Orphan Nuclear Receptor Nur77 Is Involved in Caspase-independent Macrophage Cell Death

Abstract

Activation-induced cell death in macrophages has been observed, but the mechanism remains largely unknown. Activation-induced cell death in macrophages can be independent from caspases, and the death of activated macrophages can even be triggered by the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (zVAD). Here, we show that this type of macrophage death can occur in the septic mouse model and that toll-like receptor (TLR)-2 or TLR4 signaling is required in this process. We conclude that Nur77 is involved in the macrophage death because Nur77 expression correlates with cell death, and cell death is reduced significantly in Nur77-deficient macrophages. The extracellular signal–regulated kinase pathway, which is downstream of TLR2 or TLR4, and myocyte-specific enhancer binding factor 2 (MEF2) transcription factor activity, which is up-regulated by zVAD, are required for Nur77 induction and macrophage death. Reporter gene analysis suggests that Nap, Ets, Rce, and Sp1 sites in the Nur77 promoter are regulated by TLR4 signaling and that MEF2 sites in the Nur77 promoter are regulated by zVAD treatment. MEF2 transcription factors are constitutively expressed and degraded in macrophages, and zVAD increases MEF2 transcription factor activity by preventing the proteolytic cleavage and degradation of MEF2 proteins. This paper delineates the dual signaling pathways that are required for Nur77 induction in macrophages and demonstrates a role of Nur77 in caspase-independent cell death.

MEF2C mediates the activation induced cell death (AICD) of macrophages

“…On the contrary, zVAD enhance the cell death slightly (Figure1B). Consistent with earlier reports [22], we also found that zVAD treatment could prime macrophages for subsequent cell death induced by LPS ( Figure 1B). …”

“…For example, LPS was reported to cause cell death of conA or IFN-γ primed macrophages and to enhance cell death induced by the Yersinia baccillus [17,20,21]. Intriguingly, it was found that zVAD and boc-D, pan-caspase inhibitors that inhibit apoptosis in many different cells, not only had no inhibitory effect on LPS-induced apoptosis in INF-γ primed macrophages but also by themselves induced susceptibility to LPS-induced cell death in macrophages [22]. As zVAD treatment can prime macrophages for LPSinduced cell death, a previous study by Kim et al utilized this model system to investigate the underlying molecular mechanisms [22].…”

“…Intriguingly, it was found that zVAD and boc-D, pan-caspase inhibitors that inhibit apoptosis in many different cells, not only had no inhibitory effect on LPS-induced apoptosis in INF-γ primed macrophages but also by themselves induced susceptibility to LPS-induced cell death in macrophages [22]. As zVAD treatment can prime macrophages for LPSinduced cell death, a previous study by Kim et al utilized this model system to investigate the underlying molecular mechanisms [22]. They showed that induction of the orphan nuclear receptor Nur77 is a key event.…”

“…Induction of Nur77 was found to correlate with macrophage AICD; and Nur77 deficient macrophages were insensitive to zVAD priming. Further analysis implicated a critical role of the MEF family of transcription factors, which are involved in apoptosis in several cell lines including macrophages [22,28,29]. Kim et al showed that zVAD treatment increased the protein stability of MEFs, which could then bind to the Nur77 promoter and are presumably required for induction of Nur77 during macrophage death [22].…”

“…Further analysis implicated a critical role of the MEF family of transcription factors, which are involved in apoptosis in several cell lines including macrophages [22,28,29]. Kim et al showed that zVAD treatment increased the protein stability of MEFs, which could then bind to the Nur77 promoter and are presumably required for induction of Nur77 during macrophage death [22]. The study by Kim et al strongly suggests that cell death in zVAD primed macrophages involves the MEF/Nur77 pathway; however, it remains to be determined whether a similar mechanism also applies to macrophage death under pathological conditions.…”