Remodeling of Lipid Droplets during Lipolysis and Growth in Adipocytes

• Published in: The Journal of biological chemistry Vol. 287; no. 14; pp. 11164 - 11173
• Main Authors: Paar, Margret, Jüngst, Christian, Steiner, Noemi A., Magnes, Christoph, Sinner, Frank, Kolb, Dagmar, Lass, Achim, Zimmermann, Robert, Zumbusch, Andreas, Kohlwein, Sepp D., Wolinski, Heimo
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 30.03.2012; American Society for Biochemistry and Molecular Biology
• Subjects: 3T3-L1 Cells; Adipocyte; Adipocytes - cytology; Adipocytes - metabolism; Animals; CARS; Cell Biology; Cell Survival; Fragmentation; Fusion; Humans; Lipid Droplets; Lipid Metabolism; Lipids - chemistry; Lipogenesis; Lipolysis; Lipotoxicity; Mice; Molecular Imaging; Stem Cells - cytology; Surface Properties; Time Factors; Time Lapse Imaging; CARS; Adipocyte; Time Lapse Imaging; Lipid Metabolism; Lipogenesis; Lipotoxicity; Fusion; Lipolysis; Lipid Droplets; Fragmentation
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M111.316794

Synthesis, storage, and turnover of triacylglycerols (TAGs) in adipocytes are critical cellular processes to maintain lipid and energy homeostasis in mammals. TAGs are stored in metabolically highly dynamic lipid droplets (LDs), which are believed to undergo fragmentation and fusion under lipolytic and lipogenic conditions, respectively. Time lapse fluorescence microscopy showed that stimulation of lipolysis in 3T3-L1 adipocytes causes progressive shrinkage and almost complete degradation of all cellular LDs but without any detectable fragmentation into micro-LDs (mLDs). However, mLDs were rapidly formed after induction of lipolysis in the absence of BSA in the culture medium that acts as a fatty acid scavenger. Moreover, mLD formation was blocked by the acyl-CoA synthetase inhibitor triacsin C, implicating that mLDs are synthesized de novo in response to cellular fatty acid overload. Using label-free coherent anti-Stokes Raman scattering microscopy, we demonstrate that LDs grow by transfer of lipids from one organelle to another. Notably, this lipid transfer between closely associated LDs is not a rapid and spontaneous process but rather occurs over several h and does not appear to require physical interaction over large LD surface areas. These data indicate that LD growth is a highly regulated process leading to the heterogeneous LD size distribution within and between individual cells. Our findings suggest that lipolysis and lipogenesis occur in parallel in a cell to prevent cellular fatty acid overflow. Furthermore, we propose that formation of large LDs requires a yet uncharacterized protein machinery mediating LD interaction and lipid transfer. Background: Micro-lipid droplets (mLDs) appear in adipocytes upon lipolytic stimulation. LDs may grow by spontaneous, homotypic fusion. Results: Scavenging of fatty acids prevents mLD formation. LDs grow by a slow transfer of lipids between LDs. Conclusion: mLDs form due to fatty acid overflow. LD growth is a controlled process. Significance: Novel mechanistic insights into LD remodeling are provided.