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Lipid Signaling Pathways in Physiology and Disease
Summary: Peter Tontonoz studies the regulation of gene expression by nuclear receptors and the relationship of these signaling pathways to human diseases such as obesity, diabetes, and atherosclerosis.
Obesity, diabetes, and cardiovascular disease are the leading causes of morbidity and mortality in industrialized societies. The common thread that links these disorders is dysregulation of lipid metabolism. Our long-term goal is to understand the mechanisms whereby lipids control gene expression and impact the development of metabolic disease. With the discovery of nuclear receptors that are activated by lipids, the past 10 years have seen a new paradigm emerge for the transcriptional regulation of metabolic pathways. Dissection of these pathways is advancing our understanding of basic mechanisms that control metabolism and highlighting new opportunities for therapeutic intervention.
Nuclear Receptors Are Central Regulators of Lipid Metabolism
It is well established that high serum cholesterol levels increase the risk of cardiovascular disease. A central question that we have endeavored to understand is why this is true. How do lipids control gene expression in cells of the artery wall, and how do these gene expression changes relate to the development of disease? Our work has shown that lipids regulate gene expression in macrophages through three distinct nuclear receptor pathways: PPARs (peroxisome proliferator-activated receptors), LXRs (liver X receptors), and NR4As.
LXR: a new molecular target in atherosclerosis. A principal function of LXR in macrophages is to promote cholesterol removal from the cell. Activation of LXR induces a number of proteins linked to cholesterol efflux, including ABCA1, ABCG1, and apolipoprotein E. The observation that the LXR pathway is critical for cholesterol efflux in vitro led us to explore the role of these receptors in the context of the atherosclerotic lesion. We have used both gain-of-function and loss-of-function studies to test the role of LXRs in atherosclerosis. Treatment of atherosclerosis-susceptible mice with a synthetic LXR ligand reduces lesion formation. Conversely, mice transplanted with LXR-null bone marrow developed accelerated atherosclerosis. These studies establish a central role for LXR in macrophage cholesterol homeostasis and have stimulated interest in the development of LXR agonists as cardiovascular therapeutics. Recently, we have shown that targeting LXRβ with synthetic agonists inhibits atherosclerosis in the absence of LXRα. Since the major side effects of LXR agonists (hypertriglyceridemia and fatty liver) are due to LXRα, these findings provide strong support for drug development strategies targeting LXRβ.
Nuclear Receptors as Integrators of Metabolic and Immune Signaling in Macrophages
Inflammation is an integral component of metabolic disorders, including atherosclerosis and type 2 diabetes. An unexpected insight to emerge from our study of LXR is the recognition that lipid metabolic and inflammatory signaling pathways in macrophages are intimately connected.
Crosstalk between inflammatory signaling and LXRs. In addition to its role in cholesterol metabolism, LXR is also an important modulator of inflammatory responses. Global analysis of gene expression in activated macrophages has revealed that LXRs inhibit many genes involved in inflammation while simultaneously inducing those involved in lipid metabolism. Activation of LXR inhibits the expression of inflammatory mediators in response to LPS or cytokine stimulation in vitro and ameliorates inflammatory responses in vivo.
We have also found that inflammatory stimuli inhibit LXR function. Recently we have uncovered a novel mechanism by which microbial pathogens may accelerate atherosclerotic lesion formation: interference with LXR-dependent cholesterol metabolism. Expression of LXR target genes such as ABCA1 is severely compromised during infection of macrophages with either Escherichia coli or influenza A. Dissection of the TLR signaling pathway revealed that pathogen inhibition of LXR is mediated by the viral response transcription factor IRF3. We are testing the hypothesis that LXR-TLR crosstalk is important for the development of atherosclerosis.
Nuclear receptors in the innate immune response. The macrophage LXR signaling pathway also impacts antimicrobial responses and cell survival. Mice lacking LXRs are highly susceptible to infection with the intracellular bacteria Listeria monocytogenes. LXR-null macrophages undergo accelerated apoptosis when challenged with Listeria and exhibit defective bacterial clearance. These defects are associated with loss of regulation of the anti-apoptotic factor SPα/AIM, an LXR target gene. This work provided the first evidence that the LXR pathway mediates macrophage responses to both modified lipoproteins and intracellular pathogens. An important goal over the next few years is to define the links between lipid metabolism and immunity.
A role for LXR signaling in the pathogenesis and treatment of Alzheimer's disease. Alzheimer's disease (AD) is an age-dependent neurodegenerative disease resulting in progressive cognitive impairment. The initiation and progression of AD pathology is linked to cholesterol metabolism and inflammation, processes that can be modulated by LXRs. We have recently shown that endogenous LXR signaling impacts the development of AD-related pathology. Loss of either Lxrα or Lxrβ in APP/PS1 transgenic mice results in increased amyloid plaque load, and this effect is linked to LXR-dependent alterations in both lipid metabolism and inflammation. In the brain, LXRs positively regulate expression of key genes involved in cholesterol homeostasis and act as potent inhibitors of inflammation. Ligand activation of LXRs attenuates the inflammatory response of primary mixed glial cultures to fibrillar amyloid-β peptide in a receptor-dependent manner. These results identify endogenous LXR signaling as an important determinant of AD pathogenesis in mice. Because of their ability to modulate both lipid metabolic and inflammatory gene expression in the brain, LXRs may be tractable targets for the treatment of AD.
Molecular Dissection of Adipocyte Differentiation and Gene Expression
Deciphering the mechanism of action of bioactive small molecules can bring new insight into complex biological processes. We propose that the use of chemical genetics to dissect adipocyte differentiation will uncover novel signaling pathways controlling lipid metabolism. We are conducting high-throughput phenotypic screening for chemical modulators of adipogenesis. Our long-term goal is to identify small molecules that target each of the key adipogenic signaling pathways. Although several such pathways have been identified (e.g., insulin signaling, PPAR, C/EBP), we suspect that additional pathways remain to be discovered, and we predict that small molecules will be useful in elucidating each of these pathways.
At present, the PPARγ agonist thiazolidinedione drugs are the principal class of drugs available for improvement of insulin sensitivity in obese diabetic subjects. Despite their efficacy, the development and clinical use of PPARγ ligands is limited by adverse effects. Since PPARγ agonists ameliorate obesity-related insulin resistance by inducing PPAR target genes in adipocytes, we have speculated that small molecules that promote adipocyte differentiation and/or regulate PPARγ target genes by distinct mechanisms could be candidates for novel insulin sensitizers. Using our chemical screening approach, we identified a small molecule that targets the PPARγ pathway by a distinct mechanism. This molecule, harmine, is not a ligand for the receptor; rather, it acts as a cell-type-selective regulator of PPARγ expression. Administration of harmine to diabetic mice mimics the effects of PPARγ ligands on adipocyte gene expression and insulin sensitivity. Unlike thiazolidinediones, however, harmine does not cause significant weight gain or hepatic lipid accumulation. Molecular studies indicate that harmine controls PPARγ expression through inhibition of the Wnt signaling pathway. This work validates phenotypic screening of adipocytes as a promising strategy for the identification of bioactive small molecules and suggests that regulators of PPARγ expression may be a complementary approach to PPARγ ligands in the treatment of insulin resistance.
This work is also supported by grants from the National Institutes of Health.
Last updated October 23, 2008
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