Project Summary Cancer results from dysregulated cell cycle progression and uncontrolled cell division. Most tumors alter the CDK4,6/RB/E2F pathway to promote oncogenesis, making it a promising therapeutic target. When coupled with anti-hormone therapy, CDK4/6 inhibitors significantly improve the prognosis of patients with estrogen receptor (ER)-positive/Her2-negative breast cancer. Nevertheless, the clinical use of CDK4/6 inhibitors is restricted by dose-limiting toxicities and resistance. Thus, a better understanding of the mechanism of action of CDK4/6 inhibitors is required to maximize their therapeutic efficacy. The central goal of this proposal is to examine changes in the stability of the minichromosome maintenance (MCM) complex proteins induced by CDK4/6 inhibition (CDK4/6i). MCM is an essential DNA replication protein and its dysregulation can result in replication stress, DNA damage, and cancer. To avoid this, the cellular localization of MCM is highly regulated throughout the cell cycle, however, the protein abundance remains constant. Surprisingly, we discovered that CDK4/6 inhibitors result in proteasome-dependent degradation of MCM in both untransformed epithelial cells and in breast cancer cells. To our knowledge, this represents the first known mechanism of regulating MCM abundance through active protein degradation. It thus remains unclear why a CDK4/6i-induced cellular arrest, but not other forms of arrest such as quiescence, leads to active MCM degradation. In Aim 1 of this proposal, we will determine the mechanism and consequences of CDK4/6i-induced MCM degradation by identifying the E3 ubiquitin ligase(s) that tags MCM for degradation and by defining the precise target of ubiquitination. In our initial discovery, we associated CDK4/6i-induced MCM degradation with replication stress and DNA damage, but did not directly implicate MCM degradation as the primary source. We will test the hypothesis that MCM degradation is a key source of CDK4/6i-induced replication stress and DNA damage by preventing its degradation and determining if this prevents the accumulation of these phenotypes upon release from CDK4/6 inhibition. The results of these experiments will provide insight into whether MCM degradation can be exploited to increase the cellular death-inducing capabilities of CDK4/6 inhibitors. In Aim 2, we will test the hypothesis that CDK4/6i-induded MCM degradation results from altered RB/E2F-mediated gene expression. We have already discovered that MCM degradation is RB-dependent, however, it is still unclear if it is E2F-dependent. To test this, will first determine if CDK4/6i-induced MCM degradation results directly from repressed E2F activity. If so, we will manipulate the expression of downstream E2F-regulated genes to elucidate the regulatory pathway(s) that results in MCM degradation. If MCM degradation is E2F-independent, we will probe alternative mechanisms by which RB mediates this phenotype. Taken together, these aims ...