Rat and human HARE/stabilin-2 are clearance receptors for high- and low-molecular-weight heparins

The human hyaluronic acid (HA) receptor for endocytosis (HARE/stabilin-2) is the primary clearance receptor for systemic HA, chondroitin sulfates, and heparin, but not for heparan sulfate or keratan sulfate (Harris EN, Weigel JA, Weigel PH. J Biol Chem 283: 17341-17350, 2008). HARE is expressed in the sinusoidal endothelial cells (SECs) of liver and lymph nodes where it acts as a scavenger for uptake and degradation of glycosaminoglycans, both as free chains and proteoglycan fragments. Unfractionated heparin (UFH; approximately 14 kDa) and low-molecular-weight heparin (LMWH; approximately 4 kDa) are commonly used in treatments for thrombosis and cancer and in surgical and dialysis procedures. The reported half-lives of UFH and LMWH in the blood are approximately 1 h and 2-6 h, respectively. In this study, we demonstrate that anti-HARE antibodies specifically block the uptake of LMWH and UFH by isolated rat liver SECs and by human 293 cells expressing recombinant human HARE (hHARE). hHARE has a significant affinity (K(d) = 10 microM) for LMWH, and higher affinity (K(d) = 0.06 microM) for the larger UFH. Rat liver SECs or cells expressing the recombinant 190-kDa HARE isoform internalized both UFH and LMWH, and both heparins cross-compete with each other, suggesting that they share the same binding sites. These cellular results were confirmed in ELISA-like assays using purified soluble 190-hHARE ectodomain. We conclude that both UFH and LMWH are cleared by HARE/Stab2 and that the differences in the affinities of HARE binding to LMWH and UFH likely explain the longer in vivo circulating half-life of LMWH compared with UFH.