LIPID DROPLETS AND CELLULAR LIPID HOMEOSTASIS
Our research group integrates systems-level discovery methods and mechanistic cell biology approaches to achieve a comprehensive understanding of lipid droplet biology and lipid homeostasis in health and disease.
LIPID DROPLET BIOGENESIS AND REGULATION
Lipid droplets are dynamic neutral lipid (i.e. fat and sterol esters) storage organelles that function as hubs of lipid metabolism, providing an “on demand” source of fatty acids that can be used for energy, membrane biogenesis, and lipid signaling pathways. Lipid droplets are formed at the endoplasmic reticulum (ER) in a process involving the deposition of neutral lipids between the leaflets of the ER, followed by the emergence of the lipid droplet into the cytoplasm from the outer leaflet. The precise mechanisms by which a monolayer organelle emerges from a bilayer organelle are incompletely understood. Does this occur at specific ER subdomains? Is this driven by protein and/or lipid changes? The mature lipid droplet can be degraded by lipolysis or through a selective
autophagic pathway known as lipophagy. How these pathways are regulated under different conditions and in different cell types remain to be determined. Furthermore, alterations in lipid droplet abundance are associated with a wide variety of diseases. For example, the accumulation of large hepatic lipid droplets is the pathological hallmark of fatty liver disease. Interestingly, mutations in certain genes (e.g. TM6SF2 and PNPLA3) are associated with increased risk of fatty liver. Human genetics may provide insights into the cellular mechanisms that regulate lipid droplet biogenesis and turnover.
We are especially interested in how the lipid droplet proteome is established and regulated (see http://dropletproteome.org). Under certain conditions we observe that some lipid droplet proteins are targeted for proteolysis by ER-associated degradation (ERAD), which mediates the ubiquitin and proteasome dependent degradation of proteins from the early secretory pathway. What machinery (e.g. E3 ligase) target lipid droplet proteins for proteasomal clearance? Can proteins be ubiquitinated and extracted for proteasomal degradation directly from the lipid droplet? What are the functional implications? Does ubiquitin-dependent degradation of lipid droplet proteins contribute to the regulation of lipid droplet functions during metabolic state fluctuations?
ESTABLISHING AND REMODELING THE LIPID DROPLET PROTEOME
The functions of lipid droplets are controlled by the proteins that decorated the lipid droplets surface. These proteins are collectively known as the lipid droplet proteome and include lipid metabolic enzymes (acyl transferases and lipases) as well as signaling scaffold proteins that monitor and integrate the nutrient status of the cell. It is well established that signaling proteins control several key lipid droplet proteins through phosphorylation.
Lipid droplets do not exist in isolation, but instead are in constant contact with many organelles in the cell. These contacts enable transfer of lipids and proteins, such as the transfer of fatty acids for b-oxidation in mitochondria. Our recent findings have identified multiple examples of inter-organelle communication. We found that the degradation of select lipid droplet proteins in the ER though ERAD, regulates their levels on lipid droplets. In addition, we discovered an unexpected protective role for DGAT1-dependent lipid droplets during autophagy, in which they served as lipid buffers sequestering fatty acids and preventing lipotoxic damage to other organelles. Our current studies aim to identify and characterize the physical complexes that mediate lipid droplet association with other organelles in the cell. How are these contacts regulated? Are they impacted by the metabolic state of the cell? Furthermore, how extensive is the role of lipid droplets in preventing lipotoxicity? Under what physiological conditions is this important?
INTER-ORGANELLE CROSSTALK AND LIPID HOMEOSTASIS
MECHANISMS OF LIPOTOXICITY
Lipotoxicity generally refers to the deleterious effects that lipid can have on cellular homeostasis and function. Accumulation of different types of lipids can induce distinct forms of lipotoxicity. For example, free fatty acids can induce ER stress by incorporation into ER lipids or can cause mitochondrial damage through incorporation into acylcarnitine. We and others have found that lipid droplets can prevent fatty acid toxicity under certain conditions by sequestering fatty acids as triacylglyercerol. Some lipids play direct roles in cell death pathways. High levels of ceramide are associated with apoptotic signaling and the accumulation of oxidatively damaged phospholipids
triggers the non-apoptotic cell death pathway known as ferroptosis. Lipotoxicity has been implicated in the pathogenesis of a variety of degenerative conditions and targeted induction of lipotoxic pathways is actively being pursued as a novel chemotherapeutic strategy to treat cancer. We are interested in leveraging genetic and cell biology approaches to understand the mechanisms of different types of lipotoxicity and the cellular factors that protect against these forms of damage.