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Quantitative proteomic analysis of the effect of 24(S),25-epoxycholesterol on SN4741 neuron cells. / Ian Richard Gilmore
Swansea University Author: Ian Richard, Gilmore
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Oxysterols are oxygenated derivatives of cholesterol or its precursors. One oxysterol, 24(S),25-epoxycholesterol (24(S),25-EC), which results from a shunt in the cholesterol synthesis pathway has been found at higher than expected levels in embryonic murine brain. Interestingly, the receptor that 24...
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Oxysterols are oxygenated derivatives of cholesterol or its precursors. One oxysterol, 24(S),25-epoxycholesterol (24(S),25-EC), which results from a shunt in the cholesterol synthesis pathway has been found at higher than expected levels in embryonic murine brain. Interestingly, the receptor that 24(5),25-EC is a ligand for, Liver X Receptor (LXR), has been implicated in neurogenesis in the ventral mid brain region of embryonic brain; an area with a high density of dopaminergic neurons. The mechanism by which LXR induces this effect is unclear. Therefore, proteomic and phosphoproteomic studies were performed using a stable isotope labelled in amino acid in cell culture (SILAC) approach in order to quantify changes in the proteome between different treatment groups in a mouse substantia nigra dopaminergic cell line (SN4741) SN4741 cells were cultured in SILAC media containing differentially isotope labelled arginine and lysine. For protein expression studies SN4741 cells were treated in serum free media with vehicle, 10muM 24(S),25-EC, or 1muM GW3965, a synthetic ligand of LXR, for 24 hours. For analysis of changes in the phosphoproteome SN4741 cells were treated in serum free media with vehicle, 10muM 24(5),25-EC, or 30muM 25- hydroxycholesterol for 6 hours. Cells were lysed and protein combined in a 1:1 ratio before trypsin digestion and peptide separation via strong cation exchange chromatography. Phosphopeptides were enriched using immobilised metal affinity chromatography (IMAC). Resulting fractions were analysed, using a data dependent LC-MS/MS method. Data was quantified using MaxQuant software in conjunction with Mascot using an IPl mouse database. In protein expression analysis known oxysterol regulated genes, via SREBP or LXR, were differentially expressed. Oxysterol treatment induced global changes in proteins involved in lipid (cholesterol, fatty acid, phospholipid, triglyceride) synthesis. LXR? protein expression increased after GW3965 and 24(5),25-EC treatment, though no change was seen on LXRp mRNA, implying that ligand binding protects LXR? from degradation. 24(S),25-EC induced changes in expression and localisation of the membrane protein caveolin-1. Also, phosphoethanolamine cytidylyltransferase and collagen type IV alpha-3-binding protein, 2 proteins involved in phospholipid synthesis, had an altered expression after 24(S),25-EC treatment suggesting a role for oxysterols in membrane homeostasis. A cytokine, macrophage colony stimulating factor, which is required for normal neuronal development and macrophage differentiation had an LXR independent increased expression after 24(S),25-EC treatment. Quantitative RT-PCR data demonstrated that proteomic changes were due to both transcriptional and post-transcriptional effects of oxysterol. In addition, studies examining changes in the mouse phosphoproteome identified a number of novel phosphorylation sites.
Swansea University Medical School