PIASy Represses CCAAT/Enhancer-binding Protein δ (C/EBPδ) Transcriptional Activity by Sequestering C/EBPδ to the Nuclear Periphery

CCAAT/enhancer binding proteinδ (C/EBPδ) plays a key role in mammary epithelial cell G0 growth arrest, and “loss of function” alterations in C/EBPδ have been reported in breast cancer and acute myeloid leukemia. C/EBPδ is regulated at the transcriptional, post-transcriptional, and post-translational levels, suggesting tight control of C/EBPδ content and function. Protein inhibitors of activated STATs (PIASs) regulate a growing number of transcription factors, including C/EBPs. HC11 nontransformed mammary epithelial cells express PIAS3, PIASxβ, and PIASy, and all three PIAS family members repress C/EBPδ transcriptional activity. PIASy is the most potent, however, repressing C/EBPδ transcriptional activity by >80%. PIASy repression of C/EBPδ transcriptional activity is dependent upon interaction between the highly conserved PIASy N-terminal nuclear matrix binding domain (SAPD) and the C/EBPδ transactivation domain (TAD). PIASy repression of C/EBPδ transcriptional activity is independent of histone deacetylase activity, PIASy E3 SUMO ligase activity, and C/EBPδ sumoylation status. PIASy expression is associated with C/EBPδ translocation from nuclear foci, where C/EBPδ co-localizes with p300, to the nuclear periphery. PIASy-mediated translocation of C/EBPδ is dependent upon the PIASy SAPD and C/EBPδ TAD. PIASy reduces the expression of C/EBPδ adhesion-related target genes and enhances repopulation of open areas within a cell monolayer in the in vitro “scratch” assay. These results demonstrate that PIASy represses C/EBPδ by a mechanism that requires interaction between the PIASy SAPD and C/EBPδ TAD and does not require PIASy SUMO ligase activity or C/EBPδ sumoylation. PIASy alters C/EBPδ nuclear localization, reduces C/EBPδ transcriptional activity, and enhances cell proliferation/migration.