Altered low density lipoprotein receptor regulation is associated with cholesteryl ester accumulation in Simian virus 40 transformed rodent fibroblast cell lines

Telomerase immortalization upregulates Rab9 expression and restores LDL cholesterol egress from Niemann-Pick C1 late endosomes

Niemann-Pick C (NPC) disease is a rare recessive lipidosis marked by excessive accumulation of LDL-derived free cholesterol and glycosphingolipids in the late endosomal-lysosomal (E-L) system. Here we report that ectopic expression of human telomerase reverse transcriptase (hTeRT) in human cells leads to an upregulation of the small GTPase Rab9 and its effector p40. Expression of hTeRT in NPC1 cells results in a correction of their cellular phenotype, including clearance of accumulated cholesterol from their E-L system. Specifically, in NPC1-TeRT cells, the transport of cholesterol from the E-L system to the plasma membrane is restored with a concomitant increase in cholesterol esterification. This effect is Rab9-specific since expression of Rab9 in untransformed NPC1 cells also leads to a reversal of their disease phenotype. These effects are also seen in normal TeRT-immortalized cells and it appears that TeRT expression leads to an increase in the transport of molecules, including cholesterol, from the E-L system, and may play a role in increasing cellular proliferation. These results suggest the existence of alternative endogenous therapeutic targets that can be modulated to reverse the NPC1 disease phenotype.

Overexpression of low density lipoprotein receptor on Chinese hamster ovary cells generates foam cells

The atherosclerotic lesion is characterized by the presence of cholesterol-loaded foam cells. Chinese hamster ovary (CHO) cells do not normally store cholesteryl esters because low density lipoprotein (LDL) receptors are suppressed by exposure of these cells to LDL cholesterol. We transfected LDL receptor cDNA linked to the simian virus 40 early promoter into CHO cells (CHO 29) and found that LDL receptor binding in these cells was not suppressed by an excess amount of LDL cholesterol, indicating no regulation of the LDL receptor in CHO 29 cells. Furthermore, CHO 29 cells showed a high activity of LDL uptake and intracellular accumulation of cholesteryl esters. Light-microscopic examination demonstrated the resulting formation of foam cells in CHO 29 cells in the presence of 5 micrograms LDL/ml. These results demonstrated that foam cell changes in atherosclerotic lesions can be reproduced in CHO cells, whose LDL receptor activity is overexpressed, through the mechanism of LDL receptor-mediated endocytosis of native LDL.

Regulation of low-density lipoprotein receptor expression during keratinocyte differentiation

Transformed keratinocytes (i.e., SCC-4, SCC-15, SCC-12F2, SVK14) or normal keratinocytes which differ in their differentiation program, were used to study the regulation of low-density lipoprotein (LDL)-receptor expression. The capacity of the cells to differentiate was modulated by changing the extracellular calcium concentration. We now demonstrate that LDL-receptor expression in normal and transformed keratinocytes depends on the cell type and one or more levels of regulatory control. Cells express elevated mRNA levels when cultured under low Ca++ (proliferating) conditions. In contrast, SV40-transformed keratinocytes express decreased message under similar condition. In addition, LDL-receptor protein is decreased in transformed cells when extracellular Ca++ is increased (1.6 mM) to stimulate differentiation; the decrease in protein is comparable to the decrease in mRNA expression. Under the same conditions, normal keratinocytes show markedly decreased LDL-receptor protein relative to the decrease in mRNA. Incubation with LDL-cholesterol decreases the number of cell surface-exposed LDL-receptors. The LDL-receptor in fibroblasts is regulated differently from SCC-4 cells. The addition of LDL-cholesterol to fibroblasts causes decreased LDL-receptor mRNA and protein expression whereas SCC-4 cells show the opposite effect. The addition of cholesterol in non-lipoprotein form causes decreased LDL-receptor mRNA and protein expression in both cell types. These results suggest another, yet unidentified, regulatory mechanism that affects LDL-receptor expression in these two cell types.

Decreased messenger RNA translation in herpesvirus-infected arterial cells: effects on cholesteryl ester hydrolase

Herpes simplex viruses (HSVs) contain a function that can cause the degradation of host mRNA and mediate the shutoff of host protein synthesis. Previously, we observed that HSV infection causes a 40-fold increase in cholesteryl ester (CE) accretion in arterial smooth muscle cells due, in part, to a substantial decrease in CE hydrolysis. In studies reported herein, we found that HSV infection leads to reduced immunoprecipitable lysosomal (acid) CE hydrolase (ACEH) and beta-galactosidase, another lysosomal enzyme in vascular smooth muscle cells. The HSV-induced reduction was greater with respect to ACEH than beta-galactosidase. To determine whether degradation of host cellular mRNA or inhibition of cellular translation was responsible for decreased CE hydrolysis in HSV-infected smooth muscle cells, we utilized an in vitro translation system that permitted us to compensate for any mRNA degradation during viral infection. Reduced ACEH activity was observed in the total cellular RNA translation products of HSV-infected smooth muscle cells compared to uninfected cells owing to posttranscriptional modification. We conclude that the decrease in CE hydrolysis in HSV-infected smooth muscle cells is caused primarily by decreased ACEH synthesis and activity, which can contribute to CE accretion in these vascular cells.