Deacetylase inhibitors repress STAT5-mediated transcription by interfering with bromodomain and extra-terminal (BET) protein function.
Bottom Line: We showed previously that the deacetylase inhibitor trichostatin A (TSA) inhibits STAT5-mediated transcription by preventing recruitment of the transcriptional machinery at a step following STAT5 binding to DNA.The mechanism and factors involved in this inhibition remain unknown.Instead, they induce a rapid increase in global histone acetylation apparently resulting in the delocalization of the bromodomain and extra-terminal (BET) protein Brd2 and of the Brd2-associated factor TBP to hyperacetylated chromatin.
Affiliation: Stat5 Signaling Research Group, Institute of Immunology, University of Regensburg, 93053 Regensburg, Germany.Show MeSH
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Mentions: Since the above results suggested a possible implication of Brd2 in STAT5-mediated transcription in Ba/F3 cells, we investigated whether and how TSA could interfere with Brd2 function. We first assessed the subcellular localization of Brd2 in TSA-treated and untreated Ba/F3 cells by cell fractionation and western blot analysis (Figure 10). Brd2 was exclusively detected in nuclear extracts of Ba/F3 cells (Figure 10C), within both the soluble and insoluble (i.e. mainly chromatin-associated) nuclear fractions (Figure 10A and C). Remarkably, upon TSA treatment, Brd2 protein was depleted from the soluble nuclear fraction while it was still detectable in the insoluble nuclear fraction (Figure 10A and C). The depletion of Brd2 from the soluble nuclear fraction was not due to proteasome-dependent protein degradation since it was not prevented by pre-treatment with the proteasome inhibitor MG132 (Figure 10A). MG132 activity was verified by measuring the mRNA levels of hsp70 (Figure 10B), an acknowledged MG132-induced gene (87). The drop in the level of soluble nuclear Brd2 protein in TSA-treated cells was also not the consequence of a reduced Brd2 gene expression, since Brd2 mRNA levels were not affected by TSA (Figure 10B). These observations suggest that delocalization of nuclear Brd2 from the soluble to the insoluble chromatin fraction following TSA treatment could limit its availability to support STAT5-mediated transcription.
Affiliation: Stat5 Signaling Research Group, Institute of Immunology, University of Regensburg, 93053 Regensburg, Germany.