Background: Many cancer patients experience a wasting syndrome (cachexia) characterized by weight loss and abnormalities in carbohydrate, protein, and lipid metabolism. Recent experimental studies suggest that the development of cancer cachexia involves the host's production of inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha).
Purpose: Our goal was to evaluate in rats the effects of an 8-day TNF-alpha treatment on overall protein metabolism in the liver, diaphragm, heart, and hind-leg muscles.
Methods: Four experimental groups corresponding to specific tissues (liver, diaphragm, heart, and hind-leg muscles) in female Wistar rats (100-150 g) were studied. Each group consisted of 25 TNF-alpha-treated and 25 control female Wistar rats. The TNF-alpha-treated rats were given intraperitoneal injections of recombinant-derived human TNF-alpha (0.5 mL) that was administered in two daily injections of 50 micrograms/kg (total dose of 100 micrograms/kg per day) for 8 days. Control animals followed the same injection schedule as the treatment group and received 0.5 mL of physiological saline instead of TNF-alpha. All rats were radioactively labeled with NaH14CO3 24 hours prior to TNF-alpha treatment. At 0, 1, 2, 4, and 8 days during TNF-alpha treatment, five rats per group were killed to measure the radioactive decay of labeled protein in specific tissues in order to estimate fractional protein turnover. During necropsy, the liver, hind-leg muscles (soleus muscle analyzed separately on 8th day only), heart, and diaphragm were rapidly weighed, and each was homogenized. Total protein content and total DNA were also determined. Total protein radioactivity and specific protein radioactivity (per milligram of protein) were evaluated for liver, diaphragm, heart, and hind-leg muscles. Radioactivity was counted in a liquid scintillation counter. Fractional rates of protein synthesis, protein degradation, total protein, and protein accumulation or loss were calculated.
Results: The TNF-alpha treatment administered to female Wistar rats for 8 days resulted in a transient decrease in food intake and body weight 24 hours after the beginning of the TNF-alpha treatment. In all types of tissues studied, TNF-alpha treatment resulted in increases in both the protein synthesis and protein degradation, with a greater increase in the protein degradation that resulted in a reduced protein accumulation following TNF-alpha treatment. This reduction in protein accumulation was directly associated with a decreased soleus muscle mass on day 8 of the treatment.
Conclusions: Data suggest that TNF-alpha enhances muscle degradation in experimental situations where body weight loss is not apparent.