Intracellular Modification of Human Apolipoprotein All (ApoAII) and Sites of ApoAII mRNA Synthesis: Comparison of ApoAII with ApoCII and ApoCIII Isoproteins

We have studied the intracellular modifications of human apoAII by pulse-chase labeling of HepG2 cell cultures with [³⁵S]methionine or [³H]arginine followed by two-dimensional analysis and autoradiography of the radiolabeled apoAII isoproteins. A short (5.0-min) pulse showed the presence of a precursor form of apoAII (pI = 5.75) designated proapoAII or apoAII₃. A 5–10-min chase resulted in a decrease in the relative concentration of the proapoAII coupled with an increase in the relative concentration of a new form (pI = 5.3) designated modified proapoAII or apoAII₁. Longer chase resulted in the appearance of the plasma apoAII form and at least five other acidic apoAII isoproteins in the cell lysate and the culture medium. Labeling with [³H]arginine showed that apoAII isoproteins designated 3, 1, −1, and −3 contained the prosegment whereas isoproteins designated la, 0, −la, −2a, −3a, and −4a did not. Comparison of nascent apoAII, apoCII, and apoCIII isoproteins revealed that they were distinctly different on the two-dimensional gels. Neuraminidase treatment converted the acidic apoAII isoproteins to isoproteins la and 0 (modified and plasma apoAII forms). The combined data are consistent with the following intra- and/or extracellular modifications of apoAII: (a) modification of the apoAII which results in the net loss of two positive charges; (b) glycosylation of the modified proapoAII with carbohydrate chains containing sialic acid; (c) proteolytic removal of the prosegment and cyclization of the N-terminal glutamine. Analysis of apoAII mRNA distribution in 13 fetal human tissues as well as in cell lines of human origin showed abundance of apoAII mRNA in liver and HepG2 cells and only traces in the intestine.