The Capacity of the Hypomethylating Agents Azacytidine and Decitabine to Induce Apoptosis and Cell Cycle Arrest Depends on the Activation of the DNA-Damage Response Pathway.

Background: The hypomethylating agents azacytidine (AZA) and decitabine (DEC) have shown clinical efficacy in patients (pts) with MDS. There is in vitro evidence that both agents, in addition to their hypomethylating effect, also function by inducing apoptosis, cell cycle arrest and/or the activation of a DNA damage response (DDR). However, the exact contributions of those mechanisms of action and their functional interdependence remain to be defined.

Methods: A panel of MDS (P39, MDS-1)- and AML (HL-60, KG-1)-derived cell lines were incubated with increasing dosages of AZA (1–2μM) and DEC (1–2μM) and the drugs capacity to induce apoptosis (DiOC6(3)/PI), cell cycle arrest (PI) and/or a DDR (immunoflourescence staining of P-ATM, P-Chk-1, P-Chk-2, γ-H2AX) were assessed in absence and presence of the ATM-inhibitor KU-55933 and the Chk-1 inhibitor UCN-01.

Results: We show that both drugs induced dose-dependent apoptosis in myeloid cell lines: whereas AZA increased apoptosis in KG-1 and HL-60 by about 10% (48h, 2μM) the respective incubation with DEC augmented apoptosis by about 20% (HL-60) to 30% (KG-1). P39 cells were resistant to AZA and increased apoptosis by 15% after 48h of 2μM DEC, and MDS-1 cells were resistant to both drugs. In addition, both drugs induced a G2/M-arrest in P39 (+15% after 48h with 2μM of AZA or DEC) and HL-60 (+20% after 48h with 2μM of AZA or DEC) cells, but not in KG-1 and MDS-1 cells. Noteworthy, both drugs induced a DDR in the apoptosis-sensitive KG-1 cells (but not P39 cells) as evidenced by the appearance of nuclear P-ATM and γ-H2AX foci. Surprisingly, this activation of P-ATM did not induce the nuclear translocation of P-Chk-1-Ser317 or P-Chk-2-Ser68. To more clearly define the importance of the DDR in AZA- and DEC-induced apoptosis and G2/M-arrest, experiments were recapitulated in the presence of the ATM-inhibitor KU-55933 and the Chk-1 inhibitor UCN-01. Inhibition of ATM abrogated the apoptosis-inducing activity of AZA and DEC in KG-1 cells (without influencing cell cycle progression), whereas inhibition of Chk-1 remained without effect. In contrast, in P39 and HL-60 cells, inhibition of ATM neither affected cell cycle progression, nor sensitivity towards the drugs. Nevertheless, inhibition of Chk-1 by UCN-01 completely abrogated the G2/M-arresting effect of AZA (and diminished that of DEC) in P39 and HL-60 cells.

Conclusions: We provide novel evidence for the cell-type dependent capacity of the hypomethylating agents 5-azacytidine and decitabine to induce apoptosis, cell-cycle arrest and DDR in cell lines representing different subtypes of MDS and AML. Moreover, we show the crucial role of ATM and Chk-1 activation – as part of the DDR – in mediating AZA and DEC apoptosis-inducing and cell cycle-arresting effects, respectively, providing evidence that hypomethylating agents confer their beneficial effects by employing different pathways of the DDR.