Modulation by cAMP of 1 alpha,25-dihydroxyvitamin D-3 sensitivity of murine erythroleukemia cells

As we previously reported, 1 alpha ,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3 ) dose-dependently inhibited not only proliferation of undifferentiated mur ine erythroleukemia (MEL) cells but also activin A-induced erythroid differ entiation of MEL cells. However, the effect of 1,25(OH)(2)D-3 on MEL cell p roliferation was significantly greater by one order of magnitude than that on differentiation (IC50: 9.2 vs 0.8 nM, respectively). The response of act ivin A-treated mature MEL cells to 1,25(OH)(2)D-3 in the induction of 1,25( OH)(2)D-3-24-hydroxylase (24-OHase) activity, a rapid effect of 1,25(OH)(2) D-3, was enhanced to the same degree as in untreated immature cells, sugges ting that differences in capacity of cells to inactivate 1,25(OH)(2)D-3 did not contribute to augmentation of 1,25(OH)(2)D-3 effect in activin A-treat ed mature cells. Furthermore, neither the number nor the affinity of vitami n D receptors (VDR) differed significantly between activin A-treated cells and untreated immature cells. The intracellular cAMP level, which affects 1 ,25(OH)(2)D-3-mediated induction of 24-OHase activity, was significantly le ss in activin A-treated mature cells than in immature MEL cells. The additi on of dibutyryl cAMP (dbc AMP) to activin A-treated MEL cells dose-dependen tly attenuated 1,25(OH)(2)D-3-mediated induction of 24-OHase activity, fina lly to a level comparable to that of the untreated cells at the final conce ntration of 100 nM dbcAMP, while dbcAMP itself by 100 nM did not affect MEL cell differentiation by 24 h. In summary, we have shown for the first time that 1,25(OH)(2)D-3 exerted it s effect on leukemia cells at physiological concentration and that the magn itude of this effect depended on the changes in intracellular cAMP level th rough stages of differentiation, suggesting that the cAMP-protein kinase A system may be useful as a target for clinical application of vitamin D anal ogs by improving the sensitivity of leukemic cells to 1,25(OH)(2)D-3. (C) 2 001 Academic Press.