The triple threat to nascent apolipoprotein B - Evidence for multiple, distinct degradative pathways

We previously showed that Omega -3 fatty acids reduce secretion of apolipop rotein B (apoB) from cultured hepatocytes by stimulating post-translational degradation. In this report, we now characterize this process, particularl y in regard to the two known processes that degrade newly synthesized apoB, endoplasmic reticulum (ER)-associated degradation and re-uptake from the c ell surface. First, we found that Omega -3-induced degradation preferential ly reduces the secretion of large, assembled apoB-lipoprotein particles, an d apoB polypeptide length is not a determinant. Second, based on several ex perimental approaches, ER-associated degradation is not involved. Third, re -uptake, the only process known to destroy fully assembled nascent lipoprot eins, was clearly active in primary hepatocytes, but Omega -3-induced degra dation of apoB continued even when re-uptake was blocked. Cell fractionatio n showed that Omega -3 fatty acids induced a striking loss of apoB(100) fro m the Golgi, while sparing apoB(100) in the ER, indicating a post-ER proces s. To determine the signaling involved, we used wortmannin, a phosphatidyli nositol 3-kinase (PI3K) inhibitor, which blocked most, if not all, of the O mega -3 fatty acid effect. Therefore, nascent apoB is subject to ER-associa ted degradation, re-uptake, and a third distinct degradative pathway that a ppears to target lipoproteins after considerable assembly and involves a po st-ER compartment and PI3K signaling. Physiologic, pathophysiologic, and ph armacologic regulation of net apoB secretion may involve alterations in any of these three degradative steps.