title: Epstein-Barr virus nuclear antigen 3C facilitates G1-S transition by stabilizing and enhancing the function of cyclin D1.
abstract: EBNA3C, one of the Epstein-Barr virus (EBV)-encoded latent antigens, is essential for primary B-cell transformation. Cyclin D1, a key regulator of G1 to S phase progression, is tightly associated and aberrantly expressed in numerous human cancers. Previously, EBNA3C was shown to bind to Cyclin D1 in vitro along with Cyclin A and Cyclin E. In the present study, we provide evidence which demonstrates that EBNA3C forms a complex with Cyclin D1 in human cells. Detailed mapping experiments show that a small N-terminal region which lies between amino acids 130-160 of EBNA3C binds to two different sites of Cyclin D1- the N-terminal pRb binding domain (residues 1-50), and C-terminal domain (residues 171-240), known to regulate Cyclin D1 stability. Cyclin D1 is short-lived and ubiquitin-mediated proteasomal degradation has been targeted as a means of therapeutic intervention. Here, we show that EBNA3C stabilizes Cyclin D1 through inhibition of its poly-ubiquitination, and also increases its nuclear localization by blocking GSK3β activity. We further show that EBNA3C enhances the kinase activity of Cyclin D1/CDK6 which enables subsequent ubiquitination and degradation of pRb. EBNA3C together with Cyclin D1-CDK6 complex also efficiently nullifies the inhibitory effect of pRb on cell growth. Moreover, an sh-RNA based strategy for knock-down of both cyclin D1 and EBNA3C genes in EBV transformed lymphoblastoid cell lines (LCLs) shows a significant reduction in cell-growth. Based on these results, we propose that EBNA3C can stabilize as well as enhance the functional activity of Cyclin D1 thereby facilitating the G1-S transition in EBV transformed lymphoblastoid cell lines.
authors.affiliation: Department of Microbiology and Tumor Virology Program, Abramson Comprehensive Cancer Center, University of Pennsylvania Medical School, Philadelphia, Pennsylvania, United States of America.
authors.forename: Abhik
authors.initials: A
authors.lastname: Saha
authors.forename: Sabyasachi
authors.initials: S
authors.lastname: Halder
authors.forename: Santosh K
authors.initials: SK
authors.lastname: Upadhyay
authors.forename: Jie
authors.initials: J
authors.lastname: Lu
authors.forename: Pankaj
authors.initials: P
authors.lastname: Kumar
authors.forename: Masanao
authors.initials: M
authors.lastname: Murakami
authors.forename: Qiliang
authors.initials: Q
authors.lastname: Cai
authors.forename: Erle S
authors.initials: ES
authors.lastname: Robertson
mesh_terms: D000956:Antigens, Viral / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D002472:Cell Transformation, Viral / Q000235:genetics; D002478:Cells, Cultured; D019938:Cyclin D1 / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D020031:Epstein-Barr Virus Infections / Q000235:genetics; D019309:Epstein-Barr Virus Nuclear Antigens; D016193:G1 Phase / Q000235:genetics*; D005786:Gene Expression Regulation; D004854:Herpesvirus 4, Human / Q000502:physiology; D006801:Humans; D011485:Protein Binding; D011499:Protein Processing, Post-Translational / Q000235:genetics / Q000502:physiology; D055550:Protein Stability; D017434:Protein Structure, Tertiary / Q000502:physiology; D016196:S Phase / Q000235:genetics*; D054875:Ubiquitination; D015854:Up-Regulation / Q000235:genetics
chemical_list: D000956:Antigens, Viral; C530104:CCND1 protein, human; C498774:EBNA-3C, epstein-barr virus; D019309:Epstein-Barr Virus Nuclear Antigens; D019938:Cyclin D1