The PML and PML/RARα Domains: From Autoimmunity to Molecular Oncology and from Retinoic Acid to Arsenic

Acute promyelocytic leukemia (APL) is specifically associated to a t(15; 17) translocation which fuses a gene encoding a nuclear receptor for retinoic acid, RARα, to a previously unknown gene PML. The PML protein is localized in the nucleus on a specific domain of unknown function (PML nuclear bodies, NB) previously detected with autoimmune sera from patients with primary biliary cirrhosis (PBC). These bodies are nuclear matrix-associated and all of their identified components (PML, Sp100, and NDP52) are sharply upregulated by interferons. We show that autoantibodies against both PML and Sp100 are usually associated in sera with multiple nuclear dot anti-nuclear antibodies and demonstrate that PML is an autoantigen, not only in PBC, but also in other autoimmune diseases. In APL, the PML/RARα fusion interferes with both the retinoic acid (RA) response and PML localization on nuclear bodies, but the respective contribution of each defect to leukemogenesis is unclear. RA induces the terminal differentiation of APL blasts, yielding to complete remissions, and corrects the localization of NB antigens. Arsenic trioxide (As₂O₃) also induces remissions in APL, seemingly through induction of apoptosis. We show that in APL, As₂O₃leads to the rapid reformation of PML bodies. Thus, both agents correct the defect in NB antigen localization, stressing the role of nuclear bodies in the pathogenesis of APL.     

Conjugation with the ubiquitin-related modifier SUMO-1 regulates the partitioning of PML within the nucleus.

The PML protein, identified first as part of the oncogenic PML-RARalpha chimera in acute promyelocytic leukemia (APL), concentrates within discrete subnuclear structures, corresponding to some types of nuclear bodies. These structures are disrupted in APL cells, and retinoic acid (RA) can trigger their reorganization, correlating with its therapeutic effect in this type of leukemia. Recently, arsenic trioxide (As2O3) was identified as a potent antileukemic agent which, similarly to RA, induces complete remissions in APL patients. Here we show that, in APL cells, As2O3 triggers rapid degradation of PML-RARalpha and provokes the restoration of intact nuclear bodies. In non-APL cells, the ubiquitin-like protein SUMO-1 is covalently attached to a subset of wild-type PML in a reversible and phosphorylation-dependent manner. The unmodified form of PML is found in the soluble nucleoplasmic fraction, whereas the SUMO-1-polymodified forms of PML are compartmentalized exclusively in the PML nuclear bodies. As2O3 administration strikingly increases the pool of SUMO-1-PML conjugates that, subsequently, accumulate in enlarged nuclear bodies. In contrast to PML-RARalpha, the overall amount of PML seems to remain unaltered up to 36 h following As2O3 treatment. These findings indicate that the conjugation of PML with SUMO-1 modulates its intracellular localization and suggest that post-translational modification by SUMO-1 may be more generally involved than previously suspected in the targeting of proteins to distinct subcellular structures. They provide additional evidence that the role of 'ubiquitin-like' post-translational modification is not limited to a degradation signal.