Garry D. Tan

The effects of rosiglitazone on fatty acid and triglyceride metabolism in type 2 diabetes.

Aims/hypothesisWe investigated the effects of rosiglitazone on NEFA and triglyceride metabolism in type 2 diabetes.MethodsIn a double-blind, placebo-controlled, cross-over study of rosiglitazone in diet-treated type 2 diabetic subjects, we measured arteriovenous differences and tissue blood flow in forearm muscle and subcutaneous abdominal adipose tissue, used stable isotope techniques, and analysed gene expression. Responses to a mixed meal containing [1,1,1-13C]tripalmitin were assessed.ResultsRosiglitazone induced insulin sensitisation without altering fasting NEFA concentrations (−6.6%, p=0.16). Postprandial NEFA concentrations were lowered by rosiglitazone compared with placebo (−21%, p=0.04). Adipose tissue NEFA release was not decreased in the fasting state by rosiglitazone treatment (+24%, p=0.17) and was associated with an increased fasting hormone-sensitive lipase rate of action (+118%, p=0.01). Postprandial triglyceride concentrations were decreased by rosiglitazone treatment (−26%, p<0.01) despite unchanged fasting concentrations. Rosiglitazone did not change concentrations of triglyceride-rich lipoprotein remnants. Adipose tissue blood flow increased with rosiglitazone (+32%, p=0.03). Postprandial triglyceride [13C]palmitic acid concentrations were unchanged, whilst NEFA [13C]palmitic acid concentrations were decreased (p=0.04). In muscle, hexokinase II mRNA expression was increased by rosiglitazone (+166%, p=0.001) whilst the expression of genes involved in insulin signalling was unchanged. Adipose tissue expression of FABP4, LPL and FAT/CD36 was increased.Conclusions/interpretationRosiglitazone decreases postprandial NEFA and triglyceride concentrations. This may represent decreased spillover of NEFAs from adipose tissue depots. Decreased delivery of NEFAs to the liver may lead to lowered postprandial triglyceride concentrations. Upregulation of hexokinase II expression in muscle may contribute to insulin sensitisation by rosiglitazone.

The in vivo effects of the Pro12Ala PPARγ2 polymorphism on adipose tissue NEFA metabolism: the first use of the Oxford Biobank

Aims/hypothesisTo investigate the phenotypic effects of common polymorphisms on adipose tissue metabolism and cardiovascular risk factors, we set out to establish a biobank with the unique feature of allowing a prospective recruit-by-genotype approach. The first use of this biobank investigates the effects of the peroxisome proliferator-activated receptor (PPAR) Pro12Ala polymorphism on integrative tissue-specific physiology. We hypothesised that Ala12 allele carriers demonstrate greater adipose tissue metabolic flexibility and insulin sensitivity.Materials and methodsFrom a comprehensive population register, subjects were recruited into a biobank, which was genotyped for the Pro12Ala polymorphism. Twelve healthy male Ala12 carriers and 12 matched Pro12 homozygotes underwent detailed physiological phenotyping using stable isotope techniques, and measurements of blood flow and arteriovenous differences in adipose tissue and muscle in response to a mixed meal containing [1,1,1-13C]tripalmitin.ResultsOf 6,148 invited subjects, 1,072 were suitable for inclusion in the biobank. Among Pro12 homozygotes, insulin sensitivity correlated with HDL-cholesterol concentrations, and inversely correlated with blood pressure, apolipoprotein B, triglyceride and total cholesterol concentrations. Ala12 carriers showed no such correlations. In the meal study, Ala12 carriers had lower plasma NEFA concentrations, higher adipose tissue and muscle blood flow, and greater insulin-mediated postprandial hormone-sensitive lipase suppression along with greater insulin sensitivity than Pro12 homozygotes.Conclusions/interpretationThis study shows that a recruit-by-genotype approach is feasible and describes the biobank’s first application, providing tissue-specific physiological findings consistent with the epidemiological observation that the PPAR Ala12 allele protects against the development of type 2 diabetes.

RANTES release by human adipose tissue in vivo and evidence for depot-specific differences

Obesity is associated with elevated inflammatory signals from various adipose tissue depots. This study aimed to evaluate release of regulated on activation, normal T cell expressed and secreted (RANTES) by human adipose tissue in vivo and ex vivo, in reference to monocyte chemoattractant protein-1 (MCP-1) and interleukin-6 (IL-6) release. Arteriovenous differences of RANTES, MCP-1, and IL-6 were studied in vivo across the abdominal subcutaneous adipose tissue in healthy Caucasian subjects with a wide range of adiposity. Systemic levels and ex vivo RANTES release were studied in abdominal subcutaneous, gastric fat pad, and omental adipose tissue from morbidly obese bariatric surgery patients and in thoracic subcutaneous and epicardial adipose tissue from cardiac surgery patients without coronary artery disease. Arteriovenous studies confirmed in vivo RANTES and IL-6 release in adipose tissue of lean and obese subjects and release of MCP-1 in obesity. However, in vivo release of MCP-1 and RANTES, but not IL-6, was lower than circulating levels. Ex vivo release of RANTES was greater from the gastric fat pad compared with omental (P = 0.01) and subcutaneous (P = 0.001) tissue. Epicardial adipose tissue released less RANTES than thoracic subcutaneous adipose tissue in lean (P = 0.04) but not obese subjects. Indexes of obesity correlated with epicardial RANTES but not with systemic RANTES or its release from other depots. In conclusion, RANTES is released by human subcutaneous adipose tissue in vivo and in varying amounts by other depots ex vivo. While it appears unlikely that the adipose organ contributes significantly to circulating levels, local implications of this chemokine deserve further investigation.

A "futile cycle" induced by thiazolidinediones in human adipose tissue?

To the editor: Guan et al. recently showed that glycerol kinase is induced by thiazolidinediones (TZDs) in isolated human adipocytes and in rodents1, resulting in a ‘futile metabolic cycle’ involving glycerol reutilization, thereby reducing nonesterified fatty acid (NEFA) output from adipose tissue. This was proposed as a potential explanation for insulin sensitization by thiazolidinediones. Here, we present data challenging this hypothesis in humans. We wished to test whether TZD-induced insulin sensitization in humans is associated with glycerol kinase induction. We therefore assessed glycerol output from subcutaneous abdominal adipose tissue in vivo and adipose tissue mRNA expression in 24 diettreated type 2 diabetic patients (median age 53 years; interquartile range (IQR) 45–56) who were given rosiglitazone 8 mg/d for 3 months in a randomized, double-blind, placebo-controlled crossover study. Informed written consent was obtained from all subjects. Arteriovenous differences in glycerol concentrations were measured across subcutaneous abdominal adipose tissue by cannulation of an arterialized hand vein and the superficial epigastric vein. It is well established that this venous drainage represents the outflow from adipose tissue without significant contamination from other tissues2. Using glycerol arteriovenous differences and adipose tissue blood flow measured by 133Xe washout, glycerol output from adipose tissue was quantified as previously detailed2. This technique enables the quantification of glycerol derived from adipocytes, as distinguished from glycerol from intravascular lipolysis by lipoprotein lipase2. Fasting plasma NEFAs, insulin and glucose concentrations were also measured. From adipose tissue biopsies, changes in glycerol kinase and FABP4 mRNA expression in response to rosiglitazone treatment were quantified by real-time PCR and normalized for cyclophilin, a housekeeping gene. Change in FABP4 mRNA was used as a positive control for peroxisome proliferator-activated receptor-γ induction during rosiglitazone treatment. Rosiglitazone reduced insulin concentrations by 30% (P < 0.001) and glucose by 14% (P < 0.00001). Fasting plasma NEFAs (–12%; P = 0.16) and glycerol (+15%, P = 0.09; Fig. 1a) concentrations were not significantly altered. Fasting glycerol output from adipose tissue showed a nonsignificant increase of 30% (P = 0.06) (Fig.1b). Glycerol kinase mRNA expression was unchanged by rosiglitazone (from 0.85 (IQR 0.64–1.17) to 0.85 (IQR 0.47–1.12); P = 0.54; Fig. 1c), whereas FABP4 mRNA was increased by 66% (from 880 (IQR 760–1,370) to 1,460 (IQR 1,110–2,020), P < 0.0001; Fig. 1d). The level of glycerol kinase

Transcriptional Control of Human Adipose Tissue Blood Flow

Adipose tissue is highly vascularized and expresses several genes involved in vasodilatory and vasoconstrictive regulation. We took a transcriptional approach to study the relationships between adipose tissue blood flow (ATBF) and genes involved in vasoactive processes. As ATBF is impaired in obesity, we tested whether body weight interfered with the transcriptional regulation of ATBF. The mRNA content (real‐time PCR) of 26 genes was quantified in subcutaneous adipose tissue biopsies from 28 healthy men with a wide range of BMI. ATBF was measured by 133Xe washout. None of the transcripts was related to fasting ATBF (ATBFF). However, the expression levels of two transcripts involved in vasodilation (natriuretic peptide receptor A/guanylate cyclase A (NPRA) and endothelial nitric oxide synthase (eNOS)) were positively associated with postprandial ATBF (r = 0.53 and r = 0.55, P < 0.01, respectively). Although BMI was negatively related to the mRNA content of NPRA and eNOS (r = −0.78 and r = −0.63, P < 0.01, respectively), the strong associations found between postprandial ATBF and the two transcripts were not affected by obesity. Several genes were subject to coordinated regulation of expression. This study demonstrates for the first time that ATBF responsiveness to nutrient intake is related to the transcription of two genes expressed in adipose tissue and directly involved in vasodilatory actions (eNOS and NPRA), suggesting that part of the regulation of ATBF is at a transcriptional level. Interestingly, these associations were not secondary to changes in BMI. We also found that certain genes involved in the regulation of ATBF are subject to coordinate regulation of expression suggesting physiological autoregulation.

Rosiglitazone decreases postprandial production of acylation stimulating protein in type 2 diabetics

BackgroundWe evaluated plasma ASP and its precursor C3 in type 2 diabetic men with/without rosiglitazone (ROSI) treatment compared to healthy non-obese men. We tested (1) whether plasma ASP or C3 are altered postprandially in subcutaneous adipose tissue or forearm muscle effluent assessed by arteriovenous (A-V) differences in healthy lean men and older obese diabetic men and (2) whether treatment with ROSI changes the arteriovenous gradient of ASP and/or C3.MethodsIn this ongoing placebo-controlled, crossover, double-blinded study, AV differences following a mixed meal were measured in diabetic men (n = 6) as compared to healthy men (n = 9).ResultsPostprandial arterial and adipose venous TG and venous NEFA were increased in diabetics vs. controls (p < 0.05–0.0001). ROSI treatment decreased postprandial arterial TG (p < 0.001), adipose venous NEFA (p < 0.005), reduced postprandial glucose (p < 0.0001) and insulin concentrations (p < 0.006). In healthy men, there was no change in postprandial C3, but an increase in adipose venous ASP vs. arterial ASP (p < 0.02), suggesting ASP production, with no change in forearm muscle. In older, obese diabetic subjects, arterial C3 was greater than in controls (p < 0.001). Arterial C3 was greater than venous C3 (p < 0.05), an effect that was lost with ROSI treatment. In diabetics, postprandial venous ASP was greater than arterial (p < 0.05), indicating ASP production, an effect that was lost with ROSI treatment (p < 0.01).ConclusionIncreased postprandial venous production of ASP is specific for adipose tissue (absent in forearm muscle). Increased postprandial C3 and ASP in diabetic subjects is consistent with an ASP resistant state, this state is partially normalized by treatment with ROSI.

Fatty Acid Metabolism in Patients with PPARγ Mutations

CONTEXT PPARG mutations may cause insulin resistance and dyslipidemia, but little is known about the mechanisms of the abnormalities of lipid metabolism. OBJECTIVE We hypothesized that in PPARG mutations, abnormal adipose tissue triglyceride storage causes insulin resistance. DESIGN, PATIENTS, AND MAIN OUTCOME MEASURES Whole-body and adipose tissue-specific metabolic phenotyping through arteriovenous blood sampling was made before and after a mixed meal including 13C-palmitic acid. Studies were performed in a 32-yr-old male with partial lipodystrophy and type 2 diabetes, heterozygous for the PPARG P467L mutation and in an apparently phenotypically normal 32-yr-old male heterozygous for the PPARG n.AAA553T mutation. Comparator groups were age- and sex-matched healthy participants (n=10) and type 2 diabetes sex-matched participants (n=6). RESULTS The P467L patient had elevated unmodulated fasting and postprandial plasma nonesterified fatty acid (NEFA) concentrations, despite a low adipose tissue NEFA output. Instead, NEFA appeared to originate directly from triglyceride-rich lipoproteins: 13C-palmitic acid accumulated rapidly in the NEFA fraction, as a sign of impaired fatty acid trapping in tissues. In contrast to the Pparg haploinsufficient mouse, the patient with n.AAA553T mutation did not exhibit paradoxically insulin sensitive and showed a mostly normal metabolic pattern. CONCLUSIONS The lipodystrophic PPARG P467L phenotype include excessive and uncontrolled generation of NEFA directly from triglyceride-rich lipoproteins, explaining high systemic NEFA concentrations, whereas the human PPARG haploinsufficiency is metabolically almost normal.

Removal of triacylglycerols from chylomicrons and VLDL by capillary beds: the basis of lipoprotein remnant formation

The triacylglycerol content of chylomicrons and VLDL (very-low-density lipoprotein) compete for the same lipolytic pathway in the capillary beds. Although chylomicron triacylglycerols appear to be the favoured substrate for lipoprotein lipase, VLDL particles compete in numbers. Methods to quantify the specific triacylglycerol removal from VLDL and chylomicrons may involve endogenous labelling of the triacylglycerol substrate with stable isotopes in combination with arteriovenous blood sampling in humans. Arteriovenous quantification of remnant lipoproteins suggests that adipose tissue with its high lipoprotein lipase activity is a principal site for generation of remnant lipoproteins. Under circumstances of reduced efficiency in the removal of triacylglycerols from lipoproteins, there is accumulation of remnant lipoproteins, which are potentially atherogenic.

Insulin sensitisation affects lipoprotein lipase transport in type 2 diabetes: role of adipose tissue and skeletal muscle in response to rosiglitazone

Aims/hypothesisLipoprotein lipase (LPL) is produced by adipose tissue and skeletal muscle, but acts on plasma lipoproteins after being transported to endothelial binding sites. Insulin resistance is associated with decreased plasma LPL mass. We investigated the effects of insulin sensitisation on tissue-specific LPL expression and transport in patients with type 2 diabetes.Materials and methodsArterio-venous gradients of plasma LPL activity and mass across adipose tissue and skeletal muscle were measured in 16 type 2 diabetic patients in a double-blind, placebo-controlled, cross-over randomised trial of rosiglitazone. In vivo LPL rate of action was assessed by tissue-specific arterio-venous triglyceride concentration gradients. LPL mRNA was quantified in adipose tissue and skeletal muscle biopsies.ResultsAdipose tissue released large quantities of inactive LPL (p<0.001); skeletal muscle released small amounts of active LPL (p<0.01). Rosiglitazone increased adipose tissue release of LPL mass (+35%, p=0.04) and decreased the release of active LPL from skeletal muscle (−57%, p=0.03). Rosiglitazone increased adipose tissue and skeletal muscle LPL mRNA, but did not affect adipose tissue LPL rate of action or activity. Adipose tissue release of LPL mass correlated with systemic LPL mass concentrations (r=0.47, p=0.007), suggesting that the rate of adipose tissue release of LPL mass is a major determinant of systemic LPL mass concentrations.Conclusions/interpretationLPL transport from adipose tissue and skeletal muscle are regulated differently. In adipose tissue, rosiglitazone increases LPL mRNA abundance and LPL transport rate and possibly increases endothelial binding sites for LPL, but affects neither tissue LPL activity nor LPL rate of action.