AN RXR-SELECTIVE ANALOG ATTENUATES THE RAR-ALPHA-SELECTIVE ANALOG-INDUCED DIFFERENTIATION AND NON-G(1)-RESTRICTED GROWTH ARREST OF NB4 CELLS

NB4, a human acute promyelocytic leukemia cell line expressing the pro myelocyte-retinoic acid receptor alpha (PML-RAR alpha) hybrid protein was treated with RAR- and retinoid X receptor (RXR)-selective analogs to determine their effects on cell proliferation, retinoblastoma (RE) tumor-suppressor protein phosphorylation, and differentiation. An RAR- or just RAR alpha-selective analog alone induced similar cell populat ion growth arrest, cell cycle arrest without restriction to G(1), hypo phosphorylation of RB, and myelomonocytic cell surface differentiation marker expression (CD11b). In addition, an RAR alpha antagonist could inhibit the effects of the RAR alpha agonist completely. The RAR alph a-selective analog-elicited response was attenuated by simultaneous ad dition of various RXR-selective analogs. In contrast, each of the RXR- selective analogs was unable to induce any of the cellular responses a nalyzed. The growth arrest of NB4 cells is not G(1)-restricted and occ urs at all points in the cell cycle, Cells growth arrested by treatmen t with an RAR alpha-selective analog show primarily hypophosphorylated RE. When these cells are sorted into G(1) or S + G(2)/M subpopulation s by flow cytometry, hypophosphorylated RE protein was in G(1) as well as S + G(2)/M cells. This suggests that the hypophosphorylated RE pro tein may be mediating the growth arrest of NB4 cells at all points in the cell cycle. These results are consistent with an involvement of PM L-RAR alpha and/or RAR alpha in the transduction of the retinoid signa l in NB4 cells. (C) 1997 Academic Press.