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Nutritional supplementation or pharmacological manipulation of appetite are unable to control the muscle atrophy seen in cancer cachexia. This suggests that tumour and/or host factors might be responsible for the depression in protein synthesis and the increase in protein degradation. An increased expression of the ubiquitin–proteasome proteolytic pathway is responsible for the increased degradation of myofibrillar proteins in skeletal muscle, and this may be due to tumour factors, such as proteolysis-inducing factor (PIF), or host factors such as tumour necrosis factor-α (TNF-α). In humans loss of adipose tissue is due to an increase in lipolysis rather than a decrease in synthesis, and this may be due to tumour factors such as lipid-mobilising factor (LMF) or TNF-α, both of which can increase cyclic AMP in adipocytes, leading to activation of hormone-sensitive lipase (HSL). Levels of mRNA for HSL are elevated twofold in adipose tissue of cancer patients, while there are no changes in lipoprotein lipase (LPL), involved in extraction of fatty acids from plasma lipoproteins for storage.
Treatment for cachexia
This has concentrated on increasing food intake, although that alone is unable to reverse the metabolic changes. Agents interfering with TNF-α have not been very successful to date, although more research is required in that area. The only agent tested clinically that is able to interfere with the action of PIF is eicosapentaenoic acid (EPA). EPA attenuates protein degradation in skeletal muscle by preventing the increased expression of the ubiquitin–proteasome pathway, but has no effect on protein synthesis. When used alone EPA prevents further wasting in cachectic patients, and, when it is combined with an energy- and protein-dense nutritional supplement, weight gain is seen, which is totally lean body mass. These results suggest that mechanistic studies into the causes of cancer cachexia will allow appropriate therapeutic intervention.
KeywordsCancer cachexia Proteolysis-inducing factor Muscle wasting Eicosapentaenoic acid Cytokines
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