Insulin regulates the expression of the enhancer of split- and hairy-related protein-2 gene via different pathways in 3T3-L1 adipocytes and L6 myotubes.

We investigated the effect of insulin on the expression of the enhancer of split- and hairy-related protein-2 gene in 3T3-L1 adipocytes and L6 myotubes. The level of enhancer of split- and hairy-related protein-2 mRNA was increased by insulin in both cells. While both wortmannin and LY294002 blocked the increase in 3T3-L1 adipocytes, and only PD98059 was effective in L6 myotubes. Although the increase by insulin in these cells was inhibited by treatment with actinomycin D, this was enhanced by treatment with cycloheximide. Furthermore, cyclic AMP increased the level of enhancer of split- and hairy-related protein-2 mRNA in both cells in an additive manner. Thus, we conclude that insulin and cyclic AMP induce the expression of the enhancer of split- and hairy-related protein-2 gene in both 3T3-L1 adipocytes and L6 myotubes, and that the gene expression enhanced by insulin is regulated by the cell type-specific pathway. The former requires a phosphoinositide 3-kinase pathway and the latter a mitogen-activated protein kinase pathway.     

The Effects of Propionate and Valerate on Insulin Responsiveness for Glucose Uptake in 3T3-L1 Adipocytes and C2C12 Myotubes via G Protein-Coupled Receptor 41

Since insulin resistance can lead to hyperglycemia, improving glucose uptake into target tissues is critical for regulating blood glucose levels. Among the free fatty acid receptor (FFAR) family of G protein-coupled receptors, GPR41 is known to be the Gα_i/o-coupled receptor for short-chain fatty acids (SCFAs) such as propionic acid (C3) and valeric acid (C5). This study aimed to investigate the role of GPR41 in modulating basal and insulin-stimulated glucose uptake in insulin-sensitive cells including adipocytes and skeletal muscle cells. Expression of GPR41 mRNA and protein was increased with maximal expression at differentiation day 8 for 3T3-L1 adipocytes and day 6 for C2C12 myotubes. GPR41 protein was also expressed in adipose tissues and skeletal muscle. After analyzing dose-response relationship, 300 µM propionic acid or 500 µM valeric acid for 30 min incubation was used for the measurement of glucose uptake. Both propionic acid and valeric acid increased insulin-stimulated glucose uptake in 3T3-L1 adipocyte, which did not occur in cells transfected with siRNA for GPR41 (siGPR41). In C2C12 myotubes, these SCFAs increased basal glucose uptake, but did not potentiate insulin-stimulated glucose uptake, and siGPR41 treatment reduced valerate-stimulated basal glucose uptake. Therefore, these findings indicate that GPR41 plays a role in insulin responsiveness enhanced by both propionic and valeric acids on glucose uptake in 3T3-L1 adipocytes and C2C12 myotubes, and in valerate-induced increase in basal glucose uptake in C2C12 myotubes.     

Regulation of the expression of pp160, a putative insulin receptor signal protein, by insulin, dexamethasone, and 1-methyl-3-isobutylxanthine in 3T3-L1 adipocytes.

pp160, a cytosolic protein with Mr of approximately 160,000, is phosphorylated on tyrosine in response to insulin and is considered to be involved in signaling from the insulin receptor. The expression of pp160 during the differentiation of 3T3-L1 fibroblasts to adipocytes and in adipocytes has been investigated using quantitative immunoblotting with antibodies against a peptide from pp160. Between day 6 and day 8 of differentiation induced by insulin, dexamethasone (Dex), and 1-methyl-3-isobutylxanthine (Mix), pp160 expression increased 10-20-fold over the amount present in confluent fibroblasts. Omission of either insulin or Dex resulted in reduced expression of pp160 and in incomplete adipogenesis. Chronic treatment of fully differentiated adipocytes for 24 h with either insulin, Dex, or Mix alone in the presence of serum resulted in a decrease in the expression of pp160 by 70-85%. Chronic exposure to insulin caused a significant increase in the apparent size of pp160 to 172 kDa. Alkaline phosphatase treatment lowered the Mr of pp160 from both insulin-treated and basal cells to 150,000. These results demonstrate that pp160 is expressed in 3T3-L1 adipocytes during the time when insulin receptors are expressed in large numbers and that the maintenance of pp160 concentrations in adipocytes can be regulated by insulin, Mix, and Dex. The decreased expression of pp160 caused by these factors may be related to postreceptor insulin resistance.     

A dominant-negative p38 MAPK mutant and novel selective inhibitors of p38 MAPK reduce insulin-stimulated glucose uptake in 3T3-L1 adipocytes without affecting GLUT4 translocation

Participation of p38 mitogen-activated protein kinase (p38) in insulin-induced glucose uptake was suggested using pyridinylimidazole p38 inhibitors (e.g. SB203580). However, the role of p38 in insulin action remains controversial. We further test p38 participation in glucose uptake using a dominant-negative p38 mutant and two novel pharmacological p38 inhibitors related to but different from SB203580. We present the structures and activities of the azaazulene pharmacophores A291077 and A304000. p38 kinase activity was inhibited in vitro by A291077 and A304000 (IC(50) = 0.6 and 4.7 microm). At higher concentrations A291077 but not A304000 inhibited JNK2alpha (IC(50) = 3.5 microm). Pretreatment of 3T3-L1 adipocytes and L6 myotubes expressing GLUT4myc (L6-GLUT4myc myotubes) with A291077, A304000, SB202190, or SB203580 reduced insulin-stimulated glucose uptake by 50-60%, whereas chemical analogues inert toward p38 were ineffective. Expression of an inducible, dominant-negative p38 mutant in 3T3-L1 adipocytes reduced insulin-stimulated glucose uptake. GLUT4 translocation to the cell surface, immunodetected on plasma membrane lawns of 3T3-L1 adipocytes or on intact L6-GLUT4myc myotubes, was not altered by chemical or molecular inhibition of p38. We propose that p38 contributes to enhancing GLUT4 activity, thereby increasing glucose uptake. In addition, the azaazulene class of inhibitors described will be useful to decipher cellular actions of p38 and JNK.     

Insulin induces chaperone and CHOP gene expressions in adipocytes

Adipocyte secretes bioactive proteins called adipocytokines, and biosynthesis of secretory proteins requires molecular chaperones and folding enzymes in endoplasmic reticulum (ER). ER chaperones are known to be induced by unfolded protein response (UPR) and growth factors, however, it has not been determined how ER chaperones expression is regulated in adipocytes. Here we show that insulin treatment induced GRP78 and ERO1L mRNA levels in 3T3-L1 adipocytes. Insulin also upregulated CHOP mRNA levels, but did not induce phosphorylation of eIF2α. Pretreatment with insulin protected 3T3-L1 adipocytes against thapsigargin-mediated phosphorylation of eIF2α but did not against DTT-mediated one. In vivo mice study showed that GRP78 and CHOP expressions were regulated by feeding conditions. These results suggest that insulin signaling is important to induce mRNA expressions of GRP78 and CHOP, and may have a protective role against UPR.     

R-alpha-lipoic acid action on cell redox status, the insulin receptor, and glucose uptake in 3T3-L1 adipocytes.

The insulin signaling pathway has been reported to mediate R-alpha-lipoic acid- (R-LA-)-stimulated glucose uptake into 3T3-L1 adipocytes and L6 myotubes. We investigated the role of the thiol antioxidant dihydrolipoic acid (DHLA) and intracellular glutathione (GSH) in R-LA-stimulated glucose transport and explored the hypothesis that R-LA could increase glucose uptake into 3T3-L1 adipocytes in an oxidant-mimetic manner. R-LA pretreatment of 3T3-L1 cells stimulated glucose transport at early time points (30 min - 6 h), whereas it inhibited glucose uptake at later time points. Analysis of the oxidized and reduced content of LA in cells and medium showed that >90% of lipoic acid present was in its oxidized form. Furthermore, all oxidized forms of LA (S-, R-, and racemic LA) stimulated glucose uptake, whereas the reduced form, dihydrolipoic acid, was ineffective. Intracellular GSH levels were not changed at the early time points (before 12 h), while longer preincubation (24 - 48 h) of cells with R-LA significantly increased intracellular GSH. Pretreatment of adipocytes with R-LA increased intracellular peroxide levels at early time points (30 min - 6 h), after which it was decreased (12 - 48 h). R-LA also increased tyrosine phosphorylation of immunoprecipitated insulin receptors from 3T3-L1 adipocytes. These results indicate that (i) 3T3-L1 adipocytes have a low capacity to reduce R-LA and the oxidized form of lipoic acid is responsible for stimulating glucose uptake, (ii) R-LA modulates glucose uptake by changing the intracellular redox status, and (iii) the insulin receptor is a potential cellular target for R-LA action.     

3T3-L1 adipocyte glucose transporter (HepG2 class): sequence and regulation of protein and mRNA expression by insulin, differentiation, and glucose starvation

A glucose transporter cDNA (GLUT) clone was isolated from mouse 3T3-L1 adipocytes and sequenced. The nucleotide and deduced amino acid sequences were, respectively, 95 and 99% homologous to those of the rat brain transporter. The mouse cDNA and a polyclonal antibody recognizing the corresponding in vitro translation product were used to compare changes in transporter mRNA and protein levels during differentiation, glucose starvation, and chronic insulin exposure of 3T3-L1 preadipocytes. The respective cellular content of transporter mRNA and protein were increased 6.6- and 7.8-fold during differentiation, and 3.8- and 2.5-fold from chronic insulin exposure of differentiated adipocytes. Glucose starvation increased transporter mRNA and protein levels 2.2- and 3.5-fold in undifferentiated preadipocytes and 1.8- and 3.1-fold in differentiated adipocytes. Starvation of undifferentiated cells completely converted the native transporter to an incompletely glycosylated form, while increasing basal transport rates 4.5-fold. Either full glycosylation is not required to produce a functionally active transporter, or starvation causes a unique predifferentiation induction of the normally absent "responsive" transporter. The changes in transporter protein expression elicited by differentiation were attributed primarily to increased rates of transporter synthesis, while the disproportionate changes in mRNA and protein expression from chronic insulin treatment and starvation suggested these conditions increase synthesis and decrease turnover rates in regulating transporter protein expression. Although chronic insulin exposure and glucose starvation each raised the expression of transporter protein greater than 3-fold and basal transport rates 2.5- to 4.5-fold, no significant increase in the insulin responsiveness of 3T3-L1 preadipocytes or differentiated adipocytes was observed. Thus, the changes in the transporter mRNA and protein expression observed in this study were most consistent with their being associated with the regulated expression of a basal or low level insulin-responsive transporter.     

灵芝多糖对3T3-L1胰岛素抵抗细胞模型PI-3K p85和GLUT4蛋白表达的影响

目的 研究灵芝多糖对3T3-L1胰岛素抵抗细胞模型PI-3K p85和GLUT4蛋白表达的影响,探讨灵芝多糖改善胰岛素抵抗的分子机制.方法 3T3-L1前脂肪细胞经1-甲基-3-异丁基-黄嘌呤、地塞米松、胰岛素诱导分化成3T3-L1脂肪细胞,以葡萄糖氧化酶法测定培养液中残余的葡萄糖含量.比较二甲双胍组,检测培养液中葡萄糖含量及PI-3K p85和GLUT4蛋白表达变化.结果 地塞米松联合胰岛素诱导3T3-L1脂肪细胞产生胰岛素抵抗,细胞对葡萄糖的摄取量减少.灵芝多糖可改善3T3-L1脂肪细胞胰岛素抵抗.胰岛素抵抗细胞的PI-3K p85和GLUT4蛋白表达明显减少;应用灵芝多糖后,相关蛋白表达增加.结论 灵芝多糖通过提高PI-3K p85和GLUT4蛋白的表达,参与胰岛素抵抗状态下3T3-L1细胞的葡萄糖代谢.     

An inhibitor of p38 mitogen-activated protein kinase prevents insulin-stimulated glucose transport but not glucose transporter translocation in 3T3-L1 adipocytes and L6 myotubes

The precise mechanisms underlying insulin-stimulated glucose transport still require investigation. Here we assessed the effect of SB203580, an inhibitor of the p38 MAP kinase family, on insulin-stimulated glucose transport in 3T3-L1 adipocytes and L6 myotubes. We found that SB203580, but not its inactive analogue (SB202474), prevented insulin-stimulated glucose transport in both cell types with an IC50 similar to that for inhibition of p38 MAP kinase (0.6 microM). Basal glucose uptake was not affected. Moreover, SB203580 added only during the transport assay did not inhibit basal or insulin-stimulated transport. SB203580 did not inhibit insulin-stimulated translocation of the glucose transporters GLUT1 or GLUT4 in 3T3-L1 adipocytes as assessed by immunoblotting of subcellular fractions or by immunofluorescence of membrane lawns. L6 muscle cells expressing GLUT4 tagged on an extracellular domain with a Myc epitope (GLUT4myc) were used to assess the functional insertion of GLUT4 into the plasma membrane. SB203580 did not affect the insulin-induced gain in GLUT4myc exposure at the cell surface but largely reduced the stimulation of glucose uptake. SB203580 had no effect on insulin-dependent insulin receptor substrate-1 phosphorylation, association of the p85 subunit of phosphatidylinositol 3-kinase with insulin receptor substrate-1, nor on phosphatidylinositol 3-kinase, Akt1, Akt2, or Akt3 activities in 3T3-L1 adipocytes. In conclusion, in the presence of SB203580, insulin caused normal translocation and cell surface membrane insertion of glucose transporters without stimulating glucose transport. We propose that insulin stimulates two independent signals contributing to stimulation of glucose transport: phosphatidylinositol 3-kinase leads to glucose transporter translocation and a pathway involving p38 MAP kinase leads to activation of the recruited glucose transporter at the membrane.     

Inducible disruption of autophagy in the lung causes airway hyper-responsiveness

Objective

Central adiposity and inflammation play key roles in the development of insulin resistance through the effects of pro-inflammatory adipokines such as IL-6, but the effect of infiltrating adipocytes in skeletal muscle tissues is not known. Communications between muscle cells and fat cells may contribute to the inflammatory response associated with insulin resistance.

Methods

In this study we used a co-culture system of skeletal muscle (L6) and adipocyte (3T3-L1) cell lines to study expression of the inflammatory cytokine IL-6 and changes in insulin signaling. This model could mimic the adipocytes infiltrating myocytes that is commonly seen in obese patients.

Results

When plated alone the L6 cells express IL-6 mRNA and secrete IL-6 protein, both of which are increased when the cells are challenged with the bacterial lipopolysaccharide (LPS). In contrast, the 3T3-L1 cells had very little expression of IL-6 mRNA or protein. Co-culture of 3T3-L1 pre-adipocytes with L6 cells, at a density ratio of 1:10, respectively, increased IL-6 expression significantly and decreased insulin-stimulated Akt phosphorylation. To examine the role of IL-6 in insulin sensitivity we incubated the L6 cells with IL-6. A brief challenge of L6 cells with IL-6 enhanced insulin-stimulated Akt phosphorylation. In contrast, incubation of the L6 cells with IL-6 for 96 h markedly decreased insulin-stimulated Akt phosphorylation.

Conclusion

The enhanced IL-6 mRNA expression and IL-6 release in L6 myocytes co-cultured with 3T3-L1 cells indicate an important interaction between adipocytes and myocytes. This observation may shed some light on the long-standing enigma of obesity-induced insulin resistance where infiltration of the skeletal muscle by preadipocytes/adipocytes is evident.     

Relationship between insulin stimulation and endogenous regulation of 2-deoxyglucose uptake in 3T3-L1 adipocytes

The occurrence of the endogenous regulatory response to high rates of 2-deoxyglucose (2-DG) uptake, as previously described for C6 glioma cells during incubation with 2 mM 2-DG (Lange et al.: J. Cell. Physiol., 1989), was studied in 3T3-L1 preadipocytes and adipocytes, and the influence of insulin on this endogenous uptake regulation was examined. In contrast to 3T3-L1 preadipocytes, insulin-sensitive differentiated 3T3-L1 adipocytes displayed the time-dependent cyclic pattern of 2-DG uptake rates characteristic of the membrane-limited and endogenously regulated cellular state of hexose utilization. Although insulin induced a threefold stimulation of 2-DG tracer uptake in adipocytes, the hormone did not additionally stimulate the uptake rates or affect the periodic response: maximum and minimum levels of uptake remained unchanged. Scanning electron microscopy (SEM) revealed that the acquirement of the differentiated state is accompanied by a conspicuous transformation of the smooth surface of undifferentiated 3T3-L1 cells into a surface covered by numerous microvilli of uniform size and appearance. Treatment with insulin (10 mU/ml; 10 minutes) converted these microvilli into voluminous saccular membrane protrusions of the same type as had been formed during incubation of 3T3-L1 adipocytes with 2 mM 2-DG, and which have previously been shown to be involved in the endogenous uptake regulation of C6 glioma cells (Lange et al.: J. Cell. Physiol., 1989). These insulin-induced saccated membrane areas appeared to become integrated into the cell surface. Accordingly, insulin treatment caused a twofold increase of the intracellular distribution space of 3-O-methylglucose (3-OMG) in 3T3-L1 adipocytes. This insulin-induced increase of the 3-OMG distribution space exhibited the same time (t1/2 = 2-2.5 minutes) and dose dependence (EC50 = 20 nM) as the insulin-induced stimulation of 3-OMG transport. Glucose deprivation during the differentiation period inhibited the outgrowth of microvilli from the cell surface. Glucose starvation (18 hours at less than 0.5 mM) induced a conspicuous reduction of the length of microvilli on differentiated 3T3-L1 cells. In this state, the stalks of the microvilli are almost invisible and the enlarged spherical tips of the microvilli (with an average diameter of 370 nm compared to 230 nm of fed cells) appeared to protrude directly out of the cell surface. Starvation-induced shortening of microvilli was accompanied by a threefold increase of the basal 3-OMG transport rate and a greater than twofold increase of the intracellular 3-OMG distribution space as compared to fed cells (10 mM; 18 hours).(ABSTRACT TRUNCATED AT 400 WORDS)     

MKK6/3 and p38 MAPK pathway activation is not necessary for insulin-induced glucose uptake but regulates glucose transporter expression

p38 mitogen-activated protein kinase (MAPK), which is situated downstream of MAPK kinase (MKK) 6 and MKK3, is activated by mitogenic or stress-inducing stimuli, as well as by insulin. To clarify the role of the MKK6/3-p38 MAPK pathway in the regulation of glucose transport, dominant negative p38 MAPK and MKK6 mutants and constitutively active MKK6 and MKK3 mutants were overexpressed in 3T3-L1 adipocytes and L6 myotubes using an adenovirus-mediated transfection procedure. Constitutively active MKK6/3 mutants up-regulated GLUT1 expression and down-regulated GLUT4 expression, thereby significantly increasing basal glucose transport but diminishing transport induced by insulin. Similar effects were elicited by chronic (24 h) exposure to tumor necrosis factor alpha, interleukin-1beta, or 200 mm sorbitol, all activate the MKK6/3-p38 MAPK pathway. SB203580, a specific p38 MAPK inhibitor, attenuated these effects, further confirming that both MMK6 and MMK3 act via p38 MAPK, whereas they had no effect on the increase in glucose transport induced by a constitutively active MAPK kinase 1 (MEK1) mutant or by myristoylated Akt. In addition, suppression of p38 MAPK activation by overexpression of a dominant negative p38 MAPK or MKK6 mutant did not diminish insulin-induced glucose uptake by 3T3-L1 adipocytes. It is thus apparent that activation of p38 MAPK is not essential for insulin-induced increases in glucose uptake. Rather, p38 MAPK activation leads to a marked down-regulation of insulin-induced glucose uptake via GLUT4, which may underlie cellular stress-induced insulin resistance caused by tumor necrosis factor alpha and other factors.     

Apelin stimulates glucose uptake through the PI3K/Akt pathway and improves insulin resistance in 3T3-L1 adipocytes

Apelin, a cytokine mainly secreted by adipocytes, is closely related with insulin resistance. The underlying molecular mechanisms of how apelin affects insulin resistance, however, are poorly understood. This study aimed to investigate the effect of apelin on glucose metabolism and insulin resistance in 3T3-L1 adipocytes. After 10 ng/ml TNF-α treatment for 24 h, insulin-stimulated glucose uptake was reduced by 47% in 3T3-L1 adipocytes. Apelin treatment improved glucose uptake in a time- and dose-dependent manner. Treatment of 1,000 nM apelin for 60 min maximally augmented glucose uptake in insulin-resistant 3T3-L1 adipocytes. Furthermore, apelin pre-incubation also increased adipocytes' insulin-stimulated glucose uptake, and PI3K/Akt pathway were involved in these effects. In addition, immunocytochemistry staining and western blotting analysis indicated that apelin could increase glucose transporter 4 translocation from the cytoplasm to the plasma membrane. Apelin also increased the anti-inflammatory adipokine adiponectin mRNA expression while reducing that of pro-inflammatory adipokine interleukin-6 in insulin-resistant 3T3-L1 adipocytes. These results suggest that apelin stimulates glucose uptake through the PI3K/Akt pathway, promotes GLUT4 translocation from the cytoplasm to the plasma membrane, and modulates inflammatory responses in insulin-resistant 3T3-L1 adipocytes.     

人源FGF-21在脂肪细胞糖代谢中的作用

近年来研究发现,成纤维细胞生长因子(FGF)-21是一种新的代谢调节因子.为了深入研究人源FGF-21（hFGF-21）的生物活性,本实验利用SUMO高效表达载体,高效表达成熟的hFGF-21,并利用小鼠3T3-L1脂肪细胞检测hFGF-21的糖代谢活性．实验结果表明,hFGF-21可促进脂肪细胞的葡萄糖吸收,且葡萄糖吸收效率呈剂量依赖性．hFGF-21作用4 h即可促进脂肪细胞糖吸收,其活性可持续24 h以上．hFGF-21与胰岛素共同作用的葡萄糖吸收效果,明显优于它们的单独作用结果,说明hFGF-21与胰岛素发挥协同作用．脂肪细胞经hFGF-21预处理后,显著增加了胰岛素促进脂肪细胞吸收葡萄糖的效率,说明hFGF-21可以增加胰岛素的敏感性．本实验为临床应用hFGF-21治疗糖尿病,增加胰岛素敏感性提供了依据．     

Insulin and TNF alpha induce expression of the forkhead transcription factor gene Foxc2 in 3T3-L1 adipocytes via PI3K and ERK 1/2-dependent pathways

We have recently identified the winged helix/forkhead gene Foxc2 as a key regulator of adipocyte metabolism that counteracts obesity and diet-induced insulin resistance. This study was performed to elucidate the hormonal regulation of Foxc2 in adipocytes. We find that TNF alpha and insulin induce Foxc2 mRNA in differentiated 3T3-L1 cells with the kinetics of an immediate early response (1-2 h with 100 ng/ml insulin or 5 ng/ml TNF alpha). This induction is, in both cases, attenuated by the PI3K inhibitor wortmannin as well as the MAPK kinase inhibitor PD98059. Furthermore, we show that stimulation of 3T3-L1 adipocytes with phorbol-12-myristate-13-acetate or 8-(4-chlorophenyl)thio-cAMP induces the expression of Foxc2. Interestingly, we find that the basal level of Foxc2 mRNA is down-regulated whereas hormonal responsiveness increases during differentiation of 3T3-L1 from preadipocytes to adipocytes. At the protein level, immunoblots with Foxc2 antibody demonstrated an induction of Foxc2 by insulin and TNF alpha in nuclear extracts of 3T3-L1 adipocytes. EMSA of nuclear proteins from phorbol-12-myristate-13-acetate- and TNF alpha-treated 3T3-L1 adipocytes using a forkhead consensus oligonucleotide revealed specific binding of a Foxc2/DNA complex. In conclusion, our data suggest that insulin and TNF alpha regulate the expression of Foxc2 via a PI3K- and ERK 1/2-dependent pathway in 3T3-L1 adipocytes. Also, signaling pathways downstream of PKA and PKC induce the expression of Foxc2 mRNA.