Intracellular metabolism of human apolipoprotein(a) in stably transfected Hep G2 cells

Lipoprotein(a) [Lp(a)] consists of LDL and the glycoprotein apolipoprotein(a) [apo(a)], which are covalently linked via a single disulfide bridge. The formation of Lp(a) occurs extracellularly, but an intracellular assembly in human liver cells has also been claimed. The human apo(a) gene locus is highly polymorphic due to a variable number of tandemly arranged kringle IV repeats. The size of apo(a) isoforms correlates inversely with Lp(a) plasma concentrations, which is believed to reflect different synthesis rates. To examine this association at the cellular level, we analyzed the subcellular localization and fate of apo(a) in stably transfected HepG2 cells. Our results demonstrate that apo(a) is synthesized as a precursor with a lower molecular mass which is processed into the mature, secreted form. The retention times of the precursor in the ER positively correlated with the sizes of apo(a) isoforms. The mature form was observed intracellularly at low levels and only in the Golgi apparatus. No apo(a) was found to be associated with the plasma membrane. Under temperature-blocking conditions, we did not detect any apo(a)/apoB-100 complexes within cells. This finding was confirmed in HepG2 cells transiently expressing KDEL-tagged apo(a). The precursor and the mature forms of apo(a) were found in the ER and Golgi fractions, respectively, also in human liver tissue. From our data, 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 time, possibly due to an extensive maturation process of this large protein. The assembly of Lp(a) takes place exclusively extracellularly following the separate secretion of apo(a) and apoB.