Mice deficient in phosphofructokinase-M have greatly decreased fat stores

Synthesis of triacylglycerol requires the glucose-derived glycerol component, and glucose uptake has been viewed as the rate-limiting step in glucose metabolism in adipocytes. Furthermore, adipose tissue contains all three isoforms of the glycolytic enzyme phosphofructokinase (PFK). We here report that mice deficient in the muscle isoform PFK-M have greatly reduced fat stores. Mice with disrupted activity of the PFK-M distal promoter were obtained from Lexicon Pharmaceuticals, developed from OmniBank OST#56064. Intra-abdominal fat was measured by magnetic resonance imaging of the methylene proton signal. Lipogenesis from labeled glucose was measured in isolated adipocytes. Lipolysis (glycerol and free fatty acid release) was measured in perifused adipocytes. Intra-abdominal fat in PFK-M-deficient female mice (5-10 months old) was 17 +/- 3% of that of wild-type littermates (n = 4; P < 0.02). Epididymal fat weight in 15 animals (7-9.5 months) was 34 +/- 4% of control littermate (P < 0.002), with 10-30% lower body weight. Basal and insulin-stimulated lipogenesis in PFK-M-deficient epididymal adipocytes was 40% of the rates in cells from heterozygous littermates (n = 3; P < 0.05). The rate of isoproterenol-stimulated lipolysis in wild-type adipocytes declined approximately 10% after 1 h and 50% after 2 h; in PFK-M-deficient cells it declined much more rapidly, 50% in 1 h and 90% in 2 h, and lipolytic oscillations appeared to be damped (n = 4). These results indicate an important role for PFK-M in adipose metabolism. This may be related to the ability of this isoform to generate glycolytic oscillations, because such oscillations may enhance the production of the triacylglycerol precursor alpha-glycerophosphate.

The CB1 antagonist rimonabant decreases insulin hypersecretion in rat pancreatic islets

Type 2 diabetes and obesity are characterized by elevated nocturnal circulating free fatty acids, elevated basal insulin secretion, and blunted glucose-stimulated insulin secretion (GSIS). The CB1 receptor antagonist, Rimonabant, has been shown to improve glucose tolerance and insulin sensitivity in vivo but its direct effect on islets has been unclear. Islets from lean littermates and obese Zucker (ZF) and Zucker Diabetic Fatty (ZDF) rats were incubated for 24 h in vitro and exposed to 11 mmol/l glucose and 0.3 mmol/l palmitate (GL) with or without Rimonabant. Insulin secretion was determined at basal (3 mmol/l) or stimulatory (15 mmol/l) glucose concentrations. As expected, basal secretion was significantly elevated in islets from obese or GL-treated lean rats whereas the fold increase in GSIS was diminished. Rimonabant decreased basal hypersecretion in islets from obese rats and GL-treated lean rats without decreasing the fold increase in GSIS. However, it decreased GSIS in islets from lean rats without affecting basal secretion. These findings indicate that Rimonabant has direct effects on islets to reduce insulin secretion when secretion is elevated above normal levels by diet or in obesity. In contrast, it appears to decrease stimulated secretion in islets from lean animals but not in obese or GL-exposed islets.

Glucose-dependent insulinotropic polypeptide enhances adipocyte development and glucose uptake in part through Akt activation

BACKGROUND & AIMS

In addition to its role as the primary mediator of the enteroinsular axis, glucose-dependent insulinotropic polypeptide (GIP) may play a critical role in the development of obesity. The purpose of these studies was to characterize the effects of GIP and its receptor (GIPR) in adipocyte development and signaling.

METHODS

Effects of GIP and GIPR on differentiated 3T3-L1 cells were analyzed using Western blot analysis, Oil-Red-O staining, cyclic adenosine monophosphate radioimmunoassay, immunofluorescence microscopy, and glucose uptake measurements.

RESULTS

To determine whether GIP and GIPR are important components in adipocyte development, the expression profile of GIPR during differentiation was examined. GIPR protein expression was enhanced during the differentiation process, and coincubation with its ligand GIP augmented the expression of aP2, a fat cell marker. Conversely, the suppression of GIPR expression by a specific short hairpin RNA attenuated Oil-Red-O staining and aP2 expression, suggesting that the GIPR may play a critical role in adipocyte development. To investigate specific signaling components that may mediate the effects of GIP, we analyzed Akt, glucose transporter-4, and glucose uptake, all of which are modulated by insulin in fat cells. Like insulin, GIP induced the activation of Akt in a concentration-dependent manner, promoted membrane glucose transporter-4 accumulation, and enhanced [(3)H]-2-deoxyglucose uptake.

CONCLUSIONS

These studies provide further evidence for an important physiologic role for GIP in lipid homeostasis and possibly in the pathogenesis of obesity. Furthermore, our data indicate that the GIPR might represent a suitable target for the treatment of obesity.

Tissue-dependent loss of phosphofructokinase-M in mice with interrupted activity of the distal promoter: impairment in insulin secretion

Phosphofructokinase is a key enzyme of glycolysis that exists as homo- and heterotetramers of three subunit isoforms: muscle, liver, and C type. Mice with a disrupting tag inserted near the distal promoter of the phosphofructokinase-M gene showed tissue-dependent differences in loss of that isoform: 99% in brain and 95-98% in islets, but only 50-75% in skeletal muscle and little if any loss in heart. This correlated with the continued presence of proximal transcripts specifically in muscle tissues. These data strongly support the proposed two-promoter system of the gene, with ubiquitous use of the distal promoter and additional use of the proximal promoter selectively in muscle. Interestingly, the mice were glucose intolerant and had somewhat elevated fasting and fed blood glucose levels; however, they did not have an abnormal insulin tolerance test, consistent with the less pronounced loss of phosphofructokinase-M in muscle. Isolated perifused islets showed about 50% decreased glucose-stimulated insulin secretion and reduced amplitude and regularity of secretory oscillations. Oscillations in cytoplasmic free Ca(2+) and the rise in the ATP/ADP ratio appeared normal. Secretory oscillations still occurred in the presence of diazoxide and high KCl, indicating an oscillation mechanism not requiring dynamic Ca(2+) changes. The results suggest the importance of phosphofructokinase-M for insulin secretion, although glucokinase is the overall rate-limiting glucose sensor. Whether the Ca(2+) oscillations and residual insulin oscillations in this mouse model are due to the residual 2-5% phosphofructokinase-M or to other phosphofructokinase isoforms present in islets or involve another metabolic oscillator remains to be determined.

Glucose-dependent insulin modulation of oscillatory lipolysis in perifused rat adipocytes

OBJECTIVE

We showed glucose-dependent lipolytic oscillations in adipocytes that are modulated by free fatty acids (FFAs). We hypothesized that the oscillations are driven by oscillatory glucose metabolism that leads to oscillatory formation of alpha-glycerophosphate (alpha-GP), oscillatory removal of long-chain coenzyme A (LC-CoA) by alpha-GP to form triglycerides, and oscillatory relief of LC-CoA inhibition of triglyceride lipases. This study examined the effect of insulin on this hypothesis.

RESEARCH METHODS AND PROCEDURES

Samples were collected every minute from perifused rat adipocytes during the basal state followed by insulin (+/-glucose) or isoproterenol (+/-insulin; n = 4 each).

RESULTS

Insulin caused a significant increase in glycerol release (18%), with a concomitant significant decrease in FFA release (38%). Without glucose, insulin had no effect on glycerol release while still decreasing FFA release (35%). Insulin (5 microU/mL) attenuated the response of lipolysis to isoproterenol (approximately 3-fold increase with isoproterenol vs. 2-fold increase with insulin + isoproterenol). However, 1 mU/mL insulin amplified the lipolytic response ( approximately 5-fold increase in glycerol release with insulin + isoproterenol), with a concomitant increase in FFA reesterification (no increase in FFA release compared with isoproterenol alone).

DISCUSSION

We interpret these results to be due to insulin's ability to increase glucose uptake and conversion to alpha-GP, thus removing LC-CoA inhibition of triglyceride lipases. While the physiological importance of lipolytic oscillations remains to be determined, we hypothesize that such an oscillation may play an important role in the delivery of FFAs to the liver, beta cells, and other tissues.

Glucose-dependent insulinotropic polypeptide modulates adipocyte lipolysis and reesterification

OBJECTIVE

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin released from intestinal K-cells during the postprandial period. Previous studies have suggested that GIP may play an etiologic role in obesity; thus, the GIP receptor may represent a target for anti-obesity drugs. The present studies were conducted to elucidate mechanisms by which GIP might promote obesity by examining the effect of GIP on both glycerol release (indicative of lipolysis) and free fatty acid (FFA) release (indicative of both lipolysis and reesterification), as well as the ability of a GIP-specific receptor antagonist (ANTGIP) to attenuate these effects.

RESEARCH METHODS AND PROCEDURES

Isolated rat adipocytes were perifused on a column with 10 nM GIP alone or in combination with 10 microU/mL insulin, 1 microM isoproterenol, or 1 microM ANTGIP. Samples were collected every minute and assayed for FFA, glycerol, and lactate.

RESULTS

GIP significantly increased FFA reesterification (decreased FFA release by 25%), stimulated lipolysis (increased glycerol release by 22%), and attenuated the lipolytic response to isoproterenol by 43%. These properties were similar to those of insulin in vitro, suggesting that GIP possesses insulin-like lipogenic effects on adipocytes. Finally, ANTGIP reversed the effects of GIP on both basal and stimulated adipocyte metabolism.

DISCUSSION

These studies provide further evidence for an important physiological role for GIP in lipid homeostasis and possibly in the pathogenesis of obesity. They also suggest that the GIP receptor may represent an excellent target for the prevention and treatment of obesity and obesity-related type 2 diabetes.

Acyl coenzyme a synthetase regulation: putative role in long-chain acyl coenzyme a partitioning

OBJECTIVE

Long-chain acyl coenzyme A synthetase (ACSL) converts free fatty acids (FFAs) into their metabolizable long-chain acyl coenzyme A (LC-CoA) derivatives that are essential for FFA conversion to CO(2), triglycerides, or complex lipids. ACSL-1 is highly expressed in adipose tissue with broad substrate specificity. We tested the hypothesis that ACSL localization, and resulting local generation of LC-CoA, regulates FFA partitioning.

RESEARCH METHODS AND PROCEDURES

These studies used cell fractionation of rat adipocytes to measure ACSL activity and mass and compared cells from young, mature, fed, fasted, and diabetic rats. Functional studies included measurement of FFA oxidation, complex lipid synthesis, and LC-CoA levels.

RESULTS

High ACSL specific activity was expressed in the mitochondria/nuclei (M/N), high-density microsomes (HDM), low-density microsomes (LDM), and plasma membrane (PM) fractions. We show here that, during fasting, total FFA oxidation increased, and, although total ACSL activity decreased, a greater percentage of activity (43 +/- 1.5%) was associated with the M/N fraction than in the fed state (23 +/- 0.3%). In the fed state, more ACSL activity (34 +/- 0.5%) was associated with the HDM than in the fasted state (25 +/- 0.9%), concurrent with increased triglyceride formation from FFA. Insulin increased LC-CoA and ACSL activity associated with the PM. The changes in ACSL activity in response to insulin were associated with only minor changes in mass as determined by Western blotting.

DISCUSSION

It is hypothesized that ACSL plays an important role in targeting FFA to specific metabolic pathways or acylation sites in the cell, thus acting as an important control mechanism in fuel partitioning. Localization of ACSL at the PM may serve to decrease FFA efflux and trap FFA within the cell as LC-CoA.

Free fatty acid regulation of glucose-dependent intrinsic oscillatory lipolysis in perifused isolated rat adipocytes

Free fatty acids (FFAs) and glycerol oscillate in plasma. This study examined intrinsic lipolytic oscillations within adipocytes. Rat adipocytes were perifused with Krebs-Ringer bicarbonate buffer: 1) +/- 2 mmol/l glucose; 2) +1 micromol/l isoproterenol +/- 2 mmol/l glucose; 3) + increasing oleate; and 4) + increasing percent BSA. At 2 mmol/l glucose, there were 9 +/- 1 glycerol, FFAs, and lactate pulses per hour with a pulse duration of 5 +/- 1 min. Lipolytic stimulation caused a 50-80% increase in the amplitude of lipolytic oscillations. Removal of glucose caused a 40-70% decrease in the amplitude of lipolytic oscillations and disturbed the pulsatility. Exogenous FFAs suppressed lipolysis and oscillatory amplitude, possibly because of increased cytosolic long-chain coenzyme A (LC-CoA). Increasing percent BSA increased stimulated lipolysis and oscillatory amplitude, possibly because of decreased intracellular LC-CoA. These data show, for the first time, intrinsic lipolytic oscillations, which are glucose dependent and modulated by FFAs. We hypothesize that lipolytic oscillations are driven by oscillatory glucose metabolism, which leads to oscillatory relief of LC-CoA inhibition of triglyceride lipase(s). The results contribute to the understanding of physiological and biochemical regulators of lipolysis, such as glucose and FFAs. Lipolytic oscillations may be beneficial in the delivery of FFAs to liver, pancreas, and other tissues.