Abstract
A plasmid carrying antisense human myc DNA and the gene encoding E. coli xanthine—guanine phosphoribosyltransferase (Ecogpt) was introduced into human promyelocytic leukemia cell line HL-60 by protoplast fusion. High-level expression of antisense myc RNA was obtained by selecting cells resistant to progressively higher levels of mycophenolic acid. The constitutive production of myc protein in clones producing high levels of antisense myc RNA was reduced by 90% compared with parental HL-60 cells, and these cells showed increased commitment toward monocytic differentiation. Inhibition of myc expression was seen at both the translational and the transcriptional levels, implying that antisense RNA can regulate transcription of the myc gene. However, since monocytic differentiation is associated with downregulation of c-myc expression, it cannot be ruled out that the induction of the 74 kD protein and reduction of c-myc transcription are secondary events, triggered by an initial reduction in c-myc expression by “conventional” antisense regulation at the posttranscriptional level. The TCC-CACC repeats in the c-myc leader sequence are the primary transcriptional target of the antisense RNA. The 74 kD nuclear protein, which appears to be induced in the antisense myc transformants, was able to bind to this sequence in these transformants. The suppression of endogenous myc gene expression by either antisense RNA or this 74 kD protein decreases cell proliferation and triggers monocytic differentiation.
