Adipose Lipoprotein Lipase Activity Research Articles

Postprandial exercise regulates tissue-specific triglyceride uptake through angiopoietin-like proteins.

Fuel substrate switching between carbohydrates and fat is essential for maintaining metabolic homeostasis. During aerobic exercise, the predominant energy source gradually shifts from carbohydrates to fat. While it is well known that exercise mobilizes fat storage from adipose tissues, it remains largely obscure how circulating lipids are distributed tissue-specifically according to distinct energy requirements. Here, we demonstrate that aerobic exercise is linked to nutrient availability to regulate tissue-specific activities of lipoprotein lipase (LPL), the key enzyme catabolizing circulating triglyceride (TG) for tissue uptake, through the differential actions of angiopoietin-like (ANGPTL) proteins. Exercise reduced the tissue binding of ANGPTL3 protein, increasing LPL activity and TG uptake in the heart and skeletal muscle in the postprandial state specifically. Mechanistically, exercise suppressed insulin secretion, attenuating hepatic Angptl8 transcription through the PI3K/mTOR/CEBPα pathway, which is imperative for the tissue binding of its partner ANGPTL3. Constitutive expression of ANGPTL8 hampered lipid utilization and resulted in cardiac dysfunction in response to exercise. Conversely, exercise promoted the expression of ANGPTL4 in white adipose tissues, overriding the regulatory actions of ANGPTL8/ANGPTL3 in suppressing adipose LPL activity, thereby diverting circulating TG away from storage. Collectively, our findings show an overlooked bifurcated ANGPTL-LPL network that orchestrates fuel switching in response to aerobic exercise.

Downregulation of adipose LPL by PAR2 contributes to the development of hypertriglyceridemia.

Lipoprotein lipase (LPL) hydrolyzes circulating triglycerides (TGs), releasing fatty acids (FA) and promoting lipid storage in white adipose tissue (WAT). However, the mechanisms regulating adipose LPL and its relationship with the development of hypertriglyceridemia are largely unknown. WAT from obese humans exhibited high PAR2 expression, which was inversely correlated with the LPL gene. Decreased LPL expression was also inversely correlated with elevated plasma TG levels, suggesting that adipose PAR2 might regulate hypertriglyceridemia by downregulating LPL. In mice, aging and high palmitic acid diet (PD) increased PAR2 expression in WAT, which was associated with a high level of macrophage migration inhibitory factor (MIF). MIF downregulated LPL expression and activity in adipocytes by binding with CXCR2/4 receptors and inhibiting Akt phosphorylation. In a MIF overexpression model, high-circulating MIF levels suppressed adipose LPL, and this suppression was associated with increased plasma TGs but not FA. Following PD feeding, adipose LPL expression and activity were significantly reduced, and this reduction was reversed in Par2-/- mice. Recombinant MIF infusion restored high plasma MIF levels in Par2-/- mice, and the levels decreased LPL and attenuated adipocyte lipid storage, leading to hypertriglyceridemia. These data collectively suggest that downregulation of adipose LPL by PAR2/MIF may contribute to the development of hypertriglyceridemia.

6-OR: PAR2 Downregulates Adipose LPL through MIF—A Novel Adipose Mechanism Contributing to the Development of Hypertriglyceridemia

Lipoprotein lipase (LPL) is a key enzyme that hydrolyzes circulating triglycerides to release fatty acid (FA). In adipose tissue, LPL is required for lipid storage. However, it is currently unknown whether alteration of LPL in white adipose tissue (WAT) contributes to the development of hypertriglyceridemia. Our present study indicated that WAT isolated from human obese patients have increased expression of PAR2 which is negatively associated with LPL gene. The reduced LPL expression is also negatively correlated with increased plasma TG levels, suggesting that adipose PAR2 may modulate hyperlipidemia through downregulating LPL. In mice, aging and high palmitic oil diet significantly increased PAR2 expression in adipose tissue which was associated with high plasma MIF levels. PAR2 deficiency attenuates the rise of MIF, suggesting a key role of PAR2 in regulating adipose MIF release. MIF reduced LPL expression and activity in adipocytes. In a MIF overexpressed animal model (Mif lung Tg), high circulating MIF levels inhibited adipose LPL which was associated with increased plasma triglyceride but not fatty acid. Following high palmitic oil diet feeding, adipose LPL expression and activity were also reduced, and this reduction was reversed in PAR2 knockout mice. Interestingly, PAR2 mediated LPL in adipose tissue regulates hypertriglyceridemia through controlling adipocyte lipid storage. In Par2-/- mice, recombinant MIF perfusion recovered high plasma MIF levels, which decreased LPL and attenuated adipocyte lipid storage leading to hypertriglyceridemia. These data together suggest that the downregulation of adipose LPL by PAR2/MIF is an important mechanism for the development of hypertriglyceridemia. Disclosure Y.Huang: None. L.Li: None. L.Chen: None. X.Chen: None. P.Gao: None. Y.Qi: None. D.Qi: None. Funding Canadian Institutes of Health Research (PJT156116)

Regulation of plasma triglyceride partitioning by adipose-derived ANGPTL4 in mice

Elevated plasma triglyceride levels are associated with metabolic disease. Angiopoietin-like protein 4 (ANGPTL4) regulates plasma triglyceride levels by inhibiting lipoprotein lipase (LPL). Our aim was to investigate the role of adipocyte-specific deficiency of ANGPTL4 in mice during high fat diet feeding. Adipocyte-specific ANGPTL4 deficient mice were fed a high fat diet (60% kCal from fat) for either 12 weeks or 6 months. We performed plasma metabolic measurements, triglyceride clearance and uptake assays, LPL activity assays, and assessed glucose homeostasis. Mice lacking adipocyte ANGPTL4 recapitulated the triglyceride phenotypes of whole-body ANGPTL4 deficiency, including increased adipose LPL activity, lower plasma triglyceride levels, and increased uptake of triglycerides into adipose tissue. When fed a high fat diet (HFD), these mice continued to display enhanced adipose LPL activity and initially had improved glucose and insulin sensitivity. However, after 6 months on HFD, the improvements in glucose homeostasis were largely lost. Moreover, despite higher adipose LPL activity levels, mice lacking adipocyte ANGPTL4 no longer had increased triglyceride uptake into adipose compared to littermate controls after chronic high-fat feeding. These observations suggest that after chronic high-fat feeding LPL is no longer rate-limiting for triglyceride delivery to adipocytes. We conclude that while adipocyte-derived ANGPTL4 is an important regulator of plasma triglyceride levels and triglyceride partitioning under normal diet conditions, its role is diminished after chronic high-fat feeding.

PPARγ2 Pro12Ala polymorphism is associated with improved lipoprotein lipase functioning in adipose tissue of insulin resistant obese women

Lipoprotein lipase (LPL) plays a pivotal role in lipid metabolism, contributes to metabolic disorders related to insulin action and body weight regulation, and is influenced by inflammation. The Pro12Ala polymorphism of the peroxisome proliferator-activated receptor (PPAR)γ2 gene seems to influence LPL functioning, but its role in obesity and insulin resistance status, which usually coexist in the clinical setting, has not been explored. Our aim was to analyze the association of obesity and insulin resistance with adipose LPL activity and expression, and the influence of the PPARγ2 Pro12Ala polymorphism. A cross-sectional study was conducted in 58 reproductive-age women who underwent elective abdominal surgery. Free-fatty acids, glucose, insulin, and selected adipokines were measured in fasting blood samples. DNA was isolated and the polymorphism genotyped. Biopsies of abdominal subcutaneous adipose tissue obtained during surgery were used to determine enzymatic LPL activity and expression; and expression of selected cytokines. Overweight/obese women presented lower LPL activity (P=0.022) and higher circulating TNF-α (P=0.020) than controls. Insulin resistant women also showed borderline lower LPL activity than non-resistant (P=0.052), but adiposity and inflammatory molecules were comparable. Nevertheless, LPL activity was higher in Pro12Ala carriers than in non-carriers after adjusting for obesity, insulin resistance and inflammation. Likewise, adipose LPL expression was increased in carriers while expression of cytokines was decreased. Our data suggest that insulin resistance is associated with low adipose LPL activity independently of obesity, but the PPARγ2 Pro12Ala polymorphism seems to protect the LPL functioning of obese insulin resistant women, likely through regulating inflammation in adipose tissue.

Acute Intermittent Hypoxia Inhibits Triglycerides Clearance And Has a Tissue‐Specific Effect on Lipoprotein Lipase Activity

Background We have recently shown that chronic intermittent hypoxia (IH) decreases triglycerides (TG) clearance due to the inhibition of lipoprotein lipase (LPL) activity in adipose tissue. However, the impact of acute IH on TG clearance and LpL activity remains unknown. Methods Male C57BL/6J mice were exposed to 7hs of IH with FiO2 decreasing from 20.9% to 5% once per minute or to intermittent air (IA). During the exposure, intralipid (20%, 7.5 ml/kg) was administered and serum triglyceride levels were measured over the 2 hr time course. In a separate batch of mice, fasting (5 hrs) LPL activity was measured in the epididymal fat pads, heart and skeletal muscle. Results Compared to IA, IH increased TG levels throughout the 2 hr time course of intralipid (p <0.01). Exposure to IH for 7 hrs resulted in a significant decrease in adipose LPL activity (IA: 5.83±0.51 vs. IH: 4.40±0.29 umol FFA released/g/hr; P<0.05) and a significant increase in the heart LPL activity (IA: 42.61±2.28 vs. IH: 60.70±3.05 umol FFA released/g/hr; P<0.01). No difference was observed in the skeletal muscle. Conclusions Acute IH inhibits TG clearance, possibly by suppressing adipose LpL activity. A simultaneous increase in heart LpL may be attributable to the heightened myocardial demand in fatty acids as a source of energy. Research Support: R01 HL80105, AHA 10GRNT3360001, 5P50HL084945, FAPESP 2010/11681-0

Mechanisms of ketamine action on lipid metabolism in rats

Background and objective: This study was conducted to determine the effect of ketamine on metabolic homoeostasis and particularly in lipoprotein lipase (LPL) activity in adipose tissue. Methods: Sixty male Wistar rats were divided into six groups of 10 each. Group A served as controls, while Groups B-F received, respectively, ketamine 60, 80, 100, 120 and 140 mg kg−1 intraperitoneally. The animals were sacrificed 20 min after the administration of ketamine. Insulin concentrations in plasma and total cholesterol, triglyceride, high-density lipoprotein (HDL) and free fatty acid (FFA) concentrations in serum were measured. LPL activity in adipose tissue and medium-chain acyl-CoA dehydrogenase (MCAD) content in muscle were determined. Results: FFA concentrations in serum significantly increased from the second lowest dose of ketamine. Insulin concentrations in plasma did not exhibit any significant difference between groups. MCAD levels were 0.5-fold more in Group F than in Group A, while there were no significant differences between control group and Groups B-E. Furthermore, high concentrations (120 and 140 mg kg−1) of ketamine interfered with in metabolic homoeostasis by significantly reducing LPL activity, thus elevating triglyceride concentrations in serum without affecting cholesterol and HDL metabolism. Conclusions: Ketamine induces various metabolic effects due to changes in adipose LPL activity and MCAD levels in muscles. These findings seem to be significant only at high doses.

Mechanisms of ketamine action on lipid metabolism in rats

This study was conducted to determine the effect of ketamine on metabolic homoeostasis and particularly in lipoprotein lipase (LPL) activity in adipose tissue. Sixty male Wistar rats were divided into six groups of 10 each. Group A served as controls, while Groups B-F received, respectively, ketamine 60, 80, 100, 120 and 140 mg kg(-1) intraperitoneally. The animals were sacrificed 20 min after the administration of ketamine. Insulin concentrations in plasma and total cholesterol, triglyceride, high-density lipoprotein (HDL) and free fatty acid (FFA) concentrations in serum were measured. LPL activity in adipose tissue and medium-chain acyl-CoA dehydrogenase (MCAD) content in muscle were determined. FFA concentrations in serum significantly increased from the second lowest dose of ketamine. Insulin concentrations in plasma did not exhibit any significant difference between groups. MCAD levels were 0.5-fold more in Group F than in Group A, while there were no significant differences between control group and Groups B-E. Furthermore, high concentrations (120 and 140 mg kg(-1)) of ketamine interfered with in metabolic homoeostasis by significantly reducing LPL activity, thus elevating triglyceride concentrations in serum without affecting cholesterol and HDL metabolism. Ketamine induces various metabolic effects due to changes in adipose LPL activity and MCAD levels in muscles. These findings seem to be significant only at high doses.

Transgenic Mice Expressing Lipoprotein Lipase in Adipose Tissue: ABSENCE OF THE PROXIMAL 3′-UNTRANSLATED REGION CAUSES TRANSLATIONAL UPREGULATION

Lipoprotein lipase (LPL) is a key enzyme in lipoprotein and adipocyte metabolism. Defects in LPL can lead to hypertriglyceridemia and the subsequent development of atherosclerosis. The mechanisms of regulation of this enzyme are complex and may occur at multiple levels of gene expression. Because the 3'-untranslated region (UTR) is involved in LPL translational regulation, transgenic mice were generated with adipose tissue expression of an LPL construct either with or without the proximal 3'-UTR and driven by the aP2 promoter. Both transgenic mouse colonies were viable and expressed the transgene, resulting in a 2-fold increase in LPL activity in white adipose tissue. Neither mouse colony exhibited any obvious phenotype in terms of body weight, plasma lipids, glucose, and non-esterified fatty acid levels. In the mice expressing hLPL with an intact 3'-UTR, hLPL mRNA expression approximately paralleled hLPL activity. However in the mice without the proximal 3'-UTR, hLPL mRNA was low in the setting of large amounts of hLPL protein and LPL activity. In previous studies, the 3'-UTR of LPL was critical for the inhibitory effects of constitutively expressed hormones, such as thyroid hormone and catecholamines. Therefore, these data suggest that the absence of the 3'-UTR results in a translationally unrepressed LPL, resulting in a moderate overexpression of adipose LPL activity.

Weight Gain in Gonadectomized Normal and Lipoprotein Lipase–Deficient Male Domestic Cats Results from Increased Food Intake and Not Decreased Energy Expenditure

Gonadectomy predisposes domestic cats to undesired body weight gain and obesity. The disturbance responsible for this disregulation of energy balance has not been clearly identified. Energy intake and expenditure, body composition and plasma concentrations of leptin, insulin, glucose and triacylglycerol were determined during a 36-wk period in adult male (2-5 y) gonadectomized (n = 8) and intact (n = 8) normal cats and gonadectomized (n = 8) and intact (n = 8) lipoprotein lipase (LPL)-deficient cats. Cats were housed individually in temperature- and light-controlled rooms and continuously provided a commercial dry-type diet. In normal and LPL-deficient cats, body weight increased (P < 0.05) after gonadectomy by 27 to 29%, mostly as a result of fat accretion. There was a rapid increase (P < 0.05) in food intake of approximately 12% after gonadectomy of normal and LPL-deficient cats. The metabolic rate (kJ.kg(-1).d(-1)), determined in normal intact (319 +/- 20, n = 5) and gonadectomized (332 +/- 36, n = 5) cats, did not differ after gonadectomy. After gonadectomy, plasma concentrations of glucose and triacylglycerol did not change, whereas plasma insulin and leptin concentrations increased (P < 0.05), but not coincidentally with body weight gain. A stair-step increase in energy intake, and not decreased energy expenditure, appears to drive the weight gain associated with gonadectomy. Body fat mass appears to increase until the energy intake supports no further expansion. Adiposity signaling through insulin or leptin does not appear to mediate the energy intake effect. LPL deficiency did not preclude development of the overweight body condition. Therefore, gonadectomy-induced weight gain in cats is not a result of changed adipose LPL activity, as previously suggested.

The estrogen antagonist EM-652 and dehydroepiandrosterone prevent diet- and ovariectomy-induced obesity.

EM-652 is a pure antiestrogen in human breast and uterine cancer cells that also reduces bone loss and plasma lipid levels in the rat. This study aimed to assess the ability of EM-652, alone or with dehydroepiandrosterone (DHEA), to prevent obesity and related metabolic abnormalities induced by an obesity-promoting diet and ovariectomy. Female rats were fed a high-sucrose, high-fat (HSHF) diet, were left intact or ovariectomized (OVX), and were treated with EM-652, DHEA, or both for 20 days. Variables of energy balance and determinants of lipid metabolism and insulin sensitivity were assessed. The HSHF diet (vs. chow) and OVX both increased energy intake and gain, as well as energetic efficiency. Both EM-652 and DHEA prevented diet- and OVX-induced energy gain mainly by decreasing fat deposition, without being additive. The modest EM-652-induced increase in liver triglycerides of intact rats was prevented by its combination with DHEA. EM-652, but not DHEA, decreased cholesterolemia. The HSHF diet and OVX reduced insulin sensitivity, an effect that was attenuated by EM-652 and abrogated by DHEA and EM-652+DHEA. Treatment with EM-652, DHEA, or their combination abolished the diet- and OVX-induced increase in adipose lipoprotein lipase activity that accompanied fat gain. EM-652 is an effective agent to prevent diet- and OVX-induced obesity and its associated cardiovascular risk factors such as insulin resistance. The addition of DHEA prevents hepatic lipid accumulation and further ameliorates insulin sensitivity. The beneficial metabolic effects of such combined steroid therapy may, therefore, eventually prove to be clinically relevant.

PMT13, a pyrimidone analogue of thiazolidinedione improves insulin resistance-associated disorders in animal models of type 2 diabetes.

To evaluate the antidiabetic and hypolipidaemic potential of a novel thiazolidinedione, PMT13, in different animal models of insulin resistance. PPAR transactivation study was performed in HEK293T cells using ligand binding domains of PPARalpha, gamma and delta. Insulin-resistant db/db and ob/ob mice were treated orally with different doses of PMT13 at 0.3-10 mg/kg/day for 15 and 14 days respectively. Zucker fa/fa rats were treated with 3 mg/kg (p.o.) dose of the compound. Plasma glucose, triglyceride, free fatty acid and insulin levels were measured. Liver glucose 6-phosphatase (G6-Ptase) and adipose lipoprotein lipase activity was measured in treated mice. Isolated rat aortic preparations preconstricted with phenylephrine were used to study the vascular relaxation potential of PMT13 in presence of insulin. A 28-day oral toxicity study was performed in Wistar rats. PMT13 showed similar PPARgamma activation as rosiglitazone, but failed to show any activity against PPARalpha or PPARdelta. In obese and diabetic db/db and ob/ob mice, PMT13 showed better reduction in plasma glucose, triglyceride and insulin levels than rosiglitazone and an improvement in glucose tolerance. In insulin-resistant Zucker fa/fa rat model, PMT13 treatment showed better reduction in plasma triglyceride, free fatty acid and insulin levels than that of rosiglitazone. Treated mice showed decreased G6-Ptase activity in liver. The LPL activity was increased in post-heparin plasma and epididymal fat of treated db/db mice. In an isolated, precontracted rat aortic preparation, PMT13 treatment significantly increased insulin-induced relaxation. A 28-day oral toxicity study in rats showed no treatment-related adverse effects. Our studies indicate that PMT13 is a potent activator of PPARgamma with antidiabetic, hypolipidaemic and insulin-sensitizing properties. Additionally, PMT13 inhibited liver G6-Ptase activity and increased lipoprotein lipase activity. It showed improvement in insulin-induced vasorelaxation. The compound also showed a good safety margin. Therefore, PMT13 can be a potential drug candidate for future development.

Responses of adipose and muscle lipoprotein lipase to chronic infection and subsequent acute lipopolysaccharide challenge.

Infection of male Swiss Webster mice with Toxoplasma gondii or Neospora caninum leads to long-term alterations in energy balance. Following an initial 20 to 30% weight loss in all T. gondii-infected mice, half of the animals regain most of the lost weight (gainers), whereas the others maintain their low body weight (nongainers). Infection with N. caninum does not elicit weight loss. Lipoprotein lipase (LPL), the enzyme responsible for plasma triglyceride (TG) clearance and partitioning among tissues, is under tissue-specific modulation associated with energy balance. It is also a major determinant of infection-induced hypertriglyceridemia. This study aimed to assess the long-term modulation of adipose and muscle LPL activity in mice infected with T. gondii or N. caninum, to evaluate the effects of subsequent acute lipopolysaccharide (LPS) administration, and to relate LPL modulation in these conditions with infection-related changes in body weight gain. Twenty-eight days after infection, LPL activity in muscle of both gainer and nongainer T. gondii-infected mice was reduced by 40 to 50% compared with the levels in controls and N. caninum-infected mice, whereas LPL activity in adipose depots remained unchanged in all infected groups compared to the level in controls. LPS (from Escherichia coli, 100 ng/kg) injection induced a global reduction in adipose LPL in all groups, as assessed 90 min later. In both T. gondii-infected subgroups, muscle LPL was not further reduced by LPS treatment, whereas it was decreased by 40 to 50% in muscles of control and N. caninum-infected mice. Pre-LPS TG levels in plasma were similar in all groups. LPS greatly increased TG levels in plasma in both control and N. caninum-infected animals, whereas it did not alter those of T. gondii-infected gainer or nongainer animals. These results show that (i) independently of the extent of postinfection weight gain, long-term infection with T. gondii chronically reduces muscle LPL, which becomes unresponsive to acute endotoxemia; (ii) modulation of tissue LPL activity during chronic T. gondii infection favors TG partitioning towards adipose tissue; and (iii) skeletal muscle LPL is a key determinant of the acute response of triglyceridemia to LPS.

Resistance of adipose tissue lipoprotein lipase to insulin action in rats fed an obesity-promoting diet.

This study aimed to assess whether adipose lipoprotein lipase (LPL) becomes resistant to insulin in a nutritional model of resistance of glucose metabolism to insulin. Sprague-Dawley rats were fed for 4 wk chow or a purified high-sucrose, high-fat (HSHF) diet that induced overt insulin resistance. Rats were fasted for 24 h and then refed chow for 1, 3, or 6 h. The postprandial rise in insulinemia was similar in both dietary cohorts, whereas glycemia was higher in HSHF-fed than in chow-fed animals, indicating glucose intolerance and insulin resistance. In chow-fed rats, adipose LPL activity increased two- to fourfold postprandially, but only minimally (30%) in HSHF-fed rats. Muscle LPL decreased postprandially in HSHF-fed rats, suggesting intact sensitivity to insulin, but it increased in chow-fed animals. Peak postprandial triglyceridemia was higher (+70%) in insulin-resistant than in control rats. The postprandial rate of appearance of triglycerides in the circulation was similar in control and insulin-resistant rats, indicating that hypertriglyceridemia of the latter was the result of impaired clearance. These results demonstrate that adipose LPL becomes resistant to insulin in diet-induced IR and further suggest that, under certain nutritional conditions, modifications in adipose LPL modulation associated with insulin resistance, along with low muscle LPL, heightens postprandial hypertriglyceridemia through attenuated triglyceride clearance.

Abnormal insulin and beta-adrenergic modulation of lipoprotein liipase during refeeding after prolonged fasting in the Zucker rat.

To characterise the response of tissue lipoprotein lipase to refeeding after prolonged (24 h) fasting in lean and obese Zucker rats, and to verify whether lipoprotein lipase in obese rats is resistant to the short-term action of insulin and escapes modulation by the beta-adrenergic pathway. Lean Fa/? and obese fa/fa male Zucker rats fasted for 24 h and refed at will. Lipoprotein lipase activity in adipose and muscle tissues was assessed in the freely fed and fasted states and at various times during refeeding, with or without beta-adrenergic blockade (propranolol). The 24 h fast erased the phenotype-related differences in insulinaemia and adipose lipoprotein lipase activity present in freely fed rats. Adipose lipoprotein lipase increased twofold in obese rats 1 h after refeeding, whereas no change occurred at that time in lean rats. Activity remained at that level for at least 6 h after refeeding in obese rats, whereas in lean animals it was increased fivefold after 6 h of refeeding. In muscle of obese rats, lipoprotein lipase decreased in response to refeeding, but paradoxically increased twofold in lean animals. Giving propranolol to lean rats before refeeding abolished the atypical response of muscle lipoprotein lipase to food intake and restored the early (1 h after refeeding) increase in adipose lipoprotein lipase but had no effect in obese rats. Refeeding after prolonged fasting activates the P-adrenergic pathway in lean rats, which transiently counteracts insulin-mediated modulation of lipoprotein lipase. The P-adrenergic pathway is not activated by refeeding in adipose tissue and muscle of the obese Zucker rat. In the obese Zucker rat, the early modulation of adipose lipoprotein lipase activity is abnormal upon refeeding after prolonged fasting, suggesting short-term resistance to the action of insulin.

Contribution of hyperinsulinemia to modulation of lipoprotein lipase activity on the obese Zucker rat

This study was designed to assess the contribution of hyperinsulinemia to the maintenance of high adipose and low muscle lipoprotein lipase (LPL) activity in the obese Zucker fa/fa rat. Insulinemia in obese Zucker rats was reduced for 4 days with a single injection of low-dose streptozotocin (STZ). Saline-injected intact obese (obese-INT) and STZ-injected obese (obese-STZ) rats were compared with a lean Fa/? reference group. LPL activity was assessed after a 12-hour fast, with or without a 1-hour refeeding period. Fasting serum insulin levels were 17-fold higher in obese-INT versus lean rats and were reduced to 60% of obese-INT levels in obese-STZ animals. In the postprandial state, serum insulin levels remained low in obese-STZ rats and were similar to the values in lean animals, whereas insulinemia increased in the obese-INT group to 18-fold the levels in lean rats. Serum glucose, nonesterified fatty acid (NEFA), and triglyceride levels, which were higher in obese-INT versus lean rats, were further increased in the obese-STZ group. Tissue weights of obese rats were unaffected by STZ treatment. Fasting LPL specific activity was higher in white adipose tissue ([WAT] +87%) and brown adipose tissue ([BAT] +167%) of obese-INT versus lean rats. Reducing the insulinemia in obese-STZ rats reduced fasting enzyme activity to the levels in lean animals in both WAT and BAT. Insulinemia and adipose LPL activity were positively correlated in the fasted state. Acute food intake increased WAT LPL activity in lean animals, but not in obese animals. Soleus LPL activity was lower in obese-INT compared with lean rats and was further decreased in obese-STZ animals. Heart LPL was decreased only in obese-STZ rats compared with the lean group. LPL in muscle tissue was not correlated with insulinemia, but an inverse relationship was found between serum NEFA levels and enzyme activity. It is concluded that in the obese Zucker rat, hyperinsulinemia is responsible for the maintenance of elevated basal LPL activity in adipose tissue independently of fat mass, whereas muscle enzyme activity appears to be more strongly and inversely related to the availability or tissue utilization of lipid substrates.

Lipoprotein hydrolysis and fat accumulation in chicken adipose tissues are reduced by chronic administration of lipoprotein lipase monoclonal antibodies

The lipoprotein lipase (LPL) catalyzed hydrolysis of plasma lipoproteins is a rate-limiting step in the lipid transport into peripheral tissues. The aim of the present study was to isolate monoclonal antibodies against chicken adipose LPL and to investigate whether chronic infusion of the LPL monoclonal antibodies inhibits adipose LPL activity and consequently reduces fat accumulation in broiler chickens. The LPL catalyzed very low density lipoprotein (VLDL) hydrolysis was completely inhibited by the addition of 100 microg/mL of monoclonal antibodies (CLP10, CLP14, CLP16) in the in vitro incubation with plasma VLDL and LPL. A single injection of CLP10 and CLP16 into chickens fed or starved for 24 h elevated plasma triacylglycerol concentrations for 24 h, whereas that of CLP14 was ineffective. Intravenous injection every other day and continuous infusion by osmotic minipump with CLP16 maintained higher plasma triacylglycerol concentration for 5 d than that of the control group and extensively reduced LPL activity in adipose tissues and abdominal fat pad weight. Lipoprotein lipase mRNA and protein levels in adipose tissue were not modified by chronic administration of anti-LPL antibody. The results indicate that chronic administration of anti-LPL antibodies is effective in retarding fatness in broiler chickens, and the antibodies are a proper subject for studies of lipoprotein metabolism.

Alterations of Lipid and Cholesterol Metabolism in Cachectic Tumor-Bearing Rats Are Prevented by Insulin

The ascites hepatoma Yoshida AH130 causes in the host a rapid and progressive body weight loss, associated with reduced food intake, and protein and lipid hypercatabolism. Because insulin regulates glucose as well as lipid and protein metabolism, we suggest that the observed alterations are at least in part secondary to hypoinsulinemia and/or to the increase of counterregulatory hormones in AH130-bearing rats. To verify this hypothesis, controls with free access to food (n = 4), controls with free access to food plus insulin (107 μmol · kg body wt−1 · d−1) (n = 4), controls pair-fed to the tumor-bearing rats (n = 4), pair-fed controls treated with insulin (n= 4), tumor hosts (n = 9), and tumor hosts treated with insulin (n = 6) were used. The Yoshida ascites hepatoma cells (∼108 cells/rat) were inoculated intraperitoneally. Daily food intake and body weight were measured; insulin was injected starting the day of tumor implantation for 6 d. The metabolism of both cholesterol and lipids was investigated in tumor cells, and ascitic fluid and blood serum were investigated at the end of treatment. Insulin prevented the reduction of food intake (19 ± 0.6 vs. 13 ± 0.4 g/d, P < 0.01; AH130 hosts treated and not treated with insulin, respectively), the loss of body weight (202 ± 12 vs. 135 ± 9 g, P < 0.01), lowered the circulating triglycerides (48.3 ± 4.9 vs. 84.5 ± 7.1 mmol/L, P < 0.01), and free fatty acids (561 ± 47 vs. 989 ± 54 mmol/L (P < 0.01), while corrected the decrease of adipose lipoprotein lipase activity (1,240 ± vs. 300 ± pmol FA, P < 0.01) observed in AH130 hosts. Moreover, insulin prevented the decrease in HDL cholesterol (13.2 ± 0.8 vs. 9.3. ± 0.7 mmol/L, P < 0.01) and significantly increased hepatic cholesterol synthesis as evaluated by 14C-acetate incorporation into cholesterol, in both liver (3,337 ± 245 vs. 830 ± 115 Bq/g, P < 0.01) and AH130 cells (11,676 ± 1,693 vs. 4,196 ± 527 Bq/106 cells, P < 0.01). Thus insulin treatment ameliorated many metabolic derangements, with a lengthening of rats survival time (7 ± 1 vs. 11 ± 1 d, P < 0.05) without significantly stimulating tumor growth. These data, together with our previous observations on the effectiveness of insulin on protein turnover perturbations, suggest that many metabolic alterations occurring during cancer cachexia can be avoided by the administration of this hormone.

Effects of leptin on adipose tissue lipoprotein lipase in the obese ob/ob mouse

To characterize the adaptations of lipid metabolism, with special emphasis on tissue lipoprotein lipase, to negative energy balance brought by chronic treatment of obese ob/ob mice with leptin. According to a 2 x 2 factorial analysis, lean and obese C57BL/6J mice were subcutaneously infused with leptin (100 micrograms.kg-1.day-1) or vehicle (phosphate-buffered saline) during seven days. Cumulative food intake and final body weight of vehicle-infused obese mice were twofold higher than in lean controls. Leptin decreased cumulative food intake and body weight of obese, but not lean mice. Lipoprotein lipase (LPL) activity in white inguinal and epididymal and brown interscapular adipose tissues of control obese mice was at least twofold higher than in lean mice, but comparable in the vastus lateralis muscle. Leptin treatment of obese mice significantly lowered LPL activity to that of lean mice in all tissues examined. Vehicle-infused obese mice had higher liver triglyceride content and were hypertriglyceridemic compared to lean mice, and triglyceride concentrations in plasma and liver were decreased proportionally after leptin treatment. Leptin lowered glycemia and insulinemia of obese mice to lean levels and decreased plasma corticosterone. Leptin infusion had no notable effect on tissue lipoprotein lipase nor plasma variables of lean mice. Leptin infusion abolished hyperinsulinemia in the ob/ob mouse, an effect that was probably responsible for the concomitant normalization of adipose LPL activity. This study shows that decreased LPL activity, plasma triglyceride concentrations and hepatic triglyceride production constitute some of the adaptive peripheral adaptations of lipid metabolism, which accompany the reduction in fat mass accretion brought by leptin treatment of the obese ob/ob mouse.

Insulin Stimulates Lipoprotein Lipase Activity and Synthesis in Adipocytes from Septic Rats

Gram-negative sepsis suppresses lipoprotein lipase (LPL) activity in adipose tissue which contributes, in part, to the altered clearance of triglycerides. The suppression in LPL activity occurs when plasma insulin concentrations are elevated and insulin-stimulated glucose utilization is impaired. This study was planned to evaluate whether the presence of insulin resistance was responsible for the decrease in adipose LPL activity. Adipocytes were isolated from epididymal fat pads 24 h after inducing sepsis in male Lewis rats by intravenous injection of 4 × 108colonies of liveEscherichia coli/100 g body wt. The decrease in heparin-releasable (HR) LPL activity in adipocytes from the septic rats was evident at the time of isolation and maintained in a 20-h culture. After overnight incubation with insulin (10−8M), HR LPL activity was stimulated to a greater extent in adipocytes from septic rats (298%) than in adipocytes from control rats (88%). The insulin stimulation of LPL activity during sepsis could not be attributed to insulin-like growth factor-I (IGF-I) as adipocytes from septic rats appeared to be IGF-I resistant. Insulin-treatment (10−8M) increased LPL synthesis 99% in adipocytes from control rats and 136% in adipocytes from septic rats. Insulin treatment also led to a 65 and 62% increase in LPL mass in adipocytes from control and septic rats, respectively. These findings indicate that the sepsis-induced decrease in adipose LPL is not due to insulin resistance with respect to LPL. The insulin stimulation of LPL activity in adipocytes from septic rats appears to be mediated by an increase in LPL synthesis.