Altered expression of caveolin-1 and increased cholesterol in detergent insoluble membrane fractions from liver in mice with Niemann-Pick disease type C.

ABSTRACT

Niemann-Pick type C (NPC) is an autosomal recessive disease characterized by impaired cholesterol homeostasis due to a defect in the intracellular transport of unesterified cholesterol. While accumulation of lysosomal cholesterol is the most apparent microscopic finding, cholesterol has also been shown to accumulate in the trans-cisternae of the Golgi apparatus. Caveolin-1, a cholesterol-binding protein that cycles between the Golgi apparatus and the plasma membrane, has been hypothesized to participate in cholesterol transport. Using the BALB/c murine model for NPC disease, we found that the expression of caveolin-1 in total liver homogenates from heterozygous and homozygous affected animals was altered. Immunoblot analysis of liver homogenates from heterozygous mice revealed that caveolin-1 is significantly (p < 0.005) elevated, 4.9 fold, compared to normal mice. Total liver homogenates from homozygous affected mice also had a significant (p < 0.05) increase in caveolin-1 expression, 1.6 fold, compared to normal mice. Immunohistochemical staining of liver cross-sections revealed that the increased caveolin-1 was localized to sinusoidal endothelial cells in heterozygous mice. The Triton insoluble floating fraction (TIFF) was isolated using liver from each genotype and analyzed for caveolin-1 expression. Caveolin-1 in the TIFF from heterozygous mice was significantly (p < 0.01) elevated, 1.8 fold, compared to normal and homozygous affected animals; normal and homozygous affected animals, however, were not significantly different from each other. The TIFF isolated from homozygous affected mice revealed a 15 fold increase in unesterified cholesterol compared to the TIFF isolated from heterozygous and normal mice. In summary, these findings demonstrate an altered expression of caveolin-1 in liver from heterozygous and homozygous NPC mice and increased concentration of cholesterol from TIFF in homozygous affected NPC mice. The identification of these alterations in the TIFF suggests involvement of detergent insoluble membrane structures, possibly caveolae and/or detergent insoluble glycosphingolipid-enriched complexes (DIGs), in the cholesterol trafficking defect in this disorder.