p53-Dependent and -independent transactivation by the E6 protein of human papillomavirus type 16

Human papillomavirus 18 E6 inhibits phosphorylation of p53 expressed in HeLa cells

Background

In HPV infected cells p53 function is abrogated by E6 and even ectopically expressed p53 is unable to perform tumor suppressor functions. In addition to facilitating its degradation, E6 may also inhibit p53 transactivity, though the mechanisms are still poorly understood. It has been reported that inhibition of p300, an acetyltransferase responsible for p53 acetylation is inactivated by E6. Activation of overexpressed p53 to cause cell growth inhibition is facilitated by its phosphorylation. Previously, we reported that non-genotoxically overexpressed p53 in HeLa cells needs to be phosphorylated to perform its cell growth inhibitory functions. Since over expressed p53 by itself was not activated, we hypothesized an inhibitory role for E6.

Results

Majority of reports proposes E6 mediated degradation of p53 as a possible reason for its inactivation. However, results presented here for the first time demonstrate that overexpressed p53 is not directly associated with E6 and therefore free, yet it is not functionally active in HPV positive cells. Also, the stability of overexpressed p53 does not seem to be an issue because inhibition of proteasomal degradation did not increase the half-life of overexpressed p53, which is more than endogenous p53. However, inhibition of proteasomal degradation prevents the degradation of endogenous p53. These findings suggest that overexpressed p53 and endogenous p53 are differentially subjected to proteasomal degradation and the reasons for this discrepancy remain unclear. Our studies demonstrate that p53 over expression has no effect on anchorage independent cell-growth and E6 nullifies its cell growth inhibitory effect. E6 overexpression abrogates OA induced p53 occupancy on the p21 promoter and cell death as well. E6 did not decrease p53 protein but phospho-p53 level was significantly reduced.

Conclusion

We report for the first time that E6 de-activates p53 by inhibiting its phosphorylation. This prevents p53 binding to p21 promoter and thereby restraining its cell-growth inhibitory functions. Our study provides new evidence indicating that viral protein E6 inhibits p53 transactivity by mechanism independent of degradation pathway.

Repression of HPV E6-activated RSV promoter activity by anti-cancer agents

Human papillomavirus E6 forms a complex with p53 tumor suppressor and E6-associated protein, leading to the degradation of p53 via the ubiquitination pathway, resulting in the oncogenesis of cervical carcinomas. Several viral and cellular gene promoters were shown to be transactivated by E6 oncogene. In this study, to understand the role of transcription activity of E6 related to cervical carcinogenesis, the effect of cervical cancer drugs on E6 induced transcription activity has been elucidated. Several viral promoter (RSV, CMV, SV40, and HIV)-luciferase reporter gene constructs, and eukaryotic E6 expression vector were prepared as an E6 transcription analysis system and an exogenous E6 protein source, respectively. It was shown that the promoters of RSV, SV40, and HIV, but not CMV, were transactivated by HPV 16 E6 in cervical cancer cell line. Several known cervical cancer drugs were investigated for their effects on transcription activity of E6 in SiHa stably transfected with E6-responsive promoters. Cervical cancer drugs consistently reduced luciferase activity, in transfectants with RSV-luc (SiHa/pRSV-luc, KCTC 0427BP) E6 mRNA also. Thus, in this study, we have demonstrated that the promoters of RSV, HIV, and SV40 were transactivated by E6 in cervical cancer cells. Three cervical cancer drugs downregulated RSV-luc transcription and E6 expression by a p53 independent pathway. RSV-luc promoter analysis system could be useful for understanding the role of transcription activity of E6 related to cervical cancer and also for screening drugs against cervical cancers caused by HPV infection.

The papillomavirus E6 proteins

Specific types of human papillomaviruses (HPV) are strongly associated with the development of cervical cancer. The E6 gene from cancer-related HPVs has exhibited functions in tumorigenesis, regulation of transcription, telomerase, and apoptosis. Cancer-related HPVs E6 proteins bind the tumor suppressor p53 and promotes its degradation through an ubiquitin-dependent pathway. Several additional cellular E6-binding proteins have recently been identified and implicated in playing roles in p53-independent functions of E6.

The early HPV16 proteins can regulate mRNA levels of cell cycle genes in human cervical carcinoma cells by p53-independent mechanisms

Cervical carcinoma-associated human papillomavirus type 16 (HPV16) encodes E6 and E7 oncoproteins which inactivate p53 and Rb, respectively, but these interactions are not sufficient to account for the oncogenic potential of the virus. Several viral promoters were shown to be regulated by E6 and E7. To identify genes as cellular targets of the HPV16 early proteins, we transfected a new HPV-negative and p53-mutated cervical carcinoma-derived cell line with either the HPV16 full-length genome or the HPV16 E6 gene. HPV16 clones but not 16E6 clones showed a decreased doubling time that was not related to the viral DNA and mRNA patterns. In exponentially growing cells as well as in cells synchronized by serum starvation, expression of the E6 gene was associated with upregulation of the c-fos and c-jun proto-oncogenes and with downregulation of the c-Ha-ras gene. Furthermore, a viral gene other than E6 may be involved in downregulation of p53 because a reduced mRNA level at the G1/S transition was observed only in HPV16-cells. The present study on natural host cells indicates p53-independent transcriptional modulations of cell cycle regulatory genes related to HPV16 E6 and E7 expression.

Mutational analysis of transcriptional activation by the bovine papillomavirus type 1 E6

While the bovine papillomavirus type 1 (BPV-1) E6 induces tumorigenic transformation of murine C127 cells, it does not bind or promote the degradation of p53. We recently showed the cellular protein ERC-55/E6BP binds BPV-1 E6 as well as the cancer-related human papillomavirus (HPV) E6 proteins. BPV-1 E6 also binds E6-AP, a ubiquitin ligase necessary for HPV E6-induced p53 degradation. We previously reported that the transforming activity of a set of BPV-1 E6 mutants correlated with their E6BP-binding ability. Another function of BPV-1 E6 is stimulation of transcription when targeted to a promoter, although cellular promoters responsive to BPV-1 E6 have not been identified. To examine whether its transcriptional function is required for oncogenic activity, or is related to its interactions with E6-AP or E6BP, a series of BPV-1 E6 mutants were analyzed as fusions to a sequence-specific DNA binding domain for activity in yeast and in mammalian cells. We show that some transformation defective mutants retained substantial levels of transcriptional activation activity. These mutants also distinguish transcriptional activation from E6-AP and E6BP binding. These results suggest the transcriptional activation function of BPV-1 E6 is not sufficient for cell transformation.

Human papillomavirus type 16 E6 protein transcriptionally modulates fibronectin gene expression by induction of protein complexes binding to the cyclic AMP response element

Although human papillomavirus type 16 (HPV16) E6 protein has a transcription-modulatory activity for a wide variety of viral promoters, a cellular target for this activity of E6 has not yet been identified. In this study, using differential hybridization, we identified a mouse fibronectin (FN) gene as a putative cellular target whose expression is up-regulated by E6. Chloramphenicol acetyltransferase (CAT) assays with mouse and rat FN promoter-CAT fusion constructs indicated that HPV16 E6 transactivates the FN promoters in a p53-independent manner. Deletion and site-specific mutation analyses revealed that transactivation by HPV16 E6 depends upon a cyclic AMP response element (CRE) located at -160 relative to the start site of transcription. Gel retardation assays demonstrated that nuclear extracts from the HPV16 E6-expressing cells, compared to those from parental 10T1/2 cells, have increased binding activity to the CRE. Antibodies against c-Jun and ATF-2 disrupted this binding activity. These data indicate that HPV16 E6 transcriptionally modulates FN gene expression via the CRE by inducing the binding of the protein complexes, probably including c-Jun and ATF-2, to the CRE.

Transactivation of prothymosin alpha and c-myc promoters by human papillomavirus type 16 E6 protein

The human papillomavirus type 16 E6 protein exerts a transforming activity through inactivation of tumor suppressor p53. Recently E6 has been shown to have additional transforming activities independent of p53. E6 is able to transactivate or repress several specific viral promoters. However, underlying molecular mechanisms and cellular target genes for the activity are not well understood. Using a differential hybridization technique, we identified the prothymosin alpha gene as a cellular target of E6 transactivation. E6 was able to transactivate the prothymosin alpha promoter in H358 cells lacking p53 and in C33A cells harboring a mutant p53 allele. Disruption of the E-box in intron 1 of the prothymosin alpha promoter abolished the responsiveness to E6. Then we determined if E6 up-regulates the expression of Myc, by which the prothymosin alpha promoter is transactivated through the E-box. We found that E6 is also able to transactivate the c-myc promoter in H358 cells and in C33A cells. These results suggest that E6 is able to transactivate the c-myc promoter independently of p53, and that the prothymosin alpha promoter is subsequently transactivated by Myc.