Reactive oxygen species (ROS) are transient, highly reactive intermediates or byproducts produced during oxygen metabolism. However, when innate mechanisms are unable to cope with sequestration of surplus ROS, it causes oxidative stress, where excess ROS damage biomolecules. Oxidized phosphatidylserine (PS), a pro-apoptotic “eat me” signal, is produced in response to elevated ROS, yet, little is known of its chemical composition and metabolism. Here, we report a small molecule that generates ROS in different mammalian cells, using which we detect, characterize and study oxidized PS in mammalian cells. We describe a chemical genetic screen to identify enzymes that regulate oxidized PS in mammalian cells, and find that the lipase ABHD12 hydrolyzes oxidized PS. We validate these findings in different physiological settings including primary peritoneal macrophages, and brains from Abhd12–/– knockout mice under inflammatory stress, and in the process functionally annotate an enzyme capable of regulating oxidized PS in vivo.

Phagocytosis is an evolutionarily conserved biological process where pathogens or cellular debris are cleared by engulfing them in a membrane-enclosed cellular compartment called the phagosome. The formation, maturation, and subsequent degradation of a phagosome is an important immune response essential for protection against many pathogens. Yet, the global lipid profile of phagosomes remains unknown, especially as a function of their maturation in immune cells. Here, we show using mass spectrometry based quantitative lipidomics that the ceramide class of lipids, especially very long chain ceramides, are enriched on maturing phagosomes with a concomitant decrease in the biosynthetic precursors of ceramides. We thus posit a new function for the enzyme ceramide synthase during phagocytosis in mammalian macrophages. Biochemical assays, cellular lipid feeding experiments, and pharmacological blockade of ceramide synthase together show that this enzyme indeed controls the flux of ceramides on maturing phagosomes. We also find similar results in the primitive eukaryote Dictyostelium discoideum, suggesting that ceramide enrichment may be evolutionarily conserved and likely an indispensible step in phagosome maturation.

Insulin resistance (IR) is known to precede onset of type 2 diabetes and increased oxidative stress appears to be a deleterious factor leading to IR. In this study, we evaluated ability of Pterostilbene (PTS), a methoxylated analog of Resveratrol and a known antioxidant, to reverse palmitic acid (PA)-mediated IR in HepG2 cells. PTS prevented ROS formation and subsequent oxidative lipid damage by reducing the expression of NADPH Oxidase 3 (NOX3) in PA treated HepG2 cells. Hepatic glucose production was used as a measure of IR and PTS reversed PA-mediated increase in hepatic glucose production by reducing expression of genes coding for gluconeogenic enzymes namely glucose-6-phosphatase (G6Pase), phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PC); and their transcription factors cAMP response element binding protein (CREB) and fork head class box O (FOXO1) along with its co-activator peroxisome proliferator-activated receptor gamma co-activator-1 α (PGC1α). PTS reversed PA-mediated activation of c-jun N-terminal kinase (JNK), which in turn altered insulin signaling pathway by phosphorylating IRS-1 at Ser 307, leading to inhibition of phosphorylation of Akt and GSK3β. PTS also reduced PA-mediated lipid accumulation by reducing expression of transcription factors SREBP1c and PPARα. SREBP1c activates genes involved in fatty acid and triglyceride synthesis while PPARα activates CPT1, a rate limiting enzyme for controlling entry and oxidation of fatty acids into mitochondria. PTS, however, did not influence PA uptake confirmed by using BODIPY-labeled fluorescent C16 fatty acid analog. Thus, our data provides a possible mechanistic explanation for reversal of PA-mediated IR in HepG2 cells.

Phagocytosis is an important physiological process, which, in higher organisms, is a means of fighting infections and clearing cellular debris. During phagocytosis, detrimental foreign particles (e.g. pathogens and apoptotic cells) are engulfed by phagocytes (e.g. macrophages), enclosed in membrane-bound vesicles called phagosomes, and transported to the lysosome for eventual detoxification. During this well-choreographed process, the nascent phagosome (also called early phagosome, EP) undergoes a series of spatiotemporally regulated changes in its protein and lipid composition and matures into a late phagosome (LP), which subsequently fuses with the lysosomal membrane to form the phagolysosome. While several elegant proteomic studies have identified the role of unique proteins during phagosomal maturation, the corresponding lipidomic studies are sparse. Recently, we reported a comparative lipidomic analysis between EPs and LPs and showed that ceramides are enriched on the LPs. Further, we found that this ceramide accumulation on LPs was orchestrated by ceramide synthase 2, inhibition of which hampers phagosomal maturation. Following up on this study, here, using biochemical assays, we first show that the increased ceramidase activity on EPs also significantly contributes to the accumulation of ceramides on LPs. Next, leveraging lipidomics, we show that de novo ceramide synthesis does not significantly contribute to the ceramide accumulation on LPs, while concomitant to increased ceramides, glucosylceramides are substantially elevated on LPs. We validate this interesting finding using biochemical assays and show that LPs indeed have heightened glucosylceramide synthase activity. Taken together, our studies provide interesting insights and possible new roles of sphingolipid metabolism during phagosomal maturation.

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