The differentiating agent and
histone deacetylase inhibitor,
sodium butyrate (NaB), was shown previously to cause a transient, 3-17-fold induction of human
DNA topoisomerase II alpha (
topo II alpha) gene promoter activity and a 2-fold increase in
topo II alpha
protein early in monocytic differentiation of HL-60 cells. This observation has now been extended to other
short chain fatty acids and aromatic
butyrate analogues, and evidence is presented that human
topo II alpha promoter induction correlates closely with
histone H4 acetylation status. Because increased
topo II alpha expression is associated with enhanced efficacy of
topo II-
poisoning antitumor drugs such as
etoposide, the hypothesis tested in this report was whether NaB pretreatment could sensitize HL-60
myeloid leukemia and K562
erythroleukemia cells to
etoposide-triggered DNA damage and cell death. A 24-72 h NaB treatment (0.4-0.5 mM) induced
topo II alpha 2-2.5-fold in both HL-60 and K562 cells and caused a dose-dependent enhancement of etoposidestimulated,
protein-linked
DNA complexes in both cell lines. At concentrations with minimal effects on cell cycle kinetics (0.4 mM in HL-60; 0.5 mM in K562), NaB pretreatment also modestly enhanced etoposidetriggered apoptosis in HL-60 cells, as determined morphologically after
acridine orange/
ethidium bromide staining, and substantially increased K562 growth inhibition and
poly(ADP-ribose)polymerase cleavage after
etoposide exposure. Therefore, a temporal window may exist whereby a differentiating agent may sensitize
experimental leukemias to a cytotoxic
antitumor agent. These results indicate that
histone deacetylase inhibitors should be investigated for
etoposide sensitization of other
butyrate-responsive hematopoietic and nonhematopoietic
tumor lines in vitro and in vivo.