COMMON DEFECTS OF DIFFERENT RETINOIC ACID-RESISTANT PROMYELOCYTIC LEUKEMIA-CELLS ARE PERSISTENT TELOMERASE ACTIVITY AND NUCLEAR-BODY DISORGANIZATION

The acute promyelocytic leukemia (APL) t(15;17) rearrangement fuses th e promyelocytic leukemia (PML) gene to the retinoic acid receptor-alph a (RAR alpha). There is expression of the chimeric transcript, PML/RAR alpha, in these APL cells. These clinical APL cases respond to the di fferentiation agent all-trans retinoic acid (ATRA) with complete but n ot durable remissions because ATRA resistance develops. The NB4 APL ce ll line expresses PML/RAR alpha and responds to the growth inhibitory and differentiation-inducing signals of ATRA. To identify mechanisms r esponsible for ATRA resistance in APL, ATRA-resistant NB4 cell lines w ere derived from parental NB4 cells using different strategies. These lines were resistant to the growth inhibition and differentiation effe cts of:ATRA. ATRA-resistant cells were isolated as a de novo resistant line from parental NB4 cells (NB4-R1), following chemical mutagenizat ion and selection in ATRA (NB4-R2). or after chronic selection in ATRA (NB4-R3). Common defects linked to this ATRA resistance were found. W hen cultured in ATRA, these resistant cells still express PML, RAR alp ha, and PML/RAR alpha proteins. Sequence abnormalities were not detect ed in the RAR alpha DNA binding domains cloned from a representative R A-resistant NB4 line. In ATRA-sensitive but not ATRA-resistant NB4 cel ls, ATRA down-regulated retinoid X receptor-alpha (RXR alpha) expressi on, a known marker of ATRA response in parental NB4 cells. Notably, en gineered overexpression of RXR alpha in ATRA-sensitive NB4 cells did n ot block ATRA-mediated growth suppression. ATRA treatment of these res istant NB4 lines did not signal a decline in telomerase activity or re organization of PML-associated nuclear bodies, but both events occurre d in ATRA-sensitive NB4 cells. These ATRA-resistant NB4 lines are not fully differentiation-defective, since monocytic maturation was induce d following treatment with phorbol 12-myristate 13-acetate (PMA) and 1 ,25 dihydroxy vitamin D3 (vitamin D3). Notably, induced monocytic diff erentiation of these distinct ATRA-resistant APL lines markedly repres sed telomerase activity. Thus, this study suggests that persistent tel omerase activity and nuclear body disorganization are linked to ATRA r esistance in APL.