Unexpected Outcomes of CDK4/6 Inhibition

The response of cells to extracellular mitogenic signals and commitment to enter G1 phase, are regulated by the D-type cyclins (D1, D2 and D3). Once induced they heterodimerize with and activate either cyclin dependent kinase 4 or 6 (CDK4 or CDK6). Cyclin D-CDK4 and D-CDK6 kinases phosphorylate and inactivate the retinoblastoma family of proteins, leading to release and derepression of E2F transcription factors. E2Fs, in turn, activate a transcriptional program required for G1-S transition. Cyclin D-associated kinase also affects other pathways by phosphorylating SMADs and FOXM1 transcriptional regulators. In addition, cyclin D-CDK4/6 complexes play an important non-catalytic function by sequestering cell cycle inhibitors p21 Cip1 and p27 Kip1 from CDK2 [1]. Dysregulated expression of D-cyclins and resulting CDK4/6 hyperactivation are thought to represent a driving force in tumorigenesis. Indeed, cyclin D1 and CDK4 are among the most commonly amplified genes across all human cancer types [2]. Cyclin D1 gene is amplified in up to 20% of invasive breast cancers, while the protein is overexpressed in over 50% of mammary carcinomas [1]. Cyclin D3 is highly expressed in several hematopoietic malignancies, such as T-cell acute lymphoblastic leukemias (TALL). These observations suggest that cyclin D-CDK4 and D-CDK6 kinases might represent attractive therapeutic targets in cancer treatment. Recent analyses using mouse cancer models revealed unexpected outcomes of cyclin D-CDK kinase inhibition in vivo, in tumor bearing mice. Cyclin D germline knockout animals provided some of the first evidence that the D-type cyclins are required for tumor initiation. Thus, several groups have shown that mice lacking cyclin D1, or lacking CDK4, or expressing kinase-inactive cyclin D1-CDK complexes are either completely resistant, or show significantly reduced susceptibility to HER2-driven breast cancers, depending on the genetic background used (see references in [3]). Likewise, cyclin D3-deficient mice, or animals lacking CDK6 are resistant to T-cell acute lymphoblastic leukemias (TALL) driven by the NOTCH1 oncogene, or by its downstream effector AKT (see references in [3]). It could have been argued that tumor resistance observed in cyclin-or CDK-deficient animals was due to the developmental defects caused by germline cyclin/ CDK ablation. Indeed, cyclin D1-knockout mice show defects in mammary epithelial lobuloalveolar development [4]. Likewise hematopoietic abnormalities were noted in cyclin D3-or CDK6-null mice [4]. Collectively, these observations raised a possibility that progenitor cells targeted for transformation might have been missing in knockout animals. To circumvent the developmental effects of germline cyclin ablation, we generated conditional cyclin D-knockout mouse strains. …