Caspase-dependent cleavage disrupts the ERK cascade scaffolding function of KSR1

Navigating the ERK1/2 MAPK Cascade

The RAS-ERK pathway is a fundamental signaling cascade crucial for many biological processes including proliferation, cell cycle control, growth, and survival; common across all cell types. Notably, ERK1/2 are implicated in specific processes in a context-dependent manner as in stem cells and pancreatic β-cells. Alterations in the different components of this cascade result in dysregulation of the effector kinases ERK1/2 which communicate with hundreds of substrates. Aberrant activation of the pathway contributes to a range of disorders, including cancer. This review provides an overview of the structure, activation, regulation, and mutational frequency of the different tiers of the cascade; with a particular focus on ERK1/2. We highlight the importance of scaffold proteins that contribute to kinase localization and coordinate interaction dynamics of the kinases with substrates, activators, and inhibitors. Additionally, we explore innovative therapeutic approaches emphasizing promising avenues in this field.

Dipolar coupling-based electron paramagnetic resonance method for protease enzymatic characterization and inhibitor screening

Herein, we report an EPR-based method for protease enzymatic characterization and inhibitor screening. This method utilizes dual paramagnetically-labeled probes consisting of a nitroxide spin probe and a Gd³⁺ ion flanking a peptide that could be specifically cleaved by protease caspase-3. Distance-dependent dipolar coupling between the two paramagnetic centers can be modulated by the protease cleavage activity, thus providing a straightforward and convenient method for protease activity detection using EPR spectroscopy under ambient conditions. Moreover, time-course monitoring of the protease-catalyzed cleavage reaction demonstrated that this EPR-based method could not only allow a direct quantitative enzymatic kinetic assessment, but also could be used for protease inhibitor screening, thus holding great potential in drug discovery studies.

Repeated olanzapine treatment mitigates PTSD like symptoms in rats with changes in cell signaling factors

Post Traumatic Stress Disorder is an anxiety disorder with prolonged distortion of rational behavior. In this study, we report the preclinical potential of olanzapine (OLZ) in the treatment of PTSD. Since the atypical antipsychotics have modulating effects on cell protective and destructive factors, we tested the effects of OLZ in PTSD regarding these cell modulating factors. Rats, when subjected to stress-restress (SRS) model of PTSD, showed a derangement in cell protective factors like the decline in BDNF, ERK, and CREB. While the adversarial factors like caspase-3 were enhanced. Four weeks treatment with OLZ at doses of 1 and 10 mg/kg significantly mitigated the SRS-induced derangement related to PTSD. OLZ at doses of 1 and 10 mg/kg enhanced BDNF, ERK and CREB levels which were reduced by SRS in PTSD animals. Further, at the fore mentioned doses it also inhibited the elevation of caspase-3 a downstream apoptotic factor. Besides, OLZ also showed mitigation in behavioral alterations related to anxiety and memory brought about by PTSD. These effects of OLZ were comparable to that of paroxetine a clinically approved drug for PTSD in terms of biochemical and behavioral assessments indicating its anti-PTSD potential.

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.

praja2 regulates KSR1 stability and mitogenic signaling

The kinase suppressor of Ras 1 (KSR1) has a fundamental role in mitogenic signaling by scaffolding components of the Ras/MAP kinase pathway. In response to Ras activation, KSR1 assembles a tripartite kinase complex that optimally transfers signals generated at the cell membrane to activate ERK. We describe a novel mechanism of ERK attenuation based on ubiquitin-dependent proteolysis of KSR1. Stimulation of membrane receptors by hormones or growth factors induced KSR1 polyubiquitination, which paralleled a decline of ERK1/2 signaling. We identified praja2 as the E3 ligase that ubiquitylates KSR1. We showed that praja2-dependent regulation of KSR1 is involved in the growth of cancer cells and in the maintenance of undifferentiated pluripotent state in mouse embryonic stem cells. The dynamic interplay between the ubiquitin system and the kinase scaffold of the Ras pathway shapes the activation profile of the mitogenic cascade. By controlling KSR1 levels, praja2 directly affects compartmentalized ERK activities, impacting on physiological events required for cell proliferation and maintenance of embryonic stem cell pluripotency.

Tyr728 in the kinase domain of the murine kinase suppressor of RAS 1 regulates binding and activation of the mitogen-activated protein kinase kinase

In metazoans, the highly conserved MAPK signaling pathway regulates cell fate decision. Aberrant activation of this pathway has been implicated in multiple human cancers and some developmental disorders. KSR1 functions as an essential scaffold that binds the individual components of the cascade and coordinates their assembly into multiprotein signaling platforms. The mechanism of KSR1 regulation is highly complex and not completely understood. In this study, we identified Tyr(728) as a novel regulatory phosphorylation site in KSR1. We show that Tyr(728) is phosphorylated by LCK, uncovering an additional and unexpected link between Src kinases and MAPK signaling. To understand how phosphorylation of Tyr(728) may regulate the role of KSR1 in signal transduction, we integrated structural modeling and biochemical studies. We demonstrate that Tyr(728) is involved in maintaining the conformation of the KSR1 kinase domain required for binding to MEK. It also affects phosphorylation and activation of MEK by RAF kinases and consequently influences cell proliferation. Moreover, our studies suggest that phosphorylation of Tyr(728) may affect the intrinsic kinase activity of KSR1. Together, we propose that phosphorylation of Tyr(728) may regulate the transition between the scaffolding and the catalytic function of KSR1 serving as a control point used to fine-tune cellular responses.

Anti-apoptotic role of caspase-cleaved GAB1 adaptor protein in hepatocyte growth factor/scatter factor-MET receptor protein signaling

The GRB2-associated binder 1 (GAB1) docking/scaffold protein is a key mediator of the MET-tyrosine kinase receptor activated by hepatocyte growth factor/scatter factor (HGF/SF). Activated MET promotes recruitment and tyrosine phosphorylation of GAB1, which in turn recruits multiple proteins and mediates MET signaling leading to cell survival, motility, and morphogenesis. We previously reported that, without its ligand, MET is a functional caspase target during apoptosis, allowing the generation of a p40-MET fragment that amplifies apoptosis. In this study we established that GAB1 is also a functional caspase target by evidencing a caspase-cleaved p35-GAB1 fragment that contains the MET binding domain. GAB1 is cleaved by caspases before MET, and the resulting p35-GAB1 fragment is phosphorylated by MET upon HGF/SF binding and can interact with a subset of GAB1 partners, PI3K, and GRB2 but not with SHP2. This p35-GAB1 fragment favors cell survival by maintaining HGF/SF-induced MET activation of AKT and by hindering p40-MET pro-apoptotic function. These data demonstrate an anti-apoptotic role of caspase-cleaved GAB1 in HGF/SF-MET signaling.

RAF inhibitor-induced KSR1/B-RAF binding and its effects on ERK cascade signaling

RAF kinase inhibitors can induce ERK cascade signaling by promoting dimerization of RAF family members in the presence of oncogenic or normally activated RAS. This interaction is mediated by a dimer interface region in the RAF kinase domain that is conserved in members of the ERK cascade scaffold family, kinase suppressor of RAS (KSR). In this study, we find that most RAF inhibitors also induce the binding of KSR1 to wild-type and oncogenic B-RAF proteins, including V600E B-RAF, but promote little complex formation between KSR1 and C-RAF. The inhibitor-induced KSR1/B-RAF interaction requires direct binding of the drug to B-RAF and is dependent on conserved dimer interface residues in each protein, but, unexpectedly, is not dependent on binding of B-RAF to activated RAS. Inhibitor-induced KSR/B-RAF complex formation can occur in the cytosol and is observed in normal mouse fibroblasts, as well as a variety of human cancer cell lines. Strikingly, we find that KSR1 competes with C-RAF for inhibitor-induced binding to B-RAF and, as a result, alters the effect of the inhibitors on ERK cascade signaling.

Disassembly of the JIP1/JNK molecular scaffold by caspase-3-mediated cleavage of JIP1 during apoptosis

We report here the cleavage of the c-Jun N-terminal Kinase (JNK) pathway scaffold protein, JNK Interacting Protein-1 (JIP1), by caspases during both Tumour Necrosis Factor-Related Apoptosis-Inducing Ligand (TRAIL) and staurosporine-induced apoptosis in HeLa cells. During the initiation of apoptosis, maximal JNK activation is observed when JIP1 is intact, whereas cleavage of JIP1 correlates with JNK inactivation and progression of apoptosis. JIP1 is cleaved by caspase-3 at two sites, leading to disassembly of the JIP1/JNK complex. Inhibition of JIP1 cleavage by the caspase-3 inhibitor DEVD.fmk inhibits this disassembly, and is accompanied by sustained JNK activation. These data suggest that TRAIL and staurosporine induce JNK activation in a caspase-3-independent manner and that caspase-3-mediated JIP1 cleavage plays a role in JNK inactivation via scaffold disassembly during the execution phase of apoptosis. Caspase-mediated cleavage of JIP scaffold proteins may therefore represent an important mechanism for modulation of JNK signalling during apoptotic cell death.

KSR1 is a functional protein kinase capable of serine autophosphorylation and direct phosphorylation of MEK1

The extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathway is a highly conserved signaling pathway that regulates diverse cellular processes including differentiation, proliferation, and survival. Kinase suppressor of Ras-1 (KSR1) binds each of the three ERK cascade components to facilitate pathway activation. Even though KSR1 contains a C-terminal kinase domain, evidence supporting the catalytic function of KSR1 remains controversial. In this study, we produced recombinant wild-type or kinase-inactive (D683A/D700A) KSR1 proteins in Escherichia coli to test the hypothesis that KSR1 is a functional protein kinase. Recombinant wild-type KSR1, but not recombinant kinase-inactive KSR1, underwent autophosphorylation on serine residue(s), phosphorylated myelin basic protein (MBP) as a generic substrate, and phosphorylated recombinant kinase-inactive MAPK/ERK kinase-1 (MEK1). Furthermore, FLAG immunoprecipitates from KSR1(-/-) colon epithelial cells stably expressing FLAG-tagged wild-type KSR1 (+KSR1), but not vector (+vector) or FLAG-tagged kinase-inactive KSR1 (+D683A/D700A), were able to phosphorylate kinase-inactive MEK1. Since TNF activates the ERK pathway in colon epithelial cells, we tested the biological effects of KSR1 in the survival response downstream of TNF. We found that +vector and +D683A/D700A cells underwent apoptosis when treated with TNF, whereas +KSR1 cells were resistant. However, +KSR1 cells were sensitized to TNF-induced cell loss in the absence of MEK kinase activity. These data provide clear evidence that KSR1 is a functional protein kinase, MEK1 is an in vitro substrate of KSR1, and the catalytic activities of both proteins are required for eliciting cell survival responses downstream of TNF.

Signaling dynamics of the KSR1 scaffold complex

Scaffold proteins contribute to the spatiotemporal control of MAPK signaling and KSR1 is an ERK cascade scaffold that localizes to the plasma membrane in response to growth factor treatment. To better understand the molecular mechanisms of KSR1 function, we examined the interaction of KSR1 with each of the ERK cascade components, Raf, MEK, and ERK. Here, we identify a hydrophobic motif within the proline-rich sequence (PRS) of MEK1 and MEK2 that is required for constitutive binding to KSR1 and find that MEK binding and residues in the KSR1 CA1 region enable KSR1 to form a ternary complex with B-Raf and MEK following growth factor treatment that enhances MEK activation. We also find that docking of active ERK to the KSR1 scaffold allows ERK to phosphorylate KSR1 and B-Raf on feedback S/TP sites. Strikingly, feedback phosphorylation of KSR1 and B-Raf promote their dissociation and result in the release of KSR1 from the plasma membrane. Together, these findings provide unique insight into the signaling dynamics of the KSR1 scaffold and reveal that through regulated interactions with Raf and ERK, KSR1 acts to both potentiate and attenuate ERK cascade activation, thus regulating the intensity and duration of ERK cascade signaling emanating from the plasma membrane during growth factor signaling.

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.

Role of kinase suppressor of Ras-1 in neuronal survival signaling by extracellular signal-regulated kinase 1/2

Scaffolding proteins including kinase suppressor of Ras-1 (KSR1) determine specificity of signaling by extracellular signal-regulated kinase 1/2 (ERK1/2), enabling it to couple diverse extracellular stimuli to various cellular responses. The scaffolding protein(s) that contributes to ERK1/2-mediated neuronal survival has not yet been identified. In cultured rat cortical neurons, BDNF activates ERK1/2 to enhance neuronal survival by suppressing DNA damage- or trophic deprivation-induced apoptosis. Here we report that in this system, BDNF increased KSR1 association with activated ERK1/2, whereas KSR1 knockdown with a short hairpin (sh) RNA reduced BDNF-mediated activation of ERK1/2 and protection against a DNA-damaging drug, camptothecin (CPT). In contrast, BDNF suppression of trophic deprivation-induced apoptosis was unaffected by shKSR1 although blocked by shERK1/2. Also, overexpression of KSR1 enhanced BDNF protection against CPT. Therefore, KSR1 is specifically involved in antigenotoxic activation of ERK1/2 by BDNF. To test whether KSR1 contributes to ERK1/2 activation by other neuroprotective stimuli, we used a cAMP-elevating drug, forskolin. In cortical neurons, ERK1/2 activation by forskolin was protein kinase A (PKA) dependent but TrkB (receptor tyrosine kinase B) independent and was accompanied by the increased association between KSR1 and active ERK1/2. Forskolin suppressed CPT-induced apoptosis in a KSR1 and ERK1/2-dependent manner. Inhibition of PKA abolished forskolin protection, whereas selective PKA activation resulted in an ERK1/2- and KSR1-mediated decrease in apoptosis. Hence, KSR1 is critical for the antiapoptotic activation of ERK1/2 by BDNF or cAMP/PKA signaling. In addition, these novel data indicate that stimulation of cAMP signaling is a candidate neuroprotective strategy to intervene against neurotoxicity of DNA-damaging agents.