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The human papilloma virus E7 oncoprotein inhibits transforming growth factor-beta signaling by blocking binding of
Dug Keun Lee1, Byung-Chul Kim, Isaac Yi Kim
1Laboratory of Cell Regulation and Carcinogenesis, National Institutes of Health, Bethesda, Maryland 20892, USA.
Abstract:
The human papillomavirus (HPV) oncoprotein E7 is implicated in the etiology of cervical cancer associated with infection by HPV. HPV-positive cells develop resistance to TGF-beta growth inhibitory activity through the inhibition of hypophosphorylation of pRb by papillomavirus type 16 E7 oncoprotein. In this study, we examined whether E7, in addition to its well known effects on pRb, might directly target the Smad proteins that mediate TGF-beta signaling. Here, we show that E7 significantly blocks both Smad transcriptional activity and the ability of TGF-beta to inhibit DNA synthesis. We found that E7 interacts constitutively with Smad2, Smad3, and Smad4. Confocal microscopic studies confirm that E7 and Smads co-localize in vivo. Using a canonical Smad DNA binding sequence, we found that E7 blocks Smad3 binding to its target sequence on DNA. These results suggest that suppression of Smad-mediated signaling by E7 may contribute to HPV-associated carcinogenesis.
Insights
Human papillomavirus (HPV) oncoprotein E7 targets Smad proteins, blocking TGF-beta signaling. This interaction is crucial for HPV-associated cervical cancer development.
Area of Science:
- Molecular Biology
- Oncology
- Virology
Background:
- Human papillomavirus (HPV) oncoprotein E7 is linked to cervical cancer.
- HPV-positive cells resist TGF-beta growth inhibition due to E7's effect on pRb.
- The role of E7 in targeting Smad proteins, key mediators of TGF-beta signaling, is unclear.
Purpose of the Study:
- To investigate if HPV oncoprotein E7 directly targets Smad proteins involved in TGF-beta signaling.
- To determine E7's impact on Smad transcriptional activity and TGF-beta's growth inhibitory function.
Main Methods:
- Examined E7's interaction with Smad proteins (Smad2, Smad3, Smad4).
- Utilized confocal microscopy to confirm in vivo co-localization of E7 and Smads.
- Assessed E7's effect on Smad3 binding to DNA using a canonical Smad DNA binding sequence.
Main Results:
- HPV oncoprotein E7 significantly inhibits Smad transcriptional activity.
- E7 blocks TGF-beta's ability to inhibit DNA synthesis in HPV-positive cells.
- E7 constitutively interacts with Smad2, Smad3, and Smad4, co-localizing in vivo.
Conclusions:
- HPV oncoprotein E7 directly interacts with and suppresses Smad-mediated signaling.
- E7's disruption of Smad signaling pathways may contribute to the development of HPV-associated cervical cancer.
- This study reveals a novel mechanism by which HPV oncoproteins promote carcinogenesis.