Insulin Resistance Accelerates Muscle Protein Degradation: Activation of the Ubiquitin-Proteasome Pathway by Defects in Muscle Cell Signaling

Abstract

Conditions such as acidosis, uremia, and sepsis are characterized by insulin resistance and muscle wasting, but whether the insulin resistance associated with these disorders contributes to muscle atrophy is unclear. We examined this question in db/db mice with increased blood glucose despite high levels of plasma insulin. Compared with control littermate mice, the weights of different muscles in db/db mice and the cross-sectional areas of muscles were smaller. In muscle of db/db mice, protein degradation and activities of the major proteolytic systems, caspase-3 and the proteasome, were increased. We examined signals that could activate muscle proteolysis and found low values of both phosphatidylinositol 3 kinase (PI3K) activity and phosphorylated Akt that were related to phosphorylation of serine 307 of insulin receptor substrate-1. To assess how changes in circulating insulin and glucose affect muscle protein, we treated db/db mice with rosiglitazone. Rosiglitazone improved indices of insulin resistance and abnormalities in PI3K/Akt signaling and decreased activities of caspase-3 and the proteasome in muscle leading to suppression of proteolysis. Underlying mechanisms of proteolysis include increased glucocorticoid production, decreased circulating adiponectin, and phosphorylation of the forkhead transcription factor associated with increased expression of the E3 ubiquitin-conjugating enzymes atrogin-1/MAFbx and MuRF1. These abnormalities were also corrected by rosiglitazone. Thus, insulin resistance causes muscle wasting by mechanisms that involve suppression of PI3K/Akt signaling leading to activation of caspase-3 and the ubiquitin-proteasome proteolytic pathway causing muscle protein degradation.

Knockdown of PRAS40 inhibits insulin action via proteasome-mediated degradation of IRS1 in primary human skeletal muscle cells

“…Furthermore, increased expression of Fbxo32 and Murf1 associated with reduced activation of the PI3K/Akt pathway by insulin was described in skeletal muscle of db/db mice [42]. Interestingly, restoration of insulin sensitivity in skeletal muscle of this animal model by rosiglitazone was accompanied by reductions in the expression of Fbxo32 and Murf1 [42]. Although these observations are fully in line with our study, further studies are required to assess whether FBXO32 and MURF1 function as genuine IRS1 ligases.…”

“…In this context, multiple E3-ligases, including F-box only protein 40, CBLB, and F-box/WD repeat-containing protein 8, have been linked to IRS1 degradation in response to hyperinsulinaemia, inflammation and chronic exposure to insulin-like growth factor 1 [37][38][39][40][41]. Furthermore, increased expression of Fbxo32 and Murf1 associated with reduced activation of the PI3K/Akt pathway by insulin was described in skeletal muscle of db/db mice [42]. Interestingly, restoration of insulin sensitivity in skeletal muscle of this animal model by rosiglitazone was accompanied by reductions in the expression of Fbxo32 and Murf1 [42].…”

Amino acid metabolism and regulatory effects in aging

“…Other than the obvious complications that arise from increased insulin resistance (e.g. metabolic syndrome and type 2 diabetes), a recent study [58] suggests that insulin resistance may accelerate muscle protein degradation. Thus, studies are necessary to determine not only the minimum amount of amino acid and protein intake necessary to maintain an adequate muscle mass in aging, but also the upper limit beyond which side effects could occur.…”

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

“…The area of at least 500 individual myofibers per muscle was measured using the Micro-suite Five Biological Software (Olympus, Melville, NY, USA). 43 For a count of positive fiber numbers, we used gastrocnemius muscle that was transduced with Len-EGFP.…”

“…To measure proteasome proteolytic activity, 43 hind-limb muscles were pulverized under liquid nitrogen and homogenized in lysis buffer (Tris-HCl, pH 7.4, 50 mM, MgCl 2 5 mm, sucrose 250 mM, and freshly added dithiothreitol 2 mM and ATP 2 mM). Lysates were pelleted by centrifugation (5 min, 400 g).…”