A clathrin-dependent pathway leads to KRas signaling on late endosomes en route to lysosomes

Myotubularin-related-protein-7 inhibits mutant (G12V) K-RAS by direct interaction

Inhibition of K-RAS effectors like B-RAF or MEK1/2 is accompanied by treatment resistance in cancer patients via re-activation of PI3K and Wnt signaling. We hypothesized that myotubularin-related-protein-7 (MTMR7), which inhibits PI3K and ERK1/2 signaling downstream of RAS, directly targets RAS and thereby prevents resistance. Using cell and structural biology combined with animal studies, we show that MTMR7 binds and inhibits RAS at cellular membranes. Overexpression of MTMR7 reduced RAS GTPase activities and protein levels, ERK1/2 phosphorylation, c-FOS transcription and cancer cell proliferation in vitro. We located the RAS-inhibitory activity of MTMR7 to its charged coiled coil (CC) region and demonstrate direct interaction with the gastrointestinal cancer-relevant K-RASG12V mutant, favouring its GDP-bound state. In mouse models of gastric and intestinal cancer, a cell-permeable MTMR7-CC mimicry peptide decreased tumour growth, Ki67 proliferation index and ERK1/2 nuclear positivity. Thus, MTMR7 mimicry peptide(s) could provide a novel strategy for targeting mutant K-RAS in cancers.

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.

Recent advances in the development of inhibitors targeting KRAS-G12C and its related pathways

The RAS gene, also known as the mouse sarcoma virus, includes three genes (KRAS, HRAS, and NRAS) that are associated with human tumors. Among them, KRAS has the highest incidence of mutations in cancer, accounting for around 80% of cases. At the molecular level, the RAS gene plays a regulatory role in transcription and translation, while at the cellular level, it affects cell proliferation and migration, making it crucial for cancer development. In 2021, the FDA approved AMG510, the first direct inhibitor targeting the KRAS-G12C mutation, which has shown tumor regression, prolonged survival, and low off-target activity. However, with the increase of drug resistance, a single inhibitor is no longer sufficient to achieve the desired effect on tumors. Therefore, a large number of other highly efficient inhibitors are being developed at different stages. This article provides an overview of the mechanism of action targeting KRAS-G12C in the KRASGTP-KRASGDP cycle pathway, as well as the structure-activity relationship, structure optimization, and biological activity effects of inhibitors that target the upstream and downstream pathways, or combination therapy.

Gold Nanoparticles Inhibit Macropinocytosis by Decreasing KRAS Activation

The RAS-transformed cells utilize macropinocytosis to acquire amino acids to support their uncontrolled growth. However, targeting RAS to inhibit macropinocytosis remains a challenge. Here, we report that gold nanoparticles (GNP) inhibit macropinocytosis by decreasing KRAS activation. Using surface-modified and unmodified GNP, we showed that unmodified GNP specifically sequestered both wild-type and mutant KRAS and inhibited its activation, irrespective of growth factor stimulation, while surface-passivated GNP had no effect. Alteration of KRAS activation is reflected on downstream signaling cascades, macropinocytosis and tumor cell growth in vitro, and two independent preclinical human xenograft models of pancreatic cancer in vivo. The current study demonstrates NP-mediated inhibition of macropinocytosis and KRAS activation and provides translational opportunities to inhibit tumor growth in a number of cancers where activation of KRAS plays a major role.

Chaperone-Dependent Degradation of Cdc42 Promotes Cell Polarity and Shields the Protein from Aggregation

Rho GTPases are global regulators of cell polarity and signaling. By exploring the turnover regulation of the yeast Rho GTPase Cdc42p, we identified new regulatory features surrounding the stability of the protein. We specifically show that Cdc42p is degraded at 37 °C by chaperones through lysine residues located in the C-terminus of the protein. Cdc42p turnover at 37 °C occurred by the 26S proteasome in an ESCRT-dependent manner in the lysosome/vacuole. By analyzing versions of Cdc42p that were defective for turnover, we show that turnover at 37 °C promoted cell polarity but was defective for sensitivity to mating pheromone, presumably mediated through a Cdc42p-dependent MAP kinase pathway. We also identified one residue (K16) in the P-loop of the protein that was critical for Cdc42p stability. Accumulation of Cdc42pK16R in some contexts led to the formation of protein aggregates, which were enriched in aging mother cells and cells undergoing proteostatic stress. Our study uncovers new aspects of protein turnover regulation of a Rho-type GTPase that may extend to other systems. Moreover, residues identified here that mediate Cdc42p turnover correlate with several human diseases, which may suggest that turnover regulation of Cdc42p is important to aspects of human health.

FGF-mediated establishment of left-right asymmetry requires Rab7 function in the dorsal mesoderm in Xenopus

Gastrulation denotes a very important developmental process, which includes significant structural tissue rearrangements and patterning events that shape the emerging vertebrate organism. At the end of gastrulation, the three body axes are spatially defined while the left-right axis still lacks any molecular or morphological polarity. In most vertebrates, this is established during neurulation by a symmetry breaking LR organizer. However, this mesoderm-derived structure depends on proper induction and specification of the mesoderm, which in turn requires involvement of several signaling pathways. Endocytosis and the endosomal machinery offer manifold platforms for intracellular pathway regulation, especially late endosomes claim increasing attention. The late endosomal regulator Rab7 has been linked to mesoderm specification during gastrulation. Distinct axial defects due to compromised dorsal mesoderm development in rab7-deficient Xenopus embryos suggested a requirement of Rab7 for FGF-dependent mesoderm patterning and LR asymmetry. Here we specifically addressed such a role of Rab7, demonstrating a functional requirement for LR organizer development and symmetry breakage. Using different FGF/MAPK pathway components we show that Rab7 participates in dorsal mesoderm patterning. We suggest a hierarchical classification of Rab7 upstream of MAPK-dependent mesoderm specification, most probably at the level of the small GTPase Ras. Thus, this study affords an insight on how the Rab7-regulated endosomal machinery could participate in signal transduction to enable correct mesoderm specification and left-right asymmetry.

Circulating Exosome Cargoes Contain Functionally Diverse Cancer Biomarkers: From Biogenesis and Function to Purification and Potential Translational Utility

Although diagnostic and therapeutic treatments of cancer have tremendously improved over the past two decades, the indolent nature of its symptoms has made early detection challenging. Thus, inter-disciplinary (genomic, transcriptomic, proteomic, and lipidomic) research efforts have been focused on the non-invasive identification of unique "silver bullet" cancer biomarkers for the design of ultra-sensitive molecular diagnostic assays. Circulating tumor biomarkers, such as CTCs and ctDNAs, which are released by tumors in the circulation, have already demonstrated their clinical utility for the non-invasive detection of certain solid tumors. Considering that exosomes are actively produced by all cells, including tumor cells, and can be found in the circulation, they have been extensively assessed for their potential as a source of circulating cell-specific biomarkers. Exosomes are particularly appealing because they represent a stable and encapsulated reservoir of active biological compounds that may be useful for the non-invasive detection of cancer. T biogenesis of these extracellular vesicles is profoundly altered during carcinogenesis, but because they harbor unique or uniquely combined surface proteins, cancer biomarker studies have been focused on their purification from biofluids, for the analysis of their RNA, DNA, protein, and lipid cargoes. In this review, we evaluate the biogenesis of normal and cancer exosomes, provide extensive information on the state of the art, the current purification methods, and the technologies employed for genomic, transcriptomic, proteomic, and lipidomic evaluation of their cargoes. Our thorough examination of the literature highlights the current limitations and promising future of exosomes as a liquid biopsy for the identification of circulating tumor biomarkers.

CRISPR/Cas9 Gene Editing of HeLa Cells to Tag Proteins with mNeonGreen

Subcellular localization dynamics of proteins involved in signal transduction processes is crucial in determining the signaling outcome. However, there is very limited information about the localization of endogenous signaling proteins in living cells. For example, biochemical mechanisms underlying the signaling pathway from epidermal growth factor (EGF) receptor (EGFR) to RAS-RAF and ERK1/2/MAPK are well understood, whereas the operational domains of this pathway in the cell remain poorly characterized. Tagging of endogenous components of signaling pathways with fluorescent proteins allows more accurate characterization of their intracellular dynamics at their native expression levels controlled by endogenous regulatory mechanisms, thus avoiding possible tainting effects of overexpression and mistargeting. In this study, we describe methodological approaches to label components of the EGFR-RAS-MAPK pathway, such as Grb2, KRAS, and NRAS, with the fluorescent protein mNeonGreen (mNG) using CRISPR/Cas9 gene-editing, as well as generation of homozygous single-cell clones of the edited target protein.

An emerging role of KRAS in biogenesis, cargo sorting and uptake of cancer-derived extracellular vesicles

Extracellular vesicles (EVs) are nanovesicles secreted for intercellular communication with endosomal network regulating secretion of small EVs (or exosomes) that play roles in cancer progression. As an essential oncoprotein, Kirsten rat sarcoma virus (KRAS) is tightly regulated by its endosomal trafficking for membrane attachment. However, the crosstalk between KRAS and EVs has been scarcely discussed despite its endocytic association. An overview of the oncogenic role of KRAS focusing on its correlation with cancer-associated EVs should provide important clues for disease prognosis and inspire novel therapeutic approaches for treating KRAS mutant cancers. Therefore, this review summarizes the relevant studies that provide substantial evidence linking KRAS mutation to EVs and discusses the oncogenic implication from the aspects of biogenesis, cargo sorting, and release and uptake of the EVs.

Upregulated flotillins and sphingosine kinase 2 derail AXL vesicular traffic to promote epithelial-mesenchymal transition

Altered endocytosis and vesicular trafficking are major players during tumorigenesis. Flotillin overexpression, a feature observed in many invasive tumors and identified as a marker of poor prognosis, induces a deregulated endocytic and trafficking pathway called upregulated flotillin-induced trafficking (UFIT). Here, we found that in non-tumoral mammary epithelial cells, induction of the UFIT pathway promotes epithelial-to-mesenchymal transition (EMT) and accelerates the endocytosis of several transmembrane receptors, including AXL, in flotillin-positive late endosomes. AXL overexpression, frequently observed in cancer cells, is linked to EMT and metastasis formation. In flotillin-overexpressing non-tumoral mammary epithelial cells and in invasive breast carcinoma cells, we found that the UFIT pathway-mediated AXL endocytosis allows its stabilization and depends on sphingosine kinase 2, a lipid kinase recruited in flotillin-rich plasma membrane domains and endosomes. Thus, the deregulation of vesicular trafficking following flotillin upregulation, and through sphingosine kinase 2, emerges as a new mechanism of AXL overexpression and EMT-inducing signaling pathway activation.

Seriniquinones as Therapeutic Leads for Treatment of BRAF and NRAS Mutant Melanomas

Isolated from the marine bacteria Serinicoccus sp., seriniquinone (SQ1) has been characterized by its selective activity in melanoma cell lines marked by its modulation of human dermcidin and induction of autophagy and apoptosis. While an active lead, the lack of solubility of SQ1 in both organic and aqueous media has complicated its preclinical evaluation. In response, our team turned its effort to explore analogues with the goal of returning synthetically accessible materials with comparable selectivity and activity. The analogue SQ2 showed improved solubility and reached a 30-40-fold greater selectivity for melanoma cells. Here, we report a detailed comparison of the activity of SQ1 and SQ2 in SK-MEL-28 and SK-MEL-147 cell lines, carrying the top melanoma-associated mutations, BRAFV600E and NRASQ61R, respectively. These studies provide a definitive report on the activity, viability, clonogenicity, dermcidin expression, autophagy, and apoptosis induction following exposure to SQ1 or SQ2. Overall, these studies showed that SQ1 and SQ2 demonstrated comparable activity and modulation of dermcidin expression. These studies are further supported through the evaluation of a panel of basal expression of key-genes related to autophagy and apoptosis, providing further insight into the role of these mutations. To explore this rather as a survival or death mechanism, autophagy inhibition sensibilized BRAF mutants to SQ1 and SQ2, whereas the opposite happened to NRAS mutants. These data suggest that the seriniquinones remain active, independently of the melanoma mutation, and suggest the future combination of their application with inhibitors of autophagy to treat BRAF-mutated tumors.

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.

Post-translational modification of RAS proteins

Mutations of RAS genes drive cancer more frequently than any other oncogene. RAS proteins integrate signals from a wide array of receptors and initiate downstream signaling through pathways that control cellular growth. RAS proteins are fundamentally binary molecular switches in which the off/on state is determined by the binding of GDP or GTP, respectively. As such, the intrinsic and regulated nucleotide-binding and hydrolytic properties of the RAS GTPase were historically believed to account for the entirety of the regulation of RAS signaling. However, it is increasingly clear that RAS proteins are also regulated by a vast array of post-translational modifications (PTMs). The current challenge is to understand what are the functional consequences of these modifications and which are physiologically relevant. Because PTMs are catalyzed by enzymes that may offer targets for drug discovery, the study of RAS PTMs has been a high priority for RAS biologists.

Ras Isoforms from Lab Benches to Lives-What Are We Missing and How Far Are We?

The central protein in the oncogenic circuitry is the Ras GTPase that has been under intense scrutiny for the last four decades. From its discovery as a viral oncogene and its non-oncogenic contribution to crucial cellular functioning, an elaborate genetic, structural, and functional map of Ras is being created for its therapeutic targeting. Despite decades of research, there still exist lacunae in our understanding of Ras. The complexity of the Ras functioning is further exemplified by the fact that the three canonical Ras genes encode for four protein isoforms (H-Ras, K-Ras4A, K-Ras4B, and N-Ras). Contrary to the initial assessment that the H-, K-, and N-Ras isoforms are functionally similar, emerging data are uncovering crucial differences between them. These Ras isoforms exhibit not only cell-type and context-dependent functions but also activator and effector specificities on activation by the same receptor. Preferential localization of H-, K-, and N-Ras in different microdomains of the plasma membrane and cellular organelles like Golgi, endoplasmic reticulum, mitochondria, and endosome adds a new dimension to isoform-specific signaling and diverse functions. Herein, we review isoform-specific properties of Ras GTPase and highlight the importance of considering these towards generating effective isoform-specific therapies in the future.

Blocking K-Ras Interaction With the Plasma Membrane Is a Tractable Therapeutic Approach to Inhibit Oncogenic K-Ras Activity

Ras proteins are membrane-bound small GTPases that promote cell proliferation, differentiation, and apoptosis. Consistent with this key regulatory role, activating mutations of Ras are present in ∼19% of new cancer cases in the United States per year. K-Ras is one of the three ubiquitously expressed isoforms in mammalian cells, and oncogenic mutations in this isoform account for ∼75% of Ras-driven cancers. Therefore, pharmacological agents that block oncogenic K-Ras activity would have great clinical utility. Most efforts to block oncogenic Ras activity have focused on Ras downstream effectors, but these inhibitors only show limited clinical benefits in Ras-driven cancers due to the highly divergent signals arising from Ras activation. Currently, four major approaches are being extensively studied to target K-Ras–driven cancers. One strategy is to block K-Ras binding to the plasma membrane (PM) since K-Ras requires the PM binding for its signal transduction. Here, we summarize recently identified molecular mechanisms that regulate K-Ras–PM interaction. Perturbing these mechanisms using pharmacological agents blocks K-Ras–PM binding and inhibits K-Ras signaling and growth of K-Ras–driven cancer cells. Together, these studies propose that blocking K-Ras–PM binding is a tractable strategy for developing anti–K-Ras therapies.

Axon guidance receptors: Endocytosis, trafficking and downstream signaling from endosomes

During the development of the nervous system, axons extend through complex environments. Growth cones at the axon tip allow axons to find and innervate their appropriate targets and form functional synapses. Axon pathfinding requires axons to respond to guidance signals and these cues need to be detected by specialized receptors followed by intracellular signal integration and translation. Several downstream signaling pathways have been identified for axon guidance receptors and it has become evident that these pathways are often initiated from intracellular vesicles called endosomes. Endosomes allow receptors to traffic intracellularly, re-locating receptors from one cellular region to another. The localization of axon guidance receptors to endosomal compartments is crucial for their function, signaling output and expression levels. For example, active receptors within endosomes can recruit downstream proteins to the endosomal membrane and facilitate signaling. Also, endosomal trafficking can re-locate receptors back to the plasma membrane to allow re-activation or mediate downregulation of receptor signaling via degradation. Accumulating evidence suggests that axon guidance receptors do not follow a pre-set default trafficking route but may change their localization within endosomes. This re-routing appears to be spatially and temporally regulated, either by expression of adaptor proteins or co-receptors. These findings shed light on how signaling in axon guidance is regulated and diversified - a mechanism which explains how a limited set of guidance cues can help to establish billions of neuronal connections. In this review, we summarize and discuss our current knowledge of axon guidance receptor trafficking and provide directions for future research.

Targeted Degradation of Oncogenic KRASG12C by VHL-Recruiting PROTACs

KRAS is mutated in ∼20% of human cancers and is one of the most sought-after targets for pharmacological modulation, despite having historically been considered "undruggable." The discovery of potent covalent inhibitors of the KRAS^G12C mutant in recent years has sparked a new wave of interest in small molecules targeting KRAS. While these inhibitors have shown promise in the clinic, we wanted to explore PROTAC-mediated degradation as a complementary strategy to modulate mutant KRAS. Herein, we report the development of LC-2, the first PROTAC capable of degrading endogenous KRAS^G12C. LC-2 covalently binds KRAS^G12C with a MRTX849 warhead and recruits the E3 ligase VHL, inducing rapid and sustained KRAS^G12C degradation leading to suppression of MAPK signaling in both homozygous and heterozygous KRAS^G12C cell lines. LC-2 demonstrates that PROTAC-mediated degradation is a viable option for attenuating oncogenic KRAS levels and downstream signaling in cancer cells.

FCHSD2 controls oncogenic ERK1/2 signaling outcome by regulating endocytic trafficking

The evolution of transformed cancer cells into metastatic tumors is, in part, driven by altered intracellular signaling downstream of receptor tyrosine kinases (RTKs). The surface levels and activity of RTKs are governed mainly through clathrin-mediated endocytosis (CME), endosomal recycling, or degradation. In turn, oncogenic signaling downstream of RTKs can reciprocally regulate endocytic trafficking by creating feedback loops in cells to enhance tumor progression. We previously showed that FCH/F-BAR and Double SH3 Domain-Containing Protein (FCHSD2) has a cancer-cell specific function in regulating CME in non-small-cell lung cancer (NSCLC) cells. Here, we report that FCHSD2 loss impacts recycling of the RTKs, epidermal growth factor receptor (EGFR) and proto-oncogene c-Met (MET), and shunts their trafficking into late endosomes and lysosomal degradation. Notably, FCHSD2 depletion results in the nuclear translocation of active extracellular signal-regulated kinase 1 and 2 (ERK1/2), leading to enhanced transcription and up-regulation of EGFR and MET. The small GTPase, Ras-related protein Rab-7A (Rab7), is essential for the FCHSD2 depletion-induced effects. Correspondingly, FCHSD2 loss correlates to higher tumor grades of NSCLC. Clinically, NSCLC patients expressing high FCHSD2 exhibit elevated survival, whereas patients with high Rab7 expression display decreased survival rates. Our study provides new insight into the molecular nexus for crosstalk between oncogenic signaling and RTK trafficking that controls cancer progression.

Avicin G is a potent sphingomyelinase inhibitor and blocks oncogenic K- and H-Ras signaling

K-Ras must interact primarily with the plasma membrane (PM) for its biological activity. Therefore, disrupting K-Ras PM interaction is a tractable approach to block oncogenic K-Ras activity. Here, we found that avicin G, a family of natural plant-derived triterpenoid saponins from Acacia victoriae, mislocalizes K-Ras from the PM and disrupts PM spatial organization of oncogenic K-Ras and H-Ras by depleting phosphatidylserine (PtdSer) and cholesterol contents, respectively,  at the inner PM leaflet. Avicin G also inhibits oncogenic K- and H-Ras signal output and the growth of K-Ras-addicted pancreatic and non-small cell lung cancer cells. We further identified that avicin G perturbs lysosomal activity, and disrupts cellular localization and activity of neutral and acid sphingomyelinases (SMases), resulting in elevated cellular sphingomyelin (SM) levels and altered SM distribution. Moreover, we show that neutral SMase inhibitors disrupt the PM localization of K-Ras and PtdSer and oncogenic K-Ras signaling. In sum, this study identifies avicin G as a new potent anti-Ras inhibitor, and suggests that neutral SMase can be a tractable target for developing anti-K-Ras therapeutics.

Pleiotropic Roles of Calmodulin in the Regulation of KRas and Rac1 GTPases: Functional Diversity in Health and Disease

Calmodulin is a ubiquitous signalling protein that controls many biological processes due to its capacity to interact and/or regulate a large number of cellular proteins and pathways, mostly in a Ca²⁺-dependent manner. This complex interactome of calmodulin can have pleiotropic molecular consequences, which over the years has made it often difficult to clearly define the contribution of calmodulin in the signal output of specific pathways and overall biological response. Most relevant for this review, the ability of calmodulin to influence the spatiotemporal signalling of several small GTPases, in particular KRas and Rac1, can modulate fundamental biological outcomes such as proliferation and migration. First, direct interaction of calmodulin with these GTPases can alter their subcellular localization and activation state, induce post-translational modifications as well as their ability to interact with effectors. Second, through interaction with a set of calmodulin binding proteins (CaMBPs), calmodulin can control the capacity of several guanine nucleotide exchange factors (GEFs) to promote the switch of inactive KRas and Rac1 to an active conformation. Moreover, Rac1 is also an effector of KRas and both proteins are interconnected as highlighted by the requirement for Rac1 activation in KRas-driven tumourigenesis. In this review, we attempt to summarize the multiple layers how calmodulin can regulate KRas and Rac1 GTPases in a variety of cellular events, with biological consequences and potential for therapeutic opportunities in disease settings, such as cancer.

Galectin-8 binds to the Farnesylated C-terminus of K-Ras4B and Modifies Ras/ERK Signaling and Migration in Pancreatic and Lung Carcinoma Cells

K-Ras is the most prominent driver of oncogenesis and no effective K-Ras inhibitors have been established despite decades of intensive research. Identifying new K-Ras-binding proteins and their interaction domains offers the opportunity for defining new approaches in tackling oncogenic K-Ras. We have identified Galectin-8 as a novel, direct binding protein for K-Ras4B by mass spectrometry analyses and protein interaction studies. Galectin-8 is a tandem-repeat Galectin and it is widely expressed in lung and pancreatic carcinoma cells. siRNA-mediated depletion of Galectin-8 resulted in increased K-Ras4B content and ERK1/2 activity in lung and pancreatic carcinoma cells. Moreover, cell migration and cell proliferation were inhibited by the depletion of Galectin-8. The K-Ras4B–Galectin-8 interaction is indispensably associated with the farnesylation of K-Ras4B. The lysine-rich polybasic domain (PBD), a region that is unique for K-Ras4B as compared to H- and N-Ras, stabilizes the interaction and accounts for the specificity. Binding assays with the deletion mutants of Galectin-8, comprising either of the two carbohydrate recognition domains (CRD), revealed that K-Ras4B only interacts with the N-CRD, but not with the C-CRD. Structural modeling uncovers a potential binding pocket for the hydrophobic farnesyl chain of K-Ras4B and a cluster of negatively charged amino acids for interaction with the positively charged lysine residues in the N-CRD. Our results demonstrate that Galectin-8 is a new binding partner for K-Ras4B and it interacts via the N-CRD with the farnesylated PBD of K-Ras, thereby modulating the K-Ras effector pathways as well as cell proliferation and migration.

Regulation of large and small G proteins by ubiquitination

Many sensory and chemical signal inputs are transmitted by intracellular GTP-binding (G) proteins. G proteins make up two major subfamilies: "large" G proteins comprising three subunits and "small" G proteins, such as the proto-oncogene product RAS, which contains a single subunit. Members of both subfamilies are regulated by post-translational modifications, including lipidation, proteolysis, and carboxyl methylation. Emerging studies have shown that these proteins are also modified by ubiquitination. Much of our current understanding of this post-translational modification comes from investigations of the large G-protein α subunit from yeast (Gpa1) and the three RAS isotypes in humans, NRAS, KRAS, and HRAS. Gα undergoes both mono- and polyubiquitination, and these modifications have distinct consequences for determining the sites and mechanisms of its degradation. Genetic and biochemical reconstitution studies have revealed the enzymes and binding partners required for addition and removal of ubiquitin, as well as the delivery and destruction of both the mono- and polyubiquitinated forms of the G protein. Complementary studies of RAS have identified multiple ubiquitination sites, each having distinct consequences for binding to regulatory proteins, shuttling to and from the plasma membrane, and degradation. Here, we review what is currently known about these two well-studied examples, Gpa1 and the human RAS proteins, that have revealed additional mechanisms of signal regulation and dysregulation relevant to human physiology. We also compare and contrast the effects of G-protein ubiquitination with other post-translational modifications of these proteins.

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.

Verteporfin-induced lysosomal compartment dysregulation potentiates the effect of sorafenib in hepatocellular carcinoma

Lysosomal sequestration of anti-cancer compounds reduces drug availability at intracellular target sites, thereby limiting drug-sensitivity and inducing chemoresistance. For hepatocellular carcinoma (HCC), sorafenib (SF) is the first line systemic treatment, as well as a simultaneous activator of autophagy-induced drug resistance. The purpose of this study is to elucidate how combination therapy with the FDA-approved photosensitizer verteporfin (VP) can potentiate the antitumor effect of SF, overcoming its acquired resistance mechanisms. HCC cell lines and patient-derived in vitro and in vivo preclinical models were used to identify the molecular mechanism of action of VP alone and in combination with SF. We demonstrate that SF is lysosomotropic and increases the total number of lysosomes in HCC cells and patient-derived xenograft model. Contrary to the effect on lysosomal stability by SF, VP is not only sequestered in lysosomes, but induces lysosomal pH alkalinization, lysosomal membrane permeabilization (LMP) and tumor-selective proteotoxicity. In combination, VP-induced LMP potentiates the antitumor effect of SF, further decreasing tumor proliferation and progression in HCC cell lines and patient-derived samples in vitro and in vivo. Our data suggest that combination of lysosome-targeting compounds, such as VP, in combination with already approved chemotherapeutic agents could open a new avenue to overcome chemo-insensitivity caused by passive lysosomal sequestration of anti-cancer drugs in the context of HCC.

A split fluorescent reporter with rapid and reversible complementation

Interactions between proteins play an essential role in metabolic and signaling pathways, cellular processes and organismal systems. We report the development of splitFAST, a fluorescence complementation system for the visualization of transient protein-protein interactions in living cells. Engineered from the fluorogenic reporter FAST (Fluorescence-Activating and absorption-Shifting Tag), which specifically and reversibly binds fluorogenic hydroxybenzylidene rhodanine (HBR) analogs, splitFAST displays rapid and reversible complementation, allowing the real-time visualization of both the formation and the dissociation of a protein assembly.

Induction of N-Ras degradation by flunarizine-mediated autophagy

Ras GTPases are powerful drivers for tumorigenesis, but directly targeting Ras for treating cancer remains challenging. The growth and transforming activity of the aggressive basal-like breast cancer (BLBC) are driven by N-Ras. To target N-Ras in BLBC, this study screened existing pharmacologically active compounds for the new ability to induce N-Ras degradation, which led to the identification of flunarizine (FLN), previously approved for treating migraine and epilepsy. The FLN-induced N-Ras degradation was not affected by a 26S-proteasome inhibitor. Rather, it was blocked by autophagy inhibitors. Furthermore, N-Ras can be seen co-localized with active autophagosomes upon FLN treatment, suggesting that FLN alters the autophagy pathway to degrade N-Ras. Importantly, FLN treatment recapitulated the effect of N-RAS silencing in vitro by selectively inhibiting the growth of BLBC cells, but not that of breast cancer cells of other subtypes. In addition, in vivo FLN inhibited tumor growth of a BLBC xenograft model. In conclusion, this proof-of-principle study presents evidence that the autophagy pathway can be coerced by small molecule inhibitors, such as FLN, to degrade Ras as a strategy to treat cancer. FLN has low toxicity and should be further investigated to enrich the toolbox of cancer therapeutics.

Analysis of K-Ras Interactions by Biotin Ligase Tagging

BACKGROUND

Mutations of the human K-Ras 4B (K-Ras) G protein are associated with a significant proportion of all human cancers. Despite this fact, a comprehensive analysis of K-Ras interactions is lacking. Our investigations focus on characterization of the K-Ras interaction network.

MATERIALS AND METHODS

We employed a biotin ligase-tagging approach, in which tagged K-Ras proteins biotinylate neighbor proteins in a proximity-dependent fashion, and proteins are identified via mass spectrometry (MS) sequencing.

RESULTS

In transfected cells, a total of 748 biotinylated proteins were identified from cells expressing biotin ligase-tagged K-Ras variants. Significant differences were observed between membrane-associated variants and a farnesylation-defective mutant. In pancreatic cancer cells, 56 K-Ras interaction partners were identified. Most of these were cytoskeletal or plasma membrane proteins, and many have been identified previously as potential cancer biomarkers.

CONCLUSION

Biotin ligase tagging offers a rapid and convenient approach to the characterization of K-Ras interaction networks.

The Yeast Saccharomyces cerevisiae as a Model for Understanding RAS Proteins and their Role in Human Tumorigenesis

The exploitation of the yeast Saccharomyces cerevisiae as a biological model for the investigation of complex molecular processes conserved in multicellular organisms, such as humans, has allowed fundamental biological discoveries. When comparing yeast and human proteins, it is clear that both amino acid sequences and protein functions are often very well conserved. One example of the high degree of conservation between human and yeast proteins is highlighted by the members of the RAS family. Indeed, the study of the signaling pathways regulated by RAS in yeast cells led to the discovery of properties that were often found interchangeable with RAS proto-oncogenes in human pathways, and vice versa. In this work, we performed an updated critical literature review on human and yeast RAS pathways, specifically highlighting the similarities and differences between them. Moreover, we emphasized the contribution of studying yeast RAS pathways for the understanding of human RAS and how this model organism can contribute to unveil the roles of RAS oncoproteins in the regulation of mechanisms important in the tumorigenic process, like autophagy.

Characterisation of HRas local signal transduction networks using engineered site-specific exchange factors

Ras GTPases convey signals from different types of membranes. At these locations, different Ras isoforms, interactors and regulators generate different biochemical signals and biological outputs. The study of Ras localisation-specific signal transduction networks has been hampered by our inability to specifically activate each of these Ras pools. Here, we describe a new set of site-specific tethered exchange factors, engineered by fusing the RasGRF1 CDC25 domain to sub-localisation-defining cues, whereby Ras pools at specific locations can be precisely activated. We show that the CDC25 domain has a high specificity for activating HRas but not NRas and KRas. This unexpected finding means that our constructs mainly activate endogenous HRas. Hence, their use enabled us to identify distinct pathways regulated by HRas in endomembranes and plasma membrane microdomains. Importantly, these new constructs unveil different patterns of HRas activity specified by their subcellular localisation. Overall, the targeted GEFs described herein constitute ideal tools for dissecting spatially-defined HRas biochemical and biological functions.

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.

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.

Modulation of RAB5A early endosome trafficking in response to KRas mediated macropinocytic fluxes in pancreatic cancer cells

KRAS is the key mutated gene in pancreatic ductal adenocarcinoma (PDAC). Emerging evidence indicates that KRas modulates endocytic uptake. The present study aimed to explore the fate of early endosomal trafficking under the control of KRas expression in PDAC. Surprisingly, PANC-1 cells lacking KRas exhibited significantly enlarged early and late endosomes containing internalized dextran and epidermal growth factor. Endosome enlargement was accompanied by reduced endosomal degradation. Both KRas silencing and lysosomal blockade caused an upregulation of the master regulator of early endosome biogenesis, RAB5A, which is likely responsible for the expansion of the early endosomal compartment, because simultaneous KRAS/RAB5A knockdown abolished endosome enlargement. In contrast, early endosome shrinkage was seen in MIA PaCa-2 cells despite RAB5A upregulation, indicating that distinct KRas-modulated responses operate in different metabolic subtypes of PDAC. In conclusion, mutant KRAS promotes endosomal degradation in PDAC cell lines, which is impaired by KRAS silencing. Moreover, KRAS silencing activates RAB5A upregulation and drives PDAC subtype-dependent modulation of endosome trafficking.

The G protein-coupled receptor GPR31 promotes membrane association of KRAS

Mutant KRAS drives oncogenesis when associated with the plasma membrane. Fehrenbacher et al. identify GPR31, a G protein–coupled receptor, as a secretory pathway chaperone that guides the KRAS protein to the plasma membrane.

The product of the KRAS oncogene, KRAS4B, promotes tumor growth when associated with the plasma membrane (PM). PM association is mediated, in part, by farnesylation of KRAS4B, but trafficking of nascent KRAS4B to the PM is incompletely understood. We performed a genome-wide screen to identify genes required for KRAS4B membrane association and identified a G protein–coupled receptor, GPR31. GPR31 associated with KRAS4B on cellular membranes in a farnesylation-dependent fashion, and retention of GPR31 on the endoplasmic reticulum inhibited delivery of KRAS4B to the PM. Silencing of GPR31 expression partially mislocalized KRAS4B, slowed the growth of KRAS-dependent tumor cells, and blocked KRAS-stimulated macropinocytosis. Our data suggest that GPR31 acts as a secretory pathway chaperone for KRAS4B.

Tamoxifen Induces Cytotoxic Autophagy in Glioblastoma

Glioblastomas (GBMs) are the most common and aggressive primary human malignant brain tumors. 4-Hydroxy tamoxifen (OHT) is an active metabolite of the tamoxifen (TMX) prodrug and a well-established estrogen receptor (ER) and estrogen-related receptor antagonist. A recent study from our laboratory demonstrated that OHT induced ER-independent malignant peripheral nerve sheath tumor (MPNST) cell death by autophagic degradation of the prosurvival protein Kirsten rat sarcoma viral oncogene homolog. Because both MPNST and GBM are glial in cell origin, we hypothesized that OHT could mediate similar effects in GBM. OHT induced a concentration-dependent reduction in cell viability that was largely independent of caspase activation in a human GBM cell line and 2 patient-derived xenolines. Further, OHT induced both cytotoxic autophagy and a concentration-dependent decrease in epidermal growth factor receptor (EGFR) protein levels. A GBM cell line expressing EGFR variant III (EGFRvIII) was relatively resistant to OHT-induced death and EGFRvIII was refractory to OHT-induced degradation. Thus, OHT induces GBM cell death through a caspase-independent, autophagy-related mechanism and should be considered as a potential therapeutic agent in patients with GBM whose tumors express wild-type EGFR.

KRAS-MEK Signaling Controls Ago2 Sorting into Exosomes

Secretion of RNAs in extracellular vesicles is a newly recognized form of intercellular communication. A potential regulatory protein for microRNA (miRNA) secretion is the critical RNA-induced silencing complex (RISC) component Argonaute 2 (Ago2). Here, we use isogenic colon cancer cell lines to show that overactivity of KRAS due to mutation inhibits localization of Ago2 to multivesicular endosomes (MVEs) and decreases Ago2 secretion in exosomes. Mechanistically, inhibition of mitogen-activated protein kinase kinases (MEKs) I and II, but not Akt, reverses the effect of the activating KRAS mutation and leads to increased Ago2-MVE association and increased exosomal secretion of Ago2. Analysis of cells expressing mutant Ago2 constructs revealed that phosphorylation of Ago2 on serine 387 prevents Ago2-MVE interactions and reduces Ago2 secretion into exosomes. Furthermore, regulation of Ago2 exosomal sorting controls the levels of three candidate miRNAs in exosomes. These data identify a key regulatory signaling event that controls Ago2 secretion in exosomes.

Signal processing by the endosomal system

Cells need to decode chemical or physical signals from their environment in order to make decisions on their fate. In the case of signalling receptors, ligand binding triggers a cascade of chemical reactions but also the internalization of the activated receptors in the endocytic pathway. Here, we highlight recent studies revealing a new role of the endosomal network in signal processing. The diversity of entry pathways and endosomal compartments is exploited to regulate the kinetics of receptor trafficking, and interactions with specific signalling adaptors and effectors. By governing the spatio-temporal distribution of signalling molecules, the endosomal system functions analogously to a digital-analogue computer that regulates the specificity and robustness of the signalling response.

Endocytosis separates EGF receptors from endogenous fluorescently labeled HRas and diminishes receptor signaling to MAP kinases in endosomes

Signaling from epidermal growth factor receptor (EGFR) to extracellular-stimuli-regulated protein kinase 1/2 (ERK1/2) is proposed to be transduced not only from the cell surface but also from endosomes, although the role of endocytosis in this signaling pathway is controversial. Ras is the only membrane-anchored component in the EGFR-ERK signaling axis, and therefore, its location determines intracellular sites of downstream signaling. Hence, we labeled endogenous H-Ras (HRas) with mVenus fluorescent protein using gene editing in HeLa cells. mVenus-HRas was primarily located at the plasma membrane, and in small amounts in tubular recycling endosomes and associated vesicles. EGF stimulation resulted in fast but transient activation of mVenus-HRas. Although EGF:EGFR complexes were rapidly accumulated in endosomes together with the Grb2 adaptor, very little, if any, mVenus-HRas was detected in these endosomes. Interestingly, the activities of MEK1/2 and ERK1/2 remained high beyond the point of the physical separation of HRas from EGF:EGFR complexes and down-regulation of Ras activity. Paradoxically, this sustained MEK1/2 and ERK1/2 activation was dependent on the active EGFR kinase. Cell surface biotinylation and selective inactivation of surface EGFRs suggested that a small fraction of active EGFRs remaining in the plasma membrane is responsible for continuous signaling to MEK1/2 and ERK1/2. We propose that, under physiological conditions of cell stimulation, EGFR endocytosis serves to spatially separate EGFR-Grb2 complexes and Ras, thus terminating Ras-mediated signaling. However, sustained minimal activation of Ras by a small pool of active EGFRs in the plasma membrane is sufficient for extending MEK1/2 and ERK1/2 activities.

R-Ras protein inhibits autophosphorylation of vascular endothelial growth factor receptor 2 in endothelial cells and suppresses receptor activation in tumor vasculature

Abnormal angiogenesis is associated with a broad range of medical conditions, including cancer. The formation of neovasculature with functionally defective blood vessels significantly impacts tumor progression, metastasis, and the efficacy of anticancer therapies. Vascular endothelial growth factor (VEGF) potently induces vascular permeability and vessel growth in the tumor microenvironment, and its inhibition normalizes tumor vasculature. In contrast, the signaling of the small GTPase R-Ras inhibits excessive angiogenic growth and promotes the maturation of regenerating blood vessels. R-Ras signaling counteracts VEGF-induced vessel sprouting, permeability, and invasive activities of endothelial cells. In this study, we investigated the effect of R-Ras on VEGF receptor 2 (VEGFR2) activation by VEGF, the key mechanism for angiogenic stimulation. We show that tyrosine phosphorylation of VEGFR2 is significantly elevated in the tumor vasculature and dermal microvessels of VEGF-injected skin in R-Ras knockout mice. In cultured endothelial cells, R-Ras suppressed the internalization of VEGFR2, which is required for full activation of the receptor by VEGF. Consequently, R-Ras strongly suppressed autophosphorylation of the receptor at all five major tyrosine phosphorylation sites. Conversely, silencing of R-Ras resulted in increased VEGFR2 phosphorylation. This effect of R-Ras on VEGFR2 was, at least in part, dependent on vascular endothelial cadherin. These findings identify a novel function of R-Ras to control the response of endothelial cells to VEGF and suggest an underlying mechanism by which R-Ras regulates angiogenesis.

Transport through recycling endosomes requires EHD1 recruitment by a phosphatidylserine translocase

P4-ATPases translocate aminophospholipids, such as phosphatidylserine (PS), to the cytosolic leaflet of membranes. PS is highly enriched in recycling endosomes (REs) and is essential for endosomal membrane traffic. Here, we show that PS flipping by an RE-localized P4-ATPase is required for the recruitment of the membrane fission protein EHD1. Depletion of ATP8A1 impaired the asymmetric transbilayer distribution of PS in REs, dissociated EHD1 from REs, and generated aberrant endosomal tubules that appear resistant to fission. EHD1 did not show membrane localization in cells defective in PS synthesis. ATP8A2, a tissue-specific ATP8A1 paralogue, is associated with a neurodegenerative disease (CAMRQ). ATP8A2, but not the disease-causative ATP8A2 mutant, rescued the endosomal defects in ATP8A1-depleted cells. Primary neurons from Atp8a2-/- mice showed a reduced level of transferrin receptors at the cell surface compared to Atp8a2+/+ mice. These findings demonstrate the role of P4-ATPase in membrane fission and give insight into the molecular basis of CAMRQ.

KRAS protein stability is regulated through SMURF2: UBCH5 complex-mediated β-TrCP1 degradation

Attempts to target mutant KRAS have been unsuccessful. Here, we report the identification of Smad ubiquitination regulatory factor 2 (SMURF2) and UBCH5 as a critical E3:E2 complex maintaining KRAS protein stability. Loss of SMURF2 either by small interfering RNA/short hairpin RNA (siRNA/shRNA) or by overexpression of a catalytically inactive mutant causes KRAS degradation, whereas overexpression of wild-type SMURF2 enhances KRAS stability. Importantly, mutant KRAS is more susceptible to SMURF2 loss where protein half-life decreases from >12 hours in control siRNA-treated cells to <3 hours on Smurf2 silencing, whereas only marginal differences were noted for wild-type protein. This loss of mutant KRAS could be rescued by overexpressing a siRNA-resistant wild-type SMURF2. Our data further show that SMURF2 monoubiquitinates UBCH5 at lysine 144 to form an active complex required for efficient degradation of a RAS-family E3, β-transducing repeat containing protein 1 (β-TrCP1). Conversely, β-TrCP1 is accumulated on SMURF2 loss, leading to increased KRAS degradation. Therefore, as expected, β-TrCP1 knockdown following Smurf2 siRNA treatment rescues mutant KRAS loss. Further, we identify two conserved proline (P) residues in UBCH5 critical for SMURF2 interaction; mutation of either of these P to alanine also destabilizes KRAS. As a proof of principle, we demonstrate that Smurf2 silencing reduces the clonogenic survival in vitro and prolongs tumor latency in vivo in cancer cells including mutant KRAS-driven tumors. Taken together, we show that SMURF2:UBCH5 complex is critical in maintaining KRAS protein stability and propose that targeting such complex may be a unique strategy to degrade mutant KRAS to kill cancer cells.

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.

CaM interaction and Ser181 phosphorylation as new K-Ras signaling modulators

The small G-protein Ras was the first oncogene to be identified and has a very important contribution to human cancer development (20-23% prevalence). K-RasB, one of the members of the Ras family, is the one that is most mutated and plays a prominent role in pancreatic, colon and lung cancer development. Ras proteins are membrane bound GTPases that cycle between inactive, GDP-bound and active, GTP-bound, states. Most of the research into K-RasB activity regulation has focused on the analysis of how GTP-exchange factors (GEFs) and GTPase activating proteins (GAPs) are regulated by external and internal signals. In contrast, oncogenic K-RasB has a very low GTPase activity and furthermore is not deactivated by GAPs. Consequently, the consensus was that activity of oncogenic K-RasB was not modulated. In this extra view we recapitulate some recent data showing that calmodulin binding to K-RasB inhibits phosphorylation of K-RasB at Ser181, near to the membrane anchoring domain, modulating signaling of both non-oncogenic and oncogenic K-RasB. This may be relevant to normal cell physiology, but also opens new therapeutic perspectives for the inhibition of oncogenic K-RasB signaling in tumors.

Ras history: The saga continues

Although the roots of Ras sprouted from the rich history of retrovirus research, it was the discovery of mutationally activated RAS genes in human cancer in 1982 that stimulated an intensive research effort to understand Ras protein structure, biochemistry and biology. While the ultimate goal has been developing anti-Ras drugs for cancer treatment, discoveries from Ras have laid the foundation for three broad areas of science. First, they focused studies on the origins of cancer to the molecular level, with the subsequent discovery of genes mutated in cancer that now number in the thousands. Second, elucidation of the biochemical mechanisms by which Ras facilitates signal transduction established many of our fundamental concepts of how a normal cell orchestrates responses to extracellular cues. Third, Ras proteins are also founding members of a large superfamily of small GTPases that regulate all key cellular processes and established the versatile role of small GTP-binding proteins in biology. We highlight some of the key findings of the last 28 years.

Dynamics of KRas on endosomes: involvement of acidic phospholipids in its association

The endocytic compartment is emerging as a functional platform for controlling important cellular processes. We have found that ∼10 to 15% of total KRas, a protein that is frequently mutated in cancer, is present on endosomes, independent of its activation state. The dynamics of GFP-KRas wild-type (WT) and constitutively active or inactive mutants on endosomes were analyzed by fluorescence recovery after photobleaching (FRAP) microscopy. The measurements revealed an extraordinarily fast recovery of KRas WT [half-time (HT), ∼1.3 s] compared to HRas, Rab5, and EGFR, with the active KRasG12V mutant being significantly faster and more mobile (HT, ∼1 s, and ∼82% of exchangeable fraction) than the inactive KRasS17N (HT, ∼1.6 s, and ∼60% of exchangeable fraction). KRas rapidly switches from the cytoplasm to the endosomal membranes by an electrostatic interaction between its polybasic region and the endosomal acidic phospholipids, mainly phosphatidylserine.-Gelabert-Baldrich, M., Soriano-Castell, D., Calvo, M., Lu, A., Viña-Vilaseca, A., Rentero, C., Pol, A., Grinstein, S. Enrich, C., Tebar, F. Dynamics of KRas on endosomes: involvement of acidic phospholipids in its association.

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.