Determinants of human apolipoprotein [a] secretion from mouse hepatocyte cultures

Efforts to develop an in vitro model system to analyze apolipoprotein [a] ( apo[a]) gene transcription, mRNA translation, and protein secretion have be en complicated by the limited tissue and species distribution of apo[a] and the presence of regulatory DNA sequences remote from the apo[al transcript ion start site, In the current study we examined primary hepatocytes cultur ed from apo[a] transgenic mice as a model system for analyzing apo[a] bioge nesis, Hepatocytes from mice transgenic for a yeast artificial chromosome ( YAC) encoding the entire apo[a] gene in its own genomic context (YAG-apo[a] hepatocytes) were unable to maintain apo[a] expression beyond 48 h of cult ure. This suggests that the apo[a] promoter was not active in cultured YAC- apo[a] hepatocytes, In contrast, apo[a] expression was maintained for at le ast 7 days in hepatocytes cultured from mice transgenic for an apo[a] cDNA under control of the mouse transferrin promoter (transferrin-apo[a] hepatoc ytes), Pulse-chase experiments established that more than 80% of apo[a] syn thesized by both transferrin-apo[a] and YAC-apo[a] hepatocytes was degraded prior to secretion, independently of the coexpression of human apoB. Thus, low secretion efficiency appears to be a general characteristic of human a po[a] proteins in mouse liver, Apo[a] secretion was increased somewhat (fro m 18% to 32%) in the presence of lipoprotein-containing. serum. Transformed cell lines derived from transferrin apo[a] hepatocytes retained characteri stics of apo[a] secretion similar to those observed in primary cells. Prima ry and transformed apo[a] transgenic hepatocytes may provide valuable addit ional models with which to study posttranslational mechanisms regulating ap o[a] secretion.