G(s)alpha repression of adipogenesis via Syk

The Therapeutic Potential of Piceatannol, a Natural Stilbene, in Metabolic Diseases: A Review

Metabolic disease comprises a set of risk factors highly associated with obesity and insulin resistance and is a consequence of central adiposity, hyperglycemia, and dyslipidemia. Furthermore, obesity increases the risk of the development of metabolic disease due to ectopic fat deposition, low-grade inflammation, and systemic energy disorders caused by dysregulated adipose tissue function. Piceatannol is a naturally occurring polyphenolic stilbene found in various fruits and vegetables and has been reported to exhibit anticancer and anti-inflammatory properties. In addition, recently reported beneficial effects of piceatannol on hypercholesterolemia, atherosclerosis, and angiogenesis underscore its therapeutic potential in cardiovascular disease. However, investigation of its role in metabolic disease is still in its infancy. This review intensively summarizes in vitro and in vivo studies supporting the potential therapeutic effects of piceatannol in metabolic disease, including inhibition of adipogenesis and lipid metabolism in adipocytes, and regulation of hyperlipidemia, hyperglycemia, insulin resistance, and fatty acid-induced inflammation and oxidative stress.

An adenosine receptor-Krüppel-like factor 4 protein axis inhibits adipogenesis

Adipogenesis represents a key process in adipose tissue development and remodeling, including during obesity. Exploring the regulation of adipogenesis by extracellular ligands is fundamental to our understanding of this process. Adenosine, an extracellular nucleoside signaling molecule found in adipose tissue depots, acts on adenosine receptors. Here we report that, among these receptors, the A2b adenosine receptor (A2bAR) is highly expressed in adipocyte progenitors. Activation of the A2bAR potently inhibits differentiation of mouse stromal vascular cells into adipocytes, whereas A2bAR knockdown stimulates adipogenesis. The A2bAR inhibits differentiation through a novel signaling cascade involving sustained expression of Krüppel-like factor 4 (KLF4), a regulator of stem cell maintenance. Knockdown of KLF4 ablates the ability of the A2bAR to inhibit differentiation. A2bAR activation also inhibits adipogenesis in a human primary preadipocyte culture system. We analyzed the A2bARKLF4 axis in adipose tissue of obese subjects and, intriguingly, found a strong correlation between A2bAR and KLF4 expression in both subcutaneous and visceral human fat. Hence, our study implicates the A2bAR as a regulator of adipocyte differentiation and the A2bAR-KLF4 axis as a potentially significant modulator of adipose biology.

Blockade of Syk ameliorates the development of murine sclerodermatous chronic graft-versus-host disease

BACKGROUND

Murine sclerodermatous chronic graft-versus-host disease (Scl-cGVHD) is a model for human Scl-cGVHD and systemic sclerosis (SSc). Syk is expressed in most of hematopoietic cells, fibroblasts, and endothelial cells. Syk is a protein tyrosine kinase that has an important role in transmitting signals from a variety of cell surface receptors.

OBJECTIVE

This study aims to investigate the effect of R788 (fostamatinib sodium), an oral prodrug that is rapidly converted to a potent inhibitor of Syk, R406, on Scl-cGVHD.

METHODS

R788 was orally administered twice a day to allogeneic recipients from day 14 to day 42 after bone marrow transplantation (BMT). In vitro, proliferation of GVHD-derived CD4(+) T cells and CD11b(+) cells was analyzed by R406.

RESULTS

Allogeneic BMT increased Syk phosphorylation in T, B, and CD11b(+) cells. The administration of R788 attenuated severity and fibrosis of Scl-cGVHD. The elevated expressions of CXCR4 on T cells, B cells, and CD11b(+) cells were significantly down-regulated by R788 treatment. R788 reduced memory CD4(+) T cells (CD44(hi)CD62L(-)CD4(+)). R406 inhibited proliferation of GVHD CD4(+) T cells and CD11b(+) cells in vitro. In addition, R788 treatment, inhibited proliferation of CD11b(+) cells in Scl-cGVHD mice. R788 treatment also reduced skin mRNA expressions of MCP-1, MIP-1α, IFN-γ, IL-13, IL-17A, and TGF-β1, but not influenced RANTES, CXCL12, and TFN-α.

CONCLUSION

Blockade of Syk suppressed migration factor of immune cells and antigen-specific memory CD4(+) T cells and proliferation and activation of GVHD CD4(+) T cells and CD11b(+) cells. The current studies suggested that Syk inhibitor is a potential candidate for use in treating patients with Scl-cGVHD and SSc.

G protein-coupled receptors and adipogenesis: a focus on adenosine receptors

G-protein coupled receptors (GPCRs) are a large family of proteins that coordinate extracellular signals to produce physiologic outcomes. Adenosine receptors (AR) are one class of GPCRs that have been shown to regulate functions as diverse as inflammation, blood flow, and cellular differentiation. Adenosine signals through four GPCRs that either inhibit (A1AR and A3AR) or activate (A2aAR and A2bAR) adenylyl cyclase. This review will focus on the role of GPCRs, and in particular, adenosine receptors, in adipogenesis. Preadipocytes differentiate to mature adipocytes as the adipose tissue expands to compensate for the consumption of excess nutrients. These newly generated adipocytes contribute to maintaining metabolic homeostasis. Understanding the key drivers of this differentiation process can aid the development of therapeutics to combat the growing obesity epidemic and associated metabolic consequences. Although much literature has covered the transcriptional events that culminate in the formation of an adipocyte, less focus has been on receptor-mediated extracellular signals that direct this process. This review will highlight GPCRs and their downstream messengers as significant players controlling adipocyte differentiation.

Adipose tissue gene expression analysis reveals changes in inflammatory, mitochondrial respiratory and lipid metabolic pathways in obese insulin-resistant subjects

Background

To get insight into molecular mechanisms underlying insulin resistance, we compared acute in vivo effects of insulin on adipose tissue transcriptional profiles between obese insulin-resistant and lean insulin-sensitive women.

Methods

Subcutaneous adipose tissue biopsies were obtained before and after 3 and 6 hours of intravenously maintained euglycemic hyperinsulinemia from 9 insulin-resistant and 11 insulin-sensitive females. Gene expression was measured using Affymetrix HG U133 Plus 2 microarrays and qRT-PCR. Microarray data and pathway analyses were performed with Chipster v1.4.2 and by using in-house developed nonparametric pathway analysis software.

Results

The most prominent difference in gene expression of the insulin-resistant group during hyperinsulinemia was reduced transcription of nuclear genes involved in mitochondrial respiration (mitochondrial respiratory chain, GO:0001934). Inflammatory pathways with complement components (inflammatory response, GO:0006954) and cytokines (chemotaxis, GO:0042330) were strongly up-regulated in insulin-resistant as compared to insulin-sensitive subjects both before and during hyperinsulinemia. Furthermore, differences were observed in genes contributing to fatty acid, cholesterol and triglyceride metabolism (FATP2, ELOVL6, PNPLA3, SREBF1) and in genes involved in regulating lipolysis (ANGPTL4) between the insulin-resistant and -sensitive subjects especially during hyperinsulinemia.

Conclusions

The major finding of this study was lower expression of mitochondrial respiratory pathway and defective induction of lipid metabolism pathways by insulin in insulin-resistant subjects. Moreover, the study reveals several novel genes whose aberrant regulation is associated with the obese insulin-resistant phenotype.

Piceatannol, natural polyphenolic stilbene, inhibits adipogenesis via modulation of mitotic clonal expansion and insulin receptor-dependent insulin signaling in early phase of differentiation

Piceatannol, a natural stilbene, is an analog and a metabolite of resveratrol. Despite a well documented health benefit of resveratrol in intervention of the development of obesity, the role of piceatannol in the development of adipose tissue and related diseases is unknown. Here, we sought to determine the function of piceatannol in adipogenesis and elucidate the underlying mechanism. We show that piceatannol inhibits adipogenesis of 3T3-L1 preadipocytes in a dose-dependent manner at noncytotoxic concentrations. This anti-adipogenic property of piceatannol was largely limited to the early event of adipogenesis. In the early phase of adipogenesis, piceatannol-treated preadipocytes displayed a delayed cell cycle entry into G(2)/M phase at 24 h after initiation of adipogenesis. Furthermore, the piceatannol-suppressed mitotic clonal expansion was accompanied by reduced activation of the insulin-signaling pathway. Piceatannol dose-dependently inhibited differentiation mixture-induced phosphorylation of insulin receptor (IR)/insulin receptor substrate-1 (IRS-1)/Akt pathway in the early phase of adipogenesis. Moreover, we showed that piceatannol is an inhibitor of IR kinase activity and phosphatidylinositol 3-kinase (PI3K). Our kinetics study of IR further identified a K(m) value for ATP of 57.8 μm and a K(i) value for piceatannol of 28.9 μm. We also showed that piceatannol directly binds to IR and inhibits IR kinase activity in a mixed noncompetitive manner to ATP, through which piceatannol appears to inhibit adipogenesis. Taken together, our study reveals an anti-adipogenic function of piceatannol and highlights IR and its downstream insulin signaling as novel targets for piceatannol in the early phase of adipogenesis.

The role of Syk kinase in ultraviolet-mediated skin damage

BACKGROUND

Ultraviolet (UV) irradiation is the main cause of skin photodamage; the resulting modulation of matrix metalloproteinases (MMPs) leads to collagen degradation. There is no easily accessible molecular indicator of early skin UV damage.

OBJECTIVES

In this study, we investigated the effects of Syk kinase on MMP expression and evaluated the sensitivity and usefulness of Syk as an early indicator of skin UV damage.

METHODS

Human dermal fibroblasts (HDFs) were transfected with Syk cDNA to overexpress Syk. MMP-1 expression and Syk activity were determined by Western blot after UV exposure. The effect of Syk on MMP-1 expression in HDFs was further explored by either Syk siRNA or a selective Syk inhibitor. Possible downstream molecules of Syk were also evaluated in HDFs upon UV exposure. The relationship between Syk and collagenase was further explored in vivo (MMP-13, hairless mice).

RESULTS

Our studies in HDFs demonstrated that both a Syk inhibitor and Syk siRNA were able to inhibit MMP-1 expression in HDFs exposed to UV and that overexpression of Syk increased MMP-1 expression and the activity of JNK kinase, but not p38 or Erk1/2 MAP kinase. UV exposure enhanced both expression and activity of Syk in HDFs. Experiments with hairless mice suggested that Syk expression is an earlier indicator of UV exposure than MMP-13 expression.

CONCLUSIONS

Our results demonstrate that Syk expression correlates well with increase of MMPs (MMP-1 in humans and MMP-13 in mice) in response to UV exposure. The findings suggest that Syk may be a novel target for the prevention and treatment of skin photodamage by modulating MMPs.

G(s)alpha deficiency in adipose tissue leads to a lean phenotype with divergent effects on cold tolerance and diet-induced thermogenesis

G(s)alpha, the G protein that mediates receptor-stimulated cAMP generation, has been implicated as a regulator of adipogenesis and adipose tissue function. Heterozygous G(s)alpha mutations lead to obesity in Albright hereditary osteodystrophy (AHO) patients and in mice. In this study, we generated mice with adipose-specific G(s)alpha deficiency. Heterozygotes had 50% loss of G(s)alpha expression in adipose tissue and no obvious phenotype, suggesting that adipose-specific G(s)alpha deficiency is not the cause of obesity in AHO. Homozygotes (FGsKO) had severely reduced adipose tissue, indicating that G(s)alpha is required for adipogenesis. Although FGsKO mice had impaired cold tolerance and reduced responsiveness of brown adipose tissue (BAT) to sympathetic signaling, diet-induced thermogenesis and fatty acid oxidation in skeletal muscle were increased. In normal mice, high-fat diet raised sympathetic nerve activity in muscle, but not in BAT. Our results show that cold- and diet-induced thermogenesis may occur in separate tissues, especially when BAT function is impaired.

Gsalpha signalling suppresses PPARgamma2 generation and inhibits 3T3L1 adipogenesis

Since TSH receptor (TSHR) expression increases during adipogenesis and signals via cAMP/phospho-cAMP-response element binding protein (CREB), reported to be necessary and sufficient for adipogenesis, we hypothesised that TSHR activation would induce preadipocyte differentiation. Retroviral vectors introduced constitutively active TSHR (TSHR*) into 3T3L1 preadipocytes; despite increased cAMP (RIA) and phospho-CREB (western blot) there was no spontaneous adipogenesis (assessed morphologically, using oil red O and QPCR measurement of adipogenesis markers). We speculated that Gbetagamma signalling may be inhibitory but failed to induce adipogenesis using activated Gsalpha (gsp*). Inhibition of phosphodiesterases did not promote adipogenesis in TSHR* or gsp* populations. Furthermore, differentiation induced by adipogenic medium with pioglitazone was reduced in TSHR* and abolished in gsp* expressing 3T3L1 cells. TSHR* and gsp* did not inactivate PPARgamma (PPARG as listed in the HUGO database) by phosphorylation but expression of PPARgamma1 was reduced and PPARgamma2 undetectable in gsp*. FOXO1 phosphorylation (required to inactivate this repressor of adipogenesis) was lowest in gsp* despite the activation of AKT by phosphorylation. PROF is a mediator that facilitates FOXO1 phosphorylation by phospho-Akt. Its transcript levels remained constantly low in the gsp* population. In most measurements, the TSHR* cells were between the gsp* and control 3T3L1 preadipocytes. The enhanced down-regulation of PREF1 (adipogenesis inhibitor) permits retention of some adipogenic potential in the TSHR* population. We conclude that Gsalpha signalling impedes FOXO1 phosphorylation and thus inhibits PPARgamma transcription and the alternative promoter usage required to generate PPARgamma2, the fat-specific transcription factor necessary for adipogenesis.

Dose-dependent effects of soy phyto-oestrogen genistein on adipocytes: mechanisms of action

The potential role of genistein in the prevention and treatment of obesity has attracted much attention among public and medical communities. Conversely, increasing evidence indicates that genistein as an endocrine-disrupting substance is likely to play a role in the aetiology of obesity. This review focuses on the role of soy phyto-oestrogen genistein in adipocytes and the underlying mechanisms of action. Genistein dose-dependently inhibits and stimulates adipogenesis in vitro. Increasing evidence shows that genistein dose-dependently influences obesity in both male and female animals. Dose-dependent effects of genistein on adipocytes vary with factors such as age and gender of animals. In addition, the role of developmental exposure of genistein in adult obesity has been discussed. Genistein, different from oestrogen, concurrently activates nuclear receptors, oestrogen receptors and peroxisome proliferator-activated receptors, and it inhibits various enzyme activities. The balance among these pleiotrophic effects of genistein determines its dose-dependent effects on adipocyte differentiation and function. Current data suggest that genistein could regulate adiposity. However, it remains uncertain whether genistein plays a beneficial role in the prevention and treatment of obesity. Additional evidence is required before firm conclusions showing that genistein decreases adiposity.

Syk tyrosine kinase is linked to cell motility and progression in squamous cell carcinomas of the head and neck

Syk, a non-receptor tyrosine kinase, is an important component of immunoreceptor signaling in hematopoietic cells. It has been implicated in key regulatory pathways including phosphoinositide 3-kinase and phospholipase Cgamma (PLCgamma) activation in B cells and integrin signaling in platelets and bronchial epithelial cells. Recently, potential roles in cancer have been reported. In breast cancers, reduced Syk expression was associated with invasion, and its overexpression in cell lines was shown to inhibit cell motility. In contrast, Syk has been shown to mediate chemomigration in nasopharyngeal carcinoma cells. Its role in squamous cell carcinomas of the head and neck (SCCHN) has not yet been investigated. Syk mRNA and protein expression was detected in 6 of 10 SCCHN cell lines. When Syk was transfected into Syk-negative cells (SIHN-011A), chemomigration was enhanced in vitro and this was associated with activation of PLCgamma1. Conversely, abrogation of Syk activity by pharmacologic inhibition or small interfering RNA in HN6 cells with high levels of endogenous expression inhibited migration, haptotaxis, and engagement with matrix proteins; this was accompanied by decreased levels of phosphorylated AKT. Similar effects were seen in Syk-positive CAL 27 cells but not in Syk-negative SIHN-011A cells. Immunoprecipitation suggested co-association of Syk with epidermal growth factor receptor and GRB-2. Syk expression in SCCHN patient tissues was examined by semiquantitative real-time PCR (n = 45) and immunohistochemistry (n = 38) in two independent cohorts. Higher levels of Syk expression were observed in tumors and lymph node metastases relative to normal tissues. High Syk expression significantly correlated with worse survival and may be of prognostic value in SCCHN due to its potential role in cell migration and invasion.

Oestradiol, but not genistein, inhibits the rise in food intake following gonadectomy in cats, but genistein is associated with an increase in lean body mass

The prevalence of obesity in domestic cats is increasing worldwide, and is strongly associated with gonadectomy. We have previously demonstrated the effectiveness of oestradiol in reducing food intake in both male and female neutered cats. This experiment was designed to test the hypothesis that oestradiol or genistein would prevent the increase in food intake following gonadectomy of male and female cats, and would prevent an increase in body fat mass. Three groups of eight cats each were surgically neutered then treated daily with either 0.5 mug oestradiol subcutaneously, 100 mg/kg genistein orally, or vehicle only. Effect of treatment on food intake, vaginal cytology and body weight were recorded, and body composition was assayed using the D(2)O isotopic dilution method. Neutering was followed by an increase in food intake, bodyweight and body fat mass in the control group, which were almost completely prevented by treatment with oestradiol (p < 0.001). Treatment with genistein had no effect on food intake or bodyweight increase, but was associated with a significant increase in lean body mass (p = 0.018), and significantly less body fat accumulation than the control group (p = 0.01). There were no significant differences in responses to treatment between sexes. These findings demonstrate the importance of gonadal oestrogen for the control of food intake in male and female cats, and suggest the provision of an oestrogenic compound could help prevent obesity following neutering. In addition, the findings of this study are consistent with observations in rodents of the efficacy of genistein in inhibiting adipogenesis and promoting lean body tissue development.

Galpha(s) sensitizes human SH-SY5Y cells to apoptosis independently of the protein kinase A pathway

Disturbances in Galpha(s-L) levels and function have been implicated in the pathophysiology of bipolar disorder, but the role of these changes in the development of the illness is not clear. In view of the critical role of Galpha(s)-mediated cAMP signaling in regulating cell survival, we investigated the potential role of Galpha(s-L) in modulating susceptibility to cellular stressors in human SH-SY5Y neuroblastoma cells. Overexpression of Galpha(s-L) to a level twice that of the vector-transfected cells did not directly affect cell viability but significantly increased the sensitivity to induction of cell death by serum deprivation and other apoptotic stimuli, including staurosporine, H(2)O(2), and tunicamycin. This enhanced sensitivity was associated with increased caspase-3 activation and appearance of fragmented nuclei (Hoechst 33342 staining). The broad-spectrum caspase inhibitor z-VAD-fmk completely suppressed cell death evoked by these apoptotic insults in both vector-transfected and Galpha(s-L)-overexpressing cells. The increased vulnerability conferred by increased Galpha(s-L) expression was neither mimicked by cAMP analogs 8-Br-cAMP, 8-CPT-cAMP, and 8-CPT-2Me-cAMP nor attenuated by PKA inhibitors Rp-cAMPS and KT5720. These data indicate that Galpha(s-L) may modulate apoptotic processes in a caspase-dependent manner through a signaling cascade that is independent of the cAMP/PKA or cAMP/Epac pathway. These results suggest that enhanced Galpha(s-L) expression, as was observed in post-mortem brain of bipolar patients, may impair cellular resilience in response to intracellular stress signals resulting from mitochondrial and/or endoplasmic reticulum dysfunction implicated in this disorder.

Angiotensin-converting enzyme signalling in human preadipocytes and adipocytes

Angiotensin-converting enzyme (ACE, kininase II) is a plasma membrane zinc metallopeptidase that acts as a key enzyme for the extracellular conversion of vasoactive peptides. Recently, ACE outside-in signalling in endothelial cells has been described. The present study tested the hypothesis that ACE signalling is not restricted to endothelial cells and may act as an additional peptide receptor on human preadipocytes and adipocytes. ACE protein levels were not changed during adipose conversion of human primary preadipocytes. The enzyme was primarily localized to the non-detergent-resistant fraction of the membrane and phosphorylated in non-dividing cells. Antibody arrays of whole cell lysate detected putative ACE-interacting proteins, which all share important roles in cell cycle control and/or apoptosis. These findings suggest that ACE is a versatile molecule, involved both in the regulation of extracellular peptide concentrations and direct intracellular signalling. In human adipose cells ACE may potentially influence exit from the cell cycle, differentiation, and programmed cell death signalling.

Removal of serum factors by charcoal treatment promotes adipogenesis via a MAPK-dependent pathway

In vitro differentiation of the progenitor cells or preadipocytes into adipocytes is usually achieved by adding an adipogenic mixture (isobutylmethylxanthine, dexamethasone, and insulin, IDI) to medium supplemented with fetal bovine serum (FBS). To study the effects of steroid hormones in vitro, endogenous hormones, growth factors and cytokines are removed by charcoal stripping of serum. However, the effects of charcoal-stripped serum (CS-FBS) per se on adipogenesis have been ignored. Here, we showed that alkaline phosphate activity and nodule formation of osteoprogenitor KS483 cells were lower in CS-FBS than in FBS. Concurrently, abundant amounts of adipocytes were only observed in KS483 cells cultured with CS-FBS, irrespective of the brands of serum used. Inhibition of the p42/44 MAPK pathway by its specific inhibitor PD98059 increased adipogenesis of KS483 cells with FBS, whereas activation of this signalling pathway by EGF blocked adipogenesis of these cells with CS-FBS. Furthermore, the p42/44 MAPK phosphorylation of KS483 cells cultured with CS-FBS was decreased compared with FBS. We concluded that charcoal-stripping of serum removed stimulators of the MAPK signalling pathway and in turn led to downregulation of osteogenesis and upregulation of adipogenesis. Interestingly, the adipogenic mixture IDI stimulated adipogenesis of KS483 cells cultured with CS-FBS, but not with FBS. Furthermore, differential effects of genistein on adipogenesis were observed in KS483 cells cultured with FBS or CS-FBS in combination with IDI. Our results showed that charcoal stripping of serum affected the commitment of KS483 cells and therefore differentially regulated adipogenesis influenced by IDI alone and in combination with genistein.

Isoproterenol induces actin depolymerization in human airway smooth muscle cells via activation of an Src kinase and GS

In a previous study, we showed that isoproterenol induced actin depolymerization in human airway smooth muscle cells by both protein kinase A (PKA)-dependent and -independent signaling pathways. We now investigate the signaling pathway of PKA-independent actin depolymerization induced by isoproterenol in these cells. Cells were briefly exposed to isoproterenol or PGE1 in the presence and absence of specific inhibitors of Src-family tyrosine kinases, phosphatidylinositol-3-kinase (PI3 kinase), or MAP kinase, and actin depolymerization was measured by concomitant staining of filamentous actin with FITC-phalloidin and globular actin with Texas red DNase I. Isoproterenol, cholera toxin, and PGE1 induced actin depolymerization, indicated by a decrease in the intensity of filamentous/globular fluorescent staining. Pretreatment with the Src kinase inhibitors 4-amino-5-(4-chlorophenyl)-7-( t-butyl)pyrazolo[3,4-d]pyriimidine (PP2) or geldanamycin or the PKA inhibitor Rp-cAMPS only partly inhibited isoproterenol- or PGE1-induced actin depolymerization. In contrast, PP2 and geldanamycin did not inhibit forskolin-induced actin depolymerization, and AG-213 (an EGF receptor tyrosine kinase inhibitor) did not inhibit isoproterenol- or PGE1-induced actin depolymerization. PI3 kinase or MAP kinase inhibition did not inhibit isoproterenol-induced actin depolymerization. Moreover, isoproterenol but not forskolin induced tyrosine phosphorylation of an Src family member at position 416. These results further confirm that both PKA-dependent and PKA-independent pathways mediate actin depolymerization in human airway smooth muscle cells and that the PKA-independent pathway by which isoproterenol induces actin depolymerization in human airway smooth muscle cells involves Src protein tyrosine kinases and the Gs protein.

A Novel Mode of Gleevec Binding Is Revealed by the Structure of Spleen Tyrosine Kinase

Spleen tyrosine kinase (Syk) is a non-receptor tyrosine kinase required for signaling from immunoreceptors in various hematopoietic cells. Phosphorylation of two tyrosine residues in the activation loop of the Syk kinase catalytic domain is necessary for signaling, a phenomenon typical of tyrosine kinase family members. Syk in vitro enzyme activity, however, does not depend on phosphorylation (activation loop tyrosine --> phenylalanine mutants retain catalytic activity). We have determined the x-ray structure of the unphosphorylated form of the kinase catalytic domain of Syk. The enzyme adopts a conformation of the activation loop typically seen only in activated, phosphorylated tyrosine kinases, explaining why Syk does not require phosphorylation for activation. We also demonstrate that Gleevec (STI-571, Imatinib) inhibits the isolated kinase domains of both unphosphorylated Syk and phosphorylated Abl with comparable potency. Gleevec binds Syk in a novel, compact cis-conformation that differs dramatically from the binding mode observed with unphosphorylated Abl, the more Gleevec-sensitive form of Abl. This finding suggests the existence of two distinct Gleevec binding modes: an extended, trans-conformation characteristic of tight binding to the inactive conformation of a protein kinase and a second compact, cis-conformation characteristic of weaker binding to the active conformation. Finally, the Syk-bound cis-conformation of Gleevec bears a striking resemblance to the rigid structure of the nonspecific, natural product kinase inhibitor staurosporine.

G-protein-coupled receptor signaling in Syk-deficient neutrophils and mast cells

The Syk tyrosine kinase is essential for immunoreceptor and multiple integrin functions as well as being implicated in signaling from G-protein-coupled receptors (GPCR) in cell lines, transfection systems, and pharmacologic studies. In contrast, using Syk-deficient primary cells, we show here that Syk does not play a major functional role in chemoattractant/chemokine signaling in neutrophils and mast cells. syk(-/-) neutrophils showed normal respiratory burst and degranulation in response to the bacterial peptide formyl-Met-Leu-Phe (fMLP). The migration of neutrophils toward fMLP was similarly not affected by the syk(-/-) mutation. fMLP initiated normal Ca(2+)-signal, activation of the extracellular signal-related kinase (ERK) and p38 mitogen-activated protein (MAP) kinase cascades, and polymerization of cellular actin in the absence of Syk. syk(-/-) and wild-type neutrophils also responded similarly to LTB(4), C5a, and the chemokines macrophage inflammatory protein-1 (MIP-1)alpha or MIP-2, both in functional assays and in intracellular signaling mechanisms. Furthermore, bone marrow-derived syk(-/-) mast cells showed normal activation of the Akt, ERK, and p38 MAP kinase pathways when stimulated by the GPCR ligand adenosine. We conclude that, in contrast to previous reports, Syk does not play a major role in GPCR signaling.

Genistein inhibits CCAAT/enhancer-binding protein beta (C/EBPbeta) activity and 3T3-L1 adipogenesis by increasing C/EBP homologous protein expression

The tyrosine kinase inhibitor genistein inhibits 3T3-L1 adipogenesis when present during the first 72 h of differentiation. In this report, we investigated the underlying mechanisms involved in the anti-adipogenic effects of genistein. We found that genistein blocked the DNA binding and transcriptional activity of CCAAT/enhancer-binding protein beta (C/EBPbeta) during differentiation by promoting the expression of C/EBP homologous protein, a dominant-negative member of the C/EBP family. Loss of C/EBPbeta activity was manifested as a loss of differentiation-induced C/EBPalpha and peroxisome-proliferator-activated receptor gamma protein expression and a dramatic reduction in lipid accumulation. Further, we documented for the first time that C/EBPbeta was tyrosine-phosphorylated in vivo during differentiation and in vitro by activated epidermal growth factor receptor. Genistein inhibited both of these events. Collectively, these results indicate that genistein blocks adipogenesis and C/EBPbeta activity by increasing the level of C/EBP homologous protein and possibly by inhibiting the tyrosine phosphorylation of C/EBPbeta.

Role of protein-tyrosine kinase syk in oxidative stress signaling in B cells

Oxidative stress induces the activation of multiple signaling pathways related to various cellular responses. In B cells, Syk has a crucial role in intracellular signal transduction induced by oxidative stress as well as antigen receptor engagement. Treatment of B cells with hydrogen peroxide (H(2)O(2)) induces enzymatic activation of Syk. Syk is essential for Ca(2+) release from intracellular pools through phospholipase C-gamma2 and the activation of c-Jun N-terminal kinase, p38 mitogen-activated protein kinase, and phosphatidylinositol 3-kinase-Akt survival pathway following H(2)O(2) stimulation. Oxidative stress-induced cellular responses in B cells follow different patterns, such as necrosis, apoptosis, and mitotic arrest, according to the intensity of H(2)O(2) stimulation. Syk is involved in the protection of cells from apoptosis and induction of G2/M arrest. Syk leads to the activation of the phosphatidylinositol 3-kinase-Akt survival pathway, thereby enhancing cellular resistance to oxidative stress-induced apoptosis. On the other hand, Syk-dependent phospholipase C-gamma2 activation is required for acceleration toward apoptosis following oxidative stress. These findings suggest that oxidative stress-induced Syk activation triggers the activation of several pathways, such as proapoptotic and survival pathways, and the balance among these various pathways is a key factor in determining the fate of a cell exposed to oxidative stress.

Syk expression and novel function in a wide variety of tissues

Syk protein-tyrosine kinase has been implicated in a variety of hematopoietic cell responses, in particular immunoreceptor signaling events that mediate diverse cellular responses including proliferation, differentiation, and phagocytosis. On the other hand, Syk exhibits a more widespread expression pattern in nonhematopoietic cells like fibroblasts, epithelial cells, breast tissue, hepatocytes, neuronal cells, and vascular endothelial cells and has been shown to be functionally important on these cell types. Thus, Syk appears to play a general physiological function in a wide variety of cells. In this article, we briefly review the current literature regarding the expression and novel function of Syk in various cells and tissues.

Syk expression in endothelial cells and their morphologic defects in embryonic Syk-deficient mice

Mice deficient in the Syk tyrosine kinase showed severe petechiae in utero and died shortly after birth. The mechanism of this bleeding, however, remains unknown. Here it is shown that this bleeding is caused by morphologic defects of Syk-deficient endothelial cells during embryogenesis. Immunoblot and reverse transcriptase-polymerase chain reaction Northern blot analysis indicated that Syk is expressed in several endothelial cell lines. Immunocytochemical analysis also confirmed that Syk is expressed in the normal embryonic endothelial cells and is absent in Syk-deficient mice. Furthermore, electron microscopic analysis of Syk-deficient mice revealed an abnormal morphogenesis and a decreased number of endothelial cells. The results indicate a critical role for Syk in endothelial cell function and in maintaining vascular integrity in vivo.

Actin depolymerization via the beta-adrenoceptor in airway smooth muscle cells: a novel PKA-independent pathway

Actin is a major functional and structural cytoskeletal protein that mediates such diverse processes as motility, cytokinesis, contraction, and control of cell shape and polarity. While many extracellular signals are known to mediate actin filament polymerization, considerably less is known about signals that mediate depolymerization of the actin cytoskeleton. Human airway smooth muscle cells were briefly exposed to isoproterenol, forskolin, or the cAMP-dependent protein kinase A (PKA) agonist stimulatory diastereoisomer of adenosine 3',5'-cyclic monophosphate (Sp-cAMPS). Actin polymerization was measured by concomitant staining of filamentous actin with FITC-phalloidin and globular actin with Texas red DNase I. Isoproterenol, forskolin, or Sp-cAMPS induced actin depolymerization, indicated by a decrease in the intensity of filamentous/globular fluorescent staining. The PKA inhibitor Rp diastereomer of adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS) completely inhibited forskolin-stimulated depolymerization, whereas it only partially inhibited isoproterenol-induced depolymerization. The protein tyrosine kinase inhibitors genistein or tyrphostin A23 also partially inhibited isoproterenol-induced actin depolymerization. In contrast, the combination of Rp-cAMPS and either tyrosine kinase inhibitor had an additive effect at inhibiting isoproterenol-induced actin depolymerization. These results suggest that both PKA-dependent and -independent pathways mediate actin depolymerization in human airway smooth muscle cells.

Galpha(i2) enhances insulin signaling via suppression of protein-tyrosine phosphatase 1B

Suppression of the expression of the heterotrimeric G-protein Galpha(i2) in vivo has been shown to provoke insulin resistance, whereas enhanced insulin signaling is observed when Galpha(i2) is overexpressed in vivo. The basis for Galpha(i2) regulation of insulin signaling was explored in transgenic mice with targeted expression of the GTPase-deficient, constitutively active Q205L Galpha(i2) in fat and skeletal muscle. Phosphorylation of insulin receptor and IRS-1 in response to insulin challenge in vivo was markedly amplified in fat and skeletal muscle expressing Q205L Galpha(i2). The expression and activity of the protein-tyrosine phosphatase 1B (PTP1B), but not protein-tyrosine phosphatases SHP-1, SHP-2, and LAR, were constitutively decreased in tissues expressing the Q205L Galpha(i2), providing a direct linkage between insulin signaling and Galpha(i2). The loss of PTP1B expression may explain, in part, the loss of PTP1B activity in the iQ205L transgenic mice. Activation of Galpha(i2) in mouse adipocytes with lysophosphatidic acid was shown to decrease PTP1B activity, whereas pertussis toxin inactivates Galpha(i2), blocks lysophosphatidic acid-stimulated inhibition of PTP1B activity, and blocks tonic suppression of PTP1B activity by Galpha(i2). Elevation of intracellular cAMP in fat cells is shown to increase PTP1B activity, whereas either depression of cAMP levels or direct activation of Galpha(i2) suppresses PTP1B. These data provide the first molecular basis for the interplay between Galpha(i2) and insulin signaling, i.e. activation of Galpha(i2) can suppress both the expression and activity of PTP1B in insulin-sensitive tissues.

IL-1 induced chemokine production through the association of Syk with TNF receptor-associated factor-6 in nasal fibroblast lines

The fibroblasts stimulated by cytokines released the chemokine and recruited the infiltrating cells, including eosinophils, that play a key role in the pathogenesis of airway disease. We established the human fibroblast lines showing high Syk expression and the lines showing low Syk expression from pieces of nasal polyp. IL-1 induces the interaction of TNFR-associated factor (TRAF) 6 with IL-1R-associated kinase, which is rapidly recruited to the IL-1R after IL-1 induction, whereas TRAF2 participates in TNF-alpha-signaling. In the present study, we found that Syk played a different role in IL-1- and TNF-alpha-induced chemokine production through a signaling complex involving Syk and TRAF6. Overexpression of wild-type Syk by gene transfer enhanced RANTES production from nasal fibroblasts stimulated with IL-1. The decrease of Syk expression by the administration of Syk antisense inhibited RANTES production in response to IL-1. However, the change of Syk expression did not affect RANTES production by TNF-alpha stimulation. We concluded that Syk is required for the IL-1-induced chemokine production through the association with TRAF-6 in fibroblasts of nasal polyps.

Differential effects of flavonoids on 3T3-L1 adipogenesis and lipolysis

Flavonoids, polyphenolic compounds that exist widely in plants, inhibit cell proliferation and increase cell differentiation in many cancerous and noncancerous cell lines. Because terminal differentiation of preadipocytes to adipocytes depends on proliferation of both pre- and postconfluent preadipocytes, we predicted that flavonoids would inhibit adipogenesis in the 3T3-L1 preadipocyte cell line. The flavonoids genistein and naringenin inhibited proliferation of preconfluent preadipocytes in a time- and dose-dependent manner. When added to 2-day postconfluent preadipocytes at the induction of differentiation, genistein inhibited mitotic clonal expansion, triglyceride accumulation, and peroxisome proliferator-activated receptor-gamma expression, but naringenin had no effect. The antiadipogenic effect of genistein was not due to inhibition of insulin receptor subtrate-1 tyrosine phosphorylation. When added 3 days after induction of differentiation, neither flavonoid inhibited differentiation. In fully differentiated adipocytes, genistein increased basal and epinephrine-induced lipolysis, but naringenin had no significant effects. These data demonstrate that genistein and naringenin, despite structural similarity, have differential effects on adipogenesis and adipocyte lipid metabolism.

Syk protein-tyrosine kinase is involved in neuron-like differentiation of embryonal carcinoma P19 cells

Syk has been implicated in activated immunoreceptors to downstream signaling events in hematopoietic cells. Here we report that Syk is expressed in neuron-like cells and involved in neuron-like differentiation of embryonal carcinoma P19 cells. Immunoblot, RT-PCR, and Northern analysis indicated that Syk is expressed in mouse brain, PC12 and P19 cells. In addition, Syk was found to be tyrosine phosphorylated during neuron-like differentiation of P19 cells. Furthermore, adenovirus-mediated overexpression of Syk induced supernumerary neurite formation and extracellular signal-regulated kinase (ERK) activation in P19 cells. These results suggest that Syk plays an important role in signaling steps leading to ERK activation in P19 cells.

Protein-tyrosine kinase Syk expressed in human nasal fibroblasts and its effect on RANTES production

Fibroblasts, a rich source of chemokines, interact with eosinophils and play a key role in the pathogenesis of airway disease. RANTES is produced by fibroblasts to attract and activate eosinophils. LPS is known to induce RANTES and cause protein tyrosine phosphorylation. Nonreceptor protein tyrosine kinase Syk is widely expressed and an important role in intracellular signal transduction in hemopoietic cells. In the present study, we examined whether Syk was expressed in a number of primary human nasal polyp tissue-derived fibroblast lines and whether it played some role in cellular function. Syk proteins were expressed in human nasal fibroblasts, but the expression level varied. There were positive correlations between the level of Syk expression and RANTES production induced by LPS. Overexpression of wild-type Syk by gene transfer enhanced RANTES production from nasal fibroblasts stimulated with LPS. The decrease of Syk expression by the administration of Syk antisense inhibited RANTES production. These results suggest that Syk expression affects RANTES production in fibroblasts of nasal polyps.

Apoptotic signaling through the beta -adrenergic receptor. A new Gs effector pathway

Stimulation of beta-adrenergic receptor normally results in signaling by the heterotrimeric G protein G(s), leading to the activation of adenylyl cyclase, production of cAMP, and activation of cAMP-dependent protein kinase (PKA). Here we report that cell death of thymocytes can be induced after stimulation of beta-adrenergic receptor, or by addition of exogenous cAMP. Apoptotic cell death in both cases was observed with the appearance of terminal deoxynucleotidyl transferase-mediated UTP end labeling reactivity and the activation of caspase-3 in S49 T cells. Using thymocytes deficient in either Galpha(s) or PKA, we find that engagement of beta-adrenergic receptors initiated a Galpha(s)-dependent, PKA-independent pathway leading to apoptosis. This alternative pathway involves Src family tyrosine kinase Lck. Furthermore, we show that Lck protein kinase activity can be directly stimulated by purified Galpha(s). Our data reveal a new signaling pathway for Galpha(s), distinct from the classical PKA pathway, that accounts for the apoptotic action of beta-adrenergic receptors.