Altered regulation of c-myc in an HL-60 differentiation resistant subclone, HL-60-1E3

HL-60 cells treated with phorbol dibutyrate (PDBu) differentiate into cells which functionally and morphologically resemble macrophages (G. Rovera, D. Santoli, and D. Damskey, Proc. Natl. Acad. Sci. USA, 75: 2779-2783, 1979; E. Huberman and M.F. Callahan, Proc. Natl. Acad. Sci. USA, 76:1293-1298, 1979). This differentiation involves modulation of the expression of several cellular oncogenes. However, the significance of the temporal relationships between differentiation events and specific oncogene expression are not known. Others have reported that transcriptional down regulation of c-myc occurs early in the differentiation of HL-60 cells (R.D. Dalla-Favera et al., Haematol. Blood Transfusion, 28: 247-253, 1983; L.E. Grosso and H.C. Pitot, Biochem. Biophys. Res. Commun., 119: 473-480, 1984). To determine the significance of the regulation of c-myc during HL-60 maturation, we performed parallel PDBu induction studies analyzing the kinetics of expression of c-myc, cell cycle frequency distribution, cytotoxic effector activity, and clonogenic potential in HL-60 cells and in a partial-differentiation resistant HL-60 clone (HL-60-1E3) (J. A. Leftwich, P. Carlson, B. Adelman, and R. E. Hall, Cancer Res., 47: 1319-1324, 1987). We report that PDBu stimulation results in early c-myc transcriptional down regulation in the HL-60-1E3 clone cells with the same kinetics as has been previously reported for HL-60 parental cells (R. D. Dalla-Favera et al., Haematol. Blood Transfusion, 28: 247-253, 1983; L. E. Grosso and H. C. Pitot, Biochem. Biophys. Res. Commun., 119: 473-480, 1984). However, reexpression of c-myc occurs 15 hours postinduction in HL-60-1E3 but not parental cells. This reexpression is maintained through 30 h of stimulation and correlates with a lack of terminal commitment as assessed by an increase in clonogenic potential and the inability of these cells to acquire cytotoxic function. Sequential stimulation of HL-60-1E3 cells with DMSO and PDBu overcomes the block in macrophage differentiation (J. A. Leftwich, P. Carlson, B. Adelman, and R. E. Hall, Cancer Res., 47; 1319-1324, 1987). Such treatment results in a transient reexpression of c-myc at 15 h after PDBu treatment, and the complete downregulation of c-myc 24 h postinduction. These data suggest that the reported early decrease in c-myc transcripts following PDBu stimulus in HL-60 cells is not sufficient to commit these cells to macrophage-like terminal differentiation. Late regulation of c-myc gene expression may be an important additional component of the regulatory mechanisms which allow HL-60 cells to complete this program.