Uptake of postprandial lipoproteins into bone in vivo: Impact on osteoblast function ☆ Andreas Niemeier a, ⁎, Dagmara Niedzielska b , Rukiye Secer b , Arndt Schilling c , Martin Merkel d , Carlos Enrich e , Patrick C.N. Rensen f , Joerg Heeren b a Department of Orthopaedics, University Medical Center Hamburg-Eppendorf, Hamburg, Germany b Department of Biochemistry and Molecular Biology II: Molecular Cell Biology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany c Department of Trauma- Hand- and Reconstructive Surgery, University Medical Center Hamburg-Eppendorf, Hamburg, Germany d Department of Internal Medicine, University Medical Center Hamburg-Eppendorf, Hamburg, Germany e Departament de Biologia Cellular, Facultat de Medicina, Universitat de Barcelona, Spain f Department of General Internal Medicine, Endocrinology and Metabolic Diseases, Leiden University Medical Center, The Netherlands abstract article info Article history: Received 27 November 2007 Revised 17 March 2008 Accepted 19 March 2008 Available online 10 April 2008 Edited by: Dr. R. Baron Keywords: Postprandial metabolism Lipoproteins Osteoblasts Vitamin K Osteocalcin Dietary lipids and lipophilic vitamins are transported by postprandial lipoproteins and are required for bone metabolism. Despite that, it remains unknown whether bone cells are involved in the uptake of circulating postprandial lipoproteins in vivo. The current study was performed to investigate a putative participation of bone in the systemic postprandial lipoprotein metabolism in mice, to identify potentially involved cell type populations and to analyze whether lipoprotein uptake affects bone function in vivo. As a model for the postprandial state, chylomicron remnants (CR) were injected intravenously into mice. Next to the liver and compared to other organs, bone appeared to be the second most important organ for the clearance of radiolabeled CR particles from the circulation in vivo. In addition, uptake of radiolabeled CR by primary murine osteoblasts and hepatocytes was quantified to be in a similar range in vitro. A complementary approach with fluorescently labeled CR and immunohistochemical staining for apoE proved that intact CR particles were taken up into bone and liver. Electron microscopy localization studies of bone sections revealed CR uptake into sinusoidal endothelial cells, macrophages and osteoblasts. The relative amount of radiolabeled CR uptake into femoral cortical bone, representing predominantly osteoblasts, and bone marrow, representing predominantly non-osteoblast cells, was within the same range. Most importantly, the injection of vitamin K 1 - enriched CR resulted in an increase of the degree of osteocalcin carboxylation in vivo while total osteocalcin concentrations remained unaffected, giving functional proof that osteoblasts process CR in vivo. In conclusion, here we demonstrate that bone is involved in the postprandial lipoprotein metabolism in mice. Osteoblasts participate in CR clearance from the circulation, which has a direct impact on the secretory function of osteoblasts. © 2008 Elsevier Inc. All rights reserved. Introduction The nutritional composition of lipids and lipophilic vitamins is known to influence bone metabolism, in particular bone formation. A growing body of evidence from experimental approaches and clinical investigations indicates that diet-derived lipids, in particular essential and polyunsaturated fatty acids [1–6] and lipid soluble vitamins, such as vitamin K [7–10] play an important role for osteoblast function. Chylomicrons (CM) and their remnants (CR) function as plasma carriers of these lipid constituents in the postprandial phase. CM undergo intravascular hydrolysis by the action of lipoprotein lipase (LPL), thereby generating CR, which are taken up by target organs that require CR components for specific metabolic needs [11–13]. The majority of CR is cleared by the liver via receptor-mediated endocytosis through the low density lipoprotein (LDL) receptor and LDL receptor related protein 1 (LRP1) [14,15]. Binding of CR to all known CR receptors is mediated by apolipoprotein E (apoE), a structural protein component of CR. The receptor-associated protein (RAP) is a chaperone for LDLR gene family members [16] and is widely used as a model ligand to analyze lipoprotein receptor–lipoprotein interactions. Some extra- hepatic tissues also have a capacity for CR plasma clearance. However, the molecular mechanism of CR uptake into these organs is less well understood than for the liver. We and others have previously shown that apoE and LRP1, next to their central role in CR metabolism in the liver, are expressed by osteoblasts [17–19]. ApoE expression is induced during osteoblast differentiation and apoE-deficient mice display a high bone forma- tion rate [19]. LRP1 mediates the uptake of apoE-containing CR and CR-associated vitamin K 1 (CR-K 1 ) into human osteoblasts in vitro [18]. The lipophilic vitamin K is bound exclusively to lipoproteins in Bone 43 (2008) 230–237 ☆ The work was supported by the DFG grants Ni 637/2-3 and He 3645/2-1. ⁎ Corresponding author. Department of Orthopaedics and IBMII: Molecular Cell Biology, University Medical Center Hamburg-Eppendorf, Martinistrasse 52, D — 20246 Hamburg, Germany. Fax: +49 40 42803 4592. E-mail address: niemeier@uke.uni-hamburg.de (A. Niemeier). 8756-3282/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.bone.2008.03.022 Contents lists available at ScienceDirect Bone journal homepage: www.elsevier.com/locate/bone