Translocation of both lysosomal LDL-derived cholesterol and plasma membrane cholesterol to the endoplasmic reticulum for esterification may require common cellular factors involved in cholesterol egress from the acidic compartments (lysosomes/endosomes).

Using a stable cell line 25-RA derived from wild-type Chinese hamster ovary (CHO) cells as the parental cell, this laboratory previously reported the isolation and characterization of CHO cell mutants (cholesterol-trafficking or CT) defective in transporting LDL-derived cholesterol out of the acidic compartment(s) (lysosomes/endosomes) to the endoplasmic reticulum (ER) for esterification. In this report, we show that the CT mutation can be complemented by fusion with human cells; however, attempts to complement the CT defect through DNA transfection have resulted in a collection of stable cell lines designated as ST cells. Under cholesterol starvation condition, the ST cells exhibit an elevated rate of cholesterol ester biosynthesis (by 3- to 5-fold) compared to both the parental CHO cells and the CT cells. The phenotypes of the ST cells are stable. ST cells are thus new cell lines arisen from the CT cells. When the plasma membranes of the parental, CT, and ST cells are labelled with [3H]cholesterol, ST cells show rates of [3H]cholesterol esterification much higher than that observed in CT cells but lower than that observed in the parental CHO cells. This result shows that translocation of plasma membrane cholesterol to the ER for esterification is defective in the CT cells. This result also suggests that ST cells acquire increased cholesterol trafficking activity between the lysosome and the ER without mixing the plasma membrane cholesterol pool. The characteristics of CT cells and ST cells reported here suggest that translocation of both lysosomal LDL-derived cholesterol and plasma membrane cholesterol to the ER for esterification may require common cellular factors involved in cholesterol egress from the acidic compartment(s) (lysosomes/endosomes).