The breast cell growth inhibitor, estrogen down regulated gene 1, modulates a novel functional interaction between estrogen receptor alpha and transcriptional elongation factor cyclin T1

Abstract

Estrogen receptor alpha (ERa) regulates transcription of specific genes and is believed to play a major role in breast tumorigenesis. We previously identified estrogen down regulated gene 1 (EDG1 (also known as HEXIM1)) using the C-terminus of ERa (E/F domain) as bait in yeast twohybrid screenings. Here we report on the role of EDG1 as a coregulator of ERa transcriptional activity. We observe an interaction between EDG1 and ERa. EDG1 inhibits the transcriptional activity of ERa and this is dependent upon the C-terminus of EDG1. The C-terminus of EDG1/ HEXIM1 was recently shown to inhibit the positive transcription elongation factor b (P-TEFb) by interacting with the cyclin T1 subunit. Here we show that ERa interacts with cyclin T1, cyclin T1 and ER co-occupancy on the promoter region of an ER target gene, and that this interaction plays an important role in ERa-induced gene expression. The interaction of ERa with cyclin T1 also allows ERa to compete with EDG1 for cyclin T1, and may release cyclin T1 from EDG1 repression. Conversely, increased EDG1 expression results in inhibition of cyclin T1 recruitment and ERa DNA binding. Our results support a novel functional interaction between ERa and cyclin T1 that is modulated by EDG1.

Hexim1 sequesters positive transcription elongation factor b from the class II transactivator on MHC class II promoters

“…Our observation extends already established mechanisms of repression by Hexim1, where its BR binds transcriptional activators and blocks their activity (20)(21)(22). In addition, our data reveal that Hexim1 can inhibit transcriptional activators in trans, by decoying P-TEFb away from them, which presents a conceptually previously undescribed mechanism for the inhibition of transcription by Hexim1.…”

“…Because BR in Hexim1 is necessary for the inhibition of p65, ER␣, and glucocorticoid receptors, we were interested in whether the same is true for CIITA (20)(21)(22). Surprisingly, the mutant f:NLS.Hex1(181-359) protein lacking the BR still inhibited the activity of CIITA to the same extent as the mutant f:Hex1 (150-359) protein.…”

“…In addition to its role in the inactivation of P-TEFb, Hexim1 can also inhibit the transcriptional activity of a variety of activators, such as the p65 subunit of NF-B, glucocorticoid receptor and estrogen receptor ␣ (ER␣) (20)(21)(22). In these cases, Hexim1 binds these activators by its BR and interferes with their function.…”

“…Moreover, even when Hexim1 binds ER␣, this interaction is not sufficient to inhibit completely its activity in cells. Rather, the competition between ER␣ and Hexim1 for the binding to CycT1 is responsible for decreased ER␣-mediated transcription (22).…”

Ctr9, a key subunit of PAFc, affects global estrogen signaling and drives ERα-positive breast tumorigenesis

“…6G-J). Given that ERα interacts with the positive transcription elongation factor-b (P-TEFb) complex that promotes transcriptional elongation in part by relieving the negative elongation factor (NELF) complex and stimulating RNAPII pSer2 (Wittmann et al 2005;Peterlin and Price 2006), which in turn promotes elongation-associated histone modifications (Pirngruber et al 2009), we further performed ChIP assays for CDK9 and NELF-A and observed that Ctr9 knockdown decreased the occupancy of CDK9 and increased the occupancy of NELF-A at CXCL12, GREB1, and PGR genes (Supplemental Fig. S15A-C).…”