Targeting the JNK MAPK cascade for inhibition: basic science and therapeutic potential

Conditional expression of MAP kinase phosphatase-2 protects against genotoxic stress-induced apoptosis by binding and selective dephosphorylation of nuclear activated c-jun N-terminal kinase

MAP Kinase Phosphatase-2 (MKP-2) is a dual specific nuclear phosphatase which is selective for both ERK and JNK, MAP kinases implicated in the regulation of apoptosis in response to genotoxic stress. Here we report the conditional expression of MKP-2 in human embryonic kidney cells 293. We demonstrate that Flag-WT-MKP-2 is able to rescue cells from apoptotic commitment when subjected to UV-C or cisplatin treatment. We establish that upon stimulation all three major MAP kinase families (ERK, JNK and p38 MAP kinases) are activated. However, MKP-2 is surprisingly only able to deactivate JNK in vivo. Furthermore, whilst pre-treatment of cells with either the JNK inhibitor SP600125, or the MEK-1 inhibitor PD98059, also reverses UV-C and cisplatin-induced apoptosis, the anti-apoptotic effect of MKP-2 overexpression is not additive with SP600125 but is with PD098059, suggesting that MKP-2 is involved in specifically terminating JNK activity and not ERK. The inability of MKP-2 to dephosphorylate ERK in vivo is also not due to the inability of Flag-MKP-2 to bind both ERK and JNK; phosphorylated forms of each kinase are co-precipitated with both WT and CI-MKP-2. Immunofluorescence studies however demonstrate that ERK is exclusively cytosolic in origin and not translocated to the nucleus following UV-C and cisplatin treatment whilst JNK is principally nuclear. These studies demonstrate the in vivo specificity of MKP-2 for JNK and not ERK and show that nuclear-targeted JNK is involved in genotoxic stress-induced apoptosis.

Development of a Non-Radioactive, 384-Well Format Assay to Detect Inhibitors of the Mitogen-Activated Protein Kinase Kinase 4

Mitogen-activated protein kinase (MAPK) kinases (MKKs, also called MAPK/extracellular signal-regulated kinase [ERK] kinase [MEK]) are constituents of numerous signal transduction pathways involved in growth, differentiation, and stress response. One of its members, MKK4, directly phosphorylates and activates the c-Jun terminal kinases (also called stress-activated protein kinase [SAPK]) in response to stress and pro-inflammatory cytokines. Recent evidence suggest that control of MKK4 activity may provide a novel approach for the treatment of cancer or as anti-inflammatory therapy. To screen for novel low-molecular-weight inhibitors of MKK4, we established a quantitative, non-radioactive in vitro kinase assay. Human MKK4 was expressed as fusion protein with glutathione S-transferase (GST) in Escherichia coli. Co-expression of a constitutive active fragment of the MAPK/ERK kinase kinase-1 yielded active GST-MKK4 using GST-SAPK alpha-kinase-negative (KN) mutant as substrate. We determined the kinetic constants for ATP and GST-SAPK alpha-KN. The apparent Km value for GST-SAPKalpha-KN was 3.7 microM, while the apparent Km value for ATP was 0.17 microM. Staurosporine inhibited GST-MKK4 with an IC50 of 70 nM. The kinase assay was adapted to a 384-well non-radioactive format. After the kinase reaction the phosphorylated product was captured onto a streptavidin-coated microtiter plate, and phosphorylation was detected with a europium-labeled anti-phosphotyrosine antibody, which allowed time-resolved fluorescence measurement.

Identification of promoters bound by c-Jun/ATF2 during rapid large-scale gene activation following genotoxic stress

The NH2-terminal Jun kinases (JNKs) function in diverse roles through phosphorylation and activation of AP-1 components including ATF2 and c-Jun. However, the genes that mediate these processes are poorly understood. A model phenotype characterized by rapid activation of Jun kinase and enhanced DNA repair following cisplatin treatment was examined using chromatin immunoprecipitation with antibodies against ATF2 and c-Jun or their phosphorylated forms and hybridization to promoter arrays. Following genotoxic stress, we identified 269 genes whose promoters are bound upon phosphorylation of ATF2 and c-Jun. Binding did not occur following treatment with transplatin or the JNK inhibitor SP600125 or JNK-specific siRNA. Of 89 known DNA repair genes represented on the array, 23 are specifically activated by cisplatin treatment within 3-6 hr. Thus, the genotoxic stress response occurs at least partly via activation of ATF2 and c-Jun, leading to large-scale coordinate gene expression dominated by genes of DNA repair.

Beta-defensin-2 expression is regulated by TLR signaling in intestinal epithelial cells

The intestinal epithelium serves as a barrier to the intestinal flora. In response to pathogens, intestinal epithelial cells (IEC) secrete proinflammatory cytokines. To aid in defense against bacteria, IEC also secrete antimicrobial peptides, termed defensins. The aim of our studies was to understand the role of TLR signaling in regulation of beta-defensin expression by IEC. The effect of LPS and peptidoglycan on beta-defensin-2 expression was examined in IEC lines constitutively or transgenically expressing TLRs. Regulation of beta-defensin-2 was assessed using promoter-reporter constructs of the human beta-defensin-2 gene. LPS and peptidoglycan stimulated beta-defensin-2 promoter activation in a TLR4- and TLR2-dependent manner, respectively. A mutation in the NF-kappaB or AP-1 site within the beta-defensin-2 promoter abrogated this response. In addition, inhibition of Jun kinase prevents up-regulation of beta-defensin-2 protein expression in response to LPS. IEC respond to pathogen-associated molecular patterns with expression of the antimicrobial peptide beta-defensin-2. This mechanism may protect the intestinal epithelium from pathogen invasion and from potential invaders among the commensal flora.

Signaling transduction: target in osteoarthritis

PURPOSE OF REVIEW

The pathophysiology of osteoarthritis is the result of an imbalance between anabolic and catabolic pathways. This imbalance is the result of the activation of joint cells by inflammatory mediators, matrix components, and mechanical stress. All these mediators act through specific receptors that transmit the signals to the nucleus to activate the transcription of matrix metalloproteinases and inflammatory genes. Targeting these signaling pathways in osteoarthritis is considered a novel approach to modulate this imbalance.

RECENT FINDINGS

Although many signaling pathways are necessary for physiologic cell life, it is now well established that a few are more specifically induced in an inflammatory environment. In osteoarthritis, the nuclear factor-kappaB and mitogen-activated protein kinase pathways have been shown to play a predominant role in the expression of metalloproteinases and inflammatory genes and proteins. Also involved in the activation of osteoarthritic cells are other molecules interacting with one or several signaling pathways, such as nitric oxide, peroxisome proliferator-activated receptor-gamma ligands, or C/EBP transcriptional factors. Based on this knowledge, specific inhibitors for some of these signaling pathways have been designed and include p38 mitogen-activated protein kinase or nuclear factor-kappaB inhibitors. Experimental studies evaluating cartilage degradation in arthritis models are promising, although fewer have been done specifically in osteoarthritis models.

SUMMARY

Targeting signaling pathways in osteoarthritis did not seem feasible a few years ago because of the complexity of the multiple intracellular pathways, mainly physiologic, defined by a high degree of redundancy and cross-talk. However, important advances in the knowledge of chondrocyte and synoviocyte signaling in osteoarthritis have been achieved in recent years and suggest that inhibitors of specific signaling pathways could shortly provide effective treatments for this disease.

Reverse two-hybrid screening identifies residues of JNK required for interaction with the kinase interaction motif of JNK-interacting protein-1

The development of specific inhibitors for the c-Jun N-terminal kinase (JNK) family of mitogen-activated protein kinases (MAPKs) has been a recent research focus because of the association of JNK with cell death in conditions such as stroke and neurodegeneration. We have demonstrated previously the presence of critical inhibitory residues within an 11-mer peptide (TI-JIP) based on the sequence of JNK-interacting protein-1 (JIP-1). However, the corresponding region of JNK bound by this JIP-1-based peptide was unknown. To identify this region, we used a novel reverse two-hybrid approach with TI-JIP as bait. We screened a library of JNK1 mutants that had been generated by random PCR mutagenesis and found three mutants of JNK1 that failed to interact with TI-JIP. The mutations in JNK1 were L131R, R309W, and Y320H. Of these mutated residues, Leu-131 and Tyr-320 were located on a common face of the JNK protein close to other residues implicated previously in the interactions of MAPKs with substrates, phosphatases, and scaffolds. To test whether these JNK1 mutants were thus affected in their regulation, we evaluated their activation in mammalian cells in response to hyperosmolarity or cotransfection with a constitutively active upstream kinase or their direct phosphorylation by either MAPK kinase (MKK)4 or MKK7. In each situation, all three JNK mutants were not activated or phosphorylated to the same level as wild-type JNK. Therefore, the results of our unbiased reverse two-hybrid screening approach have identified residues of JNK responsible for binding JIP-1-based peptides as well as MKK4 or MKK7.