Potential intracellular targets for anabolic/anti-catabolic therapies

Anabolic and catabolic mediators of intestinal protein turnover: a new experimental approach

Studies on regulation of protein turnover in skeletal muscle have revealed the important contributions of protein synthesis and catabolism to tissue protein balance, and have identified a host of specific anabolic and catabolic stimuli and biochemical mechanisms that regulate these processes. This knowledge is critical to current efforts designed to promote anabolism and limit atrophy. Of the tissues with a potentially large contribution to whole-body amino acid metabolism, protein turnover of the intestine stands out as being poorly understood. The intestine is subject to complexities in regulation of its metabolism that are not apparent for other tissues. The study of intestinal protein turnover also entails some important technical challenges. We recently developed an in-situ experimental system for study of intestinal mucosal protein synthesis with the following unique features: multiple observations within an animal; controlled delivery of nutritional stimuli to the apical side, basolateral side, or both; and luminal delivery of tracer in a flooding dose for determination of protein synthesis. We have begun to use the system to test the specific roles of individual luminal nutrients in regulation of mucosal protein synthesis. We have also used protease gene expression as an index of potential regulation of catabolic pathways.

Stimulation of myofibrillar protein degradation and expression of mRNA encoding the ubiquitin-proteasome system in C(2)C(12) myotubes by dexamethasone: effect of the proteasome inhibitor MG-132

Addition of the synthetic glucocorticoid, dexamethasone (Dex) to serum-deprived C(2)C(12) myotubes elicited time- and concentration-dependent changes in N(tau)-methylhistidine (3-MH), a marker of myofibrillar protein degradation. Within 24 h, 100 nM Dex significantly decreased the cell content of 3-MH and increased release into the medium. Both of these responses had increased in magnitude by 48 h and then declined toward basal values by 72 h. The increase in the release of 3-MH closely paralleled its loss from the cell protein. Furthermore, Dex also decreased the 3-MH:total cell protein ratio, suggesting that myofibrillar proteins were being preferentially degraded. Incubation of myotubes with the peptide aldehyde, MG-132, an inhibitor of proteolysis by the (ATP)-ubiquitin (Ub)-dependent proteasome, prevented both the basal release of 3-MH (>95%) and the increased release of 3-MH into the medium in response to Dex (>95%). Northern hybridization studies demonstrated that Dex also elicited similar time- and concentration-dependent increases in the expression of mRNA encoding two components (14 kDa E(2) Ub-conjugating enzyme and Ub) of the ATP-Ub-dependent pathway. The data demonstrate that Dex stimulates preferential hydrolysis of myofibrillar proteins in C(2)C(12) myotubes and suggests that the ATP-Ub-dependent pathway is involved in this response.