Nicotinamide uncouples hormone-dependent chromatin remodeling from transcription complex assembly.

Sirtuins, homologs of the yeast SIR2 family, are protein deacetylases that require nicotinamide adenosine dinucleotide as cofactor. To determine whether the sirtuin family of deacetylases is involved in progesterone receptor (PR)-mediated transcription, the effect of sirtuin inhibitor, nicotinamide (NAM), was monitored in T47D breast cancer cells. NAM suppressed hormone-dependent activation of PR-regulated genes in a dose-dependent manner. Surprisingly, NAM-mediated inhibition of PR-mediated transcription occurs independently of SIRT1 and PARP1. Chromatin immunoprecipitation experiments did not show that PR binding nor that of the coactivators CBP and SRC3 was compromised. Consistent with the recruitment of the BRG1 chromatin remodeling complex, promoter chromatin remodeling still occurs despite NAM inhibition of PR transactivation. Rather, we show that this inhibition of transcription is due to dramatic loss of recruitment of the basal transcriptional machinery to the promoter. These results show that NAM uncouples promoter chromatin remodeling from transcription preinitiation complex assembly and suggest the existence of vital NAM-regulated steps required for promoter chromatin remodeling and basal transcription complex communication.