Hijacking of the O-GlcNAcZYME complex by the HTLV-1 Tax oncoprotein facilitates viral transcription

The viral Tax oncoprotein plays a key role in both Human T-cell lymphotropic virus type 1 (HTLV-1)-replication and HTLV-1-associated pathologies, notably adult T-cell leukemia. Tax governs the transcription from the viral 5’LTR, enhancing thereby its own expression, via the recruitment of dimers of phosphorylated CREB to cAMP-response elements located within the U3 region (vCRE). In addition to phosphorylation, CREB is also the target of O-GlcNAcylation, another reversible post-translational modification involved in a wide range of diseases, including cancers. O-GlcNAcylation consists in the addition of O-linked-N-acetylglucosamine (O-GlcNAc) on Serine or Threonine residues, a process controlled by two enzymes: O-GlcNAc transferase (OGT), which transfers O-GlcNAc on proteins, and O-GlcNAcase (OGA), which removes it. In this study, we investigated the status of O-GlcNAcylation enzymes in HTLV-1-transformed T cells. We found that OGA mRNA and protein expression levels are increased in HTLV-1-transformed T cells as compared to control T cell lines while OGT expression is unchanged. However, higher OGA production coincides with a reduction in OGA specific activity, showing that HTLV-1-transformed T cells produce high level of a less active form of OGA. Introducing Tax into HEK-293T cells or Tax-negative HTLV-1-transformed TL-om1 T cells is sufficient to inhibit OGA activity and increase total O-GlcNAcylation, without any change in OGT activity. Furthermore, Tax interacts with the OGT/OGA complex and inhibits the activity of OGT-bound OGA. Pharmacological inhibition of OGA increases CREB O-GlcNAcylation as well as HTLV-1-LTR transactivation by Tax and CREB recruitment to the LTR. Moreover, overexpression of wild-type CREB but not a CREB protein mutated on a previously described O-GlcNAcylation site enhances Tax-mediated LTR transactivation. Finally, both OGT and OGA are recruited to the LTR. These findings reveal the interplay between Tax and the O-GlcNAcylation pathway and identify new key molecular actors involved in the assembly of the Tax-dependent transactivation complex.Author summary: Human T-cell lymphotropic virus type 1 (HTLV-1) is the only human retrovirus associated to a cancer. Indeed, HTLV-1 is responsible for adult T-cell leukemia, an aggressive malignant proliferation of CD4+ T lymphocytes. The regulatory protein Tax governs HTLV-1 transcription from the 5’LTR, driving expression of all viral proteins, including itself, at the exception of the antisense product HBZ. Besides this critical role in HTLV-1 expression, Tax acts as an oncoprotein able to induce T-cell immortalization in vitro and tumor formation in mice. In this study, we report that Tax interacts with the O-GlcNAczyme OGT/OGA complex that catalyzes O-GlcNAcylation, a post-translational modification often deregulated in cancers. We found that Tax interacts with the OGT/OGA complex and inhibits the activity of OGA, increasing thereby cellular O-GlcNAcylation. Strikingly, we found that O-GlcNAcylation of CREB, the cellular transcription factor recruited by Tax on the viral promoter, is increased in a Tax-dependent manner. Moreover, increased CREB O-GlcNAcylation strongly enhances Tax-induced LTR transactivation as well as CREB binding to the viral promoter. Finally, both OGT and OGA are part of the transactivation complex. These findings shed new light on the mechanism of Tax-dependent LTR transactivation and may open the way to new molecular interventions targeting HTLV-1 expression.