Lrp1 in osteoblasts controls osteoclast activity and protects against osteoporosis by limiting PDGF–RANKL signaling

Abstract

Skeletal health relies on architectural integrity and sufficient bone mass, which are maintained through a tightly regulated equilibrium of bone resorption by osteoclasts and bone formation by osteoblasts. Genetic studies have linked the gene coding for low-density lipoprotein receptor-related protein1 (Lrp1) to bone traits but whether these associations are based on a causal molecular relationship is unknown. Here, we show that Lrp1 in osteoblasts is a novel regulator of osteoclast activity and bone mass. Mice lacking Lrp1 specifically in the osteoblast lineage displayed normal osteoblast function but severe osteoporosis due to highly increased osteoclast numbers and bone resorption. Osteoblast Lrp1 limited receptor activator of NF-κB ligand (RANKL) expression in vivo and in vitro through attenuation of platelet-derived growth factor (PDGF-BB) signaling. In co-culture, Lrp1-deficient osteoblasts stimulated osteoclastogenesis in a PDGFRβ-dependent manner and in vivo treatment with the PDGFR tyrosine kinase inhibitor imatinib mesylate limited RANKL production and led to complete remission of the osteoporotic phenotype. These results identify osteoblast Lrp1 as a key regulator of osteoblast-to-osteoclast communication and bone mass through a PDGF–RANKL signaling axis in osteoblasts and open perspectives to further explore the potential of PDGF signaling inhibitors in counteracting bone loss as well as to evaluate the importance of functional LRP1 gene variants in the control of bone mass in humans.

Osteoporosis: A novel path to strength

Maintaining strong bones critically depends on a receptor (Lrp1) for low-density lipoprotein. Bones are continually remodeled, with osteoblast cells adding new bone and osteoclast cells resorbing old bone. Imbalanced growth and resorption can lead to osteoporosis. Genetic studies had previously linked Lrp1 to bone health, but the nature of the link remained unknown. Andreas Niemeier at the University Medical Center Hamburg-Eppendorf in Germany and co-workers used model mice whose osteoblasts lacked Lrp1 to investigate how the receptor is involved in bone turnover. Lrp-1-deficient mice showed severe osteoporosis. They also showed high numbers of osteoclasts but normal numbers of osteoblasts, indicating that lack of the receptor caused increased bone resorption. Treatment of the mice with a drug related to Lrp1 restored bone strength. These results may help to identify new treatments for bone loss.

References

[1] Rachner TD, Khosla S, Hofbauer LC. Osteoporosis: now and the future. Lancet, 2011, 377: 1276-1287

[2] Baron R, Ferrari S, Russell RG. Denosumab and bisphosphonates: different mechanisms of action and effects. Bone, 2011, 48: 677-692

[3] Teitelbaum SL. Bone resorption by osteoclasts. Science, 2000, 289: 1504-1508

[4] Boyle WJ, Simonet WS, Lacey DL. Osteoclast differentiation and activation. Nature, 2003, 423: 337-342

[5] Jones DH et al. Regulation of cancer cell migration and bone metastasis by RANKL. Nature, 2006, 440: 692-696

[6] Schramek D et al. Osteoclast differentiation factor RANKL controls development of progestin-driven mammary cancer. Nature, 2010, 468: 98-102

[7] Sims AM et al. Genetic analyses in a sample of individuals with high or low BMD shows association with multiple Wnt pathway genes. J. Bone Miner. Res., 2008, 23: 499-506

[8] Cao H, Lei S, Deng HW, Wang YP. Identification of genes for complex diseases using integrated analysis of multiple types of genomic data. PLoS ONE, 2012, 7: e42755

[9] Dieckmann M, Dietrich MF, Herz J. Lipoprotein receptors--an evolutionarily ancient multifunctional receptor family. Biol. Chem., 2010, 391: 1341-1363

[10] Herz J, Clouthier DE, Hammer RE. LDL receptor-related protein internalizes and degrades uPA-PAI-1 complexes and is essential for embryo implantation. Cell, 1992, 71: 411-421

[11] Rohlmann A, Gotthardt M, Hammer RE, Herz J. Inducible inactivation of hepatic LRP gene by cre-mediated recombination confirms role of LRP in clearance of chylomicron remnants. J. Clin. Invest., 1998, 101: 689-695

[12] Boucher P, Gotthardt M, Li WP, Anderson RG, Herz J. LRP: role in vascular wall integrity and protection from atherosclerosis. Science, 2003, 300: 329-332

[13] Niemeier A et al. Expression of LRP1 by human osteoblasts: a mechanism for the delivery of lipoproteins and vitamin K1 to bone. J. Bone Miner. Res., 2005, 20: 283-293

[14] Niemeier A et al. Uptake of postprandial lipoproteins into bone in vivo: impact on osteoblast function. Bone, 2008, 43: 230-237

[15] Kawata K et al. Role of the low-density lipoprotein receptor-related protein-1 in regulation of chondrocyte differentiation. J. Cell Physiol., 2010, 222: 138-148

[16] Rohlmann A, Gotthardt M, Willnow TE, Hammer RE, Herz J. Sustained somatic gene inactivation by viral transfer of Cre recombinase. Nat. Biotechnol., 1996, 14: 1562-1565

[17] Rauch A et al. Glucocorticoids suppress bone formation by attenuating osteoblast differentiation via the monomeric glucocorticoid receptor. Cell Metab., 2010, 11: 517-531

[18] Nakashima T, Hayashi M, Takayanagi H. New insights into osteoclastogenic signaling mechanisms. Trends Endocrinol. Metab., 2012, 23: 582-590

[19] Loukinova E et al. Platelet-derived growth factor (PDGF)-induced tyrosine phosphorylation of the low density lipoprotein receptor-related protein (LRP). Evidence for integrated co-receptor function betwenn LRP and the PDGF. J. Biol. Chem., 2002, 277: 15499-15506

[20] Boucher P et al. Platelet-derived growth factor mediates tyrosine phosphorylation of the cytoplasmic domain of the low Density lipoprotein receptor-related protein in caveolae. J. Biol. Chem., 2002, 277: 15507-15513

[21] Takayama Y, May P, Anderson RG, Herz J. Low density lipoprotein receptor-related protein 1 (LRP1) controls endocytosis and c-CBL-mediated ubiquitination of the platelet-derived growth factor receptor beta (PDGFR beta). J. Biol. Chem., 2005, 280: 18504-18510

[22] El Hajj Dib I et al. Imatinib mesylate (Gleevec) enhances mature osteoclast apoptosis and suppresses osteoclast bone resorbing activity. Eur. J. Pharmacol., 2006, 551: 27-33

[23] O’Sullivan S et al. Imatinib promotes osteoblast differentiation by inhibiting PDGFR signaling and inhibits osteoclastogenesis by both direct and stromal cell-dependent mechanisms. J. Bone Miner. Res., 2007, 22: 1679-1689

[24] Baron R, Kneissel M. WNT signaling in bone homeostasis and disease: from human mutations to treatments. Nat. Med., 2013, 19: 179-192

[25] Gordts PL et al. Impaired LDL receptor-related protein 1 translocation correlates with improved dyslipidemia and atherosclerosis in apoE-deficient mice. PLoS ONE, 2012, 7: e38330

[26] Okayasu M et al. Low-density lipoprotein receptor deficiency causes impaired osteoclastogenesis and increased bone mass in mice because of defect in osteoclastic cell-cell fusion. J. Biol. Chem., 2012, 287: 19229-19241

[27] Sanchez-Fernandez MA, Gallois A, Riedl T, Jurdic P, Hoflack B. Osteoclasts control osteoblast chemotaxis via PDGF-BB/PDGF receptor beta signaling. PLoS ONE, 2008, 3: e3537

[28] Bartelt A et al. Apolipoprotein E-dependent inverse regulation of vertebral bone and adipose tissue mass in C57Bl/6 mice: modulation by diet-induced obesity. Bone, 2010, 47: 736-745

[29] Choi HY, Dieckmann M, Herz J, Niemeier A. Lrp4, a novel receptor for Dickkopf 1 and sclerostin, is expressed by osteoblasts and regulates bone growth and turnover in vivo. PLoS ONE, 2009, 4: e7930

[30] Bartelt A et al. Altered endocannabinoid signalling after a high-fat diet in Apoe(-/-) mice: relevance to adipose tissue inflammation, hepatic steatosis and insulin resistance. Diabetologia, 2011, 54: 2900-2910

[31] Bartelt A et al. Brown adipose tissue activity controls triglyceride clearance. Nat. Med., 2011, 17: 200-205

[32] May P et al. Neuronal LRP1 functionally associates with postsynaptic proteins and is required for normal motor function in mice. Mol. Cell. Biol., 2004, 24: 8872-8883

[33] Dempster DW et al. Standardized nomenclature, symbols, and units for bone histomorphometry: a 2012 update of the report of the ASBMR Histomorphometry Nomenclature Committee. J. Bone Miner. Res., 2013, 28: 2-17