Analysis of Ras protein expression in mammalian cells

Ras signals principally via Erk in G1 but cooperates with PI3K/Akt for Cyclin D induction and S-phase entry

Numerous studies exploring oncogenic Ras or manipulating physiological Ras signalling have established an irrefutable role for Ras as driver of cell cycle progression. Despite this wealth of information the precise signalling timeline and effectors engaged by Ras, particularly during G1, remain obscure as approaches for Ras inhibition are slow-acting and ill-suited for charting discrete Ras signalling episodes along the cell cycle. We have developed an approach based on the inducible recruitment of a Ras-GAP that enforces endogenous Ras inhibition within minutes. Applying this strategy to inhibit Ras stepwise in synchronous cell populations revealed that Ras signaling was required well into G1 for Cyclin D induction, pocket protein phosphorylation and S-phase entry, irrespective of whether cells emerged from quiescence or G2/M. Unexpectedly, Erk, and not PI3K/Akt or Ral was activated by Ras at mid-G1, albeit PI3K/Akt signalling was a necessary companion of Ras/Erk for sustaining cyclin-D levels and G1/S transition. Our findings chart mitogenic signaling by endogenous Ras during G1 and identify limited effector engagement restricted to Raf/MEK/Erk as a cogent distinction from oncogenic Ras signalling.

Regulation of ras exchange factors and cellular localization of ras activation by lipid messengers in T cells

The Ras-MAPK signaling pathway is highly conserved throughout evolution and is activated downstream of a wide range of receptor stimuli. Ras guanine nucleotide exchange factors (RasGEFs) catalyze GTP loading of Ras and play a pivotal role in regulating receptor-ligand induced Ras activity. In T cells, three families of functionally important RasGEFs are expressed: RasGRF, RasGRP, and Son of Sevenless (SOS)-family GEFs. Early on it was recognized that Ras activation is critical for T cell development and that the RasGEFs play an important role herein. More recent work has revealed that nuances in Ras activation appear to significantly impact T cell development and selection. These nuances include distinct biochemical patterns of analog versus digital Ras activation, differences in cellular localization of Ras activation, and intricate interplays between the RasGEFs during distinct T cell developmental stages as revealed by various new mouse models. In many instances, the exact nature of these nuances in Ras activation or how these may result from fine-tuning of the RasGEFs is not understood. One large group of biomolecules critically involved in the control of RasGEFs functions are lipid second messengers. Multiple, yet distinct lipid products are generated following T cell receptor (TCR) stimulation and bind to different domains in the RasGRP and SOS RasGEFs to facilitate the activation of the membrane-anchored Ras GTPases. In this review we highlight how different lipid-based elements are generated by various enzymes downstream of the TCR and other receptors and how these dynamic and interrelated lipid products may fine-tune Ras activation by RasGEFs in developing T cells.

A potent HIV protease inhibitor, darunavir, does not inhibit ZMPSTE24 or lead to an accumulation of farnesyl-prelamin A in cells

HIV protease inhibitors (HIV-PIs) are key components of highly active antiretroviral therapy, but they have been associated with adverse side effects, including partial lipodystrophy and metabolic syndrome. We recently demonstrated that a commonly used HIV-PI, lopinavir, inhibits ZMPSTE24, thereby blocking lamin A biogenesis and leading to an accumulation of prelamin A. ZMPSTE24 deficiency in humans causes an accumulation of prelamin A and leads to lipodystrophy and other disease phenotypes. Thus, an accumulation of prelamin A in the setting of HIV-PIs represents a plausible mechanism for some drug side effects. Here we show, with metabolic labeling studies, that lopinavir leads to the accumulation of the farnesylated form of prelamin A. We also tested whether a new and chemically distinct HIV-PI, darunavir, inhibits ZMPSTE24. We found that darunavir does not inhibit the biochemical activity of ZMPSTE24, nor does it lead to an accumulation of farnesyl-prelamin A in cells. This property of darunavir is potentially attractive. However, all HIV-PIs, including darunavir, are generally administered with ritonavir, an HIV-PI that is used to block the metabolism of other HIV-PIs. Ritonavir, like lopinavir, inhibits ZMPSTE24 and leads to an accumulation of prelamin A.

HIV protease inhibitors block the zinc metalloproteinase ZMPSTE24 and lead to an accumulation of prelamin A in cells

HIV protease inhibitors (HIV-PIs) target the HIV aspartyl protease, which cleaves the HIV gag-pol polyprotein into shorter proteins required for the production of new virions. HIV-PIs are a cornerstone of treatment for HIV but have been associated with lipodystrophy and other side effects. In both human and mouse fibroblasts, we show that HIV-PIs caused an accumulation of prelamin A. The prelamin A in HIV-PI-treated fibroblasts migrated more rapidly than nonfarnesylated prelamin A, comigrating with the farnesylated form of prelamin A that accumulates in ZMPSTE24-deficient fibroblasts. The accumulation of farnesyl-prelamin A in response to HIV-PI treatment was exaggerated in fibroblasts heterozygous for Zmpste24 deficiency. HIV-PIs inhibited the endoproteolytic processing of a GFP-prelamin A fusion protein. The HIV-PIs did not affect the farnesylation of HDJ-2, nor did they inhibit protein farnesyltransferase in vitro. HIV-PIs also did not inhibit the activities of the isoprenyl-cysteine carboxyl methyltransferase ICMT or the prenylprotein endoprotease RCE1 in vitro, but they did inhibit ZMPSTE24 (IC(50): lopinavir, 18.4 +/- 4.6 microM; tipranavir, 1.2 +/- 0.4 microM). We conclude that the HIV-PIs inhibit ZMPSTE24, leading to an accumulation of farnesyl-prelamin A. The inhibition of ZMPSTE24 by HIV-PIs could play a role in the side effects of these drugs.

Effects of HIV protease inhibitor ritonavir on Akt-regulated cell proliferation in breast cancer

PURPOSE

These studies were designed to determine whether ritonavir inhibits breast cancer in vitro and in vivo and, if so, how.

EXPERIMENTAL DESIGN

Ritonavir effects on breast cancer cell growth were studied in the estrogen receptor (ER)-positive lines MCF7 and T47D and in the ER-negative lines MDA-MB-436 and MDA-MB-231. Effects of ritonavir on Rb-regulated and Akt-mediated cell proliferation were studied. Ritonavir was tested for inhibition of a mammary carcinoma xenograft.

RESULTS

ER-positive estradiol-dependent lines (IC50, 12-24 micromol/L) and ER-negative (IC50, 45 micromol/L) lines exhibit ritonavir sensitivity. Ritonavir depletes ER-alpha levels notably in ER-positive lines. Ritonavir causes G1 arrest, depletes cyclin-dependent kinases 2, 4, and 6 and cyclin D1 but not cyclin E, and depletes phosphorylated Rb and Ser473 Akt. Ritonavir induces apoptosis independent of G1 arrest, inhibiting growth of cells that have passed the G1 checkpoint. Myristoyl-Akt, but not activated K-Ras, rescues ritonavir inhibition. Ritonavir inhibited a MDA-MB-231 xenograft and intratumoral Akt activity at a clinically attainable serum Cmax of 22 +/- 8 micromol/L. Because heat shock protein 90 (Hsp90) substrates are depleted by ritonavir, ritonavir effects on Hsp90 were tested. Ritonavir binds Hsp90 (K(D), 7.8 micromol/L) and partially inhibits its chaperone function. Ritonavir blocks association of Hsp90 with Akt and, with sustained exposure, notably depletes Hsp90. Stably expressed Hsp90alpha short hairpin RNA also depletes Hsp90, inhibiting proliferation and sensitizing breast cancer cells to low ritonavir concentrations.

CONCLUSIONS

Ritonavir inhibits breast cancer growth in part by inhibiting Hsp90 substrates, including Akt. Ritonavir may be of interest for breast cancer therapeutics and its efficacy may be increased by sustained exposure or Hsp90 RNA interference.

Growth factor-dependent AKT activation and cell migration requires the function of c-K(B)-Ras versus other cellular ras isoforms

K-Ras-negative fibroblasts are defective in their steady-state expression of MMP-2. This occurs through c-K(B)-Ras dependent regulation of basal levels of AKT activity. In this report, we have extended those studies to demonstrate that in the absence of K-Ras expression, PDGF-BB fails to induce significant AKT activation, although this was not the case in N-Ras-negative cells. This phenotype was directly linked to PDGF-dependent cell migration. All of the independently immortalized K-Ras-negative cells failed to migrate upon the addition of PDGF. Only ectopic expression of c-K(B)-Ras, not c-K(A)-Ras nor oncogenic N-Ras, could restore both PDGF-dependent AKT activation and cell migration. Since most Ras binding partners can interact with all Ras isoforms, the specificity of PDGF-dependent activation of AKT and enhanced cell migration suggests that these outcomes are likely to be regulated through a c-K(B)-Ras-specific binding partner. Others have published that of the four Ras isoforms, only K(B)-Ras can form a stable complex with calmodulin (CaM). Along those lines, we provide evidence that 1) PDGF addition results in increased levels of a complex between c-K(B)-Ras and CaM and 2) the biological outcomes that are strictly dependent on c-K(B)-Ras (AKT activation and cell migration) are blocked by CaM antagonists. The PDGF-dependent activation of ERK is unaffected by the absence of K(B)-Ras and presence of CaM antagonists. This is the first example of a linkage between a specific biological outcome, cell migration, and the activity of a single Ras isoform, c-K(B)-Ras.

Prenylation is not necessary for endogenous Ras activation in non-malignant cells

Ras monomeric GTPases are pivotal to many core cellular processes such as proliferation and differentiation. The post-translational prenylation of Ras with a farnesyl or a geranylgeranyl moiety is thought to be critical for its membrane binding and consequent signaling activity. Inhibitors of Ras prenylation have an anti-proliferative effect in some Ras-transformed cells. We present a study of the effects of prenylation inhibitors on endogenous, wild-type Ras in three renal cell types, namely primary adult human renal fibroblasts, primary adult human mesangial cells, and a primate renal fibroblast cell line (Vero cells). We have previously demonstrated that Ras is necessary for normal proliferation in these cells. Here we show that Ras is farnesylated and not geranylgeranylated in all three cell types. Furthermore, inhibiting Ras farnesylation has no effect on cell proliferation or Ras activation. Although inhibiting geranylgeranylation in these cells does inhibit proliferation, this is through an Ras-independent mechanism. Non-prenylated Ras is able to localize to the plasma membrane, bind Raf when cells are stimulated by epidermal growth factor or platelet-derived growth factor, and activate the Ras downstream effectors mitogen-activated protein kinase and phosphotidylinositol 3-kinase. We conclude that in wild-type cells, endogenous Ras does not need to be prenylated to be active.

Activation of Ras Proteins by Ras Guanine Nucleotide Releasing Protein Family Members

Ras guanine nucleotide releasing proteins (RasGRPs) function as guanine nucleotide exchange factors for Ras proteins. Thus, RasGRPs are direct activators of Ras proteins and contribute an important role in various cell-signaling pathways that are regulated by the activation state of Ras proteins. RasGRPs are regulated by the second messengers diacylglycerol and intracellular calcium and are also known as CalDAG-GEFs or calcium and diacylglycerol-regulated guanine nucleotide exchange factors. RasGRPs couple signaling events that generate these second messengers in the cell into activation of signaling pathways that are regulated by Ras. RasGRPs, therefore, increase the repertoire of extracellular stimuli that lead to activation of Ras. Analyzing the regulation of RasGRP activity should continue to play an important role in understanding the mechanisms by which signal transduction pathways use RasGRP proteins to activate Ras proteins in cells.

Using Inhibitors of Prenylation to Block Localization and Transforming Activity

The proper subcellular localization and biological activity of most Ras and Rho family small GTPases are dependent on their posttranslational modification by isoprenylation. Farnesyltransferase (FTase) and geranylgeranyl transferase I (GGTase I) are the prenyltransferases that catalyze the irreversible attachment of C15 farnesyl (Ras, Rnd) or C20 (R-Ras, Ral, Rap, Rho, Rac, Cdc42) isoprenoid lipid moieties to these small GTPases and other proteins. Therefore, pharmacological inhibitors of FTase (FTIs) and GGTase I (GGTIs) have been developed to prevent these modifications and thereby to block the lipid-mediated association of Ras and Rho proteins with cellular membranes and the consequent signaling and transforming activities. In addition, other small molecule inhibitors such as farnesyl thiosalicylic acid (FTS) can compete with the isoprenoid moiety of small GTPases for membrane binding sites. Finally, endogenous regulatory proteins such as RhoGDIs can bind to and mask the prenyl groups of small GTPases, leading to their sequestration from membranes. We describe here methods to use each of these categories of prenylation inhibitors to manipulate and investigate the subcellular localization patterns and transforming potential of these Ras and Rho family GTPases.

Prevention of farnesylation of c-Ha-Ras protein enhances synergistically the cytotoxic action of doxorubicin in cycling but not in quiescent cells

Ras, the product of a proto-oncogene, is a GTP-hydrolyzing enzyme found mutated in approximately 50% of human cancers. "Gain of function" mutations of Ras lead to an escape of transformed cells from cell-cycle control, rendering them independent to stimulation by growth factors, giving them almost unlimited proliferation capacity. The cytosolic precursor isoform of Ras is biologically inactive. After several post-translational modifications, Ras is anchored to the plasma membrane and, thereby, the protein becomes activated. The finding that lipid modifications of Ras protein, particularly farnesylation, are essential for its signal transduction activity, gave rise to the concept that blocking farnesyl protein transferase (FPTase), the enzyme catalyzing the first step in the Ras modification cascade, would prevent proper membrane anchoring and provide an improved approach in the cure of tumors harboring Ras mutations. In the present study we used transformed rat cells overexpressing a temperature-sensitive p53 protein, adopting wt conformation at 32 degrees C and mutant conformation at 37 degrees C. We treated the cells growing at 32 or 37 degrees C with doxorubicin alone, or in combination with inhibitors of FPTase. Combined treatment was more efficient and the same inhibition of cell proliferation was reached at lower DOX concentrations. The treatment strongly affected the growth rate of tumor cells but only negligibly of normal cells. However, the inhibitors of FPTase prevented the membrane anchoring in both situations. These results show two striking advantages of the combined treatment: the desired cytostatic effect on tumor cells at lower drug concentrations and clearly reduced adverse effects on quiescent cells.

A Tagging‐via‐Substrate Technology for Genome‐Wide Detection and Identification of Farnesylated Proteins

Protein farnesylation is one of the most common lipid modifications and has an important role in the regulation of various cellular functions. We have recently developed a novel proteomics strategy, designated the tagging-via-substrate (TAS) approach, for the detection and proteomic analysis of farnesylated proteins. This chapter describes the principle of TAS technology and details the method for detection and enrichment of farnesylated proteins.

Differential membrane localization of ERas and Rheb, two Ras-related proteins involved in the phosphatidylinositol 3-kinase/mTOR pathway

Two Ras-related proteins, ERas and Rheb, which are involved in the phosphatidylinositol 3-kinase pathway, display high GTP affinity and have atypical CAAX motifs. The factors governing the intracellular localization of ERas and Rheb are incompletely understood. In the current study, we show by confocal microscopy that ERas is localized to the plasma membrane, whereas Rheb is confined to the endomembranes. Membrane localization of the two proteins was abolished by mutation of the cysteine of the CAAX motif. Membrane targeting was also abolished by a farnesyltransferase inhibitor but not by a geranylgeranyltransferase inhibitor. In mouse fibroblasts deficient in either Rce1 (Ras converting enzyme 1) or Icmt (isoprenylcysteine carboxyl methyltransferase), ERas was mislocalized mainly to the Golgi apparatus, whereas Rheb showed diffuse localization. Mutation of cysteines in the hypervariable region of ERas prevented the plasma membrane localization of ERas, very strongly suggesting that palmitoylation of the cysteines is essential for membrane targeting. The hypervariable region of Rheb does not contain cysteines or polybasic residues, and when it was replaced with the hypervariable region of H-Ras, Rheb displayed plasma membrane localization. These data indicate that ERas shares the same posttranslational modifications with H-Ras and N-Ras and is localized at the plasma membrane. Rheb also shares the same membrane-targeting pathway but because of the absence of palmitoylation is located on endomembranes.

Requirement for C-terminal sequences in regulation of Ect2 guanine nucleotide exchange specificity and transformation

Ect2 was identified originally as a transforming protein and a member of the Dbl family of Rho guanine nucleotide exchange factors (GEFs). Like all Dbl family proteins, Ect2 contains a tandem Dbl homology (DH) and pleckstrin homology (PH) domain structure. Previous studies demonstrated that N-terminal deletion of sequences upstream of the DH domain created a constitutively activated, transforming variant of Ect2 (designated DeltaN-Ect2 DH/PH/C), indicating that the N terminus served as a negative regulator of DH domain function in vivo. The role of sequences C-terminal to the DH domain has not been established. Therefore, we assessed the consequences of mutation of C-terminal sequences on Ect2-transforming activity. Surprisingly, in contrast to observations with other Dbl family proteins, we found that mutation of the invariant tryptophan residue in the PH domain did not impair DeltaN-Ect2 DH/PH/C transforming activity. Furthermore, although the sequences C-terminal to the PH domain lack any known functional domains or motifs, deletion of these sequences (DeltaN-Ect2 DH/PH) resulted in a dramatic reduction in transforming activity. Whereas DeltaN-Ect2 caused formation of lamellipodia, DeltaN-Ect2 DH/PH enhanced actin stress fiber formation, suggesting that C-terminal sequences influenced Ect2 Rho GTPase specificity. Consistent with this possibility, we determined that DeltaN-Ect2 DH/PH activated RhoA, but not Rac1 or Cdc42, whereas DeltaN-Ect2 DH/PH/C activated all three Rho GTPases in vivo. Taken together, these observations suggest that regions of Ect2 C-terminal to the DH domain alter the profile of Rho GTPases activated in vivo and consequently may contribute to the enhanced transforming activity of DeltaN-Ect2 DH/PH/C.

High affinity for farnesyltransferase and alternative prenylation contribute individually to K-Ras4B resistance to farnesyltransferase inhibitors

Farnesyltransferase inhibitors (FTIs) block Ras farnesylation, subcellular localization and activity, and inhibit the growth of Ras-transformed cells. Although FTIs are ineffective against K-Ras4B, the Ras isoform most commonly mutated in human cancers, they can inhibit the growth of tumors containing oncogenic K-Ras4B, implicating other farnesylated proteins or suggesting distinct functions for farnesylated and for geranylgeranylated K-Ras, which is generated when farnesyltransferase is inhibited. In addition to bypassing FTI blockade through geranylgeranylation, K-Ras4B resistance to FTIs may also result from its higher affinity for farnesyltransferase. Using chimeric Ras proteins containing all combinations of Ras background, CAAX motif, and K-Ras polybasic domain, we show that either a polybasic domain or an alternatively prenylated CAAX renders Ras prenylation, Ras-induced Elk-1 activation, and anchorage-independent cell growth FTI-resistant. The polybasic domain alone increases the affinity of Ras for farnesyltransferase, implying independent roles for each K-Ras4B sequence element in FTI resistance. Using microarray analysis and colony formation assays, we confirm that K-Ras function is independent of the identity of the prenyl group and, therefore, that FTI inhibition of K-Ras transformed cells is likely to be independent of K-Ras inhibition. Our results imply that relevant FTI targets will lack both polybasic and potentially geranylgeranylated methionine-CAAX motifs.

Critical role of the pleckstrin homology and cysteine-rich domains in Vav signaling and transforming activity

Vav family proteins are members of the Dbl family of guanine nucleotide exchange factors and activators of Rho family small GTPases. In addition to the Dbl homology (DH) domain important for guanine nucleotide exchange factor catalytic function, all Dbl family proteins contain an adjacent pleckstrin homology (PH) domain that serves to regulate DH domain activity. Although the role of the PH domain in Vav function has been evaluated extensively, its precise role and whether it serves a distinct role in different Vav proteins remain unresolved. Additionally, the precise role of an adjacent cysteine-rich domain (CRD) in regulating DH domain function is also unclear. In this study, we evaluated the contribution of these putative protein-protein or protein-lipid interaction domains to Vav signaling and transforming activity. In contrast to previous observations, we found that the PH domain is critical for Vav transforming activity. Similarly, the CRD was also essential and served a function distinct from that of the PH domain. Although mutation of either domain reduced Vav membrane association, addition of plasma membrane targeting sequences to either the CRD or PH domain mutant proteins did not restore Vav transforming activity. This result contrasts with other Dbl family proteins, where a membrane targeting sequence alone was sufficient to restore the loss of function caused by mutation of the PH domain. Furthermore, green fluorescent protein fusion proteins containing the PH domain or CRD, or both, failed to target to the plasma membrane, suggesting that these two domains also serve regulatory functions independent of promoting membrane localization. Finally, we found that phosphatidylinositol 3-kinase activation may promote Vav membrane association via phosphatidylinositol 3,4,5-triphosphate binding to the PH domain.

RasGRP4 is a novel Ras activator isolated from acute myeloid leukemia

Although a number of genetic defects are commonly associated with acute myeloid leukemia (AML), a large percentage of AML cases are cytogenetically normal. This suggests a functional screen for transforming genes is required to identify genetic mutations that are missed by cytogenetic analyses. We utilized a retrovirus-based cDNA expression system to identify transforming genes expressed in cytogenetically normal AML patients. We identified a new member of the Ras guanyl nucleotide-releasing protein (RasGRP) family of Ras guanine nucleotide exchange factors, designating it RasGRP4. Subsequently, cDNA sequences encoding rodent and human RasGRP4 proteins were deposited in GenBank. RasGRP4 contains the same protein domain structure as other members of the RasGRP family, including a Ras exchange motif, a CDC25 homology domain, a C1/diacyglycerol-binding domain, and putative calcium-binding EF hands. We show that expression of RasGRP4 induces anchorage-independent growth of Rat1 fibroblasts. RasGRP4 is a Ras-specific activator and, interestingly, is highly expressed in peripheral blood leukocytes and myeloid cell lines. Unlike other RasGRP proteins, RasGRP4 is not expressed in the brain or in lymphoid cells. We demonstrated that 32D myeloid cells expressing RasGRP4 have elevated levels of activated Ras compared with control cells, and phorbol 12-myristate 13-acetate (PMA) treatment greatly enhanced Ras activation. PMA induced membrane localization of RasGRP4 and 32D cells expressing RasGRP4 were capable of cytokine-independent proliferation in the presence of PMA. We conclude that RasGRP4 is a member of the RasGRP family of Ras guanine nucleotide exchange factors that may play a role in myeloid cell signaling growth regulation pathways that are responsive to diacylglycerol levels.

A distinct class of dominant negative Ras mutants: cytosolic GTP-bound Ras effector domain mutants that inhibit Ras signaling and transformation and enhance cell adhesion

Cytosolic GTP-bound Ras has been shown to act as a dominant negative (DN) inhibitor of Ras by sequestering Raf in non-productive cytosolic complexes. Nevertheless, this distinct class of DN mutants has been neither well characterized nor extensively used to analyze Ras signaling. In contrast, DN Ras17N, which functions by blocking Ras guanine nucleotide exchange factors, has been well characterized and is widely used. Cytosolic GTP-bound Ras mutants could be used to inhibit particular Ras effectors by introducing additional mutations (T35S, E37G or Y40C) that permit them to associate selectively with and inhibit Raf, RalGDS, or phosphoinositide 3-kinase, respectively. When the wild-type Ras effector binding region is used, cytosolic Ras should associate with all Ras effectors, even those that are not yet identified, making these DN Ras mutants effective inhibitors of multiple Ras functions. We generated cytosolic GTP-bound H-, N-, and K-Ras, and we assessed their ability to inhibit Ras-induced phenotypes. In fibroblasts, cytosolic H-, N-, and K-Ras inhibited Ras-induced Elk-1 activation and focus formation, induced a flattened cell morphology, and increased adhesion to fibronectin through modulation of a beta(1)-subunit-containing integrin, thereby demonstrating that DN activity is not limited to a subset of Ras isoforms. We also generated cytosolic GTP-bound Ras effector domain mutants (EDMs), each of which reduced the ability of cytosolic GTP-bound Ras proteins to inhibit Elk-1 activation and to induce cell flattening, implicating multiple pathways in these phenotypes. In contrast, Ras-induced focus formation, platelet-derived growth factor (PDGF)-, or Ras-induced phospho-Akt levels and cell adhesion to fibronectin were affected by T35S and Y40C EDMs, whereas PDGF- or Ras-induced phospho-Erk levels were affected only by the T35S EDM, implying that a more limited set of Ras-mediated pathways participate in these phenotypes. These data constitute the first extensive characterization of this functionally distinct class of DN Ras inhibitor proteins.

Absence of the CAAX endoprotease Rce1: effects on cell growth and transformation

After isoprenylation, the Ras proteins and other CAAX proteins undergo two additional enzymatic modifications-endoproteolytic release of the last three amino acids of the protein by the protease Rce1 and methylation of the carboxyl-terminal isoprenylcysteine by the methyltransferase Icmt. This postisoprenylation processing is thought to be important for the association of Ras proteins with membranes. Blocking postisoprenylation processing, by inhibiting Rce1, has been suggested as a potential approach for retarding cell growth and blocking cellular transformation. The objective of this study was to develop a cell culture system for addressing these issues. We generated mice with a conditional Rce1 allele (Rce1(flox)) and produced Rce1(flox/flox) fibroblasts. Cre-mediated excision of Rce1 (thereby producing Rce1(Delta/Delta) fibroblasts) eliminated Ras endoproteolytic processing and methylation and caused a partial mislocalization of truncated K-Ras and H-Ras fusion proteins within cells. Rce1(Delta/Delta) fibroblasts grew more slowly than Rce1(flox/flox) fibroblasts. The excision of Rce1 also reduced Ras-induced transformation, as judged by the growth of colonies in soft agar. The excision of Rce1 from a Rce1(flox/flox) skin carcinoma cell line also significantly retarded the growth of cells, and this effect was exaggerated by cotreatment of the cells with a farnesyltransferase inhibitor. These studies support the idea that interference with postisoprenylation processing retards cell growth, limits Ras-induced transformation, and sensitizes tumor cells to a farnesyltransferase inhibitor.

Oxidative modification of H-ras: S-thiolation and S-nitrosylation of reactive cysteines

The reactive cysteines in H-ras are subject to oxidative modifications that potentially alter the cellular function of this protein. In this study, purified H-ras was modified by thiol oxidants such as hydrogen peroxide (H(2)O(2)), S-nitrosoglutathione, diamide, glutathione disulphide (GSSG) and cystamine, producing as many as four charge-isomeric forms of the protein. These results suggest that all four reactive cysteines of H-ras are potential sites of regulatory modification reactions. S-nitrosylated and S-glutathiolated forms of H-ras were identified by protocols that depend on separation of alkylated proteins on electrofocusing gels. S-nitrosoglutathione could S-nitrosylate H-ras on four cysteine residues, while reduced glutathione (GSH) and H(2)O(2) mediate S-glutathiolation on at least one cysteine of H-ras. Either GSSG or diamide S-glutathiolated at least two cysteine residues of purified H-ras. Iodoacetic acid reacts with three cysteine residues. In intact NIH-3T3 cells, wild-type H-ras was S-glutathiolated by diamide. Similarly, cells expressing a C118S mutant or a C181S/C184S double mutant of H-ras were S-glutathiolated by diamide. These results suggest that H-ras can be S-glutathiolated on multiple thiols in vivo and that at least one of these thiols is normally lipid-modified. In cells treated with S-nitrosocysteine, evidence for both S-nitrosylated and S-glutathiolated H-ras was obtained and S-nitrosylation was the predominant modification. These results show that oxidative modification of H-ras can be extensive in vivo, that both S-nitrosylated and S-glutathiolated forms may be important, and that oxidation may occur on reactive cysteines that are normally targeted for lipid-modification reactions.

Synthesis of functional Ras lipoproteins and fluorescent derivatives

For the study of biological signal transduction, access to correctly lipidated proteins is of utmost importance. Furthermore, access to bioconjugates that embody the correct structure of the protein but that may additionally carry different lipid groups or labels (i.e., fluorescent tags) by which the protein can be traced in biological systems, could provide invaluable reagents. We report here of the development of techniques for the synthesis of a series of modified Ras proteins. These modified Ras proteins carry a number of different, natural and non-natural lipid residues, and the process was extended to also provide access to a number of fluorescently labeled derivatives. The maleimide group provided the key to link chemically synthesized lipopeptide molecules in a specific and efficient manner to a truncated form of the H-Ras protein. Furthermore, a preliminary study on the biological activity of the natural Ras protein derivative (containing the normal farnesyl and palmitoyl lipid residues) has shown full biological activity. This result highlights the usefulness of these compounds as invaluable tools for the study of Ras signal transduction processes and the plasma membrane localization of the Ras proteins.

Single cell Ras-GTP analysis reveals altered Ras activity in a subpopulation of neurofibroma Schwann cells but not fibroblasts

Neurofibromatosis type 1 (NF1) is a common genetic disorder characterized by multiple neurofibromas, peripheral nerve tumors containing mainly Schwann cells and fibroblasts. The NF1 gene encodes neurofibromin, a tumor suppressor postulated to function in part as a Ras GTPase-activating protein. The roles of different cell types and of elevated Ras-GTP in neurofibroma formation are unclear. To determine which neurofibroma cell type has altered Ras-GTP regulation, we developed an immunocytochemical assay for active, GTP-bound Ras. In NIH 3T3 cells, the assay detected overexpressed, constitutively activated K-, N-, and Ha-Ras and insulin-induced endogenous Ras-GTP. In dissociated neurofibroma cells from NF1 patients, Ras-GTP was elevated in Schwann cells but not fibroblasts. Twelve to 62% of tumor Schwann cells showed elevated Ras-GTP, unexpectedly revealing neurofibroma Schwann cell heterogeneity. Increased basal Ras-GTP did not correlate with increased cell proliferation. Normal human Schwann cells, however, did not demonstrate elevated basal Ras activity. Furthermore, compared with cells from wild type littermates, Ras-GTP was elevated in all mouse Nf1(-/-) Schwann cells but never in Nf1(-/-) mouse fibroblasts. Our results indicate that Ras activity is detectably increased in only some neurofibroma Schwann cells and suggest that neurofibromin is not an essential regulator of Ras activity in fibroblasts.

Rit, a non-lipid-modified Ras-related protein, transforms NIH3T3 cells without activating the ERK, JNK, p38 MAPK or PI3K/Akt pathways

The biological functions of Rit (Ras-like protein in tissues) and Rin (Ras-like protein in neurons), members of a novel branch of Ras-related GTP-binding proteins that are approximately 50% identical to Ras, have not been characterized. Therefore, we assessed their activity in growth control, transformation and signaling. NIH cells stably expressing a constitutively activated mutant of Rit [Rit(79L)] (analogous to the oncogenic mutant H-Ras(61L)) demonstrated strong growth transformation, proliferating rapidly in low serum and forming colonies in soft agar and tumors in nude mice. Although Rit(79L) alone did not promote morphologically transformed foci, it cooperated with both Raf and Rho A to form Rac/Rho-like foci. Rin [Rin(78L)] cooperated only with Raf. Rit(79L) but not Rin(78L) stimulated transcription from luciferase reporter constructs regulated by SRF, NF-kappaB, Elk-1 and Jun. However, neither activated ERK, JNK or p38, or PI3-K/Akt kinases in immune complex kinase assays. Interestingly, although Rit lacks any known recognition signal for C-terminal lipidation, Rit-transformed cell growth and survival in low serum is dependent on a farnesylated protein, as treatment with farnesyltransferase inhibitors caused apoptosis. Rin cooperated with Raf in focus assays but did not otherwise function in these assays, perhaps due to a lack of appropriate effector pathways in NIH3T3 fibroblasts for this neural-specific Ras family member. In summary, although Rit shares most core effector domain residues with Ras, our results suggest that Rit uses novel effector pathways to regulate proliferation and transformation.

Growth factor-dependent activation of the Ras-Raf-MEK-MAPK pathway in the human pancreatic carcinoma cell line PANC-1 carrying activated K-ras: implications for cell proliferation and cell migration

Human ductal adenocarcinoma of the pancreas frequently carry activating point mutations in the K-ras protooncogene. We have analysed the activity of the Ras-Raf-MEK-MAPK cascade in the human pancreatic carcinoma cell line PANC-1 carrying an activating K-ras mutation. Serum-starved cells and cells grown in medium with serum did not show constitutively activated c-Raf, MEK-1, or p42 MAPK. Stimulation of cells with epidermal growth factor (EGF) or fetal calf serum (FCS) resulted in activation of N-Ras, but not K-Ras, as well as activation of c-Raf, MEK-1, and p42 MAPK. Preincubation of serum-starved cells with MEK-1 inhibitor PD98059 abolished EGF- and FCS-induced MAPK activation, identifying MEK as the upstream activator of MAPK. PANC-1 cells exhibited marked serum-dependence of anchorage-dependent and -independent cell growth as well as cell migration. EGF, alone or in combination with insulin and transferrin, did not induce cell proliferation of serum-starved PANC-1 cells, indicating that activation of MAPK alone was not sufficient to induce cell proliferation. FCS-induced DNA synthesis was inhibited by 40% by the MEK-1 inhibitor. On the other hand, treatment with either FCS or EGF alone resulted in marked, MEK-dependent increase of directed cell migration. Collectively, our results show that the activating K-ras mutation in PANC-1 cells does not result in constitutively increased Raf-MEK-MAPK signaling. Signal transduction via the Ras-Raf-MEK-MAPK cascade is maintained in these cells and is required for growth factor-induced cell proliferation and directed cell migration. Oncogene (2000).

Ras regulates sympathetic neuron survival by suppressing the p53-mediated cell death pathway

In this report, we examine how the Ras protein regulates neuronal survival, focusing on sympathetic neurons. Adenovirus-expressed constitutively activated Ras (RasV12) enhanced survival and the phosphorylation of Akt (protein kinase B) and MAP kinase (MAPK), two targets of Ras activity. Functional inhibition of endogenous Ras by adenovirus-expressed dominant-inhibitory Ras (N17Ras) decreased nerve growth factor (NGF)-dependent survival and both Akt and MAPK phosphorylation as well. To determine the signaling pathways through which Ras mediates survival, we used Ras effector mutants and pharmacological inhibitors that selectively suppress phosphatidylinositol 3-kinase (PI3-K)/Akt or MAP kinase kinase (MEK)/MAPK pathways. The Ras effector mutant Ras(V12)Y40C, which selectively stimulates PI3-K and Akt, rescued survival in the absence of NGF, and the PI3-K inhibitor LY 294002 inhibited both Ras- and NGF-dependent survival. Ras(V12)T(35)S, which activates MEK/MAPK but not PI3-K/Akt, was less effective at rescuing survival, whereas the MEK inhibitor PD 098059 also partially suppressed Ras-dependent survival. To investigate the mechanisms by which Ras suppresses neuronal death, we examined whether Ras functions by inhibiting the proapoptotic p53 pathway (Jun-N-terminal kinase/p53/BAX) that is necessary for neuronal death after NGF withdrawal and p75NTR activation. We found that RasV12 suppressed c-jun, BAX, and p53 levels, whereas inhibition of NGF-induced Ras-survival activity via N17Ras increased the levels of these proteins. Furthermore, the E1B55K protein, which suppresses p53 activity, blocked N17Ras-induced neuronal death. Together, these results indicate that Ras is, in part, both necessary and sufficient for survival of sympathetic neurons and that this effect is mediated by activation of both the PI3-K- and MEK-signaling cascades, which in turn suppress a proapoptotic p53 pathway.

Involvement of NH(2)-terminal sequences in the negative regulation of Vav signaling and transforming activity

Deletion of the NH(2)-terminal 65 amino acids of proto-Vav (to form onco-Vav) activates its transforming activity, suggesting that these sequences serve a negative regulatory role in Vav function. However, the precise role of these NH(2)-terminal sequences and whether additional NH(2)-terminal sequences are also involved in negative regulation have not been determined. Therefore, we generated additional NH(2)-terminal deletion mutants of proto-Vav that lack the NH(2)-terminal 127, 168, or 186 amino acids, and assessed their abilities to cause focus formation in NIH 3T3 cells and to activate different signaling pathways. Since Vav mutants lacking 168 or 186 NH(2)-terminal residues showed a several 100-fold greater focus forming activity than that seen with deletion of 65 residues, residues spanning 66 to 187 also contribute significantly to negative regulation of Vav transforming activity. The increase in Vav transforming activity correlated with the activation of the c-Jun, Elk-1, and NF-kappaB transcription factors, as well as increased transcription from the cyclin D1 promoter. Tyrosine 174 is a key site of phosphorylation by Lck in vitro and Lck-mediated phosphorylation has been shown to be essential for proto-Vav GEF function in vitro. However, we found that an NH(2)-terminal Vav deletion mutant lacking this tyrosine residue (DeltaN-186 Vav) retained the ability to be phosphorylated by Lck in vivo and Lck still caused enhancement of DeltaN-186 Vav signaling and transforming activity. Thus, Lck can stimulate Vav via a mechanism that does not involve Tyr(174) or removal of NH(2)-terminal regulatory activity. Finally, we found that NH(2)-terminal deletion enhanced the degree of Vav association with the membrane-containing particulate fraction and that an isolated NH(2)-terminal fragment (residues 1-186) could impair DeltaN-186 Vav signaling. Taken together, these observations suggest that the NH(2) terminus may serve as a negative regulator of Vav by intramolecular interaction with COOH-terminal sequences to modulate efficient membrane association.

Disruption of the mouse Rce1 gene results in defective Ras processing and mislocalization of Ras within cells

Little is known about the enzyme(s) required for the endoproteolytic processing of mammalian Ras proteins. We identified a mouse gene (designated Rce1) that shares sequence homology with a yeast gene (RCE1) implicated in the proteolytic processing of Ras2p. To define the role of Rce1 in mammalian Ras processing, we generated and analyzed Rce1-deficient mice. Rce1 deficiency was lethal late in embryonic development (after embryonic day 15.5). Multiple lines of evidence revealed that Rce1-deficient embryos and cells lacked the ability to endoproteolytically process Ras proteins. First, Ras proteins from Rce1-deficient cells migrated more slowly on SDS-polyacrylamide gels than Ras proteins from wild-type embryos and fibroblasts. Second, metabolic labeling of Rce1-deficient cells revealed that the Ras proteins were not carboxymethylated. Finally, membranes from Rce1-deficient fibroblasts lacked the capacity to proteolytically process farnesylated Ha-Ras, N-Ras, and Ki-Ras or geranylgeranylated Ki-Ras. The processing of two other prenylated proteins, the farnesylated Ggamma1 subunit of transducin and geranylgeranylated Rap1B, was also blocked. The absence of endoproteolytic processing and carboxymethylation caused Ras proteins to be mislocalized within cells. These studies indicate that Rce1 is responsible for the endoproteolytic processing of the Ras proteins in mammals and suggest a broad role for this gene in processing other prenylated CAAX proteins.

Early biochemical events in insulin-stimulated fluid phase endocytosis

We examined the initial molecular mechanisms by which cells nonselectively internalize extracellular solutes in response to insulin. Insulin-stimulated fluid phase endocytosis (FPE) was examined in responsive cells, and the roles of the insulin receptor, insulin receptor substrate-1 (IRS-1), phosphatidylinositol 3'-kinase (PI 3'-kinase), Ras, and mitogen-activated protein kinase kinase (MEK) were assessed. Active insulin receptors were essential, as demonstrated by the stimulation of FPE by insulin in HIRc-B cells (Rat-1 cells expressing 1.2 x 10(6) normal insulin receptors/cell) but not in untransfected Rat-1 cells or in Rat-1 cells expressing the inactive A/K1018 receptor. IRS-1 expression augmented insulin-stimulated FPE, as assessed in 32D cells, a hematopoietic precursor cell line lacking endogenous IRS-1. Insulin-stimulated FPE was inhibited in mouse brown adipose tissue (BAT) cells expressing the 17N dominant negative mutant Ras and was augmented in cells expressing wild-type Ras. The MEK inhibitor PD-98059 had little effect on insulin-stimulated FPE in BAT cells. In 32D cells, but not in HIRc-B and BAT cells, insulin-stimulated FPE was inhibited by 10 nM wortmannin, an inhibitor of PI 3'-kinase. The results indicate that the insulin receptor, IRS-1, Ras, and, perhaps in certain cell types, PI 3'-kinase are involved in mediating insulin-stimulated FPE.

The Ras-related protein Rheb is farnesylated and antagonizes Ras signaling and transformation

Presently, nothing is known about the function of the Ras-related protein Rheb. Since Rheb shares significant sequence identity with the core effector domains of Ras and KRev-1/Rap1A, it may share functional similarities with these two structurally related, yet functionally distinct, small GTPases. Furthermore, since like Ras, Rheb terminates with a COOH terminus that is likely to signal for farnesylation, it may be a target for the farnesyltransferase inhibitors that block Ras processing and function. To compare Rheb function with those of Ras and KRev-1, we introduced mutations into Rheb that generate constitutively active or dominant negative forms of Ras and Ras-related proteins and were designated Rheb(64L) and Rheb(20N), respectively. Expression of wild type or mutant Rheb did not alter the morphology or growth properties of NIH 3T3 cells. Thus, aberrant Rheb function is distinct from that of Ras and fails to cause cellular transformation. Instead, similar to KRev-1, co-expression of Rheb antagonized oncogenic Ras transformation and signaling. In vitro and in vivo analyses showed that like Ras, Rheb proteins are farnesylated and are sensitive to farnesyltransferase inhibition. Thus, it is possible that Rheb function may be inhibited by farnesyltransferase inhibitors treatment and, consequently, may contribute to the ability of these inhibitors to impair Ras transformation.

Ras activation in platelets after stimulation of the thrombin receptor, thromboxane A2 receptor or protein kinase C

Several reports have indicated that the small G-protein Ras is not present immunologically in platelets. However, here we report the identification of Ras in platelets by immunoprecipitation with the Ras-specific monoclonal antibodies Y13-259 or Y13-238, followed by Western blotting. The presence of Ras was not due to contamination of samples with erythrocytes or leucocytes. Immunofluorescence studies indicated that Ras was present in a peripheral rim pattern in fixed, permeabilized platelets, suggesting an intracellular, plasma membrane location. Activation of platelets with the thrombin receptor peptide42-50, the prostaglandin H2/thromboxane A2 mimetic U46619 or phorbol 12-myristate 13-acetate induced a rapid increase in GTP-bound, activated Ras. In each case, this increase was inhibited by the protein kinase C (PKC) inhibitor bisindolylmaleimide GF 109203X, suggesting that Ras is activated downstream of PKC in platelets. Thus the activation of Ras in platelets by agonists will now allow consideration of multiple potential Ras-dependent signal transduction pathways in platelet activation processes.

Activation of a novel calcium-dependent protein-tyrosine kinase. Correlation with c-Jun N-terminal kinase but not mitogen-activated protein kinase activation

Many G protein-coupled receptors (e.g. that of angiotensin II) activate phospholipase Cbeta, initially increasing intracellular calcium and activating protein kinase C. In the WB and GN4 rat liver epithelial cell lines, agonist-induced calcium signals also stimulate tyrosine phosphorylation and subsequently increase the activity of c-Jun N-terminal kinase (JNK). We have now purified the major calcium-dependent tyrosine kinase (CADTK), and by peptide and nucleic acid sequencing identified it as a rat homologue of human PYK2. CADTK/PYK2 is most closely related to p125(FAK) and both enzymes are expressed in WB and GN4 cells. Angiotensin II, which only slightly increases p125(FAK) tyrosine phosphorylation in GN4 cells, substantially increased CADTK tyrosine autophosphorylation and kinase activity. Agonists for other G protein-coupled receptors (e.g. LPA), or those increasing intracellular calcium (thapsigargin), also stimulated CADTK. In comparing the two rat liver cell lines, GN4 cells exhibited approximately 5-fold greater angiotensin II- and thapsigargin-dependent CADTK activation than WB cells. Although maximal JNK activation by stress-dependent pathways (e.g. UV and anisomycin) was equivalent in the two cell lines, calcium-dependent JNK activation was 5-fold greater in GN4, correlating with CADTK activation. In contrast to JNK, the thapsigargin-dependent calcium signal did not activate mitogen-activated protein kinase and Ang II-dependent mitogen-activated protein kinase activation was not correlated with CADTK activation. Finally, while some stress-dependent activators of the JNK pathway (NaCl and sorbitol) stimulated CADTK, others (anisomycin, UV, and TNFalpha) did not. In summary, cells expressing CADTK/PYK2 appear to have two alternative JNK activation pathways: one stress-activated and the other calcium-dependent.

Oncogenic Ras activation of Raf/mitogen-activated protein kinase-independent pathways is sufficient to cause tumorigenic transformation

Substantial evidence supports a critical role for the activation of the Raf-1/MEK/mitogen-activated protein kinase pathway in oncogenic Ras-mediated transformation. For example, dominant negative mutants of Raf-1, MEK, and mitogen-activated protein kinase all inhibit Ras transformation. Furthermore, the observation that plasma membrane-localized Raf-1 exhibits the same transforming potency as oncogenic Ras suggests that Raf-1 activation alone is sufficient to mediate full Ras transforming activity. However, the recent identification of other candidate Ras effectors (e.g., RalGDS and phosphatidylinositol-3 kinase) suggests that activation of other downstream effector-mediated signaling pathways may also mediate Ras transforming activity. In support of this, two H-Ras effector domain mutants, H-Ras(12V, 37G) and H-Ras(12V, 40C), which are defective for Raf binding and activation, induced potent tumorigenic transformation of some strains of NIH 3T3 fibroblasts. These Raf-binding defective mutants of H-Ras induced a transformed morphology that was indistinguishable from that induced by activated members of Rho family proteins. Furthermore, the transforming activities of both of these mutants were synergistically enhanced by activated Raf-1 and inhibited by the dominant negative RhoA(19N) mutant, indicating that Ras may cause transformation that occurs via coordinate activation of Raf-dependent and -independent pathways that involves Rho family proteins. Finally, cotransfection of H-Ras(12V, 37G) and H-Ras(12V, 40C) resulted in synergistic cooperation of their focus-forming activities, indicating that Ras activates at least two Raf-independent, Ras effector-mediated signaling events.