© 2002 American Thoracic Society
Ventilator-induced CachexiaMeakins-Christie Laboratories McGill University Montreal, Quebec, Canada A major challenge for intensivists is that many mechanically ventilated patients fail to wean from the ventilator. Although the cause of weaning failure is complex and often multifactorial, ventilatory muscle dysfunction is central (1). Often no mechanism of muscle dysfunction can be identified. Evidence from animal studies, however, suggests that both short- and long-term mechanical ventilation can elicit contractile dysfunction and disuse atrophy in the ventilatory muscles (2). In this issue of AJRCCM, Shanely and coworkers (pp. 1369–1374) unravel a few of the mechanisms underlying ventilator-induced cachexia (3). Atrophy can result from decreased protein synthesis, increased proteolysis, or both. The authors measured the in vitro release of the amino acid tyrosine and documented a significant increase in proteolysis in the diaphragms of rats subjected to 18 hours of controlled mechanical ventilation. To understand the mechanisms behind this enhanced muscle proteolysis, we will summarize the types and mechanisms of action of intracellular proteases. Three main intracellular protease systems exist in mammalian cells. One, the lysosomal proteases (cathepsins), is responsible for degrading extracellular proteins and surface receptors and is not considered important for proteolysis in disuse atrophy. Two, the two ubiquitously expressed calpains (m-calpain and µ-calpains) are heterodimeric proteases that require calcium and autocleavage for activation (4). The biologic roles of calpains and their in vivo protein targets are not precisely known. Calpains partially cleave proteins in vivo and render them susceptible to the action of the proteasome. Three, the proteasome is a multisubunit multicatalytic complex that exists in two forms: the core 20S proteasome, which is either free or bound to a pair of 19S regulators to form the second form of proteasome, namely, the 26S proteasome (5). Proteins marked for degradation are first conjugated in an ATP-dependent reaction by three different enzymes (E1, E2, and E3) to a protein called ubiquitin, and this process is repeated until a polyubiquitin chain forms. The polyubiquitin-protein complex is recognized by the 19S regulator, which releases the ubiquitin chain and catalyzes protein entry into the barrel-shaped 20S core. The 26S proteasome is mainly responsible for muscular atrophy in various disease states (cancer, sepsis, acquired immunodeficiency syndrome, trauma). Shanely and coworkers (3) observed that elevated diaphragmatic proteolysis in mechanically ventilated rats was inhibited by the proteasome inhibitor lactacystin. This finding, along with the observation that controlled mechanical ventilation augmented muscle 20S proteasome activity more than fivefold, strongly implicates the proteasome pathway in ventilator-induced diaphragmatic atrophy. What is so impressive about this rise in 20S proteasome activity? Most proteolysis in cells is performed by the 26S proteasome, in an energy-dependent, ubiquitin-mediated process. The free 20S proteasome, however, is specialized in degrading proteins oxidized by oxygen radicals. Oxidative damage to a protein results in its partial unfolding, exposing hidden hydrophobic residues. Therefore, an oxidized protein does not need to be further modified by ubiquitin conjugation to confer a hydrophobic patch, nor does it require energy from ATP hydrolysis to unfold. The reported rise in the level of oxidized proteins in the diaphragm of the ventilated rats (3) could, therefore, explain the need to augment 20S proteasome activity. The development of oxidative stress in limb muscles has been observed after more than 4 days of disuse and was associated with increased lipid peroxidation and a reduction in total glutathione (6). The reported rise in protein carbonyls and 8-isoprostane in the diaphragm after only 18 hours of controlled mechanical ventilation is rather surprising. Controlled mechanical ventilation causes a special form of disuse because the diaphragm is being passively shortened by repeated lung inflations. Passive muscle shortening increases blood flow and affects muscle metabolism (7), which might partly explain the different time course of oxidative stress in the diaphragm versus limb muscles. The mechanisms of ventilator-induced oxidative stress were not explored by Shanely and coworkers (3). Limb muscle studies revealed, however, that disuse is associated with a significant upregulation of superoxide-generating xanthine oxidase (8) and elevated levels of transition metals, including iron, calcium, copper, and manganese (6). The rise in iron is expected to facilitate the generation of hydroxyl radicals from superoxide and hydrogen peroxide. Moreover, manganese and copper are capable of catalyzing the oxidation of glutathione, thereby reducing the overall antioxidant capacity. The possible rise in intracellular calcium in the diaphragm of ventilated rats could explain the observed increase in muscle calpain activity. One major consequence of augmented calpain activity would be the partial disorganization of the highly ordered intact myofibrils that are normally, unlike its individual constituent actin and myosin, resistant to proteasome proteolysis (5). Furthermore, the fact that an inhibitor of calpains (E-64d) also attenuated ventilator-induced proteolysis (3), suggests that calpain activity is required to render muscle proteins amenable to degradation by the proteasome. Is the human diaphragm as susceptible to ventilator-induced cachexia as the diaphragm of the rat? This question has not yet been answered. We speculate on the basis that the rate of disuse muscle atrophy correlates strongly and positively with body mass–specific metabolic rate (which is higher in rats than in humans) (9) and that relatively longer periods of mechanical ventilation than those reported in the study of Shanely and coworkers are required to produce disuse atrophy in humans. What could be done to prevent controlled mechanical ventilation-induced diaphragm atrophy? When feasible, modes of partial ventilator support allowing diaphragmatic contractions are an attractive, although unproven, alternative. The results of Shanely and coworkers, however, point to another possibility. Antioxidant supplementation could prevent the development of oxidative stress and consequently attenuate muscle proteolysis. This proposal is supported by the observations that vitamin E supplements attenuate immobilization-induced atrophy in limb muscles (6). In fact, this is what occurs when hibernating animals are immobilized for prolonged periods of time, yet muscle atrophy doesn't develop. This is because of a decrease in metabolic rate (and hence reduction in oxygen radical formation) and a concomitant rise in the expression of antioxidant enzymes (10). An additional useful intervention that warrants testing is the prevention of the rise in intracellular calcium with dantrolene (5). Finally, future development of tissue-specific proteasome and calpain inhibitors might ameliorate disuse atrophy. REFERENCES
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