Sea cucumber plasmalogen regulates the lipid profile in high-fat diet mouse liver via lipophagy.

Wang Z, Wang X, Liu Y, Wang X, Meng N, Cong P, Song Y, Xu J, and Xue C. (2024) Sea cucumber plasmalogen regulates the lipid profile in high-fat diet mouse liver via lipophagy.

Plasmalogens are a unique class of phospholipid that contain a vinyl-ether bond at the sn-1 position on the glycerol backbone. This double bond causes a more compact structure in this lipid which influences the structure and fluidity of the cell membrane, vesicular fusion, and provides antioxidant properties. Plasmalogens are known to be associated to various chronic diseases linked to dysfuntional lipid metabolism, however this mechanism is not known. Previously, this group demonstrated that plasmalogens (PlsEtn) were able to activate lipophagy, the breakdown of lipids by lysosomes, and that this was able to significantly reduce the accumulation of lipids in a high-fat diet cell model. Interestingly, when they also tested plasmanylcholine (PakCho), similar in structure to the PlsEtn but it does not contain the vinyl-ether bond, it was not able to demonstrate this lipophagy activation which suggested that the double bond was necessary for this lipophagy enhancement effect. In the last literature blog, the effects of PlsEtn in a high-fat diet cell culture model findings from this group were discussed. In this edition of the literature blog, we are looking at a continuation of their work, moving from in vitro work to in vivo using a mouse model.

To complete this work, PlsEtn and PakCho were isolated from Cucumaria frondosa and made into 40 ng/mL solutions with dimethyl sulfoxide. The supplementation made up 0.1% of their diet and the mice had free access to food and water throughout the study. Five-month-old C57BL/6J male mice were split into four treatment groups: normal diet, high fat diet (HFD, 60% kcal energy is fat), HFD + PlsEtn, and HFD + PakCho. There were eight animals per group and the treatment was administered daily for 8 weeks. The authors first confirmed that their mouse model had abnormal lipid accumulation through assessing weight gain and lipid levels after eight weeks on the diet. All mice on the HFD had abnormal weight gain compared to the mice fed the normal diet. The total triglycerides and total cholesterol levels in the liver of the mice were also significantly increased by the HFD. However, supplementation with PlsEtn or PakCho significantly reduced these effects, and PlsEtn was found to perform better. Oil red O staining demonstrated more lipid droplet accumulation in the livers of mice fed the HFD, but this was reduced in the mice also supplemented with PlsEtn. Histology further supported this where the HFD encouraged hepatic steatosis and larger fat vacuoles compared to the normal diet group, but the PlsEtn group demonstrated reduced the steatosis and smaller vacuoles.

Although glycerolipids were found to be increased in the HFD group, supplementation with PlsEtn was shown to significantly reduce this, and was more effective than PakCho. Similarly, total lipid levels were higher in the HFD group compared to the normal diet group, but PlsEtn decreased this. To determine whether PlsEtn or PakCho influenced lipid metabolism in these mice, lipidomic analysis was used. All lipids that were detected were classified as glycerolipids, phospholipids, sphingolipids, and they also identified 41 diglycerides and 134 triglycerides. As mentioned, glycerolipids were significantly increased by the HFD and this effect was largely reduced by PlsEtn supplementation. When looking at sphingolipids, 19 ceramides, 5 hexosylceramides, and 21 sphingomyelins were identified in addition to a couple other species. The changes seen in sphingolipids were highly significant and almost all were found to be elevated in the HFD group, while PlsEtn and PakCho were able to mitigate some of this effect and appeared to be equally capable when it came to regulating sphingolipid metabolism. For phospholipids, 50 non-ether phosphatidylcholines, 11 ether phosphatidylcholines, 30 non-ether phosphatidylethanolamines, 8 ether phosphatidylethanolamines, and 22 lyso-phospholipids among others were identified. Of these, some were found to increase, but most decreased with in the HFD group. Based on the heatmap used for the analysis, it appeared the PlsEtn supplementation was able to regulate phospholipid metabolism, but PakCho had a weaker effect. Interestingly, ether phospholipids, which would include endogenous plasmalogen levels, were distinctly reduced in the HFD group and the PlsEtn supplementation was able to restore these levels similar to that seen in the normal diet group.   

Following their work in cell culture which demonstrated that PlsEtn supplementation in particular was effective at preventing the effects of a HFD, Wang et al were interested in furthering this work into a mouse model of lipid accumulation. They showed that the HFD group had an abnormal increase in weight gain and increased levels of glycerolipids and triglycerides, supporting its use as a model of lipid accumulation. With supplementation of either PlsEtn or PakCho, these effects were reduced, but PlsEtn was found to be most effective. Liver histology also supported this with less lipid droplet accumulation and smaller lipid vacuoles being found in the liver tissue of the PlsEtn supplemented group compared to the HFD group. As well, PlsEtn was able to restore levels of most glycerolipids, spingolipids, and phospholipids to that seen in the normal diet group, or have them trend back in that direction, suggesting that it was able to regulate lipid metabolism in these animals. Their previous work in cell culture also reinforced a greater effectiveness of PlsEtn compared to PakCho which could indicate that the vinyl-ether bond, present in only PlsEtn, is necessary for the highest efficacy with preventing lipid accumulation and lipid metabolism disorder.