Arrest and Granulocytic Differentiation Promyelocytic Leukemia Cell Line NB4, Induces Cell Growth 4-Oxoretinol, a Metabolite of Retinol in the Human

all-frans-Retinoic acid (RA) is used as a differentiation therapy for acute promyelocytic leukemia. Patients can become resistant to RA, and this resistance is thought to be mediated in part by an increase in the rate of RA metabolism. We have characterized the metabolism of all-frans-retinol (ROL; vitamin A) in NB4 cells, which are human promy elocytic leukemia cells. NB4 cells metabolize ROL into a variety of com pounds, including all-frans-4-hydroxyretinol, all-/rans-4-oxoretinol (4oxoROL), 14-hydroxy-4,14-re/ro-retinol, anhydroretinol, and several ROL esters. No metabolism of ROL to RA or to RA derivatives in NB4 cells was detected. The rate of ROL metabolism increased after cell differentiation; in a 24-h period, differentiated cells metabolized 2-fold more ROL than did undifferentiated cells. The major difference in the ROL metabolism pattern between undifferentiated and differentiated cells was an approximately 10-fold increase in the production of a\\-trans4-hydroxyretinol and 4-oxoROL in differentiated cells. Furthermore, exogenously added 4-oxoROL was capable of eliciting NB4 cell differentia tion, as measured by growth inhibition, nitroblue tetrazolium reduction, nuclear body relocalization of PML, and surface expression of CD lib. In addition, 4-oxoROL synergized with IFN-y in the promotion of NB4 cell growth arrest. Following treatment of NB4 cells with 4-oxoROL to induce differentiation, the production of 4-oxoROL from ROL was observed; this indicated that 4-oxoROL induces its own synthesis in NB4 cells. In addi tion, 48 h after the administration of 1 ¿tM4-oxoROL, NB4 cells main tained a high intracellular concentration (17 /IM) of 4-oxoROL. These unique properties of 4-oxoROL may provide advantages over RA in the treatment of promyelocytic leukemia cells because it may be possible to maintain cytodifferentiating concentrations of 4-oxoROL in the cells for extended periods of time.