Abrogation of the Brd4-Positive Transcription Elongation Factor b Complex by Papillomavirus E2 Protein Contributes to Viral Oncogene Repression

Abstract

The cellular bromodomain protein Brd4 is a major interacting partner of the papillomavirus (PV) E2 protein. Interaction of E2 with Brd4 contributes to viral episome maintenance. The E2-Brd4 interaction also plays an important role in repressing viral oncogene expression from the integrated viral genome in human PV (HPV)-positive cancer cells. However, the underlying mechanism is not clearly understood. In host cells, Brd4 recruits positive transcription elongation factor b (P-TEFb) to stimulate RNA polymerase II phosphorylation during cellular and viral gene expression. P-TEFb associates with the C terminus of Brd4, which largely overlaps with the E2 binding site on Brd4. In this study, we demonstrate that E2 binding to Brd4 inhibits the interaction of endogenous Brd4 and P-TEFb. P-TEFb is essential for viral oncogene E6/E7 transcription in both HeLa and CaSki cells that contain integrated HPV genomes. E2 binding to Brd4 abrogates the recruitment of P-TEFb to the integrated viral chromatin template, leading to inactivation of P-TEFb and repression of the viral oncogene E6/E7. Furthermore, dissociation of the Brd4-P-TEFb complex from the integrated viral chromatin template using a Brd4 bromodomain dominant-negative inhibitor also hampers HPV E6/E7 oncogene expression. Our data support that Brd4 recruitment of P-TEFb to the viral chromatin template is essential for viral oncogene expression. Abrogation of the interaction between P-TEFb and Brd4 thus provides a mechanism for E2-mediated repression of the viral oncogenes from the integrated viral genomes in cancer cells.Papillomaviruses (PVs) are small DNA tumor viruses that induce a variety of benign and malignant epithelial lesions in the infected host (for a review, see reference 29). Over 140 human PV (HPV) types have been identified to date. Depending on the potential for induction of malignant transformation, these viruses are further classified into high-risk and low-risk HPVs. High-risk HPVs are strongly associated with cervical cancer (69), which is the second most common cancer among women worldwide and the leading cause of death from cancer among women in developing countries.The PVs establish long-term, persistent infections. During latent infection, the viral genomes are maintained as extrachromosomal elements (episomes) that replicate along with host DNA. The PV E2 protein is a sequence-specific DNA binding protein involved in viral DNA replication, episome maintenance, and viral transcription (26). E2 consists of an N-terminal transcriptional activation domain linked to a C-terminal DNA binding/dimerization domain by a flexible hinge (39). The multiple functions of E2 rely on its sequence-specific recognition of a number of E2 binding sites within PV genomes. E2 initiates viral genome replication by loading the viral helicase E1 to the origin of replication (4, 43). It ensures the accurate partitioning of the replicated viral genomes to daughter cells by tethering them to host mitotic chromosomes (2,31,36,48,55). In addition, E2 can either activate or...

Identification of Interaction Sites for Dimerization and Adapter Recruitment in Toll/Interleukin-1 Receptor (TIR) Domain of Toll-like Receptor 4

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Negative Elongation Factor-Mediated Suppression of RNA Polymerase II Elongation of Kaposi's Sarcoma-Associated Herpesvirus Lytic Gene Expression

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