Regulation of low density lipoprotein receptor and 3-hydroxy-3-methyl-glutaryl-CoA reductase activities are differentially affected in Niemann–Pick type C and type D fibroblasts

Abstract

Defective regulation of intracellular cholesterol metabolism has been investigated in cultured fibroblasts from two subtypes of Niemann–Pick type II disease: the panethnic Niemann–Pick type C (NPC) and the Nova Scotia type D (NPD). Cell extracts from NPC and NPD fibroblasts cultured in lipoprotein-deficient medium exhibited activities of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase that were two-fold greater than in normal cells. Addition of serum resulted in only a 15% decrease in HMG-CoA reductase activity within 6 h in these cells, compared with a decrease of 80% in normal fibroblasts. The initial rate of return to maximal values for the first 6 h after removal of serum was similar in all three cell types; thereafter, the rate was faster in the mutant fibroblasts. Binding and internalization of ¹²⁵I-labeled low density lipoprotein (LDL) was not decreased within 12 h of incubation of NPC fibroblasts with serum, while a decrease of 50% was observed for both NPD and normal fibroblasts over this time period. Northern blot analysis also indicated a slower decrease in steady-state LDL receptor mRNA in NPC relative to normal and NPD cells. In all three cell types, inhibition of HMG-CoA reductase with mevinolin had no effect on serum-stimulated cholesterol esterification, while inhibition of acyl-CoA:cholesterol acyltransferase with Sandoz 58-035 did not influence HMG-CoA reductase activity, indicating that defects in these regulatory mechanisms are independent. Together with previous observations that NPC and NPD fibroblasts exhibit different levels of cholesterol accumulation and esterification, our results suggest that NPD may result from a distinct mutation in a Niemann–Pick type II gene, in which different mutations can differentially alter the various mechanisms of cholesterol regulation.Key words: Niemann–Pick disease, cholesterol regulation, low density lipoprotein receptor, 3-hydroxy-3-methylglutaryl-CoA reductase, acyl-CoA:cholesterol acyltransferase.