The role of cytokines in the catabolic consequences of infection and injury

During infection and injury a series of metabolic events are activated that leads to a state of negative nitrogen balance and significant loss of lean body mass. This process is characterized by marked anorexia, net whole body protein breakdown, and liver anabolism. This host response initially is beneficial to the body because it helps it to fight disease and enhance healing. However, if such imbalance is maintained for long periods, it will invariably produce significant loss of lean body mass that may lead to a series of untoward clinical events. The role of the proximate cytokines, tumor necrosis factor (TNF), interleukin-1 (IL-1), and interleukin-6 (IL-6) as well as glucocorticoids as important mediators of many pathophysiological manifestations of infection and injury has been studied extensively. However, the involvement of other mediators, at least in skeletal muscle proteolysis during sepsis has been hypothesized, because blockade of glucocorticoids, TNF, IL-1, and IL-6 reduces but does not normalize protein breakdown rates nor does the direct application of these mediators to skeletal muscle in vitro enhance proteolysis. Furthermore other studies have suggested that the lymphokine, interferon-gamma (IFN-gamma, type II interferon or immune interferon), produces fever and enhances thermogenesis, body weight loss, and skeletal muscle depletion in rodents in a manner similar to that seen with TNF and IL-1. Cytokines appear to be major components of the host metabolic response during infection and injury. However, neither all the cytokines involved nor the exact mechanisms underlying their metabolic effects are completely understood. The regulation of muscle protein synthesis and breakdown, which largely determines the development of cachexia, appears to depend on the delicate balance between a number of regulatory substances including cytokines, glucocorticoids, catecholamines, insulin, and insulin-like growth factors.