Effect of methylation inhibitors on gene expression in HL-60 cells

The methylation inhibitors Neplanocin A (Nep A), 3'-deazaadenosine (dzAdo), and 3-deaza(+/-)aristeromycin (Dari) were tested for their effect on the expression of histone H2B, actin, and the protooncogenes c-myc, and v-fos. Nep A and Dari bind to the S-adenosylhomocysteine hydrolase resulting in the accumulation of S-adenosylhomocysteine, while dzAdo served as a substrate for the enzyme. With dzAdo, inordinant amounts of 3-deazaadenosylhomocysteine (dzAdoHcy) accumulated in the cell, provided L-homocysteine (Hcy) was present. When added at sublethal concentrations, the methylation inhibitors had little or no effect on c-myc, v-fos, histone H2B, or actin expression, nor did any significant number of the drug-treated cells demonstrate myeloid characteristics. However, growth and gene expression were markedly inhibited upon the addition of Hcy and dzAdo. One of the earliest effects of dzAdoHcy on HL-60 cells was the disappearance of c-myc mRNA. Within 1 h of the addition of dzAdo and Hcy, only trace amounts of c-myc mRNA were detectable. After 4-5 h v-fos, histone H2B, and actin mRNAs also decreased to about 40% of control levels. Differences in the stability of preexisting mRNAs would appear to account for these results. Within 1 h following the addition of dzAdo and Hcy, the synthesis of rRNA and mRNA were completely blocked as measured by the incorporation of [3H]uridine.