Protection from cell death by mcl-1 is mediated by membrane hyperpolarization induced by K+ channel activation

Mcl-1, a member of the Bcl-2 family, has been identified as an inhibitor of apoptosis induced by anticancer agents and radiation in myeloblastic leukemia cells. The molecular mechanism underlying this phenomenon, however, is not yet understood. In the present study, we report that hyperpolarization of the membrane potential is required for prevention of mcl-1 mediated cell death in murine myeloblastic FDC-P1 cells. In cells transfected with mcl-1, the membrane potential, measured by the whole-cell patch clamp, was hyperpolarized more than -30 mV compared with control cells. The membrane potential was repolarized by increased extracellular K+ concentration (56 mV per 10-fold change in K+ concentration). Using the cell-attached patch-clamp technique, K+ channel activity was 1.7 times higher in mcl-1 transfected cells (NPo = 22.7 +/- 3.3%) than control cells (NPo = 13.2 +/- 1.9%). Viabilities of control and mcl-1 transfected cells after treatment with the cytotoxin etoposide (20 mu g/ml), were 37.9 +/- 3.9% and 78.2 +/- 2.0%, respectively. Suppression of K+ channel activity by 4-aminopyridine (4-AP) before etoposide treatment significantly reduced the viability of mcl-1 transfected cells to 49.0 +/- 3.6%. These results indicate that as part of the prevention of cell death, mcl-1 causes a hyperpolarization of membrane potential through activation of K+ channel activity.