ABSTRACT
1. Proteins in eukaryotic cells are continually degraded and replaced under precise control mechanisms. Although this continual proteolysis may seem wasteful, it serves several important functions: cells selectively degrade proteins with abnormal sequences or conformations, the accumulation of which could be harmful; the rapid degradation of regulatory peptides and enzymes is essential for the control of metabolic pathways and the cell cycle; and the breakdown of proteins in starvation provides amino acids for gluconeogenesis and energy metabolism. 2. Protein breakdown in eukaryotic cells occurs through distinct pathways: A) lysosomal (involves cathepsins B, H, L, etc.); B) Ca(2+)-dependent (involves Ca(2+)-dependent proteases calpains I and II); C) ATP-dependent, that require or not ubiquitin (comprises at least two large cytosolic proteases, UCDEN and proteasome), and D) ATP-independent (it is not known which proteases are involved in this degradative system). Despite recent dramatic progress, the relative contributions of these pathways to the accelerated proteolysis occurring in normal and pathological states is still largely unknown. 3. In order to identify the cellular mechanisms of skeletal muscle atrophy during fasting and diabetes mellitus, we have studied protein turnover in soleus and EDL muscles from control and fasted (for 24 h) or diabetic rats (1, 3, 5 and 10 days after streptozotocin injection). 4. The increase in muscle proteolysis during fasting seems to be attributable to an enhancement of the energy-requiring process. An increase in the ATP-dependent proteolytic pathway was evident 1 day after food restriction and probably accounted for all of the increased proteolysis demonstrated in the EDL muscles. In parallel with the alterations in the ATP-dependent process, an increase in the ubiquitin-mRNA and proteasome subunit-mRNA was detected. 5. In the acute phase of diabetes (1-3 days) there was an activation of Ca(2+)-dependent (soleus and EDL) and ATP-dependent (EDL) pathways. However, after 5 and 10 days of diabetes the activity of these two pathways fell to values even below control ones. No changes in the lysosomal proteolytic system were observed during diabetes. 6. Although appreciable progress has been made in this research, a large number of important questions remain to be answered, and some of them are discussed in the present paper.