Macrophages Kill Human Papillomavirus Type 16 E6-Expressing Tumor Cells by Tumor Necrosis Factor Alpha- and Nitric Oxide-Dependent Mechanisms

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Abstract
The expression of adenovirus serotype 2 or 5 (Ad2/5) E1A sensitizes cells to killing by NK cells and activated macrophages, a property that correlates with the ability of E1A to bind the transcriptional coadaptor proteins p300-CBP. The E6 oncoproteins derived from the high-risk human papillomaviruses (HPV) interact with p300 and can complement mutant forms of E1A that cannot interact with p300 to induce cellular immortalization. Therefore, we determined if HPV type 16 (HPV16) E6 could sensitize cells to killing by macrophages and NK cells. HPV16 E6 expression sensitized human (H4 and C33A) and murine (MCA-102) cell lines to lysis by macrophages but not by NK cells. The lysis of cells that expressed E6 by macrophages was p53 independent but dependent on the production of tumor necrosis factor alpha (TNF-α) or nitric oxide (NO) by macrophages. Unlike cytolysis assays with macrophages, E6 expression did not significantly sensitize cells to lysis by the direct addition of NO or TNF-α. Like E1A, E6 has been reported to sensitize cells to lysis by TNF-α by inhibiting the TNF-α-induced activation of NF-κB. We found that E1A, but not E6, blocked the TNF-α-induced activation of NF-κB, an activity that correlated with E1A-p300 binding. In summary, Ad5 E1A and HPV16 E6 sensitized cells to lysis by macrophages. Unlike E1A, E6 did not block the ability of TNF-α to activate NF-κB or sensitize cells to lysis by NK cells, TNF-α, or NO. Thus, there appears to be a spectrum of common and unique biological activities that result as a consequence of the interaction of E6 or E1A with p300-CBP.

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