FRI0527 HIGH LDL LEVELS LESSEN BONE DESTRUCTION DURING ANTIGEN-INDUCED ARTHRITIS BY INHIBITING OSTEOCLAST FORMATION AND FUNCTION
Authors: Giuliana Ascone, Irene DI Ceglie, Ilze Bot, Fons van de Loo, Marije Koenders, Peter van der Kraan, Arjen Blom, Martijn van den Bosch, Peter van Lent
Journal: Annals of the Rheumatic Diseases (2019)

<h3>Background</h3> Rheumatoid arthritis (RA) is a chronic inflammatory disease with severe joint inflammation and bone destruction resulting from increased numbers and activity of osteoclasts as important hallmarks. RA is often associated with the metabolic syndrome, characterized by elevated levels of low-density lipoprotein (LDL), which in inflammatory environments is oxidized into oxLDL. However, the effects of high (ox)LDL levels on osteoclasts remains elusive. <h3>Objectives</h3> Here, we investigated the effects of high LDL and oxLDL levels on osteoclast differentiation and bone destruction. <h3>Methods</h3> Antigen-induced arthritis (AIA) was induced in <i>Apoe<sup>-/-</sup></i> mice that spontaneously develop high LDL levels. Bone erosion was assessed with histology and numbers of osteoclasts were determined with staining for tartrate-resistant acid phosphatase (TRAP). Numbers of CD11b<sup>+</sup>/Ly6C<sup>high</sup> and CD11b<sup>-</sup>/Ly6C<sup>high</sup> osteoclast precursors were determined by flow cytometry. <i>In vitro</i> osteoclast differentiation from bone marrow cells was induced with macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappa-B (RANK) for 7 days. Pre-osteoclasts from day 3 and mature osteoclasts from day 6 of differentiation were exposed to LDL/oxLDL. Gene expression was determined with qPCR. <h3>Results</h3> Whereas basal levels of bone resorption were comparable between WT and <i>Apoe<sup>-/-</sup></i>mice, induction of AIA resulted in significantly lower bone resorption in <i>Apoe<sup>-/-</sup></i> mice as compared to WT mice, which was associated with lower number of TRAP<sup>+</sup> osteoclasts along the bone surface. However, the absence of <i>Apoe</i> did not result in altered numbers of osteoclast precursors in the bone marrow of naïve mice, whereas even increased numbers were observed in <i>Apoe<sup>-/-</sup></i> mice during AIA. Moreover, <i>in vitro</i> osteoclastogenesis showed comparable numbers and mRNA expression of osteoclast markers, such as c-Fms, RANK, NFATc1, DC-STAMP, TRAP, CTR, ClC-7, CA-II, Cat K and MMP-9. Together, this showed that the absence of <i>Apoe</i> by itself does not affect osteoclastogenesis. Therefore, we determined whether high levels of LDL cholesterol, a characteristic of <i>Apoe<sup>-/-</sup></i> mice, or oxLDL as the result of LDL oxidation by inflammation led to decreased osteoclast differentiation. Addition of oxLDL, but not LDL, to pre-osteoclasts from day 3 and mature osteoclasts from day 6 of osteoclastogenesis strongly reduced the number of TRAP<sup>+</sup> osteoclasts and their resorptive capacity. This was accompanied by a decreased expression of various osteoclast markers. To determine the mechanism of how oxLDL stimulation might reduce the osteoclast differentiation, we determined the expression of RANK and various members of the co-stimulatory pathway that promotes osteoclast formation. Interestingly, whereas RANK expression was not affected upon oxLDL exposure, we observed decreased expression of osteoclast -associated receptor (<i>Oscar</i>) and the DNAX adaptor protein-12 encoding gene <i>Tyrobp</i>, which regulate the immunoreceptor tyrosine-based activation motif (ITAM) mediated co-stimulation signaling pathway. <h3>Conclusion</h3> <i>Apoe<sup>-/-</sup></i> mice have decreased bone resorption during experimental RA, probably via oxLDL-mediated interference in the co-stimulatory pathway during osteoclastogenesis. <h3>Disclosure of Interests</h3> None declared