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1 Internal Medicine, The Ohio State University, Columbus, OH, USA
* To whom correspondence should be addressed. E-mail: ilangovan-1{at}medctr.osu.edu.
A mild heat shock (hyperthermia) protects cells from apoptotic and necrotic deaths by inducing over expression of various heat shock proteins (Hsps). These proteins, in combination with the activation of the nitric oxide synthase (NOS) enzyme, play important roles in the protection of the myocardium against a variety of diseases. In the present work we report that the generation of potent reactive oxygen species (ROS), namely .OH in cardiac H9c2 cells, is attenuated by heat shock treatment (2 h at 42°C). Western blots showed that heat shock treatment induced over expression of Hsp70, Hsp60 and Hsp25. The observed .OH was found to be derived from the superoxide (O2.-) generated by the mitochondria. While the manganese superoxide dismutase (MnSOD) activity was increased in the heat-shocked cells, the mitochondrial aconitase activity was reduced. The mechanism of O2.- conversion into .OH in mitochondria is proposed as follows. The O2.- leaked from the electron transport chain (ETC), oxidatively damages the m-aconitase, releasing a free Fe2+. The aconitase-released Fe2+ combines with H2O2 to generate .OH via a Fenton reaction and the oxidized Fe3+ recombines with the inactivated enzyme after getting reduced to Fe2+ by other cellular reductants, turning it over to be active. However, in heat-shocked cells, because of higher MnSOD activity, the excess H2O2 causes irreversible damage to the m-aconitase enzyme inhibiting its activity. In conclusion, we propose that attenuation of .OH generation after heat shock treatment might play an important role in reducing the myocardial ischemic injury, observed in heat shock treated animals.
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