Modification of protein substrates by the addition of SUMO molecules Temsirolimus can influence protein protein interac tions and or alter protein stability, localization, or tran scriptional activity. PR SUMOylation most frequently represses PR transcriptional activity, and tends to slow the rate of ligand dependent PR downregulation via proteasome mediated turnover, but does not appreciably alter PR location. Numerous genes in our analyses behaved like MSX2, expression was sub stantially upregulated by SUMO deficient KR PR, but not WT PR. Additionally, KR PR occupied the MSX2 enhancer two to three times more than the WT receptor. The finding that increased levels of KR PR are recruited to this locus and asso ciated with increased MSX2 mRNA expression, suggests that PR SUMOylation alters co factor interactions that occur at the level of PR DNA binding.

Related to this finding, PIAS3, a PR SUMOylation E3 ligase, directly inhibits PR binding to PRE DNA sequences Inhibitors,Modulators,Libraries in vitro. Thus, PIAS3 mediated SUMO conjugation to WT PR may prevent effi cient receptor binding to selected PRE sequences, thus subsequently shifting the equilibrium away from PR occupancy at these loci. How this mechanism might be sequence specific or promoter specific remains to be determined. Promoter structure is likely to be an important deter minant of promoter selection by SUMOylated transcrip tion factors, including PR. Holmstrom et al. found Inhibitors,Modulators,Libraries that SUMOylated GR requires stable interaction with DNA containing multiple GR binding sites in order to efficiently inhibit transcription.

Interestingly, Inhibitors,Modulators,Libraries Inhibitors,Modulators,Libraries GR SUMOylation also selectively affects the transcriptional induction of linked endogenous genes. Related to this finding, recent chromatin modification mapping studies have revealed that Inhibitors,Modulators,Libraries histone H3 Lys4 mono and dimethylation at enhancers is associated with transcriptionally active genes. Indeed, regions of transcription factor accessibility to DNA response elements were first identified as DNase or MNase hypersensitive sites because these regions were relatively free from occupied nucleosomes. H3K4me2 is believed to be an epigenetic marker at functional enhancers that may recruit additional pro teins to facilitate nucleosome remodel ing and accessibility of the region for transcription factor binding.

We have not identified the pioneer factors for PR sellckchem recruitment, but in this study, we observed elevated H3K4 dimethylation at the MSX2 enhancer in cells expressing SUMO deficient KR PR, compared to WT PR. In this model, deSUMOylated PR may preferentially recruit the histone methyltransferase, MLL2, resulting in sus tained H3K4 dimethylation that allows formation of pro ductive transcriptional complexes at active sites that are normally repressed by SUMOylated receptors. Finally, DNA binding specificity for SRs is also highly dependent on sequence composition.