The acute promyelocytic leukaemia specific PML and PLZF proteins localize to adjacent and functionally distinct nuclear bodies

Abstract

Acute promyelocytic leukaemia is characterized by translocations that involve the retinoic acid receptor α (RARα) locus on chromosome 17 and the PML locus on 15 or the PLZF locus on 11. The resulting abnormal translocation products encode for PML/RARα or PLZF/RARα fusion proteins. There is increasing experimental evidence that the APL-specific fusion proteins have similar biologic activities on differentiation and survival and that both components of the fusion proteins (PML or PLZF and RARα) are indispensable for these biological activities. The physiologic function of PML or PLZF or whether PML and PLZF contribute common structural or functional features to the corresponding fusion proteins is not known. We report here immunofluorescence studies on the cellular localization of PLZF and PLZF/RARα and compare it with the localization of PML and PML/RARα. PLZF localizes to nuclear domains of 0.3-0.5 microns, approximately 14 per cell in the KG1 myeloid cell line. These PLZF-bodies are morphologically similar to the domains reported for PML (PML-NBs). There is tight spatial relationship between about 30% of PLZ-NBs and PML-NBs: they partially overlap. However, PML and PLZF do not form soluble complexes in vivo. PLZF- and PML-NBs are functionally distinct. Adenovirus E4-ORF3 protein expression alters the structure of the PML-NBs and interferon increases the number of PML-NBs and neither has any effect on PLZF NBs. The localization of PLZF/RARα is different to that of PLZF and RARα. The nuclear distribution pattern of PLZF/RARα is one of hundreds of small dots (microspeckles) less than 0.1 micron. Expression of PLZF/RARα did not provoke disruption of the PML-NBs. Co-expression of PML/RARα and PLZF/RARα in U937 cells revealed apparent colocalization. Overall the results suggest that the PML- and PLZF-NBs are distinct functional nuclear domains, but that they may share common regulatory pathways and/or targeting sequences, as revealed by the common localization of their corresponding fusion proteins.