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

Mapping the KRAS proteoform landscape in colorectal cancer identifies truncated KRAS4B that decreases MAPK signaling

The KRAS gene is one of the most frequently mutated oncogenes in human cancer and gives rise to two isoforms, KRAS4A and KRAS4B. KRAS post-translational modifications (PTMs) have the potential to influence downstream signaling. However, the relationship between KRAS PTMs and oncogenic mutations remains unclear, and the extent of isoform-specific modification is unknown. Here, we present the first top-down proteomics study evaluating both KRAS4A and KRAS4B, resulting in 39 completely characterized proteoforms across colorectal cancer cell lines and primary tumor samples. We determined which KRAS PTMs are present, along with their relative abundance, and that proteoforms of KRAS4A versus KRAS4B are differentially modified. Moreover, we identified a subset of KRAS4B proteoforms lacking the C185 residue and associated C-terminal PTMs. By confocal microscopy, we confirmed that this truncated GFP-KRAS4B^C185∗ proteoform is unable to associate with the plasma membrane, resulting in a decrease in mitogen-activated protein kinase signaling pathway activation. Collectively, our study provides a reference set of functionally distinct KRAS proteoforms and the colorectal cancer contexts in which they are present.

Targeting Glucose Metabolism Sensitizes Pancreatic Cancer to MEK Inhibition

Pancreatic ductal adenocarcinoma (PDAC) is almost universally lethal. A critical unmet need exists to explore essential susceptibilities in PDAC and to identify druggable targets to improve PDAC treatment. KRAS mutations dominate the genetic landscape of PDAC and lead to activation of multiple downstream pathways and cellular processes. Here, we investigated the requirement of these pathways for tumor maintenance using an inducible Kras^G12D -driven PDAC mouse model (iKras model), identifying that RAF-MEK-MAPK signaling is the major effector for oncogenic KRAS-mediated tumor maintenance. However, consistent with previous studies, MEK inhibition had minimal therapeutic effect as a single agent for PDAC in vitro and in vivo. Although MEK inhibition partially downregulated transcription of glycolysis genes, it failed to suppress glycolytic flux in PDAC cells, which is a major metabolic effector of oncogenic KRAS. Accordingly, an in vivo genetic screen identified multiple glycolysis genes as potential targets that may sensitize tumor cells to MEK inhibition. Inhibition of glucose metabolism with low-dose 2-deoxyglucose in combination with a MEK inhibitor induced apoptosis in Kras^G12D -driven PDAC cells in vitro. The combination also inhibited xenograft PDAC tumor growth and prolonged overall survival in a genetically engineered PDAC mouse model. Molecular and metabolic analyses indicated that co-targeting glycolysis and MAPK signaling results in apoptosis via induction of lethal endoplasmic reticulum stress. Together, our work suggests that combined inhibition of glycolysis and the MAPK pathway may serve as an effective approach to target KRAS-driven PDAC. SIGNIFICANCE: This study demonstrates the critical role of glucose metabolism in resistance to MAPK inhibition in KRAS-driven pancreatic cancer, uncovering a potential therapeutic approach for treating this aggressive disease.

LY6D-induced macropinocytosis as a survival mechanism of senescent cells

Although senescent cells display various morphological changes including vacuole formation, it is still unclear how these processes are regulated. We have recently identified the gene, lymphocyte antigen 6 complex, locus D (LY6D), to be upregulated specifically in senescent cells. LY6D is a glycosylphosphatidylinositol-anchored cell-surface protein whose function remains unknown. Here, we analyzed the functional relationship between LY6D and the senescence processes. We found that overexpression of LY6D induced vacuole formation and knockdown of LY6D suppressed the senescence-associated vacuole formation. The LY6D-induced vacuoles were derived from macropinocytosis, a distinct form of endocytosis. Furthermore, Src family kinases and Ras were found to be recruited to membrane lipid rafts in an LY6D-dependent manner, and inhibition of their activity impaired the LY6D-induced macropinocytosis. Finally, reduction of senescent-cell survival induced by glutamine deprivation was recovered by albumin supplementation to the culture media in an LY6D-dependent manner. Because macropinocytosis acts as an amino acid supply route, these results suggest that LY6D-mediated macropinocytosis contributes to senescent-cell survival through the incorporation of extracellular nutrients.

Insight into human Miro1/2 domain organization based on the structure of its N-terminal GTPase

Dysfunction in mitochondrial dynamics is believed to contribute to a host of neurological disorders and has recently been implicated in cancer metastasis. The outer mitochondrial membrane adapter protein Miro functions in the regulation of mitochondrial mobility and degradation, however, the structural basis for its roles in mitochondrial regulation remain unknown. Here, we report a 1.7Å crystal structure of N-terminal GTPase domain (nGTPase) of human Miro1 bound unexpectedly to GTP, thereby revealing a non-catalytic configuration of the putative GTPase active site. We identify two conserved surfaces of the nGTPase, the "SELFYY" and "ITIP" motifs, that are potentially positioned to mediate dimerization or interaction with binding partners. Additionally, we report small angle X-ray scattering (SAXS) data obtained from the intact soluble HsMiro1 and its paralog HsMiro2. Taken together, the data allow modeling of a crescent-shaped assembly of the soluble domain of HsMiro1/2. PDB RSEFERENCE: Crystal structure of the human Miro1 N-terminal GTPase bound to GTP, 6D71.

Secreted Glioblastoma Nanovesicles Contain Intracellular Signaling Proteins and Active Ras Incorporated in a Farnesylation-dependent Manner

Glioblastomas (GBMs) are malignant brain tumors with a median survival of less than 18 months. Redundancy of signaling pathways represented within GBMs contributes to their therapeutic resistance. Exosomes are extracellular nanovesicles released from cells and present in human biofluids that represent a possible biomarker of tumor signaling state that could aid in personalized treatment. Herein, we demonstrate that mouse GBM cell-derived extracellular nanovesicles resembling exosomes from an H-RasV12 myr-Akt mouse model for GBM are enriched for intracellular signaling cascade proteins (GO: 0007242) and Ras protein signal transduction (GO: 0007265), and contain active Ras. Active Ras isolated from human and mouse GBM extracellular nanovesicles lysates using the Ras-binding domain of Raf also coprecipitates with ESCRT (endosomal sorting complex required for transport)-associated exosome proteins Vps4a and Alix. Although we initially hypothesized a role for active Ras protein signaling in exosome biogenesis, we found that GTP binding of K-Ras was dispensable for its packaging within extracellular nanovesicles and for the release of Alix. By contrast, farnesylation of K-Ras was required for its packaging within extracellular nanovesicles, yet expressing a K-Ras farnesylation mutant did not decrease the number of nanovesicles or the amount of Alix protein released per cell. Overall, these results emphasize the primary importance of membrane association in packaging of extracellular nanovesicle factors and indicate that screening nanovesicles within human fluids could provide insight into tissue origin and the wiring of signaling proteins at membranes to predict onset and behavior of cancer and other diseases linked to deregulated membrane signaling states.

Targeting RAS Membrane Association: Back to the Future for Anti-RAS Drug Discovery?

RAS proteins require membrane association for their biologic activity, making this association a logical target for anti-RAS therapeutics. Lipid modification of RAS proteins by a farnesyl isoprenoid is an obligate step in that association, and is an enzymatic process. Accordingly, farnesyltransferase inhibitors (FTI) were developed as potential anti-RAS drugs. The lack of efficacy of FTIs as anticancer drugs was widely seen as indicating that blocking RAS membrane association was a flawed approach to cancer treatment. However, a deeper understanding of RAS modification and trafficking has revealed that this was an erroneous conclusion. In the presence of FTIs, KRAS and NRAS, which are the RAS isoforms most frequently mutated in cancer, become substrates for alternative modification, can still associate with membranes, and can still function. Thus, FTIs failed not because blocking RAS membrane association is an ineffective approach, but because FTIs failed to accomplish that task. Recent findings regarding RAS isoform trafficking and the regulation of RAS subcellular localization have rekindled interest in efforts to target these processes. In particular, improved understanding of the palmitoylation/depalmitoylation cycle that regulates RAS interaction with the plasma membrane, endomembranes, and cytosol, and of the potential importance of RAS chaperones, have led to new approaches. Efforts to validate and target other enzymatically regulated posttranslational modifications are also ongoing. In this review, we revisit lessons learned, describe the current state of the art, and highlight challenging but promising directions to achieve the goal of disrupting RAS membrane association and subcellular localization for anti-RAS drug development. Clin Cancer Res; 21(8); 1819-27. ©2015 AACR. See all articles in this CCR Focus section, "Targeting RAS-Driven Cancers."

Computational allosteric ligand binding site identification on Ras proteins

A number of computational techniques have been proposed to expedite the process of allosteric ligand binding site identification in inherently flexible and hence challenging drug targets. Some of these techniques have been instrumental in the discovery of allosteric ligand binding sites on Ras proteins, a group of elusive anticancer drug targets. This review provides an overview of these techniques and their application to Ras proteins. A summary of molecular docking and binding site identification is provided first, followed by a more detailed discussion of two specific techniques for binding site identification in ensembles of Ras conformations generated by molecular simulations.

Isoprenylcysteine carboxylmethyltransferase deficiency exacerbates KRAS-driven pancreatic neoplasia via Notch suppression

RAS is the most frequently mutated oncogene in human cancers. Despite decades of effort, anti-RAS therapies have remained elusive. Isoprenylcysteine carboxylmethyltransferase (ICMT) methylates RAS and other CaaX-containing proteins, but its potential as a target for cancer therapy has not been fully evaluated. We crossed a Pdx1-Cre;LSL-KrasG12D mouse, which is a model of pancreatic ductal adenocarcinoma (PDA), with a mouse harboring a floxed allele of Icmt. Surprisingly, we found that ICMT deficiency dramatically accelerated the development and progression of neoplasia. ICMT-deficient pancreatic ductal epithelial cells had a slight growth advantage and were resistant to premature senescence by a mechanism that involved suppression of cyclin-dependent kinase inhibitor 2A (p16INK4A) expression. ICMT deficiency precisely phenocopied Notch1 deficiency in the Pdx1-Cre;LSL-KrasG12D model by exacerbating pancreatic intraepithelial neoplasias, promoting facial papillomas, and derepressing Wnt signaling. Silencing ICMT in human osteosarcoma cells decreased Notch1 signaling in response to stimulation with cell-surface ligands. Additionally, targeted silencing of Ste14, the Drosophila homolog of Icmt, resulted in defects in wing development, consistent with Notch loss of function. Our data suggest that ICMT behaves like a tumor suppressor in PDA because it is required for Notch1 signaling.

Cancer cell growth and survival as a system-level property sustained by enhanced glycolysis and mitochondrial metabolic remodeling

Systems Biology holds that complex cellular functions are generated as system-level properties endowed with robustness, each involving large networks of molecular determinants, generally identified by “omics” analyses. In this paper we describe four basic cancer cell properties that can easily be investigated in vitro: enhanced proliferation, evasion from apoptosis, genomic instability, and inability to undergo oncogene-induced senescence. Focusing our analysis on a K-ras dependent transformation system, we show that enhanced proliferation and evasion from apoptosis are closely linked, and present findings that indicate how a large metabolic remodeling sustains the enhanced growth ability. Network analysis of transcriptional profiling gives the first indication on this remodeling, further supported by biochemical investigations and metabolic flux analysis (MFA). Enhanced glycolysis, down-regulation of TCA cycle, decoupling of glucose and glutamine utilization, with increased reductive carboxylation of glutamine, so to yield a sustained production of growth building blocks and glutathione, are the hallmarks of enhanced proliferation. Low glucose availability specifically induces cell death in K-ras transformed cells, while PKA activation reverts this effect, possibly through at least two mitochondrial targets. The central role of mitochondria in determining the two investigated cancer cell properties is finally discussed. Taken together the findings reported herein indicate that a system-level property is sustained by a cascade of interconnected biochemical pathways that behave differently in normal and in transformed cells.

The C11R gene, which encodes the vaccinia virus growth factor, is partially responsible for MVA-induced NF-κB and ERK2 activation

MVA is an attenuated strain of vaccinia virus (VACV) that is a popular vaccine vector. MVA infection activates NF-κB. For 293T cells, it is known that MVA early gene expression activates extracellular signal-regulated kinase 2 (ERK2), resulting in NF-κB activation. However, other viral and cellular mechanisms responsible for this event are ill defined. The data presented here show that the epidermal growth factor receptor (EGFR) is at least one apical trigger in this pathway: ERK2 and NF-κB activation was diminished when MVA infections occurred in cells devoid of the EGFR (CHO K1 cells) or in the presence of a drug that inhibits EGFR activation (AG1478) in 293T cells. The expression of dominant negative Ras or Raf proteins still permitted NF-κB activation, suggesting that a nonclassical EGFR-based signal transduction pathway triggered ERK2-NF-κB activation. C11R is an early gene present in MVA and other orthopoxviruses. It encodes the soluble, secreted vaccinia virus growth factor (VGF), a protein that binds to and stimulates the EGFR. Here it was observed that NF-κB was activated in 293T cells transfected with a plasmid encoding the C11R gene. Silencing by small interfering RNA (siRNA) or deletion of the C11R gene (MVAΔC11R) reduced both MVA-induced ERK2 and NF-κB activation in 293T cells or the keratinocyte line Hacat, suggesting that this mechanism of MVA-induced NF-κB activation may be common for several cell types.

Focal adhesions and Ras are functionally and spatially integrated to mediate IL-1 activation of ERK

In connective tissue cells, IL-1-induced ERK activation leading to matrix metalloproteinase (MMP)-3 expression is dependent on cooperative interactions between focal adhesions and the endoplasmic reticulum (ER). As Ras can be activated on the ER, we investigated the role of Ras in IL-1 signaling and focal adhesion formation. We found that constitutively active H-Ras, K-Ras or N-Ras enhanced focal adhesion maturation and β1-integrin activation. IL-1 promoted the accumulation of Ras isoforms in ER and focal adhesion fractions, as shown in cells cotransfected with GFP-tagged Ras isoforms and YFP-ER protein and by analysis of subcellular fractions enriched for ER or focal adhesion proteins. Dominant-negative H-Ras or K-Ras reduced accumulation of H-Ras and K-Ras in focal adhesions induced by IL-1 and also blocked ERK activation and focal adhesion maturation. Ras-GRF was enriched constitutively in focal adhesion fractions and was required for Ras recruitment to focal adhesions. We conclude that Ras activation and IL-1 signaling are interactive processes that regulate the maturation of focal adhesions, which, in turn, is required for ERK activation.

A switch I mutant of Cdc42 exhibits less conformational freedom

Cdc42 is a Ras-related small G-protein and functions as a molecular switch in signal transduction pathways linked with cell growth and differentiation. It is controlled by cycling between GTP-bound (active) and GDP-bound (inactive) forms. Nucleotide binding and hydrolysis are modulated by interactions with effectors and/or regulatory proteins. These interactions are centralized in two relatively flexible "Switch" regions as characterized by internal dynamics on multiple time scales [Loh, A. P., et al. (2001) Biochemistry 40, 4590-4600], and this flexibility may be essential for protein interactions. In the Switch I region, Thr(35) seems to be critical for function, as it is completely invariant in Ras-related proteins. To investigate the importance of conformational flexibility in Switch I of Cdc42, we mutated threonine to alanine, determined the solution structure, and characterized the backbone dynamics of the single-point mutant protein, Cdc42(T35A). Backbone dynamics data suggest that the mutation changes the time scale of the internal motions of several residues, with several resonances not being discernible in wild-type Cdc42 [Adams, P. D., and Oswald, R. E. (2007) Biomol. NMR Assignments 1, 225-227]. The mutation does not appear to affect the thermal stability of Cdc42, and chymotrypsin digestion data further suggest that changes in the conformational flexibility of Switch I slow proteolytic cleavage relative to that of the wild type. In vitro binding assays show less binding of Cdc42(T35A), relative to that of wild type, to a GTPase binding protein that inhibits GTP hydrolysis in Cdc42. These results suggest that the mutation of T(35) leads to the loss of conformational freedom in Switch I that could affect effector-regulatory protein interactions.

Relation between the conformational heterogeneity and reaction cycle of Ras: molecular simulation of Ras

Ras functions as a molecular switch by cycling between the active GTP-bound state and the inactive GDP-bound state. It is known experimentally that there is another GTP-bound state called state 1. We investigate the conformational changes and fluctuations arising from the difference in the coordinations between the switch regions and ligands in the GTP- and GDP-bound states using a total of 830 ns of molecular-dynamics simulations. Our results suggest that the large fluctuations among multiple conformations of switch I in state 1 owing to the absence of coordination between Thr-35 and Mg(2+) inhibit the binding of Ras to effectors. Furthermore, we elucidate the conformational heterogeneity in Ras by using principal component analysis, and propose a two-step reaction path from the GDP-bound state to the active GTP-bound state via state 1. This study suggests that state 1 plays an important role in signal transduction as an intermediate state of the nucleotide exchange process, although state 1 itself is an inactive state for signal transduction.

Cytosolic Ras supports eye development in Drosophila

Ras proteins associate with cellular membranes as a consequence of a series of posttranslational modifications of a C-terminal CAAX sequence that include prenylation and are thought to be required for biological activity. In Drosophila melanogaster, Ras1 is required for eye development. We found that Drosophila Ras1 is inefficiently prenylated as a consequence of a lysine in the A(1) position of its CAAX sequence such that a significant pool remains soluble in the cytosol. We used mosaic analysis with a repressible cell marker (MARCM) to assess if various Ras1 transgenes could restore photoreceptor fate to eye disc cells that are null for Ras1. Surprisingly, we found that whereas Ras1 with an enhanced efficiency of membrane targeting could not rescue the Ras1 null phenotype, Ras1 that was not at all membrane targeted by virtue of a mutation of the CAAX cysteine was able to fully rescue eye development. In addition, constitutively active Ras1(12V,C186S) not targeted to membranes produced a hypermorphic phenotype and stimulated mitogen-activated protein kinase (MAPK) signaling in S2 cells. We conclude that the membrane association of Drosophila Ras1 is not required for eye development.

Dominant negative factors in health and disease

The idea of dominant mutations that interfere with the activity of a normal gene product has been known for more than 80 years-the famous Muller's antimorphs. However, only over half a century later, the mechanistic bases of dominant negative mutations (DNMs) were defined in a systematic way by Ira Herskowitz. Most analyses of DNMs consider only intralocus (interallelic) interactions. The typical textbook explanation invokes a defective subunit, which poisons a homo-dimer or a homo-oligomer. More complex cases exist and the quantitative dimension of this phenomenon will be explored here. The basic ideas underlying DN effects can be (and should be) extended to included epistatic (interloci) interactions. Indeed, poisoning heteromeric macromolecular complexes is per se a matter of 'transdominant' negative effects. In this context, non-allelic non-complementation is also considered. Given the importance of DNMs in human disease and in the study of gene function, understanding how they work is essential for understanding pathology and for the design of effective DN molecules that can also prove useful in therapeutics. Finally, the existence and potential relevance of an increasing number of physiological DN protein isoforms is briefly discussed.

Transforming Growth Factor β Signaling via Ras in Mesenchymal Cells Requires p21-Activated Kinase 2 for Extracellular Signal-Regulated Kinase-Dependent Transcriptional Responses

Transforming growth factor beta (TGF-beta) signaling via Smad proteins occurs in various cell types. However, whereas the biological response to TGF-beta can be as distinct as growth promoting (i.e., mesenchymal cells) versus growth inhibiting (i.e., epithelial cells), few discernible differences in TGF-beta signaling have been reported. In the current study, we examined the role of Ras in the proliferative response to TGF-beta and how it might interface with Smad-dependent and Smad-independent TGF-beta signaling targets. TGF-beta stimulated Ras activity in a subset of mesenchymal, but not epithelial, cultures and was required for extracellular signal-regulated kinase (ERK)-dependent transcriptional responses. Although dominant negative Ras had no effect on TGF-beta internalization or Smad-dependent signaling (i.e., phosphorylation, nuclear translocation, or SBE-luciferase activity), it did prevent the hyperphosphorylation of the Smad transcriptional corepressor TG-interacting factor (TGIF). This was not sufficient, however, to overcome the mitogenic response stimulated by TGF-beta, which was dependent on signals downstream of p21-activated kinase 2 (PAK2). Moreover, although the initial activation of Ras and PAK2 are distinctly regulated, TGF-beta-stimulated PAK2 activity is required for Ras-dependent ERK phosphorylation and Elk-1 transcription. These findings show the requirement for crosstalk between two Smad-independent pathways in regulating TGF-beta proliferation and indicate that the mechanism(s) by which TGF-beta stimulates growth is not simply the opposite of its growth inhibitory actions.

H-ras inhibits RhoA/ROCK leading to a decrease in the basal tone in the internal anal sphincter

BACKGROUND & AIMS

The present studies evaluated the role of H-ras and its implications in the RhoA/Rho kinase (ROCK) pathway in regulating basal tone in the internal anal sphincter (IAS).

METHODS

Studies were performed in the IAS from the wild-type (H-ras(+/+)) and knock-out (H-ras(-/-)) mice. The basal tone of smooth muscle strips was measured by isometric force transducers. Length of smooth muscle cells (SMC) isolated from the IAS in the basal state was determined by phase contrast microscopy. Experiments were repeated in the presence of Y 27632, a ROCK inhibitor. Involvement of the RhoA/ROCK machinery was analyzed by reverse-transcription polymerase chain reaction, Western blot, and immunocytochemistry. Reversal of H-ras knock-out effect was evaluated by transfection of SMCs with the constitutively activated (G12V) mutant.

RESULTS

Basal tone of the H-ras(-/-) IAS was significantly higher and resistant to relaxation by Y 27632, compared with the H-ras(+/+) IAS. Similarly, the length of SMCs from H-ras(-/-) IAS was significantly shorter. Y 27632 eliminated this difference. RhoA immunoreactivity shifted from cytoplasm to plasma membrane in H-ras(-/-) SMCs, a change typically associated with contraction. Further, SMCs from H-ras(-/-) mice exhibited higher levels of the contractile proteins ROCK II, phosphorylated-MYPT(1) and phosphorylated-MLC(20). Transfection with the G12V mutant increased the length of H-ras(-/-) cells. Conversely, the dominant negative H-ras (S17N) mutant decreased the length of H-ras(+/+) cells.

CONCLUSIONS

H-ras negatively regulates basal tone in the IAS by inhibiting RhoA/Rho-kinase machinery. Studies may have significant relevance in the pathophysiology and therapy of certain anorectal motility disorders associated with the IAS dysfunction.

Structural and functional consequences of c-N-Ras constitutively associated with intact mitochondria

We demonstrate that both c-N-Ras and c-K(B)-Ras are constitutively associated with purified mitochondria. c-K(B)-Ras is associated with the mitochondrial outer membrane, and c-N-Ras is associated with both the outer membrane and inner mitochondrial compartments. The mitochondrial morphology is abnormal in both c-N-Ras negative and K-Ras negative cells. Normal mitochondrial morphology was restored by targeting N-Ras to both the inner and outer mitochondrial compartments, or by ectopically expressing c-K(B)-Ras. Impaired mitochondrial function can result in increased CHOP and NFkappaB activity, typical for a retrograde signaling response. Both are constitutively elevated in the N-Ras negative cells, but not in the K-Ras negative background, and are restored by c-N-Ras targeted exclusively to the inner mitochondrial compartment. Surprisingly, both targeting and the ability to functionally reduce retrograde transcriptional activity were found to be independent of c-N-Ras farnesylation. Overall, these data demonstrate for the first time a (1) farnesylation independent function for c-N-Ras and (2) that N-Ras within the inner mitochondrial compartment is an essential component of the retrograde signaling system between the mitochondria and nucleus.

Activation of heterotrimeric G-proteins independent of a G-protein coupled receptor and the implications for signal processing

Heterotrimeric G-proteins are key transducers for signal transfer from outside the cell, mediating signals emanating from cell-surface G-protein coupled receptors (GPCR). Many, if not all, subtypes of heterotrimeric G-proteins are also regulated by accessory proteins that influence guanine nucleotide binding, guanosine triphosphate (GTP) hydrolysis, or subunit interactions. One subgroup of such accessory proteins (activators of G-protein signaling; AGS proteins) refer to a functionally defined group of proteins that activate selected G-protein signaring systems in the absence of classical G-protein coupled receptors. AGS and related proteins provide unexpected insights into the regulation of the G-protein activation-deactivation cycle. Different AGS proteins function as guanine nucleotide exchange factors or guanine nucleotide dissociation inhibitors and may also influence subunit interactions by interaction with GBgamma. These proteins play important roles in the generation or positioning of signaling complexes and of the regulation of GPCR signaling, and as alternative binding partners for G-protein subunits. Perhaps of even broader impact is the discovery that AGS proteins provide a foundation for the concept that heterotrimeric G-protein subunits are processing signals within the cell involving intrinsic cues that do not involve the classical signal input from a cell surface GPCR.

Kallikrein-related peptidase (KLK) family mRNA variants and protein isoforms in hormone-related cancers: do they have a function?

The kallikrein-related peptidase (KLK) gene family of 15 serine proteases encodes many proteins, including prostate specific antigen (PSA or KLK3), that are well described and/or are potential biomarkers for hormone-related cancers. Variant mRNA transcripts produced by alternative splicing, polyadenylation or AUG sites, or intron retention have been found for each of the KLK genes. The predicted protein for many of these alternative transcripts is different from that of the classical kallikrein-related peptidases and would not be an active serine protease. The majority of these novel protein isoforms have not been studied in vivo. The possible function(s) of the variant transcripts/protein isoforms and potential roles that they may play in hormone-related cancers are still unknown and are the focus of this short review.

Analysis of mTOR signaling by the small G-proteins, Rheb and RhebL1

The small G protein Rheb (Ras homologue enriched in brain) is known to promote mammalian target of rapamycin (mTOR) signaling. In this study, we show that Rheb like-1 protein (RhebL1) rescues mTOR signaling during nutrient withdrawal and that tuberous sclerosis complex-1 (TSC) and TSC2 impairs RhebL1-mediated signaling through mTOR. We identify critical residues within the switch I region (N41) and 'constitutive' effector (Ec) region (Y/F54 and L56) of Rheb and RhebL1, which are required for their efficient activation of mTOR signaling. Mutation of Rheb and RhebL1 at N41 impaired their interaction with mTOR, which identifies mTOR as a common downstream target of both Rheb and RhebL1.

Inhibition of Ras oncogenic activity by Ras protooncogenes

Point mutations in ras genes have been found in a large number and wide variety of human tumors. These oncogenic Ras mutants are locked in an active GTP-bound state that leads to a constitutive and deregulated activation of Ras function. The dogma that ras oncogenes are dominant, whereby the mutation of a single allele in a cell will predispose the host cell to transformation regardless of the presence of the normal allele, is being challenged. We have seen that increasing amounts of Ras protooncogenes are able to inhibit the activity of the N-Ras oncogene in the activation of Elk in NIH 3T3 cells and in the formation of foci. We have been able to determine that the inhibitory effect is by competition between Ras protooncogenes and the N-Ras oncogene that occurs first at the effector level at the membranes, then at the processing level and lastly at the effector level in the cytosol. In addition, coexpression of the N-Ras protooncogene in thymic lymphomas induced by the N-Ras oncogene is associated with increased levels of p107, p130 and cyclin A and decreased levels of Rb. In the present report, we have shown that the N-Ras oncogene is not truly dominant over Ras protooncogenes and their competing activities might be depending on cellular context.

Ras inhibition amplifies cisplatin sensitivity of human glioblastoma

Resistance to chemotherapy is a common feature of malignant gliomas. This resistance is mediated by receptor tyrosine kinase (RTK)-regulated signaling. p21-Ras protein is pivotal in the propagation of the signal originated from many RTKs. Our aim was to investigate whether inhibition of Ras pathway affects the response to cisplatin in malignant gliomas. We found an enhanced sensitivity to cisplatin of two glioblastoma cell lines expressing dominant negative Ras. Moreover, DN-Ras expressing cells, implanted in nude mice, resulted in being extremely sensitive to cisplatin. The growth of all the tumors was significantly inhibited by combining DN-Ras adenovirus infection with cisplatin treatment. The majority of glioma cells expressing DN-Ras underwent apoptosis in response to cisplatin. In vivo, DN-Ras alone did not influence the growth of tumors, suggesting that the effects of Ras-inhibition observed in vitro could not be extrapolated in vivo. The survival signal pathway transduced by Ras was essentially mediated by inhibition of caspase-9 cleavage via PI3K/Akt.

The dark side of Ras: regulation of apoptosis

Mutational activation of Ras promotes oncogenesis by disrupting a multitude of normal cellular processes. Perhaps, best characterized and understood are the mechanisms by which oncogenic Ras promotes deregulated cell cycle progression and uncontrolled cellular proliferation. However, it is now clear that oncogenic Ras can also deregulate processes that control apoptosis. In light of the diversity of downstream effector targets known to facilitate Ras function, it is perhaps not surprising that Ras regulation of cell survival is complex, involving the balance and interplay of multiple signaling networks. While our understanding of these events is still far from complete, and is complicated by cell type and signaling context differences, several important mechanisms have begun to emerge. We review the role and mechanism of specific effectors in regulating the antiapoptotic (Raf, phosphatidylinositol 3-kinase and Tiam1) and apoptotic (Nore1 and RASSF1) actions of oncogenic Ras, and discuss the possibility that the effector actions of p120RasGAP make a significant contribution to Ras regulation of apoptotic events.

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.

Rab5 and Rab11 mediate transferrin and anti-variant surface glycoprotein antibody recycling in Trypanosoma brucei

The mammalian-infective bloodstream form of Trypanosoma brucei possesses a highly active endocytotic system. Evasion of the host immune response by T. brucei is dependent on antigenic variation of VSG (variant surface glycoprotein), but additional mechanisms for removal of surface-bound antibody also operate. Four Rab proteins, Tb (trypanosomal) RAB4, 5A, 5B and 11 are located to the endosomal system; TbRAB5A and TbRAB11 co-localize with internalized anti-VSG antibody and transferrin. A live cell assay was used to record a single cycle of endocytosis of anti-VSG IgG and transferrin, their subsequent degradation within the endosomal system and exocytosis of the products. TbRAB5A and TbRAB11 were involved in the overall process of endocytosis, degradation and exocytosis, whereas TbRAB5B and TbRAB4 were not implicated. The kinetics of anti-VSG IgG and transferrin recycling depend on the nucleotide state of TbRAB5A and TbRAB11. These data, together with previous work, suggest that IgG and transferrin initially enter a TbRAB5A sorting endosome and are most probably recycled subsequently via a TbRAB11-dependent step. Analysis of the recycled IgG and transferrin demonstrated extensive degradation of these recycled proteins. Degradation of transferrin was enhanced in cells expressing increased amounts of TbRAB5A or TbRAB11 with a Ser-->Asn mutation, but was decreased when active TbRAB11 was overexpressed. The extent of degradation of anti-VSG IgG was found to be unaffected by mutant Rab protein expression. The presence of an efficient mechanism for the removal of IgG bound to the external surface of T. brucei and its subsequent proteolysis within the recycling system suggests a role for this pathway in immune evasion.

Signal transduction mediated by the Ras/Raf/MEK/ERK pathway from cytokine receptors to transcription factors: potential targeting for therapeutic intervention

The Ras/Raf/Mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) cascade couples signals from cell surface receptors to transcription factors, which regulate gene expression. Depending upon the stimulus and cell type, this pathway can transmit signals, which result in the prevention or induction of apoptosis or cell cycle progression. Thus, it is an appropriate pathway to target for therapeutic intervention. This pathway becomes more complex daily, as there are multiple members of the kinase and transcription factor families, which can be activated or inactivated by protein phosphorylation. The diversity of signals transduced by this pathway is increased, as different family members heterodimerize to transmit different signals. Furthermore, additional signal transduction pathways interact with the Raf/MEK/ERK pathway to regulate positively or negatively its activity, or to alter the phosphorylation status of downstream targets. Abnormal activation of this pathway occurs in leukemia because of mutations at Ras as well as genes in other pathways (eg PI3K, PTEN, Akt), which serve to regulate its activity. Dysregulation of this pathway can result in autocrine transformation of hematopoietic cells since cytokine genes such as interleukin-3 and granulocyte/macrophage colony-stimulating factor contain the transacting binding sites for the transcription factors regulated by this pathway. Inhibitors of Ras, Raf, MEK and some downstream targets have been developed and many are currently in clinical trials. This review will summarize our current understanding of the Ras/Raf/MEK/ERK signal transduction pathway and the downstream transcription factors. The prospects of targeting this pathway for therapeutic intervention in leukemia and other cancers will be evaluated.

Involvement of PI3K/Akt pathway in cell cycle progression, apoptosis, and neoplastic transformation: a target for cancer chemotherapy

The PI3K/Akt signal transduction cascade has been investigated extensively for its roles in oncogenic transformation. Initial studies implicated both PI3K and Akt in prevention of apoptosis. However, more recent evidence has also associated this pathway with regulation of cell cycle progression. Uncovering the signaling network spanning from extracellular environment to the nucleus should illuminate biochemical events contributing to malignant transformation. Here, we discuss PI3K/Akt-mediated signal transduction including its mechanisms of activation, signal transducing molecules, and effects on gene expression that contribute to tumorigenesis. Effects of PI3K/Akt signaling on important proteins controlling cellular proliferation are emphasized. These targets include cyclins, cyclin-dependent kinases, and cyclin-dependent kinase inhibitors. Furthermore, strategies used to inhibit the PI3K/Akt pathway are presented. The potential for cancer treatment with agents inhibiting this pathway is also addressed.

Targeting the Raf kinase cascade in cancer therapy--novel molecular targets and therapeutic strategies

The mitogen-activated protein kinases (MAPKs) are a group of signal transducers with oncogenic potential in an assortment of cell types. Dysregulated signalling from any of the members of this family has been shown to result in development of human malignancies. Consequently, the collective goal of the scientific community is to inhibit aberrant signalling initiated from these molecules whilst minimising toxicity associated with such inhibition. This review covers events responsible for MAPK activation in detail, with an emphasis placed upon possible points of pharmacological intervention. A discussion addressing numerous chemotherapeutic approaches that have been developed over the previous decade for MAPK inhibition is also included. In addition, emphasis is placed upon the various arrays of kinase inhibitors, small molecule inhibitors, competitive inhibitors, nucleic acid aptamers and other molecules which have been proven effective in prevention of MAPK signalling. Finally, the potential therapeutic promise of many of these compounds is addressed in a manner that encompasses the complexities of MAPK signal transduction, in addition to concerns surrounding the development of drug resistance.

Isoprenoids influence expression of Ras and Ras-related proteins

Mevalonate depletion by inhibition of hydroxymethylglutaryl coenzyme A reductase impairs post-translational processing of Ras and Ras-related proteins. We have previously shown that this mevalonate depletion also leads to the upregulation of Ras, Rap1a, RhoA, and RhoB. This upregulation may result from global inhibition of isoprenylation or depletion of key regulatory isoprenoid species. Studies utilizing specific isoprenoid pyrophosphates in mevalonate-depleted cells reveal that farnesyl pyrophosphate (FPP) restores Ras processing and prevents RhoB upregulation while geranylgeranyl pyrophosphate (GGPP) restores Rap1a processing and prevents RhoA and RhoB upregulation. Either FPP or GGPP completely prevents lovastatin-induced upregulation of RhoB mRNA. Inhibition of FPP or squalene synthase allowed for the further identification of the putative regulatory species. Studies involving the specific isoprenyl transferase inhibitors FTI-277 and GGTI-286 demonstrate that selective inhibition of protein isoprenylation does not mimic lovastatin's ability to increase Ras and RhoA synthesis, decrease Ras and RhoA degradation, increase RhoB mRNA, or increase total levels of Ras, Rap1a, RhoA, and RhoB. In aggregate, these findings reveal a novel role and mechanism for isoprenoids to influence levels of Ras and Ras-related proteins.

Galectin-1 augments Ras activation and diverts Ras signals to Raf-1 at the expense of phosphoinositide 3-kinase

Ras proteins activate diverse effector molecules. Depending on the cellular context, Ras activation may have different biological consequences: induction of cell proliferation, senescence, survival, or death. Augmentation and selective activation of particular effector molecules may underlie various Ras actions. In fact, Ras effector-loop mutants interacting with distinctive effectors provide evidence for such selectivity. Interactions of active Ras with escort proteins, such as galectin-1, could also direct Ras selectivity. Here we show that in comparison with Ras transfectants, H-Ras/galectin-1 or K-Ras4B/galectin-1 co-transfectants exhibit enhanced and prolonged epidermal growth factor (EGF)-stimulated increases in Ras-GTP, Raf-1 activity, and active extracellular signal-regulated kinase. Galectin-1 antisense RNA inhibited these EGF responses. Conversely, Ras and galectin-1 co-transfection inhibited the EGF-stimulated increase in phosphoinositide 3-kinase (PI3K) activity. Galectin-1 transfection also inhibited Ras(G12V)-induced PI3K but not Raf-1 activity. Galectin-1 co-immunoprecipitated with Ras(G12V) or with Ras(G12V/T35S) that activate Raf-1 but not with Ras(G12V/Y40C) that activates PI3K. Thus, galectin-1 binds active Ras and diverts its signal to Raf-1 at the expense of PI3K. This demonstrates a novel mechanism controlling the duration and selectivity of the Ras signal. Ras gains selectivity when it is associated with galectin-1, mimicking the selectivity of Ras(T35S), which activates Raf-1 but not PI3K.

ACTH promotion of p27(Kip1) induction in mouse Y1 adrenocortical tumor cells is dependent on both PKA activation and Akt/PKB inactivation

Here we report antimitogenic mechanisms activated by the adrenocorticotropic hormone (ACTH) in the mouse Y1 adrenocortical tumor cell line. ACTH receptors activate the Galphas/adenylate cyclase cAMP/PKA pathway to promote dephosphorylation of Akt/PKB enzymes, leading to induction of the cyclin-dependent kinases' (CDKs) inhibitor p27(Kip1). Y1 cells display high constitutive levels of phosphorylated Akt/PKB dependent on chronically elevated c-Ki-Ras.GTP and PI3K activity. Expression of the dominant negative mutant RasN17 in Y1 cells results in strong reduction of both c-Ki-Ras.GTP and phosphorylated Akt/PKB, which are restored by FGF2 treatments. Inhibitors of PI3K lead to rapid dephosphorylation of Akt/PKB and block phosphorylation of Akt/PKB promoted by FGF2. ACTH rapidly promotes dephosphorylation of Akt/PKB in Y1 adrenal cells, while constitutively high levels of c-Ki-Ras.GTP remain unchanged. ACTH and cAMP elevating agents fail to cause Akt/PKB dephosphorylation in PKA-deficient clonal mutants of Y1 cells. In addition, cholera toxin, forskolin, and 8BrcAMP all mimic ACTH, causing dephosphorylation of Akt/PKB in wild-type Y1 cells. ACTH is unable to prevent Akt/PKB phosphorylation, promoted by FGF2 in clonal lines of RasN17-Y1 transfectants displaying negligible levels of c-Ki-Ras.GTP. ACTH promotes strong p27(Kip1) protein induction in wild-type Y1 adrenocortical cells but not in PKA-deficient Y1-clonal mutants nor in RasN17-Y1 transfectants. PI3K inhibitors induce p27(Kip1) protein in all cells studied, i.e., wild type and transfectants. The inverse correlation between levels of phosphorylated Akt/PKB and of p27(Kip1) protein caused by ACTH suggests a novel antimitogenic pathway activated by ACTH and mediated by cAMP/PKA in the mouse Y1 adrenocortical tumor cell line.