Blockade of the Ubiquitin Protease UBP43 Destabilizes Transcription Factor PML/RARα and Inhibits the Growth of Acute Promyelocytic Leukemia

More effective treatments for acute promyelocytic leukemia (APL) are needed. APL cell treatment with all-trans-retinoic acid (RA) degrades the chimeric, dominant-negative-acting transcription factor promyelocytic leukemia gene (PML)/RAR alpha, which is generated in APL by chromosomal translocation. The E1-like ubiquitin-activating enzyme (UBE1L) associates with interferon-stimulated gene ISG15 that binds and represses PML/RAR alpha protein. Ubiquitin protease UBP43/USP18 removes ISG15 from conjugated proteins. In this study, we explored how RA regulates UBP43 expression and the effects of UBP43 on PML/RAR alpha stability and APL growth, apoptosis, or differentiation. RA treatment induced UBE1L, ISG15, and UBP43 expression in RA-sensitive but not RA-resistant APL cells. Similar in vivo findings were obtained in a transgenic mouse model of transplantable APL, and in the RA response of leukemic cells harvested directly from APL patients. UBP43 knockdown repressed PML/RAR alpha protein levels and inhibited RA-sensitive or RA-resistant cell growth by destabilizing the PML domain of PML/RAR alpha. This inhibitory effect promoted apoptosis but did not affect the RA differentiation response in these APL cells. In contrast, elevation of UBP43 expression stabilized PML/RAR alpha protein and inhibited apoptosis. Taken together, our findings define the ubiquitin protease UBP43 as a novel candidate drug target for APL treatment. Cancer Res; 70(23); 9875-85. (C)2010 AACR.