VEGF-C/VEGFR-3 signalling in macrophages ameliorates acute lung injury.

BACKGROUND: Successful recovery from acute lung injury requires inhibition of neutrophil influx and clearance of apoptotic neutrophils. However, the mechanisms underlying recovery remain unclear. We investigated the ameliorative effects of vascular endothelial growth factor (VEGF)-C/VEGF receptor 3 (VEGFR-3) signalling in macrophages in lipopolysaccharide (LPS)-induced lung injury. METHODS: LPS was intranasally injected into wild-type and transgenic mice. Gain and loss of VEGF-C/VEGFR-3 signalling function experiments employed adenovirus-mediated intranasal delivery of VEGF-C (Ad-VEGF-C vector) and soluble VEGFR-3 (sVEGFR-3) or anti-VEGFR-3 blocking antibodies and mice with a deletion of VEGFR-3 in myeloid cells. RESULTS: The early phase of lung injury was significantly alleviated by the overexpression of VEGF-C with increased levels of bronchoalveolar lavage (BAL) fluid interleukin-10 (IL-10), but worsened in the later phase by VEGFR-3 inhibition upon administration of Ad-sVEGFR-3 vector. Injection of anti-VEGFR-3 antibodies to mice in the resolution phase inhibited recovery from lung injury. The VEGFR-3-deleted mice had a shorter survival time than littermates and more severe lung injury in the resolution phase. Alveolar macrophages in the resolution phase digested most of the extrinsic apoptotic neutrophils and VEGF-C/VEGFR-3 signalling increased efferocytosis via upregulation of integrin αv in the macrophages. We also found that incubation with BAL fluid from acute respiratory distress syndrome (ARDS) patients, but not from controls, decreased VEGFR-3 expression and the efficiency of IL-10 expression and efferocytosis in human monocyte-derived macrophages. CONCLUSIONS: VEGF-C/VEGFR-3 signalling in macrophages ameliorates experimental lung injury. This mechanism may also provide an explanation for ARDS resolution.

Lymphatic System in Cardiovascular Medicine.

The mammalian circulatory system comprises both the cardiovascular system and the lymphatic system. In contrast to the blood vascular circulation, the lymphatic system forms a unidirectional transit pathway from the extracellular space to the venous system. It actively regulates tissue fluid homeostasis, absorption of gastrointestinal lipids, and trafficking of antigen-presenting cells and lymphocytes to lymphoid organs and on to the systemic circulation. The cardinal manifestation of lymphatic malfunction is lymphedema. Recent research has implicated the lymphatic system in the pathogenesis of cardiovascular diseases including obesity and metabolic disease, dyslipidemia, inflammation, atherosclerosis, hypertension, and myocardial infarction. Here, we review the most recent advances in the field of lymphatic vascular biology, with a focus on cardiovascular disease.

Lymphatic vessel insufficiency in hypercholesterolemic mice alters lipoprotein levels and promotes atherogenesis.

OBJECTIVE: Lymphatic vessels collect extravasated fluid and proteins from tissues to blood circulation as well as play an essential role in lipid metabolism by taking up intestinal chylomicrons. Previous studies have shown that impairment of lymphatic vessel function causes lymphedema and fat accumulation, but clear connections between arterial pathologies and lymphatic vessels have not been described. APPROACH AND RESULTS: Two transgenic mouse strains with lymphatic insufficiency (soluble vascular endothelial growth factor 3 [sVEGFR3] and Chy) were crossed with atherosclerotic mice deficient of low-density lipoprotein receptor and apolipoprotein B48 (LDLR(-/-)/ApoB(100/100)) to study the effects of insufficient lymphatic vessel transport on lipoprotein metabolism and atherosclerosis. Both sVEGFR3×LDLR(-/-)/ApoB(100/100) mice and Chy×LDLR(-/-)/ApoB(100/100) mice had higher plasma cholesterol levels compared with LDLR(-/-)/ApoB(100/100) control mice during both normal chow diet (16.3 and 13.7 versus 8.2 mmol/L, respectively) and Western-type high-fat diet (eg, after 2 weeks of fat diet, 45.9 and 42.6 versus 30.2 mmol/L, respectively). Cholesterol and triglyceride levels in very-low-density lipoprotein and low-density lipoprotein fractions were increased. Atherosclerotic lesions in young and intermediate cohorts of sVEGFR3×LDLR(-/-)/ApoB(100/100) mice progressed faster than in control mice (eg, intermediate cohort mice at 6 weeks, 18.3% versus 7.7% of the whole aorta, respectively). In addition, lesions in sVEGFR3×LDLR(-/-)/ApoB(100/100) mice and Chy×LDLR(-/-)/ApoB(100/100) mice had much less lymphatic vessels than lesions in control mice (0.33% and 1.07% versus 7.45% of podoplanin-positive vessels, respectively). CONCLUSIONS: We show a novel finding linking impaired lymphatic vessels to lipoprotein metabolism, increased plasma cholesterol levels, and enhanced atherogenesis.

Angptl3 deficiency is associated with increased insulin sensitivity, lipoprotein lipase activity, and decreased serum free fatty acids.

OBJECTIVE: Angiopoietin-like 3 (Angptl3) is a regulator of lipoprotein metabolism at least by inhibiting lipoprotein lipase activity. Loss-of-function mutations in ANGPTL3 cause familial combined hypolipidemia through an unknown mechanism. APPROACH AND RESULTS: We compared lipolytic activities, lipoprotein composition, and other lipid-related enzyme/lipid transfer proteins in carriers of the S17X loss-of-function mutation in ANGPTL3 and in age- and sex-matched noncarrier controls. Gel filtration analysis revealed a severely disturbed lipoprotein profile and a reduction in size and triglyceride content of very low density lipoprotein in homozygotes as compared with heterozygotes and noncarriers. S17X homozygotes had significantly higher lipoprotein lipase activity and mass in postheparin plasma, whereas heterozygotes showed no difference in these parameters when compared with noncarriers. No changes in hepatic lipase, endothelial lipase, paraoxonase 1, phospholipid transfer protein, and cholesterol ester transfer protein activities were associated with the S17X mutation. Plasma free fatty acid, insulin, glucose, and homeostatic model assessment of insulin resistance were significantly lower in homozygous subjects compared with heterozygotes and noncarriers subjects. CONCLUSIONS: These results indicate that, although partial Angptl3 deficiency did not affect the activities of lipolytic enzymes, the complete absence of Angptl3 results in an increased lipoprotein lipase activity and mass and low circulating free fatty acid levels. This latter effect is probably because of decreased mobilization of free fatty acid from fat stores in human adipose tissue and may result in reduced hepatic very low density lipoprotein synthesis and secretion via attenuated hepatic free fatty acid supply. Altogether, Angptl3 may affect insulin sensitivity and play a role in modulating both lipid and glucose metabolism.

Serum angiopoietin-like 4 protein levels and expression in adipose tissue are inversely correlated with obesity in monozygotic twins.

Animal studies have suggested that angiopoietin-like 4 (Angptl4) regulates adiposity through central and peripheral mechanisms. The aim of this study was to investigate whether serum concentration and adipose tissue expression of Angptl4 are associated with obesity-related parameters in humans. Altogether, 75 dizygotic (DZ) and 46 monozygotic (MZ) twin pairs were studied, from the FinnTwin12 and FinnTwin16 cohorts. Among the MZ pairs, 21 were discordant for body mass index (BMI) (intra-pair BMI-difference >2.5 kg/m², age 23-33 years). Serum Angptl4 (s-Angptl4) levels were measured by ELISA, and adipose tissue gene expression was analyzed by genome-wide transcript profiling. In MZ twin pairs discordant for BMI, s-Angptl4 and adipose tissue ANGPTL4 mRNA (at-ANGPTL4) levels were significantly decreased (P = 0.04 and P = 0.03, respectively) in obese twins as compared with their nonobese cotwins. In all twins, intra-pair differences in s-Angptl4 levels were inversely correlated with intra-pair differences in BMI (r = -0.27, P = 0.003). In individual MZ twins, at-ANGPTL4 expression was inversely correlated with BMI (r = -0.44, P = 0.001) and positively correlated with at-LIPE (r = 0.24, P = 0.01) and at-ABHD5 (r = 0.41, P = 0.005) expression. Our results demonstrated that variation in Angptl4 concentration was only modestly accounted for by genetic factors and suggest a role for Angptl4 in acquired obesity in humans.

Apolipoprotein A-I stimulates cholesteryl ester transfer protein and apolipoprotein E secretion from lipid-loaded macrophages; the role of NF-κB and PKA signaling pathways.

Cholesteryl ester transfer protein (CETP) and apolipoprotein E (apoE) are secreted by macrophages. Apolipoprotein A-I (apoA-I) is a potent inducer of apoE secretion from lipid-loaded macrophages, but its effect on CETP is not known. We aimed to identify the signaling pathways involved in apoA-I and HDL-mediated regulation of CETP and apoE secretion from lipid-loaded macrophages. THP-1 macrophages were loaded with lipids by incubation with human copper-oxidized LDL. The cells were subsequently exposed to human purified apoA-I or HDL(3) with/without inhibitors of NF-κB (TPCK) or PKA (H89). CETP and apoE in the cultured cells and media were quantified by real-time PCR and Western blot. Results showed that in lipid-loaded macrophages: (i) CETP and apoE gene expression and secretion were increased in the presence of apoA-I, and further increased by inhibition of NF-kB with TPCK; (ii) CETP and apoE gene expression and secretion were reduced by the inhibition of PKA with H89; (iii) PKA-gamma subunit was activated by oxidized LDL and moreover by apoA-I. We also showed that: (i) siRNA-mediated CETP gene silencing diminished apoE secretion from both non-loaded and lipid-loaded macrophages; (ii) addition of apoA-I partially restored apoE secretion from lipid-loaded macrophages with the silenced CETP gene. In conclusion, our data suggest a new mechanism by which apoA-I stimulates CETP secretion, in addition to apoE, from lipid loaded macrophages, a process involving NF-κB inhibition and/or PKA pathway activation.

The relationship between plasma angiopoietin-like protein 4 levels, angiopoietin-like protein 4 genotype, and coronary heart disease risk.

OBJECTIVE: To investigate the relationship between angiopoietin-like protein 4 (Angptl4) levels, coronary heart disease (CHD) biomarkers, and ANGPTL4 variants. METHODS AND RESULTS: Plasma Angptl4 was quantified in 666 subjects of the Northwick Park Heart Study II using a validated ELISA. Seven ANGPTL4 single-nucleotide polymorphisms were genotyped, and CHD biomarkers were assessed in the whole cohort (N=2775). Weighted mean±SD plasma Angptl4 levels were 10.0±11.0 ng/mL. Plasma Angptl4 concentration correlated positively with age (r=0.15, P<0.001) and body fat mass (r=0.19, P=0.003) but negatively with plasma high-density lipoprotein cholesterol (r=-0.13, P=0.01). No correlation with triglycerides (TGs) was observed. T266M was independently associated with plasma Angptl4 levels (P<0.001) but was not associated with TGs or CHD risk in the meta-analysis of 5 studies (4061 cases/15 395 controls). E40K showed no independent association with plasma Angptl4 levels. In human embryonic kidney 293 and human hepatoma 7 cells compared with wild type, E40K and T266M showed significantly altered synthesis and secretion, respectively. CONCLUSIONS: Circulating Angptl4 levels may not influence TG levels or CHD risk for the following reasons: (1) Angptl4 levels were not correlated with TGs; (2) T266M, although associated with Angptl4 levels, showed no association with plasma TGs; and (3) TG-lowering E40K did not influence Angptl4 levels. These results provide new insights into the role of Angptl4 in TG metabolism.

Quantitation of serum angiopoietin-like proteins 3 and 4 in a Finnish population sample.

We have developed and validated quantitative ELISAs for human angiopoietin-like (ANGPTL)3 and 4 and correlated their serum levels with parameters of lipid and carbohydrate metabolism. For this study, we used a random subsample of the Health 2000 Health Examination Survey consisting of 125 men and 125 women, aged 30-94 years. The anthropometric and biochemical parameters of subjects were characterized in detail. ANGPTL 3 and 4 levels were determined using the developed ELISAs. The intra- and inter-assay coefficients of variation for the assays were less than 15%. The average serum concentration of ANGPTL3 was 368 +/- 168 ng/ml (mean +/- SD) and for ANGPTL4 it was 18 +/- 23 ng/ml (mean +/- SD). ANGPTL4 serum levels displayed high variability between individuals ranging from 2 to 158 ng/ml. In post-heparin plasma, both ANGPTL 3 and 4 were increased. Low levels of ANGPTL3 were associated with decreased HDL-cholesterol and increased triglyceride levels. ANGPTL4 levels were positively correlated with FFAs (P = 0.044) and waist-hip ratio (P = 0.016). The developed ELISAs will be important tools to clarify the role of ANGPTL 3 and 4 in human energy metabolism and partitioning of triglycerides between sites of storage (adipose tissue) and oxidation (skeletal and cardiac muscle).

Human apoA-I increases macrophage foam cell derived PLTP activity without affecting the PLTP mass.

BACKGROUND: phospholipid transfer protein (PLTP) plays important roles in lipoprotein metabolism and atherosclerosis and is expressed by macrophages and macrophage foam cells (MFCs). The aim of the present study was to determine whether the major protein from HDL, apoA-I, affects PLTP derived from MFCs. RESULTS: as cell model we used human THP-1 monocytes incubated with acetylated LDL, to generate MFC. The addition of apoA-I to the cell media increased apoE secretion from the cells, in a concentration dependent fashion, without affecting cellular apoE levels. In contrast, apoA-I had no effect on PLTP synthesis and secretion, but strongly induced the PLTP activity in the media. ApoA-I also increased phospholipid transfer activity of PLTP isolated from human plasma. This effect was dependent on apoA-I concentration but independent on apoA-I lipidation status. ApoE, ApoA-II and apoA-IV, but not immunoglobulins or bovine serum albumin, also increased PLTP activity. We also report that apoA-I protects PLTP from heat inactivation. CONCLUSION: apoA-I enhances the phospholipid transfer activity of PLTP secreted from macrophage foam cells without affecting the PLTP mass.

Proteolytic activation defines distinct lymphangiogenic mechanisms for VEGFC and VEGFD.

Lymphangiogenesis is supported by 2 homologous VEGFR3 ligands, VEGFC and VEGFD. VEGFC is required for lymphatic development, while VEGFD is not. VEGFC and VEGFD are proteolytically cleaved after cell secretion in vitro, and recent studies have implicated the protease a disintegrin and metalloproteinase with thrombospondin motifs 3 (ADAMTS3) and the secreted factor collagen and calcium binding EGF domains 1 (CCBE1) in this process. It is not well understood how ligand proteolysis is controlled at the molecular level or how this process regulates lymphangiogenesis, because these complex molecular interactions have been difficult to follow ex vivo and test in vivo. Here, we have developed and used biochemical and cellular tools to demonstrate that an ADAMTS3-CCBE1 complex can form independently of VEGFR3 and is required to convert VEGFC, but not VEGFD, into an active ligand. Consistent with these ex vivo findings, mouse genetic studies revealed that ADAMTS3 is required for lymphatic development in a manner that is identical to the requirement of VEGFC and CCBE1 for lymphatic development. Moreover, CCBE1 was required for in vivo lymphangiogenesis stimulated by VEGFC but not VEGFD. Together, these studies reveal that lymphangiogenesis is regulated by two distinct proteolytic mechanisms of ligand activation: one in which VEGFC activation by ADAMTS3 and CCBE1 spatially and temporally patterns developing lymphatics, and one in which VEGFD activation by a distinct proteolytic mechanism may be stimulated during inflammatory lymphatic growth.

VEGFB/VEGFR1-Induced Expansion of Adipose Vasculature Counteracts Obesity and Related Metabolic Complications.

Impaired angiogenesis has been implicated in adipose tissue dysfunction and the development of obesity and associated metabolic disorders. Here, we report the unexpected finding that vascular endothelial growth factor B (VEGFB) gene transduction into mice inhibits obesity-associated inflammation and improves metabolic health without changes in body weight or ectopic lipid deposition. Mechanistically, the binding of VEGFB to VEGF receptor 1 (VEGFR1, also known as Flt1) activated the VEGF/VEGFR2 pathway and increased capillary density, tissue perfusion, and insulin supply, signaling, and function in adipose tissue. Furthermore, endothelial Flt1 gene deletion enhanced the effect of VEGFB, activating the thermogenic program in subcutaneous adipose tissue, which increased the basal metabolic rate, thus preventing diet-induced obesity and related metabolic complications. In obese and insulin-resistant mice, Vegfb gene transfer, together with endothelial Flt1 gene deletion, induced weight loss and mitigated the metabolic complications, demonstrating the therapeutic potential of the VEGFB/VEGFR1 pathway.

VEGF-C is required for intestinal lymphatic vessel maintenance and lipid absorption.

Vascular endothelial growth factor C (VEGF-C) binding to its tyrosine kinase receptor VEGFR-3 drives lymphatic vessel growth during development and in pathological processes. Although the VEGF-C/VEGFR-3 pathway provides a target for treatment of cancer and lymphedema, the physiological functions of VEGF-C in adult vasculature are unknown. We show here that VEGF-C is necessary for perinatal lymphangiogenesis, but required for adult lymphatic vessel maintenance only in the intestine. Following Vegfc gene deletion in adult mice, the intestinal lymphatic vessels, including the lacteal vessels, underwent gradual atrophy, which was aggravated when also Vegfd was deleted. VEGF-C was expressed by a subset of smooth muscle cells adjacent to the lacteals in the villus and in the intestinal wall. The Vegfc-deleted mice showed defective lipid absorption and increased fecal excretion of dietary cholesterol and fatty acids. When fed a high-fat diet, the Vegfc-deficient mice were resistant to obesity and had improved glucose metabolism. Our findings indicate that the lymphangiogenic growth factors provide trophic and dynamic regulation of the intestinal lymphatic vasculature, which could be especially important in the dietary regulation of adiposity and cholesterol metabolism.

Blockade of VEGF-C and VEGF-D modulates adipose tissue inflammation and improves metabolic parameters under high-fat diet.

OBJECTIVE: Elevated serum levels of the lymphangiogenic factors VEGF-C and -D have been observed in obese individuals but their relevance for the metabolic syndrome has remained unknown. METHODS: K14-VEGFR-3-Ig (sR3) mice that constitutively express soluble-VEGFR-3-Ig in the skin, scavenging VEGF-C and -D, and wildtype (WT) mice were fed either chow or high-fat diet for 20 weeks. To assess the effect of VEGFR-3 blockage on adipose tissue growth and insulin sensitivity, we evaluated weight gain, adipocyte size and hepatic lipid accumulation. These results were complemented with insulin tolerance tests, FACS analysis of adipose tissue macrophages, in vitro 3T3-L1 differentiation assays and in vivo blocking antibody treatment experiments. RESULTS: We show here that sR3 mice are protected from obesity-induced insulin resistance and hepatic lipid accumulation. This protection is associated with enhanced subcutaneous adipose tissue hyperplasia and an increased number of alternatively-activated (M2) macrophages in adipose tissue. We also show that VEGF-C and -D are chemotactic for murine macrophages and that this effect is mediated by VEGFR-3, which is upregulated on M1 polarized macrophages. Systemic antibody blockage of VEGFR-3 in db/db mice reduces adipose tissue macrophage infiltration and hepatic lipid accumulation, and improves insulin sensitivity. CONCLUSIONS: These results reveal an unanticipated role of the lymphangiogenic factors VEGF-C and -D in the mediation of metabolic syndrome-associated adipose tissue inflammation. Blockage of these lymphangiogenic factors might constitute a new therapeutic strategy for the prevention of obesity-associated insulin resistance.

VEGF-B-induced vascular growth leads to metabolic reprogramming and ischemia resistance in the heart.

Angiogenic growth factors have recently been linked to tissue metabolism. We have used genetic gain- and loss-of function models to elucidate the effects and mechanisms of action of vascular endothelial growth factor-B (VEGF-B) in the heart. A cardiomyocyte-specific VEGF-B transgene induced an expanded coronary arterial tree and reprogramming of cardiomyocyte metabolism. This was associated with protection against myocardial infarction and preservation of mitochondrial complex I function upon ischemia-reperfusion. VEGF-B increased VEGF signals via VEGF receptor-2 to activate Erk1/2, which resulted in vascular growth. Akt and mTORC1 pathways were upregulated and AMPK downregulated, readjusting cardiomyocyte metabolic pathways to favor glucose oxidation and macromolecular biosynthesis. However, contrasting with a previous theory, there was no difference in fatty acid uptake by the heart between the VEGF-B transgenic, gene-targeted or wildtype rats. Importantly, we also show that VEGF-B expression is reduced in human heart disease. Our data indicate that VEGF-B could be used to increase the coronary vasculature and to reprogram myocardial metabolism to improve cardiac function in ischemic heart disease.

Angiopoietin-like 4 mediates PPAR delta effect on lipoprotein lipase-dependent fatty acid uptake but not on beta-oxidation in myotubes.

Peroxisome proliferator-activated receptor (PPAR) delta is an important regulator of fatty acid (FA) metabolism. Angiopoietin-like 4 (Angptl4), a multifunctional protein, is one of the major targets of PPAR delta in skeletal muscle cells. Here we investigated the regulation of Angptl4 and its role in mediating PPAR delta functions using human, rat and mouse myotubes. Expression of Angptl4 was upregulated during myotubes differentiation and by oleic acid, insulin and PPAR delta agonist GW501516. Treatment with GW501516 or Angptl4 overexpression inhibited both lipoprotein lipase (LPL) activity and LPL-dependent uptake of FAs whereas uptake of BSA-bound FAs was not affected by either treatment. Activation of retinoic X receptor (RXR), PPAR delta functional partner, using bexarotene upregulated Angptl4 expression and inhibited LPL activity in a PPAR delta dependent fashion. Silencing of Angptl4 blocked the effect of GW501516 and bexarotene on LPL activity. Treatment with GW501516 but not Angptl4 overexpression significantly increased palmitate oxidation. Furthermore, Angptl4 overexpression did not affect the capacity of GW501516 to increase palmitate oxidation. Basal and insulin stimulated glucose uptake, glycogen synthesis and glucose oxidation were not significantly modulated by Angptl4 overexpression. Our findings suggest that FAs-PPARdelta/RXR-Angptl4 axis controls the LPL-dependent uptake of FAs in myotubes, whereas the effect of PPAR delta activation on beta-oxidation is independent of Angptl4.

OSBP-related proteins (ORPs) in human adipose depots and cultured adipocytes: evidence for impacts on the adipocyte phenotype.

Oxysterol-binding protein (OSBP) homologues, ORPs, are implicated in lipid homeostatic control, vesicle transport, and cell signaling. We analyzed here the quantity of ORP mRNAs in human subcutaneous (s.c.) and visceral adipose depots, as well as in the Simpson-Golabi-Behmel syndrome (SGBS) adipocyte cell model. All of the ORP mRNAs were present in the s.c and visceral adipose tissues, and the two depots shared an almost identical ORP mRNA expression pattern. SGBS adipocytes displayed a similar pattern, suggesting that the adipose tissue ORP expression pattern mainly derives from adipocytes. During SGBS cell adipogenic differentiation, ORP2, ORP3, ORP4, ORP7, and ORP8 mRNAs were down-regulated, while ORP11 was induced. To assess the impacts of ORPs on adipocyte differentiation, ORP3 and ORP8, proteins down-regulated during adipogenesis, were overexpressed in differentiating SGBS adipocytes, while ORP11, a protein induced during adipogenesis, was silenced. ORP8 overexpression resulted in reduced expression of the aP2 mRNA, while down-regulation of adiponectin and aP2 was observed in ORP11 silenced cells. Furthermore, ORP8 overexpression or silencing of ORP11 markedly decreased cellular triglyceride storage. These data identify the patterns of ORP expression in human adipose depots and SGBS adipocytes, and provide the first evidence for a functional impact of ORPs on the adipocyte phenotype.

Inflammatory signaling pathways regulating ApoE gene expression in macrophages.

The atheroprotective role of apolipoprotein E (apoE) is well established. During inflammation, expression of apoE in macrophages is reduced leading to enhanced atheromatous plaque development. In the present study, we investigated the signaling pathways involved in the repression of apoE gene expression in response to lipopolysaccharide (LPS) treatment, a condition that mimics the inflammatory stress, in mouse macrophages RAW 264.7. We identified Tpl-2 and MEKK1 as the kinases that are primarily responsible for the down-regulation of apoE promoter activity by LPS. Using a dominant negative form of IkappaB, we established that Tpl-2 and MEKK1 signaling pathways converge to NF-kappaB acting on the apoE core promoter -55/+73. In addition to NF-kappaB activation, LPS also activated c-Jun via its phosphorylation by JNK. The activity of the apoE promoter was repressed by c-Jun, whereas small interference RNA-mediated inhibition of endogenous c-Jun expression reversed the inhibitory effect of Tpl-2 on the apoE promoter. Transfection experiments and DNA binding assays showed that the binding site for c-Jun is in the -55/+73 region of the apoE promoter. Finally, we showed that LPS inhibited apoE gene expression via activation of the Tpl-2/MEK/ERK pathway acting on a different apoE promoter region. In summary, LPS represses apoE gene expression in macrophages via signaling pathways that involve the upstream kinases Tpl-2 and MEKK1, the intermediate mitogen-activated protein kinases ERK and JNK, and the downstream transcription factors AP-1 and NF-kappaB that inhibit the apoE promoter activity via distinct regions.