In this report, we have found that the E3 ubiquitin ligase AIP2 facilitates T-cell activation and inhibits activation-induced T-cell apoptosis. AIP2 suppresses T-cell apoptosis by promoting the degradation of EGR2, a transcription factor that is responsible for FasL expression. These findings uncovered a previously uncharacterized mechanism underlying AIP2 in regulation of T-cell function in mammals.
Activation-induced cell death is one crucial mechanism that maintains homeostasis in the peripheral lymphoid tissues after clonal expansion. Fas and FasL-mediated apoptosis play an important role in deleting clonally expanded T cells that are no longer useful (16
). Mice deficient in Fas or FasL show defects in peripheral T-cell deletion and eventually develop autoimmune disorders (40
). Several transcription factors, including NFAT, AP-1, NF-κB, and the EGR family proteins, have been found to regulate the expression of FasL during the activation-induced death of T cells (22
). Our studies here demonstrate that AIP2 regulates FasL expression by catalyzing EGR2 for ubiquitination and degradation, which defines a new mechanism in regulating activation-induced T-cell death. EGR2 functions as a negative regulator for T-cell activation by transcription of both FasL and the E3 ubiquitin ligase Cbl-b (37
). Indeed, AIP2 appears to promote T-cell activation by downregulating the transcription of EGR2 target genes, both FasL and Cbl-b, and maybe others.
EGR2 was initially identified as a protein with three zinc fingers, whose expression is activated during G0
transition in cultured cells (4
). Recent studies have demonstrated that EGR2 regulates activation-induced T-cell death by promoting FasL expression (7
). In addition, EGR2 regulates the development and activation of T lymphocytes in mice (32
). The expression of EGR2 is highly upregulated during the CD4-CD8 stage and is involved in regulation of pre-TCR expression during T-cell development. Ectopic expression of EGR2 inhibits T-cell activation in both Jurkat and mouse primary T cells, and the knockdown of its expression enhances antigen-specific T-cell activation (17
). Upregulation of EGR2 is one of the important mechanisms in the induction of T-cell tolerance, as the reversion of T-cell tolerance by IL-2 is accompanied by a quick degradation of EGR2 (17
). Our findings demonstrate that AIP2 is involved in T-cell activation and death by promoting EGR2 ubiquitination and degradation. It will be interesting to further analyze whether AIP2-targeted EGR2 degradation is involved in T-cell tolerance.
The HECT-type E3 ubiquitin ligases, with few exceptions, recruit substrates using their WW domains. The WW domain refers to one of the distinguishing features of the domain: the presence of two highly conserved tryptophan (W) residues spaced by 20 to 22 amino acids (38
). Previous studies have demonstrated that the WW domain specifically recognizes the proline-rich, tyrosine-containing motif PPXY (26
). For example, the WW domains of Itch, another member of the NEDD4 family E3 ubiquitin ligase, specifically bind to the PPXY motif of either c-Jun or JunB (10
). There are two PPXY motifs in EGR2, and the mutation of each partially abolished its interaction with AIP2. However, the mutation of each of the EGR2 PPXY motifs reduced its interaction with AIP2 differently, suggesting that WW domains may favor one PPXY motif rather than another. The tyrosine of the PPXY motif has been found to be phosphorylated by tyrosine kinases, and this phosphorylation regulates its reorganization by WW domains (13
). Interestingly, our data here clearly show that the AIP2/EGR2 interaction is regulated by TCR/CD28 signaling. Our laboratory is currently investigating how AIP2-catalyzed EGR2 ubiquitination is regulated in T cells.
E3 ubiquitin ligases have diverse functions in the activation of T lymphocytes. Two HECT-type E3 ubiquitin ligases, Itch and Nedd4, have been shown to play crucial roles in immune regulation (10
). Unlike Itch and Nedd4, which suppress T-cell activation, AIP2 enhances IL-2 production as well as T-cell proliferation by suppressing activation-induced T-cell apoptosis. Further investigation of AIP2 in regulating T-cell function in vivo, such as using AIP2 mutant mice, will provide a better understanding of the importance of AIP2-mediated ubiquitination in normal immune responses.