synthesis over a diurnal cycle suggest that increases in breakdown are not required to explain the observed muscle loss. For example, normal turnover is 0.05% h–1 or 1.2% d–1 where synthesis and breakdown are equal and opposite. Based on data showing that muscle synthesis increases 2.5-fold after a maximally e ective feed with the increase only lasting 1.5 h, it is likely that r5 h/day is spent in ‘‘fed’’ periods where muscle is gaining protein (assuming three good meals). Thus, based on conservative assumptions from previous findings in disuse in which muscle synthesis is suppressed B50% in both post-absorptive and fed periods then diurnal protein accretion would be: (0.025 19) þ (0.025 1.25 5) ¼ 0.63% per day. If muscle protein breakdown remained constant then muscle would be lost at a rate of: 1.2 – 0.63 ¼ 0.57% per day. This is indistinguishable from the typically measured B0.6% per day over the first 30 days, which suggests that increases in muscle breakdown are not necessary to explain human disuse atrophy. Nevertheless, some work has provided some secondary evidence from static markers (e.g. increased expression of proteasomal subunits) for short-term increases in degradation. For example, increases in interstitial 3-methylhistidine (a marker of myofibrillar breakdown) was reported to occur 3 days after immobilization as was upregulation of total ubiquitinylation and mRNA for the E3 ubiquitin ligases and so-called ‘‘atrogenes’’, muscle ring-finger 1 (MuRF1) and muscle atrophy F box (MAFBx), which have been suggested as a common mechanism for muscle wasting (see ref. 35 for further detail). Additionally, other workers have shown that during short-term immobilization, for 5 days and after spinal cord transection in patients, there are increased amounts of markers of increased protein breakdown, in terms of both mRNA and protein. Furthermore, in animals, there is also evidence that acute disuse may cause damage to mitochondria, leading to the generation of ROS and oxidative stress, which themselves have been purported to increase apoptotic and proteolytic processes in muscle. One possibility that has been overlooked is that such increases in markers of breakdown are not necessarily regulating ‘‘bulk’’ increases in protein breakdown, but rather targeted degradation. For example, MAFbx targets the eukaryotic initiation factor 3 subunit 5 (eIF3-f) for ubiquitination and degradation by the proteasome and since mTOR and p70S6K1 interact directly with eIF3-f to mediate assembly of the translation pre-initiation complex this could be the explanation for reduced muscle synthesis, rather than proposed increases in protein breakdown.

5.4.3Sepsis

Sepsis due to infection induces whole body inflammation and is a major cause of comorbidity and mortality in critically ill patients and is associated with and exacerbates acute trauma and thermal injury.36,37 Wasting of body muscles is associated with sepsis and this can directly contribute to the morbidity and mortality, particularly when the respiratory muscles are involved. While sepsis has negative consequences and is associated with illness, the induction of the inflammatory state is presumably adaptive in as much as liberation of energy

from various tissues can assist in healing. As such, there is a large induction of glucocorticoids, proinflammatory cytokines and increases in cellular Ca21 in sepsis. Collectively, this results in increased protein degradation. This increase is predominantly via proteasome-mediated degradation and is associated with upregulation of MURF-1 and MAFbx mRNA, likely due to a downregulation in PI3K/Akt signaling. However, both the calpains and caspases are activated and thought to initiate breakdown of myofibers with final degradation occurring via the proteasome. As cytokines all promote degradation via the proteasome, these factors are presumably extramuscular signals responsible for triggering muscle degradation in septic patients. However, decreased protein synthesis and mitochondrial dysfunction have been observed in septic patients. Presumably the decreased protein synthesis could be the result of elevated cortisol, TNFa and/or Ca21 in septic patients as a result of degradation of key pieces of the transcriptional and translational machinery (for example transcription factors, initiation and/or elongation factors). Indeed, both glucocorticoid and Ca21 antagonists (RU38486 and dantrolene, respectively) have e cacy in reducing muscle wasting in experimental sepsis. Similarly, mitochondrial dysfunction and altered plasma membrane conductance could be due to increased membrane and/or protein damage arising from the increased ROS being present in inflamed tissue as the result of release from inflammatory cells (for example macrophages and neutrophils).

5.4.4Burns

Burn patients often enter a hypermetabolic state where energy is used in healing the wound and raising core body temperature.38 Typically this hypermetabolic state is observed for burns covering 10% or more of the total body surface area and there is a proportional relationship between size of the burn and the resting metabolic rate. When left untreated the muscle wasting associated with burn contributes to both comorbidity, for example infection, and mortality, particularly in patients with larger burn surface areas. Successful treatment modalities point to the complexity of the muscle wasting observed.

Muscle wasting appears to occur via at least four distinct mechanisms. Firstly, in the immediate post-burn period there is a hypercatabolic state, which includes increased muscle protein degradation. The initial hypercatabolic state can be eased by excision and closure of the burn, for example a 40% reduction for large burn surface areas that are excised and covered after two to three days versus after one week. This suggests that at least part of the catabolic state is directly tied to the increased requirement for heat production. While muscle is the major metabolically active tissue in the body and therefore the major producer of heat due to ine ciency of running oxidative phosphorylation, the notion that human muscle functions to produce heat is controversial and therefore the mechanisms controlling increased heat production are currently largely unknown. Presumably increased mitochondrial uncoupling occurs in muscle via increased expression of uncoupling proteins. Thyroid hormone and epinephrine which, as stress-induced hormones, are likely both elevated

immediately post-burn are both capable of inducing uncoupling protein expression in muscle. These hormones presumably also trigger a net negative protein balance (see Sections 5.2.1.2 and 5.2.1.4 for putative mechanisms) with the energy liberated being used to produce heat via less e cient oxidative phosphorylation. The ability to attenuate the hypermetabolic state immediately post-burn by use of beta blockers highlights the role of epinephrine as a key inducer of the hypermetabolic state immediately post-burn. Secondly, increased caloric intake can maintain lean body mass while loss of lean body mass results in delayed healing time. This suggests that at least part of the catabolic state is directly tied to increased caloric demand associated with the healing process. Presumably the signals regulating entry into the catabolic state due to nutritional insu ciency are both decreased plasma AA levels and, consequently, decreased insulin levels (see Section 5.3.1 for details of signals which are presumably lacking). Thirdly, sepsis is quite common after thermal injury and can elevate the metabolic rate by an additional 40% (mechanisms and interventions for sepsis are discussed in Section 5.4.3 above). Fourthly, once patients have recovered from the initial burn injury and/or sepsis stages, resistance exercise training and hormone management have both been shown to improve muscle mass (see Section 5.3.2 for putative mechanisms underlying exercise actions on muscle mass). Because insulin and testosterone levels often drop in response to the burn and recombinant growth hormone (stimulates IGF-1 production), IGF-1, insulin and testosterone (or oxandrolone, which has much less potent androgenic e ects) all can counteract muscle wasting, loss of inhibition of protein degradation by insulin/IGF-1 seems a likely contributor to longer-term muscle loss post-burn (see Sections 5.2.2.1 and 5.2.1.3 for insulin/IGF-1 and testosterone mechanisms, respectively).

5.4.5Cancer Cachexia

Patients with pancreatic or gastric cancer have the highest frequency of weight loss, while patients with non-Hodgkin’s lymphoma, breast cancer, acute nonlymphocytic leukemia and sarcomas have the lowest frequency.39,40 Myosin heavy chain is selectively degraded by the ubiquitin proteasome pathway in the cachectic state, while other core myofibrillar proteins including troponin T, tropomyosin (a- and b-forms) and sarcomeric actin remain unchanged. As with thermal injury and sepsis, glucocorticoids may play a role in the development of cancer cachexia, although adrenalectomy has been shown not to alter the course of cachexia in other animal models, which argues against this. In addition, there is considerable evidence from animal studies that TNFa and IL-6 play a role in muscle loss in cancer cachexia, although its role in the human condition may be more questionable. Further discussion of these is unwarranted as their upregulation likely follows a similar track to sepsis and burns. On the other hand, highly specific to tumors is the production of pro- teolysis-inducing factor (PIF), a 24-kDa molecular mass sulfated glycoprotein, originally isolated from the cachexia-inducing MAC16 tumor. PIF has also been shown to inhibit protein synthesis and stimulate protein degradation