Role of calnexin, calreticulin, and endoplasmic reticulum mannosidase I in apolipoprotein(a) intracellular targeting

Apolipoprotein(a) [apo(a)] is a component of atherogenic lipoprotein(a) [Lp(a)]. Differences in the extent of endoplasmic reticulum (ER) associated degradation (ERAD) of apo(a) allelic variants contribute to the >1000-fold variation in plasma Lp(a) levels. Using human apo(a) transgenic mouse hepatocytes, we analyzed the role of the ER chaperones calnexin (CNX) and calreticulin (CRT), and ER mannosidase I in apo(a) intracellular targeting. Co-immunoprecipitation and pulse-chase analyses revealed similar kinetics of apo(a) interaction with CNX and CRT, peaking 15-30 min after apo(a) synthesis. Trapping of apo(a) N-linked glycans in their monoglucosylated form, by posttranslational inhibition of ER glucosidase activity with castanospermine (CST), enhanced apo(a)-CNX/CRT interaction and prevented both apo(a) secretion and ERAD. Delay of CST addition until 20 or 30 min after apo(a) synthesis [when no apo(a) had yet undergone degradation or Golgi-specific carbohydrate modification] allowed a portion of apo(a) to be secreted or degraded. These results are consistent with a transient apo(a)-CNX/CRT association and suggest that events downstream of CNX/CRT interaction determine apo(a) intracellular targeting. Inhibition of ER mannosidase I with deoxymannojirimycin or kifunensine had no effect on apo(a) secretion, but inhibited proteasome-mediated apo(a) ERAD even under conditions where apo(a)CNX/CRT interaction was prevented. These results suggest a role for an additional, mannose-specific, ER lectin in targeting secretory proteins to the proteasome for destruction.