Pathogenesis of cancer cachexia.

Cachexia is a progressive wasting syndrome characterized by extensive loss of adipose tissue and skeletal muscle. It occurs in about half of all cancer patients. While anorexia also may be present, the energy deficit alone does not explain the pathogenesis of cachexia. The presence of an acute phase response (APR) has been linked to accelerated weight loss and a shortened survival time. The APR is thought to be initiated by cytokines such as interleukin (IL)-6 and IL-8, production of which is induced by a tumor factor, proteolysis inducing factor (PIF). Cachectic cancer patients also show an increased expression of uncoupling protein-3 in muscle, which may act as an energy sink, increasing energy expenditure. Loss of adipose tissue appears to be due to an increase in degradation of triglycerides, rather than a decrease in synthesis. One candidate for this effect is a tumor lipid mobilizing factor, which stimulates lipolysis directly through a cyclic AMP-mediated process via interaction with a beta3-adrenergic receptor. Loss of skeletal muscle arises from both a depression in protein synthesis and an increase in protein degradation. The major proteolytic pathway involved in intracellular protein breakdown in cachectic muscle is the ATP-ubiquitin-dependent proteolytic pathway. Both PIF and tumor necrosis factor-alpha, but not other cytokines, can induce expression of the key regulatory components of this pathway. Eicosapentaenoic acid, found in oily fish, effectively attenuates protein degradation in cachectic muscle by inhibiting the increased proteasome expression and can stabilize body weight in cachectic cancer patients.