Glucocorticoids use a positive liver element to repress fibrate-induced adipose transcription of the phosphoenolpyruvate carboxykinase gene

Retinoids upregulate phosphoenolpyruvate carboxykinase and glyceroneogenesis in human and rodent adipocytes

Glyceroneogenesis is an important metabolic pathway for fatty acid reesterification in adipose tissue, thereby reducing fatty acid release. Glyceroneogenesis and cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C), which is the key enzyme in this pathway, are both regulated by a series of hormones and nutrients, among which all-trans retinoic acid (all-trans RA) is a transcriptional inducer of the PEPCK-C gene (Pck1). All-trans RA binds to the retinoic acid receptor (RAR) and activates it, whereas its stereoisomer 9-cis retinoic acid (9-cis RA) is a ligand for the 9-cis RA receptor (RXR). Three RXR-binding elements [retinoic acid response element (RARE)1/PCK1, RARE2, and RARE3/PCK2] were previously located in the promoter of Pck1. Using 3T3-F442A adipocytes, we demonstrated that Pck1 expression was 10-fold more sensitive to 9-cis RA (EC(50): 10 nmol/L) than to all-trans RA. We then analyzed the respective involvement of RARE1/PCK1, RARE2, and RARE3/PCK2 in the response of Pck1 to 9-cis RA and all-trans RA in adipocytes. The response to 9-cis RA mainly involved the RARE1/PCK1 element, whereas RARE2 was mainly responsive to all-trans RA. In contrast, the full response to both RA isomers involved these 2 elements and included RARE3/PCK2 as well. Furthermore, 9-cis RA, but not all-trans RA, selectively induced PCK1 in ex-vivo-treated human adipose tissue explants, with a concomitant induction of glyceroneogenesis monitored by [1-(14)C]-pyruvate incorporation into neutral lipids. The concomitant 9-cis RA-induced reduction in fatty acid output indicates an important role for this RA stereoisomer in lipid homeostasis through stimulation of PEPCK-C and glyceroneogenesis in adipose tissue.

Proposed involvement of adipocyte glyceroneogenesis and phosphoenolpyruvate carboxykinase in the metabolic syndrome

Elevated concentration of plasma non-esterified fatty acids (NEFA) is now recognized as a key factor in the onset of insulin-resistance and type 2 diabetes mellitus. During fasting, circulating NEFAs arise from white adipose tissue (WAT) as a consequence of lipolysis from stored triacylglycerols. However, a significant part of these FAs (30-70%) is re-esterified within the adipocyte, so that a recycling occurs and net FA output is much less than << true >> lipolysis. Indeed, a balance between two antagonistic processes, lipolysis and FA re-esterification, controls the rate of net FA release from WAT. During fasting, re-esterification requires glyceroneogenesis defined as the de novo synthesis of glycerol-3-P from pyruvate, lactate or certain amino acids. The key enzyme in this process is the cytosolic isoform of phosphoenolpyruvate carboxykinase (PEPCK-C; EC 4.1.1.32). Recent advance has stressed the role of glyceroneogenesis and of PEPCK-C in FA release from WAT. Results indicate that glyceroneogenesis is indeed important to lipid homeostasis and that a disregulation in this pathway may have profound pathophysiological effects. The present review focuses on the regulation of glyceroneogenesis and of PEPCK-C gene expression and activity by FAs, retinoic acids, glucocorticoids and the hypolipidemic class of drugs, thiazolidinediones.

A single element in the phosphoenolpyruvate carboxykinase gene mediates thiazolidinedione action specifically in adipocytes

Phosphoenolpyruvate carboxykinase (PEPCK) is the key enzyme in glyceroneogenesis, an important metabolic pathway that functions to restrain the release of non-esterified fatty acids (NEFAs) from adipocytes. The antidiabetic drugs known as thiazolidinediones (TZDs) are thought to achieve some of their benefits by lowering elevated plasma NEFAs. Moreover, peroxisome proliferator activated receptor gamma (PPARgamma) mediates the antidiabetic effects of TZDs, though many TZD responses appear to occur via PPARgamma-independent pathways. PPARgamma is required for adipocyte PEPCK expression, hence PEPCK could be a major target gene for the antidiabetic actions of TZDs. Here we used tissue culture and transfection assays to confirm that the TZD, rosiglitazone, stimulates PEPCK gene transcription specifically in adipocytes. We made the novel observation that this effect was by far the most rapid and robust among several other genes expressed in adipocytes. Adipocytes were transfected with a PEPCK/chloramphenicol acetyltransferase chimeric gene, in which either of the two previously discovered PPARgamma/retinoid X receptor alpha response elements, PCK2 and gAF1/PCK1, had been inactivated by mutagenesis. We demonstrate that PCK2 alone is a bona fide thiazolidinedione response element. We show also that the regulation of PEPCK by PPARs is cell-specific and isotype-specific since rosiglitazone induces PEPCK gene expression selectively in adipocytes, and PPARalpha- and PPARbeta-specific activators are inefficient. Hence, TZDs could lower plasma NEFAs via PPARgamma and PEPCK by enhancing adipocyte glyceroneogenesis.

Peroxisome proliferator-activated receptor gamma and chicken ovalbumin upstream promoter transcription factor II negatively regulate the phosphoenolpyruvate carboxykinase promoter via a common element

A heterodimer of peroxisome proliferator-activated receptor gamma (PPARgamma) and retinoid X receptor (RXR) is required for adipocyte differentiation. The gene encoding cytosolic phosphoenolpyruvate carboxykinase (PEPCK) is a PPARgamma/RXR target gene in adipose tissue. Of the two PPARgamma response elements, gAF1/PCK1 and PCK2, only PCK2 is required for PEPCK expression and responsiveness to the PPARgamma agonist, rosiglitazone, in adipose tissue even though both elements bind PPARgamma/RXR in vitro. In contrast, gAF1/PCK1 is essential for glucocorticoid inhibition of PPARgamma-induced PEPCK gene expression in adipocytes. We report that chicken ovalbumin upstream promoter transcription factor II (COUP-TFII) is the predominant nuclear receptor bound to gAF1/PCK1 in preadipocytes. COUP-TFII declines during adipogenesis in reciprocal fashion to PPARgamma. In transiently transfected fibroblasts COUP-TFII acts at gAF1/PCK1 to inhibit PPARgamma/RXR activation via PCK2. In contrast COUP-TFs are transcriptional activators of PEPCK in hepatocytes. PPARgamma/RXR occupies gAF1/PCK1 in adipocytes, and mutation of gAF1/PCK1 enhances PEPCK promoter transactivation by PPARgamma/RXR in fibroblasts, suggesting that this element is also a negative PPARgamma response element. These results indicate that gAF1/PCK1 is a pleiotropic element through which COUP-TFII inhibits premature PEPCK expression, and perhaps adipogenesis in general, and PPARgamma/RXR uses this same element in adipocytes to participate in PEPCK modulation by glucocorticoids.

Isolation and characterization of the mouse cytosolic phosphoenolpyruvate carboxykinase (GTP) gene: evidence for tissue-specific hypersensitive sites

A 72 kilobase pair DNA fragment that contains the mouse phosphoenolpyruvate carboxykinase (PEPCK) gene locus, pck1, was isolated from a genomic bacterial artificial chromosome library. The region from approximately -5.5 to +6.6 kilobase pairs relative to the pck1 transcription start site was sequenced and exhibits a high degree of homology to the rat and human genes. Additionally, the chromatin structure of the PEPCK gene in mouse liver resembles that seen in rat. Backcross panel analysis of a microsatellite sequence confirms that the gene is located on chromosome 2. Hypersensitive site analysis was performed on nuclei isolated from the adipocyte cell line 3T3-F442A in the preadipose and adipose states. Several hypersensitive sites are present in the undifferentiated 3T3-F442A cells, before PEPCK mRNA is detected. The same sites are present after differentiation, however, the sensitivity of mHS 3 increases relative to the others. We conclude that the chromatin is open in 3T3-F442A cells and that factors are able to bind in the undifferentiated state but that something else is required for transcription.

Glucocorticoids repress induction by thiazolidinediones, fibrates, and fatty acids of phosphoenolpyruvate carboxykinase gene expression in adipocytes

Phosphoenolpyruvate carboxykinase (PEPCK) exerts a glyceroneogenic function in adipocytes in which transcription of its gene is increased by unsaturated fatty acids and fibrates. We used cultured rat adipose tissue fragments and 3T3-F442A adipocytes to show that the antidiabetic thiazolidinedione BRL 49653, a ligand and an activator of the gamma isoform of peroxisome proliferator activated receptors (PPARgamma), is a potent inducer of PEPCK mRNA. In 3T3-F442A adipocytes, the effect of BRL 49653 is rapid and concentration dependent, with a maximum reached at 1 microM and a half-maximum at 10-100 nM. PEPCK mRNA is similarly induced by the natural ligand of PPARgamma, the 15-deoxy-delta(12-14) prostaglandin J2. These observations strongly suggest that PPARgamma is a primary regulator of PEPCK gene expression in adipocytes. Dexamethasone at 10 nM repress induction of PEPCK mRNA by 1 microM BRL 49653, 0.32 mM oleate, or 1 mM clofibrate, in a cycloheximide-independent manner. The antiglucocorticoid RU 38486 prevents dexamethasone action, demonstrating involvement of the glucocorticoid receptor. Stable transfectants of 3T3-F442A adipocytes bearing -2100 to +69 base pairs of the PEPCK gene promoter fused to the chloramphenicol acetyltransferase (CAT) gene respond to 1 microM BRL 49653 or 1 mM clofibrate by a large increase in CAT activity, which is prevented by the simultaneous addition of 10 nM dexamethasone. Hence, in adipocytes, glucocorticoids act directly through the 5'-flanking region of the PEPCK gene to repress, in a dominant fashion, the stimulation of PEPCK gene transcription by thiazolidinediones and fibrates.

Transcriptional and posttranscriptional mechanisms of glucocorticoid-mediated repression of phosphoenolpyruvate carboxykinase gene expression in adipocytes

Glucocorticoids exert pleiotropic effects, among which negative regulation of transcription has been recognized as of crucial importance. While glucocorticoids induce phosphoenolpyruvate carboxykinase (PEPCK) gene expression in liver cells, it represses gene activity in adipose cells. We used the 3T3-F442A adipocytes to analyze the underlying mechanisms in these cells, the synthetic glucocorticoid dexamethasone exerts a dominant repression either on basal or on beta-agonist stimulation of PEPCK gene expression. To determine whether glucocorticoid action required protein synthesis, we employed cycloheximide, anisomycin, and puromycin, three different translation inhibitors. None of these affected induction by isoprenaline or repression by dexamethasone of isoprenaline stimulation. In contrast, dexamethasone inhibitory action on basal PEPCK mRNA was totally prevented by the three translation inhibitors. Time courses of glucocorticoid action on basal and on induction by beta-agonist were similar. Half-maximal effect of dexamethasone on isoprenaline-induced PEPCK mRNA was obtained at about 10 nM, a tenfold higher concentration than that observed for the reduction of basal mRNA. Using the transcription inhibitor DRB, we showed that dexamethasone did not alter mRNA half-life, while isoprenaline strongly stabilized mRNA. In a 3T3-F442A stable transfectant bearing -2,100 base pairs of the PEPCK promoter fused to the chloramphenicol acetyltransferase (CAT) gene, isoprenaline stimulated CAT activity, whereas dexamethasone reduced basal and isoprenaline-induced CAT expression. Hence, beta-agonists exert both transcriptional and posttranscriptional regulation, while glucocorticoid action is purely transcriptional. However, mechanisms of glucocorticoid repression of basal and of beta-agonist stimulation appear different.