C-myc expression is down-regulated in mafosfamide-treated HL-60 cells undergoing apoptosis

The expression of the proto-oncogene c-myc has been strongly implicated in the regulation of apoptosis. Since we have previously shown apoptosis to occur in HL-60 leukemic cells exposed to the prodrug Mafosfamide (ASTA Z 7557), the present study was undertaken to examine the levels of c-myc mRNA transcripts in such cells. Mafosfamide, which is a cyclophosphamide derivative that rapidly generates 4-Hydroperoxycyclophosphamide after aqueous dissolution, was employed in doses ranging from 0.1 to 20 micrograms/ml. No changes in c-myc mRNA levels were evident after 1 hour of exposure to Mafosfamide. After 6 and 24 hours of Mafosfamide-exposure, however, there was a dose- and time-dependent decrease in c-myc transcripts. c-myc mRNA expression was reduced to a greater extent than was either beta-actin of GAPDH expression. Morphological, biochemical and ultrastructural evidence of apoptosis accompanied the Mafosfamide-induced c-myc mRNA down-regulation at 24 hours. We conclude that, in the context of Mafosfamide-treated HL-60 cells, upregulation of c-myc mRNA transcription was not fundamental for the activation of the apoptotic cascade.