BRD4 as a potential target for human papillomaviruses associated cancer

Abstract

Around 99% of cervical cancer and 5%-10% of human cancer are associated with human papillomaviruses (HPV). Notably, the life-cycle of HPV begins by low-level infection of the basal cells of the stratified epithelium, where the viral genomes are replicated and passed on to the daughter proliferating basal cells. The production of new viral particles remains restricted to eventually differentiated cells. HPVs support their persistent infectious cycle by hijacking pivotal pathways and cellular processes. Bromodomain-containing protein 4 (BRD4) is one of the essential cellular factors involved in multiple stages of viral transcription and replication. In this review, we demonstrate the role of BRD4 in the multiple stages of HPV infectious cycle. Also, we provide an overview of the intense research about the cellular functions of BRD4, the mechanism of action of bromodomain and extra terminal inhibitors, and how it could lead to the development of antiviral/anticancer therapies.

Keywords: antiviral agents; cell cultures: research and analysis methods; human immunodeficiency virus: vrus classification; human papillomavirus: virus classification; oncogenesis.

Fig. 1:. BRD4 structure and its isoforms.

Protein isoforms of human (h) BRD4. Sequence lengths and specific regions are represented. BD, bromodomain; CTM, C-terminal module; ET, extra-terminal domain; GPA: glycine-proline-alanine; NLS, nuclear localization signal; SEED, Ser/Glu/Asp-rich region.

Fig 2:. BRD4 involves multiple-steps of HPV life-cycle.

Fig 3:. Proposed mechanisms of ZL0580 in suppressing HIV.

ZL0580 exhibits binding affinity towards BRD4, specifically targeting a location of the non-KAc site other than the classic KAc binding site on BRD4. This interaction results in the augmentation of binding between ZL0580 and its associated binding partners (P-TEFb/CDK9) for protein-protein interactions. Nevertheless, it inhibits the interaction between Tat and BRD4, leading to the downregulation of the ELL2 protein via a decrease in protein stability. The combination of diminished Tat-CDK9 interaction and decreased ELL2 expression results in a decrease in RNA polymerase phosphorylation at the HIV long terminal repeat (LTR) and a disruption in the elongation of HIV transcription.

Fig. 4.. BRD4 inhibitors inhibit HPV DNA amplification in patient-derived and naturally HPV infected cells under 3D cultures, not 2D cultures.

No inhibition effect of JQ1, ZL0580 and vorinostat on HPV DNA in 2D cultures of naturally HPV-18 infected, patient-derived cells (left). Cells were treated by DMSO, JQ1, ZL0580 or vorinostat (5 uM) for 24 hrs. HPV-18 DNA copies were determined by qPCR. Inhibition of HPV-18 DNA copies by JQ1, ZL0580 and vorinostat in ALI 3D cultures. ALI 3D cultures of patient-derived cells were exposed to DMSO or ZL0580. Drug treatment was added at day 11 to 18 of ALI 3D cultures. Cellular DNA was collected at day 18. HPV18 DNA copies were determined by qPCR using a HPV18 plasmid DNA as a control.

Fig. 5.

Experimental systems for studies of how HIV affects HPV life cycle and oncogenesis and discovery of dual inhibition of HIV/HPV. HPV positive CR cells from patients can be used to establish and 3D based culture systems. HPV cells will be cocultured with HPV in the presence of HIV isolates and mutants. HIV factors (proteins or RNAs) including gp120, Tat, Nef, TAR, Vpr can be used in these systems for studies of regulation and inhibition of HPV viral gene expression, replication and biological changes of host cells with naturally HPV infected cells after coculture.