Sphingolipid Storage Induces Accumulation of Intracellular Cholesterol by Stimulating SREBP-1 Cleavage

Abstract

We showed previously that the intracellular transport of sphingolipids (SLs) is altered in SL storage disease fibroblasts, due in part to the secondary accumulation of free cholesterol. In the present study we examined the mechanism of cholesterol elevation in normal human skin fibroblasts induced by treatment with SLs. When cells were incubated with various natural SLs for 44 h, cholesterol levels increased 25-35%, and cholesterol esterification was reduced. Catabolism of the exogenous SLs was not required for elevation of cholesterol because (i) a non-hydrolyzable and a degradable SL analog elevated cellular cholesterol to similar extents, and (ii) incubation of cells with various SL catabolites, including ceramide, had no effect on cholesterol levels. Elevated cholesterol was derived primarily from low density lipoproteins (LDL) and resulted from up-regulation of LDL receptors induced by cleavage of the sterol regulatory element-binding protein-1. Upon SL treatment, cholesterol accumulated with exogenous SLs in late endosomes and lysosomes. These results suggest a model in which excess SLs present in endocytic compartments serve as a "molecular trap" for cholesterol, leading to a reduction in cholesterol at the endoplasmic reticulum, induction of sterol regulatory element-binding protein-1 cleavage, and up-regulation of LDL receptors.Sphingolipid storage diseases (SLSDs) 1 are metabolic disorders that generally result from a defective lysosomal hydrolase or activator protein leading to accumulation of endogenous lipids in the lysosomes in many different cell types (1). We showed previously that a fluorescent glycosphingolipid (GSL) analog (BODIPY-lactosylceramide; LacCer) is internalized from the plasma membrane (PM) and transported to the Golgi complex in normal human skin fibroblasts (HSFs) but is mistargeted to endosomes and lysosomes in cells from multiple SLSDs, suggesting a common mechanism of cellular dysfunction in these biochemically distinct diseases (2). In a subsequent study we showed that these SLSD cells accumulate unesterified cholesterol in the late endosomes and lysosomes (3), and this cholesterol plays an important role in the altered sorting and targeting of the GSL analog in SLSD fibroblasts (3-6). Similarly, cholesterol has been reported to accumulate secondary to sphingomyelin storage in sphingomyelinase-deficient (Niemann-Pick A disease) human and mouse cells (7,8). The mechanism by which intracellular cholesterol is elevated and/or redistributes in SLSDs with primary defects in SL catabolism is unknown.In the current study, we show that incubation of normal HSFs with exogenous, naturally occurring SLs resulted in a dramatic increase and redistribution in cellular cholesterol reminiscent of that seen in SLSD cells. These observations led us to study further the relationship between SL accumulation and cellular cholesterol. We provide evidence that accumulation of SLs in endocytic vesicles serves as a molecular trap for cholesterol, binding and immobilizing the sterol as it circulates ...

Renal Glycosphingolipid Metabolism Is Dysfunctional in Lupus Nephritis

“…Elevated levels of GlcCers and LacCers are sufficient to induce SREBPactivation. 28 Thus, glycosphingolipids may be up-and downstream of SREBP activation. Aberrant activation of SREBPs and renal lipid accumulation occur in kidney diseases and were linked to increased production of inflammatory cytokines and glomerulosclerosis.…”

Membrane lipids and vesicular traffic

“…The glycolipid rafts appear to concentrate in the pathways from the early endosomes to the recycling endosomes and to the late endosomes, whence the rafts reach the Golgi [34], a transport pathway followed by glycolipid-binding toxins en route to the ER [35]. Transport of both sphingolipids and cholesterol to the Golgi is interrupted under conditions of lipid storage in endocytotic organelles (as seen in many lysosomal storage diseases) [36]. Cholesterol removal from the endosome depends on the NPC1 protein, which contains a sterol-sensing domain.…”

Sphingosine 1-Phosphate (S1P) Lyase Deficiency Increases Sphingolipid Formation via Recycling at the Expense of de Novo Biosynthesis in Neurons

“…The close interaction of sphingolipids and especially of sphingomyelin with cholesterol in cellular plasma membranes is well documented (21,22). Because our results indicated no changes in the amount of plasma membrane-associated sphingolipid species in neurons lacking S1P lyase activity, we evaluated the amounts of free cholesterol and some of its metabolites as well as of cholesterol esters in brains of S1P lyase-deficient and of wild type mice, respectively.…”