Activation of cardiac chloride conductance by the tyrosine kinase inhibitor, genistein

Genistein and daidzein prevent low potassium-dependent apoptosis of cerebellar granule cells

We have investigated the ability of certain dietary flavonoids, known to exert beneficial effects on the central nervous system, to affect neuronal apoptosis. We used cerebellar granule cells undergoing apoptosis due to potassium deprivation in a serum-free medium in either the absence or presence of the flavonoids genistein and daidzein, which are present in soy, and of catechin and epicatechin, which are present in cocoa. These compounds were used in a blood dietary concentration range. We found that genistein and daidzein, but not catechin and epicatechin, prevented apoptosis, with cell survival measured 24h after the induction of apoptosis being higher than that of the same cells incubated in flavonoid free medium (80% and 40%, respectively); there was no effect in control cells. A detailed investigation of the effect of these compounds on certain mitochondrial events that occur in cells en route to apoptosis showed that genistein and daidzein prevented the impairment of glucose oxidation and mitochondrial coupling, reduced cytochrome c release, and prevented both impairment of the adenine nucleotide translocator and opening of the mitochondrial permeability transition pore. Interestingly, genistein and daidzein were found to reduce the levels of reactive oxygen species, which are elevated in cerebellar granule cell apoptosis. These findings strongly suggest that the prevention of apoptosis depends mainly on the antioxidant properties of genistein and daidzein. This could lead to the development of a flavonoid-based therapy in neuropathies.

Involvement of tyrosine kinase and PI3K in the regulation of OAT3-mediated estrone sulfate transport in isolated rabbit renal proximal tubules

It was shown previously that OAT3 activity was differentially regulated by protein kinases including MAPK, PKA, and PKC. The present study investigated the short-term effect of tyrosine kinase and phosphatidylinositol 3-kinase (PI3K) on OAT3-mediated organic anion transport in S2 segments of renal proximal tubules. Genistein, a tyrosine kinase inhibitor, and wortmannin, a PI3K inhibitor, inhibited transport of estrone sulfate, a prototypic substrate for OAT3, in a dose-dependent manner. Previously, we showed that epidermal growth factor (EGF) stimulated OAT3 activity via the MAPK pathway. In the present study, we investigated whether EGF-stimulated OAT3 activity was dependent on tyrosine kinase and PI3K. We showed that EGF stimulation of OAT3 was reduced by inhibition of tyrosine kinase or PI3K, suggesting that they play a role in the stimulatory process. Inhibitory effects also indicated that tyrosine kinase and PI3K are involved in the MAPK pathway for EGF stimulation of OAT3 in intact renal proximal tubules, with PI3K acting upstream and tyrosine kinase acting downstream of mitogen-activated/extracellular signal-regulated kinase kinase activation.

Differential action of a protein tyrosine kinase inhibitor, genistein, on the positive inotropic effect of endothelin-1 and norepinephrine in canine ventricular myocardium

Experiments were carried out in isolated canine ventricular trabeculae and acetoxymethylester of indo-1-loaded single myocytes to elucidate the role of protein tyrosine kinase (PTK) in the inotropic effect of endothelin-1 (ET-1) induced by crosstalk with norepinephrine (NE). The PTK inhibitor genistein was used as a pharmacological tool. Genistein but not daidzein inhibited the positive inotropic effect and the increase in Ca(2+) transients induced by ET-1 by crosstalk with NE at low concentrations. Genistein and daidzein antagonized the negative inotropic effect and the decrease in Ca(2+) transients induced by ET-1 by crosstalk with NE at high concentrations, but genistein did not affect the antiadrenergic effect of carbachol. Genistein but not daidzein enhanced the positive inotropic effect and the increase in Ca(2+) transients induced by NE via beta-adrenoceptors, while the enhancing effect of genistein was abolished by the protein tyrosine phosphatase inhibitor vanadate. These findings indicate that genistein (1) induces a positive inotropic effect in association with an increase in Ca(2+) transients, (2) inhibits the positive inotropic effect of ET-1 induced by crosstalk with NE, and (3) enhances the positive inotropic effect of NE induced via beta-adrenoceptors by inhibition of PTK. In addition, genistein inhibits the negative inotropic effect of ET-1 induced by crosstalk with NE through a PTK-unrelated mechanism. PTK may play a crucial role in the receptor-mediated regulation of cardiac contractile function in canine ventricular myocardium.

Antagonistic regulation of swelling-activated Cl- current in rabbit ventricle by Src and EGFR protein tyrosine kinases

Regulation of swelling-activated Cl(-) current (I(Cl,swell)) is complex, and multiple signaling cascades are implicated. To determine whether protein tyrosine kinase (PTK) modulates I(Cl,swell) and to identify the PTK involved, we studied the effects of a broad-spectrum PTK inhibitor (genistein), selective inhibitors of Src (PP2, a pyrazolopyrimidine) and epidermal growth factor receptor (EGFR) kinase (PD-153035), and a protein tyrosine phosphatase (PTP) inhibitor (orthovanadate). I(Cl,swell) evoked by hyposmotic swelling was increased 181 +/- 17% by 100 microM genistein, and the genistein-induced current was blocked by the selective I(Cl,swell) blocker tamoxifen (10 microM). Block of Src with PP2 (10 microM) stimulated tamoxifen-sensitive I(Cl,swell) by 234 +/- 27%, mimicking genistein, whereas the inactive analog of PP2, PP3 (10 microM), had no effect. Moreover, block of PTP by orthovanadate (1 mM) inhibited I(Cl,swell) and prevented its stimulation by PP2. In contrast with block of Src, block of EGFR kinase with PD-153035 (20 nM) inhibited I(Cl,swell). Several lines of evidence argue that the PP2-stimulated current was I(Cl,swell): 1) the stimulation was volume dependent, 2) the current was blocked by tamoxifen, 3) the current outwardly rectified with both symmetrical and physiological Cl(-) gradients, and 4) the current reversed near the Cl(-) equilibrium potential. To rule out contributions of other currents, Cd(2+) (0.2 mM) and Ba(2+) (1 mM) were added to the bath. Surprisingly, Cd(2+) suppressed the decay of I(Cl,swell), and Cd(2+) plus Ba(2+) eliminated time-dependent currents between -100 and +100 mV. Nevertheless, these divalent ions did not eliminate I(Cl,swell) or prevent its stimulation by PP2. The results indicate that tyrosine phosphorylation controls I(Cl,swell), and regulation of I(Cl,swell) by the Src and EGFR kinase families of PTK is antagonistic.

Cardiac Na+-Ca2+ exchanger current induced by tyrphostin tyrosine kinase inhibitors

Tyrosine kinase (TK) inhibitors genistein and tyrphostin A23 (A23) inhibited Ca(2+) currents in guinea-pig ventricular myocytes investigated under standard whole-cell conditions (K(+)-free Tyrode's superfusate; EGTA-buffered (pCa-10.5) Cs(+) dialysate). However, the inhibitors (100 microM) also induced membrane currents that reversed between -40 and 0 mV, and the objective of the present study was to characterize these currents. Genistein-induced current behaved like Cl(-) current, and was unaffected by either the addition of divalent cations (0.5 mM Cd(2+); 3 mM Ni(2+)) that block the Na(+)-Ca(2+) exchanger (NCX), or the removal of external Na(+) and Ca(2+). A23-induced current was independent of Cl(-) driving force, and strongly suppressed by addition of Cd(2+) and Ni(2+), and by removal of either external Na(+) or Ca(2+). These and other results suggested that A23 activated an NCX current driven by submembrane Na(+) and Ca(2+) concentrations higher than those in the bulk cytoplasm. Improved control of intracellular Na(+) and Ca(2+) concentrations was obtained by suppressing cation influx (10 microM verapamil) and raising dialysate Na(+) to 7 mM and dialysate pCa to 7. Under these conditions, stimulation by A23 was described by the Hill equation with EC(50) 68 +/- 4 microM and coefficient 1.1, tyrphostin A25 was as effective as A23, and TK-inactive tyrphostin A1 was ineffective. Phosphotyrosyl phosphatase inhibitor orthovanadate (1 mM) antagonized the action of 100 microM A23. The results suggest that activation of cardiac NCX by A23 is due to inhibition of genistein-insensitive TK.

Direct block of the cystic fibrosis transmembrane conductance regulator Cl(-) channel by niflumic acid

Niflumic acid is widely used to inhibit Ca(2+) -activated Cl(-) channels. However, the chemical structure of niflumic acid resembles that of diphenylamine-2-carboxylate, a drug that inhibits the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. To investigate how niflumic acid inhibits CFTR Cl(-) channel, we studied recombinant wild-type human CFTR in excised inside-out membrane patches. When added to the intracellular solution, niflumic acid caused a concentration- and voltage-dependent decrease of CFTR Cl(-) current with half-maximal inhibitory concentration (K(i)) of 253 microM and Hill co-efficient of approximately 1, at -50 mV. Niflumic acid inhibition of single CFTR Cl(-) channels was characterized by a very fast, flickery block that decreased dramatically current amplitude without altering open-probability. Consistent with these data, spectral analysis of CFTR Cl(-) currents suggested that channel block by niflumic acid was described by the closed <--> open <--> blocked kinetic scheme with blocker on rate (k(on)) = 13.9 x 10(6) M(-1)s(-1), off rate (k(off))=3348 s(-1) and dissociation constant (K(d)) = 241 microM, at -50 mV. Based on these data, we tested the effects of niflumic acid on transepithelial Cl(-) secretion and cyst growth using type I MDCK epithelial cells. Niflumic acid (200 microM) inhibited cAMP-stimulated, bumetanide-sensitive short-circuit current by 55%. Moreover, the drug potently retarded cyst growth. We conclude that niflumic acid is an open-channel blocker of CFTR that inhibits Cl(-) permeation by plugging the channel pore. It or related agents might be of value in the development of new therapies for autosomal dominant polycystic kidney disease.

Differential effects of tyrosine kinase inhibitors on volume-sensitive chloride current in human atrial myocytes: evidence for dual regulation by Src and EGFR kinases

To determine whether protein tyrosine kinase (PTK) modulates volume-sensitive chloride current (I_Cl.vol) in human atrial myocytes and to identify the PTKs involved, we studied the effects of broad-spectrum and selective PTK inhibitors and the protein tyrosine phosphatase (PTP) inhibitor orthovanadate (VO₄⁻³). I_Cl.vol evoked by hyposmotic bath solution (0.6-times isosmotic, 0.6T) was enhanced by genistein, a broad-spectrum PTK inhibitor, in a concentration-dependent manner (EC₅₀ = 22.4 μM); 100 μM genistein stimulated I_Cl.vol by 122.4 ± 10.6%. The genistein-stimulated current was inhibited by DIDS (4,4′-diisothiocyanostilbene-2,2′-disulfonic acid, 150 μM) and tamoxifen (20 μM), blockers of I_Cl.vol. Moreover, the current augmented by genistein was volume dependent; it was abolished by hyperosmotic shrinkage in 1.4T, and genistein did not activate Cl⁻ current in 1T. In contrast to the stimulatory effects of genistein, 100 μM tyrphostin A23 (AG 18) and A25 (AG 82) inhibited I_Cl.vol by 38.2 ± 4.9% and 40.9 ± 3.4%, respectively. The inactive analogs, daidzein and tyrphostin A63 (AG 43), did not alter I_Cl.vol. In addition, the PTP inhibitor VO₄⁻³ (1 mM) reduced I_Cl.vol by 53.5 ± 4.5% (IC₅₀ = 249.6 μM). Pretreatment with VO₄⁻³ antagonized genistein-induced augmentation and A23- or A25-induced suppression of I_Cl.vol. Furthermore, the selective Src-family PTK inhibitor PP2 (5 μM) stimulated I_Cl.vol, mimicking genistein, whereas the selective EGFR (ErbB-1) kinase inhibitor tyrphostin B56 (AG 556, 25 μM) reduced I_Cl.vol, mimicking A23 and A25. The effects of both PP2 and B56 also were substantially antagonized by pretreatment with VO₄⁻³. The results suggest that I_Cl.vol is regulated in part by the balance between PTK and PTP activity. Regulation is complex, however. Src and EGFR kinases, distinct soluble and receptor-mediated PTK families, have opposing effects on I_Cl.vol, and multiple target proteins are likely to be involved.

Novel stimulatory actions of the phytoestrogen genistein: effects on the gain of cardiac excitation-contraction coupling

Genistein, a phytoestrogen found abundantly in soy products, is thought to be cardioprotective, partly through its ability to act as a natural Ca2+ channel antagonist. However, the precise nature and significance of such direct cardiac actions remain obscure. We investigated the hypothesis that genistein exerts important additional actions on cardiac excitation-contraction coupling (ECC). Genistein acutely increased cell shortening and the Ca2+ transient in field stimulated guinea-pig ventricular myocytes despite potently inhibiting the L-type Ca2+ current, I(Ca,L). The specific phosphotyrosine phosphatase inhibitor, bpV(phen), diminished the stimulatory effects of genistein on myocyte contractility, suggesting that the mechanism partly involved tyrosine kinase inhibition. Genistein increased sarcoplasmic reticulum (SR) Ca2+ load as measured with a caffeine pulse in Na+-free/ Ca2+-free solution. Furthermore, in the continued presence of caffeine, genistein increased the time constant of decline of the caffeine-induced Ca2+ transient, implying impaired sarcolemmal Na+/Ca2+ exchanger function. Tetanization studies in intact myocytes revealed that 43% of cells exhibited increased myofilament Ca2+ sensitivity in the presence of genistein. These findings demonstrate novel cardiac actions of genistein on the SR Ca2+ load, Na+/Ca2+ exchanger, and myofilament Ca2+ sensitivity, which result in an overall increase in myocyte contractility and consequently the gain of ECC.

Activation of background membrane conductance by the tyrosine kinase inhibitor tyrphostin A23 and its inactive analog tyrphostin A1 in guinea pig ventricular myocytes

ABSTRACT-The effects of the tyrosine kinase (TK) inhibitor tyrphostin A23 and its inactive analog tyrphostin Al on background membrane conductance were investigated in guinea pig ventricular myocytes. TK-inhibiting A23 reversibly increased membrane conductance under conditions designed to minimize Na+, Ca2+, K+, and Na+-K+ pump currents. Similar stimulatory action was obtained with TK-inactive Al. The tyrphostin-induced current was inhibited by omitting external Na+ or Ca+, suppressed by chelating internal Ca2+, blocked by external Cd2+ and Ni2+, and insensitive to changes in internal Cl- concentration. We conclude that tyrphostins have a direct, TK-independent action that increases membrane conductance probably by stimulating Na+-Ca2+ exchange.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Control of epithelial Cl(−) secretion by basolateral osmolality in the euryhaline teleost Fundulus heteroclitus

Euryhaline teleost fish adapt rapidly to salinity change and reduce their rate of ion secretion on entry to fresh water. Killifish (Fundulus heteroclitus) transferred from full-strength sea water to fresh water showed large reductions in plasma [Na(+)] and osmolality at 6 h which were corrected by 24 h. To mimic this in vitro, a hypotonic shock of 20–70 mosmol kg(−)(1) was applied on the basolateral side of opercular epithelia. This hypotonic shock reversibly reduced the short-circuit current (I(sc), equivalent to the rate of secretion of Cl(−)) in a dose-dependent fashion, with a 40 mosmol kg(−)(1) hypotonic shock reducing I(sc) by 58+/−4.6 % in 40 min. Similar reductions in [NaCl], but with added mannitol to maintain osmolality, were without effect, indicating that the effect was purely osmotic. Hypotonic inhibition of I(sc) was accompanied by reductions in epithelial conductance (G(t)) but no significant change in transepithelial potential (V(t)). The hypotonic inhibition was apparently not Ca(2+)-mediated because Ca(2+)-depleted salines, thapsigargin and ionomycin all failed to block the reduction in I(sc) produced by hypotonic shock. The inhibition was not mediated via a reduction in intracellular cyclic AMP level because cyclic AMP levels, measured by radioimmunoassay, were unchanged by hypotonic shock and by 1.0 micromol l(−)(1) clonidine (which inhibits I(sc) by changing intracellular [Ca(2+)]) but were increased markedly by 1.0 micromol l(−)(1) isoproterenol, a positive control. The protein tyrosine kinase inhibitor genistein (100 micromol l(−)(1)), but not its inactive analogue daidzein, inhibited I(sc) in normal osmolality but produced a stimulation of I(sc) after hypotonic shock (and after clonidine treatment). The inhibitory effects of genistein and hypotonicity were not additive, suggesting that the same portion of the I(sc) was inhibited by both treatments. These data are consistent with a model for Cl(−) transport regulation involving tyrosine phosphorylation in cell-swelling-induced inhibition of Cl(−) secretion when euryhaline teleosts adapt to fresh water.

Involvement of tyrosine phosphorylation in the positive inotropic effect produced by H(1)-receptors with histamine in guinea-pig left atrium

We investigated the effect of stimulation of H(1)-receptors with histamine on protein tyrosine phosphorylation levels in guinea-pig left atrium and evaluated the influences of tyrosine kinase inhibitors on the positive inotropic effect mediated by H(1)-receptors in this tissue. Histamine induced an increase in tyrosine phosphorylation in four main clusters of proteins with apparent molecular weights of 25, 35, 65 and 150 kDa. Tyrosine phosphorylation of these proteins attained a peak around 2 - 3 min following histamine stimulation and then declined to or below basal levels. Histamine-induced protein tyrosine phosphorylation was antagonized by the H(1)-receptor antagonists mepyramine (1 microM) and chlorpheniramine (1 microM), but not by the H(2)-receptor antagonist cimetidine (10 microM). The positive inotropic effect of histamine was depressed in a concentration-dependent manner by the tyrosine kinase inhibitors tyrphostin A25 (50 to 100 microM) and genistein (10 to 50 microM) but not by the inactive genistein analogue daidzein (50 microM). The positive inotropic effect of isoprenaline was unchanged by tyrphostin A25 and genistein. At a concentration of 1 microM histamine produced a dual-component positive inotropic response composed of an initial increasing phase and a second and late developing, greater positive inotropic phase. Treatment with tyrphostin A25 (100 microM) and genistein (50 microM), but not daidzein (50 microM), significantly attenuated the two components of the inotropic response, although genistein suppressed the initial component more markedly than the late component. We conclude that increased protein tyrosine phosphorylation may play an important role in initiating at least some part of the positive inotropic effect of H(1)-receptor stimulation in guinea-pig left atrium.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Inhibition by genistein of the hyperpolarization-activated cation current in porcine sino-atrial node cells

1 The hyperpolarization-activated cation current (If) was recorded in single pacemaker cells of the porcine sino-atrial node, and the effects of genistein, an isoflavone inhibitor of tyrosine-specific protein kinases was investigated by the whole-cell patch clamp technique. 2 Genistein (20-500 microM) decreased If in a dose-dependent manner with an IC50 value of 62.3 microM and a maximum inhibition of 45.3%. 3 The effect on If appeared without altering the half-activation potential (control, -88.3+/-2.8 mV; genistein, -87.0+/-1.8 mV) and the slope factor (control, 8.0+/-0.3 mV; genistein, 8.6+/-0.7 mV) of the steady-state activation curve. No significant voltage-dependency was detected in the fully-activated current-voltage relation measured by the double-pulse protocols. 4 The inactive form of genistein analogue, daidzein (500 microM) or genistin (200 microM), were without effect. If was not affected by another tyrosine kinase inhibitor, tryphostin-47 (100 microM), but tyrphostin-25 (100-200 microM) suppressed If in an irreversible manner. 5 Neither bath nor intracellular application of the tyrosine phosphatase inhibitor, orthovanadate, affected If, and subsequent application of genistein inhibited If significantly. 6 These data indicate that the inhibition of If by genistein is not mediated through tyrosine kinase inhibition but through nonselective block of the If channels.

Inhibition of ATPase, GTPase and adenylate kinase activities of the second nucleotide-binding fold of the cystic fibrosis transmembrane conductance regulator by genistein

In the presence of ATP, genistein, like the ATP analogue adenosine 5'-[beta,gamma-imido]triphosphate (pp[NH]pA), increases cystic fibrosis transmembrane conductance regulator (CFTR) chloride currents by prolonging open times. As pp[NH]pA is thought to increase CFTR currents by interfering with ATP hydrolysis at the second nucleotide-binding fold (NBF-2), the present study was undertaken to investigate the effects of genistein on a fusion protein comprising maltose-binding protein (MBP) and NBF-2 (MBP-NBF-2). MBP-NBF-2 exhibited ATPase, GTPase and adenylate kinase activities that were inhibited by genistein in a partial non-competitive manner with respect to ATP or GTP. Ki values for competitive and uncompetitive inhibition were respectively 20 microM and 63 microM for ATPase, 15 microM and 54 microM for GTPase, and 46 microM and 142 microM for adenylate kinase. For ATPase activity, genistein reduced Vmax by 29% and Vmax/Km by 77%. Additional evidence for complex-formation between genistein and MBP-NBF-2 was obtained by the detection of genistein-dependent alterations in the CD spectrum of MBP-NBF-2 that were consistent with the formation of a higher-ordered state. Addition of MBP-NBF-2 increased the fluorescence intensity of genistein, consistent with a change to a less polar environment. pp[NH]pA partially eliminated this enhanced fluorescence of genistein. These observations provide the first direct biochemical evidence that genistein interacts with CFTR, thus inhibiting NBF-2 activity, and suggest a similar mechanism for genistein-dependent stimulation of CFTR chloride currents.

L-type Ca2+ current in guinea pig ventricular myocytes treated with modulators of tyrosine phosphorylation

Guinea pig ventricular myocytes in whole cell configuration were treated with tyrosine kinase (TK) inhibitors [genistein (Gst), tyrphostin A23 (T23), and tyrphostin A25 (T25)] and with inactive analogs [daidzein, genistin, and tyrphostin A1 (T1)] to measure effects on L-type Ca2+ current (ICa,L). Gst inhibited ICa,L (IC50 = 47 microM) without affecting its time course or shifting the ICa, L-voltage relationship. At the highest concentration of isoflavone tested (200 microM), ICa,L was inhibited by 66 +/- 7% (Gst), 22 +/- 2% (daidzein), and 1 +/- 3% (genistin). Inhibition of ICa,L by the active tyrphostins was significantly larger than inhibition by T1; at 200 microM the inhibitions were 72 +/- 6% (T23), 71 +/- 6% (T25), and 27 +/- 6% (T1). The phosphotyrosine phosphatase inhibitor orthovanadate (1 mM) had a small stimulatory effect (6 +/- 2%) on basal ICa,L and blocked the inhibition of ICa,L by TK inhibitors. The data suggest a role for the TK-phosphotyrosine phosphatase system in the regulation of cardiac Ca2+ channels.

Activation of Cl- channel and Na+/K+/2Cl- cotransporter in renal epithelial A6 cells by flavonoids: genistein, daidzein, and apigenin

The present study investigates regulation of Cl- channels and Na+/K+/2Cl- cotransporter in a renal epithelial cell line, A6, by flavones: genistein [an inhibitor of protein tyrosine kinases (PTK)], daidzein (an inactive compound of genistein), and apigenin [an inhibitor of mitogen-activated protein (MAP) kinase]. Genistein and daidzein activated Cl- channels. Genistein and apigenin had a stimulatory effect on the bumetanide-sensitive Na+/K+/2Cl- cotransporter. Other PTK inhibitors, tyrphostin A23, lavendustin A, and herbimycin A, which do not contain a structure flavone, had no stimulatory action on Cl- channels or the Na+/K+/2Cl- cotransporter. These observations conclude that (i) genistein activates a Cl- channel and the Na+/K+/2Cl- cotransporter; and (ii) the stimulatory action is not mediated through its inhibitory action on protein tyrosine kinase, but rather the structure of flavone itself plays a crucial role in stimulatory regulation of Cl- channels and Na+/K+/2Cl- cotransporter.

Modulation of nicotinic acetylcholine receptor activity in submucous neurons by intracellular messengers

The effects on acetylcholine-induced membrane currents (ACh currents), produced by agents known to modify the activity of intracellular messengers, were studied in the neurons of the guinea-pig ileum submucous plexus (SMP) using a whole-cell patch clamp recording method. The ACh currents were not affected by forskolin, the adenylate cyclase activator, regardless of whether or not ATP and GTP were present in the intracellular solution, and by phorbol 12-myristate 13-acetate, the protein kinase C activator. The ACh currents were strongly suppressed by thapsigargin, the microsomal calcium ATPase inhibitor, and genistein, the tyrosine protein kinase inhibitor. They were also suppressed by 3-isobutyl-1-methylxanthine, the cyclic-AMP phosphodiesterase inhibitor, regardless of the presence of forskolin in the extracellular solution and ATP and GTP in the intracellular solution. In addition, the currents were suppressed by activation of P2 purinoceptors with ATP, which could not be explained by a direct effect of ATP on nicotinic acetylcholine receptors (nAChRs). Reactive blue 2, the P2y purinoceptor antagonist, did not abolish inhibition of the ACh current by ATP. Alpha,beta-Imido-ATP and adenosine caused no membrane current responses and did not influence the ACh currents. These results suggest that the activity of the nAChRs in the SMP neurons is strongly suppressed by raised intracellular Ca2+ level, without involvement of protein kinases A and C, and may involve the participation of tyrosine kinase. The activity of nAChRs is also influenced by the activity of P2 purinoceptors; the mechanisms responsible for this influence are not yet clear. So, the activity of the SMP neuronal nAChRs is relatively independent on the intracellular signaling known to influence many other groups of transmitter-gated receptors of neuronal membrane.

Pharmacology of CFTR chloride channel activity

Pharmacology of CFTR Chloride Channel Activity. Physiol. Rev. 79, Suppl.: S109-S144, 1999. - The pharmacology of cystic fibrosis transmembrane conductance regulator (CFTR) is at an early stage of development. Here we attempt to review the status of those compounds that modulate the Cl- channel activity of CFTR. Three classes of compounds, the sulfonylureas, the disulfonic stilbenes, and the arylaminobenzoates, have been shown to directly interact with CFTR to cause channel blockade. Kinetic analysis has revealed the sulfonylureas and arylaminobenzoates interact with the open state of CFTR to cause blockade. Suggestive evidence indicates the disulfonic stilbenes act by a similar mechanism but only from the intracellular side of CFTR. Site-directed mutagenesis studies indicate the involvement of specific amino acid residues in the proposed transmembrane segment 6 for disulfonic stilbene blockade and segments 6 and 12 for arylaminobenzoate blockade. Unfortunately, these compounds (sulfonylureas, disulfonic stilbenes, arylaminobenzoate) also act at a number of other cellular sites that can indirectly alter the activity of CFTR or the transepithelial secretion of Cl-. The nonspecificity of these compounds has complicated the interpretation of results from cellular-based experiments. Compounds that increase the activity of CFTR include the alkylxanthines, phosphodiesterase inhibitors, phosphatase inhibitors, isoflavones and flavones, benzimidazolones, and psoralens. Channel activation can arise from the stimulation of the cAMP signal transduction cascade, the inhibition of inactivating enzymes (phosphodiesterases, phosphatases), as well as the direct binding to CFTR. However, in contrast to the compounds that block CFTR, a detailed understanding of how the above compounds increase the activity of CFTR has not yet emerged.

Tyrosine kinase-independent extracellular action of genistein on the CFTR Cl- channel in guinea pig ventricular myocytes and CFTR-transfected mouse fibroblasts

The effects of genistein, a protein tyrosine kinase inhibitor, on the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel were studied in guinea pig ventricular myocytes and in NIH3T3 mouse fibroblasts stably transfected with CFTR cDNA by the whole-cell patch-clamp technique. Genistein did not activate whole-cell Cl- currents when applied to the intracellular (pipette) solution. In contrast, when applied to the extracellular solution, genistein alone promptly activated the Cl- current in a fully reversible manner. Also, extracellular genistein reversibly potentiated the forskolin-activated Cl- current. However, both basal and forskolin-activated Cl- currents were not affected by other protein tyrosine kinase inhibitors, including herbimycin A, lavendustin A, tyrphostin 21, tyrphostin 47, and tyrphostin 51. A nonspecific inhibitor of protein phosphatases, orthovanadate, had no effect on the genistein-induced activation of CFTR. Pretreatment with a protein kinase inhibitor, either H-89 or H-7, or with an adenylate cyclase inhibitor, SQ 22536, also had no effect on the genistein-induced response. Thus, it is concluded that genistein alone activates CFTR by a protein tyrosine kinase-independent and protein phosphatase-independent mechanism from the extracellular side, but not from the intracellular side.

Genistein increases the sensitivity of cardiac ion channels to beta-adrenergic receptor stimulation

The whole-cell patch-clamp technique was used to monitor the effects of genistein, a tyrosine kinase inhibitor, on membrane currents recorded from isolated guinea pig ventricular myocytes. Under control conditions, genistein (50 micromol/L) did not activate the latent cAMP-regulated Cl- current (ICl). However, in the presence of a subthreshold concentration (1 nmol/L) of the beta-adrenergic agonist isoproterenol (Iso), genistein caused a near-maximal activation of this current. In the absence of genistein, Iso activated ICl with an EC50 of 5 nmol/L. In the presence of genistein, Iso activated ICl with an EC50 of 0.3 nmol/L. This facilitatory effect was not observed in the presence of daidzein (50 micromol/L), an analogue of genistein that only weakly inhibits tyrosine kinase activity. Furthermore, peroxovanadate, a potent inhibitor of phosphotyrosine phosphatase activity, inhibited ICl activated by Iso alone, and it blocked the stimulatory effect of genistein in the presence of Iso. To determine whether the stimulatory effect of genistein was specific for ICl, we also studied its action on the cAMP-regulated delayed rectifier K+ current (IK) and L-type Ca2+ current (ICa-L) present in these cells. Basal IK and ICa-L were partially (approximately 30% to 40%) inhibited by genistein. However, this inhibitory effect was mimicked by daidzein, suggesting that inhibition of tyrosine kinase activity is not involved. In addition to the nonspecific inhibitory effect, genistein also caused a significant increase in the beta-adrenergic sensitivity of the unblocked cationic currents. In the absence of genistein, 1 nmol/L Iso had no effect on either IK or ICa-L. However, in the presence of genistein, 1 nmol/L Iso significantly increased the magnitude of both currents. These results suggest that tyrosine kinase activity may play an important role in regulating beta-adrenergic responsiveness of the heart.

Genistein elicits biphasic effects on L-type Ca2+ current in feline atrial myocytes

A perforated patch recording method was used to determine the effects of genistein (Gen), a protein tyrosine kinase (PTK) inhibitor, on basal L-type Ca2+ current (ICa,L) in feline atrial myocytes. Gen (50 microM) elicited biphasic changes in ICa,L: an initial inhibition (-55 +/- 4%; phase 1) followed by a secondary stimulation (34 +/- 9%; phase 2) of ICa,L. Withdrawal of Gen elicited a further potentiation of ICa,L (152 +/- 19%; phase 3) above control (n = 46). In general, phase 1 inhibition and phase 3 potentiation varied directly with Gen concentration, and phase 2 stimulation exhibited biphasic concentration-dependent changes compared with control. When cells were dialyzed using a ruptured patch recording method, Gen elicited only inhibition of ICa,L; phases 2 and 3 were abolished. Vanadate (1 mM), an inhibitor of protein tyrosine phosphatase, abolished both Gen-induced inhibition and stimulation of ICa,L. Daidzein (50 microM), a weakly active analog of Gen, exerted no significant effects on ICa,L, and withdrawal of daidzein failed to potentiate ICa,L. In a few cells, Gen elicited a prominent vanadate-sensitive stimulation of ICa,L in the absence of any significant inhibition of ICa,L. Gen-induced changes in ICa,L were unaffected by either 100 microM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-acetoxymethyl ester (AM) or 1 microM ryanodine, agents that alter intracellular Ca2+; 4 microM H-89 or 50 microM Rp diastereomer of adenosine 3',5'-monophosphothioate (RP-cAMPS), inhibitors of protein kinase A (PKA); 0.1 microM calphostin C or 2 microM chelerythrine, inhibitors of protein kinase C (PKC); or 100 microM NG-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide (NO) synthase. We conclude that in feline atrial myocytes, Gen acts via membrane-bound PTKs to inhibit ICa,L and via cytosolic PTKs to stimulate ICa,L. Gen-induced changes in ICa,L are not related to changes in intracellular Ca2+ or to secondary interactions with either PKA, PKC, or NO signaling pathways. These results indicate that in atrial myocytes ICa,L is regulated by two independent and competing PTK signaling mechanisms.

Regulation of a swelling-activated chloride current in bovine endothelium by protein tyrosine phosphorylation and G proteins

1. The role of protein tyrosine phosphorylation and of G proteins in the activation of a swelling-activated Cl- current (ICl,swell) in calf pulmonary artery endothelial (CPAE) cells was studied using the whole-cell patch clamp technique. ICl,swell was activated by reducing the extracellular osmolality by either 12.5% (mild hypotonicity) or 25% (strong hypotonicity). 2. The protein tyrosine kinase (PTK) inhibitors tyrphostin B46, tyrphostin A25 and genistein inhibited ICl,swell with IC50 values of, respectively, 9.2 +/- 0.2, 61.4 +/- 1.7 and 62.9 +/- 1.3 microM. Tyrphostin A1, a tyrphostin analogue with little effect on PTK activity, and daidzein, an inactive genistein analogue, were without effect on ICl,swell. 3. The protein tyrosine phosphatase (PTP) inhibitors Na3VO4 (200 microM) and dephostatin (20 microM) potentiated ICl,swell activated by mild hypotonicity by 47 +/- 9 and 69 +/- 15%, respectively. 4. Intracellular perfusion with GTP gamma S (100 microM) transiently activated a Cl- current with an identical biophysical and pharmacological profile to ICl,swell. This current was inhibited by the tested PTK inhibitors and potentiated by the PTP inhibitors. Hypertonicity-induced cell shrinkage completely inhibited the GTP gamma S-activated Cl- current. 5. Intracellular perfusion with GDP beta S (1 mM) caused a time-dependent inhibition of ICl,swell, which was more pronounced when the current was activated by mild hypotonicity. 6. Our results demonstrate that the activity of endothelial swelling-activated Cl- channels is dependent on tyrosine phosphorylation and suggest that G proteins regulate the sensitivity to cell swelling.

Synergistic activation of guinea-pig cardiac cystic fibrosis transmembrane conductance regulator by the tyrosine kinase inhibitor genistein and cAMP

1. The regulation of cardiac Cl- current (ICl) by tyrosine and serine/threonine phosphorylation was examined in guinea-pig and rat ventricular myocytes. The protein tyrosine kinase (PTK) inhibitor genistein (GST) and phosphotyrosine phosphatase (PTP) inhibitor sodium orthovanadate (VO4) were used to modify tyrosine phosphorylation, whereas forskolin (FSK), cAMP, and other agents were used to modify cytoplasmic cAMP concentration and protein kinase A (PKA) phosphorylation. 2. Low concentrations (0.1 microM) of FSK did not activate the PKA-regulated cystic fibrosis transmembrane regulator (CFTR) ICl in guinea-pig ventricular myocytes, but strongly potentiated activation of an ICl by 20-100 microM GST. The potentiation did not occur when GST was replaced by PTK-inactive daidzein, and it was strongly inhibited by 1 mM VO4. 3. Potentiation by 0.1 microM FSK was linked to a small stimulation of the adenylate cyclase-cAMP-PKA pathway. The potentiation was not mimicked by inactive 1,9-dideoxyforskolin, and was inhibited by muscarinic stimulation (ACh) and by a PKA inhibitor. Internal application of a cAMP solution that alone was too weak to activate CFTR ICl strongly potentiated the activation of ICl by 50 microM GST and occluded potentiation by 0.1 microM FSK. 4. The foregoing suggests that potentiated ICl flows through cAMP-dependent CFTR channels. In agreement with this interpretation, GST did not increase ICl when CFTR was maximally activated by a high concentration (5 microM) of FSK and okadaic acid, and neither GST nor GST plus FSK activated an ICl in CFTR-deficient rat myocytes. The lack of effect in rat myocytes was not due to the absence of functional, channel-relevant PKA and PTK-PTP systems, because (as in guinea-pig myocytes) L-type Ca2+ current (ICa,L) was stimulated by FSK and inhibited in a VO4-reversible manner by GST. 5. The synergistic activation of CFTR by low concentrations of FSK and GST cannot be explained by either a GST-induced elevation of cAMP concentration or inhibition of serine/threonine phosphatase. Rather, it appears to be due to tyrosine dephosphorylation that facilitates PKA-mediated phosphorylation of the channels.

Genistein directly induces cardiac CFTR chloride current by a tyrosine kinase-independent and protein kinase A-independent pathway in guinea pig ventricular myocytes

With one-suction electrode voltage-clamp technique, we demonstrated that genistein, a tyrosine kinase (TK) inhibitor, could directly activate cystic fibrosis transmembrane regulator (CFTR) chloride current in guinea pig ventricular myocytes. The activation showed concentration-dependent effect with the estimated IC50 of 39.7 microM. Tyrphostin 51, another TK inhibitor, had no effect, suggesting that genistein's effect might be unrelated to TK inhibition. After the chloride current had been activated by the maximally elevated intracellular cAMP content by saturating concentration of isoproterenol, forskolin and IBMX, genistein could further enhance the current. Pre-treatment with saturating concentration of a specific protein kinase A (PKA) inhibitor, H-89, or other protein kinase inhibitors H-8 and H-9 in the perfusate or intracellularly could not prevent the activation of the current by genistein, suggesting a PKA-independent activity. Furthermore, saturating concentration of calyculin A, a specific inhibitor of phosphotase 1 and 2A, in the perfusate or intracellularly could not block genistein's action. It is possible that genistein opens the channels directly or inhibits the dephosphorylation process of CFTR, which is not sensitive calyculin A.