Diseases of metabolic excess constitute the most prominent killers in the developed world. At the heart of these diseases lies excessive storage of triacylglycerols (triglycerides, TG), which gives rise to obesity, lipotoxicity, and associated o-morbidities, such as non-alcoholic fatty liver disease (NAFLD) and steatohepatitis (NASH). TGs are stored in cytosolic lipid droplets (LDs). Our research recently revealed two distinct classes o LDs: smaller LDs (sLD) and larger and more heterogeneous expanding LDs (eLDs). LDs of the latter class grow by local TG synthesis on the LD surface, mediated by isoforms of TG synthesis enzymes, such as GPAT4 and DGAT2. We discovered that these different LD populations are present in nearly all types of cells. However, their relative contributions to TG storage and secretion in tissues are unknown, as are their functions in physiology and metabolism. Here we propose experiments in murine hepatocytes and liver that will elucidate the functions of eLDs and sLDs in hepatic TG metabolism. We hypothesize that eLDs, conserved throughout evolution, represent the first tier of TG storage: eLD formation allows LDs to expand to a large size, accommodating the majority of synthesized TG in cells and segregating lipids from other lipid metabolic pathways and ER functions. When the capacity of eLDs for TG storage is overwhelmed, sLDs form and provide a second line of defense by esterifying these molecules. To test this model for the function of different LD classes, we will determine the physiological function of eLDs and sLDs by determining a) the relative contributions of the eLD and sLD pathways to LD formation and TG storage in hepatocytes, b) the requirements of the eLD and sLD pathways for TG secretion (as components of very low-density lipoproteins, or VLDL) in the liver, and c) the relative contributions of the eLD and sLD pathways to the development of NAFLD and NASH in the liver. Our results will provide the first insights of how different LD classes function to store or secrete TG. Moreover, they will shed light on which of these different LD classes are important for preventing the development of NASH in the setting of NAFLD. Insights from these studies should provide new information for pharmaceutical targeting of drugs to treat diseases of TG excess, such as hepatic steatosis and VLDL-related hypertriglyceridemia.
Diseases of excess lipid storage, such as obesity, type 2 diabetes, and fatty liver disease, cause huge morbidity and mortality in the developed world. At the heart of these diseases lie pathways of fat storage in cellular lipid droplets. We discovered that lipid droplets exist in two different types, and we propose to study how these different types function in fat metabolism and obesity-related diseases.
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