PMLRAR HOMODIMERS - DISTINCT DNA-BINDING PROPERTIES AND HETEROMERIC INTERACTIONS WITH RXR

Fusion proteins (named PMLRAR) between PML and the retinoic acid recep tor alpha (RARalpha) are generated as a result of the t(15;17) chromos omal translocation found in acute promyelocytic leukemia (APL). We sho w here that PMLRAR proteins exist in solution as stable homodimers who se formation is mediated by a presumptive coiled coil in the PML moiet y. In contrast to RARalpha, which requires heterodimerization with RXR for efficient DNA binding, PMLRAR homodimers can bind to target seque nces in the absence of RXR, and the binding pattern of PMLRAR homodime ric complexes to directly repeated motif (DR) response elements with 1 -5 bp spacers is different from that of RAR/RXR heterodimeric complexe s. We show that the presence of RXR induces the formation of PMLRAR/RX R heteromeric complexes which bind to DNA via one RAR DNA binding doma in (DBD) and one RXR DBD, like 'classical' RAR/RXR heterodimers. PMLRA R interaction with RXR occurs in solution and in transfected cultured Cos cells, and PMLRAR is able to sequester RXR efficiently in the cyto plasm, suggesting that dominant 'inactivation' of RXR may be a possibl e mechanism of action for PMLRAR. Accordingly, we show that PMLRAR can both prevent the binding of the vitamin D3 receptor (VDR) to a target sequence in vitro and inhibit vitamin D3-dependent activation of a VD R-reponsive reporter gene in transfected cells. These results suggest that both the distinct DNA binding properties of PMLRAR homodimers and the sequestration of RXR by PMLRARs may contribute to the molecular m echanisms which underlie the pathogenesis of APL. We also report that RXRalpha transcripts are down-regulated by RA-treatment in promyelocyt ic cells.