Statins are a family of cholesterol lowering drugs associated with undisputable cardiovascular benefits. However, their use is associated with the risk of developing skeletal muscle myopathy which remains a significant barrier to maximizing the efficacy of statin therapy. Many recognize that the instances of statin-associated myopathy reported grossly underestimate the actual frequency of these events. Statin myopathy is a major reason for why many patients are unable to comply with their prescribed statin treatments. Identifying and understanding the molecular events involved in the development of statin-induced myopathy will pave the way to mitigating the unwanted effect of statin-myopathy.
We have identified that statins induce mitochondrial loss in both C2C12 myotubes, and in gastrocnemius muscle of mice evidenced by loss of mitochondrial markers Tom70 and Cox4 by western blot analysis. C2C12 myotubes were treated with 2μM simvastatin for 24 hours, and wild-type C57BL/6 mice were given a daily i.p. dose of 20mg/kg simvastatin for 2 weeks. Pretreatment of C2C12 cells with 100nM bafilomycin for 2 hours before statin treatment attenuated autophagic flux, and prevents the observed loss of mitochondria through Tom70 and Cox4 mitochondrial markers. Thus, the loss of mitochondria is attributable to autophagy (mitophagy). Statin treatment increases the expression of Bnip3 and p62/SQSTM1 (sequestosome 1) which have been identified as important facilitators of mitophagy. We found increased Bnip3 dimerization via western blot under non-reducing conditions which implies increased activity. Furthermore, we observed increased mitochondrial translocation of p62/SQSTM1. Based on our findings, we hypothesized that statin-induced mitophagy may play a key role in the development of muscle myopathy.
To investigate the role of mitophagy in statin myopathy, we silenced p62/SQSTM1 via RNAi prior to statin treatment. Surprisingly, reducing mitophagic flux via p62/SQSTM1 silencing resulted in increased destruction of myotubes. This was associated with increased levels of cytosolic cytochrome c. Our data combined suggests that mitophagy protects against statin-mediated cell death by sequestering and eliminating decrepit mitochondria via autophagy.