A GM-COLONY-STIMULATING FACTOR (CSF) ACTIVATED RIBONUCLEASE SYSTEM TRANSREGULATES M-CSF RECEPTOR EXPRESSION IN THE MURINE FDC-P1/MAC MYELOID CELL-LINE

The murine myeloid precursor cell line FDC-PI/MAC simultaneously expresses receptors for multi-colony-stimulating factor (CSF), granulocyte-macrophage (GM)-CSF, and macrophage (M)-CSF. Growth of FDC-P1/MAC cells in either multi-CSF or GM-CSF results in the posttranscriptional suppression of M-CSF receptor (c-fms proto-oncogene) expression. We use the term transregulation to describe this control of receptor expression and have further characterized this regulatory process. The removal of FDC-P1/MAC cells from GM-CSF stimulation resulted in the re-expression of c-fms mRNA independent of M-CSF stimulation and new protein synthesis. Switching FDC-P1/MAC cells from growth in M-CSF to GM-CSF caused the selective degradation of c-fms mRNA within 6 h after factor switching. Blocking protein synthesis or gene transcription with metabolic inhibitors effectively prevented GM-CSF stimulated degradation of c-fms mRNA. These results suggest that the transregulation of c-fms transcripts by GM-CSF requires the transcriptional activation of a selective mRNA degradation factor. In vitro analysis, the use of cytoplasmic cell extracts, provided evidence that a ribonuclease is preferentially active in GM-CSF stimulated cells, although the specificity for mRNA degradation in vitro is broader than seen in vivo. Together, these data suggest that GM-CSF can dominantly transregulate the level of c-fms transcript through the transcriptional activation of a ribonuclease degradation system.