HIV-1 Transcription Inhibitor 1E7-03 Decreases Nucleophosmin Phosphorylation

Transcription activation of latent human immunodeficiency virus-1 (HIV-1) occurs due to HIV-1 rebound, the interruption of combination antiretroviral therapy, or development of drug resistance. Thus, novel HIV-1 inhibitors, targeting HIV-1 transcription are needed. We previously developed an HIV-1 transcription inhibitor, 1E7-03, that binds to the noncatalytic RVxF-accommodating site of protein phosphatase 1 and inhibits HIV-1 replication in cultured cells and HIV-1-infected humanized mice by impeding protein phosphatase 1 interaction with HIV-1 Tat protein. However, host proteins and regulatory pathways targeted by 1E7-03 that contribute to its overall HIV-1 inhibitory activity remain to be identified. To address this issue, we performed label-free quantitative proteome and phosphoproteome analyses of noninfected and HIV-1-infected CEM T cells that were untreated or treated with 1E7-03. 1E7-03 significantly reprogramed the phosphorylation profile of proteins including PPARα/RXRα, TGF-β, and PKR pathways. Phosphorylation of nucleophosmin (NPM1) at Ser-125 residue in PPARα/RXRα pathway was significantly reduced (>20-fold, p = 1.37 × 10-9), followed by the reduced phosphorylation of transforming growth factor-beta 2 at Ser-46 (TGF-β2, >12-fold, p = 1.37 × 10-3). Downregulation of NPM1's Ser-125 phosphorylation was further confirmed using Western blot. Phosphorylation mimicking NPM1 S125D mutant activated Tat-induced HIV-1 transcription and exhibited enhanced NPM1-Tat interaction compared to NPM1 S125A mutant. Inhibition of Aurora A or Aurora B kinases that phosphorylate NPM1 on Ser-125 residue inhibited HIV-1, further supporting the role of NPM1 in HIV-1 infection. Taken together, 1E7-03 reprogrammed PPARα/RXRα and TGF-β pathways that contribute to the inhibition of HIV-1 transcription. Our findings suggest that NPM1 phosphorylation is a plausible target for HIV-1 transcription inhibition.

The Host-Microbiome Response to Hyperbaric Oxygen Therapy in Ulcerative Colitis Patients

BACKGROUND & AIMS

Hyperbaric oxygen therapy (HBOT) is a promising treatment for moderate-to-severe ulcerative colitis. However, our current understanding of the host and microbial response to HBOT remains unclear. This study examined the molecular mechanisms underpinning HBOT using a multi-omic strategy.

METHODS

Pre- and post-intervention mucosal biopsies, tissue, and fecal samples were collected from HBOT phase 2 clinical trials. Biopsies and fecal samples were subjected to shotgun metaproteomics, metabolomics, 16s rRNA sequencing, and metagenomics. Tissue was subjected to bulk RNA sequencing and digital spatial profiling (DSP) for single-cell RNA and protein analysis, and immunohistochemistry was performed. Fecal samples were also used for colonization experiments in IL10^-/- germ-free UC mouse models.

RESULTS

Proteomics identified negative associations between HBOT response and neutrophil azurophilic granule abundance. DSP identified an HBOT-specific reduction of neutrophil STAT3, which was confirmed by immunohistochemistry. HBOT decreased microbial diversity with a proportional increase in Firmicutes and a secondary bile acid lithocholic acid. A major source of the reduction in diversity was the loss of mucus-adherent taxa, resulting in increased MUC2 levels post-HBOT. Targeted database searching revealed strain-level associations between Akkermansia muciniphila and HBOT response status. Colonization of IL10^-/- with stool obtained from HBOT responders resulted in lower colitis activity compared with non-responders, with no differences in STAT3 expression, suggesting complementary but independent host and microbial responses.

CONCLUSIONS

HBOT reduces host neutrophil STAT3 and azurophilic granule activity in UC patients and changes in microbial composition and metabolism in ways that improve colitis activity. Intestinal microbiota, especially strain level variations in A muciniphila, may contribute to HBOT non-response.

Myeloid Cell Mediated Immune Suppression in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDA), the most common pancreatic cancer, is a nearly universally lethal malignancy. PDA is characterized by extensive infiltration of immunosuppressive myeloid cells, including tumor-associated macrophages and myeloid-derived suppressor cells. Myeloid cells in the tumor microenvironment inhibit cytotoxic T-cell responses promoting carcinogenesis. Immune checkpoint therapy has not been effective in PDA, most likely because of this robust immune suppression, making it critical to elucidate mechanisms behind this phenomenon. Here, we review myeloid cell infiltration and cellular crosstalk in PDA progression and highlight current therapeutic approaches to target myeloid cell-driven immune suppression.

Wnt Signalling Drives Context-Dependent Differentiation or Proliferation in Neuroblastoma

Neuroblastoma is one of the commonest and deadliest solid tumours of childhood, and is thought to result from disrupted differentiation of the developing sympathoadrenergic lineage of the neural crest. Neuroblastoma exhibits intra- and intertumoural heterogeneity, with high risk tumours characterised by poor differentiation, which can be attributable to MYCN-mediated repression of genes involved in neuronal differentiation. MYCN is known to co-operate with oncogenic signalling pathways such as Alk, Akt and MEK/ERK signalling, and, together with c-MYC has been shown to be activated by Wnt signalling in various tissues. However, our previous work demonstrated that Wnt3a/Rspo2 treatment of some neuroblastoma cell lines can, paradoxically, decrease c-MYC and MYCN proteins. This prompted us to define the neuroblastoma-specific Wnt3a/Rspo2-driven transcriptome using RNA sequencing, and characterise the accompanying changes in cell biology. Here we report the identification of ninety Wnt target genes, and show that Wnt signalling is upstream of numerous transcription factors and signalling pathways in neuroblastoma. Using live-cell imaging, we show that Wnt signalling can drive differentiation of SK-N-BE(2)-C and SH-SY5Y cell-lines, but, conversely, proliferation of SK-N-AS cells. We show that cell-lines that differentiate show induction of pro-differentiation BMP4 and EPAS1 proteins, which is not apparent in the SK-N-AS cells. In contrast, SK-N-AS cells show increased CCND1, phosphorylated RB and E2F1 in response to Wnt3a/Rspo2, consistent with their proliferative response, and these proteins are not increased in differentiating lines. By meta-analysis of the expression of our 90 genes in primary tumour gene expression databases, we demonstrate discrete expression patterns of our Wnt genes in patient cohorts with different prognosis. Furthermore our analysis reveals interconnectivity within subsets of our Wnt genes, with one subset comprised of novel putative drivers of neuronal differentiation repressed by MYCN. Assessment of β-catenin immunohistochemistry shows high levels of β-catenin in tumours with better differentiation, further supporting a role for canonical Wnt signalling in neuroblastoma differentiation.

Shikonin causes cell-cycle arrest and induces apoptosis by regulating the EGFR-NF-κB signalling pathway in human epidermoid carcinoma A431 cells

Shikonin, a naphthoquinone pigment isolated from the Chinese herbal Zicao, has been shown to exhibit antioxidant and anticancer effects. In the present study, we investigated the antiproliferative and pro-apoptotic effects of shikonin on A431 cells and explored the underlying molecular mechanisms. In the present study, our results showed that shikonin significantly inhibited the growth of A431 cells in a concentration- and time-dependent manner, and caused cell cycle arrest by upregulation of p21 and p27, and downregulation of cyclins and cyclin-dependent kinases. In addition, shikonin evidently induced apoptosis due to decreasing Bcl-2 expression, increasing Bax expression, activating caspase and inactivating NF-κB, while pretreatment with a pan-caspase inhibitor Z-Asp-CH2-DCB abrogated shikonin-induced apoptosis. Moreover, EGF could significantly increase the NF-κB DNA-binding activity and reversed the shikonin-induced inactivation of NF-κB. As anticipated AG1478 (EGFR inhibitor) and Bay11-7082 (NF-κB inhibitor) blocked EGF-reversed the inactivation of NF-κB induced by shikonin. Our data also showed that EGF could evidently reverse the shikonin-induced decreases in cell viability and increases in apoptosis. Then, the NF-κB inhibitors such as Bay11-7082, SN50, Helenalin and the EGFR inhibitor AG1478 and its downstream inhibitor such as PI3K inhibitor LY294002 and STAT3 inhibitor Stattic dramatically blocked EGF-reversed decreases in cell viability and increases in apoptosis induced by shikonin. Collectively, our findings indicated that shikonin inhibited cell growth and caused cell cycle arrest of the A431 cells through the regulation of apoptosis. Moreover, these effects were mediated at least partially by suppressing the activation of the EGFR-NF-κB signaling pathways.

Heparanase mediates a novel mechanism in lapatinib-resistant brain metastatic breast cancer

Heparanase (HPSE) is the dominant mammalian endoglycosidase and important tumorigenic, angiogenic, and pro-metastatic molecule. Highest levels of HPSE activity have been consistently detected in cells possessing highest propensities to colonize the brain, emphasizing the therapeutic potential for targeting HPSE in brain metastatic breast cancer (BMBC). Lapatinib (Tykerb) is a small-molecule and dual inhibitor of human epidermal growth factor receptor1 and 2 (EGFR and HER2, respectively) which are both high-risk predictors of BMBC. It was approved by the US Food and Drug Administration for treatment of patients with advanced or metastatic breast cancer. However, its role is limited in BMBC whose response rates to lapatinib are significantly lower than those for extracranial metastasis. Because HPSE can affect EGFR phosphorylation, we examined Roneparstat, a non-anticoagulant heparin with potent anti-HPSE activity, to inhibit EGFR signaling pathways and BMBC onset using lapatinib-resistant clones generated from HER2-transfected, EGFR-expressing MDA-MB-231BR cells. Cell growth, EGFR pathways, and HPSE targets were assessed among selected clones in the absence or presence of Roneparstat and/or lapatinib. Roneparstat overcame lapatinib resistance by inhibiting pathways associated with EGFR tyrosine residues that are not targeted by lapatinib. Roneparstat inhibited the growth and BMBC abilities of lapatinib-resistant clones. A molecular mechanism was identified by which HPSE mediates an alternative survival pathway in lapatinib-resistant clones and is modulated by Roneparstat. These results demonstrate that the inhibition of HPSE-mediated signaling plays important roles in lapatinib resistance, and provide mechanistic insights to validate the use of Roneparstat for novel BMBC therapeutic strategies.

Transcriptional regulation induced by cAMP elevation in mouse Schwann cells

In peripheral nerves, Schwann cell development is regulated by a variety of signals. Some of the aspects of Schwann cell differentiation can be reproduced in vitro in response to forskolin, an adenylyl cyclase activator elevating intracellular cAMP levels. Herein, the effect of forskolin treatment was investigated by a comprehensive genome-wide expression study on primary mouse Schwann cell cultures. Additional to myelin-related genes, many so far unconsidered genes were ascertained to be modulated by forskolin. One of the strongest differentially regulated gene transcripts was the transcription factor Olig1 (oligodendrocyte transcription factor 1), whose mRNA expression levels were reduced in treated Schwann cells. Olig1 protein was localized in myelinating and nonmyelinating Schwann cells within the sciatic nerve as well as in primary Schwann cells, proposing it as a novel transcription factor of the Schwann cell lineage. Data analysis further revealed that a number of differentially expressed genes in forskolin-treated Schwann cells were associated with the ECM (extracellular matrix), underlining its importance during Schwann cell differentiation in vitro. Comparison of samples derived from postnatal sciatic nerves and from both treated and untreated Schwann cell cultures showed considerable differences in gene expression between in vivo and in vitro, allowing us to separate Schwann cell autonomous from tissue-related changes. The whole data set of the cell culture microarray study is provided to offer an interactive search tool for genes of interest.

Recent advances in pre-clinical mouse models of COPD

COPD (chronic obstructive pulmonary disease) is a major incurable global health burden and will become the third largest cause of death in the world by 2020. It is currently believed that an exaggerated inflammatory response to inhaled irritants, in particular cigarette smoke, causes progressive airflow limitation. This inflammation, where macrophages, neutrophils and T-cells are prominent, leads to oxidative stress, emphysema, small airways fibrosis and mucus hypersecretion. The mechanisms and mediators that drive the induction and progression of chronic inflammation, emphysema and altered lung function are poorly understood. Current treatments have limited efficacy in inhibiting chronic inflammation, do not reverse the pathology of disease and fail to modify the factors that initiate and drive the long-term progression of disease. Therefore there is a clear need for new therapies that can prevent the induction and progression of COPD. Animal modelling systems that accurately reflect disease pathophysiology continue to be essential to the development of new therapies. The present review highlights some of the mouse models used to define the cellular, molecular and pathological consequences of cigarette smoke exposure and whether they can be used to predict the efficacy of new therapeutics for COPD.

Nox2-derived ROS in PPARγ signaling and cell-cycle progression of lung alveolar epithelial cells

Reactive oxygen species (ROS) play important roles in peroxisome proliferator-activated receptor γ (PPARγ) signaling and cell-cycle regulation. However, the PPARγ redox-signaling pathways in lung alveolar epithelial cells remain unclear. In this study, we investigated the in vivo and in vitro effects of PPARγ activation on the levels of lung ROS production and cell-cycle progression using C57BL/6J wild-type and Nox2 knockout mice (n=10) after intraperitoneal injection of a selective PPARγ agonist (GW1929, 5 mg/kg body wt, daily) for 14 days. Compared to vehicle-treated mice, GW1929 increased significantly the levels of ROS production in wild-type lungs, and this was accompanied by significant up-regulation of PPARγ, Nox2, PCNA, and cyclin D1 and phosphorylation of ERK1/2 and p38MAPK. These effects were absent in Nox2 knockout mice. In cultured alveolar epithelial cells, GW1929 (5 μM for 24 h) increased ROS production and promoted cell-cycle progression from G0/G1 into S and G2/M phases, and these effects were abolished by (1) adding a PPARγ antagonist (BADGE, 1 μM), (2) knockdown of PPARγ using siRNA, or (3) knockout of Nox2. In conclusion, PPARγ activation through Nox2-derived ROS promotes cell-cycle progression in normal mouse lungs and in cultured normal alveolar epithelial cells.

Capacity for resolution of Ras-MAPK-initiated early pathogenic myocardial hypertrophy modeled in mice

Activation of Ras signaling in cardiomyocytes has been linked to pathogenic myocardial hypertrophy progression and subsequent heart failure. Whether cardiomyopathy can regress once initiated needs to be established more fully. A 'tet-off' system was used to regulate expression of H-Ras-G12V in myocardium to examine whether Ras-induced pathogenic myocardial hypertrophy could resolve after removal of Ras signaling in vivo. Ras activation at weaning for 2 wk caused hypertrophy, whereas activation for 4 to 8 wk led to cardiomyopathy and heart failure. Discontinuing H-Ras-G12V transgene expression after cardiomyopathy onset led to improved survival and cardiomyopathy lesion scores, with reduced heart:body weight ratios, demonstrating the reversibility of early pathogenic hypertrophy. Activation of Ras and downstream ERK 1/2 was associated with elevated expression of proliferating cell nuclear antigen and cyclins B1 and D1, indicating cell-cycle activation and reentry. Coordinate elevation of broad-spectrum cyclin-dependent kinase inhibitors (p21, p27, and p57) and Tyr15 phosphorylation of cdc2 signified the activation of cell-cycle checkpoints; absence of cell-cycle completion and cardiomyocyte replication were documented by using immunohistochemistry for mitosis and cytokinesis markers. After resolution of cardiomyopathy, cell-cycle activators and inhibitors examined returned to basal levels, a change that we interpreted as exit from the cell cycle. Cardiac cell-cycle regulation plays a role in recovery from pathogenic hypertrophy. The model we present provides a means to further explore the underlying mechanisms governing cell-cycle capacity in cardiomyocytes, as well as progression and regression of pathogenic cardiomyocyte hypertrophy.

The Friedreich's ataxia protein frataxin modulates DNA base excision repair in prokaryotes and mammals

DNA-repair mechanisms enable cells to maintain their genetic information by protecting it from mutations that may cause malignant growth. Recent evidence suggests that specific DNA-repair enzymes contain ISCs (iron-sulfur clusters). The nuclearencoded protein frataxin is essential for the mitochondrial biosynthesis of ISCs. Frataxin deficiency causes a neurodegenerative disorder named Friedreich's ataxia in humans. Various types of cancer occurring at young age are associated with this disease, and hence with frataxin deficiency. Mice carrying a hepatocyte-specific disruption of the frataxin gene develop multiple liver tumours for unresolved reasons. In the present study, we show that frataxin deficiency in murine liver is associated with increased basal levels of oxidative DNA base damage. Accordingly, eukaryotic V79 fibroblasts overexpressing human frataxin show decreased basal levels of these modifications, while prokaryotic Salmonella enterica serotype Typhimurium TA104 strains transformed with human frataxin show decreased mutation rates. The repair rates of oxidative DNA base modifications in V79 cells overexpressing frataxin were significantly higher than in control cells. Lastly, cleavage activity related to the ISC-independent repair enzyme 8-oxoguanine glycosylase was found to be unaltered by frataxin overexpression. These findings indicate that frataxin modulates DNA-repair mechanisms probably due to its impact on ISC-dependent repair proteins, linking mitochondrial dysfunction to DNA repair and tumour initiation.

Myriocin-mediated up-regulation of hepatocyte apoA-I synthesis is associated with ERK inhibition

Sphingolipids including sphingomyelin have been implicated as potential atherogenic lipids. Studies in apoE (apolipoprotein E)-null mice have revealed that the serine palmitoyltransferase inhibitor myriocin reduces plasma levels of sphingomyelin, ceramide, sphingosine-1-phosphate and glycosphingolipids and that this is associated with potent inhibition of atherosclerosis. Interestingly, hepatic apoA-I (apolipoprotein A-I) synthesis and plasma HDL (high-density lipoprotein)-cholesterol levels were also increased in apoE-null mice treated with myriocin. Since myriocin is a known inhibitor of ERK (extracellular-signal-related kinase) phosphorylation, we assessed the possibility that myriocin may be acting to increase hepatic apoA-I production via this pathway. To address this, HepG2 cells and primary mouse hepatocytes were treated with 200 muM myriocin for up to 48 h. Myriocin increased apoA-I mRNA and protein levels by approx. 3- and 2-fold respectively. Myriocin also increased apoA-I secretion up to 3.5-fold and decreased ERK phosphorylation by approx. 70%. Similar findings were obtained when primary hepatocytes were isolated from apoE-null mice that were treated with myriocin (intraperitoneal injection at a dose of 0.3 mg/kg body weight). Further experiments revealed that the MEK (mitogen-activated protein kinase/ERK kinase) inhibitor PD98059 potently inhibited ERK phosphorylation, as expected, and increased primary hepatocyte apoA-I production by 3-fold. These results indicate that ERK phosphorylation plays a role in regulating hepatic apoA-I expression and suggest that the anti-atherogenic mechanism of action for myriocin may be linked to this pathway.

Deletion of mitogen-activated protein kinase phosphatase 1 modifies the response to mechanical bone marrow ablation in a mouse model

The maintenance of bone mass results from a delicate balance between bone formation by osteoblasts and bone resorption by osteoclasts. Understanding these processes is essential for the development of effective treatments for skeletal diseases. Mechanical bone marrow ablation provides a unique animal model to study bone repair and the roles of specific genes in this process. Ablation of marrow induces the formation of intramembranous bone in the medullary cavity, which is subsequently resorbed by osteoclasts. We used this model to ask whether mitogen-activated protein kinase (MAPK) phosphatase 1 (MKP1) affects the bone formed in response to marrow ablation. MKP1 is a negative regulator of MAPK signaling, which is essential for a wide variety of cellular mechanisms, including those critical for osteoblast and osteoclast function. At 10 d after mechanical bone marrow ablation, the femurs of male mkp1(+/+) and mkp1(-/-) mice were compared with those of unoperated baseline mice by using radiography, peripheral quantitative computed tomography, and microcomputed tomography. Both genotypes developed increased bone mass after marrow ablation, but the increase was more pronounced in mkp1(-/-) mice compared with mkp1(+/+) mice. These results indicate that MKP1 affects the bone formed in response to marrow ablation and suggest encouraging possibilities for the use of inhibitors of MKP1 to modulate bone repair.

Platelets from diabetic pigs exhibit hypersensitivity to thrombin

Responses of platelets from diabetic and diabetic-hyperlipidemic pigs were studied. Pigs were made diabetic with single dose of alloxan, which acts by selectively destroying insulin-producing pancreatic beta cells thus inducing type 1 diabetes. Pigs were kept for 1 or 12 wk, during which thrombin-induced aggregation was monitored in washed platelets. The platelets showed increased sensitivity to aggregation within 1 wk of induction of diabetes. Hyperlipidemia alone for 12 wk did not increase platelet hypersensitivity, but hyperlipidemia together with diabetes significantly increased thrombin-induced platelet aggregation. Because this hypersensitivity occurred in washed platelets, this characteristic appears to be independent of any contribution by plasma factors or other blood cells. The hypersensitivity of platelets from diabetic pigs correlated with decreased activity of mitogen-activated protein kinase. These studies offer the first evidence that platelet hyperactivity occurs during the early stages (within a week) of induction of diabetes in pigs and before manifestation of other cardiovascular problems.

MptpB, a virulence factor from Mycobacterium tuberculosis, exhibits triple-specificity phosphatase activity

Bacterial pathogens have developed sophisticated mechanisms of evading the immune system to survive in infected host cells. Central to the pathogenesis of Mycobacterium tuberculosis is the arrest of phagosome maturation, partly through interference with PtdIns signalling. The protein phosphatase MptpB is an essential secreted virulence factor in M. tuberculosis. A combination of bioinformatics analysis, enzyme kinetics and substrate-specificity characterization revealed that MptpB exhibits both dual-specificity protein phosphatase activity and, importantly, phosphoinositide phosphatase activity. Mutagenesis of conserved residues in the active site signature indicates a cysteine-based mechanism of dephosphorylation and identifies two new catalytic residues, Asp165, essential in catalysis, and Lys164, apparently involved in substrate specificity. Sequence similarities with mammalian lipid phosphatases and a preference for phosphoinositide substrates suggests a potential novel role of MptpB in PtdIns metabolism in the host and reveals new perspectives for the role of this phosphatase in mycobacteria pathogenicity.

p90 RSK-1 associates with and inhibits neuronal nitric oxide synthase

Evidence is presented that RSK1 (ribosomal S6 kinase 1), a downstream target of MAPK (mitogen-activated protein kinase), directly phosphorylates nNOS (neuronal nitric oxide synthase) on Ser847 in response to mitogens. The phosphorylation thus increases greatly following EGF (epidermal growth factor) treatment of rat pituitary tumour GH3 cells and is reduced by exposure to the MEK (MAPK/extracellular-signal-regulated kinase kinase) inhibitor PD98059. Furthermore, it is significantly enhanced by expression of wild-type RSK1 and antagonized by kinase-inactive RSK1 or specific reduction of endogenous RSK1. EGF treatment of HEK-293 (human embryonic kidney) cells, expressing RSK1 and nNOS, led to inhibition of NOS enzyme activity, associated with an increase in phosphorylation of nNOS at Ser847, as is also the case in an in vitro assay. In addition, these phenomena were significantly blocked by treatment with the RSK inhibitor Ro31-8220. Cells expressing mutant nNOS (S847A) proved resistant to phosphorylation and decrease of NOS activity. Within minutes of adding EGF to transfected cells, RSK1 associated with nNOS and subsequently dissociated following more prolonged agonist stimulation. EGF-induced formation of the nNOS-RSK1 complex was significantly decreased by PD98059 treatment. Treatment with EGF further revealed phosphorylation of nNOS on Ser847 in rat hippocampal neurons and cerebellar granule cells. This EGF-induced phosphorylation was partially blocked by PD98059 and Ro31-8220. Together, these data provide substantial evidence that RSK1 associates with and phosphorylates nNOS on Ser847 following mitogen stimulation and suggest a novel role for RSK1 in the regulation of nitric oxide function in brain.

The regulation of ATF3 gene expression by mitogen-activated protein kinases

ATF3 (activating transcription factor 3) gene encodes a member of the ATF/CREB (cAMP-response-element-binding protein) family of transcription factors. Its expression is induced by a wide range of signals, including stress signals and signals that promote cell proliferation and motility. Thus the ATF3 gene can be characterized as an 'adaptive response' gene for the cells to cope with extra- and/or intra-cellular changes. In the present study, we demonstrate that the p38 signalling pathway is involved in the induction of ATF3 by stress signals. Ectopic expression of CA (constitutively active) MKK6 [MAPK (mitogen-activated protein kinase) kinase 6], a kinase upstream of p38, indicated that activation of the p38 pathway is sufficient to induce the expression of the ATF3 gene. Inhibition of the pathway indicated that the p38 pathway is necessary for various signals to induce ATF3, including anisomycin, IL-1beta (interleukin 1beta), TNFalpha (tumour necrosis factor alpha) and H2O2. Analysis of the endogenous ATF3 gene indicates that the regulation is at least in part at the transcription level. Specifically, CREB, a transcription factor known to be phosphorylated by p38, plays a role in this induction. Interestingly, the ERK (extracellular-signal-regulated kinase) and JNK (c-Jun N-terminal kinase)/SAPK (stress-activated protein kinase) signalling pathways are neither necessary nor sufficient to induce ATF3 in the anisomycin stress paradigm. Furthermore, analysis of caspase 3 activation indicated that knocking down ATF3 reduced the ability of MKK6(CA) to exert its pro-apoptotic effect. Taken together, our results indicate that a major signalling pathway, the p38 pathway, plays a critical role in the induction of ATF3 by stress signals, and that ATF3 is functionally important to mediate the pro-apoptotic effects of p38.

The histone acetyltransferases CBP/p300 are degraded in NIH 3T3 cells by activation of Ras signalling pathway

The CBP [CREB (cAMP-response-element-binding protein)-binding protein]/p300 acetyltransferases function as transcriptional co-activators and play critical roles in cell differentiation and proliferation. Accumulating evidence shows that alterations of the CBP/p300 protein levels are linked to human tumours. In the present study, we show that the levels of the CBP/p300 co-activators are decreased dramatically by continuous PDGF (platelet-derived growth factor) and Ras signalling pathway activation in NIH 3T3 fibroblasts. This effect occurs by reducing the expression levels of the CBP/p300 genes. In addition, CBP and p300 are degraded by the 26 S proteasome pathway leading to an overall decrease in the levels of the CBP/p300 proteins. Furthermore, we provide evidence that Mdm2 (murine double minute 2), in the presence of active H-Ras or N-Ras, induces CBP/p300 degradation in NIH 3T3 cells. These findings support a novel mechanism for modulating other signalling transduction pathways that require these common co-activators.

Crystal structures and inhibitor identification for PTPN5, PTPRR and PTPN7: a family of human MAPK-specific protein tyrosine phosphatases

Protein tyrosine phosphatases PTPN5, PTPRR and PTPN7 comprise a family of phosphatases that specifically inactivate MAPKs (mitogen-activated protein kinases). We have determined high-resolution structures of all of the human family members, screened them against a library of 24000 compounds and identified two classes of inhibitors, cyclopenta[c]quinolinecarboxylic acids and 2,5-dimethylpyrrolyl benzoic acids. Comparative structural analysis revealed significant differences within this conserved family that could be explored for the design of selective inhibitors. PTPN5 crystallized, in two distinct crystal forms, with a sulphate ion in close proximity to the active site and the WPD (Trp-Pro-Asp) loop in a unique conformation, not seen in other PTPs, ending in a 3(10)-helix. In the PTPN7 structure, the WPD loop was in the closed conformation and part of the KIM (kinase-interaction motif) was visible, which forms an N-terminal aliphatic helix with the phosphorylation site Thr66 in an accessible position. The WPD loop of PTPRR was open; however, in contrast with the structure of its mouse homologue, PTPSL, a salt bridge between the conserved lysine and aspartate residues, which has been postulated to confer a more rigid loop structure, thereby modulating activity in PTPSL, does not form in PTPRR. One of the identified inhibitor scaffolds, cyclopenta[c]quinoline, was docked successfully into PTPRR, suggesting several possibilities for hit expansion. The determined structures together with the established SAR (structure-activity relationship) propose new avenues for the development of selective inhibitors that may have therapeutic potential for treating neurodegenerative diseases in the case of PTPRR or acute myeloblastic leukaemia targeting PTPN7.

Bax translocates to mitochondria of heart cells during simulated ischaemia: involvement of AMP-activated and p38 mitogen-activated protein kinases

The cytosolic protein Bax plays a key role in apoptosis by migrating to mitochondria and releasing proapoptotic proteins from the mitochondrial intermembrane space. The present study investigates the movement of Bax in isolated rat neonatal cardiomyocytes subjected to simulated ischaemia (minus glucose, plus cyanide), using green fluorescent protein-tagged Bax as a means of imaging Bax movements. Simulated ischaemia induced Bax translocation from the cytosol to mitochondria, commencing within 20 min of simulated ischaemia and progressing for several hours. Under the same conditions, there was an increase in the active, phosphorylated forms of p38 MAPK (mitogen-activated protein kinase) and AMPK (AMP-activated protein kinase). The AMPK activators AICAR (5-aminoimidazole-4-carboxamide ribonucleoside) and metformin also stimulated Bax translocation. Inhibition of p38 MAPK with SB203580 attenuated the phosphorylation of the downstream substrates, MAPK-activated protein kinases 2 and 3, but not that of the upstream MAPK kinase 3, nor of AMPK. Under all conditions (ischaemia, AICAR and metformin), SB203580 blocked Bax translocation completely. It is concluded that Bax translocation to mitochondria is an early step in ischaemia and that it occurs in response to activation of p38 MAPK downstream of AMPK.

Insulin-like growth factor-binding protein-5 (IGFBP-5): a critical member of the IGF axis

The six members of the insulin-like growth factor-binding protein family (IGFBP-1-6) are important components of the IGF (insulin-like growth factor) axis. In this capacity, they serve to regulate the activity of both IGF-I and -II polypeptide growth factors. The IGFBPs are able to enhance or inhibit the activity of IGFs in a cell- and tissue-specific manner. One of these proteins, IGFBP-5, also has an important role in controlling cell survival, differentiation and apoptosis. In this review, we report on the structural and functional features of the protein which are important for these effects. We also examine the regulation of IGFBP-5 expression and comment on its potential role in tumour biology, with special reference to work with breast cancer cells.

Intracellular signalling pathways activated by leptin

Leptin is a versatile 16 kDa peptide hormone, with a tertiary structure resembling that of members of the long-chain helical cytokine family. It is mainly produced by adipocytes in proportion to fat size stores, and was originally thought to act only as a satiety factor. However, the ubiquitous distribution of OB-R leptin receptors in almost all tissues underlies the pleiotropism of leptin. OB-Rs belong to the class I cytokine receptor family, which is known to act through JAKs (Janus kinases) and STATs (signal transducers and activators of transcription). The OB-R gene is alternatively spliced to produce at least five isoforms. The full-length isoform, OB-Rb, contains intracellular motifs required for activation of the JAK/STAT signal transduction pathway, and is considered to be the functional receptor. Considerable evidence for systemic effects of leptin on body mass control, reproduction, angiogenesis, immunity, wound healing, bone remodelling and cardiovascular function, as well as on specific metabolic pathways, indicates that leptin operates both directly and indirectly to orchestrate complex pathophysiological processes. Consistent with leptin's pleiotropic role, its participation in and crosstalk with some of the main signalling pathways, including those involving insulin receptor substrates, phosphoinositide 3-kinase, protein kinase B, protein kinase C, extracellular-signal-regulated kinase, mitogen-activated protein kinases, phosphodiesterase, phospholipase C and nitric oxide, has been observed. The impact of leptin on several equally relevant signalling pathways extends also to Rho family GTPases in relation to the actin cytoskeleton, production of reactive oxygen species, stimulation of prostaglandins, binding to diacylglycerol kinase and catecholamine secretion, among others.

The vascular endothelin system in hypertension--recent patents and discoveries

The discovery of endothelin two decades ago has now evolved into an intricate vascular endothelin (ET) system. Several ET isoforms, receptors, signaling pathways, agonists, antagonists, and clinical applications have been identified and documented in first-rate patents. The role of ET as one of the most potent endothelium-derived vasoconstricting factors is now complemented by a newly discovered role in vascular relaxation. ET synthesis is initiated by the transcription of ET genes in endothelial cells and the generation of the gene products preproET and big ET, which are further cleaved by specific ET converting enzymes into ET-1, -2, -3 and -4 isoforms. ET isoforms bind with different affinities to ET(A) and ET(B2) receptors in vascular smooth muscle, and stimulate [Ca(2+)](i), protein kinase C, mitogen-activated protein kinase and other signaling mechanisms of smooth muscle contraction, growth and proliferation. ET also binds to endothelial ET(B1) receptors, which mediate the release of vasodilator substances such as nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor. Endothelial ET(B1) receptors may also function in ET re-uptake and clearance. Although the effects of ET on vascular function and growth are well-recognized, the role of ET and its receptors in the regulation of blood pressure and in the pathogenesis of hypertension is not clearly established. Salt-dependent hypertension in experimental animals and some forms of moderate to severe hypertension in human may show elevated levels of plasma or vascular ET; however, other forms of hypertension show normal ET levels. The currently available ET receptor antagonists reduce blood pressure in some forms of experimental hypertension. Careful examination of recent patents may identify more effective and specific modulators of the vascular ET system for clinical use in human hypertension.

Mirk/Dyrk1B mediates survival during the differentiation of C2C12 myoblasts

The kinase Mirk/dyrk1B is essential for the differentiation of C2C12 myoblasts. Mirk reinforces the G0/G1 arrest state in which differentiation occurs by directly phosphorylating and stabilizing p27(Kip1) and destabilizing cyclin D1. We now demonstrate that Mirk is anti-apoptotic in myoblasts. Knockdown of endogenous Mirk by RNA interference activated caspase 3 and decreased myoblast survival by 75%, whereas transient overexpression of Mirk increased cell survival. Mirk exerts its anti-apoptotic effects during muscle differentiation at least in part through effects on the cell cycle inhibitor and pro-survival molecule p21(Cip1). Overexpression and RNA interference experiments demonstrated that Mirk phosphorylates p21 within its nuclear localization domain at Ser-153 causing a portion of the typically nuclear p21 to localize in the cytoplasm. Phosphomimetic GFP-p21-S153D was pancellular in both cycling C2C12 myoblasts and NIH3T3 cells. Endogenous Mirk in myotubes and overexpressed Mirk in NIH3T3 cells were able to cause the pancellular localization of wild-type GFP-p21 but not the nonphosphorylatable mutant GFP-p21-S153A. Translocation to the cytoplasm enables p21 to block apoptosis through inhibitory interaction with pro-apoptotic molecules. Phosphomimetic p21-S153D was more effective than wild-type p21 in blocking the activation of caspase 3. Transient expression of p21-S153D also increased myoblast viability in colony forming assays, whereas the p21-S153A mutant had no effect. This Mirk-dependent change in p21 intracellular localization is a natural part of myoblast differentiation. Endogenous p21 localized exclusively to the nuclei of proliferating myoblasts but was also found in the cytoplasm of post-mitotic multinucleated myotubes and adult human skeletal myofibers.

Activation and assembly of the NADPH oxidase: a structural perspective

The NADPH oxidase of professional phagocytes is a crucial component of the innate immune response due to its fundamental role in the production of reactive oxygen species that act as powerful microbicidal agents. The activity of this multi-protein enzyme is dependent on the regulated assembly of the six enzyme subunits at the membrane where oxygen is reduced to superoxide anions. In the resting state, four of the enzyme subunits are maintained in the cytosol, either through auto-inhibitory interactions or through complex formation with accessory proteins that are not part of the active enzyme complex. Multiple inputs are required to disrupt these inhibitory interactions and allow translocation to the membrane and association with the integral membrane components. Protein interaction modules are key regulators of NADPH oxidase assembly, and the protein-protein interactions mediated via these domains have been the target of numerous studies. Many models have been put forward to describe the intricate network of reversible protein interactions that regulate the activity of this enzyme, but an all-encompassing model has so far been elusive. An important step towards an understanding of the molecular basis of NADPH oxidase assembly and activity has been the recent solution of the three-dimensional structures of some of the oxidase components. We will discuss these structures in the present review and attempt to reconcile some of the conflicting models on the basis of the structural information available.

2-arachidonoylglycerol, an endogenous cannabinoid receptor ligand, induces rapid actin polymerization in HL-60 cells differentiated into macrophage-like cells

Delta9-Tetrahydrocannabinol, a major psychoactive constituent of marijuana, interacts with specific receptors, i.e. the cannabinoid receptors, thereby eliciting a variety of pharmacological responses. To date, two types of cannabinoid receptors have been identified: the CB1 receptor, which is abundantly expressed in the nervous system, and the CB2 receptor, which is predominantly expressed in the immune system. Previously, we investigated in detail the structure-activity relationship of various cannabinoid receptor ligands and found that 2-AG (2-arachidonoylglycerol) is the most efficacious agonist. We have proposed that 2-AG is the true natural ligand for both the CB1 and CB2 receptors. Despite the potential physiological importance of 2-AG, not much information is available concerning its biological activities towards mammalian tissues and cells. In the present study, we examined the effect of 2-AG on morphology as well as the actin filament system in differentiated HL-60 cells, which express the CB2 receptor. We found that 2-AG induces rapid morphological changes such as the extension of pseudopods. We also found that it provokes a rapid actin polymerization in these cells. Actin polymerization induced by 2-AG was abolished when cells were treated with SR144528, a CB2 receptor antagonist, and pertussis toxin, suggesting that the response was mediated by the CB2 receptor and G(i/o). A phosphoinositide 3-kinase, Rho family small G-proteins and a tyrosine kinase were also suggested to be involved. Reorganization of the actin filament system is known to be indispensable for a variety of cellular events; it is possible that 2-AG plays physiologically essential roles in various inflammatory cells and immune-competent cells by inducing a rapid actin rearrangement.

PAK and other Rho-associated kinases--effectors with surprisingly diverse mechanisms of regulation

The Rho GTPases are a family of molecular switches that are critical regulators of signal transduction pathways in eukaryotic cells. They are known principally for their role in regulating the cytoskeleton, and do so by recruiting a variety of downstream effector proteins. Kinases form an important class of Rho effector, and part of the biological complexity brought about by switching on a single GTPase results from downstream phosphorylation cascades. Here we focus on our current understanding of the way in which different Rho-associated serine/threonine kinases, denoted PAK (p21-activated kinase), MLK (mixed-lineage kinase), ROK (Rho-kinase), MRCK (myotonin-related Cdc42-binding kinase), CRIK (citron kinase) and PKN (protein kinase novel), interact with and are regulated by their partner GTPases. All of these kinases have in common an ability to dimerize, and in most cases interact with a variety of other proteins that are important for their function. A diversity of known structures underpin the Rho GTPase-kinase interaction, but only in the case of PAK do we have a good molecular understanding of kinase regulation. The ability of Rho GTPases to co-ordinate spatial and temporal phosphorylation events explains in part their prominent role in eukaryotic cell biology.

Functional interaction between p75NTR and TrkA: the endocytic trafficking of p75NTR is driven by TrkA and regulates TrkA-mediated signalling

The topology and trafficking of receptors play a key role in their signalling capability. Indeed, receptor function is related to the microenvironment inside the cell, where specific signalling molecules are compartmentalized. The response to NGF (nerve growth factor) is strongly dependent on the trafficking of its receptor, TrkA. However, information is still scarce about the role of the cellular localization of the TrkA co-receptor, p75NTR (where NTR is neurotrophin receptor), following stimulation by NGF. It has been shown that these two receptors play a key role in epithelial tissue and in epithelial-derived tumours, where the microenvironment at the plasma membrane is defined by the presence of tight junctions. Indeed, in thyroid carcinomas, rearrangements of TrkA are frequently found, which produce TrkA mutants that are localized exclusively in the cytoplasm. We used a thyroid cellular model in which it was possible to dissect the trafficking of the two NGF receptors upon neurotrophin stimulation. In FRT (Fischer rat thyroid) cells, endogenous TrkA is localized exclusively on the basolateral surface, while transfected p75NTR is selectively distributed on the apical membrane. This cellular system enabled us to selectively stimulate either p75NTR or TrkA and to analyse the role of receptor trafficking in their signalling capability. We found that, after binding to NGF, p75NTR was co-immunoprecipitated with TrkA and was transcytosed at the basolateral membrane. We showed that the TrkA-p75NTR interaction is necessary for this relocation of p75NTR to the basolateral side. Interestingly, TrkA-specific stimulation by basolateral NGF loading also induced the TrkA-p75NTR interaction and subsequent p75NTR transcytosis at the basolateral surface. Moreover, specific stimulation of p75NTR by NGF activated TrkA and the MAPK (mitogen-activated protein kinase) pathway. Our data indicate that TrkA regulates the subcellular localization of p75NTR upon stimulation with neurotrophins, thus affecting the topology of the signal transduction molecules, driving the activation of a specific signal transduction pathway.

Use of three-dimensional basement membrane cultures to model oncogene-induced changes in mammary epithelial morphogenesis

The development of breast carcinomas involves a complex set of phenotypic alterations in breast epithelial cells and the surrounding microenvironment. While traditional transformation assays provide models for investigating certain aspects of the cellular processes associated with tumor initiation and progression, they do not model alterations in tissue architecture that are critically involved in tumor development. In this review, we provide examples of how three-dimensional (3D) cell culture models can be utilized to dissect the pathways involved in the development of mammary epithelial structures and to elucidate the mechanisms responsible for oncogene-induced phenotypic alterations in epithelial behavior and architecture. Many normal mammary epithelial cell lines undergo a stereotypic morphogenetic process when grown in the presence of exogenous matrix proteins. This 3D morphogenesis culminates in the formation of well-organized, polarized spheroids, and/or tubules that are highly reminiscent of normal glandular architecture. In contrast, transformed cell lines isolated from mammary tumors exhibit significant deviations from normal epithelial behavior in 3D culture. We describe the use of 3D models as a method for both reconstructing and deconstructing the cell biological and biochemical events involved in mammary neoplasia.

RLP, a novel Ras-like protein, is an immediate-early transforming growth factor-beta (TGF-beta) target gene that negatively regulates transcriptional activity induced by TGF-beta

We have described previously the use of microarray technology to identify novel target genes of TGF-beta (transforming growth factor-beta) signalling in mouse embryo fibroblasts deficient in Smad2 or Smad3 [Yang, Piek, Zavadil, Liang, Xie, Heyer, Pavlidis, Kucherlapati, Roberts and Böttinger (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 10269-10274]. Among the TGF-beta target genes identified, a novel gene with sequence homology to members of the Ras superfamily was identified, which we have designated as RLP (Ras-like protein). RLP is a Smad3-dependent immediate-early TGF-beta target gene, its expression being induced within 45 min. Bone morphogenetic proteins also induce expression of RLP, whereas epidermal growth factor and phorbol ester PMA suppress TGF-beta-induced expression of RLP. Northern-blot analysis revealed that RLP was strongly expressed in heart, brain and kidney, and below the detection level in spleen and skeletal muscles. At the protein level, RLP is approx. 30% homologous with members of the Ras superfamily, particularly in domains characteristic for small GTPases. However, compared with prototypic Ras, RLP contains a modified P-loop, lacks the consensus G2 loop and the C-terminal prenylation site and harbours amino acid substitutions at positions that render prototypic Ras oncogenic. However, RLP does not have transforming activity, does not affect phosphorylation of mitogen-activated protein kinase and is unable to bind GTP or GDP. RLP was found to associate with certain subtypes of the TGF-beta receptor family, raising the possibility that RLP plays a role in TGF-beta signal transduction. Although RLP did not interact with Smads and did not affect TGF-beta receptor-induced Smad2 phosphorylation, it inhibited TGF-beta-induced transcriptional reporter activation, suggesting that it is a novel negative regulator of TGF-beta signalling.

Noradrenaline represses PPAR (peroxisome-proliferator-activated receptor) gamma2 gene expression in brown adipocytes: intracellular signalling and effects on PPARgamma2 and PPARgamma1 protein levels

PPAR (peroxisome-proliferator-activated receptor) gamma is expressed in brown and white adipose tissues and is involved in the control of differentiation and proliferation. Noradrenaline stimulates brown pre-adipocyte proliferation and brown adipocyte differentiation. The aim of the present study was thus to investigate the influence of noradrenaline on PPARgamma gene expression in brown adipocytes. In primary cultures of brown adipocytes, PPARgamma2 mRNA levels were 20-fold higher than PPARgamma1 mRNA levels. PPARgamma expression occurred during both the proliferation and the differentiation phases, with the highest mRNA levels being found at the time of transition between the phases. PPARgamma2 mRNA levels were downregulated by noradrenaline treatment (EC50, 0.1 microM) in both proliferative and differentiating cells, with a lagtime of 1 h and lasting up to 4 h, after which expression gradually recovered. The down-regulation was beta-adrenoceptor-induced and intracellularly mediated via cAMP and protein kinase A; the signalling pathway did not involve phosphoinositide 3-kinase, Src, p38 mitogen-activated protein kinase or extracellular-signal-regulated kinases 1 and 2. Treatment of the cells with the protein synthesis inhibitor cycloheximide not only abolished the noradrenaline-induced down-regulation of PPARgamma2 mRNA, but also in itself induced PPARgamma2 hyperexpression. The down-regulation was probably the result of suppression of transcription. The down-regulation of PPARgamma2 mRNA resulted in similar down-regulation of PPARgamma2 and phosphoPPARgamma2 protein levels. Remarkably, the level of PPARgamma1 protein was similar to that of PPARgamma2 (despite almost no PPARgamma1 mRNA), and the down-regulation by noradrenaline demonstrated similar kinetics to that of PPARgamma2; thus PPARgamma1 was apparently translated from the PPARgamma2 template. It is suggested that beta-adrenergic stimulation via cAMP and protein kinase A represses PPARgamma gene expression, leading to reduction of PPARgamma2 mRNA levels, which is then reflected in down-regulated levels of PPARgamma2, phosphoPPARgamma2 and PPARgamma1.

Interaction of the tyrosine phosphatase SHP-2 with Gab2 regulates Rho-dependent activation of the c-fos serum response element by interleukin-2

Gab2 (Grb2-associated binder-2), a member of the IRS (insulin receptor substrate)/Gab family of adapter proteins, undergoes tyrosine phosphorylation in response to cytokine or growth factor stimulation and serves as a docking platform for many signal transduction effectors, including the tyrosine phosphatase SHP-2 [SH2 (Src homology 2)-domain-containing tyrosine phosphatase]. Here, we report that, following IL-2 (interleukin-2) stimulation of human T lymphocytes, SHP-2 binds tyrosine residues 614 and 643 of human Gab2 through its N- and C-terminal SH2 domains respectively. However, the sole mutation of Tyr-614 into phenylalanine is sufficient to prevent Gab2 from recruiting SHP-2. Expression of the Gab2 Tyr-614-->Phe (Y614F) mutant, defective in SHP-2 association, prevents ERK (extracellular-signal-regulated kinase) activation and expression of a luciferase reporter plasmid driven by the c-fos SRE (serum response element), indicating that interaction of SHP-2 with Gab2 is required for ERK activation in response to IL-2. Further investigation of IL-2-dependent induction of SRE showed that expression of a constitutively active mutant of the RhoA GTPase synergizes with IL-2 for SRE-driven transcription, whereas a dominant-negative mutant reduces the IL-2 response. Thus, in response to IL-2, full induction of the SRE requires ERK-dependent as well as Rho-dependent signals that target the Ets-box and the CArG-box respectively. We also report that the synergy between Gab2/SHP-2 and RhoA for IL-2-dependent CArG-box-driven transcription depends upon MEK (mitogen-activated protein kinase/ERK kinase) activation, and is likely to involve regulation of the serum response factor co-activator MAL. Our studies thus provide new insights into the role of Gab2 and SHP-2 in IL-2 signal transduction.

Combinational clustering of receptors following stimulation by bacterial products determines lipopolysaccharide responses

The innate immune system has the capacity to recognize a wide range of pathogens based on conserved PAMPs (pathogen-associated molecular patterns). In the case of bacterial LPS (lipopolysaccharide) recognition, the best studied PAMP, it has been shown that the innate immune system employs at least three cell-surface receptors: CD14, TLR4 (Toll-like receptor 4) and MD-2 protein. CD14 binds LPS from Enterobacteriaceae and then transfers it to MD-2, leading to TLR4 aggregation and signal transduction. LPS analogues such as lipid IVa seem to act as LPS antagonists in human cells, but exhibit LPS mimetic activity in mouse cells. Although TLR4 has been shown to be involved in this species-specific discrimination, the mechanism by which this is achieved has not been elucidated. The questions that remain are how the innate immune system can discriminate between LPS from different bacteria as well as different LPS analogues, and whether or not the structure of LPS affects its interaction with the CD14-TLR4-MD-2 cluster. Is it possible that the 'shape' of LPS induces the formation of different receptor clusters, and thus a different immune response? In the present study, we demonstrate using biochemical as well as fluorescence-imaging techniques that different LPS analogues trigger the recruitment of different receptors within microdomains. The composition of each receptor cluster as well as the number of TLR4 molecules that are recruited within the cluster seem to determine whether an immune response will be induced or inhibited.