New approaches to targeting cell cycle proteins for cancer treatment

Project Summary/Abstract The proliferation of mammalian cells is driven by enzymatic complexes composed of regulatory subunits, cyclin proteins and their kinase partners, the cyclin-dependent kinases (CDKs). Cyclin-CDK complexes phosphorylate cellular proteins, thereby driving cell division. The control of mammalian cell proliferation takes place mainly during the G1 phase of the cell cycle. Two classes of cyclins operate during this phase: D-type cyclins (cyclins D1, D2 and D3) which bind and activate the cyclin-dependent kinase 4 (CDK4) and CDK6, and E-type cyclins (E1 and E2) which partner primarily with CDK2. Abnormal activation of cyclin D-CDK4/6 and E- CDK2 kinases represents the driving force of uncontrolled tumor cell proliferation. Small molecule inhibitors of CDK4/6 have been approved for the treatment of women with hormone receptor-positive breast cancers and are in clinical trials for several different cancer types. Inhibitors of CDK2 are currently being tested in clinical trials, primarily in women with ovarian and breast cancers. Despite a great success of CDK inhibitors so far, there are several issues that must be addressed in order to realize the full therapeutic potential of these compounds. (1) CDK4/6 inhibitors arrest tumor cell proliferation, but they do not kill tumor cells. From a therapeutic standpoint, it is important to develop therapeutic strategies that would allow tumor cell killing upon CDK4/6 inhibition. (2) While the majority of hormone receptor-positive breast cancers initially respond to the therapy with CDK4/6 inhibitors, eventually nearly all patients develop resistance and succumb to the disease. Moreover, several malignant tumor types are intrinsically resistant to CDK4/6 inhibitors. Overcoming the resistance represents an important clinical challenge. (3) Recent work indicates that in addition to driving tumor cell proliferation, cyclin-dependent kinases regulate several other functions, such as anti-tumor immune response. Moreover, some of these functions are tumor cell-extrinsic and reflect the role of cell cycle proteins in tumor stroma. Elucidation of these tumor cell-intrinsic and -extrinsic functions is very important in order to understand the effects of CDK inhibition in cancer patients. (4) In addition to their classical roles as activators of CDKs, cyclins also perform CDK- and kinase-independent functions. With the advent of degrader compounds that can physically remove a specific protein, it becomes possible to target these kinase-independent functions for cancer treatment. Hence, it is important to delineate the CDK-independent roles of cell cycle proteins in specific tumor types. The goal of this application is to address these key unresolved issues, using mouse cancer models and patient-derived xenografts.