Clofibrate-induced apoptosis is mediated by Ca²⁺-dependent caspase-12 activation

The mechanism of fibrate-induced myopathy was investigated in this report. When clofibrate (30 to 300 μM) was applied to L6 rat skeletal myoblasts, dose-dependently apoptosis was observed within 24 h. In the apoptotic myoblasts, a caspase-12 cleavage was observed at 2 h and with following caspases-9 and -3-related cascade activation. In contrast, the neutral protease calpain, that is a key enzyme in ER stress-related apoptosis via caspase-12 activation, was significantly decreased during apoptosis. Next, the authors evaluated a role of calcium-dependent signal(s). When clofibrate was added into medium, cytosolic calcium concentration was rapidly and persistently increased. On the other hand, an addition of 10 mM EGTA depressed sustained calcium phase, and concurrent myoblasts apoptosis was completely inhibited. Taken together, our findings indicate that the clofibrate-induced myopathy is triggered by Ca²⁺ influx, then activated cytosolic caspase-12 through calpain-independent cascade, and consequently caused apoptotic DNA fragmentation.