Effect of eicosapentaenoic acid, protein and amino acids on protein synthesis and degradation in skeletal muscle of cachectic mice

Abstract

Atrophy of skeletal muscle reduces both the quality and quantity of life of patients with cancer cachexia. Loss of muscle mass is thought to arise from a reduction in protein synthesis combined with an enhanced rate of protein degradation, and few treatments are available to counteract this process. Eicosapentaenoic acid (EPA) has been shown to attenuate the enhanced protein degradation, but to have no effect on protein synthesis. This study examines the effect of EPA combined with a protein and amino-acid supplementation on protein synthesis and degradation in gastrocnemius muscle of mice bearing the cachexia-inducing MAC16 tumour. Muscles from cachectic mice showed an 80% reduction in protein synthesis and about a 50-fold increase in protein degradation compared with muscles from nontumour-bearing mice of the same age and weight. Treatment with EPA (1 g kg(-1)) daily reduced protein degradation by 88%, but had no effect on protein synthesis. Combination of EPA with casein (5.35 g kg(-1)) also had no effect on protein synthesis, but when combined with the amino acids leucine, arginine and methionine there was almost a doubling of protein synthesis. The addition of carbohydrate (10.7 g kg(-1)) to stimulate insulin release had no additional effect. The combination involving the amino acids produced almost a doubling of the ratio of protein synthesis to protein degradation in gastrocnemius muscle over that of EPA alone. No treatment had a significant effect on tumour growth rate, but the inclusion of amino acids had a more significant effect on weight loss induced by the MAC16 tumour than that of EPA alone. The results suggest that combination therapy of cancer cachexia involving both inhibition of the enhanced protein degradation and stimulation of the reduced protein synthesis may be more effective than either treatment alone.

Figure 1

Protein synthesis (□) and degradation in gastrocnemius muscle of nontumour-bearing mice (normal), cachectic mice (cachectic), cachectic mice treated with EPA (EPA), cachectic mice treated with EPA+casein (EPA+C), cachectic mice treated with EPA+casein+amino acids (EPA+C+AA), cachectic mice treated with EPA+casein+amino acids+carbohydrate (EPA+C+AA+CHO). The details of the treatments are given in the Materials and methods section. The weight loss in the various groups is shown in Figure 3. Differences from control are indicated as a, P<0.01 and b, P<0.001, while differences from cachectic are indicated as c, P<0.05 and d, P<0.001 and differences from EPA as f, P<0.05. The number of animals used for both protein synthesis and protein degradation n=6.

Figure 2

Ratio of protein synthesis to protein degradation in gastrocnemius muscle of cachectic mice bearing the MAC16 tumour after being subjected to the nutritional regimes detailed in Figure 1 for a 4-day period.

Figure 3

Effect of nutritional supplementation on body weight loss in mice bearing the MAC16 tumour. All animals were weight losing at the time of initiation of the experiment (day 1). Animals (n=12) were randomised to receive daily p.o. treatments of olive oil (X), EPA (▪), EPA+casein (○), EPA+casein+amino acids (•) or EPA+casein+amino acids+carbohydrate (□) at the concentrations indicated in the Materials and methods section. Differences from animals receiving olive oil alone are indicated as a, P<0.05, b, P<0.01 and c, P<0.001.

Figure 4

Effect of nutritional supplementation on tumour growth in mice bearing the MAC16 tumour. Tumour size has been normalised to 100% at the start of the experiment. Animals (n=12) were randomised to receive daily p.o. treatments of olive oil (X), EPA (▪), EPA+casein (○), EPA+casein+amino acids (•) or EPA+casein+amino acids+carbohydrate (□) at the concentrations indicated in the Materials and methods section. There was no significant difference between the groups.