We previously reported that MCF-7 cells were arrested in the G0/G1 phase of the cell cycle by agents known to block the activity of ATP-sensitive potassium channels (Woodfork et al., 1995, J. Cell Physiol. 162:163-171). The goal of our current study was to determine if MCF-7 cells undergo changes in membrane potential during the cell cycle that might be linked to changes in K permeability. The resting membrane potentials of unsynchronized MCF-7 cells during exponential growth phase were measured using sharp glass microelectrodes, and they ranged from -58.6 mV to -2.7 mV. The distribution of membrane potentials was best fitted by the sum of four Gaussian distributions with means of -9.0 mV, -17.4 mV, -24.6 mV, and -40.4 mV. These membrane potential groups were designated D (depolarized), ID (intermediate depolarized), IH (intermediate hyperpolarized), and H (hyperpolarized), respectively. The membrane potential was sensitive to the substitution of external K and Na but not Cl. The K:Na permeability ratio increased in proportion to the negativity of the membrane potential. MCF-7 cells pharmacologically arrested in G0/G1 phase were depolarized compared to control, with cells shifted from the H and IH groups to the D group. Tamoxifen-arrested cells chased from G0/G1 into S phase by the addition of mitogenic concentrations of 17 beta-estradiol were not depolarized, and these cells were shifted from the D group back to the IH and H groups. We conclude that MCF-7 cells hyperpolarize during passage through G0/G1 and into S phase, and this hyperpolarization probably results from an increase in the relative permeability of the plasma membrane to K.

The purpose of this study was to determine if potassium channel activity is required for the proliferation of MCF-7 human mammary carcinoma cells. We examined the sensitivities of proliferation and progress through the cell cycle to each of nine potassium channel antagonists. Five of the potassium channel antagonists produced a concentration-dependent inhibition of cell proliferation with no evidence of cytotoxicity following a 3-day or 5-day exposure to drug. The IC50 values for these five drugs, quinidine (25 microM), glibenclamide (50 microM), linogliride (770 microM), 4-aminopyridine (1.6 mM), and tetraethylammonium (5.8 mM) were estimated from their respective concentration-response curves. Four other potassium channel blockers were tested at supra-maximal channel blocking concentrations, including charybdotoxin (200 nM), iberiotoxin (100 nM), margatoxin (10 nM), and apamin (500 nM), and they had no effect on MCF-7 cell proliferation, viability, or cell cycle distribution. Of the five drugs that inhibited proliferation, only quinidine, glibenclamide, and linogliride also affected the cell cycle distribution. Cell populations exposed to each of these drugs for 3 days showed a statistically significant accumulation in G0/G1 phase and a significant proportional reduction in S phase and G2/M phase cells. The inhibition of cell proliferation correlated significantly with the extent of cell accumulation in G0/G1 phase and the threshold concentrations for inhibition of growth and G0/G1 arrest were similar. The G0/G1 arrest produced by quinidine and glibenclamide were reversed by removing the drug, and cells released from arrest entered S phase synchronously with a lag period of approximately 24 hours. Based on the differential sensitivity of cell proliferation and cell cycle progression to the nine potassium channel antagonists, we conclude that inhibition of ATP-sensitive potassium channels in these human mammary carcinoma cells, reversibly arrests the cells in the G0/G1 phase of the cell cycle, resulting in an inhibition of cell proliferation.

The coupling of receptors to heterotrimeric G proteins is determined by interactions between the receptor and the G protein ␣ subunits and by the composition of the ␤␥ dimers. To determine the role of the ␥ subunit prenyl modification in this interaction, the CaaX motifs in the ␥ 1 and ␥ 2 subunits were altered to direct modification with different prenyl groups, recombinant ␤␥ dimers expressed in the baculovirus/Sf9 insect cell system, and the dimers purified. The activity of the ␤␥ dimers was compared in two assays: formation of the high affinity agonist binding conformation of the A1 adenosine receptor and receptor-catalyzed exchange of GDP for GTP on the ␣ subunit. The ␤ 1 ␥ 1 dimer (modified with farnesyl) was significantly less effective than ␤ 1 ␥ 2 (modified with geranylgeranyl) in either assay. The ␤ 1 ␥ 1 -S74L dimer (modified with geranylgeranyl) was nearly as effective as ␤ 1 ␥ 2 in either assay. The ␤ 1 ␥ 2 -L71S dimer (modified with farnesyl) was significantly less active than ␤ 1 ␥ 2 . Using 125 I-labeled ␤␥ subunits, it was determined that native and altered ␤␥ dimers reconstituted equally well into Sf9 membranes containing A1 adenosine receptors. These data suggest that the prenyl group on the ␥ subunit is an important determinant of the interaction between receptors and G protein ␥ subunits.The membranes of all cells contain multiple receptors and transmembrane signaling systems that regulate cell function (1-5). One major unsolved problem in cell signaling is understanding how a cell selects its response to a hormone or growth factor given the possibilities available. The signaling mechanism used by receptors coupled to the heterotrimeric G proteins 1 provides an excellent example of the complexities. Current evidence suggests that specificity is determined at many levels in this pathway. In addition to the selectivity provided by the receptor itself, the interaction between the intracellular loops of the receptors and the ␣␤␥ subunits in the heterotrimer is a major determinant of specificity (6, 7). Interestingly, some receptors couple selectively to certain G protein ␣ subunits. For example, the ␤-adrenergic receptor couples primarily to members of the G s ␣ family (1, 6) and rhodopsin to the G t ␣ subunit (2). Other receptors, such as the angiotensin AT 1 or muscarinic receptors couple to multiple ␣ subunits including members of the G q and G i class of ␣ subunits (6, 8), leading to activation of multiple signaling networks by a given ligand (5).Although the receptor and the ␣ subunit provide one determinant of selectivity, the ␤␥ subunit is clearly required for the receptor to couple to the ␣ subunit (9, 10). Thus, a third level of selectivity may be provided by the type of ␤␥ subunit used to form the receptor-␣␤␥ complex. In addition, since the ␤␥ dimer can activate effectors directly (11-13), the diversity of these proteins may contribute to the specificity of signaling. Two lines of experimental evidence support this conclusion. Using recombinant ␤␥ subunits purified from Sf9 insect cel...

The mechanism of the G0/G1 arrest and inhibition of proliferation by quinidine, a potassium channel blocker, was investigated in a tissue culture cell line, MCF-7, derived from a human breast carcinoma. The earliest measurable effect of quinidine on the cell cycle was a decrease in the fraction of cells in S phase at 12 hr, followed by the accumulation of cells in G1/G0 phases at 30 hr. Arrest and release of the cell cycle established quinidine as a cell synchronization agent, with a site of arrest in early G1 preceding the lovastatin G1 arrest site by 5-6 hr. There was a close correspondence among the concentration-dependent arrest by quinidine in G1, depolarization of the membrane potential, and the inhibition of ATP-sensitive potassium currents, supporting a model in which hyperpolarization of the membrane potential and progression through G1 are functionally linked. Furthermore, the G1 arrest by quinidine was overcome by valinomycin, a potassium ionophore that hyperpolarized the membrane potential in the presence of quinidine. With sustained exposure of MCF-7 cells to quinidine, expression of the Ki67 antigen, a marker for cells in cycle, decreased, and apoptotic and necrotic cell death ensued. We conclude that MCF-7 cells that fail to progress through the quinidine-arrest site in G1 die.

The ␥ subunits of heterotrimeric G proteins undergo post-translational prenylation and carboxylmethylation after formation of the ␤␥ dimer, modifications that are essential for ␣-␤␥, ␤␥-receptor, and ␤␥-effector interactions. We have determined the specific prenyl group present on the ␤ 1 ␥ 1 , ␤ 1 ␥ 2 , and ␤ 1 ␥ 3 dimers purified from baculovirus-infected Sf9 cells by specific binding to G protein ␣ subunits immobilized on agarose. These recombinant dimers undergo the same post-translational modifications determined for ␥ 1 and ␥ 2 isolated from mammalian tissues. Furthermore, infection of Sf9 cells with a recombinant baculovirus encoding an alteration of the ␥ 1 CaaX sequence (␥ 1 -S74L) resulted in geranylgeranylation of the resulting ␥ 1 subunit, and alteration of the ␥ 2 CaaX sequence to CAIS (␥ 2 -L71S) resulted in farnesylation. Both of these altered ␥ subunits were able to associate stably with ␤ 1 , and the resulting ␤␥ dimer bound tightly to ␣-agarose and eluted specifically with aluminum fluoride. These results indicate that Sf9 insect cells properly process the CaaX motif in G protein ␥ subunits and are a useful model system to study the role of prenylation in the protein-protein interactions in which the ␤␥ subunits participate.The ␣ and ␥ subunits of heterotrimeric G proteins 1 undergo post-translational modification with lipids that are essential for their interaction with receptors and downstream effectors (1). The ␣ subunits are known to be modified with myristate and palmitate groups. The 11 known ␥ subunit isoforms all have CaaX motifs at their carboxyl terminus (2-4) which direct post-translational modifications including attachment of a prenyl group to the cysteine side chain, proteolytic cleavage of the aaX tripeptide, and methylation of the resulting carboxyl terminus (5). The identity of the prenyl group attached appears to be directed by the last amino acid in the CaaX motif (6 -8). The prenylation of the cysteine side chain not only targets the ␤␥ dimer to the membrane (8 -11), it also plays a major role in determining protein-protein interactions (12). Prenylation of the ␥ subunit is necessary for the formation of an active transducin ␣␤␥ complex (13, 14), for stimulation of phospholipase C-␤ by the ␤ 1 ␥ 1 dimer (15), and for translocation of the ␤-adrenergic receptor kinase to the plasma membrane (16). In addition, the ability of the ␤␥ subunit to support ADP-ribosylation of G␣ i subunits by pertussis toxin and to regulate adenyl cyclase activity both require the presence of the prenyl modification (17). Prenylated synthetic peptides corresponding to the carboxyl terminus of the ␥ subunit can inhibit ADP-ribosylation of ␣ subunits, but the same peptides lacking the prenyl thioether have no effect (18). Thus, the carboxyl terminus of the ␥ subunit is an important domain for the interaction between the ␣ subunit and the ␤␥ dimer.We have taken advantage of the ability of the baculovirus Sf9 cell system to produce g quantities of highly purified ␤␥ subunits to investigate further this c...