Transport of lipids from the yolk to the tissues of the chick embryo is slow during the first 2 weeks of development, but increases abruptly during the last week. Evidence suggests that the lipid traverses the cytoplasm of the yolk-sac membrane before secretion as lipoprotein into the fetal circulation. Little is known about the cytoplasmic transport of lipid in avian systems, but recently the presence of sterol carrier protein 2 (SCP2) was reported in chicken liver. Here we examine the cells of yolk-sac membrane, liver and small intestine for the presence of this protein as a function of the time of embryonic development. The quantity of SCP2 present in the embryonic cells did not appear to correlate with the rate of lipid flux in these tissues. The abrupt appearance of a high-molecular-mass form of SCP2 was detected in small intestine shortly before hatching, but the significance of this protein is not clear. The presence of SCP2 in these tissues was also confirmed by immunocytochemical techniques. Similarly to SCP2 of mammalian cells, avian SCP2 is localized in both peroxisome-like structures and mitochondria. To a lesser extent it is associated with the endoplasmic reticulum.

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The relationship of cholesteryl ester hydrolysis to the physical state of the cholesteryl ester in J774 murine macrophages was explored in cells induced to store cholesteryl esters either in anisotropic (ordered) inclusions or isotropic (liquid) inclusions. In contrast to other cell systems, the rate of cholesteryl ester hydrolysis was faster in cells containing anisotropic inclusions than in cells containing isotropic inclusions. Two contributing factors were identified. Kinetic analyses of the rates of hydrolysis are consistent with a substrate competition by co-deposited triglyceride in cells with isotropic inclusions. In addition, hydrolysis of cholesteryl esters in cells with anisotropic droplets is mediated by both cytoplasmic and lysosomal lipolytic enzymes, as shown by using the lysosomotropic agent, chloroquine, and an inhibitor of neutral cholesteryl ester hydrolase, umbelliferyl diethylphosphate. In cells containing anisotropic inclusions, hydrolysis was partially inhibited by incubation in media containing either chloroquine or umbelliferyl diethylphosphate. Together, chloroquine and umbelliferyl diethylphosphate completely inhibited hydrolysis. However, when cells containing isotropic inclusions were incubated with umbelliferyl diethylphosphate, cholesteryl ester hydrolysis was completely inhibited, but chloroquine had no effect. Transmission electron microscopy demonstrated a primarily lysosomal location for lipid droplets in cells with anisotropic droplets and both non-lysosomal and lysosomal populations of lipid droplets in cells with isotropic droplets. These results support the conclusion that there is a lysosomal component to the hydrolysis of stored cholesteryl esters in foam cells.-