Differential regulation of the lysosomal, Ca2+-dependent and ubiquitin/proteasome-dependent proteolytic pathways in fast-twitch and slow-twitch rat muscle following hyperinsulinaemia.

In order to characterize the poorly defined mechanisms that account for the anti-proteolytic effects of insulin in skeletal muscle, we investigated in rats the effects of a 3 h systemic euglycaemic hyperinsulinaemic clamp on lysosomal, Ca(2+)-dependent proteolysis, and on ubiquitin/proteasome-dependent proteolysis. Proteolysis was measured in incubated fast-twitch mixed-fibre extensor digitorum longus (EDL) and slow-twitch red-fibre soleus muscles harvested at the end of insulin infusion. Insulin inhibited proteolysis (P<0.

Insulin action on skeletal muscle protein metabolism during catabolic states.

Insulin plays a major role in the regulation of skeletal muscle protein turnover but its mechanism of action is not fully understood, especially in vivo during catabolic states. These aspects are presently reviewed. Insulin inhibits the ATP-ubiquitin proteasome proteolytic pathway which is presumably the predominant pathway involved in the breakdown of muscle protein.

Acute hyperinsulinemia fails to change GLUT-4 content in crude membranes from goat skeletal muscles and adipose tissue.

The effect of insulin on GLUT-4 protein level in samples of adipose tissue and skeletal muscles from goats was studied in vivo using an euglycemic hyperinsulinemic clamp. The clamp was maintained in conscious goats for 6 h in the presence of amino acids to prevent insulin-induced hypoaminoacidemia. GLUT-4 protein was assessed in crude membrane preparations from adipose tissue and four skeletal muscles (longissimus dorsi, tensor fasciae latae, anconeus and diaphragm) by Western blot analysis.

Ubiquitin-proteasome-dependent proteolysis in skeletal muscle.

The ubiquitin-proteasome proteolytic pathway has recently been reported to be of major importance in the breakdown of skeletal muscle proteins. The first step in this pathway is the covalent attachment of polyubiquitin chains to the targeted protein. Polyubiquitylated proteins are then recognized and degraded by the 26S proteasome complex.

Expression of subunits of the 19S complex and of the PA28 activator in rat skeletal muscle.

A precise knowledge of the role of subunits of the 19S complex and the PA28 regulator, which associate with the 20S proteasome and regulate its peptidase activities, may contribute to design new therapeutic approaches for preventing muscle wasting in human diseases. The proteasome is mainly responsible for the muscle wasting of tumor-bearing and unweighted rats. The expression of some ATPase (MSS1, P45) and non ATPase (P112-L, P31) subunits of the 19S complex, and of the two subunits of the PA28 regulator, was studied in such atrophying muscles.

Euglycemic hyperinsulinemia and hyperaminoacidemia decrease skeletal muscle ubiquitin mRNA in goats.

Insulin inhibits protein breakdown at the whole body level, but neither the tissues nor the proteolytic pathways on which insulin exerts its antiproteolytic effect are well characterized. We measured the effects of insulin on mRNA levels for cathepsin D and m-calpain (a lysosomal and Ca2(+)-dependent proteinase, respectively) and ubiquitin (a component of ubiquitin-dependent proteolysis) in skeletal muscle, skin, liver, and intestine. We used a 6-h hyperinsulinemic, euglycemic, and hyperaminoacidemic clamp in goats, a species in which insulin markedly inhibited whole body protein breakdown under similar conditions [S.

Effect of hyperinsulinemia and hyperaminoacidemia on muscle and liver protein synthesis in lactating goats.

The experiment was carried out to clarify the roles of insulin and amino acids on protein synthesis in fed lactating goats (30 days postpartum). Protein synthesis in the liver and various skeletal muscles was assessed after an intravenous injection of a large dose of unlabeled valine containing a tracer dose of L-[2,3,4-3H]valine. The animals were divided into three groups.

Increased ATP-ubiquitin-dependent proteolysis in skeletal muscles of tumor-bearing rats.

Little information is available on proteolytic pathways responsible for muscle wasting in cancer cachexia. Experiments were carried out in young rats to demonstrate whether a small (< 0.3% body weight) tumor may activate the lysosomal, Ca(2+)-dependent, and/or ATP-ubiquitin-dependent proteolytic pathway(s) in skeletal muscle.

Regulation of ATP-ubiquitin-dependent proteolysis in muscle wasting.

Protein breakdown plays a major role in muscle growth and atrophy. However, the regulation of muscle proteolysis by nutritional, hormonal and mechanical factors remains poorly understood. In this review, the methods available to study skeletal muscle protein breakdown, and our current understanding of the role of 3 major proteolytic systems that are well characterized in this tissue (ie the lysosomal, Ca(2+)-dependent and ATP-ubiquitin-dependent proteolytic pathways) are critically analyzed.