Differentiation of myeloid leukemia cells by HDAC-inhibitors

Histone deacetylase (HDAC)-inhibitors are a new class of promising anti-cancer agents. Here we show that different structural classes of HDAC-inhibitors (butyric acid, apicidin, MS-275, SAHA, novel synthesized valproic acid derivatives) exhibit anti-proliferative and erythroid differentiation inducing activities in bcr/abl positive K562 myeloid leukemia cells. Inhibition of cell proliferation and erythroid differentiation are accompanied by induction of p21 mRNA and accumulation of hyperacetylated histone H4 proteins in cells. We identified a K562 subline with resistance to erythroid differentiation by HDAC-inhibitors despite evidence of effective HDAC inhibition suggesting that signaling pathways downstream of HDAC-inhibition are defective in these subline. We therefore performed subtractive cDNA-hybridization of K562-sensitive and resistent cells in order to clone genes involved in mediating the HDAC-inhibitor effect in these cells. We identified retinaldehyde-dehydrogenase 2, an enzyme required for intracellular retinoic acid synthesis, as a differentially expressed gene. Treatment of K562 cells with retinoic acid receptor (RAR)-antagonists inhibited erythroid differentiation by HDAC-inhibitors, whereas retinoic acid increased the effect of HDAC-inhibitors on erythroid differentiation.

In summary our data suggest that HDAC-inhibitors may be useful for differentiation therapy of bcr/abl positive leukemic cells. Additionally, we identified novel valproic acid derivatives with potent activities. Intact retinoic acid signaling appears to be required for action of HDAC-inhibitors in these cells.