POTENTIATION OF RETINOIC ACID-INDUCED U-937 DIFFERENTIATION INTO RESPIRATORY BURST-COMPETENT CELLS BY NITRIC-OXIDE DONORS

All-trans retinoic acid (tretinoin) is a known inducer of differentiat ion of the human monoblastic cell line, U-937. We now report that the ability of retinoic acid (RA) to induce differentiation of U-937 cells into cells possessing respiratory burst activity is enhanced by the k nown nitric oxide-donating drugs glyceryl trinitrate, molsidomine and CAS 936, and by tetranitromethane in combination with cysteine. RA alo ne was a strong inducer of U-937 differentiation as indicated by the f ollowing responses to 12-O-tetradecanoylphorbol-13-acetate (TPA) stimu lation: (1) increase in the percentage of cells staining with nitroblu e tetrazolium (NET); (2) increase in the total amount of formazan (the product of NET reduction by O-2(<radical anion>)) as determined spect rophotometrically; (3) increase in hexose monophosphate shunt (HMPS) a ctivity as assessed by [C-14]CO2 released from D-[1-C-14]glucose. RA w as also able to increase mRNA levels for two respiratory burst-related genes and for glucose-6-phosphate dehydrogenase (G6PD), an HMPS enzym e. Other indications of differentiation were reduced cell proliferatio n, increased adherence and altered nuclear morphology. The observed in crease in formazan production and HMPS activity and the reduction of c ell proliferation due to RA were augmented by co-treatment with either glyceryl trinitrate, molsidomine, CAS 936 or tetranitromethane plus c ysteine. Glyceryl trinitrate alone increased HMPS activity and G6PD mR NA levels and also reduced cell proliferation. Glyceryl trinitrate, mo lsidomine and CAS 936 are presumed to release nitric oxide and increas e intracellular cGMP levels by stimulation of soluble guanylate cyclas e. The mechanism of action of tetranitromethane is less certain, altho ugh it may also generate reactive nitrogen intermediates. These data s uggest that a NO ./cGMP pathway may augment a retinoic acid-mediated p athway to enhance maturation of U-937 cells with respect to the respir atory burst. Glyceryl trinitrate may act additionally by another pathw ay.