H-Ras signaling and K-Ras signaling are differentially dependent on endocytosis

Exploring the role of sporadic BRAF and KRAS mutations during colorectal cancer pathogenesis: A spotlight on the contribution of the endosome-lysosome system

The highly heterogenous nature of colorectal cancer can significantly hinder its early and accurate diagnosis, eventually contributing to high mortality rates. The adenoma-carcinoma sequence and serrated polyp-carcinoma sequence are the two most common sequences in sporadic colorectal cancer. Genetic alterations in adenomatous polyposis coli (APC), v-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog (KRAS) and tumour protein 53 (TP53) genes are critical in adenoma-carcinoma sequence, whereas v-Raf murine sarcoma viral oncogene homolog B (BRAF) and MutL Homolog1 (MLH1) are driving oncogenes in the serrated polyp-carcinoma sequence. Sporadic mutations in these genes contribute differently to colorectal cancer pathogenesis by introducing distinct alterations in several signalling pathways that rely on the endosome-lysosome system. Unsurprisingly, the endosome-lysosome system plays a pivotal role in the hallmarks of cancer and contributes to specialised colon function. Thus, the endosome-lysosome system might be distinctively influenced by different mutations and these alterations may contribute to the heterogenous nature of sporadic colorectal cancer. This review highlights potential connections between major sporadic colorectal cancer mutations and the diverse pathogenic mechanisms driven by the endosome-lysosome system in colorectal carcinogenesis.

Identification of a novel endocytosis‑associated gene signature for prognostic prediction in lung adenocarcinoma

Lung cancer is one of the most common malignant solid tumors and the leading cause of cancer-associated mortality worldwide. Endocytosis is an essential physiological activity for cells to maintain membrane homeostasis, and has been reported to serve an important role in tumorigenesis and progression. In the present study, the aim was to construct a prognostic prediction model of endocytosis-associated genes for patients with lung adenocarcinoma (LUAD). The endocytosis-associated gene signature was established using Lasso Cox regression analysis using the training set of the LUAD cohort from The Cancer Genome Atlas (TCGA) database, and verified using two datasets from the Gene Expression Omnibus (GEO) database. Kaplan-Meier survival curves were used to evaluate the effectiveness of the prognostic evaluation of patients with LUAD. Differentially expressed genes were screened in the tumor tissue of patients compared with paired paracancerous tissues. A series of candidate genes associated to the prognosis of patients with LUAD was obtained using univariate Cox's regression analysis. Using the Lasso Cox regression analysis, an appropriate risk model with 18 endocytosis-associated genes was established. A high-risk score was positively correlated with a higher tumor stage and pathologic grade. Patients with LUAD and high-risk scores had shorter survival times, increased intratumor heterogeneities and immune cell infiltration into tumor tissues, compared with those patients with LUAD and low-risk scores. The endocytosis inhibitor chloroquine could repress proliferation and increase the apoptosis of lung cancer cells. In summary, a novel endocytosis-associated gene signature was constructed using TCGA and GEO datasets. Patients with LUAD and high-risk scores, as calculated by the signature, had a poor prognosis and short survival time.

Redox-Mediated Rewiring of Signalling Pathways: The Role of a Cellular Clock in Brain Health and Disease

Metazoan signalling pathways can be rewired to dampen or amplify the rate of events, such as those that occur in development and aging. Given that a linear network topology restricts the capacity to rewire signalling pathways, such scalability of the pace of biological events suggests the existence of programmable non-linear elements in the underlying signalling pathways. Here, we review the network topology of key signalling pathways with a focus on redox-sensitive proteins, including PTEN and Ras GTPase, that reshape the connectivity profile of signalling pathways in response to an altered redox state. While this network-level impact of redox is achieved by the modulation of individual redox-sensitive proteins, it is the population by these proteins of critical nodes in a network topology of signal transduction pathways that amplifies the impact of redox-mediated reprogramming. We propose that redox-mediated rewiring is essential to regulate the rate of transmission of biological signals, giving rise to a programmable cellular clock that orchestrates the pace of biological phenomena such as development and aging. We further review the evidence that an aberrant redox-mediated modulation of output of the cellular clock contributes to the emergence of pathological conditions affecting the human brain.

Ras Multimers on the Membrane: Many Ways for a Heart-to-Heart Conversation

Formation of Ras multimers, including dimers and nanoclusters, has emerged as an exciting, new front of research in the ‘old’ field of Ras biomedicine. With significant advances made in the past few years, we are beginning to understand the structure of Ras multimers and, albeit preliminary, mechanisms that regulate their formation in vitro and in cells. Here we aim to synthesize the knowledge accrued thus far on Ras multimers, particularly the presence of multiple globular (G-) domain interfaces, and discuss how membrane nanodomain composition and structure would influence Ras multimer formation. We end with some general thoughts on the potential implications of Ras multimers in basic and translational biology.

EGFR-RAS-MAPK signaling is confined to the plasma membrane and associated endorecycling protrusions

The subcellular localization of RAS GTPases defines the operational compartment of the EGFR-ERK1/2 signaling pathway within cells. Hence, we used live-cell imaging to demonstrate that endogenous KRAS and NRAS tagged with mNeonGreen are predominantly localized to the plasma membrane. NRAS was also present in the Golgi apparatus and a tubular, plasma-membrane derived endorecycling compartment, enriched in recycling endosome markers (TERC). In EGF-stimulated cells, there was essentially no colocalization of either mNeonGreen-KRAS or mNeonGreen-NRAS with endosomal EGFR, which, by contrast, remained associated with endogenous Grb2-mNeonGreen, a receptor adaptor upstream of RAS. ERK1/2 activity was diminished by blocking cell surface EGFR with cetuximab, even after most ligand-bound, Grb2-associated EGFRs were internalized. Endogenous mCherry-tagged RAF1, an effector of RAS, was recruited to the plasma membrane, with subsequent accumulation in mNG-NRAS–containing TERCs. We propose that a small pool of surface EGFRs sustain signaling within the RAS-ERK1/2 pathway and that RAS activation persists in TERCs, whereas endosomal EGFR does not significantly contribute to ERK1/2 activity.

A Historic Perspective and Overview of H-Ras Structure, Oncogenicity, and Targeting

H-Ras is a unique isoform of the Ras GTPase family, one of the most prominently mutated oncogene families across the cancer landscape. Relative to other isoforms, though, mutations of H-Ras account for the smallest proportion of mutant Ras cancers. Yet, in recent years, there have been renewed efforts to study this isoform, especially as certain H-Ras-driven cancers, like those of the head and neck, have become more prominent. Important advances have therefore been made not only in the understanding of H-Ras structural biology but also in approaches designed to inhibit and impair its signaling activity. In this review, we outline historic and present initiatives to elucidate the mechanisms of H-Ras-dependent tumorigenesis as well as highlight ongoing developments in the quest to target this critical oncogene.

KRAS regulation by small non-coding RNAs and SNARE proteins

KRAS receives and relays signals at the plasma membrane (PM) where it transmits extracellular growth factor signals to downstream effectors. SNORD50A/B were recently found to bind KRAS and inhibit its tumorigenic action by unknown mechanisms. KRAS proximity protein labeling was therefore undertaken in SNORD50A/B wild-type and knockout cells, revealing that SNORD50A/B RNAs shape the composition of proteins proximal to KRAS, notably by inhibiting KRAS proximity to the SNARE vesicular transport proteins SNAP23, SNAP29, and VAMP3. To remain enriched on the PM, KRAS undergoes cycles of endocytosis, solubilization, and vesicular transport to the PM. Here we report that SNAREs are essential for the final step of this process, with KRAS localization to the PM facilitated by SNAREs but antagonized by SNORD50A/B. Antagonism between SNORD50A/B RNAs and specific SNARE proteins thus controls KRAS localization, signaling, and tumorigenesis, and disrupting SNARE-enabled KRAS function represents a potential therapeutic opportunity in KRAS-driven cancer.

High-throughput, single-particle tracking reveals nested membrane domains that dictate KRasG12D diffusion and trafficking

Membrane nanodomains have been implicated in Ras signaling, but what these domains are and how they interact with Ras remain obscure. Here, using single particle tracking with photoactivated localization microscopy (spt-PALM) and detailed trajectory analysis, we show that distinct membrane domains dictate KRas^G12D (an active KRas mutant) diffusion and trafficking in U2OS cells. KRas^G12D exhibits an immobile state in ~70 nm domains, each embedded in a larger domain (~200 nm) that confers intermediate mobility, while the rest of the membrane supports fast diffusion. Moreover, KRas^G12D is continuously removed from the membrane via the immobile state and replenished to the fast state, reminiscent of Ras internalization and recycling. Importantly, both the diffusion and trafficking properties of KRas^G12D remain invariant over a broad range of protein expression levels. Our results reveal how membrane organization dictates membrane diffusion and trafficking of Ras and offer new insight into the spatial regulation of Ras signaling.

The Ras family of proteins play an important role in relaying signals from the outside to the inside of the cell. Ras proteins are attached by a fatty tail to the inner surface of the cell membrane. When activated they transmit a burst of signal that controls critical behaviors like growth, survival and movement. It has been suggested that to prevent these signals from being accidently activated, Ras molecules must group together at specialized sites within the membrane before passing on their message. However, visualizing how Ras molecules cluster together at these domains has thus far been challenging. As a result, little is known about where these sites are located and how Ras molecules come to a stop at these domains.

Now, Lee et al. have combined two microscopy techniques called ‘single-particle tracking’ and ‘photoactivated localization microscopy' to track how individual molecules of activated Ras move in human cells grown in the lab. This revealed that Ras molecules quickly diffuse along the inside of the membrane until they arrive at certain locations that cause them to halt. However, computer models consisting of just the ‘fast’ and ‘immobile’ state could not correctly re-capture the way Ras molecules moved along the membrane. Lee et al. found that for these models to mimic the movement of Ras, a third ‘intermediate’ state of Ras mobility needed to be included.

To investigate this further, Lee et al. created a fluorescent map that overlaid all the individual paths taken by each Ras molecule. The map showed regions in the membrane where the Ras molecules had stopped and possibly clustered together. Each of these ‘immobilization domains’ were then surrounded by an ‘intermediate domain’ where Ras molecules had begun to slow down their movement. Although the intermediate domains did not last long, they seemed to guide Ras molecules into the immobilization domains where they could cluster together with other molecules. From there, the cell constantly removed Ras molecules from these membrane domains and returned them back to their ‘fast’ diffusing state.

Mutations in Ras proteins occur in around a third of all cancers, so a better understanding of their dynamics could help with future drug discovery. The methods used here could also be used to investigate the movement of other signaling molecules.

SHOC2 complex-driven RAF dimerization selectively contributes to ERK pathway dynamics

The ERK signaling pathway is hyperactivated in a majority of cancers. However, because it mediates myriad physiological responses, the clinical efficacy of current ERK pathway inhibitors has been severely limited by toxicity. This study uncovers both SHOC2 phosphatase complex-dependent and -independent mechanisms of RAF and ERK activation that are differentially engaged in a context and spatio-temporal–dependent manner. KRAS oncogenic signaling preferentially depends on SHOC2 dependent-mechanisms, which thus presents a therapeutic opportunity. This study provides a molecular framework for how targeting the SHOC2-holophosphatase regulatory node of the RAF activation process provides a mechanism for selective inhibition of ERK signaling.

Despite the crucial role of RAF kinases in cell signaling and disease, we still lack a complete understanding of their regulation. Heterodimerization of RAF kinases as well as dephosphorylation of a conserved “S259” inhibitory site are important steps for RAF activation but the precise mechanisms and dynamics remain unclear. A ternary complex comprised of SHOC2, MRAS, and PP1 (SHOC2 complex) functions as a RAF S259 holophosphatase and gain-of-function mutations in SHOC2, MRAS, and PP1 that promote complex formation are found in Noonan syndrome. Here we show that SHOC2 complex-mediated S259 RAF dephosphorylation is critically required for growth factor-induced RAF heterodimerization as well as for MEK dissociation from BRAF. We also uncover SHOC2-independent mechanisms of RAF and ERK pathway activation that rely on N-region phosphorylation of CRAF. In DLD-1 cells stimulated with EGF, SHOC2 function is essential for a rapid transient phase of ERK activation, but is not required for a slow, sustained phase that is instead driven by palmitoylated H/N-RAS proteins and CRAF. Whereas redundant SHOC2-dependent and -independent mechanisms of RAF and ERK activation make SHOC2 dispensable for proliferation in 2D, KRAS mutant cells preferentially rely on SHOC2 for ERK signaling under anchorage-independent conditions. Our study highlights a context-dependent contribution of SHOC2 to ERK pathway dynamics that is preferentially engaged by KRAS oncogenic signaling and provides a biochemical framework for selective ERK pathway inhibition by targeting the SHOC2 holophosphatase.

RAS at the Golgi antagonizes malignant transformation through PTPRκ-mediated inhibition of ERK activation

RAS GTPases are frequently mutated in human cancer. H- and NRAS isoforms are distributed over both plasma-membrane and endomembranes, including the Golgi complex, but how this organizational context contributes to cellular transformation is unknown. Here we show that RAS at the Golgi is selectively activated by apoptogenic stimuli and antagonizes cell survival by suppressing ERK activity through the induction of PTPRκ, which targets CRAF for dephosphorylation. Consistently, in contrast to what occurs at the plasma-membrane, RAS at the Golgi cannot induce melanoma in zebrafish. Inactivation of PTPRκ, which occurs frequently in human melanoma, often coincident with TP53 inactivation, accelerates RAS-ERK pathway-driven melanomagenesis in zebrafish. Likewise, tp53 disruption in zebrafish facilitates oncogenesis driven by RAS from the Golgi complex. Thus, RAS oncogenic potential is strictly dependent on its sublocalization, with Golgi complex-located RAS antagonizing tumor development.

RAS isoforms are associated with the plasma membrane and endomembranes, but how their localization contributes to tumorigenesis is unclear. Here, the authors show that RAS signals from Golgi complex antagonize tumour formation by inducing apoptosis via ERK inhibition.

The Na/K-ATPase Signaling: From Specific Ligands to General Reactive Oxygen Species

The signaling function of the Na/K-ATPase has been established for 20 years and is widely accepted in the field, with many excellent reports and reviews not cited here. Even though there is debate about the underlying mechanism, the signaling function is unquestioned. This short review looks back at the evolution of Na/K-ATPase signaling, from stimulation by cardiotonic steroids (also known as digitalis-like substances) as specific ligands to stimulation by reactive oxygen species (ROS) in general. The interplay of cardiotonic steroids and ROS in Na/K-ATPase signaling forms a positive-feedback oxidant amplification loop that has been implicated in some pathophysiological conditions.

p27kip1 expression limits H-Ras-driven transformation and tumorigenesis by both canonical and non-canonical mechanisms

The tumor suppressor protein p27Kip1 plays a pivotal role in the control of cell growth and metastasis formation.Several studies pointed to different roles for p27Kip1 in the control of Ras induced transformation, although no explanation has been provided to elucidate these differences. We recently demonstrated that p27kip1 regulates H-Ras activity via its interaction with stathmin.Here, using in vitro and in vivo models, we show that p27kip1 is an important regulator of Ras induced transformation. In H-RasV12 transformed cells, p27kip1 suppressed cell proliferation and tumor growth via two distinct mechanisms: 1) inhibition of CDK activity and 2) impairment of MT-destabilizing activity of stathmin. Conversely, in K-Ras4BV12 transformed cells, p27kip1 acted mainly in a CDK-dependent but stathmin-independent manner.Using human cancer-derived cell lines and primary breast and sarcoma samples, we confirmed in human models what we observed in mice.Overall, we highlight a pathway, conserved from mouse to human, important in the regulation of H-Ras oncogenic activity that could have therapeutic and diagnostic implication in patients that may benefit from anti-H-Ras therapies.

GTPases Rac1 and Ras Signaling from Endosomes

The endocytic compartment is not only the functional continuity of the plasma membrane but consists of a diverse collection of intracellular heterogeneous complex structures that transport, amplify, sustain, and/or sort signaling molecules. Over the years, it has become evident that early, late, and recycling endosomes represent an interconnected vesicular-tubular network able to form signaling platforms that dynamically and efficiently translate extracellular signals into biological outcome. Cell activation, differentiation, migration, death, and survival are some of the endpoints of endosomal signaling. Hence, to understand the role of the endosomal system in signal transduction in space and time, it is therefore necessary to dissect and identify the plethora of decoders that are operational in the different steps along the endocytic pathway. In this chapter, we focus on the regulation of spatiotemporal signaling in cells, considering endosomes as central platforms, in which several small GTPases proteins of the Ras superfamily, in particular Ras and Rac1, actively participate to control cellular processes like proliferation and cell mobility.

Targeting Na/K-ATPase Signaling: A New Approach to Control Oxidative Stress

Renal and cardiac function are greatly affected by chronic oxidative stress which can cause many pathophysiological states. The Na/K-ATPase is well-described as an ion pumping enzyme involved in maintaining cellular ion homeostasis; however, in the past two decades, extensive research has been done to understand the signaling function of the Na/K-ATPase and determine its role in physiological and pathophysiological states. Our lab has shown that the Na/K-ATPase signaling cascade can function as an amplifier of reactive oxygen species (ROS) which can be initiated by cardiotonic steroids or increases in ROS. Regulation of systemic oxidative stress by targeting Na/K-ATPase signaling mediated oxidant amplification improves 5/6th partial nephrectomy (PNx) mediated uremic cardiomyopathy, renal sodium handling, as well as ameliorates adipogenesis. This review will present this new concept of Na/K-ATPase signaling mediated oxidant amplification loop and its clinic implication.

SIRT2 and lysine fatty acylation regulate the transforming activity of K-Ras4a

Ras proteins play vital roles in numerous biological processes and Ras mutations are found in many human tumors. Understanding how Ras proteins are regulated is important for elucidating cell signaling pathways and identifying new targets for treating human diseases. Here we report that one of the K-Ras splice variants, K-Ras4a, is subject to lysine fatty acylation, a previously under-studied protein post-translational modification. Sirtuin 2 (SIRT2), one of the mammalian nicotinamide adenine dinucleotide (NAD)-dependent lysine deacylases, catalyzes the removal of fatty acylation from K-Ras4a. We further demonstrate that SIRT2-mediated lysine defatty-acylation promotes endomembrane localization of K-Ras4a, enhances its interaction with A-Raf, and thus promotes cellular transformation. Our study identifies lysine fatty acylation as a previously unknown regulatory mechanism for the Ras family of GTPases that is distinct from cysteine fatty acylation. These findings highlight the biological significance of lysine fatty acylation and sirtuin-catalyzed protein lysine defatty-acylation.

The Prognostic Value of HRAS mRNA Expression in Cutaneous Melanoma

This study aimed to investigate the prognostic value of HRAS mRNA expression in cutaneous melanoma. Cutaneous melanoma is an aggressive cancer with an increasing incidence. Few studies have focused on the transcriptional level of RAS isoforms (KRAS, NRAS, and HRAS) in cutaneous melanoma. To gain further insight into RAS isoforms at transcriptional level, we obtained the cutaneous melanoma data from cBioPortal and investigated the RAS mRNA expression levels in different stages of melanoma and evaluated their correlation with clinical characteristics and patients' survival. Furthermore, we retrieved and analyzed the coexpression data and performed pathway enrichment analysis. Totally, 452 cutaneous melanoma cases were included in this study. We found that lower HRAS expression level was associated with longer patient survival. 206 genes that negatively correlated with HRAS expression were positively correlated with KRAS and NRAS expression. In contrast, no gene that positively correlated with HRAS expression was positively correlated with KRAS and NRAS expression. In conclusion, our data showed that transcriptional regulation was different for the three RAS isoforms in cutaneous melanoma. This study highlighted the prognostic value of HRAS mRNA expression and revealed that HRAS greatly differs from KRAS and NRAS at the transcriptional level.

Rab11, a vesicular trafficking protein, affects endoreplication through Ras-mediated pathway in Drosophila melanogaster

Rab11, a small monomeric GTPase associated with recycling endosomes, is a key molecule in the regulation of vesicular trafficking and is involved in the development and differentiation of many Drosophila tissues through interaction with diverse signaling pathways. In this study, we report for the first time that Rab11 affects endoreplication through a Ras-mediated pathway. Suppression of Rab11 activity in salivary glands, an endoreplicating tissue, leads to reduction in size of salivary glands with cells having a small nucleus. Endoreplication-regulating proteins, CycE, E2f1 and Gem, are also down-regulated in Rab11 knocked-down salivary glands suggesting that Rab11 has a role in the process of endoreplication, possibly indirectly through other pathways that regulate cell cycle progression. Ras signaling plays an important role in cell cycle progression through G/S phase transition. Ectopic expression of activated Ras in salivary glands of Rab11 down-regulated individuals rescues the small-sized glands to intermediate size. Furthermore, we observed altered localization of Ras in Rab11 down-regulated salivary glands. It is likely that the low level of endoreplication in the Rab11 down-regulated condition is Ras-mediated.

PI3K pathway is involved in ERK signaling cascade activation by histamine H2R agonist in HEK293T cells

BACKGROUND

Histamine, through histamine H2 receptor (H2R), modulates different biological processes, involving the modulation of PI3K/AKT/mTOR and RAS/RAF/MEK/ERK pathways. Many evidences have demonstrated the existence and importance of the crossregulation between these two signaling pathways. The aim of the present work was to determine the molecular mechanisms leading to PI3K and ERK pathways modulation induced by the H2R agonist amthamine and to evaluate the possible interplay between them.

METHODS

Phosphorylation levels of ERK and Akt were examined by Western blot in HEK293T cells expressing the human H2R, in the presence of H2R agonist and dominant negative mutants or pharmacological inhibitors of different proteins/pathways. Transcriptional activity assays were assessed to determine SRE activity. Amthamine-mediated cellular proliferation was investigated in MA-10A cells in the presence of PI3K inhibitor.

RESULTS

H2R agonist inhibits PI3K/Akt/mTOR and stimulates Ras/MEK/ERK pathways. Moreover, PI3K/Akt/mTOR signaling inhibition is necessary to achieve H2R mediated ERK activation. In the presence of a constitutive active mutant of Akt, amthamine is not able to mediate ERK activation. This crosstalk affects classical ERK downstream targets such as Elk1 phosphorylation and the transcriptional activity of the SRE, classically associated to proliferation. We further demonstrate that amthamine-induced proliferation in Leydig MA-10 tumor cells, is enhanced by LY294002, a PI3K inhibitor.

CONCLUSIONS

These results describe a crosstalk between PI3K/AKT/mTOR and Ras/MEK/ERK pathways induced by H2R stimulation with implications in cell proliferation.

GENERAL SIGNIFICANCE

This work indicates that the modulation of PI3K/AKT/mTOR pathway by H2R in turn regulates Ras/MEK/ERK activation conditioning the proliferative capacity of the cells.

ARF6 Is an Actionable Node that Orchestrates Oncogenic GNAQ Signaling in Uveal Melanoma

Activating mutations in Gαq proteins, which form the α subunit of certain heterotrimeric G proteins, drive uveal melanoma oncogenesis by triggering multiple downstream signaling pathways, including PLC/PKC, Rho/Rac, and YAP. Here we show that the small GTPase ARF6 acts as a proximal node of oncogenic Gαq signaling to induce all of these downstream pathways as well as β-catenin signaling. ARF6 activates these diverse pathways through a common mechanism: the trafficking of GNAQ and β-catenin from the plasma membrane to cytoplasmic vesicles and the nucleus, respectively. Blocking ARF6 with a small-molecule inhibitor reduces uveal melanoma cell proliferation and tumorigenesis in a mouse model, confirming the functional relevance of this pathway and suggesting a therapeutic strategy for Gα-mediated diseases.

Meet me in the cytoplasm: A role for p27(Kip1) in the control of H-Ras

The small GTPases of the Ras family play a pivotal role in the regulation of cell proliferation and motility, both in normal and transformed cells. In particular, the 3 genes encoding for the N-, H- and K-Ras are frequently mutated in human cancer and their inappropriate regulation, expression and subcellular localization can drive tumor onset and progression. Activation of the Ras-MAPK pathway directly signals on the cell cycle machinery by regulating the expression and/or localization of 2 key cell cycle player, Cyclin D1 and p27(Kip1). We recently reported that in normal fibroblasts, following mitogenic stimuli, p27(Kip1) translocates to the cytoplasm where it regulates H-Ras localization and activity. This regulatory mechanism ensures that cells pass beyond the restriction point of the cell cycle only when the proper level of stimulation is reached. Here, we comment on this new evidence that possibly represents one of the ways that cells have developed during evolution to ensure that the cell decision to divide is taken only when time and context are appropriate.

Palmitoylation pilots ras to recycling endosomes

We recently showed that palmitoylated Ras proteins (H-Ras and N-Ras) localize intracellularly at recycling endosomes (REs) and that REs act as a way-station for Ras proteins as they move along the post-Golgi exocytic pathway to the plasma membrane (PM). Palmitoylation is essential for H-Ras/N-Ras targeting to REs. H-Ras requires two palmitoyl groups for RE targeting. A lack of either or both palmitoyl groups causes H-Ras to be mislocalized to the endoplasmic reticulum (ER), the Golgi apparatus, or the PM. In this commentary, we summarize recent progress about the Ras trafficking cycle between the endomembranes (endosomes/ER/Golgi) and the PM. We further discuss (1) the critical determinants of RE targeting of lipidated proteins and (2) possible Ras-mediated signaling pathways that originate from REs.

Mechanism of uptake of ZnO nanoparticles and inflammatory responses in macrophages require PI3K mediated MAPKs signaling

The inflammatory responses after exposure to zinc oxide nanoparticles (ZNPs) are known, however, the molecular mechanisms and direct consequences of particle uptake are still unclear. Dose and time-dependent increase in the uptake of ZNPs by macrophages has been observed by flow cytometry. Macrophages treated with ZNPs showed a significantly enhanced phagocytic activity. Inhibition of different internalization receptors caused a reduction in uptake of ZNPs in macrophages. The strongest inhibition in internalization was observed by blocking clathrin, caveolae and scavenger receptor mediated endocytic pathways. However, FcR and complement receptor-mediated phagocytic pathways also contributed significantly to control. Further, exposure of primary macrophages to ZNPs (2.5 μg/ml) caused (i) significant enhancement of Ras, PI3K, (ii) enhanced phosphorylation and subsequent activation of its downstream signaling pathways via ERK1/2, p38 and JNK MAPKs (iii) overexpression of c-Jun, c-Fos and NF-κB. Our results demonstrate that ZNPs induce the generation of reactive nitrogen species and overexpression of Cox-2, iNOS, pro-inflammatory cytokines (IL-6, IFN-γ, TNF-α, IL-17 and regulatory cytokine IL-10) and MAPKs which were found to be inhibited after blocking internalization of ZNPs through caveolae receptor pathway. These results indicate that ZNPs are internalized through caveolae pathway and the inflammatory responses involve PI3K mediated MAPKs signaling cascade.

Endocytosis and cancer

Endocytosis entails selective packaging of cell-surface proteins, such as receptors for cytokines and adhesion components, in cytoplasmic vesicles (endosomes). The series of sorting events that determines the fate of internalized proteins, either degradation in lysosomes or recycling back to the plasma membrane, relies on intrinsic sequence motifs, posttranslational modifications (e.g., phosphorylation and ubiquitination), and transient assemblies of both Rab GTPases and phosphoinositide-binding proteins. This multicomponent process is enhanced and skewed in cancer cells; we review mechanisms enabling both major drivers of cancer, p53 and Ras, to bias recycling of integrins and receptor tyrosine kinases (RTKs). Likewise, cadherins and other junctional proteins of cancer cells are constantly removed from the cell surface, thereby disrupting tissue polarity and instigating motile phenotypes. Mutant forms of RTKs able to evade Cbl-mediated ubiquitination, along with overexpression of the wild-type forms and a variety of defective feedback regulatory loops, are frequently detected in tumors. Finally, we describe pharmacological attempts to harness the peculiar endocytic system of cancer, in favor of effective patient treatment.

Helminth-excreted/secreted products are recognized by multiple receptors on DCs to block the TLR response and bias Th2 polarization in a cRAF dependent pathway

Dendritic cells (DCs) recognize pathogens and initiate the T-cell response. The DC-helminth interaction induces an immature phenotype in DCs; as a result, these DCs display impaired responses to TLR stimulation and prime Th2-type responses. However, the DC receptors and intracellular pathways targeted by helminth molecules and their importance in the initiation of the Th2 response are poorly understood. In this report, we found that products excreted/secreted by Taenia crassiceps (TcES) triggered cRAF phosphorylation through MGL, MR, and TLR2. TcES interfered with the LPS-induced NFκB p65 and p38 MAPK signaling pathways. In addition, TcES-induced cRAF signaling pathway was critical for down-regulation of the TLR-mediated DC maturation and secretion of IL-12 and TNF-α. Finally, we show for the first time that blocking cRAF in DCs abolishes their ability to induce Th2 polarization in vitro after TcES exposure. Our data demonstrate a new mechanism by which helminths target intracellular pathways to block DC maturation and efficiently program Th2 polarization.

Ras-guanine-nucleotide-releasing factors 1 and 2 interact with PLCγ at focal adhesions to enable IL-1-induced Ca(2+) signalling, ERK activation and MMP-3 expression

IL (interleukin)-1 signalling in anchorage-dependent cells involves focal-adhesion-restricted and Ca2+-dependent Ras and ERK (extracellular-signal-regulated kinase) activation that leads to MMP (matrix metalloproteinase) release and extracellular matrix remodelling. Ras activity is regulated, in part, by the Ca2+-responsive Ras GRFs (guanine-nucleotide-releasing factors) 1 and 2, but the mechanisms that link and localize IL-1-induced Ca2+ signalling to focal adhesions are not defined. In the present study we characterized the role of Ras-GRF1/2 in Ca2+ and Ras→ERK signalling after IL-1 stimulation. By immunoprecipitation we found that Ras-GRF1/2 associates with PLCγ1 (phospholipase Cγ1). This association enables PLCγ1 recruitment to focal adhesions and is required for Ras signalling, ERK activation and MMP-3 release downstream of IL-1 stimulation. Depletion of PLCγ1 by siRNA (small interfering RNA) abolished IL-1-induced Ras activation and MMP-3 expression. Buffering of cytosolic Ca2+ reduced Ras interactions with Ras-GRF1/2 and blocked MMP-3 release. The results of the present study show that, in addition to their functions as Ras-exchange factors, Ras-GRF1 and -GRF2 may act as adaptors that bind PLCγ1 and restrict Ca2+ signalling to the vicinity of focal adhesions, indicating a new role for these GRFs that is required for IL-1 induction of the Ras→ERK pathway and MMP-3 expression.

Mitogen-Activated Protein (MAP) Kinase Scaffolding Proteins: A Recount

The mitogen-activated protein kinase (MAPK) pathway is the canonical signaling pathway for many receptor tyrosine kinases, such as the Epidermal Growth Factor Receptor. Downstream of the receptors, this pathway involves the activation of a kinase cascade that culminates in a transcriptional response and affects processes, such as cell migration and adhesion. In addition, the strength and duration of the upstream signal also influence the mode of the cellular response that is switched on. Thus, the same components can in principle coordinate opposite responses, such as proliferation and differentiation. In recent years, it has become evident that MAPK signaling is regulated and fine-tuned by proteins that can bind to several MAPK signaling proteins simultaneously and, thereby, affect their function. These so-called MAPK scaffolding proteins are, thus, important coordinators of the signaling response in cells. In this review, we summarize the recent advances in the research on MAPK/extracellular signal-regulated kinase (ERK) pathway scaffolders. We will not only review the well-known members of the family, such as kinase suppressor of Ras (KSR), but also put a special focus on the function of the recently identified or less studied scaffolders, such as fibroblast growth factor receptor substrate 2, flotillin-1 and mitogen-activated protein kinase organizer 1.

A comprehensive survey of Ras mutations in cancer

All mammalian cells express 3 closely related Ras proteins, termed H-Ras, K-Ras, and N-Ras, that promote oncogenesis when they are mutationally activated at codon 12, 13, or 61. Although there is a high degree of similarity among the isoforms, K-Ras mutations are far more frequently observed in cancer, and each isoform displays preferential coupling to particular cancer types. We examined the mutational spectra of Ras isoforms curated from large-scale tumor profiling and found that each isoform exhibits surprisingly distinctive codon mutation and amino-acid substitution biases. These findings were unexpected given that these mutations occur in regions that share 100% amino-acid sequence identity among the 3 isoforms. Of importance, many of these mutational biases were not due to differences in exposure to mutagens, because the patterns were still evident when compared within specific cancer types. We discuss potential genetic and epigenetic mechanisms, as well as isoform-specific differences in protein structure and signaling, that may promote these distinct mutation patterns and differential coupling to specific cancers.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

CHMP6 and VPS4A mediate the recycling of Ras to the plasma membrane to promote growth factor signaling

While Ras is well-known to function on the plasma membrane (PM) to mediate growth factor signaling, increasing evidence suggests that Ras has complex roles in the cytoplasm. To uncover these roles, we screened a cDNA library and isolated H-Ras-binding proteins that also influence Ras functions. Many isolated proteins regulate trafficking involving endosomes; CHMP6/VPS20 and VPS4A, which interact with ESCRT-III, were chosen for further study. We showed that the binding is direct and occurs in endosomes. Furthermore, the binding is most efficient when H-Ras has a functional effector-binding-loop and is GTP-bound and ubiquitylated. CHMP6 and VPS4A also bound N-Ras, but not K-Ras. Repressing CHMP6 and VPS4A blocked Ras-induced transformation, which correlated with inefficient Ras localization to the PM as measured by cell fractionation and photobleaching. Moreover, silencing CHMP6 and VPS4A also blocked EGFR recycling. These data suggest that Ras interacts with key ESCRT-III components to promote recycling of itself and EGFR back to the PM to create a positive feedback loop to enhance growth factor signaling.

Monocyte to macrophage differentiation-associated (MMD) positively regulates ERK and Akt activation and TNF-α and NO production in macrophages

Macrophage activation is modulated by both environmental cues and endogenous programs. In the present study, we investigated the role of a PAQR family protein, monocyte to macrophage differentiation-associated (MMD), in macrophage activation and unveiled its underlying molecular mechanism. Our results showed that while MMD expression could be detected in all tissues examined, its expression level is significantly up-regulated upon monocyte differentiation. Within cells, EGFP-MMD fusion protein could be co-localized to endoplasmic reticulum, mitochondria, Golgi apparatus, but not lysosomes and cytoplasm. MMD expression is up-regulated in macrophages after LPS stimulation, and this might be modulated by RBP-J, the critical transcription factor of Notch signaling. Overexpression of MMD in macrophages increased the production of TNF-α and NO upon LPS stimulation. We found that MMD overexpression enhanced ERK1/2 and Akt phosphorylation in macrophages after LPS stimulation. Blocking Erk or Akt by pharmacological agent reduced TNF-α or NO production in MMD-overexpressing macrophages, respectively. These results suggested that MMD modulates TNF-α and NO production in macrophages, and this process might involves Erk or Akt.

Regulating the regulator: post-translational modification of RAS

RAS proteins are monomeric GTPases that act as binary molecular switches to regulate a wide range of cellular processes. The exchange of GTP for GDP on RAS is regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), which regulate the activation state of RAS without covalently modifying it. By contrast, post-translational modifications (PTMs) of RAS proteins direct them to various cellular membranes and, in some cases, modulate GTP-GDP exchange. Important RAS PTMs include the constitutive and irreversible remodelling of its carboxy-terminal CAAX motif by farnesylation, proteolysis and methylation, reversible palmitoylation, and conditional modifications, including phosphorylation, peptidyl-prolyl isomerisation, monoubiquitylation, diubiquitylation, nitrosylation, ADP ribosylation and glucosylation.

A genome-wide RNAi screen in mouse embryonic stem cells identifies Mp1 as a key mediator of differentiation

Despite intense investigation of intrinsic and extrinsic factors that regulate pluripotency, the process of initial fate commitment of embryonic stem (ES) cells is still poorly understood. We used a genome-wide short hairpin RNA screen in mouse ES cells to identify genes that are essential for initiation of differentiation. Knockdown of the scaffolding protein Mek binding protein 1 (Mp1, also known as Lamtor3 or Map2k1ip1) stimulated self-renewal of ES cells, blocked differentiation, and promoted proliferation. Fibroblast growth factor 4 (FGF4) signaling is required for initial fate commitment of ES cells. Knockdown of Mp1 inhibited FGF4-induced differentiation but did not alter FGF4-driven proliferation. This uncoupling of differentiation and proliferation was also observed when oncogenic Ras isoforms were overexpressed in ES cells. Knockdown of Mp1 redirected FGF4 signaling from differentiation toward pluripotency and up-regulated the pluripotency-related genes Esrrb, Rex1, Tcl1, and Sox2. We also found that human germ cell tumors (GCTs) express low amounts of Mp1 in the invasive embryonic carcinoma and seminoma histologies and higher amounts of Mp1 in the noninvasive carcinoma in situ precursor and differentiated components. Knockdown of Mp1 in invasive GCT cells resulted in resistance to differentiation, thereby showing a functional role for Mp1 both in normal differentiation of ES cells and in germ cell cancer.

Differential Regulation of RasGAPs in Cancer

Ever since their discovery as cellular counterparts of viral oncogenes more than 25 years ago, much progress has been made in understanding the complex networks of signal transduction pathways activated by oncogenic Ras mutations in human cancers. The activity of Ras is regulated by nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs), and much emphasis has been put into the biochemical and structural analysis of the Ras/GAP complex. The mechanisms by which GAPs catalyze Ras-GTP hydrolysis have been clarified and revealed that oncogenic Ras mutations confer resistance to GAPs and remain constitutively active. However, it is yet unclear how cells coordinate the large and divergent GAP protein family to promote Ras inactivation and ensure a certain biological response. Different domain arrangements in GAPs to create differential protein-protein and protein-lipid interactions are probably key factors determining the inactivation of the 3 Ras isoforms H-, K-, and N-Ras and their effector pathways. In recent years, in vitro as well as cell- and animal-based studies examining GAP activity, localization, interaction partners, and expression profiles have provided further insights into Ras inactivation and revealed characteristics of several GAPs to exert specific and distinct functions. This review aims to summarize knowledge on the cell biology of RasGAP proteins that potentially contributes to differential regulation of spatiotemporal Ras signaling.

Ras, an actor on many stages: posttranslational modifications, localization, and site-specified events

Among the wealth of information that we have gathered about Ras in the past decade, the introduction of the concept of space in the field has constituted a major revolution that has enabled many pieces of the Ras puzzle to fall into place. In the early days, it was believed that Ras functioned exclusively at the plasma membrane. Today, we know that within the plasma membrane, the 3 Ras isoforms-H-Ras, K-Ras, and N-Ras-occupy different microdomains and that these isoforms are also present and active in endomembranes. We have also discovered that Ras proteins are not statically associated with these localizations; instead, they traffic dynamically between compartments. And we have learned that at these localizations, Ras is under site-specific regulatory mechanisms, distinctively engaging effector pathways and switching on diverse genetic programs to generate different biological responses. All of these processes are possible in great part due to the posttranslational modifications whereby Ras proteins bind to membranes and to regulatory events such as phosphorylation and ubiquitination that Ras is subject to. As such, space and these control mechanisms act in conjunction to endow Ras signals with an enormous signal variability that makes possible its multiple biological roles. These data have established the concept that the Ras signal, instead of being one single, homogeneous entity, results from the integration of multiple, site-specified subsignals, and Ras has become a paradigm of how space can differentially shape signaling.

Genes and pathways co-associated with the exposure to multiple drugs of abuse, including alcohol, amphetamine/methamphetamine, cocaine, marijuana, morphine, and/or nicotine: a review of proteomics analyses

Drug addiction is a chronic neuronal disease. In recent years, proteomics technology has been widely used to assess the protein expression in the brain tissues of both animals and humans exposed to addictive drugs. Through this approach, a large number of proteins potentially involved in the etiology of drug addictions have been identified, which provide a valuable resource to study protein function, biochemical pathways, and networks related to the molecular mechanisms underlying drug dependence. In this article, we summarize the recent application of proteomics to profiling protein expression patterns in animal or human brain tissues after the administration of alcohol, amphetamine/methamphetamine, cocaine, marijuana, morphine/heroin/butorphanol, or nicotine. From available reports, we compiled a list of 497 proteins associated with exposure to one or more addictive drugs, with 160 being related to exposure to at least two abused drugs. A number of biochemical pathways and biological processes appear to be enriched among these proteins, including synaptic transmission and signaling pathways related to neuronal functions. The data included in this work provide a summary and extension of the proteomics studies on drug addiction. Furthermore, the proteins and biological processes highlighted here may provide valuable insight into the cellular activities and biological processes in neurons in the development of drug addiction.

Ras trafficking, localization and compartmentalized signalling

Ras proteins are proto-oncogenes that are frequently mutated in human cancers. Three closely related isoforms, HRAS, KRAS and NRAS, are expressed in all cells and have overlapping but distinctive functions. Recent work has revealed how differences between the Ras isoforms in their trafficking, localization and protein-membrane orientation enable signalling specificity to be determined. We review the various strategies used to characterize compartmentalized Ras localization and signalling. Localization is an important contextual modifier of signalling networks and insights from the Ras system are of widespread relevance for researchers interested in signalling initiated from membranes.

PX-FERM proteins: A link between endosomal trafficking and signaling?

Endosomes are the primary organelle where decisions are made as to whether endocytosed proteins will be sorted into degradative trafficking pathways or recycled back to the plasma membrane. This balance between cellular uptake and recycling regulates the plasma membrane composition and is therefore critical for many cellular processes such as nutrient uptake, neuronal transmission and cell migration.1 In addition to its well-known role in membrane trafficking, the endosome is increasingly being recognized as a critical cellular domain for regulated cell signaling. We recently showed that several proteins that regulate endosomal recycling, SNX17, SNX27 and SNX31 are structurally and functionally related.2 These proteins use an unusual FERM domain to bind specific endosomal cargo molecules, and most interestingly, we also found that these proteins use the same FERM domain to associate with the activated Ras small GTPase. Here we speculate on the potential dual role of the PX-FERM proteins in endosomal transport and as scaffolds that may be involved in endosomal Ras signaling processes.

Phosphorylation of threonine 190 is essential for nuclear localization and endocytosis of the FTS (Fused Toes Homolog) protein

Fused Toes Homolog (FTS) is a member of a group of proteins termed as E2 variants and this group of proteins lacks an active cysteine residue that is required for ubiquitin transfer. We have identified the expression of this protein in early neoplastic stages of cervical cancer and its translocation into nucleus from cytoplasm upon irradiation. Here we have reported that a threonine residue at position 190 is essential for its nucleocytoplasmic shuttling and function. Upon LMB treatment we found that FTS was located in the nucleus and it suggests that direct role of nuclear export signal (NES) is required for the binding to CRM1 and facilitates nuclear export. The threonine residue was phosphorylated and promoted the phosphorylation of EGFR, p38 and JNK facilitating vesicular trafficking of early to late endosomes. Mutational change of the threonine into alanine resulted in the cytoplasmic localization of FTS and failed to phosphorylate EGFR and its downstream effector proteins. In addition the mutation also reduced the number of early endosomes formed and also resulted in the clustering of late endosomes around the perinuclear region. These data suggest that threonine residue of FTS at position 190 is not only essential for its function but also for the formation, maturation and trafficking of early endosomes to late endosome/lysosome, as well as we speculate that FTS may function at a connection point in the vesicle tethering.

Multiple cellular proteins modulate the dynamics of K-ras association with the plasma membrane

Although specific proteins have been identified that regulate the membrane association and facilitate intracellular transport of prenylated Rho- and Rab-family proteins, it is not known whether cellular proteins fulfill similar roles for other prenylated species, such as Ras-family proteins. We used a previously described method to evaluate how several cellular proteins, previously identified as potential binding partners (but not effectors) of K-ras4B, influence the dynamics of K-ras association with the plasma membrane. Overexpression of either PDEδ or PRA1 enhances, whereas knockdown of either protein reduces, the rate of dissociation of K-ras from the plasma membrane. Inhibition of calmodulin likewise reduces the rate of K-ras dissociation from the plasma membrane, in this case in a manner specific for the activated form of K-ras. By contrast, galectin-3 specifically reduces the rate of plasma membrane dissociation of activated K-ras, an effect that is blocked by the K-ras antagonist farnesylthiosalicylic acid (salirasib). Multiple cellular proteins thus control the dynamics of membrane association and intercompartmental movement of K-ras to an important degree even under basal cellular conditions.

Mechanistic principles of RAF kinase signaling

The RAF family of kinases are key components acting downstream of receptor tyrosine kinases and cells employ several distinct mechanisms to strictly control their activity. RAF transitions from an inactive state, where the N-terminal regulatory region binds intramolecularly to the C-terminal kinase domain, to an open state capable of executing the phosphoryl transfer reaction. This transition involves changes both within and between the protein domains in RAF. Many different proteins regulate the transition between inactive and active states of RAF, including RAS and KSR, which are arguably the two most prominent regulators of RAF function. Recent developments have added several new twists to our understanding of RAF regulation. Among others, dimerization of the RAF kinase domain is emerging as a crucial step in the RAF activation process. The multitude of regulatory protein-protein interactions involving RAF remains a largely untapped area for therapeutic applications.

H-ras resides on clathrin-independent ARF6 vesicles that harbor little RAF-1, but not on clathrin-dependent endosomes

Internalization of H-Ras from the cell surface onto endomembranes through vesicular endocytic pathways may play a significant role(s) in regulating the outcome of Ras signaling. However, the identity of Ras-associated subcellular vesicles and the means by which Ras localize to these internal sites remain elusive. In this study, we show that H-Ras is absent from endosomes initially derived from a clathrin-dependent endocytic pathway. Instead, both oncogenic H-Ras-61L and wild type H-Ras (basal or EGF-stimulated) bind Arf6-associated clathrin-independent endosomes and vesicles of the endosomal-recycling center (ERC). K-Ras4B-12V can also be internalized via Arf6 endosomes, and the C-terminal tails of both H-Ras and K-Ras4B are sufficient to mediate localization of GFP chimeras to Arf6-associated vesicles. Interestingly, little Raf-1 was found on these Arf6-associated endosomes even when active H-Ras was present. Instead, endogenous Raf-1 distributed primarily on EEA1-containing vesicles, suggesting that this H-Ras effector, although accessible for H-Ras interaction on the plasma membrane, appears to separate from its regulator during early stages of endocytosis. The discrete and dynamic distribution of Ras pathway components with spatio-temporal complexity may contribute to the specificity of Ras:effector interaction.

Demonstration of angiotensin II-induced Ras activation in the trans-Golgi network and endoplasmic reticulum using bioluminescence resonance energy transfer-based biosensors

Previous studies have demonstrated that molecules of the Ras signaling pathway are present in intracellular compartments, including early endosomes, the endoplasmic reticulum (ER), and the Golgi, and suggested that mitogens can regulate Ras activity in these endomembranes. In this study, we investigated the effect of angiotensin II (AngII) on intracellular Ras activity in living HEK293 cells expressing angiotensin type 1 receptors (AT(1)-Rs) using newly developed bioluminescence resonance energy transfer biosensors. To investigate the subcellular localization of AngII-induced Ras activation, we targeted our probes to various intracellular compartments, such as the trans-Golgi network (TGN), the ER, and early endosomes. Using these biosensors, we detected AngII-induced Ras activation in the TGN and ER, but not in early endosomes. In cells expressing a cytoplasmic tail deletion AT(1)-R mutant, the AngII-induced response was enhanced, suggesting that receptor internalization and β-arrestin binding are not required for AngII-induced Ras activation in endomembranes. Although we were able to demonstrate EGF-induced Ras activation in the plasma membrane and TGN, but not in other endomembranes, AG1478, an EGF receptor inhibitor, did not affect the AngII-induced response, suggesting that the latter is independent of EGF receptor transactivation. AngII was unable to stimulate Ras activity in the studied compartments in cells expressing a G protein coupling-deficient AT(1)-R mutant ((125)DRY(127) to (125)AAY(127)). These data suggest that AngII can stimulate Ras activity in the TGN and ER with a G protein-dependent mechanism, which does not require β-arrestin-mediated signaling, receptor internalization, and EGF receptor transactivation.

Small G protein signaling in neuronal plasticity and memory formation: the specific role of ras family proteins

Small G proteins are an extensive family of proteins that bind and hydrolyze GTP. They are ubiquitous inside cells, regulating a wide range of cellular processes. Recently, many studies have examined the role of small G proteins, particularly the Ras family of G proteins, in memory formation. Once thought to be primarily involved in the transduction of a variety of extracellular signals during development, it is now clear that Ras family proteins also play critical roles in molecular processing underlying neuronal and behavioral plasticity. We here review a number of recent studies that explore how the signaling of Ras family proteins contributes to memory formation. Understanding these signaling processes is of fundamental importance both from a basic scientific perspective, with the goal of providing mechanistic insights into a critical aspect of cognitive behavior, and from a clinical perspective, with the goal of providing effective therapies for a range of disorders involving cognitive impairments.

The Ras-ERK pathway: understanding site-specific signaling provides hope of new anti-tumor therapies

Recent discoveries have suggested the concept that intracellular signals are the sum of multiple, site-specified subsignals, rather than single, homogeneous entities. In the context of cancer, searching for compounds that selectively block subsignals essential for tumor progression, but not those regulating "house-keeping" functions, could help in producing drugs with reduced side effects compared to compounds that block signaling completely. The Ras-ERK pathway has become a paradigm of how space can differentially shape signaling. Today, we know that Ras proteins are found in different plasma membrane microdomains and endomembranes. At these localizations, Ras is subject to site-specific regulatory mechanisms, distinctively engaging effector pathways and switching-on diverse genetic programs to generate different biological responses. The Ras effector pathway leading to ERKs activation is also under strict, space-related regulatory processes. These findings may open a gate for aiming at the Ras-ERK pathway in a spatially restricted fashion, in our quest for new anti-tumor therapies.

Intracellular signaling: peripatetic Ras

Ras proteins traffic between the plasma membrane and endomembranes and signal from the cytosolic face of a variety of organelles. Palmitoylated N-Ras and H-Ras signal from early endosomes. A recent study reports that K-Ras resides on and signals from various types of endosomes, including late endosomes/lysosomes and multivesicular bodies.

Differential involvement of H- and K-Ras in Raf-1 activation determines the role of calmodulin in MAPK signaling

We have previously demonstrated that inhibition of calmodulin (CaM) and the concomitant reduction of PI3K interfere with H-Ras-mediated activation of Raf-1 [1]. In the present study, we show that CaM has completely opposite effects on K-Ras-mediated Raf-1 activation. The differential contribution of CaM in the regulation of Raf-1 kinase activity via K- or H-Ras correlates with the stimulatory or inhibitory effect of CaM on MAPK phosphorylation depending on the cell type analyzed. FRET microscopy and biochemical analysis show that inhibition of CaM increases K-Ras-GTP levels and consequently its association with Raf-1. Though inhibition of CaM, using the CaM antagonist W-13, significantly increased Raf-1 activation by K-Ras-GTP, MAPK activation downstream K-Ras/Raf-1 was strongly reduced in COS-1 and several other cell lines. In contrast, in other cell lines such as NIH3T3-wt8, W-13-mediated inhibition of CaM increased Raf-1 activity, but resulted in an increase in MAPK phosphorylation. These findings suggest that modulation of K-Ras activity via CaM regulates MAPK signaling only in certain cell types. In support of this hypothesis, the comparison of H- and K-Ras expression, GTP loading and Raf-1 interaction in COS-1 and NIH3T3-wt8 suggests that the overall role of CaM in MAPK signal output is determined by the ratio of activated H- and K-Ras and the cell-specific contribution of each isoform in Raf-1 activation.