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Pennington Biomedical Research Center (Z.G., X.Z., A.Z., M.L., J.Y.), Louisiana State University, Baton Rouge, Louisiana 70808; Western Human Nutrition Research Center and Department of Nutrition (D.H.), University of California, Davis, California 95616; and Diabetes Unit (M.J.Q.), National Center for Complementary and Alternative Medicine, National Institutes of Health, Bethesda, Maryland 20892-1755
Address all correspondence and requests for reprints to: Jianping Ye, Pennington Biomedical Research Center, 6400 Perkins Road, Baton Rouge, Louisiana 70808. E-mail: yej{at}pbrc.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONRESULTSDISCUSSIONMATERIALS AND METHODSREFERENCES |
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B kinase (IKK) and c-JUN NH2-terminal kinase (JNK). IKK and JNK were activated by linoleic acid and inhibition of the two kinases led to prevention of IRS-1 reduction. We demonstrate that protein kinase C (PKC) 
is expressed in adipocytes. In 3T3-L1 adipocytes and fat tissue, PKC was activated by fatty acids as indicated by its phosphorylation status, and by its protein level, respectively. Activation of PKC contributes to IKK and JNK activation as inhibition of PKC by calphostin C blocked activation of the latter kinases. Inhibition of either PKC or IKK plus JNK by chemical inhibitors resulted in protection of IRS-1 function and insulin sensitivity in 3T3-L1 adipocytes. These data suggest that: 1) activation of PKC contributes to IKK and JNK activation by FFAs; 2) IKK and JNK mediate PKC signals for IRS-1 serine phosphorylation and degradation; and 3) this molecular mechanism may be responsible for insulin resistance associated with hyperlipidemia. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONMATERIALS AND METHODSREFERENCES |
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, and TNF- contributes to insulin resistance (4). The third is that obesity increases activity of protein tyrosine phosphatases that interrupt insulin signaling by dephosphorylating the insulin receptor substrate (IRS) (5). It has been known for more than a decade that FFAs can induce insulin resistance (6). In human (6) or animals (7), hyperlipidemia generated by iv infusion of lipid/heparin consistently induces acute insulin resistance in the body. The glucose tolerance returns to the normal range after hyperlipidemia is eliminated. At the molecular level, IRS protein has been suggested as a target of FFAs for insulin resistance (7, 8). Phosphorylation of serine 307 (Ser307) in IRS-1 protein has been linked to FFA-associated insulin resistance (7). In the normal rats, infusion of a lipid emulsion results in IRS-1 Ser307 phosphorylation, and this phosphorylation correlates to a reduced PI(3)K (phosphatidylinositol-3 kinase) activity in the skeletal muscle (7). However, it is not clear how FFAs lead to Ser307 phosphorylation in IRS-1.
IRS-1 Ser307 phosphorylation is inducible and responsible for the inhibition of IRS-1 function (9, 10, 11, 12). White’s group first demonstrates that IRS-1 Ser307 phosphorylation is induced by stimuli that lead to c-JUN NH2-terminal kinase (JNK) activation (9, 13). Ser307 phosphorylation may lead to the inhibition of IRS-1 function through interrupting IRS/insulin receptor interaction (10) or promoting protein degradation of IRS-1 (14). Our previous studies suggest that in addition to JNK, inhibitor B kinase (IKK) also phosphorylates Ser307 (Ser312 in the human IRS-1) in IRS-1 protein in response to TNF- or serine phosphatase inhibitor calyculin A (11, 12).
In this study, we investigated the molecular events underlying FFA-induced insulin resistance. We observed that linoleic acid induced insulin resistance in 3T3-L1 adipocytes. This cellular model reflects insulin resistance induced by high-fat diet in C57BL/6J mice. The insulin resistance was associated with a Ser307 phosphorylation followed by IRS-1 protein reduction. Activation of IKK and JNK was induced by FFA, and activities of the two serine kinases were required for Ser307 phosphorylation and degradation of IRS-1. We show that protein kinase C (PKC) is expressed in fat tissue and activation of PKC by FFA leads to induction of IKK and JNK activities.
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RESULTS |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONMATERIALS AND METHODSREFERENCES |
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9,12) to induce insulin resistance. Insulin-induced glucose uptake was measured to determine insulin sensitivity. The result shows that insulin-induced glucose uptake was inhibited by as much as 70% after a 16 h-treatment with linoleic acid (Fig. 1A
). This is consistent with reports that FFA induces insulin resistance in cell culture (8, 15). To understand the role of IRS-1 in the mechanism of FFA-induced insulin resistance, IRS-1 protein abundance was monitored in the FFA-treated cells in a time-course study (Fig. 1B). IRS-1 protein decreased gradually. A 50% decrease was detected 6 h after addition of linoleic acid. At 24 h, 80% of IRS-1 protein was lost. Because IRS-1 protein abundance is mainly regulated by protein degradation (16, 17, 18), this result suggests that the reduction in IRS-1 protein may be a result of protein degradation in 3T3-L1 adipocytes.
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, B and C). Interestingly, GLUT1 was induced by FFA (Fig. 1C). These results suggest that inhibition of insulin-induced glucose uptake by FFA is not a result of reduction in IRS-2 or GLUT4. It is known that GLUT1 does not involve in insulin-induced glucose uptake.
FFA Induces Serine Phosphorylation of IRS-1
Because serine phosphorylation precedes IRS-1 degradation (16, 17, 18), our results suggest that FFA may induce IRS-1 serine phosphorylation. To test the hypothesis, 3T3-L1 adipocytes were treated with linoleic acid and IRS-1 phosphorylation was determined with the phospho-specific IRS-1 (Ser307) antibody by immunoblotting. The phosphorylation was induced by linoleic acid, and the induction was in a dose- and time-dependent manner. Linoleic acid was able to promote the phosphorylation at 100 µM and the strongest activity was observed at 300 µM (Fig. 2A). When FFA dose was further increased, the phosphorylation was reduced at 400 µM. FFA BSA does not have this activity (data not shown). In the time-course study, a 300 µM concentration of linoleic acid was used to treat the cells for different times (Fig. 2B). IRS-1 phosphorylation was increased at 1 h, and the signal was maintained up to 5 h before a drop at 8 h. Stearic (C18) and oleic (C189) acids were compared with linoleic acid for induction of IRS-1 phosphorylation (Fig. 2C). There is no significant difference in these FFAs (C18, C189, and C189,12) as indicated by IRS-1 Ser307 phosphorylation. This suggests that saturation status of FFAs may not play a role in the serine phosphorylation of IRS-1.
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). However, activation of the two kinases exhibited a difference in dose-dependence. IKK activation was observed at 100 µM and JNK2 activation was detected at 300 µM (Fig. 3A), suggesting that IKK is more sensitive to FFAs. It seems that JNK1 is constitutively activated in 3T3-L1 adipocytes and JNK2 activity is induced by FFA. To confirm the role of IKK and JNK in Ser307 phosphorylation of IRS-1, specific inhibitors 15-deoxy-12,14-prostaglandin J2 (15d-PGJ2) and SP600125 were used to inhibit IKK and JNK, respectively (22, 23). As expected, inhibition of IKK resulted in a reduction in Ser307 phosphorylation (Fig. 3B). Similarly, inhibition of JNK by SP600125 also blocked Ser307 phosphorylation (Fig. 3C). These data are consistent with those observed in HepG2 and 3T3-L1 preadipocytes that IKK and JNK mediate Ser307 phosphorylation (11, 12).
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(Thr538) and PKC
(Thr410) was induced by linoleic acid (Fig. 4, A and B). Phosphorylation of PKC exhibited a peak at 200 µM of the FFA. PKC phosphorylation reached the peak at 400 µM. Functional consequence of PKC activation is indicated by phosphorylation of the downstream substrate protein kinase D (PKD) (also known as PKCµ) (30, 31). Phosphorylation of PKD at Ser744/748 is dependent on PKC activity (31) and is increased in a similar pattern to that of PKC (Fig. 4, A and B). Phosphorylation of PKC/ß (Thr638/641), and PKC
(Ser643) was not changed by FFA in 3T3-L1 adipocytes (data not shown). These results suggest that PKC and PKC are activated by FFA in 3T3-L1 adipocytes.
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). Calphostin C also inhibited FFA-induced phosphorylation of IKK and JNK (Fig. 5B), suggesting that IKK and JNK activation is dependent on PKC activity. Consistently, IRS-1 (Ser307) phosphorylation was also inhibited by calphostin C in FFA-treated adipocytes (Fig. 5C). These results suggest that IKK and JNK mediate PKC activity in IRS-1 (Ser307) phosphorylation in adipocytes. Because PKC is not sensitive to Calphostin C, the result does not support that PKC is involved in IRS-1 Ser307 phosphorylation induced by FFAs.
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). The "D" mutant (IRS-1D) that mimics the phosphorylated Ser307 was unable to associate with the insulin receptor. Expression of different forms of the recombinant IRS-1 is consistent in the transfected cell (Fig. 6A). These data support that phosphorylation of Ser307 leads to an impairment of IRS-1/IR interaction. To confirm this effect, phosphorylation of IRS-1 at tyrosine 632 (Y632) was examined by immunoblotting of IRS-1 with pY632-specific antibody in the IP product. The phosphorylation was induced by insulin in 5 min. Exposure to linoleic acid led to a reduction in Y632 phosphorylation (Fig. 6B). This reduction was detected in 3 h of FFA treatment, suggesting that the inhibition of IRS-1 function is dependent on duration of FFA treatment. This time point is consistent with that observed in vivo for lipid-induced insulin resistance (34).
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). Consistently, activation of Akt was also inhibited as indicated by Thr308 phosphorylation (Fig. 6D). These results further support that serine phosphorylation of IRS-1 leads to impairment of PI(3)K signal transduction.
Restoration of Insulin Sensitivity by PKC Inhibitor
Pharmacological inhibitors of PKC, IKK, and JNK were tested to rescue IRS-1 protein from degradation. PKC inhibitor Calphostin C or combination of IKK and JNK inhibitors was used to pretreat 3T3-L1 adipocytes. IRS-1 protein abundance and glucose uptake were examined in 3T3-L1 adipocytes 16 h later after FFA-treatment. The results show that the inhibitors are able to block IRS-1 degradation completely (Fig. 7A). The inhibitors also protected insulin-induced glucose uptake in adipocytes (Fig. 7B). After pretreatment with the inhibitors, the glucose uptake was significantly restored in the FFA-treated cells. It is noted that the restoration was not complete. This may be a result of limitation of the inhibitor activity.
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in the Adipose Tissue of Dietary Obese C57BL/6J Mice can be activated by FFA in the skeletal muscle (7, 24, 26, 35, 36), it has not been reported whether this happens in the adipose tissue. To address this question, we examined PKC in the fat tissue of dietary obese mice. C57BL/6J mice were fed a high-fat diet to induce dietary insulin resistance. This insulin resistance is associated with body weight gain in the mice (Fig. 8A). As markers of insulin resistance, hyperglycemia and hyperinsulinemia were detected at 8–10 wk on high-fat diet (Fig. 8, B and C). The PKC protein level was increased dramatically in the adipose tissue when insulin resistance occurs in C57BL/6J mice on the high-fat diet (Fig. 8D), suggesting a chronic activation of the PKC serine kinase. Accordingly, the IRS-1 protein abundance was reduced in the adipose tissue of the dietary insulin-resistant mice (Fig. 7D). This observation is similar to PKC
increase in the red muscles from fat-fed rats (26). These data suggest that hyperlipidemia activates PKC in fat tissue.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONMATERIALS AND METHODSREFERENCES |
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IKK and JNK may contribute to IRS-1 serine phosphorylation in response to FFAs. In this study, we observed that both IKK and JNK were activated by linoleic acid, a FFA used in this study. The activation is associated with the Ser307 phosphorylation in IRS-1. Inhibition of the two serine kinases led to protection from Ser307 phosphorylation and degradation of IRS-1 (Figs. 3 and 6). It is known that serine phosphorylation is associated with protein degradation of IRS-1 (16, 39). Because there are about 50 serine/threonine residues in IRS-1, it is hard to determine which serine/threonine is involved in the IRS-1 degradation. Recently, it has been shown that Ser307 phosphorylation contributes to IRS-1 degradation in hepatocytes in the response to insulin (14). Our result suggests that the same mechanism contributes to FFA-induced degradation of IRS-1 in adipocytes (Fig. 1). A decrease in IRS-1 abundance leads to insulin resistance as shown in IRS-1 knockout studies (40, 41). We observed that IRS-1 protein was reduced in the fat tissue of mouse model of dietary insulin resistance (Fig. 8B). This is consistent with that IRS-1 protein is reduced in the fat tissue of type 2 diabetes patient (42). We observed that inhibition of IKK and JNK by pharmacological agents was able to protect 3T3-L1 adipocytes from insulin resistance (Fig. 6). Taken together, our data support that IKK and JNK involve in FFA signaling pathway for insulin resistance.
IKK and JNK may mediate PKC signal for insulin resistance. It was reported that activities classical PKCs and novel PKCs are negatively associated with insulin sensitivity (7, 24, 43, 44). Some reports suggest that activation of these two classes of PKCs by phorbol esters leads to activation of PI(3)K and glucose transporters (45, 46). Serine phosphorylation of IRS-1 represents a mechanism by which PKCs leads to the inhibition of insulin sensitivity (7, 35, 47, 48). However, it is not well defined how PKC promotes IRS-1 serine phosphorylation. PKC was shown to phosphorylate IRS-1 protein directly (49, 50); however, it remains to be established how PKC leads to Ser307 phosphorylation (32). In this study, we provide evidence that IKK and JNK may mediate PKC activity for Ser307 phosphorylation. The evidence includes: 1) FFAs induced activation of PKC as indicated by their phosphorylation status (Fig. 4); 2) Inhibition of PKC activities by specific inhibitor calphostin C resulted in suppression of both IKK and JNK activities (Fig. 5). The inhibition is associated with a reduction in IRS-1 serine phosphorylation (Ser307). Activation of IKK and JNK by PKC has been well established in the signaling pathways of cell membrane receptors. In B or T cells, PKCß and PKC are responsible for IKK and JNK activation, respectively. These have been demonstrated in signaling pathway of B-cell receptor and T-cell receptor (51, 52). Because IKK and JNK may act as downstream signal mediators for PKC, our data suggest that IKK and JNK mediate PKC signals in adipocytes.
PKC may be involved in the FFAs signaling pathway in adipocytes. It has been suggested that PKC is a major PKC isoenzyme in the skeletal muscle and activation of PKC by FFAs might be responsible for insulin resistance in the skeletal muscle (7, 24). Although PKC has drawn a lot of attention in the skeletal muscle (7, 24), it is not clear whether PKC plays a role in the adipose tissue. In this study, we evaluated PKC activity in adipocytes. Our result suggests that PKC is expressed in adipocytes and its phosphorylation is induced by FFAs (Fig. 4). In addition, PKC abundance is increased in the adipose tissue of dietary obese mice, suggesting a chronic activation of PKC. In addition to PKC, it was reported that PKCß (25), PKC (25), and PKC (26) could be activated by FFAs. However, these observations were made in muscle. In this study, our data suggest that PKC, but not other PKC isoforms, is activated by FFA in adipocytes (Fig. 4). This result suggests a tissue-specific effect of FFA activity.
We observed that phosphorylation of PKC can be induced in adipocytes by FFAs. It is generally believed that PKCs are constitutively phosphorylated at the activation and autophosphorylation domains in cells. However, it is not clear what is responsible for the constitutive phosphorylation. Our data suggest that the phosphorylation in some PKC isoforms is inducible by insulin. In serum-starved 3T3-L1 adipocytes, phosphorylation of PKC (Thr538), PKC (643), and PKC/
(Thr410/403) are induced by insulin (data not shown). Thus, insulin may be responsible for the constitutive phosphorylation of certain PKCs in cultured cells that are maintained in serum-containing medium. Our observation is consistent with that insulin induces membrane association of PKC in 3T3-L1 adipocytes in serum-free condition (53). Because activities of most PKC isoforms are negatively associated with insulin sensitivity in cells and in animals as shown in PKC or ß knockout mice (44, 54), it is possible that activation of PKCs involves in the negative feedback of insulin signaling. In this study, we observed that phosphorylation of PKCs was induced by FFA in serum-free condition. It is possible that FFAs contribute to insulin resistance through activation of this negative feedback mechanism. An increase in intracellular diacylglycerol was suggested to contribute to PKC activation (7).
In summary, our data suggest a signaling pathway of FFAs for insulin resistance in adipocytes (Fig. 9). In this pathway, FFAs activate PKC isoenzymes such as PKC and leads to the activation of IKK and JNK. Activation of these two serine kinases leads to Ser307 phosphorylation in IRS-1. The serine phosphorylation is responsible for a reduction in IRS-1 protein and insulin resistance in adipocytes. This molecular pathway might operate in many cell types including adipocytes, myocytes, and hepatocytes.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONMATERIALS AND METHODSREFERENCES |
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/ß II (catalog no. 9375), PKC (catalog nos. 9374, 9376), PKCµ (catalog no. 2064), PKC (catalog no. 9377), PKC (catalog no. 9378) and Akt (catalog no. 9275) were purchased from Cell Signaling Technology (Beverly, MA). Expression vectors for HA-IRS-1, HA-IRS-1A, and HA-IRS-1D were used in our previous study (11). JNK inhibitor SP600125 (catalog no. EI-305) was from Biomol (Plymouth Meeting, PA). 15dPGJ2 (catalog no. 538927) was from Calbiochem (San Diego, CA).
Dietary Obese Mice
Male C57BL/6J mice at age of 4 wk were purchased from The Jackson Laboratory (Bar Harbor, ME) and housed in the animal facility at the Pennington Biomedical Research Center with 12-h light, 12-h dark cycle and constant temperature (23 C). The mice were free to access water and diet. After a 1-wk quarantine, the mice were divided into two groups, 12 mice per group. The experimental group was fed with high-fat diet (D12331, Research Diets, New Brunswick, NJ) in which fat accounts for 58 kcal%. The control group was fed with chow diet. All procedures were performed in accordance with National Institute of Health guidelines for the care and use of animal and approved by the Institute Animal Care and Use Committee at the Pennington Biomedical Research Center.
Fasting Plasma Glucose and Insulin
Fasting glucose and insulin were determined in the plasma every 2 wk. The blood (30 µl/mouse) was collected from the tail vein using heparinized micro-hematocrit capillary tubes (catalog no. 22–362-566; Fisher Scientific, Pittsburgh, PA) after overnight (16 h) starvation. The plasma was prepared by centrifuging the blood at 4 C, 4000 rpm for 20 min. The glucose level was determined with a FreeStyle blood glucose monitoring system (TheraSense, Phoenix, AZ). The insulin level was determined with ELISA using the "Ultra Sensitive Insulin ELISA Kit" (catalog no. 90060, Crystal Chem, Chicago, IL).
3T3-L1 Adipocytes
The mouse fibroblast 3T3-L1 preadipocytes (CL-173) were purchased from the American Type Culture Collection (ATCC, Manassas, VA) and maintained in DMEM culture medium supplemented with 10% fetal calf serum, and 4 mM glutamine. For adipogenesis, 3T3-L1 preadipocytes were grown into confluence in a six-well or 100-mm plate, and then were differentiated into adipocytes using a standard protocol. The 3T3-L1 cells were incubated in the adipogenic cocktail (5 µg/ml insulin, 0.5 mM isobutylmethylxanthine, and 10 µM dexamethasone) for 2 d. This was followed by incubation in insulin-supplemented medium for additional 4 d. The normal medium was used at d 7 to maintain the adipocytes.
Fatty Acid Treatment
Stearic (S4751) was from Sigma (St. Louis, MO). Oleic (90260) and linoleic acids (90150) were purchased from Cayman Chemical (Ann Arbor, MI). These FFAs were mixed with FFA-free BSA (152401, ICN Biomedicals, Irvine, CA) at a weight ratio of 1:1 to make BSA-bound FFA. The 3T3-L1 adipocytes were serum-starved overnight in 0.1% BSA DMEM and then treated with BSA-bound FFAs.
Glucose Uptake (55)
3T3-L1 preadipocytes (5 x 105/well) were differentiated into adipocytes in a 12-well plate. After serum-starvation in 0.1% BSA DMEM for overnight, the cells were incubated in 1 ml/well PBS containing 200 nM insulin for 30 min at 37 C. After washing in PBS, the cells were incubated in 1 ml PBS containing 0.1 mM 2-deoxyglucose and 1 µCi/ml 2-deoxy-D-[3H] glucose for 5 min. Then, the cells were washed three times in ice-cold PBS, and solubilized in 0.4 ml of 1% sodium dodecyl sulfate. 3H-glucose uptake was detected in 4 ml of scintillant using a Beckman LS6500 scintillation counter (Beckman Coulter, Inc., Fullerton, CA). Nonspecific deoxyglucose uptake is measured in the presence of 20 µM cytochalasin B and is subtracted from the total uptake to get specific glucose uptake.
Immunoblotting and IP (11)
The whole cell lysate was made by sonication in lysis buffer [1% Triton X-100, 50 mM KCl, 25 mM HEPES (pH 7.8), leupeptin 10 µg/ml, aprotinin 20 µg/ml, 125 µM dithiothreitol, 1 mM phenylmethylsulfonyl fluoride, 1 mM sodium orthovanadate]. IP was conducted with 200–400 µg protein and 2–4 µg antibodies. The IP product was then subjected to immunoblotting analysis. The protein was resolved in SDS-PAGE, and transferred onto polyvinylidene difluoride membrane (162–0184, Bio-Rad, Hercules, CA). The membrane was preblotted in milk buffer for 20 min, and then immunoblotted with a primary antibody for 1–24 h followed by a secondary antibody for 30 min. Horseradish peroxidase-conjugated secondary antibodies (NA934V or NA931, Amersham Life Science, Piscataway, NJ) were used with chemiluminescence reagent for signal imaging (NEL-105, PerkinElmer, Boston, MA). To detect multiple signals from a single membrane, the blot membrane was treated with a stripping buffer (59 mM trizma hydrochloride, 2% sodium dodecyl sulfate, 0.75% 2-merthylethylenediamine) for 30 min at 42 C, washed extensively in PBS for 2 h, and then used for reblotting with a different primary antibody. The intensity of Western blot signal was quantified with an image analysis program PDQuest 7.1 (Bio-Rad), and the signal was normalized against loading control.
Data Analysis
The data of glucose, insulin, glucose uptake and signals in immunoblot are presented as mean ± SE of triplicates in a representative experiments or results of three independent experiments. Student’s t test was used with significance of P < 0.05.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Abbreviations: 15dPGJ2, 15-Deoxy- 12,14-prostaglandin J2; FFAs, free fatty acids; HA, hemagglutin; IKK, inhibitor B kinase; IP, immunoprecipitation; IRS-1, insulin receptor substrate 1; JNK, c-JUN NH2-terminal kinase; PI(3)K, phosphatidylinositol-3 kinase; PKC, protein kinase C; PKD, protein kinase D; Ser307, serine 307.
Received for publication October 1, 2003. Accepted for publication May 7, 2004.
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W. Bakker, P. Sipkema, C. D.A. Stehouwer, E. H. Serne, Y. M. Smulders, V. W.M. van Hinsbergh, and E. C. Eringa Protein Kinase C {theta} Activation Induces Insulin-Mediated Constriction of Muscle Resistance Arteries Diabetes, March 1, 2008; 57(3): 706 - 713. [Abstract] [Full Text] [PDF]
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 J.-a Kim, Y. Wei, and J. R. Sowers Role of Mitochondrial Dysfunction in Insulin Resistance Circ. Res., February 29, 2008; 102(4): 401 - 414. [Abstract] [Full Text] [PDF]
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G. Boden, W. Song, L. Pashko, and K. Kresge In Vivo Effects of Insulin and Free Fatty Acids on Matrix Metalloproteinases in Rat Aorta Diabetes, February 1, 2008; 57(2): 476 - 483. [Abstract] [Full Text] [PDF]
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J. Yin, Z. Gao, D. Liu, Z. Liu, and J. Ye Berberine improves glucose metabolism through induction of glycolysis Am J Physiol Endocrinol Metab, January 1, 2008; 294(1): E148 - E156. [Abstract] [Full Text] [PDF]
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 E. Park, V. Wong, X. Guan, A. I Oprescu, and A. Giacca Salicylate prevents hepatic insulin resistance caused by short-term elevation of free fatty acids in vivo J. Endocrinol., November 1, 2007; 195(2): 323 - 331. [Abstract] [Full Text] [PDF]
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 J. A. A. Pires, J. B. Pescara, and R. R. Grummer Reduction of Plasma NEFA Concentration by Nicotinic Acid Enhances the Response to Insulin in Feed-Restricted Holstein Cows J Dairy Sci, October 1, 2007; 90(10): 4635 - 4642. [Abstract] [Full Text] [PDF]
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R. Yang, D. M. Wilcox, D. L. Haasch, P. M. Jung, P. T. Nguyen, M. J. Voorbach, S. Doktor, S. Brodjian, E. N. Bush, E. Lin, et al. Liver-specific Knockdown of JNK1 Up-regulates Proliferator-activated Receptor {gamma} Coactivator 1beta and Increases Plasma Triglyceride despite Reduced Glucose and Insulin Levels in Diet-induced Obese Mice J. Biol. Chem., August 3, 2007; 282(31): 22765 - 22774. [Abstract] [Full Text] [PDF]
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 R. Muniyappa, M. Montagnani, K. K. Koh, and M. J. Quon Cardiovascular Actions of Insulin Endocr. Rev., August 1, 2007; 28(5): 463 - 491. [Abstract] [Full Text] [PDF]
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W. Guo, S. Wong, W. Xie, T. Lei, and Z. Luo Palmitate modulates intracellular signaling, induces endoplasmic reticulum stress, and causes apoptosis in mouse 3T3-L1 and rat primary preadipocytes Am J Physiol Endocrinol Metab, August 1, 2007; 293(2): E576 - E586. [Abstract] [Full Text] [PDF]
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A. R. Subauste and C. F. Burant Role of FoxO1 in FFA-induced oxidative stress in adipocytes Am J Physiol Endocrinol Metab, July 1, 2007; 293(1): E159 - E164. [Abstract] [Full Text] [PDF]
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A. Herschkovitz, Y.-F. Liu, E. Ilan, D. Ronen, S. Boura-Halfon, and Y. Zick Common Inhibitory Serine Sites Phosphorylated by IRS-1 Kinases, Triggered by Insulin and Inducers of Insulin Resistance J. Biol. Chem., June 22, 2007; 282(25): 18018 - 18027. [Abstract] [Full Text] [PDF]
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 M. Qatanani and M. A. Lazar Mechanisms of obesity-associated insulin resistance: many choices on the menu Genes & Dev., June 15, 2007; 21(12): 1443 - 1455. [Abstract] [Full Text] [PDF]
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J. A. A. Pires, A. H. Souza, and R. R. Grummer Induction of Hyperlipidemia by Intravenous Infusion of Tallow Emulsion Causes Insulin Resistance in Holstein Cows J Dairy Sci, June 1, 2007; 90(6): 2735 - 2744. [Abstract] [Full Text] [PDF]
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N. Bashan, K. Dorfman, T. Tarnovscki, I. Harman-Boehm, I. F. Liberty, M. Bluher, S. Ovadia, T. Maymon-Zilberstein, R. Potashnik, M. Stumvoll, et al. Mitogen-Activated Protein Kinases, Inhibitory-{kappa}B Kinase, and Insulin Signaling in Human Omental Versus Subcutaneous Adipose Tissue in Obesity Endocrinology, June 1, 2007; 148(6): 2955 - 2962. [Abstract] [Full Text] [PDF]
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H.-Y. Liu, Q. F. Collins, Y. Xiong, F. Moukdar, E. G. Lupo Jr., Z. Liu, and W. Cao Prolonged Treatment of Primary Hepatocytes with Oleate Induces Insulin Resistance through p38 Mitogen-activated Protein Kinase J. Biol. Chem., May 11, 2007; 282(19): 14205 - 14212. [Abstract] [Full Text] [PDF]
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 M. P Corcoran, S. Lamon-Fava, and R. A Fielding Skeletal muscle lipid deposition and insulin resistance: effect of dietary fatty acids and exercise Am. J. Clinical Nutrition, March 1, 2007; 85(3): 662 - 677. [Abstract] [Full Text] [PDF]
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M. Bajaj, R. Medina-Navarro, S. Suraamornkul, C. Meyer, R. A. DeFronzo, and L. J. Mandarino Paradoxical Changes in Muscle Gene Expression in Insulin-Resistant Subjects After Sustained Reduction in Plasma Free Fatty Acid Concentration Diabetes, March 1, 2007; 56(3): 743 - 752. [Abstract] [Full Text] [PDF]
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Z. Gao, Z. Wang, X. Zhang, A. A. Butler, A. Zuberi, B. Gawronska-Kozak, M. Lefevre, D. York, E. Ravussin, H.-R. Berthoud, et al. Inactivation of PKC{theta} leads to increased susceptibility to obesity and dietary insulin resistance in mice Am J Physiol Endocrinol Metab, January 1, 2007; 292(1): E84 - E91. [Abstract] [Full Text] [PDF]
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 M. A. Bogoyevitch and B. Kobe Uses for JNK: the Many and Varied Substrates of the c-Jun N-Terminal Kinases Microbiol. Mol. Biol. Rev., December 1, 2006; 70(4): 1061 - 1095. [Abstract] [Full Text] [PDF]
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K. Morino, K. F. Petersen, and G. I. Shulman Molecular Mechanisms of Insulin Resistance in Humans and Their Potential Links With Mitochondrial Dysfunction Diabetes, December 1, 2006; 55(Supplement_2): S9 - S15. [Abstract] [Full Text] [PDF]
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 G. Svegliati-Baroni, C. Candelaresi, S. Saccomanno, G. Ferretti, T. Bachetti, M. Marzioni, S. De Minicis, L. Nobili, R. Salzano, A. Omenetti, et al. A Model of Insulin Resistance and Nonalcoholic Steatohepatitis in Rats: Role of Peroxisome Proliferator-Activated Receptor-{alpha} and n-3 Polyunsaturated Fatty Acid Treatment on Liver Injury Am. J. Pathol., September 1, 2006; 169(3): 846 - 860. [Abstract] [Full Text] [PDF]
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Q. F. Collins, Y. Xiong, E. G. Lupo Jr., H.-Y. Liu, and W. Cao p38 Mitogen-activated Protein Kinase Mediates Free Fatty Acid-induced Gluconeogenesis in Hepatocytes J. Biol. Chem., August 25, 2006; 281(34): 24336 - 24344. [Abstract] [Full Text] [PDF]
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B. Draznin Molecular Mechanisms of Insulin Resistance: Serine Phosphorylation of Insulin Receptor Substrate-1 and Increased Expression of p85{alpha}: The Two Sides of a Coin. Diabetes, August 1, 2006; 55(8): 2392 - 2397. [Abstract] [Full Text] [PDF]
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H. Malhi, S. F. Bronk, N. W. Werneburg, and G. J. Gores Free Fatty Acids Induce JNK-dependent Hepatocyte Lipoapoptosis J. Biol. Chem., April 28, 2006; 281(17): 12093 - 12101. [Abstract] [Full Text] [PDF]
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 J.-a Kim, M. Montagnani, K. K. Koh, and M. J. Quon Reciprocal Relationships Between Insulin Resistance and Endothelial Dysfunction: Molecular and Pathophysiological Mechanisms Circulation, April 18, 2006; 113(15): 1888 - 1904. [Abstract] [Full Text] [PDF]
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N. Warwar, S. Efendic, C.-G. Ostenson, E. P. Haber, E. Cerasi, and R. Nesher Dynamics of Glucose-Induced Localization of PKC Isoenzymes in Pancreatic {beta}-Cells: Diabetes-Related Changes in the GK Rat Diabetes, March 1, 2006; 55(3): 590 - 599. [Abstract] [Full Text] [PDF]
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Z. Gao, Q. He, B. Peng, P. J. Chiao, and J. Ye Regulation of Nuclear Translocation of HDAC3 by I{kappa}B{alpha} Is Required for Tumor Necrosis Factor Inhibition of Peroxisome Proliferator-activated Receptor {gamma} Function J. Biol. Chem., February 17, 2006; 281(7): 4540 - 4547. [Abstract] [Full Text] [PDF]
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G. S. Hotamisligil Role of Endoplasmic Reticulum Stress and c-Jun NH2-Terminal Kinase Pathways in Inflammation and Origin of Obesity and Diabetes Diabetes, December 1, 2005; 54(suppl_2): S73 - S78. [Abstract] [Full Text] [PDF]
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M. T. A. Nguyen, H. Satoh, S. Favelyukis, J. L. Babendure, T. Imamura, J. I. Sbodio, J. Zalevsky, B. I. Dahiyat, N.-W. Chi, and J. M. Olefsky JNK and Tumor Necrosis Factor-{alpha} Mediate Free Fatty Acid-induced Insulin Resistance in 3T3-L1 Adipocytes J. Biol. Chem., October 21, 2005; 280(42): 35361 - 35371. [Abstract] [Full Text] [PDF]
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 K. M. Ajuwon and M. E. Spurlock Palmitate Activates the NF-{kappa}B Transcription Factor and Induces IL-6 and TNF{alpha} Expression in 3T3-L1 Adipocytes J. Nutr., August 1, 2005; 135(8): 1841 - 1846. [Abstract] [Full Text] [PDF]
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 J.-a Kim, K. K. Koh, and M. J. Quon The Union of Vascular and Metabolic Actions of Insulin in Sickness and in Health Arterioscler Thromb Vasc Biol, May 1, 2005; 25(5): 889 - 891. [Full Text] [PDF]
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H. Sugita, M. Fujimoto, T. Yasukawa, N. Shimizu, M. Sugita, S. Yasuhara, J. A. J. Martyn, and M. Kaneki Inducible Nitric-oxide Synthase and NO Donor Induce Insulin Receptor Substrate-1 Degradation in Skeletal Muscle Cells J. Biol. Chem., April 8, 2005; 280(14): 14203 - 14211. [Abstract] [Full Text] [PDF]
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P. A. Tataranni and E. Ortega A Burning Question: Does an Adipokine-Induced Activation of the Immune System Mediate the Effect of Overnutrition on Type 2 Diabetes? Diabetes, April 1, 2005; 54(4): 917 - 927. [Abstract] [Full Text] [PDF]
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C. Rask-Madsen and G. L. King Proatherosclerotic Mechanisms Involving Protein Kinase C in Diabetes and Insulin Resistance Arterioscler Thromb Vasc Biol, March 1, 2005; 25(3): 487 - 496. [Abstract] [Full Text] [PDF]
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N. Sato, K. Kobayashi, T. Inoguchi, N. Sonoda, M. Imamura, N. Sekiguchi, N. Nakashima, and H. Nawata Adenovirus-Mediated High Expression of Resistin Causes Dyslipidemia in Mice Endocrinology, January 1, 2005; 146(1): 273 - 279. [Abstract] [Full Text] [PDF]
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