Branched chain amino acid metabolism profiles in progressive human nonalcoholic fatty liver disease.

Nonalcoholic fatty liver disease (NAFLD) is a globally widespread disease of increasing clinical significance. The pathological progression of the disease from simple steatosis to nonalcoholic steatohepatitis (NASH) has been well defined, however, the contribution of altered branched chain amino acid metabolomic profiles to the progression of NAFLD is not known. The three BCAAs: leucine, isoleucine and valine are known to mediate activation of several important hepatic metabolic signaling pathways ranging from insulin signaling to glucose regulation. The purpose of this study is to profile changes in hepatic BCAA metabolite levels with transcriptomic changes in the progression of human NAFLD to discover novel mechanisms of disease progression. Metabolomic and transcriptomic data sets representing the spectrum of human NAFLD (normal, steatosis, NASH fatty, and NASH not fatty livers) were utilized for this study. During the transition from steatosis to NASH, increases in the levels of leucine (127% of normal), isoleucine (139%), and valine (147%) were observed. Carnitine metabolites also exhibited significantly elevated profiles in NASH fatty and NASH not fatty samples and included propionyl, hexanoyl, lauryl, acetyl and butyryl carnitine. Amino acid and BCAA metabolism gene sets were significantly enriched among downregulated genes during NASH. These cumulative alterations in BCAA metabolite and amino acid metabolism gene profiles represent adaptive physiological responses to disease-induced hepatic stress in NASH patients.

Arsenite activates NFκB through induction of C-reactive protein.

C-reactive protein (CRP) is an acute phase protein in humans. Elevated levels of CRP are produced in response to inflammatory cytokines and are associated with atherosclerosis, hypertension, cardiovascular disease and insulin resistance. Exposure to inorganic arsenic, a common environmental toxicant, also produces cardiovascular disorders, namely atherosclerosis and is associated with insulin-resistance. Inorganic arsenic has been shown to contribute to cardiac toxicities through production of reactive oxygen species (ROS) that result in the activation of NFκB. In this study we show that exposure of the hepatic cell line, HepG2, to environmentally relevant levels of arsenite (0.13 to 2 µM) results in elevated CRP expression and secretion. ROS analysis of the samples showed that a minimal amount of ROS are produced by HepG2 cells in response to these concentrations of arsenic. In addition, treatment of FvB mice with 100 ppb sodium arsenite in the drinking water for 6 months starting at weaning age resulted in dramatically higher levels of CRP in both the liver and inner medullary region of the kidney. Further, mouse Inner Medullary Collecting Duct cells (mIMCD-4), a mouse kidney cell line, were stimulated with 10 ng/ml CRP which resulted in activation of NFκB. Pretreatment with 10 nM Y27632, a known Rho-kinase inhibitor, prior to CRP exposure attenuated NFκB activation. These data suggest that arsenic causes the expression and secretion of CRP and that CRP activates NFκB through activation of the Rho-kinase pathway, thereby providing a novel pathway by which arsenic can contribute to metabolic syndrome and cardiovascular disease.

TGFß2-mediated production of hyaluronan is important for the induction of epicardial cell differentiation and invasion.

In the developing heart, the epicardium is a major source of progenitor cells that contribute to the formation of the coronary vessel system. These epicardial progenitors give rise to the different cellular components of the coronary vasculature by undergoing a number of morphological and physiological changes collectively known as epithelial to mesenchymal transformation (EMT). However, the specific signaling mechanisms that regulate epicardial EMT are yet to be delineated. In this study we investigated the role of TGFß2 and hyaluronan (HA) during epicardial EMT and how signals from these two molecules are integrated during this important process. Here we show that TGFß2 induces MEKK3 activation, which in turn promotes ERK1/2 and ERK5 phosphorylation. TGFß2 also increases Has2 expression and subsequent HA production. Nevertheless, inhibition of MEKK3 kinase activity, silencing of ERK5 or pharmacological disruption of ERK1/2 activation significantly abrogates this response. Thus, TGFß2 promotes Has2 expression and HA production through a MEKK3/ERK1/2/5-dependent cascade. Furthermore, TGFß2 is able to induce epicardial cell invasion and differentiation but not proliferation. However, inhibition of MEKK3-dependent pathways, degradation of HA by hyaluronidases or blockade of CD44, significantly impairs the biological response to TGFß2. Taken together, these findings demonstrate that TGFß2 activation of MEKK3/ERK1/2/5 signaling modulates Has2 expression and HA production leading to the induction of EMT events. This is an important and novel mechanism showing how TGFß2 and HA signals are integrated to regulate changes in epicardial cell behavior.

MEKK3 initiates transforming growth factor beta 2-dependent epithelial-to-mesenchymal transition during endocardial cushion morphogenesis.

Congenital heart defects occur at a rate of 5% and are the most prevalent birth defects. A better understanding of the complex signaling networks regulating heart development is necessary to improve repair strategies for congenital heart defects. The mitogen-activated protein 3 kinase (MEKK3) is important to early embryogenesis, but developmental processes affected by MEKK3 during heart morphogenesis have not been fully examined. We identify MEKK3 as a critical signaling molecule during endocardial cushion development. We report the detection of MEKK3 transcripts to embryonic hearts before, during, and after cardiac cushion cells have executed epithelial-to-mesenchymal transition (EMT). MEKK3 is observed to endocardial cells of the cardiac cushions with a diminishing gradient of expression into the cushions. These observations suggest that MEKK3 may function during production of cushion mesenchyme as required for valvular development and septation of the heart. We used a kinase inactive form of MEKK3 (MEKK3(KI)) in an in vitro assay that recapitulates in vivo EMT and show that MEKK3(KI) attenuates mesenchyme formation. Conversely, constitutively active MEKK3 (ca-MEKK3) triggers mesenchyme production in ventricular endocardium, a tissue that does not normally undergo EMT. MEKK3-driven mesenchyme production is further substantiated by increased expression of EMT-relevant genes, including TGFbeta(2), Has2, and periostin. Furthermore, we show that MEKK3 stimulates EMT via a TGFbeta(2)-dependent mechanism. Thus, the activity of MEKK3 is sufficient for developmental EMT in the heart. This knowledge provides a basis to understand how MEKK3 integrates signaling cascades activating endocardial cushion EMT.

Influence of arsenate and arsenite on signal transduction pathways: an update.

Arsenic has been a recognized contaminant and toxicant, as well as a medicinal compound throughout human history. Populations throughout the world are exposed to arsenic and these exposures have been associated with a number of human cancers. Not much is known about the role of arsenic as a human carcinogen and more recently its role in non-cancerous diseases, such as cardiovascular disease, hypertension and diabetes mellitus have been uncovered. The health effects associated with arsenic are numerous and the association between arsenic exposure and human disease has intensified the search for molecular mechanisms that describe the biological activity of arsenic in humans and leads to the aforementioned disease states. Arsenic poses a human health risk due in part to the regulation of cellular signal transduction pathways and over the last few decades, some cellular mechanisms that account for arsenic toxicity, as well as, signal transduction pathways have been discovered. However, given the ubiquitous nature of arsenic in the environment, making sense of all the data remains a challenge. This review will focus on our knowledge of signal transduction pathways that are regulated by arsenic.

HER2/HER3 regulates lactate secretion and expression of lactate receptor mRNA through the MAP3K4 associated protein GIT1.

One of the major features of cancer is Otto Warburg's observation that many tumors have increased extracellular acidification compared to healthy tissues. Since Warburg's observation, the importance of extracellular acidification in cancer is now considered a hallmark of cancer. Human MAP3K4 functions upstream of the p38 and JNK mitogen activated protein kinases (MAPKs). Additionally, MAP3K4 is required for cell migration and extracellular acidification of breast cancer cells in response to HER2/HER3 signaling. Here, we demonstrate that GIT1 interacts with MAP3K4 by immunoprecipitation, while cellular lactate production and the capacity of MCF-7 cells for anchorage independent growth in soft agar were dependent on GIT1. Additionally, we show that activation of HER2/HER3 signaling leads to reduced expression of lactate receptor (GPR81) mRNA and that both, GIT1 and MAP3K4, are necessary for constitutive expression of GPR81 mRNA. Our study suggests that targeting downstream proteins in the HER2/HER3-induced extracellular lactate signaling pathway may be a way to inhibit the Warburg Effect to disrupt tumor growth.

Arsenic alters vascular smooth muscle cell focal adhesion complexes leading to activation of FAK-src mediated pathways.

Chronic exposure to arsenic has been linked to tumorigenesis, cardiovascular disease, hypertension, atherosclerosis, and peripheral vascular disease; however, the molecular mechanisms underlying its pathological effects remain elusive. In this study, we investigated arsenic-induced alteration of focal adhesion protein complexes in normal, primary vascular smooth muscle cells. We demonstrate that exposure to environmentally relevant concentrations of arsenic (50 ppb As(3+)) can alter focal adhesion protein co-association leading to activation of downstream pathways. Co-associated proteins were identified and quantitated via co-immunoprecipitation, SDS-PAGE, and Western blot analysis followed by scanning densitometry. Activation of MAPK pathways in total cell lysates was evaluated using phosphor-specific antibodies. In our model, arsenic treatment caused a sustained increase in FAK-src association and activation, and induced the formation of unique signaling complexes (beginning after 3-hour As(3+) exposure and continuing throughout the 12-hour time course studied). The effects of these alterations were manifested as chronic stimulation of downstream PAK, ERK and JNK pathways. Past studies have demonstrated that these pathways are involved in cellular survival, growth, proliferation, and migration in VSMCs.

Ablation of MEKK4 kinase activity causes neurulation and skeletal patterning defects in the mouse embryo.

Skeletal disorders and neural tube closure defects represent clinically significant human malformations. The signaling networks regulating normal skeletal patterning and neurulation are largely unknown. Targeted mutation of the active site lysine of MEK kinase 4 (MEKK4) produces a kinase-inactive MEKK4 protein (MEKK4(K1361R)). Embryos homozygous for this mutation die at birth as a result of skeletal malformations and neural tube defects. Hindbrains of exencephalic MEKK4(K1361R) embryos show a striking increase in neuroepithelial cell apoptosis and a dramatic loss of phosphorylation of MKK3 and -6, mitogen-activated protein kinase kinases (MKKs) regulated by MEKK4 in the p38 pathway. Phosphorylation of MAPK-activated protein kinase 2, a p38 substrate, is also inhibited, demonstrating a loss of p38 activity in MEKK4(K1361R) embryos. In contrast, the MEK1/2-extracellular signal-regulated kinase 1 (ERK1)/ERK2 and MKK4-Jun N-terminal protein kinase pathways were unaffected. The p38 pathway has been shown to regulate the phosphorylation and expression of the small heat shock protein HSP27. Compared to the wild type, MEKK4(K1361R) fibroblasts showed significantly reduced phosphorylation of p38 and HSP27, with a corresponding heat shock-induced instability of the actin cytoskeleton. Together, these data demonstrate MEKK4 regulation of p38 and that substrates downstream of p38 control cellular homeostasis. The findings are the first demonstration that MEKK4-regulated p38 activity is critical for neurulation.

Arsenic-induced decreases in the vascular matrix.

Chronic ingestion of arsenic is associated with increased incidence of respiratory and cardiovascular diseases. To investigate the role of arsenic in early events in vascular pathology, C57BL/6 mice ingested drinking water with or without 50 ppb sodium arsenite (AsIII) for four, five, or eight weeks. At five and eight weeks, RNA from the lungs of control and AsIII-exposed animals was processed for microarray. Sixty-five genes were significantly and differentially expressed. Differential expression of extracellular matrix (ECM) gene transcripts was particularly compelling, as 91% of genes in this category, including elastin and collagen, were significantly decreased. In additional experiments, real-time RT-PCR showed an AsIII-induced decrease in many of these ECM gene transcripts in the heart and NIH3T3 fibroblast cells. Histological stains for collagen and elastin show a distinct disruption in the ECM surrounding small arteries in the heart and lung of AsIII-exposed mice. Immunohistochemical detection of alpha-smooth muscle actin in blood vessel walls was decreased in the AsIII-exposed animals. These data reveal a functional link between AsIII exposure and disruption in the vascular ECM. These AsIII-induced early pathological events may predispose humans to respiratory and cardiovascular diseases linked to chronic low-dose AsIII exposure.

The death domain kinase RIP1 is essential for tumor necrosis factor alpha signaling to p38 mitogen-activated protein kinase.

The cytokine tumor necrosis factor alpha (TNF-alpha) stimulates the NF-kappaB, SAPK/JNK, and p38 mitogen-activated protein (MAP) kinase pathways by recruiting RIP1 and TRAF2 proteins to the tumor necrosis factor receptor 1 (TNFR1). Genetic studies have revealed that RIP1 links the TNFR1 to the IkappaB kinase (IKK) complex, whereas TRAF2 couples the TNFR1 to the SAPK/JNK cascade. In transfection studies, RIP1 and TRAF2 stimulate p38 MAP kinase activation, and dominant-negative forms of RIP1 and TRAF2 inhibit TNF-alpha-induced p38 MAP kinase activation. We found TNF-alpha-induced p38 MAP kinase activation and interleukin-6 (IL-6) production impaired in rip1(-/-) murine embryonic fibroblasts (MEF) but unaffected in traf2(-/-) MEF. Yet, both rip1(-/-) and traf2(-/-) MEF exhibit a normal p38 MAP kinase response to inducers of osmotic shock or IL-1alpha. Thus, RIP1 is a specific mediator of the p38 MAP kinase response to TNF-alpha. These studies suggest that TNF-alpha-induced activation of p38 MAP kinase and SAPK/JNK pathways bifurcate at the level of RIP1 and TRAF2. Moreover, endogenous RIP1 associates with the MAP kinase kinase kinase (MAP3K) MEKK3 in TNF-alpha-treated cells, and decreased TNF-alpha-induced p38 MAP kinase activation is observed in Mekk3(-/-) cells. Taken together, these studies suggest a mechanism whereby RIP1 may mediate the p38 MAP kinase response to TNF-alpha, by recruiting the MAP3K MEKK3.

Interferon-gamma-dependent tyrosine phosphorylation of MEKK4 via Pyk2 is regulated by annexin II and SHP2 in keratinocytes.

IFNgamma (interferon-gamma) binding to its cognate receptor results, through JAK (Janus kinase), in direct activation of receptor-bound STAT1 (signal transducer and activator of transcription 1), although there is evidence for additional activation of a MAPK (mitogen-activated protein kinase) pathway. In the present paper, we report IFNgamma-dependent activation of the MEKK4 (MAPK/extracellular-signal-regulated kinase kinase kinase 4) pathway in HaCaT human keratinocytes. MEKK4 is tyrosine-phosphorylated and the IFNgamma-dependent phosphorylation requires intracellular calcium. Calcium-dependent phosphorylation of MEKK4 is mediated by Pyk2. Moreover, MEKK4 and Pyk2 co-localize in an IFNgamma-dependent manner in the perinuclear region. Furthermore, the calcium-binding protein, annexin II, and the calcium-regulated kinase, Pyk2, co-immunoprecipitate with MEKK4 after treatment with IFNgamma. Immunofluorescence imaging of HaCaT cells shows an IFNgamma-dependent co-localization of annexin II with Pyk2 in the perinuclear region, suggesting that annexin II mediates the calcium-dependent regulation of Pyk2. Tyrosine phosphorylation of MEKK4 correlates with its activity to phosphorylate MKK6 (MAPK kinase 6) in vitro and subsequent p38 MAPK activation in an IFNgamma-dependent manner. Additional studies demonstrate that the SH2 (Src homology 2)-domain-containing tyrosine phosphatase SHP2 co-immunoprecipitates with MEKK4 in an IFNgamma-dependent manner and co-localizes with MEKK4 after IFNgamma stimulation in the perinuclear region in HaCaT cells. Furthermore, we provide evidence that SHP2 dephosphorylates MEKK4 and Pyk2, terminating the MEKK4-dependent branch of the IFNgamma signalling pathway.

Cyclin-dependent kinase 11(p110) activity in the absence of CK2.

Cyclin-dependent kinase (CDK)11(p110), formerly known as PITSLRE, is a serine/threonine kinase whose catalytic activity has been associated with transcription and RNA processing. To further evaluate the regulation of CDK11(p110) catalytic activity, interacting proteins were identified by liquid chromatography and tandem mass spectrometry (LC-MS/MS). Following the immunoprecipitation of CDK11(p110) from COS-7 cells, the serine/threonine kinase CK2 was identified by LC-MS/MS. These results were extended through the observation that CDK11(p110) serves as a substrate for CK2 and the identification of a phosphorylation site on CDK11(p110) at Ser227 by LC-MS/MS. To obtain CDK11(p110) devoid of CK2, CDK11(p110) was expressed in High Five insect cells and secreted into the media due to the presence of a honeybee melittin signal sequence encoded at the amino-terminus of CDK11(p110). Recombinant CDK11(p110) was purified from the media and phosphorylation of histone H1 subsequently demonstrated. After demonstrating retention of CDK11(p110) kinase activity, it was evaluated for activity on the carboxyl-terminal domain (CTD) of the largest subunit of RNA polymerase II (RNAP II), but only CK2 was found to phosphorylate the CTD.

HER2/HER3 regulates extracellular acidification and cell migration through MTK1 (MEKK4).

Human MAP3K4 (MTK1) functions upstream of mitogen activated protein kinases (MAPKs). In this study we show MTK1 is required for human epidermal growth factor receptor 2/3 (HER2/HER3)-heregulin beta1 (HRG) induced cell migration in MCF-7 breast cancer cells. We demonstrate that HRG stimulation leads to association of MTK1 with activated HER3 in MCF-7 and T-47D breast cancer cells. Activated HER3 association with MTK1 is dependent on HER2 activation and is decreased by pre-treatment with the HER2 inhibitor, lapatinib. Moreover, we also identify the actin interacting region (AIR) on MTK1. Disruption of actin cytoskeletal polymerization with cytochalasin D inhibited HRG induced MTK1/HER3 association. Additionally, HRG stimulation leads to extracellular acidification that is independent of cellular proliferation. HRG induced extracellular acidification is significantly inhibited when MTK1 is knocked down in MCF-7 cells. Similarly, pre-treatment with lapatinib significantly decreased HRG induced extracellular acidification. Extracellular acidification is linked with cancer cell migration. We performed scratch assays that show HRG induced cell migration in MCF-7 cells. Knockdown of MTK1 significantly inhibited HRG induced cell migration. Furthermore, pre-treatment with lapatinib also significantly decreased cell migration. Cell migration is required for cancer cell metastasis, which is the major cause of cancer patient mortality. We identify MTK1 in the HER2/HER3-HRG mediated extracellular acidification and cell migration pathway in breast cancer cells.

MAP3Ks as central regulators of cell fate during development.

The cytoplasmic serine/threonine kinases transduce extracellular signals into regulatory events that impact cellular responses. The induction of one kinase triggers the activation of several downstream kinases, leading to the regulation of transcription factors to affect gene function. This arrangement allows for the kinase cascade to be amplified, and integrated according to the cellular context. An upstream mitogen or growth factor signal initiates a module of three kinases: a mitogen-activated protein (MAP) kinase kinase kinase (MAPKKK; e.g., Raf) that phosphorylates and activates a MAP kinase kinase (MAPKK; e.g., MEK) and finally activation of MAP kinase (MAPK; e.g., ERK). Thus, this MAP3K-MAP2K-MAPK module represents critical effectors that regulate extracellular stimuli into cellular responses, such as differentiation, proliferation, and apoptosis all of which function during development. There are 21 characterized MAP3Ks that activate known MAP2Ks, and they function in many aspects of developmental biology. This review summarizes known transduction routes linked to each MAP3K and highlights mouse models that provide clues to their physiological functions. This perspective reveals that some of these MAP3K effectors may have redundant functions, and also serve as unique nexus depending on the context of the signaling pathway.

Increased hexokinase II expression in the renal glomerulus of mice in response to arsenic.

Epidemiological studies link arsenic exposure to increased risks of cancers of the skin, kidney, lung, bladder and liver. Additionally, a variety of non-cancerous conditions such as diabetes mellitus, hypertension, and cardiovascular disease have been associated with chronic ingestion of low levels of arsenic. However, the biological and molecular mechanisms by which arsenic exerts its effects remain elusive. Here we report increased renal hexokinase II (HKII) expression in response to arsenic exposure both in vivo and in vitro. In our model, HKII was up-regulated in the renal glomeruli of mice exposed to low levels of arsenic (10 ppb or 50 ppb) via their drinking water for up to 21 days. Additionally, a similar effect was observed in cultured renal mesangial cells exposed to arsenic. This correlation between our in vivo and in vitro data provides further evidence for a direct link between altered renal HKII expression and arsenic exposure. Thus, our data suggest that alterations in renal HKII expression may be involved in arsenic-induced pathological conditions involving the kidney. More importantly, these results were obtained using environmentally relevant arsenic concentrations.

MEKK4 regulates developmental EMT in the embryonic heart.

Congenital heart malformations occur at a rate of one per one hundred births and are considered the most frequent birth defects. This high incidence of cardiac defects underscores the complex developmental processes required to form the first functioning organ in mammals. The molecular cues which govern heart development are poorly defined and require an improved understanding in order to advance repair strategies for heart defects. The cytoplasmic MAP kinase kinase kinase, MEKK4, is a critical effector in cellular stress responses; however, the function of MEKK4 during embryonic development and cardiogenesis is not well understood. We have identified MEKK4 as a critical signaling molecule during cardiovascular development. We report the detection of MEKK4 transcripts to early myocardium, endocardium and to cardiac cushion cells that have executed epithelial to mesenchymal transformation (EMT). These observations suggest that MEKK4 may function during production of the cushion mesenchyme as required to create valves and the septated heart. We used a kinase inactive form of MEKK4(MEKK4(KI)) in an in vitro assay that recapitulates in vivo EMT, and show that MEKK4(KI) attenuates mesenchyme production. However, addition of a constitutively active MEKK4 into ventricular explants, a system that does not normally undergo EMT, is not able to cause mesenchymal cell outgrowth. Thus, the kinase activity of MEKK4 is essential, but not sufficient, to support developmental EMT. This knowledge provides a basis to understand how MEKK4 may integrate signaling cascades controlling heart development.

Phosphorylation of serine 526 is required for MEKK3 activity, and association with 14-3-3 blocks dephosphorylation.

MAPK/ERK kinase kinase 3 (MEKK3) is a mitogen-activated protein kinase kinase kinase (MAP3K) that functions upstream of the MAP kinases and IkappaB kinase. Phosphorylation is believed to be a critical component for MEKK3-dependent signal transduction, but little is known about the phosphorylation sites of this MAP3K. To address this question, point mutations were introduced in the activation loop (T-loop), substituting alanine for serine or threonine, and the mutants were transfected into HEK293 Epstein-Barr virus nuclear antigen cells. MEKK3-dependent activation of an NF-kappaB reporter gene as well as ERK, JNK, and p38 MAP kinases correlated with a requirement for serine at position 526. Constitutively active mutants of MEKK3, consisting of S526D and S526E, were capable of activating a NF-kappaB luciferase reporter gene as well as ERK and MEK, suggesting that a negative charge at Ser526 was necessary for MEKK3 activity and implicating Ser526 as a phosphorylation site. An antibody was developed that specifically recognized phospho-Ser526 of MEKK3 but did not recognize the S526A point mutant. The catalytically inactive (K391M) mutant of MEKK3 was not phosphorylated at Ser526, indicating that phosphorylation of Ser526 occurs via autophosphorylation. Endogenous MEKK3 was phosphorylated on Ser526 in response to osmotic stress. In addition, phosphorylation of Ser526 was required for MKK6 phosphorylation in vitro, whereas dephosphorylation of Ser526 was mediated by protein phosphatase 2A and sensitive to okadaic acid and sodium fluoride. Finally, the association between MEKK3 and 14-3-3 was dependent on Ser526 and prevented dephosphorylation of Ser526. In summary, Ser526 of MEKK3 is an autophosphorylation site within the T-loop that is regulated by PP2A and 14-3-3 proteins.

Angiotensin II stimulated transcription of cyclooxygenase II is regulated by a novel kinase cascade involving Pyk2, MEKK4 and annexin II.

Although it is known that MEKK4 regulates MKK6, and p38 MAP kinase, extracellular stimuli that activate the serine/threonine kinase, MEKK4, are unknown. The aim of this study was then to identify stimuli that regulate MEKK4. By using recombinant MEKK4, as bait to attract interacting proteins, the calcium binding protein, annexin II, was identified by mass spectrometry as interacting with MEKK4, suggesting that MEKK4 might be regulated by calcium. A calcium-dependent interaction between MEKK4 and annexin II was observed when MEKK4 was immunoprecipitated from rat aortic smooth muscle cells that were treated with angiotensin II. Additional studies using recombinant MEKK4 in a Far-Western immunoblot identified a protein of 120 kDa as interacting directly with MEKK4. Prior studies indicated that MEKK4 was phosphorylated on tyrosine in vivo, and in fact, Pyk2 interacts with MEKK4 in an angiotensin II dependent manner in rat aortic smooth muscle cells. Pyk2 phosphorylates MEKK4 in vitro and Pyk2-dependent phosphorylation further regulates MEKK4-dependent phosphorylation of MKK6. Finally, dominant-negative MEKK4 inhibits angiotensin II mediated transcription of a luciferase reporter construct containing the cyclooxygenase II promoter, demonstrating that MEKK4 functions in a calcium-dependent manner as a substrate for Pyk2 and regulates transcription of cyclooxygenase II.

Cyclin-dependent kinase 11p110 and casein kinase 2 (CK2) inhibit the interaction between tyrosine hydroxylase and 14-3-3.

Tyrosine hydroxylase (TH) is regulated by the reversible phosphorylation of serines 8, 19, 31 and 40. Upon initiation of this study, serine 19 was unique due to its requirement of 14-3-3 binding after phosphorylation for optimal enzyme activity, although it has been more recently demonstrated that phosphorylated serine 40 also binds 14-3-3. To identify proteins that interact with TH following phosphorylation of serine 19, this amino acid was mutated to alanine and THS19A was used as bait in a yeast two-hybrid system. From this, mouse-derived cyclin-dependent kinase 11 (CDK11)p110 was identified as an interacting partner with THS19A. The interaction was confirmed using human CDK11p110 cDNA in a mammalian system. Previous research has demonstrated that casein kinase 2 (CK2) interacts with CDK11p110, and both were observed to phosphorylate TH in vitro. In addition, CDK11p110 overexpression was observed to inhibit the interaction between TH and 14-3-3. A mechanism contributing to disruption of the interaction between TH and 14-3-3 may be due to CK2 phosphorylation of specific 14-3-3 isoforms, i.e. 14-3-3 tau. Collectively, these results imply that CDK11p110 and CK2 negatively regulate TH catecholamine biosynthetic activity since phosphoserine 19 of TH requires 14-3-3 binding for optimal enzyme activity and a decreased rate of dephosphorylation.

Phosphorylation of the stress-activated protein kinase, MEKK3, at serine 166.

Much effort has focused on the identification of MAPK cascades that are activated by the MEKK family of protein kinases. However, direct phosphorylation and regulation of the MEKK proteins has not been shown. To address this question, we have expressed recombinant (His)6FLAG.MEKK3 in Sf9 insect cells and tethered the purified protein to Ni-Sepharose so that we could precipitate interacting proteins and then identify such proteins by liquid chromatography and mass spectrometry (LC-MS). We identified 14-3-3 proteins as interacting with MEKK3, which suggested that (His)6FLAG.MEKK3 was phosphorylated on serine since 14-3-3 proteins are known to associate with phosphorylated proteins. We identified two phosphorylated amino acids at Ser166 and Ser337 of tryptic peptides derived from (His)6FLAG.MEKK3 by using LC-MS. Antibodies were developed that recognize the specific phosphorylated amino acid and with these antibodies, we demonstrate that various stimuli (tumor necrosis factor, arsenite, forskolin, and serum) promote phosphorylation of Ser166 and Ser337. However, neither of these phosphorylated amino acids is required for association with 14-3-3 protein or regulation of MEKK3-dependent ERK and JNK activity. Nonetheless, these results suggest that MEKK3 is a convergence point of multiple upstream signaling pathways.