A dual block to cell cycle progression in HL60 cells exposed to analogues of vitamin D,

Abstract. The physiologically active form of vitamin D₃, 1,25-dihydroxy-vitamin D₃, (1,25(OH)₂, D₃), induces differentiation of several types of myeloid leukaemia cells. The acquisition of monocyte-like phenotype is accompanied by slower progression through the cell cycle, and G₁, block has been reported to be the basis of this effect. It is shown here that human promyelocytic leukaemia HL60 cells treated with analogues of vitamin D₃, which are potent inducers of monocytic differentiation, have an additional cell cycle block. Exposure to 10-⁷m 1,25(OH)₂, D₃, or 1,25-(OH)₂,-16-ene-D₃ resulted in monocytic differentiation and the expected G₁, block evident at approximately 48 h in a rapidly differentiating variant of HL60 cells (HL60-G), and at 96 h in the more slowly differentiating HL60-240 cells. In addition, a G₂,+M block was noted at approximately 72 h in HL60-G and HL60-240 cells. Exposure to vitamin D₃, analogues also markedly increased the number of dikaryons, suggesting that cytokinesis was impaired more than karyokinesis. Treatment with a third analogue 25-hydroxy-16,23-diene-D₃, produced little differentiation and had minimal effects on the cell cycle parameters. These findings indicate that vitamin D₃, analogues regulate cell proliferation by control of the transition of G₁, and G₂,+M phases, reminiscent of the cdc2/CDK2 type of cell cycle control.