OP56 – Cyp24a1 attenuation limits progression of BrafV600E-induced papillary thyroid cancer cells and sensitizes them to BRAFV600E inhibitor PLX4720

•   Shi, Yufei¹ ; Zou, Minjing¹, Baitei, Essa Y. ¹, BinEssa, Huda A.¹, Al-Mohanna, Futwan A.², Parhar, Ranjit S.², St-Arnaud, Rene³, Kimura, Shioko⁴, Pritchard, Catrin⁵, Alzahrani, Ali S.⁶, Assiri, Abdullah M. ⁷, Meyer, Brian F.¹ 1 Departments of Genetics, 2 Cell Biology, 6 Medicine,and 7 Comparative Medicine, King Faisal Specialist Hospital and Research Centre, Riyadh, Saudi Arabia 3 Departments of Surgery and Human Genetics, McGill University, Montreal, Quebec, Canada; and Research Centre, Shriners Hospitals for Children – Canada, Montreal, Quebec, Canada 4 Laboratory of Metabolism, National Cancer Institute, National Institutes of Health, Bethesda, MD, USA 5 Department of Biochemistry, University of Leicester, Leicester, UK   Background: CYP24A1, the primary inactivating enzyme for vitamin D, is often overexpressed in human cancers, potentially neutralizing the antitumor effects of calcitriol, the active form of vitamin D. However, it is unclear whether CYP24A1 expression serves as a functional contributor versus only a biomarker for tumor progression. In this study, we investigated the role of CYP24A1 on malignant progression of a murine model of BrafV600E-induced papillary thyroid cancer (PTC). Methods: Mice harboring wild-type Cyp24a1 (BVE^Cyp24a1-wt) developed PTC at 5 weeks of age. Mice harboring a homozygous deletion of Cyp24a1 (BVE^Cyp24a1-null) were obtained by several rounds of breading among LSL-Braf^V600E, TPO-Cre, and Cyp24a1+/-mice. Thyroid tumor growth and progression in BVE^Cyp24a1-null mice were evaluated both in vitro and in vivo. Results: BVE^Cyp24a1-null exhibited a 4-fold reduction in tumor growth. Notably, we found the tumorigenic potential of BVE^Cyp24a1-null-derived tumor cells to be nearly abolished in immunocompromised nude mice. This phenotype was associated with downregulation of MAPK, PI3K/Akt, and TGF-? signaling pathways and a loss of epithelial-mesenchymal transition (EMT) in BVE^Cyp24a1-null cells. Gene expression profiling showed many genes involved in EMT and tumor invasion/metastasis were significantly down-regulated following Cyp24a1 knockout.  Although calcitriol treatment alone did not decrease cell proliferation in BVE^Cyp24a1-null cells, it strengthened antitumor responses to BRAF^V600E inhibitor PLX4720 in both BVE^Cyp24a1-null and BVE^Cyp24a1-wt cells. Conclusion: Our results indicate Cyp24a1 is a proto-oncogene. Its overexpression activated multiple signaling cascades to promote thyroid cancer progression and caused resistance to PLX4720. Calcitriol synergized the therapeutic effects of PLX4720 and reduced tumor resistance. Targeting Cyp24a1 overexpression maybe a viable approach for cancer therapy.