THE PML RAR-ALPHA ONCOPROTEIN IS A DIRECT MOLECULAR TARGET OF RETINOIC ACID IN ACUTE PROMYELOCYTIC LEUKEMIA-CELLS/

Acute promyelocytic leukemia (APL) is characterized by the translocati on, t(15;17) and the expression of a PML/RAR alpha fusion protein that is diagnostic of the disease. There is evidence that PML/RAR alpha pr otein acts as a dominant negative inhibitor of normal retinoid recepto r function and myeloid differentiation. We now show that the PML/RAR a lpha fusion product is directly downregulated in response to retinoic acid (tRA) treatment in the human APL cell line, NB4. tRA treatment in duces loss of PML/RAR alpha at the protein level but not at the level of mRNA, as determined by Northern blots, by Western blots, and by lig and binding assays and in binding to RA-responsive DNA elements. We pr esent evidence that this regulation is posttranslational. This evidenc e suggests that tRA induces synthesis of a protein that selectively de grades PML/RAR alpha. We further show that this loss of PML/RAR-alpha is not limited to the unique APL cell line, NB4, because PML/RAR alpha protein is selectively downregulated by tRA when expressed in the tra nsfected myeloid cell line U937. The loss of PML/RAR alpha may be dire ctly linked to tRA-induced differentiation, because in a retinoid-resi stant subclone of NB4, tRA does not decrease PML/RAR alpha protein exp ression. In NB4 cells, the specific downregulation of the fusion prote in decreases the ratio of PML/RAR alpha to wild-type RAR alpha. Becaus e the ratio of expression of PML/RAR alpha to wild-type RAR alpha and PML may be important in maintaining the dominant negative block of mye locytic differentiation, these data suggest a molecular mechanism for restoration by tRA normal myeloid differentiation in APL cells. (C) 19 96 by The American Society of Hematology.