Wear debris inhibition of anti-osteoclastogenic signaling by interleukin-6 and interferon-γ -: Mechanistic insights and implications for periprosthetic osteolysis

Background: Wear debris challenge of macrophages provokes the generation of proinflammatory cytokines, which contribute to periprosthetic osteolysis. However, it is not known whether this effect is accompanied by reprogramming of other cytokines present within the periprosthetic tissue that may be involved in anti-osteoclastogenic activities. In the present study, we examined the ability of wear debris particles to inhibit the signaling of two such cytokines, interleukin-6 and interferon-gamma. Methods: Human osteoclast precursor cells were challenged with particles of titanium or polymethylmethacrylate bone cement prior to the addition of the cytokines interleukin-6 or interferon-gamma. Interleukin-6 signaling was determined by measuring the activation of STAT3 signal transduction with use of immunoblotting and electrophoretic mobility shift assays. Interferon-gamma signaling was determined by measuring the activation of STAT1 with use of immunoblotting and electrophoretic mobility shift assays and by measuring the expression of interferon-gamma-inducible genes with use of real-time reverse transcription-polymerise chain reaction assays. Involvement of mitogen-activated protein kinases in cytokine signaling was assessed by including mitogen-activated protein kinase inhibitors in these assays and also by means of immunoblot assessment of mitogen-activated protein kinase activation by wear debris particles. Wear debris modulation of expression of the cytokine suppressors SOCS1 and SOCS3 (as well as pro-inflammatory mediators) was assessed with use of real-time reverse transcription-polymerise chain reaction assays. Results: Both titanium and polymethyl methacrylate particles potently inhibited interleukin-6-induced STAT3 activation in human osteoclast precursor cells. Inhibition of p38 mitogen-activated protein kinase, which is activated by titanium and polymethylmethacrylate, reversed the inhibitory effects of these particles on interleukin-6 signaling, whereas inhibition of ERK and JNK mitogen-activated protein kinases (which are also activated by both types of wear debris) had no effect. Titanium and polymethylmethacrylate also both induced expression of SOCS3, an inhibitor of interleukin-6 signaling. In addition to its effects on interleukin-6 signaling, titanium also profoundly inhibited the interferon-gamma-induced activation of STAT1 and the expression of interferon-gamma-inducible genes, whereas polymethylmethacrylate had no effect on interferon-gamma signaling. Conclusions: Titanium inhibits both interferon-gamma and interleukin-6 signaling in human osteoclast precursor cells, whereas polymethylmethacrylate bone cement inhibits only the latter. Wear particle inhibition of interleukin-6 specifically involves the activation of p38 mitogen-activated protein kinase and is accompanied by substantial induction of SOCS3, an inhibitor of interleukin-6 signaling. In contrast, titanium inhibition of interferon-gamma signaling is not dependent on mitogen-activated protein kinase activation and is accompanied by only modest induction of the interferon-gamma inhibitor SOCS1. Clinical Relevance: The critical role of wear debris in the development of periprosthetic osteolysis likely involves the inhibition of anti-inflammatory/anti-osteoclastogenic cytokine signaling in addition to the well-established induction of pro-inflammatory mediators. Strategies to augment these "protective" signaling pathways may therefore have therapeutic potential for the treatment of periprosthetic osteolysis.