C-FMS MESSENGER-RNA IS REGULATED POSTTRANSCRIPTIONALLY BY 1,25(OH)(2)D-3 IN HL-60 CELLS

Macrophage colony-stimulating factor (MCSF) is required for normal ost eoclast and macrophage development. The receptor for MCSF (MCSF) is ex pressed on the pluripotent precursor and mature osteoclasts and macrop hages. We have previously shown in myelomonocytic HL-60 cells that pho rbol myristate acetate (PMA) upregulates c-fms mRNA expression. This i nduction of c-fms is inhibited by 1,25(OH)(2)D-3. The major regulatory control of c-fms mRNA levels by PMA has been identified as posttransc riptional. However, a role of transcript elongation in controlling lev els of c-fms mRNA has also been suggested. To better understand the 1, 25(OH)(2)D-3 regulation of c-fms mRNA expression we studied nuclear ru n on, mRNA stability, and transcript elongation in HL-60 cells treated with 10 ng/ml phorbol myristate acetate, 10 nM 1,25(OH)(2)D-3 alone o r combined. We demonstrated by nuclear run on that c-fms was constitut ively transcribed in 1,25(OH)(2)D-3 as well as control and PMA-treated cells. Transcript elongation was evaluated by RT-PCR for exon 2 or ex on 3. Both exons were minimally expressed in control and 1,25(OH)(2)D- 3-treated cells, and increased in PMA-treated cells; this increased ex pression was inhibited by the addition of 1,25(OH)(2)D-3. These result s fail to show differential transcript elongation. Measurement of mRNA stability demonstrated decreased mRNA half-life to 5 hours in cells t reated with PMA and 1,25(OH)(2)D-3 compared with a half-life of 8 hour s in cells treated with PMA alone. Our findings demonstrate that c-fms is regulated by 1,25(OH)(2)D-3 at the posttranscriptional level by ch anges in mRNA stability. This gives the cell the ability to respond to local signals with rapid changes in c-fms levels altering the ability of the cell to respond to MCSF.