The role of HPV oncoproteins and cellular factors in maintenance of hTERT expression in cervical carcinoma cells.
OBJECTIVE
E6 and E7 oncoproteins of high risk type HPV modulate activities of host components in cell cycle regulation. Many of these factors are also involved in the regulation of telomerase activity or the expression of hTERT, the catalytic subunit. Transcription of E6 and E7 is inhibited by the papillomavirus E2 protein, and ectopic expression of E2 in HeLa cells has been shown to cause activation of the p53-growth inhibitory pathway and downregulation of the hTERT gene. In this study, using E2 transduction in HeLa cells, the relative importance of host and viral factors in the maintenance of hTERT and telomerase activity in cervical carcinoma cells was investigated.


METHODS
Depletion of E6/E7 proteins, concomitant upregulation of p53, p21WAF1, and hypophosphorylated Rb, and downregulation of E2F1, c-Myc, and hTERT were achieved in HeLa cells through SV40-mediated E2 transduction. And, through gene transduction, E6 and E7 proteins were separately re-expressed in HeLa cells that were devoid of these proteins. As well, E2F1, c-Myc, and p53 were ectopically expressed in HeLa cells to ascertain their effect on the level of hTERT expression through RT-PCR, Western blotting, and TRAP assays.


RESULTS
Continued expression of E2F1 and c-Myc could not prevent hTERT downregulation caused by E2 transduction, but re-expression of either E6 or E7 individually reactivated hTERT expression. Finally, p53 overexpression caused repression of the hTERT gene in the presence of E6 and E7.


CONCLUSIONS
HPV E6 plays an important role in the induction and maintenance of high levels of hTERT in cervical carcinoma cells through direct stimulation of hTERT promoter and prevention of the inhibitory effects of p53. E7, but not E2F1, may contribute to high telomerase activity in cancer cells.
Jeong Seo
Eun
E
Department of Life Science, University of Seoul, Dongdaemungu, Jeonnongdong 90, Seoul 130-743, South Korea.
Jung Kim
Hyun
H
Jae Lee
Chan
C
Tae Kang
Hyun
H
Seong Hwang
Eun
E
D000818:Animals; D002417:Cattle; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000235:genetics; D015536:Down-Regulation; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; D004789:Enzyme Activation; D005260:Female; D006367:HeLa Cells; D006801:Humans; D009856:Oncogene Proteins, Viral / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D027383:Papillomaviridae; D011401:Promoter Regions, Genetic; D016271:Proto-Oncogene Proteins c-myc / Q000096:biosynthesis; D019098:Telomerase / Q000096:biosynthesis* / Q000378:metabolism; D014157:Transcription Factors / Q000096:biosynthesis / Q000235:genetics; D014161:Transduction, Genetic; D016159:Tumor Suppressor Protein p53 / Q000037:antagonists & inhibitors / Q000502:physiology; D002583:Uterine Cervical Neoplasms / Q000235:genetics / Q000378:metabolism* / Q000821:virology*; D014764:Viral Proteins / Q000235:genetics
D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; C051550:E2 protein, Bovine papillomavirus; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; C493753:E2F1 protein, human; C052603:E6 protein, Human papillomavirus type 18; C052633:E7 protein, Human papillomavirus type 18; C489427:MYC protein, human; D009856:Oncogene Proteins, Viral; D016271:Proto-Oncogene Proteins c-myc; D014157:Transcription Factors; D016159:Tumor Suppressor Protein p53; D014764:Viral Proteins; D019098:Telomerase