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• Myoglobin is not a marker of accellerated muscle protein breakdown
  Samuele M Marcora
  Published on: 23 May 2006

• Published on: 23 May 2006

  Myoglobin is not a marker of accellerated muscle protein breakdown

  • Samuele M Marcora, Lecturer in Exercise Physiology

  Dear Editor,

  RA is a systemic disease and Sarzi-Puttini and colleagues have to be praised for investigating the effects of anti-TNF therapy on the circulating IGF system rather than the usual measures of disease activity, joint damage and disability. However, their conclusions are based on false assumption that serum myoglobin is a marker of the muscle catabolism characteristic of rheumatoid cachexia.

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  Dear Editor,

  RA is a systemic disease and Sarzi-Puttini and colleagues have to be praised for investigating the effects of anti-TNF therapy on the circulating IGF system rather than the usual measures of disease activity, joint damage and disability. However, their conclusions are based on false assumption that serum myoglobin is a marker of the muscle catabolism characteristic of rheumatoid cachexia.

  In my opinion the authors have confused accelerated muscle protein breakdown (a cardinal feature of cachexia) with skeletal muscle degeneration which occurs only in the relatively few patients complicated by rheumatoid myositis. In the vast majority of patients with RA muscle wasting is caused by increased muscle proteolysis and, possibly, reduced muscle protein synthesis. As correctly described in many of the references cited by Sarzi-Puttini and colleagues, these alterations in muscle protein turnover are caused by increased expression of inflammatory cytokines such as TNF, and corticosteroid treatment. Cytokines and corticosteroids can act directly on skeletal muscle tissue or indirectly through alterations in the levels and biological actions of anabolic hormones such as IGF-I or anti-anabolic factors such as myostatin. What the authors seem to ignore is that this muscle protein breakdown is an intracellular process in which the ubiquitin-proteasome system degrades muscle proteins into their constituent amino acids which are then exported in the circulation where they can be used to synthesise acute-phase proteins in the liver (the so called muscle-liver connection).

  The presence in the circulation of relatively large and intact muscle proteins such as myoglobin and creatine kinase is, on the contrary, indicative of skeletal muscle tissue damage/necrosis. This completely different process is seen in healthy people few days after unaccustomed eccentric exercise, rabdomyolisis and degenerative muscle diseases such as muscle dystrophy and myositis. In these conditions muscle atrophy is mainly caused by loss of muscle fibres while in cachexia muscle wasting is secondary to muscle fibre atrophy with no reduction in muscle fibre number. As serum myoglobin can not be used as a marker of the actions of IGF-I on muscle protein metabolism in RA patients, the concomitant presence of high serum levels of myoglobin and IGF-I in the corticosteroid-treated patients can not be interpreted as evidence of IGF-I resistance.

  Similarly, the decrease in serum IGF-I in the context of stable levels of serum myoglobin observed in the same group of RA patients when treated with adalimumab can not be interpreted as a positive effect of this anti-TNF therapy on IGF-I resistance. Therefore, the conclusions of Sarzi-Puttini and colleagues should be revised to avoid confusion among readers. Further research on the effects of IGF-I on proper measures of muscle protein metabolism are necessary to demonstrate the presence of IGF-I resistance in RA patients and the effects of anti-TNF and other treatments.

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  Conflict of Interest:
  None declared.

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