INTRACELLULAR METABOLISM OF HUMAN APOLIPOPROTEIN(A) IN STABLY TRANSFECTED HEP G2 CELLS

Lipoprotein(a) [Lp(a)] consists of LDL and the glycoprotein apolipopro tein(a) [apo(a)], which are covalently linked via a single disulfide b ridge. The formation of Lp(a) occurs extracellularly, but an intracell ular assembly in human liver cells has also been claimed. The human ap o(a) gene locus is highly polymorphic due to a variable number of tand emly arranged kringle IV repeats. The size of apo(a) isoforms correlat es inversely with Lp(a) plasma concentrations, which is believed to re flect different synthesis rates. To examine this association at the ce llular level, we analyzed the subcellular localization and fate of apo (a) in stably transfected HepG2 cells. Our results demonstrate that ap o(a) is synthesized as a precursor with a lower molecular mass which i s processed into the mature, secreted form. The retention times of the precursor in the ER positively correlated with the sizes of apo(a) is oforms. The mature form was observed intracellularly at low levels and only in the Golgi apparatus, No apo(a) was found to be associated wit h the plasma membrane. Under temperature-blocking conditions, we did n ot detect any apo(a)/apoB-100 complexes within cells. This finding was confirmed in HepG2 cells transiently expressing KDEL-tagged apo(a). T he precursor and the mature forms of apo(a) were found in the ER and G olgi fractions, respectively, also in human liver tissue. From our dat a, we conclude that in HepG2 cells the apo(a) precursor, dependent on the apo(a) isoform, is retained in the ER for a prolonged period of ti me, possibly due to an extensive maturation process of this large prot ein. The assembly of Lp(a) takes place exclusively extracellularly fol lowing the separate secretion of apo(a) and apoB.