Key Points RA/arsenic induces proteasomal degradation of mutant NPM1, yielding AML growth arrest and apoptosis. RA/arsenic treatment restored nucleolar localization of NPM1 and significantly reduced bone marrow blasts in NPM1 mutant AML patients.

The human T-lymphotropic virus type I oncoprotein Tax is critical for T-cell transformation, acting mainly through nuclear factor kappa B essential modulator (NEMO) binding and subsequent nuclear factor-B activation. Tax localizes to Tax nuclear bodies and to the centrosome and is subjected to ubiquitylation and small ubiquitin-like modifier (SUMO)ylation, which are both necessary for complete transcriptional activation. Using the photoconvertible fluorophore Dendra-2 coupled with live video confocal microscopy, we show for the first time that the same Tax molecule shuttles among Tax nuclear bodies and between these nuclear bodies and the centrosome, depending on its posttranslational modifications. Ubiquitylation targets Tax to nuclear bodies to which NEMO is recruited and subsequently SUMOylated. We also demonstrate that Tax nuclear bodies contain the SUMOylation machinery including SUMO and the SUMO conjugating enzyme Ubc9, strongly suggesting that these nuclear bodies represent sites of active SUMOylation. IntroductionTax, the viral oncoprotein encoded by the pX region of the human T-lymphotropic virus type I (HTLV-I) genome is a key player in the pathogenesis of HTLV-I-associated diseases, mainly adult T-cell leukemia/lymphoma and tropical spastic paraparesis/HTLVassociated myelopathy. Tax deregulates the cellular machinery at multiple levels leading to the transformation of HTLV-I-infected T lymphocytes, predominantly through the activation of the nuclear factor-B (NF-B) pathway, which regulates key genes implicated in inflammation, apoptosis, and oncogenesis. 1,2 A corner stone in this NF-B activation is Tax binding to the regulatory subunit of the IB kinase (IKK) complex, IKK␥ (also known as NEMO). 3,4 Tax binding to NEMO phosphorylates both IKK␣ and IKK␤ leading to the formation of the IKK complex that phosphorylates the NF-B inhibitors IBs. This leads to the nuclear translocation of the active NF-B dimers and to transcriptional activation of target genes. 5 Tax/NEMO binding can also induce the IKK␣-dependent processing of the NF-B p100 precursor protein to its active p52 form. 6 Tax displays a dual nuclear and cytoplasmic localization with functions that are essential for cellular transformation in each compartment. 7,8 In the nucleus, Tax is predominantly located in RelA-enriched nuclear bodies. 9 In the cytoplasm, a major fraction of Tax lays in perinuclear hotspots colocalizing with the centrosome or microtubule organizing center in close association with the cis-Golgi compartment. 10,11 Posttranslational modification of proteins regulates protein functions by modifying their subcellular localization, stability, and/or network of interaction. We and others have described different forms of Tax posttranslational modifications including phosphorylation, 12,13 acetylation, 14 ubiquitylation, and small ubiquitin-like modifier (SUMO)ylation, 15,16 all of which are implicated in Tax-mediated activation of gene expression. Indeed, we showed that Tax is differentially ubiquitylated by either K-48 ubiquitin...

Key Points• Survival of ATL cells depends on continuous Tax expression.• Arsenic/interferon combination induces SUMO/PML/RNF4-mediated Tax degradation.The human T-cell lymphotropic virus type I (HTLV-1) Tax transactivator initiates transformation in adult T-cell leukemia/lymphoma (ATL), a highly aggressive chemotherapyresistant malignancy. The arsenic/interferon combination, which triggers degradation of the Tax oncoprotein, selectively induces apoptosis of ATL cell lines and has significant clinical activity in Tax-driven murine ATL or human patients. However, the role of Tax loss in ATL response is disputed, and the molecular mechanisms driving degradation remain elusive. Here we demonstrate that ATL-derived or HTLV-1-transformed cells are dependent on continuous Tax expression, suggesting that Tax degradation underlies clinical responses to the arsenic/ interferon combination. The latter enforces promyelocytic leukemia protein (PML) nuclear body (NB) formation and partner protein recruitment. In arsenic/interferon-treated HTLV-1 transformed or ATL cells, Tax is recruited onto NBs and undergoes PML-dependent hypersumoylation by small ubiquitin-like modifier (SUMO)2/3 but not SUMO1, ubiquitination by RNF4, and proteasome-dependent degradation. Thus, the arsenic/interferon combination clears ATL through degradation of its Tax driver, and this regimen could have broader therapeutic value by promoting degradation of other pathogenic sumoylated proteins. (Blood. 2015;125(3):474-482)