Physiologic and Pharmacologic Factors Influencing GlyceroneogenicContribution to Triacylglyceride Glycerol Measured by Mass IsotopomerDistribution Analysis

Abstract

An imbalance between triacylglycerol synthesis and breakdown is necessary for the development of obesity. The direct precursor for triacylglycerol biosynthesis is ␣-glycerol phosphate, which can have glycolytic and glyceroneogenic origins. We present a technique for determining the relative glyceroneogenic contribution to triacylglyceride glycerol by labeling the glycerol moiety with 2 H 2 O. The number of hydrogen atoms (n) incorporated from H 2 O into C-H bonds reflects the metabolic source of ␣-glycerol phosphate and can be calculated by combinatorial analysis of the distribution of mass isotopomers in triacylglyceride glycerol. Three physiological settings with potential effects on glyceroneogenesis and glycolysis were studied in rodents. Adipose tissue acylglyceride glycerol in mice fed a low carbohydrate diet had significantly higher values of n than in mice fed a high carbohydrate diet, suggesting an increased contribution from glyceroneogenesis of from 17 to 50% on the low carbohydrate diet. Similarly, mice administered rosiglitazone had a significant relative increase in glyceroneogenesis (from 17 to 53%), indicated by an increase in adipose acylglyceride glycerol n. Fructose infusion in overnight fasted rats rapidly lowered plasma triacylglyceride glycerol n, reflecting a decreased contribution from glyceroneogenesis (from 66 to 34%) presumably because of increased glycolytic input. In conclusion, we demonstrate that the number of C-H atoms derived from cellular H 2 O in triacylglyceride glycerol is an informative indicator of ␣-glycerol phosphate origin and, ultimately, triacylglycerol metabolism. Under certain physiological conditions, glyceroneogenesis can be upregulated in adipose (e.g. low carbohydrate diet) or down-regulated in liver (e.g. fructose infusion). Additionally, stimulation of glyceroneogenesis by rosiglitazone in adipose tissue may be an important factor in the antilipolytic actions of thiazolidinediones.

The Transcriptional Coactivator Peroxisome Proliferator–Activated Receptor (PPAR)γ Coactivator-1α and the Nuclear Receptor PPARα Control the Expression of Glycerol Kinase and Metabolism Genes Independently of PPARγ Activation in Human White Adipocytes

“…Fasting glycerol output from WAT was not decreased during a rosiglitazone treatment as would be expected if Gyk activity was upregulated (31). Moreover, elegant in vivo experiments using heavy water showed that, under rosiglitazone treatment, GyK is unlikely to contribute to triglyceride synthesis in adipose tissue (33). The lack of induction in human fat cells is probably related to the absence of PGC-1␣ upregulation because this mechanism has been proposed as essential for the induction of GyK by rosiglitazone in 3T3-L1 adipocytes (20).…”

Triglyceride Synthesis in Epididymal Adipose Tissue

“…In addition, we observed a vigorous remodeling of the fatty acid distribution profile (6). Using 2 H 2 O, Hellerstein and colleagues (7,8) have demonstrated that rates of triglyceride turnover in adipose tissue are depot-specific and sensitive to pharmacological therapy.…”

“…Although it is clear that the liver can utilize all of these pathways depending on the nutritional/hormonal status, there is an uncertainty regarding the contribution of the various pathways to the formation of ␣-glycerol-3-phosphate in white adipose tissue in vivo. Although many investigators agree that glucose plays an important role in triglyceride synthesis in adipose tissue and that glycerol makes a minor (if any) contribution to ␣-glycerol-3-phosphate (because there is low glycerol kinase activity), the potential for converting pyruvate to ␣-glycerol-3-phosphate (referred to as glyceroneogenesis) is intriguing (7,9,12). For example, investigators have suggested that there should be a glyceroneogenic flux to facilitate fatty acid (re)esterification (13), and that hypothesis is supported by experiments in which ϳ90% of triglyceride glycerol was apparently derived from glyceroneogenesis (14).…”

“…Studies have used various designs to demonstrate the importance of glucose versus glyceroneogenesis to the formation of ␣-glycerol-3-phosphate (7,9,12). For example, Chen et al (7) have suggested that following the administration of 2 H 2 O, it is possible to quantify the contribution of glucose versus glyceroneogenesis to triglyceride glycerol in vivo; this requires that one measures the proportion of triglyceride glycerol containing one and two 2 H and then applies a combinatorial method to determine the number of exchangeable hydrogens (i.e.…”

Acute and selective regulation of glyceroneogenesis and cytosolic phosphoenolpyruvate carboxykinase in adipose tissue by thiazolidinediones in type 2 diabetes

“…We showed that glyceroneogenesis was the main contributor pathway for fatty acid reesterification in rat adipose tissue in both basal and thiazolidinedione-stimulated conditions [19]. This result was reinforced by elegant in vivo experiments from Chen et al [34], in which mass isotopomer analyses in mice confirmed the major contribution of glyceroneogenesis to thiazolidinedione-induced triacylglycerol synthesis, with a very minor role of glycerol phosphorylation. One important observation of our present work is that the administration of rosiglitazone to insulin-resistant rats induced an increase in glyceroneogenesis, which was accompanied by an improvement in dyslipidaemia, as shown by the large decrease in plasma triacylglycerol.…”