Deregulation of eIF4E: 4E-BP1 in differentiated human papillomavirus-containing cells leads to high levels of expression of the E7 oncoprotein.
Infections with high-risk human papillomaviruses (HPVs) are linked to more than 95% of cervical cancers. HPVs replicate exclusively in differentiated cells and the function of the HPV E7 oncoprotein is essential for viral replication. In this study, we investigated the mechanism that regulates E7 expression in differentiated cells. The level of E7 protein was strongly induced in HPV-containing Caski, HOK-16B, and BaP-T cells during growth in methylcellulose-containing medium, a condition that induces differentiation. Enhanced expression of E7 was observed between 4 and 8 h of culturing in methylcellulose and was maintained for up to 24 h. The increase was not due to altered stability of the E7 protein or an increase in the steady-state level of the E7 mRNA. Instead, the translation of the E7 mRNA was enhanced during differentiation. More than 70 to 80% of the E7 mRNA was found in the polysome fractions in the differentiated cells. Consistent with this observation, higher levels of the phosphorylated translator inhibitor 4E-BP1 were observed in differentiated HPV-containing cells but not in differentiated non-HPV tumor cells or primary keratinocytes. The mTOR kinase inhibitor rapamycin blocked phosphorylation of 4E-BP1 and significantly decreased the level of E7 protein in Caski cells, suggesting that phosphorylation of 4E-BP1 is linked to E7 expression. Prevailing models for the molecular mechanisms underlying E7 expression have focused largely on transcriptional regulation. The results presented in this study demonstrate a significant role of the cellular translation machinery to maintain a high level of E7 protein in differentiated cells.
Oh
Kwang-Jin
KJ
Center for Molecular Biology of Oral Diseases, College of Dentistry, M/C 860, University of Illinois at Chicago, 801 South Paulina Street, 60612, USA.
Kalinina
Anna
A
Park
No-Hee
NH
Bagchi
Srilata
S
D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D000903:Antibiotics, Antineoplastic / Q000494:pharmacology; D018797:Cell Cycle Proteins; D002454:Cell Differentiation / Q000187:drug effects; D039561:Eukaryotic Initiation Factor-4E / Q000378:metabolism*; D005260:Female; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D015967:Gene Expression Regulation, Viral / Q000187:drug effects; D006367:HeLa Cells; D006801:Humans; D015603:Keratinocytes / Q000378:metabolism / Q000821:virology; D008957:Models, Genetic; D027383:Papillomaviridae / Q000235:genetics / Q000378:metabolism*; D050725:Papillomavirus E7 Proteins / Q000096:biosynthesis* / Q000235:genetics; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D014176:Protein Biosynthesis / Q000187:drug effects; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D012367:RNA, Viral / Q000096:biosynthesis / Q000235:genetics; D020123:Sirolimus / Q000494:pharmacology; D013997:Time Factors; D015854:Up-Regulation / Q000187:drug effects; D002583:Uterine Cervical Neoplasms / Q000235:genetics / Q000378:metabolism* / Q000821:virology; D014779:Virus Replication / Q000187:drug effects
D048868:Adaptor Proteins, Signal Transducing; D000903:Antibiotics, Antineoplastic; D018797:Cell Cycle Proteins; C087000:EIF4EBP1 protein, human; D039561:Eukaryotic Initiation Factor-4E; D050725:Papillomavirus E7 Proteins; D010750:Phosphoproteins; D012333:RNA, Messenger; D012367:RNA, Viral; D020123:Sirolimus