Stabilization of Kv1.5 channel protein by bepridil through its action as a chemical chaperone

Uric Acid-Induced Enhancements of Kv1.5 Protein Expression and Channel Activity via the Akt-HSF1-Hsp70 Pathway in HL-1 Atrial Myocytes

BACKGROUND

Intracellular uric acid is known to increase the protein level and channel current of atrial Kv1.5; however, mechanisms of the uric acid-induced enhancement of Kv1.5 expression remain unclear.

METHODS AND RESULTS

The effects of uric acid on mRNA and protein levels of Kv1.5, as well as those of Akt, HSF1 and Hsp70, in HL-1 cardiomyocytes were studied by using qRT-PCR and Western blotting. The uptake of uric acid was measured using radio-labeled uric acid. The Kv1.5-mediated channel current was also measured by using patch clamp techniques. Uric acid up-taken by HL-1 cells significantly increased the level of Kv1.5 proteins in a concentration-dependent manner, with this increase abolished by an uric acid transporter inhibitor. Uric acid slowed degradation of Kv1.5 proteins without altering its mRNA level. Uric acid enhanced phosphorylation of Akt and HSF1, and thereby increased both transcription and translation of Hsp70; these effects were abolished by a PI3K inhibitor. Hsp70 knockdown abolished the uric acid-induced increases of Kv1.5 proteins and channel currents.

CONCLUSIONS

Intracellular uric acid could stabilize Kv1.5 proteins through phosphorylation of Akt and HSF1 leading to enhanced expression of Hsp70.

Long-term 4-AP treatment facilitates functional expression of human Kv1.5 channel

The human Kv1.5 channel (hKv1.5) produces the ultrarapid delayed rectifier potassium current (I_Kur), which is important for determining the repolarization of action potential in the cardiac atrium. However, the expression of I_Kur is reduced in patients with chronic atrial fibrillation. 4-Aminopyridine (4-AP) can specifically suppress I_Kur, suggesting that it modifies hKv1.5 as a chaperone molecule. Herein, the effects of long-term 4-AP treatment on hKv1.5 protein expression and function were investigated in HEK cells. 4-AP treatment (24 h) improved hKv1.5 protein levels, promoted hKv1.5 glycosylation, and facilitated the hKv1.5 current in a time-dependent manner. Long-term 4-AP treatment also markedly enhanced hKv1.5 localization in the cell membrane, endoplasmic reticulum, and Golgi. Importantly, the Ile508 residue located in the hKv1.5 channel pore was found to be important for 4-AP inhibitory activity. These results provide insight into developing hKv1.5 channel blocker that can functionally rescue I_Kur in patients with chronic atrial fibrillation.

Inhibitory effects of local anesthetics on the proteasome and their biological actions

Local anesthetics (LAs) inhibit endoplasmic reticulum-associated protein degradation, however the mechanisms remain elusive. Here, we show that the clinically used LAs pilsicainide and lidocaine bind directly to the 20S proteasome and inhibit its activity. Molecular dynamic calculation indicated that these LAs were bound to the β5 subunit of the 20S proteasome, and not to the other active subunits, β1 and β2. Consistently, pilsicainide inhibited only chymotrypsin-like activity, whereas it did not inhibit the caspase-like and trypsin-like activities. In addition, we confirmed that the aromatic ring of these LAs was critical for inhibiting the proteasome. These LAs stabilized p53 and suppressed proliferation of p53-positive but not of p53-negative cancer cells.

Short- and long-term inhibition of cardiac inward-rectifier potassium channel current by an antiarrhythmic drug bepridil

Bepridil is an effective antiarrhythmic drug on supraventricular and ventricular arrhythmias, and inhibitor of calmodulin. Recent investigations have been elucidating that bepridil exerts antiarrhythmic effects through its acute and chronic application for patients. The aim of this study was to identify the efficacy and the potential mechanism of bepridil on the inward-rectifier potassium channel in neonatal rat cardiomyocytes in acute- and long-term conditions. Bepridil inhibited inward-rectifier potassium current (I K1) as a short-term effect with IC50 of 17 μM. Bepridil also reduced I K1 of neonatal cardiomyocytes when applied for 24 h in the culture medium with IC50 of 2.7 μM. Both a calmodulin inhibitor (W-7) and an inhibitor of calmodulin-kinase II (KN93) reduced I K1 when applied for 24 h as a long-term effect in the same fashion, suggesting that the long-term application of bepridil inhibits I K1 more potently than that of the short-term application through the inhibition of calmodulin kinase II pathway in cardiomyocytes.