G1 Cell-Cycle Arrest and Apoptosis by Histone Deacetylase Inhibition in MLL-AF9 Acute Myeloid Leukemia Cells Is MLL-AF9 Independent.

Acute myeloid leukemia (AML) with MLL rearrangements (MLLmut), found mainly in M5 or M4 FAB subtypes, is frequent in infants and secondary leukemias. The most common MLL translocation gives rise to MLL-AF9. MLL protein interacts with histone deacetylases (HDACs) -1 and -2 through the MLL repression domain.

We report the effects of HDAC inhibition by valproic acid (VPA) in MLL-AF9 AML-M5 cells (THP-1, MM6 and MOLM-13) and MLLmut AML-M5 blasts. VPA led to histone hyper-acetylation, strong cell-growth inhibition, G1 cell-cycle arrest and apoptosis. Combined treatment with all-trans-retinoic-acid (ATRA) did not substantially improve these effects. VPA increased MLL-AF9 transcription, indicating that VPA overcomes the cell-growth promoting activity and resistance to apoptosis conferred by MLL-AF9 in AML-M5 cells, even with increased MLL-AF9. A small number of genes were significantly affected by VPA in p53-absent THP-1 cells (GeneChip analysis), and the majority of these were up-regulated. VPA potently induced p21 and cyclin G2 (CG2) expression. p21 and CG2 targeted inhibition demonstrated that p21 acts as a key regulator in the VPA-inducted G1 cell-cycle arrest, while induction of CG2 has no effect.

These data suggest that these poor prognosis patients may benefit from HDAC inhibitor therapy.