Selective modification of pyruvate dehydrogenase kinase isoform expression in rat pancreatic islets elicited by starvation and activation of peroxisome proliferator-activated receptor-alpha: implications for glucose-stimulated insulin secretion.

The pyruvate dehydrogenase complex (PDC) has a pivotal role in islet metabolism. The pyruvate dehydrogenase kinases (PDK1-4) regulate glucose oxidation through inhibitory phosphorylation of PDC. Starvation increases islet PDK activity (Am J Physiol Endocrinol Metab 270:E988-E994, 1996). In this study, using antibodies against PDK1, PDK2, and PDK4 (no sufficiently specific antibodies are as yet available for PDK3), we identified the PDK isoform profile of the pancreatic islet and delineated the effects of starvation (48 h) on protein expression of individual PDK isoforms. Rat islets were demonstrated to contain all three PDK isoforms, PDK1, PDK2, and PDK4. Using immunoblot analysis with antibodies raised against the individual recombinant PDK isoforms, we demonstrated increased islet protein expression of PDK4 in response to starvation (2.3-fold; P < 0.01). Protein expression of PDK1 and PDK2 was suppressed in response to starvation (by 27% [P < 0.01] and 10% [NS], respectively). We demonstrated that activation of peroxisome proliferator-activated receptor-alpha (PPAR-alpha) by the selective agonist WY14,643 for 24 h in vivo leads to specific upregulation of islet PDK4 protein expression by 1.8-fold (P < 0.01), in the absence of change in islet PDK1 and PDK2 protein expression but in conjunction with a 2.2-fold increase (P < 0.01) in islet PPAR-alpha protein expression. Thus, although no changes in islet PPAR-alpha expression were observed after the starvation protocol, activation of PPAR-alpha in vivo may be a potential mechanism underlying upregulation of islet PDK4 protein expression in starvation. We evaluated the effects of antecedent changes in PDK profile and/or PPAR-alpha activation induced by starvation or PPAR-alpha activation in vivo on glucose-stimulated insulin secretion (GSIS) in isolated islets. GSIS at 20 mmol/l glucose was modestly impaired on incubation with exogenous triglyceride (1 mmol/l triolein) ( approximately 20% inhibition; P < 0.05) in islets from fed rats. Starvation (48 h) impaired GSIS in the absence of triolein (by 57%; P < 0.001), but GSIS after the further addition of triolein did not differ significantly between islets from fed or starved rats. GSIS by islets prepared from WY14,643-treated fed rats did not differ significantly from that seen with islets from control fed rats, and the response to triolein addition resembled that of islets prepared from fed rather than starved rats. PPAR-alpha activation in vivo led to increased insulin secretion at low glucose concentrations. Our results are discussed in relation to the potential impact of changes in islet PDK profile on the insulin secretory response to lipid and of PPAR-alpha activation in the cause of fasting hyperinsulinemia.

Role of peroxisome proliferator-activated receptor-alpha in the mechanism underlying changes in renal pyruvate dehydrogenase kinase isoform 4 protein expression in starvation and after refeeding.

The pyruvate dehydrogenase complex (PDC) occupies a strategic role in renal intermediary metabolism, via partitioning of pyruvate flux between oxidation and entry into the gluconeogenic pathway. Inactivation of PDC via activation of pyruvate dehydrogenase kinases (PDKs), which catalyze PDC phosphorylation, occurs secondary to increased fatty acid oxidation (FAO). In kidney, inactivation of PDC after prolonged starvation is mediated by up-regulation of the protein expression of two PDK isoforms, PDK2 and PDK4. The lipid-activated transcription factor, peroxisome proliferator-activated receptor-alpha (PPAR alpha), plays a pivotal role in the cellular metabolic response to fatty acids and is abundant in kidney. In the present study we used PPAR alpha null mice to examine the potential role of PPAR alpha in regulating renal PDK protein expression. In wild-type mice, fasting (24 h) induced marked up-regulation of the protein expression of PDK4, together with modest up-regulation of PDK2 protein expression. In striking contrast, renal protein expression of PDK4 was only marginally induced by fasting in PPAR alpha null mice. The present results define a critical role for PPAR alpha in renal adaptation to fasting, and identify PDK4 as a downstream target of PPAR alpha activation in the kidney. We propose that specific up-regulation of renal PDK4 protein expression in starvation, by maintaining PDC activity relatively low, facilitates pyruvate carboxylation to oxaloacetate and therefore entry of acetyl-CoA derived from FA beta-oxidation into the TCA cycle, allowing adequate ATP production for brisk rates of gluconeogenesis.

Fuel-sensing mechanisms integrating lipid and carbohydrate utilization.

Fuel metabolism is highly regulated to ensure adequate energy for cellular function. The contribution of the major metabolic fuels--glucose, lactate and fatty acids (FAs)--often reflects their circulating levels. In addition, regulatory cross-talk and fuel-induced hormone secretion ensures appropriate and co-ordinate fuel utilization. Because its activity can either determine or reflect fuel preference (carbohydrate versus fat), the pyruvate dehydrogenase complex (PDC) occupies a pivotal position in fuel cross-talk. Active PDC permits glucose oxidation and allows the formation of mitochondrially derived intermediates (e.g. malonyl-CoA and citrate) that reflect fuel abundance. FA oxidation suppresses PDC activity. PDC inactivation by phosphorylation is catalysed by pyruvate dehydrogenase kinases (PDKs) 1-4, which are regulated differentially by metabolite effectors. Most tissues contain at least two and often three of the PDK isoforms. We develop the hypothesis that PDK4 is a "lipid status"-responsive PDK isoform facilitating FA oxidation and signalling through citrate formation. Substrate interactions at the level of gene transcription extend glucose-FA interactions to the longer term. We discuss potential targets for substrate-mediated transcriptional regulation in relation to selective PDK isoform expression and the influence of altered PDK isoform expression in fuel sensing, selection and utilization.

The influence of improved glycaemic control with insulin and sulphonylureas on acute phase and endothelial markers in Type II diabetic subjects.

AIMS/HYPOTHESIS: Improved glycaemic control might reduce both microvascular and macrovascular complications of Type II diabetes (non-insulin-dependent) mellitus. To explore such possible mechanisms, we investigated the effects of intensive treatment on markers of endothelial dysfunction and of acute phase activation, using both sulphonylureas and insulin. METHODS: In a randomised cross-over study we gave sulphonylureas or insulin each for a period of 16 weeks to 22 poorly controlled Type II diabetic subjects who were being treated by diet. There was a 4 week washout period between each treatment. Subjects were studied at baseline and at the end of each treatment. RESULTS: Treatment with sulphonylureas and insulin resulted in similar improvements in glycaemic control (glycated haemoglobin, baseline: 11.8 [(SD 2.2)%; after sulphonylureas: 8.6 (1.2)%,p < 0.001; after insulin: 8.6 (1.2)%, p < 0.001] and in insulin sensitivity ¿metabolic clearance rate of glucose, baseline: median 1.75 [interquartile (IQ) range 1.41, 2.27] ml x kg(-1) x min(-1); after sulphonylureas: 2.41 (1.82, 3.01) ml x kg(-1) x min(-1), p = 0.001; after insulin: 2.23 (1.92, 2.75) ml x kg(-1) min(-1), p = 0.027¿. There were no significant changes in concentrations of endothelial markers von Willebrand factor, cellular fibronectin, thrombomodulin, tissue plasminogen activator, soluble E-selectin or soluble intercellular adhesion molecule-1 or in urinary albumin excretion rate after either treatment period. Concentrations of C-reactive protein were not significantly influenced by sulphonylureas but fell after insulin [baseline: median 4.50 (IQ range 1.37, 6.44) microg x ml(-1); sulphonylureas: 2.69 (0.88, 9.65) microg x ml(-1) (p = 0.53); insulin: 2.07 (1.16, 5.24) microg x ml(-1) (p = 0.017)]. There were, however, no significant effects of either treatment on circulating concentrations of fibrinogen (p = 0.28-0.34) or of the proinflammatory cytokines interleukin-6 or tumour necrosis factor-alpha (p = 0.65-0.79). CONCLUSION/INTERPRETATION: Markers of endothelial dysfunction and concentrations of proinflammatory cytokines in Type II diabetes are not influenced by improved glycaemic control over 16 weeks. Improved metabolic control with insulin could, however, be associated with reduced concentrations of the acute phase marker C-reactive protein.

beta-Adrenergic regulation of proinflammatory cytokines in humans.

OBJECTIVES: To investigate the effects of beta-adrenergic stimulation on IL-6 secretion in humans, and to determine the potential contribution to this response of adipocytes and peripheral blood cells (PBC). DESIGN: Experimental study in 8 human volunteers, and in vitro studies on murine adipocyte cell-line, 3T3.L1 and 3T3.F442A, and human PBC. MEASUREMENTS: Plasma IL-6 and TNFalpha responses to isoprenaline infusion. Cytokine secretion from differentiated adipocyte cell-lines and PBC in response to isoprenaline. RESULTS: Plasma IL-6 levels increased ninefold (median) by 180 min (baseline median 0.51 [interquartile range 0.47-1.4] vs 180 mins 4.53 [2.58-5.69] pg ml(-1), P=0.01). One hour after infusion, IL-6 levels (2.9 [1.27-3.98]) were lower than at 180 min (P=0.05), but higher than baseline (P=0.01). TNFalpha levels were unchanged. Differentiated adipocytes incubated in isoprenaline (0-0.1 microM) released significantly increased amounts of IL-6 whereas no response was elicited from PBC. CONCLUSIONS: The induction of IL-6 observed in vivo may be attributed to the beta-adrenergic stimulation of IL-6 release specifically from adipocytes, as opposed to circulating blood cells.

Physiological relationships of uncoupling protein-2 gene expression in human adipose tissue in vivo.

The physiological significance of changes in uncoupling protein-2 (UCP-2) gene expression is controversial. In this study we investigated the biochemical and functional correlates of UCP-2 gene expression in sc abdominal adipose tissue in humans in vivo. UCP-2 messenger ribonucleic acid expression was quantified by nuclease protection in adipose tissue from lean and obese humans in both the fasting and postprandial states. Plasma fatty acids, insulin, and leptin were all determined in paired samples from the superficial epigastric vein and radial artery, and local production rates were calculated from 133Xe washout. In the fasting state UCP-2 expression correlated inversely with body mass index (r = -0.45; P = 0.026), percent body fat (r = -0.41; P = 0.05), plasma insulin (r = -0.47; P = 0.02), epigastric venous fatty acids (r = -0.45; P = 0.04), and leptin (r = -0.50; P = 0.018). UCP-2 expression remained inversely related with plasma leptin after controlling for percent body (r = -0.45; P = 0.038). At 2 or 4 h postprandially, there were no significant relationships between UCP-2 expression and biochemical parameters. In conclusion, 1) UCP-2 messenger ribonucleic acid expression in sc adipose tissue is inversely related to adiposity and independently linked to local plasma leptin levels; and 2) UCP-2 expression is not acutely regulated by food intake, insulin, or fatty acids. Reduced UCP-2 expression may be a maladaptive response to sustained energy surplus and could contribute to the pathogenesis and maintenance of obesity.

Production of soluble tumor necrosis factor receptors by human subcutaneous adipose tissue in vivo.

To investigate in vivo adipose tissue production of tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), and their soluble receptors: TNF receptor type I (sTNFR-I), TNF receptor type II (sTNFR-II), and IL-6 receptor (sIL-6R), we determined arteriovenous differences in their levels across abdominal subcutaneous adipose tissue in obese subjects. Subjects had a median (interquartile range) age of 44.5 (27-51.3) yr, body mass index (BMI) of 32.9 (26. 0-46.6) kg/m(2), and %body fat of 42.5 (28.5-51.2) %. Although there was not a significant difference in the arteriovenous concentrations of TNF-alpha (P = 0.073) or sTNFR-II (P = 0.18), the levels of sTNFR-I (P = 0.002) were higher in the vein compared with artery, suggesting adipose tissue production of this soluble receptor. There was a significant arteriovenous difference in IL-6 (P < 0.001) but not in its soluble receptor (P = 0.18). There was no relationship between TNF-alpha levels and adiposity indexes (r(s) = 0.12-0.22, P = not significant); however, levels of both its soluble receptor isomers correlated significantly with BMI and %body fat (sTNFR-I r(s) = 0.42-0.72, P < 0.001; sTNFR-II r(s) = 0.36-0.65, P < 0.05- <0. 001). IL-6 levels correlated significantly with both BMI and %body fat (r(s) = 0.51, P = 0.004, and r(s) = 0.63, P < 0.001), but sIL-6R did not. In conclusion, 1) soluble TNFR-I is produced by adipose tissue, and concentrations of both soluble isoforms correlate with the degree of adiposity, and 2) IL-6, but not its soluble receptor, is produced by adipose tissue and relates to adiposity.

Lack of evidence for secretion of plasminogen activator inhibitor-1 by human subcutaneous adipose tissue in vivo.

Circulating plasminogen activator inhibitor-1 (PAI-1) levels are elevated in patients with coronary heart disease and may play an important role in atherothrombosis. Levels are also raised in obese, hypertriglyceridaemic, or insulin-resistant subjects, which predispose people to coronary heart disease. It is unclear, though, which organ is responsible for PAI-1 secretion, either in health or disease. We measured arteriovenous differences across a subcutaneous adipose tissue bed in 25 subjects without coronary heart disease, which, combined with measures of adipose tissue blood flow, provides synthetic rates. There was no net increase in levels of PAI-1 activity (median change 0.23, interquartile range -0.59, 1.21 IU x L(-1), p=0.30) or of PAI-1 antigen (mean change -0.01, SD+/-2.93 ng x mL(-1), p=0.98) in these subjects. Assuming homogeneous production of PAI-1 by all adipose tissue beds, the contribution of adipose tissue to PAI-1 activity is 3.1% (interquartile range, -11.7, +7.0%) and to PAI-1 antigen 1.6% (interquartile range -14.5, +7.3%). Arteriovenous difference of PAI-1 activity and antigen did not relate to measures of obesity, triglyceride, insulin, fatty acids, or circulating concentrations or adipose tissue production of tumour necrosis factor-alpha or interleukin-6. We conclude that, at least in healthy subjects, subcutaneous adipose tissue does not contribute significantly to circulating levels of PAI-1.

Does malnutrition in utero determine diabetes and coronary heart disease in adulthood? Results from the Leningrad siege study, a cross sectional study.

OBJECTIVE: To investigate the relation between decreased maternal food intake and risk factors for coronary heart disease in adult life. DESIGN: Cross sectional study. SUBJECTS: 169 subjects exposed to malnutrition in utero (intrauterine group) during the siege of Leningrad (now St Petersburg) in 1941-4; 192 subjects born in Leningrad just before rationing began, before the siege (infant group); and 188 subjects born concurrently with the first two groups but outside the area of the siege (unexposed group). SETTING: Ott Institute of Obstetrics and Gynaecology, St Petersburg. MAIN OUTCOME MEASURES: Development of risk factors for coronary heart disease and diabetes mellitus-obesity, blood pressure, glucose tolerance, insulin concentrations, lipids, albumin excretion rate, and clotting factors. RESULTS: There was no difference between the subjects exposed to starvation in utero and those starved during infant life in: (a) glucose tolerance (mean fasting glucose: intrauterine group 5.2 (95% confidence interval 5.1 to 5.3), infant group 5.3 (5.1 to 5.5), P = 0.94; mean 2 hour glucose: intrauterine group 6.1 (5.8 to 6.4), infant group 6.0 (5.7 to 6.3), P = 0.99); (b) insulin concentration; (c) blood pressure; (d) lipid concentration; or (e) coagulation factors. Concentrations of von Willebrand factor were raised in the intrauterine group (156.5 (79.1 to 309.5)) compared with the infant group (127.6 (63.9 to 254.8); P < 0.001), and female subjects in the intrauterine group had a stronger interaction between obesity and both systolic (P = 0.01) and diastolic (P = 0.04) blood pressure than in the infant group. Short adult stature was associated with raised concentrations of glucose and insulin 2 hours after a glucose load-independently of siege exposure. Subjects in the unexposed group had non-systematic differences in subscapular to triceps skinfold ratio, diastolic blood pressure, and clotting factors compared with the exposed groups. CONCLUSIONS: Intrauterine malnutrition was not associated with glucose intolerance, dyslipidaemia, hypertension, or cardiovascular disease in adulthood. Subjects exposed to malnutrition showed evidence of endothelial dysfunction and a stronger influence of obesity on blood pressure.

Control of acyl-CoA carboxylase activity in mycobacteria.

Acyl-CoA carboxylase activity in four pathogenic mycobacteria and Mycobacterium smegmatis was shown with both acetyl-CoA and propionyl-CoA substrates. Only very low activity was detected in mycobacteria grown in host tissues or on egg-based media rich in lecithin and avidin. This appeared to be a result of severe depression of activity, as strains which could be grown both in host tissue and egg-based media, and in the relatively simple Dubos or Sauton's media showed 8 to 120-fold higher activity in the simpler media.

Fatty acid oxidation and the beta-oxidation complex in Mycobacterium leprae and two axenically cultivable mycobacteria that are pathogens.

Intact, non-growing Mycobacterium leprae, M. avium and M. microti oxidized a wide range of 1-14C-labelled fatty acids (C8 to C24) to 14CO2. Laurate (C12) was oxidized most rapidly, and its oxidation by M. leprae was inhibited by the antileprosy agents Dapsone, clofazamine and rifampicin. Key enzymes of beta-oxidation were detected in extracts from all three mycobacteria. All these activities (both in intact mycobacteria and the enzymes) were stimulated in M. avium grown in Dubos medium plus palmitate but activities in M. microti or M. avium grown either in Dubos medium with added liposomes or triolein, or in vivo were similar to those detected in the same strain grown in Dubos medium alone. M. avium could be grown in medium in which 95% of its fatty acyl elongase activity is acetyl-CoA dependent. In this medium growing M. avium organisms oxidized [1-14C]palmitate to 14CO2 but simultaneously elongated palmitate to C24 acids and even longer. Acetyl-CoA-dependent elongase activity is similar but clearly not identical to reversed beta-oxidation, but the exact point(s) of difference have not yet been identified.

Enzymes for biosynthesis de novo and elongation of fatty acids in mycobacteria grown in host cells: is Mycobacterium leprae competent in fatty acid biosynthesis?

Fatty acid synthetase activity in extracts of Mycobacterium leprae was equivalent to 1.7 pmol malonyl-CoA incorporated into fatty acid min-1 (mg protein)-1. This activity--if representative of living M. leprae organisms--is insufficient to enable them to synthesize their lipid requirements rapidly enough to support growth. The major activity for scavenging fatty acids in extracts of Mycobacterium microti and Mycobacterium avium, as well as in extracts of M. leprae, was acetyl-CoA-dependent fatty acyl-CoA 'elongase'. This activity was about four times higher in M. avium and M. microti grown in a medium which contained lipids, or when grown in mice, than in medium without added lipids. In contrast, the de novo fatty acid synthetase activity was repressed in M. avium and M. microti when grown in medium that contained lipids, or when grown in mice. These results are consistent with the hypothesis that mycobacteria grown in vivo preferentially scavenge lipids from the host cells, and suggest that a source of lipid should be included in media for attempted axenic isolation of M. leprae.