Okadaic Acid and Anisomycin are Protective and Stimulate the SAPK/JNK Pathway

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Abstract
We report that okadaic acid (OA), a known inhibitor of Ser/Thr phosphatases, protects pig myocardium against ischemic injury in an in vivo model and stimulates the activities of stress-activated protein kinases/c-Jun N-terminal kinases (SAPKs/JNKs). When OA was directly infused into the subsequently ischemic myocardium for 60 min before a 60-min period of coronary occlusion followed by reperfusion, infarct size was reduced from a control value of 83.4 ± 2.8% of the risk region to 40.7 ± 9.1%. When OA was infused for 10 min before a 5-min occlusion and during 45 min thereafter, infarct size was reduced to 26.5%. In a separate set of similar experiments, we pretreated pig hearts in vivo with the protein-synthesis inhibitor and known activator of SAPK/JNK, anisomycin (AN), and found that this compound also significantly reduced infarct size from 83.4 ± 2.8.1% to 48.1 ± 5.1%. For in vitro assays, OA (600 nM), AN (500 μM), or solvent (KHB) were locally infused into the left ventricular myocardium, and biopsies from in situ beating hearts were obtained after 10, 30, and 60 min of infusion. The activities of Ser/Thr phosphatases (PPases), especially PP-2A, were significantly decreased after OA infusion. OA infusion increased the activity (in-gel phosphorylation of N-terminal c-Jun1-135) of both 46- and 55-kDa SAPK/JNKs (twofold to threefold, 30 and 60 min of infusion), and this increase correlated well with the observed decrease of PPase activities. Western blot analysis with a phosphospecific SAPK/JNK (Thr 183/Tyr 185) antibody showed an increased content of the phosphorylated forms after OA treatment. We observed significant stimulation of SAPK/JNK activity also after AN treatment (threefold to fourfold, after 30 min of infusion). In contrast to the SAPK/JNKs, the infusion of both OA and AN did not significantly change the activities and phosphorylation of extracellular signal-related kinases (ERKs) and p38-MAPK. The findings that the protective effect of both OA and AN correlates with increased activity of SAPK/JNKs suggest the involvement of these enzymes in the mechanism of cardioprotection.