Polyubiquitination events mediate polymethylmethacrylate (PMMA) particle activation of NF-kappaB pathway

The pathologic response to implant wear-debris constitutes a major component of inflammatory osteolysis and remains under intense investigation. Polymethylmethacrylate (PMMA) particles, which are released during implant wear and loosening, constitute a major culprit by virtue of inducing inflammatory and osteolytic responses by macrophages and osteoclasts, respectively. Recent work by several groups has identified important cellular entities and secreted factors that contribute to inflammatory osteolysis. In previous work, we have shown that PMMA particles contribute to inflammatory osteolysis through stimulation of major pathways in monocytes/macrophages, primarily NF-κB and MAP kinases. The former pathway requires assembly of large IKK complex encompassing IKK1, IKK2, and IKKγ/NEMO. We have shown recently that interfering with the NF-κB and MAPK activation pathways, through introduction of inhibitors and decoy molecules, impedes PMMA-induced inflammation and osteolysis in mouse models of experimental calvarial osteolysis and inflammatory arthritis. In this study, we report that PMMA particles activate the upstream transforming growth factor β-activated kinase-1 (TAK1), which is a key regulator of signal transduction cascades leading to activation of NF-κB and AP-1 factors. More importantly, we found that PMMA particles induce TAK1 binding to NEMO and UBC13. In addition, we show that PMMA particles induce TRAF6 and UBC13 binding to NEMO and that lack of TRAF6 significantly attenuates NEMO ubiquitination. Altogether, these observations suggest that PMMA particles induce ubiquitination of NEMO, an event likely mediated by TRAF6, TAK1, and UBC13. Our findings provide important information for better understanding of the mechanisms underlying PMMA particle-induced inflammatory responses.     

Novel phosphorylation-dependent ubiquitination of tristetraprolin by mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 1 (MEKK1) and tumor necrosis factor receptor-associated factor 2 (TRAF2)

Acute versus chronic inflammation is controlled by the accurate activation and regulation of interdependent signaling cascades. TNF-receptor 1 engagement concomitantly activates NF-κB and JNK signaling. The correctly timed activation of these pathways is the key to account for the balance between NF-κB-mediated cell survival and cell death, the latter fostered by prolonged JNK activation. Tristetraprolin (TTP), initially described as an mRNA destabilizing protein, acts as negative feedback regulator of the inflammatory response: it destabilizes cytokine-mRNAs but also acts as an NF-κB inhibitor by interfering with the p65/RelA nuclear import pathway. Our biochemical studies provide evidence that TTP contributes to the NF-κB/JNK balance. We find that the MAP 3-kinase MEKK1 acts as a novel TTP kinase that, together with the TNF receptor-associated factor 2 (TRAF2), constitutes not only a main determinate of the NF-κB-JNK cross-talk but also facilitates "TTP hypermodification": MEKK1 triggers TTP phosphorylation as prerequisite for its Lys-63-linked, TRAF2-mediated ubiquitination. Consequently, TTP no longer affects NF-κB activity but promotes the activation of JNK. Based on our data, we suggest a model where upon TNFα induction, TTP transits a hypo- to hypermodified state, thereby contributing to the molecular regulation of NF-κB versus JNK signaling cascades.