6 5. ROS Mediate Autophagy and Apoptosis ROS may trigger either autophagy or apoptosis: whether these two pathways will be activated depends on the cell context and on the availability of speciﬁc modulators of ROS activity [87]. Autophagy is one of the cellular defense mechanisms activated in response to an excessive ROS production. Indeed, ROS act as signaling molecules in the early events of autophagy induction [87]. Phosphoinositide 3 kinase (PI3K) is known to mediate, at least in part, ROS eﬀects. If the prosurvival eﬀort fails, ROS induce cell death which may involve either the autophagic or the apoptotic pathway, or both [72, 88]. ROS signaling pathways play an important role in the induction of autophagy under physio- and pathological conditions. In healthy cells, autophagy is routinely involved in organelles and proteins turnover as well as in cellular energetic balance [89]. One of its strongest and better- characterized stimuli is starvation, where mitochondrial ROS production is enhanced and autophagy increased [87]. Increased ROS generation in the mitochondria under starva- tion is known to depend, at least in part, by class III PI3K: this event is essential for the induction of autophagy [90]. Indeed, upon starvation, ROS, and in particular H2O2, oxidize and inhibit Atg4, a protease responsible for microtubule- associated protein (MAP) light chain 3 (LC3) delipidation, that is, a condition resulting in the stabilization of the lipidated forms of LC3 and promoting the autophagosome maturation [87]. Notably, the same authors reported that addition of antioxidants inhibits these eﬀects, preventing autophagosome biogenesis [91]. Thus, autophagy induced by starvation, where ROS participate in a feedforward manner, plays a prosurvival role since it contributes to the mobilization and reutilization of diverse cellular energy stores [89]. In a diﬀerent direction, it is also known that when autophagy is prolonged, it could lead to cell death indepen- dently from apoptosis [92]. Indeed in nonmuscle tissues and in speciﬁc pathological conditions, ROS-induced autophagy was often linked to cell demise and death. As to skeletal muscle, ROS have been implicated in the induction of autophagy in muscle atrophy, disuse, and aging [72, 93]. Important new evidence on the wasting eﬀect induced by increased oxidative stress on muscle phenotype was obtained by targeting a mutant SOD variant found in human amyotrophic lateral sclerosis myopathy [93, 94]. Indeed, these authors created a mouse model with a G93A mutation of SOD1 restricted to skeletal muscle [93]: accumulation of ROS in the muscles of these mice induced progressive atrophy associated with increased autophagy and forkhead transcription factors O (FoxO3) expression, a transcription factor which controls the transcription of autophagy-related genes and is required for the induction of autophagy through the lysosomal pathway in skeletal muscle in the absence of AKT repression [95–97]. In addition, NF-κB signaling has been proposed as an alternative pathway linked to ROS-mediated skeletal muscle atrophy [98]: indeed NF-κB was found to induce muscle atrophy and wasting via the lysosomal enzyme cathepsin L [93, 99] upregulation. Since Journal of Signal Transduction cathepsin L is typically upregulated by FoxO3, it might be speculated that ROS-induced NF-κB converges on the FoxO3 autophagic pathway. Increasing evidence suggests that autophagy of mito- chondria is a selective and defense-oriented response against ROS, mitochondrial dysfunction and the accumulation of somatic mutations of mtDNA with aging [72, 100]. For this reason it has been recently proposed the term “mitophagy” to emphasize the nonrandom nature of this process [100]. Damaged mitochondria are removed by mitophagy by Binp3, a BH3 proapoptotic member of the Bcl-2 family and ﬁs 1, a pro-ﬁssion mitochondrial protein that induces mitochondrial fragmentation and enhances the extent of mitophagy. Notably, inhibition/alteration of mitophagy can contribute to myoﬁber degeneration and weakness in muscle disorders characterized by accumulation of abnormal mito- chondria and inclusions [101, 102]. ROS may have various and important roles in apoptotic cell death: direct actions such as oxidation of cellular proteins and lipids, damage of nucleic acids and functional alteration of organelles; ROS may also modulate cell death processes aﬀecting various signaling cascades [103]. Indeed, ROS participate in early and late steps of the regulation of apoptosis, aﬀecting diﬀerent apoptotic signaling cascades in both intrinsic or extrinsic pathways. The extrinsic path, which involves stimulation of recept- or-mediated apoptotic pathways, can be initiated by ligand- induced (e.g., TNFα and Fas-L and TNF-related apoptosis- inducing ligand, TRAIL) binding, which promotes the activation of caspase-3 and subsequent degradation of genomic DNA [103]. Recent evidence suggests possible direct roles for ROS in mediating death receptors activation and subsequent induction of apoptosis [104]. Indeed, apoptotic signaling is induced by NOX-derived ROS at the plasma membrane level, which lead to lipid raft formation and death receptor clustering activation [104]. The physiological rele- vance and signiﬁcance of ROS-dependent receptor-mediated apoptosis as compared to the classical receptor/ligand- induced apoptotic signaling is, at present, incompletely un- derstood and warrants further investigation. ROS may act as intracellular intermediates directly dysregulating the sarcoplasmic reticulum Ca++ ﬂux and handling, which results in caspase-7 and calpain activa- tion. Furthermore, ROS may cause mitochondrial swelling and fragmentation, and/or alter the conformation of the mitochondrial permeability transition pores (MPTPs), thus facilitating their opening and the release of proapoptotic proteins such as cytochrome c (Cyt C). Independently of caspase activity, apoptosis may follow the intrinsic path, where ROS may directly cause the release of mitochondrial endonuclease G (Endo G), and/or of apoptosis inducing fac- tor (AIF), which is capable of promoting DNA fragmentation in skeletal muscle myonuclei [105]. Another protein coupled with ROS-induced apoptosis is the voltage-dependent anion selective channel protein 1 (VDAC1). This transmembrane protein has been deﬁned a ROS sensor [106] that triggers opening of the MPTP complex under conditions of oxidative stress. Indeed VDAC1 is the main channel within the mitochondrial outer membrane and