Tissue-specific lipoprotein lipase: relationships to body composition and body fat distribution in normal weight humans

Twenty-six normal weight subjects (22 female, 4 male) were studied to determine the relationships of fasting levels of lipoprotein lipase in gluteal adipose tissue (ATLPL) and skeletal muscle (SMLPL) to body composition and body fat distribution. No relationship was found between fasting gluteal ATLPL and percent (%) body fat. There was, however, an inverse relationship between fasting SMLPL (from the vastus lateralis) and %body fat (p=0.005). A strong inverse correlation was also seen between fasting ATLPL and waist/hip ratio (p=0.0006), a measurement of body fat distribution. These relationships existed with or without the male subjects included. The tissue-specific relationships of lipoprotein lipase to body composition and body fat distribution could relate to the development of obesity or the maintenance of normal body weight by the effects of the lipase on the partitioning of lipoprotein triglyceride fatty acids.

Evidence for second-phonon nuclear wobbling

The nucleus 163Lu has been populated through the reaction 139La(29Si,5n) with a beam energy of 157 MeV. Three triaxial, strongly deformed (TSD) bands have been observed with very similar rotational properties. The first excited TSD band has earlier been assigned as a one-phonon wobbling excitation built on the lowest-lying (yrast) TSD band. The large B(E2)(out)/B(E2)(in) value obtainable for one of four observed transitions between the second and first excited TSD bands is in good agreement with particle-rotor calculations for a two-phonon wobbling excitation.

Evidence for the wobbling mode in nuclei

The nucleus (163)Lu has been populated through the fusion-evaporation reaction (139)La((29)Si,5n)(163)Lu with a beam energy of 152 MeV. The electromagnetic properties of several connecting transitions between two presumably triaxial, strongly deformed (TSD) bands have been studied. Evidence is presented for the assignment of the excited TSD band as a wobbling mode built on the yrast TSD band, based on comparisons to new calculations in which an aligned particle is coupled to a strongly deformed triaxial rotor. The wobbling mode is uniquely related to triaxiality in nuclei.

Overexpressing human lipoprotein lipase in mouse skeletal muscle is associated with insulin resistance

Lipoprotein lipase (LPL) plays a rate-limiting role in triglyceride-rich lipoprotein metabolism and is expressed in most tissues. Overexpression of LPL in skeletal muscle has been linked with higher plasma glucose levels suggesting insulin resistance (Jensen et al., Am J Physiol 273:R683-R689, 1997). The aim of our study was to ascertain whether the overexpression of human LPL in skeletal muscle leads to insulin resistance and to investigate the mechanism. Respiratory quotient measurements in both transgenic (MCKhLPL) and nontransgenic mice on a high-carbohydrate diet were conducted and showed a shift in fuel usage in transgenic mice when fasting but not when actively feeding. An increase in citrate and glucose 6-phosphate levels in fasted MCKhLPL mice further supports this preferential use of lipids. When challenged with an intraperitoneal injection of glucose (1 g/kg), MCKhLPL mice had a higher plasma glycemic excursion than nontransgenic mice. No differences in insulin response were observed between the two groups. Further investigation using hyperinsulinemic-euglycemic clamps revealed insulin resistance in MCKhLPL mice. Despite signs of insulin resistance, there was no associated increase in free fatty acids, hypertriglyceridemia, or hyperinsulinemia in MCKhLPL mice. In conclusion, MCKhLPL mice are insulin resistant, presumably due to increased delivery of lipoprotein-derived fatty acids to muscle.

A self-correcting indirect calorimeter system for the measurement of energy balance in small animals

Indirect calorimetry involves measurement of CO(2) produced and O(2) consumed by an organism. These measurements are then used to calculate energy output, metabolic rate (MR), and respiratory quotient (RQ), a relative assessment of carbohydrate and lipid oxidation. By far the most difficult aspect of indirect calorimetry is measurement of O(2). Moreover, the abundance of O(2) (20.95%) relative to CO(2) (0.03%) in ambient conditions dictates that measurement errors of O(2) have greater implications on calculations of MR and RQ. Because compressed air is not feasible for use with animals in long-term experiments, changes in ambient conditions are nearly unavoidable. A self-correcting indirect calorimetry system was designed and constructed utilizing differential O(2) and CO(2) analyzers and a blank cage to monitor ambient conditions periodically. The system was validated by changing ambient O(2) and CO(2) concentrations by infusing N(2) into the system during a test butane burn. MR and RQ were largely unaffected by these changes in ambient conditions, and inclusion of a blank cage in the system accounted for slight calibration offsets. MR and RQ were measured in mice (n = 95) with and without correction for any small changes in ambient conditions measured in the blank cage. Coefficients of variation for MR and RQ were significantly decreased by taking into account ambient conditions measured in the blank cage (P < 0.001), which resulted in a 2.3% increase in precision for measurement of MR. This system will be used to more accurately assess long-term measurements of energy balance in the many murine models of leanness and obesity to gain better insights into pathophysiology and treatment of human obesity.

Seasonal variation in lipoprotein lipase and plasma lipids in physically active, normal weight humans

Adipose tissue lipoprotein lipase (ATLPL) provides free fatty acids (FFA) for storage in adipocytes, whereas in skeletal muscle LPL (SMLPL) provides FFA for oxidation. In hibernating animals, the level of SMLPL is relatively higher in summer than winter (promoting fat oxidation), whereas the opposite is seen with ATLPL. A patient-controlled study was designed to determine whether such seasonal variation occurs in normal weight humans. Eighteen subjects were studied in the summer and winter. After 2 days of a standardized diet, they underwent muscle and adipose biopsies for LPL activity, assessment of fitness by VO2 max, and determination of body composition by hydrostatic weighing. The percentages of body fat, body mass index, VO2 max, insulin, glucose, FFA, glycerol, and leptin were not affected by the season. Total cholesterol was higher in the winter than in the summer (157 +/- 5.5 vs. 148 +/- 4.2 mg/dL respectively; P = 0.03). The ATLPL activity was also higher in the winter than in the summer (4.4 +/- 0.8 vs. 2.3 +/- 0.6 nmol FFA/10(6) cells-min; P = 0.04). SMLPL activity trended to be higher in the winter than in the summer (1.9 +/- 0.5 vs. 1.0 +/- 0.1 nmol FFA/g x min; P = 0.06). In summary, ATLPL is seasonally regulated. It appears that SMLPL is similarly regulated by season. For physically active lean subjects, this increase in SMLPL may be a compensatory mechanism to help protect from seasonal weight gain.

Thyroid hormone resistance and increased metabolic rate in the RXR-gamma-deficient mouse

Vitamin A and retinoids affect pituitary-thyroid function through suppression of serum thyroid-stimulating hormone (TSH) levels and TSH-beta subunit gene expression. We have previously shown that retinoid X receptor-selective (RXR-selective) ligands can suppress serum TSH levels in vivo and TSH-beta promoter activity in vitro. The RXR-gamma isotype has limited tissue distribution that includes the thyrotrope cells of the anterior pituitary gland. In this study, we have performed a detailed analysis of the pituitary-thyroid function of mice lacking the gene for the RXR-gamma isotype. These mice had significantly higher serum T4 levels and TSH levels than did wild-type (WT) controls. Treatment of RXR-gamma-deficient and WT mice with T3 suppressed serum TSH and T4 levels in both groups, but RXR-gamma-deficient mice were relatively resistant to exogenous T3. RXR-gamma-deficient mice had significantly higher metabolic rates than did WT controls, suggesting that these animals have a pattern of central resistance to thyroid hormone. RXR-gamma, which is also expressed in skeletal muscle and the hypothalamus, may have a direct effect on muscle metabolism, regulation of food intake, or thyrotropin-releasing hormone levels in the hypothalamus. In conclusion, the RXR-gamma isotype appears to contribute to the regulation of serum TSH and T4 levels and to affect peripheral metabolism through regulation of the hypothalamic-pituitary-thyroid axis or through direct effects on skeletal muscle.

Obesity resistance and multiple mechanisms of triglyceride synthesis in mice lacking Dgat

Triglycerides (or triacylglycerols) represent the major form of stored energy in eukaryotes. Triglyceride synthesis has been assumed to occur primarily through acyl CoA:diacylglycerol transferase (Dgat), a microsomal enzyme that catalyses the final and only committed step in the glycerol phosphate pathway. Therefore, Dgat has been considered necessary for adipose tissue formation and essential for survival. Here we show that Dgat-deficient (Dgat-/-) mice are viable and can still synthesize triglycerides. Moreover, these mice are lean and resistant to diet-induced obesity. The obesity resistance involves increased energy expenditure and increased activity. Dgat deficiency also alters triglyceride metabolism in other tissues, including the mammary gland, where lactation is defective in Dgat-/- females. Our findings indicate that multiple mechanisms exist for triglyceride synthesis and suggest that the selective inhibition of Dgat-mediated triglyceride synthesis may be useful for treating obesity.

Increased intracellular triglyceride in C(2)C(12) muscle cells transfected with human lipoprotein lipase

Much of the knowledge about the cell biology of lipoprotein lipase (LPL) in vitro has been gained from adipose tissue model systems. However, the importance of skeletal muscle lipoprotein lipase (SMLPL) to both lipoprotein and muscle metabolism remains unclear. Although the production of LPL in cultured myocytes has been documented, the amount of enzyme activity produced is small. To develop a more suitable tissue culture model for SMLPL, mouse C(2)C(12) myoblasts were stably transduced with a retroviral vector encoding the full-length human LPL (hLPL) cDNA. Control cells were transduced with a vector encoding beta-galactosidase. LPL expression was assayed as a function of cell growth by measuring LPL activity on days 3, 7, 9, 11, and 14 after subculture. The hLPL-transduced myoblasts increasingly overexpressed both heparin-releasable (HR) and intracellular (IN) LPL activity compared to nontransduced myoblasts (P < 0.001 at Day 11) and myoblasts transduced with the control vector (P < 0.001 at Day 11). This increase occurred while LPL mRNA levels remained stable between days 3 and 14. As expected, IN LPL activity was also increased in the transduced cells. High levels of LPL activity were also obtained after differentiating the C(2)C(12) cells into myotubes by serum deprivation. Additionally, throughout the time course, C(2)/LPL cells had greater amounts of intracellular triglyceride than both the C(2)C(12) and the C(2)/beta-GEO cells (P = 0.005 and P < 0.001, respectively) with the largest differences seen on day 14 of the time course (P = 0.001, C(2)/LPL vs C(2)C(12) (r) or C(2)/beta-GEO cells). Thus, C(2)C(12) myoblasts stably transduced with hLPL markedly overexpressed both HR and IN LPL activity compared to control cells which, in turn, was associated with increases in intracellular triglyceride content. Because LPL regulation in tissues is mostly posttranslational, this new in vitro model will permit the in-depth study of the posttranslational regulation of SMLPL and provide new insights into the fate of lipoprotein-derived fatty acids in muscle.

Orlistat fails to alter postprandial plasma lipid excursions or plasma lipases in normal-weight male volunteers

OBJECTIVES

After 10 d of orlistat administration (120 mg three times/day), the primary objective was to determine the drug's effect on postprandial plasma lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) activities on day 10 after an oral fat-load. The secondary objectives were to determine the effects of orlistat on 12 h postprandial measures of: (1) preheparin HTGL and LPL; and (2) serum triglycerides, very-low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol and free fatty acids.

METHODS

Twenty-four normal-weight, healthy male volunteers were randomized to either 120 mg orlistat (n=12) or placebo (n=12) three times a day with meals for 10 d. Preheparin LPL and HTGL activities and LPL specific activity were measured in the fasted state on days 1, 5, and 10. On days 5 and 10 the study medication (orlistat or placebo) was taken at the beginning of a fat-rich breakfast and serum lipid and lipoprotein levels monitored for 12 h postprandially. On day 10, 15 min postheparin HTGL activity was measured 8 h after the fat-rich breakfast.

RESULTS

No differences were found between groups in fasting levels of preheparin LPL or HTGL activity or in LPL-specific activity on days 1, 5 and 10. No difference was found between the two treatment groups in postheparin HTGL activity 8 h after the fat-rich breakfast. Also, no differences were found between the two groups in plasma triglycerides or lipoproteins.

CONCLUSION

The results indicate that the oral administration of orlistat (120 mg t. i.d.) does not significantly alter plasma triglycerides or lipoproteins, and that the inhibitory effect of orlistat on lipases is limited to the gastrointestinal tract and is not manifested systemically.

Lipid and lipoprotein analysis of cats with lipoprotein lipase deficiency

BACKGROUND

We have previously described a colony of domestic cats with a naturally occurring mutation in the lipoprotein lipase (LPL) gene. We have now further characterized cats homozygous for LPL deficiency (LPL -/-, homozygotes), and have contrasted these with heterozygotes (LPL +/-) and normal cats (LPL +/+).

MATERIALS AND METHODS

Density gradient ultracentrifugation with subsequent lipid analysis, agarose and polyacrylamide gel electrophoresis was used to examine detailed liproprotein differences between the genotypes. Oral fat loading studies and breast milk fatty acid analysis were also performed to further characterize the phenotypic expression of LPL deficiency in this model system.

RESULTS

Several lipid abnormalities associated with homozygosity for LPL deficiency were evident. Triglyceride-rich lipoprotein-triglycerides (TRL-TG) and cholesterol (TRL-C) were higher (TRL-TG 2.09 +/- 1.14 vs. 0.15 +/- 0.04 mmol L-1, P < 0.001; TRL-C 0.42 +/- 0.30 vs. 0.11 +/- 0.16 mmol L-1, P < 0.05) in male -/- than in male +/+ cats, as was HDL-cholesterol (HDL-C, 1.75 +/- 0.24 vs. 1.41 +/- 0.14 mmol L-1, P < 0.05). LDL-C levels were lower in homozygous cats than in control cats, similar to what is seen in human LPL deficiency. Oral fat loading studies revealed that homozygous cats have a marked reduced ability to clear plasma TGs in terms of peak time (7 h vs. 3 h), peak height (9.36 vs. 1.1 mmol L-1), area under the TG clearance curve (AUC, 280.3 vs. 2.2 h mmol L-1) and time to return to baseline. Fasting lipid and lipoprotein levels were not significantly different between heterozygous and normal cats. However, oral fat loading in heterozygotes revealed an intermediate phenotype (peak of 2.35 mmol L-1 at 5 h, AUC 13.1 h mmol L-1), highlighting the impaired TG clearance in these animals.

CONCLUSION

Thus, LPL deficiency in the cat results in a lipid and lipoprotein phenotype that predominantly parallels human LPL deficiency, further validating the use of these animals in studies on the pathobiology of LPL.

Effect of dietary macronutrient composition on tissue-specific lipoprotein lipase activity and insulin action in normal-weight subjects

The effects of macronutrient composition on fasting and postprandial activities of adipose tissue lipoprotein lipase (ATLPL) and skeletal muscle LPL (SMLPL) and on insulin sensitivity (S(I)) were studied in 25 normal-weight subjects. Each subject was fed a high-carbohydrate (HC) diet for 16 d and a high-fat (HF) diet for 16 d, in randomized order. On day 15 of each diet, biopsies for ATLPL and SMLPL were done in the fasted state and 6 h postprandially. On day 16 of each diet, a euglycemic clamp was used to measure S(I). There was no effect of diet composition on fasting ATLPL or SMLPL. With both diets and in both tissues, LPL increased significantly from fasting to 6 h postprandially. In adipose tissue only there was a significant difference between the 2 diets in LPL meal response (HC >HF, P = 0.024). There was no effect of diet composition on S(I). After the HC diet only, there were significant correlations between fasting SMLPL and S(I), but not ATLPL. After the HF diet, associations between insulin action and LPL were evident only in the postprandial state. In summary, 16 d of HC compared with HF feeding in normal-weight subjects increased the responsiveness of ATLPL to an HC compared with an HF meal. However, the same diets had no effect on fasting ATLPL or SMLPL, the responsiveness of SMLPL to a meal, or S(I). These data suggest that in normal-weight subjects habitual dietary carbohydrate intake may have a stronger effect on subcutaneous fat storage than does dietary fat intake.

Isoproterenol and somatostatin decrease plasma leptin in humans: a novel mechanism regulating leptin secretion

In cultured adipocytes, leptin is increased by insulin and decreased by cAMP. In animal models, insulin and agents that increase intracellular cAMP have been shown to similarly affect plasma leptin in vivo. This study was undertaken to test the hypothesis that in humans increased cAMP induced by isoproterenol would decrease leptin. Five groups of normal weight subjects were studied; 1) subjects infused with isoproterenol at a rate of 24 ng/kg/min (ISO24); 2) subjects infused with isoproterenol at a rate of 8 ng/kg/min (ISO8); 3) subjects infused with somatostatin/insulin/GH followed by coinfusion with 8 ng/kg/min isoproterenol (ISO8 + SRIH); 4) subjects infused with somatostatin/insulin/GH alone (SRIH); and 5) control subjects infused with saline (NS). ISO24 infusion resulted in a 27% decrease in plasma leptin over 120 min. ISO24 also increased plasma insulin over the infusion. ISO8 resulted in a 16% decrease in leptin. Saline did not change leptin. SRIH alone decreased leptin 19% over the first 120 min, however no additional fall was seen over the next 120 min the SRIH group. Nonetheless, the addition of 8 ng/kg/min ISO during the second 120 min (ISO8 + SRIH) caused a 15% further decline in plasma leptin. Therefore both isoproterenol and somatostatin reduce plasma leptin in humans. The effect of isoproterenol is likely mediated by beta-adrenergic receptors, whereas the effect of somatostatin suggests a novel mechanism for the regulation of leptin.

Prevention of diet-induced obesity in transgenic mice overexpressing skeletal muscle lipoprotein lipase

Transgenic (Tg) FVB/N mice were produced that overexpress human lipoprotein lipase (LPL) in skeletal muscle using the muscle creatine kinase promoter and enhancers. It was hypothesized that, by overexpressing LPL in muscle, high fat feeding-induced obesity would be prevented by diverting lipoprotein-derived triglyceride fatty acids away from storage in adipose tissue to oxidation in muscle. Mice were examined both at 6 wk of age before high fat (HF) feeding and at 19 wk of age after 13 wk of HF (46.1% fat) or high carbohydrate (HC) feeding (11.5% fat). At 6 wk in heterozygous Tg mice, LPL was increased 11-fold in white muscle and 2.5-fold in red muscle, but not in cardiac muscle or spleen, brain, lung, kidney, or adipose tissue. Plasma triglycerides (mg/dl) were lower in Tg mice (87 +/- 7 vs. 117 +/- 7, P < 0.0001), and glucose increased (201 +/- 9 vs. 167 +/- 8 mg/dl, P = 0.029). There were no differences in body weight between Tg and nontransgenic (nTg) mice; however, carcass lipid content (% body wt) was significantly decreased in male Tg mice at 6 wk (7.5 +/- 1.0 vs. 9.0 +/- 1.0%, P = 0.035). Body composition was not different in female Tg mice at 6 wk. Overall, when Tg mice were fed either a HC or HF diet for 13 wk, plasma triglycerides (P < 0.001) and free fatty acids (P < 0.001) were decreased, whereas plasma glucose (P = 0.01) and insulin (P = 0.05) were increased compared with nTg mice. HF feeding increased carcass lipid content twofold in both male (10.3 +/- 1.1 vs. 21.4 +/- 2.6%, HC vs. HF, P < 0.001) and female nTg mice (6.7 +/- 0.9 vs. 12.9 +/- 1.8%, P = 0.01). However, the targeted overexpression of LPL in skeletal muscle prevented HF diet-induced lipid accumulation in both Tg male (10.2 +/- 0.7 vs. 13.5 +/- 2.2%, HC vs. HF, P = NS) and female Tg mice (6.8 +/- 0.6 vs. 10.1 +/- 1.4%, P = NS). The potential to increase LPL activity in muscle by gene or drug delivery may prove to be an effective tool in preventing and/or treating obesity in humans.

Tissue-specific regulation of lipoprotein lipase by isoproterenol in normal-weight humans

Lipoprotein lipase (LPL) is a hydrolytic enzyme, involved in lipoprotein metabolism and nutrient partitioning, that is subject to tissue-specific regulation. Evidence for divergent regulation of the lipase by insulin has been demonstrated, but alterations in the tissue-specific response of LPL to catecholamines has not been studied in humans. The regulation of LPL in gluteal adipose tissue and vastus lateralis muscle by isoproterenol (epinephrine isopropyl homologue) in humans was examined over 2 h in subjects infused with 0 (saline) or 8 or 24 ng.kg-1.min-1 isoproterenol. The infusion of normal saline into control subjects failed to alter adipose tissue or skeletal muscle LPL activity. However, in the saline-infused subjects there was a positive correlation between the percent change in plasma norepinephrine concentrations and the percent change in muscle LPL activity (r = 0.826, P < 0.05). Isoproterenol infusion did not change LPL in either adipose tissue or muscle compared with saline-infused controls, but plasma insulin levels in addition to plasma glucose, free fatty acids, and glycerol were increased. To prevent the isoproterenol-induced hyperinsulinemia, a pancreatic clamp technique was utilized. An increase in muscle LPL was demonstrated (P = 0.037) with no change in adipose tissue LPL. The change in muscle LPL activity after the 2-h infusion correlated with the change in muscle mRNA (P = 0.021). Overall, these studies indicate that in humans the response of LPL to catecholamines is tissue specific with no effect in adipose tissue but a stimulation in skeletal muscle. Endogenous regulation of LPL in muscle by catecholamines could be important in muscle fuel metabolism and could relate to effects of adenosine 3',5'-cyclic monophosphate and/or fatty acids at the level of the LPL gene.

Adipose-specific overexpression of GLUT-4 in transgenic mice alters lipoprotein lipase activity

Transgenic mice overexpressing GLUT-4 selectively in adipose tissue using the aP2 promoter/enhancer develop obesity, enhanced glucose tolerance, and increased insulin sensitivity. The current study was designed to determine whether altering glucose transport affects lipoprotein lipase (LPL) activity. Female transgenic mice (10-12 mo old) have increased parametrial fat pad weight, adipocyte size, total body lipid and fasting plasma triglycerides, fatty acids, and glycerol compared with nontransgenics. Stimulation of LPL activity by feeding is blunted in parametrial and perirenal fat from 15- and 22-fold in nontransgenic mice to three- to sevenfold in transgenics. LPL activity in the fed state in transgenic mice is reduced 60-75% in fat. In heart and skeletal muscle of transgenic mice, LPL activity in the fasted state is 55-65% lower than in nontransgenics and feeding induces an unexpected rise in LPL activity. Muscle LPL activity is strongly and inversely correlated with glucose transport in adipocytes (r = -0.942, P < 0.005), which is increased 15- to 27-fold in the basal state and 4.5- to 6.9-fold in the insulin-stimulated state in transgenics. Whereas stimulation of adipose LPL may be blunted by lower plasma insulin levels in transgenics, fasting muscle LPL may be suppressed by elevated plasma lipids. Thus altering the partitioning of glucose between adipose tissue and muscle alters a critical step for the partitioning of lipoprotein fatty acids between these tissues.

Weight regain following sustained weight reduction is predicted by relative insulin sensitivity

Ten moderately obese women (body mass index 34.9 +/- 1.1 kg/m2, mean +/- SEM), had previously been through a 3-month weight loss program followed by 3 months of weight maintenance at the reduced weight. A euglycemic clamp for determination of insulin sensitivity was performed on each subject prior to weight loss, and another at the end of the weight maintenance phase. The mean weight loss for the group was 11.4 +/- 2.2 kg. The women were then seen for follow-up weights 12 months and 18 months after the conclusion of the weight maintenance period. All of the women except one had regained their weight by the time of the 12-month visit. It was found that the amount of weight regained both at 12 months and 18 months was correlated with the change in insulin sensitivity which occurred from the baseline study to after weight loss/maintenance. The data indicate that increased insulin sensitivity following sustained weight loss in obese women predicts weight regain.

Sustained weight reduction in moderately obese women results in decreased activity of skeletal muscle lipoprotein lipase

Obesity is an increasingly prevalent problem, and long-term maintenance of the weight-reduced state is difficult for the obese individual. Following weight reduction, many metabolic changes occur. Among these is an increase in adipose tissue lipoprotein lipase (ATLPL), which predicts an alteration in lipid fuel partitioning which may then contribute to resumption of the obese state. The purpose of this study was to test whether changes in skeletal muscle LPL (SMLPL) and its response to insulin/glucose after sustained weight reduction also indicate a potential altered partitioning of lipid fuels away from oxidative pathways in muscle to storage in adipose tissue. Biopsies of vastus lateralis muscle were carried out in premenopausal obese women (n = 11, 94 +/- 4 kg, mean +/- SEM) before and after consumption of a 900 kcal day-1 diet for 3 months followed by 3 months of isocaloric maintenance of the reduced weight (n = 11, 82 +/- 4 kg). SMLPL activity was measured in the fasted state and after 6 h insulin/glucose infusion, before and after sustained weight loss. SMLPL activities were also measured in six normal weight women. Fasting SMLPL activity in obese women (3.9 +/- 0.3 nmol FFA min-1 g-1) was similar to that measured in normal weight control women (4.4 +/- 0.5). Unlike normal weight controls in whom a 6 h insulin/glucose infusion decreased SMLPL activity, in obese women the response of SMLPL was positive (normal weight vs. obese: delta -0.8 +/- 0.3 vs. delta 1.6 +/- 0.5, P = 0.002). Following maintained weight reduction, fasting SMLPL in the obese group was reduced to 1.2 +/- 0.3 (obese before weight loss vs. obese after: P = 0.0001). This change in fasting SMLPL activity following weight loss/maintenance correlated with the resultant change in percent body fat (r s = 0.663, P = 0.026).(ABSTRACT TRUNCATED AT 250 WORDS)

Change in skeletal muscle lipoprotein lipase activity in response to insulin/glucose in non-insulin-dependent diabetes mellitus

Skeletal muscle lipoprotein lipase (SMLPL) provides fatty acids to myocytes for lipoprotein triglyceride oxidation. In human obesity, an insulin-resistant state, SMLPL levels measured in the fasted state are either decreased or unchanged as compared with levels in normal-weight controls. However, insulin/glucose infusion increases SMLPL activity in obese individuals, whereas in normal-weight subjects the activity is decreased. One of the goals of this study was to determine the impact of obesity with concomitant non-insulin-dependent diabetes mellitus (NIDDM) on fasting SMLPL and on the change in SMLPL activity (delta MLPL) in response to an insulin/glucose infusion. Because NIDDM is often a more insulin-resistant state, it was hypothesized that SMLPL activity would be further increased by insulin/glucose in subjects who were obese and had NIDDM. Measurements of SMLPL were made from biopsies of vastus lateralis skeletal muscle taken before and after a 6-hour insulin/glucose infusion in the setting of a euglycemic clamp. Thirteen nondiabetic obese women (OBC) and six nondiabetic normal-weight women (NWC) were used as control subjects. SMLPL levels measured in the fasted state were significantly lower in obese NIDDM subjects as compared with either control group. Relative insulin action was determined by calculation of the mean glucose infusion rate (GIR) required to sustain euglycemia over the last 60 minutes of the infusion. For all three groups combined, representing a continuum of insulin sensitivity, there was a positive correlation between GIR and fasting SMLPL. As described earlier, in the NWC group SMLPL activity decreased significantly after 6 hours of insulin/glucose, and in the OBC group SMLPL increased after insulin/glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Macronutrient regulation of lipoprotein lipase is posttranslational

To examine the relative impact of diet and meal composition on lipoprotein lipase (LPL), high fat (60% of energy) (HF) and high carbohydrate (68%) (HC) diets were fed to Sprague-Dawley rats for 2-3 wk, followed by overnight food deprivation and a meal of the same composition. Heparin-releasable LPL activities, mass and mRNA were measured in heart, diaphragm and soleus muscle and epididymal fat after food deprivation and 1, 2, 4 and 8 h postprandially. No effect of dietary macronutrient composition on LPL activity, protein or mRNA in food-deprived rats was demonstrated. However, in cardiac and diaphragm muscle, heparin-releasable LPL activity was suppressed by HC but stimulated by HF meal-feeding at 4 h. Moreover, in adipose tissue, the HC meal increased LPL activity at 1, 2 and 4 h relative to the basal period. Although there were no consistent effects of meal composition on LPL mass or mRNA in any one tissue, overall LPL mass was generally increased by HC meal-feeding. Because there were meal composition-dependent differences in LPL activity but no detectable differences in mass or mRNA in a particular tissue, LPL regulation by meals seems to be predominantly posttranslational.

Regulation of lipoprotein lipase activity and mRNA in the mammary gland of the lactating mouse

We examined the effects of reproductive stage and fasting on lipoprotein lipase (LPL) activity and mRNA in the mouse mammary gland. Heparin-releasable and cell-associated LPL activity rose immediately after birth, followed 1-2 days later by an increase in LPL mRNA. Fasting decreased LPL activity in the mammary gland at all reproductive stages. During lactation, both milk and heparin-releasable LPL were substantially decreased by an overnight fast, whereas cell-associated LPL was less affected and LPL mRNA did not change. These studies indicate that the extracellular, heparin-releasable, fraction of mammary LPL activity responds most rapidly to alterations in physiological state, usually accompanied by smaller changes in cellular enzyme activity. Changes in the level of LPL mRNA were seen only during the transition from pregnancy to lactation, and these tended to follow, rather than precede, changes in enzyme activity. We conclude that in the mammary gland as in adipose tissue, LPL is regulated primarily at the translational and post-translational level.

Relationship between skeletal muscle lipoprotein lipase activity and 24-hour macronutrient oxidation

A low ratio of whole-body 24-h fat/carbohydrate (CHO) oxidation has been shown to be a predictor of subsequent body weight gain. We tested the hypothesis that the variability of this ratio may be related to differences in skeletal muscle metabolism. Since lipoprotein lipase (LPL) plays a pivotal role in partitioning lipoprotein-borne triglycerides to adipose (storage) and skeletal muscle (mostly oxidation), we postulated that a low ratio of fat/CHO oxidation was associated with a low skeletal muscle LPL (SMLPL) activity. As an index of substrate oxidation, 24-h RQ was measured under sedentary and eucaloric conditions in 16 healthy nondiabetic Pima males. During a 6-h euglycemic, hyperinsulinemic clamp, muscle biopsies were obtained at baseline, 3, and 6 h. Heparin-elutable SMLPL activity was 2.92 +/- 0.56 nmol free fatty acids/g.min (mean +/- SD) at baseline, was unchanged (2.91 +/- 0.51) at the third hour, and increased significantly (P < 0.05) to 3.13 +/- 0.57 at the sixth hour of the clamp. The mean (of baseline and 3-h) SMLPL activity correlated inversely with 24-h RQ (r = 0.57, P < 0.03) but not with body size, body composition, or insulin-mediated glucose uptake. Since SMLPL activity is related to the ratio of whole body fat/CHO oxidation rate, a decreased muscle LPL activity may, therefore, predispose to obesity.