Calpain-mediated proteolytic cleavage of troponin I induced by hypoxia or metabolic inhibition in cultured neonatal cardiomyocytes.

While ischemic damage to myofibrillar proteins is thought to be responsible in part for depressed cardiac function, the relation between myofilament protein breakdown and chronic hypoxia has not been defined. We previously characterized a chemical hypoxia model of neonatal cardiomyocytes mediated by 1 mM azide that exhibits features of calpain activation (Mol Cell Biochem 178:141-149, 1998). We here show that both hypoxia and azide-mediated metabolic inhibition induced heme oxygenase-1 expression, and caused cell death associated with lipid peroxidation. While blocking calcium influx or inhibiting calpain activity efficiently attenuated hypoxia-induced cell injury, it failed to prevent cell injury caused by adenoviral overexpression of the tumor suppressor protein p53. Inhibitors of caspases, on the other hand, suppressed cell injury caused by p53 overexpression. Hypoxia caused selective cleavage of troponin I (TnI), which could be suppressed by either nifedipine or calpeptin. Other myofilament proteins such as troponin T, myosin heavy chain, and actin appeared to remain largely intact. p53-mediated cell injury exhibited proteolysis of the caspase protein substrate lamin B without appreciable breakdown of TnI. We suggest that calpain-induced TnI breakdown may constitute a unique biochemical marker associated with chronically hypoxic cardiomyocytes.