Molecular regulation of plasma lipid levels during systemic inflammation and sepsis

PURPOSE OF REVIEW

Sepsis is a common syndrome of multiorgan system dysfunction caused by a dysregulated inflammatory response to an infection and is associated with high rates of mortality. Plasma lipid and lipoprotein levels and composition change profoundly during sepsis and have emerged as both biomarkers and potential therapeutic targets for this condition. The purpose of this article is to review recent progress in the understanding of the molecular regulation of lipid metabolism during sepsis.

RECENT FINDINGS

Patients who experience greater declines in high-density lipoprotein during sepsis are at much greater risk of succumbing to organ failure and death. Although the causality of these findings remains unclear, all lipoprotein classes can sequester and prevent the excessive inflammation caused by pathogen-associated lipids during severe infections such as sepsis. This primordial innate immune function has been best characterized for high-density lipoproteins. Most importantly, results from human genetics and preclinical animal studies have suggested that several lipid treatment strategies, initially designed for atherosclerosis, may hold promise as therapies for sepsis.

SUMMARY

Lipid and lipoprotein metabolism undergoes significant changes during sepsis. An improved understanding of the molecular regulation of these changes may lead to new opportunities for the treatment of sepsis.

Impact of hypertension and hyperhomocysteinemia on arterial thrombosis in primary antiphospholipid syndrome

The aim of this study was to evaluate traditional risk factors for coronary artery disease (CAD), homocysteine, anti-oxidized low-density lipoprotein (anti-oxLDL), anti-lipoprotein lipase (anti-LPL) and endothelin-1 (ET-1) in patients with primary anti-phospholipid syndrome (APS), furthermore verify possible association among these variables and arterial thrombosis. Thirty-eight women with primary APS and 30 age-and-sex-matched controls were evaluated. Patients presented higher-LDL and triglycerides levels and lower-HDL levels than controls. Anti-LPL antibodies were not detected in both groups. The mean number of risk factors was higher in patients than in controls ( P = 0.030). Anti-oxLDL antibodies, homocysteine and ET-1 mean levels were similar between groups, but abnormal homocysteine levels were found only among primary APS patients ( P = 0.031). Hypertension and the presence of at least one risk factor for CAD were more prevalent in patients with arterial involvement than those without. Homocysteine levels and mean number of risk factors for CAD were significantly higher in patients with arterial thrombosis than controls. In a multivariate analysis hypertension was the only independently associated with arterial thrombosis (OR 14.8, 95% CI = 2.1—100.0, P = 0.006). This study showed that in primary APS patients other risk factors besides anti-phospholipid antibodies contribute for the occurrence of arterial events and the most important factor was hypertension. Lupus (2007) 16, 782—787.

The effects of miR-467b on lipoprotein lipase (LPL) expression, pro-inflammatory cytokine, lipid levels and atherosclerotic lesions in apolipoprotein E knockout mice

Atherosclerosis is a lipid disorder disease characterized by chronic blood vessel wall inflammation driven by the subendothelial accumulation of macrophages. Studies have shown that lipoprotein lipase (LPL) participates in lipid metabolism, but it is not yet known whether post-transcriptional regulation of LPL gene expression by microRNAs (miRNAs) occurs in vivo. Here, we tested that miR-467b provides protection against atherosclerosis by regulating the target gene LPL which leads to reductions in LPL expression, lipid accumulation, progression of atherosclerosis and production of inflammatory cytokines in apolipoprotein E knockout (apoE(-/-)) mice. Treatment of apoE(-/-) mice with intra-peritoneal injection of miR-467b agomir led to decreased blood plasma levels of total cholesterol (TC), triglyceride (TG), low density lipoprotein cholesterol (LDL-C), high density lipoprotein cholesterol (HDL-C), tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), IL-1β and monocyte chemotactic protein-1 (MCP-1). Using Western blots and real time PCR, we determined that LPL expression in aorta and abdominal cavity macrophages were significantly down-regulated in the miR-467b agomir group. Furthermore, systemic treatment with miR-467b antagomir accelerated the progression of atherosclerosis in the aorta of apoE(-/-) mice. The present study showed that miR-467b protects apoE(-/-) mice from atherosclerosis by reducing lipid accumulation and inflammatory cytokine secretion via downregulation of LPL expression. Therefore, targeting miR-467b may offer a promising strategy to treat atherosclerotic vascular disease.

Type 1 hyperlipoproteinemia and recurrent acute pancreatitis due to lipoprotein lipase antibody in a young girl with Sjogren's syndrome

CONTEXT

Type 1 hyperlipoproteinemia (T1HLP) in childhood is most often due to genetic deficiency of lipoprotein lipase (LPL) or other related proteins.

OBJECTIVE

The aim was to report a case of marked hypertriglyceridemia and recurrent acute pancreatitis due to the presence of LPL autoantibody in a young girl who was subsequently diagnosed with Sjögren's syndrome.

SUBJECT AND METHODS

A 9-yr-old African-American girl presented with acute pancreatitis and serum triglycerides of 4784 mg/dl. Strict restriction of dietary fat reduced serum triglycerides, but she continued to experience recurrent pancreatitis. Approximately 18 months thereafter, she developed transient pauciarticular arthritis with elevated serum antinuclear antibody (>1:1280). Minor salivary gland biopsy revealed chronic sialadenitis with a dense periductal lymphocytic aggregate suggestive of Sjögren's syndrome. Genomic DNA was analyzed for LPL, GPIHBP1, APOA5, APOC2, and LMF1. Immunoblotting was performed to detect serum LPL autoantibody.

RESULTS

The patient had no disease-causing variants in LPL, GPIHBP1, APOA5, APOC2, or LMF1. Immunoblotting revealed serum LPL antibody. The patient responded to immunosuppressive therapy for Sjögren's syndrome with resolution of hypertriglyceridemia.

CONCLUSIONS

Unexplained T1HLP in childhood could be secondary to LPL deficiency induced by autoantibodies. Therefore, diagnosis of autoimmune T1HLP should be entertained if clinical features are suggestive of an autoimmune process.

Anti-lipoprotein lipase antibodies in patients with hypertriglyceridemia without associated autoimmune disease

BACKGROUND

Anti-lipoprotein lipase antibodies have been described in rare cases of patients with hypertriglyceridemia. However, no systematic study evaluating these antibodies in patients with this lipid abnormality has been undertaken.

OBJECTIVES

To analyze the correlation of anti-lipoprotein lipase (anti-LPL) antibodies with other laboratory findings in patients with hypertriglyceridemia but no autoimmune disease.

METHODS

We evaluated 44 hypertriglyceridemic patients without autoimmune disease. Clinical and laboratory evaluations included analyses of comorbidities, fasting lipid profile and anti-LPL antibodies.

RESULTS

Mean patient age was 55 +/- 10 years; 46% of the patients were female and 64% were Caucasian. The mean disease duration was 94.4 months and mean body mass index 28.7 +/- 3.6 kg/m2; 34.0% were diabetic, 25.0% were obese, 72.7% had systemic arterial hypertension, 75% were sedentary, 15.9% were smokers, 56.8% had a family history of dyslipidemia, 45.5% had a family history of coronary insufficiency, 20.5% had acute myocardial infarction, 9.0% had undergone revascularization and 11.0% angioplasty, 79.5% were being treated with statins and 43.2% were taking fibrates. Median triglyceride levels were 254 mg/dl (range 100-3781 mg/dl), and total cholesterol level was 233 t 111 mg/dl. High-density lipoprotein was 42.6 +/- 15.4 mg/dl, low-density lipoprotein 110.7 +/- 42.4 mg/dl and very low-density lipoprotein 48 +/- 15 mg/dl. Anti-LPL antibodies were identified in 2 patients (4.5%), both of whom had a family history of dyslipidemia, coronary insufficiency and acute myocardial infarction; one had undergone myocardial revascularization and percutaneous transluminal coronary angioplasty, and both were using fibrates and had normal triglyceride levels.

CONCLUSIONS

Our findings demonstrate a correlation between the immune response and dyslipoproteinemia in hypertriglyceridemic patients, suggesting that autoimmune disease contributes to the dyslipidemia process.

Lipoprotein lipase is nitrated in vivo after lipopolysaccharide challenge

Lipopolysaccharide (LPS) administration down-regulates lipoprotein lipase (LPL) activity at the posttranscriptional level. Hypertriglyceridemia is the main metabolic consequence of this fall in LPL activity and is presumably involved in the innate immune response to infection. Nitric oxide (NO) has been implicated in LPS-induced down-regulation of LPL activity, but whether its effects are direct or indirect remains unclear. Here we examined the potential nitration of LPL in vivo in response to LPS challenge in rats. We found hypertriglyceridemia, iNOS expression, NO overproduction, and a generalized decrease in LPL activity in tissues 6 h after LPS administration. LPL sensitivity to nitration was first explored by in vitro exposure of bovine LPL to peroxynitrite, a reactive nitrogen species (RNS). Nitration was confirmed by anti-nitrotyrosine Western blot and subsequent identification of specific nitrotyrosine-containing LPL sequences by tandem mass spectrometry. Further analysis by targeted mass spectrometry revealed three in vivo-nitrated tyrosine residues in heart LPL from LPS-challenged rats. This is the first study to identify nitrated tyrosine residues in LPL, both in vitro and in vivo, and it demonstrates that LPL is a target for RNS in endotoxemia. These results indicate that LPL nitration may be a new mechanism of LPL activity regulation in vivo.

Identification of a new functional domain in angiopoietin-like 3 (ANGPTL3) and angiopoietin-like 4 (ANGPTL4) involved in binding and inhibition of lipoprotein lipase (LPL)

Angiopoietin-like 3 (ANGPTL3) and angiopoietin-like 4 (ANGPTL4) are secreted proteins that regulate triglyceride (TG) metabolism in part by inhibiting lipoprotein lipase (LPL). Recently, we showed that treatment of wild-type mice with monoclonal antibody (mAb) 14D12, specific for ANGPTL4, recapitulated the Angptl4 knock-out (-/-) mouse phenotype of reduced serum TG levels. In the present study, we mapped the region of mouse ANGPTL4 recognized by mAb 14D12 to amino acids Gln(29)-His(53), which we designate as specific epitope 1 (SE1). The 14D12 mAb prevented binding of ANGPTL4 with LPL, consistent with its ability to neutralize the LPL-inhibitory activity of ANGPTL4. Alignment of all angiopoietin family members revealed that a sequence similar to ANGPTL4 SE1 was present only in ANGPTL3, corresponding to amino acids Glu(32)-His(55). We produced a mouse mAb against this SE1-like region in ANGPTL3. This mAb, designated 5.50.3, inhibited the binding of ANGPTL3 to LPL and neutralized ANGPTL3-mediated inhibition of LPL activity in vitro. Treatment of wild-type as well as hyperlipidemic mice with mAb 5.50.3 resulted in reduced serum TG levels, recapitulating the lipid phenotype found in Angptl3(-/-) mice. These results show that the SE1 region of ANGPTL3 and ANGPTL4 functions as a domain important for binding LPL and inhibiting its activity in vitro and in vivo. Moreover, these results demonstrate that therapeutic antibodies that neutralize ANGPTL4 and ANGPTL3 may be useful for treatment of some forms of hyperlipidemia.

On transient breastfeeding-related chylomicronaemia syndrome

This paper reports on two male infants with abnormally high levels of serum triacylglycerols (>15.00 mmol/l) and massive accumulation of chylomicrons. Pathological lipidograms were observed during breastfeeding only and were typical of a rare chylomicronaemia syndrome. Laboratory abnormalities were detected accidentally in the otherwise healthy infants. An unrecognized modulating factor in fresh mother's milk caused transitory decreased activity of hydrolytic complex for chylomicrons and very-low-density lipoproteins, probably due to a dysfunction of lipoprotein lipase. The normalization of lipidograms suggested that the catalytic activity of lipoprotein lipase rapidly recovered after weaning. Pathogenesis moved more towards an immune disorder, to the production of (auto)antibodies against a component of the lipolytic system. Mother's milk should be substituted with banked donor human milk, but this is not an unambiguous demand.

CONCLUSION

Fresh mother's milk caused a decrease in the activity of lipolytic enzyme (lipoprotein lipase). The massive hypertriacylglycerolaemia quickly disappeared after weaning. Two infants with this transient chylomicronaemia syndrome were asymptomatic.

Correction of feline lipoprotein lipase deficiency with adeno-associated virus serotype 1-mediated gene transfer of the lipoprotein lipase S447X beneficial mutation

Human lipoprotein lipase (hLPL) deficiency, for which there currently exists no adequate treatment, leads to excessive plasma triglycerides (TGs), recurrent abdominal pain, and life-threatening pancreatitis. We have shown that a single intramuscular administration of adeno-associated virus (AAV) serotype 1 vector, encoding the human LPL(S447X) variant, results in complete, long-term normalization of dyslipidemia in LPL(/) mice. As a prelude to gene therapy for human LPL deficiency, we tested the efficacy of AAV1-LPL(S447X) in LPL(/) cats, which demonstrate hypertriglyceridemia (plasma TGs, >10,000 mg/dl) and clinical symptoms similar to LPL deficiency in humans, including pancreatitis. Male LPL(/) cats were injected intramuscularly with saline or AAV1-LPL(S447X) (1 x 10(11)-1.7 x 10(12) genome copies [GC]/kg), combined with oral doses of cyclophosphamide (0-200 mg/m(2) per week) to inhibit an immune response against hLPL. Within 3-7 days after administration of >or=5 x 10(11) GC of AAV1-LPL(S447X) per kilogram, the visible plasma lipemia was completely resolved and plasma TG levels were reduced by >99% to normal levels (10-20 mg/dl); intermediate efficacy (95% reduction) was achieved with 1 x 10(11) GC/kg. Injection in two sites, greatly limiting the amount of transduced muscle, was sufficient to completely correct the dyslipidemia. By varying the dose per site, linear LPL expression was demonstrated over a wide range of local doses (4 x 10(10)-1 x 10(12) GC/site). However, efficacy was transient, because of an anti-hLPL immune response blunting LPL expression. The level and duration of efficacy were significantly improved with cyclophosphamide immunosuppression. We conclude that AAV1-mediated delivery of LPL(S447X) in muscle is an effective means to correct the hypertriglyceridemia associated with feline LPL deficiency.

Serological correlations with nephritis in systemic lupus erythematosus

Autoantibodies have long been thought to participate in the pathogenesis of lupus nephritis. In this regard, antibodies to double stranded (ds)DNA and ribosomal P protein have been studied the most intensively. We now report a new specificity, antibodies to lipoprotein lipase (LPL) that is strongly associated with lupus nephritis and has a powerful synergistic effect with anti-ribosomal P antibodies in its association with nephritis. The recognition of anti-LPL antibodies and their synergy with anti-P antibodies are discussed in terms of the pathogenesis of lupus nephritis.

Combination of circulating antilipoprotein lipase (Anti-LPL) antibody and heterozygous S172 fsX179 mutation of LPL gene leading to chronic hyperchylomicronemia

CONTEXT

Sporadic hyperchylomicronemia (type V hyperlipoproteinemia) results from complex interactions between genetic and environmental factors that often remain unknown.

DESIGN

Upon investigation of a patient suffering from recurrent hypertriglyceridemic pancreatitis without family history or conventional secondary cause of dyslipidemia, we identified a previously unreported nonsense heterozygous lipoprotein lipase (LPL) gene mutation S172fsX179 associated with an antihuman LPL IgG.

RESULTS

This autoantibody partially inhibited wild-type LPL activity in vitro. Furthermore, the patient's plasma triglyceride concentrations were efficiently decreased under immunosuppressive treatment, and this was confirmed by sequential withdrawal/reintroduction tests.

CONCLUSIONS

We consider that this unique combination of a genetic defect and an autoimmune disease results in chronic major hypertriglyceridemia. Because immunosuppressive treatment can improve this dyslipidemia, assessment of anti-LPL autoantibody is worthwhile in unmanageable chronic major hypertriglyceridemia, even in the presence of a heterozygous LPL deficiency.

Anti-lipoprotein lipase antibody in systemic sclerosis: association with elevated serum triglyceride concentrations

OBJECTIVE

The vascular damage systemic sclerosis (SSc) consists mainly of microvascular changes, but recently macrovascular changes with dyslipidemia were recognized. In systemic lupus erythematosus (SLE), autoantibody to lipoprotein lipase (LPL), a key enzyme that hydrolyzes triglycerides, suggested a role of autoimmunity for elevated serum triglyceride levels and atherosclerosis. We investigated the prevalence and levels of anti-LPL antibodies, their clinical correlation, and their functional significance in patients with SSc.

METHODS

Serum samples from patients with diffuse cutaneous SSc (dSSc; n = 55), limited cutaneous SSc (lSSc; n = 75), SLE (n = 21), and dermatomyositis (DM; n = 21) and healthy controls (n = 41) were examined by ELISA. The presence of anti-LPL antibody was evaluated by immunoblotting analysis using purified LPL. To determine the functional relevance of anti-LPL antibody in vivo, we assessed whether anti-LPL autoantibody was able to inhibit LPL activity using the LPL activity kit.

RESULTS

ELISA revealed that IgG or IgM anti-LPL antibodies were detected in 35% of SSc patients, while they were also positive in 67% of SLE patients and 43% of DM patients. The presence of IgG anti-LPL antibody was associated with elevated serum triglyceride levels, greater extent of skin fibrosis, and more frequent presence of lung fibrosis, heart involvement, and anti-topoisomerase I antibodies. The presence of anti-LPL autoantibody was confirmed by immunoblotting analysis. LPL activity was inhibited by IgG anti-LPL antibodies in sera from SSc patients with elevated serum triglyceride levels.

CONCLUSION

Our results suggest that anti-LPL autoantibody contributes to elevated serum triglyceride levels by inhibiting LPL enzyme activity in patients with SSc.

Fat mobilization in adipose tissue is promoted by adipose triglyceride lipase

Mobilization of fatty acids from triglyceride stores in adipose tissue requires lipolytic enzymes. Dysfunctional lipolysis affects energy homeostasis and may contribute to the pathogenesis of obesity and insulin resistance. Until now, hormone-sensitive lipase (HSL) was the only enzyme known to hydrolyze triglycerides in mammalian adipose tissue. Here, we report that a second enzyme, adipose triglyceride lipase (ATGL), catalyzes the initial step in triglyceride hydrolysis. It is interesting that ATGL contains a "patatin domain" common to plant acyl-hydrolases. ATGL is highly expressed in adipose tissue of mice and humans. It exhibits high substrate specificity for triacylglycerol and is associated with lipid droplets. Inhibition of ATGL markedly decreases total adipose acyl-hydrolase activity. Thus, ATGL and HSL coordinately catabolize stored triglycerides in adipose tissue of mammals.

Anti-lipoprotein lipase antibodies: A new player in the complex atherosclerotic process in systemic lupus erythematosus?

OBJECTIVE

The novel description of antibodies to lipoprotein lipase (anti-LPL) associated with dyslipoproteinemia prompted us to analyze the association of anti-LPL with clinical and serologic features in patients with systemic lupus erythematosus (SLE) and its link to markers of inflammation that are known to be involved in atherogenesis.

METHODS

Enzyme-linked immunosorbent assay was used to test for the presence of anti-LPL antibodies in 66 consecutive patients with SLE. Clinical and laboratory evaluation, including a fasting lipid profile, autoantibody screening, an assessment for markers of inflammation (C-reactive protein [CRP], erythrocyte sedimentation rate [ESR]), and the SLE Disease Activity Index (SLEDAI) were performed at the time of inclusion in the study. Exclusion criteria were any conditions that affect the lipid profile. SLE patients were categorized into 2 groups according to detection of these anti-LPL antibodies, as follows: anti-LPL+ and anti-LPL-.

RESULTS

Anti-LPL antibody IgG was detected in 25 SLE patients (37.8%). Triglyceride levels were significantly higher in the anti-LPL+ group (112.4 +/- 50.2 versus 89.9 +/- 54.5 mg/dl in the anti-LPL- group; P = 0.033), but no significant differences between the 2 groups were detected for total, high-density lipoprotein, and low-density lipoprotein cholesterol levels. A higher frequency of elevated CRP levels and ESRs was observed in the anti-LPL+ group compared with the anti-LPL- group (44% and 17.1%, respectively [P = 0.023] and 52% and 19.5%, respectively [P = 0.013]). Moreover, SLE patients with anti-LPL antibodies also had significantly higher levels of CRP (11.1 +/- 16.4 versus 2.4 +/- 2.6 mug/ml; P = 0.036) and higher ESRs (33.4 +/- 29.8 versus 16.5 +/- 11.8 mm/hour; P = 0.020). Anti-LPL titers had a significant positive correlation with the CRP level (r = 0.56, P < 0.001), the ESR (r = 0.55, P < 0.001), the SLEDAI score (r = 0.45, P < 0.001), anti-double-stranded DNA (anti-dsDNA; r = 0.52, P < 0.001), and anticardiolipin IgG antibodies (r = 0.25, P = 0.04), and a significant negative correlation was detected with total hemolytic complement activity (CH100) (r = -0.34, P = 0.005). Reinforcing these findings, multiple regression analysis also revealed a significant association of anti-LPL with the CRP level (P = 0.025) and anti-dsDNA (P < 0.001). Importantly, a comparison of positive and negative anti-dsDNA sera revealed similar mean CRP levels (P = 0.56) and ESRs (P = 0.102), contrasting with the SLEDAI score (P = 0.004) and CH100 (P = 0.008).

CONCLUSION

These data support the link between inflammation, immune response, and dyslipoproteinemia in SLE, introducing anti-LPL as a possible new player that may ultimately help in understanding the complex events of atherogenesis in this disease.

Autoantibodies to lipoprotein lipase and dyslipidemia in systemic lupus erythematosus

OBJECTIVE

To demonstrate the binding of bovine lipoprotein lipase (LPL) by IgG from sera obtained from patients with systemic lupus erythematosus (SLE) and other rheumatic diseases, and the relationship of anti-LPL to triglyceride levels in SLE.

METHOD

Binding of LPL by IgG from sera obtained from patients with SLE and other rheumatic diseases was measured by an enzyme-linked immunosorbent assay technique. Lipid profiles for fasting blood samples obtained from SLE patients and control subjects were determined.

RESULTS

Sera obtained from 105 patients with SLE were assessed for reactivity with LPL, and 49 (47%) of the results were positive. Sera obtained from patients with rheumatoid arthritis (RA) (n = 80), Sjögren's syndrome (n = 30), polymyositis and dermatomyositis (n = 30), and progressive systemic sclerosis (n = 31) were also studied, and 10 (13%), 3 (10%), 12 (40%), and 13 (42%), respectively, were positive for reactivity with LPL. It was determined that all affinity-purified anti-double-stranded DNA (dsDNA) antibodies and 4 of 5 monoclonal anti-dsDNA antibodies bound to LPL. The binding of IgG depleted of anti-dsDNA to LPL indicates a second anti-LPL activity in SLE. Measurements of fasting lipid levels in SLE patients with anti-LPL revealed a strong positive correlation of antibody levels and total serum triglycerides, apolipoprotein B, and apolipoprotein E concentrations.

CONCLUSION

Antibodies to LPL occurred in 47% of SLE patients and in a similar percentage of patients with polymyositis or systemic sclerosis. The prevalence of these antibodies was less in patients with RA or Sjögren's syndrome. It is hypothesized that the elevated triglyceride levels in SLE patients are in part attributable to anti-LPL, and this lipid abnormality could contribute to the premature atherosclerosis known to be present in patients with SLE.

[Autoimmune hyperchylomicronemia]

The concept of autoimmune hyperlipidemia was proposed originally by Beaumont et al. Since then, hyperlipidemic patients with autoantibodies to circulating lipoproteins as well as enzymes related to lipoprotein metabolism have been documented. However, the mechanism remains speculative except in a few cases. We described a patient with autoimmune hyperchylomicronemia due to autoantibodies against lipoproteins lipase and hepatic triglyceride lipase. We also identified a hyperlipidemic case with inhibitory monoclonal antibodies against low density lipoprotein binding to fibroblasts and multiple myeloma. These data suggest that screening of patients with nonfamilial hyperlipidemia, especially those with associated autoimmune diseases, may result in the identification of other such patients.

Detailed characterization of the binding site of the lipoprotein lipase-specific monoclonal antibody 5D2

Monoclonal antibody (MAb) 5D2 recognizes lipoprotein lipases (LPL) from different species but not related lipases. This MAb is a unique reagent, used world-wide, because it differentiates between monomeric inactive and dimeric active LPL, inhibits human LPL enzyme activity, and binds to C-terminal LPL sequences involved in interactions with lipoproteins, lipoprotein receptors, and heparin. In this study we have analyzed the fine specificity of the MAb epitope recognition in order to better understand its functional properties and species-specific LPL immune reactivity. In peptide scan assays, MAb 5D2 reacted with all, except two, 13 amino acid-long peptides located between positions 380 and 410. Peptides from the amino terminal end of this region reacted more strongly than those from the carboxyl terminal end. Furthermore, only a peptide from the amino terminal end competed effectively with the binding of MAb 5D2 to native LPL bound to microtiter plates or nitrocellulose. A systematic peptide mutagenesis study indicated that 8 amino acids of the reactive region, mainly located in the amino terminal end, are critical for binding and probably directly interact with MAb 5D2. The experimentally determined antigenicities of species-specific LPL peptides and of the corresponding denatured full-length LPL proteins on immunoblots were consistent with these findings. According to a proposed 3D-model for LPL, only the amino terminal end of the antigenic region is easily surface-accessible. These data combined with 3D-modelling of monoclonal antibody (MAb)-lipoprotein lipase (LPL) protein interaction provide new insight into the known biological effects of MAb 5D2 on LPL and the antigenic determinants that are recognized.

Rare case of autoimmune hyperchylomicronemia during pregnancy

A pregnant woman was found to have severe hypertriglyceridemia, fasting chylomicronemia, and low platelet count. The activities of serum lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) proved to be extremely low. The activities of the enzymes in normal plasma were completely inhibited by addition of the patient's plasma. We concluded that autoantibodies to lipases were responsible for this patient's hypertriglyceridemia.

Lipoprotein lipase regulates Fc receptor-mediated phagocytosis by macrophages maintained in glucose-deficient medium

During periods of intense activity such as phagocytosis, macrophages are thought to derive most of their energy from glucose metabolism under both aerobic and anaerobic conditions. To determine whether fatty acids released from lipoproteins by macrophage lipoprotein lipase (LPL) could substitute for glucose as a source of energy for phagocytosis, we cultured peritoneal macrophages from normal and LPL knockout (LPL-KO) mice that had been rescued from neonatal demise by expression of human LPL via the muscle creatine kinase promoter. Normal and LPL-KO macrophages were cultured in medium containing normal (5 mM) or low (1 mM) glucose, and were tested for their capacity to phagocytose IgG-opsonized sheep erythrocytes. LPL-KO macrophages maintained in 1 and 5 mM glucose phagocytosed 67 and 79% fewer IgG-opsonized erythrocytes, respectively, than macrophages from normal mice. Addition of VLDL to LPL-expressing macrophages maintained in 1 mM glucose enhanced the macrophages' phagocytosis of IgG-opsonized erythrocytes, but did not stimulate phagocytosis by LPL-KO macrophages. Inhibition of secreted LPL with a monoclonal anti-LPL antibody or with tetrahydrolipstatin blocked the ability of VLDL to enhance phagocytosis by LPL-expressing macrophages maintained in 1 mM glucose. Addition of oleic acid significantly enhanced phagocytosis by both LPL-expressing and LPL-KO macrophages maintained in 1 mM glucose. Moreover, oleic acid stimulated phagocytosis in cells cultured in non-glucose-containing medium, and increased the intracellular stores of creatine phosphate. Inhibition of oxidative phosphorylation, but not of glycolysis, blocked the capacity of oleic acid to stimulate phagocytosis. Receptor-mediated endocytosis of acetyl LDL by macrophages from LPL-expressing and LPL-KO mice was similar whether the cells were maintained in 5 or 1 mM glucose, and was not augmented by VLDL. We postulate that fatty acids derived from macrophage LPL-catalyzed hydrolysis of triglycerides and phospholipids provide energy for macrophages in areas that have limited amounts of ambient glucose, and during periods of intense metabolic activity.

Sepsis-induced regulation of lipoprotein lipase expression in rat adipose tissue and soleus muscle

Hypertriglyceridemia of sepsis is associated with suppressed tissue lipoprotein lipase (LPL) activities. We investigated the effect of sepsis on lipoprotein lipase gene expression in epididymal fat and soleus muscle from control and septic rats in the fasted and fed state. After 24 h of sepsis, LPL activity decreased significantly in epididymal fat from fasted rats by 45% along with a 57% reduction in LPL mRNA levels and LPL mass. Transcription rate, measured by nuclear run-on assay, decreased by 70% in epididymal fat from fasted septic rats compared to fasted control rats. The synthesis rate of LPL in epididymal fat decreased by 31% while the LPL relative synthesis declined by 50% during sepsis. The turnover rate was not altered. Sepsis in fed rats did not lead to a significant decrease in LPL mRNA in epididymal fat but did lower LPL activity and LPL mass by 45%. The sepsis-induced suppression in soleus LPL activity, LPL mRNA levels, and LPL mass in the fasted state also was observed when septic rats were fed. The results indicate that during fasting sepsis, LPL expression in epididymal fat and possibly soleus muscle involves transcriptional regulation. During sepsis in the fed state, the regulation of LPL in adipose tissue may involve posttranslational mechanisms.

Characterization of a new case of autoimmune type I hyperlipidemia: long-term remission under immunosuppressive therapy

Only a few cases of type I hyperlipidemia occurring in patients with autoimmune disease have been reported. We describe the case of a 35-yr-old woman suffering from severe type I hyperchylomicronemia. A combination of various hypolipidemic treatments, including strict hypolipidemic dietary therapy and administration of fibrates or n-3 fatty acids, was inefficient. Because of a history of familial autoimmunity, we introduced an immunosuppressive therapy that resulted in consistent long term and stable remission. Two attempts to reduce the immunosuppressor dose resulted in major relapses. To explain the defect of chylomicron hydrolysis, we investigated the postheparin plasma lipase activities. Hepatic triglyceride lipase activity was normal, whereas that of lipoprotein lipase (LPL) was reduced to about 30% of normal. Immunosuppressive therapy resulted in a complete and durable normalization of LPL activity. Using Western blot analysis, we found in the plasma of the patient a circulating IgG specifically directed against LPL, which became undetectable during immunosuppressive therapy. Western blot analysis revealed that the whole circulating anti-LPL autoantibody was bound to chylomicrons. Proteins extracted from patient's chylomicrons were able to induce a dose-related inhibition of LPL activity in vitro, whereas that of hepatic triglyceride lipase remained unchanged. These data constitute the first description of autoimmune hyperchylomicronemia due to an exclusive defect of LPL activity, and they show that a complete remission has been obtained after immunosuppressive therapy. Finally, our finding that the anti-LPL autoantibody is bound to chylomicrons emphasizes their previously unrecognized ability to transport LPL, already described for other lipoprotein fractions.

Quantification of lipoprotein lipase (LPL) by dissociation-enhanced lanthanide fluorescence immunoassay. Comparison of immunoreactivity of LPL mass and enzyme activity of LPL

Lipoprotein lipase (LPL) hydrolyses triglycerides in chylomicrons and in very low density lipoproteins. In this study, a new sensitive enzyme immunoassay, the dissociation-enhanced lanthanide fluorescence immunoassay (DELFIA), for the quantification of immunoreactive LPL mass in biological specimens was developed. In the indirect sandwich DELFIA assay polyclonal anti-human or anti-bovine LPL IgGs were used as capture antibodies, monoclonal antibody (mAb) 5D2 and Eu(3+)-labelled goat anti-mouse IgG were used as detection antibodies. In the direct sandwich DELFIA assay, mAb 5D2 was used as capture and Eu(3+)-labelled mAb 5D2 as detection antibodies. Both purified bovine and human LPL proteins served as standards in the indirect and the direct DELFIA assay. Standard curves were linear between 0.1 and 1000 ng LPL/ml, assuring the sensitivity of the DELFIAs within this range. Mean values for immunoreactive LPL mass in normal individuals were found to be 40.3 +/- 14.4 ng/ml preheparin plasma and 334.1 +/- 71 ng/ml postheparin plasma. In patients affected with type I hyperlipoproteinemia 82.4 +/- 29.3 ng/ml (postheparin plasma) were determined. Coefficients of inter- and intra-assay variation were 4.3% and 6.2% on average. The correlation coefficient between the indirect and the direct DELFIA technique was 0.9694. The correlation coefficient between immunoreactive LPL mass (estimated by DELFIA) and LPL activity (estimated by the LPL activity assay) was 0.9345. Our data are consistent with the concept that LPL is active as a dimer. Dissociation of the LPL dimer into monomers is tightly coupled to both loss of immunoreactivity and enzyme activity of LPL.

A monoclonal antibody against human lipoprotein lipase inhibiting heparin binding without affecting catalytic activity

A fragment of the human lipoprotein lipase (LPL) cDNA (405 bp, 5' terminal end) was cloned in an expression vector to produce a approximately 17 kDa fusion peptide and was used as antigen to produce a high titre anti-LPL monoclonal antibody (10C3 MAb). This antibody reacts with both native and denatured forms of LPL from different tissue and animal sources. Competition studies with heparin indicate that 10C3 MAb is specific for an epitope at a heparin binding site. The antibody does not inhibit LPL enzyme activity, indicating that the antigenic epitope is not situated within or in the proximity of the LPL catalytic region. With these characteristics, 10C3 MAb should prove to be a useful immunochemical tool in clinical as well as in fundamental investigations on the metabolism of triglyceride-rich lipoproteins and in studies on the functional anatomy of LPL.

Generation of antibodies against a human lipoprotein lipase fusion protein

Antibodies generated against specific proteins are useful tools for studying the physiology and cell biology of the protein of interest. Although antibodies have been successfully generated against lipoprotein lipase (LPL) and used to elucidate many aspects of its biology, there have been problems with the specificity, affinity and availability of these antibodies. To circumvent these problems, we have expressed a portion of human LPL as a bacterial fusion protein. The human LPL bacterial fusion protein was utilized to generate polyclonal antibodies in rabbits that recognize intact human, rat and bovine LPL. Using these antibodies, it was possible to demonstrate a direct correlation between LPL mass and LPL activity from different samples of human post-heparin plasma. In addition, these antibodies were used to develop an ELISA for the measurement of LPL in tissue or plasma. This is a useful means for obtaining polyclonal antibodies to LPL in sufficient quantity and without contaminating mammalian proteins.

Hypertriglyceridemia caused by the autoantibody to lipases for plasma lipoproteins: a case report

A 41-year-old female patient with muscle dystrophy, hepatosplenomegaly and tendinous xanthoma showed mild hypertriglyceridemia. The lipoprotein profile in blood showed increases in triglycerides in VLDL and LDL, and a marked decrease of cholesterol in HDL. Chylomicronemia was found, but was not severe. Both lipoprotein lipase and hepatic triglyceride lipase activities were reduced to a level that was only a few percent of the control. Immunoblotting study revealed that the IgG autoantibody in her serum was apparently reactable with hepatic triglyceride lipase and weakly with lipoprotein lipase. Hypertriglyceridemia in this patient is suggested to be due to the autoantibody to these lipases.

Rat liver contains a limited number of binding sites for hepatic lipase

The binding of hepatic lipase to rat liver was studied in an ex vivo perfusion model. The livers were perfused with media containing partially purified rat hepatic lipase or bovine milk lipoprotein lipase. The activity of the enzymes was determined in the perfusion media before and after passage through the liver. During perfusion with a hepatic-lipase-containing medium the lipase activity in the medium did not change, indicating that there was no net binding of lipase by the liver. In contrast, more than 80% of the lipoprotein lipase was removed from the medium. This lipoprotein lipase activity could be recovered into the perfusion medium completely by heparin perfusion of the liver. If livers, first depleted of hepatic lipase by heparin, were subsequent perfused with a hepatic-lipase-containing medium, 90 +/- 24 m-units of the lipase activity was bound per g of liver (up to 1000 m-units/total liver). However, heparin treatment of the liver decreases the ability of the liver to re-bind hepatic lipase by 80%. Perfusion of rat livers with 0.3 M NaCl released 60% of the lipase activity into the medium. Upon subsequent perfusion of these livers with hepatic-lipase-containing media, 541 +/- 164 m-units of hepatic lipase could be bound per g of liver (up to 5000 m-units/total liver). The binding of hepatic lipase was also studied in livers of corticotropin (ACTH)-pre-treated rats. In these rats also, hepatic lipase bound only to livers which had been pre-perfused with heparin or 0.3 M NaCl. After heparin pre-perfusion, 88 +/- 12 m-units of hepatic lipase could be bound per g of liver, similar to that with livers of control rats not treated with ACTH. After prior salt perfusion, however, the capacity of the livers of ACTH-pre-treated rats to bind hepatic lipase was 212 +/- 60 m-units/g of liver. This is less than in livers of control rats (541 +/- 164 m-units/g of liver). These results indicate that in rat liver the binding of hepatic lipase is heterogeneous in character and consists of heparin-resistant and heparin-sensitive components. The hepatic-lipase binding capacity of the liver is saturable and fully utilized under various conditions. The heparin-sensitive binding capacity is lowered in ACTH-treated rats, whereas the heparin-resistant binding is unaffected. We postulate that the functional hepatic lipase activity can be regulated by changes in the binding capacity of the liver.

Establishment of enzyme-linked immunosorbent assays for lipoprotein lipase with newly developed antibodies

We developed eight new antibodies against lipoprotein lipase (LPL), which included polyclonal antibodies raised against recombinant human LPL produced by transformant cells and two synthetic peptides corresponding to either amino (N)- or carboxy (C)-terminus of human LPL. With these antibodies, we established three effective sandwich enzyme-linked immunosorbent assays (ELISAs) for LPL, which enabled us to examine LPL mass not only in the postheparin plasma from human, rat, mouse, and guinea pig but also in the media and lysates of cultured cells. All of the developed antibodies showed high affinities for LPL, but their binding to LPL did not always influence the lipolytic activity of the enzyme. Interestingly, although the anti-C-terminus antibody should bind to a common epitope of human and mouse LPL, its binding selectively suppressed only human LPL activity. Because amino acid sequence surrounding the epitope is common to both LPLs, difference in the sequence outside the epitope will contribute to the selective suppression of LPL activity by the antibody. Our results also suggested that both termini of LPL would be exposed on the surface of the molecule because they were fully accessible to antibodies and that the N-terminus of LPL would be functionally less important because binding of the anti-N-terminus antibody did not affect human LPL activity. The ELISAs were further utilized to demonstrate the presence of C-terminus truncated LPL protein in the postheparin plasma of an LPL-deficient patient, to map an epitope of the anti-C-terminus antibody within residues 433-436, and to gain insight into the structure-function relationship of the LPL molecule.(ABSTRACT TRUNCATED AT 250 WORDS)

Distinct immunoreactivities suggest the existence of potential tissue variants in rat lipoprotein lipase

Lipoprotein lipases (LPL) isolated from rat cardiac muscle and bovine milk were each used as immunogens to produce polyclonal anti-LPL sera and two anti-LPL monoclonal antibodies. The immunological reactivities of these antibody sources with LPL purified from rat cardiac muscle, lung, adipose tissue, mammary gland and skeletal muscle were compared by an e.l.i.s.a. and by Western blotting. Differences between the immunoreactivities of LPL from the distinct tissue sources were revealed in both systems. A synthetic peptide with a sequence corresponding to the heparin-binding site of LPL (Ser-Arg-Thr-Asn-Thr-Lys-Val-Ser-Arg-Ile-Thr-Gly-Leu) was produced and used as an immunogen. The antiserum produced against the synthetic peptide was found to bind specifically to the region of the heparin-binding site, as determined by use of a competition e.l.i.s.a. In use against the five tissue LPL preparations, this antiserum revealed only minor variations between the tissue sources, compared with the hierarchy of reactivity observed when antibodies raised against the whole molecule were used. In combination with the outcome of previous studies on some of the physical properties of these preparations [Soteriou and Cryer (1993) Int. J. Biochem. 25, 1483-1490], the observations reported here on the distinct immunoreactivities exhibited by LPL prepared from the different tissue sources of a single species indicate the necessity to characterize fully the nature of these differences.

Mapping of the epitope on lipoprotein lipase recognized by a monoclonal antibody (5D2) which inhibits lipase activity

A monoclonal antibody, 5D2, which inhibits human lipoprotein lipase (hLPL) activity has been widely used for assessment of LPL immunoreactive mass in the clinical evaluation of patients [1] and for analysis of structure-function relationships of LPL [2,3]. We have mapped the epitope on LPL, recognized by the 5D2 antibody, within residues 396-405. Ala400 is the critical amino acid residue conferring epitope specificity. This knowledge confirms that the C-terminal domain of LPL plays a critical role in LPL activity and also provides important information for studies exploring the structure-function relationship of LPL using this antibody.

Human lipoprotein lipase: relationship of activity, heparin affinity, and conformation as studied with monoclonal antibodies

The objective of this study was to investigate how a conformational change in lipoprotein lipase (LPL) affects its molecular functions. Monoclonal antibodies (MAbs) were raised against purified bovine milk lipoprotein lipase. MAb 5D2 bound to human and bovine LPL both before and after denaturation of LPL. MAb 5F9 also recognized LPL from both species, but only after denaturation of the antigen, suggesting that a conformational change led to exposure of a previously hidden epitope. The MAbs were used in two sandwich enzyme-linked immunosorbent assays (ELISAs). One ELISA used the same MAb (5D2) to coat the plate and detect the bound antigen. This ELISA thus required the same epitope to be present in duplicate for detection (as would be the case with a dimeric antigen). The second ELISA used MAb 5F9 to coat the plate and MAb 5D2 to detect the antigen. This ELISA detected LPL only after it had been denatured. By measuring the same sample before and after denaturation with guanidine hydrochloride (GuHCl) in the 5F9 ELISA, and subtracting one from the other, a measure of native LPL was obtained. In inactivation experiments using human LPL, activity and the measure of LPL mass obtained in the 5D2 ELISA decreased and were related inversely to the measured mass obtained in the 5F9 ELISA which increased, indicating that loss of activity is closely linked to dimer dissociation and loss of native conformation. The effect of conformation and dimeric structure on LPL-heparin interaction was studied by heparin-Sepharose chromatography.(ABSTRACT TRUNCATED AT 250 WORDS)

Domain exchange: characterization of a chimeric lipase of hepatic lipase and lipoprotein lipase

Hepatic lipase and lipoprotein lipase hydrolyze fatty acids from triacylglycerols and are critical in the metabolism of circulating lipoproteins. The two lipases are similar in size and amino acid sequence but are distinguished by functional differences in substrate preference and cofactor requirement. Presumably, these distinctions result from structural differences in functional domains. To begin localization of these domains, a chimeric lipase was constructed composed of the N-terminal 329 residues of rat hepatic lipase linked to the C-terminal 136 residues of human lipoprotein lipase. The chimera hydrolyzed both monodisperse short-chain (esterase) and emulsified long-chain (lipase) triacylglycerol substrates with catalytic and kinetic properties closely resembling those of native hepatic lipase. However, monoclonal antibodies to lipoprotein lipase inhibited the lipase activity, but not the esterase function, of the chimera. Therefore, the chimeric molecule is a functional lipase and contains elements and characteristics from both parental enzymes. It is proposed that the N-terminal domain, containing the active center from hepatic lipase, governs the catalytic character of the chimera, and the C-terminal domain is essential for hydrolysis of long-chain substrates.

Lipoprotein lipase enables triacylglycerol hydrolysis by perfused newborn rat liver

Fasted 1-day-old rat liver has high heparin-releasable (endothelial) lipoprotein lipase (LPL) activity, and its hepatocytes synthesize LPL protein. To test the physiological role of this LPL, we perfused the isolated organ with a 0.8 mM triacylglycerol (TAG) (Intralipid + glycerol tri[3H]oleate) 6.3% serum medium. Samples of the recirculated perfusate were taken at different times to determine 3H in TAG, free fatty acid (FFA), and water-soluble (WS) fractions. In the medium [3H]TAG disappeared and [3H]FFA and [3H]WS fractions appeared linearly with time. This TAG hydrolysis was 1) absent when medium was recirculated without liver, 2) not affected by chloroquine addition, 3) inhibited by anti-LPL immunoglobulins, 4) absent when serum was omitted from the medium, and 5) restituted when apolipoprotein CII was added to the medium without serum. Therefore, lysosomal lipase is not involved in this TAG hydrolysis, the features of which are characteristic of LPL, not of the so-called "hepatic endothelial lipase." Thus LPL activity enables the neonatal rat liver to hydrolyze and take up circulating TAG, i.e., has the same function as extrahepatic LPL.

A sandwich-enzyme immunoassay for the quantification of lipoprotein lipase and hepatic triglyceride lipase in human postheparin plasma using monoclonal antibodies to the corresponding enzymes

We have developed a sandwich-enzyme immunoassay (EIA) for the quantification of lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) in human postheparin plasma (PHP) using monoclonal antibodies (MAbs) directed against the corresponding enzymes purified from human PHP. The sandwich-EIA for LPL was performed by using the combination of two distinct types of anti-LPL MAbs that recognize different epitopes on the LPL molecule. The immunoreactive mass of LPL was specifically measured using a beta-galactosidase-labeled anti-LPL MAb as an enzyme-linked MAb, an anti-LPL MAb linked with the bacterial cell wall as an insolubilized MAb, and purified human PHP-LPL as a standard. The sandwich-EIA for HTGL was carried out by using two distinct anti-HTGL MAbs that recognize different epitopes on HTGL. The limit of detection was 20 ng/ml for LPL and 60 ng/ml for HTGL. Each method yielded a coefficient of variation of less than 6% in intra- and inter-assays, and a high concentration of triglyceride did not interfere with the assays. The average recovery of purified human PHP-LPL and -HTGL added to human PHP samples was 98.8% and 97.5%, respectively. The immunoreactive masses of LPL and HTGL in PHP samples, obtained at a heparin dose of 30 IU/kg, from 34 normolipidemic and 20 hypertriglyceridemic subjects were quantified by the sandwich-EIA. To assess the reliability of the measured mass values, they were compared with the corresponding enzyme activities measured by selective immunoinactivation assay using rabbit anti-human PHP-LPL and -HTGL polyclonal antisera. Both assay methods yielded a highly significant correlation in either normolipidemic (r = 0.945 for LPL; r = 0.932 for HTGL) or hypertriglyceridemic subjects (r = 0.989 for LPL; r = 0.954 for HTGL). The normal mean (+/- SD) level of lipoprotein lipase mass and activity in postheparin plasma was 223 +/- 66 ng/ml and 10.1 +/- 2.9 mumol/h per ml, and that of hepatic triglyceride lipase mass and activity was 1456 +/- 469 ng/ml and 26.4 +/- 8.7 mumol/h per ml, respectively. The present sandwich-enzyme immunoassay methods make it possible to study the molecular nature of LPL and HTGL in PHP from patients with either primary or secondary hyperlipoproteinemia.

Rapid and simple isolation procedure for lipoprotein lipase from human milk

Lipoprotein lipase (LPL) is an important enzyme in lipid and energy metabolism of all vertebrates. Measurement of its activity in human postheparin plasma has become a standard procedure for diagnosis of Type I hyperlipoproteinemia and other types of hypertriglyceridemias. This paper presents a rapid and simple purification procedure for human lipoprotein lipase and the production of specific polyclonal antibodies. In the isolation procedure, the fat moiety of human milk obtained by centrifugation was delipidated and a buffer-extractable fraction chromatographed sequentially on heparin-Sepharose and phenyl-Sepharose. This three-step procedure provides a high yield of apparently pure LPL with very high specific activity against radiolabeled triacylglycerol substrates. The apparent molecular weight of LPL on SDS-PAGE was 60 kDa. Amino acid analysis and NH2-terminal sequencing proved the identity and the apparent homogeneity of the isolated enzyme. alpha-Lactoferrin and antithrombin III, common contaminants in earlier isolation procedures, were not detectable immunologically. Purified LPL was used to produce in the rabbit a specific polyclonal antiserum that inhibited LPL activity from human postheparin plasma and other tissues. In postheparin plasma from normal individuals, anti-LPL IgG was used in Western blotting to show LPL protein. In preheparin plasma, or in certain patients with Type I hyperlipoproteinemia, no specific signal was detected. The improved purification procedure presented here allows the rapid isolation of human LPL and production of antibodies to the protein, both of which will greatly facilitate future studies of this important enzyme.

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.

Sex- and age-related variations in the in vitro heparin-releasable lipoprotein lipase from mononuclear leukocytes in blood

An in vitro heparin release of lipoprotein lipase (LPL) from whole blood, mainly from monocytes, was demonstrated by (1) the time-course of lipolytic activity with the presence of 10 U/ml heparin at 37 degrees C, (2) the distribution of LPL activity in monocyte and lymphocyte fractions, (3) an immuno-inactivation with anti-LPL immunoglobulin (IgG) and (4) responses to various compounds such as NaCl, protamine sulfate, heparin, and serum activator. The in vitro heparin-releasable LPL activity from blood correlated well with the LPL activity of postheparin plasma obtained from both normolipidemic and hyperlipidemic rabbits. Studies in humans revealed sex- and age-related variations in the in vitro heparin-releasable LPL from monocytes in the blood of 134 normal subjects and 24 hypertriglyceridemic subjects: The mean LPL activity was significantly higher in normal females over the age of 30, than in the corresponding males. In the hypertriglyceridemic group, the LPL activity was also higher in females than in males, but it was not significant. The in vitro heparin-releasable LPL activity from monocytes in blood was comparable to the LPL activity derived from adipose tissue and postheparin plasma, and thus it reflects lipoprotein metabolism.

Synthesis of inactive nonsecretable high mannose-type lipoprotein lipase by cultured brown adipocytes of combined lipase-deficient cld/cld mice

Combined lipase deficiency (cld) is a recessive mutation which causes a severe deficiency of lipoprotein lipase and hepatic lipase activities and lethal hypertriacylglycerolemia within 3 days in newborn mice. The effect of this genetic defect on lipoprotein lipase was studied in primary cultures of brown adipocytes derived from tissue of newborn mice. Cells cultured from cld/cld mice replicated, accumulated triacylglycerol, and differentiated into adipocytes at normal rates. Lipoprotein lipase activity in unaffected cells was detectable on Day 0 of confluence and increased to 1.3 units/mg DNA by Day 6, while that in cld/cld cells was less than 4% of that in unaffected cells on Days 4-6. Unaffected cells released 1.2% of their lipase activity in 30 min in the absence of heparin, and 11% in 10 min in the presence of heparin, whereas cld/cld cells released no lipase activity. cld/cld cells contained 2-3 times as much lipoprotein lipase protein as unaffected cells, and released no lipase protein to the medium. Immunofluorescent lipoprotein lipase was not detectable in unaffected adipocytes unless lipase secretion was blocked with monesin, causing retention of the lipase in Golgi. cld/cld adipocytes, in contrast, contained immunofluorescent lipoprotein lipase distributed in a diffuse reticular pattern, indicating retention of lipase in endoplasmic reticulum. Lipoprotein lipase immunoprecipitated from cells incubated 1-3 h with [35S]methionine was digested with or without endoglycosidase H (endo H) or F, and resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Lipoprotein lipase in unaffected cells (Mr = 56,000-58,000) consisted of three glycosylated forms, of which the most prevalent was endo H-resistant, the next was totally endo H-sensitive, and the least was partially endo H-sensitive. In contrast, lipoprotein lipase in cld/cld cells (Mr = 56,000) consisted of a single, totally endo H-sensitive form. Lipoprotein lipase in both groups of cells contained two oligosaccharide chains. Chromatography studies with heparin-Sepharose indicated that at least some of the lipoprotein lipase in cld/cld cells was dimerized. The findings demonstrate that brown adipocytes cultured from cld/cld mice synthesize lipoprotein lipase with two high mannose oligosaccharide chains, but it is inactive and retained in endoplasmic reticulum. Whether the cld mutation affects primarily processing of oligosaccharide chains of lipoprotein lipase in endoplasmic reticulum, transport of the lipase from the reticulum, or some other process, is to be resolved.

Identification of lipoprotein lipase immunoreactive protein in pre- and postheparin plasma from normal subjects and patients with type I hyperlipoproteinemia

Postheparin plasma is a convenient source for the measurement of lipoprotein lipase (LPL) in humans. Previous studies have focused on the measurement of LPL catalytic activity, and have been unable to conveniently measure the LPL protein or identify possibly different plasma forms of the enzyme. Pre- and postheparin plasma was treated with a highly specific antibody raised against bovine milk LPL and the immunoprecipitate was analyzed by Western blotting. In normal subjects there were several species of LPL in plasma. A 56 kD protein increased after heparin injection, and likely represented active LPL. The anti-LPL antibody reacted specifically with this 56 kD protein, and also reacted specifically with proteins at 52 kD, 69 kD, as well as a 20 kD breakdown product. In addition, using peptide mapping, the 56 kD protein was structurally similar to the 52 and 69 kD LPL proteins. The antibodies were affinity purified, biotinylated, and used to quantitate LPL immunoreactive mass using an enzyme-linked immunosorbent assay (ELISA). LPL immunoreactive mass was present in all subjects in preheparin plasma. In postheparin plasma, five patients with type I hyperlipoproteinemia displayed decreased LPL immunoreactive mass when compared to normal subjects, although there was a wide range of specific activity of the small amount of enzyme present. When the LPL from the plasma of the patients was immunoprecipitated and Western blotted, there was considerable heterogeneity in the appearance of the LPL forms, and an overall decrease in LPL protein. Thus, several different immunoreactive LPL proteins were present in pre- and postheparin plasma. In preheparin plasma, as well as in patients with type I hyperlipoproteinemia, there was decreased immunoreactive LPL protein, and the LPL protein that was present was of low specific activity.

Metabolism of very low density lipoproteins in genetically lean or fat lines of chicken

Metabolism of very low density lipoproteins (VLDL) has been compared in fat (FL) and lean (LL) lines of chicken. When refed after fasting, plasma triglyceride concentration reached a significantly higher plateau in FL, although their feed consumption was lower than in LL. Newly synthesized VLDL were studied using anti-lipoprotein lipase antibodies. VLDL triglyceride (TG) concentrations were increased by antibody injection and reached a higher concentration in FL plasma than in LL. Newly synthesized VLDL exhibited a similar lipid composition. Fatty acid profiles were also similar when birds ingested a very low fat diet. Comparison of in vitro affinity of lipoprotein lipase and VLDL from both genotypes did not reveal any difference in Km and Vmax. [14C]labelled VLDL from fat or lean donors were prepared and were injected into chickens from both genotypes. Fractional rate constants did not differ between lines. However, as plasma VLDL-TG pools were very different, plasma turnover was higher in FL than in LL. About 3-fold more VLDL-TG were incorporated in abdominal fat of FL than in LL. Difference in fattening between both genotypes seem to be due to both increased VLDL secretion and VLDL removal from plasma without difference in VLDL characteristics.