Research Article: HPV-18 E7 CKII Phospho Acceptor Site for Maintaining the Transformed Phenotype of Cervical Tumour-Derived Cells

The Human Papillomavirus E7 oncoprotein plays an essential role in the development and maintenance of malignancy, which it achieves through targeting a number of critical cell con- trol pathways. An important element in the ability of E7 to contribute towards cell transforma- tion is the presence of a Casein Kinase II phospho-acceptor site within the CR2 domain of the protein. Phosphorylation is believed to enhance E7 interaction with a number of different cellular target proteins, and thereby increase the ability of E7 to enhance cell proliferation and induce malignancy. However, there is little information on how important this site in E7 is, once the tumour cells have become fully transformed. In this study, we have performed genome editing of the HPV-18 E7 CKII recognition site in C4-1 cervical tumour-derived cells. We first show that mutation of HPV18 E7 S32/S34 to A32/A34 abolishes CKII phos- phorylation of E7, and subsequently we have isolated C4-1 clones containing these muta- tions in E7. The cells continue to proliferate, but are somewhat more slow-growing than wild type cells, reach lower saturation densities, and are also more susceptible to low nutrient conditions. These cells are severely defective in matrigel invasion assays, partly due to downregulation of matrix metalloproteases (MMPs). Mechanistically, we find that phosphor- ylation of E7 plays a direct role in the ability of E7 to activate AKT signaling, which in turn is required for optimal levels of MMP secretion. These results demonstrate that the E7 CKII phospho-acceptor site thus continues to play an important role for E7’s activity in cells derived from cervical cancers, and suggests that blocking this activity of E7 could be expected to have therapeutic potential.

In this study we have used genome editing to mutate the HPV-18 E7 CKII phospho- acceptor site in cells derived from a cervical cancer. We demonstrate that this results in a decrease in cell proliferation and renders the cells particularly susceptible to low nutrient conditions. Furthermore these cells are defective in invasive potential and this appears linked to a decrease in the levels of secreted MMPs. Mechanistically this is linked directly to a role of the E7 CKII phospho-acceptor site in upregulating AKT signaling. These studies demonstrate that the E7 CKII site plays a direct role in maintaining a fully trans- formed phenotype, and indicates a novel function for this region of E7 in regulating AKT and the levels of secreted MMPs.

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