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Fbx4 is an F-box constituent of Skp-Cullin-F-box (SCF) ubiquitin ligases that directs ubiquitylation of cyclin D1. Ubiquitylation of cyclin D1 requires phosphorylation of both cyclin D1 and Fbx4 by GSK3β. GSK3β-mediated phosphorylation of Fbx4 Ser12 during the G1/S transition regulates Fbx4 dimerization, which in turn governs Fbx4-driven E3 ligase activity. In esophageal carcinomas that overexpress cyclin D1, Fbx4 is subject to inactivating mutations that specifically disrupt dimerization, highlighting the biological significance of this regulatory mechanism. In an effort to elucidate the mechanisms that regulate dimerization, we sought to identify proteins that differentially bind to wild-type Fbx4 versus a cancer-derived dimerization-deficient mutant. We provide evidence that phosphorylation of Ser12 generates a docking site for 14-3-3ɛ. 14-3-3ɛ binds to endogenous Fbx4 and this association is impaired by mutations that target either Ser8 or Ser12 in Fbx4, suggesting that this N-terminal motif in Fbx4 directs its interaction with 14-3-3ɛ. Knockdown of 14-3-3ɛ inhibited Fbx4 dimerization, reduced SCFFbx4 E3 ligase activity and stabilized cyclin D1. Collectively, the current results suggest a model wherein 14-3-3ɛ binds to Ser12-phosphorylated Fbx4 to mediate dimerization and function. [ABSTRACT FROM AUTHOR]

Protein ubiquitylation is a complex enzymatic process that results in the covalent attachment of ubiquitin, through Gly-76 of ubiquitin, to an ɛNH2 group of an internal lysine residue in a given substrate. Although E3 ligases frequently use lysines adjacent to the degron within the substrate, many substrates can be targeted to the proteasome through the polyubiquitylation of any lysine. We have assessed the role of lysine residues proximal to the cyclin D1 phosphodegron for ubiquitylation by the SCFFbx4/αB-crystallin ubiquitin ligase and subsequent proteasome-dependent degradation of cyclin D1. The work described herein reveals a requisite role for Lys-269 (K269) for the rapid polyubiquitin-mediated degradation of cyclin D1. Mutation of Lys-269, which is proximal to the phosphodegron sequence surrounding Thr-286 in cyclin D1, not only stabilizes cyclin D1 but also triggers cyclin D1 accumulation within the nucleus, thereby promoting cell transformation. In addition, D1-K269R is resistant to genotoxic stress-induced degradation, similar to non-phosphorylatable D1-T286A, supporting the critical role for the post-translational regulation of cyclin D1 in response to DNA-damaging agents. Strikingly, although mutation of lysine 269 to arginine inhibits cyclin D1 degradation, it does not inhibit cyclin D1 ubiquitylation in vivo, showing that ubiquitylation of a specific lysine can influence substrate targeting to the 26S proteasome. [ABSTRACT FROM AUTHOR]

Although cyclin D1 is overexpressed in a significant number of human cancers, overexpression alone is insufficient to promote tumorigenesis. In vitro studies have revealed that inhibition of cyclin D1 nuclear export unmasks its neoplastic potential. Cyclin D1 nuclear export depends upon phosphorylation of a C-terminal residue, threonine 286, (Thr-286) which in turn promotes association with the nuclear exportin, CRM1. Mutation of Thr-286 to a non-phosphorylatable residue results in a constitutively nuclear cyclin D1 protein with significantly increased oncogenic potential. To determine whether cyclin D1 is subject to mutations that inhibit its nuclear export in human cancer, we have sequenced exon 5 of cyclin D1 in primary esophageal carcinoma samples and in cell lines derived from esophageal cancer. Our work reveals that cyclin D1 is subject to mutations in primary human cancer. The mutations identified specifically disrupt phosphorylation of cyclin D1 at Thr-286, thereby enforcing nuclear accumulation of cyclin D1. Through characterization of these mutants, we also define an acidic residue within the C-terminus of cyclin D1 that is necessary for recognition and phosphorylation of cyclin D1 by glycogen synthase kinase-3 beta. Finally, through construction of compound mutants, we demonstrate that cell transformation by the cancer-derived cyclin D1 alleles correlates with their ability to associate with and activate CDK4. Our data reveal that cyclin D1 is subject to mutations in primary human cancer that specifically disrupt phosphorylation-dependent nuclear export of cyclin D1 and suggest that such mutations contribute to the genesis and progression of neoplastic growth.Oncogene (2006) 25, 6291–6303. doi:10.1038/sj.onc.1209644; published online 29 May 2006 [ABSTRACT FROM AUTHOR]