Lipolytic and post-lipolytic remodelling of lipid droplets in adipocytes (#29)
Lipid droplets (LDs), neutral lipid containing cytoplasmic organelles, are the major lipid storage structures in mammalian cells, the correct functioning of which is vital for many aspects of cell biology and metabolic physiology. LDs vary greatly in size from <1 to >100µm diameter and play a crucial role in maintaining the cellular levels of lipids and cholesterol by regulating the interplay between storage, hydrolysis (lipolysis) and trafficking. In adipocytes, a single, very large, central LD dominates lipid storage. However, in all cell types the number, size and distribution of LDs is tightly controlled, a fact that becomes an important consideration during lipolysis. The surface of LDs forms a platform on which complex mechanisms control the breakdown and formation of triglycerides, to both protect the cell from the cytotoxic effects of free fatty acids and to release the appropriate amounts of free fatty acids in response to physiological needs. Stimulation of lipolysis in adipocytes results in the generation of a multitude of dispersed microLDs, which are predicted to be active sites of lipolysis, and likely comprise the major component of lipolytic activity in the cell. Concomitant with this, alterations in the surface composition of the large, central LDs can also be detected. Sequential phosphorylation and translocation to LDs of the major diacylglycerol lipase, HSL, suggests a close mechanistic connection between the cell surface and the LD surface during lipolysis. Post-lipolytic remodelling of LDs is stimulated by insulin, resulting in the reversal of the microLD formation and reversion to a pre-lipolytic state through a combination of lipid transfer and LD fusion. Together these data suggest that LDs are very tightly regulated in adipocytes and that complex mechanisms are in place to ensure the appropriate magnitude and longevity of fatty acid release in response to lipolytic stimulation.