Schedule-dependent sphinganine potentiation of retinoic acid-induced differentiation, cell growth inhibition, and nucleophosmin translocation in a human leukemia cell line (HL-60)

Induction of differentiation, inhibition of cell growth, and localization of nucleophosmin in HL-60 cells under the treatment of retinoic acid (RA) were studied. Bright nucleolar fluorescence was observed in control promyelocytic growing cells. The addition of RA in the culture system resulted in time- and dose-dependent induction of differentiation, cell growth inhibition, and nucleophosmin translocation from nucleoli to nucleoplasm. Unlike the control cells, many fewer nucleophosmin-associated preribosomal ribonucleoprotein particles (pre-rRNPs) could be obtained from nucleoli of RA-treated cells. Addition of sphinganine, an inhibitor of protein kinase C, facilitated the RA-induced differentiation, nucleophosmin translocation, and cell growth inhibition. Cells treated with sphinganine were more responsive to RA. Differentiation, translocation of nucleophosmin, and inhibition of cell growth occurred with lesser doses of RA or in shorter incubation times in the presence of sphinganine. Significant numbers of HL-60 cells could be rescued from the effects of RA upon the removal of RA after 2-h drug exposure. Pretreatment but not posttreatment of HL-60 cells with sphinganine, however, modulated the reversibility of the effects induced by short-exposure RA treatment. These results indicated that RA therapy can be improved by the pretreatment or the concurrent use of a modulator of protein kinase C activity. Nucleophosmin translocation as observed by immunofluorescence may be a simple and rapid method for assessing inhibition of cellular growth in response to differentiation inducers such as RA in cancer chemotherapy.