The Fbx4 Tumor Suppressor Regulates Cyclin D1 Accumulation and Prevents Neoplastic Transformation

Abstract

Skp1-Cul1-F-box (SCF) E3 ubiquitin ligase complexes modulate the accumulation of key cell cycle regulatory proteins. Following the G₁/S transition, SCF^Fbx4 targets cyclin D1 for proteasomal degradation, a critical event necessary for DNA replication fidelity. Deregulated cyclin D1 drives tumorigenesis, and inactivating mutations in Fbx4 have been identified in human cancer, suggesting that Fbx4 may function as a tumor suppressor. Fbx4^+/− and Fbx4^−/− mice succumb to multiple tumor phenotypes, including lymphomas, histiocytic sarcomas and, less frequently, mammary and hepatocellular carcinomas. Tumors and premalignant tissue from Fbx4^+/− and Fbx4^−/− mice exhibit elevated cyclin D1, an observation consistent with cyclin D1 as a target of Fbx4. Molecular dissection of the Fbx4 regulatory network in murine embryonic fibroblasts (MEFs) revealed that loss of Fbx4 results in cyclin D1 stabilization and nuclear accumulation throughout cell division. Increased proliferation in early passage primary MEFs is antagonized by DNA damage checkpoint activation, consistent with nuclear cyclin D1-driven genomic instability. Furthermore, Fbx4^−/− MEFs exhibited increased susceptibility to Ras-dependent transformation in vitro, analogous to tumorigenesis observed in mice. Collectively, these data reveal a requisite role for the SCF^Fbx4 E3 ubiquitin ligase in regulating cyclin D1 accumulation, consistent with tumor suppressive function in vivo.

ACKNOWLEDGMENTS

We thank Petia Zamfirova, Margarita Romero, and Jesi Kim for technical assistance, Serge Fuchs for critical reading of the manuscript, the AFCRI histology core for assistance with paraffin embedding and sectioning, Oren Gilad and Eric Brown for providing p19^Arf shRNA lentivirus, and the NIH P30 Center for Molecular Studies in Digestive and Liver Diseases and its Morphology and Molecular Biology Cores.

This study was supported by grants from the National Institutes of Health (CA133154), a Leukemia and Lymphoma Scholar Award (J.A.D.), and P01-CA098101 (A.K.R. and J.A.D.).