Paracetamol inhibits cell cycling and induces apoptosis in HL-60 cells

We investigated the effects of paracetamol on cell cycle and cell death in cultured HL-60 cells. Paracetamol (0.1-3.0 mM) caused a dose-dependent inhibition of cell proliferation. Inhibition of DNA synthesis was observed at as low concentrations as 0.03 mM. When HL-60 cells were exposed to paracetamol (0.1-1.0 mM), flow cytometric analysis showed that cells accumulated in the G1/S phase, and then slowly proceeded through the S-phase. Exposure to 2.0-3.0 mM paracetamol, on the other hand, resulted in a reduction of the number of cells in S-phase and G2/M phase, and a concurrent increase in cells/bodies with a lower DNA content than that of the G1 cells. Microscopic studies revealed increased numbers of cells with nuclear condensation and fragmentation, indicating that apoptosis was the dominating mode of death in this cell line after exposure to paracetamol. However, when DNA from paracetamol-treated cells were electrophoresed in agarose, no clear ladder pattern characteristic for apoptotic cells was observed. This was most likely a result of secondary necrosis which followed incomplete apoptosis. Within 5-10 hr after start of paracetamol exposure, a marked downregulation of both c-myc and bcl-2 mRNA was observed. In conclusion, the present results show that exposure to high non-therapeutic concentrations of paracetamol cause cell cycle arrest and apoptosis in HL-60 cells in S-phase.