HBZ-mediated shift of JunD from growth suppressor to tumor promoter in leukemic cells by inhibition of ribosomal protein S25 expression
Human T-cell leukemia virus type 1 (HTLV-1) basic-leucine zipper (bZIP) factor (HBZ) is a key player in proliferation and transformation of HTLV-1-infected cells, thus contributing to adult T-cell leukemia (ATL) development. HBZ deregulates gene expression within the host cell by interacting with several cellular partners. Through its C-terminal ZIP domain, HBZ is able to contact and activate JunD, a transcription factor of the AP-1 family. JunD mRNA is intronless but can generate two protein isoforms by alternative translation initiation: JunD full-length and ΔJunD, an N-terminal truncated form unresponsive to the tumor suppressor menin. Using various cell lines and primary T-lymphocytes, we show that after serum deprivation HBZ induces the expression of ΔJunD isoform. We demonstrate that, unlike JunD, ΔJunD induces proliferation and transformation of cells. To decipher the mechanisms for ΔJunD production, we looked into the translational machinery and observed that HBZ induces nuclear retention of RPS25 mRNA and loss of RPS25 protein expression, a component of the small ribosomal subunit. Therefore, HBZ bypasses translational control of JunD uORF and favors the expression of ΔJunD. In conclusion, we provide strong evidences that HBZ induces ΔJunD expression through alteration of the cellular translational machinery and that the truncated isoform ΔJunD has a central role in the oncogenic process leading to ATL.