Phosphatidylserine-Containing Liposomes Inhibit the Differentiation of Osteoclasts and Trabecular Bone Loss

Abstract

Liposomes containing phosphatidylserine (PS) are engulfed by phagocytes including macrophages, microglia, and dendritic cells. PS liposomes (PSLs) mimic the effects of apoptotic cells on these phagocytes to induce the secretion of anti-inflammatory molecules and to inhibit the maturation of dendritic cells. However, the effects of PSLs on osteoclasts, which are also differentiated from the common myeloid precursors, remain to be determined. This study investigated the effects of PSLs on the osteoclastogenesis. In the rat bone marrow culture system, osteoclast precursors phagocytosed PSLs to secrete TGF-β1 and PGE2, which in turn inhibited osteoclastogenesis through the downregulation of receptor activator for NF-κB ligand, receptor activator of NF-κB, ICAM-1, and CD44. Consistent with these in vitro observations, i.m. injection of PSLs significantly increased the plasma level of TGF-β1 and PGE2 and decreased the expression of receptor activator for NF-κB ligand, receptor activator of NF-κB, and ICAM-1 in the skeletal tissues of ankle joints of rats with adjuvant arthritis (AA). A quantitative analysis using microcomputed tomography revealed that PSLs as well as TGF-β1 together with PGE2 significantly inhibited AA-induced trabecular bone loss. These observations strongly suggest that PSLs generate TGF-β1 and PGE2 release, leading to inhibit osteoclastogenesis and AA-induced trabecular bone loss. Because PS is a component of the cell membrane, PSLs therefore can be a potentially effective pharmacological intervention against abnormal bone loss, such as osteoporosis without deleterious side effects.

Phosphatidylserine-containing liposomes suppress inflammatory bone loss by ameliorating the cytokine imbalance provoked by infiltrated macrophages

“…The mean trabecular bone volume per unit of metaphysis (BV/TV) was significantly decreased in AA rats to a similar degree as that observed in our previous experiments. 17 Furthermore, numerous TRAP-positive osteoclasts were found in the bone cavity of the distal epiphysis of the tibia of AA rats ( Figure 1e). Treatment with PSLs significantly promoted the recovery of the AA-induced paw volume increase (Figure 1a and b), the increase in the synovial thickness of ankle joints ( Figure 1c; horizontal bars), bone destruction (Figure 1d) and the increased number of TRAP-positive osteoclasts (Figure 1e).…”

“…23,37 Third, PSLs are phagocytosed by the osteoclast precursor to directly decrease the expression of RANK by osteoclast precursor and the production of RANKL in cultured rat bone marrow cells. 17 Therefore, it is considered that the inhibitory effect of PSLs on the trabecular bone loss in AA rats completely depends on the inhibition of the RANKL/RANK pathway. However, a possibility that decreased RANKL/ RANK expression may simply be due to the decrease in levels of inflammation cannot be totally ruled out.…”

“…Therefore, in our previous study, to examine the direct effect of PSLs on osteoclastogenesis, the treatment of PSLs was started prior to the formation of osteoclasts in AA rats (14 days after CFA injection). 17 However, it is not clear whether PSLs can directly influence the infiltration of macrophages and T cells. Therefore, in this study, to elucidate the direct effect of PSLs on the infiltration of macrophages and T cells, the PSLs treatment was initiated at the beginning of their infiltration in the ankle joints (10 days after CFA injection).…”

“…2,15,16 More recently, we have demonstrated that a strong inhibitory effect on the inflammatory bone loss of PSLs is attributable to the inhibition of osteoclastogenesis through the secretion of both PGE 2 and TGF-b1 by osteoclast precursors after phagocytosis of PSLs. 17 However, it is also reasonable that infiltrated macrophages can undergo a phenotypic change from pro-to anti-inflammatory cells after phagocytosis of PSLs. This phenotypic change likely has important consequences on inflammatory bone loss, because pro-inflammatory cytokines are strong inducers of the receptor activator of nuclear factor-kB (NF-kB) ligand (RANKL) and RANK, which have essential roles in differentiation and the activity of osteoclasts.…”

Gain-of-function mutations in the phosphatidylserine synthase 1 (PTDSS1) gene cause Lenz-Majewski syndrome

“…In other related skeletal dysplasias, such as Ghosal syndrome 42 and craniometaphyseal dysplasia 43 , abnormal osteoblast-osteoclast communication seems to be the basis of the pathomechanism, which might also apply in LMS. Indeed, phosphatidylserine-containing liposomes were shown to inhibit osteoclast differentiation and prevent trabecular bone loss 44 , suggesting application of phosphatidylserine as a potential pharmacological intervention in osteoporosis. We propose, therefore, that osteoclast dysfunction, usually associated with osteopetrosis, might also be implicated in the generalized bone hyperostosis and sclerosis in individuals with LMS.…”