Differential disruption of genomic integrity and cell cycle regulation in normal human fibroblasts by the HPV oncoproteins.

Abstract

Genomic integrity is maintained by a network of cellular activities that assess the status of the genome at a given point in time, provide signals to proceed with or halt cell cycle progression, and provide for repair of damaged DNA. Mutations in any part of these pathways can have the ultimate effect of disturbing chromosomal integrity. Recent work suggests that p53 performs this integrator function in mammalian cells. Our present study demonstrates that in mortal cells, the expression of £6 and £7 viral oncoproteins of type 16 human papillomavirus each disrupts the integration of these signals by diverged pathways. Cells expressing £6 protein, which binds and degrades the p53 protein, exhibited alterations in cell cycle control when placed in drug and displayed the ability to amplify the CAD gene. The expression of £7, which binds different cellular proteins important for transformation, including Rb, led to a p53-independent alteration in cell cycle control, a widespread cytocidal response, and polyploidy as a mechanism of drug resistance. These results demonstrate that diverse perturbations of molecular pathways can have different effects on chromosomal integrity.

Polyomavirus large T antigen-dependent DNA amplification

“…Similar ®ndings were reported for human papilloma virus E7 protein (Vousden et al, 1993;Hickman et al, 1994;Demers et al, 1994;Szebos et al, 1994). On the other hand, adenovirus E1A protein was shown to convert REF52 cells to the permissive state only when mutant Ras protein was present at the same time (Perry et al, 1992) and papillomavirus E7 protein was found to induce aneuploidy in PALA treated normal human ®broblasts but due to massive cell death cell lines could only very rarely be selected (White et al, 1994). We do not have a de®nitive explanation for these di erences between the di erent viral proteins but it may be related to subtle di erences between the viral proteins in their capacity to dissociate E2F pocket protein complexes.…”

“…Important among these is the CDK inhibitor protein p21. Mutation of p53 or interference with its activity, a condition generated by proteins coded by several DNA tumor viruses, makes cells permissive for DNA ampli®cation (Perry et al, 1992;White et al, 1994). DNA ampli®cation is seen as one indication for genome destabilisation which itself is a characteristic of immortalized and transformed cells.…”

“…Proteins coded by several DNA tumor viruses (adenovirus E1B 55k protein, SV40 large T antigen, human papilloma virus protein E6) were found to bind p53 and to cause its inactivation or destruction (for a review see Shen and Shenk, 1995). Such viral proteins or mutations in p53 make cells permissive for DNA ampli®cation and it is assumed that this reaction contributes to the transforming and immortalizing activity of these viruses (Perry et al, 1992;White et al, 1994). There is one interesting exception: polyomavirus does immortalize and transform cells but none of the proteins encoded by polyomavirus DNA interacts with p53 (see the review by Pipas, 1992).…”

Induction of S phase and apoptosis by the human papillomavirus type 16 E7 protein are separable events in immortalized rodent fibroblasts

“…Next we determined the ability of the E7 proteins to trigger apoptosis. Analogously to other inducers of apoptosis, such as c-myc, Adenovirus E1A or E2F-1 (Evan et al, 1992;Hsieh et al, 1997;White et al, 1994), E7-induced apoptosis is more pronounced when the cells are deprived of serum survival factors . Therefore, NIH3T3 cells expressing the di erent E7 genes were cultured in medium containing 0.5% calf serum and the percentage of apoptotic cells was determined using time-lapse videomicroscopy.…”

“…As other stimulators of proliferation, the HPV16 E7 protein, besides the ability to deregulate the cell cycle, promotes apoptosis (Pan andGriep, 1994, 1995;White et al, 1994;Puthenveettil et al, 1996;Iglesias et al, 1998;Stoppler et al, 1998). Expression of HPV16 E7 in normal human ®broblasts (NHF) or in human keratinocytes results in a cytocidal response, which is much more evident in the absence of mitogenic signals and displays the typical features of apoptosis .…”