title: The F-box protein FBX4 targets PIN2/TRF1 for ubiquitin-mediated degradation and regulates telomere maintenance.
abstract: Pin2/TRF1 was identified previously as both a protein (TRF1) that binds to telomeric DNA repeats and as a protein (Pin2) that associates with the kinase NIMA and suppresses its mitosis inducing activity. Pin2/TRF1 negatively regulates telomere length and also plays a critical role in cell cycle checkpoint control. Pin2/TRF1 is down-regulated in many human cancers and may be degraded by the ubiquitin-proteasome pathway, but components of the pathway involved in Pin2/TRF1 turnover have not been elucidated. By using the two-hybrid system, we recently identified Pin2/TRF1-interacting proteins, PinX1-4, and we demonstrated that PinX1 is a conserved telomerase inhibitor and a putative tumor suppressor. Here we report the characterization of PinX3. PinX3 was later found to be identical to Fbx4, a member of the F-box family of proteins, which function as substrate-specific adaptors of Cul1-based ubiquitin ligases. Fbx4 interacts with both Pin2 and TRF1 isoforms and promotes their ubiquitination in vitro and in vivo. Moreover, overexpression of Fbx4 reduces endogenous Pin2/TRF1 protein levels and causes progressive telomere elongation in human cells. In contrast, inhibition of Fbx4 by RNA interference stabilizes Pin2/TRF1 and promotes telomere shortening, thereby impairing cell growth. These results demonstrate that Fbx4 is a central regulator of Pin2/TRF1 protein abundance and that alterations in the stability of Pin2/TRF1 can have a dramatic impact on telomere length. Thus, Fbx4 may play a critical role in telomere maintenance.
authors.affiliation: Cancer Biology Program, Division of Hematology/Oncology, Department of Medicine, Beth Israel Deaconess Medical Center, Boston, Massachusetts, USA.
authors.forename: Tae Ho
authors.initials: TH
authors.lastname: Lee
authors.forename: Kilian
authors.initials: K
authors.lastname: Perrem
authors.forename: J Wade
authors.initials: JW
authors.lastname: Harper
authors.forename: Kun Ping
authors.initials: KP
authors.lastname: Lu
authors.forename: Xiao Zhen
authors.initials: XZ
authors.lastname: Zhou
mesh_terms: D000255:Adenosine Triphosphate / Q000737:chemistry; D002460:Cell Line; D045744:Cell Line, Tumor; D004247:DNA / Q000737:chemistry / Q000378:metabolism; D015536:Down-Regulation; D044783:F-Box Proteins / Q000378:metabolism* / Q000502:physiology*; D005822:Genetic Vectors; D005982:Glutathione Transferase / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D015151:Immunoblotting; D047468:Immunoprecipitation; D008938:Mitosis; D011485:Protein Binding; D020033:Protein Isoforms; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D034622:RNA Interference; D012333:RNA, Messenger / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry; D013379:Substrate Specificity; D016615:Telomere / Q000648:ultrastructure*; D035321:Telomeric Repeat Binding Protein 1 / Q000378:metabolism*; D013997:Time Factors; D020798:Two-Hybrid System Techniques; D025801:Ubiquitin / Q000737:chemistry* / Q000378:metabolism
chemical_list: D044783:F-Box Proteins; C508146:FBXO4 protein, human; D020033:Protein Isoforms; D011500:Protein Synthesis Inhibitors; D012333:RNA, Messenger; D011994:Recombinant Proteins; D035321:Telomeric Repeat Binding Protein 1; D025801:Ubiquitin; D000255:Adenosine Triphosphate; D004247:DNA; D005982:Glutathione Transferase