Interactions between the retinoic acid receptor alpha (RARalpha) and coregulators play a key role in coordinating gene transcription and myeloid differentiation. In acute promyelocytic leukemia (APL), RARalpha is fused with the promyelocytic leukemia (PML) gene, resulting in the expression of the fusion protein PML/RARalpha. Here, I report that topoisomerase II beta (topoIIbeta) associates with and negatively modulates PML/RARalpha and RARalpha transcriptional activity, and increased levels and association of topoIIbeta cause resistance to retinoic acid (RA) in APL cell lines. Knock down of topoIIbeta was able to overcome resistance by permitting RA-induced differentiation and increased RA-gene expression. Overexpression of topoIIbeta, in clones from an RA-sensitive cell line, conferred resistance by a reduction in RA-induced expression of target genes and differentiation. Chromatin immunoprecipitation assays indicate that topoIIbeta is bound to an RA-response element, and inhibition of topoIIbeta causes hyper-acetylation of histone 3 at lysine 9 and activation of transcription. These results identify a novel mechanism of resistance in APL and provide further insights to the role of topoIIbeta in gene regulation and differentiation. / Studies to determine the mechanism by which topoIIbeta protein is regulated found that levels of protein kinase C delta (PKCdelta) correlated with topoIIbeta protein expression. Moreover, activation of PKCdelta, by RA or PMA, led to an increase of topoIIbeta protein levels. Most notably, in NB4-MR2 cells, we observed increased phosphorylation levels of threonine 505 on PKCdelta, a marker of activation. Inhibition of PKCdelta was able to overcome the topoIIbeta repressive effects on RA-target genes. In addition, the combination of RA and PKCdelta inhibition led to increased expression of the granulocytic marker, CD11c, in NB4 and NB4-MR2 cells. These results suggest that PKCdelta regulates topoIIbeta expression, and a constitutively active PKCdelta in the NB4-MR2 cell line leads to overexpression of topoIIbeta. / In conclusion, these studies demonstrate that topoIIbeta associates with RARalpha, binds to RAREs and plays a critical role in RA dependent transcriptional regulation and granulocytic differentiation. In addition, I show that topoIIbeta overexpression leads to RA resistance and provide evidence that topoIIbeta protein levels are regulated via a mechanism involving the PKCdelta pathway. This work has contributed to an enhanced understanding of the role of topoIIbeta in gene regulation and brings novel perspectives in the treatment of RA-resistance in APL.

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