Regulation of the secretion of lipoprotein lipase by mouse macrophages

Physiological regulation of lipoprotein lipase

The enzyme lipoprotein lipase (LPL), originally identified as the clearing factor lipase, hydrolyzes triglycerides present in the triglyceride-rich lipoproteins VLDL and chylomicrons. LPL is primarily expressed in tissues that oxidize or store fatty acids in large quantities such as the heart, skeletal muscle, brown adipose tissue and white adipose tissue. Upon production by the underlying parenchymal cells, LPL is transported and attached to the capillary endothelium by the protein GPIHBP1. Because LPL is rate limiting for plasma triglyceride clearance and tissue uptake of fatty acids, the activity of LPL is carefully controlled to adjust fatty acid uptake to the requirements of the underlying tissue via multiple mechanisms at the transcriptional and post-translational level. Although various stimuli influence LPL gene transcription, it is now evident that most of the physiological variation in LPL activity, such as during fasting and exercise, appears to be driven via post-translational mechanisms by extracellular proteins. These proteins can be divided into two main groups: the liver-derived apolipoproteins APOC1, APOC2, APOC3, APOA5, and APOE, and the angiopoietin-like proteins ANGPTL3, ANGPTL4 and ANGPTL8, which have a broader expression profile. This review will summarize the available literature on the regulation of LPL activity in various tissues, with an emphasis on the response to diverse physiological stimuli.

PPAR gamma ligands, troglitazone and pioglitazone, up-regulate expression of HMG-CoA synthase and HMG-CoA reductase gene in THP-1 macrophages

Recently it has been reported that macrophages express a nuclear receptor, peroxisome proliferator-activated receptor gamma (PPAR gamma). Using a ligand of PPAR gamma, troglitazone or pioglitazone, we have shown that the expression of two genes involved in cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase and HMG-CoA reductase, were increased by activation of PPAR gamma through a PPAR response element (PPRE) in THP-1 macrophages. In addition, treatment with troglitazone significantly increased the activity of HMG-CoA reductase and the amount of intracellular cholesterol. Thus, we conclude that PPAR gamma and its agonists increase the cholesterol content of macrophages by the increased expression of genes involved in cholesterol biosynthesis. These findings suggest that PPAR gamma may play a role in cholesterol metabolism in macrophages.

Glucose and insulin stimulate heparin-releasable lipoprotein lipase activity in mouse islets and INS-1 cells. A potential link between insulin resistance and beta-cell dysfunction

Lipoprotein lipase (LpL) provides tissues with triglyceride-derived fatty acids. Fatty acids affect beta-cell function, and LpL overexpression decreases insulin secretion in cell lines, but whether LpL is regulated in beta-cells is unknown. To test the hypothesis that glucose and insulin regulate LpL activity in beta-cells, we studied pancreatic islets and INS-1 cells. Acute exposure of beta-cells to physiological concentrations of glucose stimulated both total cellular LpL activity and heparin-releasable LpL activity. Glucose had no effect on total LpL protein mass but instead promoted the appearance of LpL protein in a heparin-releasable fraction, suggesting that glucose stimulates the translocation of LpL from intracellular to extracellular sites in beta-cells. The induction of heparin-releasable LpL activity was unaffected by treatment with diazoxide, an inhibitor of insulin exocytosis that does not alter glucose metabolism but was blocked by conditions that inhibit glucose metabolism. In vitro hyperinsulinemia had no effect on LpL activity in the presence of low concentrations of glucose but increased LpL activity in the presence of 20 mm glucose. Using dual-laser confocal microscopy, we detected intracellular LpL in vesicles distinct from those containing insulin. LpL was also detected at the cell surface and was displaced from this site by heparin in dispersed islets and INS-1 cells. These results show that glucose metabolism controls the trafficking of LpL activity in beta-cells independent of insulin secretion. They suggest that hyperglycemia and hyperinsulinemia associated with insulin resistance may contribute to progressive beta-cell dysfunction by increasing LpL-mediated delivery of lipid to islets.

Direct regulatory effect of fatty acids on macrophage lipoprotein lipase: potential role of PPARs

Atherosclerosis is a major complication of type 2 diabetes. The pathogenesis of this complication is poorly understood, but it clearly involves production in the vascular wall of macrophage (Mo) lipoprotein lipase (LPL). Mo LPL is increased in human diabetes. Peripheral factors dysregulated in diabetes, including glucose and free fatty acids (FAs), may contribute to this alteration. We previously reported that high glucose stimulates LPL production in both J774 murine and human Mo. In the present study, we evaluated the direct effect of FAs on murine Mo LPL expression and examined the involvement of peroxisome proliferator-activated receptors (PPARs) in this effect. J774 Mo were cultured for 24 h with 0.2 mmol/l unsaturated FAs (arachidonic [AA], eicosapentaenoic [EPA], and linoleic acids [LA]) and monounsaturated (oleic acid [OA]) and saturated FAs (palmitic acid [PA] and stearic acid [SA]) bound to 2% bovine serum albumin. At the end of this incubation period, Mo LPL mRNA expression, immunoreactive mass, activity, and synthetic rate were measured. Incubation of J774 cells with LA, PA, and SA significantly increased Mo LPL mRNA expression. In contrast, exposure of these cells to AA and EPA dramatically decreased this parameter. All FAs, with the exception of EPA and OA, increased extra- and intracellular LPL immunoreactive mass and activity. Intracellular LPL mass and activity paralleled extracellular LPL mass and activity in all FA-treated cells. In Mo exposed to AA, LA, and PA, an increase in Mo LPL synthetic rate was observed. To evaluate the role of PPARs in the modulatory effect of FAs on Mo LPL gene expression, DNA binding assays were performed. Results of these experiments demonstrate an enhanced binding of nuclear proteins extracted from all FA-treated Mo to the peroxisome proliferator-response element (PPRE) consensus sequence of the LPL promoter. PA-, SA-, and OA-stimulated binding activity was effectively diminished by immunoprecipitation of the nuclear proteins with anti-PPAR-alpha antibodies. In contrast, anti-PPAR-gamma antibodies only significantly decreased AA-induced binding activity. Overall, these results provide the first evidence for a direct regulatory effect of FAs on Mo LPL and suggest a potential role of PPARs in the regulation of Mo LPL gene expression by FAs.

Proliferative effect of lipoprotein lipase on human vascular smooth muscle cells

Vascular smooth muscle cell (VSMC) proliferation is a key event in the development and progression of atherosclerotic lesions. Accumulating evidence suggests that lipoprotein lipase (LPL) produced in the vascular wall may exert proatherogenic effects. The aim of the present study was to examine the effect of LPL on VSMC proliferation. Incubation of growth-arrested human VSMCs with purified endotoxin-free bovine LPL for 48 and 72 hours, in the absence of any added exogenous lipoproteins, resulted in a dose-dependent increase in VSMC growth. Addition of VLDLs to the culture media did not further enhance the LPL effect. Treatment of growth-arrested VSMCs with purified human or murine LPL (1 microg/mL) led to a similar increase in cell proliferation. Neutralization of bovine LPL by the monoclonal 5D2 antibody, irreversible inhibition, or heat inactivation of the lipase suppressed the LPL stimulatory effect on VSMC growth. Moreover, preincubation of VSMCs with the specific protein kinase C inhibitors calphostin C and chelerythrine totally abolished LPL-induced VSMC proliferation. In LPL-treated VSMCs, a significant increase in protein kinase C activity was observed. Treatment of VSMCs with heparinase III (1 U/mL) totally inhibited LPL-induced human VSMC proliferation. Taken together, these data indicate that LPL stimulates VSMC proliferation. LPL enzymatic activity, protein kinase C activation, and LPL binding to heparan sulfate proteoglycans expressed on VSMC surfaces are required for this effect. The stimulatory effect of LPL on VSMC proliferation may represent an additional mechanism through which the enzyme contributes to the progression of atherosclerosis.

Effects of platelet-derived growth factor on the synthesis of lipoprotein lipase in human monocyte-derived macrophages

Lipoprotein lipase (LPL), which is secreted by the two predominant cell types in atherosclerotic plaque, macrophages and smooth muscle cells, may be involved in atherosclerosis by generating atherogenic remnant lipoproteins. We investigated the effects of platelet-derived growth factor (PDGF)-BB on the synthesis of LPL by human monocyte-derived macrophages. These cells were cultured in the presence of PDGF-BB for 8 days, after which the enzyme activity, mass, and mRNA levels of LPL were determined. The effect of PDGF-BB was time-dependent and dose-dependent at concentrations of 1 to 10 ng/mL. At 10 ng/mL PDGF-BB enhanced twofold to 2.3-fold the secretion of LPL, and a pulse-labeling study with [35S]methionine revealed that 10 ng/mL PDGF-BB significantly increased the synthesis of LPL. Northern blotting analysis showed that the LPL mRNA level increased dose dependently in macrophages treated with PDGF-BB, and 10 ng/mL PDGF-BB enhanced twofold the expression of LPL mRNA. The protein kinase C inhibitor staurosporine suppressed the effect of PDGF-BB on LPL activity. These results indicate that PDGF-BB stimulated transcription of the LPL gene in human monocyte-derived macrophages through protein kinase C activation and resulted in an increased synthesis of LPL. Therefore, we hypothesize that the augmented synthesis of LPL by PDGF-BB modulates atherosclerosis by influencing lipoprotein metabolism in the vascular wall.

Lipoprotein lipase synergizes with interferon gamma to induce macrophage nitric oxide synthetase mRNA expression and nitric oxide production

Lipoprotein lipase (LPL) induces macrophage tumor necrosis factor-alpha (TNF-alpha) gene expression and protein secretion. Since TNF-alpha can increase interferon gamma (IFN-gamma)-dependent nitric oxide (NO) production, we studied whether LPL may synergize with IFN-gamma for the induction of macrophage NO production. Although ineffective by itself, LPL in combination with IFN-gamma increased L-arginine-dependent NO production in a dose-dependent manner. Preincubation of LPL with an anti-LPL neutralizing antibody totally suppressed this effect. Increased NO synthetase (NOS) mRNA expression was also observed after macrophage treatment with IFN-gamma and LPL. Protein synthesis was required for the induction of NOS mRNA, and a TNF-alpha-mediated effect of LPL on NOS gene expression and NO production was observed. The ability of LPL to augment IFN-gamma-dependent NOS mRNA expression was associated with an increase in the NOS gene transcriptional activity but not in the NOS mRNA stability. Finally, binding of nuclear proteins to the nuclear factor-kappa B- and TNF-alpha-responsive sequences of the macrophage NOS promotor was decreased by treatment of the cells by IFN-gamma alone or in combination with LPL. These data provide evidence for a link between LPL and arginine metabolism in macrophages and further stress the role of LPL in the regulation of macrophage activation.

Lipopolysaccharide regulation of lipoprotein lipase expression in murine macrophages

The enzyme lipoprotein lipase is expressed in a number of cell types and plays a central role in lipid metabolism. Multiple factors regulate its expression in a tissue-specific manner. In murine macrophages, lipopolysaccharide inhibits lipoprotein lipase enzyme activity. The current work examines this process in the established J774 macrophage line and primary peritoneal macrophages from endotoxin-sensitive (C3HeB/Fej) and endotoxin-resistant (C3H/Hej) murine strains. Lipopolysaccharide inhibition of macrophage lipoprotein lipase occurred at the enzyme and mRNA levels in a time- and concentration-dependent manner. Cells from endotoxin-resistant animals maintained their expression of lipoprotein lipase following treatment with lipopolysaccharide. Results of gel retention assays showed that lipopolysaccharide treatment of the J774 macrophages altered the level of nuclear proteins recognizing and binding the lipoprotein lipase promoter DNA. Nuclear extracts from resting J774 cells contained proteins which bound specifically to the octamer motif and to the CAAT box within the lipoprotein lipase promoter. Exposure of the J774 cells to lipopolysaccharide for 16 h increased the level of protein-octamer DNA complexes. Similar responses were obtained in endotoxin-sensitive, but not endotoxin-resistant, primary macrophages following in vitro treatment with lipopolysaccharide. This finding suggests that transcriptional events may contribute to the lipopolysaccharide regulation of macrophage lipoprotein lipase expression.

Expression of lipoprotein lipase mRNA and secretion in macrophages isolated from human atherosclerotic aorta

The expression of lipoprotein lipase (LPL) mRNA and the LPL activity were studied in macrophages (CD14 positive) from human atherosclerotic tissue. Macrophages were isolated after collagenase digestion by immunomagnetic isolation. About 90% of the cells were foam cells with oil red O positive lipid droplets. To analyze the mRNA expression, PCR with specific primers for LPL was used. Arterial macrophages were analyzed directly after isolation and the data showed low expression of LPL mRNA when compared with monocyte-derived macrophages. To induce the expression of LPL mRNA in macrophages, PMA was used. When incubating arterial macrophages with PMA for 24 h we could not detect any increase in LPL mRNA levels. Similarly, the cells secreted very small amounts of LPL even after PMA stimulation. In conclusion, these studies show a very low expression of LPL mRNA in the CD14-positive macrophage-derived foam cells isolated from human atherosclerotic tissue. These data suggest that the CD14-positive cells are a subpopulation of foam cells that express low levels of lipoprotein lipase, and the lipid content could be a major factor for downregulation of LPL. However, the cells were isolated from advanced atherosclerotic lesions, and these findings may not reflect the situation in early fatty streaks.

Regulation of lipoprotein lipase secretion in murine macrophages during foam cell formation in vitro. Effect of triglyceride-rich lipoproteins

Triglyceride rich-lipoproteins induce triglyceride accumulation in macrophages, leading to foam cell formation. The correlation between cell triglyceride accumulation and lipoprotein lipase (LPL) secretion in murine macrophages and the role that LPL plays in the accumulation process were examined. LPL secretion is defined as the extracellular LPL activity that accumulates during a 4-hour incubation of treated and untreated cells in a bovine serum albumin-containing RPMI-1640 medium. LPL secretion was suppressed (up to 70%) in a dose- and time-dependent manner when J774.1 cells were incubated with chylomicrons, very low density lipoproteins, and intermediate density lipoproteins but not with low or high density lipoproteins from normolipidemic and hypertriglyceridemic subjects. Oleic acid both suppressed LPL secretion and invoked triglyceride accumulation. Suppression of LPL secretion preceded gross triglyceride accumulation, was reversible, and was not the result of a reduction in LPL mRNA. P388D1 cells neither secreted LPL nor accumulated triglyceride. Inhibition of LPL secretion by tunicamycin in both peritoneal macrophages and J774.1 cells prevented a hypertriglyceridemic very low density lipoprotein-induced triglyceride accumulation, an effect that was counteracted by addition of exogenous LPL. The results suggest that 1) extracellular hydrolysis of lipoprotein triglyceride is a major factor in inducing foam cell formation and 2) LPL secretion may be regulated by cell energy needs, and when these needs are exceeded, LPL secretion is suppressed.

Regulation of lipoprotein lipase by dibutyryl cAMP, cholera toxin, Hepes and heparin in F1 heart-cell cultures

Regulation of lipoprotein lipase was studied in mesenchymal rat heart-cell cultures. Treatment of the cultures with dibutyryl cyclic AMP or with cholera toxin resulted in an increase in LPL activity and a comparable increase in LPL mRNA. When the cells were exposed to 100 mM Hepes for 24 h, total enzyme activity rose 2-fold and LPL mRNA increased 2.4-fold. After 72 h, there was a 3-fold increase in LPL mRNA and a 4-fold rise in cellular LPL activity, while medium activity increased 20-fold. Exposure of the cultures to heparin for 24 h resulted in a 3.2-fold increase in total activity and a 36-fold increase in medium activity. This increase was not accompanied by any rise in LPL mRNA. Addition of actinomycin D to control dishes for 24 h resulted in a 33% reduction in LPL mRNA and a 43% reduction in enzyme activity. These values were 71% and 56%, respectively, in Hepes-treated cells, indicating that no stabilization of LPL mRNA occurred under these conditions. It can be concluded that in mesenchymal rat heart-cells in culture cAMP and cholera toxin upregulate lipoprotein lipase at the level of transcription. The increase in LPL activity after 24 h exposure to Hepes could be compatible with transcriptional regulation, while exposure to heparin is not accompanied by a change in LPL mRNA.

Effects of human recombinant macrophage colony-stimulating factor on the secretion of lipoprotein lipase from macrophages

The effects of human recombinant macrophage colony-stimulating factor (M-CSF) on the secretion of lipoprotein lipase were studied in rat alveolar macrophages. Five nanograms per milliliter M-CSF significantly enhanced lipoprotein lipase secretion (threefold), and the maximal effect (10-fold) of M-CSF on lipoprotein lipase secretion was observed at a dose of 200 ng/ml M-CSF. The effect of M-CSF was time dependent but was not manifested during the first 8 hours of incubation. After 24 hours, its effects were evident and dose dependent. On blot hybridization of macrophage RNAs with human cDNA of lipoprotein lipase, a remarkable and dose-dependent increase in mRNA level (7.3-fold) was found in M-CSF-treated alveolar macrophages. The secretion of lipoprotein lipase was also enhanced in human monocyte-derived macrophages (2.6-fold), whereas the secretion from either THP-1 cells, P388 cells, or J774 cells was not significantly enhanced. These results indicate that the stimulation of lipoprotein lipase secretion after M-CSF treatment was evident in rat alveolar macrophages and human monocyte-derived macrophages on the basis of both enzyme activity and mRNA level; therefore, M-CSF may be involved in lipoprotein metabolism of macrophages through modulation of the secretion of lipoprotein lipase.

Lipoprotein lipase activity in neonatal-rat liver cell types

The lipoprotein lipase activity in the liver of neonatal (1 day old) rats was about 3 times that in the liver of adult rats. Perfusion of the neonatal liver with collagenase decreased the tissue-associated activity by 77%. When neonatal-rat liver cells were dispersed, hepatocyte-enriched (fraction I) and haemopoietic-cell-enriched (fraction II) populations were obtained. The lipoprotein lipase activity in fraction I was 7 times that in fraction II. On the basis of those activities and the proportion of both cell types in either fraction, it was estimated that hepatocytes contained most, if not all, the lipoprotein lipase activity detected in collagenase-perfused neonatal-rat livers. From those calculations it was also concluded that haemopoietic cells did not contain lipoprotein lipase activity. When the hepatocyte-enriched cell population was incubated at 25 degrees C for up to 3 h, a slow but progressive release of enzyme activity to the incubation medium was found. However, the total activity (cells + medium) did not significantly change through the incubation period. Cycloheximide produced a time-dependent decrease in the cell-associated activity. Heparin increased the amount of lipoprotein lipase activity released to the medium. Because the cell-associated activity was unchanged, heparin also produced a time-dependent increase in the total activity. In those cells incubated with heparin, cycloheximide did not affect the initial release of lipoprotein lipase activity to the medium, but blocked further release. The cell-associated activity was also decreased by the presence of cycloheximide in those cells. It is concluded that neonatal-rat hepatocytes synthesize active lipoprotein lipase.

Control of lipoprotein lipase secretion in mouse macrophages

The regulation of secretion of lipoprotein lipase (LPL) was studied in in vitro-derived mouse bone marrow macrophages (BMM), peritoneal exudate and resident macrophages and in the macrophage-like tumor cell line J774.1. BMM in cultures initiated with low concentrations of bone marrow cells (LC-BMC cultures) secrete more LPL per cell than BMM in cultures initiated with high concentrations of bone marrow cells (HC-BMC cultures). The suppressed state of LPL secretion in HC-BMC cultures could be alleviated by the addition of a colony-stimulating factor source (L-cell-conditioned medium; L-CM) onto the culture medium or exchanging the medium of HC-BMC cultures with medium from LC-BMC cultures for short periods (4 h). Addition of L-CM increased LPL secretion also in LC-BMC cultures. Addition of L-CM to fresh culture medium had little or no effect, suggesting that, in addition to requirement for L-CM, optimal expression depended also on factors released by the growing cells, probably providing optimal growth conditions. L-CM enhanced LPL secretion by thioglycollate-elicited peritoneal macrophages and had no effect on LPL secretion by resident peritoneal macrophages. Secretion of LPL from adherent J774.1 cells showed a biphasic effect. Secretion increased with cell density up to the point when growth inhibition was observed. In dense cultures in which cell proliferation was almost arrested, LPL secretion was remarkably suppressed (80-90%). Change of medium of dense cultures to fresh medium or medium conditioned by sparse cultures (for the last 4 h of culture) led to enhancement of LPL secretion to levels similar to those optimally expressed by sparse cultures. L-CM did not enhance LPL secretion from J774.1 cells. Dense cultures of both BMM and J774.1 cells did not contain a stable inhibitor of LPL secretion and medium from sparse cultures did not contain an inducer of LPL secretion. The data suggest that proliferating macrophages secrete large amounts of LPL, whereas in nonproliferating, quiescent cells, this activity is much reduced. L-CM enhances LPL secretion in quiescent BMM and peritoneal exudate cells to levels expressed by proliferating cells. Since this effect is already expressed after a 4 h incubation period, it is not dependent on cell cycling but could be one of the early responses to this macrophage mitogen. In J774.1 cells, a change of medium is a sufficient signal for enhancement of LPL secretion in quiescent cells.

Regulation of macrophage lipoprotein lipase secretion by the scavenger receptor

The effects of ligand binding to the scavenger receptor on the secretion of lipoprotein lipase by murine macrophages were examined. Inflammatory macrophages exposed to acetylated low-density lipoprotein (AcLDL) exhibited a dose-dependent, 40-80% increase in lipoprotein lipase secretion. This stimulation appeared to be unrelated to intracellular cholesterol and triacylglycerol levels and to phagocytosis in general. Resident and inflammatory macrophages treated with maleylated bovine serum albumin (Mal-BSA) showed a 3-fold increase in lipoprotein lipase secretion in a dose-dependent and time-dependent fashion. In contrast, dextran sulfate, which is another ligand recognized by the scavenger receptor, caused a dose-dependent decrease in lipoprotein lipase secretion. Casein, a ligand recognized by the Mal-BSA receptor, did not affect lipoprotein lipase secretion nor the ability of Mal-BSA to stimulate the enzyme, while dextran sulfate abolished the stimulatory effects of Mal-BSA. Since ethylamine, an inhibitor of receptor-mediated endocytosis, attenuated the increase in lipoprotein lipase secretion induced by AcLDL and Mal-BSA, but did not affect the inhibition induced by dextran sulfate, it is suggested that receptor-mediated endocytosis of ligands via the scavenger receptor might play a key role in the stimulation of lipoprotein lipase secretion in macrophages. This study reveals another mechanism for regulation of macrophage lipoprotein lipase secretion.