Response of adipose tissue lipoprotein lipase activity and serum lipoproteins to acute hyperinsulinaemia in man

The PNPLA3-I148M variant increases polyunsaturated triglycerides in human adipose tissue

BACKGROUND & AIMS

The I148M variant in PNPLA3 is the major genetic risk factor for non-alcoholic fatty liver disease (NAFLD). The liver is enriched with polyunsaturated triglycerides (PUFA-TGs) in PNPLA3-I148M carriers. Gene expression data indicate that PNPLA3 is liver-specific in humans, but whether it functions in adipose tissue (AT) is unknown. We investigated whether PNPLA3-I148M modifies AT metabolism in human NAFLD.

METHODS

Profiling of the AT lipidome and fasting serum non-esterified fatty acid (NEFA) composition was conducted in 125 volunteers (PNPLA3^148MM/MI , n = 63; PNPLA3^148II , n = 62). AT fatty acid composition was determined in 50 volunteers homozygous for the variant (PNPLA3^148MM , n = 25) or lacking the variant (PNPLA3^148II , n = 25). Whole-body insulin sensitivity of lipolysis was determined using [² H₅ ]glycerol, and PNPLA3 mRNA and protein levels were measured in subcutaneous AT and liver biopsies in a subset of the volunteers.

RESULTS

PUFA-TGs were significantly increased in AT in carriers versus non-carriers of PNPLA3-I148M. The variant did not alter the rate of lipolysis or the composition of fasting serum NEFAs. PNPLA3 mRNA was 33-fold higher in the liver than in AT (P < .0001). In contrast, PNPLA3 protein levels per tissue protein were three-fold higher in AT than the liver (P < .0001) and nine-fold higher when related to whole-body AT and liver tissue masses (P < .0001).

CONCLUSIONS

Contrary to previous assumptions, PNPLA3 is highly abundant in AT. PNPLA3-I148M locally remodels AT TGs to become polyunsaturated as it does in the liver, without affecting lipolysis or composition of serum NEFAs. Changes in AT metabolism do not contribute to NAFLD in PNPLA3-I148M carriers.

The role of glucose, insulin and NEFA in regulating tissue triglyceride accumulation: Substrate cooperation in adipose tissue versus substrate competition in skeletal muscle

BACKGROUND AND AIMS

Metabolic factors initiating adipose tissue expansion and ectopic triglyceride accumulation are not completely understood. We aimed to investigate the independent role of circulating glucose, NEFA and insulin on glucose and NEFA uptake, and lipogenesis in skeletal muscle and subcutaneous adipose tissue (SCAT).

METHODS AND RESULTS

Twenty-two pigs were stratified according to four protocols: 1) and 2) low NEFA + high insulin ± high glucose (hyperinsulinaemia-hyperglycaemia or hyperinsulinaemia-euglycaemia), 3) high NEFA + low insulin (fasting), 4) low NEFA + low insulin (nicotinic acid). Positron emission tomography with [¹⁸F]fluoro-2-deoxyglucose and [¹¹C]acetate, was combined with [¹⁴C]acetate and [U-¹³C]palmitate enrichment techniques to assess glucose and lipid metabolism. Hyperinsulinaemia increased glucose extraction, whilst hyperglycaemia enhanced glucose uptake in skeletal muscle and SCAT. In SCAT, during hyperglycaemia, elevated glucose uptake was accompanied by greater [U-¹³C]palmitate-TG enrichment compared to the other groups, and by a 39% increase in de novo lipogenesis (DNL) compared to baseline, consistent with a 70% increment in plasma lipogenic index. Conversely, in skeletal muscle, [U-¹³C]palmitate-TG enrichment was higher after prolonged fasting.

CONCLUSIONS

Our data show the necessary role of hyperglycaemia-hyperinsulinaemia vs euglycaemia-hyperinsulinaemia in promoting expansion of TG stores in SCAT, by the consensual elevation in plasma NEFA and glucose uptake and DNL. In contrast, skeletal muscle NEFA uptake for TG synthesis is primarily driven by circulating NEFA levels. These results suggest that a) prolonged fasting or dietary regimens enhancing lipolysis might promote muscle steatosis, and b) the control of glucose levels, in association with adequate energy balance, might contribute to weight loss.

The AGT and the GNB3 polymorphisms and insulin resistance in prehypertension

OBJECTIVE

This study surveyed the frequencies of single nucleotide polymorphisms (SNPs) M235T AGT and C825T GNB3, and their association with insulin resistance, other biochemical markers and qualitative variables in subjects with high normal blood pressure and/or prehypertension in the Greek population.

DESIGN

330 men and women of Greek origin were divided into 3 groups: a) hypertensive, b) prehypertensive and c) control group. These groups were genetically tested for these polymorphisms and insulin resistance with the HOMA index.

RESULTS

No statistically significant differences were found among the polymorphisms of the compared groups. However, the ? allele carriers (CT/TT vs. CC) of the C825T polymorphism were associated with an increased BMI in all 3 groups (p=0.004). The HOMA index was higher in the hypertensive (p=0.006) and prehypertensive (p=0.016) versus the control group, and similar results were found for insulin (hypertensive vs. control p=0.012, prehypertensive vs. control p=0.001) without statistical significance between the first 2 groups (p=0.522). Additionally, there was a statistically significant difference between the control group and the hypertensive and prehypertensive groups regarding cholesterol (control vs. hypertensive p=0.001, control vs. prehypertension p=0.018) and triglycerides (control vs. hypertensive p=0.0001, control vs. prehypertension p=0.007). Differences were also noted between the control and the hypertensive group regarding the value of HDL (p=0.005) and LDL (p=0.013).

CONCLUSION

This study failed to demonstrate a correlation between specific SNPs, blood pressure and insulin resistance in the 3 groups. However, T allele carriers of the polymorphism C825T were found to have an increased BMI. Similarly, increased insulin resistance and lipidemia were more common in the hypertensive and prehypertensive populations.

Characterization of a novel genetically obese mouse model demonstrating early onset hyperphagia and hyperleptinemia

Obesity is a critical risk factor for the development of metabolic syndrome, and many obese animal models are used to investigate the mechanisms responsible for the appearance of symptoms. To establish a new obese mouse model, we screened ∼13,000 ICR mice and discovered a mouse demonstrating spontaneous obesity. We named this mouse "Daruma" after a traditional Japanese ornament. Following the fixation of the genotype, these animals exhibited obese phenotypes according to Mendel's law of inheritance. In the Daruma mouse, the leptin receptor gene sequence carried two base mutations that are good candidates for the variation(s) responsible for the obese phenotype. The Daruma mice developed characteristic visceral fat accumulation at 4 wk of age, and the white adipose and liver tissues exhibited increases in cell size and lipid droplets, respectively. No histological abnormalities were observed in other tissues of the Daruma mice, even after the mice reached 25 wk of age. Moreover, the onset of impaired leptin signaling was early and manifested as hyperleptinemia and hyperinsulinemia. Pair feeding completely inhibited obesity, although these mice rapidly developed hyperphagia and obesity followed by hyperleptinemia when pair feeding ceased and free-access feeding was permitted. Therefore, the Daruma mice exhibited unique characteristics and may be a good model for studying human metabolic syndrome.

Insulin signalling mechanisms for triacylglycerol storage

Insulin signalling is uniquely required for storing energy as fat in humans. While de novo synthesis of fatty acids and triacylglycerol occurs mostly in liver, adipose tissue is the primary site for triacylglycerol storage. Insulin signalling mechanisms in adipose tissue that stimulate hydrolysis of circulating triacylglycerol, uptake of the released fatty acids and their conversion to triacylglycerol are poorly understood. New findings include (1) activation of DNA-dependent protein kinase to stimulate upstream stimulatory factor (USF)1/USF2 heterodimers, enhancing the lipogenic transcription factor sterol regulatory element binding protein 1c (SREBP1c); (2) stimulation of fatty acid synthase through AMP kinase modulation; (3) mobilisation of lipid droplet proteins to promote retention of triacylglycerol; and (4) upregulation of a novel carbohydrate response element binding protein β isoform that potently stimulates transcription of lipogenic enzymes. Additionally, insulin signalling through mammalian target of rapamycin to activate transcription and processing of SREBP1c described in liver may apply to adipose tissue. Paradoxically, insulin resistance in obesity and type 2 diabetes is associated with increased triacylglycerol synthesis in liver, while it is decreased in adipose tissue. This and other mysteries about insulin signalling and insulin resistance in adipose tissue make this topic especially fertile for future research.

Meal frequency differentially alters postprandial triacylglycerol and insulin concentrations in obese women

OBJECTIVE

The aim of this study was to compare postprandial lipemia, oxidative stress, antioxidant activity, and insulinemia between a three and six isocaloric high-carbohydrate meal frequency pattern in obese women.

DESIGN AND METHODS

In a counterbalanced order, eight obese women completed two, 12-h conditions in which they consumed 1,500 calories (14% protein, 21% fat, and 65% carbohydrate) either as three 500 calorie liquid meals every 4-h or six 250 calorie liquid meals every 2-h. Blood samples were taken every 30 min and analyzed for triacylglycerol (TAG), total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, oxidized low-density lipoprotein cholesterol, myeloperoxidase, paraoxonase-1 activity, and insulin.

RESULTS

The TAG incremental area under the curve (iAUC) during the three meal condition (321 ± 129 mg/dl · 12 h) was significantly lower (P = 0.04) compared with the six meal condition (481 ± 155 mg/dl · 12 h). The insulin iAUC during the three meal condition (5,549 ± 1,007 pmol/l · 12 h) was significantly higher (P = 0.05) compared with the six meal condition (4,230 ± 757 pmol/l(.) 12 h). Meal frequency had no influence on the other biochemical variables.

CONCLUSIONS

Collectively, a three and six isocaloric high-carbohydrate meal frequency pattern differentially alters postprandial TAG and insulin concentrations but has no effect on postprandial cholesterol, oxidative stress, or antioxidant activity in obese women.

Effects of hyperinsulinemia on lipoprotein lipase, angiopoietin-like protein 4, and glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 in subjects with and without type 2 diabetes mellitus

Our aims were to compare the systemic effects of insulin on lipoprotein lipase (LPL) in tissues from subjects with different degrees of insulin sensitivity. The effects of insulin on LPL during a 4-hour hyperinsulinemic, euglycemic clamp were studied in skeletal muscle, adipose tissue, and postheparin plasma from young healthy subjects (YS), older subjects with type 2 diabetes mellitus (DS), and older control subjects (CS). In addition, we studied the effects of insulin on the expression of 2 recently recognized candidate genes for control of LPL activity: angiopoietin-like protein 4 (ANGPTL4) and glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1. As an effect of insulin, LPL activity decreased by 20% to 25% in postheparin plasma and increased by 20% to 30% in adipose tissue in all groups. In YS, the levels of ANGPTL4 messenger RNA in adipose tissue decreased 3-fold during the clamp. In contrast, there was no significant change in DS or CS. Regression analysis showed that the ability of insulin to reduce the expression of ANGPTL4 was positively correlated with M-values and inversely correlated with factors linked to the metabolic syndrome. Expression of glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 tended to be higher in YS than in DS or CS, but the expression was not affected by insulin in any of the groups. Our data imply that the insulin-mediated regulation of LPL is not directly linked to the control of glucose turnover by insulin or to ANGPTL4 expression in adipose tissue or plasma. Interestingly, the response of ANGPTL4 expression in adipose tissue to insulin was severely blunted in both DS and CS.

Gastric inhibitory polypeptide: the neglected incretin revisited

After the ingestion of fat- and glucose-rich meals, gut hormones are secreted into the circulation in order to stimulate insulin secretion. This so-called "incretin effect" is primarily conferred by Glucagon-like peptide 1 (GLP-1) and Gastric Inhibitory Polypeptide (GIP). In contrast to GLP-1, GIP has lost most of its insulinotropic effect in type 2 diabetic patients. In addition to its main physiological role in the regulation of endocrine pancreatic secretion, GIP exerts various peripheral effects on adipose tissue and lipid metabolism, thereby leading to increased lipid deposition in the postprandial state. In some animal models, an influence on gastrointestinal functions has been described. However, such effects do not seem to play an important role in humans. During the last years, the major line of research has focussed on GLP-1, due to its promising potential for the treatment of type 2 diabetes mellitus. However, the physiological importance of GIP in the regulation of insulin secretion has been shown to even exceed that of GLP-1. Furthermore, work from various groups has provided evidence that GIP contributes to the pathogenesis of type 2 diabetes to a considerable degree. Recent data with modified GIP analogues further suggested a possibility of therapeutic use in the treatment of type 2 diabetes. Thus, it seems worthwhile to refocus on this important and-sometimes-neglected incretin hormone. The present work aims to review the physiological functions of GIP, to characterize its role in the pathogenesis of type 2 diabetes, and to discuss possible clinical applications and future perspectives in the light of new findings.

The effect of chronic insulin delivery via the intraperitoneal versus the subcutaneous route on hepatic triglyceride secretion rate in streptozotocin diabetic rats

Chronic intraperitoneal or subcutaneous insulin administration increases triglyceride secretion rate (TGSR) in normal rats. We wished to determine the effect of this treatment on TGSR and the hepatic lipogenic enzymes acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) in diabetic rats. Streptozotocin-diabetic rats, untreated (D), diabetic rats treated with insulin (3 U/day for 21 days) intraperitoneally (IP) or subcutaneously (SC) and non-diabetic rats (N) were studied. TGSR was determined using Triton WR-1339. Fasting glucose and triglyceride levels, high in D, were normalized by insulin treatment regardless of route. Peripheral insulin levels were lowest in D and highest in SC, portal insulin levels were lowest in D and highest in IP. Non-esterified fatty acid levels were not elevated in D, presumably due to adipose tissue depletion. TGSR was reduced in D (P<0.05) and was normalized following insulin administration, regardless of route. ACC activity was normal, but FAS was decreased in D (P<0.05). ACC and FAS were normal in both IP and SC. Thus, in streptozotocin-diabetic rats, chronic intraperitoneal or subcutaneous insulin treatment increases TGSR and FAS activity from their low levels in insulin-deficient rats to levels equal to but not higher than those in normal rats.

Insulin resistance affects the regulation of lipoprotein lipase in the postprandial period and in an adipose tissue-specific manner

AIMS

Insulin is a potent stimulator of adipose tissue lipoprotein lipase (LPL). Logically, the postprandial period is therefore a privileged time of the day for the regulation of LPL by insulin in this tissue. It is not clear to what extent a defect such as insulin resistance could affect this regulation and contribute to postprandial, as well as fasting, hypertriglyceridaemia. The aim of the present protocol was to study the relationship between insulin resistance and LPL in adipose tissue and in plasma, in the particular context of the postprandial period.

METHODS

For this study, 26 adult nondiabetic individuals (12 women and 14 men) with a wide range of whole-body insulin-mediated glucose uptake (as assessed with an insulin suppression test) were studied. An abdominal subcutaneous fat biopsy on one occasion, and post-heparin plasma on another occasion, were obtained 4 h into a standardized meal profile administered in the fasting state.

RESULTS

Postprandial triglyceride excursions (evaluated by the incremental area under the curve during the metabolic meal profile) were inversely correlated to adipose tissue LPL mRNA levels (rho = -0.43, P < 0.03) as well as to adipose tissue LPL heparin-releasable activity (rho = -0.58, P < 0.01). Steady-state plasma glucose (SSPG) concentrations during the insulin suppression test, a reflection of the degree of insulin resistance, were also negatively correlated to adipose tissue LPL mRNA (rho = -0.50, P < 0.02) and activity (rho = -0.56, P < 0.01). There was no correlation between plasma post-heparin LPL activity/mass and postprandial triglycerides nor with insulin resistance.

CONCLUSION

Regulation of adipose tissue LPL is significantly affected in insulin-resistant individuals in the postprandial period. This presumed impaired effect of insulin on LPL postprandially could be an important contributor to the atherogenic dyslipidaemia described in insulin resistance syndrome.

Effect of acute exogenous hyperinsulinaemia on very low density lipoprotein subfraction composition in normal subjects

Subtle abnormalities of very-low-density lipoprotein (VLDL) composition and distribution seem to be associated with increased cardiovascular risk. The aims of this study were first, to evaluate whether hyperinsulinaemia per se is able to produce VLDL abnormalities and second, whether this occurs through a stimulation of lipolytic enzymes. Eight normal male volunteers, age 36 +/- 7 years (M +/- SD), body mass index (BMI) 26+/-3 kg m-2, underwent a 5-h euglycaemic hyperinsulinaemic clamp (1.2 mU insulin/kg b.w. min-1). Nine sex, age and BMI comparable subjects underwent control experiments (saline infusion). Three VLDL subfractions of decreasing size were isolated by density gradient ultracentrifugation; lipoprotein lipase (LPL) and hepatic lipase (HL) post-heparin plasma activities were determined by the 3H-labelled triolein method. Hyperinsulinaemia ( approximately 65 mU mL-1) produced the expected plasma free fatty acid suppression. Triglyceride levels were reduced in total VLDL (- 27 +/- 32% vs. + 38 +/- 52% after saline, P < 0.05) and in the larger VLDL (- 56 +/- 19 vs. + 34 +/- 38, P < 0.001). Moreover the relative contribution of the larger subfraction was decreased (- 39 +/- 15% vs. - 3 +/- 21%, P < 0.01), while the percentage of smaller particles was increased (+17 +/- 20 vs. - 9 +/- 22, P < 0.05). LPL and HL activities were decreased to the same degree during either insulin or saline infusion. Exogenous hyperinsulinaemia produced lipoprotein abnormalities partially similar to those previously shown in type 1 diabetic patients, indicating that these abnormalities may be secondary to insulin therapy.

Insulin decreases plasma cholesteryl ester transfer but not cholesterol esterification in healthy subjects as well as in normotriglyceridaemic patients with type 2 diabetes

BACKGROUND

Plasma cholesterol esterification (EST) and subsequent cholesteryl ester transfer (CET) from high-density lipoproteins (HDLs) towards apolipoprotein (apo) B-containing lipoproteins are key steps in HDL metabolism.

MATERIALS AND METHODS

The effects of exogenous hyperinsulinaemia on plasma CET and EST, measured with isotope methods, were evaluated in 10 male normotriglyceridaemic (plasma triglycerides <2.0 mmol L-1) patients with type 2 diabetes and 10 individually matched healthy subjects during a two-step hyperinsulinaemic euglycaemic clamp over 6-7 h.

RESULTS

No between-group differences in baseline plasma lipid parameters were observed, but the HDL cholesteryl ester content was lower (P < 0.02) and the HDL triglyceride content was higher (P < 0.05) in diabetic patients. Baseline CET and EST were similar in the groups. In both groups, hyperinsulinaemia decreased plasma triglycerides (P < 0.01) and the HDL triglyceride content (P < 0.01) compared with saline infusion in healthy subjects, whereas the HDL cholesteryl ester content increased (P < 0.05 vs. saline infusion) in diabetic patients. CET was similarly decreased by hyperinsulinaemia in both groups (P < 0.01 vs. saline infusion). In contrast, the change in EST in either group was not different from that during saline administration. In the combined group, baseline CET was positively correlated with plasma triglycerides (Rs = 0.68, P < 0.01). The HDL cholesteryl ester content was negatively (Rs = -0.48, P < 0.05) and the HDL triglyceride content was positively (Rs = 0.64, P < 0.01) correlated with CET.

CONCLUSION

Insulin infusion decreases plasma CET in conjunction with a fall in triglycerides but does not decrease cholesterol esterification in healthy and type 2 diabetic subjects, indicating that acute hyperinsulinaemia has a different effect on these processes involved in HDL metabolism. Despite unaltered fasting plasma CET, HDL core lipid composition was abnormal in diabetic patients, suggesting that additional mechanisms may contribute to changes in HDL metabolism in diabetes mellitus.

Insulin-like growth factor-I lowers fasting and postprandial triglyceride levels without affecting chylomicron clearance in healthy men

OBJECTIVES

To study whether IGF-I treatment alters the postprandial lipid and lipoprotein metabolism.

DESIGN

Randomized, crossover study.

SETTING

University Hospital, Zürich, Switzerland.

SUBJECTS

Seven young healthy male subjects (aged 27+/-4 years, body mass index (BMI) 21.8+/-1.7 kg m(-2)).

INTERVENTIONS

Each subject was studied two times at 2-week intervals, treated with saline 0.9% (S) and IGF-I (8 microg kg(-1) h(-1)) by a continuous subcutaneous infusion. 60 h after the start of treatment a vitamin A loading test was performed after an overnight 12-h fast.

MAIN OUTCOME MEASURES

Glucose, insulin, total and free IGF-I, FFA, triglycerides and retinyl palmitate, total cholesterol, HDL and LDL cholesterol, lipoprotein (a) and apolipoprotein B were measured in serum before and after the fatty meal.

RESULTS

Total IGF-I levels rose from 29.0+/-3.3 nmol L(-1) to 113.3+/-9.0 nmol L(-1) (P<0.02) and free IGF-I from 0.24+/-0.05 to 1.08+/-0.28 nmol L(-1) (P<0.02) during IGF-I treatment. IGF-I administration reduced insulin concentrations by 50% (P<0.02), as assessed by the area under the curve. Serum triglyceride levels were significantly lower at baseline and after the fat load during IGF-I treatment (P<0.02), whereas the retinyl palmitate concentrations in chylomicron and nonchylomicron lipoprotein fractions were similar during both treatment periods.

CONCLUSIONS

IGF-I treatment reduces the triglyceride levels most probably by decreasing insulin secretion and the production of VLDL particles, and possibly by increasing their turnover. IGF-I treatment has no significant effect on the metabolism of intestine-derived triglyceride-rich lipoproteins after a high fat meal in healthy young men.

The effect of a pure antiandrogen receptor blocker, flutamide, on the lipid profile in the polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is one of the most common endocrinopathies affecting women of reproductive age; it is associated with hyperandrogenism, hyperinsulinemia, and dyslipidemia. This study was designed to assess the long term effects of a pure androgen receptor blocker, flutamide, on the lipid profile in women with PCOS and to examine the possible mechanisms by which androgens may exert their influence. Seventeen women with PCOS (10 obese and 7 lean) were studied. All subjects received a 12-week course of oral flutamide (500 mg/day). The baseline and posttreatment evaluations included lipid profile, androgen levels, insulin sensitivity, and serum catecholamine determinations. The primary outcome was the change in the ratio of low density lipoproteins (LDL) to high density lipoproteins (HDL). Treatment with flutamide was associated with a significant decrease in the LDL/HDL ratio by 23% (P = 0.005), in total cholesterol by 18% (P < 0.0001), in LDL by 13% (P = 0.002), and in triglycerides by 23% (P = 0.002). Flutamide treatment was also associated with a trend toward an increase in HDL (by 14%; P = 0.14). The effects on lipid profile were found regardless of obesity and were not associated with a change in weight. Furthermore, actions of flutamide on lipid metabolism were not associated with significant changes in circulating adrenaline or noradrenaline, glucose metabolism, or insulin sensitivity. This report has demonstrated for the first time that treatment with the pure antiandrogen, flutamide, may improve the lipid profile and that this effect may be due to direct inhibition of androgenic actions.

Suppression of plasma cholesteryl ester transfer protein activity in acute hyperinsulinemia and effect of plasma nonesterified fatty acid

Cholesteryl ester transfer protein (CETP) is a major determinant of the plasma high-density lipoprotein cholesterol (HDL-C) level and plays an important role in the reverse cholesterol transport system. The purpose of this study was to determine the effect of acute hyperinsulinemia on plasma CETP activity in normal subjects and patients with non-insulin-dependent diabetes mellitus (NIDDM). Hyperinsulinemia was achieved using the hyperinsulinemic-euglycemic clamp. CETP activity was determined as the transfer of radiolabeled cholesterol in HDL3 to acceptor lipoprotein. Mean plasma CETP activity during an insulin infusion in both subject groups was significantly decreased compared with the mean basal activity. Suppression of plasma CETP activity in the NIDDM patients was significantly less than in the normal subjects (-4.2% +/- 7.9% v -9.6% +/- 6.4%, P < .02). Regression analysis showed that this suppression was correlated with plasma nonesterified fatty acid (NEFA) levels after the clamp and with the magnitude of the NEFA decrease (r = .318, P < .02 and r = .292, P < .05, respectively). The data suggest that acute hyperinsulinemia reduces plasma CETP activity through a decrease in plasma NEFA.

Hyperglycemia-induced hyperinsulinemia acutely lowers plasma apolipoprotein B but not lipoprotein (a) in man

Acute hyperinsulinemia lowers plasma apolipoprotein B (apo B) and triglycerides by suppressing hepatic lipoprotein secretion and probably by enhancing catabolism of triglyceride-rich lipoproteins, but the effect of acute hyperinsulinemia on the plasma lipoprotein(a) (Lp(a)) level is unclear. We measured plasma triglycerides, cholesterol, apo B and Lp(a) in response to 3 h hyperglycemia-induced hyperinsulinemia (blood glucose clamped at 10 mmol/l) in 16 subjects (eight women and eight men). In a control experiment saline was infused in another group of seven men. After 3 h of hyperinsulinemia plasma triglycerides decreased by 29 +/- 14% (mean +/- S.D., P < 0.001) and this fall differed from the unchanged triglyceride level during saline infusion (P < 0.001). Plasma cholesterol fell by 8 +/- 5% (P < 0.001), which was different from the unchanged cholesterol during saline infusion (P < 0.02). Plasma apo B decreased by 9 +/- 8% (P < 0.001), which was again different from the minor fall in apo B (3 +/- 2%) during saline infusion (P < 0.02). However, plasma Lp(a) remained unchanged during hyperinsulinemia (change 8 +/- 15%, n.s.), as well as during saline infusion (change 5 +/- 15%, n.s.). The % change in apo B exceeded the % change in Lp(a) during hyperinsulinemia (P < 0.01). Baseline Lp(a) was inversely correlated with first phase insulin secretion (P < 0.05), but its level during the clamp was not related to insulin sensitivity. This study demonstrates that acute hyperglycemia-induced hyperinsulinemia has a different effect on plasma apo B and Lp(a) in healthy subjects. The present data support the notion that Lp(a) is metabolized differently from triglyceride-rich lipoproteins.

Lowering of plasma phospholipid transfer protein activity by acute hyperglycaemia-induced hyperinsulinaemia in healthy men

Human plasma contains two lipid transfer proteins involved in the remodelling of plasma lipoproteins; cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP). CETP mediates the transfer/exchange of cholesterylesters, triglycerides and phospholipids between high-density lipoproteins (HDL) and chylomicron (remnants), very low-density lipoproteins (VLDL) and low density lipoproteins (LDL). The physiological function of PLTP is unknown. It is able to transfer phospholipids (but not neutral lipids) between lipoproteins and to modulate HDL particle size in vitro. The effects of acute endogenous hyperinsulinaemia on plasma CETP and PLTP activity, as well as on lipid and lipoprotein levels, were assessed in eight healthy men during a 3-h hyperglycaemic clamp. Another group of seven men received an infusion of an equal volume of saline in order to detect possible dilution effects or effects on lipoprotein changes over time (control group). Plasma cholesterol and triglyceride concentrations fell during the clamp and the decreases were significantly different from the minor changes during saline infusion in the control group (p < 0.05 and p < 0.01, respectively). Plasma CETP activity levels did not change, but plasma PLTP activity levels decreased by 7.7 and 5.1% after 2 and 3 h of hyperglycaemia (p < 0.01 for each time-point). The hyperglycaemia-induced mean percentage change in PLTP activity levels during the 3 h of the clamp was greater than the essentially absent change during the NaCl infusion (p < 0.05). Plasma PLTP activity during the clamp was related negatively to the insulin sensitivity index (p < 0.01 by analysis of covariance). It is concluded that acute hyperglycaemia-induced hyperinsulinaemia lowers plasma PLTP, but not CETP activity levels, either directly or in conjunction with an effect on plasma lipoproteins.

The effect of acute hyperinsulinemia on plasma cholesteryl ester transfer protein activity in patients with non-insulin-dependent diabetes mellitus and healthy subjects

The effect of acute hyperinsulinemia on plasma cholesteryl ester (CE) transfer protein (CETP) activity was determined in 11 patients with non-insulin-dependent diabetes mellitus (NIDDM) and 10 healthy subjects. Plasma CETP activity was reduced significantly in NIDDM patients (-37 +/- 59 nmol/mL/h, P < .05) but not in healthy subjects (-7 +/- 37 nmol/mL/h) during insulin infusion. Saline infusion did not alter plasma CETP activity significantly. The change in plasma CETP activity was correlated significantly with the baseline plasma triglyceride (TG) concentration (r = -.523, n = 21, P = .01) and marginally with the concomitant decrease in these levels with acute hyperinsulinemia (r = .413, n = 21, P = .06) in NIDDM patients and healthy subjects combined. These data indicate that acute hyperinsulinemia reduces plasma CETP activity and probably plasma CETP concentration in NIDDM patients, and suggest coordinated regulation of CETP levels and TG metabolism by insulin.

Augmented effect of short-term pulsatile versus continuous insulin delivery on lipid metabolism but similar effect on whole-body glucose metabolism in obese subjects

The present study was designed to examine the effect of pulsatile versus continuous insulin delivery on glucose and lipid metabolism in insulin-resistant subjects. Six obese women (body mass index, 40.0 +/- 2.8 kg/m2) underwent a euglycemic glucose clamp (plasma glucose, 90 mg/dL) twice. In random order, insulin was infused intravenously for 375 minutes either at a constant rate (0.4 mU/kg/min) or in a pulsatile manner (2.4 mU/kg/min for 2 minutes followed by an off interval of 10 minutes). Endogenous insulin release was suppressed by infusion of somatostatin (250 micrograms/h). Mean circulating insulin concentrations were similar during the two protocols (pulsatile v continuous infusion, 60 +/- 10 v 56 +/- 9 mU/L), but pulsatile infusion was accompanied by oscillations with an amplitude of 120 mU/L. After 6 hours of pulsatile versus continuous insulin, isotopically determined total glucose disposal (3-3H-glucose) and hepatic glucose production (HGP) were comparable (pulsatile v continuous, 2.80 +/- 0.56 v 2.82 +/- 0.51 and 0.37 +/- 0.14 v 0.32 +/- 0.17 mg/kg/min). However, the rate of glucose oxidation (indirect calorimetry) was augmented (P < .05), whereas lipid oxidation tended to be diminished (.10 > P > .05) following pulsatile infusion. In addition, blood glycerol was more suppressed with pulsatile (31 +/- 9 nmol/L) than with continuous infusion (36 +/- 10 nmol/L, P < .05), whereas blood lactate, alanine, and 3-hydroxybutyrate were similar in the two infusion protocols.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-esterified fatty acids regulate lipid and glucose oxidation and glycogen synthesis in healthy man

We examined the interrelationship of lipid and glucose metabolism in the basal state and during insulin stimulus in 19 healthy men (27 +/- 2 years, body mass index 23.6 +/- 0.6 kg/m2). In each subject, we performed a 4-h euglycaemic (5.3 +/- 0.1 mmol/l) hyperinsulinaemic (647 +/- 21 pmol/l) insulin clamp with indirect calorimetry in the basal state and during insulin infusion, and muscle biopsies before and at the end of the clamp. In the basal state, serum non-esterified fatty acid levels correlated directly with lipid oxidation (r = 0.56, p < 0.05) and indirectly with glucose oxidation (r = -0.80, p < 0.001). Lipid and glucose oxidation rates were inversely related in the basal state (r = -0.47, p < 0.05) and during insulin infusion (r = -0.65, p < 0.01). Basal lipid oxidation and glycogen synthase total activity correlated inversely (r = -0.54, p < 0.05). Lipid oxidation both in the basal state (r = -0.61, p < 0.01) and during insulin infusion (r = -0.62, p < 0.05) was inversely related to muscle glycogen content after the insulin clamp. Fasting plasma triglyceride concentration correlated directly to fasting insulin (r = 0.55, p < 0.05) and C-peptide (r = 0.50, p < 0.03) concentrations and inversely to non-oxidative glucose disposal rate at the end of clamp (r = -0.54, p < 0.05).

IN CONCLUSION

1) Serum non-esterified fatty acid concentration enhances lipid and reduces glucose oxidation. 2) Lipid oxidation is inversely related to total glycogen synthase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

The response of hepatic lipase and serum lipoproteins to acute hyperinsulinaemia in type 2 diabetes

Hepatic lipase has a putative role in the catabolism of HDL particles and, while its activity is dependent upon insulin in the rat, no such insulin responsiveness has been demonstrated in man. We studied 21 patients with type 2 diabetes to examine whether hepatic lipase activity was influenced by hyperinsulinaemia during a 2-4 h isoglycaemic clamp study. Acute changes in lipids, lipoproteins and apolipoproteins were also documented in pre- and post-clamp serum. Hepatic lipase activity during hyperinsulinaemia was compared with activity measured after an equivalent period without insulin. For comparison, nine non-diabetic subjects (matched for age and body mass index) underwent similar clamp studies. In the control experiment without insulin, hepatic lipase activity did not change significantly (mean 9.7 (range 2.3-22.3) in the morning and 9.9 (3.0-22.5) mmol h-1 l-1 in the afternoon, NS). In contrast, after the hyperinsulinaemic clamp, hepatic lipase activity fell significantly in diabetic subjects from 12.8 (4.4-30.6) to 10.4 (3.3-31.3) mmol h-1 l-1, P less than 0.0002 along with serum triglycerides and total and LDL cholesterol. The change in hepatic lipase activity was positively related to the fasting apoprotein B concentration (Spearman r = 0.54, P = 0.016). In the normal subjects, a similar decline in hepatic lipase activity was observed during hyperinsulinaemia (from 15.1 (9.8-32.7) to 12.6 (6.3-28.3) mmol h-1 l-1, P less than 0.01) along with decreases in total, HDL and LDL cholesterol, triglycerides and apoproteins A1 and B.(ABSTRACT TRUNCATED AT 250 WORDS)

Adipose tissue metabolism in obesity: lipase action in vivo before and after a mixed meal

Physiological actions of insulin include suppression of fat mobilization from adipose tissue and activation of adipose tissue lipoprotein lipase. Here, we report measurements of adipose tissue hormone-sensitive lipase (HSL) and lipoprotein lipase (LPL) action in vivo in 10 normal and eight obese subjects, with the latter group having varying degrees of glucose intolerance. HSL and LPL actions (per gram of adipose tissue) were similar in the two groups, after an overnight fast. In the normal subjects, HSL action was suppressed after a meal (by 75% +/- 6% between 60 to 300 minutes, P less than .01), and the action of LPL was increased (clearance of circulating triacylglycerol [TAG] increased by 140% +/- 57% at 300 minutes, P less than .05). Despite hyperinsulinemia, these responses were blunted in the obese subjects (P less than .05 for each change being less than in normal group). The adipose tissue of the obese subjects showed continued nonesterified fatty acid (NEFA) release at a time when NEFA mobilization was completely suppressed in the normal group. Both impaired suppression of HSL and low fractional retention of fatty acids for reesterification within the adipose tissue contributed to this abnormal NEFA release. Impaired activation of LPL was associated with a greater absolute increase in plasma TAG concentration postprandially in the obese. In obese subjects, adipose tissue HSL and LPL fail to respond to immunoreactive insulin postprandially, which may be an important maladaptation in terms of lipoprotein metabolism and risk of coronary heart disease.

Effect of acute hyperinsulinemia on fatty acid composition of serum lipids in non-insulin-dependent diabetics and healthy men

The fatty acid pattern of serum phospholipids, cholesteryl esters, triglycerides and free fatty acids was measured before and after a 5-h two-step euglycemic hyperinsulinemic clamp (75 and 1400 microU/ml) in 21 non-insulin-dependent diabetics and 14 age-, weight-, and sex-matched healthy controls. Acute hyperinsulinemia was associated with a statistically significant increase in essential fatty acid and a decrease in non-essential fatty acid contents in triglycerides while the levels of serum triglycerides and free fatty acids dropped in both groups. The fatty acid composition of phospholipids and cholesteryl esters remained unchanged as did the levels of serum phospholipids, total cholesterol and HDL cholesterol.

The role of insulin insensitivity and hepatic lipase in the dyslipidaemia of type 2 diabetes

Fourteen male patients with Type 2 diabetes were studied to identify relationships between insulin-mediated glucose disposal, basal and glucose-stimulated insulin secretion, fasting lipoproteins and apolipoproteins, and the activities of lipoprotein lipase and hepatic lipase. Sensitivity of glucose disposal to exogenous insulin correlated positively with HDL-cholesterol (r = 0.65, p less than 0.05), HDL2-cholesterol (r = 0.59, p less than 0.05), and apolipoprotein A1 (r = 0.57, p less than 0.05) and negatively with apolipoprotein B (r = -0.53, p less than 0.05) and total: HDL-cholesterol ratio (r = -0.68, p less than 0.01). Fasting C-peptide correlated negatively with HDL-cholesterol (r = -0.76, p less than 0.01), HDL2-cholesterol (r = -0.80, p less than 0.001) and apoprotein A1 (r = -0.56, p less than 0.05) and positively with total: HDL-cholesterol ratio (r = 0.64, p less than 0.05). Neither fasting plasma glucose nor the indices of stimulated insulin secretion (glucose-stimulated plasma insulin and C-peptide) were related to any of the lipoprotein measures. Insulin insensitivity and hyperinsulinaemia were both associated with higher levels of hepatic lipase activity but did not influence lipoprotein lipase activity. In multiple linear regression analysis, hepatic lipase activity was related to HDL-cholesterol independent of insulin insensitivity. In addition, fasting C-peptide alone accounted for 70% of the variance in hepatic lipase activity and this was independent of insulin sensitivity and body mass index. We propose that the abnormalities of HDL-cholesterol in Type 2 diabetes are closely related to enhanced hepatic lipase activity brought about by increased insulin secretion which, in turn, is secondary to the defect in insulin action.

Postprandial triglyceride response in type 1 (insulin-dependent) diabetes mellitus is not altered by short-term deterioration in glycaemic control or level of postprandial insulin replacement

The effect of deteriorating glycaemic control on the lipoprotein responses to the ingestion of a high fat meal was investigated in seven normolipidaemic Type 1 (insulin-dependent) diabetic patients and the results were compared with corresponding responses in seven normolipidaemic control subjects. In addition, the importance of insulin in regulating the postprandial lipoprotein responses was examined by comparing the results obtained from the diabetic patients maintained on a basal infusion of insulin throughout the study with those obtained when a step-up, step-down insulin infusion was administered following the meal. Vitamin A was added to the test meal in all subjects to trace the metabolism of the chylomicron (Sf greater than 1000) and non-chylomicron (Sf less than 1000) fractions in the postprandial period. No differences in fasting and postprandial triglyceride levels nor in the concentration of the chylomicron and non-chylomicron fractions were observed between diabetic and control subjects. In the diabetic patients short-term (two-week) deterioration in glycaemic control did not have any adverse influence on the basal and postprandial lipid responses. However, while the amount of insulin administered after the meal in the diabetic patients did not have any effect on the postprandial triglyceride or chylomicron responses, the concentration of non-esterified fatty acids was significantly higher (p less than 0.0005) when only a basal infusion of insulin was administered.

IN CONCLUSION

1) Short-term deterioration in glycaemic control does not adversely affect lipoprotein concentrations in Type 1 diabetes. 2) Non-esterified fatty acids appear to be a more sensitive index of insulinization post-prandially than triglycerides.

Tissue-specific regulation of lipoprotein lipase activity by insulin/glucose in normal-weight humans

Eight normal-weight subjects (four men, four women) were studied to determine the relative activities of lipoprotein lipase (LPL) in adipose tissue (ATLPL) and vastus lateralis skeletal muscle (SMLPL), both in the fasting state and in response to a 6-hour insulin/glucose infusion. Mean fasting levels of ATLPL and SMLPL were not statistically different. After 6 hours of insulin/glucose infusion, mean ATLPL activity was significantly greater than the fasting level (P less than .01), while mean SMLPL activity decreased from basal (P less than .05). These tissue-specific changes in LPL responsiveness (0 to 6 hours) were significantly different (P less than .01). No differences between men and women were observed. These divergent tissue-specific LPL responses to insulin/glucose would serve to direct lipoprotein triglyceride-derived fatty acids away from muscle and to adipose tissue for storage.

Evidence of heterogeneous mechanisms in lipoprotein lipid alterations in hyperandrogenic women

Fifty-one hyperandrogenic women had their lipoprotein lipid profiles determined. Free and albumin-bound testosterone was associated with triglycerides and with high-density lipoprotein cholesterol independent of fasting insulin levels, percent ideal body weight, and waist/hip ratio. To gain insight into mechanisms of these lipid alterations, the women were subgrouped according to apparent source of androgen excess. Whereas all groups had low levels of high-density lipoprotein-2 cholesterol and high triglyceride concentrations, only in those with high luteinizing hormone-to-follicle-stimulating hormone ratios was free and albumin-bound testosterone associated with triglycerides and high-density lipoprotein cholesterol independent of fasting insulin levels. Relationships between percent ideal body weight and waist/hip ratios, free and albumin-bound testosterone, sex hormone binding globulin, fasting insulin and 2-hour insulin levels and blood pressure are not significant in all subgroups, suggesting differing endocrinological influences and differing mechanisms for lipoprotein lipid alterations.

Interrelationship between glucose and fat utilisation during simultaneous IV administration of glucose and lipid emulsion in healthy man

In order to evaluate the interaction between glucose and fat utilisation during parenteral nutrition, we studied 12 healthy volunteers under three different conditions: While Intralipid infusion does not alter the glucose utilisation, measured by metabolic clearance rate of glucose at an insulin level of about 80 uU/ml (MCR(glu)submax) 11.3 +/- 1 v.s. 11.48 +/- 0.9 ml/kg/min) and the sensitivity index, it decreases glucose utilisation at an insulin level of about 550 uU/ml (MCR(glu)max) (17.8 +/- 1.3 v.s. 15.9 +/- 0.9 ml/kg/min, p < 0.05) suggesting postreceptor alterations in insulin action. Simultaneous administration of Intralipid with glucose and insulin is associated with a smaller increase in serum triglycerides than the infusion of Intralipid alone (2.56 +/- 0.4 v.s. 5.44 +/- 0.5 mmol/lm p < 0.001). We conclude that, when infused at standard rates, Intralipid does not decrease glucose utilisation significantly and, at the same time, its own clearance is enhanced by glucose and insulin.

Effects of insulin and exercise on muscle lipoprotein lipase activity in man and its relation to insulin action

The effects of exercise and a physiological increase in plasma insulin concentration on muscle lipoprotein lipase activity (mLPLA), leg exchange of glucose, and serum lipoprotein levels were investigated in healthy young men. During euglycemic hyperinsulinemia (n = 7) at 44 mU.liter-1, m-LPLA in non-exercised muscle decreased from 30 +/- 7.4 mU.g-1 wet weight (w.w.) (mean +/- SE) to 19 +/- 3.3 (P less than 0.05). Furthermore, the decrease in m-LPLA correlated closely (r = 0.97, P less than 0.05) with the increase in leg glucose uptake. Moreover, basal m-LPLA correlated with the insulin-induced increase in leg glucose uptake (r = 0.93, P less than 0.05). In the control group (n = 6) in which saline was infused in place of insulin and glucose, m-LPLA in nonexercised muscle did not change with time. No change in m-LPLA was observed immediately after one-legged knee extension exercise, but 4 h after exercise m-LPLA was higher (P less than 0.05) in the exercised thigh (47 +/- 17.8 mU.g-1 w.w.) compared with the contralateral nonexercised thigh (29 +/- 6.3 mU.g-1 w.w.). This difference was not found 8 h after exercise. The triacylglycerol content of serum lipoproteins decreased during insulin infusion. It is concluded that in contrast to the effect on adipose tissue, physiological concentrations of insulin decrease m-LPLA in proportion to the effect of insulin on muscle glucose uptake, while muscle contractions cause a local, delayed, and transient increase in m-LPLA. Further-more, basal m-LPLA is an indicator of muscle insulin sensitivity.

Relationship between lipoprotein levels and in vivo insulin action in normal young white men

In epidemiologic studies, hyperinsulinemia has been found to be an independent risk factor for coronary heart disease (CHD). However, the mechanisms responsible for its role in atherogenesis remain unclear. We studied the relationship of in vivo insulin action and plasma lipids and lipoproteins in 44 normotriglyceridemic white men (aged 18 to 34 years). The euglycemic, hyperinsulinemic glucose clamp technique was used to quantitate insulin-mediated glucose disposal (M/I value) at a plasma insulin concentration of approximately 100 microU/mL. The M/I value correlated negatively with plasma triglycerides (r = -0.553, P less than .0001), as well as with fasting plasma insulin levels (r = -0.483, P less than .001), independent of age, body mass index, and fasting plasma glucose levels. A negative correlation of the M/I value was also observed with very low density lipoprotein (VLDL)-cholesterol (r = -0.347, P less than .05), VLDL-triglycerides (r = -0.474, P less than 0.005), and total cholesterol/high density lipoprotein (HDL)-cholesterol ratio (r = -0.431, P less than .01). The relationship between the M/I value and the total cholesterol/HDL-cholesterol ratio was independent of VLDL-cholesterol and VLDL-triglycerides, however, not independent of plasma triglycerides. No relationship was observed between insulin-mediated glucose uptake and total cholesterol, low density lipoprotein (LDL)-cholesterol, and HDL-cholesterol values. Individual differences in plasma triglycerides, fasting insulin concentration, and the total cholesterol/HDL-cholesterol ratio accounted for about half the variance observed in the M/I value.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipoprotein pattern in long-term diabetes of an at least 35 years' duration. Results of the Erfurt Study

All diabetic patients suffering from the disease for at least 20 years and living in the closed area of the Erfurt district in 1970 have been followed prospectively since that time. In 47 of them still alive in 1985, i.e. after more than 35 years of diabetes, serum lipid and apolipoprotein concentrations were measured and compared to those of non-diabetic subjects without cardiovascular diseases (n = 47) pair-matched by sex, age, and body weight. In males (n = 27) significantly (p less than 0.01) higher levels of HDL cholesterol and apolipoprotein A-I as well as lower concentrations of triglycerides and a lower total cholesterol/HDL cholesterol risk ratio than in nondiabetic control subjects could be found. In long-term diabetic females (n = 20), apolipoprotein A-I levels were also increased (p less than 0.02). Trends in HDL cholesterol and triglycerides were similar to those found in males but did not reach statistical significance. Higher concentrations of total cholesterol (p less than 0.02), LDL cholesterol (P less than 0.05), and apolipoprotein B (p less than 0.02), however, did not fit in with a beneficial lipoprotein pattern. The frequency of pathological lipoprotein patterns was not higher than among the non-diabetic control subjects (32% and 40%, respectively). According to these findings an antiatherogenic lipoprotein pattern might be considered, at least in males, as one of the determinants causing the multifactorial event of long-term survival in diabetes.

Triglyceride-rich lipoprotein metabolism during acute hyperinsulinemia in hypertriglyceridemic humans

Chronic endogenous hyperinsulinemia is associated with increased rates of triglyceride production in humans. The effect of acute exogenous hyperinsulinemia on triglyceride production was studied in seven hypertriglyceridemic men before and during six hours of hyperinsulinemic-euglycemic clamping, and in two men before and during six hours of hyperinsulinemic-hyperglycemic clamping. Apparent triglyceride production rates were assessed qualitatively by examining the rate of decline of 3H-glycerol-labeled plasma triglyceride specific activity in the preclamp period, and again when a new steady state had occurred, during the final three hours of the clamp. During the euglycemic (91.2 +/- 3.0 mg glucose/dL plasma) clamps, plasma insulin levels were increased by 700% (0.76 +/- 0.12 to 5.3 +/- 0.29 ng/mL, P less than .001) and plasma glucagon levels decreased by 19%, compared with baseline. The apparent triglyceride production rate did not increase in five of six men during the euglycemic-hyperinsulinemic clamp, or in either man during the hyperglycemic-hyperinsulinemic clamp. During the clamp period the triglyceride declined in the plasma by 23.4 +/- 3.1%; and the apolipoprotein B by 10.5 +/- 1.5%. Hyperinsulinemia with euglycemia was also associated with a decline in the ratio of triglyceride to apolipoprotein B in the triglyceride-rich lipoproteins. This was more pronounced in very low density lipoprotein (VLDL) than in intermediate density lipoprotein (IDL). In this study, hyperinsulinemia led to a decrease in the plasma glucagon concentration. This decrease was positively correlated with the decrease in the slope of the triglyceride specific activity v time curve. Hence, the changes in triglyceride production were not due to an increase in plasma glucagon concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of lipoprotein lipase immunoreactive mass in isolated human adipocytes

Previous studies of human adipose tissue lipoprotein lipase (LPL) have focused on enzyme catalytic activity, and have not measured the LPL protein directly. To study the regulation of the LPL protein, an antibody against purified bovine LPL was used. To demonstrate the specificity of the antiserum, adipose homogenates were Western blotted, and adipocytes were radiolabeled and the cell homogenates immunoprecipitated, yielding a single specific band at 53 kD. Breakdown products of LPL were demonstrated at 35 and 20 kD by Western blotting. An ELISA for human adipose LPL was established, in which LPL was sandwiched between affinity-purified antibody and biotinylated affinity-purified antibody. The standard curves for bovine LPL and human adipose LPL were parallel, and LPL activity correlated strongly with LPL immunoreactive mass. Thus, the bovine LPL standard curve was used to estimate LPL immunoreactive mass from human adipose tissue. The regulation of LPL activity and immunoreactive mass were compared in cultured adipocytes in the presence an absence of insulinlike growth factor-I/somatomedin C (IGF-I), insulin, and fetal bovine serum. IGF-I and a high insulin concentration (70 nM) stimulated only the heparin-releasable (HR) component of LPL activity and immunoreactive mass, and neither IGF-I nor insulin affected LPL specific activity. In contrast, 10% fetal bovine serum stimulated HR activity, HR mass, and cellular extractable (EXT) immunoreactive mass, with no effect on EXT activity. This resulted in a decrease in EXT specific activity in response to serum. The effects of the locally produced nucleosides adenosine and inosine were studied in a similar manner. As with serum, adenosine stimulated HR activity, HR mass, and EXT immunoreactive mass, resulting in a decrease in EXT specific activity. Inosine stimulated an increase in HR activity and HR mass, but had no effect on EXT, and thus did not change LPL specific activity. Thus, a sensitive ELISA for adipose tissue LPL has been developed using a specific, well-characterized antibody. Regulation of human LPL immunoreactive mass was demonstrated in vitro by IGF-I, serum, high concentrations of insulin, adenosine, and inosine. This method will permit further investigations into the regulation of the LPL protein.

Rapid changes in serum, plasma and erythrocyte lipid compositions, and serum transaminase levels during continuous enteral hyperalimentation by carbohydrates alone

Twelve normal men received twice their estimated basal energy requirement by a carbohydrate solution via a nasogastric catheter during 48 hours, followed by a seven-hour fast. Subsequently, in nine of them 0.5 mg epinephrine was given subcutaneously under ongoing fasting. During hyperalimentation, serum triglycerides, phospholipids, total and free cholesterol, phospholipids/free cholesterol ratio, and plasma free fatty acids decreased, whereas the percentage of free cholesterol increased. During fasting and subsequent epinephrine administration triglycerides and free fatty acids rose without reaching basal levels. Plasma and red blood cell (RBC) fatty acid composition already changed from two hours after the start of the feeding. Most markedly, a steady decrease in RBC 18:2c, omega 6, amounting to more than 17% of the basal value at the end of the observation period was found. Neither in plasma, nor in RBC a concomitant appearance of 20:3c, omega 9 was seen. In RBC, the relative amounts of the saturated fatty acids increased, whereas those of monounsaturated and polyunsaturated fatty acids decreased. RBC content of total fatty acids decreased and that of cholesterol increased. The ratios 16:1c, omega 7/16:0 and 18:3c, omega 6/18:2c, omega 6 in plasma, and 20:3c, omega 6/18:2c, omega 6 in plasma and RBC increased, whereas those of 18:1c, omega 7/16:1c, omega 7 and 20:3c, omega 6/18:3c, omega 6 in plasma decreased. After 48 hours feeding serum glutamic pyruvic transaminase and glutamic oxaloacetic transaminase levels were moderately increased and rose further during fasting. Thus, continuous enteral hyperalimentation by carbohydrates alone rapidly induces profound changes in serum-, plasma-, and RBC lipid compositions and serum parameters of hepatic function.(ABSTRACT TRUNCATED AT 250 WORDS)

Basal and postprandial lipoprotein lipase activity in adipose tissue during caloric restriction and refeeding

The effect of a ten-day caloric restriction period and of subsequent refeeding on adipose tissue lipoprotein lipase (LPL) activity was studied in 14 moderately obese women. The enzyme assays were made from subcutaneous fat taken from three separate regions (gluteal, femoral, and abdominal) after overnight fasting and from one region also after a standard meal. There was a close correlation between the activities measured from the different subcutaneous sites. The caloric restriction was followed by a decrease of the basal LPL activity to one fifth of the value recorded during the isocaloric diet. However, the relative postprandial increase of LPL activity was greater during the low-calorie diet than during the isocaloric diet. During refeeding the basal LPL activity rose but remained at a lower level than before the caloric restriction. The postprandial LPL response was markedly exaggerated after ten days of refeeding (21% increase before dieting and 250% after refeeding). The changes of LPL during caloric restriction and subsequent refeeding were not correlated to plasma insulin levels. The mechanism of the exaggerated LPL response to meal during refeeding remains obscure.

Lipoprotein lipase in diabetes

Lipoprotein lipase has a central role in the metabolism of both triglyceride-rich particles and high density lipoproteins, and it is one determinant of both serum triglyceride and HDL concentrations. In man the enzyme activity in both adipose tissue and skeletal muscle is insulin dependent, and therefore it varies in diabetes according to ambient insulin level and insulin sensitivity. In insulin deficiency (untreated Type 1 diabetes) the enzyme activity in both adipose tissue and muscle tissue is low but increases upon insulin therapy. In chronically insulin-treated patients with good control, the enzyme activity in postheparin plasma is increased. In untreated Type 2 diabetic patients, the average enzyme activity in adipose tissue and postheparin plasma is normal or subnormal. Therapy with oral agents or insulin, resulting in good glycemic control, is followed by an increase of LPL activity in both adipose tissue and postheparin plasma. In both Types 1 and 2 diabetes, changes of LPL activity are associated with relevant alterations in lipoprotein pattern. In insulin deficiency with low LPL, serum total and VLDL triglyceride levels are elevated, and HDL concentration is reduced. In chronically insulin-treated patients with high LPL activity, VLDL triglyceride concentrations are normal or subnormal, and HDL level is increased. In untreated Type 2 diabetic patients subnormal LPL activity may contribute to the elevation of serum triglycerides and to the reduction of HDL level.

Very low density lipoprotein metabolism after insulin over-treatment and during a euglycaemic clamp

The disturbance of very low density lipoprotein (VLDL) metabolism that occurs as a result of intensive insulin treatment and during a euglycaemic clamp have been investigated in a rat model. Normal rats were maintained with fed blood glucose levels below 5 mmol l-1 for 8 weeks by subcutaneous insulin injections (normal fed levels 5.8 +/- 0.4 (SD) mmol l-1). Glucose requirement to maintain a glucose clamp was significantly reduced (116 +/- 3 mumol min-1 kg-1 (SE) vs. 173 +/- 5 mumol min-1 kg-1, P less than 0.001), compared with weight-matched normal control rats. In the fasting state (blood glucose 3.5 +/- 0.2 mmol l-1 vs. 3.9 +/- 0.1 mmol l-1, NS) plasma non-esterified fatty acid levels were reduced. Fasting VLDL-triglyceride turnover, measured by bolus injection of 14C-VLDL, was also lower (3.17 +/- 0.12 mumol min-1 kg-1 vs. 3.50 +/- 0.07 mumol min-1 kg-1, P less than 0.05). Despite decreased turnover, insulin over-treated rats had normal plasma triglyceride concentrations indicating a removal defect. At the end of a 3-h euglycaemic clamp, plasma triglyceride concentrations and VLDL-triglyceride turnover were decreased in both normal control and insulin over-treated animals, and turnover remained significantly lower in the insulin over-treated rats (2.59 +/- 0.13 mumol min-1 kg-1 vs. 3.08 +/- 0.10 mumol min-1 kg-1, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Portal v peripheral hyperinsulinemia and very low density lipoprotein triglyceride kinetics

The effect of subcutaneously delivered insulin on the kinetics of rat plasma triglycerides was compared to that of intraperitoneally delivered insulin. The former route delivered insulin primarily extrahepatically and the latter, intraportally. In comparison to the intraperitoneally delivered insulin, the subcutaneously delivered insulin was associated with a higher peripheral serum insulin, lower serum glucose, lower serum FFA, lower serum triglycerides, and similar rate of triglyceride secretion. The activity of adipose tissue lipoprotein lipase was directly related to the serum insulin concentration. The pattern of serum triglycerides and lipoprotein lipase in the rats receiving subcutaneous insulin suggested that their rate of triglyceride removal exceeded that seen in the rats receiving intraperitoneal insulin. These observations indicate that the route of insulin delivery can influence the balance between the hepatic and extrahepatic effects of insulin on triglyceride kinetics.