br Introduction br Cancer associated cachexia is
Cancer-associated cachexia is a multifactorial syndrome characterized by anorexia and body weight loss, mainly due to muscle and fat wasting . Cachexia and in particular skeletal muscle loss significantly impair cancer patients' quality of life and tolerance to anti-neoplastic treatments, eventually reducing survival. Despite the relevance of this syndrome to cancer patient outcome, anti-cachexia treatments are still lacking. The currently available standard of care for cachectic cancer patients is limited to nutritional support, while specific treat-ments aimed at counteracting muscle wasting are under investigation and no clinical trial proved effective so far.
The mechanisms underlying muscle depletion have been extensively studied in the past, acceleration of protein turnover rates being the main reason for muscle mass depletion . Specifically, myofibrillar protein catabolism occurs, although reduced protein synthesis might play a role as well . As for protein degradation, two main proteolytic systems operate in the skeletal muscle: the ubiquitin–proteasome-dependent pathway (UPS) and macroautophagy (hereafter referred to as autophagy). UPS activation in cancer cachexia has been reported both in preclinical rodent models and in humans with some differences according to tumor stage and type ( and references within). However, pharmacological UPS inhibition (using bortezomib) is unable to prevent muscle wasting in two distinct experimental models of
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cancer cachexia , and no clinical trial has attempted to directly interfere with proteasomal degradation in cachectic patients.
Also, excessive autophagic degradation has been proposed to play a role in the onset of muscle depletion in cancer cachexia, based on observations in both rodents  and cachectic cancer patients [6,7]. Autophagy modulation is a double-edged sword, since basal activation is required to remove damaged proteins and organelles maintaining muscle quality, while enhanced autophagy flux favors muscle loss. As a consequence, both excessive and defective au-tophagy worsen muscle function and impair muscle mass , suggesting the importance of a finely tuned balance between protein degradation and synthesis. Even in cancer cachexia, an impaired autophagy flux, likely due to overload, has been proposed to occur, while the pharmacological reactivation rescues mus-cle homeostasis and counteracts cachexia . Those data are in conflict with others showing that the promotion of autophagy worsens muscle wasting. Specifically, the overexpression of the autophagy-regulating TP53INP2/DOR protein triggers muscle wasting in diabetic (streptozotocin-induced) mice . TP53INP2 activates basal autophagy in skeletal muscle directly interacting with LC3 and sustaining the degradation of ubiquitylated proteins. In this regard, muscle-specific TP53INP2 gain and loss of function have been shown to result in atrophy and hypertrophy, respectively .
When debating the role of autophagy in the regulation of muscle mass, a further consideration should be given to the cargo that is selectively degraded and might be different in healthy as compared to diseased conditions. Indeed, the degradation of damaged proteins and dysfunctional organelles is a premise for muscle quality, while the uncontrolled targeting of myofibrillar proteins or mitochondria negatively impact muscle mass with-out any positive effect. Such dysregulation might be the consequence of specific signals depending on inflammation and tumor-related factors, such as the increased expression of ubiquitin-ligases  and mitophagy triggers , both sufficient to cause atrophy.