PIASy represses CCAAT/enhancer-binding protein δ(C/EBPδ) transcriptional activity by sequestering C/EBPδ to the nuclear periphery

CCAAT/enhancerbindingprotein delta(C/EBP delta) plays a key role in mammary epithelial cell G(0) growth arrest, and "loss of function" alterations in C/EBP delta have been reported in breast cancer and acute myeloidleukemia. C/EBP delta isregulatedatthetranscriptional, post-transcriptional, and post-translational levels, suggesting tight control of C/EBP delta 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, PIASx beta, and PIASy, and all three PIAS family members repress C/EBP delta transcriptional activity. PIASy is the most potent, however, repressing C/EBP delta transcriptional activity by > 80%. PIASy repression of C/EBP delta transcriptional activity is dependent upon interaction between the highly conserved PIASy N-terminal nuclear matrix binding domain (SAPD) and the C/EBP delta transactivation domain (TAD). PIASy repression of C/EBP delta transcriptional activity is independent of histone deacetylase activity, PIASy E3 SUMO ligase activity, and C/EBP delta sumoylation status. PIASy expression is associated with C/EBP delta translocation from nuclear foci, where C/EBP delta co-localizes with p300, to the nuclear periphery. PIASy- mediated translocation of C/EBP delta is dependent upon the PIASy SAPD and C/EBP delta TAD. PIASy reduces the expression of C/EBP delta 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/EBP delta by a mechanism that requires interaction between the PIASy SAPD and C/EBP delta TAD and does not require PIASy SUMO ligase activity or C/EBP delta sumoylation. PIASy alters C/EBP delta nuclear localization, reduces C/EBP delta transcriptional activity, and enhances cell proliferation/ migration.