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Mechanical loading reduces inflammation-induced human osteocyte-to-osteoclast communication.

Pathak JL, Bravenboer N, Luyten FP, Verschueren P, Lems WF, Klein-Nulend J, Bakker AD - Calcif. Tissue Int. (2015)

Bottom Line: However, RA serum enhanced the RANKL/OPG expression ratio by 3.4-fold, while PFF ified this effect.Stat-CM from RA serum-pretreated osteocytes enhanced osteoclastogenesis compared with stat-CM from control serum-pretreated osteocytes, while PFF ified this effect.In conclusion, RA serum, containing inflammatory factors, did not alter the intrinsic capacity of osteocytes to sense mechanical stimuli, but upregulated osteocyte-to-osteoclast communication.

View Article: PubMed Central - PubMed

Affiliation: Department of Oral Cell Biology, Academic Centre for Dentistry Amsterdam (ACTA), University of Amsterdam and VU University Amsterdam, MOVE Research Institute Amsterdam, Amsterdam, The Netherlands.

ABSTRACT
Multiple factors contribute to bone loss in inflammatory diseases such as rheumatoid arthritis (RA), but circulating inflammatory factors and immobilization play a crucial role. Mechanical loading prevents bone loss in the general population, but the effects of mechanical loading in patients with RA are less clear. Therefore, we aimed to investigate whether mechanical stimuli reverse the stimulatory effect of RA serum on osteocyte-to-osteoclast communication. Human primary osteocytes were pretreated with 10 % RA serum or healthy control serum for 7 days, followed by 1 h ± mechanical loading by pulsating fluid flow (PFF). Nitric oxide (NO) and prostaglandin E2 were measured in the medium. Receptor activator of nuclear factor-kappaB ligand (RANKL), osteoprotegerin (OPG), interleukin-6 (IL-6), cyclooxygenase-2 (COX2), matrix-extracellular phosphoglycoprotein (MEPE), cysteine-rich protein 61 (CYR61), and SOST gene expression was quantified by qPCR. Osteoclast precursors were cultured with PFF-conditioned medium (PFF-CM) or static-conditioned medium (stat-CM), and osteoclast formation was assessed. RA serum alone did not affect IL-6, CYR61, COX2, MEPE, or SOST gene expression in osteocytes. However, RA serum enhanced the RANKL/OPG expression ratio by 3.4-fold, while PFF ified this effect. PFF enhanced NO production to the same extent in control serum (2.6-3.5-fold) and RA serum-pretreated (2.7-3.6-fold) osteocytes. Stat-CM from RA serum-pretreated osteocytes enhanced osteoclastogenesis compared with stat-CM from control serum-pretreated osteocytes, while PFF ified this effect. In conclusion, RA serum, containing inflammatory factors, did not alter the intrinsic capacity of osteocytes to sense mechanical stimuli, but upregulated osteocyte-to-osteoclast communication. Mechanical loading ified this upregulation, suggesting that mechanical stimuli could contribute to the prevention of osteoporosis in inflammatory disease.

No MeSH data available.


Related in: MedlinePlus

Pathophysiological model illustrating how mechanical loading can prevent inflammation-induced bone loss in RA. Whether mechanical loading-stimulated osteocytes produced factors that inhibit osteoclast activity was not tested in this study
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Related In: Results  -  Collection


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Fig4: Pathophysiological model illustrating how mechanical loading can prevent inflammation-induced bone loss in RA. Whether mechanical loading-stimulated osteocytes produced factors that inhibit osteoclast activity was not tested in this study

Mentions: Inflammatory cytokines such as IL-1β and TNF-α have been reported to reduce the mechanosensitivity of mouse osteocytes [29]. We found that RA serum did not affect the mechanosensitivity of human osteocytes, which might be explained by the multitude of factors present in RA serum, such as growth factors and their antagonists, pro-inflammatory as well as anti-inflammatory cytokines, cytokine receptors, and antibodies to cytokines. The combined effect of all these factors on the response of osteocytes to mechanical loading might be different from the effect of an individual recombinant cytokine. PFF ified the stimulatory effect of RA serum on the RANKL/OPG expression ratio in osteocytes. Similarly PFF treatment of RA serum-pretreated osteocytes ified the stimulatory effect of RA serum on bone cell-mediated osteoclastogenesis. Our findings are in accordance with data obtained by Kulkarni and colleagues showing that IL-1β enhances osteocyte-mediated osteoclastogenesis, and that mechanical loading reduces this effect [10]. We found that PFF enhanced IL-6, COX2, and CYR61 gene expression to a similar extent in RA serum- and control serum-treated osteocyte cultures, but it did not affect SOST and MEPE gene expression. Kulkarni and colleagues showed that PFF enhances MEPE expression in a murine osteocyte cell line (MLO-Y4) [23], while Robling and colleagues reported that high strain mechanical loading reduces sclerostin levels in mouse ulnae in vivo [25, 33]. Mechanical loading applied via oscillatory fluid flow for 2 h, but not 1 h, reduces SOST expression in UMR 106.01 osteoblasts [34]. This discrepancy might be due to differences in the cell types used, the magnitude and the type of loading applied, and the experimental set up such as in vitro 2D culture and an in vivo mouse model. We applied a peak shear stress of only 0.7 Pa for 1 h, while Papanicolaou and colleagues applied a peak shear stress of 2.0 Pa for 2 h on UMR 106.01 osteoblasts resulting in reduced SOST gene expression [34]. Moreover, we applied pulsating fluid flow on osteocytes for only 1 h, while Robling and colleagues loaded mice ulna in vivo by 360 cycles/day for two consecutive days [25]. In our study, the duration of mechanical loading and/or applied peak shear stress might not have been enough to affect SOST gene expression. We found that PFF did not inhibit the RANKL/OPG gene expression ratio in osteocytes treated with control serum. The RANKL/OPG pathway is important for osteoclastogenesis, but many other osteoclastogenesis-modulating signaling molecules are produced by osteocytes as well [23, 26]. In this study, we observed enhanced gene expression of CYR61 by osteocytes treated with control serum, which might explain the decreased osteoclastogenesis in cultures of osteocyte precursors with PFF-CM from control serum-pretreated osteocytes, since CYR61 inhibits osteoclastogenesis [26]. We found that PFF reduced both primary osteocyte-mediated osteoclastogenesis and the stimulatory effect of RA serum on primary osteocyte-mediated osteoclastogenesis. The effect of rheumatoid factor-positive and negative sera on cytokine and growth factor gene expression by osteocytes and on osteocyte-mediated osteoclastogenesis was similar. Our findings suggest an important role of mechanical loading in the inhibition of both physiological as well as inflammation-induced osteoclastogenesis. Based on our findings, we created a pathophysiological model illustrating how mechanical loading might prevent inflammation-induced bone loss in RA (Fig. 4).Fig. 4


Mechanical loading reduces inflammation-induced human osteocyte-to-osteoclast communication.

Pathak JL, Bravenboer N, Luyten FP, Verschueren P, Lems WF, Klein-Nulend J, Bakker AD - Calcif. Tissue Int. (2015)

Pathophysiological model illustrating how mechanical loading can prevent inflammation-induced bone loss in RA. Whether mechanical loading-stimulated osteocytes produced factors that inhibit osteoclast activity was not tested in this study
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4491366&req=5

Fig4: Pathophysiological model illustrating how mechanical loading can prevent inflammation-induced bone loss in RA. Whether mechanical loading-stimulated osteocytes produced factors that inhibit osteoclast activity was not tested in this study
Mentions: Inflammatory cytokines such as IL-1β and TNF-α have been reported to reduce the mechanosensitivity of mouse osteocytes [29]. We found that RA serum did not affect the mechanosensitivity of human osteocytes, which might be explained by the multitude of factors present in RA serum, such as growth factors and their antagonists, pro-inflammatory as well as anti-inflammatory cytokines, cytokine receptors, and antibodies to cytokines. The combined effect of all these factors on the response of osteocytes to mechanical loading might be different from the effect of an individual recombinant cytokine. PFF ified the stimulatory effect of RA serum on the RANKL/OPG expression ratio in osteocytes. Similarly PFF treatment of RA serum-pretreated osteocytes ified the stimulatory effect of RA serum on bone cell-mediated osteoclastogenesis. Our findings are in accordance with data obtained by Kulkarni and colleagues showing that IL-1β enhances osteocyte-mediated osteoclastogenesis, and that mechanical loading reduces this effect [10]. We found that PFF enhanced IL-6, COX2, and CYR61 gene expression to a similar extent in RA serum- and control serum-treated osteocyte cultures, but it did not affect SOST and MEPE gene expression. Kulkarni and colleagues showed that PFF enhances MEPE expression in a murine osteocyte cell line (MLO-Y4) [23], while Robling and colleagues reported that high strain mechanical loading reduces sclerostin levels in mouse ulnae in vivo [25, 33]. Mechanical loading applied via oscillatory fluid flow for 2 h, but not 1 h, reduces SOST expression in UMR 106.01 osteoblasts [34]. This discrepancy might be due to differences in the cell types used, the magnitude and the type of loading applied, and the experimental set up such as in vitro 2D culture and an in vivo mouse model. We applied a peak shear stress of only 0.7 Pa for 1 h, while Papanicolaou and colleagues applied a peak shear stress of 2.0 Pa for 2 h on UMR 106.01 osteoblasts resulting in reduced SOST gene expression [34]. Moreover, we applied pulsating fluid flow on osteocytes for only 1 h, while Robling and colleagues loaded mice ulna in vivo by 360 cycles/day for two consecutive days [25]. In our study, the duration of mechanical loading and/or applied peak shear stress might not have been enough to affect SOST gene expression. We found that PFF did not inhibit the RANKL/OPG gene expression ratio in osteocytes treated with control serum. The RANKL/OPG pathway is important for osteoclastogenesis, but many other osteoclastogenesis-modulating signaling molecules are produced by osteocytes as well [23, 26]. In this study, we observed enhanced gene expression of CYR61 by osteocytes treated with control serum, which might explain the decreased osteoclastogenesis in cultures of osteocyte precursors with PFF-CM from control serum-pretreated osteocytes, since CYR61 inhibits osteoclastogenesis [26]. We found that PFF reduced both primary osteocyte-mediated osteoclastogenesis and the stimulatory effect of RA serum on primary osteocyte-mediated osteoclastogenesis. The effect of rheumatoid factor-positive and negative sera on cytokine and growth factor gene expression by osteocytes and on osteocyte-mediated osteoclastogenesis was similar. Our findings suggest an important role of mechanical loading in the inhibition of both physiological as well as inflammation-induced osteoclastogenesis. Based on our findings, we created a pathophysiological model illustrating how mechanical loading might prevent inflammation-induced bone loss in RA (Fig. 4).Fig. 4

Bottom Line: However, RA serum enhanced the RANKL/OPG expression ratio by 3.4-fold, while PFF ified this effect.Stat-CM from RA serum-pretreated osteocytes enhanced osteoclastogenesis compared with stat-CM from control serum-pretreated osteocytes, while PFF ified this effect.In conclusion, RA serum, containing inflammatory factors, did not alter the intrinsic capacity of osteocytes to sense mechanical stimuli, but upregulated osteocyte-to-osteoclast communication.

View Article: PubMed Central - PubMed

Affiliation: Department of Oral Cell Biology, Academic Centre for Dentistry Amsterdam (ACTA), University of Amsterdam and VU University Amsterdam, MOVE Research Institute Amsterdam, Amsterdam, The Netherlands.

ABSTRACT
Multiple factors contribute to bone loss in inflammatory diseases such as rheumatoid arthritis (RA), but circulating inflammatory factors and immobilization play a crucial role. Mechanical loading prevents bone loss in the general population, but the effects of mechanical loading in patients with RA are less clear. Therefore, we aimed to investigate whether mechanical stimuli reverse the stimulatory effect of RA serum on osteocyte-to-osteoclast communication. Human primary osteocytes were pretreated with 10 % RA serum or healthy control serum for 7 days, followed by 1 h ± mechanical loading by pulsating fluid flow (PFF). Nitric oxide (NO) and prostaglandin E2 were measured in the medium. Receptor activator of nuclear factor-kappaB ligand (RANKL), osteoprotegerin (OPG), interleukin-6 (IL-6), cyclooxygenase-2 (COX2), matrix-extracellular phosphoglycoprotein (MEPE), cysteine-rich protein 61 (CYR61), and SOST gene expression was quantified by qPCR. Osteoclast precursors were cultured with PFF-conditioned medium (PFF-CM) or static-conditioned medium (stat-CM), and osteoclast formation was assessed. RA serum alone did not affect IL-6, CYR61, COX2, MEPE, or SOST gene expression in osteocytes. However, RA serum enhanced the RANKL/OPG expression ratio by 3.4-fold, while PFF ified this effect. PFF enhanced NO production to the same extent in control serum (2.6-3.5-fold) and RA serum-pretreated (2.7-3.6-fold) osteocytes. Stat-CM from RA serum-pretreated osteocytes enhanced osteoclastogenesis compared with stat-CM from control serum-pretreated osteocytes, while PFF ified this effect. In conclusion, RA serum, containing inflammatory factors, did not alter the intrinsic capacity of osteocytes to sense mechanical stimuli, but upregulated osteocyte-to-osteoclast communication. Mechanical loading ified this upregulation, suggesting that mechanical stimuli could contribute to the prevention of osteoporosis in inflammatory disease.

No MeSH data available.


Related in: MedlinePlus