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Expression of LRP1 by Human Osteoblasts: A Mechanism for the Delivery of Lipoproteins and Vitamin K 1 to Bone
Author(s) -
Niemeier Andreas,
Kassem Moustapha,
Toedter Klaus,
Wendt Dorte,
Ruether Wolfgang,
Beisiegel Ulrike,
Heeren Joerg
Publication year - 2005
Publication title -
journal of bone and mineral research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.882
H-Index - 241
eISSN - 1523-4681
pISSN - 0884-0431
DOI - 10.1359/jbmr.041102
Subject(s) - osteocalcin , osteoblast , chemistry , vitamin k2 , bone cell , endocrinology , medicine , vitamin , calcitriol receptor , biochemistry , microbiology and biotechnology , biology , receptor , in vitro , alkaline phosphatase , enzyme
Accumulating clinical and experimental data show the importance of dietary lipids and lipophilic vitamins, such as vitamin K 1 , for bone formation. The molecular mechanism of how they enter the osteoblast is unknown. Here we describe the expression of the multifunctional LRP1 by human osteoblasts in vitro and in vivo. We provide evidence that LRP1 plays an important role in the uptake of postprandial lipoproteins and vitamin K 1 by human osteoblasts. Introduction: Chylomicrons (CM) and their remnants (CR) represent the postprandial plasma carriers of dietary lipids. Dietary vitamin K 1 is known to be transported in the circulation as part of CM/CR and is required by osteoblasts as an essential co‐factor for the γ‐carboxylation of bone matrix proteins. The molecular mechanisms underlying the delivery of lipophilic substances to bone are not understood. In this study, the expression and function of CM/CR receptors was examined in human osteoblasts. Materials and Methods: Four human osteoblast‐like cell lines were analyzed: two osteosarcoma lines (MG63, SaOS‐2) and two telomerase‐immortalized human bone marrow stromal cell lines (hMSC‐TERT ‘4’ and ‘20’) after 1,25(OH) 2 vitamin D 3 induction of osteoblastic differentiation (hMSC‐TERT‐OB). Receptor expression was examined by Western blotting and immunohistochemistry of normal human bone sections. Endocytotic receptor function was analyzed by cellular uptake assays using fluorescent and radiolabeled human CR. Vitamin K 1 ‐enriched CR (CR‐K 1 ) were generated in vivo after oral vitamin administration and vitamin K 1 uptake by osteoblasts was measured by HPLC. The effect of CR‐K 1 uptake on osteocalcin carboxylation was measured by ELISA. Results: Osteoblasts exhibit high levels of protein expression of the CR receptors LRP1 and LDLR. VLDLR is expressed to a lower degree. Immunohistochemistry of normal human bone sections showed strong LRP1 expression by osteoblasts and marrow stromal cells. Uptake of fluorescent CR by osteoblasts resulted in the typical pattern of receptor‐mediated endocytosis. CR uptake was stimulated by the exogenous addition of the lipoprotein receptor ligands apolipoprotein E and lipoprotein lipase. Uptake was reduced by the known LRP1 inhibitors RAP, lactoferrin, and suramin, but not by LDL, which exclusively binds to the LDLR. Vitamin K 1 uptake by hMSC‐TERT‐OB after incubation with CR‐K 1 was also shown to be sensitive to LPL stimulation and the LRP1 specific inhibitor lactoferrin. CR‐K 1 uptake into osteoblasts stimulated the γ‐carboxylation of osteocalcin. Conclusion: Human osteoblasts express receptors of the LDLR family with a capacity for vitamin K 1 uptake through CR endocytosis, a novel mechanism for the delivery of dietary lipids and lipophilic vitamins to human bone. The current data suggest that, among the expressed receptors, LRP1 plays a predominant role.