INTRACELLULAR PROCESSING OF APO(A) IN PRIMARY BABOON HEPATOCYTES

We have developed a serum-free medium for the long-term culture of hig hly differentiated primary baboon hepatocytes. Hepatocytes isolated fr om animals with defined plasma Lp(a) levels and apo(a) glycoprotein ph enotypes were used to study the assembly of Lp(a). A combination of st eady-state and purse-chase labeling studies and endoglycosidase digest s demonstrated that apo(a) was synthesized as a lower molecular weight precursor. After a prolonged period of time in the endoplasmic reticu lum, apo(a) was converted to a mature form and secreted. A proportion of mature apo(a) also had a prolonged residence time in the trans Golg i apparatus. In all experiments, apoB coimmunoprecipitated with apo(a) from the culture medium but not from the cell lysates, supporting an extracellular association of the proteins for the formation of Lp(a). Analysis of hepatic RNA from 29 'null' Lp(a) phenotype baboons reveale d that one-third of the animals had detectable apo(a) transcripts, whe reas the remainder had no detectable apo(a) mRNA. The baboon hepatocyt e system therefore represents a valuable model to examine the effect o f allelic variation at the apo(a) locus on Lp(a) assembly.