Induction of human myeloblastic ML-1 cell G₁ arrest by suppression of K⁺ channel activity

Our previous studies have shown that a voltage-gated K⁺ channel is highly expressed in proliferating human myeloblastic ML-1 cells and is suppressed in the early stages of 12-O-tetradecanoylphorbol-13-acetate- induced ML-1 cell differentiation. In the present study, we report that inhibition of the K⁺ channel activity by 4-aminopyridine (4-AP) suppressed ML-1 cell proliferation, as measured by DNA synthesis. Cell cycle mapping indicated that ML-1 cells were arrested in G₁ phase after 24-h treatment with 4-AP. Blockade of ML-1 cells at the G₁/S boundary of the cell cycle with aphidicolin revealed that ML-1 cells past the G₁ checkpoint were capable of entering S phase and synthesizing DNA independently of the channel blockade. ML-1 cell differentiation, measured by CD14 marker protein expression, revealed that the effect of 4-AP was to cause growth arrest and that it did not cause differentiation. Dephosphorylation of retinoblastoma protein accompanied inhibition of ML-1 cell proliferation and suggested that suppression of K⁺ channel activity by 4-AP is associated with retinoblastoma protein-mediated G₁ arrest in ML-1 cells. Moreover, we found that ML-1 cell volume increased 35 ± 7% after 4-AP treatment, which could be an early event triggering inhibition of ML-1 cell proliferation. These findings suggest that a 4-AP-sensitive K⁺ channel may play an important role in the transduction of mitogenic signals in ML-1 cells.