Cyclin F (CCNF) is the substrate recognition component of the SCF complex, which functions as an E3 ubiquitin ligase. A previous report has described the downregulation of CCNF in hepatocellular carcinoma. Hence, we examined the levels of CCNF in lower grade gliomas which are characterized by mutations in the enzyme IDH1. CCNF expression levels were low in gliomas, in contrast to those of IDH1R132H. We also observed that CCNF expression is induced upon metabolic stress. We have delineated a cascade in which CCNF brings about the proteasomal degradation of RBPJ. Furthermore, we have also established that RBPJ transcriptionally regulates IDH1 expression. Here, we have confirmed the regulation of IDH1R132H expression by CCNF via RBPJ in mouse models of glioma.
BT142, a patient-derived glioma cell line which harbors the IDH1R132H mutation, was used to study the effects of the CCNF-RBPJ axis. This cell line was genetically engineered and was used for obtaining subcutaneous and orthotopic xenografts in immunodeficient mice. CCNF expression was induced by the administration of metformin to mimic metabolic stress conditions. Multiple parameters were analyzed to determine tumor growth and metastases. Immunohistochemical analyses of tumor samples from glioma patients were also performed.
We observed that implantation of CCNF-deficient cells and cells expressing the ubiquitylation-resistant RBPJ resulted in larger tumor volumes. Induction of CCNF prevented tumor growth as well as metastases in the mouse model of glioma. An inverse correlation was observed between the protein levels of CCNF and IDH1R132H in glioma samples from human patients.
CCNF causes the downregulation of IDH1R132H expression, and thereby controls tumorigenesis. It would be beneficial to induce CCNF expression using metformin as an adjuvant therapy in patients with IDH1 mutations. Alternatively, mechanisms which inhibit CCNF activity must be deciphered to identify druggable targets.
Clinical trial identification
Legal entity responsible for the study
Dr Sanjeev Das
National Institute of Immunology, India and Department of Biotechnology, Government of India
All authors have declared no conflicts of interest.