Targeting CDK1 and MEK/ERK Overcomes Apoptotic Resistance in BRAF-Mutant Human Colorectal Cancer

Abstract

The <i>BRAF</i>^V600E mutation occurs in approximately 8% of human colorectal cancers and is associated with therapeutic resistance that is due, in part, to reactivation of MEK/ERK signaling cascade. Recently, pathway analysis identified cyclin-dependent kinase 1 (CDK1) upregulation in a subset of human <i>BRAF</i>^V600E colorectal cancers. Therefore, it was determined whether CDK1 antagonism enhances the efficacy of MEK inhibition in <i>BRAF</i>^V600E colorectal cancer cells. <i>BRAF</i>^V600E colorectal cancer cell lines expressing CDK1 were sensitized to apoptosis upon siRNA knockdown or small-molecule inhibition with RO-3306 (CDK1 inhibitor) or dinaciclib (CDK1, 2, 5, 9 inhibitors). Combination of RO-3306 or dinaciclib with cobimetinib (MEK inhibitor) cooperatively enhanced apoptosis and reduced clonogenic survival versus monotherapy. Cells isogenic or ectopic for <i>BRAF</i>^V600E displayed resistance to CDK1 inhibitors, as did cells with ectopic expression of constitutively active <i>MEK</i> CDK1 inhibitors induced a <i>CASP8</i>-dependent apoptosis shown by caspase-8 restoration in deficient NB7 cells that enhanced dinaciclib-induced CASP3 cleavage. CDK inhibitors suppressed pro-CASP8 phosphorylation at S387, as shown by drug withdrawal, which restored p-S387 and increased mitosis. In a colorectal cancer xenograft model, dinaciclib plus cobimetinib produced significantly greater tumor growth inhibition in association with a caspase-dependent apoptosis versus either drug alone. The Cancer Genome Atlas (TCGA) transcriptomic dataset revealed overexpression of CDK1 in human colorectal cancers versus normal colon. Together, these data establish CDK1 as a novel mediator of apoptosis resistance in <i>BRAF</i>^V600E colorectal cancers whose combined targeting with a MEK/ERK inhibitor represents an effective therapeutic strategy.<b>Implications:</b> CDK1 is a novel mediator of apoptosis resistance in <i>BRAF</i>^V600E colorectal cancers whose dual targeting with a MEK inhibitor may be therapeutically effective. <i>Mol Cancer Res; 16(3); 378-89. ©2017 AACR</i>.