Specific inhibitors of p38 mitogen-activated protein kinase block 3T3-L1 adipogenesis

Diabetes-related changes in cAMP response element-binding protein content enhance smooth muscle cell proliferation and migration

We hypothesized that diabetes and glucose-induced reactive oxygen species lead to depletion of cAMP response element-binding protein (CREB) content in the vasculature. In primary cultures of smooth muscle cells (SMC) high medium glucose decreased CREB function but increased SMC chemokinesis and entry into the cell cycle. These effects were blocked by pretreatment with the antioxidants. High glucose increased intracellular reactive oxygen species detected by CM-H(2)DCFA. SMC exposed to oxidative stress (H(2)O(2)) demonstrated a 3.5-fold increase in chemokinesis (p < 0.05) and accelerated entry into cell cycle, accompanied by a significant decrease in CREB content. Chronic oxidative challenge similar to the microenvironment in diabetes (glucose oxidase treatment) decreases CREB content (40-50%). Adenoviral-mediated expression of constitutively active CREB abolished the increase in chemokinesis and cell cycle progression induced by either high glucose or oxidative stress. Analysis of vessels from insulin resistant or diabetic animals indicates that CREB content is decreased in the vascular stroma. Treatment of insulin-resistant animals with the insulin sensitizer rosiglitazone restores vessel wall CREB content toward that observed in normal animals. In summary, high glucose and oxidative stress decrease SMC CREB content increase chemokinesis and entry into the cell cycle, which is blocked by antioxidants or restoration of CREB content. Thus, decreased vascular CREB content could be one of the molecular mechanisms leading to increased atherosclerosis in diabetes.

Hyperglycemia-induced production of acute phase reactants in adipose tissue

Chronic elevation of systemic levels of acute phase reactants and inflammatory cytokines found in patients with diabetes and the often-associated metabolic syndrome X (hypertriglyceridemia, low serum high density lipoprotein cholesterol, hypertension, and accelerated atherosclerosis) may be responsible for the increased incidence of cardiovascular problems in this population. Here we examine the contribution of adipose tissue to the systemic elevation of acute phase reactants associated with chronic hyperglycemia. We demonstrate that adipose tissue expresses a number of acute phase reactants at high levels, including serum amyloid A3 (SAA3), alphal-acid glycoprotein, the lipocalin 24p3 as well as plasminogen activator inhibitor-1 (PAI-1). Additionally, we show SAA3 is expressed at low levels under normal conditions but in the diabetic state is dramatically up-regulated in adipose tissue while down-regulated in liver. Furthermore, pro-inflammatory stimuli and high glucose can lead to the induction of SAA3 in adipose tissue in vivo as well as in the 3T3-L1 adipocyte cell line. Adipose tissue may therefore play a major role in the pathogenic sequelae of Type II diabetes, in particular the cardiovascular problems associated with prolonged hyperglycemia.

Stimulation of glucose transport by AMP-activated protein kinase via activation of p38 mitogen-activated protein kinase

Activation of AMP-activated protein kinase (AMPK) has been recently demonstrated to be associated with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR)-stimulated glucose transport mediated by both GLUT1 and GLUT4 transporters. However, signaling events upstream and downstream of AMPK are unknown. Here we report that 1) p38 mitogen-activated protein kinase (MAPK) and mitogen-activated protein kinase kinase 3 (MKK3) were activated by AICAR in Clone 9 cells, which express only the GLUT1 transporters, and 2) activation of p38 was required for AICAR-stimulated glucose transport since treatment of the cells with p38 inhibitor SB203580 or overexpression of dominant negative p38 mutant inhibited glucose transport. Moreover, we found that overexpression of the constitutively active form of AMPK mutant also resulted in a significant activation of p38, and inhibition of p38 activity by SB203580 did not affect AICAR-stimulated activation of AMPK. These findings demonstrate that AICAR-stimulated activation of p38 is indeed mediated by AMPK, and the p38 MAPK cascade is downstream of AMPK in the signaling pathway of AICAR-stimulated glucose transport in Clone 9 cells.

Biology of leptin in the pig

The recently discovered protein, leptin, which is secreted by fat cells in response to changes in body weight or energy, has been implicated in regulation of feed intake, energy expenditure and the neuroendocrine axis in rodents and humans. Leptin was first identified as the gene product found deficient in the obese ob/ob mouse. Administration of leptin to ob/ob mice led to improved reproduction as well as reduced feed intake and weight loss. The porcine leptin receptor has been cloned and is a member of the class 1 cytokine family of receptors. Leptin has been implicated in the regulation of immune function and the anorexia associated with disease. The leptin receptor is localized in the brain and pituitary of the pig. The leptin response to acute inflammation is uncoupled from anorexia and is differentially regulated among swine genotypes. In vitro studies demonstrated that the leptin gene is expressed by porcine preadipocytes and leptin gene expression is highly dependent on dexamethasone induced preadipocyte differentiation. Hormonally driven preadipocyte recruitment and subsequent fat cell size may regulate leptin gene expression in the pig. Expression of CCAAT-enhancer binding proteinalpha (C/EBPalpha) mediates insulin dependent preadipocyte leptin gene expression during lipid accretion. In contrast, insulin independent leptin gene expression may be maintained by C/EBPalpha auto-activation and phosphorylation/dephosphorylation. Adipogenic hormones may increase adipose tissue leptin gene expression in the fetus indirectly by inducing preadipocyte recruitment and subsequent differentiation. Central administration of leptin to pigs suppressed feed intake and stimulated growth hormone (GH) secretion. Serum leptin concentrations increased with age and estradiol-induced leptin mRNA expression in fat was age and weight dependent in prepuberal gilts. This occurred at the time of expected puberty in intact contemporaries and was associated with greater LH secretion. Further work demonstrated that leptin acts directly on pituitary cells to enhance LH and GH secretion, and brain tissue to stimulate gonadotropin releasing hormone secretion. Thus, development of nutritional schemes and (or) gene therapy to manipulate leptin secretion will lead to practical methods of controlling appetite, growth and reproduction in farm animals, thereby increasing efficiency of lean meat production.

Expression of the stress-induced p8 mRNA is transiently activated after culture medium change

We report here that the mere fact of changing culture medium for fresh medium induced in several cell lines the expression of stress-activated genes including protein kinases p38, JNK and ERK1/2 and the transcription factor C/EBPbeta. As a consequence, p8, a gene induced by stress in several tissues, was strongly up-regulated. Induction did not occur after change for cell-conditioned medium. Induction was however transient, with a peak at 60 min for p38, at 15-30 min for JNK and at 15 min for ERK1/2, at 2-3 hours for C/EBPbeta and at 4-6 hours for p8. Repression of the induction was due to the secretion of thermolabile molecule(s) that progressively conditioned the medium. As low as 25% of conditioned medium added to fresh culture medium was sufficient to abolish the stress response. Taken together, our data indicate that the renewal of culture medium induces a transient cellular stress that may be a source of artifacts in experiments performed shortly after a change of culture medium.

GLUT4 translocation precedes the stimulation of glucose uptake by insulin in muscle cells: potential activation of GLUT4 via p38 mitogen-activated protein kinase

We previously reported that SB203580, an inhibitor of p38 mitogen-activated protein kinase (p38 MAPK), attenuates insulin-stimulated glucose uptake without altering GLUT4 translocation. These results suggested that insulin might activate GLUT4 via a p38 MAPK-dependent pathway. Here we explore this hypothesis by temporal and kinetic analyses of the stimulation of GLUT4 translocation, glucose uptake and activation of p38 MAPK isoforms by insulin. In L6 myotubes stably expressing GLUT4 with an exofacial Myc epitope, we found that GLUT4 translocation (t(1/2)=2.5 min) preceded the stimulation of 2-deoxyglucose uptake (t(1/2)=6 min). This segregation of glucose uptake from GLUT4 translocation became more apparent when the two parameters were measured at 22 degrees C. Preincubation with the p38 MAPK inhibitors SB202190 and SB203580 reduced insulin-stimulated transport of either 2-deoxyglucose or 3-O-methylglucose by 40-60%. Pretreatment with SB203580 lowered the apparent transport V(max) of insulin-mediated 2-deoxyglucose and 3-O-methylglucose without any significant change in the apparent K(m) for either hexose. The IC(50) values for the partial inhibition of 2-deoxyglucose uptake by SB202190 and SB203580 were 1 and 2 microM respectively, and correlated with the IC(50) for full inhibition of p38 MAPK by the two inhibitors in myotubes (2 and 1.4 microM, respectively). Insulin caused a dose- (EC(50)=15 nM) and time- (t(1/2)=3 min) dependent increase in p38 MAPK phosphorylation, which peaked at 10 min (2.3+/-0.3-fold). In vitro kinase assay of immunoprecipitates from insulin-stimulated myotubes showed activation of p38 alpha (2.6+/-0.3-fold) and p38 beta (2.3+/-0.2-fold) MAPK. These results suggest that activation of GLUT4 follows GLUT4 translocation and that both mechanisms contribute to the full stimulation of glucose uptake by insulin. Furthermore, activation of GLUT4 may occur via an SB203580-sensitive pathway, possibly involving p38 MAPK.

Tumor cell differentiation by butyrate and environmental stress

The present study shows that stress signaling plays a role in differentiation of K562, PANC1, HT29 and HL60 tumor cells: (1) Butyrate induced differentiation in K562, PANC1, and HT29 cells can be inhibited by SB203580, a specific inhibitor of p38 stress activated protein kinase. (2) Heat shock and hyperosmolarity increase expression of differentiation markers in K562, HT29, HL60 and in K562, PANC1, and HT29 cells, respectively. (3) Conversely, environmental stress induced differentiation in K562, HT29, and PANC1 cells can be inhibited by SB203580 and quercetin, a compound with heat shock pathway inhibiting activity. (4) Butyrate and environmental stress enhance either additively or synergistically differentiation of K562, HT29, PANC1 or HL60 cells, respectively. Stress signaling pathways might be an interesting pharmacologic target for differentiation therapy of malignant disease.

Inhibition of myogenesis enables adipogenic trans-differentiation in the C2C12 myogenic cell line

C2C12 cells are a well-established model system for studying myogenesis. We examined whether inhibiting the process of myogenesis via expression of dominant negative (DN) mitogen-activated protein kinase kinase-3 (MKK3) facilitated the trans-differentiation of these cells into adipocytes. Cells expressing DN MKK3 respond to rosiglitazone, resulting in adipocyte formation. The effects of rosiglitazone appear to be potentiated through peroxisome proliferator activating receptor-gamma. This trans-differentiation is inhibited by the use of the phosphoinositide-3 (PI3) kinase inhibitor, LY294002. These results indicate that preventing myogenesis through expression of DN MKK3 facilitates adipocytic trans-differentiation, and involves PI3 kinase signalling.

Semicarbazide-sensitive amine oxidase activation promotes adipose conversion of 3T3-L1 cells

Semicarbazide-sensitive amine oxidase (SSAO) is an amine oxidase related to the copper-containing amine oxidase family. The tissular form of SSAO is located at the plasma membrane, and is mainly expressed in vascular smooth muscle cells and adipocytes. Recent studies have suggested that SSAO could activate glucose transport in fat cells. In the present work, we investigated the potential role of a chronic SSAO activation on adipocyte maturation of the 3T3-L1 pre-adipose cell line. Exposure of post-confluent 3T3-L1 pre-adipocytes to methylamine, a physiological substrate of SSAO, promoted adipocyte differentiation in a time- and dose-dependent manner. This effect could be related to SSAO activation, since it was antagonized in the presence of the SSAO inhibitor semicarbazide, but not in the presence of the monoamine oxidase inhibitor pargyline. In addition, methylamine-induced adipocyte maturation was mimicked by 3T3-L1 cell treatment with other SSAO substrates. Finally, the large reversion of methylamine action by catalase indicated that hydrogen peroxide generated by SSAO was involved, at least in part, in the modulation of adipocyte maturation. Taken together, our results suggest that SSAO may contribute to the control of adipose tissue development.

Intestinal epithelial cell differentiation involves activation of p38 mitogen-activated protein kinase that regulates the homeobox transcription factor CDX2

The intracellular signaling pathways responsible for cell cycle arrest and differentiation along the crypt-villus axis of the human small intestine remain largely unknown. p38 mitogen-activated protein kinases (MAPKs) have recently emerged as key modulators of various vertebrate cell differentiation processes. In order to elucidate further the mechanism(s) responsible for the loss of proliferative potential once committed intestinal cells begin to differentiate, the role and regulation of p38 MAPK with regard to differentiation were analyzed in both intact epithelium as well as in well established intestinal cell models recapitulating the crypt-villus axis in vitro. Results show that phosphorylated and active forms of p38 were detected primarily in the nuclei of differentiated villus cells. Inhibition of p38 MAPK signaling by 2-20 microm SB203580 did not affect E2F-dependent transcriptional activity in subconfluent Caco-2/15 or HIEC cells. p38 MAPK activity dramatically increased as soon as Caco-2/15 cells reached confluence, whereas addition of SB203580 during differentiation of Caco-2/15 cells strongly attenuated sucrase-isomaltase gene and protein expression as well as protein expression of villin and alkaline phosphatase. The binding of CDX2 to the sucrase-isomaltase promoter and its transcriptional activity were significantly reduced by SB203580. Pull-down glutathione S-transferase and immunoprecipitation experiments demonstrated a direct interaction of CDX3 with p38. Finally, p38-dependent phosphorylation of CDX3 was observed in differentiating Caco-2/15 cells. Taken together, our results indicate that p38 MAPK may be involved in the regulation of CDX2/3 function and intestinal cell differentiation.

Alterations of MAPK activities associated with intestinal cell differentiation

Three distinct groups of mitogen-activated protein kinases (MAPKs) have been identified in mammalian cells (i.e., ERK, JNK, and p38) which play an important role in the differentiation and apoptosis of various cells. The purpose of our present study was to determine MAPK activity and levels associated with sodium butyrate (NaBT)-mediated differentiation and apoptosis in the human colon cancer cell lines Caco-2 and HT29. Intestinal alkaline phosphatase (IAP) activity, a marker of intestinal differentiation, was increased at 48 h after NaBT treatment followed by cell death at 72 h. ERK activity was decreased in differentiated Caco-2 cells either induced with NaBT or allowed to differentiate spontaneously and in HT29 cells treated with NaBT. The combination of the MEK inhibitor, PD98059, with NaBT further increased IAP activity and cell death compared with NaBT alone. In contrast to ERK, JNK1 activity and c-Jun phosphorylation was increased 8 h after NaBT treatment suggesting a role for the JNK pathway in intestinal cell differentiation and apoptosis. p38 activity was increased at 24 and 48 h after NaBT treatment. Taken together, our results suggest that alterations in MAPKs (i.e., ERK inhibition and JNK induction) contribute to the differentiation and apoptotic pathways in intestinal cells.

CHOP, a basic leucine zipper transcriptional factor, contributes to the antiproliferative effect of IL-1 on A375 human melanoma cells through augmenting transcription of IL-6

Interleukin-1 (IL-1) inhibits the proliferation of A375 human melanoma cells. We have demonstrated previously that p38 mitrogen-activated protein kinase (MAPK) mediated the antiproliferative effect of IL-1 partially through the downregulation of activity and protein level of ornithine decarboxylase (ODC). In this study, we investigated the role of CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP), one of the p38 MAPK target transcriptional factors. The mRNA level of CHOP was not affected by IL-1 treatment in A375-6 cells. Unexpectedly, CHOP was constitutively phosphorylated, and IL-1 or p38 MAPK inhibitor, SB203580, did not affect the phosphorylation level. However, A375-6 cells exhibited enhanced sensitivity to IL-1 by transfecting CHOP expression plasmid and reduced sensitivity to IL-1 by antisense CHOP mRNA expression plasmid. Furthermore, CHOP appeared to regulate positively IL-6 production at the transcriptional level. The experiments using CHOP muteins revealed that dimerization ability - but not p38 MAPK-dependent phosphorylation or DNA binding activity - is important for the IL-6 inducing activity of CHOP. These results indicate that CHOP contributes to the IL-1 growth-inhibitory signal through augmenting IL-6 production.

Skeletal muscle cell activation by low-energy laser irradiation: a role for the MAPK/ERK pathway

Low-energy laser irradiation (LELI) has been shown to promote skeletal muscle regeneration in vivo and to activate skeletal muscle satellite cells, enhance their proliferation and inhibit differentiation in vitro. In the present study, LELI, as well as the addition of serum to serum-starved myoblasts, restored their proliferation, whereas myogenic differentiation remained low. LELI induced mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK) phosphorylation with no effect on its expression in serum-starved myoblasts. Moreover, a specific MAPK kinase inhibitor (PD098059) inhibited the LELI- and 10% serummediated ERK1/2 activation. However, LELI did not affect Jun N-terminal kinase (JNK) or p38 MAPK phosphorylation or protein expression. Whereas a 3-sec irradiation induced ERK1/2 phosphorylation, a 12-sec irradiation reduced it, again with no effect on JNK or p38. Moreover, LELI had distinct effects on receptor phosphorylation: it caused phosphorylation of the hepatocyte growth factor (HGF) receptor, previously shown to activate the MAPK/ERK pathway, whereas no effect was observed on tumor suppressor necrosis alpha (TNF-alpha) receptor which activates the p38 and JNK pathways. Therefore, by specifically activating MAPK/ERK, but not JNK and p38 MAPK enzymes, probably by specific receptor phosphorylation, LELI induces the activation and proliferation of quiescent satellite cells and delays their differentiation.

Down-regulation of peroxisome proliferator-activated receptor-gamma gene expression by sphingomyelins

We recently demonstrated that the sphingomyelin (SM) content of adipocyte membranes was negatively correlated with the expression of peroxisome proliferator-activated receptor-gamma (PPARgamma) in the subcutaneous adipose tissue of obese women with variable degrees of insulin resistance. We have now investigated whether SM really does have an impact on the expression of PPARgamma in 3T3-F442A adipocytes. Adding SM to the culture medium for 24 h caused a significant increase in SM content of adipocyte membranes and an acyl chain length-dependent decrease in the levels of PPARgamma mRNA and protein. The longer the acyl chain of the fatty acid of SM, the greater was the decrease in PPARgamma. These data suggest that the nature of the fatty acid is important in the regulation of PPARgamma by the SM pathway.

Serum induction of the fibroblast growth factor-binding protein (FGF-BP) is mediated through ERK and p38 MAP kinase activation and C/EBP-regulated transcription

The fibroblast growth factor-binding protein (FGF-BP) modulates FGF activity through binding and release from the extracellular matrix. Consequently, the expression of FGF-BP in certain tumor types is a rate-limiting regulator of FGF-mediated angiogenesis. FGF-BP is upregulated in squamous cell carcinoma by treatment with mitogens such as EGF or TPA. In this study, we investigated the regulation of FGF-BP gene expression by serum. Treatment of serum-starved ME-180 cells with fetal bovine serum (FBS) resulted in a rapid increase in steady-state levels of FGF-BP mRNA and in the rate of FGF-BP gene transcription. Serum induction of FGF-BP mRNA was not mediated through EGF receptor activation but was dependent on PKC, as well as ERK kinase (MEK) and p38 MAP kinase activation. Promoter analysis showed that C/EBP is the main promoter element required for the serum response. Unlike EGF-activation of FGF-BP, transcriptional induction by serum is not significantly regulated through the AP-1 or E-box sites in the promoter. These results illustrate differences between the mechanism of induction in response to serum and EGF.

TNF-alpha inhibits UCP-1 expression in brown adipocytes via ERKs. Opposite effect of p38MAPK

Tumor necrosis factor-alpha (TNF-alpha) activates extracellular-regulated kinases (ERKs) and p38 mitogen-activated protein kinase (p38MAPK), and inhibits the expression of uncoupling protein-1 (UCP-1) and adipocyte-specific genes in rat fetal brown adipocytes. MEK inhibition with PD98059 abolished the inhibitory effect of TNF-alpha on UCP-1, but not on adipogenic genes. In contrast, inhibition of p38MAPK with SB203580 potentiated the negative effect of TNF-alpha on UCP-1 and adipogenic genes. The inhibitory action of TNF-alpha was partially correlated with changes in C/EBPalpha and beta protein levels and in their DNA binding activity, suggesting a role for these transcription factors. However, other transcription factors might explain the different regulation of UCP-1 and adipogenic genes by ERKs.

Molecular regulation of adipogenesis

Adipogenesis, or the development of fat cells from preadipocytes, has been one of the most intensely studied models of cellular differentiation. In part this has been because of the availability of in vitro models that faithfully recapitulate most of the critical aspects of fat cell formation in vivo. More recently, studies of adipogenesis have proceeded with the hope that manipulation of this process in humans might one day lead to a reduction in the burden of obesity and diabetes. This review explores some of the highlights of a large and burgeoning literature devoted to understanding adipogenesis at the molecular level. The hormonal and transcriptional control of adipogenesis is reviewed, as well as studies on a less well known type of fat cell, the brown adipocyte. Emphasis is placed, where possible, on in vivo studies with the hope that the results discussed may one day shed light on basic questions of cellular growth and differentiation in addition to possible benefits in human health.

Regulation of activating transcription factor-2 in early stage of the adipocyte differentiation program

p38beta mitogen-activated protein kinase activity is required for the differentiation of 3T3-L1 fibroblasts into adipocytes. Activating transcription factor-2 (ATF-2) is efficiently phosphorylated and activated by p38beta kinase. These findings led us to examine a regulatory role of ATF-2 in adipocyte differentiation. The induction of ATF-2 protein precedes the expression of the transcription factors, peroxisome proliferator-activated receptor (PPAR) gamma and CCAAT/enhancer-binding protein (C/EBP) alpha. Consistent with early activation of p38beta kinase, the phosphorylation of ATF-2 was also detected in early stage of adipocyte differentiation. ATF-2 regulated gene transcription of PPARgamma, which was synergistically enhanced by p38beta kinase and C/EBPbeta proteins expression. Ectopic expression of ATF-2 in 3T3-L1 cells induced the endogenous PPARgamma protein levels. These results suggest that ATF-2 plays a role in a primary regulator of adipocyte differentiation with C/EBPbeta through promoting adipogenesis-inducing transcription factors including PPARgamma and becomes associated earlier in the differentiation program as mitotic clonal expansion proceeds and the cells become initially differentiated.

Transcriptional control of adipogenesis

The major transcriptional factors involved in the adipogenic process include proteins belonging to the CCAAT/enhancer binding protein family, peroxisome proliferator-activated receptor gamma, and adipocyte determination and differentiation dependent factor 1, also known as sterol regulatory element-binding protein 1. This process has been characterized with the aid of cell lines that represent various stages in the path of adipocyte commitment, ranging from pluripotent mesodermal fibroblasts to preadipocytes. Molecular analyses have led to a cascade model for adipogenesis based on timed expression of CCAAT/enhancer-binding proteins and peroxisome proliferator-activated receptor gamma. Gene targeting and transgenic-mouse technologies, which allow the manipulation of endogenous genes for these transcription factors, have also contributed to the understanding of adipogenesis. This review aims to integrate this information to gain an understanding of the transcriptional regulation of fat cell formation.

Chromosomal localization, expression pattern, and promoter analysis of the mouse gene encoding adipocyte-specific secretory protein Acrp30

Acrp30 is an abundantly expressed secretory protein exclusively synthesized in adipose tissue. Due to the dysregulation in various forms of obesity in humans and mice and its strong structural similarity to TNFalpha, it is currently under study as an important molecule involved in whole body energy homeostasis. Here we describe the sequence of mouse Acrp30 locus, define the intron/exon boundaries and map the gene to the telomere of mouse chromosome 16, syntenic to the human chromosomal locus 3q27. We demonstrate that alternative polyadenylation gives rise to two distinct mRNA species. We also show that Acrp30 expression is induced only at the late stages of mouse embryonic development. Finally, we have characterized the mouse Acrp30 promoter in tissue culture cells. We propose that Acrp30 promoter has the potential to drive strong adipocyte specific heterologous gene expression in transgenic mice.

Phosphatidylinositol-3,4,5-trisphosphate is required for insulin-like growth factor 1-mediated survival of 3T3-L1 preadipocytes

Adipocyte number, a determinant of adipose tissue mass, reflects the balance between the rates of proliferation/differentiation vs. apoptosis of preadipocytes. The percentage of 3T3-L1 preadipocytes undergoing cell death following serum deprivation was reduced by 10 nM insulin-like growth factor (IGF)-1 (from 50.0 +/- 0.7% for control starved cells to 27.5 +/- 3.1%). TUNEL staining confirmed the apoptotic nature of the cell death. The protective effect of IGF-1 was blocked by phosphoinositide 3-kinase (PI3K) inhibitors, wortmannin, and LY294002, but was unaffected by rapamycin, PD98059, or SB203580, which inhibit mammalian target of rapamycin (mTOR), ERK kinase (MEK1), and p38 MAPK respectively. Exogenous PI(3,4,5)P3 (10 microM), the principal product of IGF-1-stimulated PI3K in 3T3-L1 preadipocytes, had a modest survival effect on its own, reducing cell death from 47.9 +/- 3.4% to 35.6 +/- 3.5%. When added to the combination of IGF-1 and LY294002, PI(3,4,5)P3 reversed most of the inhibitory effect of LY294002 on IGF-1-dependent cell survival, protein kinase B/Akt phosphorylation, and caspase-3 activity. Taken together, these results implicate PI(3,4,5)P3 as a necessary signal for the anti-apoptotic action of IGF-1 on 3T3-L1 preadipocytes.

p38 mitogen-activated protein kinase mediates tumor necrosis factor-alpha-induced apoptosis in rat fetal brown adipocytes

Tumor necrosis factor-alpha (TNFalpha) induces apoptosis and cell growth inhibition in primary rat fetal brown adipocytes. Here, we examine the role played by some members of the mitogen-activated protein kinase (MAPK) superfamily. TNFalpha activates extracellular regulated kinase-1/2 (ERK1/2) and p38MAPK. Inhibition of p38MAPK by either SB203580 or SB202190 highly reduces apoptosis induced by TNFalpha, whereas ERK inhibition potentiates it. Moreover, cotransfection of an active MKK3 mutant and p38MAPK induces apoptosis. p38MAPK inhibition also prevents TNFalpha-induced cell cycle arrest, whereas MEK1 inhibition enhances this effect, which correlates with changes in proliferating cell nuclear antigen expression, but not in cyclin D1. c-Jun and activating transcription factor-1 are potential downstream effectors of p38MAPK and ERKs upon TNFalpha treatment. Thus, TNFalpha-induced c-Jun messenger RNA expression requires ERKs activation, whereas p38MAPK inhibition enhances its expression. In addition, TNFalpha-induced activating transcription factor-1 phosphorylation is extensively decreased by SB203580. However, TNFalpha-induced NF-kappaB DNA-binding activity is independent of p38MAPK and ERK activation. On the other hand, C/EBP homology protein does not appear to mediate the actions of TNFalpha, because its expression is almost undetectable and even reduced by TNFalpha. Finally, although TNFalpha induces c-Jun N-terminal kinase (JNK) activation, transfection of a dominant negative of either JNK1 or JNK2 had no effect on TNFalpha-induced apoptosis. These results suggest that p38MAPK mediates TNFalpha-induced apoptosis and cell cycle arrest, whereas ERKs do the opposite, and JNKs play no role in this process of apoptosis.

Regulation of cyclooxygenase-2 induction in the mouse uterus during decidualization. An event of early pregnancy

The infertility phenotype of cyclooxygenase-2 (Cox-2)-deficient female mice establishes the important role of Cox-2 in pregnancy. Cox-2 deficiency results in defective ovulation, fertilization, implantation, and decidualization; the latter of which can be restored in part by the prostacyclin analog carbaprostacyclin. Uterine Cox-2 expression during early pregnancy shows distinct localization and kinetics in the uterine luminal epithelium and underlying stromal cells, suggesting that expression is tightly regulated. Several intracellular signaling cascades including ERK, p38, and JNK are implicated in vitro as critical components of regulated Cox-2 expression in response to mitogens, growth factors, and cytokines. We investigated the involvement of these signaling pathways during Cox-2 induction in vivo by monitoring uterine kinase activity after intraluminal application of a deciduogenic stimulus. Our results show that the ERK and p38 pathways are activated in uterine preparations as early as 5-min post-stimulation. ERK activation was sustained for several hours with a return to baseline levels by 4 h. p38 activation was rapid with a peak at 5-min post-stimulation and returned to near baseline levels after 45 min. Systemic administration of a MEK inhibitor completely inhibited ERK activation, but did not affect early (2 h) luminal epithelial or late (24 h) stromal Cox-2 expression and only modestly affected decidualization. In contrast, administration of a p38 inhibitor modestly inhibited early Cox-2 expression in the luminal epithelium, while dramatically diminishing late stromal expression. In parallel, induced stromal peroxisomal proliferator activated receptor-delta (PPARdelta) expression is blunted by p38 inhibition. p38 inhibition also significantly inhibited decidualization. These results suggest that p38, but not ERK, activation is required for induced Cox-2 and PPARdelta expression during decidualization. In addition, inhibition of p38 led to decreased decidualization suggesting that an intracrine prostanoid pathway consisting of Cox-2, prostacyclin, and PPARdelta is required for maintenance of early pregnancy.

Constitutive and growth factor-regulated phosphorylation of caveolin-1 occurs at the same site (Tyr-14) in vivo: identification of a c-Src/Cav-1/Grb7 signaling cassette

Caveolin-1 was first identified as a phosphoprotein in Rous sarcoma virus (RSV)-transformed chicken embryo fibroblasts. Tyrosine 14 is now thought to be the principal site for recognition by c-Src kinase; however, little is known about this phosphorylation event. Here, we generated a monoclonal antibody (mAb) probe that recognizes only tyrosine 14-phosphorylated caveolin-1. Using this approach, we show that caveolin-1 (Y14) is a specific tyrosine kinase substrate that is constitutively phosphorylated in Src- and Abl-transformed cells and transiently phosphorylated in a regulated fashion during growth factor signaling. We also provide evidence that tyrosine-phosphorylated caveolin-1 is localized at the major sites of tyrosine-kinase signaling, i.e. focal adhesions. By analogy with other signaling events, we hypothesized that caveolin-1 could serve as a docking site for pTyr-binding molecules. In support of this hypothesis, we show that phosphorylation of caveolin-1 on tyrosine 14 confers binding to Grb7 (an SH2-domain containing protein) both in vitro and in vivo. Furthermore, we demonstrate that binding of Grb7 to tyrosine 14-phosphorylated caveolin-1 functionally augments anchorage-independent growth and epidermal growth factor (EGF)-stimulated cell migration. We discuss the possible implications of our findings in the context of signal transduction.

Tumor necrosis factor alpha-mediated insulin resistance, but not dedifferentiation, is abrogated by MEK1/2 inhibitors in 3T3-L1 adipocytes

Tumor necrosis factor-alpha (TNFalpha) has been implicated as a contributing mediator of insulin resistance observed in pathophysiological conditions such as obesity, cancer-induced cachexia, and bacterial infections. Previous studies have demonstrated that TNFalpha confers insulin resistance by promoting phosphorylation of serine residues on insulin receptor substrate 1 (IRS-1), thereby diminishing subsequent insulin-induced tyrosine phosphorylation of IRS-1. However, little is known about which signaling molecules are involved in this process in adipocytes and about the temporal sequence of events that ultimately leads to TNFalpha-stimulated IRS-1 serine phosphorylation. In this study, we demonstrate that specific inhibitors of the MAP kinase kinase (MEK)1/2-p42/44 mitogen-activated protein (MAP) kinase pathway restore insulin signaling to normal levels despite the presence of TNFalpha. Additional experiments show that MEK1/2 activity is required for TNFalpha-induced IRS-1 serine phosphorylation, thereby suggesting a mechanism by which these inhibitors restore insulin signaling. We observe that TNFalpha requires 2.5-4 h to markedly reduce insulin-triggered tyrosine phosphorylation of IRS-1 in 3T3-L1 adipocytes. Although TNFalpha activates p42/44 MAP kinase, maximal stimulation is observed within 10-30 min. To our surprise, p42/44 activity returns to basal levels well before IRS-1 serine phosphorylation and insulin resistance are observed. These activation kinetics suggest a mechanism of p42/44 action more complicated than a direct phosphorylation of IRS-1 triggered by the early spike of TNFalpha-induced p42/44 activity. Chronic TNFalpha treatment (>> 72 h) causes adipocyte dedifferentiation, as evidenced by the loss of triglycerides and down-regulation of adipocyte-specific markers. We observe that this longer term TNFalpha-mediated dedifferentiation effect utilizes alternative, p42/44 MAP kinase-independent intracellular pathways. This study suggests that TNFalpha-mediated insulin resistance, but not adipocyte dedifferentiation, is mediated by the MEK1/2-p42/44 MAP kinase pathway.

The lipopolysaccharide-activated toll-like receptor (TLR)-4 induces synthesis of the closely related receptor TLR-2 in adipocytes

The central regulatory role of the adipocyte in whole body energy homeostasis is well established. However, recent findings suggest that preadipocytes and adipocytes may play an important physiological role in the regulation of both the innate and adaptive immune response. To systematically characterize the molecular machinery of the adipocyte that mediates the recognition of pathogens, we have focused our analysis on the recently identified Toll-like receptors (TLRs). These receptors have been implicated as mediators of the cellular response to bacterial lipopolysacharides (LPSs). Here, we report the cloning and functional characterization of mouse TLR-2 from 3T3-L1 adipocytes. TLR-2 synthesis is strongly induced in the adipocyte by LPS, TNFalpha, and the yeast cell wall extract zymosan. TLR-2 undergoes a lengthy intracellular maturation process with a half-life of exit from the ER of approximately 3 h. Furthermore, LPS treatment of adipocytes results in dramatic changes at the level of gene expression, including the synthesis of a distinct set of secretory proteins such as interleukin-6. Our studies demonstrate the presence of a fully intact pathway of innate immunity in the adipocyte that can be activated by LPS binding to the cell surface and results in the secretion of immunomodulatory molecules.

The involvement of p38 mitogen-activated protein kinase in the alpha-melanocyte stimulating hormone (alpha-MSH)-induced melanogenic and anti-proliferative effects in B16 murine melanoma cells

Activation of p38 or p44/42 mitogen-activated protein (MAP) kinases has been shown to trigger differentiation in a number of cell types. The present study has investigated the roles of these kinases in the alpha-melanocyte stimulating hormone (alpha-MSH)-induced melanogenic and proliferative responses in B16 melanoma cells. Treatment of cells with alpha-MSH led to the time-dependent phosphorylation of both p38 and p44/42 MAP kinases. However, only inhibition of p38 MAP kinase activity with SB 203580 blocked both the alpha-MSH-induced melanogenic and anti-proliferative effects. It therefore appears that activation of the p38 pathway can promote melanogenesis and inhibit growth of B16 melanoma cells.

Cooperative transformation of 32D cells by the combined expression of IRS-1 and V-Ha-Ras

32D cells expressing v-Ha-Ras fail to show a transformed phenotype. Since Ras requires an active IGF-1R for transformation of fibroblasts, we asked whether expression of IRS-1 or Shc (two of the major substrates of the IGF-1R) could co-operate with oncogenic Ras in transforming 32D cells. We find that IRS-1, but not Shc, in combination with v-Ha-Ras generates a fully transformed phenotype in 32D cells. 32D cells expressing both IRS-1 and v-Ha-Ras (32D/IRS1/Ras) survive and proliferate in the absence of IL-3, do not undergo granulocytic differentiation in the presence of G-CSF and form tumors in nu/nu and syngeneic mice. In contrast, 32D cells expressing singly IRS-1 or v-Ha-Ras exhibit only a block in differentiation capacity. Over-expression of Shc proteins, by itself, promotes differentiation of 32D cells. Concomitant expression of IRS-1 and v-Ha-Ras synergistically phosphorylates ERK-1 and ERK-2 whereas a MEK inhibitor rapidly induces death of 32D/IRS1/Ras transformed cells. Furthermore, transformed 32D/IRS1/Ras cells display high levels of PI3-K activation and undergo rapid apoptosis when exposed to PI3-K inhibitors. The data indicate that: (1) a fully transformed phenotype in 32D cells is generated when a block in differentiation (v-Ha-Ras) is coupled with another differentiation block (IRS-1); (2) PI3-K and MAPK activity are required for the survival of transformed cells; (3) the signals generated by IRS-1 and oncogenic Ras converge on ERK and PI3-K resulting in high levels of activation.

Molecular mechanisms of tumor necrosis factor alpha gene expression in monocytic cells via hyperglycemia-induced oxidant stress-dependent and -independent pathways

Increased oxidative stress has been reported in vivo in the diabetic state via the production of reactive oxygen species (ROS). Such stress is bound to play a key role on activation of circulating monocytes, leading to the accelerated atherosclerosis observed in diabetics. However the exact molecular mechanisms of monocyte activation by high glucose is currently unclear. Here, we demonstrate that chronic high glucose (CHG) causes a dramatic increase in the release of the inflammatory cytokine tumor necrosis factor alpha (TNFalpha), at least in part through enhanced TNFalpha mRNA transcription, mediated by ROS via activation of transcription factors nuclear factor kappaB (NF-kappaB) and activating protein-1 (AP-1). TNFalpha accumulation in the conditioned media was increased 10-fold and mRNA levels were increased 11.5-fold by CHG. The following observations supported that both NF-kappaB and AP-1 mediated enhanced TNFalpha transcription by CHG: 1) A 295-base pair fragment of the proximal TNFalpha promoter containing NF-kappaB and AP-1 sites reproduced the effects of CHG on TNFalpha transcription in a luciferase reporter assay, 2) mutational analyses of both NF-kappaB and the AP-1 sites abrogated 90% of the luciferase activity, 3) gel-shift analysis using the binding sites showed activation of NF-kappaB and AP-1 in CHG nuclear extracts, and 4) Western blot analyses demonstrated elevated nuclear levels of p65 and p50 and decreased cytosolic levels of IkappaBalpha in CHG-treated monocytes. That ROS acted as a key intermediate in the CHG pathway was supported by the following evidence: 1) increased superoxide levels similar to those observed with PMA or TNFalpha, 2) increased phosphorylation of stress-responsive mitogen-activated protein kinases p38 and JNK-1, 3) counteraction of the effects of CHG on TNFalpha production, the 295TNFluc reporter activity, activation of NF-kappaB, and repression of IkappaBalpha by antioxidants and p38 mitogen-activated protein kinase inhibitors. The study suggests that ROS function as key components in the regulatory pathway progressing from elevated glucose to monocyte activation.

Induction of the angiogenic modulator fibroblast growth factor-binding protein by epidermal growth factor is mediated through both MEK/ERK and p38 signal transduction pathways

Fibroblast growth factor-binding protein (FGF-BP) is a secreted protein that binds and activates fibroblast growth factors (FGF-1 and FGF-2) and induces angiogenesis in some human cancers. FGF-BP is expressed at high levels in squamous cell carcinoma (SCC) cell lines and tumor samples and has been shown to be rate-limiting for the growth of SCC tumors in vivo. In this study, we examine the regulation of FGF-BP by epidermal growth factor (EGF) and the signal transduction mechanisms that mediate this effect. We found that EGF treatment of the ME-180 SCC cell line caused a rapid induction of FGF-BP gene expression. This induction was mediated transcriptionally through the AP-1 (c-Fos/JunD) and CCAAT/enhancer-binding protein elements as well as through an E-box repressor site in the proximal regulatory region of the FGF-BP promoter. Pharmacological inhibition of protein kinase C and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1/2 (MEK1/2) completely blocked EGF induction of FGF-BP mRNA, whereas inhibition of phosphatidylinositol 3-kinase had no effect. Additionally, both EGF- and anisomycin-induced FGF-BP mRNA was abrogated by inhibition of p38 mitogen-activated protein kinase, demonstrating a role for p38 in the regulation of FGF-BP. Co-transfection of the FGF-BP promoter with dominant negative forms of MEK2, extracellular signal-regulated kinase 2, and p38 significantly decreased the level of EGF induction, whereas expression of a dominant negative c-Jun N-terminal kinase mutant or expression of c-Jun N-terminal kinase inhibitory protein had no effect. Similarly, activation of the p38 pathway by overexpression of wild-type p38 or MKK6 enhanced FGF-BP transcription. These results demonstrate that EGF induction of FGF-BP occurs selectively through dual activation of the stress-activated p38 and the MEK/extracellular signal-regulated kinase mitogen-activated protein kinase pathways, which ultimately leads to activation of the promoter through AP-1 and CCAAT/enhancer-binding protein sites.

Role of mitogen-activated protein kinase cascades in mediating lipopolysaccharide-stimulated induction of cyclooxygenase-2 and IL-1 beta in RAW264 macrophages

LPS stimulation of RAW264 macrophages triggered the activation of mitogen- and stress-activated protein kinases-1 and -2 (MSK1, MSK2) and their putative substrates, the transcription factors cyclic AMP response element-binding protein (CREB) and activating transcription factor-1 (ATF1). The activation of MSK1/MSK2 was prevented by preincubating the cells with a combination of two drugs that suppress activation of the classical mitogen-activated protein kinase cascade and stress-activated protein kinase/p38, respectively, but inhibition was only partial in the presence of either inhibitor. The LPS-stimulated activation of CREB and ATF1, the transcription of the cyclooxygenase-2 (COX-2) and IL-1 beta genes (the promoters of which contain a cyclic AMP response element), and the induction of the COX-2 protein were prevented by the same drug combination, as well as by Ro 318220 or H89, potent inhibitors of MSK1/MSK2. Two other transcription factors, C/EBP beta and NF-kappa B, have been implicated in the transcription of the COX-2 gene. However, PD 98059 and/or SB 203580 did not prevent the LPS-induced increase in the level of the transcription factor C/EBP beta, and none of the four inhibitors used in this study prevented the activation of NF-kappa B. Our results demonstrate that two different mitogen-activated protein kinase cascades are rate limiting for the LPS-induced activation of CREB/ATF1 and the transcription of the COX-2 and IL-1 beta genes. They also suggest that MSK1 and MSK2 may play a role in these processes and hence are potential targets for the development of novel antiinflammatory drugs.

Induction of terminal differentiation by constitutive activation of p38 MAP kinase in human rhabdomyosarcoma cells

MyoD inhibits cell proliferation and promotes muscle differentiation. A paradoxical feature of rhabdomyosarcoma (RMS), a tumor arising from muscle precursors, is the block of the differentiation program and the deregulated proliferation despite MyoD expression. A deficiency in RMS of a factor required for MyoD activity has been implicated by previous studies. We report here that p38 MAP kinase (MAPK) activation, which is essential for muscle differentiation, is deficient in RMS cells. Enforced induction of p38 MAPK by an activated MAPK kinase 6 (MKK6EE) restored MyoD function and enhanced MEF2 activity in RMS deficient for p38 MAPK activation, leading to growth arrest and terminal differentiation. Stress and cytokines could activate the p38 MAPK in RMS cells, however, these stimuli did not promote differentiation, possibly because they activated p38 MAPK only transiently and they also activated JNK, which could antagonize differentiation. Thus, the selective and sustained p38 MAPK activation, which is distinct from the stress-activated response, is required for differentiation and can be disrupted in human tumors.

Opposing role of mitogen-activated protein kinase subtypes, erk-1/2 and p38, in the regulation of chondrogenesis of mesenchymes

The present studies were performed to determine subtype-specific roles of mitogen-activated protein kinase in chondrogenesis. Erk-1/2 activities, downstream of protein kinase C, decreased as chondrogenesis proceeded, whereas p38 activities, independent of protein kinase C, continuously increased during chondrogenesis. Inhibition of Erk-1/2 with PD98059 enhanced chondrogenesis up to 1. 7-fold, whereas inhibition of p38 with SB203580 reduced it to about 30% of the control level. Inhibition of Erk-1/2 or p38 did not affect precartilage condensation. However, cartilage nodule formation was significantly blocked by the inhibition of p38, whereas Erk-1/2 inhibition did not affect it. Modulation of chondrogenesis by the inhibition of Erk-1/2 and p38 was accompanied by altered expression of adhesion molecules in an opposite way. Expression of N-cadherin was reduced as chondrogenesis proceeded. Inhibition of p38 caused sustained expression of N-cadherin, whereas Erk-1/2 inhibition accelerated the reduction of N-cadherin expression. Expression of integrin alpha5beta1 and fibronectin were found to transiently increase during chondrogenesis. Inhibition of p38 caused continuous increase of expression of these molecules, whereas Erk-1/2 inhibition accelerated the decrease of expression of these molecules at a later period of chondrogenesis. Because temporal expression of these adhesion molecules regulates chondrogenesis, the above results indicate that Erk-1/2 and p38 conversely regulate chondrogenesis at post-precartilage condensation stages by modulating expression of adhesion molecules.

Stimulation of sodium-dependent phosphate transport and signaling mechanisms induced by basic fibroblast growth factor in MC3T3-E1 osteoblast-like cells

Physiological and pathological observations indicate that basic fibroblast growth factor (bFGF) is an important regulator of osteoblastic cell differentiation and in particular of cranial ossification. Experimental evidence suggests that inorganic phosphate (Pi) transport could be an important function of bone matrix calcification. In the present study, we address the influence of bFGF on Pi transport activity in MC3T3-E1 osteoblast-like cells derived from mouse calvaria. The results indicate that bFGF is a potent and selective stimulator of sodium-dependent Pi transport in these cells. The change in Pi transport activity induced by bFGF depends on transcription and translation and corresponds to a change in the maximum velocity of the Pi transport system (Vmax). These observations suggest that enhanced Pi transport activity in response to bFGF may result from insertion of newly synthesized Pi transporters into the plasma membrane. A selective inhibitor of fibroblast growth factor receptor (FGFR) tyrosine kinase, SU5402, blunted the stimulation of Pi transport induced by bFGF. It also prevented the increase in protein tyrosine phosphorylation induced by bFGF, including phosphorylation of FGFR-1, FGFR-2, phospholipase C-gamma (PLC-gamma), and Shc as well as the recruitment of the Grb2/Sos signaling complex. In addition, bFGF-induced the activation of the mitogen-activated protein (MAP) kinases extracellular signal-regulated kinase (ERK) and p38, effects that were prevented by SU5402. Both the protein kinase C (PKC) inhibitor calphostin C and PKC down-regulation suppressed the stimulatory effect of bFGF on Pi transport. Selective inhibitors of ERK and p38 MAP kinases slightly reduced this cellular response with a significant effect observed with the highest concentration of the p38 MAP kinase inhibitor. In conclusion, the results of this study indicate that bFGF selectively stimulates Pi transport in calvaria-derived osteoblastic cells. The main signaling mechanism responsible for this effect involves tyrosine phosphorylation of PLC-gamma and activation of PKC, with a possible contribution of the p38 MAP kinase pathway.

The p38 signal transduction pathway: activation and function

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

Constitutively active mitogen-activated protein kinase kinase 6 (MKK6) or salicylate induces spontaneous 3T3-L1 adipogenesis

Although much has been learned regarding the importance of p38 mitogen-activated protein kinase in inflammatory and stress responses, relatively little is known concerning its role in differentiation processes. Recently, we demonstrated that p38 mitogen-activated protein kinase activity is necessary for the differentiation of 3T3-L1 fibroblasts into adipocytes (Engelman, J. A., Lisanti, M. P., and Scherer, P. E. (1998) J. Biol. Chem. 273, 32111-32120). p38 activity is high during the initial stages of differentiation but decreases drastically as the fibroblasts undergo terminal differentiation into adipocytes. However, it remains unknown whether activation of p38 is sufficient to stimulate adipogenesis and whether the down-regulation of p38 activity in mature adipocytes is critical for maintaining adipocyte homeostasis. In this report, we have directly addressed these questions by analyzing 3T3-L1 cell lines harboring a specific upstream activator of p38 (a constitutively active mitogen-activated protein kinase kinase 6 (MKK6) mutant, MKK6(Glu)) under the control of an inducible promoter. Induction of MKK6(Glu) in 3T3-L1 fibroblasts spurs adipocyte conversion in the absence of the hormonal mixture normally required for efficient differentiation of wild-type cells. However, activation of p38 in adipocytes leads to cell death. Furthermore, treatment of 3T3-L1 fibroblasts with salicylate, a potent stimulator of p38, produces adipocyte-specific changes consistent with those observed with induction of MKK6(Glu). Expression of MKK6(Glu) in NIH-3T3 fibroblasts (cells that do not differentiate into adipocytes under normal conditions) is capable of converting these fibroblasts into lipid-laden fat cells following hormonal stimulation. Thus, p38 activation has pro-adipogenic effects in multiple fibroblast cell lines.

Phosphorylation of extracellular signal-regulated kinases 1 and 2 in 3T3-L1 adipocytes by stimulation of beta(3)-adrenoceptor

Recent studies have revealed that activated extracellular signal-regulated kinases (ERKs) 1 and 2 by the stimulation of beta(3)-adrenoceptors played a critical role in cell survival in brown adipocytes. On the other hand, phosphorylation of ERK1/2 via beta(3)-adrenoceptors and its physiological and pathological significance in white adipocyte has remained uncertain despite the increasing significance of functioning white adipocytes. Accordingly, we here studied phosphorylation of ERK1/2 caused by the stimulation of beta(3)-adrenoceptors in 3T3-L1 adipocytes, and the roles of phosphorylated ERK1/2 in lipolysis. Phosphorylation of ERK1/2 was induced by a selective beta(3)-adrenoceptor agonist, DL-4-[2'-¿2-hydroxy-2-(3-chlorophenyl)ethylamino¿propyl] phenoxyacetic acid sodium salt sesquihydrate (BRL37344), in 3T3-L1 adipocytes in a time- and dose-dependent manner. The phosphorylation of ERK1/2 by BRL37344 was sensitive to the cyclic AMP (cAMP)-dependent protein kinase inhibitor, N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide (H89). To elucidate the roles of phosphorylated ERK1/2 in lipolysis, the effect of a selective inhibitor of ERK1/2 phosphorylation, 2'-amino-3'-methoxyflavone (PD98059), was examined. This inhibitor did not alter the lipolytic action caused by BRL37344, even at concentrations sufficient to block phosphorylation of ERK1/2, suggesting that ERK1/2 play no role in the lipolysis caused by BRL37344 in 3T3-L1 adipocytes.

Caveolin-1 potentiates estrogen receptor alpha (ERalpha) signaling. caveolin-1 drives ligand-independent nuclear translocation and activation of ERalpha

Estrogen receptor alpha (ERalpha) is a soluble protein that mediates the effects of the gonadal estrogens such as 17beta-estradiol. Upon ligand binding, a cytoplasmic pool of ERalpha translocates to the nucleus, where it acts as a transcription factor, driving the expression of genes that contain estrogen-response elements. The activity of ERalpha is regulated by a number of proteins, including cytosolic chaperones and nuclear cofactors. Here, we show that caveolin-1 potentiates ERalpha-mediated signal transduction. Coexpression of caveolin-1 and ERalpha resulted in ligand-independent translocation of ERalpha to the nucleus as shown by both cell fractionation and immunofluorescence microscopic studies. Similarly, caveolin-1 augmented both ligand-independent and ligand-dependent ERalpha signaling as measured using a estrogen-response element-based luciferase reporter assay. Caveolin-1-mediated activation of ERalpha was sensitive to a well known ER antagonist, 4-hydroxytamoxifen. However, much higher concentrations of tamoxifen were required to mediate inhibition in the presence of caveolin-1. Interestingly, caveolin-1 expression also synergized with a constitutively active, ligand-independent ERalpha mutant, dramatically illustrating the potent stimulatory effect of caveolin-1 in this receptor system. Taken together, our results identify caveolin-1 as a new positive regulator of ERalpha signal transduction.

Expression of leptin mRNA and CCAAT-enhancer binding proteins in response to insulin deprivation during preadipocyte differentiation in primary cultures of porcine stromal-vascular cells

The aim of this study was to examine the correlation between CCAAT-enhancer binding proteins (C/EBPs) and leptin gene expression in response to insulin deprivation in preadipocytes and adipocytes. Adipose tissue from 7 d-old pigs was digested enzymatically and stromal-vascular (S-V) cells were seeded and plated for 3 d in fetal bovine serum (FBS) with dexamethasone (DEX) followed by 6 d (Days 3-9) in serum-free medium with insulin (850 nM or 10 nM), transferrin, and selenium. During FBS+DEX treatment (Days 0-3) a large number of preadipocytes develop with no lipid accretion. In contrast, preadipocyte number does not change with lipid accretion during insulin treatment (Days 3-9). Total RNA and cells were harvested from S-V cultures after periods with and without insulin after FBS+DEX. Northern-blotting and Western blot analysis were used to study leptin mRNA and C/EBP protein expression in cultures, respectively. Insulin deprivation from Days 3-4 reduced leptin mRNA and C/EBP-alpha protein expression. Treatment with 850 nM or 10 nM insulin from Days 3-9 induced leptin mRNA and C/EBP-alpha expression at a similar level. In cultures treated with 10 nM insulin from Days 3-7, leptin and C/EBP-alpha expression were reduced markedly by insulin deprivation from Days 7-9, but were restored by insulin treatment for 6 hr before harvesting. The restoration of leptin expression by insulin was blocked by cycloheximide treatment. However, C/EBP-beta protein levels did not change regardless of insulin deprivation. Insulin deprivation from Days 7-9 in cultures treatedwith 850 nM insulin from Days 3-7 did not influence C/EBP-alpha or leptin mRNA expression, whereas C/EBP-alpha and leptin expression were reduced after treating these cultures with 1.5 uM okadaic acid for 45 min before harvesting on Day 9. However, cycloheximide treatment for 6 hr before harvesting did not reduce leptin mRNA expression. These results suggest that 1) leptin expression is positively correlated with C/EBP-alpha expression, and 2) the maintenance of leptin expression after insulin deprivation in 850 nM insulin-treated cultures on Day 9 may be associated with the presence of C/EBP-alpha expression and/or activation.

Targeted down-regulation of caveolin-3 is sufficient to inhibit myotube formation in differentiating C2C12 myoblasts. Transient activation of p38 mitogen-activated protein kinase is required for induction of caveolin-3 expression and subsequent myotube formation

Caveolin-3 is the principal structural protein of caveolae membrane domains in striated muscle cells. Caveolin-3 mRNA and protein expression are dramatically induced during the differentiation of C2C12 skeletal myoblasts, coincident with myoblast fusion. In these myotubes, caveolin-3 localizes to the sarcolemma (muscle cell plasma membrane), where it associates with the dystrophin-glycoprotein complex. However, it remains unknown what role caveolin-3 plays in myoblast differentiation and myotube formation. Here, we employ an antisense approach to derive stable C2C12 myoblasts that fail to express the caveolin-3 protein. We show that C2C12 cells harboring caveolin-3 antisense undergo differentiation and express normal amounts of four muscle-specific marker proteins. However, C2C12 cells harboring caveolin-3 antisense fail to undergo myoblast fusion and, therefore, do not form myotubes. Interestingly, treatment with specific p38 mitogen-activated protein kinase inhibitors blocks both myotube formation and caveolin-3 expression, but does not affect the expression of other muscle-specific proteins. In addition, we find that three human rhabdomyosarcoma cell lines do not express caveolin-3 and fail to undergo myoblast fusion. Taken together, these results support the idea that caveolin-3 expression is required for myoblast fusion and myotube formation, and suggest that p38 is an upstream regulator of caveolin-3 expression.

Insulin-like growth factor-I induces bcl-2 promoter through the transcription factor cAMP-response element-binding protein

Insulin-like growth factor-I (IGF-I) is known to prevent apoptosis induced by diverse stimuli. The present study examined the effect of IGF-I on the promoter activity of bcl-2, a gene with antiapoptotic function. A luciferase reporter driven by the promoter region of bcl-2 from -1640 to -1287 base pairs upstream of the translation start site containing a cAMP-response element was used in transient transfection assays. Treatment of PC12 cells with IGF-I enhanced the bcl-2 promoter activity by 2.3-fold, which was inhibited significantly (p < 0.01) by SB203580, an inhibitor of p38 mitogen-activated protein kinase (MAPK). Cotransfection of the bcl-2 promoter with MAPK kinase 6 and the beta isozyme of p38 MAPK resulted in 2-3-fold increase in the reporter activity. The dominant negative form of MAPKAP-K3, a downstream kinase activated by p38 MAPK, and the dominant negative form of cAMP-response element-binding protein, inhibited the reporter gene activation by IGF-I and p38beta MAPK significantly (p < 0.01). IGF-I increased the activity of p38beta MAPK introduced into the cells by adenoviral infection. Thus, we have characterized a novel signaling pathway (MAPK kinase 6/p38beta MAPK/MAPKAP-K3) that defines a transcriptional mechanism for the induction of the antiapoptotic protein Bcl-2 by IGF-I through the nuclear transcription factor cAMP-response element-binding protein in PC12 cells.

Leukemia inhibitory factor and its receptor promote adipocyte differentiation via the mitogen-activated protein kinase cascade

Extracellular factors and intracellular signaling pathways involved in early events of adipocyte differentiation are poorly defined. It is shown herein that expression of leukemia inhibitory factor (LIF) and LIF receptor is developmentally regulated during adipocyte differentiation. Preadipocytes secrete bioactive LIF, and an antagonist of LIF receptor inhibits adipogenesis. Genetically modified embryonic stem (ES) cells combined with culture conditions to commit stem cells into the adipocyte lineage were used to examine the requirement of LIF receptor during in vitro development of adipose cells. The capacity of embryoid bodies derived from lifr(-/-) ES cells to undergo adipocyte differentiation is dramatically reduced. LIF addition stimulates adipocyte differentiation of Ob1771 and 3T3-F442A preadipocytes and that of peroxisome proliferator-activated receptor gamma2 ligand-treated mouse embryonic fibroblasts. Expression of the early adipogenic transcription factors C/EBPbeta and C/EBPdelta is rapidly stimulated following exposure of preadipose cells to LIF. The selective inhibitors of mitogen-activated protein kinase kinase, i.e. PD98059 and U0126, inhibit LIF-induced C/EBP gene expression and prevent adipocyte differentiation induced by LIF. These results are in favor of a model that implicates stimulation of LIF receptor in the commitment of preadipocytes to undergo terminal differentiation by controlling the early expression of C/EBPbeta and C/EBPdelta genes via the mitogen-activated protein kinase cascade.

The membrane-spanning domains of caveolins-1 and -2 mediate the formation of caveolin hetero-oligomers. Implications for the assembly of caveolae membranes in vivo

The mammalian caveolin gene family consists of caveolins-1, -2, and -3. The expression of caveolin-3 is muscle-specific. In contrast, caveolins-1 and -2 are co-expressed, and they form a hetero-oligomeric complex in many cell types, with particularly high levels in adipocytes, endothelial cells, and fibroblasts. These caveolin hetero-oligomers are thought to represent the functional assembly units that drive caveolae formation in vivo. Here, we investigate the mechanism by which caveolins-1 and -2 form hetero-oligomers. We reconstituted this reciprocal interaction in vivo and in vitro using a variety of complementary approaches, including the generation of glutathione S-transferase fusion proteins and synthetic peptides. Taken together, our results indicate that the membrane-spanning domains of both caveolins-1 and -2 play a critical role in mediating their ability to interact with each other. This is the first demonstration that these unusual membrane-spanning regions found in the caveolin family play a specific role in protein-protein interactions.