Degradation Of hERG Channels Research Articles

Intracellular Mechanism of Rosuvastatin-Induced Decrease in Mature hERG Protein Expression on Membrane.

The hERG potassium channel (IKr) encoded by human ether-a-go-go-related gene plays an important role in cardiac repolarization. Decreased IKr may lead to long QT syndrome, which subsequently causes torsade de pointes and sudden cardiac death. Previous studies have shown that statins inhibit IKr and are more potent in inhibiting hERG currents when combined with other drugs. Since chemical structure of rosuvastatin is similar to that of several IKr blockers (ibutilide and E-4031), the present study aimed to reveal the mechanism that underlies rosuvastatin-induced hERG current reduction and to evaluate the possibility of cardiac toxicity. The results showed that rosuvastatin reduced hERG currents by accelerating the inactivation and prolonged action potential duration (APD) in hiPSC-CMs. Meanwhile, it was observed that rosuvastatin reduced the expression of the mature hERG. Transcription factor Sp1 was involved in hERG protein downregulation induced by rosuvastatin, and the result was verified by Sp1 siRNA and Sp1 agonist epicatechin. These results indicated that rosuvastatin could potentially inhibit transcription and reduce hERG mRNA expression. The interaction between hERG and heat shock protein was evaluated to study the mechanism of trafficking inhibition through co-immunoprecipitation. We found that rosuvastatin reduces the interaction of heat shock protein 70 (Hsp70) with the hERG protein, thereby affecting the folding of the hERG channel. Additionally, rosuvastatin significantly activates ATF6, which plays a key role in the activation of the unfolded protein response (UPR) pathway. Increased expression of the molecular chaperone calnexin and calreticulin, which are activated by ATF6 to help channel folding, further confirmed UPR activation. Meanwhile, the degradation of the hERG channel was mediated by lysosomes and proteasomes. In conclusion, Rosuvastatin reduced the expression of hERG plasma membrane by two pathways, the first is to disrupt the transport of immature hERG channels to the membrane, and the second is to increase the degradation of mature hERG channels. In addition, Rosuvastatin potently blocked hERG current, delayed cardiac repolarization, and thereby prolonged APDs and QTc intervals. Therefore, caution should be taken when rosuvastatin is used in the treatment of hyperlipidemia, especially when combined with drugs that can prolong the QT interval.

Degradation of Trafficking-defective Long QT Syndrome Type II Mutant Channels by the Ubiquitin-Proteasome Pathway

Mutations in the human ether-a-go-go-related gene (hERG) cause chromosome 7-linked long QT syndrome type II (LQT2). We have shown previously that LQT2 mutations lead to endoplasmic reticulum (ER) retention and rapid degradation of mutant hERG proteins. In this study we examined the role of the ubiquitin-proteasome pathway in the degradation of the LQT2 mutation Y611H. We showed that proteasome inhibitors N-acetyl-L-leucyl-L-leucyl-L-norleucinal and lactacystin but not lysosome inhibitor leupeptin inhibited the degradation of Y611H mutant channels. In addition, ER mannosidase I inhibitor kifunensine and down-regulation of EDEM (ER degradation-enhancing alpha-mannosidase-like protein) also suppressed the degradation of Y611H mutant channels. Proteasome inhibition but not mannosidase inhibition led to the accumulation of full-length hERG protein in the cytosol. The hERG protein accumulated in the cytosol was deglycosylated. Proteasome inhibition also resulted in the accumulation of polyubiquitinated hERG channels. These results suggest that the degradation of LQT2 mutant channels is mediated by the cytosolic proteasome in a process that involves mannose trimming, polyubiquitination, and deglycosylation of mutant channels.