Lung cancer is the most common cause of cancer-related deaths worldwide. Non-small cell lung cancer (NSCLC) accounts for 85% of all lung cancer cases and is generally diagnosed at advanced stages, requiring multimodal therapy involving radiation, chemotherapy, and targeted therapies. Despite these medical interventions, the five- year survival rates of NSCLC patients are less than 5%, highlighting the need for innovative and more effective strategies to treat NSCLC. Dysregulation of cyclin-dependent kinases (CDKs), such as CDK4 and CDK6, occurs in 70% of NSCLC patients and results in aberrant cellular proliferation and tumorigenesis. Palbociclib (PD-03329, trade name Ibrance) is the first cyclin dependent kinase 4 and 6 inhibitor to be approved for breast cancer and is currently investigated as a monotherapy for other solid tumors, including NSCLC. While palbociclib has shown initial improvements in progression-free survival in a phase II clinical trial for recurrent or metastatic NSCLC patients, over half of patients either experience adverse effects or develop resistance and disease progression after eight weeks of treatment. Palbociclib achieves its therapeutic effect by arresting cells in G1 phase and promoting an irreversible cell cycle arrest known as cellular senescence. Senescence was initially thought to suppress tumorigenesis; however, growing evidence has suggested that senescent cells can paradoxically promote tumorigenesis and cancer relapse by altering the surrounding tumor microenvironment. The use of senolytic therapies to promote synthetic lethality may bypass the negative side effects of senescence and enhance the efficacy of palbociclib by either driving palbociclib-treated cells towards apoptosis rather than senescence. Through genetic screening, we identified thrombomodulin (THBD), a potent anticoagulant endothelial receptor, as a novel senolytic target for palbociclib-induced senescence. THBD-mediated signaling was upregulated during palbociclib-induced senescence in NSCLC cancer cell lines and served as a critical regulator of NSCLC cell fate and survival, as inhibition of THBD signaling in NSCLC cells attenuated senescence and promoted apoptosis. Importantly, inhibiting the activity of THBD downstream signaling by an FDA-approved drug caused senescent NSCLC cells to apoptose under treatment of palbociclib. Built on these findings, we propose two specific aims to fully investigate the mechanism by which THBD signaling mediates the senescent program induced by palbociclib and validate this pathway as a target to induce synthetic lethality in palbociclib- treated NSCLC cells both in vitro and in vivo for combinational therapy with the ultimate goal to develop preclinical and clinical trials to improve overall NSCLC patient outcome.
These proposed studies have direct applicability to patients who fail to respond to palbociclib. Importantly, there are a variety of pharmacological inhibitors that are currently available targeting multiple components within the newly discovered signaling pathway that mediates palbociclib-induced senescence of lung cancer cells, some of which are clinically used for the treatment of non-cancer related diseases. Positive findings from the proposed research will lead to the identification of novel senolytic targets that can be used to circumvent palbociclib resistance, resulting in decreased tumor progression and improved overall survival in lung cancer patients.