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The Journal of Nutrition Vol. 128 No. 2 February 1998,
pp. 323S-327S
Endocrine Research Unit, Mayo Clinic and Foundation, Rochester, MN
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ABSTRACT |
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Abstract
Introduction
References
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Patients with insulin-dependent diabetes are in a catabolic state without insulin replacement. The mechanism of insulin's anticatabolic effect has been investigated in whole-body and regional tracer kinetic studies. Whole-body studies have demonstrated that there are increases in both protein breakdown and protein synthesis during insulin deprivation. Because the magnitude of the increase in protein breakdown is greater than the magnitude of the increase in protein synthesis, there is a net protein loss during insulin deprivation. Regional studies have shown that insulin replacement inhibits protein breakdown and synthesis in splanchnic tissue but only inhibits protein breakdown in skeletal muscle. Because the increase in protein synthesis in splanchnic tissues is greater than the increase in protein breakdown, insulin deprivation results in a net accretion of protein in the splanchnic bed. In contrast, in skeletal muscle, there is a net increase in protein breakdown during insulin deprivation, resulting in a net release of amino acids. There are no human data concerning the site of protein accretion in the splanchnic bed or the specific protein whose synthesis is increased during insulin deprivation. It appears that insulin exerts its overall anticatabolic effect in insulin-dependent diabetes mainly through the inhibition of muscle protein breakdown.
KEY WORDS: diabetes mellitus · protein metabolism
An association between diabetes mellitus and protein catabolism has been known to man for millennia. Before any of the metabolic characteristics of diabetes were known, the profound changes in body composition that occur with the onset of diabetes were recognized by the physicians of many cultures. In the ancient Sanskrit literature, diabetes mellitus was described as "honey-urine disease," associated with gross emaciation and wasting. The Greek physician Aretaeus described diabetes as a condition in which "melting of the flesh into urine" occurred. Sir William Osler, almost 100 years ago, described the disease in terms of "progressive emaciation," involving massive urinary losses of both glucose and urea. The discovery and subsequent application of insulin to the treatment of diabetes not only improved control of glucose levels but also had a profound effect on protein metabolism. The mechanism of insulin's anticatabolic effect, however, is yet to be fully elucidated. Although a role for insulin deficiency in the development of the metabolic derangements in diabetes mellitus is apparent, it has also become clear that other factors contribute to the overall diabetic state. Apart from insulin deficiency, related changes in other hormones, substrates and interactions between the two also come into play in the metabolic derangements in diabetes.
There is a relative paucity of information regarding the effects of diabetes on protein metabolism compared with our knowledge of the effect of diabetes on carbohydrate metabolism. Many of the chronic complications of diabetes involve changes in structural proteins. It is thus possible that changes in protein metabolism are responsible for many of the chronic complications of diabetes mellitus, because even a minor imbalance between protein synthesis and degradation can potentially have a profound effect over the long term on cell viability and metabolism. Alterations in protein synthesis and degradation can also adversely affect the repair of tissue after injury or infection. Changes in protein metabolism seen in diabetes have been less studied in part because of the inherent methodological difficulties in monitoring changes in protein metabolism and also because of the lack of any immediate clinical implications of acute changes in protein metabolism. By comparison, glucose levels are easy to monitor, and changes in glucose concentrations have rapid clinical effects. Because the methodology for the study of protein metabolism has been refined and the potential effect of deranged protein metabolism has been more widely appreciated, there has been an increase in focus on protein metabolism in diabetes.
Whole-body protein catabolism is the net result of increased protein breakdown, decreased protein synthesis or a combination of relative changes in both synthesis and breakdown. To investigate the mechanism of protein catabolism in insulin deficiency, it is necessary to measure protein turnover. The objective of this review is to present an overview of the current knowledge of and research into protein metabolism in insulin-dependent diabetes (IDDM).4 Information about protein metabolism in IDDM has been obtained from the study of whole-body (postabsorptive and fed state) and regional (splanchnic and cross-limb) protein dynamics as well as individual protein turnover. These methods will be discussed in turn.
The earlier studies of protein metabolism in IDDM incorporated the study of insulin's effects on whole-body protein turnover. Although whole-body protein turnover studies have some methodological constraints, whole-body protein metabolism is an important parameter to consider in understanding the mechanism of the insulin-induced fall in urinary nitrogen loss in patients with IDDM.
The effect of insulin whole-body protein turnover in the postabsorptive (fasting) state.
Protein turnover has been studied with the use of isotopically labeled amino acid tracers. The tracers most widely used for these studies have been L-[1-13C] or [1-14C] leucine (Bennet et al. 1990
The effect of insulin on whole-body protein turnover during an amino acid load.
The effects of insulin supplementation on whole-body protein dynamics during concomitant amino acid supplementation have been studied by several investigators (Bennet et al. 1990 Whole-body measurements of net protein turnover yield little information about the relative contributions of protein synthesis vs. degradation in specific tissues. There are several reasons for this. First, different tissues may have different responses to factors governing protein turnover. Insulin has been shown to have differential effects on the rates of synthesis of fibrinogen, antithrombin III, apolipoprotein B 100 and albumin (De Feo et al. 1993
There are limitations to the regional protein turnover studies discussed so far. Cross-limb and mixed muscle protein measurements represent the mean synthesis of several muscle proteins and may overlook changes in the synthesis rates of individual proteins. Insulin is known, for example, to exert a differential effect on the synthesis of hepatic proteins. The turnover of several splanchnic and muscle proteins has been investigated. De Feo and co-workers (1991) demonstrated that insulin increases the synthesis rate of albumin while decreasing the synthesis rates of fibrinogen. When the fractional synthesis rates of myosin heavy chain (MHC) protein, the principal muscle contractile protein, as well as mixed muscle protein (MMP) were recently measured in six insulin-dependent diabetic patients during insulin deprivation and during insulin treatment, acute insulin deprivation did not affect either the synthesis rate or the ratio of MHC/MMP (Charlton et al. 1997 Hormones.
The counterregulatory hormones, glucagon, growth hormone, epinephrine and cortisol, may increase during insulin deprivation. Of these, glucagon is consistently elevated during short-term insulin deprivation. Glucagon is known to play a role in glucose homeostasis and is also important in protein metabolism. Studies in healthy subjects have shown that, during insulin deficiency, glucagon increases energy expenditure (Nair 1987 Substrates.
Insulin deprivation is associated with an increase in circulating amino acids, especially the branched-chain amino acids, glucose, fatty acids and ketones. The effects of amino acid (the primary substrate for protein synthesis) availability on protein metabolism have been extensively studied. Branched-chain amino acids, especially leucine, have been shown to increase both leucine oxidation and whole-body protein synthesis while inhibiting whole-body protein breakdown (Louard et al. 1990 Insulin deficiency produces profound changes in metabolism, including whole-body protein catabolism with emaciation. The changes in metabolism and body composition that occur in IDDM are readily reversed by insulin treatment. Insulin appears to exert its anabolic effects chiefly through inhibition of muscle protein breakdown. To date, a stimulatory effect of insulin on muscle protein synthesis in the fasted state has not been shown. Across the splanchnic bed, insulin treatment is associated with decreased protein synthesis compared with insulin deficiency. Among the myriad metabolic changes that occur during insulin deficiency, increased circulating levels of glucagon are likely to contribute to the overall catabolic state of insulin deficiency. The roles of increased cytokine production, such as tumor necrosis factor- The authors are grateful for the assistance of Nancy Evans in preparing this manuscript.
INTRODUCTION
Abstract
Introduction
References
WHOLE-BODY PROTEIN TURNOVER IN IDDM
and 1991, Luzi et al. 1990, Nair 1984, Nair et al. 1987 and 1983, Pacy et al. 1989, 1991a and 1991b, Robert et al. 1985, Tessari et al. 1986 and 1990, Umpleby et al. 1986). Leucine is an essential amino acid (i.e., it is not synthesized in mammals) and comprises between 6 and 8% of the constituent proteins of the body. Leucine contains six carbon atoms. Leucine is reversibly transaminated to its ketoacid, ketoisocaproic acid (KIC), in skeletal muscle and oxidized mainly in the liver. The oxidation of KIC is irreversible and generates CO2. If the 1-carbon of leucine is labeled (with 13C or 14C) and infused as a tracer, the label will appear in CO2 breath samples. It is therefore possible to measure leucine oxidation by using 13C-KIC enrichment in plasma as its precursor. In the steady state, because the 1-carbon moiety is not synthesized in humans, dilution of labeled leucine occurs either through leucine appearing through protein breakdown (endogenous leucine flux) or through dietary leucine absorption (exogenous leucine flux). Calculations of amino acid kinetics based on leucine metabolism are performed in the steady state based on a stochastic model. If leucine is labeled with 1-13C and 15N, it is possible to measure the rate of transamination in addition to the leucine-carbon flux (Matthews et al. 1980). A summary of the results from studies investigating whole-body protein turnover with the use of these techniques is shown in Table 1.
View this table:
Table 1.
Summary of whole-body protein turnover studies in insulin-dependent diabetes mellitus (IDDM) utilizing isotopes of leucine as tracers1
, Fukagawa et al. 1985, Luck et al. 1928). It is thus possible that some of insulin's effects on protein metabolism in the above studies were due either to a direct effect of insulin or were secondary to changes in substrate (amino acid) availability.
, Flakoll et al. 1989, Inchiostro et al. 1992, Luzi et al. 1990, Matthews et al. 1980, Tessari et al. 1987). Results from these studies are consistent in showing that amino acid supplementation further decreases endogenous protein breakdown and increases leucine oxidation during insulin replacement. The effect of insulin on whole-body protein synthesis during amino acid supplementation is less clear. Three studies were performed in patients with IDDM (Bennet et al. 1991, Inchiostro et al. 1992, Luzi et al. 1990). Of these, two showed an increase in whole-body protein synthesis, as measured by nonoxidative leucine flux, with amino acid supplementation during insulin replacement (Inchiostro et al. 1992, Luzi et al. 1990), whereas the third study failed to confirm these findings (Bennet et al. 1991). A further three studies were conducted in healthy control subjects. Similarly, two of these studies indicated an increase in whole-body protein synthesis with amino acid supplementation during insulin replacement (Castellino et al. 1987, Tessari et al. 1987), and the third failed to reproduce this effect (Flakoll et al. 1989). On the basis of on the aggregate of these studies, it seems likely that the reported decrease in whole-body protein synthesis during insulin replacement in the postabsorptive state may have been largely due to diminished substrate (amino acid) availability.
REGIONAL PROTEIN METABOLISM IN IDDM
). Moreover, the synthesis rates of proteins vary in different tissues. For example, skeletal muscle protein synthesis is much slower than that of the non-skeletal muscle tissues, e.g., liver and heart (Baumann et al. 1994). On this basis, it has been estimated that skeletal muscle mass, although constituting >60% of cell mass in the body, contributes <30% to the whole-body protein synthesis. A small change in muscle protein synthesis can thus be completely negated by relatively smaller changes in the splanchnic region when whole-body measurements are done.
and 1991, Charlton et al. 1997, Nair 1984, Nair et al. 1995, Pacy et al. 1989 and 1991a, Tessari et al. 1990). In three of the leg/forearm balance studies, a decrease in whole-body protein degradation was observed, with a simultaneous decrease in skeletal muscle protein breakdown (Bennet et al. 1991, Nair et al. 1995, Tessari et al. 1990). In one study, by Pacy et al. (1991a), insulin did not inhibit muscle protein breakdown. On balance, when the data from patients with IDDM are considered together with those of healthy subjects (Denne et al. 1991, Gelfand and Barrett, 1987, McNurlan et al. 1991, Moller-Loswick et al. 1994), there is little doubt that insulin inhibits muscle protein breakdown. This effect of insulin on skeletal muscle appears to be maximally achieved at levels < 30 µU/mL (Louard et al. 1990).
View this table:
Table 2.
The effect of insulin on human skeletal muscle protein synthesis and breakdown in insulin-dependent diabetes mellitus
View larger version (45K):
[in a new window]
Fig 1.
Whole-body, muscle protein and splanchnic protein synthesis and breakdown rates in IDDM patients during insulin deprivation (I 
) and insulin treatment (I+) are shown. *Indicates a rate of synthesis or breakdown that is significantly less than during insulin deprivation (P < 0.05). Whole-body protein synthesis is higher (P < 0.01) in the insulin-deprived state than during insulin treatment, but the muscle protein synthesis rate is the same. The entire increase in whole-body protein synthesis can be accounted for by the increased synthesis of splanchnic proteins. Insulin treatment decreased protein breakdown and synthesis in the splanchnic bed, whereas it inhibited only protein breakdown in skeletal muscle. As a result, the net decline in protein loss resulted from protein conservation in skeletal muscle.
). Insulin had no effect on leg tissue protein synthesis. As a result, insulin replacement is associated with a net decrease in protein accretion across the splanchnic bed. Conversely, insulin deprivation was associated with an increase in whole-body protein synthesis. In this study, all of the changes in whole-body protein synthesis during the systemic infusion of insulin and during insulin deprivation were accounted for by changes in the splanchnic region (Fig. 1). The authors of this study hypothesized that amino acids are released by insulin-sensitive skeletal muscle during insulin deprivation and are taken up by insulin-insensitive tissues in the splanchnic bed, where they stimulate splanchnic protein synthesis. It was not clear whether the increase in splanchnic protein synthesis occurs in the intestine and/or the liver. It is also not clear how insulin exerts a differential effect on the various splanchnic proteins.
INDIVIDUAL PROTEIN TURNOVER
). The synthesis rates of these muscle proteins in patients with IDDM were not different from those of healthy control subjects.
FACTORS OTHER THAN INSULIN AFFECTING PROTEIN METABOLISM IN IDDM
), leucine oxidation, and protein breakdown (Nair et al. 1987b) and is catabolic during a protein meal (Charlton et al. 1996). The site of glucagon's catabolic action appears to be the liver. Glucagon was demonstrated in hepatic perfusion studies to increase protein degradation in hepatic parenchymal cells (Mortimore et al. 1989). Basal amounts of insulin normalize glucagon levels and thus counteract the protein catabolic effects of glucagon. However, glucagon continues to enhance leucine oxidation at circulating levels lower than those seen during insulin deficiency (Hartl et al. 1990). Catecholamines are not protein catabolic (Matthews et al. 1990). The protein catabolic effects of glucocorticoids are well established. Increases in circulating levels of cortisol within the physiologic range have been shown to increase protein breakdown and leucine oxidation (Beaufrere et al. 1989), but cortisol levels are typically unchanged during short-term insulin deficiency. Growth hormone inhibits leucine oxidation while stimulating protein synthesis but antagonizes insulin's antiproteolytic action (Horber and Haymond 1990). These actions are contradictory to those of glucagon. The relative effects (if any) of growth hormone on the protein catabolism associated with insulin deprivation have not been tested directly.
, Nair et al. 1992). Amino acid supply is likely to be critical in maintaining protein synthesis during insulin administration (Charlton et al. 1996).
). Ketoacids (
-hydroxybutyrate), however, have been shown to influence protein metabolism. Systemic infusion of -hydroxybutyrate results in decreased nitrogen loss and leucine oxidation and an increase in whole-body and skeletal muscle protein synthesis in humans (Nair et al. 1988). The infusion of nonesterified fatty acids in humans in arteriovenous difference studies inhibits protein breakdown, suggesting an overall anabolic effect (Rett et al. 1988). Nonesterified fatty acids are not known to have any effect on protein synthesis.
SUMMARY
, and lowered insulin-like growth factor-1 levels during insulin deficiency have not been established. More sophisticated techniques are required to resolve the remaining questions about the mechanism of insulin's effects on protein metabolism. These techniques should include measurements of the obligate precursors of protein synthesis, such as aminoacyl-tRNA, (or the validation of surrogate markers of precursor pool enrichment) and of the synthesis of individual proteins, such as the constituent proteins of muscle, liver proteins and intestinal mucosa. The development of this methodology is central to unraveling the nature and mechanism of the changes in protein metabolism that occur in IDDM.
ACKNOWLEDGMENT
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FOOTNOTES |
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LITERATURE CITED |
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Abstract
Introduction
References
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the effect of insulin. Diabetes (in press). This article has been cited by other articles:
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 M. J. Hamadeh and L. J. Hoffer Effect of protein restriction on sulfur amino acid catabolism in insulin-dependent diabetes mellitus Am J Physiol Endocrinol Metab, February 1, 2003; 284(2): E382 - E389. [Abstract] [Full Text] [PDF]
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Z. Liu and E. J. Barrett Human protein metabolism: its measurement and regulation Am J Physiol Endocrinol Metab, December 1, 2002; 283(6): E1105 - E1112. [Abstract] [Full Text] [PDF]
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