Dynamics and regulation of lipid droplet formation in lipopolysaccharide (LPS)-stimulated microglia

Abstract

Lipid droplets (LDs) are neutral lipid-rich organelles involved in many cellular processes. A well-known example is their accumulation in leukocytes upon activation by pro-inflammatory stimuli such as lipopolysaccharides (LPS) derived from gram-negative bacteria. A role of LDs and LD-associated proteins during inflammation in the brain is unknown, however. We have now studied their dynamics and regulation in microglia, the resident immune cells in the brain. We find that LPS treatment of microglia leads to the accumulation in them of LDs, and enhancement of the size of LDs. This induction of LDs was abolished by triacsin C, an inhibitor of triglyceride biosynthesis. LPS strongly activated c-Jun N-terminal kinase (JNK) and p38 MAPK stress signaling pathways and increased the expression of LD-associated protein perilipin-2 (ADRP) in a time-dependent manner. Immunostaining showed that perilipin-2 in LPS-treated microglia predominantly colocalized with LDs. Inhibitors of p38 α/β (SB203580) and PI3K/Akt pathway (LY294002), but not that of JNK (SP600125), reduced LPS-induced LD accumulation and eliminated the activating effect of LPS on perilipin-2. In addition, cytosolic phospholipase A 2 (cPLA 2 -α), a key enzyme for arachidonic acid release, colocalized with LPS-induced LDs. These observations suggest that LDs may play an important role in eicosanoid synthesis in activated microglia; they provide a novel insight into the regulation of LDs in inflammatory cells of the brain and point to a potential role of p38 α/β in LPS-induced LD accumulation. Collectively, our findings imply that LD formation and perilipin-2 induction could be microglial biomarkers of inflammation in the central nervous system.

Pharmacological inhibition of lipid droplet formation enhances the effectiveness of curcumin in glioblastoma

“…Our results show that pyrrolidine-2 pre-treatment dramatically increased the effectiveness of curcumin in U251N cells after 72 h, virtually abolishing cell viability at curcumin concentrations above 20 lM. Interestingly, the lipid droplet number of cells treated with curcumin following pyrrolidine-2 sensitization was higher than that of cells treated with curcumin alone -likely due to the drug-induced cellular stress, previously reported to induce lipid droplet formation [19,69,70]. Sensitization using BSO was comparatively less effective, likely due to the diversity of antioxidants and enzymes that can reduce ROS levels.…”

Simultaneous activation of p38 and JNK by arachidonic acid stimulates the cytosolic phospholipase A2-dependent synthesis of lipid droplets in human monocytes

“…Likewise, cells depleted of cPLA 2 ␣ by RNA silencing result in the appearance of tubulo-vesicular profi les of the smooth endoplasmic reticulum, compatible with a role of cPLA 2 ␣ in regulating the structure of this organelle (11). cPLA 2 ␣ has recently been reported to translocate to LD after cell activation (24,25,63). Studies on cPLA 2 ␣ targeting to membranes have indicated that, in addition to increased Ca 2+ availability, there are other factors regulating the association of the enzyme to membranes, including the local concentration of phosphoinositides in the membrane (64,65) and the phosphorylation state of the cPLA 2 ␣ (66).…”

Phospholipase A2 regulation of lipid droplet formation

“…Several enzymes implicated in the biosynthesis of eicosanoids have been found to be associated with LDs under activation conditions, including cPLA 2 α [11][12][13]. Extracellular signal-regulated kinases-1 and -2, which can phosphorylate and activate cPLA 2 α, have also been found in LDs [11], as well as the major AA-converting enzymes 5-lipoxygenase, 15-lipoxygenase, 5-lipoxygenase-activating protein, and cyclooxygenase-2 [16,42].…”

“…Prominent proteins of mammalian LDs are the so-called PAT family, which includes perilipin, adipophilin and TIP47 (TailInteracting Protein of 47 kDa) [5]. Additionally, numerous enzymes related to lipid metabolism have been described, including adipose triglyceride lipase, hormone-sensitive lipase, CGI-58 (comparative gene identification-58) [6,7], CTP:phosphocholine cytidylyltransferase [8], lipin-1 [9,10], and group IVA phospholipase A 2 (cytosolic phospholipase A 2 α, cPLA 2 α) [11][12][13].…”