Leukemia-associated retinoic acid receptor α fusion partners, PML and PLZF, heterodimerize and colocalize to nuclear bodies

Abstract

In acute promyelocytic leukemia (APL), the typical t(15;17) and the rare t(11;17) translocations express, respectively, the PML͞RAR␣ and PLZF͞RAR␣ fusion proteins (where RAR␣ is retinoic acid receptor ␣). Herein, we demonstrate that the PLZF and PML proteins interact with each other and colocalize onto nuclear bodies (NBs). Furthermore, induction of PML expression by interferons leads to a recruitment of PLZF onto NBs without increase in the levels of the PLZF protein. PML͞RAR␣ and PLZF͞RAR␣ localize to the same microspeckled nuclear domains that appear to be common targets for the two fusion proteins in APL. Although PLZF͞RAR␣ does not affect the localization of PML, PML͞ RAR␣ delocalizes the endogenous PLZF protein in t(15;17)-positive NB4 cells, pointing to a hierarchy in the nuclear targeting of these proteins. Thus, our results unify the molecular pathogenesis of APL with at least two different RAR␣ gene translocations and stress the importance of alterations of PLZF and RAR␣ nuclear localizations in this disease.Acute promyelocytic leukemia (APL) represents approximately 10% of all adult acute myeloid leukemias (1). The molecular pathogenesis of APL is, at least in part, associated with the disruption of the retinoic acid receptor ␣ (RAR␣) gene through its fusion to one of four different loci (2-7). These translocations result in the expression of chimeric RAR␣ fusion proteins that retain the DNA and ligand binding domains of the receptor and gain a dimerization domain from the fusion partner. Paradoxically, APL is the first human malignancy that may undergo complete remission in response to differentiation therapy with all-trans-retinoic acid (RA). The molecular basis of these remissions is still disputed.The majority of APL cases, and all cases that consistently respond to RA treatment, possess the t(15;17) translocation that fuses the PML and RAR␣ genes (3,(8)(9)(10)(11)(12). PML is a member of a functionally diverse gene family that encodes proteins characterized by the presence of a N-terminal C 3 HC 4 RING-finger motif (13), followed by one or two cysteine-rich regions (B boxes) and a coiled-coil protein dimerization interface. The function of PML is unknown, but up-regulation of its expression by interferons (IFNs) (14-16) and its negative effect on cell growth and cellular transformation by cooperating oncogenes (17-19) suggest a role in growth control. The product of the wild-type PML gene is a phosphoprotein (20) that localizes both in the nucleoplasm and in the specific multiprotein structures called PML nuclear bodies (NBs) (20-24). The PML͞RAR␣ fusion protein, which is expressed in APL cells as a result of t(15;17), contains all predicted PML structural motifs and is able to delocalize the wild-type PML and other NB components onto discrete microspeckled nuclear structures (21-24). It is still unclear which role, if any, disruption of NBs and͞or establishment of microspeckled structures play in cellular transformation. Nevertheless, complete restoration of NBs upon RA treatment in NB4 ...

Deregulation of NPM and PLZF in a variant t(5;17) case of acute promyelocytic leukemia

“…Analysis of bone marrow cells from the t(5;17) patient (J700) dually treated with NPM plus RARa/RXRa antibodies demonstrates that NPM localization is completely disrupted. Note that NPM, NPM-RARa and RXRa co-localize in a dispersed nuclear manner t(11;17)(q23;q21) and t(15;17) APL cells, and that the nuclear localization of PLZF is disrupted in these forms of APL as a direct result of PLZF/PLZF-RARa and PLZF/PML-RARa heterodimers, respectively (Koken et al, 1997). Consistent with the above data we show that PLZF expression appears to be similarly upregulated in a microspeckled pattern in the nucleus of t(5;17) cells.…”

“…Although the molecular mechanisms of action of PML-RARa are still unclear, it has been shown to exert its transforming e ects by sequestering hematopoietic factors like RXRa, PML, and PLZF (Dyck et al, 1994;Weis et al, 1994;Koken et al, 1997;He et al, 1998); or by acquiring altered transactivating properties that a ect retinoic acid (RA) and PML responsive genes (de The et al, 1991;Kakizuka et al, 1991). Much like RARa (Chen and Evans, 1995;Nagy et al, 1997), PML-RARa retains the ability to recruit the transcriptional co-repressors N-CoR and SMRT to RA responsive genes (Hong et al, 1997).…”

Human endogenous retrovirus protein cORF supports cell transformation and associates with the promyelocytic leukemia zinc finger protein

“…Like cORF, epsin 1 is a shuttle protein (Magin et al, 1999;Boese et al, 2000;Hyman et al, 2000). PML/PLZF interaction may be important for the maintenance of the non-transformed phenotype of certain cell types; furthermore, the oncogenic PMLRARa hybrid proteins that are involved in more than 98% of APL are still able to bind PLZF and to translocate PLZF away from its nuclear domains to other nuclear sites (Koken et al, 1997). It is thus conceivable that cORF may act oncogenic through the disruption of subcellular localization or of normal PLZF functions such as transcription repression.…”

Expression of Tip60, an androgen receptor coactivator, and its role in prostate cancer development

“…The importance of the subnuclear organization of proteins is revealed by the effects of mislocalization of nuclear proteins (discussed in Koken et al, 1997). Tip60 has been reported to exhibit a nuclear punctuate pattern (Yamamota and Horikoshi, 1997;Ran and PereiraSmith, 2000), when transiently overexpressed in cell lines.…”