Description
Statins are widely used drugs which reduce levels of circulating cholesterol, limiting atherosclerosis and the development of cardiovascular disease. Statins inhibit HMG-CoA reductase, the rate-limiting enzyme in cholesterol synthesis. Although statins have cardiac benefit, they can cause skeletal muscle myopathy through an unidentified mechanism. We recently showed that acute cardioprotection by statins requires selective autophagic clearance of mitochondria (mitophagy). Autophagy is a homeostatic process for recycling protein aggregates and damaged organelles via lysosomes. Interestingly, it has been observed by others that, the regulation of mitochondrial turnover, in response to statin challenge, differs in heart and skeletal muscle: while promoting mitochondrial biogenesis through PGC1_ in the heart, statins inhibit its expression in skeletal muscles. Therefore, we hypothesized that mitophagy, which is beneficial in heart, may contribute to statin-mediated myopathy. To investigate this, we used C2C12 myoblasts (a skeletal muscle cell line) which were differentiated for 6 days into myotubes. Cells were treated with 2µM simvastatin for 24h. Statins attenuated Akt/mTOR/ULK1 signaling and increased levels of the autophagic marker LC3-II, indicating increased autophagy. Besides cholesterol, statins can reduce synthesis of coenzyme Q10, which is required for normal mitochondrial function. The combination of upregulated autophagy and mitochondrial dysfunction sets the stage for statin-mediated mitophagy. To assess mitophagy and autophagic flux, we measured accumulation of p62/SQSTM1 in the mitochondrial fraction in the presence and absence of the lysosomal inhibitor bafilomycin. Blockade of autophagic flux with bafilomycin revealed increased levels of p62/SQSTM1 in the mitochondrial fraction, consistent with increased mitophagy. Attenuating mitophagy by silencing p62/SQSTM1 exacerbated statin-mediated apoptosis and cell death as evidenced by increased lactate dehydrogenase release and caspase 3 cleavage in statin-treated cells. Increased FoxO3 transcriptional activity is known to induce expression of numerous genes which are involved in the ubiquitin proteosome system and autophagy. Thus FoxO3 is considered a key regulator of the muscle wasting process. FoxO transcription factors can be activated in low energy states through AMPK. Because we found decreased energy levels, we investigated the activation of FoxO and found that indeed statins increased their transcriptional activity. Our unexpected results suggest that mitophagy protects cells from the myotoxic effects of statins.
