Arsenic Trioxide Arrests Cells Early in Mitosis Leading to Apoptosis

Arsenic trioxide (As2O3) is highly effective in the treatment of acute promyelocytic leukemias that express the promyelocytic leukemia-retinoic acid receptor-alpha (PML-RAR alpha) fusion protein. However, evidence has accumulated that As2O3 induces apoptosis regardless of PML-RAR alpha status. Here we show that, at clinically relevant concentrations, As2O3 causes S and G(2)M phase arrest of both PML-RAR alpha-positive and -negative leukemia cell lines, thus inhibiting their growth. Apoptotic cells are generated predominately from the G(2)M fraction. Using several independent methods, we demonstrate that the cells accumulated in the G(2)M peak consist primarily of cells arrested in the early stages of mitosis, prophase, prometaphase and metaphase. In mitotic cells, there was significant activation of caspases, PARP cleavage, and morphological changes characteristic of apoptosis. Unlike microtubule-active drugs that arrest cells in metaphase, arsenic trioxide did not affect the architecture of microtubules. Our data suggest that the antileukemic activities of arsenic may be a result of mitotic arrest which culminates in apoptosis.