Cell surface "blanket" of apolipoprotein E on rat adrenocortical cells

Apolipoprotein (apo) E is expressed at high levels by adrenocortical cells. In the present study, an affinity-purified antibody to rat apoE was used in combination with immunogold visualization at both the light and electron microscopic levels to determine the cellular and subcellular distribution of apoE within the rat adrenal cortex. At the light microscopic level, apoE was found primarily in z. fasciculata and z. reticularis with little or none detected in z. glomerulosa and medulla. Within the z. fasciculata and z. reticularis, apoE was present in the cytoplasm of all parenchymal cells. ApoE also was found on the cell surface both on the sinusoidal front and in regions well removed from the subendothelial space. Electron microscopic examination of the z. fasciculata showed that apoE on the sinusoidal front was on the parenchymal cell surface but not the endothelial cell. Cell surface apoE was prominent on microvilli as well as non-microvillar regions of plasma membrane in the subendothelial space. ApoE was also associated with the cell surface in intercellular spaces continuous with but well removed from the subendothelial space. These findings at the light and electron microscopic levels suggest that the z. fasciculata cell is encircled or covered with apoE on all faces of the cell. These results are consistent with the idea that this cell surface "blanket" of apoE participates in the uptake of lipoprotein cholesterol by either the endocytic or selective uptake pathways.