A DUAL BLOCK TO CELL-CYCLE PROGRESSION IN HL-60 CELLS EXPOSED TO ANALOGS OF VITAMIN-D-3

The physiologically active form of vitamin D-3, 1,25-dihydroxy-vitamin D-3 (1,25(OH)(2)D-3), 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 G(1) 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 D-3 which are potent inducers of monocytic differentiation have an additional cell cycle block. Exposure to 10(-7)M 1,25(OH)(2)D-3 or 1,25-(OH)(2)-16-ene-D-3 resulted in monocytic differentiation and the expected G(1) 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 G(2)+M block was noted at approximately 72 h in HL60-G and HL60-240 cells. Exposure to vitamin D-3 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-D-3 produced little differentiation and had minimal effects on the cell cycle parameters. These findings indicate that vitamin D-3 analogues regulate cell proliferation by control of the transition of G(1) and G(2)+M phases, reminiscent of the cdc2/CDK2 type of cell cycle control.