Role of Lipid Droplets in Obesity and Metabolic Syndrome Related Cardiovascular Diseases

Background

Lipid droplets (LDs) are cytosolic organelles composed of a neutral lipid core (which may contain triacylglycerols, cholesteryl esters, retinyl esters) surrounded by a phospholipid monolayer, with many proteins coating the surface. Traditionally, LDs were considered inert organelles whose sole purpose was to store lipids. This view has been overturned in the past decade, with better understanding of the biogenesis, composition, modification, and regulation of LDs. LDs, also called “intracellular lipoproteins”, are now viewed as dynamic organelles with diverse properties and functions across cell types, playing a crucial role in lipid metabolism and energy homeostasis, metabolic gene expression, intracellular lipid and membrane trafficking, viral infection, and inflammatory responses.

Current research is mostly focused on the basic biology of LDs and elucidating their roles in energy storage. Although much emphasis has been placed on understanding how LDs function in adipocytes and hepatocytes, LDs are known to be present in almost every cell type where they may interact with various proteins to exert broader functions beyond lipid storage, which may be cell-type dependent. Indeed, the composition and turnover of lipids within the LD is now recognized as an active component of metabolic signaling within both cardiomyocytes and endothelial cells which mediates the cellular response to pathological stress. Therefore, it is timely for NHLBI to leverage knowledge gained in other fields and explore the roles of LDs in cardiovascular diseases (CVDs). Specifically, a better understanding of key regulatory pathways and functions of LDs in different cell types may yield mechanistic insights into pathological processes leading to ectopic lipid storage, insulin resistance, inflammation, and associated CVD. Among the CVDs that might be affected by lipid droplet biology are atherosclerosis and heart failure, two major focuses of NHLBI.

Obesity increases the risk for many serious diseases, including hypertension, dyslipidemia, type 2 diabetes, atherosclerosis, coronary artery disease, cardiomyopathy, and atrial fibrillation. However, not all fats are equal in their contribution to CVD. Depending on when, where, and how the fat is stored in cells, it may have varying degrees of adverse effects on cardiovascular health. Elucidation of the mechanisms regulating intracellular lipid homeostasis in cardiovascular tissues and the consequences of its disruption may shed light on the molecular and physiological basis of CVD consequences associated with obesity. Understanding the differences among populations in their capacity to tolerate and adapt to obesity and resistance to CVD, as well as sex- and age-dependent changes, will provide new insights into determinants of CVD resistance.

Technical advances over the past decade in mass spectrometry, electron microscopy, high resolution live-cell imaging, nuclear magnetic resonance spectroscopy, and in particular, flux and turnover measurements using both stable isotope and radioisotope kinetics, may present an opportunity for LDs researchers to interrogate cell type and stimulus-specific regulation and function of LDs.

Discussions

The WG Presentations and discussions focused on the following areas:

• LD formation and dynamics
• Biology of LD regulation and disruption
• LDs and diseases
• Methods to assess LD biology

The WG identified several important challenges and barriers to better understanding the roles of LDs in CVD:

• Lack of dynamic imaging techniques sufficient to track LD formation in real-time

• Lack of reliable methodologies for purification and identification of intracellular lipids and their associated proteins

• Lack of concerted efforts in structural biology and enzymology needed to understand the structure and function of key molecules regulating LD formation, growth and shrinkage

• Lack of studies that integrate mechanistic studies with epidemiological findings and human genetics

• Imbalance in the knowledge base for triglyceride-rich vs. cholesteryl ester (CE)-rich LDs, with more gaps for the latter, e.g., in macrophages.

Recommendations

The WG identified the following research opportunities:

• Study biological functions and physiological roles of LDs in the cardiovascular system

  • Understand the capacity of cells and organs to store metabolic energy and how the capacity is regulated under physiological conditions

  • Comprehensively assess the key players involved in LD biology across cell types using unbiased screening approaches

  • Understand sex, race/ethnic, and age-dependent differences in LD biology that influence cardiovascular health

  • Understand how individual variations in LD biology may contribute to differences in cardiovascular health among populations and resilience to CVD

  • Understand the interaction of LDs with other intracellular organelles, such as how LDs are coupled with mitochondria to maintain energy homeostasis in cardiomyocytes

  • Understand the impact of diet and life-style on LD biology, such as the differential effects of saturated and unsaturated fatty acids on LD composition and function

  • Understand tissue-specific fluxes in energy under physiological conditions

• Investigate pathophysiological roles of LDs

  • Understand what happens when a cell exceeds its capacity to store lipid in LDs

  • Understand how abnormalities in LD function contribute to atherosclerosis, hypertension, cardiomyopathy, heart failure, and obesity- or metabolic syndrome-related CVD

  • Understand the mechanisms linking LDs to inflammation and insulin resistance

  • Understand the role of LDs in regulating gene transcription and RNA epigenetics

  • Understand how genetic and environmental factors alter LDs homeostasis and lead to pathological conditions

  • Understand how individual variations in LD biology contribute to differences in onset and progression of CVD, as well as to different responses to medications

  • Understand changes in LD composition and dynamics in response to metabolic stress

• Develop methodologies, workforce and shared resources

  • Support the development and validation of novel imaging techniques and probes that permit monitoring of spatial and temporal lipid trafficking in cells or tissues in situ in real-time with minimal perturbation to the physiological actions of the lipids

  • Support the establishment of resource centers to provide services to all investigators and develop research protocols on characterization of lipid content within LDs and proteins associated with LDs using multidisciplinary approaches. These may include innovative biochemical methods for LD extraction and purification, high throughput mass spectrometry methods for screening and identification of lipid species, and novel imaging tools to visualize the interactions of LDs with other intracellular organelles and trafficking of lipids inside and outside of cells.

  • Explore mechanisms by which NHLBI could foster collaboration and resource sharing among investigators with diverse expertise, including biochemistry and chemistry, structural biology, mass spectrometry, NMR, cardiovascular physiology, epidemiology, genetics, and bioinformatics.

  • Support training and workforce development in areas of lipidomics, imaging, mass spectrometry, and structural biology, specializing in identification and characterization of intracellular lipid trafficking in the cardiovascular system.

Publication Plans

The working group plans to prepare a manuscript for publication in a peer-reviewed journal.

NHLBI Contacts

• Jue Chen, PhD
• Michelle Olive, PhD
• Zorina Galis, PhD
• Lisa Schwartz Longacre, PhD

Division of Cardiovascular Sciences
National Heart, Lung, and Blood Institute
National Institutes of Health
Phone: 301-435-0550
Email: jue.chen@nih.gov

Working Group Participants

Co-chairs

• Ira Goldberg, MD, New York University
• Karen Reue, PhD, University of California, Los Angeles

Members

• Tobias Walther, PhD, Harvard University
• Sarah Cohen, PhD, National Institute of Child Health and Human Development
• Nada Ahumrad, PhD, Washington University in St. Louis
• Perry Bickel, MD, University of Texas Southwestern Medical Center
• James Granneman, PhD, Wayne State University
• Jean Schaffer, MD, Washington University in St. Louis
• Gerald Shulman, MD, PhD, Yale University
• Edward Fisher, MD, MPH, PhD, New York University
• Robert Murphy, PhD, University of Colorado Denver
• Douglas Lewandowski, PhD, Sanford Burnham Prebys Medical Discovery Institute

Participants from NIH

• Li-Shin Huang, PhD, NHLBI
• Ye Yan, PhD, NHLBI
• Eser Tolunay, PhD, NHLBI
• Marc Charette, PhD, NHLBI
• Jean Chin, PhD, NIGMS