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 ( approximately 50-100%). 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 (DeltaR608) in CHO cells revealed that DeltaR608 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.