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The role of JNK in the development of hepatocellular carcinoma

Wed, 03/09/2016 - 1:03pm

The cJun NH(2)-terminal kinase (JNK) signal transduction pathway has been implicated in the growth of carcinogen-induced hepatocellular carcinoma. However, the mechanism that accounts for JNK-regulated tumor growth is unclear. Here we demonstrate that compound deficiency of the two ubiquitously expressed JNK isoforms (JNK1 and JNK2) in hepatocytes does not prevent hepatocellular carcinoma development. Indeed, JNK deficiency in hepatocytes increased the tumor burden. In contrast, compound JNK deficiency in hepatocytes and nonparenchymal cells reduced both hepatic inflammation and tumorigenesis. These data indicate that JNK plays a dual role in the development of hepatocellular carcinoma. JNK promotes an inflammatory hepatic environment that supports tumor development, but also functions in hepatocytes to reduce tumor development.

Requirement of c-Jun NH(2)-terminal kinase for Ras-initiated tumor formation

Wed, 03/09/2016 - 1:03pm

The c-Jun NH(2)-terminal kinase (JNK) signal transduction pathway causes increased gene expression mediated, in part, by members of the activating transcription factor protein (AP1) group. JNK is therefore implicated in the regulation of cell growth and cancer. To test the role of JNK in Ras-induced tumor formation, we examined the effect of compound ablation of the ubiquitously expressed genes Jnk1 plus Jnk2. We report that JNK is required for Ras-induced transformation of p53-deficient primary cells in vitro. Moreover, JNK is required for lung tumor development caused by mutational activation of the endogenous KRas gene in vivo. Together, these data establish that JNK plays a key role in Ras-induced tumorigenesis.

JNK regulates FoxO-dependent autophagy in neurons

Wed, 03/09/2016 - 1:03pm

The cJun N-terminal kinase (JNK) signal transduction pathway is implicated in the regulation of neuronal function. JNK is encoded by three genes that play partially redundant roles. Here we report the creation of mice with targeted ablation of all three Jnk genes in neurons. Compound JNK-deficient neurons are dependent on autophagy for survival. This autophagic response is caused by FoxO-induced expression of Bnip3 that displaces the autophagic effector Beclin-1 from inactive Bcl-XL complexes. These data identify JNK as a potent negative regulator of FoxO-dependent autophagy in neurons.

AKAP-Lbc enhances cyclic AMP control of the ERK1/2 cascade

Wed, 03/09/2016 - 1:02pm

Mitogen-activated protein kinase (MAPK) cascades propagate a variety of cellular activities. Processive relay of signals through RAF-MEK-ERK modulates cell growth and proliferation. Signalling through this ERK cascade is frequently amplified in cancers, and drugs such as sorafenib (which is prescribed to treat renal and hepatic carcinomas) and PLX4720 (which targets melanomas) inhibit RAF kinases. Natural factors that influence ERK1/2 signalling include the second messenger cyclic AMP. However, the mechanisms underlying this cascade have been difficult to elucidate. We demonstrate that the A-kinase-anchoring protein AKAP-Lbc and the scaffolding protein kinase suppressor of Ras (KSR-1) form the core of a signalling network that efficiently relay signals from RAF, through MEK, and on to ERK1/2. AKAP-Lbc functions as an enhancer of ERK signalling by securing RAF in the vicinity of MEK1 and synchronizing protein kinase A (PKA)-mediated phosphorylation of Ser 838 on KSR-1. This offers mechanistic insight into cAMP-responsive control of ERK signalling events.

Keep the 'phospho' on MAPK, be happy

Wed, 03/09/2016 - 1:02pm

Depression ruins the lives of millions of people, causing dysphoria and anguish. New findings in rodents and human brain shed light on the mechanisms of this disease, uncovering a phosphatase as a new target to treat depressive behaviors. Comment on: A negative regulator of MAP kinase causes depressive behavior. [Nat Med. 2010]

Hippocampal c-Jun-N-terminal kinases serve as negative regulators of associative learning

Wed, 03/09/2016 - 1:02pm

In the adult mouse, signaling through c-Jun N-terminal kinases (JNKs) links exposure to acute stress to various physiological responses. Inflammatory cytokines, brain injury and ischemic insult, or exposure to psychological acute stressors induce activation of hippocampal JNKs. Here we report that exposure to acute stress caused activation of JNKs in the hippocampal CA1 and CA3 subfields, and impaired contextual fear conditioning. Conversely, intrahippocampal injection of JNKs inhibitors sp600125 (30 mum) or D-JNKI1 (8 mum) reduced activity of hippocampal JNKs and rescued stress-induced deficits in contextual fear. In addition, intrahippocampal administration of anisomycin (100 mug/mul), a potent JNKs activator, mimicked memory-impairing effects of stress on contextual fear. This anisomycin-induced amnesia was abolished after cotreatment with JNKs selective inhibitor sp600125 without affecting anisomycin's ability to effectively inhibit protein synthesis as measured by c-Fos immunoreactivity. We also demonstrated milder and transient activation of the JNKs pathway in the CA1 subfield of the hippocampus during contextual fear conditioning and an enhancement of contextual fear after pharmacological inhibition of JNKs under baseline conditions. Finally, using combined biochemical and transgenic approaches with mutant mice lacking different members of the JNK family (Jnk1, Jnk2, and Jnk3), we provided evidence that JNK2 and JNK3 are critically involved in stress-induced deficit of contextual fear, while JNK1 mainly regulates baseline learning in this behavioral task. Together, these results support the possibility that hippocampal JNKs serve as a critical molecular regulator in the formation of contextual fear.

Requirement of JIP1-mediated c-Jun N-terminal kinase activation for obesity-induced insulin resistance

Wed, 03/09/2016 - 1:01pm

The c-Jun NH(2)-terminal kinase (JNK) interacting protein 1 (JIP1) has been proposed to act as a scaffold protein that mediates JNK activation. However, recent studies have implicated JIP1 in multiple biochemical processes. Physiological roles of JIP1 that are related to the JNK scaffold function of JIP1 are therefore unclear. To test the role of JIP1 in JNK activation, we created mice with a germ line point mutation in the Jip1 gene (Thr(103) replaced with Ala) that selectively blocks JIP1-mediated JNK activation. These mutant mice exhibit a severe defect in JNK activation caused by feeding of a high-fat diet. The loss of JIP1-mediated JNK activation protected the mutant mice against obesity-induced insulin resistance. We conclude that JIP1-mediated JNK activation plays a critical role in metabolic stress regulation of the JNK signaling pathway.

The p38 MAPK pathway is essential for skeletogenesis and bone homeostasis in mice

Wed, 03/09/2016 - 1:01pm

Nearly every extracellular ligand that has been found to play a role in regulating bone biology acts, at least in part, through MAPK pathways. Nevertheless, much remains to be learned about the contribution of MAPKs to osteoblast biology in vivo. Here we report that the p38 MAPK pathway is required for normal skeletogenesis in mice, as mice with deletion of any of the MAPK pathway member-encoding genes MAPK kinase 3 (Mkk3), Mkk6, p38a, or p38b displayed profoundly reduced bone mass secondary to defective osteoblast differentiation. Among the MAPK kinase kinase (MAP3K) family, we identified TGF-beta-activated kinase 1 (TAK1; also known as MAP3K7) as the critical activator upstream of p38 in osteoblasts. Osteoblast-specific deletion of Tak1 resulted in clavicular hypoplasia and delayed fontanelle fusion, a phenotype similar to the cleidocranial dysplasia observed in humans haploinsufficient for the transcription factor runt-related transcription factor 2 (Runx2). Mechanistic analysis revealed that the TAK1-MKK3/6-p38 MAPK axis phosphorylated Runx2, promoting its association with the coactivator CREB-binding protein (CBP), which was required to regulate osteoblast genetic programs. These findings reveal an in vivo function for p38beta and establish that MAPK signaling is essential for bone formation in vivo. These results also suggest that selective p38beta agonists may represent attractive therapeutic agents to prevent bone loss associated with osteoporosis and aging.

Distinct roles of c-Jun N-terminal kinase isoforms in neurite initiation and elongation during axonal regeneration

Wed, 03/09/2016 - 1:00pm

c-Jun N-terminal kinases (JNKs) (comprising JNK1-3 isoforms) are members of the MAPK (mitogen-activated protein kinase) family, activated in response to various stimuli including growth factors and inflammatory cytokines. Their activation is facilitated by scaffold proteins, notably JNK-interacting protein-1 (JIP1). Originally considered to be mediators of neuronal degeneration in response to stress and injury, recent studies support a role of JNKs in early stages of neurite outgrowth, including adult axonal regeneration. However, the function of individual JNK isoforms, and their potential effector molecules, remained unknown. Here, we analyzed the role of JNK signaling during axonal regeneration from adult mouse dorsal root ganglion (DRG) neurons, combining pharmacological JNK inhibition and mice deficient for each JNK isoform and for JIP1. We demonstrate that neuritogenesis is delayed by lack of JNK2 and JNK3, but not JNK1. JNK signaling is further required for sustained neurite elongation, as pharmacological JNK inhibition resulted in massive neurite retraction. This function relies on JNK1 and JNK2. Neurite regeneration of jip1(-/-) DRG neurons is affected at both initiation and extension stages. Interestingly, activated JNKs (phospho-JNKs), as well as JIP1, are also present in the cytoplasm of sprouting or regenerating axons, suggesting a local action on cytoskeleton proteins. Indeed, we have shown that JNK1 and JNK2 regulate the phosphorylation state of microtubule-associated protein MAP1B, whose role in axonal regeneration was previously characterized. Moreover, lack of MAP1B prevents neurite retraction induced by JNK inhibition. Thus, signaling by individual JNKs is differentially implicated in the reorganization of the cytoskeleton, and neurite regeneration.

cJun NH2-terminal kinase 1 (JNK1): roles in metabolic regulation of insulin resistance

Wed, 03/09/2016 - 1:00pm

The cJun NH(2)-terminal kinase isoform JNK1 is implicated in the mechanism of obesity-induced insulin resistance. Feeding a high-fat diet causes activation of the JNK1 signaling pathway, insulin resistance, and obesity in mice. Germ-line ablation of Jnk1 prevents both diet-induced obesity and insulin resistance. Genetic analysis indicates that the effects of JNK1 on insulin resistance can be separated from effects of JNK1 on obesity. Emerging research indicates that JNK1 plays multiple roles in the regulation of insulin resistance, including altered gene expression, hormone/cytokine production, and lipid metabolism. Together, these studies establish JNK1 as a potential pharmacological target for the development of drugs that might be useful for the treatment of insulin resistance, metabolic syndrome, and type 2 diabetes.

JNK-mediated phosphorylation of Cdc25C regulates cell cycle entry and G(2)/M DNA damage checkpoint

Wed, 03/09/2016 - 1:00pm

c-Jun NH(2)-terminal Kinases (JNKs) play a central role in the cellular response to a wide variety of stress signals. After their activation, JNKs induce phosphorylation of substrates, which control proliferation, migration, survival, and differentiation. Recent studies suggest that JNKs may also play a role in cell cycle control, although the underlying mechanisms are largely unexplored. Here we show that JNK directly phosphorylates Cdc25C at serine 168 during G(2) phase of the cell cycle. Cdc25C phosphorylation by JNK negatively regulates its phosphatase activity and thereby Cdk1 activation, enabling a timely control of mitosis onset. Unrestrained phosphorylation by JNK, as obtained by a cell cycle-stabilized form of JNK or as seen in some human tumors, results in aberrant cell cycle progression. Additionally, UV irradiation-induced G(2)/M checkpoint requires inactivation of Cdc25C by JNK phosphorylation. JNK phosphorylation of Cdc25C as well as Cdc25A establishes a novel link between stress signaling and unperturbed cell cycle and checkpoint pathways.