CONTROL OF CELL-CYCLE REGULATORY PROTEIN EXPRESSION BY 1,25-DIHYDROXYVITAMIN D-3 IN HUMAN PROMYELOCYTIC HL-60 LEUKEMIC-CELLS CULTURED IN SERUM-FREE MEDIUM

1,25-Dihydroxyvitamin D-3 (herein referred to as vitamin D-3), the nat ural vitamin D-3 formed by successive hydroxylation of cholecalciferol at the 25 and 1 alpha position, and numerous vitamin D-3 analogs, hav e been reported to decrease proliferation and promote terminal differe ntiation from several types of human malignant cells, including the hu man promyelocytic HL-60 leukemic cells. The purpose of this study was to determine if and to what extent the cell culture conditions affect the sensitivity of the HL-60 cells to vitamin D-3, both in terms of ce ll growth, differentiation, and changes in expression of specific prot eins. Addition of 10 nM and 100 nM vitamin D-3 to HL-60 cells cultured in the serum-free, chemically defined medium of insulin/transferrin/s elenium (ITS) effected cell growth differently than cells maintained i n a fetal bovine serum-supplemented medium. In addition to the greater degree of growth suppression by 100 nM vitamin D-3, cells maintained in serum-free medium also displayed significantly higher levels of mon ocytic differentiation. Furthermore, Western blot analysis showed that a pronounced arrest of cell cycling at the G(1)-to-S-phase transition , concomitant with a corresponding 36% down-regulation of cyclin D1 an d, in parallel, a similar decreased hyperphosphorylation of pRb, was e licited by 100 nM vitamin D-3. These results indicate that the sensiti vity of HL-60 cells to vitamin D-3 is dependent on the availability of serum.