Proteasome inhibition and ventilator-induced diaphragmatic dysfunction: is the glass half full or half empty?

2525 Twenty-five years ago, Anzueto et al (1) reported for the first time that mechanical ventilation could impair diaphragmatic contractility. Since then, a solid body of experimental data has supported the existence of what has come to be known as ventilator-induced diaphragmatic dysfunction (VIDD) (2). VIDD is not limited to laboratory animals alone (3), but it can also occur in patients (4). Myocyte damage, increased protein degradation, and decreased protein synthesis are some of the important pathways implicated in the pathogenesis of VIDD (5). The growing understanding of those pathways has spurred interest in the development of therapies designed to prevent/limit VIDD (6, 7). To this novel arena of inquiry belongs the investigation of Agten et al (8), published in this issue of Critical Care Medicine. Based on the known contribution of proteasome activity to protein degradation during VIDD, Agten et al (8) tested whether administering one dose of the proteasome inhibitor bortezomib to rats would protect the diaphragm against VIDD. Following 24 hrs of controlled mechanical ventilation (CMV), the investigators determined that bortezomib only partially prevented the decrease in diaphragmatic force and atrophy of diaphragm type IIx/b fibers. In addition, diaphragmatic specific force was greater in the CMV-plus-bortezomib group than in the CMV-only group. Why were the results of Agten et al (8) only partially positive? Is bortezomib not a sufficiently potent proteasome inhibitor? Is proteasome activation during VIDD less important than what we thought? Is the prevention of VIDD with proteasome inhibitors doomed to failure? The most upstream phenomenon in the pathogenesis of VIDD is the release of reactive oxygen species (9). These, in turn, can activate calpain (that causes myofilament release) and caspase-3 (that causes actin–myosin degradation and myonuclear apoptosis). In addition, reactive oxygen species can increase the transcription of E2 and E3 ligases and the oxidation of myofilaments. The last two phenomena enhance myofilaments degradation by the proteasome system (9). The proteasome system, however, cannot catabolize intact myofilaments (10, 11). That is, the proteasome relies on the action of reactive oxygen species, caspase, and calpain to obtain the suitable substrates for proteolysis (10). The observation that the proteasome can act only on myofilaments that are already damaged suggests that pharmacologic targeting of this downstream system should have no substantial effect on diaphragmatic function in VIDD. As such, what is truly surprising in the investigation of Agten et al (8) is not so much that bortezomib did not prevent VIDD. What is surprising is that bortezomib had any effect at all. As noted by the investigators, bortezomib’s partial protection of the diaphragm from VIDD was likely caused by the reported capacity of this compound to inhibit caspase-3 activity under some (12) but no all conditions (10). Such inhibition might have better preserved the sarcomere’s architecture during CMV, and this, in turn, could have decreased the number of abnormal myofibrils in bortezomib-treated rats (3). In the current animal model of VIDD (8), the increase in diaphragmatic 20S proteasome activity and ubiquitinated proteins was nonsignificant. This result contrasts with previous reports from the same group of investigators (6, 7) as well as from others (13, 14). The incongruous results cannot be ascribed to the animal species used (6, 13, 14), type of anesthesia (6, 13), mode of mechanical ventilation (6, 13, 14), or statistical power (8). Whatever the cause, if the increase in proteasome activity during CMV was nonsignificant, it would seem reasonable—as reported by the Belgian investigators (8)—that bortezomib administration would not decrease proteasome activity and, consequently, would not increase ubiquitinated proteins, the substrate of this proteolytic system. The study was not designed to answer the question of whether bortezomib is a sufficiently potent proteasome inhibitor in the diaphragm. This possibility, however, seems unlikely considering the positive results of Supinski et al (10) and van Hees et al (12). At this point it is premature to state whether prevention of VIDD with proteasome inhibitors is doomed to failure. For example, it is impossible to exclude whether carfilzomib and marizomib, the new generation of proteasome inhibitors, may yield different results than bortezomib. A more fundamental consideration, however, is that the proteasome pathway acts on already damaged myocellular components (10, 11). This consideration raises two interconnected points. First, this pathway is probably too downstream in the pathogenesis of VIDD to be a viable target for therapies designed to prevent or ameliorate VIDD. Second, more upstream pathways, including reactive oxygen species (6, 14), caspase (15), and calpain (16), should be the target in the treatment of VIDD. The latter consideration is supported by several observations. Agten et al (8) recently reported that the antioxidant N-acetylcysteine completely protects against ventilator-induced activation of calpain and ventilator-induced activation of ubiquitin-proteasome and prevents VIDD. Other positive results have been reported with the use of trolox (antioxidant) (17), leupeptin (inhibitor of lysosomal proteases and calpain) (16), methylprednisolone (calpain inhibition and, to a lesser extent, caspase-3 inhibition) (18), and DEVD-CHO (caspase inhibitor) (15). *See also p. 2449.