APOLIPOPROTEIN(A) SYNTHESIS AND SECRETION FROM HEPATOMA-CELLS IS COUPLED TO TRIGLYCERIDE SYNTHESIS AND SECRETION

Apolipoprotein(a) (apo(a)) is synthesized and secreted from liver cell s and represents one of the two major protein components of the athero genic lipoprotein, Lp(a), Little is known, however, of the factors tha t regulate the secretion of this protein. We have undertaken an analys is of the response to oleate supplementation in stable clones of HepG2 and McA-RH7777 cells express ing either a 6 K-IV or 17 K-IV isoform o f apo(a), These cell lines were examined by pulse chase analysis and e ach demonstrated an increase (range 2-6-fold) in apo(a) secretion foll owing supplementation with 0.8 mM oleate, Microsomal membranes, prepar ed from HepG2 cells expressing a 6 K-TV apo(a) isoform, demonstrated t hat oleate supplementation increased the apparent protection of apo(a) from protease digestion, suggesting that alterations in the transloca tion efficiency of apo(a) may accompany the addition of oleate, Cells incubated with brefeldin A demonstrated increased recovery of the prec ursor form of apo(a) with oleate supplementation, suggesting that alte rations in post-translational degradation may also contribute to the o bserved in crease in apo(a) secretion following oleate addition. To fu rther characterize the oleate-dependent increase in apo(a) secretion, cells were incubated with an inhibitor of the microsomal triglyceride transfer protein. These experiments demonstrated a dose dependent decr ease in apo(a) secretion from both cell lines. Furthermore, addition o f either the microsomal triglyceride transfer protein inhibitor or tri acsin C, an inhibitor of acyl-CoA synthase, completely abrogated the o leate-dependent increase in apo(a) secretion. Taken together, these da ta provide evidence that apo(a) secretion from hepatoma cells may be l inked to elements of cellular triglyceride assembly and secretion.