Human Merkel cell polyomavirus small T antigen is an oncoprotein targeting the 4E-BP1 translation regulator
Masahiro Shuda, … , Yuan Chang, Patrick S. Moore
Masahiro Shuda, … , Yuan Chang, Patrick S. Moore
Published August 15, 2011
Citation Information: J Clin Invest. 2011;121(9):3623-3634. https://doi.org/10.1172/JCI46323.
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Research Article Oncology

Human Merkel cell polyomavirus small T antigen is an oncoprotein targeting the 4E-BP1 translation regulator

Abstract

Merkel cell polyomavirus (MCV) is the recently discovered cause of most Merkel cell carcinomas (MCCs), an aggressive form of nonmelanoma skin cancer. Although MCV is known to integrate into the tumor cell genome and to undergo mutation, the molecular mechanisms used by this virus to cause cancer are unknown. Here, we show that MCV small T (sT) antigen is expressed in most MCC tumors, where it is required for tumor cell growth. Unlike the closely related SV40 sT, MCV sT transformed rodent fibroblasts to anchorage- and contact-independent growth and promoted serum-free proliferation of human cells. These effects did not involve protein phosphatase 2A (PP2A) inhibition. MCV sT was found to act downstream in the mammalian target of rapamycin (mTOR) signaling pathway to preserve eukaryotic translation initiation factor 4E–binding protein 1 (4E-BP1) hyperphosphorylation, resulting in dysregulated cap-dependent translation. MCV sT–associated 4E-BP1 serine 65 hyperphosphorylation was resistant to mTOR complex (mTORC1) and mTORC2 inhibitors. Steady-state phosphorylation of other downstream Akt-mTOR targets, including S6K and 4E-BP2, was also increased by MCV sT. Expression of a constitutively active 4E-BP1 that could not be phosphorylated antagonized the cell transformation activity of MCV sT. Taken together, these experiments showed that 4E-BP1 inhibition is required for MCV transformation. Thus, MCV sT is an oncoprotein, and its effects on dysregulated cap-dependent translation have clinical implications for the prevention, diagnosis, and treatment of MCV-related cancers.

Authors

Masahiro Shuda, Hyun Jin Kwun, Huichen Feng, Yuan Chang, Patrick S. Moore

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Figure 6

MCV sT promotes 4E-BP1 S65 hyperphosphorylation and cap-dependent translation.

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MCV sT promotes 4E-BP1 S65 hyperphosphorylation and cap-dependent transl...
(A) MCV sT promoted δ 4E-BP1 hyperphosphorylation at S65; this phosphorylation was mediated by mTORC1 and could be inhibited by long-term raptor knockdown. 293 cells were stably transduced with Raptor shRNA lentivirus (shRap) and transfected with sT or empty vector on day 5 after transduction. (B) MCV sT prevented loss of hyperphosphorylated 4E-BP1 S65 during short-term mTORC1 inhibition with rapamycin. 293 cells transfected with empty vector or MCV sT were treated with 50 nM rapamycin for up to 1 hour. Basal phosphorylation at T37 and T46 was unaffected by rapamycin treatment or sT expression. (C) MCV sT prevented loss of 4E-BP1 during short-term mTORC1 and mTORC2 inhibition with Torin1 and PP242. 293 cells, with or without sT expression, were treated with Torin1 (500 nM) or PP242 (5 μM) for 6 hours. 4E-BP1 was almost completely dephosphorylated after drug treatment in the absence of sT expression. When sT was expressed, the δ S65 form was preserved. (D) Both wild-type sT and PP2A-binding sT mutants promoted rapamycin-resistant 4E-BP1 phosphorylation in 293 cells treated with 50 nM rapamycin for 1 hour.