Human Merkel cell polyomavirus small T antigen is an oncoprotein targeting the 4E-BP1 translation regulator
Masahiro Shuda, Hyun Jin Kwun, Huichen Feng, Yuan Chang, Patrick S. Moore
Masahiro Shuda, Hyun Jin Kwun, Huichen Feng, Yuan Chang, Patrick S. Moore
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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 5

MCV sT promotes serum-independent human BJ-TERT cell proliferation.

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MCV sT promotes serum-independent human BJ-TERT cell proliferation. (A) ...
(A) sT protein was transduced by lentiviral vector into immortalized BJ-TERT cells, and cell proliferation was determined in 10% FCS or no serum using a Wst-1 colorimetric cell proliferation assay. sT accelerated BJ-TERT cell growth in the presence of serum and prevented proliferation arrest in the absence of serum (normalized by mean OD values on day 1 for 2 independent experiments performed in triplicate). Mutation of the sT binding site to PP2A (sT.L142A) retained cell proliferation in both the presence and the absence of serum. (B) Cell cycle profiles for BJ-TERT cells expressing sT antigens in 0% and 10% FCS. In the absence of serum, greater than 95% of BJ-TERT cells transduced with the empty vector arrested in G1, whereas substantial fractions of cells expressing wild-type sT (7%) or sT.L142A (11%) continued to transit through the cell cycle.