A MAP kinase targeted by endotoxin and hyperosmolarity in mammalian cells

Mammalian cells respond to endotoxic lipopolysaccharide (LPS) by activation of protein kinase cascades that lead to new gene expression. A protein kinase, p38, that was tyrosine phosphorylated in response to LPS, was cloned. The p38 enzyme and the product of the Saccharomyces cerevisiae HOG1 gene, which are both members of the mitogen-activated protein (MAP) kinase family, have sequences at and adjacent to critical phosphorylation sites that distinguish these proteins from most other MAP kinase family members. Both HOG1 and p38 are tyrosine phosphorylated after extracellular changes in osmolarity. These findings link a signaling pathway in mammalian cells with a pathway in yeast that is responsive to physiological stress.

Pro-inflammatory cytokines and environmental stress cause p38 mitogen-activated protein kinase activation by dual phosphorylation on tyrosine and threonine

Protein kinases activated by dual phosphorylation on Tyr and Thr (MAP kinases) can be grouped into two major classes: ERK and JNK. The ERK group regulates multiple targets in response to growth factors via a Ras-dependent mechanism. In contrast, JNK activates the transcription factor c-Jun in response to pro-inflammatory cytokines and exposure of cells to several forms of environmental stress. Recently, a novel mammalian protein kinase (p38) that shares sequence similarity with mitogen-activated protein (MAP) kinases was identified. Here, we demonstrate that p38, like JNK, is activated by treatment of cells with pro-inflammatory cytokines and environmental stress. The mechanism of p38 activation is mediated by dual phosphorylation on Thr-180 and Tyr-182. Immunofluorescence microscopy demonstrated that p38 MAP kinase is present in both the nucleus and cytoplasm of activated cells. Together, these data establish that p38 is a member of the mammalian MAP kinase group.

Independent human MAP-kinase signal transduction pathways defined by MEK and MKK isoforms

Mammalian mitogen-activated protein (MAP) kinases include extracellular signal-regulated protein kinase (ERK), c-Jun amino-terminal kinase (JNK), and p38 subgroups. These MAP kinase isoforms are activated by dual phosphorylation on threonine and tyrosine. Two human MAP kinase kinases (MKK3 and MKK4) were cloned that phosphorylate and activate p38 MAP kinase. These MKK isoforms did not activate the ERK subgroup of MAP kinases, but MKK4 did activate JNK. These data demonstrate that the activators of p38 (MKK3 and MKK4), JNK (MKK4), and ERK (MEK1 and MEK2) define independent MAP kinase signal transduction pathways.

The p38 signal transduction pathway: activation and function

The p38 signalling transduction pathway, a Mitogen-activated protein (MAP) kinase pathway, plays an essential role in regulating many cellular processes including inflammation, cell differentiation, cell growth and death. Activation of p38 often through extracellular stimuli such as bacterial pathogens and cytokines, mediates signal transduction into the nucleus to turn on the responsive genes. p38 also transduces signals to other cellular components to execute different cellular responses. In this review, we summarize the characteristics of the major components of the p38 signalling transduction pathway and highlight the targets of this pathway and the physiological function of the p38 activation.

Fate of the mammalian cranial neural crest during tooth and mandibular morphogenesis

Neural crest cells are multipotential stem cells that contribute extensively to vertebrate development and give rise to various cell and tissue types. Determination of the fate of mammalian neural crest has been inhibited by the lack of appropriate markers. Here, we make use of a two-component genetic system for indelibly marking the progeny of the cranial neural crest during tooth and mandible development. In the first mouse line, Cre recombinase is expressed under the control of the Wnt1 promoter as a transgene. Significantly, Wnt1 transgene expression is limited to the migrating neural crest cells that are derived from the dorsal CNS. The second mouse line, the ROSA26 conditional reporter (R26R), serves as a substrate for the Cre-mediated recombination. Using this two-component genetic system, we have systematically followed the migration and differentiation of the cranial neural crest (CNC) cells from E9.5 to 6 weeks after birth. Our results demonstrate, for the first time, that CNC cells contribute to the formation of condensed dental mesenchyme, dental papilla, odontoblasts, dentine matrix, pulp, cementum, periodontal ligaments, chondrocytes in Meckel's cartilage, mandible, the articulating disc of temporomandibular joint and branchial arch nerve ganglia. More importantly, there is a dynamic distribution of CNC- and non-CNC-derived cells during tooth and mandibular morphogenesis. These results are a first step towards a comprehensive understanding of neural crest cell migration and differentiation during mammalian craniofacial development. Furthermore, this transgenic model also provides a new tool for cell lineage analysis and genetic manipulation of neural-crest-derived components in normal and abnormal embryogenesis.

Identification of Lps2 as a key transducer of MyD88-independent TIR signalling

In humans, ten Toll-like receptor (TLR) paralogues sense molecular components of microbes, initiating the production of cytokine mediators that create the inflammatory response. Using N-ethyl-N-nitrosourea, we induced a germline mutation called Lps2, which abolishes cytokine responses to double-stranded RNA and severely impairs responses to the endotoxin lipopolysaccharide (LPS), indicating that TLR3 and TLR4 might share a specific, proximal transducer. Here we identify the Lps2 mutation: a distal frameshift error in a Toll/interleukin-1 receptor/resistance (TIR) adaptor protein known as Trif or Ticam-1. Trif(Lps2) homozygotes are markedly resistant to the toxic effects of LPS, and are hypersusceptible to mouse cytomegalovirus, failing to produce type I interferons when infected. Compound homozygosity for mutations at Trif and MyD88 (a cytoplasmic TIR-domain-containing adaptor protein) loci ablates all responses to LPS, indicating that only two signalling pathways emanate from the LPS receptor. However, a Trif-independent cell population is detectable when Trif(Lps2) mutant macrophages are stimulated with LPS. This reveals that an alternative MyD88-dependent 'adaptor X' pathway is present in some, but not all, macrophages, and implies afferent immune specialization.

Role of substrates and products of PI 3-kinase in regulating activation of Rac-related guanosine triphosphatases by Vav

Mitogen stimulation of cytoskeletal changes and c-jun amino-terminal kinases is mediated by Rac small guanine nucleotide-binding proteins. Vav, a guanosine diphosphate (GDP)-guanosine triphosphate (GTP) exchange factor for Rac that stimulates the exchange of bound GDP for GTP, bound to and was directly controlled by substrates and products of phosphoinositide (PI) 3-kinase. The PI 3-kinase substrate phosphatidylinositol-4,5-bisphosphate inhibited activation of Vav by the tyrosine kinase Lck, whereas the product phosphatidylinositol-3,4,5-trisphosphate enhanced phosphorylation and activation of Vav by Lck. Control of Vav in response to mitogens by the products of PI 3-kinase suggests a mechanism for Ras-dependent activation of Rac.

Activation of the transcription factor MEF2C by the MAP kinase p38 in inflammation

For cells of the innate immune system to mount a host defence response to infection, they must recognize products of microbial pathogens such as lipopolysaccharide (LPS), the endotoxin secreted by Gram-negative bacteria. These cellular responses require intracellular signalling pathways, such as the four MAP kinase (MAPK) pathways. In mammalian cells the MAPK p38 is thought to play an important role in the regulation of cellular responses during infection through its effects on the expression of proinflammatory molecules. One means of understanding the role of p38 in these responses is to identify proteins with functions regulated by p38-catalysed phosphorylation. Here we demonstrate a link between the p38 pathway and a member of the myocyte-enhancer factor 2 (MEF2) group of transcription factors. We found that in monocytic cells, LPS increases the transactivation activity of MEF2C through p38-catalysed phosphorylation. One consequence of MEF2C activation is increased c-jun gene transcription. Our results show that p38 may influence host defence and inflammation by maintaining the balance of c-Jun protein consumed during infection.

Cardiac muscle cell hypertrophy and apoptosis induced by distinct members of the p38 mitogen-activated protein kinase family

p38 mitogen-activated protein (MAP) kinase activities were significantly increased in mouse hearts after chronic transverse aortic constriction, coincident with the onset of ventricular hypertrophy. Infection of cardiomyocytes with adenoviral vectors expressing upstream activators for the p38 kinases, activated mutants of MAP kinase kinase 3b(E) (MKK3bE) and MAP kinase kinase 6b(E) (MKK6bE), elicited characteristic hypertrophic responses, including an increase in cell size, enhanced sarcomeric organization, and elevated atrial natriuretic factor expression. Overexpression of the activated MKK3bE in cardiomyocytes also led to an increase in apoptosis. The hypertrophic response was enhanced by co-infection of an adenoviral vector expressing wild type p38 beta, and was suppressed by the p38 beta dominant negative mutant. In contrast, the MKK3bE-induced cell death was increased by co-infection of an adenovirus expressing wild type p38 alpha, and was suppressed by the dominant negative p38 alpha mutant. This provides the first evidence in any cell system for divergent physiological functions for different members of the p38 MAP kinase family. The direct involvement of p38 pathways in cardiac hypertrophy and apoptosis suggests a significant role for p38 signaling in the pathophysiology of heart failure.

Rho family GTPases regulate p38 mitogen-activated protein kinase through the downstream mediator Pak1

The stress-activated p38 mitogen-activated protein (MAP) kinase defines a subgroup of the mammalian MAP kinases that appear to play a key role in regulating inflammatory responses. Co-expression of constitutively active forms of Rac and Cdc42 leads to activation of p38 while dominant negative Rac and Cdc42 inhibit the ability of interleukin-1 to increase p38 activity. p21-activated kinase 1 (Pak1) is a potential mediator of Rac/Cdc42 signaling, and we observe that Pak1 stimulates p38 activity. A dominant negative Pak1 suppresses both interleukin-1- and Rac/Cdc42-induced p38 activity. Rac and Cdc42 appear to regulate a protein kinase cascade initiated at the level of Pak and leading to activation of p38 and JNK.

Characterization of the structure and function of a new mitogen-activated protein kinase (p38beta)

Mitogen-activated protein (MAP) kinase cascades represent one of the major signal systems used by eukaryotic cells to transduce extracellular signals into cellular responses. Four MAP kinase subgroups have been identified in humans: ERK, JNK (SAPK), ERK5 (BMK), and p38. Here we characterize a new MAP kinase, p38beta. p38beta is a 372-amino acid protein most closely related to p38. It contains a TGY dual phosphorylation site, which is required for its kinase activity. Like p38, p38beta is activated by proinflammatory cytokines and environmental stress. A comparison of events associated with the activation of p38beta and p38 revealed differences, most notably in the preferred activation of p38beta by MAP kinase kinase 6 (MKK6), whereas p38 was activated nearly equally by MKK3, MKK4, and MKK6. Moreover, in vitro and in vivo experiments showed a strong substrate preference by p38beta for activating transcription factor 2 (ATF2). Enhancement of ATF2-dependent gene expression by p38beta was approximately20-fold greater than that of p38 and other MAP kinases tested. The data reported here suggest that while closely related, p38beta and p38 may be regulated by differing mechanisms and may exert their actions on separate downstream targets.

Separation of long DNA molecules in a microfabricated entropic trap array

A nanofluidic channel device, consisting of many entropic traps, was designed and fabricated for the separation of long DNA molecules. The channel comprises narrow constrictions and wider regions that cause size-dependent trapping of DNA at the onset of a constriction. This process creates electrophoretic mobility differences, thus enabling efficient separation without the use of a gel matrix or pulsed electric fields. Samples of long DNA molecules (5000 to approximately 160,000 base pairs) were efficiently separated into bands in 15-millimeter-long channels. Multiple-channel devices operating in parallel were demonstrated. The efficiency, compactness, and ease of fabrication of the device suggest the possibility of more practical integrated DNA analysis systems.

BMK1/ERK5 regulates serum-induced early gene expression through transcription factor MEF2C

Big MAP kinase 1 (BMK1), also known as ERK5, is a mitogen-activated protein (MAP) kinase member whose biological role is largely undefined. We have shown previously that the activity of BMK1 in rat smooth muscle cells is up-regulated by oxidants. Here, we describe a constitutively active form of the MAP kinase kinase, MEK5(D), which selectively activates BMK1 but not other MAP kinases in vivo. Through utilization of MEK5(D), we have determined that a member of the MEF2 transcription factor family, MEF2C, is a protein substrate of BMK1. BMK1 dramatically enhances the transactivation activity of MEF2C by phosphorylating a serine residue at amino acid position 387 in this transcription factor. Serum is also a potent stimulator of BMK1-induced MEF2C phosphorylation, since a dominant-negative form of BMK1 specifically inhibits serum-induced activation of MEF2C. One consequence of MEF2C activation is increased transcription of the c-jun gene. Taken together, these results strongly suggest that in some cell types the MEK5/BMK1 MAP kinase signaling pathway regulates serum-induced early gene expression through the transcription factor MEF2C.

Characterization of the structure and function of a novel MAP kinase kinase (MKK6)

Mitogen-activated protein (MAP) kinases require dual phosphorylation on threonine and tyrosine residues in order to gain enzymatic activity. This activation is carried out by a family of enzymes known as MAP kinase kinases (MKKs or MEKs). It appears that there are at least four subgroups in this family; MEK1/MEK2 subgroup that activates ERK1/ERK2, MEK5 that activates ERK5/BMK1, MKK3 that activates p38, and MKK4 that activates p38 and Jun kinase. Here we describe the characteristics of a new MKK termed MKK6. The clones we isolated encode two splice isoforms of human MKK6 comprised of 278 and 334 amino acids, respectively, and one murine MKK6 with 237 amino acids. Sequence information derived from cDNA cloning indicated that MKK6 is most closely related to MKK3. The functional data revealed from co-transfection assays suggests that MKK6, like MKK3, selectively phosphorylates p38. Unlike the previously described MKKs (or MEKs), MKK6 exists in a variety of alternatively spliced isoforms with distinct patterns of tissue expression. This suggests novel mechanisms regulating activation and/or function of various forms of MKK6.

Vaccination of chimpanzees against infection by the hepatitis C virus

A high incidence of community-acquired hepatitis C virus infection that can lead to the progressive development of chronic active hepatitis, liver cirrhosis, and primary hepatocellular carcinoma occurs throughout the world. A vaccine to control the spread of this agent that represents a major cause of chronic liver disease is therefore needed. Seven chimpanzees (Pan troglodytes) have been immunized with both putative envelope glycoproteins [E1 (gp33) and E2 (gp72)] that were copurified from HeLa cells infected with a recombinant vaccinia virus expression vector. Despite the induction of a weak humoral immune response to these viral glycoproteins in experimentally infected chimpanzees, a strong humoral immune response was obtained in all vaccines. The five highest responders showed complete protection against an i.v. challenge with homologous hepatitis C virus 1. The remaining two vaccines became infected, but both infection and disease may have been ameliorated in comparison with four similarly challenged control chimpanzees, all of which developed acute hepatitis and chronic infections. These results provide considerable encouragement for the eventual control of hepatitis C virus infection by vaccination.

Endotoxin-responsive sequences control cachectin/tumor necrosis factor biosynthesis at the translational level

The biosynthesis of cachectin/TNF is largely regulated at a post-transcriptional level. Bacterial endotoxin, which strongly induces cachectin/TNF production, thus seems to elicit at least some of its effects by altering the macrophage cytoplasmic milieu. It has previously been shown that the 3'-untranslated TTATTTAT element present in numerous cytokines and proto-oncogenes is capable of repressing the translation of mRNA molecules in which it is represented. Using constructs in which the CAT coding sequence is followed by varying segments of the cachectin/TNF 3'-untranslated region, we now demonstrate that downstream sequences present in the cachectin/TNF mRNA are sufficient to mediate greater than 200-fold induction of CAT synthesis in response to activation by endotoxin. Induction of CAT activity is not attributable to a change in cytoplasmic mRNA concentration, but to a marked enhancement of translational efficiency. The response to endotoxin represents "derepression," and is conferred chiefly by the translationally repressive TTATTTAT element, acting in concert with essential flanking sequences.

Characterization of the structure and function of the fourth member of p38 group mitogen-activated protein kinases, p38delta

We have cloned and characterized a new member of the p38 group of mitogen-activated protein kinases here termed p38delta. Sequence comparisons revealed that p38delta is approximately 60% identical to the other three p38 isoforms but only 40-45% to the other mitogen-activated protein kinase family members. It contains the TGY dual phosphorylation site present in all p38 group members and is activated by a group of extracellular stimuli including cytokines and environmental stresses that also activate the other three known p38 isoforms. However, unlike the other p38 isoforms, the kinase activity of p38delta is not blocked by the pyridinyl imidazole, 4-(4-fluorophenyl)-2-2(4-hydroxyphenyl)-5-(4-pyridyl)-imidazole (identicalto SB202190). p38delta can be activated by MKK3 and MKK6, known activators of the other isoforms. Nonetheless, in-gel kinase assays provide evidence for additional activators. The data presented herein show that p38delta has many properties that are similar to those of other p38 group members. Nonetheless important differences exist among the four members of the p38 group of enzymes, and thus each may have highly specific, individual contributions to biologic events involving activation of the p38 pathways.

Dexamethasone and pentoxifylline inhibit endotoxin-induced cachectin/tumor necrosis factor synthesis at separate points in the signaling pathway

The induction of cachectin/tumor necrosis factor (TNF) synthesis by bacterial endotoxins is a process that entails activation at several levels. Cachectin/TNF gene transcription is accelerated, leading to rapid accumulation of mRNA within the macrophage cytosol. In addition, translational derepression occurs, leading to far more efficient message utilization. Through the use of posttranscriptional reporter constructs, we now demonstrate that certain agents capable of inhibiting cachectin/TNF biosynthesis operate through different mechanisms. In RAW 264.7 macrophages, pentoxifylline blocks cachectin/TNF mRNA accumulation but has no effect upon the efficiency of reporter mRNA translation. Dexamethasone, on the other hand, has only a modest effect on cachectin/TNF mRNA accumulation, but strongly impedes translational derepression. Combined application of dexamethasone and pentoxifylline to macrophages causes a greater suppression of cachectin/TNF biosynthesis that can be achieved by either agent alone. These findings suggest that the signaling pathway activated by endotoxin is branched, and that selective inhibition of different parts of the pathway may be achieved through the use of distinct agents.

Regulation of the MEF2 family of transcription factors by p38

Members of the MEF2 family of transcription factors bind as homo- and heterodimers to the MEF2 site found in the promoter regions of numerous muscle-specific, growth- or stress-induced genes. We showed previously that the transactivation activity of MEF2C is stimulated by p38 mitogen-activated protein (MAP) kinase. In this study, we examined the potential role of the p38 MAP kinase pathway in regulating the other MEF2 family members. We found that MEF2A, but not MEF2B or MEF2D, is a substrate for p38. Among the four p38 group members, p38 is the most potent kinase for MEF2A. Threonines 312 and 319 within the transcription activation domain of MEF2A are the regulatory sites phosphorylated by p38. Phosphorylation of MEF2A in a MEF2A-MEF2D heterodimer enhances MEF2-dependent gene expression. These results demonstrate that the MAP kinase signaling pathway can discriminate between different MEF2 isoforms and can regulate MEF2-dependent genes through posttranslational activation of preexisting MEF2 protein.

Expression of bbc3, a pro-apoptotic BH3-only gene, is regulated by diverse cell death and survival signals

BH3-only proteins function at a proximal point in a conserved cell death pathway by binding, through their BH3 domains, to other Bcl-2 family members and triggering mitochondrial events associated with apoptosis. Here, we describe a strongly pro-apoptotic BH3-only protein, designated Bbc3, whose expression increases in response to diverse apoptotic stimuli. bbc3 mRNA levels were induced by exposure to DNA-damaging agents and by wild-type p53, which mediates DNA damage-induced apoptosis. p53 transactivated bbc3 through consensus p53 binding sites within the bbc3 promoter region, indicating that bbc3 is a direct target of p53. Additionally, bbc3 mRNA was induced by p53-independent apoptotic stimuli, including dexamethasone treatment of thymocytes, and serum deprivation of tumor cells. Insulin-like growth factor-1 and epidermal growth factor, growth factors with broad anti-apoptotic activity, were each sufficient to suppress Bbc3 expression in serum-starved tumor cells. These results suggest that the transcriptional regulation of bbc3 contributes to the transduction of diverse cell death and survival signals.

Interim analysis of the effects of exenatide treatment on A1C, weight and cardiovascular risk factors over 82 weeks in 314 overweight patients with type 2 diabetes

AIM

Exenatide, an incretin mimetic for the adjunct treatment of type 2 diabetes (DM2), reduced A1C and weight in 30-week placebo-controlled trials. This analysis examined the effects of exenatide on glycaemic control and weight over an 82-week period in patients with DM2 unable to achieve adequate glycaemic control with sulphonylurea (SU) and/or metformin (MET).

METHODS

This interim analysis is of 314 patients who received exenatide in the 30-week placebo-controlled trials and subsequently in 52 weeks of open-label uncontrolled extension studies for 82 weeks of exenatide in total. Patients continued their SU and/or MET regimens throughout.

RESULTS

Patients completed 82 weeks of exenatide treatment [n = 314, 63% M, age 56 +/- 10 years, weight 99 +/- 21 kg, body mass index 34 +/- 6 kg/m2, A1C 8.3 +/- 1.0% (mean +/- SD)]. Reduction in A1C from baseline to week 30 [-0.9 +/- 0.1% (mean +/- SE)] was sustained to week 82 (-1.1 +/- 0.1%), with 48% of patients achieving A1C < or = 7% at week 82. At week 30, exenatide reduced body weight (a secondary endpoint) from baseline (-2.1 +/- 0.2 kg), with progressive reduction at week 82 (-4.4 +/- 0.3 kg). Similar results were observed for the intent-to-treat population (n = 551), with reductions in A1C and weight at week 82 of -0.8 +/- 0.1% and -3.5 +/- 0.2 kg respectively. The 82-week completer cohort showed statistically significant improvement in some cardiovascular risk factors. The most frequent adverse events were generally mild-to-moderate nausea and hypoglycaemia.

CONCLUSION

In summary, 82 weeks of adjunctive exenatide treatment in patients with DM2 treated with SU and/or MET resulted in sustained reduction in A1C and progressive reduction in weight, as well as improvement in some cardiovascular risk factors.

The primary structure of p38 gamma: a new member of p38 group of MAP kinases

We have identified a third member of the p38 group of MAP kinase termed p38 gamma. The cDNA for this MAP kinase encodes an 367 amino acid polypeptide that is slightly greater than 60% identical to p38 and p38 beta. Expression of its mRNA is primarily in muscle with no detectable expression in Northern blots using RNA from other tissues. In contrast to p38 and p38 beta, p38 gamma, fails to phosphorylate ATF-2 or MAPKAP kinase 2 but does like other MAP kinases phosphorylate MBP. In vivo kinase assays using protein extracts from skeletal muscle reveal that a 7-kDa protein is phosphorylated by p38 gamma but not by other members of this group of kinases. We suggest p38 gamma may have unique functions when compared with other members of the p38 group due to its restricted tissue expression and apparent substrate preferences.

Genetic variation in XRCC1, sun exposure, and risk of skin cancer

The XRCC1 gene is involved in the base excision repair pathway. We assessed the associations of polymorphisms and haplotypes in XRCC1 with skin cancer risk in a nested case–control study within the Nurses' Health Study (219 melanoma, 286 squamous cell carcinoma (SCC) and 300 basal cell carcinoma (BCC), and 873 controls). We genotyped four haplotype-tagging single-nucleotide polymorphisms (Arg194Trp, C26602T, Arg399Gln, and Gln632Gln). There was no significant difference in frequency distribution between cases and controls for any of the five inferred common haplotypes. We observed that the 399Gln allele was inversely associated with SCC risk. This inverse association was only seen among those who had five or more lifetime sunburns, those with a family history of skin cancer, and those in the highest tertile of cumulative sun exposure in a bathing suit, but not among those with low risk defined by these risk factors. We also observed a significant association of the carriage of 194Trp allele with increased SCC risk, which was modified by family history of skin cancer. These two polymorphisms were not associated with BCC or melanoma risk. Our data suggest that the Arg194Trp and Arg399Gln polymorphisms may be differently associated with skin cancer risk according to exposure dose and skin cancer type.