Targeting Heme Function and Mitochondrial Respiration in Non-Small Cell Lung Cancer
Each year lung cancer causes more morbidities than cancers of colon, breast, and prostate combined. The American Cancer Society estimates that lung cancer will claim about 150,000 lives in 2018. Conventional and targeted therapies are reported to have reached the plateau in effectively improving the survival of lung cancer patients. Despite the advent of advanced therapies like immunotherapy, 5-year survival rates remain abysmal at 30% and 10% for Stage III and Stage IV respectively. Therefore, it is imperative to explore different strategies to effectively treat lung cancer and improve survival outcomes. There are two major histological types of lung cancer: small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC). NSCLC accounts for about 85% of the lung cancer cases. Several studies demonstrate that enhanced mitochondrial respiration or oxidative phosphorylation (OXPHOS) is a key feature of NSCLC. Therefore, targeting OXPHOS could be an effective strategy for intervention in NSCLC. Previous studies from our lab showed that hedgehog pathway antagonist, cyclopamine tartrate (CycT), significantly reduced OXPHOS and proliferation in NSCLC cell lines. However, in vitro models do not offer reliable evidence of therapeutic efficacy, thereby, necessitating studies on in vivo models. Previous studies from our lab demonstrated intensified heme uptake and synthesis in NSCLC cell lines compared to normal cell line. Since heme is a central factor in oxygen consumption, this study also probes the effect of CycT on heme metabolism. The objective of this study is two-fold: (i) to test the efficacy of targeting OXPHOS in NSCLC in vivo, and (ii) to investigate the therapeutic efficacy and the mechanism of action of CycT in growth and progression of NSCLC in vivo. We utilized subcutaneous and lung orthotopic xenografts of NSCLC cell lines with luciferase to track the growth and progression of NSCLC in immunodeficient mouse model, NOD/SCID (Non-obese diabetic/ severe combined immunodeficiency), via bioluminescence imaging. The lung tissues of the mice were probed using immunohistochemistry to discern the mechanisms of action. We found that CycT effectively hampered the growth and progression of subcutaneous and lung orthotopic xenografts of NSCLC cell lines. CycT significantly reduced proteins involved in heme metabolism and OXPHOS in addition to other pro-oncogenic hemoproteins and regulators of OXPHOS. In vitro studies demonstrated that the effects of CycT on heme metabolism and OXPHOS are independent of its antagonist properties on the hedgehog pathway. The significance of this study is that it shows that CycT acts via diminishing heme metabolism, hemoproteins involved in oxygen consumption, and oxygen consumption in NSCLC. This is the first study to demonstrate the effect of CycT on heme metabolism and OXPOS in vivo. This novel mechanism of action of CycT is independent of its previously known antagonistic effect on hedgehog signaling. This study demonstrates that CycT has the potential to be an effective therapeutic agent in treating NSCLC. This study provides compelling evidence to further assess the feasibility of using CycT in treating NSCLC.