A New View of Ras Isoforms in Cancers (original) (raw)
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Ras isoform abundance and signalling in human cancer cell lines
Oncogene, 2008
The ubiquitously expressed major Ras isoforms: H-, Kand N-Ras, are highly conserved, yet exhibit different biological outputs. We have compared the relative efficiencies with which epidermal or hepatocyte growth factor activates Ras isoforms and the requirement for specific isoforms in the activation of downstream pathways. We find that the relative coupling efficiencies to each Ras isoform are conserved between stimuli. Furthermore, in both cases, inhibition of receptor endocytosis led to reduced N-and H-Ras activation, but K-Ras was unaffected. Acute knockdown of each isoform with siRNA allows endogenous Ras isoform function and abundance to be probed. This revealed that there is significant variation in the contribution of individual isoforms to total Ras across a panel of cancer cell lines although typically KXNbH. Intriguingly, cancer cell lines where a significant fraction of endogenous Ras is oncogenically mutated showed attenuated activation of canonical Ras effector pathways. We profiled the contribution of each Ras isoform to the total Ras pool allowing interpretation of the effect of isoform-specific knockdown on signalling outcomes. In contrast to previous studies indicating preferential coupling of isoforms to Raf and PtdIns-3kinase pathways, we find that endogenous Ras isoforms show no specific coupling to these major Ras pathways.
The role of wild type RAS isoforms in cancer
Seminars in Cell & Developmental Biology, 2016
Mutationally activated RAS proteins are critical oncogenic drivers in nearly 30% of all human cancers. As with mutant RAS, the role of wild type RAS proteins in oncogenesis, tumour maintenance and metastasis is context-dependent. Complexity is introduced by the existence of multiple RAS genes (HRAS, KRAS, NRAS) and protein "isoforms" (KRAS4A, KRAS4B), by the ever more complicated network of RAS signaling, and by the increasing identification of numerous genetic aberrations in cancers that do and do not harbour mutant RAS. Numerous mouse model carcinogenesis studies and examination of patient tumours reveal that, in RASmutant cancers, wild type RAS proteins are likely to serve as tumour suppressors when the mutant RAS is of the same isoform. This evidence is particularly robust in KRAS mutant cancers, which often display suppression or loss of wild type KRAS, but is not as strong for NRAS. In contrast, although not yet fully elucidated, the preponderance of evidence indicates that wild type RAS proteins play a tumour promoting role when the mutant RAS is of a different isoform. In non-RAS mutant cancers, wild type RAS is recognized as a mediator of oncogenic signaling due to chronic activation of upstream receptor tyrosine kinases that feed through RAS. Additionally, in the absence of mutant RAS, activation of wild type RAS may drive cancer upon the loss of negative RAS regulators such as NF1 GAP or SPRY proteins. Here we explore the current state of knowledge with respect to the roles of wild type RAS proteins in human cancers.
The Hypervariable Region of K-Ras4B Is Responsible for Its Specific Interactions with Calmodulin
Biochemistry, 2009
K-Ras4B belongs to the family of p21 Ras GTPases, which play an important role in cell proliferation, survival and motility. The p21 Ras proteins such as K-Ras4B, K-Ras4A, H-Ras, and N-Ras, share 85% sequence homology and activate very similar signaling pathways. Only the C-terminal hypervariable regions differ significantly. A growing body of literature demonstrates that each Ras isoform possesses unique functions in normal physiological processes as well as in pathogenesis. One of the central questions in the field of Ras biology is how these very similar proteins achieve such remarkable specificity in protein-protein interactions that regulate signal transduction pathways. Here we explore specific binding of K-Ras4B to calmodulin. Using NMR techniques and isothermal titration calorimetry we demonstrate that the hypervariable region of K-Ras contributes in a major way to the interaction with calmodulin while the catalytic domain of K-Ras4B provides a way to control the interaction by nucleotide binding. The hypervariable region of K-Ras4B binds specifically to the C-terminal domain of Ca 2+ -loaded calmodulin with micromolar affinity, while the GTP-γ-S loaded catalytic domain of K-Ras4B may interact with the N-terminal domain of calmodulin.
Targeting RAS signalling pathways in cancer therapy
Nature Reviews Cancer, 2003
RAS proteins: diversity and processing. The RAS proteins are members of a large superfamily of lowmolecular-weight GTP-binding proteins, which can be divided into several families according to the degree of sequence conservation. Different families are important for different cellular processes -the RAS family controls cell growth and the RHO family controls the actin cytoskeleton. Three members of the RAS family -HRAS, KRAS and NRAS -are found to be activated by mutation in human tumours 6 . These three members are very closely related, having 85% amino acid sequence identity and, although they function in very similar ways, some indications of subtle differences between them have recently come to light. The HRAS, KRAS and NRAS proteins are widely expressed, with KRAS being expressed in almost all cell types. Knockout studies have shown that Hras and Nras, either alone or in combination, are not required for normal development in the mouse, whereas Kras is essential 7 . This might reflect different molecular functions of the three proteins, but is more likely to reflect the more ubiquitous expression of KRAS.
Significance of Ras Signaling in Cancer and Strategies for its Control
Oncology & Hematology Review (US), 2015
Ras is a GTP-binding protein and is the most widely studied oncoprotein. To achieve its biological activity, it must undergo post-translation modification. Ras acts as a typical molecular switch. The GTP-bound Ras can activate several downstream effector pathways. Ras signaling regulates many important physiologic processes within a cell, such as cell cycle progression, survival, apoptosis, etc. Several studies have found mutation in Ras or its effectors in various types of tumors. Therefore, Ras or its downstream effectors can be attractive drug targets against various types of tumors in cancer therapeutics. Some therapeutic agents against Ras effectors, such as Raf, MEK1/2, PI3K, AKT etc., have successfully managed to enter into phase I and II trials. This targeted drug design could be envisaged in mainly four ways, such as prevention of Ras-GTP formation, covalent locking of the GDP-bound Ras, inhibition of Ras-effector interactions, or impairment of post-translational modificati...
The Hypervariable Region of K-Ras4B Governs Molecular Recognition and Function
International Journal of Molecular Sciences, 2019
The flexible C-terminal hypervariable region distinguishes K-Ras4B, an important proto-oncogenic GTPase, from other Ras GTPases. This unique lysine-rich portion of the protein harbors sites for post-translational modification, including cysteine prenylation, carboxymethylation, phosphorylation, and likely many others. The functions of the hypervariable region are diverse, ranging from anchoring K-Ras4B at the plasma membrane to sampling potentially auto-inhibitory binding sites in its GTPase domain and participating in isoform-specific protein–protein interactions and signaling. Despite much research, there are still many questions about the hypervariable region of K-Ras4B. For example, mechanistic details of its interaction with plasma membrane lipids and with the GTPase domain require further clarification. The roles of the hypervariable region in K-Ras4B-specific protein–protein interactions and signaling are incompletely defined. It is also unclear why post-translational modific...
Journal of Hepatology, 2011
Background & Aims: Aberrant activation of the RAS pathway is ubiquitous in human hepatocarcinogenesis, but the molecular mechanisms leading to RAS induction in the absence of RAS mutations remain under-investigated. We defined the role of Ras GTPase activating proteins (GAPs) in the constitutive activity of Ras signaling during human hepatocarcinogenesis. Methods: The mutation status of RAS genes and RAS effectors was assessed in a collection of human hepatocellular carcinomas (HCC). Levels of RAS GAPs (RASA1-4, RASAL1, nGAP, SYNGAP1, DAB2IP, and NF1) and the RASAL1 upstream inducer PITX1 were determined by real-time RT-PCR and immunoblotting. The promoter and genomic status of RASAL1, DAB2IP, NF1, and PITX1 were assessed by methylation assays and microsatellite analysis. Effects of RASAL1, DAB2IP, and PITX1 on HCC growth were evaluated by transfection and siRNA analyses of HCC cell lines. Results: In the absence of Ras mutations, downregulation of at least one RAS GAP (RASAL1, DAB2IP, or NF1) was found in all HCC samples. Low levels of DAB2IP and PITX1 were detected mostly in a HCC subclass from patients with poor survival, indicating that these proteins control tumor aggressiveness. In HCC cells, reactivation of RASAL1, DAB2IP, and PITX1 inhibited proliferation and induced apoptosis, whereas their silencing increased proliferation and resistance to apoptosis. Conclusions: Selective suppression of RASAL1, DAB2IP, or NF1 RAS GAPs results in unrestrained activation of Ras signaling in the presence of wild-type RAS in HCC. Published by Elsevier B.V. on behalf of the European Association for the Study of the Liver.
Ras Dimer Formation as a New Signaling Mechanism and Potential Cancer Therapeutic Target
Mini-reviews in Medicinal Chemistry, 2016
The K-, N-, and HRas small GTPases are key regulators of cell physiology and are frequently mutated in human cancers. Despite intensive research, previous efforts to target hyperactive Ras based on known mechanisms of Ras signaling have been met with little success. Several studies have provided compelling evidence for the existence and biological relevance of Ras dimers, establishing a new mechanism for regulating Ras activity in cells additionally to GTP-loading and membrane localization. Existing data also start to reveal how Ras proteins dimerize on the membrane. We propose a dimer model to describe Ras-mediated effector activation, which contrasts existing models of Ras signaling as a monomer or as a 5-8 membered multimer. We also discuss potential implications of this model in both basic and translational Ras biology.
Differential Regulation of RasGAPs in Cancer
Genes & Cancer, 2011
Ever since their discovery as cellular counterparts of viral oncogenes more than 25 years ago, much progress has been made in understanding the complex networks of signal transduction pathways activated by oncogenic Ras mutations in human cancers. The activity of Ras is regulated by nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs), and much emphasis has been put into the biochemical and structural analysis of the Ras/GAP complex. The mechanisms by which GAPs catalyze Ras-GTP hydrolysis have been clarified and revealed that oncogenic Ras mutations confer resistance to GAPs and remain constitutively active. However, it is yet unclear how cells coordinate the large and divergent GAP protein family to promote Ras inactivation and ensure a certain biological response. Different domain arrangements in GAPs to create differential protein-protein and protein-lipid interactions are probably key factors determining the inactivation of the 3 Ras isoforms H-, K-, and N-Ras and their effector pathways. In recent years, in vitro as well as cell-and animal-based studies examining GAP activity, localization, interaction partners, and expression profiles have provided further insights into Ras inactivation and revealed characteristics of several GAPs to exert specific and distinct functions. This review aims to summarize knowledge on the cell biology of RasGAP proteins that potentially contributes to differential regulation of spatiotemporal Ras signaling.