Multiple components in an epidermal growth factor-stimulated protein kinase cascade

The role of the type 7 adenylyl cyclase isoform in alcohol use disorder and depression

The translation of extracellular signals to intracellular responses involves a number of signal transduction molecules. A major component of this signal transducing function is adenylyl cyclase, which produces the intracellular "second messenger," cyclic AMP. What was initially considered as a single enzyme for cyclic AMP generation is now known to be a family of nine membrane-bound enzymes, and one cytosolic enzyme. Each member of the adenylyl cyclase family is distinguished by factors that modulate its catalytic activity, by the cell, tissue, and organ distribution of the family members, and by the physiological/behavioral functions that are subserved by particular family members. This review focuses on the Type 7 adenylyl cyclase (AC7) in terms of its catalytic characteristics and its relationship to alcohol use disorder (AUD, alcoholism), and major depressive disorder (MDD). AC7 may be part of the inherited system predisposing an individual to AUD and/or MDD in a sex-specific manner, or this enzyme may change in its expression or activity in response to the progression of disease or in response to treatment. The areas of brain expressing AC7 are related to responses to stress and evidence is available that CRF1 receptors are coupled to AC7 in the amygdala and pituitary. Interestingly, AC7 is the major form of the cyclase contained in bone marrow-derived cells of the immune system and platelets, and in microglia. AC7 is thus, poised to play an integral role in both peripheral and brain immune function thought to be etiologically involved in both AUD and MDD. Both platelet and lymphocyte adenylyl cyclase activity have been proposed as markers for AUD and MDD, as well as prognostic markers of positive response to medication for MDD. We finish with consideration of paths to medication development that may selectively modulate AC7 activity as treatments for MDD and AUD.

Discovery of Raf Family Is a Milestone in Deciphering the Ras-Mediated Intracellular Signaling Pathway

The Ras-Raf-MEK-ERK signaling pathway, the first well-established MAPK pathway, plays essential roles in cell proliferation, survival, differentiation and development. It is activated in over 40% of human cancers owing to mutations of Ras, membrane receptor tyrosine kinases and other oncogenes. The Raf family consists of three isoforms, A-Raf, B-Raf and C-Raf. Since the first discovery of a truncated mutant of C-Raf as a transforming oncogene carried by a murine retrovirus, forty years of extensive studies have provided a wealth of information on the mechanisms underlying the activation, regulation and biological functions of the Raf family. However, the mechanisms by which activation of A-Raf and C-Raf is accomplished are still not completely understood. In contrast, B-Raf can be easily activated by binding of Ras-GTP, followed by cis-autophosphorylation of the activation loop, which accounts for the fact that this isoform is frequently mutated in many cancers, especially melanoma. The identification of oncogenic B-Raf mutations has led to accelerated drug development that targets Raf signaling in cancer. However, the effort has not proved as effective as anticipated, inasmuch as the mechanism of Raf activation involves multiple steps, factors and phosphorylation of different sites, as well as complex interactions between Raf isoforms. In this review, we will focus on the physiological complexity of the regulation of Raf kinases and their connection to the ERK phosphorylation cascade and then discuss the role of Raf in tumorigenesis and the clinical application of Raf inhibitors in the treatment of cancer.

ERK1b, a 46-kDa ERK Isoform That Is Differentially Regulated by MEK

The extracellular signal‐regulated kinases (ERK) 1 and 2 (ERK1/2) are members of the mitogen‐activated protein kinase family. Using various stimulated rodent cells and kinase activation techniques, we identified a 46‐kDa ERK. The kinetics of activation of this ERK isoform was similar to that of ERK1 and ERK2 under most but not all circumstances. We purified this isoform from rat cells followed by its cloning. The sequence of this isoform revealed that it is an alternatively spliced version of the 44‐kDa ERK1 and therefore we termed it ERK1b. Interestingly, this isoform had a 26‐amino acid insertion between residues 340 and 341 of ERK1, which results from Intron 7 insertion to the sequence. Examining the expression pattern, we found that ERK1b is detected mainly in rat and particularly in Ras‐transformed Rat1 cells. In this cell line, ERK1b was more sensitive to extracellular stimulation than ERK1 and ERK2. Moreover, unlike ERK1 and ERK2, ERK1b had a very low binding affinity to MEK1. This low interaction led to nuclear localization of this isoform when expressed together with MEK1 under conditions in which ERK1 and ERK2 are retained in the cytoplasm. In addition, ERK1b was not coimmunoprecipitated with MEK1. We identified a new, 46‐kDa ERK alternatively spliced isoform. Our results indicate that this isoform is the major one to respond to exogenous stimulation in Ras‐transformed cells, probably due to its differential regulation by MAPK/ERK kinase and by phosphatases.

A peptide toxin in ant venom mimics vertebrate EGF-like hormones to cause long-lasting hypersensitivity in mammals

The targeting of mammalian ErbB receptor signaling by a venom toxin to cause hypersensitivity is a mode of action that has not previously been described. Natural selection of a defensive toxin to target ErbB signaling provides compelling independent evidence for a fundamental role of this receptor and its ligands in mammalian pain. The evolution of a toxin in ant venom to mimic a vertebrate nociceptive hormone serves as an example of both convergent evolution and molecular mimicry, illustrating how natural selection can shape the gene product of one organism to resemble that of another.

Venoms are excellent model systems for studying evolutionary processes associated with predator–prey interactions. Here, we present the discovery of a peptide toxin, MIITX₂-Mg1a, which is a major component of the venom of the Australian giant red bull ant Myrmecia gulosa and has evolved to mimic, both structurally and functionally, vertebrate epidermal growth factor (EGF) peptide hormones. We show that Mg1a is a potent agonist of the mammalian EGF receptor ErbB1, and that intraplantar injection in mice causes long-lasting hypersensitivity of the injected paw. These data reveal a previously undescribed venom mode of action, highlight a role for ErbB receptors in mammalian pain signaling, and provide an example of molecular mimicry driven by defensive selection pressure.

Ras Isoforms from Lab Benches to Lives-What Are We Missing and How Far Are We?

The central protein in the oncogenic circuitry is the Ras GTPase that has been under intense scrutiny for the last four decades. From its discovery as a viral oncogene and its non-oncogenic contribution to crucial cellular functioning, an elaborate genetic, structural, and functional map of Ras is being created for its therapeutic targeting. Despite decades of research, there still exist lacunae in our understanding of Ras. The complexity of the Ras functioning is further exemplified by the fact that the three canonical Ras genes encode for four protein isoforms (H-Ras, K-Ras4A, K-Ras4B, and N-Ras). Contrary to the initial assessment that the H-, K-, and N-Ras isoforms are functionally similar, emerging data are uncovering crucial differences between them. These Ras isoforms exhibit not only cell-type and context-dependent functions but also activator and effector specificities on activation by the same receptor. Preferential localization of H-, K-, and N-Ras in different microdomains of the plasma membrane and cellular organelles like Golgi, endoplasmic reticulum, mitochondria, and endosome adds a new dimension to isoform-specific signaling and diverse functions. Herein, we review isoform-specific properties of Ras GTPase and highlight the importance of considering these towards generating effective isoform-specific therapies in the future.

A walk-through MAPK structure and functionality with the 30-year-old yeast MAPK Slt2

Mitogen-activated protein kinases (MAPKs) are evolutionarily conserved signaling proteins involved in the regulation of most eukaryotic cellular processes. They are downstream components of essential signal transduction pathways activated by the external stimuli, in which the signal is conveyed through phosphorylation cascades. The excellent genetic and biochemical tractability of simple eukaryotes such as Saccharomyces cerevisiae has significantly contributed to gain fundamental information into the physiology of these key proteins. The budding yeast MAPK Slt2 was identified 30 years ago and was later revealed as a fundamental element of the cell wall integrity (CWI) pathway, one of the five MAPK routes of S. cerevisiae. As occurs with other MAPKs, whereas Slt2 displays the core typical structural traits of eukaryotic protein kinases, it also features conserved domains among MAPKs that allow an exquisite spatio-temporal regulation of their activity and binding to activating kinases, downregulatory phosphatases, or nuclear transcription factors. Additionally, Slt2 bears a regulatory extra C-terminal tail unique among S. cerevisiae MAPKs. Here, we review the structural and functional basis for the signaling role of Slt2 in the context of the molecular architecture of this important family of protein kinases.

RAF-MEK-ERK pathway in cancer evolution and treatment

The RAF-MEK-ERK signaling cascade is a well-characterized MAPK pathway involved in cell proliferation and survival. The three-layered MAPK signaling cascade is initiated upon RTK and RAS activation. Three RAF isoforms ARAF, BRAF and CRAF, and their downstream MEK1/2 and ERK1/2 kinases constitute a coherently orchestrated signaling module that directs a range of physiological functions. Genetic alterations in this pathway are among the most prevalent in human cancers, which consist of numerous hot-spot mutations such as BRAF^V600E. Oncogenic mutations in this pathway often override otherwise tightly regulated checkpoints to open the door for uncontrolled cell growth and neoplasia. The crosstalk between the RAF-MEK-ERK axis and other signaling pathways further extends the proliferative potential of this pathway in human cancers. In this review, we summarize the molecular architecture and physiological functions of the RAF-MEK-ERK pathway with emphasis on its dysregulations in human cancers, as well as the efforts made to target the RAF-MEK-ERK module using small molecule inhibitors.

Differential Role of Threonine and Tyrosine Phosphorylation in the Activation and Activity of the Yeast MAPK Slt2

The Mitogen-Activated Protein Kinase (MAPK) Slt2 is central to signaling through the yeast Cell Wall Integrity (CWI) pathway. MAPKs are regulated by phosphorylation at both the threonine and tyrosine of the conserved TXY motif within the activation loop (T190/Y192 in Slt2). Since phosphorylation at both sites results in the full activation of MAPKs, signaling through MAPK pathways is monitored with antibodies that detect dually phosphorylated forms. However, most of these antibodies also recognize monophosphorylated species, whose relative abundance and functionality are diverse. By using different phosphospecific antibodies and phosphate-affinity (Phos-tag) analysis on distinct Slt2 mutants, we determined that Y192- and T190-monophosphorylated species coexist with biphosphorylated Slt2, although most of the Slt2 pool remains unphosphorylated following stress. Among the monophosphorylated forms, only T190 exhibited biological activity. Upon stimulation, Slt2 is first phosphorylated at Y192, mainly by the MAPKK Mkk1, and this phosphorylation is important for the subsequent T190 phosphorylation. Similarly, dephosphorylation of Slt2 by the Dual Specificity Phosphatase (DSP) Msg5 is ordered, with dephosphorylation of T190 depending on previous Y192 dephosphorylation. Whereas Y192 phosphorylation enhances the Slt2 catalytic activity, T190 is essential for this activity. The conserved T195 residue is also critical for Slt2 functionality. Mutations that abolish the activity of Slt2 result in a high increase in inactive Y192-monophosphorylated Slt2. The coexistence of different Slt2 phosphoforms with diverse biological significance highlights the importance of the precise detection of the Slt2 phosphorylation status.

Comparative transcriptome analysis reveals Dusp1 as a critical regulator of inflammatory response to fly ash particle exposure in mouse

Exposure to outdoor concentrations of fine particulate matter (PM2.5) is a leading global health concern. Waste incineration emission has been recognized as a potential major contributor of ambient PM2.5. Respiratory inflammation is a central feature induced by PM2.5 exposure by inhalation. However, the molecular mechanisms are not fully understood. Dual-specificity phosphatase 1 (Dusp1) plays an instrumental role in the regulation of airway inflammation. In this study, fly ash particles (20 mg/kg BW) collected from a municipal waste incinerator in China were given to BALB/c wild-type (WT) and Dusp1^-/- mice by intranasal administration daily for three consecutive days. While these particles induced mild inflammation in both genotypes, a significantly higher level of serum interleukin-6 (665 pg/ml) was measured in Dusp1^-/- mice challenged with fly ash particles than in their WT counterparts. Genome-wide transcriptome profiling of pulmonary coding genes in response to the exposure were performed in both genotypes by RNA sequencing. We identified 487 differentially-expressed genes (DEGs) in fly ash-challenged Dusp1^-/- mice versus their WT counterparts with a log₂fold-change >1.5 and p < 0.05. Functional enrichment and molecular pathway mapping of the DEGs specific to Dusp1^-/- mice exposed to the particles revealed that the top 10 perturbed molecular pathways were associated with the immune response. Our study demonstrates the anti-inflammatory role of Dusp1 in protecting the lung against insults by fly ash particles, suggesting that Dusp1 might be a therapeutic target for the treatment of PM2.5-induced respiratory diseases.

Identification and characterization of c-raf from orange-spotted grouper (Epinephelus coioides)

C-Raf proto-oncogene serine/threonine kinase is a mitogen-activated protein kinase (MAP) kinase kinase, which can initiate a mitogen-activated protein kinase (MAPK) cascade by phosphorylating the dual-specific MAP kinase kinases (MEK1/2), and in turn activate the extracellular signal-regulated kinases (ERK1/2). To study the function of c-Raf in teleost fish, a c-Raf cDNA sequence from orange-spotted grouper (Epinephelus coioides) was cloned. Ecc-Raf shared 81%-99% amino acid identity with other vertebrate c-Raf molecules, and shared the highest amino acid identity (99%) with Lates calcarifer c-Raf. Genomic structure analysis revealed that grouper c-Raf shared a conserved exon structure with other vertebrates. Tissue distribution showed that Ecc-Raf was mainly transcribed in systemic immune organs. Ecc-Raf was distributed throughout the cytoplasm of transfected GS cells and the overexpression of Ecc-Raf only slightly enhanced the activation of Activator protein 1. The phosphorylation levels of Ecc-Raf can be induced by PMA and H₂O₂ treatment, in contrast to DMSO or untreated HKLs. Moreover, the phosphorylation level of the Raf-MEK-ERK axis was downregulated after 24 h of SGIV infection. On the other hand, the total level and phosphorylation level of c-Raf significantly increased post C. irritans infection and showed an enhanced level post immunization. The results of this study suggested that the Raf-MEK-ERK cascade was involved in the response to viral or parasitic infections.

Evolution of a dynamic molecular switch

Eukaryotic protein kinases (EPKs) regulate almost every biological process and have evolved to be dynamic molecular switches; this is in stark contrast to metabolic enzymes, which have evolved to be efficient catalysts. In particular, the highly conserved active site of every EPK is dynamically and transiently assembled by a process that is highly regulated and unique for every protein kinase. We review here the essential features of the kinase core, focusing on the conserved motifs and residues that are embedded in every kinase. We explore, in particular, how the hydrophobic core architecture specifically drives the dynamic assembly of the regulatory spine and consequently the organization of the active site where the γ-phosphate of ATP is positioned by a convergence of conserved motifs including a conserved regulatory triad for transfer to a protein substrate. In conclusion, we show how the flanking N- and C-terminal tails often classified as intrinsically disordered regions, as well as flanking domains, contribute in a highly kinase-specific manner to the regulation of the conserved kinase core. Understanding this process as well as how one kinase activates another remains as two of the big challenges for the kinase signaling community. © 2019 IUBMB Life, 71(6):672-684, 2019.

The Role of PI3K/Akt and ERK in Neurodegenerative Disorders

Disruption of Akt and Erk-mediated signal transduction significantly contributes in the pathogenesis of various neurodegenerative diseases (NDs), such as Parkinson's disease, Alzheimer's diseases, Huntington's disease, and many others. These regulatory proteins serve as the regulator of cell survival, motility, transcription, metabolism, and progression of the cell cycle. Therefore, targeting Akt and Erk pathway has been proposed as a reasonable approach to suppress ND progression. This review has emphasized on involvement of Akt/Erk cascade in the neurodegeneration. Akt has been reported to regulate neuronal toxicity through its various substrates like FOXos, GSK3β, and caspase-9 etc. Akt is also involved with PI3K in signaling pathway to mediate neuronal survival. ERK is another kinase which also regulates proliferation, differentiation, and survival of the neural cell. There has also been much progress in developing a therapeutic molecule targeting Akt and Erk signaling. Therefore, improved understanding of the molecular mechanism behind the regulatory aspect of Akt and Erk networks can make strong impact on exploration of the neurodegenerative disease pathogenesis.

Characterization and functional analysis of grouper (Epinephelus coioides) MEK1 and MEK2

MEK dual-specificity protein kinases are a group of mitogen-activated protein kinase kinases, which act as an integration point by transferring extracellular signals to the nucleus. To investigate the function of MEK in teleost fish, we cloned MEK1 and MEK2 cDNA sequences from the orange-spotted grouper (Epinephelus coioides). EcMEK1 and EcMEK2 shared 80% amino acid identity with each other. EcMEK1 had 89-99% amino acid identity with teleosts or mammals, whereas EcMEK2 shared 85-97% amino acid identity. The exon structures of the grouper MEK1/2 genes were conserved with zebrafish and human MEK1/2. Tissue distribution analysis showed that EcMEK1 and EcMEK2 had a similar expression pattern in grouper tissues and was mainly transcribe in systemic immune organs. Both EcMEK1 and EcMEK2 were distributed throughout the cytoplasm of transfected GS or HEK293T cells. Overexpression of EcMEK1 or EcMEK2 activated Activator protein 1 dependent luciferase. The phosphorylation levels of EcMEK1/2 and EcERK1/2 were significantly increased in head kidney leukocytes by stimulation with PMA treatment. The grouper MEK1/2-ERK1/2 axis was activated in Cryptocaryon irritans infection and showed an enhanced phosphorylation after immunization.

Mechanical strain induced phospho-proteomic signaling in uterine smooth muscle cells

Mechanical strain associated with the expanding uterus correlates with increased preterm birth rates. Mechanical signals result in a cascading network of protein phosphorylation events. These signals direct cellular activities and may lead to changes in contractile phenotype and calcium signaling. In this study, the complete phospho-proteome of uterine smooth muscle cells subjected to mechanical strain for 5 min was compared to un-strained controls. Statistically significant, differential phosphorylation events were annotated by Ingenuity Pathway Analysis to elucidate mechanically induced phosphorylation networks. Mechanical strain leads to the direct activation of ERK1/2, HSPB1, and MYL9, in addition to phosphorylation of PAK2, vimentin, DOCK1, PPP1R12A, and PTPN11 at previously unannotated sites. These results suggest a novel network reaction to mechanical strain and reveal proteins that participate in the activation of contractile mechanisms leading to preterm labor.

Extracellular-Regulated Kinases: Signaling From Ras to ERK Substrates to Control Biological Outcomes

The extracellular-regulated kinases ERK1 and ERK2 are evolutionarily conserved, ubiquitous serine-threonine kinases that are involved in regulating cellular signaling in both normal and pathological conditions. Their expression is critical for development and their hyperactivation is a major factor in cancer development and progression. Since their discovery as one of the major signaling mediators activated by mitogens and Ras mutation, we have learned much about their regulation, including their activation, binding partners and substrates. In this review I will discuss some of what has been discovered about the members of the Ras to ERK pathway, including regulation of their activation by growth factors and cell adhesion pathways. Looking downstream of ERK activation I will also highlight some of the many ERK substrates that have been discovered, including those involved in feedback regulation, cell migration and cell cycle progression through the control of transcription, pre-mRNA splicing and protein synthesis.

Regulatory Mechanisms and Novel Therapeutic Targeting Strategies for Protein Tyrosine Phosphatases

An appropriate level of protein phosphorylation on tyrosine is essential for cells to react to extracellular stimuli and maintain cellular homeostasis. Faulty operation of signal pathways mediated by protein tyrosine phosphorylation causes numerous human diseases, which presents enormous opportunities for therapeutic intervention. While the importance of protein tyrosine kinases in orchestrating the tyrosine phosphorylation networks and in target-based drug discovery has long been recognized, the significance of protein tyrosine phosphatases (PTPs) in cellular signaling and disease biology has historically been underappreciated, due to a large extent to an erroneous assumption that they are largely constitutive and housekeeping enzymes. Here, we provide a comprehensive examination of a number of regulatory mechanisms, including redox modulation, allosteric regulation, and protein oligomerization, that control PTP activity. These regulatory mechanisms are integral to the myriad PTP-mediated biochemical events and reinforce the concept that PTPs are indispensable and specific modulators of cellular signaling. We also discuss how disruption of these PTP regulatory mechanisms can cause human diseases and how these diverse regulatory mechanisms can be exploited for novel therapeutic development.

Role and regulation of MKP-1 in airway inflammation

Mitogen-activated protein kinase (MAPK) phosphatase 1 (MKP-1) is a protein with anti-inflammatory properties and the archetypal member of the dual-specificity phosphatases (DUSPs) family that have emerged over the past decade as playing an instrumental role in the regulation of airway inflammation. Not only does MKP-1 serve a critical role as a negative feedback effector, controlling the extent and duration of pro-inflammatory MAPK signalling in airway cells, upregulation of this endogenous phosphatase has also emerged as being one of the key cellular mechanism responsible for the beneficial actions of clinically-used respiratory medicines, including β₂-agonists, phosphodiesterase inhibitors and corticosteroids. Herein, we review the role and regulation of MKP-1 in the context of airway inflammation. We initially outline the structure and biochemistry of MKP-1 and summarise the multi-layered molecular mechanisms responsible for MKP-1 production more generally. We then focus in on some of the key in vitro studies in cell types relevant to airway disease that explain how MKP-1 can be regulated in airway inflammation at the transcriptional, post-translation and post-translational level. And finally, we address some of the potential challenges with MKP-1 upregulation that need to be explored further to fully exploit the potential of MKP-1 to repress airway inflammation in chronic respiratory disease.

A Genome-wide Analysis of Human Pluripotent Stem Cell-Derived Endothelial Cells in 2D or 3D Culture

A defined protocol for efficiently deriving endothelial cells from human pluripotent stem cells was established and vascular morphogenesis was used as a model system to understand how synthetic hydrogels influence global biological function compared with common 2D and 3D culture platforms. RNA sequencing demonstrated that gene expression profiles were similar for endothelial cells and pericytes cocultured in polyethylene glycol (PEG) hydrogels or Matrigel, while monoculture comparisons identified distinct vascular signatures for each cell type. Endothelial cells cultured on tissue-culture polystyrene adopted a proliferative phenotype compared with cells cultured on or encapsulated in PEG hydrogels. The proliferative phenotype correlated to increased FAK-ERK activity, and knockdown or inhibition of ERK signaling reduced proliferation and expression for cell-cycle genes while increasing expression for “3D-like” vasculature development genes. Our results provide insight into the influence of 2D and 3D culture formats on global biological processes that regulate cell function.

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Defined, high-efficiency differentiation of human PSCs to endothelial cell

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Comprehensive genome-wide comparisons of 2D and 3D cell-culture formats

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Gene expression profiles for endothelial cells and pericytes in 3D cell culture

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Highly proliferative phenotypes on tissue-culture polystyrene surfaces

Allosteric modulators of MEK1: drug design and discovery

Mitogen-activated protein kinase kinase (MAPKK, MEK) mediates signal transduction, controlling cell proliferation and survival. MEK occupies a key downstream position in the Ras-Raf-MEK-ERK signaling pathway, implying that inhibition of MEK will potently suppress tumor cell growth, with potential applications in cancer therapy. Based on the promising therapeutic effects of MEK modulators, continued efforts have been made in this class. Here, we review the discovery and development of MEK1 allosteric modulators, classifying them into four structural groups. The allosteric mechanisms and recent clinical progress involving these modulators are also reviewed.

Regulatory roles of conserved phosphorylation sites in the activation T-loop of the MAP kinase ERK1

The catalytic domains of most eukaryotic protein kinases are highly conserved in their primary structures. Their phosphorylation within the well-known activation T-loop, a variable region between protein kinase catalytic subdomains VII and VIII, is a common mechanism for stimulation of their phosphotransferase activities. Extracellular signal-regulated kinase 1 (ERK1), a member of the extensively studied mitogen-activated protein kinase (MAPK) family, serves as a paradigm for regulation of protein kinases in signaling modules. In addition to the well-documented T202 and Y204 stimulatory phosphorylation sites in the activation T-loop of ERK1 and its closest relative, ERK2, three additional flanking phosphosites have been confirmed (T198, T207, and Y210 from ERK1) by high-throughput mass spectrometry. In vitro kinase assays revealed the functional importance of T207 and Y210, but not T198, in negatively regulating ERK1 catalytic activity. The Y210 site could be important for proper conformational arrangement of the active site, and a Y210F mutant could not be recognized by MEK1 for phosphorylation of T202 and Y204 in vitro. Autophosphorylation of T207 reduces the catalytic activity and stability of activated ERK1. We propose that after the activation of ERK1 by MEK1, subsequent slower phosphorylation of the flanking sites results in inhibition of the kinase. Because the T207 and Y210 phosphosites of ERK1 are highly conserved within the eukaryotic protein kinase family, hyperphosphorylation within the kinase activation T-loop may serve as a general mechanism for protein kinase down-regulation after initial activation by their upstream kinases.

MEK1/2 Inhibitors: Molecular Activity and Resistance Mechanisms

Aberrant activation of the three-layered protein kinase cascade, Raf/MEK/ERK, is often detected in human cancer, which is mainly attributed to the oncogenic alterations of RAF, or its upstream activators RAS or cell surface receptor tyrosine kinases. Deregulated activity of the Raf/MEK/ERK pathway drives uncontrolled tumor cell proliferation and survival, thus providing a rational therapeutic target for the treatment of many cancers. While Raf, MEK1/2, and ERK1/2 are equally important targets for the design of therapeutic small molecular weight inhibitors, the effort to develop MEK1/2-specific inhibitors has been greatly successful. Particularly, MEK1/2 have been relatively advantageous for the design of highly selective adenosine triphosphate (ATP)-noncompetitive inhibitors. Indeed, a plethora of highly selective and potent MEK1/2 inhibitors are now available and many of those inhibitors have been evaluated for their therapeutic potential. Herein, we review different MEK1/2 inhibitors that have been studied for their inhibitory mechanisms and therapeutic potential in cancer. Some of the key structural features of MEK1/2 that are important for the efficacy of these inhibitors are also discussed. In addition, we discuss current challenges and future prospective in using these advanced MEK1/2 inhibitors for cancer therapy.

MAP kinase modules: the excursion model and the steps that count

MAP kinase modules propagate diverse extracellular signals to downstream effectors. The two dual phosphorylation reactions catalyzed by the modules are thought to control the switch behavior of the pathway. Here we review recent approaches to understand these pathways through signal-to-response studies in cells and in vitro. These data are reconciled with physical models as well as predictions made on mathematical and theoretical grounds. Biochemical analysis has shown recently that the dual phosphorylation reactions catalyzed by MAP kinase modules are sequential at both levels of the cascade. The observed order of phosphorylation events suggests an excursion from the Ser/Thr kinase activity of the MAP3K into Tyr kinase activity of the central dual specificity MAP2K. How the order of events might be encoded in the structures and interactions is discussed. The ordered mechanism confirms predictions that reactions should be sequential to generate the steep signal-to-response curves and delayed responses observed in cells.

MEK guards proteome stability and inhibits tumor-suppressive amyloidogenesis via HSF1

Signaling through RAS/MAP kinase pathway is central to biology. ERK has long been perceived as the only substrate for MEK. Here, we report that HSF1, the master regulator of the proteotoxic stress response, is a new MEK substrate. Beyond mediating cell-environment interactions, the MEK-HSF1 regulation impacts malignancy. In tumor cells, MEK blockade inactivates HSF1 and thereby provokes proteomic chaos, presented as protein destabilization, aggregation, and, strikingly, amyloidogenesis. Unlike their non-transformed counterparts, tumor cells are particularly susceptible to proteomic perturbation and amyloid induction. Amyloidogenesis is tumor suppressive, reducing in vivo melanoma growth and contributing to the potent anti-neoplastic effects of proteotoxic stressors. Our findings unveil a key biological function of the oncogenic RAS-MEK signaling in guarding proteostasis and suppressing amyloidogenesis. Thus, proteomic instability is an intrinsic feature of malignant state, and disrupting the fragile tumor proteostasis to promote amyloidogenesis may be a feasible therapeutic strategy.

ERK as a model for systems biology of enzyme kinetics in cells

A key step towards a chemical picture of enzyme catalysis was taken in 1913, when Leonor Michaelis and Maud Menten published their studies of sucrose hydrolysis by invertase. Based on a novel experimental design and a mathematical model, their work offered a quantitative view of biochemical kinetics well before the protein nature of enzymes was established and complexes with substrates could be detected. Michaelis-Menten kinetics provides a solid framework for enzyme kinetics in vitro, but what about kinetics in cells, where enzymes can be highly regulated and participate in a multitude of interactions? We discuss this question using the Extracellular Signal Regulated Kinase (ERK), which controls a myriad functions in cells, as a model of an important enzyme for which we have crystal structures, quantitative in vitro assays, and a vast list of binding partners. Despite great progress, we still cannot quantitatively predict how the rates of ERK-dependent reactions respond to genetic and pharmacological perturbations. Achieving this goal, which is important from both fundamental and practical standpoints, requires measuring the rates of enzyme reactions in their native environment and interpreting these measurements using simple but realistic mathematical models--the two elements which served as the cornerstones for Michaelis' and Menten's seminal 1913 paper.

Identification, cloning and characterization of an extracellular signal-regulated kinase (ERK) from Chinese shrimp, Fenneropenaeus chinensis

Extracellular signal-regulated kinase (ERK) is a serine/threonine-specific protein kinase, which participates in signaling transduction pathways that control intracellular events, including resumption of meiosis, embryogenesis, cell differentiation, cell proliferation, cell death and response to radiation. Some virus species evolved the ability to hijack the host cell ERK signaling transduction pathway for viral replications and gene expressions. To obtain a better understanding of ERK, we cloned a cDNA encoding ERK from the muscle of Fenneropenaeus chinensis (FcERK). The FcERK contained a 1098 bp open reading frame (ORF) encoding a protein of 365 amino acid residues with a conserved phosphorylation motif TEY in the kinase activation loop. Pair-wise and multiple sequence alignment revealed that ERK is highly conserved across taxa. The FcERK gene expressions in the hepatopancreas and gill were noticeably higher than the expression observed in the muscle. A challenge test was performed to reveal the responses of FcERK in different tissues to white spot syndrome virus (WSSV) infection. Post WSSV challenge, the FcERK expression in the gill significantly increased during the early stage of the viral infection, the FcERK expression in the muscle increased later than that in the gill, and the FcERK expression in the hepatopancreas significantly decreased. The FcERK gene expression profile accorded with the results that the virus primarily infects tissues originating from the ectoderm, with less infection of the tissues originating from the mesoderm, and hardly any infection in the tissues originating from the entoderm. Two single nucleotide polymorphisms (SNPs) were identified in the FcERK gene, involving C/T transition. The SNP genotypes of two groups of shrimps, respectively comprising 96 WSSV-resistant shrimps and 96 WSSV-susceptible shrimps were obtained using a high-resolution melting (HRM) method. In the two groups, the MAFs of both sites were greater than 0.05, and no site departed significantly (P < 0.05) from HWE. The genotype distributions of both mutation sites between the two groups were not significantly different. These results lead to a better understanding of the molecular mechanisms of the host-virus interaction and provide useful information for disease control.

Precisely ordered phosphorylation reactions in the p38 mitogen-activated protein (MAP) kinase cascade

The MAP kinase cascades, composed of a MAP3K, a MAP2K, and a MAPK, control switch responses to extracellular stimuli and stress in eukaryotes. The most important feature of these modules is thought to be the two double phosphorylation reactions catalyzed by MAP3Ks and MAP2Ks. We addressed whether the reactions are sequential or random in the p38 MAP kinase module. Mass spectrometry was used to track the phosphorylation of the MAP2K MEK6 by two MAP3Ks, TAO2 and ASK1, and the subsequent phosphorylation of p38α by MEK6/S*T* (where S (Ser) and T (Thr) are the two phosphorylation sites and * denotes phosphorylation). Both double phosphorylation reactions are precisely ordered. MEK6 is phosphorylated first on Thr-211 and then on Ser-207 by both MAP3Ks. This is the first demonstration of a precise reaction order for a MAP2K. p38α is phosphorylated first on Tyr-182 and then on Thr-180, the same reaction order observed previously in ERK2. Thus, intermediates were MEK6/ST* and p38α/TY*. Similarly, the phosphorylation of the p38α transcription factor substrate ATF2 occurs in a precise sequence. Progress curves for the appearance of intermediates were fit to kinetic models. The models confirmed the reaction order, revealed processivity in the phosphorylation of MEK6 by ASK1, and suggested that the order of phosphorylation is dictated by both binding and catalysis rates.

New small-molecule inhibitors of mitogen-activated protein kinase kinase

BACKGROUND

Overexpression of the Ras/Raf/MEK/ERK (extracellular-signal-regulated kinase) pathway is associated with the formation, progression and survival of tumours and has also been implicated in a diverse range of therapeutic areas such as arthritis, organ transplant rejection, asthma and developmental disorders. One approach to down regulation of this pathway is through the inhibition of mitogen-activated protein kinase kinase 1/2 (MEK1/2).

OBJECTIVE

The importance of the mitogen-activated protein kinase (MAPK) pathway, MEK1/2 as a therapeutic target and early MEK1/2 inhibitors is discussed, followed by an overview of recent patent activity in the area.

METHODS

The patent literature was searched for inhibitors of MEK1/2 published within the last three years; these results are described. Other relevant publications that provide further insight into the discovery and development of these compounds are also discussed.

CONCLUSION

The determination of a crystal structure with inhibitor bound has allowed the design of exquisitely selective and potent inhibitors of MEK1/2. Several allosteric inhibitors have advanced to clinical trial and shown some efficacy in cancer as single agents, but the future application of MEK1/2 inhibitors is likely to be either in combination with other therapies or in disorders which are genetically defined as being dependent on the MAPK pathway.

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

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

MOLECULAR DYNAMICS AND FREE ENERGY ANALYSES OF ERK2–PYRAZOLYLPYRROLE INHIBITORS INTERACTIONS: INSIGHT INTO STRUCTURE-BASED LIGAND DESIGN

The extracellular signal-regulated protein kinase 2 (ERK2) is a pivotal member involving in Ras/Raf/MEK/ERK signal transduction pathway, acting as a central point where multiple signaling pathways coalesce to drive transcription. The pyrazolylpyrrole compounds as ATP competitive inhibitors of ERK2 can bind target with a special binding mode and have higher inhibitory potency than other ERK2-inhibitors. We investigated the interaction mode of ERK2-inhibitor using molecular dynamics simulation. The molecular mechanics Poisson–Boltzmann surface area approach is used to calculate the binding free energy of ERK2 with pyrazolylpyrrole inhibitors to analyze the factors of improving the affinity. The results indicated that the electrostatic interactions play the most important role in keeping the stabilization of ERK2-inhibitor. The structural analyses showed that the protein motions can be controlled by changing the structures of inhibitors; furthermore, the full use of available space in the binding site by improving the flexibilities of inhibitors and introducing hydrophobic groups can increase the inhibitory effect.

Fused thiophene derivatives as MEK inhibitors

A number of novel fused thiophene derivatives have been prepared and identified as potent inhibitors of MEK. The SAR data of selected examples and the in vivo profiling of compound 13 h demonstrates the functional activity of this class of compounds in HT-29 PK/PD models.

The process of reversible phosphorylation: the work of Edmond H. Fischer

Edmond H. Fischer was awarded the 1992 Nobel Prize in Physiology or Medicine for his joint research with Edwin G. Krebs on reversible protein phosphorylation. The two Classics reprinted here relate some of Fischer and Krebs' early discoveries in their phosphorylase research

Phosphorylase Activity of Skeletal Muscle Extracts (Krebs, E. G., and Fischer, E. H. (1955) J. Biol. Chem. 216, 113–120 )

Conversion of Phosphorylase b to Phosphorylase a in Muscle Extracts (Fischer, E. H., and Krebs, E. G. (1955) J. Biol. Chem. 216, 121–132 )

The MAP kinase signaling cascades: a system of hundreds of components regulates a diverse array of physiological functions

Sequential activation of kinases within the mitogen-activated protein (MAP) kinase (MAPK) cascades is a common, and evolutionary-conserved mechanism of signal transduction. Four MAPK cascades have been identified in the last 20 years and those are usually named according to the MAPK components that are the central building blocks of each of the cascades. These are the extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-Terminal kinase (JNK), p38, and ERK5 cascades. Each of these cascades consists of a core module of three tiers of protein kinases termed MAPK, MAPKK, and MAP3K, and often two additional tiers, the upstream MAP4K and the downstream MAPKAPK, which can complete five tiers of each cascade in certain cell lines or stimulations. The transmission of the signal via each cascade is mediated by sequential phosphorylation and activation of the components in the sequential tiers. These cascades cooperate in transmitting various extracellular signals and thus control a large number of distinct and even opposing cellular processes such as proliferation, differentiation, survival, development, stress response, and apoptosis. One way by which the specificity of each cascade is regulated is through the existence of several distinct components in each tier of the different cascades. About 70 genes, which are each translated to several alternatively spliced isoforms, encode the entire MAPK system, and allow the wide array of cascade's functions. These components, their regulation, as well as their involvement together with other mechanisms in the determination of signaling specificity by the MAPK cascade is described in this review. Mis-regulation of the MAPKs signals usually leads to diseases such as cancer and diabetes; therefore, studying the mechanisms of specificity-determination may lead to better understanding of these signaling-related diseases.

Determination of ERK activity: anti-phospho-ERK antibodies and in vitro phosphorylation

The ERK signaling cascade is composed of several protein kinases that sequentially activate each other by phosphorylation. This pathway is a central component of a complex signaling network that regulates important cellular processes including proliferation, differentiation, and survival. In most of these cases, the ERK cascade is activated downstream of the small GTPase Ras that, upon activation, recruits and activates the first tier in the cascade, which contains the Raf kinases. Afterward the signal is further transmitted by MEKs, ERKs, and often RSKs in the MAPKK, MAPK, and MAPKAPKs tiers of the cascade, respectively. ERKs and RSKs can further disseminate the signal by phosphorylating and modulating the activity of a large number of regulatory proteins including transcription factors and chromatin modifying enzymes. Understanding the mechanisms of activation and the regulation of the various components of this cascade will enhance our insight into the regulation of the ERK-dependent cellular processes in normal cells or of their malfunctioning in various diseases, including cancer. In this chapter, we describe methods used to determine the activity of ERKs, which upon slight modifications can also be used for the study of other signaling kinases, either within the cascade or in other pathways. These methods have been successfully applied to study the ERK signaling cascades in a variety of tissue-cultured cell lines, homo-genized animal organs, and lower organisms. As such, the use of these methods should expand our knowledge on the regulation of many distinct systems and upon induction of various stimulations.

Oncogenic signaling from the hematopoietic growth factor receptors c-Kit and Flt3

Signal transduction in response to growth factors is a strictly controlled process with networks of feedback systems, highly selective interactions and finely tuned on-and-off switches. In the context of cancer, detailed signaling studies have resulted in the development of some of the most frequently used means of therapy, with several well established examples such as the small molecule inhibitors imatinib and dasatinib in the treatment of chronic myeloid leukemia. Impaired function of receptor tyrosine kinases is implicated in various types of tumors, and much effort is put into mapping the many interactions and downstream pathways. Here we discuss the hematopoietic growth factor receptors c-Kit and Flt3 and their downstream signaling in normal as well as malignant cells. Both receptors are members of the same family of tyrosine kinases and crucial mediators of stem-and progenitor-cell proliferation and survival in response to ligand stimuli from the surrounding microenvironment. Gain-of-function mutations/alterations render the receptors constitutively and ligand-independently activated, resulting in aberrant signaling which is a crucial driving force in tumorigenesis. Frequently found mutations in c-Kit and Flt3 are point mutations of aspartic acid 816 and 835 respectively, in the activation loop of the kinase domains. Several other point mutations have been identified, but in the case of Flt3, the most common alterations are internal tandem duplications (ITDs) in the juxtamembrane region, reported in approximately 30% of patients with acute myeloid leukemia (AML). During the last couple of years, the increasing understanding of c-Kit and Flt3 signaling has also revealed the complexity of these receptor systems. The impact of gain-of-function mutations of c-Kit and Flt3 in different malignancies is well established and shown to be of clinical relevance in both prognosis and therapy. Many inhibitors of both c-Kit or Flt3 or of their downstream substrates are in clinical trials with encouraging results, and targeted therapy using a combination of such inhibitors is considered a promising approach for future treatments.

Enzymatic activity and substrate specificity of mitogen-activated protein kinase p38alpha in different phosphorylation states

The mitogen-activated protein (MAP) kinases are essential signaling molecules that mediate many cellular effects of growth factors, cytokines, and stress stimuli. Full activation of the MAP kinases requires dual phosphorylation of the Thr and Tyr residues in the TXY motif of the activation loop by MAP kinase kinases. Down-regulation of MAP kinase activity can be initiated by multiple serine/threonine phosphatases, tyrosine-specific phosphatases, and dual specificity phosphatases (MAP kinase phosphatases). This would inevitably lead to the formation of monophosphorylated MAP kinases. However, the biological functions of these monophosphorylated MAP kinases are currently not clear. In this study, we have prepared MAP kinase p38alpha, a member of the MAP kinase family, in all phosphorylated forms and characterized their biochemical properties. Our results indicated the following: (i) p38alpha phosphorylated at both Thr-180 and Tyr-182 was 10-20-fold more active than p38alpha phosphorylated at Thr-180 only, whereas p38alpha phosphorylated at Tyr-182 alone was inactive; (ii) the dual-specific MKP5, the tyrosine-specific hematopoietic protein-tyrosine phosphatase, and the serine/threonine-specific PP2Calpha are all highly specific for the dephosphorylation of p38alpha, and the dephosphorylation rates were significantly affected by different phosphorylated states of p38alpha; (iii) the N-terminal domain of MPK5 has no effect on enzyme catalysis, whereas deletion of the MAP kinase-binding domain in MKP5 leads to a 370-fold decrease in k(cat)/K(m) for the dephosphorylation of p38alpha. This study has thus revealed the quantitative contributions of phosphorylation of Thr, Tyr, or both to the activation of p38alpha and to the substrate specificity for various phosphatases.

The detection of MAPK signaling

Mitogen-activated protein kinase (MAPK) cascades are central pathways that participate in the intracellular transmission of extracellular signals. Each of the MAPK signaling cascades seems to consist of three to five tiers of protein kinases that sequentially activate each other by phosphorylation. Since the majority of MAPK cascade components are kinases, the methods used to detect their activation involve determining phosphorylation state and protein kinase activities. The primary method describes the use of immunoblotting with specific anti-phospho antibody to detect activation of MAPK components. Alternative methods described are immunoprecipitation of desired protein kinases followed by phosphorylation of specific substrates and the use of an in-gel kinase assay. These methods have proven useful in the study of the MAPK signaling cascades.

Asbestos-induced MKP-3 expression augments TNF-alpha gene expression in human monocytes

TNF-alpha is associated with the development of interstitial fibrosis. We have demonstrated that the p38 mitogen-activated protein (MAP) kinase regulates TNF-alpha expression in monocytes exposed to asbestos. In this report, we asked if extracellular signal-regulated kinase (ERK) was also involved in TNF-alpha expression in monocytes exposed to asbestos. We found that p38 and ERK were differentially activated in alveolar macrophages obtained from patients with asbestosis compared with normal subjects. More specifically, p38 was constitutively active and ERK activation was suppressed. Since the upstream pathway leading to ERK was intact, we hypothesized that an ERK-specific phosphatase was, in part, responsible for the decreased ERK activity. We evaluated whether the dual specificity phosphatase MAP kinase phosphatase (MKP)-3, which is highly expressed in the lung and specifically dephosphorylates ERK, was increased after exposure to asbestos. We found that MKP-3 increased after exposure to asbestos, and its expression was regulated by p38. We found that p38 and ERK negatively regulated one another, and MKP-3 had a role in this differential activation. We also found that p38 was a positive regulator and ERK was a negative regulator of TNF-alpha gene expression. Cells overexpressing MKP-3 had a significant increase in TNF-alpha gene expression, suggesting than an environment favoring p38 MAP kinase activation is necessary for TNF-alpha production in monocytes exposed to asbestos. Taken together, these data demonstrate that the p38 MAP kinase down-regulates ERK via activation of MKP-3 in human monocytes exposed to asbestos to enhance TNF-alpha gene expression.

Glucocorticoids shift arachidonic acid metabolism toward endocannabinoid synthesis: a non-genomic anti-inflammatory switch

Glucocorticoids are capable of exerting both genomic and non-genomic actions in target cells of multiple tissues, including the brain, which trigger an array of electrophysiological, metabolic, secretory and inflammatory regulatory responses. Here, we have attempted to show how glucocorticoids may generate a rapid anti-inflammatory response by promoting arachidonic acid-containing endocannabinoids biosynthesis. According to our hypothesized model, non-genomic action of glucocorticoids results in the global shift of membrane lipid metabolism, subverting metabolic pathways toward the synthesis of the anti-inflammatory endocannabinoids, anandamide (AEA) and 2-arachidonoyl-glycerol (2-AG), and away from arachidonic acid production. Post-transcriptional inhibition of cyclooxygenase-2 (COX(2)) synthesis by glucocorticoids assists this mechanism by suppressing the synthesis of pro-inflammatory prostaglandins as well as endocannabinoid-derived prostanoids. In the central nervous system (CNS) this may represent a major neuroprotective system, which may cross-talk with leptin signaling in the hypothalamus allowing for the coordination between energy homeostasis and the inflammatory response.

The detection of MAPK signaling

Mitogen-activated protein kinase (MAPK) cascades are central pathways that participate in the intracellular transmission of extracellular signals. Each of the MAPK signaling cascades seems to consist of three to five tiers of protein kinases that sequentially activate each other by phosphorylation. Since the majority of MAPK cascade components are kinases, the methods used to detect their activation involve determining phosphorylation state and protein kinase activities. The Basic Protocol describes the use of immunoblotting with specific anti-phospho antibody to detect activation of MAPK components. Alternative methods described are immunoprecipitation of desired protein kinases followed by phosphorylation of specific substrates and the use of an in-gel kinase assay. These methods have proven useful in the study of the MAPK signaling cascades.

Growth factor-stimulated phosphorylation cascades: activation of growth factor-stimulated MAP kinase

Protein phosphorylation is an important mechanism in the response of cells to growth factors by which signals can be conveyed from cell surface receptors to intracellular targets. In addition to stimulation of protein tyrosine phosphorylation, activation of growth factor receptors having protein tyrosine kinase activity leads to dramatic alterations in the levels of protein serine/threonine phosphorylation. Several growth factor-stimulated serine/threonine-specific kinases have been identified as potential mediators of such signalling. MAP (microtubule-associated protein) kinase has emerged as a very interesting member of this group, because it activates a separate kinase, pp90rsk, which is also growth factor-stimulated. MAP kinase itself appears to be regulated by protein phosphorylation, because it can be inactivated by protein phosphatases. We have identified two 60 kDa proteins that promote the phosphorylation and full activation of MAP kinase in a manner paralleling its activation by growth factors in intact cells. These 'MAP kinase activators' are themselves stimulated by growth factors, suggesting that they function as intermediates between the MAP kinase and cell surface receptors in a growth factor-stimulated kinase cascade. Identification of the components of this protein kinase cascade reveals a mechanism by which at least some of the effects of receptor tyrosine kinases can be mediated through serine/threonine phosphorylation.

Phase I targeted combination trial of sorafenib and erlotinib in patients with advanced solid tumors

PURPOSE

Sorafenib and erlotinib are potent, orally administered receptor tyrosine kinase inhibitors with antiproliferative and antiangiogenic activities. Given their inhibitory target profile and efficacy as single agents, the combination of these drugs is of considerable interest in solid malignancies. This study aimed to determine the recommended phase II dose of this targeted combination, their toxicity profile, pharmacokinetic interaction, and preliminary clinical activities.

EXPERIMENTAL DESIGN

Sorafenib was administered alone for a 1-week run-in period, and then both drugs were given together continuously, with every 28 days considered as a cycle. Three dose levels were assessed.

RESULTS

Seventeen patients with advanced solid tumors received 75 cycles of treatment. The most frequent adverse events of all grades were constitutional and gastrointestinal in nature followed by electrolytes and dermatologic toxicities. Fatigue was the most common adverse event (17 patients; 100%) followed by diarrhea (15 patients; 88%), hypophosphatemia (13 patients; 76%), and acneiform rash (12 patients; 71%). These adverse events were predominantly mild to moderate. The recommended phase II dose of this combination was determined as 400 mg twice daily sorafenib and 150 mg daily erlotinib. Pharmacokinetic analysis revealed no significant effect of erlotinib on the pharmacokinetic profile of sorafenib. Among 15 evaluable patients, 3 (20%) achieved a confirmed partial response and 9 (60%) had stable disease as best response.

CONCLUSIONS

Sorafenib and erlotinib are well tolerated and seem to have no pharmacokinetic interactions when administered in combination at their full single-agent recommended doses. This well tolerated combination resulted in promising activity that needs further validation in phase II studies.

Histone H3 phosphorylation at serine 10 and serine 28 is mediated by p38 MAPK in rat hepatocytes exposed to ethanol and acetaldehyde

Ethanol modulates mitogen-activated protein kinases (MAPKs). We have now investigated the influence of ethanol and its metabolite, acetaldehyde on histone H3 phosphorylation to ascertain downstream targets of MAPKs. In primary culture of rat hepatocytes, ethanol and acetaldehyde increased phosphorylation of nuclear histone H3 at serine 10 and serine 28. Specific inhibitors of p38 MAPK, SB203580, PD169316 and SB202190 blocked this phosphorylation. The inactive analogue, SB202474 had no effect. In contrast, c-Jun N-terminal kinase (JNK) inhibitor, SP600125 or MAP/ERK kinase (MEK) 1/2 inhibitor, PD98059 had no effect on the histone H3 phosphorylation. The p38 MAPK activation correlated with upstream activation of MAPK kinase (MKK) 3/6 but was independent of protein synthesis. In the nuclear fraction, the phosphorylation of p38 MAPK and its protein level increased with peak activation at 24 h by ethanol and at 30 min by acetaldehyde. These responses were ethanol and acetaldehyde dose dependent. Surprisingly, the phosphorylation of p38 MAPK was undetectable in the cytosolic fraction suggesting a subcellular selectivity of p38 MAPK signaling. The phosphorylation of JNK and p42/44 MAPK and their protein levels also increased in the nuclear fraction. Although ethanol caused translocation of all three major MAPKs (p42/44 MAPK, JNK, p38 MAPK) into the nucleus, histone H3 phosphorylation at serine 10 and serine 28 was mediated by p38 MAPK. This histone H3 phosphorylation had no influence on ethanol and acetaldehyde induced apoptosis. These studies demonstrate for the first time that ethanol and acetaldehyde stimulated phosphorylation of histone H3 at serine 10 and serine 28 are downstream nuclear response mediated by p38 MAPK in hepatocytes.

MAP kinase pathways: the first twenty years

The MAP kinases, discovered approximately 20 years ago, together with their immediate upstream regulators, are among the most highly studied signal transduction molecules. This body of work has shaped many aspects of our present views of signal transduction by protein kinases. The effort expended in this area reflects the extensive participation of these regulatory modules in the control of cell fate decisions, i.e., proliferation, differentiation and death, across all eukaryotic phylla and in all tissues of metazoans. The discovery of these kinases is reviewed, followed by a discussion of some of the features of this signaling module that account for its broad impact on cell function and its enormous interest to many investigators.

The MEK/ERK cascade: from signaling specificity to diverse functions

The ERK signaling cascade is a central MAPK pathway that plays a role in the regulation of various cellular processes such as proliferation, differentiation, development, learning, survival and, under some conditions, also apoptosis. The ability of this cascade to regulate so many distinct, and even opposing, cellular processes, raises the question of signaling specificity determination by this cascade. Here we describe mechanisms that cooperate to direct MEK-ERK signals to their appropriate downstream destinations. These include duration and strength of the signals, interaction with specific scaffolds, changes in subcellular localization, crosstalk with other signaling pathways, and presence of multiple components with distinct functions in each tier of the cascade. Since many of the mechanisms do not function properly in cancer cells, understanding them may shed light not only on the regulation of normal cell proliferation, but also on mechanisms of oncogenic transformation.