Regulation of lipoprotein lipase in primary cultures of isolated human adipocytes

Marked Variation in Peritoneal Insulin Absorption in Peritoneal Dialysis

Objective

To assess both peritoneal insulin absorption and adsorption of insulin to the plastic delivery system, and to assess whether variation in peritoneal absorption is related to transport status.

Design

Eight insulin-dependent diabetic patients on continuous ambulatory peritoneal dialysis were studied. Insulin was added to the dialysate. The amount adsorbed by the bag tubing was determined; peritoneal absorption was measured by complete spent dialysate collection.

Results

Only 14% ± 5% of added insulin was adsorbed onto the dialysate delivery system. Marked variation in peritoneal absorption occurred (38% ± 14%, range 17% – 66%). Variations in membrane transport status did not account for variation in absorption.

Conclusion

Marked variation in peritoneal absorption of insulin occurs, accounting for some of the variation in intraperitoneal insulin requirements. This is not related to membrane transport status.

Lipid Hydrolase Enzymes: Pragmatic Prolongevity Targets for Improved Human Healthspan?

Compelling evidence suggests that lipid metabolism, which plays critical roles in fat storage, cell membrane maintenance, and cell signaling, is intricately linked to aging. Lipid hydrolases are important enzymes that catalyze the hydrolysis of more complex lipids into simpler lipids. Diverse interventions targeting lipid hydrolases can prolong or shorten life in model organisms. For example, the genetic removal of or RNAi knockdown against a phospholipase can reduce lifespan in Caenorhabditis elegans, Drosophila melanogaster, and Mus musculus. The removal of lysosomal acid lipase results in premature death in mice, while its overexpression in nematodes generates lean, long-lived individuals. The overexpression or inhibition of diacylglycerol lipase leads to enhanced or reduced longevity, respectively, in both worms and flies. Lifespan can also be extended by knocking down triacylglycerol lipases in yeast, overexpressing fatty acid amide hydrolase in worms, or removing hepatic lipase in a mouse model of coronary disease. Conversely, flies lacking the triacylglycerol lipase Brummer are obese and short lived. Linking sphingolipids and aging, removing the sphingomyelinase inositol phosphosphingolipid phospholipase shortens chronological lifespan in Saccharomyces cerevisiae, while inhibiting an acid sphingomyelinase in worms or inactivating alkaline ceramidase in flies extends lifespan. The clinical potential of manipulating these enzymes is highlighted by the FDA-approved obesity drug orlistat, which is an inhibitor of pancreatic and hepatic lipases that induces weight loss and improves insulin/glucose homeostasis. Additional research is warranted to better understand how these lipid hydrolases impact aging and to determine if clinical interventions targeting them are capable of improving human healthspan.

Basic study of soft tissue augmentation by adipose-inductive biomaterial

Reconstructive surgery for tumor resection, trauma, and congenital anomaly involves volume augmentation with autologous tissue transfer. However, a healthy region is damaged as a donor site, and the autologous tissue is transferred like a patchwork to the recipient site. We have attempted to induce adipogenesis activity in artificial biomaterial that is injectable with an injection needle for soft tissue augmentation. First of all, the optimal dose of pioglitazone hydrochloride was examined with adipo-precursor cells in terms of the proliferator-activated receptor-γ mRNA expression levels affected by reagent in vitro. Then, salmon collagen with pioglitazone was adjusted in terms of the dose and the salmon collagen was injected into mouse back using an injection needle in vivo. At 4 weeks after implantation, the pioglitazone collagen gel was substituted by mature adipocytes in comparison with the case for control collagen gel without pioglitazone. These results are indicative of the possibility of promoting adipogenesis using collagen with pioglitazone as an adipose-inductive substance.

Calcium is involved in formation of high molecular weight adiponectin

BACKGROUND

Adiponectin, an adipocyte-specific secretory protein, is known to circulate as different isoforms in the blood stream.

METHODS

Using sucrose gradients and Western blotting on nondenaturing gels, adiponectin isoforms were examined in human serum, plasma, adipose tissue, and cells. The medium from human adipose tissue and human and mouse adipocytes were also examined for changes in isoform formation upon treatment with EGTA.

RESULTS

Comparison of adiponectin complexes revealed distinct differences in distribution of high molecular weight (HMW) forms between human serum and plasma, with an apparent difference in molecular weight. Variation in molecular weight suggested a probable dissociation of the HMW isoforms in the presence of EDTA in the plasma. Examination of human serum samples treated with EDTA or EGTA showed a partial dissociation of the HMW isoform, while the addition of excess calcium, but not magnesium, to human plasma resulted in partial restoration of HMW adiponectin. When human adipose tissue-secreted adiponectin was treated with EGTA, there was a decrease in the HMW isoform by 61% (+/- 1.89%) and a corresponding increase in low molecular weight (LMW) and middle molecular weight (MMW) isoforms, compared to untreated samples. Analysis of mouse and human adipocytes also showed a reduction in HMW isoforms with a corresponding increase in MMW and LMW isoforms upon treatment with EGTA. The Simpson-Golabi-Behmel syndrome (SGBS) human adipocyte cell line, which primarily synthesizes LMW isoforms, produced increasing amounts of HMW adiponectin upon treatment with calcium in a dose-dependent manner.

CONCLUSION

These data indicate that calcium promotes the formation of HMW adiponectin, and calcium sequestration decreases HMW adiponectin. Because of the importance of HMW adiponectin in insulin sensitivity, these data demonstrate the importance of assay conditions and sample preparation in the measurement of adiponectin isoforms.

Identification of endocannabinoids and related compounds in human fat cells

OBJECTIVE

Recently, an activation of the endocannabinoid system during obesity has been reported. More particularly, it has been demonstrated that hypothalamic levels of both endocannabinoids, 2-arachidonoylglycerol and anandamide (N-arachidonoylethanolamine), are up-regulated in genetically obese rodents. Circulating levels of both endocannabinoids were also shown to be higher in obese compared with lean women. Yet, the direct production of endocannabinoids by human adipocytes has never been demonstrated. Our aim was to evaluate the ability of human adipocytes to produce endocannabinoids.

RESEARCH METHODS AND PROCEDURES

The production of endocannabinoids by human adipocytes was investigated in a model of human white subcutaneous adipocytes in primary culture. The effects of leptin, adiponectin, and peroxisome proliferator-activated receptor (PPAR)-gamma activation on endocannabinoid production by adipocytes were explored. Endocannabinoid levels were determined by high-performance liquid chromatography (HPLC)-atmospheric pressure chemical ionization (APCI)-mass spectrometry (MS) analysis, leptin and adiponectin secretion measured by enzyme-linked immunosorbent assay (ELISA), and PPAR-gamma protein expression examined by Western blotting.

RESULTS

We show that 2-arachidonoylglycerol, anandamide, and both anandamide analogs, N-palmitoylethanolamine and N-oleylethanolamine, are produced by human white subcutaneous adipocytes in concentrations ranging from 0.042+/-0.004 to 0.531+/-0.048 pM/mg lipid extract. N-palmitoylethanolamine is the most abundant cannabimimetic compound produced by human adipocytes, and its levels are significantly down-regulated by leptin but not affected by adiponectin and PPAR-gamma agonist ciglitazone. N-palmitoylethanolamine itself does not affect either leptin or adiponectin secretion or PPAR-gamma protein expression in adipocytes.

DISCUSSION

This study has led to the identification of human adipocytes as a new source of endocannabinoids and related compounds. The biological significance of these adipocyte cannabimimetic compounds and their potential implication in obesity should deserve further investigations.

Pioglitazone increases secretion of high-molecular-weight adiponectin from adipocytes

Adiponectin is an adipocyte-derived serum protein that plays important roles in energy homeostasis, obesity, and insulin sensitivity. Using sucrose gradients and Western blotting of nondenaturing gels, we examined the adiponectin isoforms secreted from human adipose tissue, human and mouse adipocytes, and cell lines in response to pioglitazone added in vitro. The predominant form secreted from adipose tissue in vitro was the high-molecular-weight (HMW) isoform, with small amounts of low-molecular-weight (LMW) forms present. The addition of pioglitazone (1-3 micromM) in vitro increased the secretion of the HMW isoform, with no significant effect on the other isoforms. Human adipose tissue was also examined for changes in adiponectin mRNA levels upon pioglitazone treatment. No difference was detected, suggesting that the effect of pioglitazone is not at the transcriptional level but, rather, at a posttranscriptional phase of the secretory pathway. Additional experiments were conducted to determine whether adiponectin expression was mechanistically similar in other adipose cells. Examination of primary human adipocytes revealed an increase in intracellular HMW isoform with a decline in LMW forms following pioglitazone treatment, with a corresponding increase in the secreted HMW form. Similar results were observed with primary mouse adipocytes, 3T3-F422A cells, and SGBS human adipocyte cells, although differences in the distribution of HMW and LMW isoforms were apparent between cell types. Although there are differences in isoforms between species, in all cases pioglitazone served to increase the secretion of the HMW form of adiponectin.

Leptin-deficient mice backcrossed to the BALB/cJ genetic background have reduced adiposity, enhanced fertility, normal body temperature, and severe diabetes

A deficiency of leptin synthesis in mice results in a complex phenotype characterized by morbid obesity, diabetes, sterility, and defective thermogenesis. To determine whether the genetic background could alter the pleiotropic effects of leptin deficiency, we backcrossed the ob mutation for 10 generations from the C57BL/6J to the BALB/cJ genetic background. Compared with C57BL/6J ob/ob mice, BALB/cJ ob/ob mice showed at 27 wk of age a 35-40% reduction in body weight attributed to a 60% decrease in white adipose tissue mass. Food intake was not significantly different between the two obese strains, suggesting distinct utilization of energy intake. In the fed state, BALB/cJ ob/ob mice had elevated insulin and triglycerides levels, demonstrating a worsening effect on diabetes. At the reproductive level and in contrast to sterile C57BL/6J ob/ob mice, male and female BALB/cJ ob/ob mice were capable of reproducing after a mating period of 16 and 32 wk, respectively. At thermoneutrality, the body temperature of BALB/cJ ob/ob mice was 2.9 C higher than that of C57BL/6J ob/ob mice, whereas exposure of both groups to 4 C demonstrated a prolonged cold tolerance of BALB/cJ ob/ob mice. These studies show that the abnormalities caused by leptin deficiency can be genetically dissected and separated from each other, suggesting discrete pathways controlled by leptin modifier genes.

The translational regulation of lipoprotein lipase in diabetic rats involves the 3'-untranslated region of the lipoprotein lipase mRNA

Adipose tissue lipoprotein lipase (LPL) activity is decreased in patients with poorly controlled diabetes, and this contributes to the dyslipidemia of diabetes. To study the mechanism of this decrease in LPL, we studied adipose tissue LPL expression in male rats with streptozotocin-induced diabetes. Heparin releasable and extractable LPL activity in the epididymal fat decreased by 75-80% in the diabetic group and treatment of the rats with insulin prior to sacrifice reversed this effect. Northern blot analysis indicated no corresponding change in LPL mRNA levels. However, LPL synthetic rate, measured using [(35)S]methionine pulse labeling, was decreased by 75% in the diabetic adipocytes, and insulin treatment reversed this effect. These results suggested regulation of LPL at the level of translation. Diabetic adipocytes demonstrated no change in the distribution of LPL mRNA associated with polysomes, suggesting no inhibition of translation initiation. Addition of cytoplasmic extracts from control and diabetic adipocytes to a reticulocyte lysate system demonstrated the inhibition of LPL translation in vitro. Using different LPL mRNA transcripts in this in vitro translation assay, we found that the 3'-untranslated region (UTR) of the LPL mRNA was important in controlling translation inhibition by the cytoplasmic extracts. To identify the specific region involved, gel shift analysis was performed. A specific shift in mobility was observed when diabetic cytoplasmic extract was added to a transcript containing nucleotides 1818-2000 of the LPL 3'-UTR. Thus, inhibition of translation is the predominant mechanism for the decreased adipose tissue LPL in this insulin-deficient model of diabetes. Translation inhibition involves the interaction of a cytoplasmic factor, probably an RNA-binding protein, with specific sequences of the LPL 3'-UTR.

Thiazolidinediones inhibit lipoprotein lipase activity in adipocytes

The thiazolidinediones troglitazone and BRL 49653 improve insulin sensitivity in humans and animals with insulin resistance. Adipose tissue lipoprotein lipase is an insulin-sensitive enzyme. We examined the effects of thiazolidinediones on lipoprotein lipase expression in adipocytes. When added to 3T3-F442A, 3T3-L1, and rat adipocytes in culture, troglitazone and BRL 49653 inhibited lipoprotein lipase activity. This inhibition was observed at concentrations as low as 0.1 microM and within 2 h after addition of the drug. Lipoprotein lipase activity was inhibited in differentiated adipocytes as well as the differentiating cells. Despite this decrease in enzyme activity, these drugs increased mRNA levels in undifferentiated 3T3-F442A and 3T3-L1 cells and had no effect on mRNA expression or synthesis of lipoprotein lipase in differentiated cells. Western blot analysis showed that these drugs did not affect the mass of the enzyme protein. Lipoprotein lipase activity in cultured Chinese hamster ovary cells was not inhibited by troglitazone. Glucose transport, biosynthesis of lipids from glucose or the biosynthesis of proteins were unaffected by thiazolidinediones in differentiated cells, whereas glucose transport and lipid biosynthesis were increased when these drugs were added during differentiation. These results show that troglitazone and BRL 49653 have a specific, post-translational inhibitory effect on lipoprotein lipase in adipocytes, yet they promote lipid accumulation and differentiation in preadipocytes.

Increased abundance of insulin/IGF-I hybrid receptors in adipose tissue from NIDDM patients

Insulin/IGF-I hybrid receptors composed of an insulin receptor (IR) alphabeta-hemireceptor and a type 1 IGF receptor (IGF-IR) alphabeta-hemireceptor are formed in tissues expressing both molecules. To date there is a limited information about the proportion of hybrids in tissues of normal or diabetic subjects. In this study, we determined the abundance of hybrids in fat from control and NIDDM subjects by using a microwell-based immunoassay. Microwells coated with MA-20 anti-IR or alpha-IGF-IR-PA anti-IGF-IR antibody were incubated with tissue extracts. Immunoadsorbed receptors were incubated with 125I-insulin or 125I-IGF-I in the presence or absence of unlabeled ligands, and hybrids were quantitated as the fraction of 125I-IGF-I binding immunoadsorbed with MA-20. Abundance of hybrids was increased in NIDDM patients as compared with controls (B/T = 1.29 +/- 0.18 and 0.52 +/- 0.06%; P < 0.008, respectively), and it was inversely correlated with both IR number (r = -0.65; P < 0.002), and in vivo insulin sensitivity measured by insulin tolerance test (r = -0.75; P < 0.005), whereas it was positively correlated with insulinemia (r = 0.63; P < 0.003). Insulin binding affinity was lower in NIDDM subjects than in controls (ED50 = 1.87 +/- 0.32 and 0.54 +/- 0.20 nmol/l; P < 0.009, respectively), and was correlated with the percentage of hybrids. Maximal IGF-I binding was significantly greater in NIDDM patients than controls and was positively correlated with the percentage of hybrids whereas IGF-I binding affinity did not differ between the two groups. Results show that expression of hybrids is increased in fat of NIDDM patients compared to control subjects and is correlated with in vivo insulin sensitivity thus raising the possibility that alterations in expression of hybrids which bind IGF-I with higher affinity than insulin may contribute, at least in part, to insulin resistance.

Impaired secretion of heart lipoprotein lipase in cyclophosphamide-treated rabbit

Cyclophosphamide administration into fasted rabbits induces a hypertriglyceridaemia and a defect in vascular lipoprotein lipase. Heart LPL activity was more than 50% decreased after antimitotic treatment in fasted animals. The tissue distribution of lipoprotein lipase activity was followed in heart using recycling perfusion. Cyclophosphamide administration resulted in a profound decline in the heparin-releasable lipoprotein lipase activity, concordant with a higher recovery in the residual heart tissue. The effects were more pronounced in fasted than in fed animals. In agreement, the proportion of neosynthesized [35S]methionine-labelled lipoprotein lipase released by heparin was decreased by 50% following antimitotic treatment. The lipolysis of very low density lipoprotein-labelled triacylglycerols was found 2.5-fold reduced in hearts from cyclophosphamide-treated rabbits as compared to controls. These results suggest that a defective secretion of lipoprotein lipase may contribute to the poor expression of lipolytic activity in the vascular bed and to the occurrence of hypertriglyceridaemia during cyclophosphamide treatment.

Fat cells

The adipocyte is a metabolically active cell that functions to store energy for times of energy deprivation or enhanced need. Obesity is characterized by increased lipid accumulation and turnover compared with the nonobese state. Both triglyceride synthesis and lipolysis are regulated metabolic processes in the adipocyte. Current research on the metabolic activities of the human adipocyte focus on plasma triglyceride hydrolysis and uptake of fatty acids by LPL, esterification of these fatty acids, and the subsequent triglyceride breakdown by hormone-sensitive lipase in response to stimulation of adrenergic receptors. These topics are discussed in relationship to the development of obesity.

The expression of tumor necrosis factor in human adipose tissue. Regulation by obesity, weight loss, and relationship to lipoprotein lipase

A previous study reported the increased expression of the cytokine TNF in the adipose tissue of genetically obese rodents. To examine this paradigm in humans, we studied TNF expression in lean, obese, and reduced-obese human subjects. TNF mRNA was demonstrated in human adipocytes and adipose tissue by Northern blotting and PCR. TNF protein was quantitated by Western blotting and ELISA in both adipose tissue and the medium surrounding adipose tissue. Using quantitative reverse transcriptase PCR (RT-PCR), TNF mRNA levels were examined in the adipose tissue of 39 nondiabetic subjects, spanning a broad range of body mass index (BMI). There was a significant increase in adipose TNF mRNA levels with increasing adiposity. There was a significant correlation between TNF mRNA and percent body fat (r = 0.46, P < 0.05, n = 23). TNF mRNA tended to decrease in very obese subjects, but when subjects with a BMI > 45 kg/m2 were excluded, there was a significant correlation between TNF mRNA and BMI (r = 0.37, P < 0.05, n = 32). In addition, there was a significant decrease in adipose TNF with weight loss. In 11 obese subjects who lost between 14 and 66 kg (mean 34.7 kg, or 26.6% of initial weight), TNF mRNA levels decreased to 58% of initial levels after weight loss (P < 0.005), and TNF protein decreased to 46% of initial levels (P < 0.02). TNF is known to inhibit LPL activity. When fasting adipose LPL activity was measured in these subjects, there was a significant inverse relationship between TNF expression and LPL activity (r = -0.39, P < 0.02, n = 39). With weight loss, LPL activity increased to 411% of initial levels. However, the magnitude of the increase in LPL did not correlate with the decrease in TNF. Thus, TNF is expressed in human adipocytes. TNF is elevated in most obese subjects and is decreased by weight loss. In addition, there is an inverse relationship between TNF and LPL expression. These data suggest that endogenous TNF expression in adipose tissue may help limit obesity in some subjects, perhaps by increasing insulin resistance and decreasing LPL.

Regulatory function of glucose and insulin on high-density lipoprotein cholesterol in normolipidemic subjects

A decreased plasma concentration of high-density lipoprotein (HDL) cholesterol is associated with a higher incidence of coronary artery disease in populations. Therefore, there is intense investigation into the mechanisms responsible for the regulation of HDL cholesterol concentration in plasma. Insulin has a potent effect on HDL cholesterol, but it is unclear whether this is mediated by the primary effect insulin has on plasma triglycerides (TG). In this study, the question of the relationship between glucose, insulin, and HDL cholesterol has been addressed by investigating a cohort of nondiabetic normolipidemic men living in the Venice, Italy, area. One hundred twenty-eight men aged 30 to 69 years were initially recruited. The following parameters were measured: fasting plasma cholesterol, TG, HDL cholesterol, glucose, and insulin. One hundred seventeen of these subjects underwent an oral glucose tolerance test (OGTT), and the glucose and insulin responses were assessed. The final statistical analysis was performed on 98 nondiabetic individuals with plasma lipid levels within the 75th percentile for cholesterol and TG concentrations of the general population of the same age. The insulin response was a positive independent variable for plasma TG (P < .005) and HDL cholesterol (P < .005). On the other hand, HDL cholesterol was negatively associated with plasma TG. This relationship remained significant (P < .0001) also after controlling for age, body mass index (BMI), and glucose- and insulin-related measurements. Consistent with these results, both a stepwise variable selection analysis and a stratification analysis of the data indicated that the plasma TG concentration is the major determinant of HDL cholesterol level.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the metabolic effects of recombinant human insulin-like growth factor-I and insulin. Dose-response relationships in healthy young and middle-aged adults

The actions of recombinant human insulin-like growth factor-I (rhIGF-I) and insulin were compared in 21 healthy young (24 +/- 1 yr) and 14 healthy middle-aged (48 +/- 2 yr) subjects during 3-h paired euglycemic clamp studies using one of three doses (rhIGF-I 0.2, 0.4, and 0.8 micrograms/kg.min and insulin 0.2, 0.4, and 0.8 mU/kg.min, doses chosen to produce equivalent increases in glucose uptake). In younger subjects, rhIGF-I infusions suppressed insulin by 19-33%, C-peptide by 47-59% and glucagon by 33-47% (all, P < 0.02). The suppression of C-peptide was less pronounced with insulin than with rhIGF-I (P < 0.007). The metabolic responses to rhIGF-I and insulin were remarkably similar: not only did both hormones increase glucose uptake and oxidation in a nearly identical fashion, but they also produced similar suppression of glucose production, free fatty acid levels, and fat oxidation rates. In contrast, rhIGF-I had a more pronounced amino acid-lowering effect than did insulin (P < 0.004). In middle-aged subjects, basal IGF-I levels were 44% lower (P < 0.0001) whereas basal insulin and C-peptide were 20-25% higher than in younger subjects. Age did not alter the response to rhIGF-I. However, insulin-induced stimulation of glucose uptake was blunted in older subjects (P = 0.05). Our data suggest that absolute IGF-I and relative insulin deficiency contribute to adverse metabolic changes seen in middle age.

Phenobarbital treatment enhances insulin-mediated glucose metabolism and improves lipid metabolism in the diabetic rat

Previous studies with healthy volunteers and non-insulin-dependent diabetic (NIDDM) patients have shown a strong association between overall glucose metabolism and hepatic microsomal enzyme activity. In this study, the effects of 10-day oral administration of phenobarbital (PB), a potent inducer of the hepatic microsomal mixed-function oxidase system, on carbohydrate and lipid metabolism in the basal state and on glucose kinetics during submaximal hyperinsulinemic (5 mU.kg-1.min-1 insulin) clamps were investigated in nondiabetic rats and in rats made diabetic by the intravenous (IV) administration of either low-dose (40 mg/kg) or high-dose (55 mg/kg) streptozocin (STZ). In control rats receiving PB in drinking water (0.5 mg/mL), serum insulin and triglyceride levels were diminished without any change in glucose and cholesterol concentrations in the fed state. Administration of PB in drinking water (0.25 mg/mL) to both groups of diabetic rats decreased their water intake and serum triglyceride levels in the absence of an effect on glucose, insulin, and cholesterol concentrations in the fed state. However, fasting serum glucose levels and basal glucose turnover rates were lower in both groups of diabetic rats receiving PB. PB treatment increased the heparin-releasable lipoprotein lipase (LPL) activity of epididymal fat in both control and low-dose diabetic groups; this was not assessed in the high-dose diabetic group. Neither peripheral glucose utilization nor hepatic glucose production during submaximal insulin clamps was modified by PB treatment in nondiabetic rats. In contrast, PB administration enhanced insulin-mediated peripheral glucose utilization, as well as suppression of hepatic glucose production, in both low-dose and high-dose diabetic groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-chain fatty acids decrease lipoprotein lipase activity of cultured rat adipocyte precursors

The effect of fatty acids on rat adipocyte precursor lipoprotein lipase (LPL) activity was examined. Cellular LPL activity in cultured perirenal precursors reached a maximum after 6 days. At day 6, addition of 10(-8) mol/L oleic acid to the culture medium for 6 hours resulted in a significant reduction of LPL activity. Exposing cultured precursors to 10(-4) mol/L oleic acid caused more than a 50% decrease of intracellular LPL activity measured in either acetone-ether or detergent extracts and more than a 60% decrease of heparin-releasable LPL activity. These reductions were evident within 2.5 hours of exposure to oleic acid, and exposure to oleic acid for as little as 15 minutes caused a subsequent decrease in LPL activity. LPL activity recovered 48 hours after removal of oleic acid from culture medium. Decreased LPL activity after oleic acid exposure was also noted in epididymal cells and in differentiated adipocyte precursors. The extent of decrease of LPL activity upon fatty acid exposure was dependent on the presence of the carboxyl group and was affected by acyl chain length. Although oleic acid did not affect protein synthesis estimated by [3H]-leucine incorporation, LPL mRNA levels were decreased following exposure of cells to oleic acid. Glycerol-3-phosphate dehydrogenase (G3PD) activity and mRNA levels were not affected by oleic acid exposure. Hence, fatty acids cause a dose-, acyl chain-, and carboxyl group-dependent specific decrease of heparin-releasable and intracellular LPL activities in cultured rat adipocyte precursors; this effect is associated with and is likely caused at least in part by a decrease in LPL mRNA levels.

Lipoprotein lipase regulation by insulin and glucocorticoid in subcutaneous and omental adipose tissues of obese women and men

There are marked variations in the activity of lipoprotein lipase (LPL) among adipose depots, particularly in women. Consistent with data on LPL activity, the level of expression of LPL mRNA was lower in omental (OM) than subcutaneous (SQ) adipose tissue of women. To investigate the cellular basis of these differences, OM and SQ adipose tissues obtained at surgery from obese men and women were placed in organ culture for 7 d with varying concentrations of insulin and dexamethasone. Insulin increased levels of LPL mRNA and LPL activity in abdominal SQ but not OM adipose tissue. Dexamethasone also increased LPL mRNA and LPL activity, and these effects were more marked in the OM adipose tissue, particularly in men. When insulin and dexamethasone were added together, synergistic increases in LPL activity were seen in both depots, and this was in part explained at the level of LPL mRNA. The SQ depot was more sensitive to the effects of submaximal doses of dexamethasone in the presence of insulin. The maximum activity of LPL induced by insulin or insulin plus dexamethasone was higher in the SQ than in the OM depot of women, and this was associated with higher levels of LPL mRNA. Rates of LPL synthesis paralleled LPL mRNA levels. These data show that insulin and glucocorticoids influence human adipose tissue LPL activity at the level of LPL gene expression, as well as posttranslationally, and that responsiveness to these hormonal effects is dependent on adipose depot and gender.

Effects of interferon alpha on insulin binding and glucose transport in human adipocytes

We have previously demonstrated that interferon administration impairs glucose tolerance and causes insulin resistance in healthy man. Whether this is a direct effect of interferon is not known. The present study was undertaken to examine directly the effect of interferon alpha on insulin binding and action on glucose transport in isolated human adipocytes. Different concentrations of interferon alpha (range 10(-3)-10(5) IU ml-1) and different incubation times (0-5-24 h) with interferon were employed. Acute and 5-h and 24-h exposure of human adipocytes to 10(-2)-10 IU ml-1 of interferon increased the high affinity binding of 125I-insulin (P less than 0.05). In contrast, human interferon alpha had no effect on insulin binding in rat adipocytes. In short-term studies interferon had no effect on 14C-glucose transport clearance. 24-h preincubation of human adipocytes with 10(-2), 10, 10(4) IU ml-1 interferon increased maximally-insulin stimulated 14C-glucose transport clearance (P less than 0.05) and glucose transport responsiveness to insulin was enhanced by 24% (P less than 0.05) in cells exposed to 10(-2) IU ml-1 interferon. After 5 and 24-h preincubations with interferon we observed modest changes in glucose transport sensitivity to moderate concentrations of insulin (50-100 pM) with upregulation in the presence of 10(-2)-10 IU ml-1 interferon and downregulation in the presence of 10(4)-10(5) IUm ml-1 interferon (P less than 0.05). The insulin sensitivity index (ED50) did not change.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic actions of insulin-like growth factor-I in cultured hepatocytes from adult rats

Short-term and long-term regulation of hepatic carbohydrate metabolism by insulinlike growth factor-I was studied in primary cultures of adult rat hepatocytes and compared with the metabolic potency of insulin. Insulinlike growth factor-I stimulated the formation of [14C]lactate from [14C]glucose up to three-fold with a half-maximally effective concentration of approximately 50 nmol/L. Basal glycogenolysis was inhibited by about 20%, and glucagon-activated glycogenolysis was blocked completely by insulinlike growth factor-I with half-maximally effective concentrations of about 1.5 to 2 nmol/L. The activity of the key glycolytic enzymes glucokinase and pyruvate kinase were induced twofold. The glucagon-dependent induction of phosphoenolpyruvate carboxykinase--the key gluconeogenic enzyme--was antagonized with a half-maximally effective concentration of about 5 nmol/L. This inhibition of the glucagon-dependent induction of the enzyme was accompanied by a similar reduction of the increase in phosphoenolpyruvate carboxykinase-mRNA level as assessed by Northern blot analysis. The potency of insulinlike growth factor-I at half-maximally effective concentrations was approximately 2% to 4% that of insulin. Because binding studies demonstrated a comparably low affinity of insulinlike growth factor-I to the insulin receptor, it is suggested that in adult liver--in contrast to fetal and regenerating liver--insulinlike growth factor-I could exert short-term and long-term metabolic effects on parenchymal cells only through interaction with the insulin receptor.

Synthesis and secretion of active lipoprotein lipase in Chinese-hamster ovary (CHO) cells

Cultured Chinese-hamster ovary cells (CHO cells) were found to produce and secrete a lipase, which was identified as a lipoprotein lipase by the following criteria. Its activity was stimulated by serum and apolipoprotein CII, and was inhibited by high salt concentration. The lipase bound to heparin-agarose and co-eluted with 125I-labelled bovine lipoprotein lipase in a salt gradient. A chicken antiserum to bovine lipoprotein lipase inhibited the activity and precipitated a labelled protein of the same apparent size as bovine lipoprotein lipase from media of CHO cells labelled with [35S]methionine. The lipase activity and secretion were similar in growing cells and in cells that had reached confluency. Hence, lipoprotein lipase appears to be expressed constitutively in CHO cells and is not linked to certain growth conditions, as in pre-adipocyte and macrophage cell lines. At 37 degrees C, but not at 4 degrees C, heparin increased the release of lipase to the medium 2-4-fold. This increased release occurred without depletion of cell-associated lipase activity, suggesting that heparin enhanced release of newly synthesized lipase.

The effects of weight loss on the activity and expression of adipose-tissue lipoprotein lipase in very obese humans

Lipoprotein lipase is an enzyme in adipose tissue that hydrolyzes circulating triglycerides and thereby generates the fatty acids used in the synthesis of triglyceride in fat cells. To determine whether the activity and expression of lipoprotein lipase are affected by weight loss, we studied lipoprotein lipase in the adipose tissue of nine very obese subjects before and after a program of weight reduction. The subjects' mean (+/- SEM) initial weight was 136 +/- 7.3 kg, and the body-mass index (weight in kilograms divided by the square of the height in meters) ranged from 33.3 to 52.8 (mean, 43.0 +/- 2.5). Biopsies of adipose tissue were performed before weight loss and after it, when weight had been stable for three months. The weight reduction was achieved by a very-low-calorie diet (mean weight loss, 42.5 +/- 6.8 kg). After weight loss, the level of heparin-releasable lipoprotein lipase activity increased in all patients, from 3.8 +/- 1.1 to 7.1 +/- 1.6 neq of free fatty acid released per minute per 10(6) cells (P less than 0.05). In addition, the amount of lipoprotein lipase immunoreactive protein increased from 6.3 +/- 1.7 to 24.4 +/- 6.9 ng per 10(6) cells (P less than 0.05), and there was also an increase in the level of lipoprotein lipase messenger RNA as measured by Northern blotting. There was a strongly positive correlation between the initial body-mass index and the magnitude of the increase in lipoprotein lipase activity (r = 0.80, P less than 0.01) and immunoreactive protein (r = 0.92, P less than 0.01). We conclude that weight loss in very obese subjects leads to the increased activity and expression of lipoprotein lipase, thereby potentially enhancing lipid storage and making further weight loss more difficult.

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.

Familial lipoprotein lipase-activity deficiency: study of total body fatness and subcutaneous fat tissue distribution

Total body fatness and subcutaneous fat tissue distribution were evaluated in 19 hyperchylomicronemic patients. Eleven were males, aged 10 to 57 years, and eight were females, aged 13 to 46 years. Familial lipoprotein-lipase-activity deficiency was diagnosed by the absence of lipoprotein-lipase activity in the plasma withdrawn ten and 20 minutes after intravenous injection of ten units of heparin per kilogram of body weight. The 19 patients had skin-fold measurements for evaluation of subcutaneous fat distribution. Fifteen also underwent body density measurements by underwater weighing. Percent body fat was calculated from body density. These anthropometric data were plotted against the regression curves of 1638 normal controls of both sexes (aged 10 to 54 years) for fat tissue weight, percent body fat, subcutaneous fat/total fat mass ratio and trunk/extremity skin-fold ratio. Impairments in the process of building fat tissue reserves could not be shown in the 19 hyperchylomicronemic patients, in spite of the absence of lipoprotein-lipase activity in their postheparin plasma. It is hypothesized that normal fat tissue mass in these patients could be due partly to de novo synthesis of fatty acids by adipocytes, hydrolysis of plasma triglycerides by hepatic lipase, and/or contribution of a specific fat-tissue lipase to the catabolism of plasma triglyceride-rich lipoproteins.

Effect of feeding and obesity on lipoprotein lipase activity, immunoreactive protein, and messenger RNA levels in human adipose tissue

Previous studies have demonstrated higher levels of adipose tissue lipoprotein lipase (LPL) catalytic activity in obese subjects, and in response to a meal. To examine the cellular mechanism of this increase in activity, LPL activity, immunoreactive mass, and mRNA level were measured in lean and obese subjects both before and 4 h after a carbohydrate-rich meal. Heparin-releasable (HR) LPL activity was approximately 2.5-fold higher in the 15 obese subjects, when compared with six lean subjects. However, there was no difference in LPL immunoreactive mass between the lean and obese subjects. In response to the meal, there was a 2.2-fold increase in total adipose tissue LPL activity in the lean subjects due to an increase in both the HR fraction, as well as the adipose fraction extracted with detergents. However, no increase in LPL immunoreactive mass was observed in any adipose tissue LPL fraction, resulting in an increase in LPL specific activity in response to the meal. In the obese subjects, there was no significant increase in LPL activity in response to feeding, and also no increase in immunoreactive mass or specific activity. After extraction of RNA, there was no difference in either the relative proportion of the 3.6- and 3.4-kb human LPL mRNA transcripts, nor in the quantity of LPL mRNA in response to feeding. Thus, these data suggest that the increase in LPL activity under these conditions occurs through a posttranslational activation of a previously inactive LPL precursor.

Fetal hypophysectomy causes a decrease in preadipocyte growth and insulin like growth factor-1 in pigs

Fetal pigs in one uterine horn of each of five gilts were hypophysectomized (HX) in utero by electrical cauterization at 72-74 days of gestation and sera collected at 110 days of gestation. Sera from HX fetuses had lower levels of insulin-like growth factor-1 compared to control littermates (P less than .05). Sera were tested for their effects on primary cultures of stromal-vascular cells from adipose tissue. The soluble protein concentration/dish was lower when pig cells were cultured in sera from HX fetuses compared to sera from control fetuses (P less than .01). Sera from HX fetuses inadequately supported growth of stromal-vascular cells so subsequent experiments utilized pooled sera from normal and HX adult pigs. Sera from HX and control fetuses were mixed with sera from the two adult pools and tested for incorporation of tritiated thymidine into rat preadipocytes and the appearance of adipocytes (determined histochemically) in pig stromal-vascular cultures. In cultures fed sera from HX fetuses there was a lower (P less than .05) number of pig fat cells/culture and a lower level (P less than .06) of preadipocyte proliferation in rat cell cultures when compared to control fetal sera. Fetal pig serum contains factors (adipogenic) which promote the proliferation and differentiation of adipocytes in culture. Serum from HX fetuses has a lower level of adipogenic factors.

Rapid modulation of rat adipocyte lipoprotein lipase: effect of calcium, A23187 ionophore, and thrombin

The effect of calcium ions on the lipoprotein lipase (LPL) activity in rat adipocytes has been investigated. Incubation of the cells in the absence of extracellular calcium produced a rapid decline of LPL activity in the cells. The enzyme, however, could be immediately reactivated in less than 3 min by the addition of calcium. The degree of reactivation was proportional to the concentration of extracellular calcium. alpha 1 agonists phenylephrine and methoxamine affected LPL activity only slightly, as did vasopressin and angiotensin II. In contrast, calcium ionophore A23187 elicited a quick and transient enzyme activation which reached its peak 4 min after the addition of the drug. Thrombin (0.1 U/ml) produced the most rapid and intense response. The effect of thrombin was already evident 10 s after its addition, and the enzyme activity almost doubled above the basal level. Extracellular calcium was necessary to achieve thrombin activation. Contrary to previous thought, these data support the conclusion that LPL may undergo rapid activation, and that calcium ions are critically involved in this activation process. Thrombin rapidly raises LPL activity and may be one of its physiological activators in vivo.

Permissive action of glucocorticoids on catecholamine-induced lipolysis: direct "in vitro" effects on the fat cell beta-adrenoreceptor-coupled-adenylate cyclase system

Exposure of adipose tissue fragments to dexamethasone leads to enhanced lipolytic and cyclic AMP responses of isolated fat cells to isoproterenol. This permissive effect of the steroid is dose-dependent, prevented by the glucocorticoid receptor antagonist RU 38486, maximum after 48 h exposure to 10 nM dexamethasone and affects only the amplitude of the maximal response (+50%). Exposure to dexamethasone also induces an increase in both the number of beta-adrenergic receptors (+30%), and the adenylate cyclase-catalytic activity (+64%) and - responses to GTP (+114%) and isoproterenol (+55%). These data strongly suggest that the permissive effect of glucocorticoids towards lipolysis "in vivo" results at least in part from a glucocorticoid-receptor mediated action of these hormones on the fat cell membranous components involved in the beta-adrenergic control of lipolysis.

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.

Long-term effect of insulin on glucose transport and insulin binding in cultured adipocytes from normal and obese humans with and without non-insulin-dependent diabetes

We have tested the hypothesis that in vitro exposure of insulin-resistant adipocytes with insulin results in improved insulin action. A primary culture system of adipocytes from obese subjects with or without non-insulin-dependent diabetes mellitus (NIDDM) and nonobese control subjects has been developed. The adipocytes when cultured in serum-free medium do not lose their original characteristics in regard to insulin binding and glucose transport. The adipocytes from three groups were incubated with insulin (0, 10(-10) M, and 10(-7) M) for 24 h at 37 degrees C, receptor-bound insulin was dissociated, and basal and insulin (1 X 10(-11)-10(-7) M)-stimulated glucose transport and 125I-insulin binding were determined. The 24-h insulin exposure of adipocytes from control subjects decreased basal and insulin-stimulated glucose transport. The effects of 1 X 10(-7) M insulin were more pronounced than 1 X 10(-10) M insulin. Similarly, insulin exposure decreased insulin sensitivity and responsiveness of cultured adipocytes from obese and NIDDM patients. The insulin-induced reduction in insulin sensitivity and responsiveness for glucose transport in three groups were due to alterations at insulin binding and postbinding levels. In conclusion, insulin induces insulin resistance in control adipocytes and further worsens the insulin resistance of adipocytes from obese and NIDDM subjects. For insulin to improve the insulin resistance of adipocytes from NIDDM patients, either more prolonged in vitro insulin exposure and/or other hormonal factors might be required.

Antibody against human lipoprotein lipase

A polyclonal antibody against human lipoprotein lipase (LPL) was prepared. LPL from post-heparin plasma was first purified by heparin Sepharose 4B affinity chromatography. Protein impurities co-eluted with LPL were then eliminated by electrophoresis in the presence of ampholytes. Antithrombin III was identified in this fraction of protein impurities by immunodiffusion against a human antithrombin antiserum, while no antithrombin III could be detected in the purified LPL fraction. Immunodiffusion revealed a single line of precipitation between this antibody and human post-heparin plasma LPL. When pre-incubated with a constant activity of highly purified post-heparin plasma LPL (2.7 mU/75 microliters), an equal volume of the anti-LPL antiserum, either pure or diluted to 1/32 caused complete inhibition of the enzyme activity. Half maximal inhibition was observed at a dilution of approximately 1/200. By using a secondary antibody, it was shown that antiserum inhibited LPL activity by means of its immunoglobulins. This antibody was able to inhibit LPL from human adipose tissue, indicating that human LPL released from endothelial cell membranes has common antigenic determinants with adipose tissue LPL.