Regulation of the Human Papillomavirus E7 Oncoprotein

Trejo Cerro, Oscar (2023). Regulation of the Human Papillomavirus E7 Oncoprotein. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00015683

Abstract

HPV-induced carcinogenesis is mainly associated with the uncontrolled expression of the oncoproteins E6 and E7. HPV E7 lacks enzymatic activity, however, it can target several cellular proteins to induce cell transformation and promote unregulated proliferation. Although numerous cellular target proteins have been reported to interact with the HPV E7 oncoprotein, the mechanisms involved in HPV-induced malignancy and the maintenance of cell transformation are still lacking. In this work, using a small HPV16 E7 peptide in a proteomic approach, we report Memo1 as a new E7 binding partner, interacting through the aspartic and glutamic acid residues (ED80-81) in the C-terminal region of HPV16 E7. The levels and localization of Memo1 in HPV-positive cell lines are regulated by HPV E7 and high expression of Memo1 correlates with reduced cell proliferation. In addition, we show that overexpression of Memo1 decreases HPV16 E7-induced transformation and is involved in the invasive capacity of transformed cells. Mechanistically, knock-down of Memo1 correlates with activation of the Akt pathway in HPV-positive cell lines, which seems to be regulated through a mechanism that is downstream of phosphatidylinositol 3-kinase (PI3K) activation. Although the E7-Memo1 interaction seems to be conserved across different HPV E7 types, including high and low-risk, we observed reduction of Memo1 levels only in the context of the high-risk HPV types. Our results show Memo1 as a new E7 binding partner and describe novel functions of Memo1 in the context of HPV-induced carcinogenesis.

Furthermore, we found that the CKI and GSK3 kinases phosphorylate HPV16 E7 at threonine 5, 7 and 19-20 residues. The novel HPV16 E7 TT19-20 phosphorylation site seems to be important for E7-induced transformation and cell proliferation. Negative charges at this site enhance the binding of pRb to E7. Moreover, using a phosphorylated peptide of this region of HPV16 E7, we found new, potentially specific, phospho-E7 interactors. This data reveals novel phosphorylation sites in HPV16 E7, which modulate its interaction with pRb and might regulate the binding/function of other potential phospho-E7 interactors.

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