Differential regulation of Raf isozymes by growth versus differentiation inducing factors in PC12 pheochromocytoma cells

A computationally engineered RAS rheostat reveals RAS-ERK signaling dynamics

Synthetic protein switches controlled with user-defined inputs are powerful tools for studying and controlling dynamic cellular processes. To date, these approaches have relied primarily on intermolecular regulation. Here, we report a computationally-guided framework for engineering intramolecular regulation of protein function. We utilize this framework to develop Chemically Inducible Activator of RAS (CIAR), a single-component RAS rheostat that directly activates endogenous RAS in response to a small molecule. Using CIAR, we show that direct RAS activation elicits markedly different RAS/ERK signaling dynamics compared to growth factor stimulation, and that these dynamics differ between cell types. We also found that the clinically-approved RAF inhibitor vemurafenib potently primes cells to respond to direct wild-type RAS activation. These results demonstrate the utility of CIAR for quantitatively interrogating RAS signaling. Finally, we demonstrate the general utility of our approach to design intramolecularly-regulated protein tools by applying this methodology to the Rho Family GEFs.

Acquired Resistance to Clinical Cancer Therapy: A Twist in Physiological Signaling

Although modern therapeutic strategies have brought significant progress to cancer care in the last 30 years, drug resistance to targeted monotherapies has emerged as a major challenge. Aberrant regulation of multiple physiological signaling pathways indispensable for developmental and metabolic homeostasis, such as hyperactivation of pro-survival signaling axes, loss of suppressive regulations, and impaired functionalities of the immune system, have been extensively investigated aiming to understand the diversity of molecular mechanisms that underlie cancer development and progression. In this review, we intend to discuss the molecular mechanisms of how conventional physiological signal transduction confers to acquired drug resistance in cancer patients. We will particularly focus on protooncogenic receptor kinase inhibition-elicited tumor cell adaptation through two major core downstream signaling cascades, the PI3K/Akt and MAPK pathways. These pathways are crucial for cell growth and differentiation and are frequently hyperactivated during tumorigenesis. In addition, we also emphasize the emerging roles of the deregulated host immune system that may actively promote cancer progression and attenuate immunosurveillance in cancer therapies. Understanding these mechanisms may help to develop more effective therapeutic strategies that are able to keep the tumor in check and even possibly turn cancer into a chronic disease.

X-linked inhibitor of apoptosis protein negatively regulates neuronal differentiation through interaction with cRAF and Trk

X-linked Inhibitor of apoptosis protein (XIAP) has been classically identified as a cell death regulator. Here, we demonstrate a novel function of XIAP as a regulator of neurite outgrowth in neuronal cells. In PC12 cells, XIAP overexpression prevents NGF-induced neuronal differentiation, whereas NGF treatment induces a reduction of endogenous XIAP levels concomitant with the induction of neuronal differentiation. Accordingly, downregulation of endogenous XIAP protein levels strongly increases neurite outgrowth in PC12 cells as well as axonal and dendritic length in primary cortical neurons. The effects of XIAP are mediated by the mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinases (ERKs) pathway since blocking this pathway completely prevents the neuritogenesis mediated by XIAP downregulation. In addition, we found that XIAP binds to cRaf and Trk receptors. Our results demonstrate that XIAP plays a new role as a negative regulator of neurotrophin-induced neurite outgrowth and neuronal differentiation in developing neurons.

Regulation of MEK/ERK pathway output by subcellular localization of B-Raf

The strength and duration of intracellular signalling pathway activation is a key determinant of the biological outcome of cells in response to extracellular cues. This has been particularly elucidated for the Ras/Raf/MEK [mitogen-activated growth factor/ERK (extracellular-signal-regulated kinase) kinase]/ERK signalling pathway with a number of studies in fibroblasts showing that sustained ERK signalling is a requirement for S-phase entry, whereas transient ERK signalling does not have this capability. A major unanswered question, however, is how a cell can sustain ERK activation, particularly when ERK-specific phosphatases are transcriptionally up-regulated by the pathway itself. A major point of ERK regulation is at the level of Raf, and, to sustain ERK activation in the presence of ERK phosphatases, sustained Raf activation is a requirement. Three Raf proteins exist in mammals, and the activity of all three is induced following growth factor stimulation of cells, but only B-Raf activity is maintained at later time points. This observation points to B-Raf as a regulator of sustained ERK activation. In the present review, we consider evidence for a link between B-Raf and sustained ERK activation, focusing on a potential role for the subcellular localization of B-Raf in this key physiological event.

A-RAF kinase functions in ARF6 regulated endocytic membrane traffic

BACKGROUND

RAF kinases direct ERK MAPK signaling to distinct subcellular compartments in response to growth factor stimulation.

METHODOLOGY/PRINCIPAL FINDINGS

Of the three mammalian isoforms A-RAF is special in that one of its two lipid binding domains mediates a unique pattern of membrane localization. Specific membrane binding is retained by an N-terminal fragment (AR149) that corresponds to a naturally occurring splice variant termed DA-RAF2. AR149 colocalizes with ARF6 on tubular endosomes and has a dominant negative effect on endocytic trafficking. Moreover actin polymerization of yeast and mammalian cells is abolished. AR149/DA-RAF2 does not affect the internalization step of endocytosis, but trafficking to the recycling compartment.

CONCLUSIONS/SIGNIFICANCE

A-RAF induced ERK activation is required for this step by activating ARF6, as A-RAF depletion or inhibition of the A-RAF controlled MEK-ERK cascade blocks recycling. These data led to a new model for A-RAF function in endocytic trafficking.

Positive regulation of A-RAF by phosphorylation of isoform-specific hinge segment and identification of novel phosphorylation sites

In mammals the RAF family of serine/threonine kinases consists of three members, A-, B-, and C-RAF. Activation of RAF kinases involves a complex series of phosphorylations. Although the most prominent phosphorylation sites of B- and C-RAF are well characterized, little is known about regulatory phosphorylation of A-RAF. Using mass spectrometry, we identified here a number of novel in vivo phosphorylation sites in A-RAF. In particular, we found that Ser-432 participates in MEK binding and is indispensable for A-RAF signaling. On the other hand, phosphorylation within the activation segment does not contribute to epidermal growth factor-mediated activation. Furthermore, we show that the potential 14-3-3 binding domains in A-RAF are phosphorylated independently of its activation status. Of importance, we identified a novel regulatory domain in A-RAF (referred to as IH-segment) positioned between amino acids 248 and 267 that contains seven putative phosphorylation sites. Three of these sites, serines 257, 262, and 264, regulate A-RAF activation in a stimulatory manner. The spatial model of the A-RAF fragment, including residues between Ser-246 and Glu-277, revealed a switch of charge at the molecular surface of the IH-region upon phosphorylation, suggesting a mechanism in which the high accumulation of negative charges may lead to an electrostatic destabilization of protein-membrane interaction resulting in depletion of A-RAF from the plasma membrane. Together, we provide here for the first time a detailed analysis of in vivo A-RAF phosphorylation status and demonstrate that regulation of A-RAF by phosphorylation exhibits unique features compared with B- and C-RAF.

Deficiency in the LIM-only protein FHL2 impairs assembly of extracellular matrix proteins

We have described the scaffolding protein FHL2 as a component of focal adhesion structures, to which it is recruited via binding to both alpha- or beta-integrin subunits. Using mesenchymal stem cells from wild-type and FHL2-knockout mice, we show here that inactivation of FHL2 leads to impaired assembly of extracellular matrix proteins on the cell surface and to impaired bundling of focal adhesions. Both altered properties can be restored by reexpression of recombinant FHL2 protein in FHL2-null cells. Molecular analysis of integrin-mediated signaling revealed a higher phosphorylation of FAK at tyrosine 925 in FHL2-knockout cells compared to their wild-type counterpart. Consequently, the activation of the mitogenic kinase ERK was more pronounced in knockout cells on cell adhesion. The growth factor-induced activation of ERK, however, was not altered. The perturbed organization of extracellular matrix on FHL2-null cells was improved when the increased activation of MAPK was inhibited. Our findings point to a role of FHL2 in bundling of focal adhesion structures, in integrin-mediated ERK activation, and subsequently in proper allocation of matrix proteins on the cell surface.

Thrombopoietin inhibits nerve growth factor-induced neuronal differentiation and ERK signalling

Thrombopoietin (TPO), a hematopoietic growth factor regulating platelet production, and its receptor (TPOR) were recently shown to be expressed in the brain where they exert proapoptotic activity. Here we used PC12 cells, an established model of neuronal differentiation, to investigate the effects of TPO on neuronal survival and differentiation. These cells expressed TPOR mRNA. TPO increased cell death in neuronally differentiated PC12 cells but had no effect in undifferentiated cells. Surprisingly, TPO inhibited nerve growth factor (NGF)-induced differentiation of PC12 cells in a dose- and time-dependent manner. This inhibition was dependent on the activity of Janus kinase-2 (JAK2). Using phospho-kinase arrays and Western blot we found downregulation of the NGF-stimulated phosphorylation of the extracellular signal-regulated kinase p42ERK by TPO with no effect on phosphorylation of Akt or stress kinases. NGF-induced phosphorylation of ERK-activating kinases, MEK1/2 and C-RAF was also reduced by TPO while NGF-induced RAS activation was not attenuated by TPO treatment. In contrast to its inhibitory effects on NGF signalling, TPO had no effect on epidermal growth factor (EGF)-stimulated ERK phosphorylation or proliferation of PC12 cells. Our data indicate that TPO via activation of its receptor-bound JAK2 delays the NGF-dependent acquisition of neuronal phenotype and decreases neuronal survival by suppressing NGF-induced ERK activity.

Unique N-region Determines Low Basal Activity and Limited Inducibility of A-RAF Kinase

In mammals the RAF family of serine/threonine kinases consists of three members, A-, B-, and C-RAF. A prominent feature of RAF isoforms regards differences in basal and inducible kinase activities. To elucidate the nature of these differences, we studied the role of the nonconserved residues within the N-region (Negative-charge regulatory region). The nonconserved amino acids in positions -3 and +1 relative to the highly conserved serine 299 in A-RAF and serine 338 in C-RAF have so far not been considered as regulatory residues. Here we demonstrate the essential role of these residues in the RAF activation process. Substitution of tyrosine 296 in A-RAF to arginine led to a constitutively active kinase. In contrast, substitution of glycine 300 by serine (mimicking B- and C-RAF) acts in an inhibitory manner. Consistent with these data, the introduction of glycine in the analogous position of C-RAF (S339G mutant) led to a constitutively active C-RAF kinase. Based on the three-dimensional structure of the catalytic domain of B-RAF and using the sequences of the N-regions of A- and C-RAF, we searched by molecular modeling for the putative contact points between these two moieties. A tight interaction between the N-region residue serine 339 of C-RAF and arginine 398 of the catalytic domain was identified and proposed to inhibit the kinase activity of RAF proteins, because abrogation of this interaction contributes to RAF activation. Furthermore, tyrosine 296 in A-RAF favors a spatial orientation of the N-region segment, which enables a tighter contact to the catalytic domain, whereas a glutamine residue at this position in C-RAF abrogates this interaction. Considering this observation, we suggest that tyrosine 296, which is unique for A-RAF, is a major determinant of the low activating potency of this RAF isoform.

Ras oncogenes and their downstream targets

RAS proteins are small GTPases, which serve as master regulators of a myriad of signaling cascades involved in highly diverse cellular processes. RAS oncogenes have been originally discovered as retroviral oncogenes, and ever since constitutively activating RAS mutations have been identified in human tumors, they are in the focus of intense research. In this review, we summarize the biochemical properties of RAS proteins, trace down the evolution of RAS signaling and present an overview of the spatio-temporal activation of major RAS isoforms. We further discuss RAS effector pathways, their role in normal and transformed cell physiology and summarize ongoing attempts to interfere with aberrant RAS signaling. Finally, we comment on the role of micro RNAs in modulating RAS expression, contribution of RAS to stem cell function and on high-throughput analyses of RAS signaling networks.

Cortical migration defects in mice expressing A-RAF from the B-RAF locus

We have previously shown that mice lacking the protein kinase B-RAF have defects in both neural and endothelial cell lineages and die around embryonic day 12 (E12). To delineate the function of B-RAF in the brain, B-RAF KIN/KIN mice lacking B-RAF and expressing A-RAF under the control of the B-RAF locus were created. B-RAF KIN/KIN embryos displayed no vascular defects, no endothelial and neuronal apoptosis, or gross developmental abnormalities, and a significant proportion of these animals survived for up to 8 weeks. Cell proliferation in the neocortex was reduced from E14.5 onwards. Newborn cortical neurons were impaired in their migration toward the cortical plate, causing a depletion of Brn-2-expressing pyramidal neurons in layers II, III, and V of the postnatal cortex. Our data reveal that B-RAF is an important mediator of neuronal survival, migration, and dendrite formation and that A-RAF cannot fully compensate for these functions.

Forebrain-specific knockout of B-raf kinase leads to deficits in hippocampal long-term potentiation, learning, and memory

Raf kinases are downstream effectors of Ras and upstream activators of the MEK-ERK cascade. Ras and MEK-ERK signaling play roles in learning and memory (L&M) and neural plasticity, but the roles of Raf kinases in L&M and plasticity are unclear. Among Raf isoforms, B-raf is preferentially expressed in the brain. To determine whether B-raf has a role in synaptic plasticity and L&M, we used the Cre-LoxP gene targeting system to derive forebrain excitatory neuron B-raf knockout mice. This conditional knockout resulted in deficits in ERK activation and hippocampal long-term potentiation (LTP) and impairments in hippocampus-dependent L&M, including spatial learning and contextual discrimination. Despite the widespread expression of B-raf, this mutation did not disrupt other forms of L&M, such as cued fear conditioning and conditioned taste aversion. Our findings demonstrate that B-raf plays a role in hippocampal ERK activation, synaptic plasticity, and L&M.

The statistical mechanics of complex signaling networks: nerve growth factor signaling

The inherent complexity of cellular signaling networks and their importance to a wide range of cellular functions necessitates the development of modeling methods that can be applied toward making predictions and highlighting the appropriate experiments to test our understanding of how these systems are designed and function. We use methods of statistical mechanics to extract useful predictions for complex cellular signaling networks. A key difficulty with signaling models is that, while significant effort is being made to experimentally measure the rate constants for individual steps in these networks, many of the parameters required to describe their behavior remain unknown or at best represent estimates. To establish the usefulness of our approach, we have applied our methods toward modeling the nerve growth factor (NGF)-induced differentiation of neuronal cells. In particular, we study the actions of NGF and mitogenic epidermal growth factor (EGF) in rat pheochromocytoma (PC12) cells. Through a network of intermediate signaling proteins, each of these growth factors stimulates extracellular regulated kinase (Erk) phosphorylation with distinct dynamical profiles. Using our modeling approach, we are able to predict the influence of specific signaling modules in determining the integrated cellular response to the two growth factors. Our methods also raise some interesting insights into the design and possible evolution of cellular systems, highlighting an inherent property of these systems that we call 'sloppiness.'

B-Raf and C-Raf signaling investigated in a simplified model of the mitogenic kinase cascade

Signaling pathways based on the reversible phosphorylation of proteins control most aspects of cellular life in higher organisms. Extracellular stimuli can induce growth, differentiation, survival and the stress response through a number of highly conserved signaling pathways. We discuss how the intensity and duration of signals may have dramatic consequences on the way cells respond to stimuli. Picking the central Ras-Raf-MEK-ERK signal cascade, we developed a mathematical model of how stimuli induce different signal patterns and thereby different cellular responses, depending on cell type and the ratio between B-Raf and C-Raf. Based on biochemical data for activation and dephosphorylation, as well as the differential equations of our model, we suggest a different signaling pattern and response result for B-Raf (strong activation, sustained signal) and C-Raf (steep activation, transient signal). We further support the significance of such differential modulatory signaling by showing different Raf isoform expression in various cell lines and experimental testing of the predicted kinase activities in B-Raf, C-Raf and mutated versions.

Bcl-2 proteins: master switches at the intersection of death signaling and the survival control by Raf kinases

Bcl-2 family members are central to the control of cell survival. Work of the last years has established that the function of these proteins can be regulated by mitogenic signaling cascades. Within the scope of this review, we will discuss the contribution of Bcl-2-dependent signaling pathways to cell survival by Raf kinases and also address the underlying mechanisms.

Signalling mechanisms for survival of lesioned motoneurons

Mechanisms controlling neuronal survival play an important role both during development and after birth, in particular when the nervous system is lesioned. Isolated embryonic motoneurons and other types of primary neurons have been a useful tool for studying basic mechanisms underlying neuronal cell death during development and under pathophysiological conditions after neurotrauma. These studies have led to the identification of neurotrophic factors which under physiological conditions regulate survival and functional properties, and after neurotrauma promote regeneration and plasticity. Functional analysis of these molecules, in particular by generation of gene knockout mice, has led to a more detailed understanding of complex requirements of individual types of neurons for their survival and also paved the way for a better understanding of the signalling pathways in lesioned neurons which decide on cell death or survival after axotomy and other pathophysiological conditions. These findings could ultimately lead to a rational basis for therapeutic approaches aiming at improving neuronal survival and regeneration after neurotrauma.

Dynamic changes in C-Raf phosphorylation and 14-3-3 protein binding in response to growth factor stimulation: differential roles of 14-3-3 protein binding sites

Phosphorylation events play a crucial role in Raf activation. Phosphorylation of serines 259 and 621 in C-Raf and serines 364 and 728 in B-Raf has been suggested to be critical for association with 14-3-3 proteins. To study the functional consequences of Raf phosphorylations at these positions, we developed and characterized phosphospecific antibodies directed against 14-3-3 binding epitopes: a monoclonal phosphospecific antibody (6B4) directed against pS621 and a polyclonal antibody specific for B-Raf-pS364 epitope. Although 6B4 detected both C- and B-Raf in Western blots, it specifically recognizes the native form of C-Raf but not B-Raf. Contrary to B-Raf, a kinase-dead mutant of C-Raf was found to be only poorly phosphorylated in the Ser-621 position. Moreover, serine 259 to alanine mutation prevented the Ser-621 phosphorylation suggesting an interdependence between these two 14-3-3 binding domains. Direct C-Raf.14-3-3 binding studies with purified proteins combined with competition assays revealed that the 14-3-3 binding domain surrounding pS621 represents the high affinity binding site, whereas the pS259 epitope mediates lower affinity binding. Raf isozymes differ in their 14-3-3 association rates. The time course of endogenous C-Raf activation in mammalian cells by nerve growth factor (NGF) has been examined using both phosphospecific antibodies directed against 14-3-3 binding sites (6B4 and anti-pS259) as well as phosphospecific antibodies directed against the activation domain (anti-pS338 and anti-pY340/pY341). Time course of Ser-621 phosphorylation, in contrast to Ser-259 phosphorylation, exhibited unexpected pattern reaching maximal phosphorylation within 30 s of NGF stimulation. Phosphorylation of tyrosine 340/341 reached maximal levels subsequent to Ser-621 phosphorylation and was coincident with emergence of kinase activity. Taken together, we found substantial differences between C-Raf.14-3-3 binding epitopes pS259 and pS621 and visualized for the first time the sequence of the essential C-Raf phosphorylation events in mammalian cells in response to growth factor stimulation.

Raf and the road to cell survival: a tale of bad spells, ring bearers and detours

Research of the last years has demonstrated the absolute requirement of mitogenic signaling pathways for the control of cell survival. As reviewed here for the members of the Raf kinase family, apoptosis suppression proceeds through diverse mechanisms. They include the recruitment of novel effectors such as IAP and Bcl-2 proteins, key molecules in cell survival control, which interfere with the executions of the cell death at various levels, but also direct effects on metabolic events.

Identification of interaction between MEK2 and A-Raf-1

Mitogen-activated protein (MAP) kinases are activated by dual-specificity kinases, termed MEKs. Using MEK2 as bait in yeast two-hybrid screening, besides c-Raf and KSR, A-Raf was identified as a novel partner that interacts with MEK2. This interaction was confirmed by in vitro binding assay. Further investigation indicates that regions critical for this interaction were located between residues 255 and 606 that represent the kinase domain of A-Raf.

SB203580 promotes EGF-stimulated early morphological differentiation in PC12 cell through activating ERK pathway

MAP kinases have important role in PC12 cell differentiation, since the activities of both extracellular regulated protein kinase (ERK) and p38 have been indicated as necessary signal for PC12 cell differentiation. Epidermal growth factor (EGF) and NGF both activate ERK and p38 in PC12 cells, but only NGF trigger differentiation. It has been proposed that the duration of ERK activation determines the switch from proliferation to differentiation, since EGF causes more transient activation of ERK than NGF in PC12 cells. Here we report that treatment of PC12 cells with EGF in the presence of SB203580, a widely used p38 inhibitor, caused differentiation. The pro-differentiation effect of SB203580 in EGF-treated PC12 cells was found to be independent of its function of p38 inhibition but was through an effect on the ERK pathway that has been recently reported (Kalmes et al. [1999] FEBS Lett. 444: 71-74; Hall-Jackson et al. [1999] Onc. 18: 2047-2054). We found that SB203580 by itself did not affect the activity of ERK1/2 but significantly extended EGF-induced ERK activation in PC12 cells, which resulted in early morphological differentiation. Our data indicated that although both ERK and p38 are required for PC12 cell differentiation, activation of p38 is not required when ERK is superactivated. Our data provided further evidence for the threshold theory that differentiation is determined by the duration of ERK activation.

Rapid activation of ERK1/2 mitogen-activated protein kinase by corticosterone in PC12 cells

Although the nongenomic effects of glucocorticoids have been well acknowledged, its precise intracellular signal transduction pathway remains to be elucidated. The present study using Western immunoblot and protein kinase activity assay, for the first time, showed that corticosterone (B) can induce a rapid activation of Erk1/2 mitogen-activated protein kinase (MAPK) in PC12 cells. The dose-response curve was bell shaped, with the maximal activation at 10(-9) M in 15 min. The results from immunofluorescence staining also revealed that the activated Erk1/2 MAPK was translocated from cytoplasm to nucleus of PC12 cells in 15 min. Activation of Erk1/2 MAPK by B was apparently not mediated by the classical cytosolic steroid receptors, for B-BSA can induce the phosphorylation of Erk1/2 MAPK, but the antagonist (RU38486) cannot block the phosphorylation of Erk1/2 MAPK induced by B. Phosphorylation of Erk1/2 MAPK induced by B was not affected by a tyrosine kinase inhibitor (genistein), suggesting that the pathway did not involve the tyrosine kinase activity. On the other hand, protein kinase C activator (PMA) can activate and protein kinase C inhibitor (Gö6976) can block the activation of Erk1/2 MAPK induced by B. Taken together, these data clearly demonstrated that B might act via putative membrane receptor and rapidly activate Erk1/2 MAPK through protein kinase C alpha in PC12 cells.

Identification of the mechanisms regulating the differential activation of the mapk cascade by epidermal growth factor and nerve growth factor in PC12 cells

In PC12 cells, epidermal growth factor (EGF) transiently stimulates the mitogen-activated protein (MAP) kinases, ERK1 and ERK2, and provokes cellular proliferation. In contrast, nerve growth factor (NGF) stimulation leads to the sustained activation of the MAPKs and subsequently to neuronal differentiation. It has been shown that both the magnitude and longevity of MAPK activation governs the nature of the cellular response. The activations of MAPKs are dependent upon two distinct small G-proteins, Ras and Rap1, that link the growth factor receptors to the MAPK cascade by activating c-Raf and B-Raf, respectively. We found that Ras was transiently stimulated upon both EGF and NGF treatment of PC12 cells. However, EGF transiently activated Rap1, whereas NGF stimulated prolonged Rap1 activation. The activation of the ERKs was due almost exclusively (>90%) to the action of B-Raf. The transient activation of the MAPKs by EGF was a consequence of the formation of a short lived complex assembling on the EGF receptor itself, composed of Crk, C3G, Rap1, and B-Raf. In contrast, NGF stimulation of the cells resulted in the phosphorylation of FRS2. FRS2 scaffolded the assembly of a stable complex of Crk, C3G, Rap1, and B-Raf resulting in the prolonged activation of the MAPKs. Together, these data provide a signaling link between growth factor receptors and MAPK activation and a mechanistic explanation of the differential MAPK kinetics exhibited by these growth factors.

Ras-independent activation of the Raf/MEK/ERK pathway upon calcium-induced differentiation of keratinocytes

MAPKs are crucially involved in the regulation of growth and differentiation of a variety of cells. To elucidate the role of MAPKs in keratinocyte differentiation, activation of ERK, JNK, and p38 in response to stimulation with extracellular calcium was analyzed. We provide evidence that calcium-induced differentiation of keratinocytes is associated with rapid and transient activation of the Raf/MEK/ERK pathway. Stimulation of keratinocytes with extracellular calcium resulted in activation of Raf isozymes and their downstream effector ERK within 10-15 min, but did not increase JNK or p38 activity. Calcium-induced ERK activation differed in kinetics from mitogenic ERK activation by epidermal growth factor and could be modulated by alterations of intracellular calcium levels. Interestingly, calcium stimulation led to down-regulation of Ras activity at the same time that ERK activation was initiated. Expression of a dominant-negative mutant of Ras also did not significantly impair calcium-induced ERK activation, indicating that calcium-mediated ERK activation does not require active Ras. Despite the transient nature of ERK activation, calcium-induced expression of the cyclin-dependent kinase inhibitor p21/Cip1 and the differentiation marker involucrin was sensitive to MEK inhibition, which suggests a role for the Raf/MEK/ERK pathway in early stages of keratinocyte differentiation.

Nerve growth factor activation of the extracellular signal-regulated kinase pathway is modulated by Ca(2+) and calmodulin

Nerve growth factor is a member of the neurotrophin family of trophic factors that have been reported to be essential for the survival and development of sympathetic neurons and a subset of sensory neurons. Nerve growth factor exerts its effects mainly by interaction with the specific receptor TrkA, which leads to the activation of several intracellular signaling pathways. Once activated, TrkA also allows for a rapid and moderate increase in intracellular calcium levels, which would contribute to the effects triggered by nerve growth factor in neurons. In this report, we analyzed the relationship of calcium to the activation of the Ras/extracellular signal-regulated kinase pathway in PC12 cells. We observed that calcium and calmodulin are both necessary for the acute activation of extracellular signal-regulated kinases after TrkA stimulation. We analyzed the elements of the pathway that lead to this activation, and we observed that calmodulin antagonists completely block the initial Raf-1 activation without affecting the function of upstream elements, such as Ras, Grb2, Shc, and Trk. We have broadened our study to other stimuli that activate extracellular signal-regulated kinases through tyrosine kinase receptors, and we have observed that calmodulin also modulates the activation of such kinases after epidermal growth factor receptor stimulation in PC12 cells and after TrkB stimulation in cultured chicken embryo motoneurons. Calmodulin seems to regulate the full activation of Raf-1 after Ras activation, since functional Ras is necessary for Raf-1 activation after nerve growth factor stimulation and calmodulin-Sepharose is able to precipitate Raf-1 in a calcium-dependent manner.

Overlapping and specific functions of Braf and Craf-1 proto-oncogenes during mouse embryogenesis

The three mammalian Raf serine/threonine protein kinases mediate the transduction of proliferative and differentiative signals from cell surface receptors to the nucleus. In vertebrates, Raf signaling has been implicated in the progression of mouse embryos through the two-cell stage and in the induction of posterior mesoderm. However, mouse embryos mutant for each of the Raf genes exhibit no developmental defects before mid-gestation. Here we describe the phenotype of mouse mutants with different combinations of mutant Craf-1 and Braf alleles. Our results show that Raf signaling is indeed indispensable for normal development beyond the blastocyst stage. However, due to a significant redundancy between Craf-1 and Braf, either gene is sufficient for normal development until mid-gestation. The molecular and developmental mechanisms for this redundancy were investigated by monitoring the expression of Raf genes throughout embryogenesis and by biochemical studies in mutant cell lines.

Ras protein signalling

Ras proteins were identified through their association with cell transformation. Since then they have been shown to regulate cell growth, differentiation and apoptosis, as well as influencing processes such as cell migration and neuronal activity. Ras regulates a number of signalling molecules by translocating them to the plasma membrane for activation. An emerging concept is that Ras acts as a branchpoint in signal transduction because it orchestrates the activity of multiple signalling pathways to regulate diverse cellular functions. This implies a degree of selectivity in the ability of Ras to activate particular arms of each pathway, but the mechanisms by which this is achieved are not known. Ras is also an important regulator of immune function and in this review, we summarise current understanding of Ras regulation and function and discuss some new aspects of Ras signalling where understanding is less clear.

Mitogenic signaling of Ras is regulated by differential interaction with Raf isozymes

In the mitogenic signaling cascade interaction of Ras with Raf represents a critical step for the regulation of cell growth and differentiation. The major effector of Ras, the serine/threonine kinase Raf exists as three isoforms with different tissue distributions. We demonstrate that transient transfection of oncogenic Ha-Ras leads to a preferential activation of endogenous c-Raf-1 in HEK 293 cells as opposed to A-Raf. In vitro binding studies using purified Ras binding domains of Raf as well as in vivo bindings tests with full length molecules reveals significantly lower binding affinities of A-Raf to Ha-Ras as compared to other Raf isoforms. The Ras-binding interface of c-Raf differs from A-Raf by a conservative Arg to Lys exchange at residue 59 or 22 respectively. Mutational analysis reveals that this residue represents a point of isozyme discrimination: c-Raf-R59K binds Ha-Ras weaker than the wildtype, likewise A-Raf-K22R increases its affinity to Ha-Ras in vivo and in vitro. Differential binding affinities are reflected in downstream signaling. Immunecomplex kinase assays reveal that Ha-Ras mediated Raf activation is decreased for c-Raf-R59K and increased for A-Raf-K22R when compared to the respective wildtype forms. Thus our observations introduce a new level of isoform discrimination in Ras/Raf signaling as a functional consequence of a conservative amino acid exchange in the Ras binding domains.

Isotype-specific functions of Raf kinases

The family of Raf-protein kinases consisting of A-Raf, B-Raf, and c-Raf-1 is involved in cellular processes which regulate proliferation, differentiation, and apoptosis. Cell-culture experiments and the knockout of individual Raf genes suggested that the three Raf isoforms have overlapping and unique regulatory functions. However, it is not known how these isotype-specific functions of Raf kinases occur in the cell. Published data suggest that Raf proteins might differ in the regulation of their activation as well as in their ability to connect to downstream signaling pathways. Since Raf is part of a multiprotein complex and protein-protein interactions are important for Raf signaling, we propose that isotype-specific functions can be achieved by isotype-restricted protein binding. Recently we were able to identify candidates for such Raf-isoform-specific interaction partners.

Raf-1 and B-Raf proteins have similar regional distributions but differential subcellular localization in adult rat brain

The Raf kinases play an important and specific role in the activation of extracellular signal-regulated kinases (ERK) cascade. Beside its role in the control of proliferation and differentiation, the ERK cascade has also been implicated in neuron-specific functions. In order to gain clues on the function of Raf kinases in the adult central nervous system (CNS), we performed a comparative analysis of the distribution and subcellular localization of the different Raf kinases in rat brain with antibodies specific for the different Raf kinases. We show that B-Raf and Raf-1 proteins are present in most brain areas, whereas A-Raf is not detected. Interestingly, the two Raf proteins have an approximately similar pattern of distribution with a rostro-caudal decreasing gradient of expression. These two kinases are colocalized in neurons but they are differentially located in subcellular compartments. Raf-1 is localized mainly in the cytosolic fraction around the nucleus, whereas B-Raf is widely distributed in the cell bodies and in the neuritic processes. In addition, we demonstrated that numerous B-Raf isoforms are present in the brain. These isoforms have a differential pattern of distribution, some of them being ubiquitously expressed whereas others are localized to specific brain areas. These isoforms also have a clear differential subcellular localization, specially in Triton-insoluble fractions, but also in synaptosomal, membrane and cytosolic compartments. Altogether these results suggest that each Raf protein could have a distinct signalling regulatory function in the brain with regard to its subcellular localization.

Cot protooncoprotein activates the dual specificity kinases MEK-1 and SEK-1 and induces differentiation of PC12 cells

Mitogenic signals initiated at the plasma membrane are transmitted to the nucleus through an intricate signalling network. We identified the protooncoprotein Cot as a new component of mitogenic signalling cascades, which activates both the classic cytoplasmic cascade and the SAPK stress pathway. Wildtype and activated Cot phosphorylate and activate MEK-1 and SEK-1 in vitro. These findings are consistent with the sequence homology between Cot and the rat gene Tpl-2. Expression of oncogenic Cot in 293, NIH3T3 and PC12 cells leads to in vivo phosphorylation of endogenous c-Jun and Erk-1/2 suggesting that the serine/threonine kinase Cot functions beside c-Raf-1 and Mos as a direct activator of MEK-1. Furthermore, we have examined the biological effects of Cot on the phenotype of fibroblastic and neuronal cells. In order to test a potential c-Raf-1 dependency of Cot transformation, the effect of oncogenic Cot on Raf revertant CHP25 cells was determined. Cot could restore the transformed phenotype indicating that Cot transformation is not dependent on active c-Raf-1 and that Cot is not a target for the putative Raf inhibitor, which is presumably active in the revertant cell line. Expression of oncogenic versions of Raf as well as v-Mos leads to differentiation of PC12 cells. Cot also induces neurite outgrowth of PC12 cells. These data are consistent with the role of Cot in the classic mitogenic cascade and suggest that the simultaneously activated JNK/SAPK stress pathway has no antagonistic effects in this context.

Calcium influx activates extracellular-regulated kinase/mitogen-activated protein kinase pathway through a calmodulin-sensitive mechanism in PC12 cells

Evidence suggests that membrane depolarization is able to promote neuronal survival through a sustained, although moderate, increase in the intracellular calcium. We have used the PC12 cell line to study the possible intracellular pathways that can be activated by calcium influx. Previously, we observed that membrane depolarization-induced calcium influx was able to activate the extracellular-regulated kinase (ERK)/mitogen-activated protein kinase pathway and most of this activation was calmodulin-dependent. We demonstrated that a part of the ERK activation is due to the phosphorylation of the epidermal growth factor receptor. Here, we show that both the epidermal growth factor receptor phosphorylation and the Shc-Grb2-Ras activation are not calmodulin-modulated. Moreover, dominant negative mutant Ha-ras (Asn-17) prevents the activation on ERKs by membrane depolarization, suggesting that Ras and calmodulin are both necessaries to activate ERKs by membrane depolarization. We failed to observe any significant induction and/or modulation of the A-Raf, B-Raf or c-Raf-1 kinase activities, thus suggesting the existence of a MEK kinase different from the classical Raf kinases that directly or indirectly can be modulated by Ca2+/calmodulin.

Activity of Rap1 is regulated by bombesin, cell adhesion, and cell density in NIH3T3 fibroblasts

Rap1 and Ras are homologous GTPases that are implicated in cell proliferation and differentiation. At present, little is known about the regulation of Rap1 activity. Using a recently developed assay with activation-specific probes, we found increased activity of endogenous Rap1 in NIH3T3 cells after stimulation with the neuropeptide growth factor bombesin in a concentration- and time-dependent manner. The activity of endogenous Ras was unaffected. Analysis of putative effectors showed no activation of c-Raf-1 or B-Raf after bombesin stimulation. However, MAPK/Erk-phosphorylation and the proliferation rate was increased. In addition, Rap1 was activated during cell adhesion to coated and uncoated tissue culture plates, as well as in response to various mitogens. Surprisingly, the basal Rap1 activity was observed to be cell density-dependent, with low levels when cells were reaching confluency. The results suggest that Rap1 acts as an important mediator of mitogenic signals distinct to Ras activation.

The cyclic adenosine monophosphate-dependent protein kinase (PKA) is required for the sustained activation of mitogen-activated kinases and gene expression by nerve growth factor

Induction of neuronal differentiation of the rat pheochromocytoma cell line, PC12 cells, by nerve growth factor (NGF) requires activation of the mitogen-activated protein (MAP) kinase or extracellular signal-regulated kinase (ERK). cAMP-dependent protein kinase (protein kinase A (PKA)) also can induce differentiation of these cells. Like NGF, the ability of PKA to differentiate PC12 cells is associated with a sustained activation of ERKs. Here we show that maximal sustained activation of ERK1 by NGF requires PKA. Inhibitors of PKA partially blocked activation of ERK1 by NGF but had no effect on activation of ERK1 by EGF. Inhibition of PKA also reduced the ability of NGF and cAMP, but not EGF, to activate the transcription factor Elk-1, reduced the induction of both immediate early and late genes after NGF treatment, and blocked the nuclear translocation of ERK1 induced by NGF. We propose that PKA is an important contributor to the activation of ERK1 by NGF and is required for maximal induction of gene expression by NGF.

p21Ras downstream effectors are increased in activity or expression in mouse liver tumors but do not differ between ras-mutated and ras-wild-type lesions

Mouse liver tumors frequently harbor activating ras gene mutations. Downstream effector molecules of p21Ras include Raf-1 kinase which mediates external signals via kinase signaling pathways to nuclear transcription factors including c-Fos and c-Jun. Mouse liver tumors with differing ras-mutational status were analyzed for alterations in Ras/Raf-1 signal transduction. Tumors were characterized with respect to the presence of base substitutions in the 3 known hot-spot positions at codons 12, 13, and 61 of Ha-ras, Ki-ras, and N-ras. Ha-ras codon 61 or Ki-ras codon 13 mutations, but no N-ras mutations, were detected in 23 out of 33 tumors analyzed, while no ras-mutations were found in 10 of the tumors. There was no significant difference in the expression of p21RaS proteins between ras-mutated tumors and tumors without detectable ras mutations. To allow for determination of Raf-1 kinase activity in tumors, a sensitive and specific assay was developed for measurements with tissue homogenates. Raf-1 kinase activity was increased about four-fold in liver tumors as compared with normal liver tissue. No significant differences in kinase activity, however, were evident between ras-mutated and ras-wild-type tumors. The same was true with respect to the levels of c-fos and c-jun mRNAs. Moreover, there were no significant differences in cell division (5-bromo-2'-deoxyuridine-labeling indices) of hepatocytes from ras-mutated and ras-wild-type tumors. The similar degree of constitutive activation of the Ras/Raf-1 signaling pathway in liver tumors, with and without detectable ras mutations, suggests that other molecules within the signaling pathway may substitute for ras-mutations during oncogenic conversion of ras-wild-type hepatocytes.

The mitogen-activated protein (MAP) kinase cascade can either stimulate or inhibit DNA synthesis in primary cultures of rat hepatocytes depending upon whether its activation is acute/phasic or chronic

Bailie et al. [In Vitro Cell Dev. Biol. (1992) 28A, 621-624] reported that primary cultures of rat hepatocytes possess low affinity binding sites for nerve growth factor (NGF). NGF treatment of primary cultures of rat hepatocytes with a maximally effective concentration of NGF (20 ng/ml, 0.8 nM) caused acute phasic activation of Raf-1 and p42(MAPkinase), and a smaller sustained activation of B-Raf. The transient increase in Raf-1 and p42(MAPkinase) activity returned to baseline within approximately 30 min. NGF treatment of hepatocytes did not induce expression of cyclin dependent kinase (cdk) inhibitor proteins, but instead stimulated cdk2 activity and increased [3H]thymidine incorporation into DNA. In contrast to hepatocytes, NGF treatment of PC12 pheochromocytoma cells caused large sustained activations of B-Raf and p42(MAPkinase), and a lower phasic activation of Raf-1. The sustained activations of B-Raf and p42(MAPkinase) were for more than 5 h. Treatment of PC12 cells with NGF increased p21(Cip1/WAF-1) expression, reduced cdk2 activity and inhibited DNA synthesis, the opposite to the effects of NGF treatment of hepatocytes. However when p42(MAPkinase) was chronically activated in hepatocytes, via infection with an inducible oestrogen receptor-Raf-1 fusion protein, expression of p21(Cip-1/WAF1) and p16(INK4a) cdk inhibitor proteins increased, cdk2 activity decreased, and DNA synthesis decreased. Equally, treatment of hepatocytes with 50 mM ethanol elevated the basal activity of p42(MAPkinase) and temporally extended the ability of NGF treatment to activate p42(MAPkinase). Ethanol and NGF co-treatment increased expression of p21(Cip-1/WAF1) and p16(INK4a) cdk inhibitor proteins and decreased hepatocyte DNA synthesis. These data demonstrate that NGF can cause either acute/phasic or sustained activation of the MAP kinase cascade in different cell types. Acute activation of the MAP kinase cascade correlated with increased DNA synthesis. In contrast, sustained activation of the MAP kinase cascade correlated with increased expression of cdk inhibitor proteins, a reduction in cdk activity, and an inhibition of DNA synthesis. These data suggest a general mechanism exists where acute activation of the MAP kinase cascade promotes G1 progression/S phase entry and that chronic activation of the MAP kinase cascade inhibits this process.

Regulation of the ERK subgroup of MAP kinase cascades through G protein-coupled receptors

The extracellularly-responsive kinase (ERK) subfamily of mitogen-activated protein kinases (MAPKs) has been implicated in the regulation of cell growth and differentiation. Activation of ERKs involves a two-step protein kinase cascade lying upstream from ERK, in which the Raf family are the MAPK kinase kinases and the MEK1/MEK2 isoforms are the MAPK kinases. The linear sequence of Raf --> MEK --> ERK constitutes the ERK cascade. Although the ERK cascade is activated through growth factor-regulated receptor protein tyrosine kinases, they are also modulated through G protein-coupled receptors (GPCRs). All four G protein subfamilies (Gq/11 Gi/o, Gs and G12/13) influence the activation state of ERKs. In this review, we describe the ERK cascade and characteristics of its activation through GPCRs. We also discuss the identity of the intervening steps that may couple agonist binding at GPCRs to activation of the ERK cascade.

Listeria monocytogenes infection of HeLa cells results in listeriolysin O-mediated transient activation of the Raf-MEK-MAP kinase pathway

In this study we investigated the effect of Listeria monocytogenes infection on the activation of the Raf-MEK-MAP kinase pathway in eukaryotic host cells. HeLa cell infection with L. monocytogenes EGD resulted in a rapid, but transient, phosphorylation of the MAP kinases erk-1 and erk-2, a transient phosphorylation of the MAP kinase kinase MEK-1, and a transient activation of the MAP kinase kinase kinase Raf. In parallel to the transient phosphorylation of the MAP kinases, we detected induced expression of the MAP kinase phosphatase MKP-1. Additionally we present evidence that listeriolysin O is the inducing agent for activation of the Raf-MEK-MAP kinase pathway.