Identification of proteins involved in neutral lipid metabolism in hepatic stellate cells: potential therapeutic targets for liver fibrosis

Abstract

Chronic liver diseases, like liver fibrosis comprise a significant clinical problem. Activation of hepatic stellate cells (HSCs) is a critical step in the development of liver fibrosis. During activation, HSCs lose their lipid droplets (LDs) containing triacylglycerol (TAG), cholesteryl esters (CEs) and retinyl esters (REs). One of the unresolved issues in the field of HCS research is whether this change in lipid metabolism is causally related to the activation process. In other words, can HSC activation be altered when formation or breakdown of lipid droplets is disturbed? The main objective of the experiments described in this thesis was to unravel the (retinoid)lipid-related pathways involved in the transformation of hepatic stellate cells into a pathological “activated state”, and to investigate whether disruption of these lipid pathways would prevent or reverse the pathological “activated state” of hepatic stellate cells. We have found evidence for metabolically different LD pools in HSCs. We here propose that activated HSCs contain an original pool of pre-existing/“old” LDs, and a relatively rapid recycling pool of “new” LDs. The “old” LDs, containing predominantly TAGs and REs, are localized around the nucleus, and are degraded with a half time of a few days, partly in the lysosome by lysosomal acid lipase (LAL/Lipa), and partly by an unidentified lipase that is sensitive to the lipase inhibitor atglistatin and a cytosolic RE-esterase. The TAGs in the “new” LDs are degraded by the lipase ATGL and are made at the periphery of the cells, preferential by the acyltransferase DGAT1, in combination with the poly unsaturated fatty acid (PUFA)-specific CoA-ligase ACSL4, and are thus enriched in PUFAs. The PUFA-enriched TAGs may serve as a buffer for PUFAs, important substrates for the synthesis of eicosanoids, a subclass of bioactive lipids. In line with a role of eicosanoids in HSC activation, we observed changes in the expression of a number of enzymes involved in the synthesis of prostaglandins in activated HSCs. The identification of the above mentioned proteins involved in lipid metabolism enabled us to investigate whether interference with LD homeostasis influences HSC activation. In general, we found a good correlation between the effect of various inhibitors on lipid levels and the effect on activation, suggesting that a rather large disturbance in lipid metabolism is required to inhibit HSC activation. As inhibition of TAG and/or RE formation had the same effect as inhibiting their breakdown, HSCs seem to require an available lipid pool for optimal functioning.Therefore, investigations to the effects of lipid metabolism altering treatments on liver fibrosis in whole animal models are warranted.