X-chromosome linked inhibitor of apoptosis protein inhibits muscle proteolysis in insulin-deficient mice

Mitochondrial Apoptotic Signaling Involvement in Remodeling During Myogenesis and Skeletal Muscle Atrophy

Mitochondria play a major role in apoptotic signaling. In addition to its role in eliminating dysfunctional cells, mitochondrial apoptotic signaling is implicated as a key component of myogenic differentiation and skeletal muscle atrophy. For example, the activation of cysteine-aspartic proteases (caspases; CASP's) can aid in the initial remodeling stages of myogenic differentiation by cleaving protein kinases, transcription factors, and cytoskeletal proteins. Precise regulation of these signals is needed to prevent excessive cell disassemble and subsequent cell death. During skeletal muscle atrophy, the activation of CASP's and mitochondrial derived nucleases participate in myonuclear fragmentation, a potential loss of myonuclei, and cleavage of contractile structures within skeletal muscle. The B cell leukemia/lymphoma 2 (BCL2) family of proteins play a significant role in regulating myogenesis and skeletal muscle atrophy by governing the initiating steps of mitochondrial apoptotic signaling. This review discusses the role of mitochondrial apoptotic signaling in skeletal muscle remodeling during myogenic differentiation and skeletal muscle pathological states, including aging, disuse, and muscular dystrophy.

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Exogenous miR-29a Attenuates Muscle Atrophy and Kidney Fibrosis in Unilateral Ureteral Obstruction Mice

Renal fibrosis leads to end-stage renal disease, but antifibrotic drugs are difficult to develop. Chronic kidney disease often results in muscle wasting, and thereby increases morbidity and mortality. In this work, adeno-associated virus (AAV)-mediated overexpressing miR-29a was hypothesized to counteract renal fibrosis and muscle wasting through muscle-kidney crosstalk in unilateral ureteral obstruction (UUO) mice. miR-29a level was downregulated in the kidney and skeletal muscle of UUO mice. The secretion of exosome-encapsulated miR-29a increased in cultured skeletal muscle satellite cells and HEK293 renal cells after stimulation with serum from UUO mice. This result was confirmed by qPCR and microRNA deep sequencing in the serum exosomes of mice with obstructed ureters. A recombinant AAV-miR-29a was generated to overexpress miR-29a and injected into the tibialis anterior muscle of the mice 2 weeks before UUO surgery. AAV-miR-29a abrogated the UUO-induced upregulation of YY1 and myostatin in skeletal muscles. Renal fibrosis was also partially improved in the UUO mice with intramuscular AAV-miR-29a transduction. AAV-miR-29a overexpression reversed the increase in transforming growth factor β, fibronectin, alpha-smooth muscle actin, and collagen 1A1 and 4A1 levels in the kidney of UUO mice. AAV-green fluorescent protein was applied to trace the AAV route in vivo, and fluorescence was significantly visible in the injected/uninjected muscles and in the kidneys. In conclusion, intramuscular AAV-miR-29a injection attenuates muscle wasting and ameliorates renal fibrosis by downregulating several fibrotic-related proteins in UUO mice.

miRNA-23a/27a attenuates muscle atrophy and renal fibrosis through muscle-kidney crosstalk

BACKGROUND

The treatment of muscle wasting is accompanied by benefits in other organs, possibly resulting from muscle-organ crosstalk. However, how the muscle communicates with these organs is less understood. Two microRNAs (miRs), miR-23a and miR-27a, are located together in a gene cluster and regulate proteins that are involved in the atrophy process. MiR-23a/27a has been shown to reduce muscle wasting and act as an anti-fibrotic agent. We hypothesized that intramuscular injection of miR-23a/27a would counteract both muscle wasting and renal fibrosis lesions in a streptozotocin-induced diabetic model.

METHODS

We generated an adeno-associated virus (AAV) that overexpresses the miR-23a∼27a∼24-2 precursor RNA and injected it into the tibialis anterior muscle of streptozotocin-induced diabetic mice. Muscle cross-section area (immunohistology plus software measurement) and muscle function (grip strength) were used to evaluate muscle atrophy. Fibrosis-related proteins were measured by western blot to monitor renal damage. In some cases, AAV-GFP was used to mimic the miR movement in vivo, allowing us to track organ redistribution by using the Xtreme Imaging System.

RESULTS

The injection of AAV-miR-23a/27a increased the levels of miR-23a and miR-27a as well as increased phosphorylated Akt, attenuated the levels of FoxO1 and PTEN proteins, and reduced the abundance of TRIM63/MuRF1 and FBXO32/atrogin-1 in skeletal muscles. It also decreased myostatin mRNA and protein levels as well as the levels of phosphorylated pSMAD2/3. Provision of miR-23a/27a attenuates the diabetes-induced reduction of muscle cross-sectional area and muscle function. Curiously, the serum BUN of diabetic animals was reduced in mice undergoing the miR-23a/27a intervention. Renal fibrosis, evaluated by Masson trichromatic staining, was also decreased as were kidney levels of phosphorylated SMAD2/3, alpha smooth muscle actin, fibronectin, and collagen. In diabetic mice injected intramuscularly with AAV-GFP, GFP fluorescence levels in the kidneys showed linear correlation with the levels in injected muscle when examined by linear regression. Following intramuscular injection of AAV-miR-23a∼27a∼24-2, the levels of miR-23a and miR-27a in serum exosomes and kidney were significantly increased compared with samples from control virus-injected mice; however, no viral DNA was detected in the kidney.

CONCLUSIONS

We conclude that overexpression of miR-23a/27a in muscle prevents diabetes-induced muscle cachexia and attenuates renal fibrosis lesions via muscle-kidney crosstalk. Further, this crosstalk involves movement of miR potentially through muscle originated exosomes and serum distribution without movement of AAV. These results could provide new approaches for developing therapeutic strategies for diabetic nephropathy with muscle wasting.

MicroRNA-23a and MicroRNA-27a Mimic Exercise by Ameliorating CKD-Induced Muscle Atrophy

Muscle atrophy is a frequent complication of CKD, and exercise can attenuate the process. This study investigated the role of microRNA-23a (miR-23a) and miR-27a in the regulation of muscle mass in mice with CKD. These miRs are located in a gene cluster that is regulated by the transcription factor NFAT. CKD mice expressed less miR-23a in muscle than controls, and resistance exercise (muscle overload) increased the levels of miR-23a and miR-27a in CKD mice. Injection of an adeno-associated virus encoding the miR-23a/27a/24-2 precursor RNA into the tibialis anterior muscles of normal and CKD mice led to increases in mature miR-23a and miR-27a but not miR-24-2 in the muscles of both cohorts. Overexpression of miR-23a/miR-27a in CKD mice attenuated muscle loss, improved grip strength, increased the phosphorylation of Akt and FoxO1, and decreased the activation of phosphatase and tensin homolog (PTEN) and FoxO1 and the expression of TRIM63/MuRF1 and FBXO32/atrogin-1 proteins. Provision of miR-23a/miR-27a also reduced myostatin expression and downstream SMAD-2/3 signaling, decreased activation of caspase-3 and -7, and increased the expression of markers of muscle regeneration. Lastly, in silico miR target analysis and luciferase reporter assays in primary satellite cells identified PTEN and caspase-7 as targets of miR-23a and FoxO1 as a target of miR-27a in muscle. These findings provide new insights about the roles of the miR-23a/27a-24-2 cluster in CKD-induced muscle atrophy in mice and suggest a mechanism by which exercise helps to maintain muscle mass.

Chronic kidney disease induces autophagy leading to dysfunction of mitochondria in skeletal muscle

Chronic kidney disease (CKD) causes loss of lean body mass by multiple mechanisms. This study examines whether autophagy-mediated proteolysis contributes to CKD-induced muscle wasting. We tested autophagy in the muscle of CKD mice with plantaris muscle overloading to mimic resistance exercise or with acupuncture plus low-frequency electrical stimulation (Acu/LFES) treatment. In CKD muscle, Bnip3, Beclin-1, and LC3II mRNAs and proteins were increased compared with those in control muscle, indicating autophagosome-lysosome formation induction. Acu/LFES suppressed the CKD-induced upregulation of autophagy. However, overloading increased autophagy-related proteins in normal and CKD muscle. Serum from uremic mice induces autophagy formation but did not increase the myosin degradation or actin break down in cultured muscle satellite cells. We examined mitochondrial biogenesis, copy number, and ATP production in cultured myotubes, and found all three aspects to be decreased by uremic serum. Inhibition of autophagy partially reversed this decline in cultured myotubes. In CKD mice, the mitochondrial copy number, biogenesis marker peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), mitochondrial transcription factor A (TFAM), and mitochondrial fusion marker Mitofusin-2 (Mfn2) are decreased. Both muscle overloading and Acu/LFES increased mitochondrial copy number, and reversed the CKD-induced decreases in PGC-1α, TFAM, and Mfn2. We conclude that the autophagy is activated in the muscle of CKD mice. However, myofibrillar protein is not directly broken down through autophagy. Instead, CKD-induced upregulation of autophagy leads to dysfunction of mitochondria and decrease of ATP production.

Curcumin ameliorates skeletal muscle atrophy in type 1 diabetic mice by inhibiting protein ubiquitination

NEW FINDINGS

What is the central question of this study? We sought to examine whether curcumin could ameliorate skeletal muscle atrophy in diabetic mice by inhibiting protein ubiquitination, inflammatory cytokines and oxidative stress. What is the main finding and its importance? We found that curcumin ameliorated skeletal muscle atrophy in streptozotocin-induced diabetic mice by inhibiting protein ubiquitination without affecting protein synthesis. This favourable effect of curcumin was possibly due to the inhibition of inflammatory cytokines and oxidative stress. Curcumin may be beneficial for the treatment of muscle atrophy in type 1 diabetes mellitus. Skeletal muscle atrophy develops in patients with diabetes mellitus (DM), especially in type 1 DM, which is associated with chronic inflammation. Curcumin, the active ingredient of turmeric, has various biological actions, including anti-inflammatory and antioxidant properties. We hypothesized that curcumin could ameliorate skeletal muscle atrophy in mice with streptozotocin-induced type 1 DM. C57BL/6 J mice were injected with streptozotocin (200 mg kg(-1) i.p.; DM group) or vehicle (control group). Each group of mice was randomly subdivided into two groups of 10 mice each and fed a diet with or without curcumin (1500 mg kg(-1) day(-1)) for 2 weeks. There were significant decreases in body weight, skeletal muscle weight and cellular cross-sectional area of the skeletal muscle in DM mice compared with control mice, and these changes were significantly attenuated in DM+Curcumin mice without affecting plasma glucose and insulin concentrations. Ubiquitination of protein was increased in skeletal muscle from DM mice and decreased in DM+Curcumin mice. Gene expressions of muscle-specific ubiquitin E3 ligase atrogin-1/MAFbx and MuRF1 were increased in DM and inhibited in DM+Curcumin mice. Moreover, nuclear factor-κB activation, concentrations of the inflammatory cytokines tumour necrosis factor-α and interleukin-1β and oxidative stress were increased in the skeletal muscle from DM mice and inhibited in DM+Curcumin mice. Curcumin ameliorated skeletal muscle atrophy in DM mice by inhibiting protein ubiquitination, inflammatory cytokines and oxidative stress. Curcumin may be beneficial for the treatment of muscle atrophy in type 1 DM.

Acupuncture plus Low-Frequency Electrical Stimulation (Acu-LFES) Attenuates Diabetic Myopathy by Enhancing Muscle Regeneration

Mortality and morbidity are increased in patients with muscle atrophy resulting from catabolic diseases such as diabetes. At present there is no pharmacological treatment that successfully reverses muscle wasting from catabolic conditions. We hypothesized that acupuncture plus low frequency electric stimulation (Acu-LFES) would mimic the impact of exercise and prevent diabetes-induced muscle loss. Streptozotocin (STZ) was used to induce diabetes in mice. The mice were then treated with Acu-LFES for 15 minutes daily for 14 days. Acupuncture points were selected according to the WHO Standard Acupuncture Nomenclature guide. The needles were connected to an SDZ-II electronic acupuncture device delivering pulses at 20Hz and 1mA. Acu-LFES prevented soleus and EDL muscle weight loss and increased hind-limb muscle grip function in diabetic mice. Muscle regeneration capacity was significantly increased by Acu-LFES. The expression of Pax7, MyoD, myogenin and embryo myosin heavy chain (eMyHC) was significantly decreased in diabetic muscle vs. control muscle. The suppressed levels in diabetic muscle were reversed by Acu-LFES. The IGF-1 signaling pathway was also upregulated by Acu-LFES. Phosphorylation of Akt, mTOR and p70S6K were downregulated by diabetes leading to a decline in muscle mass, however, Acu-LFES countered the diabetes-induced decline. In addition, microRNA-1 and -206 were increased by Acu-LFES after 24 days of treatment. We conclude that Acu-LFES is effective in counteracting diabetes-induced skeletal muscle atrophy by increasing IGF-1 and its stimulation of muscle regeneration.

Low-frequency electrical stimulation attenuates muscle atrophy in CKD--a potential treatment strategy

Effective therapeutic strategies to treat CKD-induced muscle atrophy are urgently needed. Low-frequency electrical stimulation (LFES) may be effective in preventing muscle atrophy, because LFES is an acupuncture technique that mimics resistance exercise by inducing muscle contraction. To test this hypothesis, we treated 5/6-nephrectomized mice (CKD mice) and control mice with LFES for 15 days. LFES prevented soleus and extensor digitorum longus muscle weight loss and loss of hind-limb muscle grip in CKD mice. LFES countered the CKD-induced decline in the IGF-1 signaling pathway and led to increases in markers of protein synthesis and myogenesis and improvement in muscle protein metabolism. In control mice, we observed an acute response phase immediately after LFES, during which the expression of inflammatory cytokines (IFN-γ and IL-6) increased. Expression of the M1 macrophage marker IL-1β also increased acutely, but expression of the M2 marker arginase-1 increased 2 days after initiation of LFES, paralleling the change in IGF-1. In muscle cross-sections of LFES-treated mice, arginase-1 colocalized with IGF-1. Additionally, expression of microRNA-1 and -206, which inhibits IGF-1 translation, decreased in the acute response phase after LFES and increased at a later phase. We conclude that LFES ameliorates CKD-induced skeletal muscle atrophy by upregulation of the IGF-1 signaling pathway, which improves protein metabolism and promotes myogenesis. The upregulation of IGF-1 may be mediated by decreased expression of microRNA-1 and -206 and/or activation of M2 macrophages.

Muscle wasting from kidney failure-a model for catabolic conditions

PURPOSE

Muscle atrophy is a frequent complication of chronic kidney disease (CKD) and is associated with increased morbidity and mortality. The processes causing loss of muscle mass are also present in several catabolic conditions. Understanding the pathogenesis of CKD-induced muscle loss could lead to therapeutic interventions that prevent muscle wasting in CKD and potentially, other catabolic conditions.

MAJOR FINDINGS

Insulin or IGF-1 resistance caused by CKD, acidosis, inflammation, glucocorticoids or cancer causes defects in insulin-stimulated intracellular signaling that suppresses IRS-1 activity leading to decreased phosphorylation of Akt (p-Akt). A low p-Akt activates caspase-3 which provides muscle proteins substrates of the ubiquitin-proteasome system (UPS). A low p-Akt also leads to decreased phosphorylation of forkhead transcription factors which enter the nucleus to stimulate the expression of atrogin-1/MAFbx and MuRF1, E3 ubiquitin ligases that can be associated with proteolysis of muscle cells by the UPS. Caspase-3 also stimulates proteasome-dependent proteolysis in muscle.

SUMMARY

In CKD, diabetes, inflammatory conditions or in response to acidosis or excess glucocorticoids, insulin resistance develops, initiating reduced IRS-1/PI3K/Akt signaling. In CKD, this reduces p-Akt which stimulates muscle proteolysis by activating caspase-3 and the UPS. Second, caspase-3 cleaves actomyosin yielding substrates for the UPS and increased proteasome-mediated proteolysis. Third, p-Akt down-regulation suppresses myogenesis in CKD. Fourth, exercise in CKD stimulates insulin/IGF-1 signaling to reduce muscle atrophy. Lastly, there is evidence that microRNAs influence insulin signaling providing a potential opportunity to design therapeutic interventions. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Recent progress toward understanding the molecular mechanisms that regulate skeletal muscle mass

The maintenance of muscle mass is critical for health and issues associated with the quality of life. Over the last decade, extensive progress has been made with regard to our understanding of the molecules that regulate skeletal muscle mass. Not surprisingly, many of these molecules are intimately involved in the regulation of protein synthesis and protein degradation [e.g. the mammalian target of rapamycin (mTOR), eukaryotic initiation factor 2B (eIF2B), eukaryotic initiation factor 3f (eIF3f) and the forkhead box O (FoxO) transcription factors]. It is also becoming apparent that molecules which sense, or control, the energetic status of the cell play a key role in the regulation of muscle mass [e.g. AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor gamma coactivator-1 α (PGC1α)]. In this review we will attempt to summarize the current knowledge of how these molecules regulate skeletal muscle mass.

Muscle apoptotic response to denervation, disuse, and aging

Apoptosis is a tightly regulated biological process that plays an important role in coordinating cellular proliferation and differentiation. The pathological consequences of aberrant regulation of apoptosis have been widely demonstrated in carcinogenesis, neurodegenerative diseases, autoimmune diseases, viral infections, and acquired immunodeficiency syndrome. The study of apoptosis has been initiated in skeletal muscle biology. Consistent data have indicated the activation of apoptotic events in muscle atrophic conditions including neuromuscular diseases, muscle disuse, and sarcopenia. Although these results seem to link apoptosis to muscle atrophy, the causative role of apoptosis in this process still needs to be established. Further perspective studies are desired to reveal the precise mechanism and the exact physiologic role of apoptosis in muscle adaptation. This article aims to stimulate research into apoptosis in skeletal muscle. It reviews the apoptotic response of skeletal muscle to the atrophic conditions, namely, denervation, disuse, and aging, and discusses the proposed potential physiological links of apoptosis with muscle loss.

Mitochondrial biogenesis and fragmentation as regulators of muscle protein degradation

Mitochondria form a dynamic network that rapidly adapts to cellular energy demand. This adaptation is particularly important in skeletal muscle because of its high metabolic rate. Indeed, muscle energy level is one of the cellular checkpoints that lead either to sustained protein synthesis and growth or protein breakdown and atrophy. Mitochondrial function is affected by changes in shape, number, and localization. The dynamics that control the mitochondrial network, such as biogenesis and fusion, or fragmentation and fission, ultimately affect the signaling pathways that regulate muscle mass. Regular exercise and healthy muscles are important players in the metabolic control of human body. Indeed, a sedentary lifestyle is detrimental for muscle function and is one of the major causes of metabolic disorders such as obesity and diabetes. This article reviews the rapid progress made in the past few years regarding the role of mitochondria in the control of proteolytic systems and in the loss of muscle mass and function.

XIAP reduces muscle proteolysis induced by CKD

X-chromosome-linked inhibitor of apoptosis protein (XIAP) is an endogenous caspase inhibitor. Caspase-3 contributes to the muscle wasting associated with chronic kidney disease (CKD) and other systemic illnesses, but whether XIAP modulates muscle wasting in CKD is unknown. Here, overexpression of XIAP in cultured skeletal muscle cells decreased protein degradation induced by serum deprivation, suggesting that caspase-mediated proteolysis contributes to muscle atrophy. We generated transgenic mice that overexpress human XIAP specifically in skeletal muscle (mXIAP) and evaluated muscle protein degradation induced by CKD. mXIAP mice with normal kidney function exhibited mild skeletal muscle hypertrophy. Muscle weights of mXIAP mice with CKD (mXIAP-CKD) were indistinguishable from wild-type mice, suggesting that overexpression of XIAP in skeletal muscle protects from CKD-induced muscle atrophy. The rate of total protein degradation, proteasome chymotrypsin-like activity, and caspase-3-mediated actin cleavage all were lower in muscle isolated from mXIAP-CKD mice compared with wild-type CKD mice. Concomitant with the reduction in overall proteolysis, mRNA levels of ubiquitin, muscle-specific ring finger 1, and atrogin-1/muscle atrophy F-box were lower in mXIAP-CKD mice, suggesting that decreased expression of the ubiquitin-proteasome pathway components may contribute to the protein-sparing effects of XIAP. In summary, these results demonstrate that XIAP inhibits multiple aspects of protein degradation in skeletal muscle during CKD.

XIAP reduces muscle proteolysis induced by CKD

X-chromosome-linked inhibitor of apoptosis protein (XIAP) is an endogenous caspase inhibitor. Caspase-3 contributes to the muscle wasting associated with chronic kidney disease (CKD) and other systemic illnesses, but whether XIAP modulates muscle wasting in CKD is unknown. Here, overexpression of XIAP in cultured skeletal muscle cells decreased protein degradation induced by serum deprivation, suggesting that caspase-mediated proteolysis contributes to muscle atrophy. We generated transgenic mice that overexpress human XIAP specifically in skeletal muscle (mXIAP) and evaluated muscle protein degradation induced by CKD. mXIAP mice with normal kidney function exhibited mild skeletal muscle hypertrophy. Muscle weights of mXIAP mice with CKD (mXIAP-CKD) were indistinguishable from wild-type mice, suggesting that overexpression of XIAP in skeletal muscle protects from CKD-induced muscle atrophy. The rate of total protein degradation, proteasome chymotrypsin-like activity, and caspase-3-mediated actin cleavage all were lower in muscle isolated from mXIAP-CKD mice compared with wild-type CKD mice. Concomitant with the reduction in overall proteolysis, mRNA levels of ubiquitin, muscle-specific ring finger 1, and atrogin-1/muscle atrophy F-box were lower in mXIAP-CKD mice, suggesting that decreased expression of the ubiquitin-proteasome pathway components may contribute to the protein-sparing effects of XIAP. In summary, these results demonstrate that XIAP inhibits multiple aspects of protein degradation in skeletal muscle during CKD.

Caspase-3 cleaves specific 19 S proteasome subunits in skeletal muscle stimulating proteasome activity

With muscle wasting, caspase-3 activation and the ubiquitin-proteasome system act synergistically to increase the degradation of muscle proteins. Whether proteasome activity is also elevated in response to catabolic conditions is unknown. We find that caspase-3 increases proteasome activity in myotubes but not in myoblasts. This difference is related to the cleavage of specific 19 S proteasome subunits. In mouse muscle or myotubes, caspase-3 cleaves Rpt2 and Rpt6 increasing proteasome activity. In myoblasts, caspase-3 cleaves Rpt5 to decrease proteasome activity. To confirm the caspase-3 dependence, caspase-3 cleavage sites in Rpt2, Rpt6, or Rpt5 were mutated. This prevented the cleavage of these subunits by caspase-3 as well as the changes in proteasome activity. During differentiation of myoblasts to myotubes, there is an obligatory, transient increase in caspase-3 activity, accompanied by a corresponding increase in proteasome activity and cleavage of Rpt2 and Rpt6. Therefore, differentiation changes the proteasome type from sensitivity of Rpt5 to caspase-3 in myoblasts to sensitivity of Rpt2 and Rpt6 in myotubes. This novel mechanism identifies a feed-forward amplification that augments muscle proteolysis in catabolic conditions. Indeed, we found that in mice with a muscle wasting condition, chronic kidney disease, there was cleavage of subunits Rpt2 and Rpt6 and stimulation of proteasome activity.

Cardiac muscle protein catabolism in diabetes mellitus: activation of the ubiquitin-proteasome system by insulin deficiency

Protein degradation is increased by both insulin deficiency and insulin resistance in humans and animal models. In skeletal muscle this insulin-dependent increase in protein degradation involves activation of both caspase-3 and the ubiquitin-proteasome system. The influence of abnormal insulin signaling on protein metabolism in cardiac muscle is not well understood; therefore, we measured protein degradation in cardiac muscle of mice with streptozotocin-induced diabetes. Insulin deficiency increased both total muscle proteolysis (measured as tyrosine release in muscle slices or extracts) and the degradation of the myofibrillar protein actin (measured as the appearance of a 14-kDa actin fragment). Expression of ubiquitin mRNA and chymotrypsin-like activity in the proteasome were increased, indicating activation of the ubiquitin-proteasome system in diabetic mouse heart. We also evaluated possible signaling pathways that might regulate cardiac muscle proteolysis. Insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, and Akt phosphorylation were decreased. Insulin replacement prevented the decrease in IRS-1/Akt phosphorylation, the increase in proteolysis, and attenuated the increase in ubiquitin mRNA. We conclude that insulinopenia accelerates proteolysis in cardiac muscle by reducing IRS-1/Akt signaling, which leads to activation of the ubiquitin-proteasome proteolytic pathway.

Regulation of caspase-3 activity by insulin in skeletal muscle cells involves both PI3-kinase and MEK-1/2

A hallmark of skeletal muscle atrophy is increased activities of several proteolytic systems, including caspase-3. We have previously shown that conditions involving insulin deficiency or insulin resistance increase both overall protein degradation and caspase-3-mediated actin cleavage. In the present experiments, we examined how insulin regulates caspase-3 activity in L6 myotubes. Reducing the serum concentration in the culture media from 2 to 0.5% overnight increased caspase-3 activity and actin cleavage. Addition of insulin to proteolytically active cells attenuated both responses within 4 h. Individually, inhibitors of either phosphatidylinositide 3-kinase (PI3K) or MEK1/2 partially blocked the insulin-induced reduction in caspase-3 activity; in combination, the inhibitors completely prevented insulin from attenuating caspase-3 activity. Insulin suppressed caspase-3 activity by a complex mechanism that included direct inhibition due to an increased interaction between caspase-3 and cellular inhibitor of apoptosis-1 and indirect inhibition via phosphorylation (i.e., inactivation) of the proapoptotic protein Bad, which participates in the intrinsic (i.e., mitochondrial) apoptosis activation cascade. Unlike other cell types, the phosphorylation of Bad Ser112 was mediated by the PI3K/Akt pathway rather than the MEK/ERK/ribosomal S6 protein kinase pathway. In summary, our findings indicate that insulin regulates caspase-3 activity by a multistep process that is unique to skeletal muscle, thus providing insights about the muscle-specific nature of the atrophy process.

Signaling in muscle atrophy and hypertrophy

Muscle performance is influenced by turnover of contractile proteins. Production of new myofibrils and degradation of existing proteins is a delicate balance, which, depending on the condition, can promote muscle growth or loss. Protein synthesis and protein degradation are coordinately regulated by pathways that are influenced by mechanical stress, physical activity, availability of nutrients, and growth factors. Understanding the signaling that regulates muscle mass may provide potential therapeutic targets for the prevention and treatment of muscle wasting in metabolic and neuromuscular diseases.

Muscle wasting in diabetic and in tumor-bearing rats: role of oxidative stress

Cachexia is a debilitating syndrome characterized by body weight loss, muscle wasting, and anemia. Muscle wasting results from an altered balance between protein synthesis and degradation rates. Reactive oxygen species are indicated as crucial players in the onset of muscle protein hypercatabolism by upregulating elements of the ubiquitin-proteasome pathway. The present study has been aimed at evaluating comparatively the involvement of oxidative stress in the pathogenesis of skeletal muscle wasting in two different experimental models: rats rendered hyperglycemic by treatment with streptozotocin and rats bearing the Yoshida AH-130 ascites hepatoma. For this purpose, both tumor bearers and diabetic animals have been treated with dehydroepiandrosterone (DHEA), a multifunctional steroid endowed with multitargeted antioxidant properties. We show that diabetic rats and AH-130 rats share several features, hypoinsulinemia, occurrence of oxidative stress, and positive response to DHEA administration, although the extent of the effects of DHEA largely differs between diabetic animals and tumor-bearing rats. The hypercatabolism, evaluated in terms of proteasome activity and expression of atrogin-1 and MuRF1, is activated in AH-130 rats, whereas it is lacking in streptozotocin-treated rats. Moreover, we demonstrate that the role of oxidative stress can interfere with muscle wasting through different mechanisms, not necessarily involving NF-kappaB activation. In conclusion, the present results show that, although skeletal muscle wasting occurs in both diabetic rats and tumor-host rats, the underlying mechanisms are different. Moreover, despite oxidative stress being detectable in both experimental models, its contribution to muscle wasting is not comparable.