Intracellular Modifications of Human Apolipoprotein E

We have used pulse-chase techniques to study the synthesis, intracellular modification, and secretion of human apolipoprotein E by cultures of HepG2 cells and peripheral blood human monocyte macrophages. We have found that modified apoE isoproteins are detectable intracellularly after 14 min of pulse, and their relative concentration increases linearly over a 2-h pulse-chase period. At the same time, the relative concentration of unmodified apoE decreases. All the major modified apoE isoproteins appear simultaneously, and they correspond to the sialo apoE forms apooEs2, apoEs4, and apoEs6. ApoE secretion is first detected after a 30-min pulse. Secreted apoE consists of 92% of the same sialated apoE forms observed intracellularly. ApoE sialation and secretion were not affected by treatment of the HepG2 cultures with tunicamycin, whereas monensin inhibited both the intracellular sialation and secretion of this protein. These findings suggest that apoE is secreted in the form of three major isoproteins generated by intracellular modification of this protein with one or more O-linked oligosaccharide chains containing sialic acid.