Kinase suppressor of Ras-1 (KSR-1) is an essential scaffolding protein that coordinates the assembly of the mitogen-activated protein kinase (MAPK) module, consisting of the MAPK kinase kinase Raf, the MAPK kinase MEK (mitogen-activated or extracellular signal–regulated protein kinase kinase), and the MAPK ERK (extracellular signal–regulated kinase) to facilitate activation of MEK and thus ERK. Although KSR-1 is targeted to the cell membrane in part by its atypical C1 domain, which binds to phospholipids, other domains may be involved. We identified another domain in KSR-1 that we termed CC-SAM, which is composed of a coiled coil (CC) and a sterile α motif (SAM). The CC-SAM domain targeted KSR-1 to specific signaling sites at the plasma membrane in growth factor–treated cells, and it bound directly to various micelles and bicelles in vitro, indicating that the CC-SAM functioned as a membrane-binding module. By combining nuclear magnetic resonance spectroscopy and experiments in cultured cells, we found that membrane binding was mediated by helix α3 of the CC motif and that mutating residues in α3 abolished targeting of KSR-1 to the plasma membrane. Thus, in addition to the atypical C1 domain, the CC-SAM domain is required to target KSR-1 to the plasma membrane.
Signaling by transforming growth factor–β (TGF-β) is critical for various developmental processes and culminates in the activation of the transcription factors Smad2 and Smad3. MAN1, an integral protein of the inner nuclear membrane, inhibits TGF-β signalling by binding to Smad2 and Smad3. Depletion of the gene LEMD3 encoding MAN1 leads to developmental anomalies in mice, and heterozygous loss-of-function mutations in LEMD3 in humans cause sclerosing bone dysplasia. We modeled the three-dimensional structure of the MAN1-Smad2 complex from nuclear magnetic resonance and small angle x-ray scattering data. As predicted by this model, we found that MAN1 competed in vitro and in cells with the transcription factor FAST1 (forkhead activin signal transducer 1) for binding to Smad2. The model further predicted that MAN1 bound to activated Smad2-Smad4 or Smad3-Smad4 complexes, which was confirmed by in vitro experiments; however, in cells, MAN1 bound only to Smad2 and Smad3, and not to the Smad4-containing complexes. Overexpression of MAN1 led to dephosphorylation of Smad2 and Smad3, thus hindering their recognition by Smad4, and MAN1 bound directly in vitro to the phosphatase PPM1A, which catalyzes the dephosphorylation of Smad2/3. These results demonstrate a nuclear envelope-localized mechanism of inactivating TGF-β signaling in which MAN1 competes with transcription factors for binding to Smad2 and Smad3 and facilitates their dephosphorylation by PPM1A.
The guanine nucleotide exchange factor (GEF) Vav1 synergizes with the adapter SLP-76 to support T cell development and activation. Here, we demonstrate that Vav1 controls the stability and movement T cell receptor-induced SLP-76 microclusters. The SH2 domain enables the recruitment of Vav1 into SLP-76 microclusters, whereas the SH3 domains of Vav1 cooperate to enhance microcluster stability and function. Although the amino-terminus of Vav1 is essential for downstream signaling, it possesses novel scaffolding functions that are unaffected by the inactivation of the Vav1 GEF or by the constitutive GEF activation that accompanies the mutation of the regulatory tyrosine residues 142, 160, and 174. In contrast, GEF-inactivating point mutations predicted to perturb the structural integrity of the Vav1 GEF abolish these scaffolding functions. Paradoxically, the excision of catalytic Dbl-homology (DH) / pleckstrin homology (PH) cassette produces a relatively mild scaffolding defect, indicating that the L213A and L278Q point mutations antagonize scaffolding functions mediated by adjacent domains. A deletion mutant lacking the CH domain potently inhibits calcium responses, but also exhibits mild scaffolding defects. We conclude multiple GEF-independent scaffolding functions contained within the amino-terminus of Vav1 contribute to T cell activation by acting synergistically to increase the stability and functionality of SLP-76 microclusters.
Wnt proteins are critical to mammalian brain development and function. The canonical Wnt signaling pathway involves the stabilization and nuclear translocation of β-catenin; however, Wnt also signals through alternative, noncanonical pathways. To gain a systems-level, genome-wide view of Wnt signaling, we analyzed Wnt1-stimulated changes in gene expression by transcriptional microarray analysis in cultured human neural progenitor (hNP) cells at multiple time points over a 72-hour time course. We observed a widespread oscillatory-like pattern of changes in gene expression, involving components of both the canonical and the noncanonical Wnt signaling pathways. A higher-order, systems-level analysis that combined independent component analysis, waveform analysis, and mutual information–based network construction revealed effects on pathways related to cell death and neurodegenerative disease. Wnt effectors were tightly clustered with presenilin1 (PSEN1) and granulin (GRN), which cause dominantly inherited forms of Alzheimer’s disease and frontotemporal dementia (FTD), respectively. We further explored a potential link between Wnt1 and GRN and found that Wnt1 decreased GRN expression by hNPs. Conversely, GRN knockdown increased WNT1 expression, demonstrating that Wnt and GRN reciprocally regulate each other. Finally, we provided in vivo validation of the in vitro findings by analyzing gene expression data from individuals with FTD. These unbiased and genome-wide analyses provide evidence for a connection between Wnt signaling and the transcriptional regulation of neurodegenerative disease genes.
The cytotoxic effects of natural killer (NK) cells and their ability to secrete cytokines require the induction of synergistic signals from co-activation receptors, such as CD314 (NKG2D) and CD244 (2B4), which bind to ligands expressed on target cells. Synergy is required to overcome inhibition of the guanine nucleotide exchange factor (GEF) Vav1, a central regulator of NK cell activation, by the E3 ubiquitin ligase c-Cbl. However, the molecular basis for this synergy is unknown. Here, we showed that the adaptor protein Src homology 2 (SH2) domain–containing leukocyte phosphoprotein of 76 kD (SLP-76) was required for this synergy, and that distinct tyrosine residues in SLP-76 were phosphorylated by each receptor of a synergistic pair. Selective phosphorylation of tyrosine 113 or tyrosine 128 in SLP-76, each of which enables binding of SLP-76 to Vav1, was unique to receptors that stimulate ligand-dependent target cell killing, because antibody-dependent stimulation by Fc receptor CD16 promoted phosphorylation at both sites. Knockdown and reconstitution experiments with SLP-76 showed the distinct role of each tyrosine in the synergistic mobilization of Ca2+, revealing an unexpected degree of selectivity in the phosphorylation of SLP-76 by NK cell co-activation receptors. Together, these data suggest that complementation of separate phospho-tyrosine targets in SLP-76 forms the basis of synergistic NK cell activation.
The proapoptotic proteins BAX and BAK constitute the mitochondrial apoptotic gateway that executes cellular demise after integrating death signals. The lethal BAK is kept in check by voltage-dependent anion channel 2 (VDAC2), a mammalian-restricted VDAC isoform. Here, we provide evidence showing a critical role for the VADC2-BAK complex in determining thymocyte survival in vivo. Genetic depletion of Vdac2 in the thymus resulted in excessive cell death and hypersensitivity to diverse death stimuli including engagement of the T cell receptor. These phenotypes were completely rescued by the concurrent deletion of Bak but not that of Bax. Thus, the VDAC2-BAK axis provides a mechanism that governs the homeostasis of thymocytes. Our study reveals a sophisticated built-in rheostat that likely fine-tunes immune competence to balance autoimmunity and immunodeficiency.
The fetal brain is highly plastic and is not only receptive to but requires cues from its environment to develop properly. Based on an understanding of evolutionary biology, developmental plasticity, and life history theory, one can predict that stressors are an important environmental condition that may influence brain development. In fact, the available empirical evidence appears to support the notion that exposure to excess stress in intrauterine life has the potential to adversely affect short- and long-term neurodevelopmental outcomes with implications for altered susceptibility for mental health disorders in childhood and adult life. In this presentation, we provide a rationale for proposing that endocrine and inflammatory stress mediators are key candidate pathways for programming brain development. These mediators are responsive to a diverse set of intrauterine perturbations and alter key signaling pathways critical for brain development, including but not limited to mammalian target of rapamycin, Wnt (wingless), Sonic hedgehog, and reelin signaling. We suggest that recent advances in neuroimaging and other methods now afford us an unprecedented opportunity to advance our understanding of this important topic. Additionally, we provide empirical evidence from two recently published papers for fetal programming of human brain development. We conclude by suggesting some future directions for expanding this field of research.
The fission yeast Schizosaccharomyces pombe has more metazoan-like features than the budding yeast Saccharomyces cerevisiae, yet it has similarly facile genetics. Here, we present a large-scale verified binary protein-protein interactome network, “StressNet”, based on high-throughput yeast two-hybrid screens of interacting proteins classified as part of stress-response and signal transduction pathways in S. pombe. We performed systematic, cross-species interactome mapping using StressNet and a protein interactome network of orthologous proteins in S. cerevisiae. With cross-species comparative network studies, we detected a previously unidentified component (Snr1) of the S. pombe mitogen-activated protein kinase Sty1 pathway. Coimmunoprecipitation experiments showed that Snr1 interacted with Sty1 and that deletion of snr1 increased the sensitivity of S. pombe cells to stress. Comparison of StressNet with the interactome network of orthologous proteins in S. cerevisiae showed that the majority of interactions among these stress-response and signaling proteins are not conserved between species, but are “rewired;” orthologous proteins have different binding partners in both species. In particular, transient interactions connecting proteins in different functional modules were more likely to be rewired than conserved. By directly testing interactions between proteins in one yeast species and their corresponding binding partners in the other yeast species with yeast two-hybrid assays, we found that about half of the interactions traditionally considered “conserved” form modified interaction interfaces that may potentially accommodate novel functions.
Duchenne muscular dystrophy (DMD) is a fatal X-linked degenerative muscle disease caused by the absence of the microtubule-associated protein dystrophin, which results in a disorganized and denser microtubule cytoskeleton. In addition, mechanotransduction-dependent activation of calcium (Ca2+) and reactive oxygen species (ROS) signaling underpins muscle degeneration in DMD. We show that in muscle from adult mdx mice, a model of DMD, a brief physiologic stretch elicited microtubule-dependent activation of NADPH (reduced-form nicotinamide adenine dinucleotide phosphate) oxidase–dependent production of ROS, termed X-ROS. Further, X-ROS amplified Ca2+ influx through stretch-activated channels in mdx muscle. Consistent with the importance of the microtubules to the dysfunction in mdx muscle, muscle cells with dense microtubule structure, such as those from adult mdx mice or from young wild-type mice treated with Taxol, showed increased X-ROS production and Ca2+ influx, whereas cells with a less dense microtubule network, such as young mdx or adult mdx muscle treated with colchicine or nocodazole, showed little ROS production or Ca2+ influx. In vivo treatments that disrupted the microtubule network or inhibited NADPH oxidase 2 reduced contraction-induced injury in adult mdx mice. Furthermore, transcriptome analysis identified increased expression of X-ROS–related genes in human DMD skeletal muscle. Together, these data show that microtubules are the proximate element responsible for the dysfunction in Ca2+ and ROS signaling in DMD and could be effective therapeutic targets for intervention.
Fibroblast growth factor 2 (FGF2) induces endothelial cell migration and angiogenesis through two classes of receptors: receptor tyrosine kinases, such as FGF receptor 1 (FGFR1), and heparan sulfate proteoglycans, such as syndecan 4 (S4). We examined the distinct contributions of FGFR1 and S4 in shaping the endothelial response to FGF2. S4 determined the kinetics and magnitude of FGF2-induced mitogen-activated protein kinase (MAPK) signaling by promoting the macropinocytosis of the FGFR1-S4-FGF2 signaling complex. Internalization of the S4 receptor complex was independent of clathrin and dynamin, proceeded from lipid raft–enriched membranes, and required activation of the guanosine triphosphatases RhoG and Rab5. Genetic knockout of S4, disruption of S4 function, or inhibition of Rab5 led to increased endocytosis and MAPK signaling. These data define the mechanism by which FGFR1 and S4 coordinate downstream signaling upon FGF2 stimulation: FGFR1 initiates MAPK signaling, whereas S4-dependent FGFR1 macropinocytosis modulates the kinetics of MAPK activation. Our studies identify S4 as a regulator of MAPK signaling and address the question of how distinct classes of FGFRs individually contribute to signal transduction in endothelial cells.
eEF2K is a kinase that controls the rate of peptide chain elongation by phosphorylating eukaryotic Elongation Factor 2 (eEF2), the protein that mediates the movement of the ribosome along the mRNA by promoting translocation from the A to the P site. eEF2K-mediated phosphorylation of eEF2 on Thr56 decreases its affinity for the ribosome, thereby inhibiting elongation. Here we show that in response to genotoxic stress, eEF2K is activated by AMPK-mediated phosphorylation on Ser398. Activated eEF2K phosphorylates eEF2 and induces a temporary ribosomal slowdown at the stage of elongation. Subsequently, during checkpoint silencing, eEF2K is degraded by the ubiquitin-proteasome system via the SCFβTrCP ubiquitin ligase to allow rapid resumption of translation elongation. This event requires eEF2K autophosphorylation on a canonical βTrCP-binding domain. The inability to degrade eEF2K during checkpoint silencing caused sustained phosphorylation of eEF2 on Thr56 and delayed resumption of translation elongation. Our study establishes an important link between DNA damage signaling and translation elongation.
Mitosis is a process involving a complex series of events that require careful coordination. Protein phosphorylation by a small number of kinases, in particular Aurora A, Aurora B, the cyclin-dependent kinase–cyclin complex Cdk1/cyclinB, and Polo-like kinase 1 (Plk1), orchestrates almost every step of cell division, from entry into mitosis to cytokinesis. To discover more about the functions of Aurora A, Aurora B, and kinases of the Plk family, we mapped mitotic phosphorylation sites to these kinases through the combined use of quantitative phosphoproteomics and selective targeting of kinase activities by small-molecule inhibitors. Using this integrated approach, we connected 778 phosphorylation sites on 562 proteins with these enzymes in cells arrested in mitosis. By connecting the kinases to protein complexes, we associated these kinases with functional modules. In addition to predicting previously unknown functions, this work establishes additional substrate-recognition motifs for these kinases and provides an analytical template for further use in dissecting kinase signaling events in other areas of cellular signaling and systems biology.
Environmental and internal conditions expose cells to a multiplicity of stimuli whose consequences are difficult to predict. Here, we investigate the response to mating pheromone of yeast cells adapted to high osmolarity. Events downstream of pheromone binding involve two mitogen-activated protein kinase (MAPK) cascades: the pheromone response (PR) and the cell-wall integrity response (CWI). Although these MAPK pathways share components with each and a third MAPK pathway, the high osmolarity response (HOG), they are normally only activated by distinct stimuli, a phenomenon called insulation. We found that in cells adapted to high osmolarity, PR activated the HOG pathway in a pheromone- and osmolarity- dependent manner. Activation of HOG by the PR was not due to loss of insulation, but rather a response to a reduction in internal osmolarity, which resulted from an increase in glycerol release caused by the PR. By analyzing single-cell time courses, we found that stimulation of HOG occurred in discrete bursts that coincided with the “shmooing” morphogenetic process. Activation required the polarisome, the cell wall integrity MAPK Slt2, and the aquaglyceroporin Fps1. HOG activation resulted in high glycerol turnover that improved adaptability to rapid changes in osmolarity. Our work shows how a differentiation signal can recruit a second, unrelated sensory pathway to enable responses to yeast to multiple stimuli.
Myoblast fusion is a critical process that contributes to the growth of muscle during development and to the regeneration of myofibers upon injury. Myoblasts fuse with each other as well as with multinucleated myotubes to enlarge the myofiber. Initial studies demonstrated that myoblast fusion requires extracellular calcium and changes in cell membrane topography and cytoskeletal organization. More recent studies have identified several cell-surface and intracellular proteins that mediate myoblast fusion. Furthermore, emerging evidence suggests that myoblast fusion is also regulated by the activation of specific cell-signaling pathways that lead to the expression of genes whose products are essential for the fusion process and for modulating the activity of molecules that are involved in cytoskeletal rearrangement. Here, we review the roles of the major signaling pathways in mammalian myoblast fusion.
A new technology has emerged that will facilitate the presentation of dynamic or otherwise inaccessible data on posters at scientific meetings. Video, audio, or other digital files hosted on mobile-friendly sites can be linked to through a quick response (QR) code, a two-dimensional barcode that can be scanned by smartphones, which then display the content. This approach is more affordable than acquiring tablet computers for playing dynamic content and can reach many users at large conferences. This resource details how to host videos, generate QR codes, and view the associated files on mobile devices.
The Hedgehog (Hh) and Wnt signal transduction pathways are master regulators of embryogenesis and tissue renewal and represent anticancer therapeutic targets. Using genome-wide RNA interference screening in murine cultured cells, we established previously unknown associations between these signaling pathways and genes linked to developmental malformations, diseases of premature tissue degeneration, and cancer. We identified functions in both pathways for the multitasking kinase Stk11 (also known as Lkb1), a tumor suppressor implicated in lung and cervical cancers. We found that Stk11 loss resulted in disassembly of the primary cilium, a cellular organizing center for Hh pathway components, thus dampening Hh signaling. Loss of Stk11 also induced aberrant signaling through the Wnt pathway. Chemicals that targeted the Wnt acyltransferase Porcupine or that restored primary cilia length by inhibiting the tubulin deacetylase HDAC6 (histone deacetylase 6) countered deviant pathway activities driven by Stk11 loss. Our study demonstrates that Stk11 is a critical mediator in both the Hh and the Wnt pathways, and our approach provides a platform to support the development of targeted therapeutic strategies.
Enhanced signaling by the small guanosine triphosphatase Ras is common in T cell acute lymphoblastic leukemia/lymphoma (T-ALL, but the underlying mechanisms are unclear. Here, we identified the guanine nucleotide exchange factor RasGRP1 (Rasgrp1 in mice) as a Ras activator that contributes to leukemogenesis. We found increased RasGRP1 expression in many pediatric T-ALL patients, which we did not observe in rare early T cell precursor (ETP) T-ALL patients with KRAS and NRAS mutations, such as K-RasG12D. Leukemia screens in wild-type mice, but not in mice expressing the mutant K-RasG12D that encodes a constitutively active Ras, yielded frequent retroviral insertions that led to increased Rasgrp1 expression. Rasgrp1 and oncogenic K-RasG12D promoted T-ALL through distinct mechanisms. In K-RasG12D T-ALLs, we found that enhanced Ras activation did not lead to cell cycle arrest. In mouse T-ALL cells with increased Rasgrp1 expression, we found that Rasgrp1 contributed to a previously uncharacterized cytokine receptor–activated Ras pathway that stimulated the proliferation of T-ALL cells in vivo, which was accompanied by dynamic patterns of activation of effector kinases downstream of Ras in individual T-ALLs. Reduction of Rasgrp1 abundance reduced cytokine-stimulated Ras signaling and decreased the proliferation of T-ALL in vivo, suggesting that patients with this cancer should be screened for increased abundance of RasGRP1 to customize treatment.
The clinical efficacy of tyrosine kinase inhibitors supports the dependence of distinct subsets of cancers on specific driver mutations for survival, a phenomenon called “oncogene addiction.” We demonstrate that PUMA and BIM are the key apoptotic effectors of tyrosine kinase inhibitors in breast cancers with amplification of the gene encoding human epidermal growth factor receptor 2 (HER2) and lung cancers with epidermal growth factor receptor (EGFR) mutants. The BH3 domain containing proteins BIM and PUMA can directly activate the proapoptotic proteins BAX and BAK to permeabilize mitochondria, leading to caspase activation and apoptosis. We delineated the signal transduction pathways leading to the induction of BIM and PUMA by tyrosine kinase inhibitors. Inhibition of the mitogen-activated or extracellular signal–regulated protein kinase kinase (MEK)–extracellular signal–regulated kinase (ERK) pathway caused increased abundance of BIM, whereas antagonizing the phosphoinositide 3-kinase (PI3K)–AKT pathway triggered nuclear translocation of the FOXO transcription factors, which directly activated the PUMA promoter. In a mouse breast tumor model, the abundance of PUMA and BIM was increased after inactivation of HER2. Moreover, deficiency of Bim or Puma impaired caspase activation and reduced tumor regression caused by inactivation of HER2. Similarly, deficiency of Puma impeded the regression of EGFRL858R-driven mouse lung tumors upon inactivation of the EGFR-activating mutant. Overall, our study identified PUMA and BIM as the sentinels that interconnect kinase signaling networks and the mitochondrion-dependent apoptotic program, which offers therapeutic insights for designing novel cell death mechanism–based anticancer strategies.
The Polycomb group protein Bmi1 is a transcriptional silencer of the Ink4a-Arf locus, which encodes the cell cycle regulator p16Ink4a and the tumor suppressor p19Arf. Bmi1 plays a key role in oncogenesis and stem cell self-renewal. We report that phosphorylation of human Bmi1 at Ser316 by Akt impaired its function by triggering its dissociation from the Ink4a-Arf locus, which resulted in decreased ubiquitylation of histone H2A and the inability of Bmi1 to promote cellular proliferation and tumor growth. Moreover, Akt-mediated phosphorylation of Bmi1 also inhibited its ability to promote self-renewal of hematopoietic stem and progenitor cells. Our study provides a mechanism for the increased abundance of p16Ink4a and p19Arf seen in cancer cells with an activated phosphoinositide 3-kinase to Akt signaling pathway and identifies crosstalk between phosphorylation events and chromatin structure.
The mitogen-activated protein kinase (MAPK) ERK2 is ubiquitously expressed in mammalian tissues and is involved in a wide range of biological processes. Although MAPKs have been intensely studied, identification of their substrates remains challenging. We have optimized a chemical genetic system using analog-sensitive ERK2,a form of ERK2 engineered to utilize an analog of ATP, to tag and isolate ERK2 substrates in vitro. This approach identified 80 proteins phosphorylated by ERK2, 13 of which are known ERK2 substrates. The 80 substrates are associated with diverse cellular processes, including regulation of transcription and translation, and mRNA processing, as well as regulation of the activity of the Rho-family guanosine triphosphatases. We found that one of the newly identified substrates, ETV3 (a member of the E-twenty six family of transcriptional regulators) was extensively phosphorylated on sites within canonical and non-canonical ERK motifs. Phosphorylation of ETV3 regulated transcription by preventing its binding to DNA at promoters for several thousand genes, including some involved in negative feedback regulation of itself and of upstream signals.
Organisms face unforeseen short- and long-term changes in the environment (stressors). To defend against these changes, organisms have developed a stress system that includes the hypothalamic-pituitary-adrenal (HPA) axis, which employs glucocorticoids and the glucocorticoid receptor (GR) for signal transduction. In addition, organisms live under the strong influence of day-night cycles and, hence, have also developed a highly conserved circadian clock system for adjusting their activities to recurring environmental changes. This regulatory system creates and maintains internal circadian rhythmicity by employing a self-oscillating molecular pacemaker composed of the Clock-Bmal1 heterodimer and other transcription factors. The circadian clock consists of a central master clock in the suprachiasmatic nucleus of the brain hypothalamus and peripheral slave clocks in virtually all organs and tissues. The HPA axis and the circadian clock system communicate with each other at multiple levels. The central clock controls the HPA axis, creating the diurnal oscillation of circulating adrenocorticotropic hormone and cortisol, and the HPA axis adjusts the circadian rhythmicity of the peripheral clocks in response to various stressors through the GR. Further, Clock-Bmal1 regulates the response to glucocorticoids in peripheral tissues through acetylation of the GR, possibly antagonizing the biologic actions of diurnally fluctuating circulating cortisol. Importantly, dysregulation in the clock system and the HPA axis may cause similar pathologic manifestations—including obesity, metabolic syndrome, and cardiovascular disease—by uncoupling circulating cortisol concentrations from tissue sensitivity to glucocorticoids.
In the intrinsic pathway of apoptosis, cell-damaging signals promote the release of cytochrome c from mitochondria, triggering activation of the Apaf-1 and caspase-9 apoptosome. The ubiquitin E3 ligase MDM2 decreases the stability of the proapoptotic factor p53. We show that it also coordinated apoptotic events in a p53-independent manner by ubiquitylating the apoptosome activator CAS and the ubiquitin E3 ligase HUWE1. HUWE1 ubiquitylates the antiapoptotic factor Mcl-1, and we found that HUWE1 also ubiquitylated PP5 (protein phosphatase 5), which indirectly inhibited apoptosome activation. Breast cancers that are positive for the tyrosine receptor kinase HER2 (human epidermal growth factor receptor 2) tend to be highly aggressive. In HER2-positive breast cancer cells treated with the HER2 tyrosine kinase inhibitor lapatinib, MDM2 was degraded and HUWE1 was stabilized. In contrast, in breast cancer cells that acquired resistance to lapatinib, the abundance of MDM2 was not decreased and HUWE1 was degraded, which inhibited apoptosis, regardless of p53 status. MDM2 inhibition overcame lapatinib resistance in cells with either wild-type or mutant p53 and in xenograft models. These findings demonstrate broader, p53-independent roles for MDM2 and HUWE1 in apoptosis and specifically suggest the potential for therapy directed against MDM2 to overcome lapatinib resistance.
T cell immunoglobulin and mucin domain (TIM) proteins are cell-surface signaling receptors in T cells and scavenger receptors in antigen-presenting cells and kidney tubular epithelia. Here, we demonstrated a function for TIM proteins in mediating the degradation of NUR77, a nuclear receptor implicated in the cellular response to injury. TIM proteins interacted with and mediated the lysosomal degradation of NUR77 in a phosphoinositide 3-kinase–dependent pathway. We also showed dynamic cycling of TIM-1 to and from the cell surface through clathrin-dependent constitutive endocytosis. Blocking this process or mutating the phosphatidylserine-binding pocket in TIM-1 abrogated TIM-1–mediated degradation of NUR77. In an in vitro model of kidney injury, silencing TIM-1 increased NUR77 abundance and decreased epithelial cell survival. These results show that TIM proteins may affect immune cell function and the response of the kidney to injury.
Analysis of high-throughput data increasingly relies on pathway annotation and functional information derived from Gene Ontology. This approach has limitations, in particular for the analysis of network dynamics over time or under different experimental conditions, in which modules within a network rather than complete pathways might respond and change. We report an analysis framework based on protein complexes, which are at the core of network reorganization. We generated a protein complex resource for human, Drosophila, and yeast from the literature and databases of protein-protein interaction networks, with each species having thousands of complexes. We developed COMPLEAT (http://www.flyrnai.org/compleat), a tool for data mining and visualization for complex-based analysis of high-throughput data sets, as well as analysis and integration of heterogeneous proteomics and gene expression data sets. With COMPLEAT, we identified dynamically regulated protein complexes among genome-wide RNA interference data sets that used the abundance of phosphorylated extracellular signal–regulated kinase in cells stimulated with either insulin or epidermal growth factor as the output. The analysis predicted that the Brahma complex participated in the insulin response.
Metazoans respond to various forms of environmental stress by inducing the phosphorylation of the α subunit of the translation initiation factor eIF2 at serine 51 (eIF2αP), a modification that leads to a global inhibition of mRNA translation. Herein, we demonstrate that eIF2αP is induced by pharmacological inhibition of the phosphoinositide-3-kinase (PI3K)-Akt pathway as well as by genetic or small interfering (si)RNA-mediated ablation of Akt. Increased eIF2αP is an evolutionary conserved process that involves the endoplasmic reticulum (ER)-resident protein kinase PERK, which is negatively regulated by Akt-dependent phosphorylation at threonine 799. PERK activity and eIF2αP are downregulated by activated Akt in mouse mammary gland tumors as well as in cells exposed to ER stress or oxidative stress leading to the induction of cell survival or death respectively. In unstressed cells, the PERK-eIF2αP pathway guards survival and facilitates adaptation to the deleterious effects of PI3K or Akt inactivation. As such, inactivation of the PERK-eIF2αP arm increases the susceptibility of tumor cells to death by pharmacological inhibitors of PI3K or Akt. Thus, in addition to mTOR the PERK-eIF2αP pathway provides a link between Akt signaling and translational control with implications in tumor formation and treatment.
translational control; PERK; eIF2α phosphorylation; Akt/PKB; endoplasmic reticulum stress; oxidative stress; chemotherapeutic drugs