Project Summary/Abstract: The anaphase-promoting complex/Cyclosome (APC/C) is a well-defined multi-subunit E3 ubiquitin ligase that regulates targeted cell cycle regulators for degradation by the Ubiquitin Proteasome Pathway (UPP), promoting cell cycle progression from metaphase to anaphase and being involved in G1 phase maintenance. The APC/C E3 ligase complex is evolutionarily conserved and relies on two adaptor proteins, Cdc20 and Cdh1, to recognize different target proteins and regulate cell cycle progression. However, compared to Cdc20 that is subjected to Cdh1-mediated destruction, regulation of the E3 ligase activity of Cdh1 is not well known yet. Previous study has shown that there were 19 serine and threonine residues on Cdh1 that can be phosphorylated by multi-kinases in vivo, indicating that the phosphoregulation of Cdh1 is much more complex. In the present proposal, I found that CDK4 can phosphorylates Cdh1 in vitro and modulates its E3 ligase activity. Furthermore, we found that the phosphorylation of Cdh1 by CDK4 can be recognized by the Pin1 proline isomerase, facilitating Cdh1-Pin1 complex formation. In keeping with this notion, employment of the CDK4/6 inhibitor or mutating the phosphorylation sites can disrupt the Cdh1-Pin1 interaction. Consequently, Cdh1 can mediate Pin1 for polyubiquitination and degradation. As such, depletion of endogenous Cdh1 abolished the Pin1 inhibitor treatment induced Pin1 degradation in cells. Importantly, the Pin1 inhibitor-induced cell proliferation suppression was also abolished in Cdh1-null MEFs, suggesting the functional presence of Cdh1 is required for Pin1 inhibitor-induced cell proliferation suppression. In addition, combination of the CDK4/6 inhibitor and Pin1 inhibitor exhibits significantly enhanced suppressing effect in breast cancer cells. In the first Aim of this proposal, I am going to explore the role of CDK4 kinase in regulating the E3 ligase activity of Cdh1 (Aim #1). Therefore, the second Aim in this proposal will be exploring the potential role of Cdh1 in mediating Pin1 inhibitor treatment induced Pin1 protein destruction (Aim 2). Together, these results implicate a functional role of the CDK4/Cdh1/Pin1 signaling axis in regulating cell proliferation, and provide rational for combining the CDK4/6 inhibitor and Pin1 inhibitor to treat breast cancer.