Specific interaction of oxidized low-density lipoprotein with thrombospondin-1 inhibits transforming growth factor-beta from its activation.

Oxidized LDL (Ox-LDL) plays atherogenic roles, whereas thrombospondin-1 (TSP-1) is thought to be anti-atherogenic through activation of TGF-beta that contributes to plaque stabilization. Ox-LDL was prepared by incubating of human LDL with CuSO4. Effect of Ox-LDL on TSP-1-induced TGF-beta activation was examined in the present study. Incubation of Ox-LDL with mouse peritoneal macrophages for 3 days resulted in reduction in amounts of active TGF-beta in the culture medium by 70-78% when compared with that of parallel incubation without Ox-LDL. TSP-1 could enhance conversion of latent TGF-beta1 into active TGF-beta1 in a cell-free system. This TSP-1-mediated latent TGF-beta1 activation was inhibited by 30% by Ox-LDL, suggesting the possible interaction of Ox-LDL with TSP-1. Incubation of TSP-1 with [125I]Ox-LDL or [125I]LDL, followed by immunoprecipitation with an anti-TSP-1 antibody demonstrated that a significant amount of [125I]Ox-LDL was co-precipitated with TSP-1 while precipitation of [125I]LDL was negligible. Furthermore, upon TSP-1-conjugated Sepharose 4B affinity chromatography, both [125I]Ox-LDL and [125I]latent TGF-beta1 bound to the affinity gel were eluted by unlabeled Ox-LDL. These findings indicate that Ox-LDL interacts with TSP-1 and suppresses subsequent TSP-1-dependent TGF-beta activation, revealing a novel atherogenic function of Ox-LDL.