c-Maf increases apoptosis in peripheral CD8 cells by transactivatingCaspase 6

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Abstract

In addition to transactivation of interleukin-4 (IL-4), cellular muscular aponeurotic fibrosarcoma (c-Maf) enhances CD4 cell apoptosis by limiting Bcl-2 expression. The CD8 cells also express c-Maf and peripheral CD8 cell numbers are reduced in c-Maf transgenic mice, suggesting that c-Maf may influence CD8 cell survival in a manner similar to CD4 cells. Here we confirm that, similar to CD4 cells, c-Maf enhances CD8 cell susceptibility to apoptosis induced by multiple stimuli, independent of IL-4. However, unlike CD4 cells, c-Maf enhancement of apoptosis is independent of Bcl-2, suggesting that c-Maf uses other mechanisms to regulate CD8 cell apoptosis. Real-time reverse transcription-polymerase chain reaction reveals that the pro-apoptotic geneCaspase 6is upregulated in c-Maf transgenic CD8 cells, suggesting thatCaspase 6is a novel c-Maf target gene. Luciferase reporter assays and site-directed mutagenesis reveal a functional c-Maf recognition element (MARE) within the first intron ofCaspase 6.Binding of c-Maf to the MARE site is detectable by chromatin immunoprecipitation using non-transgenic T-cell lysates, so c-Maf can interact with theCaspase 6MARE site in normal T cells. Furthermore, caspase 6 activity is increased among CD8 cells from c-Maf transgenic mice following T-cell receptor engagement. As expected, activity of the downstream caspases 3 and 7 is also increased. Consistent with the ability of caspase 6 to participate in positive feedback loops, cytochromecrelease and caspase 8 activation are also increased. Together these results indicated that c-Maf increases CD8 cell sensitivity to apoptotic stimuli, at least in part, by direct transactivation ofCaspase 6,providing increased substrate forCaspase 6-dependent apoptosis pathways.

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