Decreased rate of protein synthesis, caspase-3 activity, and ubiquitin–proteasome proteolysis in soleus muscles from growing rats fed a low-protein, high-carbohydrate diet

Abstract

The aim of this study was to investigate the changes in the rates of both protein synthesis and breakdown, and the activation of intracellular effectors that control these processes in soleus muscles from growing rats fed a low-protein, high-carbohydrate (LPHC) diet for 15 days. The mass and the protein content, as well as the rate of protein synthesis, were decreased in the soleus from LPHC-fed rats. The availability of amino acids was diminished, since the levels of various essential amino acids were decreased in the plasma of LPHC-fed rats. Overall rate of proteolysis was also decreased, explained by reductions in the mRNA levels of atrogin-1 and MuRF-1, ubiquitin conjugates, proteasome activity, and in the activity of caspase-3. Soleus muscles from LPHC-fed rats showed increased insulin sensitivity, with increased levels of insulin receptor and phosphorylation levels of AKT, which probably explains the inhibition of both the caspase-3 activity and the ubiquitin-proteasome system. The fall of muscle proteolysis seems to represent an adaptive response that contributes to spare proteins in a condition of diminished availability of dietary amino acids. Furthermore, the decreased rate of protein synthesis may be the driving factor to the lower muscle mass gain in growing rats fed the LPHC diet.

Higher insulin sensitivity in EDL muscle of rats fed a low-protein, high-carbohydrate diet inhibits the caspase-3 and ubiquitin-proteasome proteolytic systems but does not increase protein synthesis

“…The proteolysis was reduced mainly due to inhibition of both the ubiquitin-proteasome system and caspase-3 activity [14]. The results also indicated higher insulin sensitivity in the soleus muscles of the LPHC rats, which was evidenced by increases in the insulin receptor content and in insulinstimulated Akt phosphorylation [14], suggesting that the higher insulin sensitivity in the soleus muscle of the LPHC rats was responsible for the inhibition of the proteolytic processes.…”

“…Even when the increase in the food intake was accounted for, the LPHC rats ingested 60% less protein than the rats fed the C diet at the end of 15 days, which was confirmed by the reduced post-prandial amino acid concentration in the blood of these rats [14]. The protein-deficient state in LPHC rats could also be indicated by the hypoproteinemia and impairment in the body growth (evaluated by Lee index) compared to the condition of the rats receiving a C diet [13].…”

“…A previous study on soleus muscles of LPHC rats showed that the reductions in mass and protein content resulted from decreases in protein synthesis [14] and overall proteolysis. The proteolysis was reduced mainly due to inhibition of both the ubiquitin-proteasome system and caspase-3 activity [14].…”

“…The EDL muscles were rapidly dissected, weighed and fixed by the tendon in appropriate supports (to maintain the muscles at the resting length) and The in vitro basal protein synthesis was evaluated as previously described [14,28]. After a 1-h pre-incubation period at 37°C with shaking, the EDL muscles were incubated in the same Krebs-Ringer bicarbonate buffer described above containing all amino acids at concentrations similar to those of the rat plasma during fed period [29] and L-[U- 14 C] tyrosine (0.05 µCi/mL) for 2 h. After the incubation, the specific activity of the intracellular tyrosine pool in each muscle was estimated by measuring the radioactivity and the concentration of free tyrosine.…”

“…Studies from our research group showed that rats that were fed a low-protein, high-carbohydrate diet (LPHC; 6% protein and 74% carbohydrate) for 15 days soon after weaning showed an increase in the diet and calorie intake with a reduction in the body weight gain compared to the corresponding factors in the rats fed a control diet (C; 17% carbohydrate and 63% protein) [14]. Even when the increase in the food intake was accounted for, the LPHC rats ingested 60% less protein than the rats fed the C diet at the end of 15 days, which was confirmed by the reduced post-prandial amino acid concentration in the blood of these rats [14].…”