CELLULAR INTERNALIZATION AND DEGRADATION OF ANTITHROMBIN III-THROMBIN, HEPARIN-COFACTOR II-THROMBIN, AND ALPHA(1)-ANTITRYPSIN-TRYPSIN COMPLEXES IS MEDIATED BY THE LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN

The inhibition of proteinase activity by members of the serine protein ase inhibitor (serpin) family is a critical regulatory mechanism for a variety of biological processes. Once formed, the serpin enzyme compl exes (SECs) are removed from the circulation by a hepatic receptor. Th e present study suggests that this receptor is very likely the low den sity lipoprotein receptor-related protein (LRP), a prominent liver rec eptor. In vitro binding studies revealed that antithrombin III (ATIII) thrombin, heparin cofactor II (HCII). thrombin, and alpha(1)-antitryp sin (alpha(1)-AT). trypsin bound to purified LRP, and their binding wa s inhibited by the 39-kDa receptor-associated protein (RAP), an antago nist of LRP-ligand binding activity, In contrast, native or modified f orms of the inhibitors were unable to bind to LRP. Mouse embryonic fib roblasts, which express LRP, mediate the cellular internalization lead ing to degradation of these SECs, while mouse fibroblasts genetically deficient in LRP showed no capacity to internalize and degrade these c omplexes, SECs were also degraded by HepG2 cells, and this process was inhibited by LRP antibodies, RAP, and chloroquine. The cellular-media ted uptake and degradation was specific for SECs; native or modified f orms of the inhibitors were not internalized and degraded. Finally, in vivo clearance studies in rats demonstrated that RAP inhibited the cl earance of ATIII I-125-thrombin complexes from the circulation. Togeth er, these results indicate that LRP functions as a liver receptor resp onsible for the plasma clearance of SECs.