Chronic inflammation is a hallmark of cancer cachexia in both patients and preclinical models. Cachexia is prevalent in roughly 80% of cancer patients and accounts for up to 20% of all cancer-related deaths. Proinflammatory cytokines IL-6, TNF-α, and TGF-β have been widely examined for their regulation of cancer cachexia. An established characteristic of cachectic skeletal muscle is a disrupted capacity for oxidative metabolism, which is thought to contribute to cancer patient fatigue, diminished metabolic function, and muscle mass loss. This review's primary objective is to highlight emerging evidence linking cancer-induced inflammation to the dysfunctional regulation of mitochondrial dynamics, mitophagy, and biogenesis in cachectic muscle. The potential for either muscle inactivity or exercise to alter mitochondrial dysfunction during cancer cachexia will also be discussed.

While cancer-induced skeletal muscle wasting has been widely investigated, the drivers of cancer-induced muscle functional decrements are only beginning to be understood. Decreased muscle function impacts cancer patient quality of life and health status, and several potential therapeutics have failed in clinical trials due to a lack of functional improvement. Furthermore, systemic inflammation and intrinsic inflammatory signaling's role in the cachectic disruption of muscle function requires further investigation. We examined skeletal muscle functional properties during cancer cachexia and determined their relationship to systemic and intrinsic cachexia indices. Male Apc (MIN) mice were stratified by percent body weight loss into weight stable (WS; <5% loss) or cachectic (CX; >5% loss). Age-matched C57BL/6 littermates served as controls. Tibialis anterior (TA) twitch properties, tetanic force, and fatigability were examined in situ. TA protein and mRNA expression were examined in the nonstimulated leg. CX decreased muscle mass, tetanic force (P), and specific tetanic force (P). Whole body and muscle fatigability were increased in WS and CX. CX had slower contraction rates, +dP/d t and -dP/d t, which were inversely associated with muscle signal transducer and activator of transcription 3 ( STAT3) and p65 activation. STAT3 and p65 activation were also inversely associated with P. However, STAT3 was not related to P or fatigue. Muscle suppressor of cytokine signaling 3 mRNA expression was negatively associated with TA weight, P, and P but not fatigue. Our study demonstrates that multiple functional deficits that occur with cancer cachexia are associated with increased muscle inflammatory signaling. Notably, muscle fatigability is increased in the MIN mouse before cachexia development. NEW & NOTEWORTHY Recent studies have identified decrements in skeletal muscle function during cachexia. We have extended these studies by directly relating decrements in muscle function to established cachexia indices. Our results demonstrate that a slow-fatigable contractile phenotype is developed during the progression of cachexia that coincides with increased muscle inflammatory signaling. Furthermore, regression analysis identified predictors of cancer-induced muscle dysfunction. Last, we report the novel finding that whole body and muscle fatigability were increased before cachexia development.

Cancer cachexia is characterized by the progressive loss of skeletal muscle mass. While mouse skeletal muscle's response to an acute bout of stimulated low-frequency concentric muscle contractions is disrupted by cachexia, gaps remain in our understanding of cachexia's effects on eccentric contraction-induced muscle growth. The purpose of this study was to determine whether repeated bouts of stimulated high-frequency eccentric muscle contractions [high-frequency electrical muscle stimulation (HFES)] could stimulate myofiber growth during cancer cachexia progression, and whether this training disrupted muscle signaling associated with wasting. Male Apc(Min/+) mice initiating cachexia (N = 9) performed seven bouts of HFES-induced eccentric contractions of the left tibialis anterior muscle over 2 wk. The right tibialis anterior served as the control, and mice were killed 48 h after the last stimulation. Age-matched C57BL/6 mice (N = 9) served as wild-type controls. Apc(Min/+) mice lost body weight, muscle mass, and type IIA, IIX, and IIB myofiber cross-sectional area. HFES increased myofiber cross-sectional area of all fiber types, regardless of cachexia. Cachexia increased muscle noncontractile tissue, which was attenuated by HFES. Cachexia decreased the percentage of high succinate dehydrogenase activity myofibers, which was increased by HFES, regardless of cachexia. While cachexia activated AMP kinase, STAT3, and ERK1/2 signaling, HFES decreased AMP kinase phosphorylation, independent of the suppression of STAT3. These results demonstrate that cachectic skeletal muscle can initiate a growth response to repeated eccentric muscle contractions, despite the presence of a systemic cachectic environment.

BackgroundSkeletal muscle responds to eccentric contractions (ECC) with an anabolic response that involves the induction of protein synthesis through the mechanistic target of rapamycin complex 1. While we have reported that repeated ECC bouts after cachexia initiation attenuated muscle mass loss and inflammatory signalling, cachectic muscle's capacity to induce protein synthesis in response to ECC has not been determined. Therefore, we examined cachectic muscle's ability to induce mechano‐sensitive pathways and protein synthesis in response to an anabolic stimulus involving ECC and determined the role of muscle signal transducer and activator of transcription 3 (STAT3)/nuclear factor kappa‐light‐chain‐enhancer of activated B cells (NFκB) signalling on ECC‐induced anabolic signalling.MethodsMechano‐sensitive pathways and anabolic signalling were examined immediately post or 3 h after a single ECC bout in cachectic male Apc Min/+ mice (n = 17; 16 ± 1% body weight loss). Muscle STAT3/NFκB regulation of basal and ECC‐induced anabolic signalling was also examined in an additional cohort of Apc Min/+ mice (n = 10; 16 ± 1% body weight loss) that received pyrrolidine dithiocarbamate 24 h prior to a single ECC bout. In all experiments, the left tibialis anterior performed ECC while the right tibialis anterior served as intra‐animal control. Data were analysed by Student's t‐test or two‐way repeated measures analysis of variance with Student‐Newman‐Keuls post‐hoc when appropriate. The accepted level of significance was set at P < 0.05 for all analysis.Results Apc Min/+ mice exhibited a cachectic muscle signature demonstrated by perturbed proteostasis (Ribosomal Protein S6 (RPS6), P70S6K, Atrogin‐1, and Muscle RING‐finger protein‐1 (MuRF1)), metabolic (adenosine monophosphate‐activated protein kinase, Peroxisome proliferator‐activated receptor gamma coactivator 1‐alpha (PGC‐1α), and Cytochrome c oxidase subunit IV (COXIV)), and inflammatory (STAT3, NFκB, extracellular signal‐regulated kinases 1 and 2, and P38) signalling pathway regulation. Nonetheless, mechano‐sensitive signalling pathways (P38, extracellular signal‐regulated kinases 1 and 2, and Protein kinase B (AKT)) were activated immediately post‐ECC irrespective of cachexia. While cachexia did not attenuate ECC‐induced P70S6K activation, the protein synthesis induction remained suppressed compared with healthy controls. However, muscle STAT3/NFκB inhibition increased basal and ECC‐induced protein synthesis in cachectic Apc Min/+ mice.ConclusionsThese studies demonstrate that mechano‐sensitive signalling is maintained in cachectic skeletal muscle, but chronic STAT3/NFκB signalling serves to attenuate basal and ECC‐induced protein synthesis.

BackgroundThe purpose of this study was to determine the effect of two clinically relevant radiation doses on the susceptibility of mouse skeletal muscle to remodeling.Materials and methods.Alterations in muscle morphology and regulatory signaling were examined in tibialis anterior and gastrocnemius muscles after radiation doses that differed in total biological effective dose (BED). Female C57BL/6 (8-wk) mice were randomly assigned to non-irradiated control, four fractionated doses of 4 Gy (4x4 Gy; BED 37 Gy), or a single 16 Gy dose (16 Gy; BED 100 Gy). Mice were sacrificed 2 weeks after the initial radiation exposure.ResultsThe 16 Gy, but not 4x4 Gy, decreased total muscle protein and RNA content. Related to muscle regeneration, both 16 Gy and 4x4 Gy increased the incidence of central nuclei containing myofibers, but only 16 Gy increased the extracellular matrix volume. However, only 4x4 Gy increased muscle 4-hydroxynonenal expression. While both 16 Gy and 4x4 Gy decreased IIB myofiber mean cross-sectional area (CSA), only 16 Gy decreased IIA myofiber CSA. 16 Gy increased the incidence of small diameter IIA and IIB myofibers, while 4x4 Gy only increased the incidence of small diameter IIB myofibers. Both treatments decreased the frequency and CSA of low succinate dehydrogenase activity (SDH) fibers. Only 16 Gy increased the incidence of small diameter myofibers having high SDH activity. Neither treatment altered muscle signaling related to protein turnover or oxidative metabolism.ConclusionsCollectively, these results demonstrate that radiation dose differentially affects muscle remodeling, and these effects appear to be related to fiber type and oxidative metabolism.

New Findings What is the central question of this study?Interleukin‐6 has been associated with muscle mass and metabolism in both physiological and pathological conditions. A causal role for interleukin‐6 in the induction of fatigue and disruption of mitochondrial function has not been determined. What is the main finding and its importance?We demonstrate that chronically elevated interleukin‐6 increased skeletal muscle fatigability and disrupted mitochondrial content and function independent of changes in fibre type and mass. Abstract Interleukin‐6 (IL‐6) can initiate intracellular signalling in skeletal muscle by binding to the IL‐6‐receptor and interacting with the transmembrane gp130 protein. Circulating IL‐6 has established effects on skeletal muscle mass and metabolism in both physiological and pathological conditions. However, the effects of circulating IL‐6 on skeletal muscle function are not well understood. The purpose of this study was to determine whether chronically elevated systemic IL‐6 was sufficient to disrupt skeletal muscle force, fatigue and mitochondrial function. Additionally, we examined the role of muscle gp130 signalling during overexpression of IL‐6. Systemic IL‐6 overexpression for 2 weeks was achieved by electroporation of an IL‐6 overexpression plasmid or empty vector into the quadriceps of either C57BL/6 (WT) or skeletal muscle gp130 knockout (KO) male mice. Tibialis anterior muscle in situ functional properties and mitochondrial respiration were determined. Interleukin‐6 accelerated in situ skeletal muscle fatigue in the WT, with a 18.5% reduction in force within 90 s of repeated submaximal contractions and a 7% reduction in maximal tetanic force after 5 min. There was no difference between fatigue in the KO and KO+IL‐6. Interleukin‐6 reduced WT muscle mitochondrial respiratory control ratio by 36% and cytochrome c oxidase activity by 42%. Interleukin‐6 had no effect on either KO respiratory control ratio or cytochrome c oxidase activity. Interleukin‐6 also had no effect on body weight, muscle mass or tetanic force in either genotype. These results provide evidence that 2 weeks of elevated systemic IL‐6 is sufficient to increase skeletal muscle fatigability and decrease muscle mitochondrial content and function, and these effects require muscle gp130 signalling.

Muscle contraction may protect against the effects of chemotherapy to cause skeletal muscle atrophy, but the mechanisms underlying these benefits are unclear. To address this question, we utilized in vitro modeling of contraction and mechanotransduction in C2C12 myotubes treated with doxorubicin (DOX; 0.2 μM for 3 days). Myotubes expressed contractile proteins and organized these into functional myofilaments, as electrical field stimulation (STIM) induced intracellular calcium (Ca2+) transients and contractions, both of which were prevented by inhibition of membrane depolarization. DOX treatment reduced myotube myosin content, protein synthesis, and Akt (S308) and forkhead box O3a (FoxO3a; S253) phosphorylation and increased muscle RING finger 1 (MuRF1) expression. STIM (1 h/day) prevented DOX-induced reductions in myotube myosin content and Akt and FoxO3a phosphorylation, as well as increases in MuRF1 expression, but did not prevent DOX-induced reductions in protein synthesis. Inhibition of myosin-actin interaction during STIM prevented contraction and the antiatrophic effects of STIM without affecting Ca2+ cycling, suggesting that the beneficial effect of STIM derives from mechanotransductive pathways. Further supporting this conclusion, mechanical stretch of myotubes recapitulated the effects of STIM to prevent DOX suppression of FoxO3a phosphorylation and upregulation of MuRF1. DOX also increased reactive oxygen species (ROS) production, which led to a decrease in mitochondrial content. Although STIM did not alter DOX-induced ROS production, peroxisome proliferator-activated receptor-γ coactivator-1α and antioxidant enzyme expression were upregulated, and mitochondrial loss was prevented. Our results suggest that the activation of mechanotransductive pathways that downregulate proteolysis and preserve mitochondrial content protects against the atrophic effects of chemotherapeutics.

The IL-6 cytokine family activates intracellular signaling pathways through glycoprotein-130 (gp130), and this signaling has established regulatory roles in muscle glucose metabolism and proteostasis. Although the IL-6 family has been implicated as myokines regulating the muscles' metabolic response to exercise, gp130's role in mitochondrial quality control involving fission, fusion, mitophagy, and biogenesis is not well understood. Therefore, we examined gp130's role in basal and exercise-trained muscle mitochondrial quality control. Muscles from C57BL/6, skeletal muscle-specific gp130 knockout (KO) mice, and CC myotubes, were examined. KO did not alter treadmill run-to-fatigue or indices of mitochondrial content [cytochrome- c oxidase (COX) activity] or biogenesis (AMPK, peroxisome proliferator-activated receptor-γ coactivator-1α, mitochondrial transcription factor A, and COX IV). KO increased mitochondrial fission 1 protein (FIS-1) while suppressing mitofusin-1 (MFN-1), which was recapitulated in myotubes after gp130 knockdown. KO induced ubiquitin-binding protein p62, Parkin, and ubiquitin in isolated mitochondria from gastrocnemius muscles. Knockdown of gp130 in myotubes suppressed STAT3 and induced accumulation of microtubule-associated protein-1 light chain 3B (LC3)-II relative to LC3-I. Suppression of myotube STAT3 did not alter FIS-1 or MFN-1. Exercise training increased muscle gp130 and suppressed STAT3. KO did not alter the exercise-training induction of COX activity, biogenesis, FIS-1, or Beclin-1. KO increased MFN-1 and suppressed 4-hydroxynonenal after exercise training. These findings suggest a role for gp130 in the modulation of mitochondrial dynamics and autophagic processes. NEW & NOTEWORTHY Although the IL-6 family of cytokines has been implicated in the regulation of skeletal muscle protein turnover and metabolism, less is understood about its role in mitochondrial quality control. We examined the glycoprotein-130 receptor in the regulation of skeletal muscle mitochondria quality control in the basal and exercise-trained states. We report that the muscle glycoprotein-130 receptor modulates basal mitochondrial dynamics and autophagic processes and is not necessary for exercise-training mitochondrial adaptations to quality control.