MECHANISMS OF CYTOKINE INJURY TO MYOCARDIUM IN SURGERY

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Depressed myocardial contractile function is often a critical determinant of outcome in patients after cardiac surgery and cardiopulmonary bypass, transplant rejection, sepsis, and inflammatory myocarditis. Although the etiologies of myocardial dysfunction in these various states is undoubtedly multifactorial, there is increasing evidence that one important common factor is the local production of high concentrations of cytokines by activated lymphocytes and macrophages. Cytokines exert their effects upon target cells (usually considered to be immune cells) by triggering specific intracellular signaling pathways that result in the activation of potent regulatory enzymes. Protein kinase C (PKC) is one such regulatory enzyme that has been shown to be central to the signal transduction pathway of many cytokines such as tumor necrosis factor, interleukin-1beta, interleukin-2, and interleukin-8. PKC is capable of exerting profound effects upon numerous aspects of cellular metabolism, including calcium regulation, substrate metabolism and mitochondrial function, and gene transcription and translation. The role of PKC in myocardial function is poorly understood, but there is substantial evidence to suggest that increased PKC activity can impair contractility and oxidative metabolism, as well as contribute to the development of myocardial hypertrophy. At present, the mechanisms for these effects are uncertain. We have recently shown that PKC translocation and activation occurs in the heart in response to pro-inflammatory cytokines, and that this is accompanied by profound contractile dysfunction, early oxygen wastage, accelerated conversion of glucose to lactate, and finally by inhibition of oxidative metabolism. These effects were prevented by inhibitors of PKC activation and by inhibitors of PKC enzyme activity. In isolated myocytes, cytokines such as lL-2, IL-8, or TNF stimulate translocation of the epsilon-isoform of PKC in a dose-dependent fashion and generate an active cleavage product that has been found in other tissues to have higher enzyme activity and reduced target specificity. We therefore hypothesize that PKC contributes in an important but poorly defined way to the genesis of inflammatory myocardial injury. Specifically, this project will use isolated working rabbit hearts and freshly isolated cultured rabbit myocytes to test the hypothesis that cytokines stimulate signal transduction mechanisms producing PKC activation, and that PKC-mediated phosphorylation of critical intracellular targets results in contractile protein and mitochondrial dysfunction. Our overall goal is to develop a more complete understanding of the role PKC plays in myocardial injury, and thereby develop rational and effective treatment strategies.