Acetoacetylated lipoproteins used to distinguish fibroblasts from macrophages in vitro by fluorescence microscopy.

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Abstract
We have developed a procedure for labeling lipoproteins with the fluorescent probe 3,3'-dioctadecylindocarbocyanine (Dil) and have used Dil-labeled native and acetoacetylated lipoproteins to differentiate macrophages from fibroblasts in mixed cell culture. Lipoproteins labeled with this probe were suitable for the direct viewing of their binding and internalization by cells in vitro. The labeling technique has been applied to human low density lipoproteins (LDL) and to two canine cholesterol-induced lipoproteins: apo-E HDLc, which contain only the E apoprotein (apo-E), and beta-migrating, very low density lipoproteins (beta-VLDL), which contain apo-B and apo-E. The Dil-labeled lipoproteins showed specific high affinity binding to human fibroblasts via the LDL (apo-B, -E) receptors. The equilibrium dissociation constant for the binding of Dil-labeled apo-E HDLc and LDL were the same as for the respective native lipoproteins. The specific binding of Dil-labeled LDL and apo-E HDLc was further substantiated b...