Ras activation in response to phorbol ester proceeds independently of the EGFR via an unconventional nucleotide-exchange factor system in COS-7 cells

Fluorescent Biosensor for Measuring Ras Activity in Living Cells

The small GTPase Ras is a critical regulator of cell growth and proliferation. Its activity is frequently dysregulated in cancers, prompting decades of work to pharmacologically target Ras. Understanding Ras biology and developing effective Ras therapeutics both require probing Ras activity in its native context, yet tools to measure its activities in cellulo are limited. Here, we developed a ratiometric Ras activity reporter (RasAR) that provides quantitative measurement of Ras activity in living cells with high spatiotemporal resolution. We demonstrated that RasAR can probe live-cell activities of all the primary isoforms of Ras. Given that the functional roles of different isoforms of Ras are intimately linked to their subcellular distribution and regulation, we interrogated the spatiotemporal regulation of Ras utilizing subcellularly targeted RasAR and uncovered the role of Src kinase as an upstream regulator to inhibit HRas. Furthermore, we showed that RasAR enables capture of KRasG12C inhibition dynamics in living cells upon treatment with KRasG12C covalent inhibitors, including ARS1620, Sotorasib, and Adagrasib. We found in living cells a residual Ras activity lingers for hours in the presence of these inhibitors. Together, RasAR represents a powerful molecular tool to enable live-cell interrogation of Ras activity and facilitate the development of Ras inhibitors.

Evidence for functional selectivity in TUDC- and norUDCA-induced signal transduction via α5β1 integrin towards choleresis

Functional selectivity is the ligand-specific activation of certain signal transduction pathways at a receptor and has been described for G protein-coupled receptors. However, it has not yet been described for ligands interacting with integrins without αI domain. Here, we show by molecular dynamics simulations that four side chain-modified derivatives of tauroursodeoxycholic acid (TUDC), an agonist of α₅β₁ integrin, differentially shift the conformational equilibrium of α₅β₁ integrin towards the active state, in line with the extent of β₁ integrin activation from immunostaining. Unlike TUDC, 24-nor-ursodeoxycholic acid (norUDCA)-induced β₁ integrin activation triggered only transient activation of extracellular signal-regulated kinases and p38 mitogen-activated protein kinase and, consequently, only transient insertion of the bile acid transporter Bsep into the canalicular membrane, and did not involve activation of epidermal growth factor receptor. These results provide evidence that TUDC and norUDCA exert a functional selectivity at α₅β₁ integrin and may provide a rationale for differential therapeutic use of UDCA and norUDCA.

JunB defines functional and structural integrity of the epidermo-pilosebaceous unit in the skin

Transcription factors ensure skin homeostasis via tight regulation of distinct resident stem cells. Here we report that JunB, a member of the AP-1 transcription factor family, regulates epidermal stem cells and sebaceous glands through balancing proliferation and differentiation of progenitors and by suppressing lineage infidelity. JunB deficiency in basal progenitors results in a dermatitis-like syndrome resembling seborrheic dermatitis harboring structurally and functionally impaired sebaceous glands with a globally altered lipid profile. A fate switch occurs in a subset of JunB deficient epidermal progenitors during wound healing resulting in de novo formation of sebaceous glands. Dysregulated Notch signaling is identified to be causal for this phenotype. In fact, pharmacological inhibition of Notch signaling can efficiently restore the lineage drift, impaired epidermal differentiation and disrupted barrier function in JunB conditional knockout mice. These findings define an unprecedented role for JunB in epidermal-pilosebaceous stem cell homeostasis and its pathology.

Epidermal homeostasis is maintained by the activity of stem cells. Here, the authors show that deficiency of the transcription factor JunB leads to altered Notch signaling in stem cells, resulting in a cell fate switch and de novo formation of aberrant sebaceous glands, altered epidermal differentiation and impaired barrier function.

RasGRP1 overexpression in T-ALL increases basal nucleotide exchange on Ras rendering the Ras/PI3K/Akt pathway responsive to protumorigenic cytokines

Ras GTPases are activated by RasGEFs and inactivated by RasGAPs, which stimulate the hydrolysis of RasGTP to inactive RasGDP. GTPase-impairing somatic mutations in RAS genes, such as KRAS(G12D), are among the most common oncogenic events in metastatic cancer. A different type of cancer Ras signal, driven by overexpression of the RasGEF RasGRP1 (Ras guanine nucleotide-releasing protein 1), was recently implicated in pediatric T-cell acute lymphoblastic leukemia (T-ALL) patients and murine models, in which RasGRP1 T-ALLs expand in response to treatment with interleukins (ILs) 2, 7 and 9. Here, we demonstrate that IL-2/7/9 stimulation activates Erk and Akt pathways downstream of Ras in RasGRP1 T-ALL but not in normal thymocytes. In normal lymphocytes, RasGRP1 is recruited to the membrane by diacylglycerol (DAG) in a phospholipase C-γ (PLCγ)-dependent manner. Surprisingly, we find that leukemic RasGRP1-triggered Ras-Akt signals do not depend on acute activation of PLCγ to generate DAG but rely on baseline DAG levels instead. In agreement, using three distinct assays that measure different aspects of the RasGTP/GDP cycle, we established that overexpression of RasGRP1 in T-ALLs results in a constitutively high GTP-loading rate of Ras, which is constantly counterbalanced by hydrolysis of RasGTP. KRAS(G12D) T-ALLs do not show constitutive GTP loading of Ras. Thus, we reveal an entirely novel type of leukemogenic Ras signals that is based on a RasGRP1-driven increased in flux through the RasGTP/GDP cycle, which is mechanistically very different from KRAS(G12D) signals. Our studies highlight the dynamic balance between RasGEF and RasGAP in these T-ALLs and put forth a new model in which IL-2/7/9 decrease RasGAP activity.

Ras palmitoylation is necessary for N-Ras activation and signal propagation in growth factor signalling

Ras GTPases undergo post-translational modifications that govern their subcellular trafficking and localization. In particular, palmitoylation of the Golgi tags N-Ras and H-Ras for exocytotic transport and residency at the PM (plasma membrane). Following depalmitoylation, PM-Ras redistributes to all subcellular membranes causing an accumulation of palmitate-free Ras at endomembranes, including the Golgi and endoplasmic reticulum. Palmitoylation is unanimously regarded as a critical modification at the crossroads of Ras activity and trafficking control, but its precise relevance to native wild-type Ras function in growth factor signalling is unknown. We show in the present study by use of palmitoylation-deficient N-Ras mutants and via the analysis of palmitate content of agonist-activated GTP-loaded N-Ras that only palmitoylated N-Ras becomes activated by agonists. In line with an essential role of palmitoylation in Ras activation, dominant-negative RasS17N loses its blocking potency if rendered devoid of palmitoylation. Live-cell Ras-GTP imaging shows that N-Ras activation proceeds only at the PM, consistent with activated N-Ras-GTP being palmitoylated. Finally, palmitoylation-deficient N-Ras does not sustain EGF (epidermal growth factor) or serum-elicited mitogenic signalling, confirming that palmitoylation is essential for signal transduction by N-Ras. These findings document that N-Ras activation proceeds at the PM and suggest that depalmitoylation, by removing Ras from the PM, may contribute to the shutdown of Ras signalling.

Regulation of ras exchange factors and cellular localization of ras activation by lipid messengers in T cells

The Ras-MAPK signaling pathway is highly conserved throughout evolution and is activated downstream of a wide range of receptor stimuli. Ras guanine nucleotide exchange factors (RasGEFs) catalyze GTP loading of Ras and play a pivotal role in regulating receptor-ligand induced Ras activity. In T cells, three families of functionally important RasGEFs are expressed: RasGRF, RasGRP, and Son of Sevenless (SOS)-family GEFs. Early on it was recognized that Ras activation is critical for T cell development and that the RasGEFs play an important role herein. More recent work has revealed that nuances in Ras activation appear to significantly impact T cell development and selection. These nuances include distinct biochemical patterns of analog versus digital Ras activation, differences in cellular localization of Ras activation, and intricate interplays between the RasGEFs during distinct T cell developmental stages as revealed by various new mouse models. In many instances, the exact nature of these nuances in Ras activation or how these may result from fine-tuning of the RasGEFs is not understood. One large group of biomolecules critically involved in the control of RasGEFs functions are lipid second messengers. Multiple, yet distinct lipid products are generated following T cell receptor (TCR) stimulation and bind to different domains in the RasGRP and SOS RasGEFs to facilitate the activation of the membrane-anchored Ras GTPases. In this review we highlight how different lipid-based elements are generated by various enzymes downstream of the TCR and other receptors and how these dynamic and interrelated lipid products may fine-tune Ras activation by RasGEFs in developing T cells.

TCR-induced activation of Ras proceeds at the plasma membrane and requires palmitoylation of N-Ras

Ras transmits manifold signals from the TCR at various crossroads in the life of a T cell. For example, selection programs in the thymus or the acquisition of a state of hypo-responsiveness known as anergy are just some of the T cell features known to be controlled by TCR-sparked signals that are intracellularly propagated by Ras. These findings raise the question of how Ras can transmit such a variety of signals leading to the shaping of equally many T cell traits. Because Ras proteins transit through endomembrane compartments on their way to the plasma membrane (PM), compartmentalized Ras activation at distinct subcellular sites represents a potential mechanism for signal diversification in TCR signaling. This hypothesis has been nurtured by studies in T cells engineered to overexpress Ras that reported distinct activation of Ras at the PM and Golgi. Contrary to this scenario, we report in this study that activation of endogenous Ras, imaged in live Jurkat T cells using novel affinity probes for Ras-GTP, proceeds only at the PM even upon enforced signal flux through the diacylglycerol/RasGRP1 pathway. Physiological engagement of the TCR at the immunological synapse in primary T cells caused focalized Ras-GTP accumulation also only at the PM. Analysis of palmitoylation-deficient Ras mutants, which are confined to endomembranes, confirmed that the TCR does not activate Ras in that compartment and revealed a critical function for palmitoylation in N-Ras/H-Ras activation. These findings identify the PM as the only site of TCR-driven Ras activation and document that endomembranes are not a signaling platform for Ras in T cells.

Impaired TrkB-mediated ERK1/2 activation in huntington disease knock-in striatal cells involves reduced p52/p46 Shc expression

Altered neurotrophic support as a result of reduced brain-derived neurotrophic factor (BDNF) expression and trafficking has been revealed as a key factor in Huntington disease (HD) pathology. BDNF binds to and activates the tyrosine kinase receptor TrkB, leading to activation of intracellular signaling pathways to promote differentiation and cell survival. In order to design new neuroprotective therapies based on BDNF delivery, it is important to define whether BDNF-mediated TrkB signaling is affected in HD. Here, we demonstrate reduced TrkB-mediated Ras/MAPK/ERK1/2 signaling but unchanged phosphatidylinositol 3-kinase/Akt and phospholipase Cgamma activation in knock-in HD striatal cells. Altered BDNF-mediated ERK1/2 activation in mutant huntingtin cells is associated with reduced expression of p52/p46 Shc docking proteins. Notably, reduced BDNF-induced ERK1/2 activation increases the sensitivity of mutant huntingtin striatal cells to oxidative damage. Accordingly, pharmacological activation of the MAPK pathway with PMA prevents cell death induced by oxidative stress. Taken together, our results suggest that in addition to reduced BDNF, diminished Ras/MAPK/ERK1/2 activation is involved in neurotrophic deficits associated with HD pathology. Therefore, pharmacological approaches aimed to directly modulate the MAPK/ERK1/2 pathway may represent a valuable therapeutic strategy in HD.

Down-regulation of c-Cbl by morphine accounts for persistent ERK1/2 signaling in delta-opioid receptor-expressing HEK293 cells

Opioids display ligand-specific differences in the time course of ERK1/2 signaling. Whereas full agonists, like etorphine, induce only transient activation of ERK1/2, the partial agonist morphine mediates persistent stimulation of mitogenic signaling. Here we report that in stably delta-opioid receptor (DOR)-expressing HEK293 (HEK/DOR) cells, the transient nature of etorphine-induced ERK1/2 signaling is due to desensitization of epidermal growth factor (EGF) receptor-mediated activation of the Ras/Raf-1/ERK1/2 cascade. Desensitization of ERK1/2 activity by etorphine is associated with down-regulation of EGF receptors, an effect mediated by the ubiquitin ligase c-Cbl. In contrast, chronic morphine treatment failed to desensitize EGF receptors, resulting in unimpeded ERK1/2 signaling. The failure of morphine to desensitize ERK1/2 signaling is mediated by persistent activation of c-Src, which induces degradation of c-Cbl. The role of c-Src in opioid-specific ERK1/2 signaling is further demonstrated by pretreatment of the cells with PP2 and SKI-I as well as overexpression of a dominant negative c-Src mutant (c-Src(dn)) or a c-Src-resistant c-Cbl mutant (CblY3F), both of which facilitate desensitization of ERK1/2 signaling by morphine. Conversely, overexpression of c-Src as well as down-regulation of c-Cbl by small interfering RNA results in persistent etorphine-induced stimulation of ERK1/2 activity. Subcellular fractionation experiments finally attributed the ability of morphine to persistently activate c-Src to its redistribution from Triton X-100-insensitive membrane rafts to DOR and EGF receptor containing high density membrane compartments implicated in ERK1/2 signaling. These results demonstrate that agonist-specific differences in the temporal and spatial pattern of c-Src activation determine the kinetics of DOR-mediated regulation of ERK1/2 signaling.

Positive- and negative-feedback regulations coordinate the dynamic behavior of the Ras-Raf-MEK-ERK signal transduction pathway

The Ras-Raf-MEK-ERK pathway (or ERK pathway) is an important signal transduction system involved in the control of cell proliferation, survival and differentiation. However, the dynamic regulation of the pathway by positive- and negative-feedback mechanisms, in particular the functional role of Raf kinase inhibitor protein (RKIP) are still incompletely understood. RKIP is a physiological endogenous inhibitor of MEK phosphorylation by Raf kinases, but also participates in a positive-feedback loop in which ERK can inactivate RKIP. The aim of this study was to elucidate the hidden dynamics of these feedback mechanisms and to identify the functional role of RKIP through combined efforts of biochemical experiments and in silico simulations based on an experimentally validated mathematical model. We show that the negative-feedback loop from ERK to SOS plays a crucial role in generating an oscillatory behavior of ERK activity. The positive-feedback loop in which ERK functionally inactivates RKIP also enhances the oscillatory activation pattern of ERK. However, RKIP itself has an important role in inducing a switch-like behavior of MEK activity. When overexpressed, RKIP also causes delayed and reduced responses of ERK. Thus, positive- and negative-feedback loops and RKIP work together to shape the response pattern and dynamical characteristics of the ERK pathway.

Epidermal growth factor receptor lacking C-terminal autophosphorylation sites retains signal transduction and high sensitivity to epidermal growth factor receptor tyrosine kinase inhibitor

Constitutively active mutations of epidermal growth factor receptor (EGFR) (delE746_A750) activate downstream signals, such as ERK and Akt, through the phosphorylation of tyrosine residues in the C-terminal region of EGFR. These pathways are thought to be important for cellular sensitivity to EGFR tyrosine kinase inhibitors (TKI). To examine the correlation between phosphorylation of the tyrosine residues in the C-terminal region of EGFR and cellular sensitivity to EGFR TKI, we used wild-type (wt) EGFR, as well as the following constructs: delE746_A750 EGFR; delE746_A750 EGFR with substitution of seven tyrosine residues to phenylalanine in the C-terminal region; and delE746_A750 EGFR with a C-terminal truncation at amino acid 980. These constructs were transfected stably into HEK293 cells and designated HEK293/Wt, HEK293/D, HEK293/D7F, and HEK293/D-Tr, respectively. The HEK293/D cells were found to be 100-fold more sensitive to EGFR TKI (AG1478) than HEK293/Wt. Surprisingly, the HEK293/D7F and HEK293/D-Tr cells, transfected with EGFR lacking the C-terminal autophosphorylation sites, retained high sensitivity to EGFR TKI. In these three high-sensitivity cells, the ERK pathway was activated without ligand stimulation, which was inhibited by EGFR TKI. In addition, although EGFR in the HEK293/D7F and HEK293/D-Tr cells lacked significant tyrosine residues for EGFR signal transduction, phosphorylation of Src homology and collagen homology (Shc) was spontaneously activated in these cells. Our results indicate that tyrosine residues in the C-terminal region of EGFR are not required for cellular sensitivity to EGFR TKI, and that an as-yet-unknown signaling pathway of EGFR may exist that is independent of the C-terminal region of EGFR.

Regulation of the Ras-GTPase activating protein neurofibromin by C-tail phosphorylation: implications for protein kinase C/Ras/extracellular signal-regulated kinase 1/2 pathway signaling and neuronal differentiation

PKC, Ras, and ERK1/2 signaling is pivotal to differentiation along the neuronal cell lineage. One crucial protein that may play a central role in this signaling pathway is the Ras GTPase-activating protein, neurofibromin, a PKC substrate that may exert a positive role in neuronal differentiation. In this report, we studied the dynamics of PKC/Ras/ERK pathway signaling, during differentiation of SH-SY5Y neuroblastoma cells upon treatment with the PKC agonist, phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Surprisingly, we observed that, among other PKC-dependent signaling events, TPA induced a rapid and sustained decrease of neurofibromin immunoreactivity which was not due to proteolysis. Instead, we identified a specific phosphorylation event at the C-tail of neurofibromin. This phosphorylation was acute and correlated perfectly with the signaling dynamics of the Ras/ERK pathway. Moreover, it persisted throughout prolonged treatment and TPA-induced differentiation of SH-SY5Y cells, concurrently with sustained activation of ERK1/2. Most importantly, C-tail phosphorylation of neurofibromin correlated with a shift of neurofibromin localization from the nucleus to the cytosol. We propose that PKC-dependent, sustained C-tail phosphorylation is a requirement for prolonged recruitment of neurofibromin from the nucleus to the cytosol in order for a fine regulation of Ras/ERK pathway activity to be achieved during differentiation.

Lck-dependent tyrosine phosphorylation of diacylglycerol kinase alpha regulates its membrane association in T cells

TCR engagement triggers phospholipase Cgamma1 activation through the Lck-ZAP70-linker of activated T cell adaptor protein pathway. This leads to generation of diacylglycerol (DAG) and mobilization of intracellular Ca(2+), both essential for TCR-dependent transcriptional responses. TCR ligation also elicits transient recruitment of DAG kinase alpha (DGKalpha) to the lymphocyte plasma membrane to phosphorylate DAG, facilitating termination of DAG-regulated signals. The precise mechanisms governing dynamic recruitment of DGKalpha to the membrane have not been fully elucidated, although Ca(2+) influx and tyrosine kinase activation were proposed to be required. We show that DGKalpha is tyrosine phosphorylated, and identify tyrosine 335 (Y335), at the hinge between the atypical C1 domains and the catalytic region, as essential for membrane localization. Generation of an Ab that recognizes phosphorylated Y335 demonstrates Lck-dependent phosphorylation of endogenous DGKalpha during TCR activation and shows that pY335DGKalpha is a minor pool located exclusively at the plasma membrane. Our results identify Y335 as a residue critical for DGKalpha function and suggest a mechanism by which Lck-dependent phosphorylation and Ca(2+) elevation regulate DGKalpha membrane localization. The concerted action of these two signals results in transient, receptor-regulated DGKalpha relocalization to the site at which it exerts its function as a negative modulator of DAG-dependent signals.

ErbB receptors, their ligands, and the consequences of their activation and inhibition in the myocardium

The epidermal growth factor (EGF) receptor (or ErbB1) and the related ErbB4 are transmembrane receptor protein tyrosine kinases which bind extracellular ligands of the EGF family. ErbB2 and ErbB3 are "co-receptors" structurally related to ErbB1/ErbB4, but ErbB2 is an "orphan" receptor and ErbB3 lacks tyrosine kinase activity. However, both are important in transmembrane signalling. All ErbB receptors/ligands are intimately involved in the regulation of cell growth, differentiation and survival, and their dysregulation contributes to some human malignancies. After extracellular ligand binding, receptor dimerisation and transautophosphorylation of intracellular C-terminal tyrosine residues, they bind signalling proteins which recognise specific tyrosine-phosphorylated motifs. This leads to activation of multiple signalling pathways, notably the extracellular signal-regulated kinase 1/2 (ERK1/2) cascade and the phosphoinositide 3-kinase (PI3K)/protein kinase B [PKB/(Akt)] pathway. In heart, targeted deletion of ErbB2, ErbB3, ErbB4 and some ErbB receptor extracellular ligands leads to embryonic lethality resulting from cardiovascular defects. ErbB receptor ligands improve cardiac myocyte viability and are hypertrophic, partly because of activation of ERK1/2 and/or PI3K/PKB(Akt). Furthermore, ErbB transactivation by Gq protein-coupled receptor (GqPCR) signalling may mediate the hypertrophic effects of GqPCR agonists. The utility of anthracyclines in cancer chemotherapy can be limited by their cardiotoxic side effects and these may be counteracted by ErbB receptor ligands. ErbB2 is the target of anti-cancer monoclonal antibody trastuzumab (Herceptin), and its myocardial downregulation may account for the occasional cardiotoxicity of this therapy. Here, we review the basic biochemistry of ErbB receptors/ligands, and emphasise their particular roles in the myocardium.

RasGAPs: a crucial regulator of extracellular stimuli for homeostasis of cellular functions

Ras and its GTPase activating proteins (GAPs) are among the crucial regulators of extracelluar ligands. Information about these regulators has been elucidated during the course of studies in signal transduction over the last two decades. RasGAPs such as p120GAP and neurofibromin have been studied extensively for their roles as either "negative" regulators or effectors of Ras. Accumulating evidence suggests that these molecules are crucial regulators of extracellular stimuli that serve to maintain the homeostasis of cellular functions. This compendium highlights cellular functions of RasGAPs and their signaling characteristics from the viewpoint of homeostasis, including our recent finding of the phenotype of R-RasGAP mutant mice whose GAP activity is down-regulated.