Abstract-Metabolic abnormalities develop in various chronic diseases and lead to progressive catabolism with decrements in the skeletal musculature that result in muscle atrophy. We investigated pathways of skeletal muscle proteolysis using an experimental model of chronic left-ventricular dysfunction. Skeletal muscle atrophy developed in wild-type mice 12 weeks following myocardial infarction accompanied by an increase in total protein ubiquitination and enhanced proteasome activity, activation of Foxo transcription factors, and robust induction of the ubiquitin-protein ligase atrogin-1/MAFbx. Further studies identified skeletal muscle myosin as a specific target of ubiquitin-mediated degradation in muscle atrophy. In contrast, transgenic overexpression of a local isoform of insulin-like growth factor-1 prevented muscle atrophy and increased proteasome activity, inhibited skeletal muscle activation primarily of Foxo4, and blocked the expression of atrogin-1/MAFbx. These results suggest that skeletal muscle atrophy occurs through increased activity of the ubiquitin-proteasome pathway. The inhibition of muscle atrophy by local insulin-like growth factor-1 provides a promising therapeutic avenue for the prevention of skeletal muscle wasting in chronic heart failure and potentially other chronic diseases associated with skeletal muscle atrophy. Key Words: insulin-like growth-1 Ⅲ chronic heart failure Ⅲ skeletal muscle Ⅲ atrophy Ⅲ gene expression P rogressive metabolic abnormalities leading to a catabolic state have been observed in various chronic diseases, including heart and renal failure, AIDS, and cancer. 1-3 A common characteristic is the development of specific myopathic changes and a loss of lean muscle mass. 4 In chronic heart failure, alterations in multiple anabolic and catabolic systems result in progressive catabolism leading to cardiac cachexia in advanced stages of the disease. 1,5 Early stages of chronic heart failure-related muscle atrophy correlate with reduced expression of insulin-like growth factor (IGF)-1, 6 activation of systemic, 5 and local 6 markers of inflammation, as well as increased oxidative stress. 7 However, the specific mechanisms underlying skeletal muscle dysfunction and progressive muscle atrophy in chronic heart failure are still incompletely understood.Catabolic loss of muscle protein occurs primarily through enhanced protein breakdown mediated by the ubiquitinproteasome pathway. 8 The critical regulators of protein ubiquitination are ubiquitin-protein ligases E3 that catalyze the transfer of activated ubiquitin from an ubiquitin-conjugating enzyme (E2). Individual ubiquitin ligases selectively target specific proteins for ubiquitination and exhibit tissue-specific and inducible expression patterns. 9,10 Proteins marked by chains of ubiquitin are rapidly degraded through the 26S proteasome in a ATP-dependent process that yields peptides and intact ubiquitin. 8 The crucial role of the ubiquitinproteasome pathway has been demonstrated in several models of muscle atrophy that incl...

Background— Although cellular redox balance plays an important role in mechanically induced cardiac hypertrophy, the mechanisms of regulation are incompletely defined. Because thioredoxin is a major intracellular antioxidant and can also regulate redox-dependent transcription, we explored the role of thioredoxin activity in mechanically overloaded cardiomyocytes in vitro and in vivo. Methods and Results— Overexpression of thioredoxin induced protein synthesis in cardiomyocytes (127±5% of controls, P <0.01). Overexpression of thioredoxin-interacting protein (Txnip), an endogenous thioredoxin inhibitor, reduced protein synthesis in response to mechanical strain (89±5% reduction, P <0.01), phenylephrine (80±3% reduction, P <0.01), or angiotensin II (80±4% reduction, P <0.01). In vivo, myocardial thioredoxin activity increased 3.5-fold compared with sham controls after transverse aortic constriction ( P <0.01). Aortic constriction did not change thioredoxin expression but reduced Txnip expression by 40% ( P <0.05). Gene transfer studies showed that cells that overexpress Txnip develop less hypertrophy after aortic constriction than control cells in the same animals (28.1±5.2% reduction versus noninfected cells, P <0.01). Conclusions— Thus, even though thioredoxin is an antioxidant, activation of thioredoxin participates in the development of pressure-overload cardiac hypertrophy, demonstrating the dual function of thioredoxin as both an antioxidant and a signaling protein. These results also support the emerging concept that the thioredoxin inhibitor Txnip is a critical regulator of biomechanical signaling.

Inherited mutations cause approximately 30% of all dilated cardiomyopathy cases, with autosomal dominant mutations in the LMNA gene accounting for more than one third of these. The LMNA gene encodes the nuclear envelope proteins lamins A and C, which provide structural support to the nucleus and also play critical roles in transcriptional regulation. Functional deletion of a single allele is sufficient to trigger dilated cardiomyopathy in humans and mice. However, whereas Lmna −/− mice develop severe muscular dystrophy and dilated cardiomyopathy and die by 8 weeks of age, heterozygous Lmna +/− mice have a much milder phenotype, with changes in ventricular function and morphology only becoming apparent at one year of age. Here, we studied 8 to 20 week old Lmna +/− mice and wild-type littermates in a pressure overload model to examine whether increased mechanical load can accelerate or exacerbate myocardial dysfunction in the heterozygotes. While overall survival was similar between genotypes, Lmna +/− animals had a significantly attenuated hypertrophic response to pressure overload as evidenced by reduced ventricular mass and myocyte size. Analysis of pressure-overload induced transcriptional changes suggested that the reduced hypertrophy in the Lmna +/− mice was accompanied by impaired activation of the mechanosensitive gene Egr-1. In conclusion, our findings provide further support for a critical role of lamins A and C in regulating the cellular response to mechanical stress in cardiomyocytes and demonstrate that haploinsufficiency of lamins A and C alone is sufficient to alter hypertrophic responses and cardiac function in the face of pressure overload in the heart.