INDUCTION OF DIFFERENTIATION IN HL-60 CELLS BY THE REDUCTION OF EXTRACHROMOSOMALLY AMPLIFIED C-MYC

Oncogene amplification in turner cells results in the overexpression o f proteins that confer a growth advantage in vitro and in vivo. Amplif ied oncogenes can reside intrachromosomally, within homogeneously stai ning regions (HSRs), or extrachromosomally, within double minute chrom osomes (DMs). Since previous studies have shown that low concentration s of hydroxyurea (HU) can eliminate DMs, we studied the use of HU as a gene-targeting agent in tumor cells containing extrachromosomally amp lified oncogenes. In a neuroendocrine cell line (COLO 320), we have sh own that HU can eliminate amplified copies of c-myc located on DMs, le ading to a reduction in tumorigenicity in vitro and in vivo. To determ ine whether the observed reduction in tumorigenicity was due to differ entiation, we next investigated whether HU could induce differentiatio n in HL60 cells containing extrachromosomally amplified c-myc. We comp ared the effects of HU, as well as two other known differentiating age nts (dimethyl sulfoxide and retinoic acid), on c-myc gene copy number, c-myc expression, and differentiation in HL60 cells containing amplif ied c-myc genes either on DMs or HSRs. We discovered that HU and dimet hyl sulfoxide reduced both c-myc gene copy number and expression and i nduced differentiation in cells containing c-myc amplified on DMs. The se agents failed to have similar effects on HL60 cells with amplified c-myc in HSRs. By contrast, retinoic acid induced differentiation inde pendent of the localization of amplified c-myc. These data illustrate the utility of targeting extrachromosomal DNA to modulate tumor phenot ype and reveal that both HU and dimethyl sulfoxide induce differentiat ion in HL60 cells through DM elimination.