Adipose tissue glyceride synthesis in patients with hyperapobetalipoproteinemia

Low density lipoprotein delays clearance of triglyceride-rich lipoprotein by human subcutaneous adipose tissue

Delayed clearance of triglyceride-rich lipoprotein (TRL) by white adipose tissue (WAT) promotes hypertriglyceridemia and elevated apoB-lipoproteins, which are primarily in the form of LDL. This study examines whether LDL promotes delayed clearance of TRL by WAT. Following the ingestion of a (13)C-triolein-labeled high-fat meal, obese women with high plasma apoB (> median 0.93 g/l, N = 11, > 98% as IDL/LDL) had delayed clearance of postprandial (13)C-triglyceride and (13)C-NEFA over 6 h compared with controls. AUC6 h of plasma (13)C-triglyceride and (13)C-NEFA correlated with plasma apoB but not with LDL diameter or adipocyte area. There was no group difference in (13)C-triolein oxidation rate, which suggests lower (13)C-NEFA storage in peripheral tissue in women with high apoB. Ex vivo/in vitro plasma apoB correlated negatively with WAT (3)H-lipid following a 4 h incubation of women's WAT with synthetic (3)H-triolein-TRL. LDL-differentiated 3T3-L1 adipocytes had lower (3)H-TRL hydrolysis and (3)H-NEFA storage. Treatment of women's WAT with their own LDL decreased (3)H-TRL hydrolysis and (3)H-NEFA uptake. Finally, LDL, although not an LPL substrate, reduced LPL-mediated (3)H-TRL hydrolysis as did VLDL and HDL. Exposure to LDL decreases TRL clearance by human WAT ex vivo. This may promote production of apoB-lipoproteins and hypertriglyceridemia through a positive-feedback mechanism in vivo.

Adipocyte triglyceride turnover is independently associated with atherogenic dyslipidemia

BACKGROUND

Inappropriate storage of fatty acids as triglycerides in adipocytes and their removal from adipocytes through lipolysis and subsequent oxidation may cause the atherogenic dyslipidemia phenotype of elevated apolipoprotein B levels and subsequent hypertriglyceridemia. We tested whether turnover of triglycerides in fat cells was related to dyslipidemia.

METHODS AND RESULTS

The age of triglycerides (reflecting removal) and triglyceride storage in adipocytes was determined under free living conditions by measuring incorporation of atmospheric (14)C into these lipids within the adipocytes in 47 women and 26 men with a large interindividual variability in body mass index. Because limited (14)C data were available, triglyceride age was also determined in 97 men and 233 women by using an algorithm based on adipocyte lipolysis, body fat content, waist-to-hip ratio, and insulin sensitivity. This cohort consisted of nonobese subjects since obesity per se is related to all components in the algorithm. Triglyceride turnover (age and storage) was compared with plasma levels of apolipoproteins and lipids. Plasma levels of apolipoprotein B and triglycerides were positively related to triglyceride age in adipocytes, when measured directly using radiocarbon analyses (r=0.45 to 0.47; P<0.0001). This effect was independent of subject age, waist circumference measures, and insulin sensitivity (partial r=0.29 to 0.45; P from 0.03 to <0.0001). Triglyceride storage showed no independent correlation (partial r=0.02 to 0.11; P=0.42 to 0.91). Algorithm-based values for adipocyte removal of triglycerides were positively associated with plasma triglycerides and apolipoprotein B (r=0.44 to 0.45; P<0.0001) and (also positively) with the inflammation status of adipose tissue (r=0.39 to 0.47; P<0.05). These correlations were statistically independent of subject age and observed in men and women as well as in lean and overweight subjects when subgroups were examined separately.

CONCLUSIONS

Decreased removal of adipocyte triglycerides (as indicated by a high triglyceride age in fat cells) is independently associated with circulating apolipoprotein B and triglycerides. This suggests a hitherto unknown role of triglyceride turnover in adipocytes for the development and/or maintenance of atherogenic dyslipidemia.

Type 2 diabetes and cardiovascular disease: getting to the fat of the matterThis paper is one of a selection of papers published in this Special Issue, entitled Young Investigators' Forum

The increasing national prevalence of obesity is a major public health concern and a substantial burden on the health care resources of Canada. In addition to the direct health impact of obesity, this condition is a well-established risk factor for the development of various prevalent comorbidities including type 2 diabetes, hypertension, and cardiovascular disease. Historically, adipose tissue has been regarded primarily as an organ for energy storage. However, the discovery of leptin in the mid 1990’s revolutionized our understanding of this tissue and has focused attention on the endocrine function of adipose tissue as a source of secreted bioactive peptides. These compounds, collectively termed adipokines, regulate a number of biological functions including appetite and energy balance, insulin sensitivity, lipid metabolism, blood pressure, and inflammation. The physiological importance of adipokines has led to the hypothesis that changes in the synthesis and secretion of these compounds in the obese are a causative factor contributing to the development of obesity and obesity-related diseases in these individuals. Following from this it has been proposed that pharmacologic manipulation of adipokine levels may provide novel effective therapeutic strategies to treat and prevent obesity, type 2 diabetes, and cardiovascular disease.

Protease Inhibitor Effects on Triglyceride Synthesis and Adipokine Secretion in Human Omental and Subcutaneous Adipose Tissue

Objective

Significant advances in the treatment of the morbidity and mortality associated with AIDS are also associated with undesirable side-effects in fat redistribution (lipodystrophy), insulin resistance and cardiovascular risk, which is directly linked to protease inhibitor (PI) treatment.

Methods

The effects of four different PIs on triglyceride (TG) storage and adipokine production (leptin, adiponectin, and acylation stimulating protein [ASP]) in omental (OM) and subcutaneous (SC) adipose tissues were examined.

Results

Initial results demonstrated that saquinivir (SQV) and ritonivir (RTV) had little observed effect on de novo TG synthesis ([3H]glucose incorporation into TG) or fatty acid re-esterification ([14C]oleate incorporation into TG), whereas amprenivir (APV) and indinivir (IDV) reduced TG synthesis, especially in SC tissue up to 30 ±5.8% P<0.05 and 46 ±7.8% P<0.001, at 20 μM, respectively. There was no observed effect on phospholipid synthesis, tissue protein or toxicity. Only APV and IDV decreased leptin and adiponectin secretion in SC tissue, in a time- and concentration-dependent manner: at 18 h, leptin was inhibited by 54 ±3.1% ( P<0.001) and 44 ±6.4% ( P<0.001) by APV and IDV (40 μM), respectively, and adiponectin was inhibited by 35 ±5.6%( P<0.001) and 25 ±12.3% ( P<0.05) by APV and IDV (40 μM), respectively. By contrast, both IDV and APV decreased ASP secretion in OM tissues by a maximum of 53 ±7.8% ( P<0.001) and 59 ±5.9% ( P<0.001), respectively, and by a maximum of 86 ±1.6% ( P<0.001) and 72 ±4% ( P<0.001), respectively, in SC tissues.

Conclusion

PI have a direct effect on human adipose tissue which are site, PI and adipokine specific; these effects may contribute to the overall adipose imbalance and development of lipodystrophy, and metabolic syndrome in HIV-positive individuals.

Serum C3 but not plasma acylation-stimulating protein is elevated in Finnish patients with familial combined hyperlipidemia

A trapping defect of fatty acids due to impaired function of acylation-stimulating protein (ASP) has been suggested as one mechanism underlying the metabolic abnormalities in familial combined hyperlipidemia (FCHL). The study aimed at defining the role of ASP and complement C3 in 35 Finnish FCHL families. There was no difference in plasma ASP levels between the 66 hypertriglyceridemic FCHL patients and their 84 normotriglyceridemic relatives. No response in plasma ASP could be observed after a fatty meal in 10 FCHL patients or in 10 control subjects. In familial correlation analyses, C3 exhibited a significant sibling-sibling correlation. The FCHL patients had higher serum C3 levels than their unaffected relatives (P<0.001). Furthermore, serum C3 levels correlated significantly with several lipid parameters. The correlations between ASP and lipid variables were weaker than those of C3. These analyses suggest that common genes might contribute to the regulation of serum C3, triglycerides, HDL-C, free fatty acids, and insulin. The present data do not support the hypothesis that defects of the ASP pathway are reflected in plasma lipoproteins or in impaired plasma lipid clearance postprandially.

A critical evaluation of the putative role of C3adesArg (ASP) in lipid metabolism and hyperapobetalipoproteinemia

The acylation stimulating protein, ASP is a small, basic serum protein capable of stimulating triglyceride synthesis in cultured fibroblasts and adipocytes. Sequence analysis of ASP has shown that ASP is identical to C3adesArg the inactive fragment of the complement anaphylatoxin peptide, C3a. It has been proposed that C3adesArg (ASP) can be generated by mature adipocytes secreting the three complement proteins: complement protein C3, factor B and factor D (adipsin). There have also been indications that adipocytes may express a specific C3adesArg (ASP)-receptor that is distinct from the recently cloned C3a-receptor. This suggests that C3adesArg (ASP) acts as an adipocyte autocrine and that it plays a central role in the metabolism of adipose tissue. Based on these observations a hypothesis for the etiology of hyperapobetalipoproteinemia (hyperapoB) has been proposed. Hyperapobetalipoproteinemia (hyperapoB), is a familial lipoprotein disorder characterized by increased hepatic secretion of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) particles. If C3adesArg (ASP) function in the adipose tissue is impaired, a reduced rate of triglyceride synthesis will follow, generating an increased flux of fatty acids to the liver. In response to an increased flow of fatty acids, the liver will increase its production of VLDL particles yielding the phenotype of hyperapoB. This review critically assesses this hypothesis and the potential role of C3adesArg (ASP) as a major determinant for triglyceride synthesis in the light of data collected in vitro and in vivo.

Genetic deficiency of acylation stimulating protein (ASP(C3ades-Arg)) does not cause hyperapobetalipoproteinemia in mice

The acylation stimulating protein (ASP) is a 76-amino acid peptide that has been proposed as a potent mediator of triglyceride synthesis and, when functionally impaired, as a major cause of hyperapobetalipoproteinemia (HyperapoB). Purification and sequence analysis of ASP from human sera have revealed that ASP is identical to the complement C3-derived activation peptide C3ades-Arg. Because C3 is the precursor for C3ades-Arg and therefore ASP, a deficiency in C3 would be predicted to result in a phenotype characteristic of HyperapoB. To test this hypothesis in vivo, the current study was undertaken in which ASP(C3ades-Arg)-deficient mice were used as a model system. No significant differences were found in the triglyceride, cholesterol, or free fatty acid concentrations in the plasma of fasted normal and ASP(C3ades-Arg)-deficient animals. In addition, plasma lipoprotein analyses indicated that the very low density lipoprotein, low density lipoprotein, and high density lipoprotein cholesterol and triglyceride concentrations as well as the apolipoprotein B-48 and B-100 levels were not significantly different in the plasma of ASP(C3ades-Arg)-deficient and wild type mice. Furthermore, when challenged with an oral fat load, the ASP(C3ades-Arg)-deficient mice showed no impaired ability to clear triglycerides and free fatty acids from their circulation when compared with their wild-type littermates. Collectively, these results indicate that ASP(C3ades-Arg) deficiency does not cause HyperapoB in mice and that the physiological importance of impaired ASP(C3ades-Arg) function as a cause of hyperapobetalipoproteinemia needs to be reevaluated.

Differential effect of genistein on the stimulation of cholesterol production by basic protein II in normal and hyperapoB fibroblasts

We studied further the basis for the abnormal effect of human serum basic protein II (BP II) on cholesterol production in hyperapobetalipoproteinemia (hyperapoB) fibroblasts and whether this effect involves protein tyrosine kinase phosphorylation (TKP). Genistein, a specific inhibitor of TKP was used as a probe. Compared with normal cells, BP II stimulated significantly the cellular mass of total cholesterol (6.4-fold), unesterified cholesterol (3.6-fold), and esterified cholesterol (6.7-fold) in hyperapoB fibroblasts. The addition of genistein to BP II in hyperapoB cells markedly inhibited these abnormal stimulatory effects of BP II on cell sterol mass. In normal cells, the addition of genistein to BP II produced an opposite effect: a marked stimulation in the mass of total (5.5-fold) and esterified (18.3-fold) cholesterol and a decrease in unesterified cholesterol (3.4-fold). These effects of genistein on the formation of cellular cholesterol by BP II were both time and concentration dependent. The inhibition of the stimulatory effect of BP II on cholesterol production by genistein in hyperapoB cells may be mediated through 3-hydroxy 3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme of cholesterol biosynthesis, since the rate of incorporation of [14C]acetate, but not [3H]mevalonate, into unesterified cholesterol was decreased by genistein in the hyperapoB cells. When the mass of cell total cholesterol in the cells treated with BP II was subtracted from those treated with BP II plus genistein, a negative number was produced in each of the six hyperapoB cell lines, while each of the normal cell lines retained a positive number. The mean difference for the mass of total cholesterol between the hyperapoB and normal fibroblasts under these conditions was 128.2 nmol/mg cell protein, a difference that was separated by >3 SD. This study supports further the tenet that there is a defect in the response of hyperapoB cells to BP II and that this defect results in an abnormality in cholesterol metabolism that appears mediated through a protein TKP-mediated process.

Adipose tissue lipoprotein lipase and hormone-sensitive lipase. Contrasting findings in familial combined hyperlipidemia and insulin resistance syndrome

The metabolism of free fatty acids (FFA) is altered in two common atherosclerosis-promoting disorders: familial combined hyperlipidemia (FCHL) and insulin resistance syndrome (IRS). It has been suggested that these two conditions may have a common etiology. The enzymes lipoprotein lipase (LPL) and hormone-sensitive lipase (HSL) are rate-limiting steps for the turnover of fatty acids in adipose tissue, because they hydrolyze extracellular triglycerides in lipoproteins (LPL) and intracellular triglycerides in adipocytes (HSL). The present study was undertaken to simultaneously determine the activities of LPL and HSL in subcutaneous adipose tissue from male patients with FCHL and IRS. LPL and HSL activity was investigated in 10 nonobese FCHL patients and compared with 10 matched healthy nonobese subjects, and in 8 essentially normolipidemic IRS patients (who did not have overt diabetes mellitus) and compared with 9 nonobese matched control subjects. LPL activity was 43% lower in patients with IRS (P < .0005), as compared with control subjects, but HSL activity was not significantly different in the two groups, On the other hand, HSL activity was decreased by 45% in FCHL patients (P < .01), as compared with control subjects, but LPL activity was not significantly different in FCHL patients and the control group. In conclusion, triglyceride metabolism in adipose tissue is altered in both FCHL and IRS. However, the abnormalities observed involve impaired function of LPL in IRS and impaired function of HSL in FCHL, suggesting separate etiologies for the altered lipolysis in these conditions, at least in male subjects.

Abnormal protein tyrosine phosphorylation in fibroblasts from hyperapobetalipoproteinemia subjects

The stimulatory effects of three normal human serum basic proteins (BP), BP I (Mr 14,000, pI 9.10), BP II (Mr 27, 500, pI 8.48), and BP III (Mr 55,000, pI 8.73) on cellular triglyceride and cholesterol formation require intact protein-tyrosine kinase phosphorylation (TKP). Here we examined whether there is an abnormality in TKP in cultured fibroblasts from 11 patients with hyperapobetalipoproteinemia (hyperapoB) that manifest two acylation-stimulatory defects, decreased stimulation of triglyceride synthesis by BP I but enhanced formation of cholesterol by BP II. Soluble and insoluble proteins in Triton X-100 extracts were isolated by immunoprecipitation with a monoclonal anti-phosphotyrosine antibody (MAPA) bound to agarose beads and by ultracentrifugation, respectively, from confluent fibroblasts after incubation for 24 h in supplemented serum-free and lipid-free medium (DMEM/F12). Western blots of insoluble proteins showed that group (Gp) II (Mr 36,000-55,000) and Gp III (Mr 14,000-35,000) from hyperapoB cells, grown in DMEM/F12 medium without BP, had significantly decreased reactivity to MAPA. No significant differences in reactivity to MAPA were detected between normal and hyperapoB cells for Gp I (Mr 97-120,000). BP II, but not BP I or BP III, reversed the decreased reactivity of Gp II and Gp III to MAPA in hyperapoB cells. Sodium vanadate, an inhibitor of phosphotyrosine phosphatases, did not reverse the deficiency in TKP or the 50% deficiency in the stimulation of mass triglyceride by BP I in hyperapoB cells. Tyrosine-phosphorylated Erk-2, a mitogen-activated protein kinase, identified as one of the proteins in Gp II, was significantly decreased in hyperapoB cells. These results provide further evidence for abnormal protein TKP in hyperapoB cells and suggest a possible link between atherosclerotic changes in hyperapoB patients and growth factors upstream from mitogen-activated protein kinase.

Plasma Acylation-Stimulating Protein in Coronary Artery Disease

To date, plasma levels of acylation-stimulating protein (ASP) have been determined only in normal and obese individuals. Accordingly, ASP was measured in fasting samples obtained from 59 age-matched controls and 208 patients with documented coronary artery disease (CAD). Overall, plasma ASP was significantly higher in the CAD subjects compared to the control subjects (55.3±1.8 nmol/L CAD versus 32.0±2.6 nmol/L control, P <.QQQ5). In the control group, the distribution of plasma ASP values was unimodal whereas in the coronary group it was significantly skewed to the right. The coronary group was subdivided into those with pronounced elevation of apoB (a marked type II phenotype, n=13), those with hypertriglyceridemia with a normal apoB (n=17), and the remaining CAD subjects (n=178). In the first two groups, ASP did not differ significantly from control subjects (43±2.8 nmol/L and 49±4.4 nmol/L respectively). By contrast, in the remaining CAD subjects, both the mean ASP level (56.8±2.0 nmol/L, P <.001 by ANOVA) and the proportion of patients with a markedly elevated ASP (25.3% were >95th percentile, P <.005 versus control by χ 2 ) w e r e significantly increased. When this third group was divided into tertiles by plasma apoB and triglyceride, there was a direct relationship between plasma ASP and these two parameters. Linear regression analysis demonstrated an association between plasma ASP and plasma triglyceride ( P <.05), VLDL cholesterol ( P <.025), and VLDL apoB ( P <.05). Finally, when all of the CAD subjects were divided by apoE phenotype, there appeared to be a relationship between plasma ASP and apoE phenotype such that ASP was higher in E2 subjects, intermediate in E3 subjects, and lower in E4 subjects. The present data document plasma ASP levels in a number of dyslipoproteinemic states and suggest a relation between the adipsin-ASP pathway and other metabolic determinants of lipoprotein metabolism. ( Arterioscler Thromb Vase Biol. 1997;17:1239-1244.)

The acylation stimulating protein pathway: clinical implications

OBJECTIVES

The present review will focus particularly on acylation stimulating protein (ASP) and its role in adipose tissue. Two issues will be addressed (1) in vitro biochemical characterization of ASP in cell culture studies, and (2) in vivo clinical relevance for normal physiology and in pathological conditions.

CONCLUSIONS

Fat is In! There can be no question that in recent years fat tissue has become recognized as more than just a passive storage site. It is a metabolically active tissue that, under normal conditions, allows the efficient clearance of triglyceride and glucose for storage as energy. Under abnormal conditions, adipose tissue dysfunction is associated with obesity, diabetes and coronary heart disease. Adipose tissue function may be controlled by many factors.

No evidence of linkage between familial combined hyperlipidemia and genes encoding lipolytic enzymes in Finnish families

Familial combined hyperlipidemia (FCHL) is characterized by different lipid phenotypes (IIa, IIb, IV) and elevated apolipoprotein B (apo B) levels in affected family members. Despite intensive research, the genes involved in the expression of this complex disorder have not been identified, probably because of problems associated with phenotype definition, unknown mode of inheritance, and most probably genetic heterogeneity. To explore the genetics of FCHL in the genetically homogeneous Finnish population, we collected 14 well-documented Finnish pedigrees with premature coronary heart disease and FCHL-like dyslipidemia. The lipolytic enzymes lipoprotein lipase (LPL), hepatic lipase (HL), and hormone-sensitive lipase (HSL) were selected as initial candidate genes because of their central roles in apo B and triglyceride metabolism. On the basis of the pedigree structures, a dominant mode of inheritance was adopted for linkage analyses, and serum total cholesterol and/or triglyceride levels exceeding the 90th percentile level were set as diagnostic criteria (criterion 1). In pairwise linkage analyses with intragenic markers, no evidence for linkage was found. Instead, the significantly negative LOD scores suggested exclusion of all three loci for single major gene effect. LOD scores were -14.63, -5.03, and -5.70 for the three LPL polymorphisms (theta=0.00); -9.40, -6.30, and -4.74 for the three HL polymorphisms (theta=0.00); and -15.29 for the HSL polymorphism (theta=0.00). The results were very similar when apo B levels over the 90th percentile were used as criteria for affected status (criterion 2). Also, when linkage calculations were carried out using an intermediate or recessive mode of inheritance, the results of pairwise linkage analysis remained negative. Furthermore, when haplotypes were constructed from multiple polymorphisms of the LPL and HL genes, no segregation with the FCHL phenotype could be observed in the 14 Finnish families. Data obtained by the affected sib-pair method supported these findings, suggesting that the LPL, HL, or HSL genes do not represent major loci influencing the expression of the FCHL phenotype.

Acylation-stimulating protein (ASP) regulates glucose transport in the rat L6 muscle cell line

Acylation-stimulating protein (ASP), a human plasma protein, is a potent stimulator of triglyceride synthesis and glucose transport in both human adipocytes and fibroblasts. The purpose of the present in vitro study was to examine the effect of ASP on glucose transport in muscle cells. ASP stimulated 2-deoxy-glucose transport (2-DG) in differentiated rat L6 myotubes in a time (30 min to 24 h) and concentration dependent manner (97% increase). The magnitude of the ASP effect on glucose transport was comparable to the time- and concentration-dependent effects seen with insulin (125% increase), but was additive to insulin, pointing to involvement of differential signalling pathways. ASP stimulation was dependent on cell differentiation in that glucose transport increased by only 12% in myoblasts, comparable to the effect of insulin in myoblasts (15% increase) demonstrating selective responsiveness of the differentiated myotubes to ASP and insulin. The mechanism for the ASP induced increase in glucose transport was also examined. ASP increased the Vmax for 2-DG transport by 183% (4.02 vs. 1.42 nmol/mg cell protein/30 s; ASP vs. Control, respectively). This could be explained by an increased translocation of glucose transporters (GLUT 1, GLUT 4 and GLUT 3) to the plasma membrane surface as demonstrated by Western analysis (+43% P < 0.05, +30% P < 0.05, and +49% P < 0.05, respectively). The effects of ASP were equal to those of insulin (+47%, +26% and +53% for GLUT 1, GLUT 4 and GLUT 3, respectively) and in all cases were paralleled by comparable glucose transport increases under the same incubation conditions. After long-term stimulation (24 h), Western analysis indicated that ASP had a permissive effect on insulin stimulated increases in total GLUT3 and GLUT4 cellular transporter content. These results suggest that muscle is also responsive to ASP and that ASP may play a role in glucose metabolism in both muscle and adipose tissue.

Impaired activation of adipocyte lipolysis in familial combined hyperlipidemia

The pathophysiology of familial combined hyperlipidemia (FCHL) is unknown, but altered lipid turnover in peripheral tissues as well as hepatic overproduction of apolipoprotein B have been suggested as possible causes. In the present study, we explored whether a change in triglyceride breakdown by lipolysis in fat cells is present in FCHL. Lipolysis activation by catecholamines was examined in isolated subcutaneous adipocytes from 10 patients with FCHL and 22 healthy control subjects. Lipolysis rate was linear for at least 3 h in both groups. However, a marked (approximately 65%) decrease in the lipolytic response to noradrenaline was found in FCHL. This was also true when lipolysis was maximally stimulated at the receptor level with isoprenaline (nonselective beta-adrenergic agonist), at the adenylyl cyclase level with forskolin, or at the level of the protein kinase hormone-sensitive lipase complex with dibutyryl cAMP. The maximum enzymatic activity of hormone-sensitive lipase was decreased by approximately 40% in FCHL. On the other hand, the lipolytic sensitivity of alpha 2-, beta 1-, and beta 2-adrenoceptors was normal in this condition, as was the number and affinity of beta 1- and beta 2-adrenoceptors. Variations in the maximum lipolysis rate correlated significantly with the variations in hormone-sensitive lipase activity in the whole material, and with the serum values for triglycerides, HDL cholesterol and apoB lipoprotein within the control group, but the serum triglyceride values in FCHL were higher than this correlation predicted. In conclusion, the data demonstrate a marked resistance to the lipolytic effect of catecholamines in fat cells from patients with FCHL, in spite of normal adrenoceptor function. The lipolytic defect appears predominantly to be due to a defect in hormone-sensitive lipase, and may be of importance in the pathophysiology of FCHL.

Increase in intracellular triglyceride synthesis induced by gemfibrozil

Gemfibrozil is an effective hypotriglyceridemic agent. Its mechanism of action has not been determined with certainty, but it is generally thought to act by virtue of an effect on lipoprotein lipase. However, this study was designed to test the hypothesis that gemfibrozil directly affects intracellular triglyceride synthesis in peripheral cells, and indeed, when added to the medium, gemfibrozil markedly stimulated triglyceride synthesis in cultured human skin fibroblasts. The effect was concentration-dependent, with a maximum effect at a gemfibrozil concentration of 40 micrograms/mL. A marked increase in triglyceride synthesis was observed when either labeled glucose or labeled fatty acid were added to the medium, indicating that the stimulation was substrate-coordinated. The effect was sustained for hours, with an onset of action within minutes. Lineweaver-Burk plots were constructed, and these revealed no apparent change in Km for either oleate or glucose (8.18 +/- 2.97 v 6.40 +/- 2.10 mumol/L for oleate and 0.113 +/- 0.004 v 0.107 +/- 0.051 mmol/L for glucose, control v gemfibrozil, P = NS). By contrast, there was a significant increase in the maximum triglyceride synthetic rate (Vmax) for both oleate and glucose (14.57 +/- 0.01 v 32.66 +/- 1.85 nmol/mg/4 h for oleate and 67.72 +/- 1.22 v 156.77 +/- 17.61 nmol/mg/4 h for glucose, control v gemfibrozil, P < .025). Given the rapid onset in action, these data suggest that gemfibrozil altered the active state of a limiting step in the triglyceride synthetic pathway. The effect of gemfibrozil on triglyceride synthesis was also determined on differentiated human preadipocytes and HepG2 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of three serum basic proteins on the mass of lipids in normal and hyperapoB fibroblasts

We studied whether serum basic proteins (BPs) produce abnormal changes in the mass of cellular lipids in fibroblasts from patients with hyperapobetalipoproteinemia (hyperapoB) and if inhibition or stimulation of protein kinase C affects these processes. In normal cells, BP I increased the mean mass of triglycerides about twofold, whereas there was significantly less stimulation in hyperapoB cells (P < .005). The increase in the mass of cell cholesteryl esters seen in normal cells with BP I was also significantly reduced in hyperapoB cells (P < .005). In contrast, BP II abnormally stimulated the mass of cell cholesteryl esters sixfold in hyperapoB cells (P < .005). BP I also stimulated the mass of total phospholipids about twofold in normal cells, an effect that was reduced by about one third in hyperapoB cells (P = .08). No abnormality was found in hyperapoB cells with BP III. H-7, an inhibitor of protein kinase C, decreased the effects of BP I and BP II in normal and hyperapoB cells. C:8, an analogue of diacylglycerol, activated protein kinase C and stimulated triglyceride formation in normal (fourfold) and hyperapoB (fivefold) cells in the absence of BP I. When added with C:8, BP I further increased triglyceride production 1.5-fold in normal cells but not in hyperapoB cells. Two cellular abnormalities in lipid metabolism in hyperapoB fibroblasts were found, one with BP I, another with BP II. Protein kinase C activity was not deficient in hyperapoB cells, and the defect(s) may occur at another, perhaps earlier, step in the pathway.

The adipsin-acylation stimulating protein system and regulation of intracellular triglyceride synthesis

We have previously characterized an activity from human plasma that markedly stimulates triglyceride synthesis in cultured human skin fibroblasts and human adipocytes. Based on its in vitro activity we named the active component acylation stimulating protein (ASP). The molecular identity of the active serum component has now been determined. NH2-terminal sequence analysis, ion spray ionization mass spectroscopy, and amino acid composition analysis all indicate that the active purified protein is a fragment of the third component of plasma complement, C3a-desArg. As well, reconstitution experiments with complement factors B, D, and complement C3, the components necessary to generate C3a, have confirmed the identity of ASP as C3a. ASP appears to be the final effector molecule generated by a novel regulatory system that modulates the rate of triglyceride synthesis in adipocytes.

Impaired response of fibroblasts from patients with hyperapobetalipoproteinemia to acylation-stimulating protein

Acylation-stimulating protein (ASP) is a small, basic, human plasma protein that markedly stimulates triglyceride synthesis in human adipocytes and cultured human skin fibroblasts. The present studies examine the response to ASP of cultured skin fibroblasts from normal subjects patients with hyperapobetalipoproteinemia, patients with familial hypercholesterolemia, and patients with hypertriglyceridemia without hyperapobetalipoproteinemia. Triglyceride synthesis induced by ASP did not differ significantly among the normals, the patients with familial hypercholesterolemia, and the patients with hypertriglyceridemia with normal low density lipoprotein (LDL) apolipoprotein B levels; however, on average, it was markedly reduced in the patients with hyperapobetalipoproteinemia. In all groups studied, evidence of specific saturable binding of radioiodinated ASP was present. Binding, however, was significantly reduced in the groups with hyperapobetalipoproteinemia whereas the other three groups were indistinguishable. By contrast, LDL-specific binding was reduced only in the patients with familial hypercholesterolemia. There was a significant direct relation between the degree of ASP binding and the triglyceride synthesis inducible by ASP. In addition, with the exception of the patients with familial hypercholesterolemia, there was an inverse relation between both ASP-specific binding and ASP-induced triglyceride synthesis in fibroblasts to LDL levels in plasma whereas no relation was evident to plasma high density lipoprotein and very low density lipoprotein.

The effect of ASP on the adipocyte of the morbidly obese

The control of triglyceride synthesis within the adipocyte is not fully understood. Insulin is considered to be the most potent stimulant of triglyceride synthesis. In this paper, we report on the effect of a small (14000 Da), basic (pI 9.0) protein isolated from human serum. This protein has been called acylation stimulating protein (ASP). It is a potent stimulant of triglyceride synthesis in adipocytes from both normal weight and morbidly obese subjects. Its stimulatory effect on adipocytes is both rapid, occurring between 15-30 min after the start of incubation, and prolonged, lasting for up to 3 hr. Compared to insulin, it is sixfold more potent in its effect on triglyceride synthesis. As well as acting on isolated cells, ASP also has a fourfold stimulatory effect on triglyceride synthesis in human adipose microsomes at a concentration of 25 micrograms/ml. This study indicates that ASP is a potent stimulant of triglyceride synthesis and therefore may play a role in the pathogenesis of morbid obesity.