MicroRNA-Mediated Suppression of the TGF-β Pathway Confers Transmissible and Reversible CDK4/6 Inhibitor Resistance.

CDK4/6 inhibition is now part of the standard armamentarium for patients with estrogen receptor-positive (ER) breast cancer, so that defining mechanisms of resistance is a pressing issue. Here, we identify increased CDK6 expression as a key determinant of acquired resistance after palbociclib treatment in ER breast cancer cells. CDK6 expression is critical for cellular survival during palbociclib exposure. The increased CDK6 expression observed in resistant cells is dependent on TGF-β pathway suppression via miR-432-5p expression. Exosomal miR-432-5p expression mediates the transfer of the resistance phenotype between neighboring cell populations. Levels of miR-432-5p are higher in primary breast cancers demonstrating CDK4/6 resistance compared to those that are sensitive. These data are further confirmed in pre-treatment and post-progression biopsies from a parotid cancer patient who had responded to ribociclib, demonstrating the clinical relevance of this mechanism. Finally, the CDK4/6 inhibitor resistance phenotype is reversible in vitro and in vivo by a prolonged drug holiday.