Defective activity of acyl-CoA:cholesterol O-acyltransferase in Niemann-Pick type C and type D fibroblasts.

The activity of acyl-CoA:cholesterol acyltransferase (ACAT; EC 2.3.1.26) was measured in fibroblast homogenates from Niemann-Pick Type C (NPC) and Type D (NPD) patients to determine whether these cells exhibit similar defects in the regulation of cholesterol esterification. ACAT activity in normal cells cultured in the absence of serum lipoproteins responded rapidly (within 6 h) to the addition of serum and reached peak levels at 12-24 h, whereas little stimulation of activity in NPC cells was observed. In contrast, ACAT activity in NPD fibroblasts (cell lines from four different patients) began to increase between 6 and 12 h after serum addition, reaching levels up to 50% of normal values at 24 h. ACAT activity in NPC and NPD cell extracts could not be stimulated by preincubation with normal cell homogenates, nor was complementation between NPC and NPD homogenates observed. Addition of 25-hydroxycholesterol to fibroblasts cultured in delipidated serum increased ACAT activity for all three cell types, although stimulation in NPD cells was less than that observed in NPC cells. ACAT activity of deoxycholate-solubilized homogenates reconstituted into phosphatidylcholine vesicles was independent of the presence of serum lipoproteins during culture and dependent on cholesterol present in the vesicles for all cell types. However, ACAT activities of mutant fibroblasts in vesicles plus cholesterol were significantly (about 40%) lower than control levels. These results suggest that the metabolic lesions in NPC and NPD cells are biochemically distinct and that both may involve factors in addition to the availability of cholesterol substrate for the ACAT enzyme.