Soluble Low Density Lipoprotein Receptor-related Protein (LRP) Circulates in Human Plasma*
- Kathryn A. Quinn‡,
- Philip G. Grimsley‡,
- Yang-Ping Dai‡,
- Michael Tapner§,
- Colin N. Chesterman‡ and
- Dwain A. Owensby‡¶‖
- From the ‡Center for Thrombosis and Vascular Research, University of New South Wales, Sydney 2052, Australia, the§Storr Liver Unit, Department of Medicine, University of Sydney at Westmead Hospital, Westmead 2145, Australia, and the¶Illawarra Regional Hospital, Wollongong, New South Wales 2500, Australia
Abstract
Our studies have identified a soluble molecule in normal human plasma and serum with the characteristics of the α-chain of the low density lipoprotein receptor-related protein (LRP). LRP is a large multifunctional receptor mediating the clearance of diverse ligands, including selected lipoproteins, various protease inhibitor complexes, and thrombospondin. A soluble molecule (sLRP) has been isolated from plasma using an affinity matrix coupled with methylamine-activated α2-macroglobulin, the ligand uniquely recognized by LRP, and eluted with EDTA. This eluate contains a protein that co-migrates on SDS-polyacrylamide gel electrophoresis with authentic human placental LRP α-chain, is recognized by anti-LRP α-chain monoclonal antibodies, and binds the 39-kDa receptor-associated protein (RAP) and tissue plasminogen activator-inhibitor complexes. A similar RAP-binding molecule was detected in medium conditioned for 24 h by primary cultures of rat hepatocytes, suggesting that the liver may be the in vivosource of sLRP. In contrast, immunoprecipitation experiments failed to detect the production of sLRP by cultured HepG2 hepatoma and primary human fibroblast cells. Addition of a soluble form of LRP to cultured HepG2 cells resulted in a significant inhibition of capacity of these cells to degrade tPA, a process that has been demonstrated to be mediated by cell surface LRP. Preliminary data indicate that the concentration of sLRP is altered in the plasma of patients with liver disease. Increased levels of sLRP may antagonize the clearance of ligands by cell bound LRP perturbing diverse processes including lipid metabolism, cell migration and extracellular proteinase activity.
Footnotes
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↵* This work was supported by a grant from the National Health and Medical Research Council of Australia.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‖ To whom correspondence should be addressed: Center for Thrombosis and Vascular Research, School of Pathology, Wallace Wurth Bldg., University of New South Wales, Kensington 2052, Australia. Fax: 61-2-9385-1389.
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↵1 The abbreviations used are: LRP, low density lipoprotein receptor related protein; sLRP, soluble LRP-like molecule; BSA, bovine serum albumin; mAb, monoclonal antibody; RAP, receptor-associated protein; PAGE, polyacrylamide gel electrophoresis; PAI-1, plasminogen activator inhibitor-1; tPA, tissue plasminogen activator; ABTS, 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid; α2M, α2-macroglobulin; Mes, 4-morpholineethanesulfonic acid; EMEM, Earle’s modified Eagle’s medium; HFF, human foreskin fibroblasts; FBS, fetal bovine serum; NIDDM, non-insulin-dependent diabetes mellitus.
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↵2 P. G. Grimsley, K. A. Quinn, and D. A. Owensby, manuscript in preparation.
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- Received March 17, 1997.
- Revision received June 9, 1997.
