MODULATION OF HOST CELL APOPTOTIC RESPONSES BY HPVE7

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In normal cells, the processes of cellular growth, differentiation, and death are intricately coupled. In contrast, these processes are imbalanced in cancer cells. During differentiation of squamous epithelia, differentiation is tightly linked to cellular proliferation in that the keratinocytes withdraw permanently from the cell division cycle while they undergo differentiation. The terminally differentiated keratinocytes undergo cell death and are sloughed off. Human papillomaviruses (HPVs) infect keratinocytes and their life cycle is connected to the differentiation program of the host epithelial cells. To allow their replication, the HPVs have to maintain a replication-competent milieu in the infected host cell. Hence, the HPVs encode proteins that allow replication in differentiating keratinocytes. During carcinogenic progression the viral genome is often integrated into the cellular genome. This results in the dysregulated expression of these viral regulatory proteins. The two viral proteins E6 and E7 are consistently expressed in HPV-positive cancers. We have previously shown that the HPV E7 oncoprotein can inactivate cell cycle checkpoints by inactivating the retinoblastoma tumor suppressor protein, pRB, and the cyclin dependent kinase inhibitor p21. Here we present preliminary results that demonstrate, that E7 also has the capacity to modulate cell death pathways. Depending on the stimulus, HPV E7 can either enhance or inhibit the apoptotic program. HPV E7-expressing cells are predisposed to undergo apoptosis in response to growth factor deprivation while they are protected from undergoing apoptosis in response to the cytokine tumor necrosis factor alpha (TNF). The capacity of E7 to modulate cell death pathways correlates with ability to destabilize the retinoblastoma tumor suppressor protein, pRB and the stabilization of the p53 tumor suppressor. Here I propose an integrated plan to determine whether the regulation of the stabilities of pRB and p53 are coupled, and to analyze the relative importance of p53 and/or pRB in these apoptosis pathways. Furthermore I will determine the point of action of E7 in the TNF pathway. Our preliminary results indicate that this may be at the level of coupling of caspase 8 (FLICE) to the TNF-receptor. In concert, these two aims will provide insight into critical regulatory circuits of cell death. Since they may also be dysfunctional in other, non-HPV associated cancers they should provide useful targets for anticancer therapy.