Increased sphingomyelin content impairs HDL biogenesis and maturation in human Niemann-Pick disease type B

Abstract

We previously reported that human Niemann-Pick Disease type B (NPD-B) is associated with low HDL. In this study, we investigated the pathophysiology of this HDL deficiency by examining both HDL samples from NPD-B patients and nascent high density lipoprotein (LpA-I) generated by incubation of lipid-free apolipoprotein A-I (apoA-I) with NPD-B fibroblasts. Interestingly, both LpA-I and HDL isolated from patient plasma had a significant increase in sphingomyelin (SM) mass (z50-100%). Analysis of LCAT kinetics parameters (V max and K m ) revealed that either LpA-I or plasma HDL from NPD-B, as well as reconstituted HDL enriched with SM, exhibited severely decreased LCAT-mediated cholesterol esterification. Importantly, we documented that SM enrichment of NPD-B LpA-I was not attributable to increased cellular mass transfer of SM or unesterified cholesterol to lipid-free apoA-I. Finally, we obtained evidence that the conditioned medium from HUVEC, THP-1, and normal fibroblasts, but not NPD-B fibroblasts, contained active secretory sphingomyelinase (S-SMase) that mediated the hydrolysis of [ 3 H]SM-labeled LpA-I and HDL 3 . Furthermore, expression of mutant SMase (DR608) in CHO cells revealed that DR608 was synthesized normally but had defective secretion and activity. Our data suggest that defective S-SMase in NPD leads to SM enrichment of HDL that impairs LCAT-mediated nascent HDL maturation and contributes to HDL deficiency. Thus, S-SMase and LCAT may act in concert and play a crucial role in the biogenesis and maturation of nascent HDL particles. Supplementary key words high density lipoprotein . nascent LpA-I . phospholipids . sphingomyelin phosphodiesterase-1 gene . sphingomyelinase Sphingomyelin (SM) plays an important role in the structural integrity of the cellular membranes. The constitutive degradation of SM occurs in the lysosomal compartments of the cell. Fragments of the plasma membrane containing SM targeted for degradation are endocytosed and traffic through the endosomal compartments to reach the lysosomes, where they are hydrolyzed by lysosomal sphingomyelinase. The cleaved fragments (i.e., the choline head group, the fatty acids, and the sphingoid bases) then leave the lysosome and reenter the biosynthetic pathway or can be further degraded. In Niemann-Pick type I disease, which includes types A and B (NPD-A/B), lysosomal SMase is deficient, resulting in a lysosomal accumulation of SM and a secondary increase in cholesterol (1).Earlier studies by Tall and colleagues (2) investigated the crucial role of phospholipids (PLs) and their plasma transfer proteins in the metabolic pathway of HDL. It is well accepted that SM plays an important role in plasma lipoprotein metabolism. SM is the second most abundant PL in mammalian plasma. It appears in all major lipoproteins, where it is part of the monolayer of polar lipids and cholesterol that surrounds a core of neutral lipids. Up to 18% of total plasma PL occurs as SM, and the ratio of SM-to-phosphatidylcholine (PC) varies widely among lipoprotein su...

Regulation of the Activity and Fatty Acid Specificity of Lecithin-Cholesterol Acyltransferase by Sphingomyelin and Its Metabolites, Ceramide and Ceramide Phosphate

“…Depletion of SM by bacterial SMase C stimulates the reaction significantly in both native lipoproteins and synthetic substrates (18), supporting the inhibitory role of SM in the lipoproteins. A recent report by Lee et al (10) showed that HDL from Niemann-Pick Disease patients, which contains high concentration of SM, also exhibits impaired cholesterol esterification by LCAT, and that this is mostly attributable to the lack of secretory SMase in these patients. However, studies with various phospholipases showed that ceramide, the product of SMase C reaction, is an independent activator of the phospholipases (12)(13)(14)(15)(16).…”

“…On the basis of small angle x-ray refraction studies, Koumanov et al (13) suggested that the long chain polyunsaturated PCs concentrate in the intermediate structures between the hexagonal and lamellar phases created by the ceramide-induced phase separation, and that these structures are more susceptible to the enzymatic attack. It is conceivable that the enrichment of lipoproteins with ceramide, either through SMase C action or through direct secretion from the tissues (7,8,10) could facilitate interaction of more polyunsaturated PCs with LCAT. Our previous studies also showed that the action of LCAT on 16:0-20:4 PC results in the formation of 20:4 lyso PC, due to an alteration in its positional specificity (25).…”

“…Holopainen et al (9) also reported the presence of an LDL-associated SMase that may be derived from the apoprotein B of LDL, and which could therefore hydrolyze LDL SM in the circulation. More recently Lee et al (10) reported that cultured endothelial cells and fibroblasts secrete SMase in association with nascent lipoprotein particles, supporting the formation of ceramides in the circulation. The concentration of ceramides is increased in sepsis patients, and the ceramide/SM ratio correlates positively with mortality in the patients (11).…”

Regulated Secretion of Acid Sphingomyelinase

“…Although this region is distant from the predicted catalytic region (amino acids 199 -421) (46,63), the C terminus of aSMase has previously been implicated in the trafficking of both S-SMase and L-SMase (47). A common type B Niemann-Pick disease mutation (⌬R608) leads to both loss of L-SMase function and impaired secretion of S-SMase (64). Given that a variety of other mutations in the C-terminal domain of aSMase are sufficient to induce total loss both S-SMase and L-SMase activity (47), the S508A mutant represents a unique and novel tool to address S-SMase-and L-SMase-specific metabolic, and ultimately biological, functions.…”

Effects of sepsis on the metabolism of sphingomyelin and cholesterol in mice with liver dysfunction

“…For example, studies by Ding et al (31) and Yan et al (32) demonstrated that SMS overexpression may cause SM accumulation in cells and lipid rafts and increase the cholesterol content. Furthermore, patients with Niemann-Pick disease cannot hydrolyze SM due to defective SMase, resulting in the accumulation of SM and cholesterol in the liver and the nervous system (33). Additionally, because ABCA1 is a transmembrane protein that is located in or near the lipid raft, some researchers have implied that the enhancement of SM and cholesterol may also increase the ABCA1 expression in cells (32).…”