Palbociclib enhances activin-SMAD-induced cytostasis in estrogen receptor-positive breast cancer.

Cyclin-dependent kinase (CDK) 4 and CDK6 inhibitors are effective therapeutic options for hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer. Although CDK4/6 inhibitors mainly target the cyclin D-CDK4/6-retinoblastoma tumor suppressor protein (RB) axis, little is known about the clinical impact of inhibiting phosphorylation of other CDK4/6 target proteins. Here, we focused on other CDK4/6 targets, SMAD proteins. We showed that a CDK4/6 inhibitor palbociclib and activin-SMAD2 signaling cooperatively inhibited cell cycle progression of a luminal-type breast cancer cell line T47D. Palbociclib enhanced SMAD2 binding to the genome by inhibiting CDK4/6-mediated linker phosphorylation of the SMAD2 protein. We also showed that cyclin G2 plays essential roles in SMAD2-dependent cytostatic response. Moreover, comparison of the SMAD2 ChIP-seq data of T47D cells with those of Hs578T (triple-negative breast cancer cells) indicated that palbociclib augmented different SMAD2-mediated functions based on cell type, and enhanced SMAD2 binding to the target regions on the genome without affecting its binding pattern. In summary, palbociclib enhances the cytostatic effects of the activin-SMAD2 signaling pathway, whereas it possibly strengthens the tumor-promoting aspect in aggressive breast cancer.