The nuclear protein PDLIM2 acts as an E3 ubiquitin ligase for the p65 subunit of nuclear factor-B and promotes its degradation.
The transcription factor NF-B (nuclear factor-B) is an important regulator of inflammatory responses. However, NF-B activation must be terminated in a timely manner as excessive inflammation can cause tissue damage and disease. Now, Takashi Tanaka and colleagues report a new way of terminating NF-B activation: PDLIM2 (a nuclear protein containing a PDZ domain and a LIM domain) acts as an E3 ubiquitin ligase, promoting ubiquitylation of the p65 subunit of NF-B and leading to nuclear degradation of p65.
Prior to activation, NF-B components are retained in the cytoplasm in an inactive complex in association with inhibitor of NF-B (IB) proteins. Following activation, IB is ubiquitylated and degraded, allowing the p50 and p65 subunits of NF-B to translocate to the nucleus and induce expression of pro-inflammatory genes such as those encoding the cytokines interleukin-6 (IL-6) and IL-12. IB is transcribed in an NF-B-dependent manner, and this mediates the export of NF-B back into the cytoplasm again, thereby terminating the NF-B response. However, NF-B activation can be terminated in the absence of IB, so other mechanisms must exist. Ubiquitylation and proteasomal degradation of p65 has been proposed as an alternative termination mechanism, and the authors sought to identify a ubiquitin ligase that might be responsible for this.
Polyubiquitylation of p65 was detected in the nucleus of mouse embryonic fibroblasts stimulated with lipopolysaccharide (which activates cells through Toll-like receptors (TLRs)). As PDLIM2 is known to interact with STAT (signal transducer and activator of transcription) molecules in the nucleus and promote their degradation, and as it is expressed in macrophages, dendritic cells and T cells, the authors assessed whether it might also act on p65. PDLIM2 co-precipitated with endogenous p65 and when its effect was assessed in a reporter assay, PDLIM2 suppressed p65-dependent luciferase activity in a dose-dependent manner. Using mutants of PDLIM2 lacking either the LIM domain or the PDZ domain, the authors showed that the LIM domain was essential for polyubiquitylation of p65 molecules. Visualization experiments indicated that PDLIM2 mediates intranuclear trafficking of p65 to insoluble nuclear structures where it is then degraded by the proteasome. Finally, the authors investigated TLR-induced inflammatory responses in Pdlim2-/- mice and observed a twofold to fourfold increase in the production of IL-6 and IL-12 in response to lipopolysaccharide in these mice compared with wild-type mice.
This study shows that nuclear degradation mediated by PDLIM2 is an important mechanism for terminating NF-B responses. Further work will be required to determine how PDLIM2 activity itself is regulated.
Elaine Bell
References
Tanaka, T. et al. PDLIM2-mediated termination of transcription factor NF-κB activation by intranuclear sequestration and degradation of the p65 subunit. Nature Immunol.8, 584–591 (2007) | Article | PubMed |
B and promotes its degradation.
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