Inhibition of DNA synthesis by an inducer of differentiation of leukemic cells, 1 alpha, 25 dihydroxy vitamin D3, precedes down regulation of the c-myc gene.

Transient monocytic differentiation was induced in cultures of HL 60 cells by a four hour exposure to 1 alpha, 25-dihydroxyvitamin D3 (1,25(OH)2D3). This treatment resulted in a rapid, selective, inhibition of DNA synthesis, which was accompanied by reduced cellular levels of c-myc mRNA, and a more gradual appearance of c-fos mRNA. After removal of the inducer from the cultures, DNA synthesis and c-myc mRNA levels returned rapidly to near-normal levels, but the expression of c-fos gene continued to increase for 24 hr and then declined slowly. Studies with isolated nuclei showed that the inhibition of DNA synthesis can be detected earlier than the changes in transcriptional rates of the oncogenes studied, and that 1,25(OH)2D3 directly inhibits the DNA synthesis in isolated nuclei. Autoradiographic studies of [3H]thymidine incorporation showed that 1,25(OH)2D3 does not immediately block the progression of the cells into the S phase of the cell cycle, but that those cells which become differentiated as the result of a brief exposure to this inducer do have such a block. It is concluded that 1,25(OH)2D3 produces both an immediate and a delayed inhibition of DNA synthesis in HL 60 cells, that the immediate inhibition is not preceded by detectable changes in oncogene expression, and that the delayed inhibition is accompanied by an elevated expression of the c-fos gene, and may be related to the monocytic differentiation of HL 60 cells.