Andreu Ferrer-Martínez

β3-adrenoceptor agonist prevents alterations of muscle diacylglycerol and adipose tissue phospholipids induced by a cafeteria diet

BackgroundInsulin resistance induced by a high fat diet has been associated with alterations in lipid content and composition in skeletal muscle and adipose tissue. Administration of β3-adrenoceptor (β3-AR) agonists was recently reported to prevent insulin resistance induced by a high fat diet, such as the cafeteria diet. The objective of the present study was to determine whether a selective β3-AR agonist (ZD7114) could prevent alterations of the lipid profile of skeletal muscle and adipose tissue lipids induced by a cafeteria diet.MethodsMale Sprague-Dawley rats fed a cafeteria diet were treated orally with either the β3-AR agonist ZD7114 (1 mg/kg per day) or the vehicle for 60 days. Rats fed a chow diet were used as a reference group. In addition to the determination of body weight and insulin plasma level, lipid content and fatty acid composition in gastronemius and in epididymal adipose tissue were measured by gas-liquid chromatography, at the end of the study.ResultsIn addition to higher body weights and plasma insulin concentrations, rats fed a cafeteria diet had greater triacylglycerol (TAG) and diacylglycerol (DAG) accumulation in skeletal muscle, contrary to animals fed a chow diet. As expected, ZD7114 treatment prevented the excessive weight gain and hyperinsulinemia induced by the cafeteria diet. Furthermore, in ZD7114 treated rats, intramyocellular DAG levels were lower and the proportion of polyunsaturated fatty acids, particularly arachidonic acid, in adipose tissue phospholipids was higher than in animals fed a cafeteria diet.ConclusionsThese results show that activation of the β3-AR was able to prevent lipid alterations in muscle and adipose tissue associated with insulin resistance induced by the cafeteria diet. These changes in intramyocellular DAG levels and adipose tissue PL composition may contribute to the improved insulin sensitivity associated with β3-AR activation.

Long-term osmotic regulation of amino acid transport systems in mammalian cells.

SummaryMammalian cells accumulate organic osmolytes, either to adapt to permanent osmotic changes or to mediate cell volume increase in cell cycle progression. Amino acids may serve as osmolytes in a great variety of cells. System A, a transport system for neutral amino acids, is induced after hypertonic shock by a mechanism which requires protein synthesis and gene transcription. Indirect evidence supports the view that system A activity increases due to the interaction of pre-existing A carriers with putative activating proteins. The intracellular accumulation of most neutral amino acids after hypertonic shock depends, exclusively, on the increase in system A activity. Long-term activation of system A is dependent on the integrity of cytoskeletal structures, but in a different way depending on whether cells are polarized or not.

Up-regulation of liver system A for neutral amino acid transport in euglycemic hyperinsulinemic rats.

To determine the role of insulin on the in vivo modulation of liver system A activity, we used the euglycemic hyperinsulinemic clamp coupled to the measurement of solute uptakes into plasma membrane vesicles partially purified from livers of hyperinsulinemic rats and their saline-infused controls. The clamp was performed in chronically catheterized rats, either in the fasted state, 24 h after surgery (Group I), or after 3 days of recovery (Group II). System A activity, measured as the MeAIB-inhibitable L-alanine uptake, was selectively induced by hyperinsulinemia, although the effect was much greater in Group II than in Group I rats (137% vs. 24% over the basal values, respectively). This might be explained by the higher basal levels found in those liver plasma membrane vesicles from Group I fasted animals. Hyperinsulinemia also decreased blood amino acids but to a similar extent in both experimental groups. This suggests that amino acid depletion by itself may not cause up-regulation of system A. Other transport activities involved in neutral amino acid transport (Systems ASC, N and L) were not modified by the clamp. The induction of system A cannot be explained by changes in the dissipation rate of the Na+ transmembrane gradient, because the differences between insulin- and saline-infused rats remained even when the electrochemical Na+ gradient was disrupted in the presence of monensin. Thus, hyperinsulinemia might induce an increase in the number of transporters inserted into the plasma membrane.

Coordinate induction of Na(+)-dependent transport systems and Na+,K(+)-ATPase in the liver of obese Zucker rats.

Solute uptake into liver plasma membrane vesicles from either lean or obese Zucker rats was monitored. D-Glucose and L-leucine uptakes at physiological concentrations of the substrate were not different in lean and obese Zucker rats. In agreement with a previous report (Ruiz et al. (1991) Biochem. J. 280, 367-372) L-alanine uptake was significantly enhanced in those preparations from obese animals. Na(+)-coupled uridine transport was markedly enhanced also in obese rats. The effect was due to an increase in Vmax (5.5 +/- 0.6 vs. 2.1 +/- 0.2 pmol/mg protein per 3 s, P < 0.01) without any significant change in Km (11.0 +/- 2.8 vs. 9.0 +/- 2.7 microM for obese and lean rats, respectively). Na+,K(+)-ATPase activity was also higher in liver plasma membrane vesicles from rat liver and it correlated with a higher amount of alpha 1-subunit protein in both, plasma membrane vesicles and homogenates from obese rat livers. In summary, in the hypertrophic liver of obese Zucker rats a coordinate induction of several Na(+)-dependent transport systems occurs and, in order to sustain the metabolic pressure associated with this adaptation, a significant induction of the Na+,K(+)-ATPase expression is also found. These data also provide new evidence for regulation of the recently characterized Na(+)-dependent nucleoside transporter.

Effect of sucrose and saturated-fat diets on mRNA levels of genes limiting muscle fatty acid and glucose supply in rats.

In this study, we examined whether the increased availability of lipids in blood resulting from two types of diet manipulation regulated metabolic gene expression in the skeletal muscle of rats. Feeding for 4 wk on an isocaloric-sucrose or a hypercaloric-fat diet increased plasma TAG in the fed condition by increments of 70 and 40%, respectively, and increased fasting insulinemia (approximately 3-fold) compared with a starch diet. The fat diet impaired glucose tolerance and caused obesity, whereas sucrose-fed rats maintained their normal weight. We analyzed the expression of genes that regulate the exogenous FA supply (LPL, FAT/CD36, FATP1), synthesis (ACC1), glucose (GLUT4, GLUT1, HK2, GRAT1, glycogen phosphorylase) or glycerol (glycerol kinase) provision, or substrate choice for oxidation (PDK4) in gastrocnemius and soleus muscles at the end of the glucose tolerance test. LPL, FAT/CD36, FATP1, PDK4, and GLUT4 mRNA as well as glycogen phosphorylase and glycerol kinase activity levels in both muscles were unchanged by the diets. Increased mRNA levels of GLUT1 (1.6- and 2.6-fold, respectively) and GFAT1 (about 1.7-fold) in gastrocnemius, and of ACC1 (about 1.5-fold) in soleus, were found in both the sucrose and fat groups. In the fat group, HK2 mRNA was also higher (1.8-fold) in the gastrocnemius. Both sucrose and saturated-fat diets prompted hyperinsulinemia and hyperlipemia in rats. These metabolic disturbances did not alter the expression of LPL, FAT/CD36, FATP1, PDK4, and GLUT4 genes or glycogen phosphorylase and glycerol kinase activity levels in either analyzed muscle. Instead, they were linked to the coordinated upregulation in gastrocnemius of genes that govern glucose uptake and the hexosamine pathway, namely, GLUT1 and GFAT1, which might contribute to insulin resistance.