Targeting the α4-α5 interface of RAS results in multiple levels of inhibition

Targeting KRAS in Pancreatic Ductal Adenocarcinoma: The Long Road to Cure

Pancreatic ductal adenocarcinoma (PDAC) remains an important cause of cancer-related mortality, and it is expected to play an even bigger part in cancer burden in the years to come. Despite concerted efforts from scientists and physicians, patients have experienced little improvement in survival over the past decades, possibly because of the non-specific nature of the tested treatment modalities. Recently, the discovery of potentially targetable molecular alterations has paved the way for the personalized treatment of PDAC. Indeed, the central piece in the molecular framework of PDAC is starting to be unveiled. KRAS mutations are seen in 90% of PDACs, and multiple studies have demonstrated their pivotal role in pancreatic carcinogenesis. Recent investigations have shed light on the differences in prognosis as well as therapeutic implications of the different KRAS mutations and disentangled the relationship between KRAS and effectors of downstream and parallel signaling pathways. Additionally, the recognition of other mechanisms involving KRAS-mediated pathogenesis, such as KRAS dosing and allelic imbalance, has contributed to broadening the current knowledge regarding this molecular alteration. Finally, KRAS G12C inhibitors have been recently tested in patients with pancreatic cancer with relative success, and inhibitors of KRAS harboring other mutations are under clinical development. These drugs currently represent a true hope for a meaningful leap forward in this dreadful disease.

KRAS G12D targeted therapies for pancreatic cancer: Has the fortress been conquered?

KRAS mutations are among the most commonly occurring mutations in cancer. After being deemed undruggable for decades, KRAS G12C specific inhibitors showed that small molecule inhibitors can be developed against this notorious target. At the same time, there is still no agent that could target KRAS G12D which is the most common KRAS mutation and is found in the majority of KRAS-mutated pancreatic tumors. Nevertheless, significant progress is now being made in the G12D space with the development of several compounds that can bind to and inhibit KRAS G12D, most notably MRTX1133. Exciting advances in this field also include an immunotherapeutic approach that uses adoptive T-cell transfer to specifically target G12D in pancreatic cancer. In this mini-review, we discuss recent advances in KRAS G12D targeting and the potential for further clinical development of the various approaches.

Mutations in the α4-α5 allosteric lobe of RAS do not significantly impair RAS signaling or self-association

Mutations in one of the three RAS genes (HRAS, KRAS, and NRAS) are present in nearly 20% of all human cancers. These mutations shift RAS to the GTP-loaded active state due to impairment in the intrinsic GTPase activity and disruption of GAP-mediated GTP hydrolysis, resulting in constitutive activation of effectors such as RAF. Because activation of RAF involves dimerization, RAS dimerization has been proposed as an important step in RAS-mediated activation of effectors. The α4-α5 allosteric lobe of RAS has been proposed as a RAS dimerization interface. Indeed, the NS1 monobody, which binds the α4-α5 region within the RAS G domain, inhibits RAS-dependent signaling and transformation as well as RAS nanoclustering at the plasma membrane. Although these results are consistent with a model in which the G domain dimerizes through the α4-α5 region, the isolated G domain of RAS lacks intrinsic dimerization capacity. Furthermore, prior studies analyzing α4-α5 point mutations have reported mixed effects on RAS function. Here, we evaluated the activity of a panel of single amino acid substitutions in the α4-α5 region implicated in RAS dimerization. We found that these proposed "dimerization-disrupting" mutations do not significantly impair self-association, signaling, or transformation of oncogenic RAS. These results are consistent with a model in which activated RAS protomers cluster in close proximity to promote the dimerization of their associated effector proteins (e.g., RAF) without physically associating into dimers mediated by specific molecular interactions. Our findings suggest the need for a nonconventional approach to developing therapeutics targeting the α4-α5 region.

Identification of the nucleotide-free state as a therapeutic vulnerability for inhibition of selected oncogenic RAS mutants

RAS guanosine triphosphatases (GTPases) are mutated in nearly 20% of human tumors, making them an attractive therapeutic target. Following our discovery that nucleotide-free RAS (apo RAS) regulates cell signaling, we selectively target this state as an approach to inhibit RAS function. Here, we describe the R15 monobody that exclusively binds the apo state of all three RAS isoforms in vitro, regardless of the mutation status, and captures RAS in the apo state in cells. R15 inhibits the signaling and transforming activity of a subset of RAS mutants with elevated intrinsic nucleotide exchange rates (i.e., fast exchange mutants). Intracellular expression of R15 reduces the tumor-forming capacity of cancer cell lines driven by select RAS mutants and KRAS(G12D)-mutant patient-derived xenografts (PDXs). Thus, our approach establishes an opportunity to selectively inhibit a subset of RAS mutants by targeting the apo state with drug-like molecules.

Khan et al. develop a high-affinity monobody to nucleotide-free RAS that, when expressed intracellularly, inhibits oncogenic RAS-mediated signaling and tumorigenesis. This study reveals the feasibility of targeting the nucleotide-free state to inhibit tumors driven by oncogenic RAS mutants that possess elevated nucleotide exchange activity.

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

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

Targeting the KRAS α4-α5 allosteric interface inhibits pancreatic cancer tumorigenesis

RAS is the most frequently mutated oncogene in human cancer with nearly ~20% of cancer patients possessing mutations in one of three RAS genes (K, N or HRAS). However, KRAS is mutated in nearly 90% of pancreatic ductal carcinomas (PDAC). Although pharmacological inhibition of RAS has been challenging, KRAS(G12C)-specific inhibitors have recently entered the clinic. While KRAS(G12C) is frequently expressed in lung cancers, it is rare in PDAC. Thus, more broadly efficacious RAS inhibitors are needed for treating KRAS mutant-driven cancers such as PDAC. A RAS-specific tool biologic, NS1 Monobody, inhibits HRAS- and KRAS-mediated signalling and oncogenic transformation both in vitro and in vivo by targeting the α4-α5 allosteric site of RAS and blocking RAS self-association. Here, we evaluated the efficacy of targeting the α4-α5 interface of KRAS as an approach to inhibit PDAC development using an immunocompetent orthotopic mouse model. Chemically regulated NS1 expression inhibited ERK and AKT activation in KRAS(G12D) mutant KPC PDAC cells and reduced the formation and progression of pancreatic tumours. NS1-expressing tumours were characterized by increased infiltration of CD4 + T helper cells. These results suggest that targeting the #x3B1;4-#x3B1;5 allosteric site of KRAS may represent a viable therapeutic approach for inhibiting KRAS-mutant pancreatic tumours.

Targeting small GTPases and their downstream pathways with intracellular macromolecule binders to define alternative therapeutic strategies in cancer

The RAS superfamily of small GTPases regulates major physiological cellular processes. Mutation or deregulation of these small GTPases, their regulators and/or their effectors are associated with many diseases including cancer. Hence, targeting these classes of proteins is an important therapeutic strategy in cancer. This has been recently achieved with the approval of the first KRASG12C covalent inhibitors for the clinic. However, many other mutants and small GTPases are still considered as 'undruggable' with small molecule inhibitors because of a lack of well-defined pocket(s) at their surface. Therefore, alternative therapeutic strategies have been developed to target these proteins. In this review, we discuss the use of intracellular antibodies and derivatives - reagents that bind their antigen inside the cells - for the discovery of novel inhibitory mechanisms, targetable features and therapeutic strategies to inhibit small GTPases and their downstream pathways. These reagents are also versatile tools used to better understand the biological mechanisms regulated by small GTPases and to accelerate the drug discovery process.

RAS Dimers: The Novice Couple at the RAS-ERK Pathway Ball

Signals conveyed through the RAS-ERK pathway constitute a pivotal regulatory element in cancer-related cellular processes. Recently, RAS dimerization has been proposed as a key step in the relay of RAS signals, critically contributing to RAF activation. RAS clustering at plasma membrane microdomains and endomembranes facilitates RAS dimerization in response to stimulation, promoting RAF dimerization and subsequent activation. Remarkably, inhibiting RAS dimerization forestalls tumorigenesis in cellular and animal models. Thus, the pharmacological disruption of RAS dimers has emerged as an additional target for cancer researchers in the quest for a means to curtail aberrant RAS activity.

Targeting KRAS in pancreatic cancer: new drugs on the horizon

Kirsten Rat Sarcoma (KRAS) is a master oncogene involved in cellular proliferation and survival and is the most commonly mutated oncogene in all cancers. Activating KRAS mutations are present in over 90% of pancreatic ductal adenocarcinoma (PDAC) cases and are implicated in tumor initiation and progression. Although KRAS is a critical oncogene, and therefore an important therapeutic target, its therapeutic inhibition has been very challenging, and only recently specific mutant KRAS inhibitors have been discovered. In this review, we discuss the activation of KRAS signaling and the role of mutant KRAS in PDAC development. KRAS has long been considered undruggable, and many drug discovery efforts which focused on indirect targeting have been unsuccessful. We discuss the various efforts for therapeutic targeting of KRAS. Further, we explore the reasons behind these obstacles, novel successful approaches to target mutant KRAS including G12C mutation as well as the mechanisms of resistance.

Targeting the "undruggable" RAS with biologics

RAS proteins represent critical drivers of tumor development and thus are the focus of intense efforts to pharmacologically inhibit these proteins in human cancer. Although recent success has been attained in developing clinically efficacious inhibitors to KRASG12C, there remains a critical need for developing approaches to inhibit additional mutant RAS proteins. A number of anti-RAS biologics have been developed which reveal novel and potentially therapeutically targetable vulnerabilities in oncogenic RAS. This review will discuss the growing field of anti-RAS biologics and potential development of these reagents into new anti-RAS therapies.

RAS-inhibiting biologics identify and probe druggable pockets including an SII-α3 allosteric site

RAS mutations are the most common oncogenic drivers across human cancers, but there remains a paucity of clinically-validated pharmacological inhibitors of RAS, as druggable pockets have proven difficult to identify. Here, we identify two RAS-binding Affimer proteins, K3 and K6, that inhibit nucleotide exchange and downstream signaling pathways with distinct isoform and mutant profiles. Affimer K6 binds in the SI/SII pocket, whilst Affimer K3 is a non-covalent inhibitor of the SII region that reveals a conformer of wild-type RAS with a large, druggable SII/α3 pocket. Competitive NanoBRET between the RAS-binding Affimers and known RAS binding small-molecules demonstrates the potential to use Affimers as tools to identify pharmacophores. This work highlights the potential of using biologics with small interface surfaces to select unseen, druggable conformations in conjunction with pharmacophore identification for hard-to-drug proteins.

Oncogenic RAS mutants remain difficult to target with small molecules. Here, the authors show that RAS-binding Affimer proteins inhibit RAS signaling while binding diverse regions on the RAS surface, suggesting the potential to use Affimers as tools to identify new binding pockets and pharmacophores.

Diffusion-based determination of protein homodimerization on reconstituted membrane surfaces

The transient interactions between cellular components, particularly on membrane surfaces, are critical in the proper function of many biochemical reactions. For example, many signaling pathways involve dimerization, oligomerization, or other types of clustering of signaling proteins as a key step in the signaling cascade. However, it is often experimentally challenging to directly observe and characterize the molecular mechanisms such interactions—the greatest difficulty lies in the fact that living cells have an unknown number of background processes that may or may not participate in the molecular process of interest, and as a consequence, it is usually impossible to definitively correlate an observation to a well-defined cellular mechanism. One of the experimental methods that can quantitatively capture these interactions is through membrane reconstitution, whereby a lipid bilayer is fabricated to mimic the membrane environment, and the biological components of interest are systematically introduced, without unknown background processes. This configuration allows the extensive use of fluorescence techniques, particularly fluorescence fluctuation spectroscopy and single-molecule fluorescence microscopy. In this review, we describe how the equilibrium diffusion of two proteins, K-Ras4B and the PH domain of Bruton’s tyrosine kinase (Btk), on fluid lipid membranes can be used to determine the kinetics of homodimerization reactions.

RAS Nanoclusters: Dynamic Signaling Platforms Amenable to Therapeutic Intervention

RAS proteins are mutated in approximately 20% of all cancers and are generally associated with poor clinical outcomes. RAS proteins are localized to the plasma membrane and function as molecular switches, turned on by partners that receive extracellular mitogenic signals. In the on-state, they activate intracellular signal transduction cascades. Membrane-bound RAS molecules segregate into multimers, known as nanoclusters. These nanoclusters, held together through weak protein-protein and protein-lipid associations, are highly dynamic and respond to cellular input signals and fluctuations in the local lipid environment. Disruption of RAS nanoclusters results in downregulation of RAS-mediated mitogenic signaling. In this review, we discuss the propensity of RAS proteins to display clustering behavior and the interfaces that are associated with these assemblies. Strategies to therapeutically disrupt nanocluster formation or the stabilization of signaling incompetent RAS complexes are discussed.

Probing RAS Function with Monobodies

RAS is frequently mutated in human cancers with nearly 20% of all cancers harboring mutations in one of three RAS isoforms (KRAS, HRAS, or NRAS). Furthermore, RAS proteins are critical oncogenic drivers of tumorigenesis. As such, RAS has been a prime focus for development of targeted cancer therapeutics. Although RAS is viewed by many as undruggable, the recent development of allele-specific covalent inhibitors to KRAS(G12C) has provided significant hope for the eventual pharmacological inhibition of RAS (Ostrem et al., Nature 503(7477):548-551, 2013; Patricelli et al., Cancer Discov 6(3):316-329, 2016; Janes et al., Cell 172(3):578-589.e17, 2018; Canon et al., Nature 575(7781):217-223, 2019; Hallin et al., Cancer Discov 10(1):54-71, 2020). Indeed, these (G12C)-specific inhibitors have elicited promising responses in early phase clinical trials (Canon et al., Nature 575(7781):217-223, 2019; Hallin et al., Cancer Discov 10(1):54-71, 2020). Despite this success in pharmacologically targeting KRAS(G12C), the remaining RAS mutants lack readily tractable chemistries for development of covalent inhibitors. Thus, alternative approaches are needed to develop broadly efficacious RAS inhibitors. We have utilized Monobody (Mb) technology to identify vulnerabilities in RAS that can potentially be exploited for development of novel RAS inhibitors. Here, we describe the methods used to isolate RAS-specific Mbs and to define their inhibitory activity.

Natural Products Attenuating Biosynthesis, Processing, and Activity of Ras Oncoproteins: State of the Art and Future Perspectives

RAS genes encode signaling proteins, which, in mammalian cells, act as molecular switches regulating critical cellular processes as proliferation, growth, differentiation, survival, motility, and metabolism in response to specific stimuli. Deregulation of Ras functions has a high impact on human health: gain-of-function point mutations in RAS genes are found in some developmental disorders and thirty percent of all human cancers, including the deadliest. For this reason, the pathogenic Ras variants represent important clinical targets against which to develop novel, effective, and possibly selective pharmacological inhibitors. Natural products represent a virtually unlimited resource of structurally different compounds from which one could draw on for this purpose, given the improvements in isolation and screening of active molecules from complex sources. After a summary of Ras proteins molecular and regulatory features and Ras-dependent pathways relevant for drug development, we point out the most promising inhibitory approaches, the known druggable sites of wild-type and oncogenic Ras mutants, and describe the known natural compounds capable of attenuating Ras signaling. Finally, we highlight critical issues and perspectives for the future selection of potential Ras inhibitors from natural sources.

RAS Function in cancer cells: translating membrane biology and biochemistry into new therapeutics

The three human RAS proteins are mutated and constitutively activated in ∼20% of cancers leading to cell growth and proliferation. For the past three decades, many attempts have been made to inhibit these proteins with little success. Recently; however, multiple methods have emerged to inhibit KRAS, the most prevalently mutated isoform. These methods and the underlying biology will be discussed in this review with a special focus on KRAS-plasma membrane interactions.

Targeting Endogenous K-RAS for Degradation through the Affinity-Directed Protein Missile System

K-RAS is known as the most frequently mutated oncogene. However, the development of conventional K-RAS inhibitors has been extremely challenging, with a mutation-specific inhibitor reaching clinical trials only recently. Targeted proteolysis has emerged as a new modality in drug discovery to tackle undruggable targets. Our laboratory has developed a system for targeted proteolysis using peptidic high-affinity binders, called “AdPROM.” Here, we used CRISPR/Cas9 technology to knock in a GFP tag on the native K-RAS gene in A549 adenocarcinoma (A549^GFPKRAS) cells and constructed AdPROMs containing high-affinity GFP or H/K-RAS binders. Expression of GFP-targeting AdPROM in A549^GFPKRAS led to robust proteasomal degradation of endogenous GFP-K-RAS, while expression of anti-HRAS-targeting AdPROM in different cell lines resulted in the degradation of both GFP-tagged and untagged K-RAS, and untagged H-RAS. Our findings imply that endogenous RAS proteins can be targeted for proteolysis, supporting the idea of an alternative therapeutic approach to these undruggable targets.

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Generation of A549 cells with a homozygous knockin of GFP tag on the KRAS gene

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Proteasomal degradation of endogenous GFP-K-RAS using a VHL-GFP-nanobody fusion

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Proteasomal degradation of endogenous H/K-RAS using VHL-H/K-RAS-monobody fusion

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.

Therapeutic targeting of RAS: New hope for drugging the "undruggable"

RAS is the most frequently mutated oncogene in cancer and a critical driver of oncogenesis. Therapeutic targeting of RAS has been a goal of cancer research for more than 30 years due to its essential role in tumor formation and maintenance. Yet the quest to inhibit this challenging foe has been elusive. Although once considered "undruggable", the struggle to directly inhibit RAS has seen recent success with the development of pharmacological agents that specifically target the KRAS(G12C) mutant protein, which include the first direct RAS inhibitor to gain entry to clinical trials. However, the limited applicability of these inhibitors to G12C-mutant tumors demands further efforts to identify more broadly efficacious RAS inhibitors. Understanding allosteric influences on RAS may open new avenues to inhibit RAS. Here, we provide a brief overview of RAS biology and biochemistry, discuss the allosteric regulation of RAS, and summarize the various approaches to develop RAS inhibitors.

Organization of Farnesylated, Carboxymethylated KRAS4B on Membranes

Mutations of the Ras proteins HRAS, KRAS4A, KRAS4B, and NRAS are associated with a high percentage of all human cancers. The proteins are composed of highly homologous N-terminal catalytic or globular domains, plus C-terminal hypervariable regions (HVRs). Post-translational modifications of all RAS HVRs helps target RAS proteins to cellular membrane locations where they perform their signaling functions. For the predominant KRAS4 isoform, KRAS4B, post-translational farnesylation and carboxymethylation, along with a patch of HVR basic residues help foster membrane binding. Recent investigations implicate membrane-bound RAS dimers, oligomers, and nanoclusters as landing pads for effector proteins that relay RAS signals. The details of these RAS signaling platforms have not been elucidated completely, in part due to the difficulties in preparing modified proteins. We have employed properly farnesylated and carboxymethylated KRAS4B in lipid monolayer incubations to examine how the proteins assemble on membranes. Our results reveal novel insights into to how KRAS4B may organize on membranes.

Drugging the Small GTPase Pathways in Cancer Treatment: Promises and Challenges

Small GTPases are a family of low molecular weight GTP-hydrolyzing enzymes that cycle between an inactive state when bound to GDP and an active state when associated to GTP. Small GTPases regulate key cellular processes (e.g., cell differentiation, proliferation, and motility) as well as subcellular events (e.g., vesicle trafficking), making them key participants in a great array of pathophysiological processes. Indeed, the dysfunction and deregulation of certain small GTPases, such as the members of the Ras and Arf subfamilies, have been related with the promotion and progression of cancer. Therefore, the development of inhibitors that target dysfunctional small GTPases could represent a potential therapeutic strategy for cancer treatment. This review covers the basic biochemical mechanisms and the diverse functions of small GTPases in cancer. We also discuss the strategies and challenges of inhibiting the activity of these enzymes and delve into new approaches that offer opportunities to target them in cancer therapy.

Progress in targeting RAS with small molecule drugs

RAS proteins have traditionally been deemed undruggable, as they do not possess an active site to which small molecules could bind but small molecules that target one form of oncogenic RAS, KRAS G12C, are already in preclinical and clinical trials, and several other compounds that bind to different RAS proteins at distinct sites are in earlier stage evaluation. KRAS is the major clinical target, as it is by far the most significant form of RAS in terms of cancer incidence. Unfortunately, KRAS exists in two isoforms, each with unique biochemical properties. This complicates efforts to target KRAS specifically. KRAS is also a member of a family of closely related proteins, which share similar effector-binding regions and G-domains, further increasing the challenge of specificity. Nevertheless, progress is being made, driven by new drug discovery technologies and creative science.

Targeting the α4-α5 dimerization interface of K-RAS inhibits tumor formation in vivo

RAS genes are the most commonly mutated oncogenes in human cancers. Despite tremendous efforts over the past several decades, however, RAS-specific inhibitors remain elusive. Thus, targeting RAS remains a highly sought after goal of cancer research. Previously, we reported a new approach to inhibit RAS-dependent signaling and transformation in vitro through targeting the α4-α5 dimerization interface with a novel RAS-specific monobody, termed NS1. Expression of NS1 inhibits oncogenic K-RAS and H-RAS signaling and transformation in vitro. Here, we evaluated the efficacy of targeting RAS dimerization as an approach to inhibit tumor formation in vivo. Using a doxycycline (DOX) regulated NS1 expression system, we demonstrate that DOX-induced NS1 inhibited oncogenic K-RAS driven tumor growth in vivo. Furthermore, we observed context-specific effects of NS1 on RAS-mediated signaling in 2D vs 3D growth conditions. Finally, our results highlight the potential therapeutic efficacy of targeting the α4-α5 dimerization interface as an approach to inhibit RAS-driven tumors in vivo.

Oncogenic Ras Isoforms Signaling Specificity at the Membrane

How do Ras isoforms attain oncogenic specificity at the membrane? Oncogenic KRas, HRas, and NRas (K-Ras, H-Ras, and N-Ras) differentially populate distinct cancers. How they selectively activate effectors and why is KRas4B the most prevalent are highly significant questions. Here, we consider determinants that may bias isoform-specific effector activation and signaling at the membrane. We merge functional data with a conformational view to provide mechanistic insight. Cell-specific expression levels, pathway cross-talk, and distinct interactions are the key, but conformational trends can modulate selectivity. There are two major pathways in oncogenic Ras-driven proliferation: MAPK (Raf/MEK/ERK) and PI3Kα/Akt/mTOR. All membrane-anchored, proximally located, oncogenic Ras isoforms can promote Raf dimerization and fully activate MAPK signaling. So why the differential statistics of oncogenic isoforms in distinct cancers and what makes KRas so highly oncogenic? Many cell-specific factors may be at play, including higher KRAS mRNA levels. As a key factor, we suggest that because only KRas4B binds calmodulin, only KRas can fully activate PI3Kα/Akt signaling. We propose that full activation of both MAPK and PI3Kα/Akt proliferative pathways by oncogenic KRas4B-but not by HRas or NRas-may help explain why the KRas4B isoform is especially highly populated in certain cancers. We further discuss pharmacologic implications. Cancer Res; 78(3); 593-602. ©2017 AACR.

Oncogenic Ras Isoforms Signaling Specificity at the Membrane

How do Ras isoforms attain oncogenic specificity at the membrane? Oncogenic KRas, HRas, and NRas (K-Ras, H-Ras, and N-Ras) differentially populate distinct cancers. How they selectively activate effectors and why is KRas4B the most prevalent are highly significant questions. Here, we consider determinants that may bias isoform-specific effector activation and signaling at the membrane. We merge functional data with a conformational view to provide mechanistic insight. Cell-specific expression levels, pathway cross-talk, and distinct interactions are the key, but conformational trends can modulate selectivity. There are two major pathways in oncogenic Ras-driven proliferation: MAPK (Raf/MEK/ERK) and PI3Kα/Akt/mTOR. All membrane-anchored, proximally located, oncogenic Ras isoforms can promote Raf dimerization and fully activate MAPK signaling. So why the differential statistics of oncogenic isoforms in distinct cancers and what makes KRas so highly oncogenic? Many cell-specific factors may be at play, including higher KRAS mRNA levels. As a key factor, we suggest that because only KRas4B binds calmodulin, only KRas can fully activate PI3Kα/Akt signaling. We propose that full activation of both MAPK and PI3Kα/Akt proliferative pathways by oncogenic KRas4B-but not by HRas or NRas-may help explain why the KRas4B isoform is especially highly populated in certain cancers. We further discuss pharmacologic implications. Cancer Res; 78(3); 593-602. ©2017 AACR.

Direct inhibition of RAS: Quest for the Holy Grail?

RAS GTPases (H-, K-, and N-RAS) are the most frequently mutated oncoprotein family in human cancer. However, the relatively smooth surface architecture of RAS and its picomolar affinity for nucleotide have given rise to the assumption that RAS is an "undruggable" target. Recent advancements in drug screening, molecular modeling, and a greater understanding of RAS function have led to a resurgence in efforts to pharmacologically target this challenging foe. This review focuses on the state of the art of RAS inhibition, the approaches taken to achieve this goal, and the challenges of translating these discoveries into viable therapeutics.