SMRT (silencing mediator of retinoic acid and thyroid hormone receptors) and N-CoR (nuclear receptor corepressor) are large corepressor proteins that mediate repression of various transcription factors, including nuclear hormone receptors. Both corepressors are required for normal mammalian development and each corepressor plays unique roles in mediating transcriptional repression in response to intracellular signaling. Signaling pathways regulating corepressors have been shown to be important in several diseases, including cancer.
Here we delineate a novel pathway controlling steady-state levels SMRT that involves Cyclin-dependent kinase 2 (Cdk2), the peptidyl-prolyl isomerase Pin1 and the membrane tyrosine kinase receptor ErbB2. We show that Cdk2 interacts with and phosphorylates SMRT at several sites in vitro and in vivo to generate binding sites for Pin1. SMRT is targeted for destruction by a Pin1 isomerization-dependent mechanism. We further show that the oncoprotein ErbB2 controls SMRT destabilization through a Cdk2- and Pin1-dependent pathway. This pathway affects both cell proliferation and gene expression as mediated through estrogen receptor α (ERα) and may provide several novel targets for therapy in tamoxifen-resistant breast cancers.
In addition, we show that the cyclin-dependent kinase Cdk1 also interacts with and phosphorylates SMRT. Like Cdk2, Cdk1 destabilizes SMRT steady-state levels. Together, these data indicate that SMRT may be targeted for destruction during the cell cycle. We have also identified the corepressor Daxx as a SMRT-interacting protein. The sumo-interacting motif (SIM) of Daxx is not required for this interaction; furthermore, SMRT and Daxx do not interact in vitro, indicating post-translational modifications of both proteins may be required for interaction. Together, our data identifies several new SMRT-interacting proteins indicating several new potential targets for cancer therapies.