Requirement of mitogen-activated protein kinase kinase 3 (MKK3) for activation of p38alpha and p38delta MAPK isoforms by TGF-beta 1 in murine mesangial cells

Transforming growth factor-beta1 (TGF-beta1) is a potent inducer of extracellular matrix (ECM) synthesis that leads to renal fibrosis. Intracellular signaling mechanisms involved in this process remain incompletely understood. Mitogen-activated protein kinase (MAPK) is a major stress signal-transducing pathway, and we have previously reported activation of p38 MAPK by TGF-beta1 in rat mesangial cells and its role in the stimulation of pro-alpha1(I) collagen. In this study, we further investigated the mechanism of p38 MAPK activation by TGF-beta1 and the role of MKK3, an upstream MAPK kinase of p38 MAPK, by examining the effect of targeted disruption of the Mkk3 gene. We first isolated glomerular mesangial cells from MKK3-null (Mkk3-/-) and wild-type (Mkk3+/+) control mice. Treatment with TGF-beta1 induced rapid phosphorylation of MKK3 as well as p38 MAPK within 15 min in cultured wild-type (Mkk3+/+) mouse mesangial cells. In contrast, TGF-beta1 failed to induce phosphorylation of either MKK3 or p38 MAPK in MKK3-deficient (Mkk3-/-) mouse mesangial cells, indicating that MKK3 is required for TGF-beta1-induced p38 MAPK activation. TGF-beta1 selectively activated the p38 MAPK isoforms p38alpha and p38delta in wild-type (Mkk3+/+) mesangial cells, but not in MKK3-deficient (Mkk3-/-) mesangial cells. Thus, activation of p38alpha and p38delta is dependent on the activation of upstream MKK3 by TGF-beta1. Furthermore, MKK3 deficiency resulted in a selective disruption of TGF-beta1-stimulated up-regulation of pro-alpha1(I) collagen expression but not TGF-beta1 induction of fibronectin and PAI-1. These data demonstrate that the MKK3 is a critical component of the TGF-beta1 signaling pathway, and its activation is required for subsequent p38alpha and p38delta MAPK activation and collagen stimulation by TGF-beta1.     

Physiological roles of MKK4 and MKK7: insights from animal models

c-Jun NH2-terminal protein kinase (JNK) is a mitogen-activated protein kinase (MAPK) involved in the regulation of numerous physiological processes during development and in response to stress. Its activity is increased upon phosphorylation by the MAPK kinases, MKK4 and MKK7. Similar to the early embryonic death of mice caused by the targeted deletion of the jnk genes, mice lacking mkk4 or mkk7 die before birth. The inability of MKK4 and MKK7 to compensate for each other's functions in vivo is consistent with their synergistic effect in mediating JNK activation. However, the phenotypic analysis of the mutant mouse embryos indicates that MKK4 and MKK7 have specific roles that may be due to their selective regulation by extracellular stimuli and their distinct tissue distribution. MKK4 and MKK7 also have different biochemical properties. For example, whereas MKK4 can activate p38 MAPK, MKK7 functions as a specific activator of JNK. Here we summarize the studies that have shed light on the mechanism of activation of MKK4 and MKK7 and on their physiological functions.     

The interaction of p38 with its upstream kinase MKK6

Mitogen‐activated protein kinase (MAPK; p38, ERK, and JNK) cascades are evolutionarily conserved signaling pathways that regulate the cellular response to a variety of extracellular stimuli, such as growth factors and interleukins. The MAPK p38 is activated by its specific upstream MAPK kinases, MKK6 and MKK3. However, a comprehensive molecular understanding of how these cognate upstream kinases bind and activate p38 is still missing. Here, we combine NMR spectroscopy and isothermal titration calorimetry to define the binding interface between full‐length MKK6 and p38. It was shown that p38 engages MKK6 not only via its hydrophobic docking groove, but also influences helix αF, a secondary structural element that plays a key role in organizing the kinase core. It was also shown that, unlike MAPK phosphatases, the p38 conserved docking (CD) site is much less affected by MKK6 binding. Finally, it was demonstrated that these interactions with p38 are conserved independent of the MKK6 activation state. Together, the results revealed differences between specificity markers of p38 regulation by upstream kinases, which do not effectively engage the CD site, and downstream phosphatases, which require the CD site for productive binding.     

Differential involvement of p38 mitogen-activated protein kinase kinases MKK3 and MKK6 in T-cell apoptosis

The p38 mitogen-activated protein kinase (p38MAPK) is activated in response to various stimuli, including cellular stress, inflammatory cytokines and cell surface receptors. The activation of p38MAPK is predominantly mediated by the two upstream MAPK kinases MKK3 and MKK6. To study the role of the p38MAPK pathway in vivo, we generated Mkk6^–/– mice. We examined whether T-cell apoptosis is affected in these mice and in our previously reported Mkk3^–/– mice. Strikingly, in vivo deletion of double positive thymocytes in Mkk6^–/– mice was impaired, whereas Mkk3^–/– mice showed no apparent abnormality. Conversely, CD4⁺T cells from Mkk3^–/– but not from Mkk6^–/– mice were resistant to activation-induced cell death and cytokine-withdrawal-induced apoptosis. In peripheral CD4⁺T cells, MKK3 is induced upon stimulation, whereas MKK6 is downregulated. These results suggest a novel mechanism regulating T-cell apoptosis differentially through the p38MAPK pathway by MKK3 and MKK6.     

The Parkinson disease-associated protein kinase LRRK2 exhibits MAPKKK activity and phosphorylates MKK3/6 and MKK4/7, in vitro

Autosomal dominant mutations in the human Leucine-Rich Repeat Kinase 2 ( LRRK2 ) gene represent the most common monogenetic cause of Parkinson disease (PD) and increased kinase activity observed in pathogenic mutants of LRRK2 is most likely causative for PD-associated neurotoxicity. The sequence of the LRRK2 kinase domain shows similarity to MAP kinase kinase kinases. Furthermore, LRRK2 shares highest sequence homology with mixed linage kinases which act upstream of canonical MAPKK and are involved in cellular stress responses. Therefore, we addressed the question if LRRK2 exhibits MAPKKK activity by systematically testing MAPKKs as candidate substrates, in vitro . We demonstrate that LRRK2 variants phosphorylate mitogen-activated protein kinase kinases (MAPKK), including MKK3 -4, -6 and -7. MKKs act upstream of the MAPK p38 and JNK mediating oxidative cell stress, neurotoxicity and apoptosis. The disease-associated LRRK2 G2019S and I2020T mutations show an increased phosphotransferase activity towards MKKs correlating with the activity shown for its autophosphorylation. Our findings present evidence of a new class of molecular targets for mutant LRRK2 that link to neurotoxicity, cellular stress, cytoskeletal dynamics and vesicular transport.     

Differential regulation of HSP70 expression by the JNK kinases SEK1 and MKK7 in mouse embryonic stem cells treated with cadmium

JNK, a member of the mitogen-activated protein kinases (MAPKs), is activated by the MAPK kinases SEK1 and MKK7 in response to environmental stresses. In the present study, the effects of CdCl2 treatment on MAPK phosphorylation and HSP70 expression were examined in mouse embryonic stem (ES) cells lacking the sek1 gene, the mkk7 gene, or both. Following CdCl2 exposure, the phosphorylation of JNK, p38, and ERK was suppressed in sek1-/- mkk7-/- cells. When sek1-/- or mkk7-/- cells were treated with CdCl2, JNK phosphorylation, but not the phosphorylation of either p38 or ERK, was markedly reduced, while a weak reduction in p38 phosphorylation was observed in sek1-/- cells. Thus, both SEK1 and MKK7 are required for JNK phosphorylation, whereas their role in p38 and ERK phosphorylation could overlap with that of another kinase. We also observed that CdCl2-induced HSP70 expression was abolished in sek1-/- mkk7-/- cells, was reduced in sek1-/- cells, and was enhanced in mkk7-/- cells. Similarly, the phosphorylation of heat shock factor 1 (HSF1) was decreased in sek1-/- mkk7-/- and sek1-/- cells, but was increased in mkk7-/- cells. Transfection with siRNA specific for JNK1, JNK2, p38, ERK1, or ERK2 suppressed CdCl2-induced HSP70 expression. In contrast, silencing of p38 or p38 resulted in further accumulation of HSP70 protein. These results suggest that HSP70 expression is up-regulated by SEK1 and down-regulated by MKK7 through distinct MAPK isoforms in mouse ES cells treated with CdCl2.     

The mixed lineage kinase DLK utilizes MKK7 and not MKK4 as substrate

Mixed lineage kinases DLK (dual leucine zipper-bearing kinase) and MLK3 have been proposed to function as mitogen-activated protein kinase kinase kinases in pathways leading to stress-activated protein kinase/c-Jun NH2-terminal kinase activation. Differences in primary protein structure place these MLK (mixed lineage kinase) enzymes in separate subfamilies and suggest that they perform distinct functional roles. Both DLK and MLK3 associated with, phosphorylated, and activated MKK7 in vitro. Unlike MLK3, however, DLK did not phosphorylate or activate recombinant MKK4 in vitro. In confirmatory experiments performed in vivo, DLK both associated with and activated MKK7. The relative localization of endogenous DLK, MLK3, MKK4, and MKK7 was determined in cells of the nervous system. Distinct from MLK3, which was identified in non-neuronal cells, DLK and MKK7 were detected predominantly in neurons in sections of adult rat cortex by immunocytochemistry. Subcellular fractionation experiments of cerebral cortex identified DLK and MKK7 in similar nuclear and extranuclear subcellular compartments. Concordant with biochemical experiments, however, MKK4 occupied compartments distinct from that of DLK and MKK7. That DLK and MKK7 occupied subcellular compartments distinct from MKK4 was confirmed by immunocytochemistry in primary neuronal culture. The dissimilar cellular specificity of DLK and MLK3 and the specific substrate utilization and subcellular compartmentation of DLK suggest that specific mixed lineage kinases participate in unique signal transduction events.     

Differential requirement of MKK4 and MKK7 in JNK activation by distinct scaffold proteins

Different scaffold proteins play distinct roles in various signaling pathways by recruiting different downstream molecules. Here, using MKK4(-/-) and MKK4(-/-)/7(-/-) murine embryonic fibroblast cells, we examined differential employment of MKK4 and MKK7 by scaffold proteins Axin, Dvl, and Epstein-Barr virus latent membrane protein-1 (LMP-1) in mediating JNK activation. We present evidence that Axin depends mainly on MKK7 for activation of JNK, while Dvl depends almost equally on MKK4 and MKK7 for JNK activation, In contrast, LMP-1-induced JNK activation is primarily dependent on MKK4. Our results demonstrate that Axin, Dvl, and LMP-1 differentially utilize MKK4 and MKK7 for JNK activation.     

A Protoberberine Derivative HWY336 Selectively Inhibits MKK4 and MKK7 in Mammalian Cells: The Importance of Activation Loop on Selectivity

A protoberberine derivative library was used to search for selective inhibitors against kinases of the mitogen-activated protein kinase (MAPK) cascades in mammalian cells. Among kinases in mammalian MAPK pathways, we identified a compound (HWY336) that selectively inhibits kinase activity of mitogen-activated protein kinase kinase 4 and 7 (MKK4 and MKK7). The IC₅₀ of HWY336 was 6 µM for MKK4 and 10 µM for MKK7 in vitro. HWY336 bound to both kinases reversibly via noncovalent interactions, and inhibited their activity by interfering with access of a protein substrate to its binding site. The binding affinity of HWY336 to MKK4 was measured by surface plasmon resonance to determine a dissociation constant (K_d) of 3.2 µM. When mammalian cells were treated with HWY336, MKK4 and MKK7 were selectively inhibited, resulting in inhibition of c-Jun NH₂-terminal protein kinases in vivo. The structural model of HWY336 bound to either MKK4 or MKK7 predicted that HWY336 was docked to the activation loop, which is adjacent to the substrate binding site. This model suggested the importance of the activation loop of MKKs in HWY336 selectivity. We verified this model by mutating three critical residues within this loop of MKK4 to the corresponding residues in MKK3. The mutant MKK4 displayed similar kinase activity as wild-type kinase, but its activity was not inhibited by HWY336 compared to wild-type MKK4. We propose that the specific association of HWY336 to the activation loop of MKK4/MKK7 is responsible for its selective inhibition.     

MEK kinase 3 directly activates MKK6 and MKK7, specific activators of the p38 and c-Jun NH2-terminal kinases.

Mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase kinase kinase 3 (MEKK3) activates the c-Jun NH2-terminal kinase (JNK) pathway, although no substrates for MEKK3 have been identified. We have examined the regulation by MEKK3 of MAPK kinase 7 (MKK7) and MKK6, two novel MAPK kinases specific for JNK and p38, respectively. Coexpression of MKK7 with MEKK3 in COS-7 cells enhanced MKK7 autophosphorylation and its ability to activate recombinant JNK1 in vitro. MKK6 autophosphorylation and in vitro activation of p38alpha were also observed following coexpression of MKK6 with MEKK3. MEKK2, a closely related homologue of MEKK3, also activated MKK7 and MKK6 in COS-7 cells. Importantly, immunoprecipitates of either MEKK3 or MEKK2 directly activated recombinant MKK7 and MKK6 in vitro. These data identify MEKK3 as a MAPK kinase kinase specific for MKK7 and MKK6 in the JNK and p38 pathways. We have also examined whether MEKK3 or MEKK2 activates p38 in intact cells using MAPK-activated protein kinase-2 (MAPKAPK2) as an affinity ligand and substrate. Anisomycin, sorbitol, or the expression of MEKK3 in HEK293 cells enhanced MAPKAPK2 phosphorylation, whereas MEKK2 was less effective. Furthermore, MAPKAPK2 phosphorylation induced by MEKK3 or cellular stress was abolished by the p38 inhibitor SB-203580, suggesting that MEKK3 is coupled to p38 activation in intact cells.     

Differential activation of p38MAPK isoforms by MKK6 and MKK3

All four members of the mammalian p38 mitogen-activated protein kinase (MAPK) family (p38α, p38β, p38γ and p38δ) are activated by dual phosphorylation in the TGY motif in the activation loop. This phosphorylation is mediated by three kinases, MKK3, MKK6 and MKK4, at least in vitro. The role of these MKK in the activation of p38α has been demonstrated in studies using fibroblasts that lack MKK3 and/or MKK6. Nonetheless, the physiological upstream activators of the other p38MAPK isoforms have not yet been reported using MKK knockout cells. In this study, we examined p38β, γ and δ activation by MKK3 and MKK6, in cells lacking MKK3, MKK6 or both. We show that MKK3 and MKK6 are both essential for the activation of p38γ and p38β induced by environmental stress, whereas MKK6 is the major p38γ activator in response to TNFα. In contrast, p38δ activation by ultraviolet radiation, hyperosmotic shock, anisomycin or by TNFα is mediated by MKK3. Moreover, in response to osmotic stress, MKK3 and MKK6 are crucial in regulating the phosphorylation of the p38γ substrate hDlg and its activity as scaffold protein. These data indicate that activation of distinct p38MAPK isoforms is regulated by the selective and synchronized action of two kinases, MKK3 and MKK6, in response to cell stress.     

Effect of siRNA targeted against MKK4 on myostatin-induced downregulation of differentiation marker gene expression

The c-Jun N-terminal kinase (JNK) pathway was reported to be involved in myostatin signaling and MKK4 was suggested as the only upstream kinase for myostatin-induced JNK activation, implying that MKK4 is a suitable target of RNA interference (RNAi) for blocking myostatin activity. The aim of this study was to evaluate the effect of small interfering RNA (siRNA) targeted against MKK4 on myostatin-induced downregulation of differentiation marker gene expression. Real-time quantitative PCR revealed that the level of MKK4 expression was efficiently reduced by MKK4-specific siRNA. Western blot assays showed that knockdown of MKK4 attenuated the myostatin-induced downregulation of MyoD and myogenin expression.     

MKK7-mediated phosphorylation of JNKs regulates the proliferation and apoptosis of human spermatogonial stem cells

BACKGROUNDHuman spermatogonial stem cells (SSCs) are the basis of spermatogenesis. However, little is known about the developmental regulatory mechanisms of SSC due to sample origin and species differences.AIMTo investigates the mechanisms involved in the proliferation of human SSC.METHODSThe expression of mitogen-activated protein kinase kinase 7 (MKK7) in human testis was identified using immunohistochemistry and western blotting (WB). MKK7 was knocked down using small interfering RNA, and cell proliferation and apoptosis were detected by WB, EdU, cell counting kit-8 and fluorescence-activated cell sorting. After bioinformatic analysis, the interaction of MKK7 with c-Jun N-terminal kinases ( JNKs ) was verified by protein co-immunoprecipitation and WB. The phosphorylation of JNKs was inhibited by SP600125, and the phenotypic changes were detected by WB, cell counting kit-8 and fluorescence-activated cell sorting.RESULTSMKK7 is mainly expressed in human SSCs, and MKK7 knockdown inhibits SSC proliferation and promotes their apoptosis. MKK7 mediated the phosphorylation of JNKs, and after inhibiting the phosphorylation of JNKs, the phenotypic changes of the cells were similar to those after MKK7 downregulation. The expression of MKK7 was significantly downregulated in patients with abnormal spermatogenesis, suggesting that abnormal MKK7 may be associated with spermatogenesis impairment.CONCLUSIONMKK7 regulates the proliferation and apoptosis of human SSC by mediating the phosphorylation of JNKs.     

Requirements for PKC-augmented JNK activation by MKK4/7

The c-Jun N-terminal kinases (JNKs) are activated in response to stress, DNA damage, and cytokines by MKK4 and MKK7. We recently demonstrated that PKC can augment the degree of JNK activation by phosphorylating JNK, which requires the adaptor protein RACK1. Here we report on the conditions required for PKC-dependent JNK activation. In vitro kinase assays reveal that PKC phosphorylation of JNK is not sufficient for its activation but rather augments JNK activation by canonical JNK upstream kinases MKK4 or MKK7 alone or in combination. Further, to enhance JNK activity, PKC phosphorylation of JNK should precede its phosphorylation by MKK4/7. Inhibition of PKC phosphorylation of JNK affects both early and late phases of JNK activation following UV-irradiation and reduces the apoptotic response mediated by JNK. These data provide important insight into the requirements for PKC activation of JNK signaling.     

Transforming growth factor-beta1 (TGF-beta)-induced apoptosis of prostate cancer cells involves Smad7-dependent activation of p38 by TGF-beta-activated kinase 1 and mitogen-activated protein kinase kinase 3

The inhibitory Smad7, a direct target gene for transforming growth factor-beta (TGF-beta), mediates TGF-beta1-induced apoptosis in several cell types. Herein, we report that apoptosis of human prostate cancer PC-3U cells induced by TGF-beta1 or Smad7 overexpression is caused by a specific activation of the p38 mitogen-activated protein kinase pathway in a TGF-beta-activated kinase 1 (TAK1)- and mitogen-activated protein kinase kinase 3 (MKK3)-dependent manner. Expression of dominant negative p38, dominant negative MKK3, or incubation with the p38 selective inhibitor [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole], prevented TGF-beta1-induced apoptosis. The expression of Smad7 was required for TGF-beta-induced activation of MKK3 and p38 kinases, and endogenous Smad7 was found to interact with phosphorylated p38 in a ligand-dependent manner. Ectopic expression of wild-type TAK1 promoted TGF-beta1-induced phosphorylation of p38 and apoptosis, whereas dominant negative TAK1 reduced TGF-beta1-induced phosphorylation of p38 and apoptosis. Endogenous Smad7 was found to interact with TAK1, and TAK1, MKK3, and p38 were coimmunoprecipitated with Smad7 in transiently transfected COS1 cells. Moreover, ectopically expressed Smad7 enhanced the coimmunoprecipitation of HA-MKK3 and Flag-p38, supporting the notion that Smad7 may act as a scaffolding protein and facilitate TAK1- and MKK3-mediated activation of p38.     

Multiple activation mechanisms of p38alpha mitogen-activated protein kinase

The p38alpha MAPK participates in a variety of biological processes. Activation of p38alpha is mediated by phosphorylation on specific regulatory tyrosine and threonine sites, and the three dual kinases, MAPK kinase 3 (MKK3), MKK4, and MKK6, are known to be the upstream activators of p38alpha. In addition to activation by upstream kinases, p38alpha can autoactivate when interacting with transforming growth factor-beta-activated protein kinase 1-binding protein 1 (TAB1). Here we used MKK3 and MKK6 double knock-out (MKK3/6 DKO) and MKK4/7 DKO mouse embryonic fibroblast (MEF) cells to examine activation mechanisms of p38alpha. We confirmed that the MKK3/6 pathway is a primary mechanism for p38alpha phosphorylation in MEF cells, and we also showed the presence of other p38alpha activation pathways. We show that TAB1-mediated p38alpha phosphorylation in MEF cells did not need MKK3/4/6, and it accounted for a small portion of the total p38alpha phosphorylation that was induced by hyperosmolarity and anisomycin. We observed that a portion of peroxynitrite-induced phospho-p38alpha is associated with an approximately 85-kDa disulfide complex in wild-type MEF cells. Peroxynitrite-induced phosphorylation of p38alpha in the approximately 85-kDa complex is independent from MKK3/6 because only phospho-p38alpha not associated with the disulfide complex was diminished in MKK3/6 DKO cells. In addition, our data suggest interference among different pathways because TAB1 had an inhibitory effect on p38alpha phosphorylation in the peroxynitrite-induced approximately 85-kDa complex. Mutagenesis analysis of the cysteines in p38alpha revealed that no disulfide bond forms between p38alpha and other proteins in the approximately 85-kDa complex, suggesting it is a p38alpha binding partner(s) that forms disulfide bonds, which enable it to bind to p38alpha. Therefore, multiple mechanisms of p38alpha activation exist that can influence each other, be simultaneously activated by a given stimulus, and/or be selectively used by different stimuli in a cell type-specific manner.     

Stress Kinase MKK7: Saviour of Cell Cycle Arrest and Cellular Senescence

The c-Jun N-terminal kinase (JNK/SAPK) signaling cascade controls a spectrum ofcellular processes, including cell growth, differentiation, transformation, and apoptosis.We recently demonstrated that stress kinase MKK7, a direct activator of JNKs, couplesstress signaling to G2/M cell cycle progression, CDC2 expression, and cellularsenescence. We further explored other molecules involved in JNK pathway and foundthat both MKK4, another direct activator of JNK, and c-Jun, a direct substrate of JNK,have similar roles to MKK7. Here we discuss the importance of the MKK4/MKK7-JNKc-Jun pathway linking stress and developmental signals to cell proliferation, cell cycleprogression, cellular senescence, and apoptosis including recent unpublished data fromour lab.