Apolipoprotein(a) secretion from hepatoma cells is regulated in a size-dependent manner by alterations in disulfide bond formation

Fatiha Nassir, Yan Xie, and Nicholas O. Davidson

Medicine, Washington University Medical School, St. Louis, MO 63110

Corresponding Author: nod{at}im.wustl.edu

Apolipoprotein(a), [apo(a)], is a large, disulfide linked glycoprotein synthesized by hepatocytes. We have examined the role of disulfide bond formation in the processing of apo(a) using human and rat hepatoma cells expressing apo(a) isoforms containing varying numbers of K4 domains, following treatment with DTT. Hepatoma cells expressing 6- or 9-K4 isoforms revealed ~90% inhibition of apo(a) secretion following DTT treatment although larger isoforms, containing 13- or 17-K4 domains demonstrated continued secretion, up to 30% of control values, suggesting that a fraction of the larger isoforms is at least partially DTT-resistant. Wash-out experiments demonstrated that these effects were completely reversible, for all isoforms studied, with no enhanced degradation associated with prolonged intracellular retention. DTT treatment was associated with enhanced binding of apo(a) with the endoplasmic reticulum-associated chaperone proteins calnexin, calreticulin and BiP, which was reversible upon DTT removal. The chemical chaperone 6-aminohexanoic acid, previously demonstrated by others to rescue defective apo(a) secretion associated with alterations in glycosylation, failed to alter the secretion of apo(a) following DTT treatment. The demonstration that DTT modulates apo(a) secretion, in a manner influenced by both the type and number of K4 repeats, extends understanding of the mechanisms that regulate its exit from the endoplasmic reticulum.

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