Placental lipoprotein lipase activity is positively associated with newborn adiposity.

INTRODUCTION: Recent data suggest that in addition to glucose, fetal growth is related to maternal triglycerides (TG). To reach the fetus, TG must be hydrolyzed to free fatty acids (FFA) and transported across the placenta, but regulation is uncertain. Placental lipoprotein lipase (pLPL) hydrolyzes TG, both dietary chylomicron TG (CM-TG) and very-low density lipoprotein TG (VLDL-TG), to FFA. This may promote fetal fat accretion by increasing the available FFA pool for placental uptake. We tested the novel hypothesis that pLPL activity, but not maternal adipose tissue LPL activity, is associated with newborn adiposity and higher maternal TG. METHODS: Twenty mothers (n = 13 normal-weight; n = 7 obese) were prospectively recruited. Maternal glucose, insulin, TG (total, CM-TG, VLDL-TG), and FFA were measured at 14-16, 26-28, and 36-37 weeks, and adipose tissue LPL was measured at 26-28 weeks. At term delivery, placental villous biopsies were immediately analyzed for pLPL enzymatic activity. Newborn percent body fat (newborn %fat) was assessed by skinfolds. RESULTS: Placental LPL activity was positively correlated with birthweight (r = 0.48;P = 0.03) and newborn %fat (r = 0.59;P = 0.006), further strengthened by correcting for gestational age at delivery (r = 0.75;P = 0.0001), but adipose tissue LPL was not. Maternal TG and BMI were not correlated with pLPL activity. Additionally, pLPL gene expression, while modestly correlated with enzymatic activity (r = 0.53;P < 0.05), was not correlated with newborn adiposity. DISCUSSION: This is the first study to show a positive correlation between pLPL activity and newborn %fat. Placental lipase regulation and the role of pLPL in pregnancies characterized by nutrient excess and fetal overgrowth warrant further investigation.

Continuous glucose profiles in obese and normal-weight pregnant women on a controlled diet: metabolic determinants of fetal growth.

OBJECTIVE: We sought to define 24-h glycemia in normal-weight and obese pregnant women using continuous glucose monitoring (CGM) while they consumed a habitual and controlled diet both early and late in pregnancy. RESEARCH DESIGN AND METHODS: Glycemia was prospectively measured in early (15.7 ± 2.0 weeks' gestation) and late (27.7 ± 1.7 weeks' gestation) pregnancy in normal-weight (n = 22) and obese (n = 16) pregnant women on an ad libitum and controlled diet. Fasting glucose, triglycerides (early pregnancy only), nonesterified fatty acids (FFAs), and insulin also were measured. RESULTS: The 24-h glucose area under the curve was higher in obese women than in normal-weight women both early and late in pregnancy despite controlled diets. Nearly all fasting and postprandial glycemic parameters were higher in the obese women later in pregnancy, as were fasting insulin, triglycerides, and FFAs. Infants born to obese mothers had greater adiposity. Maternal BMI (r = 0.54, P = 0.01), late average daytime glucose (r = 0.48, P < 0.05), and late fasting insulin (r = 0.49, P < 0.05) correlated with infant percentage body fat. However, early fasting triglycerides (r = 0.67, P < 0.001) and late fasting FFAs (r = 0.54, P < 0.01) were even stronger correlates. CONCLUSIONS: This is the first study to demonstrate that obese women without diabetes have higher daytime and nocturnal glucose profiles than normal-weight women despite a controlled diet both early and late in gestation. Body fat in infants, not birth weight, was related to maternal BMI, glucose, insulin, and FFAs, but triglycerides were the strongest predictor. These metabolic findings may explain higher rates of infant macrosomia in obese women, which might be targeted in trials to prevent excess fetal growth.

Skeletal muscle-specific deletion of lipoprotein lipase enhances insulin signaling in skeletal muscle but causes insulin resistance in liver and other tissues.

OBJECTIVE: Skeletal muscle-specific LPL knockout mouse (SMLPL(-/-)) were created to study the systemic impact of reduced lipoprotein lipid delivery in skeletal muscle on insulin sensitivity, body weight, and composition. RESEARCH DESIGN AND METHODS: Tissue-specific insulin sensitivity was assessed using a hyperinsulinemic-euglycemic clamp and 2-deoxyglucose uptake. Gene expression and insulin-signaling molecules were compared in skeletal muscle and liver of SMLPL(-/-) and control mice. RESULTS: Nine-week-old SMLPL(-/-) mice showed no differences in body weight, fat mass, or whole-body insulin sensitivity, but older SMLPL(-/-) mice had greater weight gain and whole-body insulin resistance. High-fat diet feeding accelerated the development of obesity. In young SMLPL(-/-) mice, insulin-stimulated glucose uptake was increased 58% in the skeletal muscle, but was reduced in white adipose tissue (WAT) and heart. Insulin action was also diminished in liver: 40% suppression of hepatic glucose production in SMLPL(-/-) vs. 90% in control mice. Skeletal muscle triglyceride was 38% lower, and insulin-stimulated phosphorylated Akt (Ser473) was twofold greater in SMLPL(-/-) mice without changes in IRS-1 tyrosine phosphorylation and phosphatidylinositol 3-kinase activity. Hepatic triglyceride and liver X receptor, carbohydrate response element-binding protein, and PEPCK mRNAs were unaffected in SMLPL(-/-) mice, but peroxisome proliferator-activated receptor (PPAR)-gamma coactivator-1alpha and interleukin-1beta mRNAs were higher, and stearoyl-coenzyme A desaturase-1 and PPARgamma mRNAs were reduced. CONCLUSIONS: LPL deletion in skeletal muscle reduces lipid storage and increases insulin signaling in skeletal muscle without changes in body composition. Moreover, lack of LPL in skeletal muscle results in insulin resistance in other key metabolic tissues and ultimately leads to obesity and systemic insulin resistance.

Plasma alpha-melanocyte-stimulating hormone: sex differences and correlations with obesity.

Rodent experiments raise the possibility of a regulatory role of peripheral alpha-melanocyte-stimulating hormone (alpha-MSH) in obesity and metabolism, but human data on peripheral alpha-MSH levels remain fragmentary. Because of the possible relationship between alpha-MSH and obesity, we endeavored to test the hypothesis that higher levels of alpha-MSH in obese patients would correlate with leptin levels and with other markers of obesity. Sixty normal-weight to obese healthy men and women participated. Weight, measures of body composition, and diet diaries were obtained; fasting blood was analyzed for alpha-MSH, lipids, glucose, insulin, leptin, and adiponectin. To begin to understand the source of peripherally measured hormones, alpha-MSH was also measured in serum samples from 5 individuals with untreated Addison disease. Levels of alpha-MSH were higher in men vs women (10.1 +/- 4.3 vs 7.6 +/- 3.4 pmol/L, P = .019), and alpha-MSH levels were higher in patients with Addison disease vs controls (17.7 +/- 2.3 vs 8.7 +/- 0.52 pmol/L, P < .001). Measures of adiposity correlated with insulin and leptin in men and women, and with adiponectin in women. alpha-Melanocyte-stimulating hormone levels did not correlate significantly with any parameter of adiposity or diet composition. The elevated alpha-MSH levels in patients with untreated Addison disease suggest possible pituitary secretion of alpha-MSH to the periphery. The lack of correlation between peripheral alpha-MSH and parameters of adiposity suggests that endogenous plasma alpha-MSH levels are not a metric for body composition per se.

Increased thermoregulation in cold-exposed transgenic mice overexpressing lipoprotein lipase in skeletal muscle: an avian phenotype?

LPL is an enzyme involved in the breakdown and uptake of lipoprotein triglycerides. In the present study, we examined how the transgenic (Tg) overexpression of human LPL in mouse skeletal muscle affected tolerance to cold temperatures, cold-induced thermogenesis, and fuel utilization during this response. Tg mice and their nontransgenic controls were placed in an environmental chamber and housed in metabolic chambers that monitored oxygen consumption and carbon dioxide production with calorimetry. When exposed to 4 degrees C, an attenuation in the decline in body temperature in Tg mice was accompanied by an increased metabolic rate (15%; P < 0.001) and a reduction in respiratory quotient (P < 0.05). Activity levels, the expression of uncoupling proteins in brown fat and muscle, and lean mass failed to explain the enhanced cold tolerance and thermogenesis in Tg mice. The more oxidative type IIa fibers were favored over the more glycolytic type IIb fibers (P < 0.001) in the gastrocnemius and quadriceps muscles of Tg mice. These data suggest that Tg overexpression of LPL in skeletal muscle increases cold tolerance by enhancing the capacity for fat oxidation, producing an avian-like phenotype in which skeletal muscle contributes significantly to the thermogenic response to cold temperatures.

CD36-facilitated fatty acid uptake inhibits leptin production and signaling in adipose tissue.

Leptin plays an important role in regulating energy expenditure in response to food intake, but nutrient regulation of leptin is incompletely understood. In this study using in vivo and in vitro approaches, we examined the role of fatty acid uptake in modulating leptin expression and production. Leptin levels are doubled in the CD36-null mouse, which has impaired cellular fatty acid uptake despite a 40% decrease in fat mass. The CD36-null mouse is protected from diet-induced weight gain but not from that consequent to leptin deficiency. Leptin secretion in the CD36-null mouse is strongly responsive to glucose intake, whereas a blunted response is observed in the wild-type mouse. This indicates that leptin regulation integrates opposing influences from glucose and fatty acid and loss of fatty acid inhibition allows unsuppressed stimulation by glucose/insulin. Fatty acid inhibition of basal and insulin-stimulated leptin release is linked to CD36-facilitated fatty acid flux, which is important for fatty acid activation of peroxisome proliferator-activated receptor gamma and likely contributes to the nutrient sensing function of adipocytes. Fatty acid uptake also may modulate adipocyte leptin signaling. The ratio of phosphorylated to unphosphorylated signal transducer and activator of transcription 3, an index of leptin activity, is increased in CD36-null fat tissue disproportionately to leptin levels. In addition, expression of leptin-sensitive fatty acid oxidative enzymes is enhanced. Targeting adipocyte CD36 may offer a way to uncouple leptin production and adiposity.

CCAAT/enhancer-binding protein beta deletion reduces adiposity, hepatic steatosis, and diabetes in Lepr(db/db) mice.

CCAAT/enhancer-binding protein beta (C/EBPbeta) plays a key role in initiation of adipogenesis in adipose tissue and gluconeogenesis in liver; however, the role of C/EBPbeta in hepatic lipogenesis remains undefined. Here we show that C/EBPbeta inactivation in Lepr(db/db) mice attenuates obesity, fatty liver, and diabetes. In addition to impaired adipogenesis, livers from C/EBPbeta(-/-) x Lepr(db/db) mice had dramatically decreased triglyceride content and reduced lipogenic enzyme activity. C/EBPbeta deletion in Lepr(db/db) mice down-regulated peroxisome proliferator-activated receptor gamma2 (PPARgamma2) and stearoyl-CoA desaturase-1 and up-regulated PPARalpha independent of SREBP1c. Conversely, C/EBPbeta overexpression in wild-type mice increased PPARgamma2 and stearoyl-CoA desaturase-1 mRNA and hepatic triglyceride content. In FAO cells, overexpression of the liver inhibiting form of C/EBPbeta or C/EBPbeta RNA interference attenuated palmitate-induced triglyceride accumulation and reduced PPARgamma2 and triglyceride levels in the liver in vivo. Leptin and the anti-diabetic drug metformin acutely down-regulated C/EBPbeta expression in hepatocytes, whereas fatty acids up-regulate C/EBPbeta expression. These data provide novel evidence linking C/EBPbeta expression to lipogenesis and energy balance with important implications for the treatment of obesity and fatty liver disease.

Reduction of plasma triglycerides in apolipoprotein C-II transgenic mice overexpressing lipoprotein lipase in muscle.

LPL and its specific physiological activator, apolipoprotein C-II (apoC-II), regulate the hydrolysis of triglycerides (TGs) from circulating TG-rich lipoproteins. Previously, we developed a skeletal muscle-specific LPL transgenic mouse that had lower plasma TG levels. ApoC-II transgenic mice develop hypertriglyceridemia attributed to delayed clearance. To investigate whether overexpression of LPL could correct this apoC-II-induced hypertriglyceridemia, mice with overexpression of human apoC-II (CII) were cross-bred with mice with two levels of muscle-specific human LPL overexpression (LPL-L or LPL-H). Plasma TG levels were 319 +/- 39 mg/dl in CII mice and 39 +/- 5 mg/dl in wild-type mice. Compared with CII mice, apoC-II transgenic mice with the higher level of LPL overexpression (CIILPL-H) had a 50% reduction in plasma TG levels (P = 0.013). Heart LPL activity was reduced by approximately 30% in mice with the human apoC-II transgene, which accompanied a more modest 10% decrease in total LPL protein. Overexpression of human LPL in skeletal muscle resulted in dose-dependent reduction of plasma TGs in apoC-II transgenic mice. Along with plasma apoC-II concentrations, heart and skeletal muscle LPL activities were predictors of plasma TGs. These data suggest that mice with the human apoC-II transgene may have alterations in the expression/activity of endogenous LPL in the heart. Furthermore, the decrease of LPL activity in the heart, along with the inhibitory effects of excess apoC-II, may contribute to the hypertriglyceridemia observed in apoC-II transgenic mice.

Fasting decreases free fatty acid turnover in mice overexpressing skeletal muscle lipoprotein lipase.

Skeletal muscle lipoprotein lipase (LPL) overexpression in mice results in whole-body insulin resistance and increased intramuscular triglyceride stores, but decreased plasma triglyceride concentration and unchanged plasma free fatty acid (FFA) concentration. The effects of skeletal muscle LPL overexpression and fasting duration on FFA kinetics are unknown. Transgenic mice with muscle-specific LPL overexpression (MCKhLPL) and control mice (Con) were studied at rest during a 50-minute constant infusion of [9,10- 3H]palmitate to determine FFA kinetics after both 4 and 16 hours of fasting. FFA concentration was not different between groups after the 4-hour (Con, 0.80 +/- 0.06 mmol/L; MCKhLPL, 0.83 +/- 0.07 mmol/L) and 16-hour (Con, 0.83 +/- 0.04 mmol/L; MCKhLPL, 0.80 +/- 0.07 mmol/L) fast. FFA turnover (Ra) was not significantly different between MCKhLPL and Con groups after the 4-hour fast (Con Ra = 2.52 +/- 0.36 micromol/min; MCKhLPL Ra = 2.37 +/- 0.27 micromol/min). However, FFA turnover was significantly decreased after the 16-hour fast in MCKhLPL mice vs controls (Con Ra = 2.89 +/- 0.52 micromol/min; MCKhLPL Ra = 1.64 +/- 0.17 micromol/min; P < .05). The significantly lower FFA Ra in MCKhLPL vs control mice was due to a decrease in MCKhLPL FFA turnover from the 4- to 16-hour fast, whereas FFA turnover was unchanged in controls. The changes in FFA appearance after the 16-hour fast in MCKhLPL mice are most likely explained by increased reliance by skeletal muscle on plasma triglyceride as a fuel. These data suggest increased skeletal muscle LPL expression decreases dependence on plasma FFA during prolonged fasting in mice.

Deficiency of interleukin-18 in mice leads to hyperphagia, obesity and insulin resistance.

Here we report the presence of hyperphagia, obesity and insulin resistance in knockout mice deficient in IL-18 or IL-18 receptor, and in mice transgenic for expression of IL-18 binding protein. Obesity of Il18-/- mice resulted from accumulation of fat tissue based on increased food intake. Il18-/- mice also had hyperinsulinemia, consistent with insulin resistance and hyperglycemia. Insulin resistance was secondary to obesity induced by increased food intake and occurred at the liver level as well as at the muscle and fat-tissue level. The molecular mechanisms responsible for the hepatic insulin resistance in the Il18-/- mice involved an enhanced expression of genes associated with gluconeogenesis in the liver of Il18-/- mice, resulting from defective phosphorylation of STAT3. Recombinant IL-18 (rIL-18) administered intracerebrally inhibited food intake. In addition, rIL-18 reversed hyperglycemia in Il18-/- mice through activation of STAT3 phosphorylation. These findings indicate a new role of IL-18 in the homeostasis of energy intake and insulin sensitivity.

Obesity and metabolic syndrome in circadian Clock mutant mice.

The CLOCK transcription factor is a key component of the molecular circadian clock within pacemaker neurons of the hypothalamic suprachiasmatic nucleus. We found that homozygous Clock mutant mice have a greatly attenuated diurnal feeding rhythm, are hyperphagic and obese, and develop a metabolic syndrome of hyperleptinemia, hyperlipidemia, hepatic steatosis, hyperglycemia, and hypoinsulinemia. Expression of transcripts encoding selected hypothalamic peptides associated with energy balance was attenuated in the Clock mutant mice. These results suggest that the circadian clock gene network plays an important role in mammalian energy balance.

Retinoid X receptor gamma-deficient mice have increased skeletal muscle lipoprotein lipase activity and less weight gain when fed a high-fat diet.

Retinoids, derivatives of vitamin A, induce hypertriglyceridemia through decreased clearance of very low-density lipoprotein by a lipoprotein lipase (LPL)-dependent pathway. The retinoid X receptor (RXR) gamma isotype, which is highly expressed in skeletal muscle, may be important in mediating the effects of retinoids on skeletal muscle metabolism and triglyceride (TG) clearance. RXRgamma-deficient (-/-) mice had lower fasting plasma TG levels compared with wild-type littermates (33.1 +/- 2.0 vs. 51.7 +/- 6.3 mg/dl, respectively; P < 0.05). Skeletal muscle LPL activity was higher in RXRgamma mice (18.7 +/- 2.2 vs. 13.3 +/- 1.3 nmol free fatty acids/min.g; P = 0.03), but LPL activity was not different in adipose and cardiac tissue, suggesting a specific effect of RXRgamma in skeletal muscle. In addition, when exposed to a 14-wk high-fat diet, RXRgamma -/- mice had less weight gain, which was entirely due to lower fat mass (11.9 +/- 1.8 vs. 14.4 +/- 1.1 g; P = 0.01), and leptin levels were also lower in the RXRgamma -/- mice (17.6 +/- 5.0 vs. 30.9 +/- 6.4 ng/ml; P = 0.03). These data suggest that RXRgamma -/- mice are resistant to gain in fat mass in response to high-fat feeding. This occurs, at least in part, through up-regulation of LPL activity in skeletal muscle. An understanding of the mechanisms governing the role of RXR in TG disposal and metabolism may lead to the rational design of RXR-selective agonists and antagonists that may be useful in common disorders such as dyslipidemia and obesity.

Obesity resistance and enhanced glucose metabolism in mice transplanted with white adipose tissue lacking acyl CoA:diacylglycerol acyltransferase 1.

Recent studies have identified the white adipose tissue (WAT) as an important endocrine organ that regulates energy and glucose metabolism via a number of secreted factors. Mice lacking acyl CoA:diacylglycerol acyltransferase 1 (DGAT1), a key enzyme in mammalian triglyceride synthesis, are protected against diet-induced obesity and glucose intolerance because of increased energy expenditure and enhanced insulin sensitivity. Because DGAT1 is highly expressed in WAT, we hypothesized that DGAT1 deficiency affects the expression of adipocyte-derived factors that regulate energy and glucose metabolism. Here we show that the transplantation of DGAT1-deficient WAT decreases adiposity and enhances glucose disposal in wild-type mice. Analysis of DGAT1-deficient WAT revealed a twofold increase in the expression of adiponectin, a molecule that enhances fatty acid oxidation and insulin sensitivity, and this increase may account in part for the transplantation-induced metabolic changes. Our results highlight the importance of the endocrine function of WAT and suggest that an alteration in this function contributes to the increased energy expenditure and insulin sensitivity in DGAT1-deficient mice.

Increased insulin and leptin sensitivity in mice lacking acyl CoA:diacylglycerol acyltransferase 1.

Acyl coenzyme A:diacylglycerol acyltransferase 1 (DGAT1) is one of two known DGAT enzymes that catalyze the final step in mammalian triglyceride synthesis. DGAT1-deficient mice are resistant to diet-induced obesity through a mechanism involving increased energy expenditure. Here we show that these mice have decreased levels of tissue triglycerides, as well as increased sensitivity to insulin and to leptin. Importantly, DGAT1 deficiency protects against insulin resistance and obesity in agouti yellow mice, a model of severe leptin resistance. In contrast, DGAT1 deficiency did not affect energy and glucose metabolism in leptin-deficient (ob/ob) mice, possibly due in part to a compensatory upregulation of DGAT2 expression in the absence of leptin. Our results suggest that inhibition of DGAT1 may be useful in treating insulin resistance and leptin resistance in human obesity.

Human placental growth hormone causes severe insulin resistance in transgenic mice.

OBJECTIVE: The insulin resistance of pregnancy is considered to be mediated by human placental lactogen, but the metabolic effects of human placental growth hormone have not been well defined. Our aim was to evaluate the effect of placental growth hormone on insulin sensitivity in vivo using transgenic mice that overexpress the human placental growth hormone gene. STUDY DESIGN: Glucose and insulin tolerance tests were performed on 5 transgenic mice that overexpressed the human placental growth hormone variant gene and 6 normal littermate controls. The body composition of the mice was assessed by dual-energy radiograph absorptiometry, and free fatty acid levels were measured as a marker of lipolysis. RESULTS: The human placental growth hormone levels in the transgenic mice were comparable to those attained in the third trimester of pregnancy. These mice were nearly twice as heavy as the control mice, and their body composition differed by a significant increase in bone density and a small decrease in percentage of body fat. Fasting insulin levels in the transgenic mice that overexpressed placental growth hormone were approximately 4-fold higher than the control mice (1.57 +/- 0.22 ng/mL vs 0.38 +/- 0.07 ng/mL; P <.001) and 7 times higher 30 minutes after glucose stimulation (4.17 +/- 0.54 ng/mL vs 0.62 +/- 0.10 ng/mL; P <.0001) with no significant difference in either fasting or postchallenge glucose levels. Insulin sensitivity was markedly decreased in the transgenic mice, as demonstrated by an insignificant decline in glucose levels after insulin injection compared with the control mice, which demonstrated more than a 65% reduction in glucose levels (P <.001). CONCLUSION: Human placental growth hormone causes insulin resistance as manifested by fasting and postprandial hyperinsulinemia and minimal glucose lowering in response to insulin injection. Human placental growth hormone is a highly likely candidate to mediate the insulin resistance of pregnancy.