pcly-/- fibroblasts were more sensitive than wild-type fibroblasts to growth inhibition when prenylcysteines were added to the cell culture medium. To determine if the reduced Pcly enzyme activity levels led to an accumulation of prenylcysteines within cells, mass spectrometry was used to measure farnesylcysteine and geranylgeranylcysteine levels in the tissues of Pcly-/- mice and wild-type controls. These studies revealed a striking accumulation of both farnesylcysteine and geranylgeranylcysteine in the brain and liver of Pcly-/- mice. This accumulation did not appear to be accompanied by significant pathologic consequences. Pcly-/- mice were healthy and fertile, and surveys of more than 30 tissues did not uncover any abnormalities. We conclude that prenylcysteine lyase does play a physiologic role in cleaving prenylcysteines in mammals, but the absence of this activity does not lead to major pathologic consequences.

Role of isoprenylcysteine carboxyl methyltransferase in tumor necrosis factor-alpha stimulation of expression of vascular cell adhesion molecule-1 in endothelial cells

We have previously shown that cytokine stimulation of the expression of vascular cell adhesion molecule-1 (VCAM-1), but not that of intercellular adhesion molecule-1 (ICAM-1), is redox sensitive in endothelial cells. Here, we investigated the role of isoprenylcysteine carboxyl methyltransferase (ICMTase), which methylates isoprenylated CAAX (where C indicates cysteine; A, aliphatic amino acids; and X, almost any other amino acid) proteins, including Rac1, a component of superoxide-generating NAD(P)H oxidase, in the expression of VCAM-1. Pretreatment of endothelial cells with N-acetyl-S-farnesyl-L-cysteine (AFC) or N-acetyl-S-geranylgeranyl-L-cysteine (AGGC), specific inhibitors of ICMTase, inhibited the tumor necrosis factor-alpha (TNF-alpha) stimulation of mRNA expression of VCAM-1 but not that of ICAM-1. Endothelial cells expressed constitutively active ICMTase, as suggested by the presence of methylated Rac1 and the methylation of AFC by the cells. TNF-alpha stimulation of the cells significantly increased the methylation of AFC and Rac1 in endothelial cells. That ICMTase was a component of the redox-sensitive signaling pathway was also suggested by the AFC inhibition of the generation of reactive oxygen species by TNF-alpha. Interestingly, the dominant-negative isoform of Rac1 was not selective but inhibited the TNF-alpha stimulation of the mRNA expression of VCAM-1 and ICAM-1. Thus, ICMTase is a critical component of the redox-sensitive VCAM-1-selective signaling pathway, and it appears to activate a discrete inflammatory signaling pathway, at least in part, through the methylation of Rac1.

Pheromone-Related Inhibitors of Ustilago hordei Mating and Tilletia tritici Teliospore Germination

ABSTRACT Ustilago hordei, the causal agent of barley covered smut, produces mating pheromones that break down to smaller peptide compounds that act as potent inhibitors of mating and germination in several fungi. The pheromones are members of the farnesylated family of proteins. Synthetic peptide analogs of the pheromone derivatives, ranging in size from 4 mers to full length pheromones, were farnesylated, methyl esterified, or both and tested for mating or teliospore germination inhibition with U. hordei or Tilletia tritici, respectively. N-Acetyl-S-farnesylcysteine, which inhibits processing of Ras, and other sulfur-containing compounds such as homocysteine or methionine, were likewise modified and tested. The most potent inhibitors were methionine methyl ester and modified 4-mer peptides from both pheromones. Alanine scanning of the inhibitory 4 mers determined that the native amino acid sequence was specific for a high level of activity. The sulfur amino acids appear to be required for inhibition. Glasshouse studies using selected antagonists of mating and teliospore germination as seed treatments inhibited covered smut of barley and common bunt of wheat, although the level of control was inconsistent. The use of pheromone-related antagonists to mating or teliospore germination is a promising, novel strategy for control of smut and bunt diseases.

Antioxidants N-acetylcysteine (NAC) and 2-mercaptoethanol (2-ME) affect the survival and differentiative potential of cholinergic precursors from the embryonic septal nuclei and basal forebrain: involvement of ras signaling

We investigated the effects of antioxidants N-acetylcysteine (NAC) and 2-mercaptoethanol (2-ME) on the expression of choline acetyltransferase (ChAT) in cultured cholinergic precursors from the embryonic rat septal nuclei and basal forebrain. Carboxy-dichlorofluorescein fluorescence confirmed that 2-ME inhibited intracellular oxidation. Low micromolar concentrations of 2-ME produce as much as a 12-fold increase in ChAT; this is enhanced further by inclusion of nerve growth factor (NGF). NAC effects are biphasic: 0.15 mM produces profound increases in ChAT while 1.5 mM has no effect. Immature (E16) cultures respond with increases in ChAT while more highly differentiated cultures (E18) do not. Labeling of single precursors with a lacZ-expressing retrovirus reveals that the increase in ChAT is due primarily to an increased number and size of clones, not an increase in cholinergic neurons per clone, suggesting an effect on precursor survival. Inhibition of ras farnesylation inhibits both 2-ME and NAC induction of ChAT suggesting a ras-mediated pathway. Inclusion of the MEK inhibitor PD98059 does not affect low doses of NAC, but at doses of NAC that fail to increase ChAT activity, inhibition of the pathway actually raises ChAT. Immunocytochemical investigation of the cultures indicates that cells exposed to low doses of NAC develop healthy neuronal arbors in the apparent absence of glial support. At higher concentrations of NAC, neurons were found in association with astrocytes, making contact via elaborate varicose fibers. Treatment of the cultures with PD98059 to inhibit MEK returned cultures to a 'low-dose' phenotype. These data suggest that redox status of basal forebrain precursors affect both their survival and differentiative potential.

Distinct signaling pathways involved in multiple effects of basic fibroblast growth factor on cultured rat hippocampal neurons

We investigated possible involvement of voltage-dependent Ca(2+) channels (VDCCs) and several intracellular signaling mechanisms in multiple actions of basic fibroblast growth factor (bFGF), such as survival promotion, induction of calbindin D(28k) expression as well as acceleration of neuritic branch formation of cultured rat hippocampal neurons. Immunocytochemical staining with anti-gamma-aminobutyric acid (GABA) antibody showed that the promotion of neuron survival by bFGF in high cell-density cultures were exerted exclusively on GABA-negative neurons. Nicardipine (5 microM) attenuated the effect of bFGF on neuronal survival and formation of neurite branches, suggesting that the activity of L-type VDCCs is required for these effects. In contrast, stimulation of calbindin expression by bFGF was not attenuated by nicardipine. A phospholipase C inhibitor U73122 (1 microM) prevented the effect of bFGF on neurite branch formation, but not on neuronal survival or calbindin expression. On the other hand, chronic application of phorbol-12-myristate-13-acetate (1 microM) inhibited the effect of bFGF on neuronal survival, without inhibiting the other bFGF actions. Forskolin (100 microM) attenuated the effect of bFGF on neuronal survival and neurite branch formation, indicating that cyclic AMP plays negative regulatory roles in these actions of bFGF. Taken together, these results suggest that multiple biological actions of bFGF on hippocampal neurons are exerted through, and modulated by, distinct signaling pathways.

Targeting the Ras signaling pathway: a rational, mechanism-based treatment for hematologic malignancies?

A series of alterations in the cellular genome affecting the expression or function of genes controlling cell growth and differentiation is considered to be the main cause of cancer. These mutational events include activation of oncogenes and inactivation of tumor suppressor genes. The elucidation of human cancer at the molecular level allows the design of rational, mechanism-based therapeutic agents that antagonize the specific activity of biochemical processes that are essential to the malignant phenotype of cancer cells. Because the frequency of RAS mutations is among the highest for any gene in human cancers, development of inhibitors of the Ras–mitogen-activated protein kinase pathway as potential anticancer agents is a very promising pharmacologic strategy. Inhibitors of Ras signaling have been shown to revert Ras-dependent transformation and cause regression of Ras-dependent tumors in animal models. The most promising new class of these potential cancer therapeutics are the farnesyltransferase inhibitors. The development of these compounds has been driven by the observation that oncogenic Ras function is dependent upon posttranslational modification, which enables membrane binding. In contrast to many conventional chemotherapeutics, farnesyltransferase inhibitors are remarkably specific and have been demonstrated to cause no gross systemic toxicity in animals. Some orally bioavailable inhibitors are presently being evaluated in phase II clinical trials. This review presents an overview on some inhibitors of the Ras signaling pathway, including their specificity and effectiveness in vivo. Because Ras signaling plays a crucial role in the pathogenesis of some hematologic malignancies, the potential therapeutic usefulness of these inhibitors is discussed.

Targeting the Ras signaling pathway: a rational, mechanism-based treatment for hematologic malignancies?

A series of alterations in the cellular genome affecting the expression or function of genes controlling cell growth and differentiation is considered to be the main cause of cancer. These mutational events include activation of oncogenes and inactivation of tumor suppressor genes. The elucidation of human cancer at the molecular level allows the design of rational, mechanism-based therapeutic agents that antagonize the specific activity of biochemical processes that are essential to the malignant phenotype of cancer cells. Because the frequency of RAS mutations is among the highest for any gene in human cancers, development of inhibitors of the Ras–mitogen-activated protein kinase pathway as potential anticancer agents is a very promising pharmacologic strategy. Inhibitors of Ras signaling have been shown to revert Ras-dependent transformation and cause regression of Ras-dependent tumors in animal models. The most promising new class of these potential cancer therapeutics are the farnesyltransferase inhibitors. The development of these compounds has been driven by the observation that oncogenic Ras function is dependent upon posttranslational modification, which enables membrane binding. In contrast to many conventional chemotherapeutics, farnesyltransferase inhibitors are remarkably specific and have been demonstrated to cause no gross systemic toxicity in animals. Some orally bioavailable inhibitors are presently being evaluated in phase II clinical trials. This review presents an overview on some inhibitors of the Ras signaling pathway, including their specificity and effectiveness in vivo. Because Ras signaling plays a crucial role in the pathogenesis of some hematologic malignancies, the potential therapeutic usefulness of these inhibitors is discussed.

Identification of prenylcysteine carboxymethyltransferase in bovine adrenal chromaffin cells

Chromaffin cells from bovine adrenal medulla were examined for the presence of a specific prenylcysteine carboxymethyltransferase by using N-acetyl-S-farnesyl-L-cysteine and N-acetyl-S-geranylgeranyl-L-cysteine as artificial substrates and a crude cell homogenate as the enzyme source. From Michaelis-Menten kinetics the following constants were calculated: K(m) 90 microM and V(max) 3 pmol/min per mg proteins for N-acetyl-S-farnesyl-L-cysteine; K(m) 52 microM and V(max) 3 pmol/min per mg proteins for N-acetyl-S-geranylgeranyl-L-cysteine. Both substrates were methylated to an optimal extent at the pH range 7. 4-8.0. Methylation activity increased linearly up to 20 min incubation time and was dose dependent up to at least 160 microg of protein. Sinefungin and S-adenosylhomocysteine both caused pronounced inhibition, as also to a lesser extent did farnesylthioacetic acid, deoxymethylthioadenosine and 3-deaza-adenosine. Effector studies showed that the methyltransferase activity varied depending on the concentration and chemical nature of the cations present. Monovalent cations were slightly stimulatory, while divalent metallic ions displayed diverging inhibitory effects. The inhibition by cations was validated by the stimulatory effect of the chelators EDTA and EGTA. Sulphydryl reagents inhibited methylation but to different degrees: Hg(2+)-ions: 100%, N-ethylmaleimide: 30%, dithiothreitol: 0% and mono-iodoacetate: 20%. Due to the hydrophobicity of the substrates dimethyl sulfoxide had to be included in the incubation mixture (<4%; still moderate inhibition at more elevated concentrations). The detergents tested affected the methyltransferase activity to a varying degree. The membrane bound character of the methyltransferase was confirmed.

Targeted inactivation of the isoprenylcysteine carboxyl methyltransferase gene causes mislocalization of K-Ras in mammalian cells

After isoprenylation and endoproteolytic processing, the Ras proteins are methylated at the carboxyl-terminal isoprenylcysteine. The importance of isoprenylation for targeting of Ras proteins to the plasma membrane is well established, but the importance of carboxyl methylation, which is carried out by isoprenylcysteine carboxyl methyltransferase (Icmt), is less certain. We used gene targeting to produce homozygous Icmt knockout embryonic stem cells (Icmt-/-). Lysates from Icmt-/- cells lacked the ability to methylate farnesyl-K-Ras4B or small-molecule Icmt substrates such as N-acetyl-S-geranylgeranyl-L-cysteine. To assess the impact of absent Icmt activity on the localization of K-Ras within cells, wild-type and Icmt-/- cells were transfected with a green fluorescent protein (GFP)-K-Ras fusion construct. As expected, virtually all of the GFP-K-Ras fusion in wild-type cells was localized along the plasma membrane. In contrast, a large fraction of the fusion in Icmt-/- cells was trapped within the cytoplasm, and fluorescence at the plasma membrane was reduced. Also, cell fractionation/Western blot studies revealed that a smaller fraction of the K-Ras in Icmt-/- cells was associated with the membranes. We conclude that carboxyl methylation of the isoprenylcysteine is important for proper K-Ras localization in mammalian cells.

Structural requirements for activity of the pheromones of Ustilago hordei

Ustilago hordei, the cause of barley-covered smut, initiates mating with pheromones. Gene sequence analysis suggested that these pheromones, Uhmfa1 and Uhmfa2, would be farnesylated peptides. Although isolation of mating-type-specific activity was rarely possible, chromatographic separations of culture supernatants yielded fractions that stimulated or inhibited mating. Based on predicted amino acid sequences and mass spectra of stimulating fractions, a series of pheromone analogs were synthesized and their activities were determined. Underivatized Uhmfa1 (PGKSGSGLGYSTC) or Uhmfa2 (EGKGEPAPYC) peptides were inactive, while peptides that were farnesylated and/or methyl esterified specifically induced conjugation tubes by cells of the opposite mating type. Uhmfa1 truncated from the amino terminus beyond the lysine lost activity, while truncated Uhmfa2 remained partially active. In mating bioassays, a pheromone concentration-dependent default mating response was observed. In competition studies, shorter Uhmfa1 peptides lacking pheromone activity inhibited activity of full-length peptides most effectively when both had the same functional groups.

The specific binding of small molecule isoprenoids to rhoGDP dissociation inhibitor (rhoGDI)

The activities of small G-proteins are in part regulated by their interactions with GDI proteins. This binding is thought to be dependent on the C-terminal isoprenoid modification (geranylgeranyl or farnesyl) of these proteins. G-proteins are generally isoprenylated/methylated at their C-terminal cysteine residues. A quantitative fluorescence assay is reported here to evaluate the specificity of binding of rhoGDI. A rhodamine-labeled geranylgeranylated/methylated cysteine derivative is used to measure its binding to rhoGDI. Saturable binding in the low micromolar range is found with various geranylgeranylated/farnesylated analogues. Interestingly, the carboxymethylated derivatives bound significantly better than their free acid counterparts, suggesting that the state of methylation of the analogues is important for binding. The binding is also selective with respect to isoprenoid. Analogues containing hydrophobic modifications other than geranylgeranyl or farnesyl do not bind with significant affinities. These data demonstrate a substantial degree of specificity in the binding of isoprenoids to a protein important in signal transduction.

Concepts in Ras-directed therapy

Ras proteins are key transducers of growth signals regulated by cell surface receptors. They are anchored to the inner surface of the cell membrane where receptor-mediated signalling induces Ras activation (GDP/GTP exchange) and inactivation (stimulation of Ras GTPase activity). Ras-GTP in turn activates a multitude of signalling cascades controlling cell growth and differentiation. Aberrant Ras function (mostly constitutive activation) contributes to the development of many types of neoplastic human diseases. Activating mutations in ras genes, leading to the expression of Ras proteins insensitive to Ras-GTPase activating proteins, are found in as many as 30% of all human tumours. This suggests that Ras is an appropriate target for drug design. Remarkable improvements in the understanding of post-translational modifications in Ras that promote Ras-membrane anchorage, in the mechanisms of activation and inactivation of Ras, and in the interactions of Ras with a plethora of effector molecules have led to the development of new concepts for Ras-directed therapy. The most advanced approach has been that of farnesyltransferase inhibitors (FTIs) designed to inhibit the farnesylation of Ras required for membrane anchorage and transforming activity. FTIs now in clinical trials have been extensively reviewed. Here we review the progress in the development of FTIs and in the development of other promising concepts for Ras-directed therapy. These include compounds such as S-farnesylthiosalicylic acid (FTS), which disrupt the proper anchorage of Ras with the cell membrane and inhibit human tumour growth in animal models, and compounds that interfere with interactions of Ras with its downstream effectors. We conclude with a description of a recently described novel drug concept that could restore the defective GTPase activity of oncogenic Ras and with the interesting results of reovirus-induced tumour regression observed in animal models of human tumours containing an intact Ras signalling pathway.

Isoprenylcysteine-O-carboxyl methyltransferase regulates aldosterone-sensitive Na(+) reabsorption

The Xenopus laevis distal tubule epithelial cell line A6 was used as a model epithelia to study the role of isoprenylcysteine-O-carboxyl methyltransferase (pcMTase) in aldosterone-mediated stimulation of Na(+) transport. Polyclonal antibodies raised against X. laevis pcMTase were immunoreactive with a 33-kDa protein in whole cell lysate. These antibodies were also reactive with a 33-kDa product from in vitro translation of the pcMTase cDNA. Aldosterone application increased pcMTase activity resulting in elevation of total protein methyl esterification in vivo, but pcMTase protein levels were not affected by steroid, suggesting that aldosterone increased activity independent of enzyme number. Inhibition of pcMTase resulted in a reduction of aldosterone-induced Na(+) transport demonstrating the necessity of pcMTase-mediated transmethylation for steroid induced Na(+) reabsorption. Transfection with an eukaryotic expression construct containing pcMTase cDNA increased pcMTase protein level and activity. This resulted in potentiation of the natriferic actions of aldosterone. However, overexpression did not change Na(+) reabsorption in the absence of steroid, suggesting that pcMTase activity is not limiting Na(+) transport in the absence of steroid, but that subsequent to aldosterone addition, pcMTase activity becomes limiting. These results suggest that a critical transmethylation is necessary for aldosterone-induction of Na(+) transport. It is likely that the protein catalyzing this methylation is isoprenylcysteine-O-carboxyl methyltransferase and that aldosterone activates pcMTase without affecting transferase expression.

The carboxyl methyltransferase modifying G proteins is a metalloenzyme

The prenylated protein carboxyl methyltransferase (PPMT) catalyzes the posttranslational methylation of isoprenylated C-terminal cysteine residues found in many signaling proteins such as the small monomeric G proteins and the gamma subunits of heterotrimeric G proteins. Here we report that both membrane-bound PPMT from rat kidney and the recombinant bacterially expressed form of the enzyme required divalent cations for catalytic activity. Unlike EDTA and EGTA, the metal chelator 1,10-phenanthroline strongly inhibited the PPMT activity of kidney intracellular membranes in a dose- and time-dependent manner. 1,10-Phenanthroline was found to inhibit the methylation of the prenylcysteine analog N-acetyl-S-all-trans-geranylgeranyl-l-cysteine, a synthetic substrate for PPMT, with an IC(50) of 2.2 mM. Gel electrophoretic analysis demonstrated that 1,10-phenanthroline almost totally abolished the labeling of methylated proteins in kidney intracellular membranes. Immunoblotting analysis showed that one of the two major peaks of (3)H-methylated proteins in intracellular membranes comigrated with the small G proteins Ras, Cdc42, RhoA, and Rab1. In addition, the methylation of immunoprecipitated Ras and RhoA from kidney intracellular membranes was strongly inhibited when 1,10-phenanthroline was present. Treatment of kidney intracellular membranes with 1,10-phenanthroline increased the proteolytic degradation of PPMT by exogenous trypsin, compared to untreated membranes. We conclude from these data that metal ions are essential for the activity and the stabilization of PPMT. The finding that PPMT is a metalloenzyme may provide new insights into the functions played by this methyltransferase in signal transduction processes.

Modulation of tumor necrosis factor-alpha-mediated cytotoxicity by changes of the cellular methylation state: mechanism and in vivo relevance

A combination of adenosine (Ado) and homocysteine (Homo) enhances tumor necrosis factor (TNF)-alpha cytotoxicity in vitro and in vivo in several tumor cells. Ado and Homo at concentrations that enhanced TNF-alpha-mediated cytotoxicity accumulated S-adenosylhomocysteine (AdoHcy) and as consequence decreased the cellular methylation state, i.e. the ratio of S-adenosylmethionine to AdoHcy. This decrease led to inhibition of the isoprenylcysteine carboxyl methyltransferase (MTase), an enzyme that catalyzes carboxyl methylation of C-terminal cysteine residues on isoprenylated proteins. The effect of Ado and Homo on TNF-alpha cytotoxicity was at least partly mimicked by S-farnesylthioacetic acid, a selective inhibitor of the isoprenylcysteine carboxyl MTase, suggesting involvement of methylations of prenylated proteins in TNF-alpha-mediated cytotoxicity. Blockage of methylation reactions was associated with an enhancement of the TNF-alpha-induced disruption of the mitochondrial membrane potential (delta psi(m)). In nude mice, a combination of Ado, Homo and TNF-alpha led to TNF-alpha-induced hemorrhagic necrosis and growth inhibition of TNF-sensitive L929 tumors, whereas little effect was observed with TNF-alpha alone. Even more important, the TNF-resistant L929 M1 tumors were rendered TNF-sensitive by the combined action of Ado and Homo. We conclude that Ado and Homo together enhance the effectiveness of TNF-alpha in vitro and in vivo, results that may have therapeutic implications.