Raf-1 and B-Raf promote protein kinase C θ interaction with BAD

SIRT4 as a novel interactor and candidate suppressor of C-RAF kinase in MAPK signaling

Cellular responses leading to development, proliferation, and differentiation depend on RAF/MEK/ERK signaling, which integrates and amplifies signals from various stimuli for downstream cellular responses. C-RAF activation has been reported in many types of tumor cell proliferation and developmental disorders, necessitating the discovery of potential C-RAF protein regulators. Here, we identify a novel and specific protein interaction between C-RAF among the RAF kinase paralogs, and SIRT4 among the mitochondrial sirtuin family members SIRT3, SIRT4, and SIRT5. Structurally, C-RAF binds to SIRT4 through the N-terminal cysteine-rich domain, whereas SIRT4 predominantly requires the C-terminus for full interaction with C-RAF. Interestingly, SIRT4 specifically interacts with C-RAF in a pre-signaling inactive (serine 259-phosphorylated) state. Consistent with this finding, the expression of SIRT4 in HEK293 cells results in an up-regulation of pS259-C-RAF levels and a concomitant reduction in MAPK signaling as evidenced by strongly decreased phospho-ERK signals. Thus, we propose an additional extra-mitochondrial function of SIRT4 as a cytosolic tumor suppressor of C-RAF-MAPK signaling, besides its metabolic tumor suppressor role of glutamate dehydrogenase and glutamate levels in mitochondria.

Hidden Targets in RAF Signalling Pathways to Block Oncogenic RAS Signalling

Oncogenic RAS (Rat sarcoma) mutations drive more than half of human cancers, and RAS inhibition is the holy grail of oncology. Thirty years of relentless efforts and harsh disappointments have taught us about the intricacies of oncogenic RAS signalling that allow us to now get a pharmacological grip on this elusive protein. The inhibition of effector pathways, such as the RAF-MEK-ERK pathway, has largely proven disappointing. Thus far, most of these efforts were aimed at blocking the activation of ERK. Here, we discuss RAF-dependent pathways that are regulated through RAF functions independent of catalytic activity and their potential role as targets to block oncogenic RAS signalling. We focus on the now well documented roles of RAF kinase-independent functions in apoptosis, cell cycle progression and cell migration.

Targeting Aberrant RAS/RAF/MEK/ERK Signaling for Cancer Therapy

The RAS/RAF/MEK/ERK (MAPK) signaling cascade is essential for cell inter- and intra-cellular communication, which regulates fundamental cell functions such as growth, survival, and differentiation. The MAPK pathway also integrates signals from complex intracellular networks in performing cellular functions. Despite the initial discovery of the core elements of the MAPK pathways nearly four decades ago, additional findings continue to make a thorough understanding of the molecular mechanisms involved in the regulation of this pathway challenging. Considerable effort has been focused on the regulation of RAF, especially after the discovery of drug resistance and paradoxical activation upon inhibitor binding to the kinase. RAF activity is regulated by phosphorylation and conformation-dependent regulation, including auto-inhibition and dimerization. In this review, we summarize the recent major findings in the study of the RAS/RAF/MEK/ERK signaling cascade, particularly with respect to the impact on clinical cancer therapy.

Activated k-ras, but not h-ras or N-ras, regulates brain neural stem cell proliferation in a raf/rb-dependent manner

Neural stem cells (NSCs) give rise to all the major cell types in the brain, including neurons, oligodendrocytes, and astrocytes. However, the intracellular signaling pathways that govern brain NSC proliferation and differentiation have been incompletely characterized to date. Since some neurodevelopmental brain disorders (Costello syndrome and Noonan syndrome) are caused by germline activating mutations in the RAS genes, Ras small GTPases are likely critical regulators of brain NSC function. In the mammalian brain, Ras exists as three distinct molecules (H-Ras, K-Ras, and N-Ras), each with different subcellular localizations, downstream signaling effectors, and biological effects. Leveraging a novel series of conditional-activated Ras molecule-expressing genetically engineered mouse strains, we demonstrate that activated K-Ras, but not H-Ras or N-Ras, expression increases brain NSC growth in a Raf-dependent, but Mek-independent, manner. Moreover, we show that activated K-Ras regulation of brain NSC proliferation requires Raf binding and suppression of retinoblastoma (Rb) function. Collectively, these observations establish tissue-specific differences in activated Ras molecule regulation of brain cell growth that operate through a noncanonical mechanism.

B-Raf regulation of integrin α4β1-mediated resistance to shear stress through changes in cell spreading and cytoskeletal association in T cells

The regulation of integrin-mediated adhesion is of vital importance to adaptive and innate immunity. Integrins are versatile proteins and mediate T cell migration and trafficking by binding to extracellular matrix or other cells as well as initiating intracellular signaling cascades promoting survival or activation. The MAPK pathway is known to be downstream from integrins and to regulate survival, differentiation, and motility. However, secondary roles for canonical MAPK pathway members are being discovered. We show that chemical inhibition of RAF by sorafenib or shRNA-mediated knockdown of B-Raf reduces T cell resistance to shear stress to α4β1 integrin ligands vascular cell adhesion molecule 1 (VCAM-1) and fibronectin, whereas inhibition of MEK/ERK by U0126 had no effect. Microscopy showed that RAF inhibition leads to significant inhibition of T cell spreading on VCAM-1. The association of α4β1 integrin with the actin cytoskeleton was shown to be dependent on B-Raf activity or expression, whereas α4β1 integrin affinity for soluble VCAM-1 was not. These effects were shown to be specific for α4β1 integrin and not other integrins, such as α5β1 or LFA-1, or a variety of membrane proteins. We demonstrate a novel role for B-Raf in the selective regulation of α4β1 integrin-mediated adhesion.

It takes two to tango--signalling by dimeric Raf kinases

Raf kinases function downstream of Ras proteins to activate the MEK-ERK pathway which is deregulated in a large number of human cancers. Raf inhibitors are clinically highly effective for the treatment of cancer and melanoma in particular, but have unexpected side effects that include a paradoxical activation of the ERK pathway. These effects seem to be related to the heterodimerization of Raf-1 and B-Raf kinases. Here, we discuss the role of Raf dimerization as part of the physiological activation mechanism of Raf kinases, the mechanism of Raf dimerization induced by drugs, and the implications of dimerization for drug therapies targeting Raf kinases.

AMP-activated protein kinase mediates T cell activation-induced expression of FasL and COX-2 via protein kinase C theta-dependent pathway in human Jurkat T leukemia cells

AMP-activated protein kinase (AMPK), an important regulator of energy homeostasis, is known to be activated during T cell activation. T cell activation by T cell receptor (TCR) engagement or its pharmacological mimics, PMA plus ionomycin (PMA/Io), induces immunomodulatory FasL and cyclooxygenase-2 (COX-2) expression. In this study, we examined the role and mechanisms of AMPK in PMA/Io-induced expression of FasL and COX-2 in Jurkat T human leukemic cells. Inhibition of AMPK by a pharmacological agent, compound C, or AMPKα1 siRNA suppressed expression of FasL and COX-2 mRNAs and proteins in PMA/Io-activated Jurkat cells. It also reduced secretion of FasL protein and prostaglandin E2, a main product of COX-2, in Jurkat cells and peripheral blood lymphocytes activated with PMA/Io or monoclonal anti-CD3 plus anti-CD28. Consistently, inhibition of AMPK blocked promoter activities of FasL and COX-2 in activated Jurkat cells. As protein kinase C theta (PKCθ) is a central molecule for TCR signaling, we examined any possible cross-talk between AMPK and PKCθ in activated T cells. Of particular importance, we found that inhibition of AMPK blocked phosphorylation and activation of PKCθ, suggesting that AMPK is an upstream kinase of PKCθ. Moreover, we showed that AMPK was directly associated with PKCθ and phosphorylated Thr538 of PKCθ in PMA/Io-stimulated Jurkat cells. We also showed that inhibition of PKCθ by rottlerin or dominant negative PKCθ reduced AMPK-mediated transcriptional activation of NF-AT and AP-1 in activated Jurkat cells. Taken together, these results suggest that AMPK regulates expression of FasL and COX-2 via the PKCθ and NF-AT and AP-1 pathways in activated Jurkat cells.

PKCθ activation in pancreatic acinar cells by gastrointestinal hormones/neurotransmitters and growth factors is needed for stimulation of numerous important cellular signaling cascades

The novel PKCθ isoform is highly expressed in T-cells, brain and skeletal muscle and originally thought to have a restricted distribution. It has been extensively studied in T-cells and shown to be important for apoptosis, T-cell activation and proliferation. Recent studies showed its presence in other tissues and importance in insulin signaling, lung surfactant secretion, intestinal barrier permeability, platelet and mast-cell functions. However, little information is available for PKCθ activation by gastrointestinal (GI) hormones/neurotransmitters and growth factors. In the present study we used rat pancreatic acinar cells to explore their ability to activate PKCθ and the possible interactions with important cellular mediators of their actions. Particular attention was paid to cholecystokinin (CCK), a physiological regulator of pancreatic function and important in pathological processes affecting acinar function, like pancreatitis. PKCθ-protein/mRNA was present in the pancreatic acini, and T538-PKCθ phosphorylation/activation was stimulated only by hormones/neurotransmitters activating phospholipase C. PKCθ was activated in time- and dose-related manner by CCK, mediated 30% by high-affinity CCK(A)-receptor activation. CCK stimulated PKCθ translocation from cytosol to membrane. PKCθ inhibition (by pseudostrate-inhibitor or dominant negative) inhibited CCK- and TPA-stimulation of PKD, Src, RafC, PYK2, p125(FAK) and IKKα/β, but not basal/stimulated enzyme secretion. Also CCK- and TPA-induced PKCθ activation produced an increment in PKCθ's direct association with AKT, RafA, RafC and Lyn. These results show for the first time the PKCθ presence in pancreatic acinar cells, its activation by some GI hormones/neurotransmitters and involvement in important cell signaling pathways mediating physiological responses (enzyme secretion, proliferation, apoptosis, cytokine expression, and pathological responses like pancreatitis and cancer growth).

Raf family kinases: old dogs have learned new tricks

First identified in the early 1980s as retroviral oncogenes, the Raf proteins have been the objects of intense research. The discoveries 10 years later that the Raf family members (Raf-1, B-Raf, and A-Raf) are bona fide Ras effectors and upstream activators of the ubiquitous ERK pathway increased the interest in these proteins primarily because of the central role that this cascade plays in cancer development. The important role of Raf in cancer was corroborated in 2002 with the discovery of B-Raf genetic mutations in a large number of tumors. This led to intensified drug development efforts to target Raf signaling in cancer. This work yielded not only recent clinical successes but also surprising insights into the regulation of Raf proteins by homodimerization and heterodimerization. Surprising insights also came from the hunt for new Raf targets. Although MEK remains the only widely accepted Raf substrate, new kinase-independent roles for Raf proteins have emerged. These include the regulation of apoptosis by suppressing the activity of the proapoptotic kinases, ASK1 and MST2, and the regulation of cell motility and differentiation by controlling the activity of Rok-α. In this review, we discuss the regulation of Raf proteins and their role in cancer, with special focus on the interacting proteins that modulate Raf signaling. We also describe the new pathways controlled by Raf proteins and summarize the successes and failures in the development of efficient anticancer therapies targeting Raf. Finally, we also argue for the necessity of more systemic approaches to obtain a better understanding of how the Ras-Raf signaling network generates biological specificity.

T-cell fate and function: PKC-theta and beyond

The serine/threonine-specific protein kinase C-theta (PKC-theta) is a core component of the immunological synapse that was shown in vitro to play a central role in the activation of T cells after T cell receptor (TCR) and co-stimulatory molecule engagement. In recent years, a series of in vivo studies have shown that the situation is far more complex; specifically, PKC-theta signaling is differentially required for Th1, Th2, Th17 and CD8+ cytotoxic T-cell responses. These studies highlight the combination of signals that directly regulate T-cell differentiation and effector responses. In this review, we highlight recent in vivo studies investigating PKC-theta function and discuss this in the context of how the integration of extrinsic signals determines T cell fate and function.

JWA is required for arsenic trioxide induced apoptosis in HeLa and MCF-7 cells via reactive oxygen species and mitochondria linked signal pathway

Arsenic trioxide, emerging as a standard therapy for refractory acute promyelocytic leukemia, induces apoptosis in a variety of malignant cell lines. JWA, a novel retinoic acid-inducible gene, is known to be involved in apoptosis induced by various agents, for example, 12-O-tetradecanoylphorbol 13-acetate, N-4-hydroxy-phenyl-retinamide and arsenic trioxide. However, the molecular mechanisms underlying how JWA gene is functionally involved in apoptosis remain largely unknown. Herein, our studies demonstrated that treatment of arsenic trioxide produced apoptosis in HeLa and MCF-7 cells in a dose-dependent manner and paralleled with increased JWA expression. JWA expression was dependent upon generation of intracellular reactive oxygen species induced by arsenic trioxide. Knockdown of JWA attenuated arsenic trioxide induced apoptosis, and was accompanied by significantly reduced activity of caspase-9, enhanced Bad phosphorylation and inhibited MEK1/2, ERK1/2 and JNK phosphorylations. Arsenic trioxide induced loss of mitochondrial transmembrane potential was JWA-dependent. These findings suggest that JWA may serve as a pro-apoptotic molecule to mediate arsenic trioxide triggered apoptosis via a reactive oxygen species and mitochondria-associated signal pathway.

Alpha-1-antitrypsin mutant Z protein content in individual hepatocytes correlates with cell death in a mouse model

Alpha-1-antitrypsin (a1AT) deficiency is caused by homozygosity for the a1AT mutant Z gene and occurs in 1 in 2000 births. The Z mutation confers an abnormal conformation on the protein, resulting in an accumulation within the endoplasmic reticulum of hepatocytes rather than appropriate secretion. The accumulation of the mutant protein is strikingly heterogeneous within the liver. Homozygous ZZ children and adults have an increased risk of chronic liver disease, which is thought to result from this variable intracellular accumulation of the a1AT mutant Z protein. Previous reports have suggested that autophagy, mitochondrial injury, apoptosis, and other pathways may be involved in the mechanism of hepatocyte injury, although the interplay of these mechanisms in vivo is unclear. In this study, we examine a well-characterized in vivo model of a1AT mutant Z liver injury, the PiZ mouse, to better understand the pathways involved in this disease. The results show an increase in the stimulation of the apoptotic cascade in hepatocytes, the magnitude of which strongly correlates to the absolute amount of the a1AT mutant Z protein accumulated within the individual cell. Increases in apoptotic regulatory proteins are also detected.

CONCLUSION

These data, combined with previous work, permit for the first time the construction of a hypothetical hepatocellular injury cascade for this disease involving mitochondrial injury, caspase activation, and apoptosis, which takes into account the heterogeneous nature of the mutant Z protein accumulation within the liver. Further development of this hypothetical cascade will focus future research on this and other metabolic liver diseases.