Cinobufagin induces acute promyelocytic leukaemia cell apoptosis and PML-RARA degradation in a caspase-dependent manner by inhibiting the β-catenin signalling pathway

CONTEXT

Acute promyelocytic leukaemia (APL) is a malignant hematological tumour characterized by the presence of promyelocytic leukaemia-retinoic acid receptor A (PML-RARA) fusion protein. Cinobufagin (CBG) is one of the main effective components of toad venom with antitumor properties. However, only a few reports regarding the CBG treatment of APL are available.

OBJECTIVE

We explored the effect and mechanism of action of CBG on NB4 and NB4-R1 cells.

MATERIALS AND METHODS

We evaluated the viability of NB4 and NB4-R1 cells treated with 0, 20, 40, and 60 nM CBG for 12, 24, and 48 h. After treatment with CBG for 24 h, Bcl-2 associated X (Bax), B-cell lymphoma 2 (Bcl-2), β-catenin, cyclin D1, and c-myc expression was detected using western blotting and real-time polymerase chain reaction. Caspase-3 and PML-RARA expression levels were detected using western blotting.

RESULTS

CBG inhibited the viability of NB4 and NB4-R1 cells. The IC₅₀ values of NB4 and NB4-R1 cells treated with CBG for 24 h were 45.2 nM and 37.9 nM, respectively. CBG induced NB4 and NB4-R1 cell apoptosis and PML-RARA degradation in a caspase-dependent manner and inhibited the β-catenin signalling pathway.

DISCUSSION AND CONCLUSION

CBG induced NB4 and NB4-R1 cell apoptosis and PML-RARA degradation in a caspase-dependent manner by inhibiting the β-catenin signalling pathway. This study proposes a novel treatment strategy for patients with APL, particularly those with ATRA-resistant APL.

Allosteric effects of mutations in the extracellular S5-P loop on the gating and ion permeation properties of the hERG potassium channel

The hERG channel has an unusually long (39 amino acids) extracellular loop between the transmembrane S5 segment and the pore region that may play a role in channel function. We explored this possibility by mutating two histidine residues in this region (H578 and H587, referred to as H1 and H2) to various residues and examined the resulting changes in channel function. Both positions could tolerate drastic changes in side-chain properties (proline, cysteine, glutamate and lysine), indicating that they are solvent exposed. None of the H1 mutations affected hERG channel function. On the other hand, although replacing H2 with glutamate had little or no effect on hERG properties, putting a proline or lysine at this position disrupted the C-type inactivation process and the pore's K selectivity. There was also a hyperpolarizing shift in the voltage dependence of activation. The phenotype of the H2C mutant was similar to that of H2P or H2K. However, dithiothreitol (DTT, a thiol-reducing agent) treatment converted the H2C phenotype to that of the wild-type channel. These observations suggest that the peptide backbone conformation around position 587 in the extracellular S5-P loop of hERG channel can affect the channel's gating and ion selectivity functions.

Use-dependent 'agonist' effect of azimilide on the HERG channel

Azimilide (AZ) is a class III antiarrhythmic drug that has voltage-dependent dual effects on the HERG channel: 1) increasing current amplitude at low-voltage depolarization (agonist effect), and 2) suppressing current at more depolarized voltages (antagonist effect). We examined the mechanism for the agonist effect of AZ on HERG expressed in Xenopus oocytes. The agonist effect resulted from an AZ-induced 'prepulse potentiation: a strong depolarization prepulse increased the rate and degree of channel activation induced by subsequent depolarization to -50 or -40 mV. The potentiated state decayed slowly in an exponential fashion (time constant, 60-80 s). Degrees of potentiation were proportional to degrees of channel activation during prepulses; hence, the agonist effect of AZ was use dependent. AZ exerted its agonist effect from outside the cell membrane, and the effect did not depend on intracellular G-protein or protein kinase activity. Mutations made in the outer mouth or an extracellular loop connecting the S5 and P regions of HERG, which could hinder or modify conformational changes in the pore region during membrane depolarization, reduced or abolished AZ-induced prepulse potentiation. Importantly, these same mutations also increased the rate and degree of channel activation in the negative voltage range, and the degree of change in the activation properties was inversely correlated with the degree of AZ-induced prepulse potentiation. We propose that conformational changes in the outer mouth and neighboring extracellular domain of HERG during membrane depolarization can affect the process of channel activation. In the presence of AZ, channel activation allowed drug modification of these conformational changes, which subsequently facilitated HERG activation by low-voltage depolarization.

Mechanism for the effects of extracellular acidification on HERG-channel function

Human ether-à-go-go-related gene (HERG) encodes a K channel similar to the rapid delayed rectifier channel current (I(Kr)) in cardiac myocytes. Modulation of I(Kr) by extracellular acidosis under pathological conditions may impact on cardiac electrical activity. Therefore, we studied the effects of extracellular acidification on I(Kr) function and the underlying mechanism, using HERG expressed in Xenopus oocytes as a model. Acidification [extracellular pH (pH(o)) 8.5-6.5] accelerated HERG deactivation (at -80 mV, the time constant tau of the major component of deactivation was 253 +/- 17, 158 +/- 10, and 65 +/- 5 ms at pH(o) 8.5, 7.5, and 6.5, respectively; n = 7-10 each), with no effects on other gating kinetics except a modest acceleration of recovery from inactivation (at -80 mV, tau of recovery was 4.7 +/- 0.3, 3.8 +/- 0.3, and 1.3 +/- 0.2 ms at pH(o) 8. 5, 7.5, and 6.5, respectively; n = 4-7 each). The following were ruled out as the underlying mechanisms: 1) voltage shift in channel activation, 2) pore blockade by protons, 3) protonation of histidines on the extracellular domain of HERG, 4) acceleration of recovery from C-type inactivation, and 5) interaction between an external H(+) binding site and the cytoplasmic NH(2)-terminal domain (a key determinant of HERG deactivation rate). Extracellular application of diethylpyrocarbonate caused an irreversible acceleration of HERG deactivation and prevented further acceleration by external acidification. Our data suggest that side chains accessible to the extracellular solution mediated the effects of elevating extracellular H(+) concentration on channel deactivation.

Inhibition of myocardial inward rectifier potassium current by propylbutyldopamine

AIM

To study the effects of propylbutyldopamine (PBDA) on the inward rectifier potassium current (Ik1).

METHODS

The quasi-steady state current-voltage relationship from the isolated guinea pig ventricular cells were measured using whole-cell patch-clamp techniques with a slow ramp depolarization (8 mV.s-1).

RESULTS

PBDA 5, 50, and 100 mumol.L-1 concentration-dependently reduced the inward rectifier potassium current. PBDA blocked Ik1 in guinea pig ventricular cells. The effect of PBDA was not blocked by the selective dopamine D2-receptor blocker, domperidone.

CONCLUSION

PBDA inhibited Ik1 directly, independent of the dopamine D2-receptor.

Effects of outer mouth mutations on hERG channel function: a comparison with similar mutations in the Shaker channel

The fast-inactivation process in the hERG channel can be affected by mutations in the pore or S6 domain, similar to the C-type inactivation in the Shaker channel. However, differences in the kinetics and voltage dependence of inactivation between these two channels suggest that different structural determinants may be involved. To explore this possibility, we mutated a serine in the outer mouth region of hERG (S631) to residues of different physicochemical properties and compared the resulting changes in the channel's inactivation process with those resulting from mutations of an equivalent position in the Shaker channel (T449). The most dramatic differences are seen when this position is occupied by a charged residue: S631K and S631E disrupted C-type inactivation in hERG, whereas T449K and T449E facilitate C-type inactivation in Shaker. S631K and S631E also disrupted the K selectivity of hERG pore, a change not seen in T449K or T449E of Shaker. To further study why there are such differences, we replaced S631 with cysteine. This allowed us to manipulate the properties of thiol groups at position 631 and correlate side-chain properties here with changes in channel function. S631C behaved like the wild-type channel when the thiol groups were in the reduced state. Oxidizing thiol groups with H2O2 or modifying them with MTSET or MTSES disrupted C-type inactivation and K selectivity, similar to the phenotype of S631K and S631E. The same thiol-modifying maneuvers did not affect the wild-type channel function. Our results suggest differences in the outer mouth structure between hERG and Shaker, and we propose a "molecular spring" hypothesis to explain these differences.

Ischemic preconditioning mediated by activation of KATP channels in rat small intestine

AIM

To study whether the protective effects of ischemic preconditioning against rat small intestine ischemia/reperfusion injury could be mediated by KATP channel opener.

METHODS

Preconditioning (Pc) was induced by 3 cycles of 8-min superior mesenteric artery (SMA) occlusion and 10-min reperfusion before prolonged ischemia. Cromakalim (Cro 75 micrograms.kg-1) and glibenclamide (Gli 8 mg.kg-1) were injected i.v. 10 min before prolonged ischemia and Pc, respectively.

RESULTS

Compared with ischemic reperfusion (IR) group, Pc before prolonged ischemia (Pc + IR) decreased LDH release [(380 +/- 55) vs (559 +/- 49) U.L-1, P < 0.05], attenuated intestinal edema [wet weight/dry weight (WW/DW), 5.6 +/- 0.6 vs 6.34 +/- 0.29, P < 0.05], ameliorated intestinal histological damage (grading scale, 3.4 vs 5.7, P < 0.01), and improved reperfusion-induced hypotension. These effects of Pc were mimicked by Cro [LDH, (298 +/- 40) vs (559 +/- 49) U.L-1, P < 0.05; WW/DW, 5.6 +/- 0.4 vs 6.34 +/- 0.29, P < 0.05; grading scale, 3.6 vs 5.7, P < 0.01] and abolished in the presence of Gli [LDH, (624 +/- 44) vs (559 +/- 49) U.L-1; WW/DW, 6.6 +/- 0.6 vs 6.34 +/- 0.29; grading scale, 5.7 vs 5.7; P > 0.05] compared with IR group, respectively.

CONCLUSION

Ischemic preconditioning on the rat small intestine is mediated by activation of KATP channels.

Effects of norepinephrine and isopentenyladenosine on Na+/Ca2+ exchange currents in isolated guinea pig ventricular myocytes

AIM

To study the effects of norepinephrine (NE) and isopentenyladenosine (Iso) on Na+/Ca2+ exchange currents and the receptor mechanism.

METHODS

The quasi-steady state current-voltage relationship from the isolated guinea pig ventricular myocytes was measured using whole-cell voltage-clamp techniques with a ramp pulse protocol.

RESULTS

At potential of +50 mV, NE 0.005, 0.05, and 5 mumol.L-1 increased the Ni(2+)-sensitive current by 29% +/- 9%, 72% +/- 11%, and 124% +/- 31.4%, respectively; Iso 1.5, 150, and 1500 nmol.L-1 caused increases in the Ni(2+)-sensitive current by 2.8% +/- 2.8%, 56% +/- 13%, and 102% +/- 12%, respectively. Propranolol 10 mumol.L-1 completely inhibited the current changes induced by NE and Iso while phentolamine 50 mumol.L-1 showed no effects.

CONCLUSION

NE and Iso increased the Na+/Ca2+ exchange currents via stimulation of cardiac beta-adrenoceptor.

Blocking effects of phentolamine on L-type calcium current and ATP-sensitive potassium current in guinea pig ventricular myocytes

AIM

To study the effect of phentolamine on L-type calcium currents (ICa) and ATP-sensitive K+ currents (IK,ATP) in ventricular myocytes.

METHODS

ICa and IK,ATP were observed using patch clamp techniques in whole-cell recording configuration.

RESULTS

Phentolamine reduced ICa of ventricular myocytes in concentration-dependent and voltage-independent manners. Phentolamine 5, 25, and 100 mumol.L-1 decreased ICa from 370 +/- 99 nA to 310 +/- 95 nA (17% block, n = 6, P < 0.01), from 230 +/- 98 nA to 180 +/- 73 nA (23% block, n = 5, P < 0.05), and from 293 +/- 66 nA to 206 +/- 44 nA (30% block, n = 5, P < 0.01), respectively, without affecting the current-voltage relationship. Prazosin 100 mumol.L-1 and yohimbine 100 mumol.L-1, which were specific blockers of alpha 1 and alpha 2 adrenoceptors respectively, did not show the inhibitory effect on ICa. Phentolamine 100 mumol.L-1 also inhibited the IK,ATP induced by 2, 4-dinitrophenol (DNP) at 0 mV from 3.2 +/- 0.6 nA to 0.8 +/- 0.5 nA (75% block, n = 4, P < 0.01).

CONCLUSION

Phentolamine directly inhibits ICa and IK,ATP in guinea pig ventricular myocytes.

Effects of dopexamine hydrochloride on calcium channels in isolated guinea pig ventricular myocytes

The mechanisms responsible for the cardiac positive inotropic effects of dopexamine hydrochloride, a combined dopamine receptor agonist at both D1-receptors and beta 2-adrenoceptors, were studied. The calcium channel currents were recorded using whole-cell patch clamp technique in isolated guinea pig ventricular myocytes. At a holding potential of -40 mV, cells were depolarized to 0 mV for 400 ms at a frequency of 0.2 Hz. Dopexamine hydrochloride at doses of 5, 50 and 100 microM increased the verapamil-sensitive Ca2+ inward current by 109, 147 and 194%, respectively. The effects of dopexamine hydrochloride on Ca2+ current reached its maximum at 5 min and partially recovered after washout of the drugs. The increased Ca2+ current induced by dopexamine hydrochloride was completely inhibited by 20 microM propranolol, a beta-adrenoceptor antagonist, and was not antagonized by 20 microM of SCH23390, a highly selective D1-receptor antagonist. These results suggest that the cardiac positive inotropic effects of dopexamine hydrochloride are brought about by the increase of Ca2+ current via stimulation of beta 2-adrenoceptors.

Protection of methylflavonolamine against acute cerebral ischemia reperfusion injury in rats

AIM

To examine the possible beneficial action of methylflavonolamine (MFA) on cerebral ischemia/reperfusion injury.

METHODS

Acute cerebral ischemia-reperfusion injury was produced by 4-vessel occlusion and subsequent 1-h release. MFA, 20 mg kg-1, was injected intravenously 5 min before occlusion and again before release.

RESULTS

The brain water content in the reperfusion group (Rep) was elevated (82.7% +/- 1.1% vs control 79.7% +/- 0.5%, P < 0.01), while MFA alleviated the brain edema (80.9% +/- 0.9% vs Rep, P < 0.01). The CK level of brain tissue in Rep decreased (4.7 +/- 1.4 vs control 8.4 +/- 1.2 U/mg protein, P < 0.01), but MFA restored it (7.2 +/- 1.1 U/mg protein vs Rep, P < 0.01). Reperfusion caused the rise of lipid peroxides (2.3 +/- 0.5 vs control 1.5 +/- 0.4 nmol/mg protein, P < 0.01) and weakened the superoxide dismutase (SOD) (3.1 +/- 1.6 vs control 10.5 +/- 3.9 U/mg protein P < 0.01), MFA reduced the rise of lipid peroxides (1.6 +/- 0.4 nmol/mg protein vs Rep, P < 0.05) and protected the activity of SOD (7.9 +/- 1.6 U/mg protein vs Rep, P < 0.01) in brain.

CONCLUSION

MFA has the protective effects on cerebral ischemia/reperfusion, and these effects are relative to scavenging free radicals and anti-lipid peroxidation.

[Anti-arrhythmia and anti-lipid peroxidation effects of methylflavonolamine]

Effects of methylflavonolamine (MFA) on arrhythmias induced by myocardial reperfusion were studied with rat hearts in situ and in vitro. In pentobarbital-anesthetized rats, MFA (20 mg.kg-1, i.v.) pretreatment reduced the incidence of reperfusion-induced ventricular fibrillation after left descending coronary artery ligation (15 min) and reperfusion (3 min) (28.6% vs 85.7% in control, P less than 0.05). Malondialdehyde (MDA) production (85 +/- 9 nmol/g wet wt) was inhibited in myocardium from the reperfused area in comparison with control (133 +/- 15 nmol/g wet wt). In isolated rat hearts with local ischemia (15 min) and reperfusion (1 min), MFA 5 mumol.L-1 (perfused 10 min prior to coronary artery ligation) prevented reperfusion-induced arrhythmias (0% vs 85.7% in control, P less than 0.01). In myocardium from the reperfused area, superoxide dismutase (SOD) and catalase (Cat) activity was increased and xanthine oxidase (XOD) activity, MDA production and nonesterified fatty acids (NEFA) contents were decreased. The results show that MFA prevents reperfusion-induced arrhythmia by inhibiting lipid peroxidation and regulating the metabolism of NEFA.