CK1 modulates the transcriptional activity of ER via AIB1 in an estrogen-dependent manner and regulates ERAIB1 interactions (Publication with Expression of Concern. See vol. 49, pg. 3602, 2021)

Oncogenesis in breast cancer often requires the overexpression of the nuclear receptor coactivator AIB1/SRC-3 acting in conjunction with estrogen receptor- (ER). Phosphorylation of both ER and AIB1 has been shown to have profound effects on their functions. In addition, proteasome-mediated degradation plays a major role by regulating their stability and activity. CK1, a member of the ubiquitous casein kinase-1 family, is implicated in the progression of breast cancer. In this study, we show that both ER and AIB1 are substrates for CK1 in vitro, and identify a novel AIB1 phosphorylation site (S601) targeted by CK1, significant for the co-activator function of AIB1. CK1 is able to interact with ER and AIB1 in vivo, while overexpression of CK1 in breast cancer cells results in an increased association of ER with AIB1 as confirmed by co-immunoprecipitation assays from cell lysates. Using an siRNA-based approach, luciferase reporter assays and qRT-PCR, we observe that silencing of CK1 leads to reduced ER transcriptional activity, despite increased ER levels, similarly to proteasome inhibition. We provide evidence that AIB1 protein levels are reduced by CK1 silencing, in an estradiol-dependent manner; such destabilization can be inhibited by pre-treatment with the proteasome inhibitor MG132. We propose that differing activities adopted by ER and AIB1 as a consequence of their interactions with and phosphorylation by CK1, particularly AIB1 stabilization, influence the transcriptional activity of ER, and therefore have a role in breast cancer development.