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.

Author Summary

Epstein-Barr virus (EBV), a ubiquitous human herpesvirus, is linked to the development of multiple cancers, including lymphomas and epithelial carcinomas. EBNA3C, one of its essential latent antigens encoded by EBV, is expressed in EBV-associated lymphomas and contributes to aberrant cell growth after EBV infection. Cyclin D1 over-expression is associated with numerous cancers and is crucial for the transition from G1 to S phase in the mammalian cell-cycle. This study demonstrates that EBNA3C can enhance the functional activity of the Cyclin D1/CDK6 complex which in turn facilitates the G1 to S transition by neutralizing the growth inhibitory effects of pRb. Thus, manipulation of Cyclin D1 functions by EBNA3C provides a favorable environment to promote malignant transformation of EBV infected B-cells.