Kuivenhoven, J. A. | Search bioRxiv

bioRxiv2023

The endosomal sorting complex Retromer has a central role insystemic cholesterol homeostasis by controlling endo-lysosomal cholesterol transport in hepatocytes

BackgroundDisturbed hepatic cholesterol homeostasis is associated with multiple diseases, including atherosclerotic cardiovascular disease and metabolic dysfunction- associated steatotic liver disease. The endo-lysosomal system is essential for cholesterol uptake and intracellular distribution, yet the mechanisms governing these processes remain incompletely understood. Here, we investigated the impact of hepatic VPS35, a subunit of the endosomal sorting complex retromer, on hepatocellular and whole-body cholesterol homeostasis. MethodsWe generated a liver-specific Vps35 knockout mouse model (Vps35HepKO) and applied biochemical analyses, proteomics, and stable-isotope-labeled tracers to quantify critical processes of cholesterol metabolism. Human iPSC-derived liver organoids and CRISPR technology were used to translate our findings to humans. Mechanistic studies were performed in precision-cut liver slices from WT and Vps35HepKO mice. ResultsHepatic VPS35 deficiency led to an increase in endo-lysosomal degradative compartments and a marked reduction in specific lysosomal proteins, including lysosomal acid lipase (LAL), Scavenger Receptor Class B Member 2 (SCARB2), and Niemann-Pick type C1 (NPC1). Using pathway-specific inhibitors, we showed that VPS35 loss impairs the translation of SCABR2 and NPC1. Consistently, human iPSC-derived liver organoids lacking VPS35 also exhibited reduced expression of NPC1 and SCARB2 proteins. The decrease in these lysosomal proteins correlated with increased cholesterol levels in the plasma and liver of Vps35HepKO mice. This was likely explained by the disrupted cholesterol trafficking through the endo-lysosomal system, delayed plasma cholesterol turnover, and upregulated cholesterol biosynthesis. ConclusionThese findings uncover a previously unknown role for the hepatic retromer complex in maintaining systemic cholesterol homeostasis. Beyond its established function in endosomal cargo transport, we now demonstrate that retromer is also essential for lysosomal cholesterol handling. This role is mediated by regulating key lysosomal proteins involved in cholesterol metabolism, including LAL, NPC1, and SCARB2.

molecular biology↗

bioRxiv2018

Agent-Based Modeling Predicts HDL-independent Pathway of Removal of Excess Surface Lipids from Very Low Density Lipoprotein

A hallmark of the metabolic syndrome is low HDL-cholesterol coupled with high plasma triglycerides (TG), but it is unclear what drives this close association. Plasma triglycerides and HDL cholesterol are thought to communicate through two distinct mechanisms. Firstly, excess surface lipids from VLDL released during lipolysis are transferred to HDL, thereby contributing to HDL directly but also indirectly through providing substrate for LCAT. Secondly, high plasma TG increases clearance of HDL through core-lipid exchange between VLDL and HDL via CETP and subsequent hydrolysis of the TG in HDL, resulting in smaller HDL and thus increased clearance rates.\n\nTo test our understanding of how high plasma TG induces low HDL-cholesterol, making use of established knowledge, we developed a comprehensive agent-based model of lipoprotein metabolism which was validated using monogenic disorders of lipoprotein metabolism.\n\nBy perturbing plasma TG in the model, we tested whether the current theoretical framework reproduces experimental findings. Interestingly, while increasing plasma TG through simulating decreased lipolysis of VLDL resulted in the expected decrease in HDL cholesterol, perturbing plasma TG through simulating increased VLDL production rates did not result in the expected HDL-TG relation at physiological lipid fluxes. However, model perturbations and experimental findings can be reconciled if we assume a pathway removing excess surface-lipid from VLDL that does not contribute to HDL cholesterol ester production through LCAT. In conclusion, our model simulations suggest that excess surface lipid from VLDL is cleared in part independently from HDL.\n\nAuthor summaryWhile it has long been known that high plasma triglycerides are associated with low HDL cholesterol, the reason for this association has remained unclear. One of the proposed mechanisms is that during catabolism of VLDL, lipoproteins rich in triglyceride, the excess surface of these particles become a source for the production of HDL cholesterol, and that therefore decreased catabolism of VLDL will lead to both higher plasma triglyceride and low HDL cholesterol. Another proposed mechanism is that during increased production of VLDL, there will be increased exchange of core lipids between VLDL and HDL, with subsequent hydrolysis of the triglyceride in HDL, leading to smaller HDL that is cleared more rapidly. To investigate these mechanisms further we developed a computational model based on established knowledge concerning lipoprotein metabolism and validated the model with known findings in monogenetic disorders. Upon perturbing the plasma triglycerides within the model by increasing the VLDL production rate, we unexpectedly found an increase in both triglyceride and HDL cholesterol. However, upon assuming that less excess surface lipid is available to HDL, HDL decreases in response to increased VLDL production. We therefore propose that there must be a pathway removing excess surface lipids that is independent from HDL.\n\nAbbreviations

systems biology↗