Live cell, image-based high-throughput screen to quantitate p53 stabilization and viability in human papillomavirus positive cancer cells

HPV-induced cancers: preclinical therapeutic advancements

INTRODUCTION

High-risk HPV infections are related to several epithelial cancers. Despite the availability of prophylactic vaccines, HPV infections are still responsible for about 5% of all human malignancies worldwide. While therapeutic vaccines are ongoing clinical trials, genotoxic agents and surgical interventions represent current clinical treatments, with no specific anti-HPV drugs yet available in the clinics.

AREAS COVERED

We offer a comprehensive report of small molecules in preclinical studies proposed as potential anticancer agents against HPV-driven tumors. Given the importance of HPV oncoproteins for cancer maintenance, particularly E6 and E7, we present a classification of both non-targeted and targeted agents, with a further subdivision of the latter into two categories according to their either direct or indirect activity against viral protein functions.

EXPERT OPINION

Prophylactic vaccines can prevent the insurgence of HPV-related cancers, but have no effect against pre-existing infections. Moreover, their high cost, genotype-restricted effect and the growing worldwide distrust for vaccines make the availability of a specific drug an unmet medical need. Different viral early proteins emerge as ideal candidates for drug development. We highlight the most promising strategies and address future challenges in this field to herald the prospect of a specific therapeutic regimen against HPV-related cancers.

Identification of microRNAs that stabilize p53 in HPV-positive cancer cells

Etiologically, 5% of all cancers worldwide are caused by the high-risk human papillomaviruses (hrHPVs). These viruses encode two oncoproteins (E6 and E7) whose expression is required for cancer initiation and maintenance. Among their cellular targets are the p53 and the retinoblastoma tumor suppressor proteins. Inhibition of the hrHPV E6-mediated ubiquitylation of p53 through the E6AP ubiquitin ligase results in the stabilization of p53, leading to cellular apoptosis. We utilized a live cell high throughput screen to determine whether exogenous microRNA (miRNA) transfection had the ability to stabilize p53 in hrHPV-positive cervical cancer cells expressing a p53-fluorescent protein as an in vivo reporter of p53 stability. Among the miRNAs whose transfection resulted in the greatest p53 stabilization was 375-3p that has previously been reported to stabilize p53 in HeLa cells, providing validation of the screen. The top 32 miRNAs in addition to 375-3p were further assessed using a second cell-based p53 stability reporter system as well as in non-reporter HeLa cells to examine their effects on endogenous p53 protein levels, resulting in the identification of 23 miRNAs whose transfection increased p53 levels in HeLa cells. While a few miRNAs that stabilized p53 led to decreases in E6AP protein levels, all targeted HPV oncoprotein expression. We further examined subsets of these miRNAs for their abilities to induce apoptosis and determined whether it was p53-mediated. The introduction of specific miRNAs revealed surprisingly heterogeneous responses in different cell lines. Nonetheless, some of the miRNAs described here have potential as therapeutics for treating HPV-positive cancers. Importance Human papillomaviruses cause approximately 5% of all cancers worldwide and encode genes that contribute to both the initiation and maintenance of these cancers. The viral oncoprotein E6 is expressed in all HPV-positive cancers and functions by targeting the degradation of p53 through the engagement of the cellular ubiquitin ligase E6AP. Inhibiting the degradation of p53 leads to apoptosis in HPV-positive cancer cells. Using a high throughput live cell assay we identified several miRNAs whose transfection stabilize p53 in HPV-positive cells. These miRNAs have the potential to be used in the treatment of HPV-positive cancers.