Skeletal muscle atrophy is a debilitating response to starvation and many systemic diseases including diabetes, cancer, and renal failure. We had proposed that a common set of transcriptional adaptations underlie the loss of muscle mass in these different states. To test this hypothesis, we used cDNA microarrays to compare the changes in content of specific mRNAs in muscles atrophying from different causes. We compared muscles from fasted mice, from rats with cancer cachexia, streptozotocin-induced diabetes mellitus, uremia induced by subtotal nephrectomy, and from pair-fed control rats. Although the content of >90% of mRNAs did not change, including those for the myofibrillar apparatus, we found a common set of genes (termed atrogins) that were induced or suppressed in muscles in these four catabolic states. Among the strongly induced genes were many involved in protein degradation, including polyubiquitins, Ub fusion proteins, the Ub ligases atrogin-1/MAFbx and MuRF-1, multiple but not all subunits of the 20S proteasome and its 19S regulator, and cathepsin L. Many genes required for ATP production and late steps in glycolysis were down-regulated, as were many transcripts for extracellular matrix proteins. Some genes not previously implicated in muscle atrophy were dramatically up-regulated (lipin, metallothionein, AMP deaminase, RNA helicase-related protein, TG interacting factor) and several growth-related mRNAs were down-regulated (P311, JUN, IGF-1-BP5). Thus, different types of muscle atrophy share a common transcriptional program that is activated in many systemic diseases.

Muscle wasting is a debilitating consequence of fasting, inactivity, cancer, and other systemic diseases that results primarily from accelerated protein degradation by the ubiquitin-proteasome pathway. To identify key factors in this process, we have used cDNA microarrays to compare normal and atrophying muscles and found a unique gene fragment that is induced more than ninefold in muscles of fasted mice. We cloned this gene, which is expressed specifically in striated muscles. Because this mRNA also markedly increases in muscles atrophying because of diabetes, cancer, and renal failure, we named it atrogin-1. It contains a functional F-box domain that binds to Skp1 and thereby to Roc1 and Cul1, the other components of SCF-type Ub-protein ligases (E3s), as well as a nuclear localization sequence and PDZ-binding domain. On fasting, atrogin-1 mRNA levels increase specifically in skeletal muscle and before atrophy occurs. Atrogin-1 is one of the few examples of an F-box protein or Ub-protein ligase (E3) expressed in a tissuespecific manner and appears to be a critical component in the enhanced proteolysis leading to muscle atrophy in diverse diseases.

We previously identified a common set of genes, termed atrogenes, whose expression is coordinately induced or suppressed in muscle during systemic wasting states (fasting, cancer cachexia, renal failure, diabetes). To determine whether this transcriptional program also functions during atrophy resulting from loss of contractile activity and whether atrogene expression correlates with the rate of muscle weight loss, we used cDNA microarrays and RT-polymerase chain reaction to analyze changes in mRNA from rat gastrocnemius during disuse atrophy induced by denervation or spinal cord isolation. Three days after Den or SI, the rate of muscle weight loss was greatest, and 78% of the atrogenes identified during systemic catabolic states were induced or repressed. Of particular interest were the large inductions of key ubiquitin ligases, atrogin-1 (35- to 44-fold) and MuRF1 (12- to 22-fold), and the suppression of PGC-1alpha and PGC-1beta coactivators (15-fold). When atrophy slowed (day 14), the expression of 92% of these atrogenes returned toward basal levels. At 28 days, the atrophy-inducing transcription factor, FoxO1, was still induced and may be important in maintaining the "atrophied" state. Thus, 1) the atrophy associated with systemic catabolic states and following disuse involves similar transcriptional adaptations; and 2) disuse atrophy proceeds through multiple phases corresponding to rapidly atrophying and atrophied muscles that involve distinct transcriptional patterns.

A robust grading system incorporating the independent prognostic significance of both BMI and %WL was developed.