The macrophage LBP gene is an LXR target that promotes macrophage survival and atherosclerosis

The liver X receptors (LXRs) are members of the nuclear receptor superfamily that regulate sterol metabolism and inflammation. We sought to identify previously unknown genes regulated by LXRs in macrophages and to determine their contribution to atherogenesis. Here we characterize a novel LXR target gene, the lipopolysaccharide binding protein (LBP) gene. Surprisingly, the ability of LXRs to control LBP expression is cell-type specific, occurring in macrophages but not liver. Treatment of macrophages with oxysterols or loading with modified LDL induces LBP in an LXR-dependent manner, suggesting a potential role for LBP in the cellular response to cholesterol overload. To investigate this further, we performed bone marrow transplant studies. After 18 weeks of Western diet feeding, atherosclerotic lesion burden was assessed revealing markedly smaller lesions in the LBP(-/-) recipients. Furthermore, loss of bone marrow LBP expression increased apoptosis in atherosclerotic lesions as determined by terminal deoxynucleotidyl transferase dUTP nick end labeling staining. Supporting in vitro studies with isolated macrophages showed that LBP expression does not affect cholesterol efflux but promotes the survival of macrophages in the setting of cholesterol loading. The LBP gene is a macrophage-specific LXR target that promotes foam cell survival and atherogenesis.

Abstract 619: A Role for Macrophage Lipopolysaccharide Binding Protein in Atherosclerosis Development

Introduction: The liver X receptors (LXRs) are members of the nuclear receptor superfamily that regulate sterol metabolism and inflammation. Knockout of LXRs in macrophages greatly accelerates the development of atherosclerosis whereas pharmacological LXR activation has potent athero-protective effects. Previous studies have shown that deletion of LXR target genes in hematopoietic cells can have opposing effects on atherosclerosis development. For example, deletion of ABCA1 using bone marrow transfer in atherogenic mouse models showed increases atherosclerotic plaque burden where as knockout studies of ABCG1 and Aim1 decreased lesion formation in bone marrow transplantation experiments owing to an increase in macrophage apoptosis.

Rationale: We sought to identify previously unknown genes regulated by LXRs in macrophages and to determine their contribution to atherogenesis.

Methods & Results: We demonstrate that the lipopolysaccharide binding protein (LBP), a secreted glycoprotein, is an LXR target gene in macrophages. Treatment of mouse peritoneal macrophages with modified LDL or oxysterols induces LBP expression, an effect that was lost in LXRαβ-/- macrophages, consistent with a role for LBP in the cellular response to cholesterol overload. To investigate this further, we performed bone marrow transplant studies using WT or LBP-/- donors and LDLR-/- recipients. After 18 weeks of western diet feeding atherosclerotic lesion burden was assessed by en face and aortic root section analysis. LDLR-/- mice receiving LBP-/- bone marrow had markedly smaller lesions compared to those receiving WT (P<0.0001 in en face analysis; P<.01 in root section analysis). Furthermore, loss of bone marrow LBP expression led to a strong increase in apoptosis in atherosclerotic lesions as determined by TUNEL staining. In vitro studies with isolated macrophages and stable cell lines showed that LBP expression does not affect cholesterol efflux but promotes the survival of macrophages in the setting of cholesterol loading.

Conclusions: The LBP gene is a macrophage LXR target that promotes foam cell survival and atherogenesis. Our study identifies LBP as a potential diagnostic and therapeutic target in human cardiovascular disease.

ZIP4 is a novel diagnostic and prognostic marker in human pancreatic cancer: a systemic comparison between EUS-FNA and surgical specimens

Aberrant expression of a zinc transporter ZIP4 in pancreatic ductal adenocarcinoma (PDAC) has been shown to contribute to tumor progression and is a potential target for individualized therapy. The overall objective of this study was to determine whether ZIP4 could serve as a novel diagnostic and prognostic marker in human PDAC, and if it can be assessed by minimally invasive sampling using endoscopic ultrasound guided fine needle aspiration (EUS-FNA). Immunohistochemistry was performed to compare ZIP4 expression in the PDAC samples obtained from EUS-FNA and matched surgical tumors (parallel control). Samples were reported by sensitivity, specificity, and predictive values, all with 95% confidence intervals (CI). A total of 23 cases with both FNA and surgical specimens were evaluated. We found that ZIP4 was significantly overexpressed in tumor cells from both sets of samples. The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of ZIP4 for the diagnosis of PDAC were 72.9%, 72.5%, 76.1%, and 69.0% in EUS-FNA samples, and were 97.9%, 65.4%, 83.9%, and 94.4% in surgical specimens, respectively. The association between the positive rate of ZIP4 expression in FNA and surgical samples is statistically significant (P=0.0216). Both the intensity and percentage of ZIP4 positive cells from the surgical samples correlated significantly with tumor stage (P=0.0025 and P=0.0002). ZIP4 intensity level in FNA samples was significantly associated with tumor differentiation and patient survival. These results indicate that EUS-FNA is capable of non-operative detection of ZIP4, thus offering the potential to direct pre-operative detection and targeted therapy of PDAC.

Multi-valued logic gates based on ballistic transport in quantum point contacts

Multi-valued logic gates, which can handle quaternary numbers as inputs, are developed by exploiting the ballistic transport properties of quantum point contacts in series. The principle of a logic gate that finds the minimum of two quaternary number inputs is demonstrated. The device is scalable to allow multiple inputs, which makes it possible to find the minimum of multiple inputs in a single gate operation. Also, the principle of a half-adder for quaternary number inputs is demonstrated. First, an adder that adds up two quaternary numbers and outputs the sum of inputs is demonstrated. Second, a device to express the sum of the adder into two quaternary digits [Carry (first digit) and Sum (second digit)] is demonstrated. All the logic gates presented in this paper can in principle be extended to allow decimal number inputs with high quality QPCs.

LXRs regulate ER stress and inflammation through dynamic modulation of membrane phospholipid composition

The fatty acyl composition of phospholipids determines the biophysical character of membranes and impacts the function of membrane proteins. Here, we define a nuclear receptor pathway for the dynamic modulation of membrane composition in response to changes in cellular lipid metabolism. Ligand activation of liver X receptors (LXRs) preferentially drives the incorporation of polyunsaturated fatty acids into phospholipids through induction of the remodeling enzyme Lpcat3. Promotion of Lpcat3 activity ameliorates endoplasmic reticulum (ER) stress induced by saturated free fatty acids in vitro or by hepatic lipid accumulation in vivo. Conversely, Lpcat3 knockdown in liver exacerbates ER stress and inflammation. Mechanistically, Lpcat3 modulates inflammation both by regulating inflammatory kinase activation through changes in membrane composition and by affecting substrate availability for inflammatory mediator production. These results outline an endogenous mechanism for the preservation of membrane homeostasis during lipid stress and identify Lpcat3 as an important mediator of LXR effects on metabolism.

Early epigenetic downregulation of WNK2 kinase during pancreatic ductal adenocarcinoma development

Pancreatic ductal adenocarcinoma (PDAC) is usually incurable. Contrary to genetic mechanisms involved in PDAC pathogenesis, epigenetic alterations are ill defined. Here, we determine the contribution of epigenetically silenced genes to the development of PDAC. We analyzed enriched, highly methylated DNAs from PDACs, chronic pancreatitis (CP) and normal tissues using CpG island microarrays and identified WNK2 as a prominent candidate tumor suppressor gene being downregulated early in PDAC development. WNK2 was further investigated in tissue microarrays, methylation analysis of early pancreatic intraepithelial neoplasia (PanIN), mouse models for PDAC and pancreatitis, re-expression studies after demethylation, and cell growth assays using WNK2 overexpression. Demethylation assays confirmed the link between methylation and expression. WNK2 hypermethylation was higher in tumor than in surrounding inflamed tissues and was observed in PanIN lesions as well as in a PDAC mouse model. WNK2 mRNA and protein expressions were lower in PDAC and CP compared with normal tissues both in patients and mouse models. Overexpression of WNK2 led to reduced cell growth, and WNK2 expression in tissues correlated negatively with pERK1/2 expression, a downstream target of WNK2 responsible for cell proliferation. Downregulation of WNK2 by promoter hypermethylation occurs early in PDAC pathogenesis and may support tumor cell growth via the ERK-MAPK pathway.

The nuclear receptor LXRα controls the functional specialization of splenic macrophages

Macrophages are professional phagocytic cells that orchestrate innate immune responses and display remarkable phenotypic diversity at different anatomical locations. However, the mechanisms that control the heterogeneity of tissue macrophages are not well characterized. Here, we report that the nuclear receptor LXRα is essential for the differentiation of macrophages in the marginal zone (MZ) of the spleen. LXR deficient mice are defective in the generation of MZ and metallophilic macrophages, resulting in abnormal responses to blood-borne antigens. Myeloid specific expression of LXRα or adoptive transfer of wild-type monocytes rescues the MZ microenvironment in LXRα deficient mice. These results demonstrate that LXRα signaling in myeloid cells is crucial for the generation of splenic MZ macrophages and reveal an unprecedented role for a nuclear receptor in the generation of specialized macrophage subsets.

Abstract 61: LXR Agonist Treatment of Nonhuman Primates Increases LDL Cholesterol due to Decreased Hepatic LDL Receptor Expression

Reverse cholesterol transport (RCT) is a process whereby cholesterol from foam cells in atherosclerotic plaques is removed by HDL, transported to the liver, secreted into bile, and excreted in the feces. Treatment of mice with LXR agonist can significantly increase RCT and inhibit atherosclerosis. Therefore, it would be anticipated that LXR agonists would also increase RCT and be anti-atherogenic in humans. However, cynomolgus monkeys treated with the LXR agonist GW3965 had significant increases in LDLc, which could be due to elevated hepatic expression of the inducible degrader of the LDL receptor (IDOL). IDOL is an E3 ubiquitin ligase that promotes the ubiquitination of the LDL receptor (LDLR) on its cytoplasmic domain, thereby targeting it for degradation. We recently discovered that GW3965 treatment of mice caused an ~3-fold increase in IDOL expression in the small intestine and consequently increased the turnover of the LDLR. In livers of GW3965 treated mice, LDLR protein was unchanged and IDOL expression was only increased by ~20%. However, treatment of human hepatoma cells with GW3965 increased IDOL 7-fold resulting in very low levels of LDLR protein expression. Therefore, we hypothesized that in humans and nonhuman primates, LXR agonist treatment would increase LDLc by inducing IDOL-dependent degradation of hepatic LDLR. After treating cynomolgus monkeys with GW3965 for 7 days, we found that HDLc was increased by ~20% while LDLc was increased by ~100%. The increase in LDLc was accompanied by a dramatic increase in plasma apoB100 and apoE concentration and a significant decrease in hepatic LDLR protein. We are currently determining the potential role of hepatic IDOL expression on these effects. We also found that unlike mice, monkeys treated with GW3965 did not display an increase in hepatic lipids, biliary cholesterol, and fecal neutral sterol excretion. These findings show that LXR activation has very different effects on lipoprotein metabolism and RCT in monkeys compared to mice. Our data suggest that LXR agonists are likely to exert both pro- and anti-atherosclerotic effects in primates and that these complex effects must be considered in the development of LXR agonists as drugs.

Adipose subtype-selective recruitment of TLE3 or Prdm16 by PPARγ specifies lipid storage versus thermogenic gene programs

Transcriptional effectors of white adipocyte-selective gene expression have not been described. Here we show that TLE3 is a white-selective cofactor that acts reciprocally with the brown-selective cofactor Prdm16 to specify lipid storage and thermogenic gene programs. Occupancy of TLE3 and Prdm16 on certain promoters is mutually exclusive, due to the ability of TLE3 to disrupt the physical interaction between Prdm16 and PPARγ. When expressed at elevated levels in brown fat, TLE3 counters Prdm16, suppressing brown-selective genes and inducing white-selective genes, resulting in impaired fatty acid oxidation and thermogenesis. Conversely, mice lacking TLE3 in adipose tissue show enhanced thermogenesis in inguinal white adipose depots and are protected from age-dependent deterioration of brown adipose tissue function. Our results suggest that the establishment of distinct adipocyte phenotypes with different capacities for thermogenesis and lipid storage is accomplished in part through the cell-type-selective recruitment of TLE3 or Prdm16 to key adipocyte target genes.

Associations between coronary heart disease and individual components of the metabolic syndrome according to glucose tolerance status

OBJECTIVE

To assess whether the contributions of individual metabolic syndrome components to coronary heart disease (CHD) risk vary in patients with different glucose tolerance.

METHODS

A total of 1619 patients were included in this cross-sectional study. CHD, metabolic syndrome and glucose tolerance were assessed using coronary angiography, anthropometric and biochemical parameters, and an oral glucose tolerance test, respectively. Associations between CHD and components of metabolic syndrome were determined using logistic regression analysis.

RESULTS

Low high-density lipoprotein-cholesterol (HDL-C) was the only CHD risk factor in patients with both CHD and metabolic syndrome who had normal glucose tolerance, after adjustments for age, smoking and low-density lipoprotein-cholesterol (LDL-C) concentration. In patients with CHD plus metabolic syndrome and prediabetes, the most important risk factor was hypertension; additional risk factors were high postprandial blood glucose (PBG) and low HDL-C. In patients with CHD plus metabolic syndrome and diabetes, high PBG was the strongest risk factor, followed by hypertension, high FBG and high waist circumference.

CONCLUSIONS

Individual components of metabolic syndrome contributed variously to CHD across different glucose tolerance statuses.

Bone marrow NR4A expression is not a dominant factor in the development of atherosclerosis or macrophage polarization in mice

The formation of the atherosclerotic lesion is a complex process influenced by an array of inflammatory and lipid metabolism pathways. We previously demonstrated that NR4A nuclear receptors are highly induced in macrophages in response to inflammatory stimuli and modulate the expression of genes linked to inflammation in vitro. Here we used mouse genetic models to assess the impact of NR4A expression on atherosclerosis development and macrophage polarization. Transplantation of wild-type, Nur77^−/−, or Nor1^−/− null hematopoetic precursors into LDL receptor (LDLR)^−/− recipient mice led to comparable development of atherosclerotic lesions after high-cholesterol diet. We also observed comparable induction of genes linked to M1 and M2 responses in wild-type and Nur77-null macrophages in response to lipopolysaccharides and interleukin (IL)-4, respectively. In contrast, activation of the nuclear receptor liver X receptor (LXR) strongly suppressed M1 responses, and ablation of signal transductor and activator of transcription 6 (STAT6) strongly suppressed M2 responses. Recent studies have suggested that alterations in levels of Ly6C^lo monocytes may be a contributor to inflammation and atherosclerosis. In our study, loss of Nur77, but not Nor1, was associated with decreased abundance of Ly6C^lo monocytes, but this change was not correlated with atherosclerotic lesion development. Collectively, our results suggest that alterations in the Ly6C^lo monocyte population and bone marrow NR4A expression do not play dominant roles in macrophage polarization or the development of atherosclerosis in mice.

Skeletal muscle Nur77 expression enhances oxidative metabolism and substrate utilization

Mitochondrial dysfunction has been implicated in the pathogenesis of type 2 diabetes. Identifying novel regulators of mitochondrial bioenergetics will broaden our understanding of regulatory checkpoints that coordinate complex metabolic pathways. We previously showed that Nur77, an orphan nuclear receptor of the NR4A family, regulates the expression of genes linked to glucose utilization. Here we demonstrate that expression of Nur77 in skeletal muscle also enhances mitochondrial function. We generated MCK-Nur77 transgenic mice that express wild-type Nur77 specifically in skeletal muscle. Nur77-overexpressing muscle had increased abundance of oxidative muscle fibers and mitochondrial DNA content. Transgenic muscle also exhibited enhanced oxidative metabolism, suggestive of increased mitochondrial activity. Metabolomic analysis confirmed that Nur77 transgenic muscle favored fatty acid oxidation over glucose oxidation, mimicking the metabolic profile of fasting. Nur77 expression also improved the intrinsic respiratory capacity of isolated mitochondria, likely due to the increased abundance of complex I of the electron transport chain. These changes in mitochondrial metabolism translated to improved muscle contractile function ex vivo and improved cold tolerance in vivo. Our studies outline a novel role for Nur77 in the regulation of oxidative metabolism and mitochondrial activity in skeletal muscle.

LXRα is uniquely required for maximal reverse cholesterol transport and atheroprotection in ApoE-deficient mice

The liver X receptor (LXR) signaling pathway is an important modulator of atherosclerosis, but the relative importance of the two LXRs in atheroprotection is incompletely understood. We show here that LXRα, the dominant LXR isotype expressed in liver, plays a particularly important role in whole-body sterol homeostasis. In the context of the ApoE^−/− background, deletion of LXRα, but not LXRβ, led to prominent increases in atherosclerosis and peripheral cholesterol accumulation. However, combined loss of LXRα and LXRβ on the ApoE^−/− background led to an even more severe cholesterol accumulation phenotype compared to LXRα^−/−ApoE^−/− mice, indicating that LXRβ does contribute to reverse cholesterol transport (RCT) but that this contribution is quantitatively less important than that of LXRα. Unexpectedly, macrophages did not appear to underlie the differential phenotype of LXRα^−/−ApoE^−/− and LXRβ^−/−ApoE^−/− mice, as in vitro assays revealed no difference in the efficiency of cholesterol efflux from isolated macrophages. By contrast, in vivo assays of RCT using exogenously labeled macrophages revealed a marked defect in fecal sterol efflux in LXRα^−/−ApoE^−/− mice. Mechanistically, this defect was linked to a specific requirement for LXRα^−/− in the expression of hepatic LXR target genes involved in sterol transport and metabolism. These studies reveal a previously unrecognized requirement for hepatic LXRα for optimal reverse cholesterol transport in mice.