Effects of novel RAR- and RXR-selective retinoids on myeloid leukemic proliferation and differentiation in vitro

Retinoids such as all-trans-retinoic acid (ATRA) and 9-cis-retinoic acid (9 -cis-RA) have an important role in many aspects of proliferation and differ entiation of hematopoietic cells. They exert their effects by binding to re tinoic acid receptors (RARs) and/or retinoid X receptors (RXRs). We studied the effects of novel retinoids on proliferation and differentiation of HL- 60 and NB4 myeloid leukemic cells, as well as acute promyelocytic leukemia (APL) cells from patients. RXR-selective SR11345 (Retinoid C) had little ab ility to inhibit the clonal growth and to induce the differentiation of eit her HL-60 or NB4 cells. However, SR11276 (Retinoid E), which activated both the RAR and RXR classes, and SR11278 (Retinoid D), which activated the RAR subtypes alpha, beta, and gamma, could inhibit clonal growth of both cell types, as well as leukemic cells from APL patients. The combination of ATRA and either SR11276 or SR11278 additively inhibited APL cell proliferation. SR11302 (Retinoid A), with reported anti-AP-1 activity and no activation o f RARs and RXR and SR11363 (Retinoid B), which selectively activated RAR be ta and gamma, were inactive. The clonal proliferation of both HL-60 and NB4 cells that were pulse-exposed to 10(-9) mol/L ATRA, SR11276, SR11278, or S R11345 for 3 days, washed, and plated in methylcellulose culture were inhib ited by 0%, 51%, 21%, and 1% for HL-60 cells and 43%, 41%, 35%, and 1% for NB4, respectively, compared with nontreated control cells. When the HL-60 c ells were pulse-exposed to 10-9 mol/L of either SR11278 or SR11276, plus 10 -9 mol/L ATRA for 3 days, colony numbers were reduced by 46% and 64%, respe ctively. Induction of leukemic cell differentiation as determined by the ni troblue tetrazolium (NBT) assay showed that the combination of 10(-7) mol/L of either SR11278 or SR11276 with 10(-7) mol/L ATRA had additive effects o n HL-60 cells, NB4 cells, and fresh APL cells. Induction of CD11b expressio n on both HL-60 and NB4 cells occurs during their differentiation. Expressi on of this antigen was synergistically augmented by the combination of eith er 10(-7) to 10(-8) mol/L SR11278 or 10(-7) to 10(-9) mol/L SR11276 with 10 (-9) mol/L ATRA compared with either analog alone in HL-60 cells. Expressio n of the novel myeloid specific transcription factor C/EBP epsilon was incr eased by SR11278 and SR11276 in both the HL-60 and NB4 cell lines. We concl ude that retinoids or combination of retinoids with specificities for both RAR and RXR may markedly enhance the ability of ATRA to inhibit clonal grow th and induce differentiation of HL-60 and NB4 leukemic cells. This occurs in the absence of continuous contact with retinoids. (C) 1999 by The Americ an Society of Hematology.