Transthyretin in high density lipoproteins: association with apolipoprotein A-I

Previous studies have revealed the presence of transthyretin (TTR) on lipop roteins, To further address this issue, we fractionated plasma Lipoproteins from 9 normal individuals, 10 familial amyloidotic polyneuropathy (FAP) pa tients, and 19 hyperlipidemic subjects using gel filtration. In the majorit y of the subjects, as well as in 9 of the 10 FAP patients and 11 of the 19 patients with hyperlipidemia, TTR was detected by ELISA in the high density Lipoprotein (HDL) fraction. The presence of TTR in HDL was con firmed by d irect sequencing and by immunoblotting; using non-reducing conditions, TTR was found by immunoblotting in a high molecular weight complex, which react ed also for apolipoprotein A-I (apoA-I), The amount of TTR present in HDL ( HDL-TTR), as quantified by ELISA corresponded to 1-2% of total plasma TTR, However, no detectable TTR levels were found in HDL fraction from 6 of the hyperlipidemic subjects. No correlation was found between the lack of TTR i n HDL and plasma levels of total, LDL-, or HDL-associated cholesterol as we ll as levels of apoA-I and total plasma TTR, Ligand binding experiments sho wed that radiolabeled TTR binds to the HDL fraction of individuals with KDL -TTR but not to the corresponding fractions of individuals devoid of HDL-TT R, suggesting that HDL composition may interfere with TTR binding. The comp onent(s) to which TTR binds in the HDL fraction were investigated. Polyclon al antibody against apoA-I was able to block the interaction of TTR with HD L, suggesting that the interaction of TTR with the HDL particle occurs via apoA-I, This hypothesis was further demonstrated by showing the formation o f a complex of TTR with HDL and apoA-I by crosslinking experiments. Further more, anti-apoA-I immunoblot under native conditions suggested the existenc e of differences in HDL particle properties and/or stability between indivi duals with and without HDL-TTR.