1α,25-dihydroxyvitamin D3 stimulates phosphorylation of IκBα and synergizes with TPA to induce nuclear translocation of NFκB during monocytic differentiation of NB4 leukemia cells

Treatment of NB4 acute promyelocytic leukemia cells with 1,25-dihydroxyvitamin D-3 (1,25D(3)) or analogs 20-epi-22-oxa-24a,26a,27a-trihomo-1alpha,25-dihydroxyvitamin D-3, 1,24-dihydroxy-22-ene-24-cyclopropylvitamin D-3, 1alpha,25-dihydroxylumisterol(3), or 1alpha,25(OH)(2)-d(5)-previtamin D-3 in combination with TPA induces monocytic differentiation. The role of 1,25D(3) in the induction of maturation has been shown to be a priming effect. Differentiation in response to these agents requires VDR-independent signaling of 1,25D(3), PKC signaling, intracellular calcium, and calpain activity. In this study we identify the NFkappaB/IkappaB signaling pathway as a target of 1,25D(3) and TPA action. One of the priming effects of 1,25D(3) appears to be the rapid phosphorylation of serine residues on IkappaBalpha. On their own, 1,25D(3), its analogs, and TPA do not alter IkappaBalpha expression; however, combinations of analogs with TPA result in a synergistic decrease in IkappaBalpha expression. Decreased expression of IkappaBalpha likely results from enhanced degradation, which allows the observed subsequent nuclear translocation of NkappaKB subunit p65. Since nuclear-localized NFkappaB was observed only in combination-treated cells, it is proposed that nuclear targets of NFkappaB are required for monocytic differentiation. Intracellular calcium and proteolytic activity are both necessary for the induction Of IkappaB regulation and translocation of NFkappaB and are critical components of the nongenomic signaling cascades of the 1,25D(3)-induced differentiation pathway. (C) 2001 Elsevier Science.