Ion currents and proliferation in MCF-7 human breast cancer cells.

Previous work in our laboratory has demonstrated that in order to proliferate MCF-7 human breast cancer cells have to undergo a change of the membrane potential from a very depolarized to a more hyperpolarized. That change occurs at a hypothetical regulatory point located in early G1 phase, D control point. Potassium (K) channel antagonists quinidine, linogliride, glibenclamide, 4-aminopyridine (4-AP) and tetraethylammonium (TEA) inhibited MCF-7 cell proliferation. Among those blockers three (quinidine, linogliride and glibenclamide) also produced a reversible G0/G1 arrest. On the basis that these three drugs are antagonists of ATP-sensitive K (KA{dollar}\\sb{lcub}\\rm ATP{rcub}{dollar}) channels, we concluded that in MCF-7 cells it is the activation of K{dollar}\\sb{lcub}\\rm ATP{rcub}{dollar} channels at the D control point that promotes progression through G1 phase. The goal of our work was to identify in MCF-7 cells ion current(s) whose activation is required for passage through early G1 phase. To achieve this goal we used whole-cell and single-channel configurations of patch-clamp technique. We expected that the ion current-regulator of the G1 progression would: (1) have a reversal (i.e., zero current) potential near the equilibrium potential for K{dollar}\\sp+{dollar} ({dollar}-{dollar}84 mV); (2) be blocked by physiological concentrations of intracellular ATP; (3) be blocked by linogliride, glibenclamide and quinidine, but not by TEA. Our study demonstrated that MCF-7 cells contain ATP-sensitive K{dollar}\\sp+{dollar} current with the pharmacological profile of the regulator of G1 progression. The conductance density of the current was very low in cells arrested in early G1 phase (prior to D control point) with quinidine that further confirmed the involvement into G1 progression. We also identified in MCF-7 cells a small-conductance (8.5 pS) channel whose reversal potential was close to the equilibrium potential for K{dollar}\\sp+.{dollar} Furthermore, the channel was inhibited by quinidine. We concluded that this channel underlies the current-regulator of G1 progression