Effects of all-trans retinoic acid(retinoic acid) on DNA replication, H2B histone and DNA topoisopmerase I(Topo I) gene expression have been investigated in human promyelocytic leukemia cell line HL-60. DNA synthesis decreased at 24 hours after exposure of retinoic acid in the HL-60 cells. H2B histone mRNA rapidly reduced at 48 hours and Topo I and c-myc mRNA at 24 hours in retinoic acid-exposured HL-60 cells, respectively. The levels of c-myc, H2B his- tone and Topo I gene expression were reduced in proportion to the concentration of retinoic acid. The H2B histone mRNA in retinoic acid-exposured cells was elevated by cycloheximide treatment, while the level of Topo I mRNA was constant. These results suggest that regulations of H2B histone, c-myc and Topo I gene expression are different from one another, and repression of Topo I gene is closely correlated with c-myc gene during retinoic acid-induced differentiation of HL-60 cells.
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