The molecular scaffold kinase suppressor of Ras 1 is a modifier of RasV12-induced and replicative senescence

KSR1 mediates small-cell lung carcinoma tumor initiation and cisplatin resistance

Small-cell lung cancer (SCLC) has a dismal five-year survival rate of less than 7%, with limited advances in first line treatment over the past four decades. Tumor-initiating cells (TICs) contribute to resistance and relapse, a major impediment to SCLC treatment. Here, we identify Kinase Suppressor of Ras 1 (KSR1), a molecular scaffold for the Raf/MEK/ERK signaling cascade, as a critical regulator of SCLC TIC formation and tumor initiation in vivo . We further show that KSR1 mediates cisplatin resistance in SCLC. While 50-70% of control cells show resistance after 6-week exposure to cisplatin, CRISPR/Cas9-mediated KSR1 knockout prevents resistance in >90% of SCLC cells in ASCL1, NeuroD1, and POU2F3 subtypes. KSR1 KO significantly enhances the ability of cisplatin to decrease SCLC TICs via in vitro extreme limiting dilution analysis (ELDA), indicating that KSR1 disruption enhances the cisplatin toxicity of cells responsible for therapeutic resistance and tumor initiation. The ability of KSR1 disruption to prevent cisplatin resistant in H82 tumor xenograft formation supports this conclusion. Previous studies indicate that ERK activation inhibits SCLC tumor growth and development. We observe a minimal effect of pharmacological ERK inhibition on cisplatin resistance and no impact on TIC formation via in vitro ELDA. However, mutational analysis of the KSR1 DEF domain, which mediates interaction with ERK, suggests that ERK interaction with KSR1 is essential for KSR1-driven cisplatin resistance. These findings reveal KSR1 as a key regulatory protein in SCLC biology and a potential therapeutic target across multiple SCLC subtypes.

Statement of Implication

Genetic manipulation of the molecular scaffold KSR1 in small-cell lung cancer cells reveals its contribution to cisplatin resistance and tumor initiation.

SOS1 and KSR1 modulate MEK inhibitor responsiveness to target resistant cell populations based on PI3K and KRAS mutation status

KRAS is the most commonly mutated oncogene. Targeted therapies have been developed against mediators of key downstream signaling pathways, predominantly components of the RAF/MEK/ERK kinase cascade. Unfortunately, single-agent efficacy of these agents is limited both by intrinsic and acquired resistance. Survival of drug-tolerant persister cells within the heterogeneous tumor population and/or acquired mutations that reactivate receptor tyrosine kinase (RTK)/RAS signaling can lead to outgrowth of tumor-initiating cells (TICs) and drive therapeutic resistance. Here, we show that targeting the key RTK/RAS pathway signaling intermediates SOS1 (Son of Sevenless 1) or KSR1 (Kinase Suppressor of RAS 1) both enhances the efficacy of, and prevents resistance to, the MEK inhibitor trametinib in KRAS-mutated lung (LUAD) and colorectal (COAD) adenocarcinoma cell lines depending on the specific mutational landscape. The SOS1 inhibitor BI-3406 enhanced the efficacy of trametinib and prevented trametinib resistance by targeting spheroid-initiating cells in KRASG12/G13-mutated LUAD and COAD cell lines that lacked PIK3CA comutations. Cell lines with KRASQ61 and/or PIK3CA mutations were insensitive to trametinib and BI-3406 combination therapy. In contrast, deletion of the RAF/MEK/ERK scaffold protein KSR1 prevented drug-induced SIC upregulation and restored trametinib sensitivity across all tested KRAS mutant cell lines in both PIK3CA-mutated and PIK3CA wild-type cancers. Our findings demonstrate that vertical inhibition of RTK/RAS signaling is an effective strategy to prevent therapeutic resistance in KRAS-mutated cancers, but therapeutic efficacy is dependent on both the specific KRAS mutant and underlying comutations. Thus, selection of optimal therapeutic combinations in KRAS-mutated cancers will require a detailed understanding of functional dependencies imposed by allele-specific KRAS mutations.

Kinase Suppressor of RAS 1 (KSR1) Maintains the Transformed Phenotype of BRAFV600E Mutant Human Melanoma Cells

Kinase Suppressor of RAS 1 (KSR1) is a scaffolding protein for the RAS-RAF-MEK-ERK pathway, which is one of the most frequently altered pathways in human cancers. Previous results have shown that KSR1 has a critical role in mutant RAS-mediated transformation. Here, we examined the role of KSR1 in mutant BRAF transformation. We used CRISPR/Cas9 to knock out KSR1 in a BRAFV600E-transformed melanoma cell line. KSR1 loss produced a complex phenotype characterised by impaired proliferation, cell cycle defects, decreased transformation, decreased invasive migration, increased cellular senescence, and increased apoptosis. To decipher this phenotype, we used a combination of proteomic ERK substrate profiling, global protein expression profiling, and biochemical validation assays. The results suggest that KSR1 directs ERK to phosphorylate substrates that have a critical role in ensuring cell survival. The results further indicate that KSR1 loss induces the activation of p38 Mitogen-Activated Protein Kinase (MAPK) and subsequent cell cycle aberrations and senescence. In summary, KSR1 function plays a key role in oncogenic BRAF transformation.

Scaffold coupling: ERK activation by trans-phosphorylation across different scaffold protein species

RAS-ERK (extracellular signal-regulated kinase) pathway signals are modulated by scaffold proteins that assemble the components of different kinase tiers into a sequential phosphorylation cascade. In the prevailing model scaffold proteins function as isolated entities, where the flux of phosphorylation events progresses downstream linearly, to achieve ERK phosphorylation. We show that different types of scaffold proteins, specifically KSR1 (kinase suppressor of Ras 1) and IQGAP1 (IQ motif-containing guanosine triphosphatase activating protein 1), can bind to each other, forming a complex whereby phosphorylation reactions occur across both species. MEK (mitogen-activated protein kinase kinase) bound to IQGAP1 can phosphorylate ERK docked at KSR1, a process that we have named "trans-phosphorylation." We also reveal that ERK trans-phosphorylation participates in KSR1-regulated adipogenesis, and it also underlies the modest cytotoxicity exhibited by KSR-directed inhibitors. Overall, we identify interactions between scaffold proteins and trans-phosphorylation as an additional level of regulation in the ERK cascade, with broad implications in signaling and the design of scaffold protein-aimed therapeutics.

KSR2-14-3-3ζ complex serves as a biomarker and potential therapeutic target in sorafenib-resistant hepatocellular carcinoma

Background

Kinase suppressor of Ras 2 (KSR2) is a regulator of MAPK signaling that is overactivated in most hepatocellular carcinoma (HCC). We sought to determine the role of KSR2 in HCC pathogenesis.

Methods

We tested the level of KSR2 in HCC tissues and cell lines by tissue microarray, qPCR, and western blotting. Functionally, we determined the effects of KSR2 on the proliferation, migration, and invasion of HCC cells through colony formation assays, scratch assays, transwell migration assays, and xenograft tumor models. Co-immunoprecipitation (co-IP) experiments were used to assess the interaction of phospho-serine binding protein 14–3-3ζ and KSR2, and the effects of this interaction on growth and proliferation of human HCC cells were tested by co-overexpression and knockdown experiments. Additionally, we used flow cytometry to examine whether the KSR2 and 14–3-3ζ interaction conveys HCC resistance to sorafenib.

Results

KSR2 was significantly upregulated in HCC tissues and cell lines, and high KSR2 expression associated with poor prognosis in HCC patients. KSR2 knockdown significantly suppressed HCC cell proliferation, migration, and invasion in vitro and in vivo. Mechanistically, co-IP experiments identified that 14–3-3ζ complexed with KSR2, and elevated 14–3-3ζ increased KSR2 protein levels in HCC cells. Importantly, Kaplan–Meier survival analysis showed that patients with both high KSR2 and high 14–3-3ζ expression levels had the shortest survival times and poorest prognoses. Interestingly, HCC cells overexpressing both KSR2 and 14–3-3ζ, rather than either protein alone, showed hyperactivated MAPK signaling and resistance to sorafenib.

Conclusions

Our results provide new insights into the pro-tumorigenic role of KSR2 and its regulation of the MAPK pathway in HCC. The KSR2–14–3-3ζ interaction may be a therapeutic target to enhance the sorafenib sensitivity of HCC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40364-022-00361-9.

Conformational control and regulation of the pseudokinase KSR via small molecule binding interactions

Pseudokinases often operate through functionally related enzymes and receptors. A prime example is the pseudokinase KSR (Kinase Suppressor of RAS), which can act as both an amplifier and inhibitor of members in the RAS-MAPK (Mitogen Activated Protein Kinase) signaling pathway. KSR is structurally related to the active RAF kinases over multiple domains; moreover, the pseudokinase domain of KSR forms physical and regulatory complexes with both RAF and MEK through distinct interfaces. Characterization of small molecule interactions on KSR has been used to uncover novel chemical tools and understand the mechanism of action of clinical drugs. Here, we elaborate on assays and structural methods for measuring binding at orthosteric and interfacial binding sites on KSR. These distinct small molecule pockets provide therapeutic paths for targeting KSR1 and KSR2 pseudokinases in disease, including in RAS and RAF mutant cancers.

Molecular dissection on inhibition of Ras-induced cellular senescence by small t antigen of SV40

Simian virus 40 (SV40) is a potentially oncogenic virus of monkey origin. Transmission, prevalence, and pathogenicity rates of SV40 are unclear, but infection can occur in humans, for example individuals with high contact with rhesus macaques and individuals that received contaminated early batches of polio vaccines in 1950-1963. In addition, several human polyomaviruses, proven carcinogenic, are also highly common in global populations. Cellular senescence is a major mechanism of cancer prevention in vivo. Hyperactivation of Ras usually induces cellular senescence rather than cell transformation. Previous studies suggest small t antigen (ST) of SV40 may interfere with cellular senescence induced by Ras. In the current study, ST was demonstrated to inhibit Ras-induced cellular senescence (RIS) and accumulation of DNA damage in Ras-activated cells. In addition, ST suppressed the signal transmission from BRaf to MEK and thus blocked the downstream transmission of the activated Ras signal. B56γ knockdown mimicked the inhibitory effects of ST overexpression on RIS. Furthermore, KSR1 knockdown inhibited Ras activation and the subsequent cellular senescence. Further mechanism studies indicated that the phosphorylation level of KSR1 rather than the levels of the total protein regulates the activation of Ras signaling pathway. In sum, ST inhibits the continuous hyperactivation of Ras signals by interfering with the normal functions of PP2A-B56γ of dephosphorylating KSR1, thus inhibiting the occurrence of cellular senescence. Although the roles of SV40 in human carcinogenesis are controversial so far, our study has shown that ST of polyomaviruses has tumorigenic potential by inhibiting oncogene-induced senescence (OIS) as a proof of concept.

IKBKE-driven TPL2 and MEK1 phosphorylations sustain constitutive ERK1/2 activation in tumor cells

IKBKE have been associated with numerous cancers. As a result, IKBKE have emerged as potential target for cancer therapy. Accumulating evidence support that IKBKE orchestrate tumor cell survival in cancers. Here we evaluated the possible link between IKBKE and ERK phosphorylation. The effects of IKBKE silencing on MAPK activation in tumor vs. normal cells were evaluated via WB and RT-PCR. Ectopically expressed IKBKE, TPL2 or MEK1 constructs were used to examine the possible interactions among them via co-IP. In vitro kinase assays were performed to understand nature of the observed interactions. In tumors, IKBKE regulates MEK/ERK constitutive activations in vitro and in vivo. IKBKE and TPL2 physically interact and this interaction leads to TPL2 phosphorylation. We describe here a novel regulatory link between IKBKE and constitutive ERK1/2 activation in tumor cells. This new circuitry may be relevant for tumor cell survival in various malignancies.

KSR1- and ERK-dependent translational regulation of the epithelial-to-mesenchymal transition

The epithelial-to-mesenchymal transition (EMT) is considered a transcriptional process that induces a switch in cells from a polarized state to a migratory phenotype. Here, we show that KSR1 and ERK promote EMT-like phenotype through the preferential translation of Epithelial-Stromal Interaction 1 (EPSTI1), which is required to induce the switch from E- to N-cadherin and coordinate migratory and invasive behavior. EPSTI1 is overexpressed in human colorectal cancer (CRC) cells. Disruption of KSR1 or EPSTI1 significantly impairs cell migration and invasion in vitro, and reverses EMT-like phenotype, in part, by decreasing the expression of N-cadherin and the transcriptional repressors of E-cadherin expression, ZEB1 and Slug. In CRC cells lacking KSR1, ectopic EPSTI1 expression restored the E- to N-cadherin switch, migration, invasion, and anchorage-independent growth. KSR1-dependent induction of EMT-like phenotype via selective translation of mRNAs reveals its underappreciated role in remodeling the translational landscape of CRC cells to promote their migratory and invasive behavior.

A Gene Expression High-Throughput Screen (GE-HTS) for Coordinated Detection of Functionally Similar Effectors in Cancer

Genome-wide, loss-of-function screening can be used to identify novel vulnerabilities upon which specific tumor cells depend for survival. Functional Signature Ontology (FUSION) is a gene expression-based high-throughput screening (GE-HTS) method that allows researchers to identify functionally similar proteins, small molecules, and microRNA mimics, revealing novel therapeutic targets. FUSION uses cell-based high-throughput screening and computational analysis to match gene expression signatures produced by natural products to those produced by small interfering RNA (siRNA) and synthetic microRNA libraries to identify putative protein targets and mechanisms of action (MoA) for several previously undescribed natural products. We have used FUSION to screen for functional analogues to Kinase suppressor of Ras 1 (KSR1), a scaffold protein downstream of Ras in the Raf-MEK-ERK kinase cascade, and biologically validated several proteins with functional similarity to KSR1. FUSION incorporates bioinformatics analysis that may offer higher resolution of the endpoint readout than other screens which utilize Boolean outputs regarding a single pathway activation (i.e., synthetic lethal and cell proliferation). Challenges associated with FUSION and other high-content genome-wide screens include variation, batch effects, and controlling for potential off-target effects. In this review, we discuss the efficacy of FUSION to identify novel inhibitors and oncogene-induced changes that may be cancer cell-specific as well as several potential pitfalls within FUSION and best practices to avoid them.

Growth Inhibitory Signaling of the Raf/MEK/ERK Pathway

In response to extracellular stimuli, the Raf/MEK/extracellular signal-regulated kinase (ERK) pathway regulates diverse cellular processes. While mainly known as a mitogenic signaling pathway, the Raf/MEK/ERK pathway can mediate not only cell proliferation and survival but also cell cycle arrest and death in different cell types. Growing evidence suggests that the cell fate toward these paradoxical physiological outputs may be determined not only at downstream effector levels but also at the pathway level, which involves the magnitude of pathway activity, spatial-temporal regulation, and non-canonical functions of the molecular switches in this pathway. This review discusses recent updates on the molecular mechanisms underlying the pathway-mediated growth inhibitory signaling, with a major focus on the regulation mediated at the pathway level.

Syndecan-Mediated Ligation of ECM Proteins Triggers Proliferative Arrest of Disseminated Tumor Cells

Systemic dissemination of tumor cells often begins long before the development of overt metastases, revealing the inefficient nature of the metastatic process. Thus, already at the time of initial clinical presentation, many patients with cancer harbor a myriad disseminated tumor cells (DTC) throughout the body, most of which are found as mitotically quiescent solitary cells. This indicates that the majority of DTCs fail, for still unknown reasons, to initiate rapid proliferation after entering foreign tissue, which likely contributes significantly to the inefficiency of metastasis formation. Here, we showed that extracellular matrix (ECM) components of the host parenchyma prevented proliferation of DTCs that had recently infiltrated foreign tissue by binding to syndecan receptors expressed on the surface of these cells. This led to the recruitment of the Par-3:Par-6:atypical PKC protein complex, a critical regulator of cell polarity, to the plasma membrane and release of Par-1 kinase into the cytosol. Cytosolic Par-1 bound, phosphorylated, and inactivated KSR scaffolding proteins ultimately inhibited Ras/ERK signaling and, in turn, cell proliferation. Inhibition of the syndecan-mediated signaling restored the proliferation of otherwise dormant DTCs, enabling these cells to efficiently colonize foreign tissues. Intriguingly, naturally aggressive cancer cells overcame the antiproliferative effect of syndecan-mediated signaling either by shutting down this signaling pathway or by activating a proproliferative signaling pathway that works independent of syndecan-mediated signaling. Collectively, these observations indicate that the proliferative arrest of DTCs is attributable, in part, to the syndecan-mediated ligation of ECM proteins. SIGNIFICANCE: This study identifies a novel signaling pathway that regulates the proliferative dormancy of individual disseminated tumor cells.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/23/5944/F1.large.jpg.

Oncogenic RAS isoforms show a hierarchical requirement for the guanine nucleotide exchange factor SOS2 to mediate cell transformation

About a third of tumors have activating mutations in HRAS, NRAS, or KRAS, genes encoding guanosine triphosphatases (GTPases) of the RAS family. In these tumors, wild-type RAS cooperates with mutant RAS to promote downstream effector activation and cell proliferation and transformation, suggesting that upstream activators of wild-type RAS are important modulators of mutant RAS-driven oncogenesis. The guanine nucleotide exchange factor (GEF) SOS1 mediates KRAS-driven proliferation, but little is understood about the role of SOS2. We found that RAS family members have a hierarchical requirement for the expression and activity of SOS2 to drive cellular transformation. In mouse embryonic fibroblasts (MEFs), SOS2 critically mediated mutant KRAS-driven, but not HRAS-driven, transformation. Sos2 deletion reduced epidermal growth factor (EGF)-dependent activation of wild-type HRAS and phosphorylation of the kinase AKT in cells expressing mutant RAS isoforms. Assays using pharmacological inhibitors revealed a hierarchical requirement for signaling by phosphoinositide 3-kinase (PI3K) in promoting RAS-driven cellular transformation that mirrored the requirement for SOS2. KRAS-driven transformation required the GEF activity of SOS2 and was restored in Sos2^-/- MEFs by expression of constitutively activated PI3K. Finally, CRISPR/Cas9-mediated deletion of SOS2 reduced EGF-stimulated AKT phosphorylation and synergized with MEK inhibition to revert the transformed phenotype of human KRAS mutant pancreatic and lung tumor cells. These results indicate that SOS2-dependent PI3K signaling mediates mutant KRAS-driven transformation, revealing therapeutic targets in KRAS-driven cancers. Our data also reveal the importance of three-dimensional culture systems in investigating the mediators of mutant KRAS.

Homozygous KSR1 deletion attenuates morbidity but does not prevent tumor development in a mouse model of RAS-driven pancreatic cancer

Given the frequency with which MAP kinase signaling is dysregulated in cancer, much effort has been focused on inhibiting RAS signaling for therapeutic benefit. KSR1, a pseudokinase that interacts with RAF, is a potential target; it was originally cloned in screens for suppressors of constitutively active RAS, and its deletion prevents RAS-mediated transformation of mouse embryonic fibroblasts. In this work, we used a genetically engineered mouse model of pancreatic cancer to assess whether KSR1 deletion would influence tumor development in the setting of oncogenic RAS. We found that Ksr1^-/- mice on this background had a modest but significant improvement in all-cause morbidity compared to Ksr1^+/+ and Ksr1^+/- cohorts. Ksr1^-/- mice, however, still developed tumors, and precursor pancreatic intraepithelial neoplastic (PanIN) lesions were detected within a similar timeframe compared to Ksr1^+/+ mice. No significant differences in pERK expression or in proliferation were noted. RNA sequencing also did not reveal any unique genetic signature in Ksr1^-/- tumors. Further studies will be needed to determine whether and in what settings KSR inhibition may be clinically useful.

Oncogenic RAS-Induced Perinuclear Signaling Complexes Requiring KSR1 Regulate Signal Transmission to Downstream Targets

The precise characteristics that distinguish normal and oncogenic RAS signaling remain obscure. Here, we show that oncogenic RAS and BRAF induce perinuclear relocalization of several RAS pathway proteins, including the kinases CK2 and p-ERK1/2 and the signaling scaffold KSR1. This spatial reorganization requires endocytosis, the kinase activities of MEK-ERK and CK2, and the presence of KSR1. CK2α colocalizes with KSR1 and Rab11, a marker of recycling endosomes, whereas p-ERK associates predominantly with a distinct KSR1-positive endosomal population. Notably, these perinuclear signaling complexes (PSC) are present in tumor cell lines, mouse lung tumors, and mouse embryonic fibroblasts undergoing RAS-induced senescence. PSCs are also transiently induced by growth factors (GF) in nontransformed cells with delayed kinetics (4-6 hours), establishing a novel late phase of GF signaling that appears to be constitutively activated in tumor cells. PSCs provide an essential platform for RAS-induced phosphorylation and activation of the prosenescence transcription factor C/EBPβ in primary MEFs undergoing senescence. Conversely, in tumor cells, C/EBPβ activation is suppressed by 3'UTR-mediated localization of Cebpb transcripts to a peripheral cytoplasmic domain distinct from the PSC region. Collectively, our findings indicate that sustained PSC formation is a critical feature of oncogenic RAS/BRAF signaling in cancer cells that controls signal transmission to downstream targets by regulating selective access of effector kinases to substrates such as C/EBPβ.Significance: In addressing the long-standing question of the difference between normal and oncogenic RAS pathway signaling, this study shows that oncogenic RAS specifically triggers constitutive endocytosis-dependent movement of effector kinases to a perinuclear region, thereby creating connections to unique downstream targets such as the core prosenescence and the inflammatory regulatory transcription factor C/EBPβ. Cancer Res; 78(4); 891-908. ©2017 AACR.

Coordinating ERK signaling via the molecular scaffold Kinase Suppressor of Ras

Many cancers, including those of the colon, lung, and pancreas, depend upon the signaling pathways induced by mutated and constitutively active Ras. The molecular scaffolds Kinase Suppressor of Ras 1 and 2 (KSR1 and KSR2) play potent roles in promoting Ras-mediated signaling through the Raf/MEK/ERK kinase cascade. Here we summarize the canonical role of KSR in cells, including its central role as a scaffold protein for the Raf/MEK/ERK kinase cascade, its regulation of various cellular pathways mediated through different binding partners, and the phenotypic consequences of KSR1 or KSR2 genetic inactivation. Mammalian KSR proteins have a demonstrated role in cellular and organismal energy balance with implications for cancer and obesity. Targeting KSR1 in cancer using small molecule inhibitors has potential for therapy with reduced toxicity to the patient. RNAi and small molecule screens using KSR1 as a reference standard have the potential to expose and target vulnerabilities in cancer. Interestingly, although KSR1 and KSR2 are similar in structure, KSR2 has a distinct physiological role in regulating energy balance. Although KSR proteins have been studied for two decades, additional analysis is required to elucidate both the regulation of these molecular scaffolds and their potent effect on the spatial and temporal control of ERK activation in health and disease.

KSR as a therapeutic target for Ras-dependent cancers

INTRODUCTION

Targeting downstream effectors required for oncogenic Ras signaling is a potential alternative or complement to the development of more direct approaches targeting Ras in the treatment of Ras-dependent cancers. Areas covered: Here we review literature pertaining to the molecular scaffold Kinase Suppressor of Ras (KSR) and its role in promoting signals critical to tumor maintenance. We summarize the phenotypes in knockout models, describe the role of KSR in cancer, and outline the structure and function of the KSR1 and KSR2 proteins. We then focus on the most recent literature that describes the crystal structure of the kinase domain of KSR2 in complex with MEK1, KSR-RAF dimerization particularly in response to RAF inhibition, and novel attempts to target KSR proteins directly. Expert opinion: KSR is a downstream effector of Ras-mediated tumorigenesis that is dispensable for normal growth and development, making it a desirable target for the development of novel therapeutics with a high therapeutic index. Recent advances have revealed that KSR can be functionally inhibited using a small molecule that stabilizes KSR in an inactive conformation. The efficacy and potential for this novel approach to be used clinically in the treatment of Ras-driven cancers is still being investigated.

Phosphorylation of Rap1 by cAMP-dependent Protein Kinase (PKA) Creates a Binding Site for KSR to Sustain ERK Activation by cAMP

Cyclic adenosine monophosphate (cAMP) is an important mediator of hormonal stimulation of cell growth and differentiation through its activation of the extracellular signal-regulated kinase (ERK) cascade. Two small G proteins, Ras and Rap1 have been proposed to mediate this activation. Using HEK293 cells as a model system, we have recently shown that both Ras and Rap1 are required for cAMP signaling to ERKs. However, cAMP-dependent Ras signaling to ERKs is transient and rapidly terminated by PKA phosphorylation of the Raf isoforms C-Raf and B-Raf. In contrast, cAMP-dependent Rap1 signaling to ERKs and Rap1 is potentiated by PKA. We show that this is due to sustained binding of B-Raf to Rap1. One of the targets of PKA is Rap1 itself, directly phosphorylating Rap1a on serine 180 and Rap1b on serine 179. We show that these phosphorylations create potential binding sites for the adaptor protein 14-3-3 that links Rap1 to the scaffold protein KSR. These results suggest that Rap1 activation of ERKs requires PKA phosphorylation and KSR binding. Because KSR and B-Raf exist as heterodimers within the cell, this binding also brings B-Raf to Rap1, allowing Rap1 to couple to ERKs through B-Raf binding to Rap1 independently of its Ras-binding domain.

Kinase Suppressor of Ras 2 (KSR2) expression in the brain regulates energy balance and glucose homeostasis

Objective

Kinase Suppressor of Ras 2 (KSR2) is a molecular scaffold coordinating Raf/MEK/ERK signaling that is expressed at high levels in the brain. KSR2 disruption in humans and mice causes obesity and insulin resistance. Understanding the anatomical location and mechanism of KSR2 function should lead to a better understanding of physiological regulation over energy balance.

Methods

Mice bearing floxed alleles of KSR2 (KSR2^fl/fl) were crossed with mice expressing the Cre recombinase expressed by the Nestin promoter (Nes-Cre) to produce Nes-CreKSR2^fl/fl mice. Growth, body composition, food consumption, cold tolerance, insulin and free fatty acid levels, glucose, and AICAR tolerance were measured in gender and age matched KSR2^−/− mice

Results

Nes-CreKSR2^fl/fl mice lack detectable levels of KSR2 in the brain. The growth and onset of obesity of Nes-CreKSR2^fl/fl mice parallel those observed in KSR2^−/− mice. As in KSR2^−/− mice, Nes-CreKSR2^fl/fl are glucose intolerant with elevated fasting and cold intolerance. Male Nes-CreKSR2^fl/fl mice are hyperphagic, but female Nes-CreKSR2^fl/fl mice are not. Unlike KSR2^−/− mice, Nes-CreKSR2^fl/fl mice respond normally to leptin and AICAR, which may explain why the degree of obesity of adult Nes-CreKSR2^fl/fl mice is not as severe as that observed in KSR2^−/− animals.

Conclusions

These observations suggest that, in the brain, KSR2 regulates energy balance via control of feeding behavior and adaptive thermogenesis, while a second KSR2-dependent mechanism, functioning through one or more other tissues, modulates sensitivity to leptin and activators of the energy sensor AMPK.

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Brain-specific KSR2 knockout causes obesity and glucose intolerance in both genders, but hyperphagia only in male mice.

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Brain-specific KSR2 knockout suppresses body temperature, before obesity.

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KSR2 in the brain regulates energy balance via control of feeding behavior and adaptive thermogenesis.

KSR1 and EPHB4 Regulate Myc and PGC1β To Promote Survival of Human Colon Tumors

Identification and characterization of survival pathways active in tumor cells but absent in normal tissues provide opportunities to develop effective anticancer therapies with reduced toxicity to the patient. We show here that, like kinase suppressor of Ras 1 (KSR1), EPH (erythropoietin-producing hepatocellular carcinoma) receptor B4 (EPHB4) is aberrantly overexpressed in human colon tumor cell lines and selectively required for their survival. KSR1 and EPHB4 support tumor cell survival by promoting the expression of downstream targets, Myc and the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1β (PGC1β). While KSR1 promotes the aberrant expression of Myc and the PGC1β protein via a posttranscriptional mechanism, EPHB4 has a greater effect on Myc and PGC1β expression via its ability to elevate mRNA levels. Subsequent analysis of the posttranscriptional regulation demonstrated that KSR1 promotes the translation of Myc protein. These findings reveal novel KSR1- and EPHB4-dependent signaling pathways supporting the survival of colorectal cancer cells through regulation of Myc and PGC1β, suggesting that inhibition of KSR1 or EPHB4 effectors may lead to selective toxicity in colorectal tumors.

Differential localization of A-Raf regulates MST2-mediated apoptosis during epithelial differentiation

A-Raf belongs to the family of oncogenic Raf kinases that are involved in mitogenic signaling by activating the MEK-ERK pathway. Low kinase activity of A-Raf toward MEK suggested that A-Raf might have alternative functions. We recently identified A-Raf as a potent inhibitor of the proapoptotic mammalian sterile 20-like kinase (MST2) tumor suppressor pathway in several cancer entities including head and neck, colon, and breast. Independent of kinase activity, A-Raf binds to MST2 thereby efficiently inhibiting apoptosis. Here, we show that the interaction of A-Raf with the MST2 pathway is regulated by subcellular compartmentalization. Although in proliferating normal cells and tumor cells A-Raf localizes to the mitochondria, differentiated non-carcinogenic cells of head and neck epithelia, which express A-Raf at the plasma membrane. The constitutive or induced re-localization of A-Raf to the plasma membrane compromises its ability to efficiently sequester and inactivate MST2, thus rendering cells susceptible to apoptosis. Physiologically, A-Raf re-localizes to the plasma membrane upon epithelial differentiation in vivo. This re-distribution is regulated by the scaffold protein kinase suppressor of Ras 2 (KSR2). Downregulation of KSR2 during mammary epithelial cell differentiation or siRNA-mediated knockdown re-localizes A-Raf to the plasma membrane causing the release of MST2. By using the MCF7 cell differentiation system, we could demonstrate that overexpression of A-Raf in MCF7 cells, which induces differentiation. Our findings offer a new paradigm to understand how differential localization of Raf complexes affects diverse signaling functions in normal cells and carcinomas.

Regulation of RAF protein kinases in ERK signalling

RAF family kinases were among the first oncoproteins to be described more than 30 years ago. They primarily act as signalling relays downstream of RAS, and their close ties to cancer have fuelled a large number of studies. However, we still lack a systems-level understanding of their regulation and mode of action. The recent discovery that the catalytic activity of RAF depends on an allosteric mechanism driven by kinase domain dimerization is providing a vital new piece of information towards a comprehensive model of RAF function. The fact that current RAF inhibitors unexpectedly induce ERK signalling by stimulating RAF dimerization also calls for a deeper structural characterization of this family of kinases.

Caveolin-1 is required for kinase suppressor of Ras 1 (KSR1)-mediated extracellular signal-regulated kinase 1/2 activation, H-RasV12-induced senescence, and transformation

The molecular scaffold kinase suppressor of Ras 1 (KSR1) regulates the activation of the Raf/MEK/extracellular signal-regulated kinase (ERK) signal transduction pathway. KSR1 disruption in mouse embryo fibroblasts (MEFs) abrogates growth factor-induced ERK activation, H-Ras(V12)-induced replicative senescence, and H-Ras(V12)-induced transformation. Caveolin-1 has been primarily described as a major component of the coating structure of caveolae, which can serve as a lipid binding adaptor protein and coordinates the assembly of Ras, Raf, MEK, and ERK. In this study, we show that KSR1 interacts with caveolin-1 and is responsible for MEK and ERK redistribution to caveolin-1-rich fractions. The interaction between KSR1 and caveolin-1 is essential for optimal activation of ERK as a KSR1 mutant unable to interact with caveolin-1 does not efficiently mediate growth factor-induced ERK activation at the early stages of pathway activation. Furthermore, abolishing the KSR1-caveolin-1 interaction increases growth factor demands to promote H-Ras(V12)-induced proliferation and has adverse effects on H-Ras(V12)-induced cellular senescence and transformation. These data show that caveolin-1 is necessary for optimal KSR1-dependent ERK activation by growth factors and oncogenic Ras.

Key role of ERK1/2 molecular scaffolds in heart pathology

The ability of cardiomyocytes to detect mechanical and humoral stimuli is critical for adaptation of the myocardium in response to new conditions and for sustaining the increased workload during stress. While certain stimuli mediate a beneficial adaptation to stress conditions, others result in maladaptive remodelling, ultimately leading to heart failure. Specific signalling pathways activating either adaptive or maladaptive cardiac remodelling have been identified. Paradoxically, however, in a number of cases, the transduction pathways involved in such opposing responses engage the same signalling proteins. A notable example is the Raf-MEK1/2-ERK1/2 signalling pathway that can control both adaptive and maladaptive remodelling. ERK1/2 signalling requires a signalosome complex where a scaffold protein drives the assembly of these three kinases into a linear pathway to facilitate their sequential phosphorylation, ultimately targeting specific effector molecules. Interestingly, a number of different Raf-MEK1/2-ERK1/2 scaffold proteins have been identified, and their role in determining the adaptive or maladaptive cardiac remodelling is a promising field of investigation for the development of therapeutic strategies capable of selectively potentiating the adaptive response.

αB-crystallin promotes oncogenic transformation and inhibits caspase activation in cells primed for apoptosis by Rb inactivation

The retinoblastoma (Rb) tumor suppressor gene is frequently inactivated in cancer, resulting in deregulated activation of E2F transcription factors, which promote S-phase entry, p53-dependent and p53-independent apoptosis. Transformed cells evade p53-dependent apoptosis initiated by Rb inactivation by TP53 mutation. However, the mechanisms by which cancer cells circumvent p53-independent apoptosis in this context are poorly understood. Because Rb inactivation primes cells for apoptosis by p53-independent induction of procaspases, we postulated that αB-crystallin, an inhibitor of procaspase-3 activation, would suppress caspase activation in cells with combined Rb and p53 inactivation. Notably, αB-crystallin is commonly expressed in ER/PR/HER2 "triple-negative" breast carcinomas characterized by frequent Rb loss and TP53 mutation. We report that αB-crystallin (-/-) knock out (KO) MEFs immortalized by dominant negative (DN) p53 are resistant to transformation by the adenovirus E1A oncoprotein, which inactivates Rb, while wild-type (WT) MEFs are readily transformed by DN p53 and E1A. αB-crystallin (-/-) KO MEFs stably expressing DN p53 and E1A were more sensitive to chemotherapy-induced caspase-3 activation and apoptosis than the corresponding WT MEFs, despite comparable induction of procaspases by E1A. Similarly, silencing Rb in WT and αB-crystallin (-/-) KO MEFs immortalized by DN p53 increased procaspase levels and sensitized αB-crystallin (-/-) KO MEFs to chemotherapy. Furthermore, silencing αB-crystallin in triple-negative breast cancer cells, which lack Rb and express mutant p53, enhanced chemotherapy sensitivity compared to non-silencing controls. Our results indicate that αB-crystallin inhibits caspase activation in cells primed for apoptosis by Rb inactivation and plays a novel oncogenic role in the context of combined Rb and p53 inactivation.

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

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

VRK2 anchors KSR1-MEK1 to endoplasmic reticulum forming a macromolecular complex that compartmentalizes MAPK signaling

The spatial and temporal regulation of intracellular signaling is determined by the spatial and temporal organization of complexes assembled on scaffold proteins, which can be modulated by their interactions with additional proteins as well as subcellular localization. The scaffold KSR1 protein interacts with MAPK forming a complex that conveys a differential signaling in response to growth factors. The aim of this work is to determine the unknown mechanism by which VRK2A downregulates MAPK signaling. We have characterized the multiprotein complex formed by KSR1 and the Ser-Thr kinase VRK2A. VRK2A is a protein bound to the endoplasmic reticulum (ER) and retains a fraction of KSR1 complexes on the surface of this organelle. Both proteins, VRK2A and KSR1, directly interact by their respective C-terminal regions. In addition, MEK1 is also incorporated in the basal complex. MEK1 independently interacts with the CA5 region of KSR1 and with the N-terminus of VRK2A. Thus, VRK2A can form a high molecular size (600-1,000 kDa) stable complex with both MEK1 and KSR1. Knockdown of VRK2A resulted in disassembly of these high molecular size complexes. Overexpression of VRK2A increased the amount of KSR1 in the particulate fraction and prevented the incorporation of ERK1/2 into the complex after stimulation with EGF. Neither VRK2A nor KSR1 interact with the VHR, MKP1, MKP2, or MKP3 phosphatases. The KSR1 complex assembled and retained by VRK2A in the ER can have a modulatory effect on the signal mediated by MAPK, thus locally affecting the magnitude of its responses, and can explain differential responses depending on cell type.

Kinase suppressor of Ras 2 (KSR2) regulates tumor cell transformation via AMPK

Kinase suppressor of Ras 1 (KSR1) and KSR2 are scaffolds that promote extracellular signal-regulated kinase (ERK) signaling but have dramatically different physiological functions. KSR2(-/-) mice show marked deficits in energy expenditure that cause obesity. In contrast, KSR1 disruption has inconsequential effects on development but dramatically suppresses tumor formation by activated Ras. We examined the role of KSR2 in the generation and maintenance of the transformed phenotype in KSR1(-/-) mouse embryo fibroblasts (MEFs) expressing activated Ras(V12) and in tumor cell lines MIN6 and NG108-15. KSR2 rescued ERK activation and accelerated proliferation in KSR1(-/-) MEFs. KSR2 expression alone induced anchorage-independent growth and synergized with the transforming effects of Ras(V12). Similarly, RNA interference (RNAi) of KSR2 in MIN6 and NG108-15 cells inhibited proliferation and colony formation, with concomitant defects in AMP-activated protein kinase (AMPK) signaling, nutrient metabolism, and metabolic capacity. While constitutive activation of AMPK was sufficient to complement the loss of KSR2 in metabolic signaling and anchorage-independent growth, KSR2 RNAi, MEK inhibition, and expression of a KSR2 mutant unable to interact with ERK demonstrated that mitogen-activated protein (MAP) kinase signaling is dispensable for the transformed phenotype of these cells. These data show that KSR2 is essential to tumor cell energy homeostasis and critical to the integration of mitogenic and metabolic signaling pathways.

Suppression of replicative senescence by rapamycin in rodent embryonic cells

The TOR (target of rapamycin) pathway is involved in aging in diverse organisms from yeast to mammals. We have previously demonstrated in human and rodent cells that mTOR converts stress-induced cell cycle arrest to irreversible senescence (geroconversion), whereas rapamycin decelerates or suppresses geroconversion during cell cycle arrest. Here, we investigated whether rapamycin can suppress replicative senescence of rodent cells. Mouse embryonic fibroblasts (MEFs) gradually acquired senescent morphology and ceased proliferation. Rapamycin decreased cellular hypertrophy, and SA-β-Gal staining otherwise developed by 4-6 passages, but it blocked cell proliferation, masking its effects on replicative lifespan. We determined that rapamycin inhibited pS6 at 100-300 pM and inhibited proliferation with IC(50) around 30 pM. At 30 pM, rapamycin partially suppressed senescence. However, the gerosuppressive effect was balanced by the cytostatic effect, making it difficult to suppress senescence without causing quiescence. We also investigated rat embryonic fibroblasts (REFs), which exhibited markers of senescence at passage 7, yet were able to slowly proliferate until 12-14 passages. REFs grew in size, acquired a large, flat cell morphology, SA-β-Gal staining and components of DNA damage response (DDR), in particular, γH2AX/53BP1 foci. Incubation of REFs with rapamycin (from passage 7 to passage 10) allowed REFs to overcome the replicative senescence crisis. Following rapamycin treatment and removal, a fraction of proliferating REFs gradually increased and senescent phenotype disappeared completely by passage 24.

The role of pseudokinases in cancer

Kinases play a critical role in regulating many cellular functions including development, differentiation and proliferation. To date, over 518 proteins with kinase activity, comprising ~2-3% of total cellular proteins, have been identified from within the human kinome. Interestingly, approximately 10% of kinases are categorised as pseudokinases since they lack one or more conserved catalytic residues within their kinase domain and were originally thought to have no enzymatic activity. Recently, there has been strong evidence to suggest that some pseudokinsases can not only function as scaffold proteins, but may also possess kinase activity leading to modulation of cell signalling pathways. Altered activity of these pseudokinases can result in impaired cellular function, particularly in malignancies. In this review we are discussing recent evidence that apart from a scaffolding role, pseudokinases also orchestrate cellular processes as active kinases per se in signalling pathways of malignant cells.

Monomeric and dimeric models of ERK2 in conjunction with studies on cellular localization, nuclear translocation, and in vitro analysis

Extracellular signal-regulated protein kinase 2 (ERK2) plays many vital roles in cellular signal regulation. Phosphorylation of ERK2 leads to propagation and execution of various extracellular stimuli, which influence cellular responses to stress. The final response of the ERK2 signaling pathway is determined by localization and duration of active ERK2 at specific target cell compartments through protein-protein interactions of ERK2 with various cytoplasmic and nuclear substrates, scaffold proteins, and anchoring counterparts. In this respect, dimerization of phosphorylated ERK2 has been suggested to be a part of crucial regulating mechanism in various protein-protein interactions. After the report of putative dimeric structure of active ERK2 (Canagarajah et al., 1997), dimeric model was employed to explain many in vivo and in vitro experimental results. But more recently, many reports have been presented questioning the validity of dimer hypothesis of active ERK2. In this review, we summarize the various in vitro and in vivo studies concerning the Monomeric or the dimeric forms of ERK2 and the validity of the dimer hypothesis.

Suppression of Ras/Mapk pathway signaling inhibits Myc-induced lymphomagenesis

Although the Myc transcription factor has been shown necessary for the oncogenic function of Ras, the contribution of Ras pathway signaling to the oncogenic function of Myc remains unresolved. We report the novel findings that Myc alone induced Ras/Mapk pathway signaling, and increased signaling following growth factor stimulation. Deletion of the scaffold protein kinase suppressor of Ras 1 (Ksr1) attenuated signaling through the Ras/Mapk pathway, including activation following Myc induction. B cells that lacked Ksr1 exhibited reduced proliferation and increased cytokine deprivation-induced apoptosis. Overexpression of Myc rescued the proliferation defect of Ksr1-null B cells, but loss of Ksr1 increased sensitivity of B cells to Myc-induced apoptosis. Notably, there was a significant delay in lymphoma development in Ksr1-null mice overexpressing Myc in B cells (Eμ-myc transgenic mice). There was an elevated frequency of p53 inactivation, indicative of increased selective pressure to bypass the p53 tumor suppressor pathway, in Ksr1-null Eμ-myc lymphomas. Therefore, loss of Ksr1 inhibits Ras/Mapk pathway signaling leading to increased Myc-induced B-cell apoptosis, and this results in reduced B-cell transformation and lymphoma development. Our data indicate that suppression of Myc-induced Ras/Mapk pathway signaling significantly impairs Myc oncogenic function. These results fill a significant gap in knowledge about Myc and should open new avenues of therapeutic intervention for Myc-overexpressing malignancies.

Regulation of glucose homeostasis by KSR1 and MARK2

Protein scaffolds control the intensity and duration of signaling and dictate the specificity of signaling through MAP kinase pathways. KSR1 is a molecular scaffold of the Raf/MEK/ERK MAP kinase cascade that regulates the intensity and duration of ERK activation. Relative to wild-type mice, ksr1^-/- mice are modestly glucose intolerant, but show a normal response to exogenous insulin. However, ksr1^-/- mice also demonstrate a three-fold increase in serum insulin levels in response to a glucose challenge, suggesting a role for KSR1 in insulin secretion. The kinase MARK2 is closely related to C-TAK1, a known regulator of KSR1. Mice lacking MARK2 have an increased rate of glucose disposal in response to exogenous insulin, increased glucose tolerance, and are resistant to diet-induced obesity. mark2^-/-ksr1^-/- (DKO) mice were compared to wild type, mark2^-/-, and ksr1^-/- mice for their ability to regulate glucose homeostasis. Here we show that disruption of KSR1 in mark2^-/- mice reverses the increased sensitivity to exogenous insulin resulting from MARK2 deletion. DKO mice respond to exogenous insulin similarly to wild type and ksr1^-/- mice. These data suggest a model whereby MARK2 negatively regulates insulin sensitivity in peripheral tissue through inhibition of KSR1. Consistent with this model, we found that MARK2 binds and phosphorylates KSR1 on Ser392. Phosphorylation of Ser392 is a critical regulator of KSR1 stability, subcellular location, and ERK activation. These data reveal an unexpected role for the molecular scaffold KSR1 in insulin-regulated glucose metabolism.

Raf family kinases: old dogs have learned new tricks

First identified in the early 1980s as retroviral oncogenes, the Raf proteins have been the objects of intense research. The discoveries 10 years later that the Raf family members (Raf-1, B-Raf, and A-Raf) are bona fide Ras effectors and upstream activators of the ubiquitous ERK pathway increased the interest in these proteins primarily because of the central role that this cascade plays in cancer development. The important role of Raf in cancer was corroborated in 2002 with the discovery of B-Raf genetic mutations in a large number of tumors. This led to intensified drug development efforts to target Raf signaling in cancer. This work yielded not only recent clinical successes but also surprising insights into the regulation of Raf proteins by homodimerization and heterodimerization. Surprising insights also came from the hunt for new Raf targets. Although MEK remains the only widely accepted Raf substrate, new kinase-independent roles for Raf proteins have emerged. These include the regulation of apoptosis by suppressing the activity of the proapoptotic kinases, ASK1 and MST2, and the regulation of cell motility and differentiation by controlling the activity of Rok-α. In this review, we discuss the regulation of Raf proteins and their role in cancer, with special focus on the interacting proteins that modulate Raf signaling. We also describe the new pathways controlled by Raf proteins and summarize the successes and failures in the development of efficient anticancer therapies targeting Raf. Finally, we also argue for the necessity of more systemic approaches to obtain a better understanding of how the Ras-Raf signaling network generates biological specificity.

KSR2 is an essential regulator of AMP kinase, energy expenditure, and insulin sensitivity

Kinase suppressors of Ras 1 and 2 (KSR1 and KSR2) function as molecular scaffolds to potently regulate the MAP kinases ERK1/2 and affect multiple cell fates. Here we show that KSR2 interacts with and modulates the activity of AMPK. KSR2 regulates AMPK-dependent glucose uptake and fatty acid oxidation in mouse embryonic fibroblasts and glycolysis in a neuronal cell line. Disruption of KSR2 in vivo impairs AMPK-regulated processes affecting fatty acid oxidation and thermogenesis to cause obesity. Despite their increased adiposity, ksr2(-/-) mice are hypophagic and hyperactive but expend less energy than wild-type mice. In addition, hyperinsulinemic-euglycemic clamp studies reveal that ksr2(-/-) mice are profoundly insulin resistant. The expression of genes mediating oxidative phosphorylation is also downregulated in the adipose tissue of ksr2(-/-) mice. These data demonstrate that ksr2(-/-) mice are highly efficient in conserving energy, revealing a novel role for KSR2 in AMPK-mediated regulation of energy metabolism.

KSR1 modulates the sensitivity of mitogen-activated protein kinase pathway activation in T cells without altering fundamental system outputs

Mitogen-activated protein kinase (MAPK) cascades are evolutionarily conserved signaling pathways that regulate cell fate decisions. They generate a wide range of signal outputs, including graded and digital responses. In T cells, MAPK activation is digital in response to T-cell-receptor stimulation; however, whether other receptors on T cells that lead to MAPK activation are graded or digital is unknown. Here we evaluate MAPK activation in T cells at the single-cell level. We show that T cells responded digitally to stimulation with superantigen-loaded antigen-presenting cells, whereas they responded in a graded manner to the chemokine SDF-1, demonstrating that the system output of the MAPK module is highly plastic and determined by components upstream of the MAPK module. These findings also confirm that different MAPK system outputs are used by T cells to control discrete biological functions. Scaffold proteins are essential for proper MAPK signaling and function as they physically assemble multiple components and regulators of MAPK cascades. We found that the scaffold protein KSR1 regulated the threshold required for MAPK activation in T cells without affecting the nature of the response. We conclude that KSR1 plays a central role in determining the sensitivity of T-cell responses and is thus well positioned as a key control point.

KSR1 is required for cell cycle reinitiation following DNA damage

KSR1 (kinase suppressor of Ras 1) is a molecular scaffold and positive regulator of the Raf/MEK/ERK phosphorylation cascade. KSR1 is required for maximal ERK activation induced by growth factors and by some cytotoxic agents. We show here that KSR1 is also required for maximal ERK activation induced by UV light, ionizing radiation, or the DNA interstrand cross-linking agent mitomycin C (MMC). We further demonstrate a role for KSR1 in the reinitiation of the cell cycle and proliferation following cell cycle arrest induced by MMC. Cells lacking KSR1 underwent but did not recover from MMC-induced G(2)/M arrest. Expression of KSR1 allowed KSR1(-/-) cells to re-enter the cell cycle following MMC treatment. However, cells expressing a mutated form of KSR1 unable to bind ERK did not recover from MMC-induced cell cycle arrest, demonstrating the requirement for the KSR1-ERK interaction. In addition, constitutive activation of ERK was not sufficient to promote cell cycle reinitiation in MMC-treated KSR1(-/-) cells. Only cells expressing KSR1 recovered from MMC-induced cell cycle arrest. Importantly, MMC-induced DNA damage was repaired in KSR1(-/-) cells, as determined by resolution of gamma-H2AX-containing foci. These data indicate that cell cycle reinitiation is not actively signaled in the absence of KSR1, even when DNA damage has been resolved. These data reveal a specific role for the molecular scaffold KSR1 and KSR1-mediated ERK signaling in the cellular response to DNA interstrand cross-links.

Ras subcellular localization defines extracellular signal-regulated kinase 1 and 2 substrate specificity through distinct utilization of scaffold proteins

Subcellular localization influences the nature of Ras/extracellular signal-regulated kinase (ERK) signals by unknown mechanisms. Herein, we demonstrate that the microenvironment from which Ras signals emanate determines which substrates will be preferentially phosphorylated by the activated ERK1/2. We show that the phosphorylation of epidermal growth factor receptor (EGFr) and cytosolic phospholipase A(2) (cPLA(2)) is most prominent when ERK1/2 are activated from lipid rafts, whereas RSK1 is mainly activated by Ras signals from the disordered membrane. We present evidence indicating that the underlying mechanism of this substrate selectivity is governed by the participation of different scaffold proteins that distinctively couple ERK1/2, activated at defined microlocalizations, to specific substrates. As such, we show that for cPLA(2) activation, ERK1/2 activated at lipid rafts interact with KSR1, whereas ERK1/2 activated at the endoplasmic reticulum utilize Sef-1. To phosphorylate the EGFr, ERK1/2 activated at lipid rafts require the participation of IQGAP1. Furthermore, we demonstrate that scaffold usage markedly influences the biological outcome of Ras site-specific signals. These results disclose an unprecedented spatial regulation of ERK1/2 substrate specificity, dictated by the microlocalization from which Ras signals originate and by the selection of specific scaffold proteins.

Characterization of kinase suppressor of Ras-1 expression and anticancer drug sensitivity in human cancer cell lines

Previous studies have indicated that the ERK1/2 MAP kinase signaling pathway plays an important role not only in cell growth, cell cycle regulation, and differentiation, but also in determining the sensitivity of cells to anticancer agents as well. Furthermore, expression of kinase suppressor of Ras-1 (KSR1), a molecular scaffold that modulates signaling through the ERK1/2 MAP kinase pathway, has been shown to influence the cellular sensitivity to the anticancer agent cisplatin. To further define the role of KSR1 expression on drug sensitivity, the expression of KSR1 was examined in the NCI60 anticancer drug screen, a panel of cancer cell lines representing nine tissue types, established by the Developmental Therapeutics Program (DTP) at the National Cancer Institute (NCI). The expression of thousands of molecular targets has been examined in the NCI60 panel as well as the cellular toxicity for greater than 400,000 compounds. KSR1 expression varied almost 30-fold difference between the highest and lowest expressing cell lines in the NCI60. Using the COMPARE analysis algorithm, KSR1 expression was correlated with sensitivity of the compounds screened by DTP and several novel agents were identified whose sensitivity correlated with KSR1 expression in the NCI60 panel. Cytotoxicity of two agents, cytochalasin H and tunicamycin, identified through the COMPARE analysis of KSR1 expression and drug sensitivity, was also examined in wild type (KSR(+/+)) mouse embryo fibroblasts (MEFs) and MEFs deficient in KSR1 expression (KSR1(-/-)). These studies demonstrated enhanced sensitivity, as well as increased ERK activation, in KSR(-/-) MEFs following exposure to tunicamycin or cytochalasin H compared to KSR(+/+) MEFs. Furthermore, restoration of KSR1 expression in KSR(-/-) MEFs following stable transduction of cells with a KSR1 expression vector, enhanced sensitivity of cells to tunicamycin and cytochalasin H and decreased ERK1/2 activation following exposure to these drugs. In addition, the sensitivity to cytochalasin H and tunicamycin of breast cancer cell lines with low KSR1 expression, (HS578T and MDA-MB-231/ATCC), was increased relative to the sensitivity of breast cancer cells with higher levels of KSR1 (MCF7). These studies indicate that KSR1 may play an important role in the determination of cellular sensitivity to anticancer agents.

Scaffold proteins confer diverse regulatory properties to protein kinase cascades

The assembly of multiple signaling proteins into a complex by a scaffold protein guides many cellular decisions. Despite recent advances, the overarching principles that govern scaffold function are not well understood. We carried out a computational study using kinetic Monte Carlo simulations to understand how spatial localization of kinases on a scaffold may regulate signaling under different physiological conditions. Our studies identify regulatory properties of scaffold proteins that allow them to both amplify and attenuate incoming signals in different biological contexts. These properties are not caused by the well established prozone or combinatorial inhibition effect. These results bring coherence to seemingly paradoxical observations and suggest that cells have evolved design rules that enable scaffold proteins to regulate widely disparate cellular functions.

The role of scaffold proteins in MEK/ERK signalling

Signal transduction networks allow cells to recognize and respond to changes in the extracellular environment. All eukaryotic cells have MAPK (mitogen-activated protein kinase) pathways that participate in diverse cellular functions, including differentiation, survival, transformation and movement. Five distinct groups of MAPKs have been characterized in mammals, the most extensively studied of which is the Ras/Raf/MEK [MAPK/ERK (extracellular-signal-regulated kinase) kinase]/ERK cascade. Numerous stimuli, including growth factors and phorbol esters, activate MEK/ERK signalling. How disparate extracellular signals are translated by MEK/ERK into different cellular functions remains obscure. Originally identified in yeast, scaffold proteins are now recognized to contribute to the specificity of MEK/ERK pathways in mammalian cells. These scaffolds include KSR (kinase suppressor of Ras), beta-arrestin, MEK partner-1, Sef and IQGAP1. Scaffolds organize multiprotein signalling complexes. This targets MEK/ERK to specific substrates and facilitates communication with other pathways, thereby mediating diverse functions. The adaptor proteins regulate the kinetics, amplitude and localization of MEK/ERK signalling, providing an efficient mechanism that enables an individual extracellular stimulus to promote a specific biological response.