Constitutive degradation of PML/RAR alpha through the proteasome pathway mediates retinoic acid resistance

PML/RAR alpha is the leukemogenetic protein of acute promyelocytic leukemia (APL), Treatment with retinoic acid (RA) induces degradation of PML/RAR al pha, differentiation of leukaemic blasts, and disease remission. However, R A resistance arises during RA treatment of APL patients. To investigate the phenomenon of RA resistance in APL, we generated RA-resistant sublines fro m APL-derived NB4 cells. The NB4.007/6 RA-resistant subline does not expres s the PML/RAR alpha protein, although its mRNA is detectable at levels comp arable to those of the parental cell line. In vitro degradation assays show ed that the half-life of PML/RAR alpha is less than 30 minutes in NB4.007/6 and longer than 3 hours in NB4, Treatment of NB4.007/6 cells with the prot easome inhibitors LLnL and lactacystin partially restored PML/RAR alpha pro tein expression and resulted in a partial release of the HA-resistant pheno type. Similarly, forced expression of PML/RAR alpha, but not RAR alpha, int o the NB4/007.6 cells restored sensitivity to RA treatment to levels compar able to those of the NB4 cells. These results indicate that constitutive de gradation of PML/RAR alpha protein may lead to RA resistance and that PML/R AR alpha expression is crucial to convey RA sensitivity to APL cells. (C) 1 999 by The American Society of Hematology.