Prenylation-dependent association of Ki-Ras with microtubules. Evidence for a role in subcellular trafficking

Deciphering the role of KRAS gene in oncogenesis: Focus on signaling pathways, genetic alterations in 3'UTR, KRAS specific miRNAs and therapeutic interventions

Cancer is a significant cause of death after cardiovascular disease. The genomic, epigenetic and environmental factors have been found to be the risk factor for the disease. The most important genes that develop cancer are oncogenes and tumor suppressor genes. Among oncogenes, KRAS has emerged as a significant player in the development of many cancers. Dysregulation of the RAS signaling pathway either on account of mutation in significant genes involved in the pathway or aberrant expression of different miRNAs targeting these genes including KRAS. The focus is also on the alterations in 3'UTR of the KRAS gene sequence as well as the changes in the miRNA encoding genes especially the one targeting the KRAS gene. Efforts are also being put in to target the dysregulated KRAS gene as a therapeutic approach to treat different cancers. However, there are some challenges like resistance to KRAS inhibitors that need to be addressed.

Oncogenic KRAS: Signaling and Drug Resistance

RAS proteins play a role in many physiological signals transduction processes, including cell growth, division, and survival. The Ras protein has amino acids 188-189 and functions as GTPase. These proteins are switch molecules that cycle between inactive GDP-bound and active GTP-bound by guanine nucleotide exchange factors (GEFs). KRAS is one of the Ras superfamily isoforms (N-RAS, H-RAS, and K-RAS) that frequently mutate in cancer. The mutation of KRAS is essentially performing the transformation in humans. Since most RAS proteins belong to GTPase, mutated and GTP-bound active RAS is found in many cancers. Despite KRAS being an important molecule in mostly human cancer, including pancreatic and breast, numerous efforts in years past have persisted in cancer therapy targeting KRAS mutant. This review summarizes the biological characteristics of these proteins and the recent progress in the exploration of KRAS-targeted anticancer, leading to new insight.

Biology, pathology, and therapeutic targeting of RAS

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

Depletion of phosphatidylinositol 4-phosphate at the Golgi translocates K-Ras to mitochondria

Ras proteins are small GTPases localized to the plasma membrane (PM), which regulate cellular proliferation, apoptosis and differentiation. After a series of post-translational modifications, H-Ras and N-Ras traffic to the PM from the Golgi via the classical exocytic pathway, but the exact mechanism of K-Ras trafficking to the PM from the ER is not fully characterized. ATP5G1 (also known as ATP5MC1) is one of the three proteins that comprise subunit c of the F ₀ complex of the mitochondrial ATP synthase. In this study, we show that overexpression of the mitochondrial targeting sequence of ATP5G1 perturbs glucose metabolism, inhibits oncogenic K-Ras signaling, and redistributes phosphatidylserine (PtdSer) to mitochondria and other endomembranes, resulting in K-Ras translocation to mitochondria. Also, it depletes phosphatidylinositol 4-phosphate (PI4P) at the Golgi. Glucose supplementation restores PtdSer and K-Ras PM localization and PI4P at the Golgi. We further show that inhibition of the Golgi-localized PI4-kinases (PI4Ks) translocates K-Ras, and PtdSer to mitochondria and endomembranes, respectively. We conclude that PI4P at the Golgi regulates the PM localization of PtdSer and K-Ras.This article has an associated First Person interview with the first author of the paper.

A Direct Podocalyxin-Dynamin-2 Interaction Regulates Cytoskeletal Dynamics to Promote Migration and Metastasis in Pancreatic Cancer Cells

The sialoglycoprotein podocalyxin is absent in normal pancreas but is overexpressed in pancreatic cancer and is associated with poor clinical outcome. Here, we investigate the role of podocalyxin in migration and metastasis of pancreatic adenocarcinomas using SW1990 and Pa03c as cell models. Although ezrin is regarded as a cytoplasmic binding partner of podocalyxin that regulates actin polymerization via Rac1 or RhoA, we did not detect podocalyxin-ezrin association in pancreatic cancer cells. Moreover, depletion of podocalyxin did not alter actin dynamics or modulate Rac1 and RhoA activities in pancreatic cancer cells. Using mass spectrometry, bioinformatics analysis, coimmunoprecipitation, and pull-down assays, we discovered a novel, direct binding interaction between the cytoplasmic tail of podocalyxin and the large GTPase dynamin-2 at its GTPase, middle, and pleckstrin homology domains. This podocalyxin-dynamin-2 interaction regulated microtubule growth rate, which in turn modulated focal adhesion dynamics and ultimately promoted efficient pancreatic cancer cell migration via microtubule- and Src-dependent pathways. Depletion of podocalyxin in a hemispleen mouse model of pancreatic cancer diminished liver metastasis without altering primary tumor size. Collectively, these findings reveal a novel mechanism by which podocalyxin facilitates pancreatic cancer cell migration and metastasis. SIGNIFICANCE: These findings reveal that a novel interaction between podocalyxin and dynamin-2 promotes migration and metastasis of pancreatic cancer cells by regulating microtubule and focal adhesion dynamics.

Plinabulin, an inhibitor of tubulin polymerization, targets KRAS signaling through disruption of endosomal recycling

Constitutive activation of Kirsten rat sarcoma viral oncogene homolog (KRAS) is the most common oncogenic event in certain types of human cancer and is associated with poor patient survival. Small molecule signaling inhibitors have improved the clinical outcomes of patients with various cancer types but attempts to target KRAS have been unsuccessful. Plinabulin represents a novel class of agents that inhibit tubulin polymerization with a favorable safety profile in clinical trials. In the present study, the potency of plinabulin to inhibit tubulin polymerization and growth of KRAS-driven cancer cells was characterized. In vivo efficacy of plinabulin was tested in two different mouse models; one being the RCAS/t-va gene transfer system and the other being a xenograft model. In vitro cell culture tubulin polymerization assays were used to complement the mouse models. There was improved survival in a KRAS-driven mouse gene transfer glioma model, but lack of benefit in a similar model, without constitutively active KRAS, which supports the notion of a KRAS-specific effect. This survival benefit was mediated, at least in part, by the ability of plinabulin to inhibit tubulin polymerization and disrupt endosomal recycling. It was proposed a mechanism of compromised endosomal recycling of displaced KRAS through targeting microtubules that yields inhibition of protein kinase B, but not extracellular signal regulated kinase (ERK) signaling, therefore lending rationale to combination treatments of tubulin- and ERK-targeting agents in KRAS-driven cancer.

Perspectives of RAS and RHEB GTPase Signaling Pathways in Regenerating Brain Neurons

Cellular activation of RAS GTPases into the GTP-binding “ON” state is a key switch for regulating brain functions. Molecular protein structural elements of rat sarcoma (RAS) and RAS homolog protein enriched in brain (RHEB) GTPases involved in this switch are discussed including their subcellular membrane localization for triggering specific signaling pathways resulting in regulation of synaptic connectivity, axonal growth, differentiation, migration, cytoskeletal dynamics, neural protection, and apoptosis. A beneficial role of neuronal H-RAS activity is suggested from cellular and animal models of neurodegenerative diseases. Recent experiments on optogenetic regulation offer insights into the spatiotemporal aspects controlling RAS/mitogen activated protein kinase (MAPK) or phosphoinositide-3 kinase (PI3K) pathways. As optogenetic manipulation of cellular signaling in deep brain regions critically requires penetration of light through large distances of absorbing tissue, we discuss magnetic guidance of re-growing axons as a complementary approach. In Parkinson’s disease, dopaminergic neuronal cell bodies degenerate in the substantia nigra. Current human trials of stem cell-derived dopaminergic neurons must take into account the inability of neuronal axons navigating over a large distance from the grafted site into striatal target regions. Grafting dopaminergic precursor neurons directly into the degenerating substantia nigra is discussed as a novel concept aiming to guide axonal growth by activating GTPase signaling through protein-functionalized intracellular magnetic nanoparticles responding to external magnets.

KRAS-specific Amino Acid Substitutions are Associated With Different Responses to Chemotherapy in Advanced Non-small-cell Lung Cancer

BACKGROUND

Emerging data highlight different clinical behaviors according to KRAS amino acid substitutions (AASs) in patients with non-small-cell lung cancer (NSCLC). We aimed to evaluate whether different KRAS AASs were associated with different responses to chemotherapy.

PATIENTS AND METHODS

We retrospectively reviewed data from 1190 patients with KRAS mutations who underwent first-line platinum-based chemotherapy for stage IV NSCLC. The response to different chemotherapy regimens was evaluated using the Response Evaluation Criteria In Solid Tumors criteria (v 1.1). Overall survival and time to progression (TTP) were secondary endpoints.

RESULTS

Taxane was associated with the best response in the entire cohort (odds ratio, 2.52; 95% confidence interval [CI], 1.82-3.48; P < .001), especially in G12V patients (odds ratio, 2.15; 95% CI, 1.05-4.41; P = .036). Taxane was associated with improved TTP in the entire cohort (hazard ratio [HR], 0.31; 95% CI, 0.26-0.38; P < .001), especially in G13D patients (HR, 0.47; 95% CI, 0.22-1.01; P = .054). Pemetrexed was associated with the worst TTP in the entire cohort, particularly in G12V patients, who had the worst response rates (HR, 0.55; 95% CI, 0.30-0.99; P = .049). No impact on overall survival was observed according to different chemotherapy regimens and AASs.

CONCLUSION

KRAS-specific AAS appears to induce different responses to chemotherapy regimens after first-line platinum-based chemotherapy in advanced NSCLC.

Regulation of E-cadherin localization by microtubule targeting agents: rapid promotion of cortical E-cadherin through p130Cas/Src inhibition by eribulin

Microtubule targeting agents (MTAs) are some of the most effective anticancer drugs used to treat a wide variety of adult and pediatric cancers. Building evidence suggests that these drugs inhibit interphase signaling events and that this contributes to their anticancer actions. The effects of diverse MTAs were evaluated following a 2 hour incubation with clinically relevant concentrations to test the hypothesis that these drugs rapidly and differentially disrupt epithelial-to-mesenchymal transition (EMT)-related signaling. The MTAs rapidly promoted the cortical localization of internal pools of E-cadherin in HCC1937 breast cancer cells, with the most robust effects observed with the microtubule destabilizers eribulin and vinorelbine. Cortical E-cadherin localization was also promoted by the Src kinase inhibitor dasatinib or by siRNA-mediated depletion of the p130Cas scaffold. Mechanistic studies demonstrate that eribulin disrupts the interaction between p130Cas and Src, leading to decreased cortical Src phosphorylation that precedes the accumulation of cortical E-cadherin. These results suggest that microtubules can be actively co-opted by cancer cells to inhibit cortical E-cadherin localization, a hallmark of EMT, and provide a direct link between the initial disruption of the microtubule network and reversal of EMT phenotypes demonstrated by eribulin in long-term studies.

Efficient farnesylation of an extended C-terminal C(x)3X sequence motif expands the scope of the prenylated proteome

Protein prenylation is a post-translational modification that has been most commonly associated with enabling protein trafficking to and interaction with cellular membranes. In this process, an isoprenoid group is attached to a cysteine near the C terminus of a substrate protein by protein farnesyltransferase (FTase) or protein geranylgeranyltransferase type I or II (GGTase-I and GGTase-II). FTase and GGTase-I have long been proposed to specifically recognize a four-amino acid CAAX C-terminal sequence within their substrates. Surprisingly, genetic screening reveals that yeast FTase can modify sequences longer than the canonical CAAX sequence, specifically C(x)₃X sequences with four amino acids downstream of the cysteine. Biochemical and cell-based studies using both peptide and protein substrates reveal that mammalian FTase orthologs can also prenylate C(x)₃X sequences. As the search to identify physiologically relevant C(x)₃X proteins begins, this new prenylation motif nearly doubles the number of proteins within the yeast and human proteomes that can be explored as potential FTase substrates. This work expands our understanding of prenylation's impact within the proteome, establishes the biologically relevant reactivity possible with this new motif, and opens new frontiers in determining the impact of non-canonically prenylated proteins on cell function.

Gas-Phase Fragmentation Behavior of Oxidized Prenyl Peptides by CID and ETD Tandem Mass Spectrometry

Farnesylation and geranylgeranylation are the two types of prenyl modification of proteins. Prenylated peptides are highly hydrophobic and their abundances in biological samples are low. In this report, we studied the oxidized prenylated peptides by electrospray ionization mass spectrometry and identified them by collision-induced dissociation (CID) and electron-transfer dissociation (ETD) tandem mass spectrometry. Modified prenyl peptides were generated utilizing strong and low strength oxidizing agents to selectively oxidize and epoxidize cysteine sulfur and prenyl side chain. We selected three peptides with prenyl motifs and synthesized their prenylated versions. The detailed characteristic fragmentations of oxidized and epoxidized farnesylated and geranylgeranylated peptides were studied side by side with two popular fragmentation techniques. CID and ETD mass spectrometry clearly distinguished the modified version of these peptides. ETD mass spectrometry provided sequence information of the highly labile modified prenyl peptides and showed different characteristic fragmentations compared with CID. A detailed fragmentation of modified geranylgeranylated peptides was compared by CID and ETD mass spectrometry for the first time. Graphical Abstract ᅟ.

Cell cycle restriction is more important than apoptosis induction for RASSF1A protein tumor suppression

The Ras association domain family protein 1A (RASSF1A) is arguably one of the most frequently inactivated tumor suppressors in human cancer. RASSF1A modulates apoptosis via the Hippo and Bax pathways but also modulates the cell cycle. In part, cell cycle regulation appears to be dependent upon the ability of RASSF1A to complex with microtubules and regulate their dynamics. Which property of RASSF1A, apoptosis induction or microtubule regulation, is responsible for its tumor suppressor function is not known. We have identified a short conserved motif that is essential for the binding of RASSF family proteins with microtubule-associated proteins. By making a single point mutation in the motif, we were able to generate a RASSF1A variant that retains wild-type apoptotic properties but completely loses the ability to bind microtubule-associated proteins and complex with microtubules. Comparison of this mutant to wild-type RASSF1A showed that, despite retaining its proapoptotic properties, the mutant was completely unable to induce cell cycle arrest or suppress the tumorigenic phenotype. Therefore, it appears that the cell cycle/microtubule effects of RASSF1A are key to its tumor suppressor function rather than its apoptotic effects.

The chaperone protein SmgGDS interacts with small GTPases entering the prenylation pathway by recognizing the last amino acid in the CAAX motif

Ras family small GTPases localize at the plasma membrane, where they can activate oncogenic signaling pathways. Understanding the mechanisms that promote membrane localization of GTPases will aid development of new therapies to inhibit oncogenic signaling. We previously reported that SmgGDS splice variants promote prenylation and trafficking of GTPases containing a C-terminal polybasic region and demonstrated that SmgGDS-607 interacts with nonprenylated GTPases, whereas SmgGDS-558 interacts with prenylated GTPases in cells. The mechanism that SmgGDS-607 and SmgGDS-558 use to differentiate between prenylated and nonprenylated GTPases has not been characterized. Here, we provide evidence that SmgGDS-607 associates with GTPases through recognition of the last amino acid in the CAAX motif. We show that SmgGDS-607 forms more stable complexes in cells with nonprenylated GTPases that will become geranylgeranylated than with nonprenylated GTPases that will become farnesylated. These binding relationships similarly occur with nonprenylated SAAX mutants. Intriguingly, farnesyltransferase inhibitors increase the binding of WT K-Ras to SmgGDS-607, indicating that the pharmacological shunting of K-Ras into the geranylgeranylation pathway promotes K-Ras association with SmgGDS-607. Using recombinant proteins and prenylated peptides corresponding to the C-terminal sequences of K-Ras and Rap1B, we found that both SmgGDS-607 and SmgGDS-558 directly bind the GTPase C-terminal region, but the specificity of the SmgGDS splice variants for prenylated versus nonprenylated GTPases is diminished in vitro. Finally, we present structural homology models and data from functional prediction software to define both similar and unique features of SmgGDS-607 when compared with SmgGDS-558.

Inhibitors of K-Ras plasma membrane localization

Oncogenic mutant K-Ras is highly prevalent in multiple human tumors. Despite significant efforts to directly target Ras activity, no K-Ras-specific inhibitors have been developed and taken into the clinic. Since Ras proteins must be anchored to the inner leaflet of the plasma membrane (PM) for full biological activity, we devised a high-content screen to identify molecules with ability to displace K-Ras from the PM. Here we summarize the biochemistry and biology of three classes of compound identified by this screening method that inhibit K-Ras PM targeting: staurosporine and analogs, fendiline, and metformin. All three classes of compound significantly abrogate cell proliferation and Ras signaling in K-Ras-transformed cancer cells. Taken together, these studies provide an important proof of concept that blocking PM localization of K-Ras is a tractable therapeutic target.

Staurosporines disrupt phosphatidylserine trafficking and mislocalize Ras proteins

Oncogenic mutant Ras is frequently expressed in human cancers, but no anti-Ras drugs have been developed. Since membrane association is essential for Ras biological activity, we developed a high content assay for inhibitors of Ras plasma membrane localization. We discovered that staurosporine and analogs potently inhibit Ras plasma membrane binding by blocking endosomal recycling of phosphatidylserine, resulting in redistribution of phosphatidylserine from plasma membrane to endomembrane. Staurosporines are more active against K-Ras than H-Ras. K-Ras is displaced to endosomes and undergoes proteasomal-independent degradation, whereas H-Ras redistributes to the Golgi and is not degraded. K-Ras nanoclustering on the plasma membrane is also inhibited. Ras mislocalization does not correlate with protein kinase C inhibition or induction of apoptosis. Staurosporines selectively abrogate K-Ras signaling and proliferation of K-Ras-transformed cells. These results identify staurosporines as novel inhibitors of phosphatidylserine trafficking, yield new insights into the role of phosphatidylserine and electrostatics in Ras plasma membrane targeting, and validate a new target for anti-Ras therapeutics.

TRPV1-tubulin complex: involvement of membrane tubulin in the regulation of chemotherapy-induced peripheral neuropathy

Existence of microtubule cytoskeleton at the membrane and submembranous regions, referred as 'membrane tubulin' has remained controversial for a long time. Since we reported physical and functional interaction of Transient Receptor Potential Vanilloid Sub Type 1 (TRPV1) with microtubules and linked the importance of TRPV1-tubulin complex in the context of chemotherapy-induced peripheral neuropathy, a few more reports have characterized this interaction in in vitro and in in vivo condition. However, the cross-talk between TRPs with microtubule cytoskeleton, and the complex feedback regulations are not well understood. Sequence analysis suggests that other than TRPV1, few TRPs can potentially interact with microtubules. The microtubule interaction with TRPs has evolutionary origin and has a functional significance. Biochemical evidence, Fluorescence Resonance Energy Transfer analysis along with correlation spectroscopy and fluorescence anisotropy measurements have confirmed that TRPV1 interacts with microtubules in live cell and this interaction has regulatory roles. Apart from the transport of TRPs and maintaining the cellular structure, microtubules regulate signaling and functionality of TRPs at the single channel level. Thus, TRPV1-tubulin interaction sets a stage where concept and parameters of 'membrane tubulin' can be tested in more details. In this review, I critically analyze the advancements made in biochemical, pharmacological, behavioral as well as cell-biological observations and summarize the limitations that need to be overcome in the future.

Functional specificity of ras isoforms: so similar but so different

H-ras, N-ras, and K-ras are canonical ras gene family members frequently activated by point mutation in human cancers and coding for 4 different, highly related protein isoforms (H-Ras, N-Ras, K-Ras4A, and K-Ras4B). Their expression is nearly ubiquitous and broadly conserved across eukaryotic species, although there are quantitative and qualitative differences of expression depending on the tissue and/or developmental stage under consideration. Extensive functional studies have determined during the last quarter century that these Ras gene products are critical components of signaling pathways that control eukaryotic cell proliferation, survival, and differentiation. However, because of their homology and frequent coexpression in various cellular contexts, it remained unclear whether the different Ras proteins play specific or overlapping functional roles in physiological and pathological processes. Initially, their high degree of sequence homology and the observation that all Ras isoforms share common sets of downstream effectors and upstream activators suggested that they were mostly redundant functionally. In contrast, the notion of functional specificity for each of the different Ras isoforms is supported at present by an increasing body of experimental observations, including 1) the fact that different ras isoforms are preferentially mutated in specific types of tumors or developmental disorders; 2) the different transforming potential of transfected ras genes in different cell contexts; 3) the distinct sensitivities exhibited by the various Ras family members for modulation by different GAPs or GEFs; 4) the demonstration that different Ras isoforms follow distinct intracellular processing pathways and localize to different membrane microdomains or subcellular compartments; 5) the different phenotypes displayed by genetically modified animal strains for each of the 3 ras loci; and 6) the specific transcriptional networks controlled by each isoform in different cellular settings.

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.

Clinical relevance of KRAS in human cancers

The KRAS gene (Ki-ras2 Kirsten rat sarcoma viral oncogene homolog) is an oncogene that encodes a small GTPase transductor protein called KRAS. KRAS is involved in the regulation of cell division as a result of its ability to relay external signals to the cell nucleus. Activating mutations in the KRAS gene impair the ability of the KRAS protein to switch between active and inactive states, leading to cell transformation and increased resistance to chemotherapy and biological therapies targeting epidermal growth factor receptors. This review highlights some of the features of the KRAS gene and the KRAS protein and summarizes current knowledge of the mechanism of KRAS gene regulation. It also underlines the importance of activating mutations in the KRAS gene in relation to carcinogenesis and their importance as diagnostic biomarkers, providing clues regarding human cancer patients' prognosis and indicating potential therapeutic approaches.

Isoform-specific ras functions in development and cancer

The three closely related mammalian ras genes, Hras, Nras and Kras, have each been implicated in human tumorigenesis by virtue of mutational activation. However, while these genes encode proteins with very similar biochemical properties, activating ras alleles corresponding to the various isoforms have been linked to particular malignancies. Accumulating evidence suggests that these proteins exert distinct activities in a tissue-specific context, apparently reflecting developmental lineage-specific roles for the various ras isoforms. Some of these distinct functions appear to reflect differences in their C-termini, which determine distinct subcellular localization, thereby suggesting a role for compartmentalized signaling. In this review, we discuss the biological functions of the ras isoforms in the context of tissue-specific function as it relates to ras function in development and human cancer.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Topology of mammalian isoprenylcysteine carboxyl methyltransferase determined in live cells with a fluorescent probe

Isoprenylcysteine carboxyl methyltransferase (Icmt) is a highly conserved enzyme that methyl esterifies the alpha carboxyl group of prenylated proteins including Ras and related GTPases. Methyl esterification neutralizes the negative charge of the prenylcysteine and thereby increases membrane affinity. Icmt is an integral membrane protein restricted to the endoplasmic reticulum (ER). The Saccharomyces cerevisiae ortholog, Ste14p, traverses the ER membrane six times. We used a novel fluorescent reporter to map the topology of human Icmt in living cells. Our results indicate that Icmt traverses the ER membrane eight times, with both N and C termini disposed toward the cytosol and with a helix-turn-helix structure comprising transmembrane (TM) segments 7 and 8. Several conserved amino acids that map to cytoplasmic portions of the enzyme are critical for full enzymatic activity. Mammalian Icmt has an N-terminal extension consisting of two TM segments not found in Ste14p and therefore likely to be regulatory. Icmt is a target for anticancer drug discovery, and these data may facilitate efforts to develop small-molecule inhibitors.

Genetic analyses of the role of RCE1 in RAS membrane association and transformation

Proteins terminating with a CAAX motif, such as the nuclear lamins and the RAS family of proteins, undergo post-translational modification of a carboxyl-terminal cysteine with an isoprenyl lipid--a process called protein prenylation. After prenylation, the last three residues of CAAX proteins are clipped off by an endoprotease of the endoplasmic reticulum. RCE1 is responsible for the endoproteolytic processing of the RAS proteins and is likely responsible for endoproteolytic processing of the vast majority of CAAX proteins. Prenylation has been shown to be essential for the proper intracellular targeting and function of several CAAX proteins, but the physiologic importance of the endoprotease step has remained less certain. Here, we will review methods that have been used to define the physiologic importance of the endoproteolytic processing step of CAAX protein processing.

Structural model of a complex between the heterotrimeric G protein, Gsalpha, and tubulin

A number of studies have demonstrated interplay between the cytoskeleton and G protein signaling. Many of these studies have determined a specific interaction between tubulin, the building block of microtubules, and G proteins. The alpha subunits of some heterotrimeric G proteins, including Gsalpha, have been shown to interact strongly with tubulin. Binding of Galpha to tubulin results in increased dynamicity of microtubules due to activation of GTPase of tubulin. Tubulin also activates Gsalpha via a direct transfer of GTP between these molecules. Structural insight into the interaction between tubulin and Gsalpha was required, and was determined, in this report, through biochemical and molecular docking techniques. Solid phase peptide arrays suggested that a portion of the amino terminus, alpha2-beta4 (the region between switch II and switch III) and alpha3-beta5 (just distal to the switch III region) domains of Gsalpha are important for interaction with tubulin. Molecular docking studies revealed the best-fit models based on the biochemical data, showing an interface between the two molecules that includes the adenylyl cyclase/Gbetagamma interaction regions of Gsalpha and the exchangeable nucleotide-binding site of tubulin. These structural models explain the ability of tubulin to facilitate GTP exchange on Galpha and the ability of Galpha to activate tubulin GTPase.

10 Genetic approaches to understanding the physiologic importance of the carboxyl methylation of isoprenylated proteins

This chapter examines recent studies on the physiologic importance of the carboxyl methylation of isoprenylated proteins, focusing largely on what has been learned from cells lacking the Icmt methyltransferase. Proteins terminating with a CaaX motif (e.g., the nuclear lamins, the Ras family of proteins) undergo posttranslational modification of a carboxyl-terminal cysteine with an isoprenyl lipid (a process generally called protein isoprenylation or protein prenylation). Following this lipidation step, CaaX proteins generally undergo two additional processing steps: endoproteolytic release of the last three residues of the protein (i.e., the -aaX of the CaaX motif) and methylesterification of the newly exposed isoprenylcysteine a-carboxyl group. The CaaX proteins are not, however, the only prenylated proteins that undergo carboxyl methylation. A subset of the Rab family of proteins, those terminating with a CXC motif, undergo methylesterification of a carboxyl-terminal geranylgeranylcysteine. The methylation of CaaX proteins and the CXC Rab proteins is carried out by a single membrane methyltransferase of the endoplasmic reticulum, Icmt (for isoprenylcysteine carboxyl methyltransferase). Many studies have shown that protein prenylation is essential for the proper intracellular targeting and function of numerous intracellular proteins, but the physiologic importance of the carboxyl methylation step has remained less certain. Here, we review recent studies that have shed light on the importance of carboxyl methylation of prenylated proteins.

Identification and characterisation of novel tubulin-binding motifs located within the C-terminus of TRPV1

Previously, we reported that TRPV1, the vanilloid receptor, interacts with soluble alphabeta-tubulin dimers as well as microtubules via its C-terminal cytoplasmic domain. The interacting region of TRPV1, however, has not been defined. We found that the TRPV1 C-terminus preferably interacts with beta-tubulin and less with alpha-tubulin. Using a systematic deletion approach and biotinylated-peptides we identified two tubulin-binding sites present in TRPV1. These two sequence stretches are highly conserved in all known mammalian TRPV1 orthologues and partially conserved in some of the TRPV1 homologues. As these sequence stretches are not similar to any known tubulin-binding sequences, we conclude that TRPV1 interacts with tubulin and microtubule through two novel tubulin-binding motifs.

Ras oncogenes and their downstream targets

RAS proteins are small GTPases, which serve as master regulators of a myriad of signaling cascades involved in highly diverse cellular processes. RAS oncogenes have been originally discovered as retroviral oncogenes, and ever since constitutively activating RAS mutations have been identified in human tumors, they are in the focus of intense research. In this review, we summarize the biochemical properties of RAS proteins, trace down the evolution of RAS signaling and present an overview of the spatio-temporal activation of major RAS isoforms. We further discuss RAS effector pathways, their role in normal and transformed cell physiology and summarize ongoing attempts to interfere with aberrant RAS signaling. Finally, we comment on the role of micro RNAs in modulating RAS expression, contribution of RAS to stem cell function and on high-throughput analyses of RAS signaling networks.

Cyclodextrins but not compactin inhibit the lateral diffusion of membrane proteins independent of cholesterol

Cholesterol and glycosphingolipid-enriched membrane domains, termed lipid rafts, were proposed to play important roles in trafficking and signaling events. These functions are inhibited following putative disruption of rafts by cholesterol depletion, commonly induced by treatment with methyl-beta-cyclodextrin (MbetaCD). However, several studies showed that the lateral diffusion of membrane proteins is inhibited by MbetaCD, suggesting that it may have additional effects on membrane organization unrelated to cholesterol removal. Here, we investigated this possibility by comparison of the effects of cholesterol depletion by MbetaCD and by metabolic inhibition (compactin), and of treatment with alpha-CD, which does not bind cholesterol. The studies employed two series of proteins (Ras and influenza hemagglutinin), each containing as internal controls related mutants that differ in raft association. Mild MbetaCD treatment retarded the lateral diffusion of both raft and non-raft mutants, whereas similar cholesterol reduction (30-33%) by metabolic inhibition enhanced selectively the diffusion of the raft-associated mutants. Moreover, alpha-CD also inhibited the diffusion of raft and non-raft mutants, despite its lack of effect on cholesterol content. These findings suggest that the widely used treatment with CD to reduce cholesterol has additional, cholesterol-independent effects on membrane protein mobility, which do not necessarily distinguish between raft and non-raft proteins.

Nonconventional Trafficking of Ras Associated with Ras Signal Organization

Ras signaling to its downstream effectors appears to include combinations of extracellular-signal-regulated Ras activation at the plasma membrane (PM) and endomembranes, dynamic lateral segregation in the PM, and translocation of Ras from the PM to intracellular compartments. These processes are governed by the C-terminal polybasic farnesyl domain in K-Ras 4B and by the cysteine-palmitoylated C-terminal farnesyl domains in H-Ras and N-Ras. K-Ras 4B has no palmitoylated cysteines. Depalmitoylation/repalmitoylation of H-/N-Ras proteins promotes their cellular redistribution and signaling by mechanisms as yet unknown, possibly involving chaperones. Palmitoylation of H-/N-Ras also promotes their association with 'rasosomes', randomly diffusing nanoparticles that apparently provide a means by which multiple copies of activated Ras and its signal can spread rapidly. Ubiquitination of H-Ras evidently targets it to the endosomes. The polybasic farnesyl domain of K-Ras 4B was shown to act as a target for Ca++/calmodulin, which sequesters the active protein from the PM, thereby facilitating its trafficking to Golgi apparatus and early endosomes. Protein kinase C-dependent phosphorylation of S181 in K-Ras 4B was shown to provide a regulated farnesyl-electrostatic switch on K-Ras 4B, which promotes its translocation to the mitochondria. All these translocation events are characterized by nonconventional trafficking of the farnesyl-modified Ras proteins and seem to govern the selectivity and probably also the robustness of the Ras signal. In this review, we discuss the various modifications and interactions of the farnesylated C-terminus, the trafficking of Ras proteins in the PM and between the PM and the endomembranes, and the relevance of the subcellular localization of Ras for Ras function.

Spatio-temporal segregation of Ras signals: one ship, three anchors, many harbors

Dynamic assembly of spatially separated signaling platforms enables a cell to tune cellular outputs in response to different input stimuli. Understanding how a vast diversity in signaling responses can be generated from a limited protein repertoire requires knowledge of how cells maintain the segregation of proteins and thereby orchestrate their local activities. Ras proteins are subject to this type of precise regulation of localization, and thus activity, in space and time. A model emerges where different lipid anchors dynamically shuttle Ras between specific membrane compartments, where differences in the accessibility of signaling environments and in the residence time of Ras therein account for isoform-specific signaling responses.

Microarray analysis of the differential transformation mediated by Kirsten and Harvey Ras oncogenes in a human colorectal adenocarcinoma cell line

Colorectal cancer arises after a series of mutational events in the colon epithelia and is often used as a model of the multistep progression of tumorigenesis. Mutations in Ki-Ras have been detected in some 50% of cases and are thought to occur at an early stage. Almost never do mutations arise in the loci of other Ras isoforms (Ha- and N-), leading to the assumption that Ki-Ras plays a unique role in tumorigenesis. In order to examine the distinctive function that Ki-Ras plays in cancer development in the colon, we introduced constitutively active mutant Ki- and Ha-Ras genes into an intermediate-stage colon adenoma cell line (Caco-2). We found that mutant active Ha-RasV12 was more efficient at transforming these colon epithelial cells as assessed by anchorage-independent growth, tumor formation in SCID mice and the development of mesenchymal morphology compared to transformation by Ki-RasV12. We conducted microarray analysis in an attempt to reveal the genes whose aberrant expression is a direct result of overexpression of either Ki-RasV12 or Ha-RasV12. We used Clontech's Atlas cancer cDNA (588 genes) and RZPD's Onco Set 1 (1,544 genes) arrays. We identified fewer genes that were commonly regulated than were differentially expressed between Ki- and Ha-RasV12 isoforms. Specifically, we found that Ki-RasV12 regulated genes involved in cytokine signaling, cell adhesion and colon development, whereas Ha-RasV12 mainly regulated genes involved in controlling cell morphology, correlating to an epithelial-mesenchymal transition only observed in these cells. Our results demonstrate how 2 Ras isoforms regulate disparate biologic processes, revealing a number of genes whose deregulated expression may influence colon carcinogenesis (supplementary material for this article can be found on the International Journal of Cancer website at http://www.interscience.wiley.com/jpages/0020-7136/suppmat/index.html).

The growth and tumor suppressor NORE1A is a cytoskeletal protein that suppresses growth by inhibition of the ERK pathway

NORE1A is a growth and tumor suppressor that is inactivated in a variety of cancers. NORE1A has been shown to bind to the active Ras oncogene product. However, the mechanism of NORE1A-induced growth arrest and tumor suppression remains unknown. Using anchorage-independent growth assays, we mapped the NORE1A effector domain (the minimal region of the protein responsible for its growth-suppressive effects) to the fragment containing the central and Ras association domains of NORE1A (amino acids 191-363). Expression of the NORE1A effector domain in A549 lung adenocarcinoma cells resulted in the selective inhibition of signal transduction through the ERK pathway. The full-length NORE1A (416 amino acids) and its fragments capable of growth suppression were localized to centrosomes and microtubules in normal and transformed human cells in a Ras-independent manner. A mutant that was deficient in binding to centrosomes and microtubules was also deficient in inducing cell cycle arrest. This suggests that cytoskeletal localization is required for growth-suppressive effects of NORE1A. Ras binding function was required for growth-suppressive effects of the full-length NORE1A but not for the growth-suppressive effects of the effector domain. Our studies suggest that association of NORE1A with cytoskeletal elements is essential for NORE1A-induced growth suppression and that the ERK pathway is a target for NORE1A growth-suppressive activities.

Visualizing Ras signalling in real-time

Ras GTPases are universal molecular switches that act as kinetic timers of signal transduction events. They are post-translationally modified by the addition of lipid groups to their hypervariable carboxyl termini, which plug the proteins to membranes and influence their dynamic sorting and trafficking. For the past twenty years, the plasma membrane has been considered to be the predominant platform from which Ras operates. Recent work using live-cell imaging and novel probes to visualize where and when Ras is active has supported this long-held belief. However, an equally fascinating aspect of these imaging studies has been the discovery of dynamic Ras activity, as well as distinct signal output, from intracellular organelles. Activation of Ras on the Golgi exhibits kinetics different from Ras activation on the plasma membrane, and compartmentalized Ras signalling seems particularly prominent in lymphocytes. However, data on the spatial and temporal regulation of Ras activity has frequently differed depending on the nature of the probe, the cell type and the stimulus. Nevertheless, because Ras traffics through endomembranes en route to the plasma membrane, it seems likely that Ras can signal from such compartments. The burning question in this field concerns the significance of this observation for endogenous Ras signalling output.

Molecular targets for altering radiosensitivity: lessons from Ras as a pre-clinical and clinical model

Ras activation has been correlated with malignant and metastatic cancer phenotypes and poor prognosis for cancer patients. In the preclinical setting, Ras activation by mutation or EGFR amplification results in increased clonogenic cell survival and decreased tumor growth delay following irradiation. Activation of the Ras pathway has also been associated with increased risk of local failure and decreased overall survival in patients receiving radiotherapy. Prenyltransferase inhibitors target the post-translational processing of Ras and have been shown to increase the radiosensitivity of human cancer cell lines. In the clinical setting, these inhibitors have been used with concurrent radiotherapy in a small number of phase I clinical trials with acceptable toxicity. Therefore, inhibiting Ras activation represents a promising molecular approach for radiosensitization in cancer therapy.

Oncogenic Ras in tumour progression and metastasis

The ras genes give rise to a family of related GTP-binding proteins that exhibit potent transforming potential. Mutational activation of Ras proteins promotes oncogenesis by disturbing a multitude of cellular processes, such as gene expression, cell cycle progression and cell proliferation, as well as cell survival, and cell migration. Ras signalling pathways are well known for their involvement in tumour initiation, but less is known about their contribution to invasion and metastasis. This review summarises the role and mechanisms of Ras signalling, especially the role of the Ras effector cascade Raf/MEK/ERK, as well as the phosphatidylinositol 3-kinase/Akt pathway in Ras-mediated transformation and tumour progression. In addition, it discusses the impact of Rho GTPases on Ras-mediated transformation and metastasis.

Reversible intracellular translocation of KRas but not HRas in hippocampal neurons regulated by Ca2+/calmodulin

The Ras/MAPK pathway regulates synaptic plasticity and cell survival in neurons of the central nervous system. Here, we show that KRas, but not HRas, acutely translocates from the plasma membrane (PM) to the Golgi complex and early/recycling endosomes in response to neuronal activity. Translocation is reversible and mediated by the polybasic-prenyl membrane targeting motif of KRas. We provide evidence that KRas translocation occurs through sequestration of the polybasic-prenyl motif by Ca²⁺/calmodulin (Ca²⁺/CaM) and subsequent release of KRas from the PM, in a process reminiscent of GDP dissociation inhibitor–mediated membrane recycling of Rab and Rho GTPases. KRas translocation was accompanied by partial intracellular redistribution of its activity. We conclude that the polybasic-prenyl motif acts as a Ca²⁺/CaM-regulated molecular switch that controls PM concentration of KRas and redistributes its activity to internal sites. Our data thus define a novel signaling mechanism that differentially regulates KRas and HRas localization and activity in neurons.

Expression of Ras GTPase isoforms in normal and diseased pancreas

BACKGROUND

Ki-Ras is well studied in its oncogenic form in relation to pancreatic pathologies. However, the individual contribution of each of the wild-type Ras isoforms (Ha-, Ki-, and N-) in pancreatic cells in health and disease is unknown.

METHODS

Archival formalin-fixed, paraffin-embedded specimens of normal (n = 6) and malignant pancreas (n = 35) were used for immuno-histochemical detection of Ras isoforms using a modified polymer system. In addition, immunogold labelling for Ras isoforms was done for subcellular localisation under electron microscopy.

RESULTS

Pancreatic ductal cells expressed Ha-Ras in the cytoplasm, with Ki-Ras in the apical region and N-Ras (50% of cases) in a supranuclear distribution. Pancreatic acinar cells express all three isoforms with some nuclear expression of Ki-Ras and supranuclear expression of N-Ras. Islets show Ki- and Ha-Ras mainly with differential expression of Ha-Ras (beta cells showing less Ha-Ras and more Ki-Ras than alpha cells). Electron microscopy shows that Ha-Ras is mainly localised in the endoplasmic reticulum and Golgi apparatus of the acinar cells with some plasma membrane localisation of Ki-Ras in the ductal cells. There was no change in any of the Ras isoform expression in the ductal or acinar cells in various malignancies studied (Mann-Whitney U test, p > 0.1).

CONCLUSIONS

Ras isoforms have distinct and separate cellular and subcellular distribution that may persist even in the malignantly transformed state. Understanding this distinct functional distribution patterns in detail is an essential step if mutant Ki-Ras is to be targeted in the pancreas by genetic or molecular therapeutic tools.

Macropinocytosis Is the Endocytic Pathway That Mediates Macrophage Foam Cell Formation with Native Low Density Lipoprotein

Previously, we reported that fluid-phase endocytosis of native LDL by PMA-activated human monocytederived macrophages converted these macrophages into cholesterol-enriched foam cells (Kruth, H. S., Huang, W., Ishii, I., and Zhang, W. Y. (2002) J. Biol. Chem. 277, 34573-34580). Uptake of fluid by cells can occur either by micropinocytosis within vesicles (<0.1 microm diameter) or by macropinocytosis within vacuoles ( approximately 0.5-5.0 microm) named macropinosomes. The current investigation has identified macropinocytosis as the pathway for fluid-phase LDL endocytosis and determined signaling and cytoskeletal components involved in this LDL endocytosis. The phosphatidylinositol 3-kinase inhibitor, LY294002, which inhibits macropinocytosis but does not inhibit micropinocytosis, completely blocked PMA-activated macrophage uptake of fluid and LDL. Also, nystatin and filipin, inhibitors of micropinocytosis from lipid-raft plasma membrane domains, both failed to inhibit PMA-stimulated macrophage cholesterol accumulation. Time-lapse video phase-contrast microscopy and time-lapse digital confocal-fluorescence microscopy with fluorescent DiI-LDL showed that PMA-activated macrophages took up LDL in the fluid phase by macropinocytosis. Macropinocytosis of LDL depended on Rho GTPase signaling, actin, and microtubules. Bafilomycin A1, the vacuolar H+-ATPase inhibitor, inhibited degradation of LDL and caused accumulation of undegraded LDL within macropinosomes and multivesicular body endosomes. LDL in multivesicular body endosomes was concentrated >40-fold over its concentration in the culture medium consistent with macropinosome shrinkage by maturation into multivesicular body endosomes. Macropinocytosis of LDL taken up in the fluid phase without receptor-mediated binding of LDL is a novel endocytic pathway that generates macrophage foam cells. Macropinocytosis in macrophages and possibly other vascular cells is a new pathway to target for modulating foam cell formation in atherosclerosis.

A role for the RASSF1A tumor suppressor in the regulation of tubulin polymerization and genomic stability

The high frequency with which the novel tumor suppressor RASSF1A is inactivated by promoter methylation suggests that it plays a key role in the development of many primary human tumors. Yet the mechanism of RASSF1A action remains unknown. We now show that RASSF1A associates with microtubules and that this association is essential for RASSF1A to mediate its growth inhibitory effects. Overexpression of RASSF1A promotes the formation of stable microtubules, whereas a dominant-negative fragment of RASSF1A destabilizes microtubule networks. The RASSF1 protein is expressed as two main isoforms, 1A and 1C. The smaller 1C isoform also associates with microtubules but is less effective at stabilizing them. Because RASSF1A and RASSF1C localize to the mitotic spindle, we examined their effects upon genomic instability. RASSF1A and RASSF1C block activated Ras-induced genomic instability. However, a point mutant of RASSF1C, identified in human tumors, was severely defective for stabilizing tubulin and was unable to block the genomic destabilizing effects of Ras. Thus, we identify a role for RASSF1A/C in the control of microtubule polymerization and potentially in the maintenance of genomic stability.

H-Ras and phosphoinositide 3-kinase cooperate to induce alpha(1,3)-fucosyltransferase VII expression in Jurkat T cells

The alpha(1,3)-fucosyltransferase FucT-VII is essential for the biosynthesis of selectin ligands, but the signaling pathways mediating FucT-VII induction in T cells and other lymphocytes are poorly understood. We have shown previously that sustained activation of Ras in Jurkat T cells induces FucT-VII transcription, which requires the Raf-MEK-ERK pathway. In this study we report that FucT-VII induction is specific to the H-Ras isoform. Jurkat T cells retrovirally transduced with constitutively active H-Ras but not N- or K-Ras up-regulated expression of FucT-VII. Pharmacological inhibition studies also revealed that phosphoinositide 3-kinase (PI3K) activity is required for H-Ras-mediated FucT-VII induction. However, the ability of H-Ras to selectively induce FucT-VII is not a function of the inability of the N- or K-Ras isoforms to activate Raf or PI3K pathways. The use of effector-loop domain mutants of H-Ras, which are impaired for their ability to interact selectively with individual effectors alone or in combination with active Raf, indicated that induction of FucT-VII requires the concomitant activation of at least three signaling pathways. These studies show that H-Ras mediates FucT-VII induction in Jurkat T cells via the activation of the Raf, PI3K, and a distinct, H-Ras-specific effector signaling pathway.

Ras activation in Jurkat T cells following low-grade stimulation of the T-cell receptor is specific to N-Ras and occurs only on the Golgi apparatus

Ras activation is critical for T-cell development and function, but the specific roles of the different Ras isoforms in T-lymphocyte function are poorly understood. We recently reported T-cell receptor (TCR) activation of ectopically expressed H-Ras on the the Golgi apparatus of T cells. Here we studied the isoform and subcellular compartment specificity of Ras signaling in Jurkat T cells. H-Ras was expressed at much lower levels than the other Ras isoforms in Jurkat and several other T-cell lines. Glutathione S-transferase-Ras-binding domain (RBD) pulldown assays revealed that, although high-grade TCR stimulation and phorbol ester activated both N-Ras and K-Ras, low-grade stimulation of the TCR resulted in specific activation of N-Ras. Surprisingly, whereas ectopically expressed H-Ras cocapped with the TCRs in lipid microdomains of the Jurkat plasma membrane, N-Ras did not. Live-cell imaging of Jurkat cells expressing green fluorescent protein-RBD, a fluorescent reporter of GTP-bound Ras, revealed that N-Ras activation occurs exclusively on the Golgi apparatus in a phospholipase Cgamma- and RasGRP1-dependent fashion. The specificity of N-Ras signaling downstream of low-grade TCR stimulation was dependent on the monoacylation of the hypervariable membrane targeting sequence. Our data show that, in contrast to fibroblasts stimulated with growth factors in which all three Ras isoforms become activated and signaling occurs at both the plasma membrane and Golgi apparatus, Golgi-associated N-Ras is the critical Ras isoform and intracellular pool for low-grade TCR signaling in Jurkat T cells.