Publications by authors named "Zhibo An"

14 Publications

  • Page 1 of 1

The role of β cell glucagon-like peptide-1 signaling in glucose regulation and response to diabetes drugs.

Cell Metab 2014 Jun 15;19(6):1050-7. Epub 2014 May 15.

Division of Endocrinology, Department of Internal Medicine, University of Cincinnati, Cincinnati, OH 45267, USA; Cincinnati Veterans Affairs Medical Center, Cincinnati, OH 45237, USA. Electronic address:

Glucagon-like peptide-1 (GLP-1), an insulinotropic gut peptide released after eating, is essential for normal glucose tolerance (GT). To determine whether this effect is mediated directly by GLP-1 receptors (GLP1R) on islet β cells, we developed mice with β cell-specific knockdown of Glp1r. β cell Glp1r knockdown mice had impaired GT after intraperitoneal (i.p.) glucose and did not secrete insulin in response to i.p. or intravenous GLP-1. However, they had normal GT after oral glucose, a response that was impaired by a GLP1R antagonist. β cell Glp1r knockdown mice had blunted responses to a GLP1R agonist but intact glucose lowering with a dipeptidylpeptidase 4 (DPP-4) inhibitor. Thus, in mice, β cell Glp1rs are required to respond to hyperglycemia and exogenous GLP-1, but other factors compensate for reduced GLP-1 action during meals. These results support a role for extraislet GLP1R in oral glucose tolerance and paracrine regulation of β cells by islet GLP-1.
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http://dx.doi.org/10.1016/j.cmet.2014.04.005DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4109713PMC
June 2014

Improved glycemic control enhances the incretin effect in patients with type 2 diabetes.

J Clin Endocrinol Metab 2013 Dec 3;98(12):4702-8. Epub 2013 Oct 3.

MD, 2170 East Galbraith Road, Room E-319, University of Cincinnati, Cincinnati, OH 45237.

Background And Aims: Impairment of the incretin effect is one of the hallmarks of type 2 diabetes mellitus (T2DM). However, it is unknown whether this abnormality is specific to incretin-stimulated insulin secretion or a manifestation of generalized β-cell dysfunction. The aim of this study was to determine whether improved glycemic control restores the incretin effect.

Methods: Fifteen T2DM subjects were studied before and after 8 weeks of intensified treatment with insulin. The incretin effect was determined by comparing plasma insulin and C-peptide levels at clamped hyperglycemia from iv glucose, and iv glucose plus glucose ingestion.

Results: Long-acting insulin, titrated to reduce fasting glucose to 7 mM, lowered hemoglobin A1c from 8.6% ± 0.2% to 7.1% ± 0.2% over 8 weeks. The incremental C-peptide responses and insulin secretion rates to iv glucose did not differ before and after insulin treatment (5.6 ± 1.0 and 6.0 ± 0.9 nmol/L·min and 0.75 ± 0.10 and 0.76 ± 0.11 pmol/min), but the C-peptide response to glucose ingestion was greater after treatment than before (10.9 ± 2.2 and 7.1 ± 0.9 nmol/L·min; P = .03) as were the insulin secretion rates (1.11 ± 0.22 and 0.67 ± 0.07 pmol/min; P = .04). The incretin effect computed from plasma C-peptide was 21.8% ± 6.5% before insulin treatment and increased 40.9% ± 3.9% after insulin treatment (P < .02).

Conclusion: Intensified insulin treatment to improve glycemic control led to a disproportionate improvement of insulin secretion in response to oral compared with iv glucose stimulation in patients with type 2 diabetes. This suggests that in T2DM the impaired incretin effect is independent of abnormal glucose-stimulated insulin secretion.
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http://dx.doi.org/10.1210/jc.2013-1199DOI Listing
December 2013

Effects of intraportal exenatide on hepatic glucose metabolism in the conscious dog.

Am J Physiol Endocrinol Metab 2013 Jul 14;305(1):E132-9. Epub 2013 May 14.

Department of Molecular Physiology and Biophysics, Vanderbilt University Medical Center, Nashville, TN 37232, USA.

Incretins improve glucose metabolism through multiple mechanisms. It remains unclear whether direct hepatic effects are an important part of exenatide's (Ex-4) acute action. Therefore, the objective of this study was to determine the effect of intraportal delivery of Ex-4 on hepatic glucose production and uptake. Fasted conscious dogs were studied during a hyperglycemic clamp in which glucose was infused into the hepatic portal vein. At the same time, portal saline (control; n = 8) or exenatide was infused at low (0.3 pmol·kg⁻¹·min⁻¹, Ex-4-low; n = 5) or high (0.9 pmol·kg⁻¹·min⁻¹, Ex-4-high; n = 8) rates. Arterial plasma glucose levels were maintained at 160 mg/dl during the experimental period. This required a greater rate of glucose infusion in the Ex-4-high group (1.5 ± 0.4, 2.0 ± 0.7, and 3.7 ± 0.7 mg·kg⁻¹·min⁻¹ between 30 and 240 min in the control, Ex-4-low, and Ex-4-high groups, respectively). Plasma insulin levels were elevated by Ex-4 (arterial: 4,745 ± 428, 5,710 ± 355, and 7,262 ± 1,053 μU/ml; hepatic sinusoidal: 14,679 ± 1,700, 15,341 ± 2,208, and 20,445 ± 4,020 μU/ml, 240 min, area under the curve), whereas the suppression of glucagon was nearly maximal in all groups. Although glucose utilization was greater during Ex-4 infusion (5.92 ± 0.53, 6.41 ± 0.57, and 8.12 ± 0.54 mg·kg⁻¹·min⁻¹), when indices of hepatic, muscle, and whole body glucose uptake were expressed relative to circulating insulin concentrations, there was no indication of insulin-independent effects of Ex-4. Thus, this study does not support the notion that Ex-4 generates acute changes in hepatic glucose metabolism through direct effects on the liver.
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http://dx.doi.org/10.1152/ajpendo.00160.2013DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3725568PMC
July 2013

Liver glycogen loading dampens glycogen synthesis seen in response to either hyperinsulinemia or intraportal glucose infusion.

Diabetes 2013 Jan 24;62(1):96-101. Epub 2012 Aug 24.

Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA.

The purpose of this study was to determine the effect of liver glycogen loading on net hepatic glycogen synthesis during hyperinsulinemia or hepatic portal vein glucose infusion in vivo. Liver glycogen levels were supercompensated (SCGly) in two groups (using intraportal fructose infusion) but not in two others (Gly) during hyperglycemic-normoinsulinemia. Following a 2-h control period during which fructose infusion was stopped, there was a 2-h experimental period in which the response to hyperglycemia plus either 4× basal insulin (INS) or portal vein glucose infusion (PoG) was measured. Increased hepatic glycogen reduced the percent of glucose taken up by the liver that was deposited in glycogen (74 ± 3 vs. 53 ± 5% in Gly+INS and SCGly+INS, respectively, and 72 ± 3 vs. 50 ± 6% in Gly+PoG and SCGly+PoG, respectively). The reduction in liver glycogen synthesis in SCGly+INS was accompanied by a decrease in both insulin signaling and an increase in AMPK activation, whereas only the latter was observed in SCGly+PoG. These data indicate that liver glycogen loading impairs glycogen synthesis regardless of the signal used to stimulate it.
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http://dx.doi.org/10.2337/db11-1773DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3526057PMC
January 2013

A cyclic guanosine monophosphate-dependent pathway can regulate net hepatic glucose uptake in vivo.

Diabetes 2012 Oct 11;61(10):2433-41. Epub 2012 Jun 11.

Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA.

We previously showed that hepatic nitric oxide regulates net hepatic glucose uptake (NHGU), an effect that can be eliminated by inhibiting hepatic soluble guanylate cyclase (sGC), suggesting that the sGC pathway is involved in the regulation of NHGU. The aim of the current study was to determine whether hepatic cyclic guanosine monophosphate (cGMP) reduces NHGU. Studies were performed on conscious dogs with transhepatic catheters. A hyperglycemic-hyperinsulinemic clamp was established in the presence of portal vein glucose infusion. 8-Br-cGMP (50 µg/kg/min) was delivered intraportally, and either the glucose load to the liver (CGMP/GLC; n = 5) or the glucose concentration entering the liver (CGMP/GCC; n = 5) was clamped at 2× basal. In the control group, saline was given intraportally (SAL; n = 10), and the hepatic glucose concentration and load were doubled. 8-Br-cGMP increased portal blood flow, necessitating the two approaches to glucose clamping in the cGMP groups. NHGU (mg/kg/min) was 5.8 ± 0.5, 2.7 ± 0.5, and 4.8 ± 0.3, whereas the fractional extraction of glucose was 11.0 ± 1, 5.5 ± 1, and 8.5 ± 1% during the last hour of the study in SAL, CGMP/GLC, and CGMP/GCC, respectively. The reduction of NHGU in response to 8-Br-cGMP was associated with increased AMP-activated protein kinase phosphorylation. These data indicate that changes in liver cGMP can regulate NHGU under postprandial conditions.
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http://dx.doi.org/10.2337/db11-1816DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3447895PMC
October 2012

Regulation of hepatic glucose uptake and storage in vivo.

Adv Nutr 2012 May 1;3(3):286-94. Epub 2012 May 1.

Department of Molecular Physiology & Biophysics, Vanderbilt University School of Medicine, Nashville, TN.

In the postprandial state, the liver takes up and stores glucose to minimize the fluctuation of glycemia. Elevated insulin concentrations, an increase in the load of glucose reaching the liver, and the oral/enteral/portal vein route of glucose delivery (compared with the peripheral intravenous route) are factors that increase the rate of net hepatic glucose uptake (NHGU). The entry of glucose into the portal vein stimulates a portal glucose signal that not only enhances NHGU but concomitantly reduces muscle glucose uptake to ensure appropriate partitioning of a glucose load. This coordinated regulation of glucose uptake is likely neurally mediated, at least in part, because it is not observed after total hepatic denervation. Moreover, there is evidence that both the sympathetic and the nitrergic innervation of the liver exert a tonic repression of NHGU that is relieved under feeding conditions. Further, the energy sensor 5'AMP-activated protein kinase appears to be involved in regulation of NHGU and glycogen storage. Consumption of a high-fat and high-fructose diet impairs NHGU and glycogen storage in association with a reduction in glucokinase protein and activity. An understanding of the impact of nutrients themselves and the route of nutrient delivery on liver carbohydrate metabolism is fundamental to the development of therapies for impaired postprandial glucoregulation.
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http://dx.doi.org/10.3945/an.112.002089DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3649460PMC
May 2012

Hepatic glycogen supercompensation activates AMP-activated protein kinase, impairs insulin signaling, and reduces glycogen deposition in the liver.

Diabetes 2011 Feb;60(2):398-407

Department of Molecular Physiology and Biophysics, Vanderbilt University Medical Center, Nashville, Tennessee, USA.

Objective: The objective of this study was to determine how increasing the hepatic glycogen content would affect the liver's ability to take up and metabolize glucose.

Research Design And Methods: During the first 4 h of the study, liver glycogen deposition was stimulated by intraportal fructose infusion in the presence of hyperglycemic-normoinsulinemia. This was followed by a 2-h hyperglycemic-normoinsulinemic control period, during which the fructose infusion was stopped, and a 2-h experimental period in which net hepatic glucose uptake (NHGU) and disposition (glycogen, lactate, and CO(2)) were measured in the absence of fructose but in the presence of a hyperglycemic-hyperinsulinemic challenge including portal vein glucose infusion.

Results: Fructose infusion increased net hepatic glycogen synthesis (0.7 ± 0.5 vs. 6.4 ± 0.4 mg/kg/min; P < 0.001), causing a large difference in hepatic glycogen content (62 ± 9 vs. 100 ± 3 mg/g; P < 0.001). Hepatic glycogen supercompensation (fructose infusion group) did not alter NHGU, but it reduced the percent of NHGU directed to glycogen (79 ± 4 vs. 55 ± 6; P < 0.01) and increased the percent directed to lactate (12 ± 3 vs. 29 ± 5; P = 0.01) and oxidation (9 ± 3 vs. 16 ± 3; P = NS). This change was associated with increased AMP-activated protein kinase phosphorylation, diminished insulin signaling, and a shift in glycogenic enzyme activity toward a state discouraging glycogen accumulation.

Conclusions: These data indicate that increases in hepatic glycogen can generate a state of hepatic insulin resistance, which is characterized by impaired glycogen synthesis despite preserved NHGU.
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http://dx.doi.org/10.2337/db10-0592DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3028338PMC
February 2011

Insulin-induced hypoglycemia increases hepatic sensitivity to glucagon in dogs.

J Clin Invest 2010 Dec 15;120(12):4425-35. Epub 2010 Nov 15.

Department of Molecular Physiology, Vanderbilt University School of Medicine, Nashville, Tennessee, USA.

In individuals with type 1 diabetes, hypoglycemia is a common consequence of overinsulinization. Under conditions of insulin-induced hypoglycemia, glucagon is the most important stimulus for hepatic glucose production. In contrast, during euglycemia, insulin potently inhibits glucagon's effect on the liver. The first aim of the present study was to determine whether low blood sugar augments glucagon's ability to increase glucose production. Using a conscious catheterized dog model, we found that hypoglycemia increased glucagon's ability to overcome the inhibitory effect of insulin on hepatic glucose production by almost 3-fold, an effect exclusively attributable to marked enhancement of the effect of glucagon on net glycogen breakdown. To investigate the molecular mechanism by which this effect comes about, we analyzed hepatic biopsies from the same animals, and found that hypoglycemia resulted in a decrease in insulin signaling. Furthermore, hypoglycemia and glucagon had an additive effect on the activation of AMPK, which was associated with altered activity of the enzymes of glycogen metabolism.
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http://dx.doi.org/10.1172/JCI40919DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2993579PMC
December 2010

A soluble guanylate cyclase-dependent mechanism is involved in the regulation of net hepatic glucose uptake by nitric oxide in vivo.

Diabetes 2010 Dec 7;59(12):2999-3007. Epub 2010 Sep 7.

Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee. USA.

Objective: We previously showed that elevating hepatic nitric oxide (NO) levels reduced net hepatic glucose uptake (NHGU) in the presence of portal glucose delivery, hyperglycemia, and hyperinsulinemia. The aim of the present study was to determine the role of a downstream signal, soluble guanylate cyclase (sGC), in the regulation of NHGU by NO.

Research Design And Methods: Studies were performed on 42-h-fasted conscious dogs fitted with vascular catheters. At 0 min, somatostatin was given peripherally along with 4× basal insulin and basal glucagon intraportally. Glucose was delivered at a variable rate via a leg vein to double the blood glucose level and hepatic glucose load throughout the study. From 90 to 270 min, an intraportal infusion of the sGC inhibitor 1H-[1,2,4] oxadiazolo[4,3-a] quinoxalin-1-one (ODQ) was given in -sGC (n = 10) and -sGC/+NO (n = 6), whereas saline was given in saline infusion (SAL) (n = 10). The -sGC/+NO group also received intraportal SIN-1 (NO donor) to elevate hepatic NO from 180 to 270 min.

Results: In the presence of 4× basal insulin, basal glucagon, and hyperglycemia (2× basal ), inhibition of sGC in the liver enhanced NHGU (mg/kg/min; 210-270 min) by ∼55% (2.9 ± 0.2 in SAL vs. 4.6 ± 0.5 in -sGC). Further elevating hepatic NO failed to reduce NHGU (4.5 ± 0.7 in -sGC/+NO). Net hepatic carbon retention (i.e., glycogen synthesis; mg glucose equivalents/kg/min) increased to 3.8 ± 0.2 in -sGC and 3.8 ± 0.4 in -sGC/+NO vs. 2.4 ± 0.2 in SAL (P < 0.05).

Conclusions: NO regulates liver glucose uptake through a sGC-dependent pathway. The latter could be a target for pharmacologic intervention to increase meal-associated hepatic glucose uptake in individuals with type 2 diabetes.
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http://dx.doi.org/10.2337/db10-0138DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2992759PMC
December 2010

A physiological increase in the hepatic glycogen level does not affect the response of net hepatic glucose uptake to insulin.

Am J Physiol Endocrinol Metab 2009 Aug 26;297(2):E358-66. Epub 2009 May 26.

Department of Molecular Physiology and Biophysics, Vanderbilt University Medical Center, Nashville, Tennessee 37232-6015, USA.

To determine the effect of an acute increase in hepatic glycogen on net hepatic glucose uptake (NHGU) and disposition in response to insulin in vivo, studies were performed on two groups of dogs fasted 18 h. During the first 4 h of the study, somatostatin was infused peripherally, while insulin and glucagon were replaced intraportally in basal amounts. Hyperglycemia was brought about by glucose infusion, and either saline (n = 7) or fructose (n = 7; to stimulate NHGU and glycogen deposition) was infused intraportally. A 2-h control period then followed, during which the portal fructose and saline infusions were stopped, allowing NHGU and glycogen deposition in the fructose-infused animals to return to rates similar to those of the animals that received the saline infusion. This was followed by a 2-h experimental period, during which hyperglycemia was continued but insulin infusion was increased fourfold in both groups. During the initial 4-h glycogen loading period, NHGU averaged 1.18 +/- 0.27 and 5.55 +/- 0.53 mg x kg(-1) x min(-1) and glycogen synthesis averaged 0.72 +/- 0.24 and 3.98 +/- 0.57 mg x kg(-1) x min(-1) in the saline and fructose groups, respectively (P < 0.05). During the 2-h hyperinsulinemic period, NHGU rose from 1.5 +/- 0.4 and 0.9 +/- 0.2 to 3.1 +/- 0.6 and 2.5 +/- 0.5 mg x kg(-1) x min(-1) in the saline and fructose groups, respectively, a change of 1.6 mg x kg(-1) x min(-1) in both groups despite a significantly greater liver glycogen level in the fructose-infused group. Likewise, the metabolic fate of the extracted glucose (glycogen, lactate, or carbon dioxide) was not different between groups. These data indicate that an acute physiological increase in the hepatic glycogen content does not alter liver glucose uptake and storage under hyperglycemic/hyperinsulinemic conditions in the dog.
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http://dx.doi.org/10.1152/ajpendo.00043.2009DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2724107PMC
August 2009

Portal infusion of escitalopram enhances hepatic glucose disposal in conscious dogs.

Eur J Pharmacol 2009 Apr;607(1-3):251-7

Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.

To examine whether escitalopram enhances net hepatic glucose uptake during a hyperinsulinemic hyperglycemic clamp, studies were performed in conscious 42-h-fasted dogs. The experimental period was divided into P1 (0-90 min) and P2 (90-270 min). During P1 and P2 somatostatin (to inhibit insulin and glucagon secretion), 4x basal intraportal insulin, basal intraportal glucagon, and peripheral glucose (2x hepatic glucose load) were infused. Saline was infused intraportally during P1 in all groups. In one group saline infusion was continued in P2 (SAL, n = 11), while escitalopram was infused intraportally at 2 microg/kg/min (L-ESC, n = 6) or 8 microg/kg/min (H-ESC, n = 7) during P2 in two other groups. The arterial insulin concentrations rose approximately four fold (to 123 +/- 8, 146 +/- 13 and 148 +/- 15 pmol/L) while glucagon concentrations remained basal (41 +/- 3, 44 +/- 9 and 40 +/- 3 ng/L) in all groups. The hepatic glucose load averaged 216 +/- 13, 223 +/- 19 and 202 +/- 12 micromol/kg/min during the entire experimental period (P1 and P2) in the SAL, L-ESC and H-ESC groups, respectively. Net hepatic glucose uptake was 11.6 +/- 1.4, 10.1 +/- 0.9 and 10.4 +/- 2.3 micromol/kg/min in P1 and averaged 16.9 +/- 1.5, 15.7 +/- 1.3 and 22.6 +/- 3.7 (P < 0.05) in the SAL, L-ESC and H-ESC groups, respectively during the last hour of P2 (210-270 min). Net hepatic carbon retention (glycogen storage) was 15.4 +/- 1.3, 14.9 +/- 0.6 and 20.9 +/- 2.6 (P < 0.05) micromol/kg/min in SAL, L-ESC and H-ESC respectively during the last hour of P2. Escitalopram enhanced net hepatic glucose uptake and hepatic glycogen deposition, showing that it can improve hepatic glucose clearance under hyperinsulinemic hyperglycemic conditions. Its use in individuals with diabetes may, therefore, result in improved glycemic control.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2775505PMC
http://dx.doi.org/10.1016/j.ejphar.2009.01.042DOI Listing
April 2009

Effects of the nitric oxide donor SIN-1 on net hepatic glucose uptake in the conscious dog.

Am J Physiol Endocrinol Metab 2008 Feb 20;294(2):E300-6. Epub 2007 Nov 20.

Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN 37232-0615, USA.

To determine the role of nitric oxide in regulating net hepatic glucose uptake (NHGU) in vivo, studies were performed on three groups of 42-h-fasted conscious dogs using a nitric oxide donor [3-morpholinosydnonimine (SIN-1)]. The experimental period was divided into period 1 (0-90 min) and period 2 (P2; 90-240 min). At 0 min, somatostatin was infused peripherally, and insulin (4-fold basal) and glucagon (basal) were given intraportally. Glucose was delivered intraportally (22.2 mumol.kg(-1).min(-1)) and peripherally (as needed) to increase the hepatic glucose load twofold basal. At 90 min, an infusion of SIN-1 (4 mug.kg(-1).min(-1)) was started in a peripheral vein (PeSin-1, n = 10) or the portal vein (PoSin-1, n = 12) while the control group received saline (SAL, n = 8). Both peripheral and portal infusion of SIN-1, unlike saline, significantly reduced systolic and diastolic blood pressure. Heart rate rose in PeSin-1 and PoSin-1 (96 +/- 5 to 120 +/- 10 and 88 +/- 6 to 107 +/- 5 beats/min, respectively, P < 0.05) but did not change in response to saline. NHGU during P2 was 31.0 +/- 2.4 and 29.9 +/- 2.0 mumol.kg(-1).min(-1) in SAL and PeSin-1, respectively but was 23.7 +/- 1.7 in PoSin-1 (P < 0.05). Net hepatic carbon retention during P2 was significantly lower in PoSin-1 than SAL or PeSin-1 (21.4 +/- 1.2 vs. 27.1 +/- 1.5 and 26.1 +/- 1.0 mumol.kg(-1).min(-1)). Nonhepatic glucose uptake did not change in response to saline or SIN-1 infusion. In conclusion, portal but not peripheral infusion of the nitric oxide donor SIN-1 inhibited NHGU.
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http://dx.doi.org/10.1152/ajpendo.00380.2007DOI Listing
February 2008

Nicotine-induced activation of AMP-activated protein kinase inhibits fatty acid synthase in 3T3L1 adipocytes: a role for oxidant stress.

J Biol Chem 2007 Sep 16;282(37):26793-26801. Epub 2007 Jul 16.

Vascular Biology Laboratory, Department of Surgery, Graduate School of Medicine, University of Tennessee, Knoxville, Tennessee 37922; Division of Endocrinology and Diabetes, Department of Medicine, University of Oklahoma Health Science Center, Oklahoma City, Oklahoma 73104. Electronic address:

Recent studies suggest that the AMP-activated protein kinase (AMPK) acts as a major energy sensor and regulator in adipose tissues. The objective of this study was to investigate the role of AMPK in nicotine-induced lipogenesis and lipolysis in 3T3L1 adipocytes. Exposure of 3T3L1 adipocytes to smoking-related concentrations of nicotine increased lipolysis and inhibited fatty acid synthase (FAS) activity in a time- and dose-dependent manner. The effects of nicotine on FAS activity were accompanied by phosphorylation of both AMPK (Thr(172)) and acetyl-CoA carboxylase (ACC; Ser(79)). Nicotine-induced AMPK phosphorylation appeared to be mediated by reactive oxygen species based on the finding that nicotine significantly increased superoxide anions and 3-nitrotyrosine-positive proteins, exogenous peroxynitrite (ONOO(-)) mimicked the effects of nicotine on AMPK, and N-acetylcysteine (NAC) abolished nicotine-enhanced AMPK phosphorylation. Inhibition of AMPK using either pharmacologic (insulin, compound C) or genetic means (overexpression of dominant negative AMPK; AMPK-DN) abolished FAS inhibition induced by nicotine or ONOO(-). Conversely, activation of AMPK by pharmacologic (nicotine, ONOO(-), metformin, and AICAR) or genetic (overexpression of constitutively active AMPK) means inhibited FAS activity. Notably, AMPK activation increased threonine phosphorylation of FAS, and this effect was blocked by adenovirus encoding dominant negative AMPK. Finally, AMPK-dependent FAS phosphorylation was confirmed by (32)P incorporation into FAS in adipocytes. Taken together, our results strongly suggest that nicotine, via ONOO(-) activates AMPK, resulting in enhanced threonine phosphorylation and consequent inhibition of FAS.
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http://dx.doi.org/10.1074/jbc.M703701200DOI Listing
September 2007