Abstract 3571: Inhibition of Cdk9 blocks ligand induced phosphorylation of Estrogen receptor alpha at Ser294.

Abstract

Abstract In response to ligand binding, the estrogen receptor (ER, alpha isoform) becomes phosphorylated on several serines including Ser294 and Ser118 to initiate an estrogenic transcriptional program that has been well studied. On the other hand, growth factor stimulation of ER also promotes serine phosphorylation at several sites including Ser305 and Ser118, but not Ser294, to drive a transcriptional program different from that induced by estrogenic ligands. Our previous work demonstrated that following estrogen or tamoxifen stimulation, cyclin dependent kinase (Cdk) mediated phosphorylation of the ER hinge region at Ser294 was essential for full transcriptional activation of ER. Current studies, using a recently developed rabbit monoclonal antibody to phospho-Ser294 ER, show that the Cdk9 inhibitor SCH727956/Dinaciclib, now in phase II clinical trials, completely prevents Ser294 phosphorlyation induced by estrogen stimulation of MCF-7 cells without influencing phospho-Ser118 formation which remains robust. Immunoprecipitation of ER from MCF-7 cells following estrogen treatment results in the co-immunoprecipitaton of cdk9. Further SCH727965/Dinaciclib has no influence upon epidermal growth factor (EGF) induced ER phosphorylation at Ser305, consistent with previous studies linking phospho-Ser305 formation to the activated kinases PKA and PAK. Ongoing studies evaluating siRNA induced knockdown of Cdk9 in MCF-7 cells followed by estrogen and/or EGF stimulation are designed to confirm the specific role of Cdk9 in mediating ligand-induced ER phosphorylation at Ser294. As Cdk9 is essential to promote transcriptional elongation by RNA polymerase II, the Ser294 specific recruitment of Cdk9 to ER following ligand stimulation offers rationale for the clinical use of Cdk9 inhibitors such as SCH727965/Dinaciclib in the treatment of ER-positive breast cancers. In particular, for those ER-positive breast cancers responsive to endocrine therapy the combined use of a Cdk9 inhibitor with antiestrogenic therapy is likely to prove clinically beneficial. Citation Format: Gary K. Scott, Ravneet Kaur, Daniel Rothschild, C. C. Benz. Inhibition of Cdk9 blocks ligand induced phosphorylation of Estrogen receptor alpha at Ser294. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3571. doi:10.1158/1538-7445.AM2013-3571