Interleukin-12 expression in response to apoptotic cells can also be regulated by a new mechanism involving the novel zinc-finger nuclear factor GC-BP.
The removal of apoptotic cells by phagocytes usually results in an anti-inflammatory state that involves the inhibition of pro-inflammatory cytokines, such as interleukin-12 (IL-12). In a recent issue of Immunity, Kim et al. describe a new mechanism that regulates IL-12 expression in response to apoptotic cells, involving a novel zinc-finger nuclear factor, GC-BP.
Previous studies have focused on the production of anti-inflammatory cytokines, such as IL-10 and transforming growth factor- (TGF-), in regulating responses after phagocytosis of apoptotic cells. However, here, Kim et al. show that the inhibition of IL-12 production by macrophages in the presence of apoptotic cells is direct and independent of increased IL-10 or TGF- production. Inhibition of IL-12 production was dependent on macrophage contact with apoptotic cells, because liposomes containing phosphatidylserine (which is exposed on the surface of apoptotic cells) similarly inhibited IL-12 production by macrophages.
IL-12 is a heterodimeric cytokine composed of two chains, p40 and p35; analysis of the levels of mRNA encoding each IL-12 chain indicated that the presence of apoptotic cells mainly reduced the levels of IL-12p35 mRNA in responding macrophages. This prompted the authors to focus on the transcriptional mechanisms that are involved in inhibition of IL-12p35 gene expression. In the IL-12p35 promoter, they identified a GC dinucleotide at position +17–+18 that is crucial for mediating the response to apoptotic cells, and they showed that this region is specifically bound by a novel zinc-finger nuclear factor, which they named GC-binding protein (GC-BP). Overexpression of GC-BP in a mouse macrophage cell line selectively inhibited IL-12p35 expression in response to interferon- and lipopolysaccharide. And specific knockdown of GC-BP expression by small interfering RNAs confirmed its inhibitory role.
Because GC-BP was found to be constitutively expressed by macrophages, the authors assessed whether post-translational modifications might regulate its activity. They detected a decrease in tyrosine phosphorylation levels of GC-BP in macrophages that were exposed to apoptotic but not necrotic cells. Mutation of a tyrosine residue at position 15 (Tyr15) abolished the inhibitory effect of GC-BP on IL-12 production, indicating that the activity of GC-BP is likely to be regulated by phosphorylation of Tyr15.
These results uncover a new signalling pathway and further our understanding of cytokine regulation, which is a crucial mechanism for avoiding autoreactivity to apoptotic cells.
Lucy Bird
References
Kim, S. , Elkon, K. B. & Ma, X. Transcriptional suppression of interleukin-12 gene expression following phagocytosis of apoptotic cells. Immunity21, 643–653 (2004). | Article | PubMed |
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and lipopolysaccharide. And specific knockdown of GC-BP expression by small interfering RNAs confirmed its inhibitory role.