MLL-AF9 Positive Acute Myeloid Leukemia Cells Are Sensitive To All-Trans Retinoic Acid

Among the subtypes of acute myeloid leukemia (AML), acute promyelocytic leukemia (APL) responds dramatically to differentiation therapy with all-trans retinoic acid (ATRA). But, ATRA is not sufficient to induce differentiation in non-APL AML. Herein, we first evaluated whether MLL fusion partners, such as MLL-AF9 and MLL-AF4/AF5q31, affect the sensitivity of ATRA in human and murine MLL fusion positive AML cell lines. In addition, we also assessed the level of H3K4me2 modification for the RARα gene in human AML cell lines, and whether the LSD1 inhibitor affected the ATRA-resistant MLLfusion positive AML cell lines.

Three human AML cell lines with MLL fusion (THP1 and MOLM13 expressing MLL-AF9, and KOCL48 expressing MLL-AF4) and two murine leukemic cell lines derived from murine Lin- hematopoietic progenitors transduced by retroviral vector expressing MLL fusion genes, such as MLL-AF9 and MLL-AF5q31 were used in this study. To test the sensitivity of ATRA, all cell lines were treated with 1 μM ATRA for three days. Monocytic differentiation was assessed by morphological analysis, NBT reduction test and flow cytometric analysis (FCM) of CD11b expression. Quantitative RT-PCR (qRT-PCR) analysis and western blotting was carried out to measure the RARα, C/EBPα, C/EBPε, and PU.1 expressions. Cytotoxicity assay was performed to determine the IC50 of ATRA in these cell lines and whether ATRA could decrease the IC50 of cytarabine in MLL-AF9positive cells by using WST assays. Chromatin immunoprecipitation (ChIP) assay was performed to determine the value of H3K4me2 status using RARα-specific primers. To determine whether tranylcypromine (TCP), which is a nonreversible LSD1 inhibitor, could reactivate ATRA sensitivity, we treated KOCL48 with 10 μM TCP and 1μM ATRA.

We first determined that morphological changes characteristic of monocytic differentiation, CD11b expression and NBT reduction are more readily induced by ATRA in human and murine MLL-AF9 positive cells than MLL-AF4/AF5q31 positive cells. The NBT reduction percentage was 12.5±3.77 in THP1, 13.1±2.02 in MOLM13, but 7.00±2.64 in KOCL48 cells (p<0.05). The ATRA treatment also induced growth inhibition and increased gene expression related to monocytic differentiation through retinoic acid (RA) pathway, more efficiency in MLL-AF9 positive cells than MLL-AF4/AF5q31 cells. The IC50 of ATRA for THP1, MOLM13 and murine MLL-AF9 cells was 3.85, 1.24 and 1.95 μM, but over 10 μM for KOCL48 and murine MLL-AF5q31 cells. Furthermore, qRT-PCR and western blot revealed that ATRA increased expression level of RARα, C/EBPα, C/EBPε, and PU.1 in MLL-AF9 positive cells, but not in MLL-AF4/AF5q31 positive cells. Collectively, RA pathway is more impaired in MLL-AF4/AF5q31 positive cells than MLL-AF9 positive cells. In addition, the increase in RARα, C/EBPα, C/EBPε, and PU.1 mRNA expressions were observed in two primary MLL-AF9 positive AML cells treated with ATRA. Next, we also carried out ChIP assay and the H3K4me2/ H3 on the RARα promoter in MLL-AF9 positive cells were higher than MLL-AF4 positive cell. Furthermore, ATRA and TCP combination treatment in KOCL48 induced morphological changes, CD11b expression, and increased the expression level of RARα, C/EBPα, C/EBPε, and PU.1, suggesting that inhibition of LSD1 restores ATRA sensitivity. Finally, ATRA in combination with cytarabine treatment in MLL-AF9 positive cells enhanced cytarabine sensitivity: the IC50 of cytarabine in THP1, MOLM13, and murine MLL-AF9cells was 4.18, 0.04, and0.065 μM without ATRA and 0.13, 0.0005, and 0.015 μM with ATRA, respectively.

Our data demonstrated that RA pathway was more profoundly impaired in MLL-AF4/AF5q31 positive cells than MLL-AF9 positive cells, suggesting type of MLL fusion protein contributes inactivation of RA pathway. Our data also identified the sensitivity of ATRA was correlated with the ratio of H3K4me2/ H3 on the RARα promoter, and TCP restore the sensitivity of ATRA in KOCL48, suggesting the decrease of the H3K4me2/H3 plays a role in inactivation of RA pathway. Additionally, we revealed that synergistic antileukemic activity of ATRA in combination with cytarabine in MLL-AF9 positive cells. Therefore, ATRA in combination with cytarabine might be novel therapeutic option for the ATRA sensitive AML cells, especially for MLL-AF9 positive cells.

No relevant conflicts of interest to declare.