Submitted on December 22, 2008
Revised on March 24, 2009
Accepted on April 7, 2009
RAT AND HUMAN HARE/STABILIN-2 ARE CLEARANCE RECEPTORS FOR HIGH- AND LOW-MOLECULAR WEIGHT HEPARINS
Edward Noel Harris1*, Bruce Anthony Baggenstoss2, and Paul H. Weigel3
1 University of Oklahoma Health Sciences Center
2 University of Oklahoma HSC
3 The University of Oklahoma
* To whom correspondence should be addressed. E-mail: edward-harris{at}ouhsc.edu.
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 et al, J. Biol Chem. 283, 17341, 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; ~14 kDa) and low-molecular weight heparin (LMWH; ~4 kDa) are commonly used in treatments for thrombosis, cancer, and in surgical and dialysis procedures. The reported half-lives of UFH and LMWH in the blood are about 1 hr and 2-6 hr, 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. Human HARE has a significant affinity (Kd = 10 µM) for LMWH, and higher affinity (Kd =0.06 µM) 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 ecto-domain. 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 to UFH.
