Constitutive Degradation of PML/RARα Through the Proteasome Pathway Mediates Retinoic Acid Resistance

PML/RARα is the leukemogenetic protein of acute promyelocytic leukemia (APL). Treatment with retinoic acid (RA) induces degradation of PML/RARα, differentiation of leukaemic blasts, and disease remission. However, RA resistance arises during RA treatment of APL patients. To investigate the phenomenon of RA resistance in APL, we generated RA-resistant sublines from APL-derived NB4 cells. The NB4.007/6 RA-resistant subline does not express the PML/RARα protein, although its mRNA is detectable at levels comparable to those of the parental cell line. In vitro degradation assays showed that the half-life of PML/RARα is less than 30 minutes in NB4.007/6 and longer than 3 hours in NB4. Treatment of NB4.007/6 cells with the proteasome inhibitors LLnL and lactacystin partially restored PML/RARα protein expression and resulted in a partial release of the RA-resistant phenotype. Similarly, forced expression of PML/RARα, but not RARα, into the NB4/007.6 cells restored sensitivity to RA treatment to levels comparable to those of the NB4 cells. These results indicate that constitutive degradation of PML/RARα protein may lead to RA resistance and that PML/RARα expression is crucial to convey RA sensitivity to APL cells.