Proteasome production in human muscle during nutritional inhibition of myofibrillar protein degradation.

Protein undernutrition inhibits adenosine triphosphate (ATP)-dependent muscle protein degradation-a hallmark of the proteasome system. Here we report decreased myofibrillar protein degradation during dietary protein restriction without a concomitant decrease in proteasome gene expression, proteasome protein abundance, or proteasome in vivo fractional synthesis rate. Healthy human subjects consuming the average minimum adult protein requirement (0.71 g x kg(-1) fat-free mass x d(-1)) exhibited substantially lower (68%) excretion of 3-methylhistidine, an indicator of myofibrillar protein breakdown, when compared with subjects consuming an ample, American-style protein intake (1.67 g x kg(-1) fat-free mass x d(-1)). However, they displayed no difference in the expression of mRNA for proteasome subunits C2 or C3, in the content of C2 protein, or in the rate of incorporation of stable isotopically labeled l-[1-(13)C]-leucine into proteasome proteins. The results demonstrate that nutritional inhibition of myofibrillar protein degradation does not involve suppression in vivo of proteasome production in man. This suggests that other elements of the ubiquitin-proteasome system, such as ubiquitination pathways, are more important than proteasome abundance in the nutritional regulation of skeletal muscle mass.

Isoenergetic dietary protein restriction decreases myosin heavy chain IIx fraction and myosin heavy chain production in humans.

The synthesis of muscle protein is restrained during dietary protein restriction. This is widely understood to vary quantitatively with the degree of nutritional deprivation, but there has been little discussion of qualitative changes in muscle protein deriving from dietary protein restriction. We studied 14 healthy subjects in a 2-sample study. Subjects were randomly assigned to a diet providing an ample, American-style protein intake (1.67 g. kg fat-free mass(-1). d(-1)) or a diet approximating the mean minimum adult protein requirement (0.71 g. kg fat-free mass(-1). d(-1)). We found that consumption of an isoenergetic diet at the mean adult minimum protein requirement for 4 wk produced an 81% lower fractional synthesis rate of myosin heavy chain (MHC) proteins in vastus lateralis muscle than did consumption of an ample protein diet (P = 0.05). Protein deprivation altered the skeletal muscle myosin composition such that the proportion of the total myosin content represented by fast-twitch MHC IIx was 51% lower than with ample intake (P = 0.013). The steady state content of MHC IIx messenger RNA (mRNA) did not differ in subjects consuming the minimum requirement of protein, suggesting that the reduced proportion of MHC IIx arises from posttranscriptional events. A 68% lower rate of 3-methylhistidine excretion with protein restriction (P < 0.01) suggests that myofibrillar protein degradation was lower. We conclude that dietary amino acid scarcity produces a change in myosin isoform distribution via posttranscriptional mechanisms. The relative contribution of inhibited myosin synthesis and inhibited degradation to the altered myosin isoform composition remains unknown. This has implications for the mechanisms by which amino acids govern muscle protein composition in vivo, and further exploration is required.