Methionine restriction effects on 11b-HSD1 activity and lipogenic/lipolytic balance in F344 rat adipose tissue

Methionine restriction (MR) limits age-related adiposity in Fischer 344 (F344) rats. To assess the mechanism of adiposity resistance, the effect of MR on adipose tissue (AT) 11b-hydroxysteroid dehydrogenase-1 (11bHSD1) was examined. MR induced 11b-HSD1 activity in all ATs, correlating with increased tissue corticosterone. However, an inverse relationship between 11b-HSD1 activity and adipocyte size was observed. Because dietary restriction controls lipogenic and lipolytic rates, MR’s effects on lipogenic and lipolytic enzymes were evaluated. MR increased adipose triglyceride lipase and acetyl-coenzyme A carboxylase (ACC) protein levels but induced ACC phosphorylation at serine residues that render the enzyme inactive, suggesting alterations of basal lipolysis and lipogenesis. In contrast, no changes in basal or phosphorylated hormone-sensitive lipase levels were observed. ACCphosphorylated sites were specific for AMP-activated protein kinase (AMPK); therefore, AMPK activation was evaluated. Significant differences in AMPKa protein, phosphorylation, and activity levels were observed only in retroperitoneal fat from MR rats. No differences in protein kinase A phosphorylation and intracellular cAMP levels were detected. In vitro studies revealed increased lipid degradation and a trend toward increased lipid synthesis, suggesting the presence of a futile cycle. In conclusion, MR disrupts the lipogenic/lipolytic balance, contributing importantly to adiposity resistance in F344 rats.—Perrone, C. E., D. A. L. Mattocks, G. Hristopoulos, J. D. Plummer, R. A. Krajcik, and N. Orentreich. Methionine restriction effects on 11b-HSD1 activity and lipogenic/lipolytic balance in F344 rat adipose tissue. J. Lipid Res. 2008. 49: 12–23. Supplementary key words adiposity & glucocorticoid metabolism & signal transduction pathways & 11b-hydroxysteroid dehydrogenase-1 Dietary restriction of the essential amino acid methionine increases longevity in Fischer 344 (F344) rats and BaLB/cl3C57BL/6J F1 mice (1, 2). Like other dietary regimens leading to increased lifespan (3, 4), methionine restriction (MR) imposed early in life reduces body weight gain rates (5). Also, MR limits age-related increases in adipose tissue (AT) mass, as indicated by reduced serum leptin and increased adiponectin levels (5), which are tightly correlated with adiposity (6, 7). Moreover, no increase in AT weight is observed in rats fed an energy-dense MR diet (1), suggesting that MR rats are resistant to obesity. AT mass is tightly modulated by diet, energy expenditure, hormones, and tissue-specific regulators (reviewed in Refs. 8–10). Among the hormones involved in adiposity are glucocorticoids, which induce the transcription of the CCAAT enhancer binding proteins a, b, and y, leading to increased expression of the peroxisome proliferatoractivated receptor g (PPARg), an important transcription factor involved in adipogenesis and lipogenesis (11). Glucocorticoid-mediated effects in target tissues depend on both circulating and tissue concentrations of active hormone (12), which are regulated by the 11bhydroxysteroid dehydrogenase (11b-HSD) enzymes (13, 14). 11b-HSD1 is a reductase involved in the conversion of inactive glucocorticoids (cortisone and dehydrocorticosterone in humans and rodents, respectively) to their respective active forms (cortisol and corticosterone); 11bHSD2 exerts the opposite effect (13, 14). Inactivation of 11b-HSD2, leading to increased active serum glucocorticoid levels (as in Cushing’s syndrome), or overexpression of 11-bHSD1 in AT results in obesity (13, 15–17) and implicates glucocorticoids in the onset of central obesity and the metabolic syndrome (18, 19). Adiposity is a complex process involving preadipocyte proliferation and differentiation as well as adipocyte hypertrophy. Moreover, adipocyte hypertrophy is determined by the balance between lipogenic and lipolytic enzymes. Insulin promotes the expression of lipogenic enzymes by activating sterol-regulatory element binding Manuscript received 23 April 2007 and in revised form 27 August 2007 and in re-revised form 25 September 2007. Published, JLR Papers in Press, October 1, 2007. DOI 10.1194/jlr.M700194-JLR200 Abbreviations: ACC, acetyl-coenzyme A carboxylase; AMPK, AMPactivated protein kinase; AT, adipose tissue; ATGL, adipose triglyceride lipase; CF, control fed; F344, Fischer 344; 11b-HSD, 11b-hydroxysteroid dehydrogenase; HSL, hormone-sensitive lipase; IGF-1, insulin-like growth factor-1; MR, methionine restriction; PKA, protein kinase A; PPARg, peroxisome proliferator-activated receptor g; SAMS peptide, substrate for AMPK-activated protein kinase; Ser, serine; SREBP-1c, sterol-regulatory element binding protein-1c; Thr172, threonine 172. 1 To whom correspondence should be addressed. e-mail: [email protected] Copyright D 2008 by the American Society for Biochemistry and Molecular Biology, Inc. This article is available online at http://www.jlr.org 12 Journal of Lipid Research Volume 49, 2008 by gest, on O cber 9, 2017 w w w .j.org D ow nladed fom protein-1c (SREBP-1c), inhibits the expression of lipolytic enzymes (20), and prevents lipolysis by inhibiting adenylate cyclase (reviewed in Ref. 21). In contrast to insulin, glucagon and catecholamines activate the as subunit of G-protein, which stimulates adenylate cyclase activity and cAMP synthesis (reviewed in Ref. 22). cAMP activates protein kinase A (PKA), which phosphorylates hormonesensitive lipase (HSL) and perilipin, allowing their association to the lipid vacuole and the catalysis of triacylglycerides into diacylglycerides (reviewed in Ref. 23). Although HSL has been considered the key lipolytic enzyme, HSL knockout experiments revealed the presence of another lipolytic enzyme involved in the breakdown of triacylglycerides, adipose triglyceride lipase (ATGL) (24). Unlike HSL, ATGL expression is altered by nutritional regulation (20), and its inactivation increases AT mass and renders mice incapable of cold adaptation, suggesting the importance of this enzyme in the maintenance of energy balance (25). Another mechanism controlling the lipogenic/lipolytic balance involves the allosteric regulation of acetyl-coenzyme A carboxylase (ACC). The ACC-1 isoform located in the cytosol synthesizes malonyl-CoA, which is required for fatty acid synthesis (26, 27). Malonyl-CoA produced by the ACC-2 isoform, which is located on the outer mitochondria membrane, inhibits fatty acid oxidation through inhibition of the mitochondrial carnitine palmitoyl transferase-1 (26, 27). Thus, downregulation of ACC protein levels or activity, either by allosteric ligands or by AMP-activated protein kinase (AMPK)-mediated phosphorylation, shifts lipogenesis to fatty acid oxidation (26, 27). Although glucocorticoids have been associated extensively with increased obesity, they can also play a role in lipolysis. Administration of glucocorticoids to adult rats was reported to reduce FAS and ACC activity in AT (28). Exposure of AT to the synthetic glucocorticoid dexamethasone amplifies cAMP responses (29) and induces the phosphorylation and activation of HSL (30, 31). Moreover, ATGL activity can be induced by dexamethasone through a cAMP-independent mechanism in 3T3-L1 cells (32). Thus, glucocorticoids could potentially change AT mass by regulating signaling mechanisms that control lipogenic and lipolytic enzymes. The objective of this study was to assess the mechanism(s) involved in MR-mediated adiposity resistance. Specifically, the correlation between 11b-HSD1 activity, glucocorticoid levels, and adipocyte size was evaluated. Furthermore, the effects of MR on lipogenic and lipolytic enzymes and on the activation of regulatory proteins that control the lipogenic/lipolytic balance were examined.