Presecretory degradation of apolipoprotein[a] is mediated by the proteasome pathway

Abstract

Plasma levels of atherogenic lipoprotein [a] (Lp[a]) vary over a 1000-fold range and are largely determined by the gene for its unique glycoprotein, apolipoprotein [a] (apo[a]). The apo[a] locus comprises more than 100 alleles, encoding proteins from Ͻ 300 to Ͼ 800 kDa. Using primary baboon hepatocyte cultures, we previously demonstrated that differences in the secretion efficiency of apo[a] allelic variants contribute to the variation in plasma Lp[a] levels. In the current study, we investigated the mechanism of apo[a] presecretory degradation. The proteasome inhibitors, acetyl-leucyl-leucyl-norleucinal and lactacystin, prevented apo[a] degradation and increased apo[a] secretion. Transfection with an HA-tagged ubiquitin construct demonstrated the accumulation of ubiquitinated apo[a] in the presence of lactacystin. These results suggest a role for the cytoplasmic proteasome in apo[a] proteolysis. Apo[a] that accumulated intracellularly in the presence of lactacystin remained sensitive to endo-B-N-glucosaminidase H, and apo[a] degradation was reversibly inhibited by brefeldin A, suggesting that transport to a post-endoplasmic reticulum (ER) pre-medial Golgi compartment is required for apo[a] degradation. Newly synthesized apo[a] bound to the ER chaperone calnexin and conditions that enhanced this interaction prevented apo[a] degradation, suggesting that calnexin can protect apo[a] from proteolysis. These studies provide further support for the role of the proteasome in endoplasmic reticulum quality control, and expand this role to one that influences plasma levels of the atherogenic lipoprotein Lp[a].-White, A. L., B. Guerra, J. Wang, and R. E. Lanford. Presecretory degradation of apolipoprotein[a] is mediated by the proteasome pathway.

Apolipoprotein [a] genotype influences isoform dominance pattern differently in African Americans and Caucasians

“…The exact causes for these differences have not been clarified, and they may involve both regulation of the apo[a] gene, and factors involved in translation and/or secretion. Previous in vitro findings have suggested that the length of the apo[a] protein, i.e., the number of K4 repeats, could be a major determinant of the efficiency of apo[a] secretion in vivo (18)(19)(20)(21)(22)(23)(24). However, our results argue against the possibility that apo[a] size is the sole predictor of dominance, suggesting that additional mechanisms are involved.…”

“…Using primary hepatocytes from baboons, White and co-workers demonstrated that larger size apo[a] isoforms are secreted from hepatocytes at a slower rate than smaller size apo[a] isoforms, and that a greater proportion of larger apo[a] isoforms is targeted for intracellular degradation (18)(19)(20)(21). Similar results have been obtained by other investigators using transfected cells (22,23).…”

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

“…The development of mice transgenic for a 17 K4 human apo[a] cDNA under the control of the mouse transferrin promoter (23,52), and subsequently of mice transgenic for both apo[a] and human apoB (Lp[a] transgenic mice) (53,54), has provided in vivo models to study some aspects of Lp[a] metabolism. The characteristics of human apo[a] synthesis and secretion and Lp[a] assembly in hepatocytes cultured from Lp[a] transgenic mice (55)(56)(57) are similar to those documented for endogenous apo[a] in primary baboon hepatocytes (20,21,30,31,33) and for transfected human apo [a] proteins in transformed liver cell lines (24,32,58). One important difference between baboon apo[a] and human apo[a] expressed in mouse hepatocytes, however, is that the extent of human apo[a] presecretory degradation ( Ͼ 80%) (55,56) is much greater than expected for the relatively small (17 K4) apo[a] protein expressed in mice (30).…”

“…Posttranscriptional mechanisms are also important. Apo[a] is inefficiently secreted by the hepatocyte and a portion of the protein is retained in the endoplasmic reticulum (ER) and degraded by the proteasome (30,31). Large apo[a] isoforms tend to be degraded to a greater extent than small isoforms (24,30,32), accounting at least partially for the inverse correlation between apo[a] size and plasma Lp[a] level.…”

Role of N-linked carbohydrate processing and calnexin in human hepatic lipase secretion

“…5B). Because trimming of glucose residues from nascent N-linked glycans occurs very rapidly, CST added only to the chase medium can be used to trap radiolabeled proteins in their monoglucosylated form (40). Under these conditions, a 2.2-fold increase in association of HL with calnexin was observed compared to control cells (Fig.…”