A Possible Mechanism of Cellular Cholesterol Efflux*

After in vitro incubation with radiolabeled amino acids, extracts of various chicken tissues were reacted with antiserum against apolipoprotein AI (apo-AI) of plasma high density lipoprotein. Radiolabeled apo-AI was found in liver and intestine as well as in the kidney and a variety of peripheral tissues including aorta, peripheral arteries, veins, and skeletal muscle. The immunoreactive apo-AI synthesized by peripheral tissues had the same mobility as plasma apo-AI and newly synthesized liver apo-AI upon sodium dodecyl sulfatepolyacrylamide gel electrophoresis. High resolution two-dimensional gel analysis showed that newly synthesized apo-AI exists in at least four isoforms in each of the tissues examined. Comparisons of isoform patterns as well as mixing experiments suggest that newly synthesized apo-AI isoforms have identical charge properties in each tissue examined. Plasma apo-AI also shows four isoforms with the same isoelectric points, but the quantitative distribution among the isoforms is different. The two basic isoforms predominate in newly synthesized apo-AI, while plasma apo-AI consists primarily of the two acidic isoforms. Messenger RNA from liver, intestine, kidney, and skeletal muscle directed the synthesis of apo-AI in a wheat germ extract. The apo-AI cell-free product directed by each tissue RNA had the same molecular weight on sodium dodecyl sulfate-polyacrylamide gels. These products are 2000 daltons larger than mature apo-AI synthesized by liver tissue. Liver apo-AI is a secretory protein, and rat liver mRNA encodes an apoAI precursor that can be processed in vitro to the size of mature apo-AI (Lin-Su, M.-H., Lin-Lee, Y.-C., Bradley, W. A., and Chan, L. (1981) Biochemistry 20, 2470-2475). Since the apo-AI synthesized in response to skeletal muscle and kidney RNA is larger than mature apo-AI but the same size as the cell-free product directed by liver RNA, it is likely that the apo-Al synthesized in peripheral chicken tissues is destined for secretion. Newly synthesized apo-AI was secreted from cultures of differentiated chick embryo muscle cells. Analysis of secreted apo-AI by isopycnic density gradient ultracentrifugation showed a substantial portion of the apo-AI in the density range <1.22 g/ml. This result implies an association of muscle apo-AI with lipid. These results are discussed with respect to the potential role of peripheral apo-AI in cellular cholesterol efflux and/or the movement of cholesterol from peripheral tissues to the liver.