Evidence for a Cholesterol Transport Pathway from Lysosomes to Endoplasmic Reticulum That Is Independent of the Plasma Membrane*
Abstract
We have studied the movement of low density lipoprotein (LDL)-derived cholesterol in cultured Chinese hamster ovary cells. Our hypothesis is that when LDL cholesterol is effluxed from lysosomes, the bulk of LDL cholesterol is mobilized to the plasma membrane, while another pathway delivers LDL cholesterol from lysosomes to acyl-CoA/cholesterol acyltransferase (ACAT) in the endoplasmic reticulum. Three lines of evidence support this model. First, LDL cholesterol transport to ACAT can be blocked without inhibiting the movement of cholesterol from lysosomes to plasma membrane or from plasma membrane to endoplasmic reticulum. Second, LDL cholesterol transport to ACAT is normal in a Chinese hamster ovary mutant with defective plasma membrane-to-ACAT movement. Third, LDL cholesterol is not diluted by the plasma membrane cholesterol pool before reaching ACAT. Our evidence supports a vesicular model of cholesterol transport from lysosomes to the endoplasmic reticulum that is independent of the plasma membrane.
Footnotes
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↵* This work was supported by National Institutes of Health Grant DK49564. Fluorescence microscopy was made possible by the Center for Gastroenterology Research on Absorptive and Secretory Processes (NIDDK, National Institutes of Health, Grant P30 DK34928).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ Supported by National Institutes of Health Training Grant DK07542. Present address: Genetics Institute, Cambridge, MA 02140.
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↵§ Supported by a student fellowship from the American Liver Foundation and National Institutes of Health Training Grant DK07704.
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↵¶ Supported by the Ara Parseghian Medical Research Foundation.
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↵‖ To whom correspondence should be addressed: Dept. of Physiology, Tufts University School of Medicine, 136 Harrison Ave., Boston, MA 02111. Tel.: 617-636-6945; Fax: 617-636-0445.
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↵1 The abbreviations used are: LDL, low density lipoprotein; ER, endoplasmic reticulum; ACAT, acyl-CoA/cholesterol acyltransferase; CHO, Chinese hamster ovary; U18666A, 3-β-[2-(diethylamino)-ethoxy]-androst-5-en-17-one; FITC, fluorescein isothiocyanate; 25-HC, 25-hydroxycholesterol; BSA, bovine serum albumin; CL, cholesteryl linoleate; HBSS, Hanks’ balanced salt solution; IC50, 50% inhibitory concentration; TBS, Tris-buffered saline; PBS, phosphate-buffered saline; CD, 2-hydroxypropyl-β-cyclodextrin; NCS, newborn calf serum; LPDS, lipoprotein-deficient serum; MTT, 3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyl tetrazolium bromide.
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- Received October 7, 1997.
- Revision received November 25, 1997.
- The American Society for Biochemistry and Molecular Biology, Inc.
