The function of Shoc2: A scaffold and beyond

The expression of congenital Shoc2 variants induces AKT-dependent crosstalk activation of the ERK1/2 pathway

The Shoc2 scaffold protein is crucial in transmitting signals within the Epidermal Growth Factor Receptor (EGFR)-mediated Extracellular signal-Regulated Kinase (ERK1/2) pathway. While the significance of Shoc2 in this pathway is well-established, the precise mechanisms through which Shoc2 governs signal transmission remain to be fully elucidated. Hereditary variants in Shoc2 are responsible for Noonan Syndrome with Loose anagen Hair (NSLH). However, due to the absence of known enzymatic activity in Shoc2, directly assessing how these variants affect its function is challenging. ERK1/2 phosphorylation is used as a primary parameter of Shoc2 function, but the impact of Shoc2 mutants on the pathway activation is unclear. This study investigates how the NSLH-associated Shoc2 variants influence EGFR signals in the context of the ERK1/2 and AKT downstream signaling pathways. We show that when the ERK1/2 pathway is a primary signaling pathway activated downstream of EGFR, Shoc2 variants cannot upregulate ERK1/2 phosphorylation to the level of the WT Shoc2. Yet, when the AKT and ERK1/2 pathways were activated, in cells expressing Shoc2 variants, ERK1/2 phosphorylation was higher than in cells expressing WT Shoc2. In cells expressing the Shoc2 NSLH mutants, we found that the AKT signaling pathway triggers the PAK activation, followed by phosphorylation of Raf-1/MEK1/2 and activation of the ERK1/2 signaling axis. Hence, our studies reveal a previously unrecognized feedback regulation downstream of the EGFR and provide additional evidence for the role of Shoc2 as a "gatekeeper" in controlling the selection of downstream effectors within the EGFR signaling network.

SHOC2 mediates the drug-resistance of triple-negative breast cancer cells to everolimus

Aberrant activation of the mTOR pathway is a characteristic alteration in triple-negative breast cancer, but the mTOR pathway inhibitor everolimus is not effective for the triple-negative breast cancer (TNBC) patients. Presently, we showed that the activation of ERK pathway was an important mechanism of resistance to everolimus in TNBC cells in this study. SHOC2, a key protein mediating the Ras-Raf-ERK pathway, could act as a scaffolding protein to facilitate the activation of the pathway by mediating the interaction of key components of the pathway. Our results showed that everolimus activated the Raf-ERK pathway by promoting the interaction between SHOC2 and c-Raf and that knockdown of SHOC2 significantly inhibited the Raf-ERK pathway induced by everolimus. We further demonstrated that SHOC2 expression levels were closely related to the sensitivity of TNBC cells to everolimus and that interference with SHOC2 expression in combination with everolimus had significant effects on the cell cycle progression and apoptosis in vitro experiments. Western blotting analysis showed that cell cycle regulators and apoptosis-related proteins were significantly altered by the combination treatment. Xenograft model also demonstrated that knockdown of SHOC2 significantly increased the sensitivity of tumor to everolimus in nude mice. In conclusion, our study showed that SHOC2 is a key factor in regulating the sensitivity of TNBC cells to everolimus and that combined therapy may be a more effective therapeutic approach for TNBC patients.

The expression of congenital Shoc2 variants induces AKT-dependent feedback activation of the ERK1/2 pathway

The Shoc2 scaffold protein is crucial in transmitting signals within the Epidermal Growth Factor Receptor (EGFR)-mediated Extracellular signal-regulated Kinase (ERK1/2) pathway. While the significance of Shoc2 in this pathway is well-established, the precise mechanisms through which Shoc2 governs signal transmission remain to be fully elucidated. Hereditary mutations in Shoc2 are responsible for Noonan Syndrome with Loose anagen Hair (NSLH). However, due to the absence of known enzymatic activity in Shoc2, directly assessing how these mutations affect its function is challenging. ERK1/2 phosphorylation is used as a primary parameter of Shoc2 function, but the impact of Shoc2 mutants on the pathway activation is unclear. This study investigates how the NSLH-associated Shoc2 variants influence EGFR signals in the context of the ERK1/2 and AKT downstream signaling pathways. We show that when the ERK1/2 pathway is a primary signaling pathway activated downstream of EGFR, Shoc2 variants cannot upregulate ERK1/2 phosphorylation to the level of the WT Shoc2. Yet, when the AKT and ERK1/2 pathways were activated, in cells expressing Shoc2 variants, ERK1/2 phosphorylation was higher than in cells expressing WT Shoc2. We found that, in cells expressing the Shoc2 NSLH mutants, the AKT signaling pathway triggers the PAK activation, followed by phosphorylation and Raf-1/MEK1/2 /ERK1/2 signaling axis activation. Hence, our studies reveal a previously unrecognized feedback regulation downstream of the EGFR and provide evidence for the Shoc2 role as a "gatekeeper" in controlling the selection of downstream effectors within the EGFR signaling network.

The Sag-Shoc2 axis regulates conversion of mPanINs to cystic lesions in Kras pancreatic tumor model

SAG/RBX2 is an E3 ligase, whereas SHOC2 is a RAS-RAF positive regulator. In this study, we address how Sag-Shoc2 crosstalk regulates pancreatic tumorigenesis induced by Kras^G12D. Sag deletion increases the size of pancreas and causes the conversion of murine pancreatic intraepithelial neoplasms (mPanINs) to neoplastic cystic lesions with a mechanism involving Shoc2 accumulation, suggesting that Sag determines the pathological process via targeting Shoc2. Shoc2 deletion significantly inhibits pancreas growth, mPanIN formation, and acinar cell transdifferentiation, indicating that Shoc2 is essential for Kras^G12D-induced pancreatic tumorigenesis. Likewise, in a primary acinar 3D culture, Sag deletion inhibits acinar-to-ductal transdifferentiation, while Shoc2 deletion significantly reduces the duct-like structures. Mechanistically, SAG is an E3 ligase that targets SHOC2 for degradation to affect both Mapk and mTorc1 pathways. Shoc2 deletion completely rescues the phenotype of neoplastic cystic lesions induced by Sag deletion, indicating physiological relevance of the Sag-Shoc2 crosstalk. Thus, the Sag-Shoc2 axis specifies the pancreatic tumor types induced by Kras^G12D.

Shoc2 controls ERK1/2-driven neural crest development by balancing components of the extracellular matrix

The extracellular signal-regulated kinase (ERK1/2) pathway is essential in embryonic development. The scaffold protein Shoc2 is a critical modulator of ERK1/2 signals, and mutations in the shoc2 gene lead to the human developmental disease known as Noonan-like syndrome with loose anagen hair (NSLH). The loss of Shoc2 and the shoc2 NSLH-causing mutations affect the tissues of neural crest (NC) origin. In this study, we utilized the zebrafish model to dissect the role of Shoc2-ERK1/2 signals in the development of NC. These studies established that the loss of Shoc2 significantly altered the expression of transcription factors regulating the specification and differentiation of NC cells. Using comparative transcriptome analysis of NC-derived cells from shoc2 CRISPR/Cas9 mutant larvae, we found that Shoc2-mediated signals regulate gene programs at several levels, including expression of genes coding for the proteins of extracellular matrix (ECM) and ECM regulators. Together, our results demonstrate that Shoc2 is an essential regulator of NC development. This study also indicates that disbalance in the turnover of the ECM may lead to the abnormalities found in NSLH patients.

Nuclear p120 catenin is a component of the perichromosomal layer and coordinates sister chromatid segregation during mitosis in lung cancer cells

Abnormal expression of p120 catenin is associated with the malignant phenotype in human lung cancer. Numerous studies have focused on the function of p120 catenin in the juxta-membrane compartment. However, the role of nuclear p120 catenin remains unclear. In this study, the dynamic changes in nuclear p120 catenin localization during cell cycle progression were investigated. Immunofluorescent staining, FACS analysis, and western blotting revealed that nuclear p120 catenin is a major architectural constituent of the chromosome periphery during mitosis. During mitosis, granule-like p120 catenin dispersed into a cloudy-like structure and formed cordon-like structures surrounding the condensed chromosomes to create the peri-chromosomal layer. Interestingly, lumican and p120 catenin colocalized at the spindle fiber where the perichromosomal layer connects to the condensed chromosomes during mitosis. Furthermore, downregulation of p120 catenin using a specific siRNA induced cell cycle stalling in the G2/M phase and promoted aneuploidy. This study validates the role of nuclear p120 catenin in the formation of the chromosome periphery and reveals the p120 catenin-lumican interaction may couple orientation of cell division with the segregation of sister chromatids during mitosis. Our data suggest the protective role of p120 catenin in maintaining the integrity of chromosomes, and also warrants further studies to evaluate the contribution of the loss of p120 catenin to the creation of gene rearrangement in cancer evolution and tumor progression.

The adducin saga: pleiotropic genomic targets for precision medicine in human hypertension-vascular, renal, and cognitive diseases

Hypertension is a leading risk factor for stroke, heart disease, chronic kidney disease, vascular cognitive impairment, and Alzheimer's disease. Previous genetic studies have nominated hundreds of genes linked to hypertension, and renal and cognitive diseases. Some have been advanced as candidate genes by showing that they can alter blood pressure or renal and cerebral vascular function in knockout animals; however, final validation of the causal variants and underlying mechanisms has remained elusive. This review chronicles 40 years of work, from the initial identification of adducin (ADD) as an ACTIN-binding protein suggested to increase blood pressure in Milan hypertensive rats, to the discovery of a mutation in ADD1 as a candidate gene for hypertension in rats that were subsequently linked to hypertension in man. More recently, a recessive K572Q mutation in ADD3 was identified in Fawn-Hooded Hypertensive (FHH) and Milan Normotensive (MNS) rats that develop renal disease, which is absent in resistant strains. ADD3 dimerizes with ADD1 to form functional ADD protein. The mutation in ADD3 disrupts a critical ACTIN-binding site necessary for its interactions with actin and spectrin to regulate the cytoskeleton. Studies using Add3 KO and transgenic strains, as well as a genetic complementation study in FHH and MNS rats, confirmed that the K572Q mutation in ADD3 plays a causal role in altering the myogenic response and autoregulation of renal and cerebral blood flow, resulting in increased susceptibility to hypertension-induced renal disease and cerebral vascular and cognitive dysfunction.

Shoc2 recognizes bacterial flagellin and mediates antibacterial Erk/Stat signaling in an invertebrate

Flagellin is a key bacterial virulence factor that can stimulate molecular immune signaling in both animals and plants. The detailed mechanisms of recognizing flagellin and mounting an efficient immune response have been uncovered in vertebrates; however, whether invertebrates can discriminate flagellin remains largely unknown. In the present study, the homolog of human SHOC2 leucine rich repeat scaffold protein in kuruma shrimp (Marsupenaeus japonicus), designated MjShoc2, was found to interact with Vibrio anguillarum flagellin A (FlaA) using yeast two-hybrid and pull-down assays. MjShoc2 plays a role in antibacterial response by mediating the FlaA-induced expression of certain antibacterial effectors, including lectin and antimicrobial peptide. FlaA challenge, via MjShoc2, led to phosphorylation of extracellular regulated kinase (Erk), and the subsequent activation of signal transducer and activator of transcription (Stat), ultimately inducing the expression of effectors. Therefore, by establishing the FlaA/MjShoc2/Erk/Stat signaling axis, this study revealed a new antibacterial strategy in shrimp, and provides insights into the flagellin sensing mechanism in invertebrates.

Flagellin sensing has been proven as a general antibacterial strategy. Recognition of bacterial flagellin by the transmembrane receptor toll like receptor 5 (TLR5) leads to the activation of nuclear factor kappa B (NF-κB) pathway and induction of proinflammatory cytokines, while recognition by the intracellular nucleotide-binding leucine-rich (NLR) receptor leads to caspase-activation and cytokines-expression. Although flagellin is an effective immune stimulator that induces antimicrobial peptides in Drosophila and in crustaceans, how an invertebrate host senses flagellin and mounts an immune response is poorly understood. Here, we used the flagellin (FlaA) from Vibrio anguillarum, a pathogen of shrimp, as a bait protein to screen a yeast two-hybrid library derived from kuruma shrimp (Marsupenaeus japonicus). We found a scaffold protein, MjShoc2, able to interact with FlaA. We also found that FlaA could effectively induce the expression of certain recognized antibacterial effectors in shrimp depending on MjShoc2. We revealed that extracellular regulated kinase (Erk) phosphorylation occurred downstream of FlaA/MjShoc2, and led to signal transducer and activator of transcription (Stat) activation, resulting in transcription of certain effectors. Therefore our study provides new insights into the FlaA-induced molecular immunity in invertebrates.

The role of USP7 in the Shoc2-ERK1/2 signaling axis and Noonan-like syndrome with loose anagen hair

The ERK1/2 (also known as MAPK3 and MAPK1, respectively) signaling pathway is critical in organismal development and tissue morphogenesis. Deregulation of this pathway leads to congenital abnormalities with severe developmental dysmorphisms. The core ERK1/2 cascade relies on scaffold proteins, such as Shoc2 to guide and fine-tune its signals. Mutations in SHOC2 lead to the development of the pathology termed Noonan-like Syndrome with Loose Anagen Hair (NSLAH). However, the mechanisms underlying the functions of Shoc2 and its contributions to disease progression remain unclear. Here, we show that ERK1/2 pathway activation triggers the interaction of Shoc2 with the ubiquitin-specific protease USP7. We reveal that, in the Shoc2 module, USP7 functions as a molecular 'switch' that controls the E3 ligase HUWE1 and the HUWE1-induced regulatory feedback loop. We also demonstrate that disruption of Shoc2-USP7 binding leads to aberrant activation of the Shoc2-ERK1/2 axis. Importantly, our studies reveal a possible role for USP7 in the pathogenic mechanisms underlying NSLAH, thereby extending our understanding of how ubiquitin-specific proteases regulate intracellular signaling.

Signaling Microdomains in the Spotlight: Visualizing Compartmentalized Signaling Using Genetically Encoded Fluorescent Biosensors

How cells muster a network of interlinking signaling pathways to faithfully convert diverse external cues to specific functional outcomes remains a central question in biology. Through their ability to convert dynamic biochemical activities to rapid and precise optical readouts, genetically encoded fluorescent biosensors have become instrumental in unraveling the molecular logic controlling the specificity of intracellular signaling. In this review, we discuss how the use of genetically encoded fluorescent biosensors to visualize dynamic signaling events within their native cellular context is elucidating the different strategies employed by cells to organize signaling activities into discrete compartments, or signaling microdomains, to ensure functional specificity.

A Leucine-Rich Repeat Protein Provides a SHOC2 the RAS Circuit: a Structure-Function Perspective

SHOC2 is a prototypical leucine-rich repeat protein that promotes downstream receptor tyrosine kinase (RTK)/RAS signaling and plays important roles in several cellular and developmental processes. Gain-of-function germ line mutations of SHOC2 drive the RASopathy Noonan-like syndrome, and SHOC2 mediates adaptive resistance to mitogen-activated protein kinase (MAPK) inhibitors. Similar to many scaffolding proteins, SHOC2 facilitates signal transduction by enabling proximal protein interactions and regulating the subcellular localization of its binding partners. Here, we review the structural features of SHOC2 that mediate its known functions, discuss these elements in the context of various binding partners and signaling pathways, and highlight areas of SHOC2 biology where a consensus view has not yet emerged.

Unified control of amoeboid pseudopod extension in multiple organisms by branched F-actin in the front and parallel F-actin/myosin in the cortex

The trajectory of moving eukaryotic cells depends on the kinetics and direction of extending pseudopods. The direction of pseudopods has been well studied to unravel mechanisms for chemotaxis, wound healing and inflammation. However, the kinetics of pseudopod extension-when and why do pseudopods start and stop- is equally important, but is largely unknown. Here the START and STOP of about 4000 pseudopods was determined in four different species, at four conditions and in nine mutants (fast amoeboids Dictyostelium and neutrophils, slow mesenchymal stem cells, and fungus B.d. chytrid with pseudopod and a flagellum). The START of a first pseudopod is a random event with a probability that is species-specific (23%/s for neutrophils). In all species and conditions, the START of a second pseudopod is strongly inhibited by the extending first pseudopod, which depends on parallel filamentous actin/myosin in the cell cortex. Pseudopods extend at a constant rate by polymerization of branched F-actin at the pseudopod tip, which requires the Scar complex. The STOP of pseudopod extension is induced by multiple inhibitory processes that evolve during pseudopod extension and mainly depend on the increasing size of the pseudopod. Surprisingly, no differences in pseudopod kinetics are detectable between polarized, unpolarized or chemotactic cells, and also not between different species except for small differences in numerical values. This suggests that the analysis has uncovered the fundament of cell movement with distinct roles for stimulatory branched F-actin in the protrusion and inhibitory parallel F-actin in the contractile cortex.

The leucine-rich repeat signaling scaffolds Shoc2 and Erbin: cellular mechanism and role in disease

Leucine-rich repeat-containing proteins (LRR proteins) are involved in supporting a large number of cellular functions. In this review, we summarize recent advancements in understanding functions of the LRR proteins as signaling scaffolds. In particular, we explore what we have learned about the mechanisms of action of the LRR scaffolds Shoc2 and Erbin and their roles in normal development and disease. We discuss Shoc2 and Erbin in the context of their multiple known interacting partners in various cellular processes and summarize often unexpected functions of these proteins through analysis of their roles in human pathologies. We also review these LRR scaffold proteins as promising therapeutic targets and biomarkers with potential application across various pathologies.

Advancing RAS/RASopathy therapies: An NCI-sponsored intramural and extramural collaboration for the study of RASopathies

RASopathies caused by germline pathogenic variants in genes that encode RAS pathway proteins. These disorders include neurofibromatosis type 1 (NF1), Noonan syndrome (NS), cardiofaciocutaneous syndrome (CFC), and Costello syndrome (CS), and others. RASopathies are characterized by heterogenous manifestations, including congenital heart disease, failure to thrive, and increased risk of cancers. Previous work led by the NCI Pediatric Oncology Branch has altered the natural course of one of the key manifestations of the RASopathy NF1. Through the conduct of a longitudinal cohort study and early phase clinical trials, the MEK inhibitor selumetinib was identified as the first active therapy for the NF1-related peripheral nerve sheath tumors called plexiform neurofibromas (PNs). As a result, selumetinib was granted breakthrough therapy designation by the FDA for the treatment of PN. Other RASopathy manifestations may also benefit from RAS targeted therapies. The overall goal of Advancing RAS/RASopathy Therapies (ART), a new NCI initiative, is to develop effective therapies and prevention strategies for the clinical manifestations of the non-NF1 RASopathies and for tumors characterized by somatic RAS mutations. This report reflects discussions from a February 2019 initiation meeting for this project, which had broad international collaboration from basic and clinical researchers and patient advocates.

Single-domain antibodies for functional targeting of the signaling scaffold Shoc2

The accurate transmission of signals by the canonical ERK1/2 kinase pathway critically relies on the proper assembly of an intricate multiprotein complex by the scaffold protein Shoc2. However, the details of the mechanism by which Shoc2 guides ERK1/2 signals are not clear, in part, due to the lack of research tools targeting specific protein binding moieties of Shoc2. We report generation and characterization of single domain antibodies against human Shoc2 using a universal synthetic library of humanized nanobodies. Our results identify eight synthetic single-domain antibodies and show that two evaluated antibodies have binding affinities to Shoc2 in the nanomolar range. High affinity antibodies were uniquely suited for the analysis of the Shoc2 complex assembly. Selected single-domain antibodies were also functional in intracellular assays. This study illustrates that Shoc2 single-domain antibodies can be used to understand functional mechanisms governing complex multiprotein signaling modules and have promise in application for therapies that require modulation of the ERK1/2-associated diseases.

SHOC2 is associated with the survival of breast cancer cells and has prognostic value for patients with breast cancer

SHOC2 leucine rich repeat scaffold protein (SHOC2) has been identified as a positive regulator of the Ras pathway; however, the function of SHOC2 in breast cancer has rarely been explored. The current study investigated the effects of SHOC2 on breast cancer cell growth and evaluated its prognostic value in patients with breast cancer. The effects of SHOC2 on MCF‑7 and MDA‑MB‑231 breast cancer cells were studied using short hairpin RNA. In total, 120 pairs of formalin‑fixed, paraffin‑embedded breast cancer tissue specimens were compared to normal tissue using immunohistochemical staining. SHOC2 knockdown significantly inhibited MCF‑7 and MDA‑MB‑231 breast cancer cell proliferation, and induced cell apoptosis and cell cycle arrest. Additionally, the RAS‑MAPK/PI3K pathway was inhibited by SHOC2 knockdown. In a clinical study, the results revealed that high SHOC2 expression was associated with more aggressive clinical characteristics of breast cancer. Moreover, Kaplan‑Meier and Cox regression analyses indicated that SHOC2 expression was an independent prognostic factor for survival, suggesting that increased SHOC2 expression predicted a worse overall survival. This indicated that SHOC2 knockdown could affect breast cancer cell survival, and SHOC2 upregulation may be associated with a poor prognosis in patients with breast cancer.

SUR-8 interacts with PP1-87B to stabilize PERIOD and regulate circadian rhythms in Drosophila

Circadian rhythms are generated by endogenous pacemakers that rely on transcriptional-translational feedback mechanisms conserved among species. In Drosophila, the stability of a key pacemaker protein PERIOD (PER) is tightly controlled by changes in phosphorylation status. A number of molecular players have been implicated in PER destabilization by promoting PER progressive phosphorylation. On the other hand, there have been few reports describing mechanisms that stabilize PER by delaying PER hyperphosphorylation. Here we report that the protein Suppressor of Ras (SUR-8) regulates circadian locomotor rhythms by stabilizing PER. Depletion of SUR-8 from circadian neurons lengthened the circadian period by about 2 hours and decreased PER abundance, whereas its overexpression led to arrhythmia and an increase in PER. Specifically SUR-8 promotes the stability of PER through phosphorylation regulation. Interestingly, downregulation of the protein phosphatase 1 catalytic subunit PP1-87B recapitulated the phenotypes of SUR-8 depletion. We found that SUR-8 facilitates interactions between PP1-87B and PER. Depletion of SUR-8 decreased the interaction of PER and PP1-87B, which supports the role of SUR-8 as a scaffold protein. Interestingly, the interaction between SUR-8 and PER is temporally regulated: SUR-8 has more binding to PER at night than morning. Thus, our results indicate that SUR-8 interacts with PP1-87B to control PER stability to regulate circadian rhythms.

Circadian clocks govern daily rhythms in physiology and behavior. Conserved molecular machinery drives circadian clocks among animals. PERIOD is a key pacemaker protein in fruit flies that undergoes a series of post-translational modifications. Several kinases have been identified in destabilizing PER. Here we identify the role of SUR-8 in circadian locomotor rhythms. Depletion of SUR-8 in pacemaker neurons slows down circadian rhythms and reduces PER abundance. Indeed, SUR-8 promotes the stability of PER. Finally we characterize SUR-8 as a scaffold protein to bridge PER and a phosphatase (PP1-87B) together to regulate PER phosphorylation and abundance.

Hematopoietic and neural crest defects in zebrafish shoc2 mutants: a novel vertebrate model for Noonan-like syndrome

The extracellular signal-related kinase 1 and 2 (ERK1/2) pathway is a highly conserved signaling cascade with numerous essential functions in development. The scaffold protein Shoc2 amplifies the activity of the ERK1/2 pathway and is an essential modulator of a variety of signaling inputs. Germline mutations in Shoc2 are associated with the human developmental disease known as the Noonan-like syndrome with loose anagen hair. Clinical manifestations of this disease include congenital heart defects, developmental delays, distinctive facial abnormalities, reduced growth and cognitive deficits along with hair anomalies. The many molecular details of pathogenesis of the Noonan-like syndrome and related developmental disorders, cumulatively called RASopathies, remain poorly understood. Mouse knockouts for Shoc2 are embryonic lethal, emphasizing the need for additional animal models to study the role of Shoc2 in embryonic development. Here, we characterize a zebrafish shoc2 mutant, and show that Shoc2 is essential for development, and that its loss is detrimental for the development of the neural crest and for hematopoiesis. The zebrafish model of the Noonan-like syndrome described here provides a novel system for the study of structure-function analyses and for genetic screens in a tractable vertebrate system.

SHOC2 complex-driven RAF dimerization selectively contributes to ERK pathway dynamics

The ERK signaling pathway is hyperactivated in a majority of cancers. However, because it mediates myriad physiological responses, the clinical efficacy of current ERK pathway inhibitors has been severely limited by toxicity. This study uncovers both SHOC2 phosphatase complex-dependent and -independent mechanisms of RAF and ERK activation that are differentially engaged in a context and spatio-temporal–dependent manner. KRAS oncogenic signaling preferentially depends on SHOC2 dependent-mechanisms, which thus presents a therapeutic opportunity. This study provides a molecular framework for how targeting the SHOC2-holophosphatase regulatory node of the RAF activation process provides a mechanism for selective inhibition of ERK signaling.

Despite the crucial role of RAF kinases in cell signaling and disease, we still lack a complete understanding of their regulation. Heterodimerization of RAF kinases as well as dephosphorylation of a conserved “S259” inhibitory site are important steps for RAF activation but the precise mechanisms and dynamics remain unclear. A ternary complex comprised of SHOC2, MRAS, and PP1 (SHOC2 complex) functions as a RAF S259 holophosphatase and gain-of-function mutations in SHOC2, MRAS, and PP1 that promote complex formation are found in Noonan syndrome. Here we show that SHOC2 complex-mediated S259 RAF dephosphorylation is critically required for growth factor-induced RAF heterodimerization as well as for MEK dissociation from BRAF. We also uncover SHOC2-independent mechanisms of RAF and ERK pathway activation that rely on N-region phosphorylation of CRAF. In DLD-1 cells stimulated with EGF, SHOC2 function is essential for a rapid transient phase of ERK activation, but is not required for a slow, sustained phase that is instead driven by palmitoylated H/N-RAS proteins and CRAF. Whereas redundant SHOC2-dependent and -independent mechanisms of RAF and ERK activation make SHOC2 dispensable for proliferation in 2D, KRAS mutant cells preferentially rely on SHOC2 for ERK signaling under anchorage-independent conditions. Our study highlights a context-dependent contribution of SHOC2 to ERK pathway dynamics that is preferentially engaged by KRAS oncogenic signaling and provides a biochemical framework for selective ERK pathway inhibition by targeting the SHOC2 holophosphatase.

VCP/p97 controls signals of the ERK1/2 pathway transmitted via the Shoc2 scaffolding complex: novel insights into IBMPFD pathology

Valosin-containing protein (VCP), also named p97, is an essential hexameric AAA+ ATPase with diverse functions in the ubiquitin system. Here we demonstrate that VCP is critical in controlling signals transmitted via the essential Shoc2-ERK1/2 signaling axis. The ATPase activity of VCP modulates the stoichiometry of HUWE1 in the Shoc2 complex as well as HUWE1-mediated allosteric ubiquitination of the Shoc2 scaffold and the RAF-1 kinase. Abrogated ATPase activity leads to augmented ubiquitination of Shoc2/RAF-1 and altered phosphorylation of RAF-1. We found that in fibroblasts from patients with inclusion body myopathy with Paget’s disease of bone and frontotemporal dementia (IBMPFD) that harbor germline mutations in VCP, the levels of Shoc2 ubiquitination and ERK1/2 phosphorylation are imbalanced. This study provides a mechanistic basis for the critical role of VCP in the regulation of the ERK1/2 pathway and reveals a previously unrecognized function of the ERK1/2 pathway in the pathogenesis of IBMPFD.

ERK: A Key Player in the Pathophysiology of Cardiac Hypertrophy

Cardiac hypertrophy is an adaptive and compensatory mechanism preserving cardiac output during detrimental stimuli. Nevertheless, long-term stimuli incite chronic hypertrophy and may lead to heart failure. In this review, we analyze the recent literature regarding the role of ERK (extracellular signal-regulated kinase) activity in cardiac hypertrophy. ERK signaling produces beneficial effects during the early phase of chronic pressure overload in response to G protein-coupled receptors (GPCRs) and integrin stimulation. These functions comprise (i) adaptive concentric hypertrophy and (ii) cell death prevention. On the other hand, ERK participates in maladaptive hypertrophy during hypertension and chemotherapy-mediated cardiac side effects. Specific ERK-associated scaffold proteins are implicated in either cardioprotective or detrimental hypertrophic functions. Interestingly, ERK phosphorylated at threonine 188 and activated ERK5 (the big MAPK 1) are associated with pathological forms of hypertrophy. Finally, we examine the connection between ERK activation and hypertrophy in (i) transgenic mice overexpressing constitutively activated RTKs (receptor tyrosine kinases), (ii) animal models with mutated sarcomeric proteins characteristic of inherited hypertrophic cardiomyopathies (HCMs), and (iii) mice reproducing syndromic genetic RASopathies. Overall, the scientific literature suggests that during cardiac hypertrophy, ERK could be a “good” player to be stimulated or a “bad” actor to be mitigated, depending on the pathophysiological context.

Systematic identification of Celastrol-binding proteins reveals that Shoc2 is inhibited by Celastrol

Colorectal cancer (CRC) is the third most commonly diagnosed cancer. Celastrol exhibits anti-tumor activities in a variety of cancers. However, the effect of Celastrol on human CRC and the underlying mechanisms still need to be elucidated. The present study aimed to use in vitro and in vivo methods to clarify the anti-tumor effect of Celastrol and use protein microarrays to explore its mechanisms. We demonstrated that Celastrol effectively inhibited SW480 CRC cell proliferation. Two weeks of Celastrol gavage significantly inhibited the growth of xenografts in nude mice. A total of 69 candidate proteins were identified in the protein microarray experiment, including the most highly enriched protein Shoc2, which is a scaffold protein that modulates cell motility and metastasis through the ERK pathway. Celastrol significantly inhibited ERK1/2 phosphorylation in cell lines and xenograft tumors. Down-regulation of Shoc2 expression using Shoc2 siRNA also inhibited ERK1/2 phosphorylation. Furthermore, down-regulation of Shoc2 expression also significantly inhibited proliferation, colony formation, and migration functions of tumor cells. In addition, the LD0 of Celastrol by gavage is equal or more than 80 mg/kg in C57 male mice. In summary, we unraveled the anti-CRC function of Celastrol and confirmed for the first time that it inhibited the ERK1/2 pathway through binding to Shoc2.

The RASopathy Family: Consequences of Germline Activation of the RAS/MAPK Pathway

Noonan syndrome [NS; Mendelian Inheritance in Men (MIM) #163950] and related syndromes [Noonan syndrome with multiple lentigines (formerly called LEOPARD syndrome; MIM #151100), Noonan-like syndrome with loose anagen hair (MIM #607721), Costello syndrome (MIM #218040), cardio-facio-cutaneous syndrome (MIM #115150), type I neurofibromatosis (MIM #162200), and Legius syndrome (MIM #611431)] are a group of related genetic disorders associated with distinctive facial features, cardiopathies, growth and skeletal abnormalities, developmental delay/mental retardation, and tumor predisposition. NS was clinically described more than 50 years ago, and disease genes have been identified throughout the last 3 decades, providing a molecular basis to better understand their physiopathology and identify targets for therapeutic strategies. Most of these genes encode proteins belonging to or regulating the so-called RAS/MAPK signaling pathway, so these syndromes have been gathered under the name RASopathies. In this review, we provide a clinical overview of RASopathies and an update on their genetics. We then focus on the functional and pathophysiological effects of RASopathy-causing mutations and discuss therapeutic perspectives and future directions.