Promyelocytic leukemia (PML) is the organizer of nuclear matrix domains, PML nuclear bodies (NBs), with a proposed role in apoptosis control. In acute promyelocytic leukemia, PML/retinoic acid receptor (RAR) α expression disrupts NBs, but therapies such as retinoic acid or arsenic trioxide (As₂O₃) restore them. PML is conjugated by the ubiquitin-related peptide SUMO-1, a process enhanced by As₂O₃ and proposed to target PML to the nuclear matrix. We demonstrate that As₂O₃ triggers the proteasome-dependent degradation of PML and PML/RARα and that this process requires a specific sumolation site in PML, K160. PML sumolation is dispensable for its As₂O₃-induced matrix targeting and formation of primary nuclear aggregates, but is required for the formation of secondary shell-like NBs. Interestingly, only these mature NBs harbor 11S proteasome components, which are further recruited upon As₂O₃ exposure. Proteasome recruitment by sumolated PML only likely accounts for the failure of PML-K160R to be degraded. Therefore, studying the basis of As₂O₃-induced PML/RARα degradation we show that PML sumolation directly or indirectly promotes its catabolism, suggesting that mature NBs could be sites of intranuclear proteolysis and opening new insights into NB alterations found in viral infections or transformation.

Acute promyelocytic leukemia (APL) is associated with the t(15;17) translocation, which generates a PML/RARα fusion protein between PML, a growth suppressor localized on nuclear matrix-associated bodies, and RARα, a nuclear receptor for retinoic acid (RA). PML/RARα was proposed to block myeloid differentiation through inhibition of nuclear receptor response, as does a dominant negative RARα mutant. In addition, in APL cells, PML/RARα displaces PML and other nuclear body (NB) antigens onto nuclear microspeckles, likely resulting in the loss of PML and/or NB functions. RA leads to clinical remissions through induction of terminal differentiation, for which the respective contributions of RARα (or PML/RARα) activation, PML/RARα degradation, and restoration of NB antigens localization are poorly determined. Arsenic trioxide also leads to remissions in APL patients, presumably through induction of apoptosis. We demonstrate that in non-APL cells, arsenic recruits the nucleoplasmic form of several NB antigens onto NB, but induces the degradation of PML only, identifying a powerful tool to approach NB function. In APL cells, arsenic targets PML and PML/RARα onto NB and induces their degradation. Thus, RA and arsenic target RARα and PML, respectively, but both induce the degradation of the PML/RARα fusion protein, which should contribute to their therapeutic effects. The difference in the cellular events triggered by these two agents likely stems from RA-induced transcriptional activation and arsenic effects on NB proteins.

Analyzing the pathways by which retinoic acid (RA) induces promyelocytic leukemia/retinoic acid receptor α (PML/RAR α ) catabolism in acute promyelocytic leukemia (APL), we found that, in addition to caspase-mediated PML/RAR α cleavage, RA triggers degradation of both PML/RAR α and RAR α . Similarly, in non-APL cells, RA directly targeted RAR α and RAR α fusions to the proteasome degradation pathway. Activation of either RAR α or RXR α by specific agonists induced degradation of both proteins. Conversely, a mutation in RAR α that abolishes heterodimer formation and DNA binding, blocked both RAR α and RXR α degradation. Mutations in the RAR α DNA-binding domain or AF-2 transcriptional activation region also impaired RAR α catabolism. Hence, our results link transcriptional activation to receptor catabolism and suggest that transcriptional up-regulation of nuclear receptors by their ligands may be a feedback mechanism allowing sustained target-gene activation.