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. The discovery of nuclear receptors that are activated by lipids defined a new paradigm 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 in disease.
Transcriptional Control of Lipid Homeostasis
The liver X receptors (LXRs) maintain cholesterol homeostasis by coordinating the tissue-specific expression of genes involved in sterol transport and metabolism. Loss of LXRs in mice dramatically accelerates atherosclerosis. This increased disease burden results from both the inhibition of cholesterol efflux from peripheral cells to the liver and the inhibition of hepatic cholesterol excretion. Efforts to elucidate the molecular mechanisms by which LXRs modulate metabolism have uncovered novel signaling pathways and potential opportunities for drug development. Continuing studies are aimed at delineating the functions and mechanisms of action of new metabolic effectors in lipid homeostasis and disease. Examples of such effectors include the E3 ubiquitin ligase Idol (inducible degrader of the low-density lipoprotein receptor [LDLR]) and the phospholipid-remodeling enzyme Lpcat3.
Lipid-Dependent Protein Degradation in Metabolic Control
Analysis of the LXR pathway led to the discovery of a previously unrecognized mechanism for the regulation of lipoprotein uptake. LXR inhibits the LDLR pathway through the induction of the E3 ubiquitin ligase Idol. We have dissected the molecular interactions underlying the LXR-Idol-LDLR axis and begun to delineate the role of this circuit in physiology. We showed that Idol interacts with UBE2D1 to accomplish LDLR ubiquitination, and with John Schwabe's group (University of Leicester), we solved the structure of the Idol-UBE2D1 complex. We further showed that the Idol FERM domain binds a conserved recognition motif in its targets and that ubiquitinated LDLR is sorted to the lysosome through the multivesicular body (MVB) pathway. Supporting the relevance of Idol for human metabolism, we identified a single nucleotide polymorphism (SNP) in the human IDOL gene that is associated with LDL levels. Consistent with our own work, others independently reported that a complete loss-of-function IDOL mutation is correlated with low plasma LDL levels and that the IDOL locus is linked with response to statin treatment.
Knockout of Idol expression in cells confirmed its importance in determining LDLR protein levels. Surprisingly, however, the activity of the hepatic LXR-Idol pathway in vivo is species-specific. Idol-deficient mice show minimal change in LDL levels. By contrast, treatment of monkeys with an LXR agonist reduces LDLR protein and raises plasma LDL. Knockdown of Idol in monkeys with an antisense oligonucleotide blunts these effects. Our results highlight critical differences in metabolic responses between species and support further investigation into Idol inhibition as a strategy for LDL lowering in humans.
Recent work suggests that Idol may play an unexpected role in lipoprotein receptor pathways in the brain. We are working to identify the signals that activate Idol in the brain and to define Idol's function in development and memory. These studies may provide insight into links between lipid metabolism and neuronal biology and diseases such as Alzheimer's.
Dynamic Control of Phospholipid Metabolism Integrates Lipid and Inflammatory Signaling
Phospholipids are important structural components of biological membranes and precursors of numerous signaling molecules. We have uncovered a novel mechanism whereby changes in sterol metabolism are linked to membrane composition. LXRs control phospholipid acylation through induction of the gene encoding lysophosphatidylcholine acyltransferase 3 (Lpcat3). This enzyme esterifies unsaturated fatty acids at the sn-2 position of phospholipids. Our elucidation of LXR-Lpcat3 regulation provides an unprecedented opportunity to study the physiological and pathophysiological consequences of manipulation of membrane phospholipid composition in vivo. It is expected that modifications of the fatty acyl composition of cell membranes would affect cellular processes; however, there is little understanding of regulatory pathways that control membrane composition. Furthermore, there have heretofore been few if any experimental strategies to acutely change membrane phospholipid composition in living animals. Our work has the potential to elucidate how manipulation of membrane composition can be used as a regulatory mechanism to modulate metabolic pathways.
We find that loss of Lpcat3 has profound effects on lipid metabolism in cells and mice. Lpcat3-dependent membrane remodeling facilitates cholesterol efflux by ABC transporters. Furthermore, loss of Lpcat3 activity in vivo inhibits hepatic and intestinal lipoprotein production. Study of the LXR-Lpact3 axis also has the potential to revise the accepted paradigm of how LXRs regulate inflammation. Our data indicate that alteration of phospholipid composition is a major mechanism by which LXRs antagonize inflammatory responses. We expect that understanding the molecular mechanisms by which the LXR-Lpcat3 pathway integrates sterol, triglyceride, and phospholipid metabolism will bring insights relevant for the understanding of metabolic disease. Since Lpcat3 is an enzyme, it would theoretically be possible to target it with a small-molecule inhibitor. Thus, our studies could also provide initial validation for a new strategy for intervention in metabolic disease.
LXR-Dependent Lipid Metabolism and the Pathogenesis of Insulin Resistance
We endeavor to integrate our study of discrete regulatory pathways with an analysis of the impact of these circuits on complex disease phenotypes. As one example, excessive hepatic lipogenesis is a hallmark feature of diabetes, although the causal relationship between tissue lipid accumulation and insulin resistance is unclear. Given the ability of LXR signaling to affect hepatic lipid levels we hypothesized that loss of LXRs would impact the development of metabolic disease in mice. Surprisingly, mice deficient in both LXRa and LXRb, when bred onto the obese (leptin-deficient) background (LOKO mice), are not protected from obesity. They are, however, completely protected from hepatic steatosis and show marked improvement in insulin sensitivity. Impaired hepatic lipogenesis in LOKO mice is accompanied by reciprocal increases in adipose lipid storage, reflecting tissue-selective effects on lipogenic pathways. LXRs are required for obesity-driven SREBP-1c and ChREBP activity in liver, but not fat. Loss of LXRs actually promotes adipose PPARg and ChREBP activity, leading to improved insulin sensitivity. This work illustrates how sterol sensing by LXRs in obesity is critically linked with lipid and glucose homeostasis and insulin signaling.
This work is also supported by grants from the National Institutes of Health.
As of October 15, 2014