Chimeric Leukemic Protein PLZF/RARα Functions as an Adoptor for Cul3-Based E3 Ubiquitin Ligase To Regulate Proteosomal Degradation of RXRα.

Typical acute promyelocytic leukemia (APL) is associated with expression of the PML/RARα fusion protein resulting from chromosomal translocation t(15;17) and responsiveness to treatment with all-trans retinoic acid (ATRA). A few population of APL is associated with variant chromosomal translocations, t(11;17), t(5;17) and t(17;17). PLZF/RARα is the chimeric fusion protein resulting from the chromosomal translocation, t(11;17)(q23;q21). APL cells with PLZF/RARα have been reported to be unresponsive to ATRA-induced terminal differentiation clinically and experimentally. The molecular basis of unresponsiveness in PLZF/RARα-derived APL cells against ATRA explained by a rigid interaction between BTB/POZ domain of PLZF and a transcriptional co-repressor, N-CoR. PLZF/RARα contains BTB/POZ domain in its N-terminus. BTB/POZ domain is developmentally conserved among various species, and recently several BTB/POZ-containing molecules have been reported to function as substrate-specific adaptors for Cul3-based E3 ubiquitin ligase. Here we examined the possibility that PLZF/RARα functions as an E3 ligase.

We performed the series of transient transfection analysis. By immunoprecipitation assay, PLZF/RARα associated with Cul3, and PLZF/RARα also associated with RXRα. PLZF/RARα accelerated an ubiquitin-dependent degradation of RXRα, and resulted in the decreased expression of RXRα. This degradation of RXRα was dependent on the expression level of PLZF/RARα. On the contrary, co-expression of dominant negative form of Cul3 with PLZF/RARα resulted in the restored expression of RXRα. When we expressed PML/RARα, PLZF or PLZF/RARαΔBTB, the accelerated degradation of RXRα was not observed.

In RARα-responsive luciferase assay, PLZF/RARα repressed ATRA response. Consistent with the result that PLZF/RARαΔBTB did not down-regulate the expression of RXRα, PLZF/RARαΔBTB did not repress ATRA response. In addition, the transduction of recombinant RXRα molecule into PLZF/RARα expressing cells partially restored ATRA-responsiveness. Collectively, we suggest that ATRA resistance in PLZF/RARα-positive cells is explained by the novel function of PLZF/RARα molecule.