Abstract
Extracellular vesicles (EVs) are well-known mediators in intercellular communication playing pivotal roles in promoting liver inflammation and fibrosis, events associated to hepatic lipotoxicity caused by saturated free fatty acid overloading. However, despite the importance of lipids in EV membrane architecture which, in turn, affects EV biophysical and biological properties, little is known about the lipid asset of EVs released under these conditions. Here, we analyzed phospholipid profile alterations of EVs released by hepatocarcinoma Huh-7 cells under increased membrane lipid saturation induced by supplementation with saturated fatty acid palmitate or Δ9 desaturase inhibition, using oleate, a nontoxic monounsaturated fatty acid, as control. As an increase of membrane lipid saturation induces endoplasmic reticulum (ER) stress, we also analyzed phospholipid rearrangements in EVs released by Huh-7 cells treated with thapsigargin, a conventional ER stress inducer. Results demonstrate that lipotoxic and/or ER stress conditions induced rearrangements not only into cell membrane phospholipids but also into the released EVs. Thus, cell membrane saturation level and/or ER stress are crucial to determine which lipids are discarded via EVs and EV lipid cargos might be useful to discriminate hepatic lipid overloading and ER stress.
Highlights
Extracellular vesicles (EVs) are well-known mediators in intercellular communication playing pivotal roles in promoting liver inflammation and fibrosis, events associated to hepatic lipotoxicity caused by saturated free fatty acid overloading
To characterize EVs released under membrane lipid saturation and/or endoplasmic reticulum (ER) stress conditions, Huh-7 cells were treated for 16 h with PA (400 μM) or with a SCD1 inhibitor (CAY 10,566, 5 μM)[24] to increase the saturation level of membrane phospholipids, or Tg (2.5 nM), to induce classical ER s tress[25]
Treatment of Huh-7 cells with the monounsaturated fatty acid OA did not induce relevant changes in lipid profiles of released EVs. These results suggest that the level of cell membrane saturation and/or ER stress are crucial to determine which lipids are discarded via EVs
Summary
Extracellular vesicles (EVs) are well-known mediators in intercellular communication playing pivotal roles in promoting liver inflammation and fibrosis, events associated to hepatic lipotoxicity caused by saturated free fatty acid overloading. A functional role is played by phosphatidic acid (PtdA), a small negatively charged headgroup phospholipid that favors the invagination of endosomal membranes forming ILVs, and cholesterol, that regulates MVBs transfer towards the plasma m embranes[5]. Another cone-shaped lipid that favors the invagination of the endosomal membrane is ceramide (Cer)[7]. The presence of ether lipids in EVs might have important implications for both fusion efficiency and stability in the extracellular space[12] Another characteristic of EVs is the enrichment in membrane phospholipids containing saturated fatty acids compared to originating cells[8,9,13]. A mean to protect these mediators from degradation and exchange these molecules between different cell types, playing a pivotal role in their transcellular b iosynthesis[14]
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