Role of γ-glutamyl transpeptidase in redox regulation of K⁺ channel remodeling in postmyocardial infarction rat hearts

Abstract

-␥-Glutamyl transpeptidase (␥-GT) is a key enzyme in GSH metabolism that regulates intracellular GSH levels in response to extracellular GSH (GSHo). The objective of this study was to identify the role of ␥-GT in reversing pathogenic K ϩ channel remodeling in the diseased heart. Chronic ventricular dysfunction was induced in rats by myocardial infarction (MI), and studies were done after 6 -8 wk. Biochemical assays of tissue extracts from post-MI hearts revealed significant increases in ␥-GT activity in left ventricle (47%) and septum (28%) compared with sham hearts, which paralleled increases in protein abundance and mRNA. Voltage-clamp studies of isolated left ventricular myocytes from post-MI hearts showed that downregulation of transient outward K ϩ current (Ito) was reversed after 4 -5 h by 10 mmol/l GSHo or N-acetylcysteine (NACo), and that the effect of GSHo but not NACo was blocked by the ␥-GT inhibitors, acivicin or S-hexyl-GSH. Inhibition of ␥-glutamylcysteine synthetase by buthionine sulfoximine did not prevent upregulation of Ito by GSHo, suggesting that intracellular synthesis of GSH was not directly involved. However, pretreatment of post-MI myocytes with an SOD mimetic [manganese (III) tetrapyridylporphyrin] and catalase completely blocked recovery of Ito by GSHo. Confocal microscopy using the fluorogenic dye 2Ј,7Ј-dichlorodihydrofluorescein diacetate confirmed that GSHo increased reactive oxygen species (ROS) generation by post-MI myocytes and to a lesser extent in myocytes from sham hearts. Furthermore, GSHomediated upregulation of Ito was blocked by inhibitors of tyrosine kinase (genistein, lavendustin A, and AG1024) and thioredoxin reductase (auranofin and 13-cis-retinoic acid). These data suggest that GSHo elicits ␥-GT-and ROS-dependent transactivation of tyrosine kinase signaling that upregulates K ϩ channel activity or expression via redox-mediated mechanisms. The signaling events stimulated by ␥-GT catalysis of GSHo may be a therapeutic target to reverse pathogenic electrical remodeling of the failing heart. glutathione; voltage-dependent K ϩ channel; thioredoxin; transient outward current CHRONIC MYOCARDIAL INFARCTION (MI)-induced ventricular dysfunction elicits a pathogenic process of electrical remodeling that is characterized at the myocyte level by downregulation of K ϩ channel expression. This change in electrophysiological phenotype is proposed to contribute to arrhythmogenic abnormalities in repolarization (5) and cellular alterations in Ca 2ϩ handling that impact contractile function (45). Recent studies from our laboratory suggest that redox-mediated mechanisms underlie K ϩ channel remodeling by oxidative stress in the rat heart with chronic MI (36, 37). It is proposed that redox control of K ϩ channels in heart involves endogenous thiol oxidoreductase systems, particularly 1) the thioredoxin (Trx) system, which operates with thioredoxin reductase (TrxR) and NADPH to catalyze the reduction of protein disulfides (20,30,32,40,47), and 2) the glutaredoxin (Grx) system, which uses re...

Elevated γ-glutamyltransferase in implantable cardioverter defibrillator patients

“…Based on fi ndings in rats that GGT interacts with redox regulation of potassium channels in the postmyocardial infarction heart, we speculated that elevated GGT may predispose to ventricular tachyarrhythmia [16]. Our study, however, does not support this notion, as appropriate ICD therapy did not correlate with GGT levels.…”

Role of Apoptosis Signal-Regulating Kinase-1-c-Jun NH2-Terminal Kinase-p38 Signaling in Voltage-Gated K+Channel Remodeling of the Failing Heart: Regulation by Thioredoxin

“…6), and thus the precise relationship between the Trx system and Kv4 channel expression is unclear. We reported previously that short-term (4-6 h) treatment of isolated ventricular myocytes from sham hearts with AF did not significantly decrease I to density (46), but in another study we showed that normal rats treated for 3 days with buthionine sulfoximine (a blocker of GSH synthesis) plus 1,3-bis-(2-chloroethyl)-1-nitrosourea (an inhibitor of TrxR and glutathione reductase) had marked decreases in Kv4 channel mRNA, protein abundance, and I to density in the LV (21). In agreement with the present study, downregulation of I to density with oxidoreductase inhibition was also reversed in vitro by stimulation of receptor tyrosine kinase signaling with the insulin-mimetic, bpV(phen) (21).…”

Pyruvate restores β-adrenergic sensitivity of L-type Ca2+channels in failing rat heart: role of protein phosphatase

“…In previous studies, we also documented the pathogenic condition of oxidative stress in this MI model. In particular, there is marked impairment of the thioredoxin and glutaredoxin systems in the surviving regions of the left ventricle (24,31,32), and isolated myocytes from post-MI hearts are characterized by significantly increased ROS levels and lower concentrations of reduced glutathione (GSH) (23,42).…”